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TECHNICAL

WWW.SPIE.ORG/PW16 Conferences & Courses 13–18 February 2016 BiOS EXPO 13–14 February 2016 Photonics West Exhibition 16–18 February 2016

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The Moscone Center San Francisco, California, USA

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The Moscone Center San Francisco, California, USA DATES Conferences & Courses 13–18 February 2016

Contents

9726: Solid State Lasers XXV: Technology and Devices

. . . . . . . . . . . .3

9727: Laser Resonators, Microresonators, and Beam Control XVIII

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .23

9728: Fiber Lasers XIII: Technology, Systems, and Applications

. . .38

9729: High Energy/Average Power Lasers and Intense Beam Applications IX

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

9730: Components and Packaging for Laser Systems II

. . . . . . . . . . .74

9731: Nonlinear Frequency Generation and Conversion: Materials, Devices, and Applications XV

. . . . . . . . . . . . . . . . . . 86

9732: Real-time Measurements, Rogue Events, and Emerging Applications

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98

9733: High-Power Diode Laser Technology and Applications XIV

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .106

SYMPOSIUM CHAIRS:

Guido Hennig,

Daetwyler Graphics AG (Switzerland)

Yongfeng Lu,

Univ. of Nebraska- Lincoln (USA)

SYMPOSIUM CO-CHAIRS:

Reinhart Poprawe

Fraunhofer-Institut für Lasertechnik (Germany)

Koji Sugioka

RIKEN (Japan)

9735: Laser Applications in Microelectronic and Optoelectronic Manufacturing (LAMOM) XXI

. . . . . . . . . . . . . . . . . . . . . . . . . . . . 126

9736: Laser-based Micro- and Nanoprocessing X

. . . . . . . . . . . . . . . 139

9737: Synthesis and Photonics of Nanoscale Materials XIII

. . . . . . . 155

9738: Laser 3D Manufacturing III

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .160

9739: Free-Space Laser Communication and Atmospheric Propagation XXVIII

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170

9740: Frontiers in Ultrafast Optics: Biomedical, Scientific, and Industrial Applications XVI

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181

9741: High-Power Laser Materials Processing: Lasers, Beam Delivery, Diagnostics, and Applications V

. . . . . . . . . . . . . . . . 196

SPIE is the international society for optics and photonics, a not-for-profit organization founded in 1955 to advanced light-based technologies. The Society serves nearly 225,000 constituents from approximately 150 countries, offering conferences, continuing educa tion, books, journals, and a digital library in support of interdisciplinary information ex change, professional growth, and patent precedent. SPIE provided $3.4 million in support of education and outreach programs in 2014.

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Conference 9726: Solid State Lasers XXV: Technology and Devices

Monday - Thursday 15–18 February 2016 Part of Proceedings of SPIE Vol . 9726 Solid State Lasers XXV: Technology and Devices 9726-1, Session 1

Mid-IR laser source using hollow waveguide beam combining

Ian F . Elder, Daniel H . Thorne, Robert A . Lamb, Selex ES Ltd . (United Kingdom); Richard M . Jenkins, HollowGuide Ltd . (United Kingdom) High brightness laser sources operating in the mid-infrared (wavelengths 2-5 µ m) region of the electromagnetic spectrum are of interest for a wide range of applications encompassing medical, defence and remote sensing.

A compact, rugged, low-loss beam combiner for combining four wavelengths covering the wavelength range 2.1 substrate.

µ m to 4.6 µ m has been designed, manufactured and characterised. The beam combiner consisted of a series of dichroic components integrated into a hollow waveguide optical circuit for confining, combining and directing the optical beams from four discrete waveguide input ports to a common output port. By coupling each wavelength into the fundamental mode of the waveguide circuit, common boresight of the combined wavelengths was achieved. Precision CNC milling techniques have been used to manufacture the hollow waveguide circuit and the alignment slots for the dichroic components, in a common Macor 4.2 W of combined output power from three quantum cascade lasers (QCLs) and a thulium fibre laser pumped Ho:YAG laser was demonstrated, with an overall transmission for the hollow waveguide beam combiner (HWBC) optical circuit of 93%. Propagation losses in the waveguide sections were negligible. The major contributor to the overall loss was the performance of the dichroic optics for the near diffraction-limited beams used here.

The four wavelengths were co-boresighted to better than 20 µ rad in the common output beam exiting the HWBC optical circuit. The performance of the HWBC optical circuit was measured to be insensitive to lateral and angular misalignments typical for operation over a wide temperature range.

resonator configuration, allowing for stable operation from 0-10% duty cycle. To demonstrate the possibilities, a fiber coupling by standard optics into a 200 µ m, 0.2NA GeO2-fiber has been shown. A high optical average power of 30W and energy of up to 200mJ within 300 was shown to be limited only by the fiber itself.

µ s pulses were sent through the fiber without any damage. The coupling efficiency of about 70%

9726-3, Session 1

High peak power ultrafast Cr:ZnSe oscillator and power amplifier

Evgeny Slobodchikov, IPG Photonics Corp . (United States); Logan R . Chieffo, Kevin F . Wall, Q-Peak, Inc . (United States) Q-Peak Inc. has developed a Cr:ZnSe based femtosecond oscillator – power amplifier laser operating in the 2.5 ?m region. The system generates 1 mJ per pulse at a 1-kHz repetition rate with a pulse duration of 184 fs, corresponding to a peak power of 5 GW. To the best of our knowledge this represents a record power for this spectral region. The high-peak power source utilizes a hybrid laser architecture, combining efficient fiber-laser pumping of solid state crystals. A Tm:fiber laser pumped, SEASAM initiated, Cr:ZnSe femtosecond oscillator provides a seed for chirped pulse amplification. 50-fs pulses from the oscillator are stretched in a grating pulse stretcher and then amplified in a chain consisting of a regenerative amplifier and two stages of linear amplifiers all based on Cr:ZnSe. The pump power for the amplification is provided by a Q-switched, high repetition rate, Ho:YLF laser, which in turn, is pumped by a high power Tm:fiber laser. The amplified pulses are compressed by a grating pulse compressor, resulting in 1 W of average power at a 1-kHz repetition rate. This laser system represents the state-of-the-art in short-pulse duration, pulse energy, and beam quality in this IR spectral range.

9726-2, Session 1

High brightness diode pumped Er:YAG laser system at 2.94

µ

m with nearly 1kW peak power

Manuel Messner, Arne Heinrich, Clemens Hagen, Pantec Engineering AG (Liechtenstein); Karl Unterrainer, Technische Univ . Wien (Austria) Over the last two decades, the need for diode pumped solid state (DPSS) lasers has increased drastically due to their obvious advantages of mainly stability and ruggedness, compared to conventional flash-lamp systems. Of special interest is Er:YAG as a crystal material since light at 2.94

glass cutting are now within reach.

µ m is emitted, coinciding with a major water absorption line. This allows the presented laser system to efficiently cut hard and soft biological tissue for future medical applications, and, furthermore, industrial applications like We demonstrated a monolithic high-power DPSS Er:YAG laser at 2.94

in 300 µ µ m with average output power of up to 50W and pulse energy beyond 300mJ s pulses. The high peak power of nearly 1kW is delivered in a high quality beam (M?<15), maintained over a large cooling water temperature range of 18-25°C. These results present a twofold increase in average output power and a threefold increase in pulse energy over the prior laser design as well as a decrease in laser threshold. The enabling step was an optimized FEM simulation of the thermal lensing effect resulting in an improved

9726-4, Session 1

Compact laser for high power eyesafe illumination

Chris DePriest, Nadia Baranova, Ken Stebbins, Ilya Bystryak, Michael Rayno, Rui Pu, Kevin Ezzo, Q-Peak, Inc . (United States) Q-Peak has demonstrated a novel, compact, pulsed eyesafe laser architecture operating with >15 mJ pulses at repetition rates over 150 Hz. The design leverages an end-pumped solid-state laser geometry to produce better eyesafe beam quality (<7 mm-mrad), and also provides a path to more scalable laser system architectures, covering a wide range of eyesafe applications. The laser consists of an actively Q-switched oscillator cavity which utilizes an end-pumped Nd:YAG gain medium, and a Rubidium Titanyl Phosphate (RTP) electro-optical crystal to produce pulse-widths <20 ns. The oscillator provides an effective front-end-seed to the optical parametric oscillator (OPO), which utilizes Potassium Titanyl Arsenate (KTA) in a ring-cavity OPO geometry. This OPO geometry efficiently converts pump photons into the eyesafe band with minimal degradation in beam quality, allowing this laser architecture to provide an attractive path to high-power eyesafe pulse energies compared to traditional systems utilizing Potassium Titanyl Phosphate (KTP). The laser system has been designed to fit within a volume of less than 2450 cc.

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Conference 9726: Solid State Lasers XXV: Technology and Devices

9726-5, Session 1

Er/Yb glass laser with compact mechanical Q-switch

Brian Cole, Alan D . Hays, Nathaniel Hough, John Nettleton, Lew Goldberg, U .S . Army RDECOM CERDEC NVESD (United States) Compact, low cost Q-switched lasers in the eye-safe 1.5

µ m regime are required for LADAR, range-finding and laser pointing. In this paper, we describe a compact, side-pumped, Er/Yb glass laser with a low cost mechanical Q-switch. The Q-switch uses a mirror or reflecting prism mounted on a cantilever resonant spring that is driven by a small electromagnetic coil. The electromagnetic drive principle is the same as used in haptic devices that produce a tactile response in cell phones. The mechanical Q-switch offers the advantages of negligible optical loss, low cost, and operation with a prism reflector to achieve resonant cavity stability. The demonstrated laser used a 5 mm long Er/Yb glass gain element with AR coating on one facet and a 90% reflective coating on the second facet for the cavity output coupler. The glass was side-pumped by a 940 nm, 5 mm wide diode bar generating up to 100 W peak power, and operated for a duration of 3 ms at 1-5 Hz. Laser cavity lengths were in the 25-40 mm range. Target energies of 5mJ have been realized, with pulse widths of 20-25ns, and an optical-to-optical efficiency of greater than 2%. Lasers were operated in TEM00 mode with a near-diffraction limited beam divergence of 2.5 mRad. Laser operation was explored as a function of pulse repetition frequency; and modeling was performed to understand the interplay between the thermal loading of the glass during optical pumping and the resultant time dependent thermal lensing and cavity mis-alignment.

9726-7, Session 2

High power and widely tunable Raman fiber laser around 1.7

µ

m

Yongguang Zhao, Weichao Yao, Deyuan Shen, Jiangsu Key Lab . of Advanced Laser Materials (China) Extension of laser wavelength is the direction of researcher’s consistent efforts, especially for the eye-safe band due to its unique applications in in lidar, remote sensing, guidance, and free-space optical communication. Now, widely tunable Raman fiber laser from ~1620 nm to ~1720 nm has been demonstrated using a home-made Er,Yb fiber as pump source and volume Bragg grating as wavelength selector. The output power in the whole wave-band could easily reach ten watt level, which means this scheme is an effect way to obtain high power output with high enough pump power and suitable thermal management. This Raman fiber laser would have promising applications in various fields, such as remote sensing, medicine treatment, photoacoustic imaging system for deep tissue imaging, as an effect pump source for Cr2+, or Dy3+ doped laser gain medium, etc.

9726-8, Session 2

Watt level Er:Lu2O3 and Er:Y2O3 ceramic lasers at ~2.7 ?m with optimized Er3+ concentration

Wei Zhou, Li Wang, Haitao Huang, Yongguang Zhao, Deyuan Shen, Jian Zhang, Dingyuan Tang, Jiangsu Key Lab . of Advanced Laser Material (China)

9726-6, Session 1

Comparative study of broadband, narrowband, and multi-wavelength resonant pumping of Er:YAG lasers

Haro Fritsche, DirectPhotonics Industries GmbH (Germany) and Technische Univ . Berlin (Germany); Oliver Lux, Martin Gaertner, Technische Univ . Berlin (Germany); Andreas Grohe, Wolfgang Gries, DirectPhotonics Industries GmbH (Germany); Hans Joachim Eichler, Technische Univ . Berlin (Germany) In recent years, compact diode pumped ~3-?m solid-state laser have attracted considerable attention due to their powerful laser radiation directly from handheld device without using costly IR-fibers and applications in military countermeasures, remote sensing, atmosphere pollution monitoring, medical. We have recently reported continue-wave (CW) laser operations with Er:Lu2O3 and Er:Y2O3 ceramics as gain mediums. With a simple plan parallel cavity, maximum output powers of of 610 mW and 320 mW have been achieved respectively. In some specific biomedical applications, watt level CW radiation is required. Now, a systematic investigation on a series of Er-doped Lu2O3 and Y2O3 ceramics with different dopant concentrations (CEr = 3at.%–15at.%) was presented. By designing laser resonator and selecting reasonable ceramic lengths, the maximum output powers of 1.4 W and 2.1 W have been achieved with Er:Lu2O3 and Er:Y2O3 ceramic, respectively. The corresponding slope efficiencies were 11.9% and 11.0%. The prospects for improvement in output power and lasing efficiency via further optimization in Er3+-doping concentration are considered. Resonantly pumped Er:YAG lasers at 1645 nm or 1617 nm are of interest for developing laser transmitters that are suitable for CO2 and CH4 LIDAR applications. These laser sources are characterized by a high quantum efficiency, since the corresponding pump and lasing processes involve different Stark sub-levels of the 4I13/2 and 4I15/2 electronic states. We report the characterization of resonantly pumped Er:YAG lasers that were pumped by both broadband and narrowband diode lasers at 1470 and 1532 nm. As the erbium absorption lines are very narrow, higher efficiency of the Er:YAG laser is demonstrated when using narrowband pump lasers. Further increase in efficiency was realized by utilizing a pump source at 1532 nm instead of 1470 nm due to the lower quantum defect. Here, we measured a slope efficiency of about 70 % which is comparable to fiber laser pumping. In Q-switched mode, pulse durations of about 60 ns were obtained which is ideal for LIDAR applications. In addition, two narrowband diode laser modules incorporating volume Bragg gratings were combined to a dual wavelength pump source, providing a power of 80 W out of a 200 µ m fiber. Compared to single-wavelength pumping, we observed a significant increase in output power as well as a reduction of the laser threshold. Hence, multi-wavelength pumping employing wavelength-stabilized diode lasers offers the possibility to develop a compact and highly efficient laser transmitter which meets the strict requirements of a satellite-borne methane LIDAR system in terms of weight and volume while ensuring very low power consumption.

9726-10, Session 3

100W class compact Yb:YAG single crystal fiber amplifier for femtosecond lasers without CPA

Vesna Markovic, Andreas Rohrbacher, Peter Hofmann, Wolfgang Pallmann, Simonette Pierrot, Lumentum (Switzerland); Bojan Resan, Lumentum (Switzerland) and Univ . of Applied Sciences and Arts Northwestern (Switzerland) Single crystal fibers (SCF) represent an alternative technology for ultrashort pulse amplification to high average power in a simple architecture. SCF have an aspect ratio of a short rod fiber or a thin and long crystal and benefit several advantages from the both concepts. Relatively short interaction length and large signal beam diameter mitigate the nonlinear effects and allow direct amplification of femtosecond pulses avoiding the standard

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Conference 9726: Solid State Lasers XXV: Technology and Devices

chirped pulse amplification (CPA) technique.

In this paper, we demonstrate the amplification of femtosecond pulses up to 160 W of average power using a compact and simple two-stage SCF amplifier without CPA. High brightness pumping and a double-pass signal configuration of the first stage allowed us to reach the small signal gain value of 32.5 dB, i.e. almost 2000, which is the highest reported value. Additionally, we implemented for the first time the bidirectional pumping scheme in the second stage. With the total pump power of 300 W, we achieved the highest average power of femtosecond pulses and the highest extraction efficiency from the SCF, i.e. 160 W and 42 %, respectively. The pulse duration at the maximum output power was measured to be 800 fs assuming sech2 temporal shape. The amplification details and spectral, temporal and spatial characterization of the output beam will be presented.

guided inside, thus flood–illuminating the gain medium, but the signal is in a free propagation mode inside it. Presented here are the new laser experimental results with the diode-pumped Yb:YAG SCFs fabricated by laser-heated pedestal growth (LHPG) technique. The YAG SCFs Yb-doped at 10%, 6% and 1% level with diameters of 100 µ m and 50 µ m and maximum length of 100 mm were studied. A 200-W diode laser module (DLM) was used to pump the SCFs. The DLM‘s output wavelength was tunable from 960 nm to 980 nm, and was selected for each experiment depending on the SCF length and Yb doping concentration. The absorption and emission spectrum of each fiber were characterized and the insertion loss and small signal gain of each fiber were measured. The observed difference in lasing performance between the SCF laser in a free propagation mode and the SCF laser in a waveguiding mode of operation is thoroughly discussed.

9726-11, Session 3

Yb:YAG single-crystal fiber amplifiers for picosecond lasers using divided pulse amplification technic

Fabien Lesparre, Jean-Thomas Gomes, Xavier Delen, Institut d’Optique Graduate School (France); Igor Marial, Julien Didierjean, FiberCryst S .A .S . (France); Wolfgang Pallmann, Lumentum (Switzerland); Bojan Resan, Lumentum (Switzerland) and Univ . of Applied Sciences and Arts Northwestern (Switzerland); Frederic Druon, François Balembois, Patrick Georges, Institut d’Optique Graduate School (France) Yb-doped diode-pumped solid-state lasers in MOPA configuration clearly dominate the field of high average-power ultrafast lasers. Among the geometries used so far for high-power Yb-doped DPSSL as slabs, rods and thin disks, the single-crystal-fiber technology (SCF) was recently shown to have a high potential for the amplification of high average and peak power pulses thanks to a very efficient thermal management and high optical efficiencies. We present the use of Yb:YAG SCFs for high-energy picosecond-pulse amplification. As a seed, we used a passively modelocked Yb-YAG oscillator delivering 8.5-ps pulses at 500 kHz with an average power of 25 mW at 1030 nm. The amplifier consists of two double-pass SCF stages. The first amplification stage is optimized to provide high gain. Up to 8 W are obtained after amplification in a 30 mm long 2 at. % doped Yb:YAG SCF pumped with a 100 W high brightness laser diode, corresponding to a high optical gain of 25 dB. In order to achieve higher extraction levels, a 969 nm laser diode delivering 200 W is used for the second stage. Furthermore, to reduce the peak power and prevent Kerr lensing, the divided-pulse amplification technic is implemented. It consists of creating several temporal replicas seeded in a double-pass amplifier and passively recombined into single, high power pulses. The output power extracted from the second amplifier is 45 W and the pulse energy reaches 90 µ J with a combining efficiency above 96% and a peak power of 11 MW.

9726-12, Session 3

Laser properties of LHPG-grown, diode pumped, Yb:YAG single crystal fiber

Jun Zhang, U .S . Army Research Lab . (United States); Youming Chen, U .S . Army Research Lab (United States); Mark Dubinskii, U .S . Army Research Lab . (United States); Gisele Maxwell, Shasta Crystals (United States) Single crystal fiber (SCF), as most often used in the literature today, is the term referring to a pretty thin (usually < 800 µ m dia.) air-clad single crystalline gain element in laser configuration where the pump power is

9726-13, Session 3

Cladding rare-earth doped single-crystal YAG fiber optics

Subhabrata Bera, Craig D . Nie, James A . Harrington, Rutgers, The State Univ . of New Jersey (United States); Stephen C . Rand, Ayan A . Chakrabarty, Theresa Chick, James Chapman, Stephen Trembath-Reichert, Univ . of Michigan (United States) Rare-earth doped single-crystal (SC) Yttrium Aluminum Garnet (YAG) fibers are excellent candidates for high power lasers. These SC fiber optics combine the favorable low Stimulated Brillouin Scattering (SBS) gain coefficient and excellent thermal properties to make them an attractive alternative to glass fiber lasers and amplifiers. Various rare-earth doped SC fibers have been grown using the laser heated pedestal growth (LHPG) technique. Several cladding methods, including in-situ and post-growth cladding techniques, are discussed in this paper. A rod-in-tube approach has been used by to grow a fiber with an Erbium doped SC YAG fiber core inserted in a SC YAG tube. The result is a radial gradient in the distribution of rare-earth ions. Post cladding methods include sol-gel deposited polycrystalline cladding and the growth of a crystalline cladding layer using a hydrothermal technique.

9726-14, Session 3

High power Nd:YAG single crystal fiber laser emitting at 1064 nm

Zhaojun Liu, Sasa Zhang, Xingyu Zhang, Yang Liu, Zhenhua Cong, Shandong Univ . (China) High power continuous wave Nd:YAG single crystal fiber laser is demonstrated with an output wavelength of 1064 nm. A continuous wave 808-nm diode was employed as the end pumping source. The gain medium was a Nd:YAG crystal fiber (0.2 at.% Nd-doped) with a diameter of 1 mm and a length of 50 mm. It was welded in copper that was water-cooled with the temperature setting at 18 oC. The beam waist diameter of the pumping wave was 800 um inside the crystal fiber. We studied the absorption rate of the crystal fiber with the highest incident diode power of 162 W. Several input mirrors and output couplers with different radius of curvature (ROC) were examined for pursuit of higher output power. Also output couplers with different transmissions were tried. The resonating cavity was also simulated with LASCAD. As a result, the highest output power of up to 72.3 W was obtained from a concave-concave cavity. The input mirror was a plano-concave mirror with a ROC of 500 mm. The output coupler was also a plano-concave mirror with a ROC of 100 mm. It had a transmission of 50% at 1064 nm. The optical to optical conversion efficiency was up to 61.8% with an absorbed diode power of 117 W. The beam quality factors (M2) were determined to be 20 and 22 in horizontal and vertical directions, respectively. Through intracavity frequency doubling by KTiOPO4 crystal, we also obtained 9.8 W of 532 nm green laser.

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Conference 9726: Solid State Lasers XXV: Technology and Devices

9726-15, Session 3

Micro-pulling-down furnace modification and single crystal fibers growth

Dongsheng Yuan, Zhitai Jia, Yang Li, Baiyi Wu, Xutang Tao, Shandong Univ . (China)

9726-17, Session 4

Ruggedized SLM Nd:YAG laser for airborne Doppler lidar applications

Wade Collins, Ryan Feeler, Faming Xu, Mark Kushina, Northrop Grumman Cutting Edge Optronics (United States) Single crystal fiber (SCF) combines the excellent instinct properties of conventional bulk laser crystals, and the special geometry advantage of active optical fibers. YAG and LuAG belong to cubic crystal structure, with good thermal conductivity of 10.7 and 8 W·m?1·K?1, respectively. Despite lower thermal conductivity, LuAG crystal is easier to obtain homogeneous optical quality. Both of them are proper host candidates for single crystal fiber. Recently there are basically two different methods for fabricating crystal fibers: laser heated pedestal growth (LHPG) and micro-pulling down (?-PD). In comparison with LHPG, ?-PD has much smaller thermal gradient, and here we use ?-PD to carry out high quality SCFs. Through the ?-PD setup manufactured by ourselves, crystal fibers with diameters of ?1 mm and ?800 ?m have been grown successfully. Moreover, some furnace modifications were made to realize the constant crystal diameter for thinner fibers. The crystalline quality and homogeneity along the whole fiber were investigated. The influences of different growth conditions on the crystal fibers will also be discussed.

9726-16, Session 4

Development of a conductively-cooled, tripled-pulsed 2-micron solid-state laser for simultaneous and independent measurements of water vapor and carbon dioxide from an airborne platform

Upendra N . Singh, Mulugeta Petros, Tamer F . Refaat, NASA Langley Research Ctr . (United States); Hyung R . Lee, Karl D . Reithmaier, Science System & Applications, Inc . (United States); Charles W . Antill Jr ., Jirong Yu, NASA Langley Research Ctr . (United States) Water vapor and carbon dioxide are the most dominant greenhouse gases directly contributing to the Earth’s radiation budget and global warming. Researchers at NASA Langley Research Center are developing a state of-the-art fully conductively-cooled, end-pumped, triple-pulse 2-micron Ho:Tm:YLF laser transmitter, as part of the integrated path differential absorption (IPDA) lidar system, for simultaneous and independent monitoring of atmospheric water vapor and carbon dioxide column from an airborne platform. The Ho:Tm:YLF crystal is chosen for its capability to produce a succession of Q-switched pulses from a single pump pulse. For 50 Hz pump pulse repetition rate, the 2 ?m triple-pulse rate is equivalent to 150 Hz. The benefit of generating multiple pulses is facilitated by the combination of long thulium (Tm) upper lifetime and the co-doping of Tm and Holmium (Ho) in YLF. The 792 nm pump laser excites the Tm from 3H6 to 3H4 level. The 3F4, which pumps the Ho, gets two atoms excitation for every excited atom that decays from the 3H4 level. This “two for one” efficiency is one of the features that make this crystal attractive for triple-pulsing. The 2 ?m laser is produced by the Ho 5I7 to 5I8 transition pumped by Tm. The long lifetime of Tm results in the repopulation of Ho after every Q-switch pulse. The energy for the second and third pulse is then extracted by simply reopening the Q-switch every 200 microseconds after the first Q-switch pulse. For a single pump pulse of 5 milliseconds, the laser is targeted to produce three successive injection locked pulses of three different wavelengths, separated by 200 microseconds, with respective pulse energy of 50, 15, and 5 mJ at 50 Hz. Detail design and performance of an end-pumped, tripled-pulsed, injection seeded, conductively-cooled 2-micron Ho:Tm:YLF laser will be presented. We describe the design, development and testing of an injection seeded, pulsed, diode pumped, third harmonic Nd:YAG laser that is ruggedized for high energy Doppler LIDAR applications from ground based and airborne platforms. The laser described is specifically designed to provide stable Single Longitudinal Mode (SLM) performance while operating in challenging thermal and vibration environments. Test data is shown of the third harmonic (355 nm) output of > 65 mJ per pulse, operating at 30 Hz, while undergoing thermal and vibrational environmental testing. Data is also shown to support the scalability of this laser technology to 355 nm pulse energies > 100 mJ at laser repetition rates of > 100 Hz.

9726-18, Session 4

Narrow linewidth UV laser transmitter for ozone dial remote sensing application

Ti Chuang, Joe Hansell, Tim Shuman, Tom Schum, Kent Puffenberger, Ralph Burnham, Fibertek, Inc . (United States) Fibertek has demonstrated a dual-wavelength narrow linewidth UV laser source for NASA airborne ozone DIAL remote sensing application (Global Ozone Lidar Demonstrator). The application requires two narrow linewidth lasers in the UV region between 300 nm and 320 nm with at least 12 nm separation between the two wavelengths. Each UV laser was based on a novel ring structure incorporating an optical parametric oscillator (OPO) and a sum frequency generator (SFG). The fundamental pump source of the UV laser was a single frequency 532 nm laser, which was frequency doubled from a diode-pumped, injection-seeded single frequency Nd:YAG laser operating at 1064 nm and 50 Hz repetition rate. The ring frequency converters generated UV wavelengths at 304 nm and 316 nm respectively. The demonstrated output energies were 2.6 mJ for 304 nm and 2.3 mJ for 316 nm UV lines, with rooms to potentially achieve more energy for each laser. Linewidth narrowing was achieved using a volume Bragg grating as the output coupler of the OPO in each ring oscillator. We obtained spectral linewidths (FWHM) of 0.12 nm for the 304 nm line and 0.1 nm for the 316 nm line. Fibertek is now building an airborne DIAL transmitter based on the reported demonstration, which is a single optical module with dual wavelength output at the demonstrated wavelengths. NASA plans to field the DIAL transmitter as a key component of the Global Ozone Lidar Demonstrator high altitude airborne instrument to perform autonomous global ozone DIAL remote sensing field campaigns.

9726-19, Session 4

Diode-pumped alexandrite ring laser for lidar applications

Alexander Munk, Bernd Jungbluth, Michael Strotkamp, Hans-Dieter Hoffmann, Reinhart Poprawe, Fraunhofer Institut für Lasertechnik (Germany); Josef Höffner, Leibniz Institut für Atmosphärenphysik e .V . (Germany) In recent years scientific climate investigations have gained increasing importance. To validate numerical simulations of the Earth’s climate, temperature distributions in the atmosphere at altitudes between 80 and 110 km have to be investigated. Lidarsystems based on flashlamp pumped Alexandrite ring lasers in single frequency operation represent

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9726-20, Session 4

Conference 9726: Solid State Lasers XXV: Technology and Devices

a powerful source for these investigations but provide low efficiency and high maintenance effort. To overcome these drawbacks diode-pumped Alexandrite lasers are required.

The 7 mm-long Alexandrite crystal was longitudinally pumped by a commercial diode bar module in the red spectral region and the resulting thermal lens was investigated at different operating points. A linear resonator was designed for operation in quasi-cw mode at 35 Hz and yielded to 4 mJ pulse burst energy at 770 nm with an optical efficiency of 21%. The wavelength could be tuned via temperature tuning of the Alexandrite crystal. In Q-switched operation 0.8 mJ pulse energy were demonstrated at 770 nm and 35 Hz with a pulse duration of 300 ns. Based on the results of the linear cavity a diode-pumped Alexandrite ring laser was realized. Two diode modules were used to longitudinally pump two Alexandrite crystals which were implemented in one oscillator. The ring laser was operated at 100 Hz in quasi-cw operation and 6.2 mJ pulse burst energy could be demonstrated at 770 nm. The resulting optical efficiency was 15%. Further experiments should demonstrate Q-switched, unidirectional and single frequency operation of the Alexandrite ring laser. Power scaling can be carried out by subsequent amplification.

Laser transmitter design and performance for the slope imaging multi-polarization photon-counting lidar (SIMPL) instrument

Anthony W . Yu, David J . Harding, Philip W . Dabney, NASA Goddard Space Flight Ctr . (United States)

9726-21, Session 4

A single-frequency double-pulse Ho:YLF laser for CO2-lidar

Philipp Kucirek, Ansgar Meissner, Patrick Eiselt, Marco Höfer, Hans-Dieter Hoffmann, Fraunhofer-Institut für Lasertechnik (Germany) For measuring trace-gas abundances with the differential absorption lidar technique a pair of single-frequency laser pulses with highly stable emission wavelengths adjusted to the specific trace-gas of interest is required. In order to reduce mass and volume requirements for the laser beam source for a prospective space-borne system it is advantageous if the required pair of laser pulses is generated in only one optical assembly. For CO2 a detection wavelength of 2051 nm can be used, but the requirements on spectral stability are very challenging.

A q-switched Ho:YLF laser oscillator with a bow-tie ring resonator, specifically designed for high-spectral stability, is reported. It is pumped with gain-switched pulses from a Tm:YLF laser at 1.9 a temporal separation of 750 µ 1.5 MHz over 2 seconds of integration time.

µ m. The ramp-and-fire method with a DFB-diode laser as a reference is employed for generating single-frequency emission. With a repetition rate of 50 Hz, pulse pairs with s are produced. The measured pulse energy is 2 mJ and the measured pulse duration is 15 ns for each of the two pulses in the burst. The standard deviation of the emission wavelength of the laser pulses of 2051 nm is measured with the heterodyne technique to be below Future work will be in further increasing the spectral stability of the laser pulses and in building up an INNOSLAB pulse amplifier for pulse energy scaling towards 40 mJ (online pulse) and 15 mJ (offline pulse).

The Slope Imaging Multi-polarization Photon-counting Lidar (SIMPL) instrument is a polarimetric, two-color, multi-beam push broom laser altimeter developed through the NASA Earth Science Technology Office Instrument Incubator Program and has been flown successfully on multiple airborne platforms since 2008. The SIMPL transmitter is based on a high repetition rate (~12 kHz), short-pulse (~1 ns), linearly-polarized microchip laser centered at 1064 nm. Part of the near infrared (NIR) beam at 1064 nm is frequency doubled to 532 nm (Green). Each of the NIR and Green beams is split into four push-broom beams. The output of the SIMPL instrument has 4 linearly-polarized beams each with co-aligned wavelengths in order to have co-incident NIR and Green footprints at the surface. A KTP crystal is used for the frequency doubling with temperature control used to vary conversion efficiency between 37% and 6% in order to equalize the signal strength of the two wavelengths as a function of surface type. A dichroic filter is used to separate the two colors into individual beam paths. In each of the beam paths, two tandem calcite crystals with different lengths and waveplates are used to separate the single input beam into four equally spaced beams. An array of half waveplates are used after the last calcite crystal to correct the alternating polarization of the four output beams so they have the same state of polarization (SOP). The NIR and Green beams are then recombined so they are co-aligned with the same SOP before entering a lens array for beam shaping and divergence control. A recent addition for this laser transmitter is a transmit echo pulse (TEP) feature for each color to quantify the pulse shape and amplitude for use in correcting received signal range biases. A receiver dichroic divides the two wavelengths into separate paths which are further divided using polarizing beam splitting cubes into signals parallel and perpendicular to the transmit beam. Sixteen single-photon counting modules (SPCM) and timing electronics with 0.1 nsec precision are used to determine range to the target for the four color/polarization states on the four beams. The short pulse width and high timing precision achieves an 8 cm range precision per single detected photon. Upon aggregation of the signal photons into a range histogram a measurement with a few cm resolution of pulse broadening is achieved. The broadening is due to surface slope and roughness within the footprint and light transmission into the target resulting in volume scattering. In this talk we will discuss the laser transmitter performance and present recent science data collected over the Greenland ice sheet and sea ice in support of the NASA Ice Cloud and land Elevation Satellite 2 (ICESat-2) mission to be launched in 2017.

9726-22, Session 4

Post-flight test results of diode laser bar subjected to space exposure

Narasimha S . Prasad, NASA Langley Research Ctr . (United States) This paper discusses the recently completed post-flight test results of a G-Stack diode laser unit that was sent on NASA’s MISSE 7 mission. The 5 bar G stack operating around 808 nm with 1 kW survived the harsh space environment with some reduction in performance due to contamination. The objective of the Materials International Space Station Experiment (MISSE) is to study the performance of novel materials when subjected to the synergistic effects of the harsh space environment for several months. MISSE missions provided an opportunity for developing space qualifiable materials. This laser stack was a part of lidar component package on the MISSE 7 box that was transported to the international space station (ISS) via STS 129 and returned to the Earth via STS 134. The STS 129 mission was launched on Nov 16, 2009 and the MISSE 7 package was brought back to the earth via the STS 134 that landed on June 1, 2011. This package that was in space environment for more than one and a half year included fiber laser, solid-state laser gain materials, coherent receiver, and semiconductor laser diode. The post-flight testing of several MISSE 7 materials that were recently returned back after more than one year of exposure on the International Space Station (ISS) is underway. This paper will present the comparison of pre-flight and post-flight performance characteristics and discuss the effect of space exposure as well as contamination on the diode laser stack.

9726-53, Session 4

Demonstration of a 500 mJ InnoSlab amplifier for future lidar applications

Jens Löhring, Michael Strotkamp, Florian Elsen, Raphael Kasemann, Jürgen Klein, Martin Traub, Gerd Kochem,

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Conference 9726: Solid State Lasers XXV: Technology and Devices

Ansgar Meisssner, Marco Höfer, Hans-Dieter Hoffmann, Fraunhofer-Institut für Lasertechnik (Germany) discuss the overall design of the laser system, the current status, and further applications of this laser.

In the field of atmospheric research lidar is a powerful technology to measure remotely different parameters like gas or aerosol concentrations, wind speed or temperature profiles. For global coverage spaceborne systems are advantageous. To achieve highly accurate measurements over long distances high pulse energies are required.

A Nd:YAG-MOPA system consisting of a stable oscillator and two subsequent InnoSlab-based amplifier stages was designed and built as a breadboard demonstrator. Overall, more than 500 mJ of pulse energy at 100 Hz pulse repetition frequency at about 30 ns pulse duration in single longitudinal mode was demonstrated. Being seeded with 75 mJ pulses, the 2nd amplifier stage achieved an optical efficiency (pump energy to extracted energy) of more than 23% at excellent beam quality.

Recently, different MOPA systems comprising a single InnoSlab amplifier stage in the 100 mJ regime were designed and built for current and future airborne and spaceborne lidar missions. Amplification factors of about 10 at optical efficiencies of about 23% were achieved. In order to address the 500 mJ regime the established InnoSlab design was scaled geometrically in a straight forward way. Hereby, the basic design properties like stored energy densities, fluences and thermal load densities could be retained. The InnoSlab concept has demonstrated the potential to fulfill the strong requirements of spaceborne instruments concerning high efficiency at low optical loads, excellent beam quality at low system complexity. Therefore, it was chosen as baseline concept for the MERLIN mission, currently in phase B.

9726-24, Session 5

Megawatt-level peak-power from a passively Q-switched hybrid fiber-bulk amplifier and its applications

Axel Reiser, Juraj Bdzoch, Sven Höfer, Sina Riecke, Daniel Seitz, Nicolas Kugler, Peter Genter, ROFIN-SINAR Laser GmbH (Germany) A novel laser system with optical parameters that fill the gap between Q-switched and modelocked lasers has been developed. It consists of a high gain hybrid fiber-bulk amplifier seeded by a low power SESAM Q-switched oscillator. The mW level output power of the seed oscillator is preamplified by a single mode fiber which is limited by SRS effects. The final amplification stage is realized by two, longitudinal pumped, Nd:YVO4 crystals in a double pass setup.

This MOPA configuration delivers sub-300ps pulses at repetition rates up to 1 MHz with an output power exceeding 60 Watt. Nonlinear frequency conversion to 532nm and 355nm is achieved with efficiencies of >75% and >45%, respectively. Due to the high peak power, high repetition rate and high beam quality of this system, applications formerly only addressable at lower pulse repetition frequencies or with complex modelocked laser systems are now possible with high speed and lower cost of ownership. Application results that take benefit of these new laser parameters will be shown.

Furthermore, the reduction of the pulse duration to sub-100ps and power scaling to output powers >100 Watt by the use of the Innoslab concept are being presented.

9726-23, Session 5

100-J UV laser for dynamic compression research

Jason S . Zweiback, Logos Technologies, Inc . (United States); Scott Fochs, Univ . of Rochester (United States); Jake Bromage, Douglas Broege, Robert Cuffney, Laboratory for Laser Energetics, University of Rochester (United States); Zachary Currier, Logos Technologies Inc (United States); Christopher Dorrer, Laboratory for Laser Energetics, University of Rochester (United States); Brian Ehrich, Univ . of Rochester (United States); Jim Engler, Logos Technologies Inc (United States); Mark Guardalben, Laboratory for Laser Energetics, University of Rochester (United States); Nick Kephalos, Logos Technologies Inc (United States); John Marozas, Richard Roides, Laboratory for Laser Energetics, University of Rochester (United States); Jon Zuegel, Univ . of Rochester (United States) A 100 J, 351 nm laser is under construction for the Dynamic Compression Sector located at the Advanced Photon Source. This laser will drive shocks in solid-state materials which will be probed by picosecond x-ray pulses available from the synchrotron source. Using proven technology, the laser is designed for reliability and ease of use. A state-of-the-art fiber front end provides pulse lengths up to 20 ns with pulse shapes tailored to optimize shock trajectories. A diode-pumped Nd:glass regenerative amplifier is followed by a four-pass, flash-lamp-pumped rod amplifier. The regenerative amplifier is designed to produce up to 20 mJ with high stability. The final amplifier uses a six-pass 15-cm Nd:glass disk amplifier based on an OMEGA laser design. A KDP Type-II/Type-II frequency tripler configuration converts the 1053-nm laser output to a wavelength of 351 nm and the ultraviolet beam is image relayed to the target chamber. Smoothing by spectral dispersion and polarization smoothing have been optimized to produce uniform shocks in the materials to be tested. Modeling shows that better than 6% RMS uniformity should be achieved in a 500 mm diameter (FWHM) far-field spot. Custom control software collects all diagnostic information and provides a central control for all aspects of laser operation. We will

9726-25, Session 5

High energy pulsewidth tunable CPA free picosecond source

Julien Pouysegur, Florent Guichard, Institut d’Optique Graduate School (France); Yoann Zaouter, Amplitude Systèmes (France); Marc Hanna, Frédéric Druon, Institut d’Optique Graduate School (France); Clemens Hönninger, Eric Mottay, Amplitude Systèmes (France); Patrick Georges, Institut d’Optique Graduate School (France) Work on a Fourier transform limited picosecond source composed of a seeder system tunable in pulse duration and a hybrid fiber – bulk amplifier able to provide large gain, high pulse energy, and high average power is presented. Spectral compression effect induced by self-phase modulation (SPM) in an optical fiber is exploited to obtain the pulsewidth tunability. To increase pulse energy beyond self-focusing threshold both in the fiber amplifier and Yb:YAG amplifier, Divided Pulse Amplification architecture is investigated. This had led to a source able to delivering 3ps 350 ps), energy (10 – 350 µ particularly versatile source. A maximum pulse energy of 720 hundred the output peak power of fiber systems available.

µ µ J pulses at 50 kHz of repetition rate, corresponding to an average power of 17.5W. These performances are obtained in a compact and robust in a CPA free setup, and can be easily adjusted in a large range of pulsewidth (3 – 20 J), and average power (10 – 50 W), making it a J has been demonstrated with 20ps pulse duration without any use of divided pulse scheme, corresponding to a peak power of 36MW per pulse. 116MW peak power has been made by reducing the pulse duration down to 3ps together with employment of divided pulse scheme generating 4 temporal replicas. This hybrid configuration is well useful to improve by a factor of one

8 SPIE Photonics West 2016 · www.spie.org/pw

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9726-26, Session 5

Conference 9726: Solid State Lasers XXV: Technology and Devices Multi-joule lasers operating at 60+ Hz for industrial and scientific applications

Ryan Feeler, Chris Briggs, Wade Collins, Jay Doster, Faming Xu, Northrop Grumman Cutting Edge Optronics (United States) A new generation of diode-pumped solid-state lasers has been developed that enables operation at high energies (1-10 J/pulse) and high repetition rates (tens to hundreds of Hz). Data for two separate lasers operating at 60Hz is presented – one a 1.2J, 532nm laser and the other a 1.5J, 1064nm, single longitudinal mode (SLM) laser. Data for a 10J, 1053nm, 20Hz, SLM system is also presented. The impact of wavelength seeding and oscillator design on the final output parameter of the laser is discussed.

A discussion of the suitability of these lasers in their applications (Raman spectroscopy, material inspection, and laser peening) is also discussed. Special attention is paid to the design of the relay imaging system in these lasers, in order to generate the flat-top output beams that are required by these applications. Design details are presented and the path to higher pulse energies and repetition rates is discussed. Life test data for the pump diodes is also presented, demonstrating an expected operating lifetime in excess of 10 billion pulses before replacement diodes are required.

9726-27, Session 5

A compact solid state laser

Bhabana Pati, Eric D . Park, Kenneth Stebbins, Q-Peak, Inc . (United States) Compact, lightweight, and efficient lasers are essential for space based and unmanned aerial vehicle (UAV) applications where there are constraints on size weight and power (SWaP). We have developed a compact, passively Q-switched, intra-cavity frequency doubled Nd:YLF laser that produces 1-mJ of energy in a 10-ns pulse at a 1-30 Hz repetition rate. In order to obtain a high energy per pulse at repetition rates ~ 30 Hz, we chose Nd:YLF as the laser material as it has a longer upper state lifetime compared with the more common material, Nd:YAG. The laser is side-pumped by a semiconductor laser and passively Q-switched by a saturable absorber. A KTP crystal is placed inside the laser resonator to double the laser frequency to generate green light at 523 nm. The resonator was optimized to obtain near diffraction limited beam. The overall volume of the laser head is < 8 cm3 and the weight is < 80 gm. The unique pumping geometry makes the laser power insensitive to the temperature over +/- 5 °C. The laser head is designed to be insensitive to mechanical or thermal misalignment.

Laser Science (Germany); Franz X . Kärtner, Deutsches Elektronen-Synchrotron (Germany) and The Hamburg Ctr . for Ultrafast Imaging (Germany) and Ctr . for Free-Electron Laser Science (Germany); Ingmar Hartl, Deutsches Elektronen-Synchrotron (Germany); R . J . Dwayne Miller, Max-Planck-Institut für Struktur und Dynamik der Materie (Germany) and Ctr . for Free-Electron Laser Science (Germany) and The Hamburg Ctr . for Ultrafast Imaging (Germany) We present, to the best of our knowledge, record high output pulse energies from Ho:YLF regenerative amplifier (RA) seeded by a Ho:Fiber oscillator. The system delivers pulse energies of above 10 mJ at 100 Hz and above 4 mJ at 1 kHz, with less than 1.5 % rms fluctuation. The output spectrum of the pulses supports a Fourier limited duration of 2.8 ps. This laser system is ideally suited for ultrafast Mid-IR OPCPAs, high energy THz generation and strong-field experiments.

Due to the high energy storage capacity and long upper state life time of Ho:YLF, Ho:YLF RAs are highly susceptible to show pulse instability, in the form of bi- and multifurcation, when seeded with low energy seed pulses. We show both theoretically and experimentally that apart from a first operation region, a second operation point exists at a larger round trip number exhibiting almost one order of magnitude less pulse fluctuations. This operation point is, to the best of our knowledge, demonstrated experimentally for the first time.

The experimental results are supported by simulations that reveal this system behavior under the conditions of a high gain build-up and high gain depletion for consecutive pump and amplification cycles. Here, the effects of seed and pump noise are negligible or significantly suppressed, respectively.

Operation at high pump powers and high pulse energies require a careful system design. We will also discuss general design guidelines for operation at the second stability point to suppress pulse instabilities without sacrificing high pulse energy.

9726-29, Session 6

High-gain, high-energy picosecond Nd:YVO4 amplifier end-pumped at 880 nm

Xavier Delen, Institut d’Optique Graduate School (France); Loic Deyra, ALPhANOV (France) and Spark Lasers (France); Simon Salort, ALPhANOV (France); Pascal Dupriez, ALPhANOV (France) and Spark Lasers (France); François Balembois, Patrick Georges, Institut d’Optique Graduate School (France)

9726-28, Session 5

High energetic and highly stable pulses from a Ho:YLF regenerative amplifier

Peter Kroetz, Max-Planck-Institut für Struktur und Dynamik der Materie (Germany) and Deutsches Elektronen-Synchrotron (Germany); Axel Ruehl, Deutsches Elektronen-Synchrotron (Germany); Anne-Laure Calendron, Huseyin Cankaya, Deutsches Elektronen Synchrotron (Germany) and Ctr . for Free-Electron Laser Science (Germany) and The Hamburg Ctr . for Ultrafast Imaging (Germany); Krishna Murari, Deutsches Elektronen Synchrotron (Germany) and The Hamburg Ctr . for Ultrafast Imaging (Germany) and Univ . Hamburg (Germany); Gourab Chatterjee, Max-Planck-Institut für Struktur und Dynamik der Materie (Germany) and Ctr . for Free-Electron We demonstrate a high-gain, high-energy bulk amplifier in picosecond regime. The seed laser system is carefully optimized in order to obtained a central wavelength at 1064.3 nm and a spectral linewidth of 0.33 nm ideally matching the main emission line of Nd:YVO4. The fiber-based seed laser emits pulses with an energy of 5 nJ at repetition rates ranging from 1 kHz to 10 MHz and a pulse duration of 7 ps. The gain medium is a 20 mm long 0.2 at. % doped Nd:YVO4 crystal longitudinally pumped with a high brightness, VBG locked 60 W fiber coupled laser diode emitting at 880 nm. The amplifier is operated in a double pass configuration using a Faraday rotator to extract the signal beam after the second pass. Different pumping schemes are compared experimentally showing that two sides pumping results in higher output power than one side pumping. The output power of the amplifier ranges from 14.4 W to 25.5 W for seed powers between 20 µ W to 56 mW. For a repetition rate of 200 kHz, the output energy reaches up to 93 µ J with a pulse duration of 8 ps which gives a peak power of 11 MW. It corresponds to an output power of 18.6 W and a gain of 42 dB. The influence of the pumping scheme on the peak power limitation due to self phase modulation is also studied. This source is well suited to meet specific requirements of micro-machining applications.

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Conference 9726: Solid State Lasers XXV: Technology and Devices

9726-30, Session 6

VCSEL-pumped passively Q-switched monolithic solid-state lasers

Robert van Leeuwen, Bing Xu, Tong Chen, Qing Wang, Jean-Francois Seurin, Princeton Optronics, Inc . (United States); Guoyang Xu, Delai Zhou, Princeton Optronics Inc (United States); Chuni L . Ghosh, Princeton Optronics, Inc . (United States) stage Nd:YVO4 -amplifier up to the microjoule level ( up to >2 µ compact setup.

µ J pulse energy). We have demonstrated a simple laser system which generates J-level, sub-10-ps pulses with a repetition rate tuneable from 0.2 to 1.4 MHz without active switching components and which can be integrated in a very

9726-32, Session 6

Efficient broadband TW level OPCPA pumped by a rectangular pulse

Yuriy Stepanenko, Pawel Wnuk, Tomasz Kardas, Michal Nejbauer, Czeslaw Radzewicz, Univ . of Warsaw (Poland) Compact, low-cost, Q-switched diode-pumped solid-state lasers (DPPS) with high pulse energy are needed for many applications, such as laser range finders, laser designators, laser breakdown spectroscopy, and laser ignition. In many of those applications the lasers need to operate at high temperatures where typical edge-emitting laser diode pump lasers show poor reliability. Recently, high power vertical-cavity surface-emitting laser (VCSEL) arrays have been demonstrated as excellent pump sources for diode-pumped solid-state lasers. Their key advantages over the existing edge-emitter technology include simpler coupling optics, reduced wavelength sensitivity to temperature, and increased reliability, especially at high temperatures, low-cost manufacturing, and two-dimensional planar scalability. These features make VCSEL technology very well suited for constructing low-cost DPSS lasers with high pulse energy.

Here we report on a very compact VCSEL end-pumped, passively Q-switched Nd:YAG laser with high pulse energy. The laser only comprises 3 components: a high power VCSEL pump module, an aspheric condenser lens as a pump optic, and a diffusion bonded composite laser rod consisting of an Nd:YAG gain medium and a Cr:YAG saturable absorber. The laser rods are coated with high damage threshold dielectric coatings on each end to form the laser end mirrors. A thermal lens stabilizes the laser cavity. The laser pulse energy, q-switch delay time, and optical efficiency of the passively Q-switched monolithic solid state lasers were measured as a function of VCSEL power for various rod lengths, Cr doping levels, and VCSEL heatsink temperatures. Up to 23.6 mJ laser pulse energy was achieved with 11.9% optical efficiency.

Currently, there are two viable routes towards high peak-power laser systems. A traditional one relies on laser amplifiers in some variation of the Chirped Pulse Amplification (CPA) scheme. Its major limitation comes from the thermal effects in the amplifying medium severely limiting the repetition rate laser systems. An alternative approach uses optical parametric amplification in a nonlinear crystal to transfer the energy directly from the pump beam to the amplified ultrashort pulse beam. We will present the results on a table top multi-TW ns-pumped OPCPA system with time shear. The results of theoretical considerations show that, with proper design, the system can potentially achieve extremely high efficiency as well as good temporal and spatial characteristics of the pulses. The possibility of usage of alternative, biaxial crystals (KTP, BiBO) with the pump and signal beams not necessary propagating within the major crystal planes will be discussed.

We will demonstrate experimental results of a system consisting of a femtosecond oscillator followed by a stretcher and a two-stage optical parametric amplifier. The amplifier is pumped with 532nm beam with a rectangular temporal pulse shape of 4 ns duration. The three-pass preamplifier boosts the nJ seed pulse energy by a factor of approximately 106. The power amplifier uses three BBO crystals enhancing the efficiency of the system to 35%. The spectrum of the output pulses is broad enough to support 15 fs pulse duration. This, together with hundreds of mJ of the output pulse energy shows that the system is capable of multi-TW operation.

9726-31, Session 6

Simple ps microchip Nd:YVO4 laser with 3.3 ps pulses at 0.2 - 1.4 MHz and single stage amplification to the microjoule level

Erdal Türkyilmaz, MONTFORT Laser GmbH (Austria); Christian Guenther, Eva Mehner, Technische Hochschule Nürnberg Georg Simon Ohm (Germany); Daniel Kopf, MONTFORT Laser GmbH (Austria); Harald Giessen, Univ . Stuttgart (Germany); Bernd Braun, Technische Hochschule Nürnberg Georg Simon Ohm (Germany)

9726-33, Session 7

High repetition rate (100 Hz), high peak power, high contrast femtosecond laser chain

Raphael Clady, Vadim I . Tcheremiskine, Yasmina Azamoum, Laurent Charmasson, Nicolas Sanner, Olivier P . Uteza, Marc L . Sentis, Lasers, Plasmas et Procédés Photoniques (France) Commercial picosecond sources have found widespread applications. Typical system parameters are pulse widths below 20 ps, repetition rates between 0.1 to 2 MHz, and micro Joule level pulse energies. Many systems are based on short pulse modelocked oscillators, regenerative amplifiers, and pockel cells as active beam switches. In contrast we present a completely passive system, consisting of a passively Q-switched microchip laser, a single-stage amplifier, and a pulse compressor. The Q-switched microchip laser has a 50 µ m long Nd:YVO4-gain material optically bonded to a thick undoped YVO4-crystal. It delivers pulse widths of 40 ps and repetition rates of 0.2 - 1.4 MHz at a wavelength of 1.064 µ m. The pulse energy is a few nJ. These 40-ps pulses are spectrally broadened in a standard single mode fiber and then compressed in a 24mm long CBG (chirped Bragg grating) to as low as 3.3 ps. The repetition rate can be tuned from app. 0.2 to 1.4 MHz by changing the pump power while the pulse width and the pulse energy from the microchip laser are unchanged. The spectral broadening in the fiber is observed throughout the pulse repetition rate, supporting sub-10-ps pulses. Finally, the pulses are amplified in a single High intensity femtosecond laser are now routinely used to produce energetic particles and photons via interaction with solid targets. However, the relatively low conversion efficiency of such processes requires the use of high repetition rate laser to increase the average power of the laser-induced secondary source. Furthermore, for high intensity laser-matter interaction, a high temporal contrast is of primary importance as the presence of an ASE (Amplified Spontaneous Emission) pedestal and/or various prepulses may significantly affect the governing interaction processes by creating a pre plasma on the target surface. We present the characterization of a laser chain based on Ti:Sa technology and CPA technique, which presents unique laser characteristics in the world: a high repetition rate (100 Hz), a high peak power (10 TW) and a high contrast ratio (ASE <10-10, prepulses <10-8) obtained thanks to a specific design with 2 saturable absorbers inserted in the amplification chain. A deformable mirror placed before the focusing parabolic mirror allows focusing the beam almost at the limit of diffraction. In these conditions,

10 SPIE Photonics West 2016 · www.spie.org/pw

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we obtained peak intensity above 1019W.cm-2 on target, which allows the study of phenomena in the relativistic optic domain at an unprecedented repetition rate. Preliminary results on the generation of a sub-picosecond laser-driven hard x-ray Kalpha source with a 100 Hz repetition rate are also presented, with a particular emphasis on the influence of the different laser parameters.

9726-34, Session 7

Blue-diode-pumped SESAM modelocked Ti:sapphire oscillator generating 5 nJ 88 fs pulses

Andreas Rohrbacher, Vesna Markovic, Wolfgang Pallmann, Simonette Pierrot, Peter Hofmann, Lumentum (Switzerland); Bojan Resan, Lumentum (Switzerland) and Univ . of Applied Sciences and Arts Northwestern (Switzerland) Ti:sapphire oscillators are the proven technology to generate sub-100 fs (even sub-10 fs) pulses in the near infrared and are widely used in many high impact scientific fields. However, the need for a bulky, expensive and complex pump source, typically a frequency-doubled multi-watt solid-state laser, represents the main obstacle to more widespread use. The recent development of blue diodes emitting over 1 W has opened up the possibility of directly diode-laser-pumped Ti:sapphire oscillators. Beside much lower cost and smaller footprint, direct diode pumping provides better reliability, higher efficiency and better pointing stability. The associated challenges are lower absorption of Ti: sapphire at available diode wavelengths and lower brightness of blue diodes compared to typical solid-state green pump lasers. For applications such as biomedical imaging and nano-structuring, output powers in excess of 100 mW and sub-100 fs pulses are required. In this paper, we demonstrate a high pulse energy directly blue-diode-pumped Ti:sapphire oscillator without watercooling. The SESAM modelocking ensures reliable self-starting and robust operation. We will present two configurations emitting 460 mW average power with 88 fs pulses and 350 mW average power with 75 fs pulses, both operating at 92 MHz pulse repetition rate. The maximum obtained pulse energy reaches 5 nJ. A double sided pumping scheme with two high power blue diode lasers, emitting 2.9 W each, was used for the output power scaling. The cavity design and the experimental results will be discussed in more details.

9726-35, Session 7

Conference 9726: Solid State Lasers XXV: Technology and Devices Performance of Yb:Lu2O3 crystal in diode pumped femtosecond oscillator and high power regenerative amplifier

Etienne Caracciolo, Univ . degli Studi di Pavia (Italy) and Spectra-Physics (Austria); Federico Pirzio, Univ . degli Studi di Pavia (Italy); Matthias Kemnitzer, Spectra-Physics (Austria); Annalisa Guandalini, Spectra-Physics (Austria); Florian Kienle, Spectra-Physics (Austria); Antonio Agnesi, Univ . degli Studi di Pavia (Italy); Juerg Aus der Au, Spectra-Physics (Austria) 4-mm long and 1.5% doped. The low-power oscillator was pumped by two single-mode fiber-coupled laser diodes, emitting 400 mW maximum power at 976 nm. Up to 65 mW average output power, 78-fs long mode locked pulses with 18 nm FWHM spectral bandwidth centered at 1075 nm were obtained in a resonator employing a single prism for intracavity GDD compensation. In high-power amplification experiments, we pumped two Yb:Lu2O3 crystals, each with a 60-W fiber-coupled laser diode. The seeder was a Yb:KYW oscillator delivering 250-fs long, almost Fourier transform limited pulses at 1035 nm. Before injection in the regenerative amplifier, pulses were stretched to about 30-ps. Average output powers up to 27.8 W at 500 kHz were achieved before compression and at maximum pump power. Pulse energies up to 44.4 µ J and pulse durations of 638 fs were demonstrated after a standard grating compressor. The 2.8-nm FWHM wide spectrum was centered at 1034 nm, resulting in a time-bandwidth product of 0.5. We measured a M2<1.3 in both axes at full pump power.

9726-36, Session 7

Modeling and simulation of ultra-short pulse amplification

Christoph Pflaum, Friedrich-Alexander-Univ . Erlangen Nürnberg (Germany); Rainer Hartmann, Friedrich Alexander-Univ . Erlangen-Nürnberg (Germany) and Erlangen Graduate School in Advanced Optical Technologies (Germany); Zhabiz Rahimi, ASLD GmbH (Germany) Ultrashort pulses with high average power are required for a variety of technical and medical applications. To increase the power of ultrashort lasers, single, multi-pass, and regenerative amplifiers are used. Typical laser crystals for such amplifiers include the Ti:Sapphire or Yb:YAG laser crystals. Difficulties in the amplification of ultrashort pulses include gain narrowing effects and dispersion effects in the laser crystal. In particular these complications arise, when a pulse stretcher is needed before amplification of the laser beam. We present a technique to model and simulate the amplification of ultrashort pulses. This technique allows to model both gain narrowing effects and decrease of beam quality caused by amplification of the laser beam. This requires a detailed 3-dimensional simulation of population inversion. Gain narrowing effects are taken into account by analyzing the gain of the spectrum of the laser beam. Important is to distinguish amplifiers with one or only two passes and a regenerative amplifier. These two different kind of amplifiers are modeled by different approaches. A regenerative amplifier is modeled by a set of time dependent rate equations. However, a single pass amplifier is modeled by a set of spatial dependent rate equations. In both cases, a system of rate equations arises from spectral discretization of the laser beam. Detailed simulation results are presented.

9726-37, Session 7

High contrast broadband seeder for multi PW laser system

Olivier J . Chalus, Alain Pellegrina, Olivier Casagrande, Christophe Derycke, Laurent Boudjemaa, Christophe Simon-Boisson, Sébastien Laux, François Lureau, Thales Optronique S .A .S . (France) Yb:Lu2O3 is a very promising material for high-power ultrashort pulse generation and amplification owing to a favorable combination of good thermo-mechanical properties, comparable to those of Yb:YAG, and a relatively broad emission bandwidth, able to sustain sub 100-fs pulses. Here we report, to the best of our knowledge, the shortest pulse duration ever demonstrated in a Yb:Lu2O3 oscillator and the highest average power from a bulk Yb:Lu2O3 regenerative amplifier. Both for low-power oscillator and regenerative amplification experiments, the Yb:Lu2O3 samples were In the frame work of the ELI-NP project, we have developed a broadband high contrast (>1:1014) 10mJ seeder for 2 x 10PW laser. This hybrid Ti:Sapphire/OPCPA Front End is usable for any ultra-short pulse PetaWatt laser system, or above. It consists of an oscillator developed in collaboration with Venteon GmbH providing the seed for the system as well as an optically synchronized seed for the OPCPA pump. The 800nm seed is first amplified in a regenerative amplifier and is then compressed. This

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+1 360 676 3290 · [email protected] 11

compressed pulse is injected in a XPW filter allowing enhancement of the contrast of four orders of magnitude. This pulse is then stretched in bulk to a few tens of picosecond and amplified in a double stage OPCPA at 10Hz. A gain of 3orders of magnitude in the OPCPA also allows a contrast gain of equivalent amount at 100ps. The OPCPA design is compatible with pump lasers working at 1064nm using the mature and reliable technology of flashlamp pumped Nd:YAG, allowing future scaling to higher energies and with pulse duration of twenty five picoseconds. A record optical efficiency of the OPCPA of 20% has been reached without any spatial and temporal shaping device within the pump laser. The total bandwidth at the output of the frontend exceeds 120nm therefore allowing ultrashort pulse after compression. R.M.S stability better than 1.5% over more than one hour has been demonstrated.

9726-38, Session 7

Ultrafast laser with an average power of 120 W at 515 nm and a highly dynamic repetition rate in the MHz range for novel applications in micromachining

Florian Harth, Melissa C . Piontek, Thomas Herrmann, Johannes L’huillier, Photonik-Zentrum Kaiserslautern e .V . (Germany) A new generation of resonant scanners in the kHz-range shows ultra high deflection speeds of more than 1000 m/s but suffer from an inherent nonlinear mirror oscillation. A typical bitmap, written point by point, would be strongly distorted because of the decreasing spot distance at the turning point of the scanning mirror. Furthermore, laser micromachining with constant pulse repetition frequency (PRF) even in narrow vector geometries can cause severe heat accumulation with resulting degradation of the machining quality. This can be avoided by a dynamic adaption of the PRF of the ultrafast laser. However, in the case of the resonant scanner this means that the PRF has to be continuously swept up to several 10.000 times per second between e.g. 5 MHz and 10 MHz. Even today this is still a challenging task. We present a laser system which is capable of sweeping the PRF more than 32.000 times per second between 5 MHz and 10 MHz at an average output power of up to 120 W at 515 nm with a pulse duration of about 40 ps. The laser consists of a semiconductor oscillator, a 3-stage fiber pre amplifier, a solid state power amplifier (AMPHOS 200) and a SHG stage. We systematically analyzed the dynamic of the laser system as well as the spectral and temporal behavior of the optical pulses. Switching the repetition rate typically causes a varying pulse energy, which could affect the machining quality over one scanning line. This effect will be analyzed and discussed. Possible techniques to compensate or avoid this effect will be considered.

9726-39, Session 7

Conference 9726: Solid State Lasers XXV: Technology and Devices Latest results of 10 petawatt laser beamline for ELi nuclear physics infrastructure

François Lureau, Sébastien Laux, Olivier Casagrande, Olivier Chalus, Pierre-Antoine Duvochelle, Sandrine Herriot, Guillaume Matras, Christophe Radier, Christophe Simon-Boisson, Thales Optronique S .A .S . (France); Alexandru Boianu, Horia Hulubei National Institute of Physics and Nuclear Engineering (Romania); Ioan Dancus, Razvan Dabu, National Institute for Laser, Plasma and Radiation Physics (Romania) beamline of HPLS (High Power Laser System) built by Thales for ELI NP.

Each beamline is seeded by a common front end combining Titanium Sapphire CPA and BBO based OPCPA stage in order to improve the temporal contrast and to enlarge the spectral bandwidth. The OPCPA output is split into 2 beams which are launched into beamlines based on amplification within Ti:Sa crystals including spectral filters to compensate for gain narrowing and spectral shifting effects. Each beamline will provide a main 10 PW output at the rate of 1 shot per minute and two compressed intermediate outputs of respectively 100 TW at 10 Hz and 1 PW at 1 Hz.

The front end has been entirely built and the OPCPA stage has been able to deliver 10.5 mJ for a 532 nm pump energy of 55 mJ. On the first beamline, 2 of the 3 amplification sections have been built and characterized. At the PW intermediate output, an energy per pulse of 39 J before compression has been demonstrated and the related spectral bandwidth is 75 nm FWHM, suggesting future compression below 20 fs. In addition to this, 5 of the 8 pump modules of the final amplifier have been already built. Each module based on flashlamp-pumped Nd-doped Phosphate Glass delivers after second harmonic generation in a LBO crystal more than 100 J of green light at 527 nm while the energy stability is better than 0.6% rms

9726-40, Session 8

High-energy ultra-short pulse thin-disk lasers: new developments and applications

Knut Michel, Sandro Klingebiel, Marcel Schultze, Catherine Y . Tesseit, Christoph Wandt, Stefan Prinz, Robert Bessing, Matthias Häfner, Thomas Metzger, TRUMPF Scientific Laser GmbH + Co ., KG (Germany); Dirk Sutter, TRUMPF Laser GmbH (Germany) We report on the latest developments at TRUMPF Scientific Lasers in the field of ultra-short pulse lasers with highest output energies and powers. All systems are based on the mature and industrialized thin-disk technology of TRUMPF. Thin Yb-YAG disks provide a reliable and efficient solution for power and energy scaling to Joule- and kW-class picosecond laser systems. Due to its efficient one dimensional heat removal, the thin-disk exhibits low distortions and lensing even when pumped under extremely high pump densities of 10kW/cm?. Currently TRUMPF Scientific Lasers develops regenerative amplifiers with highest average powers, optical parametric amplifiers and synchronization schemes. The first few-ps kHz multi-mJ thin-disk regenerative amplifier based on the TRUMPF thin-disk technology was developed at the LMU Munich in 2007. Since, the average power and energy have continuously been increased, reaching more than 300W (10kHz repetition rate) and 200mJ (1kHz repetition rate) at pulse durations below 2ps. First experiments have shown that the current thin-disk technology supports ultra-short pulse laser solutions >1kW of average power.

Based on few-picosecond thin-disk regenerative amplifiers few-cycle optical parametric chirped pulse amplifiers (OPCPA) can be realized. These systems have proven to be the only method for scaling few-cycle pulses to the multi-mJ energy level. OPA based few-cycle systems will allow for many applications such as attosecond spectroscopy, laser wake field acceleration, table-top few-fs accelerators and laser-driven coherent X-ray undulator sources.

Furthermore, high-energy picosecond sources can directly be used for a variety of applications such as X-ray generation or in atmospheric research.

9726-41, Session 8

Recent development of disk lasers at TRUMPF

(Invited Paper)

Sven-Silvius Schad, Tina Gottwald, Vincent Kuhn, Matthias Ackermann, Dominik Bauer, Michael Scharun, Alexander Killi, TRUMPF Laser GmbH (Germany) We present the latest results obtained on the first 10 PetaWatt laser

12 SPIE Photonics West 2016 · www.spie.org/pw

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The disk laser is one of the most important laser concepts for today’s industrial market. Offering high brilliance at low cost, high optical efficiency and great application flexibility the disk laser paved the way to many industrial laser applications. Over the past years power and brightness increased and the disk laser turned out to be a very versatile laser source, not only for welding but also for cutting. Both, the quality of the cut and speed are superior to previous CO2-based lasers for a vast majority of metals, and, most important, through a broad thickness range. In addition, the insensitivity against back reflections makes the disk laser suited for cutting highly reflective metal such as brass or copper. These advantages entail versatile cutting machines and explain the high and growing demand for disk lasers besides the classic welding application as can be observed today.

For the time being, the disk principle has not reached any fundamental limits regarding output power per disk or beam quality, and offers numerous advantages over other high power resonator concepts, especially over fiber lasers or even direct diode lasers. This paper will give insight in the latest progress in kW-disk laser technology at TRUMPF and will discuss recent results as well as power scaling strategies.

9726-42, Session 8

Conference 9726: Solid State Lasers XXV: Technology and Devices Innoslab and thin-disk amplifier system with 1.5 kW average power at 710 fs pulse duration

Thomas Sartorius, Peter Rußbüldt, Fraunhofer-Institut für Lasertechnik (Germany); Dominik Bauer, Dirk Sutter, TRUMPF Laser GmbH (Germany); Hans-Dieter Hoffmann, Fraunhofer-Institut für Lasertechnik (Germany) For many industrial and scientific applications, stable picosecond pulse laser operation at high repetition rate from 1kHz to 100kHz with good spatial beam quality is essential. Development of such kW-class Yb:YAG thin disk beamlines is going to be finished in 2015 at the Hilase facility.

A compact high repetition rate (100kHz) zero-phonon-line-pumped regenerative amplifier (footprint only 120x80cm incl. a pulse compressor) is reliably operated with 1mJ pulse energy in fundamental spatial mode. Output pulse was successfully compressed bellow 2 ps by a tiny CVBG compressor and converted to second and fourth harmonic with >65% and 18% conversion efficiency, respectively. Upgrade of the regen to 5mJ will be demonstrated soon. The cavity of the upgrade amplifier has already been tested in CW regime with 560W output power and no sign of roll over. Results of the seeded operation, efficiency of pulse compression at 0.5kW by a CVBG, and results of improved conversion to 2nd and 4th harmonics will be presented. Other beamline which is being developed is a double stage 1kHz beamline with 0.5J expected energy in a picosecond pulse. The first stage, a QCW-pumped Yb:YAG regenerative amplifier, is now used in application experiments with 45mJ pulse energy after compression by a grating-based compressor. Pulse length is <3ps and beam quality M2 =1.25. The second stage, a regenerative amplifier with a ring cavity, is being assembled now. Until end of 2015 we expect operation with multi-100mJ pulse energy. An overview and the latest results of all thin-disk beamlines will be presented.

9726-44, Session 8

High-gain Yb:YAG amplifier for ultrashort pulse laser at high-average power

John Vetrovec, Drew A . Copeland, Amardeep S . Litt, Aqwest, LLC (United States); Detao Du, General Atomics Aeronautical Systems, Inc . (United States) We present an Ytterbium based amplifier system for fs-pulses that combines the high gain of Innoslab amplifiers and the high average power of disk amplifiers. The system delivers an average power of 1.5 kW at a repetition rate of 40 MHz. The energy of the pulses results in 37.5 µ J. The pulse duration was measured to be 710 fs. The beam quality is M? = 1.5 x 2.0.

The seed source of the amplifier system is a commercial fiber MOPA with 7 W average power. The power is increased by a two-stage Innoslab amplifier to 720 W. After spatial filtering a beam quality of M? = 1.24 x 1.14 is achieved at an average power of 630 W.

The last stage of the amplifier system is a thin-disk multipass amplifier. The Yb:YAG disk module is pumped with 4.2 kW at a pump spot diameter of approximately 10 mm. The laser beam is folded 18 times over the disk within a total beam path of 22 m using flat mirrors and three lenses with a focal length f = -8 m to compensate the focal power of the disk at 4.2 kW pump power. The total gain is currently limited by the number of reflections off the disk, and by thermal issues in the multi-pass arrangement. An improvement of the output beam quality should be possible via better mode matching to the pumped spot on the disk.

9726-43, Session 8

Progress in kW-class picosecond thin-disk lasers development at the HiLASE

Martin Smr?, Taisuke Miura, HiLASE Ctr . (Czech Republic); Michal Chyla, Jiri Muzik, Siva S . Nagisetty, HiLASE Ctr . (Czech Republic) and Czech Technical Univ . in Prague (Czech Republic); Ondrej Novák, Hana Turcicova, Jens Linnemann, HiLASE Ctr . (Czech Republic); Jaroslav Huynh, Patricie Severová, Pawel Sikocinski, HiLASE Ctr . (Czech Republic) and Czech Technical Univ . in Prague (Czech Republic); Akira Endo, Tomá? Mocek, HiLASE Ctr . (Czech Republic) We report on a Yb:YAG laser amplifier for ultrashort pulse applications at kW-class average power. The laser uses two large aperture, disk-type gain elements fabricated from composite ceramic YAG material, and a multi passing architecture to obtain high gain in a chirped-pulse amplification system. The disks are edge-pumped [1, 2, 3], thus allowing for reduced doping of crystals with laser ions, which translates to lower lasing threshold and lower heat dissipation in the Yb:YAG material. The latter makes it possible to amplify a near diffraction-limited seed without significant thermo-optical distortions.

This work presents results of testing the laser amplifier with relay optics configured for energy extraction with up to 40 passes through the disks. This work was in-part supported by the US Army ARDEC Contract Number W15QKN09C0156. Applications for the ultrashort pulse laser amplifier include laser induced plasma channel, laser material ablation, and laser acceleration of atomic particles.

1. J. Vetrovec et al., “Ytterbium–based disk amplifier for an ultra–short pulse laser,” SPIE vol. 7578 (2010).

2. J. Vetrovec et al., “Initial Testing of Edge-Pumped Yb:YAG Disk Laser with Multi-Passed Extraction,” SPIE vol. 8235 (2012).

3. J. Vetrovec et al., “Erbium-Based Edge-Pumped Disk Laser,” SPIE vol. 8599 (2013).

9726-45, Session 8

Yb:YAG ceramic-based laser driver for inertial confinement fusion

John Vetrovec, Drew A . Copeland, Amardeep S . Litt, Aqwest, LLC (United States) We report on a new class of laser amplifiers for inertial confinement fusion (ICF) drivers based on a Yb:YAG ceramic disk in an edge-pumped configuration cooled by a high-velocity gas flow. The Yb lasant offers

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+1 360 676 3290 · [email protected] 13

very high efficiency and low waste heat. The ceramic host material has a thermal conductivity nearly 15-times higher than glass and it is producible in sizes suitable for a typical 10 to 20 kJ driver beam line. The combination of high lasant efficiency, low waste heat, edge-pumping, and host thermal conductivity, enable operation at 10 to 20 Hz at over 20% wall plug efficiency.

This work shows that the edge-pumped Yb:YAG driver operating at ambient temperature offers superior wall-plug efficiency with comparably smaller and less costly pump diodes than recently considered facepumped alternative drivers using Nd;glass, Yb:S-FAP, and cryogenic Yb:YAG gain elements [1].

Scalability of the laser driver over a broad range of sizes is presented. Also included is a design of a subscale 400-J laser amplifier in a configuration directly scalable to a full-size driver. When equipped with Yb:sesquioxide ceramic gain elements for improved bandwidth, the 400-J amplifier can be used in a driver for laser acceleration of nuclear particles. This work was in-part supported by the US Department of Energy Grant Number DE SC0011916.

1. A.C. Erlandson, et al., “Comparison of Nd:phosphate glass, Yb:YAG, and Yb:S-FAP laser beamlines for laser inertial fusion energy (LIFE), Optical materials Express, vol. 1, no. 7, November 1, 2011.

9726-9, Session PTue

Conference 9726: Solid State Lasers XXV: Technology and Devices Laser-diode pumped dysprosium-doped lead thiogallate laser output wavelength temporal evolution and tuning possibilities at 4.3-4.7 um

Helena Jelínková, Czech Technical Univ . in Prague (Czech Republic); Maxim E . Doroshenko, A . M . Prokhorov General Physics Institute (Russian Federation); Jan ?ulc, Michal N?mec, Michal Jelínek, Czech Technical Univ . in Prague (Czech Republic); Vjatcheslav V . Osiko, A . M . Prokhorov General Physics Institute (Russian Federation); Valerii V . Badikov, Dmitri V . Badikov, Kuban State Technological Univ . (Russian Federation)

9726-68, Session PTue

Continuous-wave generation and tunability of eye-safe resonantly diode-pumped Er:YAG laser

Michal N?mec, Lukás Indra, Jan ?ulc, Helena Jelínková, Czech Technical Univ . in Prague (Czech Republic) Laser sources generating radiation in the spectral range from 1.5 to 1.7 um are very attractive for many applications such as satellite communication, range finding, spectroscopy, and atmospheric sensing. The goal of our research was an investigation of continuous-wave generation and wavelength tuning possibility of diode pumped eye-safe Er:YAG laser emitting radiation around 1645 nm. We used two 0.5 at. % doped Er:YAG active media with lengths of 10 mm and 25 mm (diameter 5 mm). As a pumping source, a fibre-coupled 1452 nm laser-diode was utilized, which giving possibility of the in-band pumping with small quantum defect and low thermal stress of the active bulk laser material.

The 150 mm long resonator was formed by the pumping mirror (HT @ 1450 nm, HR @ 1610 - 1660 nm) and output coupler with 96 % reflectivity at 1610 - 1660 nm. For continuous-wave generation, the maximal output powers were 1 W and 0.7 W for 10 mm and 25 mm long laser crystals, respectively. The corresponding slope efficiencies with respect to absorbed pump power for these Er:YAG lasers were 26.5 % and 37.8 %, respectively. The beam spatial structure was close to the fundamental Gaussian mode. The wavelength tunability was realized by a birefringent plate and four local spectral maxima at 1616, 1632, 1645, and 1656 nm were reached.

The output characteristics of the designed and realized resonantly diode pumped eye-safe Er:YAG laser show that this compact system has a potential for usage mainly in spectroscopic fields.

9726-69, Session PTue

Effect of cryogenic temperature on spectroscopic and laser properties of Er,La:SrF2-CaF2 crystal

Richard ?vejkar, Jan ?ulc, Michal N?mec, Helena Jelínková, Czech Technical Univ . in Prague (Czech Republic); Maxim E . Doroshenko, Pavel P . Fedorov, Vjatcheslav V . Osiko, A . M . Prokhorov General Physics Institute (Russian Federation) On the basis of our previous Dy3+:PbGa2S4 laser study, the laser output wavelength temporal evolution as well as tuning possibilities in the range 4.3–4.7um were investigated. Active crystal 12x6x2mm3 was pumped by a fiber-coupled Brightlase Ultra-50 diode laser (1.7um, max. power 7.5W). The laser resonator was formed by a flat dichroic pumping mirror (T = 70%@1.7um, R~100%@3.5 - 5um) and a concave (r = 200mm) output coupler with R~99%@3.5 - 5um. In order to decrease thermal loading to the active crystal, the laser was operated in the pulsed regime at 10 Hz with various pump pulse duration. The laser output wavelength dependence on the pump pulse duration and its evolution within the pump pulse was investigated at first without any spectrally-selective filter in the resonator. At pump pulse duration of 1ms, generation just at 4.35um has been observed. Prolongation to 5ms led to generation at 4.35um in the first millisecond, followed by simultaneous generation at 4.35 and 4.38um in the second millisecond, and further followed by generation almost just at 4.6um till the end of the pump pulse. Prolongation to 10ms led to similar behavior as in the previous case followed by generation at 4.6um only. Furthermore, output wavelength tuning possibilities using MgF2 birefringent filter inside the resonator were investigated under 10ms pumping. Almost continuous tuning without any significant dip has been observed in the wavelength range 4.3 up to 4.7um. The mean output power was in the order of 400uW corresponding to the power amplitude of 4mW.

We present laser and spectroscopic properties of crystal Er,La:SrF2-CaF2 at temperature range 80 - 320 K, which is appropriate for generation of radiation around 2.7 _m. The sample of Er,La:SrF2-CaF2 (concentration Er(0.04), La(0.12):Ca(0.77)Sr(0.07)) had plan-parallel face-polished faces without anti-reflection coatings (thickness 8.2 mm). During spectroscopy and laser experiments the Er,La:SrF2-CaF2 was attached to temperature controlled copper holder and it was placed in vacuum chamber. The transmission and emission spectra of Er,La:SrF2-CaF2 together with the fluorescence decay time were measured in dependence on temperature. The excitation of Er,La:SrF2-CaF2 was carried out by a laser diode radiation (pulse duration 5 ms, repetition rate 20 Hz, pump wavelength 973 nm). Laser resonator was hemispherical, 140 mm in length with flat pumping mirror (HR @ 2.7 um) and spherical output coupler (r = 150 mm, R = 95 % @ 2.5 - 2.8 um). Tunability of laser at 80 K in range 2690 - 2765 nm was obtained using MgF2 birefringent filter. With decreasing temperature of sample the fluorescence lifetime of manifold 4I11/2 (upper laser level) became shorter and intensity of upconversion radiation was increasing. The highest slope efficiency with respect to absorbed power was 2.3 % at 80 K. The maximum output of peak amplitude power was 0.3 W at 80 K, i.e. 1.5 times higher than at 300 K. The wavelength generated by Er,La:SrF2-CaF2 laser (2.7 um) is relatively close to absorption peak of water (3 um) and so, one of the possible use should be in medicine.

14 SPIE Photonics West 2016 · www.spie.org/pw

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Conference 9726: Solid State Lasers XXV: Technology and Devices

9726-70, Session PTue

Characteristics and performance of a two lens slit spatial filter for high power lasers

Xiao Yuan, Han Xiong, Fan Gao, Xiang Zhang, Soochow Univ . (China)

9726-72, Session PTue

Yb doping concentration and temperature influence on Yb:LuAG thermal lensing

Karel Veselsk?, Jan ?ulc, Helena Jelínková, Czech Technical Univ . in Prague (Czech Republic); Karel Nejezchleb, Václav ?koda, CRYTUR spol s .r .o . (Czech Republic) Laser beams will inevitably encounter modulations of spatial frequencies when propagating in optical paths, resulting in small-scale self-focusing and degrading the beam quality, especially in inertial confinement fusion lasers that have higher intensities. Spatial filter (SF) is an effective equipment to clean off the rapidly growing spatial frequencies, which consists of two convex lenses and a pinhole placed at their common focus. Nonetheless, the pinhole aperture should be less than a specific size to efficiently clean off the rapidly growing spatial frequencies, thus the pinhole aperture in large laser systems has to bear higher intense laser irradiation, which may induce pinhole closure. Besides, the SFs in large laser systems have to be placed in high vacuum to avoid breakdown in air. In order to solve the above problems, the conventional pinhole SF has to be improved. By introducing cylindrical lenses into SFs, the laser beams can be focused into a line instead of the original spot, thus the focal area can be significantly enlarged and the focal intensity greatly lowered. We propose a new-type two-lens slit SF with two complex cylindrical lenses and two crossed slits. The characteristics of this slit filter on image relay, aperture match and spatial filtering are discussed by both theoretical calculations and numerical simulation. In simulation, we use this new-type SF to replace the original transmission spatial filter (TSF) in NIF system, and found that the focal intensity on the slit drops by about 3 orders of magnitude, which greatly improves the pinhole closure for large laser systems and could reduce the vacuum degree in SF by about one order of magnitude, which is helpful in building and maintaining the large laser systems.

9726-71, Session PTue

1-W level diode pumped Pr:YLF orange laser

Martin Fibrich, Jan ?ulc, Helena Jelínková, Czech Technical Univ . in Prague (Czech Republic) The aim of this study was to investigate whether refractive power of thermal lens for Yb:LuAG crystal at cryogenic temperatures depends on Yb doping concentration which has not been examined yet. The three measured Yb:LuAG laser rods samples (length of 3 mm, diameter 3 mm, AR @ 0.94 µ m and 1.03 (HR @ 1.03 µ µ m, doping concentration 5.4, 8.4 and 16.6 at. % Yb/Lu) were mounted in the temperature controlled copper holder of the liquid nitrogen cryostat. Samples were longitudinally pumped with fiber coupled CW laser diode at 0.930 µ m with the focal point 0.2 mm in diameter. The 38 mm long semi-hemispherical laser resonator consisted of a flat pump mirror m and HT 0.94 µ m) and curved output coupler (r=500 mm) of reflectivity 94 % @ 1.06 µ m. The refractive power of thermal lens was estimated indirectly by measuring of change in the position of focused laser beam focal point. The measurement was performed for constant absorbed power of 10 W in temperature range from 80 up to 240 K. It was observed that cryogenic cooling caused reduction of thermal lens power, which increased linearly with increasing temperature and was the same for each sample of different dopant concentration. Presented study shows that application of cryogenic temperature leads to reduction of thermal effect even for high dopant concentration in Yb:LuAG crystal. This is essential for reaching of high output power while maintaining high beam quality.

9726-73, Session PTue

Diode pumped compact cryogenic Yb:YAG/Cr:YAG pulsed laser

Petr Navratil, Venkatesan Jambunathan, Lucie Horackova, Antonio Lucianetti, Tomas Mocek, Institute of Physics of the ASCR, v .v .i . (Czech Republic) In this contribution, 1 W of the output power in the orange spectral range (607 nm) under blue laser diode pumping of the Pr:YLF crystal is reported. To reduce reabsorption losses at this particular 3P0 ? 3H6 laser transition (originating from the higher laying level of the ground state manifold to the 1D2 manifold), the laser system was designed for operation at cryogenic temperature. In our experiment, 0.4 at. % doped 4 mm long non-antireflection coated Pr:YLF crystal mounted on a liquid-nitrogen-cooled copper finger in the vacuum chamber of the cryostat was employed. Temperature of the sample was kept at 80 K. As a pump source, a fiber-coupled (core diameter 400 um, NA 0.22) blue laser diode module from Necsel company was used. The module provided 442 nm laser radiation with maximal output power of 10 W which was collimated and subsequently focused into the laser crystal by two antireflection coated achromatic doublets having focal lengths of 75 mm both. Pump absorption efficiency of the crystal was measured to be only 31 %, due to the non-perfect overlapping of the diode emission spectrum and the 444 nm absorption peak of the Pr:YLF crystal. Using 70% output coupler reflectivity at the designed laser wavelength, more than 1 W of the output power at 607 nm was reached from the Pr:YLF sample. The corresponding slope efficiency related to absorbed pump power was 37 %. To our best knowledge, this is the first demonstration of diode pumped 1-W class Pr-ion based orange emitting laser system.

Development of compact diode pumped passive Q-switched solid state lasers with high peak power has become scientific interest because of their vast technological applications such as laser ignition, material processing, nonlinear optical studies, optical communications, etc. Especially for industrial applications compact, low maintenance, robust, stable and alignment free pulsed lasers are inevitable.

In this aspect, we are developing compact pulsed laser based on Yb:YAG/ Cr:YAG. It consists of monolithic crystal having cylinder shape with 5mm in diameter that houses both the active medium Yb:YAG (3at.% Yb/Y, 3mm length) and Cr:YAG saturable absorber (initial transmission 90%, 1mm length) which are diffusion bonded. The laser resonator is formed by depositing the dielectric mirrors on the surface with Yb-doped segment having high reflection for laser wavelength and high transmission for pump wavelength, while the reflection of laser wavelength on the Cr-doped side is 80%, meaning that 20% of generated light is extracted. The laser is pumped longitudinally by fiber coupled VBG stabilized laser diode emitting at 969nm and the monolithic crystal is cooled down to cryogenic temperatures.

In the initial experiment for temperature 140K we attained stable pulse operation with pulse energy over 60 µ J and pulse width of 1ns which leads to a peak output power better than 60kW.

Present work is focused on optimizing the thickness, doping and transmission of output coupling to extract more energy. Both the experimental results and theoretical modelling based on this compact cryogenic laser will be presented in detail.

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+1 360 676 3290 · [email protected] 15

9726-75, Session PTue

Conference 9726: Solid State Lasers XXV: Technology and Devices

9726-74, Session PTue

Microchip laser based on Yb:YAG/V:YAG monolith crystal

Karel Nejezchleb, CRYTUR spol s .r .o . (Czech Republic); Jan ?ulc, Helena Jelínková, Czech Technical Univ . in Prague (Czech Republic); Václav ?koda, CRYTUR spol s .r .o . (Czech Republic) V:YAG crystal was investigated as a passive Q-switch of longitudinally diode-pumped microchip laser, emitting radiation at wavelength 1030.5 nm. This laser was based on diffusion bonded monolith crystal (diameter 3 mm) which combines in one piece an active laser part (Yb:YAG crystal, 10 at.%Yb/Y, 3 mm long) and saturable absorber (V:YAG crystal, 2 mm long, initial transmission 86 % @ 1031 nm). The microchip resonator consisted of dielectric mirrors directly deposited on the monolith surfaces (pump mirror HT @ 968 nm and HR @ 1031 nm on Yb:YAG part, output coupler with reflection 55 % @ 1031 nm on the V:YAG part). For longitudinal CW pumping of Yb:YAG part, a fibre coupled (core diameter 100 mm, NA =0.22, emission @ 968 nm) laser diode was used. The laser threshold was 3.8 W. The laser slope efficiency for output mean in respect to incident pumping was 16 %. The linearly polarized generated transversal intensity beam profile was close to the fundamental Gaussian mode. The generated pulse length, stable and mostly independent on pumping power, was equal to 1.3 ns (FWHM). The single pulse energy was increasing with the pumping power and for the maximum pumping 9.7 W it was 78 mJ which corresponds to the pulse peak-power 56 kW. The maximum Yb:YAG/V:YAG microchip laser mean output power of 1 W was reached without observable thermal roll-over. The corresponding Q-switched pulses repetition rate was 13.1 kHz.

Q-switched microchip MOPA generating 100 ps pulses at 532 nm

Jari Nikkinen, Antti Härkönen, Iiro Leino, Ville-Markus Korpijärvi, Tampere Univ . of Technology (Finland); Gabriela Salamu, National Institute for Laser, Plasma and Radiation Physics (Romania); Mircea Guina, Tampere Univ . of Technology (Finland) We report a master oscillator-power amplifier (MOPA) system based on 100 ps SESAM Q-switched microchip laser, Nd:YVO4 amplifier and frequency doubling in bulk LBO crystal. An average power of more than 100 mW is demonstrated at 532 nm corresponding to a repetition rate of ~50 kHz. Typical conversion efficiency from 1064 nm to 532 nm exceeds 30%. The system has small footprint and does not contain any moving parts. The monolithic master oscillator microchip laser was comprised of a GaAs-based semiconductor saturable absorber mirror and a Nd:YVO4 crystal with a thickness of 100 µm. It delivered a typical average output power in the range of 5 mW at 1064 nm. The seed was subsequently amplified in a Nd:YVO4 bulk amplifier using double-pass configuration increasing the average power beyond 300 mW. Both the seed laser and the amplifier were pumped with low-cost 808 nm laser diodes. A 10-mm long LBO crystal was used to convert the infrared radiation to 532 nm with over 30% efficiency in single pass. The advantages of the demonstrated light source include short pulse duration, high peak power and simple design. Such compact visible light source could be highly useful for applications including two-photon microscopy, additive manufacturing (two-photon polymerization) and fluorescence lifetime imaging, for example.

9726-76, Session PTue

Direct generation of eye-safe vortex laser with opposite helicity

Yongguang Zhao, QIyao Liu, Deyuan Shen, Jiangsu Key Lab . of Advanced Laser Materials (China) ~4 W continue wave (cw) and ~0.8 mJ pulsed Laguerre-Gaussian (LG) vortex beam at ~1.6 ?m was generated in a simple Er:YAG ceramic solid-state laser resonator pumped with annular beam. A center punched mirror with high-reflected coating at pump wavelength was designed to reshape Gauss pump beam into annular intensity distribution with unaltered M2 parameter and ~68% mode conversion efficiency. After investigating the interference patterns of the doughnut-like output beam by a home-made Mach-Zehnder interferometer, two pure LG01 modes with opposite handedness were found to exist alternately over time, and the dynamical process was detailed analysis. The generated cw and pulsed eye-safe vortex laser with opposite helicity may have novel applications in microparticle manipulation, such as generation of opposite atomic rotational states, as an optical spanner to transmit opposite orbital angular momentum, high-filed laser physics, etc.

9726-78, Session PTue

Generation of Vis-NIR light within the first biological optical window via frequency upconversion in Tm3+- and Tm3+/Er3+ doped tellurite glass excited at 1319 nm

Artur S . Gouveia-Neto, Marcos V . D . Vermelho, Carlos Jacinto de Silva, Evandro J . T . A . Gouveia, Univ . Federal de Alagoas (Brazil); Fabia C . Cassanjes, Univ . Federal de Alfenas (Brazil) Rare-earth doped frequency up-converters have drawn much scientific and technological interest lately owing to their potential application in color displays technology, optical sensing devices, visible solid-state lasers, biological markers, biomedicine, amongst many. Particularly, for applications in biomedicine such as fluorescence based nano-thermometers and fluorescence-based bio-imaging, the penetration depth is a major issue to be considered. Bearing that in mind, it is mostly desirable to produce and/or employ light sources within the two biological windows. Generation of visible and NIR frequency upconversion emission light within the first biological window spectral region using excitation at 1319 nm in the second biological window in Tm3+/Er3+-codoped TeO2-based glass, is demonstrated. Efficient energy upconversion of cw radiation at 1319 nm into near infrared 800 nm emission in Tm3+ single-doped 60TeO2 10GeO2-10K2O-10Li2O-10Nb2O5 glass is observed. The 800 nm signal is the sole light emission observed in the entire UV-VIS-NIR spectral region. Luminescence emission around 1480 nm, and 1800 nm was also observed. The proposed excitation mechanism for the 800 nm thulium emitting level is assigned to a multiphonon-assisted excitation from the ground-state 3H6 to the 3H5 excited-state level, a rapid relaxation to the 3H4 level and followed by an excited-state absorption of the pump photons mediated by multiphonons connecting the 3H4 level to the 3F4 emitting level. For Tm3+/ Er3+-codoped tellurite glass samples, green(550 nm), red(660 nm) and NIR(980 nm) owing to erbium ions in addition to the main 800 nm signal originating form thulium ions is readily observed. Here, energy-transfer from Tm3+ (3H6 - 3F4) to Er3+(4I9/2) producing the NIR emission, followed by excited-state absorption of 1319 nm photons populating 2H11/2, 4S3/2, and 4F9/2 visible emitting levels. Erbium single-doped samples did not exhibited visible detectable emissions for excitation powers as high as 1.8 W of cw radiation at 1319 nm. The dependence of the emission signals upon the excitation power, erbium and thulium content and sample temperature is also analysed

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Conference 9726: Solid State Lasers XXV: Technology and Devices

9726-79, Session PTue

Optical properties and upconversion emission in Yb3+-sensitized Er3+- and Pr3+-codoped PbGeO3:PbF2:xF2 (x = Mg, Ba) glass

Artur S . Gouveia-Neto, Univ . Federal de Alagoas (Brazil); Alexandre O . Silva, Univ . Federal Rural de Pernambuco (Brazil); Luciano A . Bueno, Univ . de Sorocaba (Brazil); Carlos Eduardo N Oliveira, Universidade Federal Rural de Pernambuco (Brazil) dry pressing of spray-dried powders and tape casting, all sintered under high vacuum. The selected geometry of materials was based on numerical simulations. Microstructure of the produced materials was characterized by SEM and EDX with a particular attention to the dopant content across the layers. The optical quality of produced ceramics was characterized and discussed in connection with the microstructure and laser emission results. Output power of about 7 W and slope efficiency 60 % were obtained in QCW regime from bilayered 0-10 %Yb:YAG. In the case of multilayered materials higher scattering losses were observed. The comparison between the two processing methods highlighted that the tape-cast materials provided higher optical uniformity and the diffusion zone between the single layers with different dopant content was about 150 nm compared to about 250 nm in samples produced by pressing of powders. Novel glass materials suitable for the development of infrared excited up-converters have drawn much attention during the last few years. Fluorogermanate glasses have recently emerged as a viable alternative for photonics and bio-photonics applications owing to the better mechanical strength, chemical durability, and thermal stability when compared to fluoride-based glasses. In addition, these glasses possess the durability and mechanical properties of an oxide glass and the maximum vibrational energy is intermediate (?800 cm-1) between those of silicate (?1100 cm-1) and fluoride (?500 cm-1) based glasses. The optical properties and energy-transfer upconversion luminescence of Er3+- and Pr3+/Yb3+-codoped PbGeO3-PbF2-xF2(Mg, Ba) glass under excitation at 980 nm is investigated. In Er3+/Yb3+-codoped samples, green(525, and 550 nm), and red(662 nm), luminescence corresponding to the 2H11/2?4I15/2, 4S3/2?4I15/2 e 4F9/2?4I15/2, respectively, was observed. A blue signal at 410 nm assigned to the 2H9/2, ? 4I15/2 transition was also detected. The green signals are originated from a nearly resonant energy-transfer involving 2F5/2+4I15/2?2F7/2+4I11/2 process which sustain population of the erbium 4I11/2 level. The erbium green emitting levels were populated through excited-state absorption from the 4I11/2 to the 4F7/2, cross-relaxation 4I11/2+ 4I15/2 ? 4I11/2 + 4F7/2, and energy-transfer 2F5/2+4I11/2 ?2F7/2 +4F7/2. Multiphonon relaxation sustains population in the 2H11/2, and 4S3/2, excited-states, from where radiative decays to the ground-state yield the recorded signals. In the Pr3+/Yb3+ system, emissions around 485, 530, 610, 645, and 725 nm ascribed to the 3P0 - 3HJ (J=4, 5, and 6), and 3P0 – 3FJ (J=2, and 3,4), transitions, respectively, were observed. The population of the praseodymium upper 3P0 emitting level was accomplished through a combination of ground-state absorption of Yb3+ ions at the 2F7/2, energy-transfer Yb3+(2F5/2) – Pr3+(3H4), and excited-state absorption of Pr3+ ions provoking the 1G4 – 3P0 transition. The dependence of the upconversion emission upon glass composition, pump power, heat treatment, and doping contents was also examined

9726-46, Session 9

Layered Yb:YAG ceramics produced by two different methods: processing, characterization and comparison

Jan Hostasa, Laura Esposito, Valentina Biasini, Andreana Piancastelli, Istituto di Scienza e Tecnologia dei Materiali Ceramici (Italy); Matteo Vannini, Guido Toci, Istituto Nazionale di Ottica (Italy) The use of Yb:YAG ceramic gain media in solid state lasers has been of growing interest for high repetition rate and high power lasers. Probably the most important advantage of ceramic production technology in comparison with that of single crystals is the flexibility of shaping methods that allow the production of near-net-shape components with a well-defined internal structure. In the case of Yb:YAG with dopant distribution designed accordingly to the pumping and cooling geometry the efficiency of the laser device can be enhanced by mitigating thermal lensing effects.

The presented work reports on Yb:YAG transparent ceramics composed of layers with different Yb doping produced by two shaping methods:

9726-47, Session 9

First laser operation and spectroscopic characterization of mixed garnet Yb:LuYAG ceramics

Guido Toci, Istituto Nazionale di Ottica (Italy); Angela Pirri, Istituto di Fisica Applicata Nello Carrara (Italy); Jiang Li, Tengfei Xie, Yubai Pan, Shanghai Institute of Ceramics (China); Vladimir Babin, Alena Beitlerová, Martin Nikl, Institute of Physics of the ASCR, v .v .i . (Czech Republic); Matteo Vannini, Istituto Nazionale di Ottica (Italy) Mixed garnets represent an interesting class of laser materials, because their disordered structure broadens the emission spectrum of the lasing ion with respect to their parent materials, resulting in a wider tuning range and an increased capability to generate or amplify ultrashort laser pulses. In this sense, the mixed Lutetium-Yttrium Aluminum Garnet ((LuxY1 x)3Al5O12, LuYAG) was recently proposed in its crystalline form as host for several lasing ions.

We present the optical and spectroscopic characterization and the first example of laser operation of Yb doped LuYAG ceramics, with two different compositions, namely (Lu0.25Y0.75)3Al5O12 and (Lu0.50Y0.50)3Al5O12, both with 15% Yb doping.

Ceramic samples were prepared by reactive sintering from high purity ?-Al2O3, Lu2O3, Y2O3, Yb2O3 powders using Tetraethoxysilane and MgO as sintering aids. After ball milling, the slurry was dried, uniaxially pressed into 20 mm diameter pellets at 20 MPa, and cold isostatically pressed at 200 MPa. Sintering was conducted at 1850 °C for 30 h under vacuum, followed by annealing in air (1500 °C, 10 h) to remove the oxygen vacancies.

Laser test were carried out in a laser cavity end pumped by a fiber coupled diode laser emitting at 936 nm. A slope efficiency as high as 50% with a maximum output power of 6.7 W (in quasi-CW conditions) was obtained from the sample with composition (Lu0.25Y0.75)3Al5O12, whereas the sample with composition (Lu0.50Y0.50)3Al5O12 had a maximum slope efficiency of 34% (due to the higher scattering losses), 3.2 W output power. The tuning range of the two samples has been characterized.

9726-48, Session 9

Spectroscopic investigation of Yb,Ho,Pr:YAG as a 3

(United States) µ

m laser source

Ronald Stites, Thomas Harris, Air Force Research Lab . In addition to the well-established 5I7 to 5I8 transition at 2.09 from the 5I6 level to 5I7 at 2.95 µ in interest and applications for 3.0 µ µ m in holmium doped laser materials, there also exists a less energetic transition m. As there has been a recent increase m light, this material stands to be a viable alternative to other rare earth doped laser systems. Unfortunately,

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Conference 9726: Solid State Lasers XXV: Technology and Devices

the wavelength required to directly pump the 5I6 level at 1.13 µ m is not convenient for commercial laser diodes. Furthermore, the emission lifetime of the 5I7 state is longer than the 5I6 level, leading to a suppression of lasing due to “bottlenecking” in the material. To overcome these effects, we investigated the activation and deactivation of holmium doped yttrium aluminum garnet (YAG) using ytterbium and praseodymium respectively. By including ytterbium ions in the host material, readily available 940 nm diode light can be used to resonantly excite the 5I6 level in holmium. Similarly, the presence of praseodymium resonantly de-excites the 5I7 state, reducing its lifetime, and making the material more suitable for lasing. Here, we report the absorption and photoluminescence spectra of this triply doped Yb,Ho,Pr:YAG crystal. In addition, the emission lifetime for both the 2.09 µ m and 2.95 µ m transitions are reported and compared to a Yb,Ho:YAG control sample. Finally, we report progress toward 2.95 µ m laser operation of this Yb,Ho,Pr:YAG crystal.

9726-49, Session 9

Laser and optical properties of Yb:YAG ceramics with layered doping distribution: design, characterization and evaluation of different production processes

Guido Toci, Antonio Lapucci, Marco Ciofini, Istituto Nazionale di Ottica (Italy); Laura Esposito, Jan Hostasa, Istituto di Scienza e Tecnologia dei Materiali Ceramici (Italy); Leonida A . Gizzi, Luca Labate, Paolo Ferrara, Istituto Nazionale di Ottica (Italy); Angela Pirri, Istituto di Fisica Applicata Nello Carrara (Italy); Matteo Vannini, Istituto Nazionale di Ottica (Italy) Solid hosts doped with elements from the lanthanide series remain as the most heavily utilized method for high energy laser gain. The unique 4f orbitals and the forbidden electronic transitions between the f orbitals (selection by the Laporte rule) make several lanthanide rare-earth elements (Ln) attractive for laser emission. The forbidden transitions in the trivalent Ln ions can be activated by pumping and the decay of electrons from the excited state to the ground state is comparatively slow. This makes it possible to achieve the necessary population inversion required for laser emission. The trivalent Er ion can produce direct emission into the 1540 nm wavelength, thus, it is the rare-earth emitter of choice for many eye-safe applications. In recent years, the need for high beam quality under passive operation in open air applications have renewed interest in Er-doped bulk glasses as the gain material of choice for solid-state eye-safe lasers. With this target in mind, a study was initiated in order to strengthen a commercially available laser glass by adding modifiers that increase the short-range order in the starting phosphate glass structure. However, it is well known that the addition of the elements that drive the glass stability will decrease the laser gain obtained. It is also known that Er3+ emission is significantly impacted by the host glass phonon energy. Thus, the main goal of the study is to produce a laser gain material that would have an increased thermo-mechanical figure of merit (FOM) while maintaining or increasing any laser FOM’s. This report details the experimental work and the results obtained for optimizing all desired glass properties from a glass laser gain element perspective.

9726-51, Session 10

High-power single-longitudinal mode and tunable diamond Raman laser at 1240 nm

Soumya Sarang, Oliver Lux, Ondrej Kitzler, Robert Williams, Aaron McKay, Richard Mildren, Macquarie Univ . (Australia) Ceramic-based laser materials are by now a widespread alternative to single crystals, in view of the development of high power laser applications. The flexibility of the ceramic process can be exploited to control the doping distribution, in order to optimize the thermal management of the lasing element. The laser, optical and spectroscopic properties of multilayer Yb:YAG ceramic structures, differently activated, were investigated. The structures were designed by means of Finite Element Modeling, adjusting the doping distributions to reduce peak temperature, surface deformation and thermally induced stresses, depending on the pump and cooling geometry.

Two ceramic processes were used, i.e. dry pressing of spray-dried powders (SD) and tape casting (TC), resulting in different defect density and size distribution: TC gives a more uniform transmission, whereas SD results in larger, unevenly scattered defects. The spectroscopic properties were found independent from the production process.

The laser performance has been characterized under high intensity pumping in a longitudinally diode pumped laser cavity, comparing the behavior of the different structures (including homogeneous doping) in terms of slope efficiency, stability under increasing thermal load, spatial uniformity of laser emission. Slope efficiency as high as 60% in Quasi-CW pumping conditions and 55% in CW conditions was measured in bi-layer structures. The production process and the number of layers influenced the behavior of the samples, in particular the one regarding the spatial uniformity of the laser emission. Samples made by tape casting have shown overall a better thermal stability with respect to samples made by spray drying.

Owing to the high gain coefficient and excellent thermal properties, diamond has enabled efficient Raman frequency conversion of CW lasers in external Raman cavity configurations. While the potential for high power has received much attention, the detailed spectral properties of diamond Raman lasers have not been thoroughly investigated. One particular area of interest is in generating single-longitudinal mode (SLM) output for highly wavelength-specific applications such as atom cooling and spectroscopy. Till now, SLM Raman lasers have been realized either in low-power semiconductor spherical microcavities and waveguides or rather complex ring cavities and linear cavities including linewidth narrowing elements. Here, we report a high-power standing-wave external cavity DRL pumped by a tunable fiber-amplified diode laser of linewidth 50 MHz. Despite the broad Raman gain linewidth (45 GHz) which is about 35 times larger than cavity mode spacing, SLM Stokes output at powers up to 1.2 W was obtained without any intracavity frequency-selective element. This can be attributed to absence of spatial hole burning in Raman lasers as opposed to inversion lasers. Nevertheless, strong mode competition has been observed at higher output powers, leading to multimode operation. The pump wavelength was tunable from 1062.9 to 1064.4 nm by changing the diode seed laser temperature, providing a tuning range of Stokes wavelength from 1237.8 to 1240.2 nm. In addition, since the Raman shift changes with diamond temperature, active temperature control was employed to stabilize and tune the Stokes wavelength with an accuracy of 1.5 pm, thus suppressing mode hopping.

9726-50, Session 9

Structurally enhanced phosphate glasses for eye-safe lasers

Simi A . George, SCHOTT North America, Inc . (United States)

9726-52, Session 10

100W class green 10ps 280

µ

J laser with M2 1.4 using Z-slab amplifier

Simon P . Chard, Cristtel Y . Ramírez-Corral, Powerlase Photonics Ltd . (United Kingdom); Michael Bass, Ying Chen, CREOL, The College of Optics and Photonics, Univ . of

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Conference 9726: Solid State Lasers XXV: Technology and Devices

Central Florida (United States); Young K . Kwon, Powerlase Photonics Ltd . (United Kingdom) High-power visible and UV ultrashort pulse lasers are increasingly in demand for industrial micromachining. For many applications, high energy and low M2 are essential for achieving clean processing by cold ablation. However, energy scaling of ultrafast laser sources is often limited by nonlinear self-focusing. In this work we present a high-power, high-energy picosecond laser system based on a slab amplifier which has been designed to minimize this effect.

A master oscillator power amplifier system was built using a 5 W passively modelocked seed laser at 1064 nm and multiple ‘Z-slab’ power amplifiers. The Z-slab is a high aspect ratio Nd:YAG zigzag slab, edge-pumped by a pair of diode arrays. A multi-pass Z-slab was designed for improved high-energy performance by careful optimization of the slab dimensions and beam width to reduce both the B-integral and the effect of thermal distortions on the beam.

The system produced up to 120 W average power at 1064 nm, which was frequency doubled to 84 W at 532 nm. The repetition rate of the seed laser was varied from 250 to 1000 kHz with high beam quality (M2 < 1.4) throughout this range. A maximum energy of 400 µ J was generated in infrared, giving 280 µ J at 532 nm. The pulse duration after the amplifiers was 9.4 ps. The seed laser was also operated in burst mode with bursts of 1 - 10 pulses separated by 33 ns. Over 1 mJ total burst energy was demonstrated at 100 kHz repetition rate at 1064 nm.

The integration of micro-electro-mechanical systems (MEMS) into solid-state laser systems enables extra functionalities for temporal, spectral or spatial tuning of laser characteristics, while simultaneously offering miniaturised, low-cost alternatives to conventional bulk optics used in solid-state lasers.

We will present results showing the Q-switch temporal characteristics of a novel Nd:YAG laser system incorporating two intracavity MEMS micromirrors. This new arrangement extends the functionality of single micromirror based Q-switched solid-state lasers that we, and other research groups, have previously reported. The micromirrors used in our dual micromirror cavity configuration have different actuation mechanisms: one is electrostatically-controlled for fast resonant scanning at 10 kHz; the other is electrothermally-controlled for slower resonant scanning at 2 kHz or quasi-static displacement. This dual-micromirror technique enables the “fast scan” mirror to control the Q-switch pulse duration. Pulse durations ranging from 30 ns (cavity-limited) up to multiple microseconds can be achieved, with average output powers exceeding 50 mW. Further functionalities, such as pulse burst control or pulse-on-demand control, are achievable through simultaneous actuation of the “slow scan” micromirror. MEMS can also allow spectral control of solid-state laser outputs. Using an intracavity micromirror and a prism or diffraction grating, the output wavelength of a Yb:KGW laser platform can be tuned by controlling the angular position of the micromirror. We will present initial results on the spectral tuning characteristics, and discuss combining spectral and temporal control using MEMS micromirrors. This combined control could provide avenues for a miniaturised, flexible laser system impacting defence and industrial applications.

9726-54, Session 10

Random anti-reflection structures on large optics for high energy laser applications

Jesse A . Frantz, Lynda E . Busse, Jasbinder S . Sanghera, U .S . Naval Research Lab . (United States); Kevin J . Major, Gopal Sapkota, Menelaos K . Poutous, The Univ . of North Carolina at Charlotte (United States); Ishwar D . Aggarwal, Sotera Defense Solutions, Inc . (United States) Random anti-reflection surface structures (rARSSs) have been shown to increase the transmission of an optical surface to >99.9%. They are an attractive alternative to traditional thin film anti-reflection (AR) coatings for several reasons: They provide AR performance over a larger spectral and angular range; and unlike thin film coatings, they are patterned directly into the optic rather than deposited on its surface. As a result, they are not prone to delamination under thermal cycling that can occur with thin film coatings, and their laser damage thresholds can be considerably higher. In this work, an optimized reactive ion etch procedure was used to pattern rARSSs on fused silica windows, with performance optimized for high energy laser (HEL) applications at 1.06 thresholds at 1.06 µ µ m. We have demonstrated scale up of this processing technique for windows up to 12” in size. This work represents what we believe to be the largest diameter nanostructured surface on an inorganic material. The windows have been shown to have a laser damage m of >100 J/cm2 – approaching those of the substrate, and approximately five times higher than those of comparable, high quality thin film AR coatings. We present results for the AR properties and uniformity of these large windows.

9726-55, Session 11

Spectral and temporal control of Q-switched solid-state lasers using intracavity MEMS

Alan Paterson, Ralf Bauer, Ran Li, Univ . of Strathclyde (United Kingdom); Caspar Clark, Helia Photonics Ltd . (United Kingdom); Walter Lubeigt, Deepak Uttamchandani, Univ . of Strathclyde (United Kingdom)

9726-56, Session 11

Compact single-frequency polarization maintaining CW single stage fiber amplifier

Enkeleda Balliu, Magnus Engholm, Mid Sweden Univ . (Sweden); Lars Norin, Acreo FiberLab (Sweden); Gunnar Elgcrona, Jonas Hellström, Håkan Karlsson, Cobolt AB (Sweden) Our objective in this work is to demonstrate a highly compact, single frequency, polarization maintaining CW laser system (MOPA) at 1064nm (and 532/355 nm by frequency conversion). Our motivation is to reduce the complexity (and cost) of the overall laser system by using only one amplification stage and a few optical components. The overall system is based on a hybrid solid state laser (SSL)/fiber amplifier where a key component is a compact ring-cavity Nd:YAG solid state laser (SSL) operating in single frequency (10 kHz linewidth), with an ultra-low noise and an excellent beam quality. In the present configuration the SSL provides up to 500mW and with a PER of > 30dB but can be designed for power levels up to 3W. The single stage fiber amplifier is constructed by using PM fiber optic components with relatively small core/cladding dimensions of 10/125 um. A short (less than 2 m), custom made, highly Yb-doped fiber (pump absorption >3 dB/m at 920nm) with high photodarkening resistance is used, allowing for reduced non-linear effects (Stimulated Brillouin Scattering). In the present configuration, a (2+1)x1 PM combiner and two pump LD’s at 976nm (25W) provides SBS free output power levels up to 30W at 1064nm with a PER of more than 20dB. A custom made, anti-reflective coated fused silica endcap (2 x 4mm) is spliced to end of the active fiber. Single-pass, extra-cavity frequency conversion to 532nm (and 355nm) is demonstrated by using different non-linear crystals (LBO, KTP and PPKTP) with conversion efficiencies up to 40%.

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9726-57, Session 11

gratings Conference 9726: Solid State Lasers XXV: Technology and Devices High brightness sub-nanosecond Q-switched laser using volume Bragg

Brian M . Anderson, Evan Hale, George Venus, Daniel Ott, Ivan Divliansky, Leonid Glebov, CREOL, The College of Optics and Photonics, Univ . of Central Florida (United States) The design of Q-switched lasers capable of producing pulse widths of 100’s of picoseconds necessitates the cavity length be shorter than a few centimeters. Increasing the amount of energy extracted per pulse requires increasing the mode area of the resonator that for the same cavity length causes exciting higher order transverse modes and decreasing the brightness of the output radiation. To suppress the higher order modes of these multimode resonators while maintaining the compact cavity requires the use of intra-cavity angular filters.

A novel Q-switched laser design is presented using volume transmitting Bragg gratings (TBGs) as angular filters to suppress the higher order transverse modes. The laser consists of a 5 mm thick slab of Nd:YAG, a 3 mm thick slab of Cr:YAG with a 20% transmission, two orthogonally aligned TBGs to suppress the higher order modes, and a 40% output coupler. The gratings are recorded in photo-thermo-refractive (PTR) glass, which has a high damage threshold that can withstand both the high peak powers and high average powers present within the resonator. The TBGs recorded in PTR glass have narrow angular selectivity ranging from 0.1 to 10 mrad, and can spatially filter beams with a diameter ranging from 0.1 to 10 mm. Experimental results are presented demonstrating the Q-switched laser with a cavity length of 1.5 cm, millijoule level output, sub-nanosecond pulse width, and diffraction limited beam quality.

9726-58, Session 11

Solid-state lasers directly pumped by InGaN diode lasers: Ti:sapphire and Pr3+:LiYF4 lasers

Hiroki Tanaka, Ryosuke Kariyama, Kodai Iijima, Ryota Sawada, Fumihiko Kannari, Keio Univ . (Japan) We report our recent progress of solid-state lasers directly pumped by InGaN diode lasers of blue and green. Diode-laser-pumped mode-locked Ti:sapphire laser and praseodymium-doped LiYF4 lasers are demonstrated. A blue and green diode lasers deliver output powers of >3.5 W and >1 W. We applied these diode lasers to a Ti:sapphire laser which have been pumped by a frequency-doubled neodymium-doped lasers, and a mode locked Ti:sapphire laser is successfully demonstrated. We confirmed that the green-diode-laser pumping is advantageous compared with blue-diode laser pumping not only due to the higher Stokes efficiency but also the lack of additional absorption loss which is induced by shorter wavelength pumping.

The praseodymium-doped laser is one of the most successful lasers oscillating in visible region. It can be efficiently and directly pumped by the blue diode laser because it has absorption band around 440 nm. We report a CW operation at 640, 607 and 523 nm from the praseodymium doped LiYF4 laser, and slope efficiency of >40 % is achieved at 640 and 523 nm. In addition to the CW operation, we demonstrate a Q-switching of the laser at 640 and 607 nm by employing a Cr4+:YAG crystal as a saturable absorber, which have been recognized as a conventional passive Q-switching element for Nd:YAG laser. We finally report a mode-locking of the laser with a semiconductor saturable absorber mirror (SESAM) at 640 nm.

9726-59, Session 11

Tunability of the highly stable single frequency mode of a hybrid laser

Mamoun Wahbeh, Raman Kashyap, Ecole Polytechnique de Montréal (Canada) A hybrid laser, based on a C-band semiconductor optical amplifier (SOA) coupled to a long fiber external cavity, is carefully engineered to operate with high spectral purity. The fiber cavity is made of a piece of PM erbium doped fiber, whose one end is sculpted into a biconic lens with an FBG directly written on the other end. The fiber lens has an asymmetric focus, designed to couple the diverging light beam from the SOA into the fiber core with high efficiency. Also, the waveguide of the SOA is tilted relative to the AR facet to suppress axial mode instabilities. Thus combining the attributes of an FBG, erbium doped fiber and semiconductor optical amplifier, a highly stable 2-kHz linewidth single external-cavity mode operating with a SMSR of > 42 dB is demonstrated. Fine tunability of this mode within a single mode-spacing is experimentally achieved. The ability to scan 218 MHz with a frequency-current coefficient of ~2.18 MHz/mA is reported. Moreover by changing temperature of the external fiber Bragg grating, a range of 1.8 GHz is smoothly scanned at a resolution of ~170 MHz and with a frequency-temperature coefficient of 3.1GHz/K. Consequently, the desired operating wavelength can be roughly set by tuning Bragg wavelength and exactly reached via the bias current. A frequency accuracy of ±11 Hz is estimated while the output power at the peak lasing wavelength is measured to be 13.3 dBm. This compact laser with such simple structure and high spectral quality is sought by many segments of industry.

9726-60, Session 11

11.5W Yb:YAG planar waveguide laser grown by pulsed laser deposition

Stephen J Beecher, James A Grant-Jacob, Tina L Parsonage, Ping Hua, Jacob I Mackenzie, Dave P Shepherd, Robert W Eason, Univ of Southampton (United Kingdom) We present details on the fabrication, characterization and laser performance of a Yb:YAG planar waveguide grown by pulsed laser deposition. The 8mm long waveguide features a 15 µ m-thick-core of Yb:YAG grown onto a <100> YAG substrate. In a simple oscillator formed of a mirror highly transmissive for the pump light and highly reflecting for the signal and a 50% reflectivity output coupler, both proximity coupled to opposing waveguide facets, 11.5W of output power was observed with a threshold of 3.0W and a slope efficiency of 47% with respect to absorbed pump power. The waveguide also oscillated with feedback provided from the Fresnel reflection of ~8% from one of the uncoated facets, but with a significantly higher threshold of 7W. This performance, with output coupling of 92%, highlights the potential suitability of these devices for use as compact high gain amplifiers with very large mode areas, in our case ~20,000 µ m2. Further improvements in performance are expected by increasing the dopant concentration from 1.4 at.% to 3.0 at.% and increasing the pump power.

9726-61, Session 11

Donut beam generation in a hybrid fiber laser-pumped Ho:YAG laser

A C Butler, P C Shardlow, R T Uren, W A Clarkson, Optoelectronics Research Centre (United Kingdom) Laguerre-Gaussian (LG0m) beams are characterised by a donut-shaped intensity profile and have a wide range of applications. One emerging application area is in the field of laser micromachining of materials where the sharper transverse intensity gradient of the ring-shaped beam profile

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compared to a Gaussian fundamental (TEM00) mode yields better edge definition and edge quality.

In this paper we describe a power-scalable laser architecture for generating LG0m modes directly in a solid-state laser. Our approach employs a hybrid fiber-laser-pumped Ho:YAG laser, where the pump beam from a Tm fiber laser at 1908 nm is re-shaped to yield a near-field annular profile for the purpose of directly exciting the desired LG0m mode in an end-pumped Ho:YAG laser. The beam shaping element is a simple silica capillary fiber with air:silica radius ratio of 50%. The output from the capillary fiber is re imaged using an appropriate arrangement of lenses to spatially match the intensity profile of the required LG0m mode in the Ho:YAG laser to achieve preferential lasing on this mode. The combination of a low quantum defect pumping cycle and the ring-shaped pump deposition profile leads to weak thermal lensing opening up the prospect of high lasing efficiency and high output power. We report on preliminary results for direct generation of the first order (LG01) donut mode at 2.1?m and discuss a simple strategy for extending operation to higher order LG0m donut modes.

9726-62, Session 12

Frequency extension of a high power diamond Raman laser by intracavity second harmonic generation

Hadiya Jasbeer, Robert Williams, Aaron McKay, Richard Mildren, Macquarie Univ . (Australia) The excellent thermal properties of diamond have recently been shown to enable frequency conversion of high-power CW beams with high efficiency and high output beam quality. To date, up to 380 W at efficiencies above 60% have been demonstrated in the infra-red. The typically high intra-cavity Stokes intensities in these lasers indicates potential for further efficient frequency conversion via second-order parametric processes as a method to generate high power and high brightness visible and UV output at wavelengths important for applications such as laser guide-stars, medical treatments and remote sensing. Here we report intra-cavity frequency doubling of an external cavity diamond Raman laser (EC-DRL) pumped using a Nd:YAG laser at 1064 nm. A lithium triborate crystal was used in a near-concentric EC-DRL to convert the intracavity 1240 nm Stokes field to 620 nm. Pumping of the EC-DRL at power up to 300 W was used with on-time durations limited to 250 micro-seconds to provide a convenient method for demonstrating high power CW conversion. The period is sufficiently long to achieve steady-state thermal gradients in the diamond, thus providing results indicative of genuine CW operation (assuming negligible heat deposition in LBO). To date, we have achieved 2.1 W (on time power) of 620 nm with 1.4% power conversion efficiency with respect to 1064 nm pump. It is found that careful management of polarization properties of optical elements and nonlinear output coupling are crucial for increasing output power and efficiency, opening up prospects for further power scaling.

9726-63, Session 12

Conference 9726: Solid State Lasers XXV: Technology and Devices A 7.5-mJ, 21-ns, 7-kHz green rotary disk laser with diffraction limited beam quality

Santanu Basu, Basu Labs Inc . (United States) Visible lasers with high pulse energy and high repetition rate are required for several important applications such as high-precision material processing and adaptive optics. Fiber lasers are unable to produce high pulse energy due to mode size limitation. In this paper, we will present results of a Q-switched rotary disk laser that produces 7.5 mJ pulses at 515 nm at 7 kHz repetition rate. The peak power of the green laser is 350 kW. Due to the absence of aberrations in a rotary disk laser, the beam quality is measured to be diffraction limited.

9726-64, Session 12

Compact side-pumped passively Q-switched Yb:YAG laser with frequency conversion to UV

Brian Cole, Chris McIntosh, Alan D . Hays, Tom DiLazaro, Lew Goldberg, U .S . Army RDECOM CERDEC NVESD (United States) Laser sources in the UV have found utility for Raman based remote sensing applications. For this application, we have explored using a Yb:YAG passively Q-switched laser, side-pumped by a single bar diode, as a source for the fourth harmonic generation (FHG) at 257.5 nm. Side pumping of Yb:YAG with a pulsed, high peak power (100 W) laser diode bar offers a number of advantages: large pump intensities (>5kW/cm2) for efficient laser operation, a simplicity for resonator design, and compact size. The compact 30 mm long laser cavity, Q-switched by a Cr:YAG saturable absorber, was operated in a burst mode with the 5 mm pump diode pulsed for 2-4 ms at low duty cycles. Q-switched pulse repetition frequencies varied from 5-20 kHz depending on the Cr:YAG transmission, which was varied from 70% to 85%. Pump duration, pulse repetition frequency and output coupler reflectivity were optimized to yield maximum Yb:YAG laser average power and laser efficiency, while providing sufficient peak intensity, typically 0.3-1 MW, to enable efficient FHG. Pulse energies and durations were in ranges of 1-2 mJ and 2-3ns ranges, respectively. We achieved an optical efficiency of greater than 15% for the Yb:YAG laser. Extra-cavity 515 nm second harmonic generation (SHG) was achieved using a 5mm long KTP crystal. The 515 nm light was then frequency doubled by focusing it into a 7mm long BBO crystal, resulting in a 20% conversion efficiency from 1030nm to 257.5 nm, with an average UV power greater than 50 mW.

9726-66, Session 12

Development of high coherence high power 193nm laser

Satoshi Tanaka, Masaki Arakawa, Atsushi Fuchimukai, Yoichi Sasaki, Takashi Onose, Yasuhiro Kamba, Hironori Igarashi, Chen Qu, Mitsuru Tamiya, Shinji Ito, Koji Kakizaki, Gigaphoton Inc . (Japan); Hongwen Xuan, Yohei Kobayashi, The Univ . of Tokyo (Japan); Hakaru Mizoguchi, Gigaphoton Inc . (Japan) There remains a huge frontier in laser applications because of the constraints of a light source. It is the high power DUV region, in which single photon energy matches the covalent energy levels of various carbon compounds. Considering the absorption spectrum of oxygen, 193 nm is the shortest wavelength suitable for laser processing in air. At 193 nm, however, solid state lasers are difficult to produce even 1 W output, because there is not any durable nonlinear crystal available in the short wavelength region.

As a solution for this frontier, we have been developing a unique hybrid 193 nm ArF laser system which consists of a solid state laser for seeding and an ArF excimer laser for power-boosting. The high coherence quality of the solid state laser can be maintained even after the excimer boosting, enabling superior performance in the fields of interference exposure pattering, ablation process and so forth. The advantage of a solid state laser and that of an excimer compensate each other in this DUV hybrid system.

The solid state laser consists of Yb and Er fiber laser systems and wavelength mixing units [1]. We have got more than 0.3 W output at 6 kHz repetition rate. By seeding the 193 nm laser light into the ArF excimer laser, we have obtained 100 W averaged power with high coherence. In this paper, we will present details of the hybrid laser system and output performance as well as some feasibility test results for various applications.

[1] Hongwen Xuan et al. Optics Express 23, 10564 (2015).

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Conference 9726: Solid State Lasers XXV: Technology and Devices

9726-67, Session 12

White random lasing in mixture of ZnSe, CdS and CdSSe micropowders

Ahmed Y . Alyamani, King Abdulaziz City for Science and Technology (Saudi Arabia); Maksim S . Leanenia, National Academy of Sciences of Belarus (Belarus); Lafi M . Alanazi, Maher M . Aljohani, Abdulaziz A . Aljariwi, King Abdulaziz City for Science and Technology (Saudi Arabia); Nikolay V . Rzheutskij, Evgeniy V . Lutsenko, Gennadiy P . Yablonskii, National Academy of Sciences of Belarus (Belarus) Room temperature random lasing with “white” light emission in a mixture of AIIBVI semiconductor powders was achieved for the first time. The scattering gain media was formed by the mixture of closely packed active micron sized crystallites of ZnSe, CdS and CdSSe semiconductors. The micropowders were produced by grinding bulk crystals of each compound. Optical excitation was performed by 10-nanosecond pulses of tuned Ti:Al2O3-laser at 390 nm. The lasing in the mixture of semiconductor powders was achieved simultaneously at four wavelengths in blue, green, yellow and red spectral regions after exceeding the threshold excitation power density. A drastic integral intensity increase, spectrum narrowing and appearance of mode structure accompanied the laser action. ZnSe crystallites produce the laser light at about 460 nm while CdS particles – at about 520 nm. Two types of CdSSe semiconductor micropowders with different sulfur content lase at 580 nm and 660 nm. The threshold excitation power densities for all laser lines in the emission spectrum are approximately the same of about 900 kW/cm2. The sum of the emission spectrum of the mixture of the micropowders forms “white” light with high brightness. Lasing is due to an appearance of random feedback for amplified radiation in the active medium of closely packed light scattering crystallites. The presented results may find their applications in visualization systems, lighting technology, data transmission, medicine as biosensors and in identification systems. The key feature of random lasers is low cost of its production and possibility to be deposited on any type of surface.

22 SPIE Photonics West 2016 · www.spie.org/pw

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Conference 9727: Laser Resonators, Microresonators, and Beam Control XVIII

Monday - Thursday 15–18 February 2016 Part of Proceedings of SPIE Vol . 9727 Laser Resonators, Microresonators, and Beam Control XVIII 9727-1, Session 1

Solvent diffusion measurements using polymeric resonators based on whispering gallery modes

Amir R . Ali, The German Univ . in Cairo (Egypt) and Southern Methodist Univ . (United States); Catherine Elias, Sara Iskander, Khalid Al-Agha, The German Univ . in Cairo (Egypt); Tindaro Ioppolo, Southern Methodist Univ . (United States) This paper presents an interaction study of solvent with the polymeric spheres. When a cross-linked polymer interacts with a solvent the polymer swells. This effect is due to diffusion of polymers and solvent molecules into each other. In turn, that leads to change in the optical and mechanical properties of the polymeric spheres. Several experiments were carried out to study the solvent induced whispering gallery modes (WGM) shift using microsphere immersed in a solvent atmosphere. In the preliminary experiments the microsphere is placed above the surface of three different solvents (Methanol, Ethanol and Hexane) and the measurements of the WGM shift signal are acquired using a 16 bit DAQ card and recorded on a PC. Results show the observed WGM shift is mainly due to three effects: (1) change in the index of refraction of the medium surrounding the sphere; (2) change in the index of refraction of the polymer due to solvent diffusion and (3) change in the microsphere radius due to swelling. This behavior has been examined using preliminary experiments to fully investigate this behavior.

9727-2, Session 1

Long period gratings based frequency selective interrogation of micro-resonators along the same fiber

Daniele Farnesi, Istituto di Fisica Applicata “Nello Carrara” (Italy) and Museo Storico della Fisica e Ctr . Studi e Ricerche “Enrico Fermi” (Italy); Francesco Chiavaioli, Francesco Baldini, Istituto di Fisica Applicata “Nello Carrara” (Italy); Giancarlo C . Righini, Centro Studi e Ricerche “E . Fermi” (Italy) and Museo Storico della Fisica e Ctr . Studi e Ricerche “Enrico Fermi” (Italy); Silvia Soria, Cosimo Trono, Gualtiero Nunzi Conti, Istituto di Fisica Applicata “Nello Carrara” (Italy) Fiber optics sensors are ideal transducers for applications requiring devices that are durable, stable and insensitive to external perturbations. Additionally, they typically provide distributed or quasi-distributed measurement capability at multiple points. The goal of this work was to find an all-in-fiber coupling method to implement a quasi-distributed interrogation of whispering gallery mode (WGM) micro-optical resonators, which are known to exhibit unique properties for sensing. In a previous work, we proposed a configuration based on a long period grating (LPG) written in silica fiber followed by a thick fiber taper (with waist diameters in excess of 15 um), where coupling of cladding modes to WGMs occurs. This configuration is more robust than the standard fiber taper coupler but it does not allow interrogating more spheres coupled to the same fiber by monitoring the transmitted light. The new approach demonstrated in this work consists of replicating more times the same structure, which includes a second LPG (identical to the first one) that couples back the modulated light into the core. Typical Q-factors of the order of 10^8 and total coupling efficiency up to 60% were measured collecting the resonances of high-Q silica microspheres or microbubbles at the fiber end. Independent addressing of two different resonators at two different wavelengths (1519 nm and 1613 nm) along the same fiber was demonstrated.

9727-3, Session 1

Development of packaged silica microspheres coupled with tapered optical microfibres

Pengfei Wang, Harbin Engineering Univ . (China); Ramgopal Madugani, Okinawa Institute of Science and Technology Graduate Univ . (Japan); Haoyu Zhao, College of Science, Harbin Engineering University (China); Jonathan Ward, Yong Yang, Light-Matter Interactions Unit, OIST Graduate University (Japan); Gerald Farrell, Dublin Institute of Technology (Ireland); Gilberto Brambilla, Univ . of Southampton (United Kingdom); Síle G . NicChormaic, Okinawa Institute of Science and Technology Graduate Univ . (Japan) Previously, microresonator-based add-drop filters have been developed, with silica fiber tapers used to efficiently couple evanescent fields to and from microresonators for their characterization and use. However, it is difficult to maintain stable alignment between microresonators and fiber tapers for an extended period, which is a disadvantage when fabricating add-drop devices for practical, real-world applications. In order to increase the mechanical stability of the microsphere resonator coupling system, in this research, a high quality silica microsphere was first coupled and packaged with two low-loss optical tapered fibers, and their relative positioning was optimized under an optical microscope and then fixed on a microscope slide using a low refractive index UV curable epoxy. The use of a coating epoxy significantly increased the mechanical alignment stability of the microsphere-fiber tapers system. At wavelengths near 1550 nm, a high-Q mode of up to 0.9?105 can be efficiently excited via evanescent coupling from the input tapered silica fiber. The temperature dependence of the packaged silica microsphere add-drop filter has also been investigated. In order to simplify the fabrication, a one-step fabrication process has been developed using a microsphere coupled to and then packaged with an optical microfiber coupler. Both of the packaging techniques offer the potential to develop low-cost, robustly-assembled fully integrated applications including WDM, sensors, ultra-small optical tunable filters and integrated micro-lasers due to the simplicity of the fabrication process compared with a conventional, costly photolithographic technique.

9727-4, Session 1

Observation of optically induced transparency in a micro-cavity

(Invited Paper)

Yuanlin Zheng, Jianjun Cao, Wenjie Wan, Shanghai Jiao Tong Univ . (China) Electromagnetically induced transparency (EIT) allows for rendering optical opaque transmission windows to a transparent one under illumination of a second coherent light beam. It was first observed in an atomic gas; intermediately, much attention has been drawn to its unique optical properties including: slowing light, all-optical switching, photon storage,

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9727-5, Session 1

Conference 9727: Laser Resonators, Microresonators, and Beam Control XVIII

etc. This opens up tremendous opportunities for information science. However, atomic gas systems requires optical cooling or vapor heating techniques combined with vacuum isolation, making it difficult for chip scale integrations. Alternatively, classical analogies mimicking EIT effect are under active pursuit in various physical systems, including coupled resonators, photonic crystals, and plasmonic meta-materials. However, most of them relying on the linear coupling between the resonances, can only exhibit EIT-like spectra but lacking of active-controlled transparency. Until recently, successful attempts have cloned the idea of EIT in an opto mechanical micro-cavity to induce a narrow transparency with the aid of mechanical oscillations excited through Brillouin scattering nonlinearity, which provide phonons to couple some hybrid optical-mechanical resonances, similar to their counterparts: photons in the EIT. Here we report an optically induced transparency (OIT) scheme in a compact micro-cavity in ambient environment by exploring cavity-enhanced four-wave mixing gain to introduce a transparency window in an opaque resonance dip, which directly results from the interference between two resonances coupled nonlinearly through FWM process. Active-controlling of the OIT can be achieved by varying a strong pump beam. Furthermore, OIT observed here is a non-reciprocal one, since FWM gain is a unidirectional one owing to the conservation law of momentum.

Deterministic photon-atom and photon photon interactions based on single photon Raman interaction

Institute of Science (Israel)

(Invited Paper)

Orel Bechler, Serge Rosenblum, Itay Shomroni, Yulia Lovsky, Gabriel Guendelman, Barak Dayan, Weizmann

9727-6, Session 2

High-Q GRIN resonators

(Invited Paper)

Andrea M . Armani, Soheil Soltani, Hyungwoo Choi, Vinh Diep, Andre Kovach, Kelvin Kuo, The Univ . of Southern California (United States) High and ultra-quality factor (Q) optical resonators have been used in numerous applications, ranging from biodetection and gyroscopes to nonlinear optics. In the majority of the measurements, the fundamental optical mode is used as it is easy to predict its behavior and subsequent response. However, there are numerous other modes which could give improved performance or offer alternative measurement opportunities. For example, by using a mode located farther from the device surface, the optical field becomes less susceptible to changes in the environment. However, selectively exciting a pre-determined, non-fundamental mode or, alternatively, creating a “designer” mode which has one’s ideal properties is extremely challenging. One approach which will be presented is based on engineering a gradient refractive index (GRIN) cavity. We use a silica ultra-high-Q toroidal cavity as a starting platform device. On top of this structure, we can controllably deposit, layer or grow different materials of different refractive indices, with nm-scale precision, creating resonators with a GRIN region co-located with the optical field. Slight adjustments in the thicknesses or indices of the films result in large changes in the mode which is most easily excited. Even in this architected structure, we have maintained Q>1 million. Using this approach, we have demonstrated the ability to tune the properties of the device. For example, we have changed the thermal response and the UV response of a device by over an order of magnitude.

9727-7, Session 2

Laser nanofabrication for advanced micro cavities

(Invited Paper)

Hong-Bo Sun, Huai-Liang Xu, Xue-Peng Zhan, Qi-Dai Chen, Jilin Univ . (China) I will present our progress toward the demonstration of a completely passive scheme for deterministic state transfer between a single photon and a single atom and vice versa. Based on a series of theoretical works [1-5], this scheme relies on a 3-level system coupled to a single mode waveguide (a single 87Rb atom interacting with a fibre-coupled microsphere resonator in our case). This scheme swaps a flying qubit, encoded in the two possible modes of the photon, with a stationary qubit, encoded in the two ground states of the atom: the state of the incoming photon is mapped to the state of the atom, and the state of the atom is mapped to the state of the outgoing photon [2]. Beyond performing as a passive photonic quantum memory, this scheme can in principle be modified to perform a universal quantum gate [3]. The scheme is completely passive, requiring no control fields beyond the single photons pulses.

In the first experimental realization of this scheme [6], the state of the atom was shown to be determined by the direction of an in-coming single-photon pulse sent through the fiber. The state of the atom was read by a subsequent photon, thereby realizing all-optical switching of single photons by single photons. We then applied this scheme for demonstrating deterministic single-photon extraction from an incoming pulse with arbitrary number of photons [7]. This scheme, which can be applied with any atom-like 3-level L system, provides a building block for scalable quantum networks based on completely passive nodes interconnected and activated solely by single photons.

[1] D. Pinotsi & A. Imamoglu, Phys. Rev. Lett. 100, 093603 (2008) [2] G. Lin, X. Zou, X. Lin, and G. Guo, Europhysics Letters 86, 30006 (2009) [3] K. Koshino, S. Ishizaka & Y. Nakamura, Phys. Rev. A 82, 010301(R) (2010) [4] S. Rosenblum, A.S. Parkins & B. Dayan, Phys. Rev. A 84, 033854 (2011) [5] S. Rosenblum & B. Dayan, arXiv: quant-ph 1412.0604 (2014) ) [6] I. Shomroni, S. Rosenblum, Y. Lovsky, O. Bechler, G. Guendelman & B. Dayan, Science 345, 903 (2014) [7] S. Rosenblum, O. Bechler, Y. Lovski, I. Shomroni, G. Guendelman, and B. Dayan, under review (2015) In summary, we have designed and fabricated 3D deformed polymer microcavity by FsLDW via two-photon polymerization, and demonstrated that these microlasers enable high Q-factor and single-mode lasing output with a low lasing threshold at room temperature. From the far field intensity distribution pattern, the deformed microcavities show that they can operate either highly unidirectional emission or single mode output, or both depending on the designs. Because the precise 3D fabrication capability of FsLDW and the good compatibility of polymer to other materials, the realization of the creation of active unidirectional lasers in polymer in this work provides an important step towards the functional integrated organic optoelectronic devices.

9727-8, Session 2

Elastomer photonics

(Invited Paper)

Diederik S . Wiersma, Karlsruher Institut für Technologie (Germany) and European Lab . for Non-linear Spectroscopy (Italy) and Consiqlio Nazionale delle Ricerche-istituto Nazionale di Ottica (Italy) In this contribution we report on the realization of photonic components with embedded elastomers that deform when they are illuminated with light. This allows to create tunable structures where a light beam influences the optical response of the structure and hence can influence the response to a signal beam. In particular, one can tune efficiently this way the resonance frequency of a high Q resonator, but also create strong (but slow) optical non-linearities

24 SPIE Photonics West 2016 · www.spie.org/pw

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Conference 9727: Laser Resonators, Microresonators, and Beam Control XVIII

9727-9, Session 2

III-V-semiconductor vertically-coupled whispering-gallery mode resonators made by selective lateral oxidation

Stephane Calvez, Gaël Lafleur, Clément Arlotti, Alexandre Larrue, Pierre-Francois Calmon, Alexandre Arnoult, Guilhem Almuneau, Olivier Gauthier-Lafaye, Lab . d’Analyse et d’Architecture des Systèmes (France) it is possible to realize all-optical modulation on-chip based on the Kerr effect at a record low power of 36 uW. I will also discuss the generation of optical Kerr combs, and talk about the dependence of the input power on the transition from modulation instability, multi-free-spectral range (FSR) mode locking, and 1-FSR mode locking. The hysteresis behavior caused by the optical bistability of a nonlinear cavity plays an important role at the transition between the states; and due to this hysteresis nature, the mode locking state in a low-order FSR is achieved only when we reduce the pump power after strong pumping. I will also describe the competition between four-wave mixing and Raman gain, and will show that the Raman comb becomes dominant during the transition between multi-FSR mode locking states.

Integrated whispering-gallery mode resonators are attractive devices which have found applications as selective filters, low-threshold lasers, high-speed modulators, high-sensitivity sensors and even as nonlinear converters. Their performance is governed by the level of detrimental (scattering, bulk, bending) loss incurred and the usable loss represented by the coupling rate between the resonator and its access waveguide. Practically, the latter parameter can be more accurately controlled when the resonator lies above the access waveguide, in other words, when the device uses a vertical integration scheme. So far, when using such an integration technique, the process involved a rather technically challenging step being either a planarization or a substrate transfer step. In this presentation, we propose and demonstrate an alternative method to fabricate vertically-coupled whispering-gallery mode resonators on III-V semiconductor epitaxial structures which has the benefit of being planarization-free and performed as single-side top-down process. The approach relies on a selective lateral thermal oxidation of aluminium-rich AlGaAs layers to define the buried access waveguide and enhance the vertical confinement of the whispering-gallery mode into the resonator. As a first experimental proof-of-principle of this approach, 75 conference.

µ m-diameter micro-disk devices exhibiting quality factor reaching ~4500 have been successfully made. Further characterization results will be presented at the

9727-12, Session 3

To be announced

(Invited Paper)

Alexander L . Gaeta, Columbia Univ . (United States) No Abstract Available

9727-13, Session 4

Soliton induced Cherenkov radiation based chipscale frequency combs

(Invited Paper)

Victor Brasch, Michael Geiselmann, Martin H . P . Pfeiffer, Arne Kordts, Maxim Karpov, Hairun Guo, Michail Zervas, Junqiu Liu, Ecole Polytechnique Fédérale de Lausanne (Switzerland); Michael L . Gorodetsky, Lomonosov Moscow State Univ . (Russian Federation) and Russian Quantum Ctr . (Romania); Tobias J . Kippenberg, Ecole Polytechnique Fédérale de Lausanne (Switzerland)

9727-10, Session 3

High-Q resonators for soliton combs and optical gyros

States)

(Invited Paper)

Kerry J . Vahala, California Institute of Technology (United Two recent applications of nonlinear optics in high-Q microcavity systems will be described. First, the generation of highly coherent light in Brillouin microlaser systems will be examined. Experimental results in which this process is used for rotation sensing is then discussed. Rotation sensitivity of 20 degrees per hour is demonstrated. Second, the generation of frequency combs using dissipative solitons is discussed. Solitons are formed in silica disk resonators at a repetition frequency of 22 GHz. The solitons have a pulse width of 130 fs and are readily broadened. The phase noise of the detected pulse train is low in in the range of a good K-band oscillator.

9727-11, Session 3

Harmonic mode locking in a high-Q whispering gallery mode microcavity

(Invited Paper)

Takasumi Tanabe, Takumi Kato, Tomoya Kobatake, Ryo Suzuki, Akitoshi C Jinnai, Keio Univ . (Japan) Discovered in 2007, microresonator (Kerr) frequency combs have emerged as an alternative and widely investigated method to synthesize optical frequency combs offering compact form factor, chip-scale integration, multi-gigahertz repetition rates, broad spectral bandwidth and high power per frequency comb line. Since their discovery there has been substantial progress in fundamental understanding and experimental realizations. Yet, in no demonstration could two key properties of optical frequency combs, broad spectral bandwidth and coherence, be achieved simultaneously. Here we overcome this challenge by accessing, for the first time, soliton induced Cherenkov radiation in an optical microresonator. By continuous wave pumping of a dispersion engineered, planar silicon nitride microresonator, continuously circulating, sub-30 fs short temporal dissipative Kerr solitons are generated that constitute a coherent optical frequency comb in the spectral domain. Emission of soliton induced Cherenkov radiation caused by higher order dispersion broadens the spectral bandwidth to 2/3 of an octave, sufficient for self-referencing and the broadest coherent microresonator frequency comb generated to date. Once generated it is shown that the frequency comb can be fully phase stabilized. The overall relative accuracy of the generated comb with respect to a reference fiber laser frequency comb is measured to be 3*10e-15.

Our findings mark a critical milestone in the development of planar optical frequency combs, enabling applications in e.g. coherent communications, broadband dual comb spectroscopy and Raman spectral imaging.

Our results underscore the utility and effectiveness of planar microresonator frequency comb technology, that offers the potential to make frequency metrology accessible beyond specialized laboratories.

A cavity with an ultrahigh quality factor (Q) and a small size is attractive because it can confine light in a tiny space and allow the strong integration of light and matter. A toroidal microcavity made of silica is of particular interest because it exhibits an ultrahigh Q/V, where V is the mode volume of the cavity. In addition, it can employ the Kerr effect. I will show that

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Conference 9727: Laser Resonators, Microresonators, and Beam Control XVIII

9727-14, Session 4

On-chip diamond frequency combs and Raman lasers

(Invited Paper)

Marko Loncar, Pawel M . Latawiec, Vivek Venkataraman, Michael J . Burek, Harvard School of Engineering and Applied Sciences (United States) Owing to its wide bandgap, large linear & nonlinear refractive index, and excellent thermal properties, diamond is well suited for applications in nonlinear and high-power photonics. I will present our work on on-chip diamond frequency combs operating in telecom wavelength range, as well as the efforts aimed at realization of combs operating in the visible. In addition, I will discuss on-chip Raman lasers that emit at ~2 micron wavelength.

9727-17, Session 5

Mode control for square resonator microlasers

Yong-Zhen Huang, Ming-Ying Tang, Yue-De Yang, Jin-Long Xiao, Yun Du, Institute of Semiconductors (China) Mode selection in square resonator semiconductor microlasers is demonstrated by adjusting the width of the output waveguide coupled to the midpoint of one side. The simulation and experimental results reveal that widely tunable single mode lasing can be realized in square resonator microlasers. Through adjusting the width of the output waveguide, the mode interval of the high-Q modes can reach four times of the longitudinal mode interval. Therefore, mode hopping can be efficiently avoided and the lasing wavelength can be tuned continuously by tuning the injection current. For a 17.8-?m-side-length square microlaser with a 1.4-?m-width output waveguide, mode-hopping-free single-mode operation is achieved with a continuous tuning range of 9.2 nm.

9727-15, Session 4

Dynamics and generation of microresonator frequency combs

(Invited Paper)

Chee Wei Wong, Shu-Wei Huang, Jinkang Lim, Abhinav K . Vinod, Jinghui Yang, Univ . of California, Los Angeles (United States); Heng Zhou, Univ . of Electronic Science and Technology of China (China)

9727-18, Session 5

On the phase noise of Kerr comb RF photonic oscillators

Andrey B . Matsko, Wei Liang, Danny Eliyahu, Vladimir Ilchenko, Anatoliy A . Savchenkov, Lute Maleki, OEwaves, Inc . (United States) Recent advances in sub-wavelength nanoscale platforms have afforded the control of light from first principles, with impact to ultrafast sciences, optoelectronics and precision measurements. In this talk I will describe recent advances in chip-scale Kerr frequency comb oscillators. Coherent mode-locking is observed in the normal dispersion regime, verified by phase-resolved ultrafast spectroscopy at sub-100-attojoule sensitivities. A phase-locked frequency comb is also achieved over 3,600 modes spanning 65 THz at 18 GHz spacing. The normal dispersion architecture uncovers the mode-locking mechanisms in Kerr frequency combs, matched with first principles coupled-mode theory, complementing our efforts on precision optical clocks.

Miniature radiofrequency (RF) photonic oscillators based on microresonator Kerr frequency combs generate the highest spectral purity RF signals compared with other electronic and photonic devices with similar size and power consumption. The low spectral frequency phase noise of the Kerr comb oscillators is comparable with the best frequency multiplied ovenized quartz oscillators, while the high spectral purity is similar to the phase noise of the lab scale RF photonic devices. Understanding fundamental limitations on the performance of Kerr comb oscillators is critically important for further development of the technology. In this presentation we review the major known noise sources of the Kerr comb oscillator and discuss possible methods for its improvement.

9727-16, Session 4

Optical frequency comb and spectroscopy with crystalline resonators in MIR

(Invited Paper)

Nan Yu, Jet Propulsion Lab . (United States) High-Q whispering gallery mode resonators have been mostly studied in the visible and near-IR wavelength regions for optical frequency comb and spectroscopy. With crystalline materials, their use can be extended to the mid-IR beyond 2 µ m where molecular gases not only have very rich characteristic spectral lines but also very large absorption cross sections. In this paper, we describe our continued efforts of pushing whispering gallery mode resonator applications in the MIR wavelength region, including Kerr comb generation and molecular absorption spectroscopy. With a variety of MIR transmitting crystalline materials, we have investigated their Q and limiting factors, dispersion and spectral engineering, parametric oscillation and comb generation. We have also explored the utility and limitation of using high Q resonators for ringdown molecular absorption measurements.

9727-19, Session 5

Third order nonlinear phenomena in silica solid and hollow whispering gallery mode resonators

Silvia Soria Huguet, Istituto di Fisica Applicata Nello Carrara (Italy); Daniele Farnesi, Istituto di Fisica Applicata Nello Carrara (Italy) and Museo Storico della Fisica e Centro Studi e Ricerche “Enrico Fermi” (Italy); Andrea Barucci, Franco Cosi, Istituto di Fisica Applicata Nello Carrara (Italy); Giancarlo C . Righini, Istituto di Fisica Applicata Nello Carrara (Italy) and Museo Storico della Fisica e Centro Studi e Ricerche “Enrico Fermi” (Italy); Gualtiero Nunzi Conti, Istituto di Fisica Applicata Nello Carrara (Italy) Dielectric microspheres and microbubbles can confine light and sound for a length of time through high quality factor whispering gallery modes (WGM). We report efficient generation of nonlinear phenomena related to third order optical non-linear susceptibility ?(3) interactions in resonant silica microspheres and microbubbles in the regime of normal dispersion. The interactions here reported are: Stimulated Raman Scattering (SRS), and four wave mixing processes comprising Stimulated Anti-stokes Raman

26 SPIE Photonics West 2016 · www.spie.org/pw

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9727-20, Session 5

Conference 9727: Laser Resonators, Microresonators, and Beam Control XVIII

Scattering (SARS) and comb generation. SARS is always detected in presence of SRS, and never in the absence of SRS, in agreement with the theory of Bloembergen and Shen. Unusually strong anti-Stokes components and extraordinarily symmetric spectra have been observed. Resonant SARS and SRS corresponding to different Raman bands were also observed. The anti-Stokes Raman components are known to be coupled to the Stokes Raman ones, even for large dispersion. This coupling results in the growth of the anti-Stokes wave along the microresonator in direct proportion to the Stokes wave through an effectively phase-matched hyper-parametric process. A proof of the cavity-enhanced phenomenon is given by the lack of correlation among the pump, signal and idler: a resonant mode has to exist in order to obtain the pair of signal and idler.

Multi-scale nonlinear effects in whispering gallery mode resonators

Guoping Lin, Souleymane Diallo, Yanne K . Chembo, FEMTO-ST (France) with only ~85 mW pump threshold power in the feeding waveguide is shown along with continuous, mode-hop-free tuning over ~7.5 GHz in a compact, integrated-optics platform.

9727-22, Session 5

Novel ultrafast sources on chip: filter driven four wave mixing lasers, from high repetition rate to burst mode operation

(Invited Paper)

Alessia Pasquazi, Marco Peccianti, Univ . of Sussex (United Kingdom); Sai T . Chu, City Univ . of Hong Kong (Hong Kong, China); David J . Moss, RMIT Univ . (Australia); Roberto Morandotti, Institut National de la Recherche Scientifique (Canada) Whispering-gallery mode resonators are known to host a wide variety of nonlinear behaviors. In particular, a large body of literature has focused on Kerr optical frequency combs in recent years. Here we investigate a wider range of nonlinear behaviors, including Brillouin, Raman, and thermo-optical effects. We first perform an experimental investigation in order to evidence the various nonlinear phenomena of interest. Basically, these phenomena involve nonlinear scattering at widely spaced spatial scales, including the electronic (for Kerr), molecular (for Raman), lattice (for Brillouin) and resonator (for thermal) scales. In our study, the host materials are fluoride crystals with a quality factor of the order of one billion at one micron. We then develop various spatiotemporal formalisms in order to investigate these nonlinear phenomena. Our theoretical analysis enables us to explore and understand how extreme events such as thermo-optical relaxation oscillations can be triggered in the resonator. Our study is complemented with numerical simulations that are in full agreement with the experimental findings.

Passive fiber mode-locked lasers enable the excitation of multiple pulses per round trip representing a potential solutions for the increasing demand of practical optical sources with repetition rates of hundreds of GHz or higher. The control of such high repetition rate regimes is however a challenge. To this purpose, linear filters have been used in an “intracavity” configuration to force the repetition rate of the laser. This design is known as dissipative four wave mixing (DFWM) but it is usually unstable and hence marginally suitable for practical applications. We explore the use of nonlinear intracavity filters, such as integrated micro ring resonators, capable of “driving” the FWM interaction in the laser. We term this approach as Filter-Driven FWM.

With a proper choice of the filter properties in terms of free spectral range (FSR) and Q factor, we could observe stable regimes over a wide range of operating conditions, from high repetition rate oscillation at a 200GHz to the formation of two stable spectral comb replicas separated by the FSR of the main cavity (65MHz). High order filters, moreover, allow achieving nonlinear operation over large passbands. With an 11th order filter we achieve low-frequency mode-locking between the main cavity modes that oscillate within each resonance of the filter, producing burst pulsed operation. A stable mode-locked pulse train at 655GHz with an envelope of 42ps at 6.45MHz is achieved.

9727-21, Session 5

Diamond microresonator-based Raman laser at 2 ?m

Pawel Latawiec, Vivek Venkataraman, Michael J . Burek, Harvard Univ . (United States); Birgit J . M . Hausmann, Lawrence Berkeley National Lab . (United States); Irfan Bulu, Schlumberger-Doll Research Ctr . (United States); Marko Lon?ar, Harvard Univ . (United States)

9727-23, Session 6

Using mechanics to convert between microwave and optical frequencies

(Invited Paper)

Amit Vainsencher, Kevin J Satzinger, Greg A Peairs, University of California - Santa Barbara (United States); Andrew N . Cleland, Univ . of California, Santa Barbara (United States) Fully integrated microresonator Raman lasers offer the ability to generate unique wavelengths of light on-chip. To date, device demonstrations have been confined to silica and silicon, limiting the scope of generated wavelengths. Here, we introduce an integrated Raman laser based on high quality-factor (>400,000) diamond racetrack resonators. Synthetic single crystal diamond is a promising platform for Raman lasers due to its giant Raman shift (~40 THz), large transparency window (from UV to THz) and excellent thermal properties yielding a greatly enhanced figure-of-merit compared to conventional materials. A polished diamond plate was thinned to specification (<1 ?m) and bonded to a SiO2/Si substrate, after which ~600 ?m path length racetrack resonators were defined via electron-beam lithography. The pattern was transferred to the diamond with an oxygen based etch chemistry, and polymer coupling pads were aligned to the adiabatically tapered diamond bus waveguides. The demonstrated Raman laser shows a Stokes output discretely tunable over a ~100 nm bandwidth centered around 2 ?m with output powers >250 ?W, extending the functionality of diamond Raman lasers to a wavelength range at the edge of the mid-infrared useful for telecommunication. Continuous-wave operation We are building nanoscale interfaces to convert coherently between optical and microwave frequencies, ultimately to provide a quantum interface between different qubits operating at microwave frequencies, linked by an optical telecommunications channel. The design is based on piezoelectric optomechanical crystals that co-locate a high quality factor optical and a mechanical resonance in the same sub-micron scale volume, allowing very strong parametric interactions between the two modes. Building these structures from piezoelectric materials allows a strong electromechanical transduction, thus coupling an electric signal at a few GHz to the mechanical resonance at the same frequency. This then allows bilateral parametric coupling between the microwave electrical signal and a 1550 nm telecommunications signal. I will report on progress in this effort.

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+1 360 676 3290 · [email protected] 27

Conference 9727: Laser Resonators, Microresonators, and Beam Control XVIII

9727-24, Session 6

Droplet’s acoustics

Raphael Dahan, Technion-Israel Institute of Technology (Israel) Droplets represent a basic liquid structure that is contained by interfacial tension while bounded almost completely by free surfaces. Such droplets can host three types of resonances: optical-, capillary- and acoustical ones. Contrary to their capillary resonances (Rayleigh, 1879) and to optical resonances (Ashkin, 1977), droplets’ acoustical resonances were rarely considered. The challenge lies in the fact that ?droplets’ acoustics requires modulating forces at MHz rates. Here we rely on optical forces (that can act sufficiently quickly) to experimentally excite acoustical resonances at 37 MHz that starts vibrating at an optical threshold of 68 ?W. The optical modes that we are using as a mechanical exciter are circulating in the 40 ?m drop with a ?108 quality-factor. Our results open an experimental access to the acoustical resonances of droplets, which were rarely considered, neither theoretically nor experimentally. The native shape of the single-mode laser beam used for high power material processing applications is circular with a Gaussian intensity profile. Manufacturers are now demanding the ability to transform the intensity profile and shape to be compatible with a new generation of advanced processing applications that require much higher precision and control. We describe the design, fabrication and application of a dual-optic, beam shaping system for single-mode laser sources, that transforms a Gaussian laser beam by remapping – hence field mapping - the intensity profile to create a wide variety of spot shapes including discs, donuts, squares and rectangles. The optic pair transform the intensity distribution and subsequently flatten the phase of the beam, to generate spot sizes and depth of focus close to that of a diffraction limited beam. The field mapping approach to beam-shaping is a refractive solution that does not add speckle to the beam, making it ideal for use with single mode laser sources.

We describe a manufacturing process for refractive optics in fused silica that uses a freeform direct-write process that is especially suited for the fabrication of this type of freeform optic. The beam-shaper described above was manufactured in conventional UV-fused silica using this process. The fabrication process generates a smooth surface (<1nm RMS), leading to laser damage thresholds of greater than 100J/cm2; which is well matched to high power laser sources. Experimental verification of the dual-optic beam shaper will be presented.

9727-25, Session 6

Optical binding in white light

Shai Maayani, Technion-Israel Institute of Technology (Israel) We experimentally demonstrate, for the first time, binding of aeroslols with a variety of sizes and shapes in white light. The optomechancial interaction between particles is long range and at the underdamped regime. Incoherency allows mitigation of interference fringes to enable monotonically changing the distance between particles from 60 micron to touching - constituting a parametrically controlled testbed for transition studies at new scales.

9727-28, Session 7

Novel specialty fiber delivering flat-top beams with on-demand beam parameter product

Clémence Jollivet, Kevin F . Farley, Michael Conroy, Jaroslaw Abramczyk, Nufern (United States); Steffen Belke, Frank Becker, ROFIN-SINAR Laser GmbH (Germany); Kanishka Tankala, Nufern (United States)

9727-26, Session 6

Tweezers controlled resonator

Leopoldo L . Martin, Samuel Kaminski, Technion-Israel Institute of Technology (Israel) We experimentally demonstrate trapping a micro-droplet with an optical tweezers and then functionalize it as a micro-resonator by bringing it close to a tapered fiber coupler. Our tweezers facilitated tuning of the coupling from the under-coupled to the critical coupling regime with an optical Q of 12 million and micro-resonator size at the 85 um scale. We prove the concept of using an optical trap for activating oil droplets as fiber-coupled micro-resonators. We believe that our technique will extend to several resonators and then to an optical circuit where the shape and position of many optical devices will be controlled.

Our long-term vision includes optical circuits where a multi-minima optical trap shapes and positions multiple resonators. Being practical, we start here with modestly proving this concept by activating one drop as a resonator, and using an optical trap to hold and position it next to a tapered-fiber coupler.

The performance of an ever-increasing number of applications directly depends on the spatial properties of the optical beam such as the shape of its intensity profile and/or its angular divergence. More recently, the strong need for efficient all-fiber beam control techniques has been motivated by the progressive generalization of optical fiber-based systems.

The concepts of transverse mode up-conversion and beam control in all fiber devices are discussed. The shape of the beam emerging the fiber is defined by the shape of the individual transverse modes and their relative fractional power. On the other end, the angular divergence of the output beam, also called the Beam Parameter Product (BPP) depends on the order of the highest transverse mode excited. Therefore, to simultaneously control the beam intensity profile and the BPP, a specific mode scrambling must be achieved in the fiber.

Here, we report a novel specialty fiber designed to tailor transverse mode scrambling in a low-maintenance and cost-effective single-fiber device is reported. Numerical analysis of controlled mode mixing by tailoring the fiber design will be discussed with an emphasis on the high degree of scalability. Furthermore, the beam control performances will be experimentally demonstrated using a fiber specially designed to transform a single mode (SM) beam into a flat-top beam with targeted BPP values. Results for fibers with 50, 100 and 200 um core diameters transforming SM laser beams into flat-top beam profiles with BPP values around 2, 4 and 8 mm x mrad respectively will be discussed.

9727-27, Session 7

Field mappers for laser material processing

Paul Blair, Matthew O . Currie, Natalia Trela, Howard J . Baker, Eoin Murphy, Duncan Walker, Roy McBride, PowerPhotonic Ltd . (United Kingdom)

9727-29, Session 7

Real time M-square and beam parameter product measurement using GigE CMOS sensors

Michael J . Scaggs, Gilbert J . Haas, Haas Laser

28 SPIE Photonics West 2016 · www.spie.org/pw

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Technologies, Inc . (United States)

Conference 9727: Laser Resonators, Microresonators, and Beam Control XVIII

The ISO 11146-1 standard for measurement of a laser’s M-square requires the minimum measurement of five (5) spatial profiles within the first Rayleigh range and an addition five (5) outside the second Rayleigh range. The first five spatial profiles within the first Rayleigh range establish the beam waist and its location; the second five beyond the second Rayleigh range establish the divergence or convergence from the focusing lens for the M-square computation. The majority of methods used to date are all time averaged and as such are incapable of a real time M-square measurement. We present an ISO 11146-1 compliant method for measuring single shot M-square or beam parameter product values or the measurement of continuous wave sources at rates greater than five frames per second utilizing a pair of GigE based CMOS sensors.

One GigE CMOS sensor is setup to measure the minimum of five spots within the first Rayleigh range for the establishment of the beam waist and its location. A second GigE CMOS sensor is setup to measure the five spatial profiles beyond the second Rayleigh range for the determination of the beam divergence from the focusing lens. Both GigE cameras utilize optics that passively create multiple spatial time slices of the beam and superimpose these time slices on the CMOS sensor in real time resulting in the ability to make single pulse measurements or continuous wave measurements at speeds of greater than five frames per second with full ISO 11146-1 compliance.

9727-31, Session 8

Flexible assembly module for beam shaping product families based on support structures

Sebastian Haag, Fraunhofer-Institut für Produktionstechnologie IPT (Germany); Olaf Ruebenach, INGENERIC GmbH (Germany); Andreas Beleke, Fraunhofer-Institut für Produktionstechnologie IPT (Germany); Tobias Haverkamp, INGENERIC GmbH (Germany); Tobias Müller, Daniel Zontar, Fraunhofer Institut für Produktionstechnologie IPT (Germany); Christian Wenzel, Innolite GmbH (Germany) and Fraunhofer-Institut für Produktionstechnologie IPT (Germany); Christian Brecher, Fraunhofer-Institut für Produktionstechnologie IPT (Germany)

9727-30, Session 8

Monolithic fiber coupler for high power diode laser bars

Jens Meinschien, Thomas Mitra, Klaus Bagschik, LIMO Lissotschenko Mikrooptik GmbH (Germany) A novel micro optical element is introduced allowing coupling of light from several emitters of a laser diode bar into an optical fiber at high brightness. The approach is unique in terms of a simple design of the laser system in combination with its high brightness. Fiber coupled laser modules of up to 50 W output power from a 100 µ m fiber and NA 0.15 are feasible with just a single mini bar of 10 emitters and the monolithic fiber coupler as only optical element for beam shaping.

Standard configurations for fiber coupling of laser diodes bars require typically multiple optical elements a) for collimation in the fast axis, b) for collimation in the slow axis, c) for rearrangement of beam lets and d) for focusing to the fiber facet. The novel monolithic fiber coupler comprises all these functions into a single element. Special emphasis is drawn on the rearrangement of beam lets to generate a symmetrized beam parameter product which suits to that of the optical fiber.

The monolithic fiber coupler is designed with individual segments for each emitter of the laser diode bar providing two refractive surfaces for each emitter. In principle, beam shaping of the fast axis is done by the first surface whereas beam shaping of the slow axis is done by the second surface. The originally horizontally arranged emitters are rearranged by displacements/tilts of the segments to end up with a vertical stacking of the emitters at the fiber facet. By means of the monolithic fiber coupler, very cost effective fiber coupled laser diode modules based on bars are feasible. There are several advantages compared to current designs due to the very small number of pieces per laser modules and their small size. Thus, approaches based on laser diodes bars can also compete with single emitter solutions for pumping application due their superior power per pump module combined with inexpensive cost structure with very small numbers of pieces and mounting steps, consequently. Further applications of laser modules with monolithic fiber couplers may also be for direct material processing or as component in projection and illumination systems.

The versatility of products in the market of high-power diode lasers (HPDL) is high. HPDL modules vary in characteristics such as wavelength, form factor, number of emitters, and divergence angles in slow and fast axis. Most applications require the collimation of the beam emitted by the diode laser. Fiber-coupled laser modules usually require beam homogenization using a beam-tilting unit. Therefore, standard beam-shaping modules for HPDL consist of fast-axis collimators (FAC), slow-axis collimators (SAC), and a beam-tilting element. The architectures of laser modules from different manufacturers require different arrangements of the optical elements. It becomes obvious that providers of optical modules for HPDL are confronted with a high number of product variants. One flexible way of arranging optical elements for industrial laser production is the use of support structures – sometimes referred to as bottom tabs. The paper presents a flexible module used for the production of beam-shaping optical modules using bottom tabs. The assembly module has been designed for flexibility in order to support the efficient production of beam-shaping product families. Results regarding the achievable precision considering the process capability index and cycle time will be dicsussed.

9727-55, Session PTue

Thermal lensing measurement from the coefficient of defocus aberration using Shack-Hartmann wavefront sensor

Lebohang T . Bell, Council for Scientific and Industrial Research (South Africa); Darryl Naidoo, CSIR National Laser Ctr . (South Africa); Sandile Ngcobo, Council for Scientific and Industrial Research (South Africa); Andrew Forbes, Univ . of KwaZulu-Natal (South Africa) The effects of a temperature gradient in a laser gain medium in an end pumped configuration in a solid-state laser resonator, results in thermally induced aberrations. For our experiment, we measured the thermally induced lens from the coefficient of defocus aberration using a Shack Hartmann wavefront sensor. A collimated Gaussian beam at 633 nm was used as a probe beam to an Nd:YAG medium under active pumping, at room temperature. This result is compared to thermal lens determination where the end mirror curvature of an unstable solid state digital laser cavity is varied.

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+1 360 676 3290 · [email protected] 29

9727-57, Session PTue

Conference 9727: Laser Resonators, Microresonators, and Beam Control XVIII

9727-56, Session PTue

White light emission from high density GaN-based nano-umbrellas acting as whispering gallery mode resonators

Tetsuya Kouno, Shizuoka Univ . (Japan); Katsumi Kishino, Sophia Univ . (Japan); Masaru Sakai, Univ . of Yamanashi (Japan); Kazuhiko Hara, Shizuoka Univ . (Japan) High density GaN-based nano-umbrellas with an optical nanocavity were demonstrated. The GaN-based nano-umbrellas consist of the InGaN curbed hexagonal nanoplates; the diameter and thickness were approximately 200-800 nm and 50 nm, respectively, and GaN nanocolumns supporting the nanoplates; the diameter and height were approximately 50-200 nm and 1500 nm, respectively. As described, nano-umbrellas with various sizes appeared in the arrays. The GaN-based nano-umbrellas were grown on a nitrogen polarity c-plane GaN template by crystal growth tech nique of radio-frequency plasma-assisted molecular beam epitaxy. The photoluminescence (PL) measurement was carried out at room- (RT-) and low- (t = 4K) (LT) temperature. Each nano-umbrella emitted different colors in the arrays with the RT-PL measurement, resulting that the high density nano-umbrella arrays emitted wide range from approximately 365 to 750 nm wavelengths, and the emission corresponded to white light. The emission color of the nano-umbrellas probably depended on the diameter of the InGaN curbed hexagonal nanoplates. With the LT-PL measurement, many sharp peaks appeared in the wide range emissions of the LT-PL spectra from the high density nano-umbrellas, and the intensities of the sharp peaks nonlinearly increased with increasing the power density of the excitation laser. These sharp peaks were caused by the whispering gallery mode in each InGaN curbed hexagonal nanoplate. In these points, the sharp peaks were probably caused by the lasing actions. These results indicate that high density GaN-based nano-umbrellas are potentially used to white light lasers.

Investigations of a dual seeded 1178nm Raman laser system

Matthew K . Block, Leidos, Inc . (United States); Leanne J . Henry, Air Force Research Lab . (United States); Michael Klopfer, Ravinder Jain, The Univ . of New Mexico (United States) There is interest in frequency doubling linearly polarized narrow linewidth 1178 nm for sodium guidestar laser applications. One way of achieving this is through a Raman laser system seeded with both the pump at 1069 nm and narrow linewidth 1178 nm. In this system, 1069 nm is Raman converted to 1121 nm in a resonator cavity having high reflector fiber Bragg gratings (FBGs) centered at 1121 nm. Linewidth broadening in the resonator cavity has been found to result in significant power leakage around the FBGs, decreased 1121 nm power buildup in the resonator cavity, and decreased amplification of 1178 nm. In order to study what impacts this for the lower power stage in polarization maintaining (PM) 10/125 germanosilicate fiber, the effect of cavity length for a fixed FBG bandwidth of 3 nm was investigated. It was found that the effective reflectivity of the FBGs decreased as cavity length increased for a given intracavity 1121 nm power level with saturation at a minimum value occurring for cavity lengths greater than 90 m. Also, for fixed cavity length of 90 m, FBGs having bandwidths of 1, 2, 3, and 5 nm were investigated. It was found that the effective reflectivity of the FBGs decreased for a given 1121 nm intracavity power level as the bandwidth of the FBGs decreased. These results as well as the achieved 1178 nm output power levels and characteristics of both the lower and higher power stages in PM 10/125 and 20/400 germanosilicate fiber, respectively, will be reported.

9727-58, Session PTue

The finite-difference matrix for beam propagation: Eigenvalues and eigenvectors

Alan H . Paxton, Air Force Research Lab . (United States) Closed form solutions are known for the eigenvalues of matrices of the type resulting from the finite-difference approximation to the paraxial wave equation. We discuss the implications of these eigenvalues and the corresponding eigenvectors for obtaining solutions to the wave equation and for obtaining a feel for the form of these solutions.

9727-59, Session PTue

Intra-cavity generation of laser modes in a diode-pumped digital laser using incomplete amplitude masks

Sandile Ngcobo, Teboho Bell, CSIR National Laser Ctr . (South Africa) No Abstract Available

9727-60, Session PTue

Femtosecond laser-induced two-photon polymerization of whispering gallery mode microresonators

Nathália B . Tomazio, Instituto de Física de São Carlos IFSC-USP (Brazil); Xavier Roselló, Univ . de València (Spain); Adriano J . G . Otuka, Gustavo F . B . Almeida, Instituto de Física de São Carlos IFSC-USP (Brazil); Antonio D . Cremades, Miguel V . Andrés, Univ . de València (Spain); Cleber R . Mendonça, Instituto de Física de São Carlos IFSC-USP (Brazil) Whispering gallery modes microresonators have been attracting increasing interest due to their ability to strongly confine light within small dielectric volumes. This property is quite useful for basic research envolving light-matter interaction and nonlinear optics, but their applications go beyond. The ease of fabrication, on-chip integration and operation at telecommunication frequencies make them suitable for a variety of practical applications, including photonic filters and sensing. In the current work, we demonstrate the fabrication of such resonators via two-photon polymerization. Using this technique, complex 3D structures with submicrometer feature size can be produced. Besides, the flexibility of geometry and the possibility of incorporating a variety of additional materials, such as organic compounds make it a powerful tool for the fabrication of microresonators.

The microstructures we have fabricated are 45 ?m outer diameter hollow microcylinders with sidewall roughness estimated in 100 nm. Light from a 1540 nm-centered broadband source was coupled into the fabricated microresonators via evanescent coupling using a 1.5 ?m waist tapered fiber. The transmitted light is then guided to an optical spectral analyzer, where it was possible to measure resonances, represented as attenuation peaks, with free spectral range of about 9.6 nm. Such microresonators were subsequently functionalized with biopolymers in order to be used as sensors for biological applications.

30 SPIE Photonics West 2016 · www.spie.org/pw

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9727-62, Session PTue

Conference 9727: Laser Resonators, Microresonators, and Beam Control XVIII

9727-61, Session PTue

Sugar sensor based on GaN nanoring lasers grown by nanocrystal growth technique

Hoshi Takeshima, Tetsuya Kouno, Shizuoka Univ . (Japan); Katsumi Kishino, Sophia Univ . (Japan); Masaru Sakai, Univ . of Yamanashi (Japan); Kazuhiko Hara, Shizuoka Univ . (Japan) The GaN nanorings were fabricated via nanocrystal growth technique using radio-frequency plasma-assisted molecular beam epitaxy. The outside diameter, ring-width, and height of the nanorings were approximately 900 nm, 150 nm, and 900 nm, respectively. With the room temperature photoluminescence characterization using a nitrogen laser as an excitation source, the sharp peak appeared at 374.5 nm (FWHM: 1.3 nm) and the peak nonlinearly increased with increasing the excitation power. The result indicated that the lasing action was obtained from the nanorings, and the lasing action was caused by the whispering-gallery-mode (WGM); the resonant light propagated in a nanoring with total internal reflection at the boundary of the outside wall of the nanoring. The nanoring lasers were potentially used to the biosensors, because the evanescent component of the WGM is strongly affected by the ambient condition of a nanoring. Subsequently, the length of the effective resonant path may be changed, resulting in the changing in the lasing wavelength. We examined a sugar sensor based on the GaN nanorings lasers. In the experiment, the lasing wavelengths obtained from the nanorings were investigated with the aqueous solutions with various sucrose concentrations. With the result, the lasing peaks were red-shifted with 2.2 nm where the concentration of aqueous sucrose solution was increased from 31.2 % to 53.7 %. Note that the refractive index of the aqueous sucrose solution increases with increasing its concentration. The results evince that the GaN nanorings potentially used to label-free sugar sensors.

Effect of the properties of a microresonator with a surface layer on the resonance frequency

Gustav Schweiger, Thomas Weigel, Andreas Ostendorf, Ruhr-Univ . Bochum (Germany) the light-matter interaction is enhanced inside it. In terms of science and engineering, an interesting use of a WGM cavity is as a coupled cavities system. When two cavity modes are strongly coupled, they are split in the frequency domain and photons are transferred cyclically between the two modes in the time domain. Recently, the time-domain observation and control of the coupling states were reported with photonic crystal nanocavities, and this technology is essential for developing a quantum node and a quantum network. However, such experiments have not yet been achieved with ultra-high Q modes despite the potential benefit to be gained from the use of ultra-high Q cavities.

In this study, we observed strong coupling between ultra-high Q modes in the time domain for the first time. We employed two counter-propagating modes that coupled with each other via surface scattering in a silica toroid microcavity. We employed two tapered fibers (add-drop configuration), one for excitation and the other for observing the energy oscillation between two cavities, which is a necessary technique for directly observing energy in a cavity. The results revealed clear oscillatory behavior, which was induced by the strong coupling. In addition, the oscillation period in the time domain precisely matched that inferred from the mode splitting in the frequency domain, and the measured results showed excellent agreement with those calculated with the developed numerical model.

9727-64, Session PTue

Random laser action in Al nanoparticle/ Rh6G-doped silica gel

Chao Yang, Guoying Feng, Hong Zhang, Sichuan University (China); Shouhuan Zhou, North China Research Institute of Electro-Optics (China) In this work, we prepared Al nanoparticles/Rh6G doped silica gel and observed random laser action by pumping it with nanosecond laser pulses for the first time. The random laser threshold, the dependence of emission spectra with the outputting direction and the bulk sample’ physical and chemical stability was also experimentally studied. A significant lasing mode centered at 559.7 nm has been observed above the threshold. The intensity of laser emission varies from different directions. The specific output wavelength and remarkable machinability as well as the physical and chemical stability both make it a novel solid-state random laser source for potential applications in biosensors or medical imaging.

Optical microresonators have attracted much interest due to their potential as highly sensitive sensors in a variety of fields, for example as label free biological sensors. For this application the resonator - often a highly transparent microsphere - has to be functionalized by additional surface layers. We present a straight forward theoretical model using geometrical optics to analyze the effect of changes in the resonator properties and the surrounding medium on the resonance wavelength. It is shown that the interaction of the sensor with the surroundings can be expressed by the magnitude of the phase change by total reflection on the resonator surface. This holds for naked resonators as well as for a resonator with a layer.

9727-65, Session PTue

Problems of uniform focal spot formation by means of deformable mirror

Alexis V Kudryashov, Julia Sheldakova, Alexey Rukosuev, Vadim Samarkin, Anna Lylova, Moscow State Univ . of Mechanical Engineering (Russian Federation) In this paper we present recent results of formation of different beam intensity distribution by means of bimorph deformable mirrors. We discuss the results of such formation as well as the problems that one faces on this way. A new method for beam structure modification is suggested based on the use of Shack-Hartmann wavefront sensor with the combination of standard M2 meter.

9727-63, Session PTue

Time-domain observation of strong coupling between counter-propagating ultra-high Q whispering gallery modes

Wataru Yoshiki, Zhelun Chen, Keio Univ . (Japan); Tomohiro Tetsumoto, Shun Fujii, Keio University (Japan); Takasumi Tanabe, Keio Univ . (Japan)

9727-66, Session PTue

Nonperturbative monitoring of dissipative Kerr solitons in microresonators

Michael L . Gorodetsky, Lomonosov Moscow State Univ . (Russian Federation) and Russian Quantum Ctr . (Russian An ultra-high Q whispering gallery mode (WGM) cavity is attractive because

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Federation); Maxim Karpov, Hairun Guo, Erwan Lucas, Ecole Polytechnique Fédérale de Lausanne (Switzerland); G . Lihachev, Lomonosov Moscow State Univ . (Russian Federation) and Russian Quantum Ctr . (Russian Federation); Tobias J . Kippenberg, Ecole Polytechnique Fédérale de Lausanne (Switzerland) Dissipative Kerr solitons that can be generated in optical microresonators provide stable optical frequency combs and low-jitter femtosecond optical pulses demanded in many applications. We demonstrate a method for nonperturbative real time tracking of dynamical behavior of solitons in microresonators using weak phase modulation of the pump driven by vector network analyzer (VNA) at sweeping frequency. The response function in soliton state demonstrates two resonances. Theoretical analysis using Lagrangian perturbation approach and numerical simulations reveal that this two-lobe function characterizes detuning of the pump from resonance, the number of solitons, existence domains and proximity to switching points. Experimental results obtained for two different platforms (integrated SiN microrings and crystalline MgF2 microresonators are in good agreement with theoretical predictions.

9727-32, Session 9

Conference 9727: Laser Resonators, Microresonators, and Beam Control XVIII Nanofiber Bragg grating cavities for quantum optical devices

Andreas W . Schell, Hideaki Takashima, Yasuko Oe, Shinjiro Fujita, Shigeki Takeuchi, Kyoto Univ . (Japan) To efficiently address quantum optical emitters, e.g., nitrogen vacancy centers in diamond or quantum dots, a variety of different micro and nano cavities has been introduced – all of them with specific pros and cons. Main requirement for the cavities are to have a reasonable high quality factor, good and reliable ways to address and read out the emitter, and the tunability of the system over a broad wavelength range. Here, we will show Nanofiber Bragg grating cavities (NFBCs), which are cavities directly produced on the thin region of tapered optical fibers. Using focused ion beam milling, two Bragg mirrors are created by milling a grating into the fiber, which modulates the effective index of refraction for the guided mode. The so produced cavities have high quality factors, small mode volumes, allow for highly efficient coupling of the photons directly to the guided mode of the optical fiber, and are easily tunable. We will show experiments of controlled coupling of quantum emitters to NFBCs and show efficient addressing and read out. Performing numerical simulations, we gain insight into the behavior of the cavities and evaluate potential improvements to the cavity design, such as one-sided cavities, in order to further optimize and adjust the NFBCs to different quantum emitters. challenging because a high degree of indistinguishability requires resonant excitation schemes while electrical pumping using pin-diode structures is intrinsically non-resonant. To tackle this issue we propose an on-chip quantum optics concept where an integrated microlaser resonantly excites a QD-micropillar cavity acting as non-classical light source. As such, our approach combines two very active but so far independent routes of cavity quantum electrodynamics (cQED), namely high-? lasing and single QD light matter interaction at the quantum in a novel, integrated device concept. To be more specific, our on-chip device concept utilize an electrically pumped whispering gallery mode microlaser as in-plane coherent light source which is used for p-shell and strict resonant excitation of an integrated QD-micropillar operating in the weak coupling regime of cQED. Using this concept, we demonstrate electrically triggered emission of non classical light from the QD-micropillar. To this end, our concept also allows for on-chip photon detection and electro-optical feedback, to pave the way for integrated ultra-high bit-rate random number generators.

9727-34, Session 9

On forced Rayleigh scattering observed in crystalline whispering gallery mode resonator

Anatoliy Savchenkov, Andrey B . Matsko, Vladimir S . Ilchenko, Lute Maleki, OEwaves, Inc . (United States) We describe theoretically, and demonstrate experimentally, power dependent stimulated Rayleigh scattering in monolithic optical resonators. The effect originates from nonlinear interaction of nearly degenerate high-Q optical modes from two different mode families. It results in total coherent back-reflection of the light coupled to the resonator when the optical power exceeds certain threshold. The effect can be used for tracking and stabilizing optical power as well as for controllable optical gating and modulation.

9727-35, Session 9

Efficient second harmonic generation in an on-chip high-Q crystalline microresonator fabricated by femtosecond laser

Jintian Lin, Shanghai Institute of Optics and Fine Mechanics (China); Yingxin Xu, Zhejiang Univ . (China); Zhiwei Fang, ShanghaiTech Univ . (China); Min Wang, Shanghai Institute of Optics and Fine Mechanics (China); Wei Fang, Zhejiang Univ . (China); Ya Cheng, ShanghaiTech Univ . (China)

9727-33, Session 9

On-chip quantum optics using integrated quantum dot microlasers

Pierce Munnelly, Matthias M . Karow, Tobias Heindel, Technische Univ . Berlin (Germany); Martin Kamp, Sven Höfling, Christian Schneider, Julius-Maximilians-Univ . Würzburg (Germany); Stephan Reitzenstein, Technische Univ . Berlin (Germany) The prospect of studying quantum optics effects in the solid state and the development of quantum light sources for applications in the field of quantum communication has triggered enormous efforts in the fabrication of micro- and nanocavity systems with embedded quantum dots (QDs). The further development of such quantum devices towards applications in quantum technology focusses to a large extend on the realization of triggered sources of indistinguishable photons and efficient schemes for electrical pumping. Combining these two goals has turned out to be highly Recently, high-Q microresonators have been fabricated in lithium niobate (LN) wafer by femtosecond laser direct writing, followed by focused ion beam (FIB) polishing and thermal treatment. The Q factor has reached 2.45?10^6 in the fabricated LN microresonators which is only about one order of magnitude lower than the theoretical limit. Due to the high-Q factor, small mode volume, and large second-order nonlinear susceptibility, the LN microresonators has allowed for second harmonics generation (SHG) at ultra-low pump powers. It is well known that in the nonlinear frequency conversion processes, phase matching often plays a determining role in terms of the achievable conversion efficiency. However, in a WGM microresonator, it is not straightforward to achieve phase matching because not only the fundamental and second waves should be at least partially phase matched but also their wavelengths must be tuned into resonance with the modes in the microresonators. Here, we overcome the difficulty by controlling the thickness of the LN microdisk and selectively exciting high-order mode of the fundamental wave in the microresonator. Thanks to the low optical absorption and the high nonlinear optical coefficient of LN crystal, we achieve a normalized conversion efficiency of 1.1?10^(-3)/mW.

32 SPIE Photonics West 2016 · www.spie.org/pw

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9727-37, Session 10 9727-39, Session 10

Conference 9727: Laser Resonators, Microresonators, and Beam Control XVIII

9727-36, Session 9

PT-symmetric microdisk lasers

(Invited Paper)

Qinghai Song, Harbin Institute of Technology Shenzhen Graduate School (China) Recent developments in parity-time (PT) symmetric systems have ushered in unique photonic devices with enhanced functionalities. While single mode laser emission has been demonstrated in such systems, the current designs face severe challenges in applications, either due to their stringent requirement on fabrication precision or non-scalability to larger devices. Here we demonstrate a general mechanism to achieve single-mode lasing in coupled cavities, which relies on external mode coupling and overcomes these drawbacks. We find significant gain enhancement for selected modes by external coupling, and our experiments have confirmed the resulting single-mode laser emission in size-mismatched photonic molecules (PMs), when only one constituent cavity is pumped. This behavior persists for a wide range of pump power, from transparent threshold to gain saturation, and it is highly tolerant of fabrication imprecisions. In addition, the output intensity of such single-mode lasers also displays enhancement when compared with the same PMs under uniformly pumping. We believe our results will both advance the understanding of different coupling scenarios in coupled cavities and improve the characteristics of on-chip laser sources for practical applications.

Micro-resonator-based electric field sensors with long durations of sensitivity

Amir R . Ali, German Univ . in Cairo (Egypt); Nada El Ghandoor, Shaimaa El Baklish, The German Univ . in Cairo (Egypt); Ben Wise, Volkan Ötügen, Tindaro Ioppolo, Southern Methodist Univ . (United States) In this paper, we present a new fabrication method for the whispering gallery mode (WGM) micro-sphere based electric field sensor based which allows for longer time periods of sensitivity. Recently, a WGM-based photonic electric field sensor was proposed using a coupled dielectric microsphere-beam. The external electric field imposes an electrtrostriction force on the dielectric beam, deflecting it. The beam, in turn compresses the sphere causing a shift in its WGM. As part of the fabrication process, the PDMS micro-beams and the spheres are curied at high-temperature (100oC) and subsequently poled by exposing to strong external electric field (~8 MV/m) for two hours. The poling process allows for the deposition of surface charges thereby increasing the electrostriction effect. This methodology is called curing-then-poling (CTP). Although the sensors do become sufficiently sensitive to electric field, they start de-poling after a short period (within ~ 10 minutes) after poling, hence losing sensitivity. In an attempt to mitigate this problem and to lock the polarization for a longer period, we use an alternate methodology whereby the beam is poled and cured simultaneously (curing-while-poling or CWP). The new fabrication method allows for the retention of polarization (and hence, sensitivity to electric field) longer (~ 1500 minutes). An analysis is carried out along with preliminary experiments. Results show that electric fields as small as ~ 100 V/m can be detected with a 300 um diameter sphere sensor a day after poling.

Whispering gallery mode plasmonics and biosensing

(Invited Paper)

Frank Vollmer, Max-Planck-Institut für die Physik des Lichts (Germany) The structure of biopolymers is known to change with temperature, affecting for example the activity of enzymes and the folding of proteins. We have now shown that such structural change can be monitored using highly sensitive “whispering gallery mode” (WGM) optical biosensors. We devised a technique to temperature-stabilize the WGM sensor system, by adding glycerol to a WGM glass microsphere placed in an aqueous sensing environment. The temperature stabilized WGM sensor was utilized to track structural changes in biopolymers, by monitoring frequency shifts associated with optical polarizability change. Using this sensing method, structural changes in albumin protein (figure) could be resolved for only one degree in temperature variation. Such highly sensitive detection of structural change can have many important applications: in single molecule studies, for thermodynamic investigations of biological systems, and for monitoring temperature-dependent reaction kinetics. We have improved the time resolution of our biosensing platform, potentially resolving the kinetics of complex molecular systems on timescales ranging from few nanoseconds to several hours.

Asymmetric microspheres allow for free-space coupling and enable “stand off” biosensing and whispering-gallery mode measurements in the far field.

Various unpublished examples for novel single molecule measurements will be shown.

E. Kim, M.R. Foreman, M.D. Baaske, F. Vollmer, “Thermal characterisation of (bio)polymers with a temperature-stabilised whispering gallery mode microsensor” Applied Physics Letters 106, 161101 (2015) S. Rosenblum, Y. Lovsky, L. Arazi, F. Vollmer, B. Dayan, “Cavity ring-up spectroscopy for ultrafast sensing with optical microresonators” Nature Communications accepted (2015) Z. Ballard, M.D. Baaske, F. Vollmer, “Stand-Off Biodetection with Free-Space Coupled Asymmetric Microsphere Cavities” Sensors 15, 8968-8980 (2015) M.R. Foreman, J. Swaim, F.Vollmer “Whispering Gallery Mode Sensors” Adv. Opt. Photon. 7(2), 168-240 (2015)

9727-40, Session 10

Optofluidic ring resonator technology platform for label-free biosensing, optomechanics, optofluidic lasers, and micro-gas chromatography

(Invited Paper)

Xudong Fan, Univ . of Michigan (United States) The optofluidic ring resonator (OFRR) is a thin-walled micron-sized glass capillary that integrates high Q-factor ring resonator (>106) and capillary based microfluidics. In the OFRR, the circular shaped capillary cross section forms the optical ring resonator that supports the optical whispering gallery modes (WGMs) traveling along the circumference. The WGM has an evanescent field extended into the capillary core, and therefore, interacts with molecules inside the capillary. This presentation sits at the intersection of photonics, micro/nanofluidics, mechanics, biomedicine, nanobiotechnology, and analytical chemistry, as well as nanoelectronics and fundamental physics. I will start with the introduction of the OFRR, describing its optical and mechanical properties. Then I will discuss how to utilize the OFRR technology platform in the following vastly different areas: (1) label-free biosensing; (2) microfluidic optomechanics; (3) optofluidic laser; and (4) multi-dimensional micro gas chromatography. Finally, if time permits, I will discuss how we expand the OFRR concept, i.e., synergy of photonics and microfluidics, to detect analytes in vapor and liquid phases at the molecular or cellular level for various applications, and to develop novel photonic devices whose performance can be precisely controlled by biological processes and interactions.

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Conference 9727: Laser Resonators, Microresonators, and Beam Control XVIII

9727-67, Session 10

Whispering-gallery mode resonator sensors based on liquid droplets

Rosa Zullo, Antonio Giorgini, Saverio Avino, Pietro Malara, Paolo De Natale, Gianluca Gagliardi, Istituto Nazionale di Ottica (Italy)

9727-42, Session 11

Microfluidic determination of vitamin D3 and its binding protein using a fluorescent cylindrical microcavity

Stephen Lane, Peter West, Univ . of Alberta (Canada); Alexandre François, The Univ . of Adelaide (Australia); Al Meldrum, Univ . of Alberta (Canada) Over the last decade, optical whispering-gallery modes (WGMs) have been observed in solid micro-cavities of various geometries and materials showing Q factors higher than 109. These resonators were studied for laser emission, non-linear optics, comb generation and recently proved ultra-sensitive bio-chemical probes. The peculiarity of WGMs supported by dielectric microspheres and toroids is that light travels along closed paths at the interface between the surface of the resonator and the surrounding environment. Unfortunately, most of the light circulates inside the resonator and only a small fraction is actually used for light-matter interaction, thereby reducing the effective cavity enhancement. Here, we propose to use liquid droplets as micro-resonators for sensing applications. The droplet itself serves as the cavity and the sample at the same time, where the internal optical field is directly used to probe dissolved analytes or particles. We demonstrate free-space excitation and laser frequency locking on whispering-gallery modes in vertically-suspended liquid droplets. The Q-factor limit is investigated by means of cavity photon lifetime measurements performed with cavity ring-down techniques. Q-factors ranging from 105 to > 107 are observed in the near-infrared and visible spectral regions. Mixtures made from different liquids are also used as a proof-of-concept of chemical sensing. The droplet system appears very promising for applications to spectroscopy, biosensing, material characterization and non-linear optics.

We developed a microfluidic optical resonance biosensor and demonstrated its refractometric and biosensing performance. The device uses the whispering gallery modes (WGMs) that form within a glass capillary whose channel walls are coated with a high-index layer of fluorescent silicon quantum dots (QDs). These cylindrical resonances appear as periodic maxima in the fluorescence spectrum emitted by the quantum dots. Here we utilize the WGMs of a fluorescent core microcapillary for protein detection. The device is comprised of a glass microcapillary with a 50-?m-diameter inner channel coated with a thin film of silicon QDs. Most of the resonant electromagnetic field of the WGMs is contained within the QD layer but a fraction extends into the capillary channel where it samples a fluidic analyte pumped inside. Changes in the local analyte refractive index cause shifts in the resonant WGM wavelengths, providing the sensing transduction mechanism. The refractometric sensitivity of this device was found to be between 3 and 24 nm per refractive index unit depending on the QD film thickness. Biosensing was demonstrated using the biotin-avidin system, by first functionalizing the channel surface (i.e., the QD film) with amine-terminated polyelectrolyte layers. This fluorescent device showed concentration detection limits on the order of 10 nM, and an equilibrium association constant of 1.1 x 106 M-1 for the biotin-neutravidin interaction. We then demonstrated the nonspecific binding of vitamin D binding protein as the first step toward developing a microfluidic competition assay for vitamin D.

9727-41, Session 11

Quantum dot optofluidic lasers and their prospects for biochemical sensing

(Invited Paper)

Alper Kiraz, Koç Univ . (Turkey) and Univ . of Michigan (United States); Qiushu Chen, Univ . of Michigan (United States); Mehdi Aas, Koç Univ . (Turkey); Alexandr Jonas, Istanbul Technical Univ . (Turkey); Xudong Fan, Univ . of Michigan (United States) We achieved three types of laser emissions with aqueous quantum dots (QDs) using the same high-Q-factor optofluidic ring resonator (OFRR) platform. In the first type, 2 ?M QDs were in bulk buffer solution that filled the entire OFRR cavity volume. The lasing threshold was 0.1 ?J/mm2, over 3 orders of magnitude lower than the state-of-the-art. In the second type, the QDs were immobilized as a single layer on the interface between the OFRR inner wall and buffer solution with a surface density as low as 3 ? 109–1010 cm–2. The lasing threshold of 60 ?J/mm2 was achieved. In the third type, we achieved optofluidic FRET lasing using QDs as FRET donors and Cy5 dye molecules as acceptors. We observed lasing from Cy5 emission band in QD Cy5 pair when excited at QD absorption band, far away from Cy5 absorption maximum. The demonstrated capability of QDs as donors in FRET lasers greatly improves the versatility for optofluidic laser operation due to the broad and large absorption cross-section of QDs in the blue and UV range.

I will also discuss the comprehensive theoretical analysis of optofluidic FRET lasers that we have performed based on a Fabry-Perot microcavity using a rate equation model. By comparing FRET lasing-based sensors with conventional sensors using FRET signals obtained by spontaneous fluorescence emission, we show that for optimal pump fluence and FRET pair concentration, FRET lasing can lead to more than 100-fold enhancement in detection sensitivities of conformation changes for linker lengths in the Förster radius range.

9727-43, Session 11

Flow sensor using a hollow WGM microlaser

Jonathan M . Ward, Yong Yang, Síle G . NicChormaic, Okinawa Institute of Science and Technology Graduate Univ . (Japan) Optical sensing of flow in microfluidic systems is an important area of study. Flow sensing using the concept of a “hot WGM microlaser” is presented. Silica microcapillary or microbubble whispering gallery resonators were coated with a layer of laser glass, in this case Yb:Er doped phosphate glass. This was realised by the fact that the two glasses have different melting points. A CO2 laser was used to melt a small piece of doped glass wire onto an 80 µ m silica capillary. The power of the CO2 laser was controlled to flow the doped glass around the capillary. The resulting geometry is a hollow microbottle shaped resonator. The Er:Yb doped glass outer layer was pumped at 980 nm via a tapered optical fiber and whispering gallery mode (WGM) lasing was recorded at 1535 nm.

Gas was then passed through the capillary and the WGMs were observed to shift towards shorter wavelengths due to the cooling effect of the gas flow. In this way thermal tuning of the lasing modes over 70 GHz was achieved. The end of the capillary was connected to a mass flow sensor and the WGM shift rate as a function of flow rate and pump laser power was measured. Results were fitted using the theory of hot wire anemometry.

Flow sensing can also be realized when the cavity is passively probed at 780 nm. At this wavelength the Q factor of the WGMs was estimated to be in excess of 105.

34 SPIE Photonics West 2016 · www.spie.org/pw

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9727-44, Session 11

Localized biomolecules immobilization in optical microbubble resonators

Simone Berneschi, Francesco Baldini, Andrea Barucci, Istituto di Fisica Applicata “Nello Carrara” (Italy); Alessandro Cosci, Museo Storico della Fisica e Ctr . Studi e Ricerche “Enrico Fermi” (Italy) and Istituto di Fisica Applicata “Nello Carrara” (Italy); Franco Cosi, Istituto di Fisica Applicata “Nello Carrara” (Italy); Daniele Farnesi, Museo Storico della Fisica e Ctr . Studi e Ricerche “Enrico Fermi” (Italy) and Istituto di Fisica Applicata “Nello Carrara” (Italy); Gualtiero Nunzi Conti, Istituto di Fisica Applicata “Nello Carrara” (Italy) and Museo Storico della Fisica e Ctr . Studi e Ricerche “Enrico Fermi” (Italy); Giancarlo C . Righini, Museo Storico della Fisica e Ctr . Studi e Ricerche “Enrico Fermi” (Italy); Silvia Soria, Sara Tombelli, Cosimo Trono, Istituto di Fisica Applicata “Nello Carrara” (Italy); Stefano Pelli, Istituto di Fisica Applicata “Nello Carrara” (Italy) and Museo Storico della Fisica e Ctr . Studi e Ricerche “Enrico Fermi” (Italy); Ambra Giannetti, Istituto di Fisica Applicata “Nello Carrara” (Italy) Optical microbubble resonators (OMBRs) are gaining more and more interest as suitable platform for sensing applications because they combine the whispering gallery mode (WGM) resonator properties with an embedded microfluidics. In particular, their operation as optical biosensors is based on the fact that, for a suitable value of the OMBR wall thickness, the WGM optical field extends on both sides of the wall. Therefore the inner surface can be used for the interaction with the analyte inside the flowing fluid and the outer one for the resonance excitation by an external guiding structure (i.e.: a tapered fiber or an optical waveguide). Due to the morphological dependence of the WGMs, any change on the OMBR inner surface related to any biochemical bond causes a shift of the resonances and produces a resonance linewidth broadening with a decrease of the Q factor value. By measuring these changes, information about the concentration of the analyte to be detected can be achieved. A crucial step for the development of an OMBR – based biosensor is constituted by the functionalization of its inner surface. Here we report on the development of an ad-hoc spatially selective photo-chemical procedure, concerning a localized immobilization of fluorescent biomolecules on the OMBR inner surface, able both to maintain high Q factor (> 10^5) for the optical transducer in buffer solution and to guarantee that the OMBR is the unique biosensing element of the overall optical device. The OMBR characterization involves fluorescence microscopy and real time measurement of the resonance broadening.

9727-45, Session 11

Conference 9727: Laser Resonators, Microresonators, and Beam Control XVIII Optical heterodyne detection for ultra high Q micro-disk laser sensor

Myung-Gi Ji, Byung-Hee Son, Tae-Ryong Kim, Mi Jung, Chung-Ang Univ . (Korea, Republic of); Hong-Seung Kim, Chil-Min Kim, Daegu Gyeongbuk Institute of Science & Technology (Korea, Republic of); Kwang Ryong Oh, Electronics and Telecommunications Research Institute (Korea, Republic of); Young-Wan Choi, Chung-Ang Univ . (Korea, Republic of) In this paper, we propose a bio-sensing method using optical heterodyne detection based on optical technique for ultra-high Q micro-disk laser (MDL) as sensor platform. MDL structure with ultra-high Q-factor(~108) has advantages in detecting a small variation of the lasing wavelength.

For example, when a single molecule is attached to sidewall of MDL, the lasing wavelength is changed by sub-pico meter. Optical spectrum analyzer(OSA) has limits to detect sub-pico meter variation in the resonant wavelength because of the spectral resolution. In order to overcome this limitation, we used a heterodyne detection method which needs two MDLs with the same characteristics.

One is used as a reference laser which has a 1550 nm lasing wavelength(?1), and the other laser is used as a sensor for the detection of molecules attached to sidewall of MDL structure. In that case, sub-pico meter variation in lasing wavelength ?1 +?? (sub-pico meter) can be expected.

We performed a heterodyne detection using light beating, and measured ?? changing a sub-pico meter wavelength band to beating frequency ranging from several hundred MHz to several GHz band.

9727-46, Session 11

Water-walled microfluidics

Shai Maayani, Technion-Israel Institute of Technology (Israel) Liquids serve microcavity research ever since Ashkin’s studies on optical resonances in levitating droplets to recent optofluidic resonators. Droplets can provide optical quality factor (Q) in proximity to the limit restricted by water absorption and radiation loss. However, water micro-drops vaporize quickly due to their large area/volume ratio. Here we fabricate a water-air interface that almost entirely surrounds our device, allowing for >1,000,000 recirculations of light (finesse). We sustain the droplets for >16 hours using a nano-water-bridge that extends from the droplet to a practically-unlimited distant-reservoir that compensates for evaporation. Our device exhibits surface tension 8000-times stronger than gravity that self-stabilizes its shape to a degree sufficient to maintain critical coupling as well as to resolve split modes. Our device has 98% of their surrounding walls made strictly of water-air interfaces with concave, convex or saddle geometries, suggesting an arbitrary-shape microfluidic technology with water-walls almost all-over.

9727-47, Session 11

Spinning optical resonator sensor for torsional vibrational applications measurements

Amir R . Ali, The German Univ . in Cairo (Egypt) and Southern Methodist Univ . (United States); Andrew Gatherer, Rice Univ . (United States) Spinning spherical resonators in the torsional vibrational applications could cause a shift in its whispering gallery mode (WGM). The centripetal force acting on the spinning micro sphere resonator will leads to these WGM shifts. An analysis and experiment were carried out in this paper to investigate and demonstrate this effect using different polymeric resonators. In this experiment, centripetal force exerted by the DC-Motor on the sphere induces an elastic deformation of the resonator. This in turn induces a shift in the whispering gallery modes of the sphere resonator. Materials used for the sphere are polydimethylsiloxane (PDMS 60:1 where 60 parts base silicon elastomer to 1 part polymer curing agent by volume) with shear modulus (G=1kPa), (PDMS 10:1) with shear modulus (G=300kPa), polymethylmethacrylate (PMMA, G=2.6*10^9GPa) and silica (G=3*10^10 GPa). The sphere size was kept constant with 1mm in diameter for all above materials. The optical modes of the sphere exit using a tapered single mode optical fiber that is coupled to a distributed feedback laser. The transmission spectrum through the fiber is monitored to detect WGM shifts. The results showed the resonators with smaller shear modulus G experience larger WGM shift due to the larger mechanical deformation induced by the applied external centripetal force. Also, the results show that angular velocity sensors used in the torsional vibrational applications could be designed using this principle.

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Conference 9727: Laser Resonators, Microresonators, and Beam Control XVIII

9727-48, Session 12

A beam quality measure for cylindrical vector beams

(Invited Paper)

Andrew Forbes, Bienvenu I . Ndagano, Melanie G . McLaren, Univ . of the Witwatersrand (South Africa) In the 1990s statistical techniques were applied to laser beams with the result that new measures were introduced to define the beam quality. The moment analysis could be applied to any beam, returning a quantitative measure of its ?quality?. The value of the approach was that the quality could be described by a single number. In this work we apply tools from quantum mechanics to find a single quality factor for how ?vector? a vector beam really is. The applicability of quantum tools to these beams is due to the inherent non-separability of vector beams, a trait they share with entangled quantum states. We show how to practically implement this in the laboratory and show how the approach may be used to monitor degradation in vector beams from lasers, for example, radially polarized laser beams.

Brillouin gain spectrum, the SBS threshold will be enhanced significantly and the power scaling for these narrow linewidth lasers is easier accordingly. In this paper, we report a kilowatts-level, narrow linewidth, all-fiber format, linearly polarized laser source at 1064 nm in master oscillator-power amplifier (MOPA) configuration. The laser source consists of a linearly polarized, narrow linewidth (~20 GHz) double cladding (DC) fiber laser oscillator and two stages of linearly polarized DC fiber amplifiers. A linearly polarized laser beam with >1kW average power, ~ 35 GHz linewidth, >14.4 dB polarization extinction ratio (PER), and diffraction-limited beam quality (M2 < 1.1), was achieved. The high power narrow linewidth, transform-limited laser are pretty suited for coherent beam combining, seeding high power Nd:YAG laser, nonlinear laser frequency conversion, etc.

9727-51, Session 12

Beam control through nonlinear propagation

(Invited Paper)

Jean Claude M . Diels, Ladan Arissian, The Univ . of New Mexico (United States)

9727-49, Session 12

Selective generation of Laguerre Gaussian mode output in double resonator configuration

Ji Won Kim, Dong Joon Kim, Eun Jee Park, Hanyang Univ . (Korea, Republic of); Minjee Jeon, Hanyang Univ . (Korea, Republic of) and Korea Institute of Industrial Technology (Korea, Republic of); Hoon Jeong, Korea Institute of Industrial Technology (Korea, Republic of) We report a simple technique to allow selective generation of the Laguerre Gaussian mode (LG0n) output in an end-pumped Nd:YAG laser. Our approach employs the double resonator configuration, which is composed of two cavities sharing a single gain medium and an input coupling mirror but having two resonating beams of different output coupling with the aid of an intracavity polarizer. Since the population inversion density in the gain medium is determined by the threshold of the first excited cavity (secondary cavity), the other cavity (the primary cavity) using a higher transmittance output coupler cannot lase on the same transverse mode due to the higher threshold level. As a result, we can select the excited transverse mode of the primary cavity simply by controlling the lasing condition of the secondary cavity. Based on this approach, we already demonstrated selective excitation of the fundamental Gaussian TEM00 or the first-order Laguerre Gaussian LG01 mode and, moreover, generation of the laser output with a tailored beam profile in an end-pumped Nd:YAG laser. Here, we extend our technique to generate a higher order LG mode output in an Nd:YAG laser yielding the LG01, LG02 or LG03 mode output with well-determined helical wavefronts. The prospects of power scaling and further improvement will be discussed along with the underlying mechanism and the previous results.

Diffraction and atmospheric distortion limits the propagation of laser beams. “Filamentation was presented as a solution, by self-trapping intense beams, with the transmission limited “only” by nonlinear effects. The “only” evolved in a plethora of parameters to control . . . and understand.

In this field of light matter interaction it becomes difficult to distinguish the controlling from the controlled parameter. The power of the original beam is drained by emission at various wavelengths.

Two types of mechanisms are involved in generating these wavelengths: (i) emission from individual molecules (self-phase modulation, shock, fluorescence), and (ii) collective mechanisms (four wave mixing, inversion gain) where the new wavelengths are amplified by a group of molecules. With ultrashort pulses, the amplification at a particular wavelength proceeds along a direction group velocity matching. Resonant amplification occurs when a population inversion is created between electronic, vibrational and rotational levels; inversion affected by molecular orientation, which is in turn controlled by the beam polarization. The filamented beam can be controlled through the seeding process (involving single molecules) and the amplification process. Seeding of new wavelengths is considerably reduced by starting with long, bandwidth limited pulses, and/or a beam waist in vacuum as initial condition. Filaments produced with GHz bandwidth UV pulses are presented, which do not loose energy to “conical emission”. It will be shown how control of the amplification process is made through manipulation of the initial beam polarization. The continuum spectrum, as well spectral lines can be enhanced through polarization control.

9727-52, Session 13

Largest in the world bimorph deformable mirror for high-power laser beam correction

(Invited Paper)

Alexis V . Kudryashov, Moscow State Univ . of Mechanical Engineering (Russian Federation)

9727-50, Session 12

1 kW monolithic linearly polarized narrow linewidth single-mode fiber laser

Wei Shi, Tianjin Univ . (China); Qiang Fang, HFB Photonics, Inc . (China) Here we report the design and installation of the largest bimorph deformable mirror (420x480 mm). This mirror is intended to correct for the aberrations of the final amplifier in FIREX laser complex in Osaka University (Japan). Initial surface and main optical properties of such a mirror are presented. Response functions of all 120 electrodes were investigated. The results of mirror test showed the possibility to build of such kind of mirrors with the size of 500 mm and larger apertures.

High power narrow linewidth linearly polarized fiber lasers have been widely utilized in various applications, such as the nonlinear frequency conversion, coherent laser beam combining etc. The power scaling for the narrow linewidth fiber lasers is challenging mainly due to the limitation from the stimulated Brillouin scattering (SBS). However, when the laser linewidth is broadened to GHz level, which is much greater than the bandwidth of the

36 SPIE Photonics West 2016 · www.spie.org/pw

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Conference 9727: Laser Resonators, Microresonators, and Beam Control XVIII

9727-53, Session 13

Discrete excitation of mode pulses using a diode-pumped solid-state digital laser

Sandile Ngcobo, Teboho Bell, CSIR National Laser Ctr . (South Africa) No Abstract Available

9727-54, Session 13

Intracavity generation of low-loss radial order Laguerre-Gaussian modes using digital holograms

Lebohang T . Bell, Council for Scientific and Industrial Research (South Africa); Kamel Äit-Ameur, ENSICAEN (France); Andrew Forbes, Univ . of KwaZulu-Natal (South Africa); Sandile Ngcobo, Council for Scientific and Industrial Research (South Africa) Laguerre-Gaussian, LGpl, modes of radial-order p and azimuthal order l=0 were generated inside the resonator. By using amplitude mask encoded on digital holograms, that were displayed on a spatial light modulator, acting as an end-mirror of the resonator. The digital holograms were encoded to act as an amplitude mask that contained absorbing rings that matched the zeros of the desired Laguerre-Gaussian mode. We show that we can generate LGp0 of p=0 to p=4, when using full circular absorbing rings and also when using half circular absorbingrings. We will demonstrate the advantages associated with using half circular absorbing rings. We observed that the laser resonator will have a lower threshold, while at the same time maintain the same laser characteristics. The characteristics such as; slope efficiency, mode purity and beam quality factor.

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Conference 9728: Fiber Lasers XIII: Technology, Systems, and Applications

Monday - Thursday 15–18 February 2016 Part of Proceedings of SPIE Vol . 9728 Fiber Lasers XIII: Technology, Systems, and Applications 9728-1, Session 1

Multi-kilowatt power scaling and coherent beam combining of narrow-linewidth fiber lasers

(Invited Paper)

Iyad Dajani, Angel Flores, Roger H . Holten, Thomas Ehrenreich, Air Force Research Lab . (United States) In contrast to stimulated Brillouin scattering (SBS) where larger mode field areas can lead to greater suppression in fiber amplifiers, the modal instability (MI) threshold is typically reduced with increased core sizes. Consequently, the fiber SBS/modal instability threshold tradeoff plays a key role for further power scaling. We report results from two ~1.5kW, all-fiber Yb-doped amplifiers with comparable optical to optical efficiencies and effective linewidths; one with a 25 µ m core fiber from Nufern and one with a 20 µ m core fiber from LEIKKI. SBS suppression in both amplifiers was achieved through pseudo-random bit sequence (PRBS) phase modulation which has proved to be more effective than white noise source. Both amplifiers can provide fringe visibilities >90% when a sample of the output is coherently combined with a low power channel. While the power output from the 25 20 µ µ m core fiber was MI limited at ~1.5 kW, no sign of MI was observed in the smaller core fiber. Furthermore, there were strong indications that the Brillouin gain coefficient was substantially lower in the m core fiber. Overall, this may allow us to utilize the higher MI threshold 20 µ m fiber to scale further in power while maintaining sufficiently narrow linewidth for either coherent or spectral beam combination. Finally, as a demonstration of the combinability of multiple kW class amplifiers driven by PRBS phase modulation, we coherently combined 5 commercial amplifiers using a 1x5 diffractive optical element leading to a 5 kW beam with a measured M2 value of 1.06. The combining efficiency was 82%.

9728-3, Session 1

Kilowatt high-efficiency narrow-linewidth monolithic fiber amplifier operating at 1034 nm

Nader A . Naderi, Angel Flores, Kenneth B . Rowland Jr ., Iyad Dajani, Air Force Research Lab . (United States) Development of Yb-doped fiber lasers has seen rapid progress with emerging wide range of applications. Operating at shorter wavelengths of the gain bandwidth is desired due to lower quantum defect heating which may be beneficial in suppressing the modal instability (MI), and in order to expand the wavelength range in spectral beam combining architectures. To date, high-power narrow-linewidth monolithic fiber amplifiers operating at wavelengths <1040 nm are much less developed compared to their 1060 nm counterparts. We report here on a kilowatt narrow-linewidth monolithic fiber amplifier operating at 1034 nm. To suppress SBS, the single-frequency master oscillator was phase modulated using pseudo-random bit sequence (PRBS) format. By utilizing a band pass filter and a WDM, as well as by optimizing the length of fiber used in the pre-amplifier stages, we were able to appreciably suppress unwanted lasing. The pre-amplifier was spliced onto a mode field adapter (MFA) followed by a (6+1)?1 tapered fiber bundle (TFB). The high-power stage was powered by six ?250 W pump diodes centered at 976 nm. The final stage consisted of ~8 meters of a conventional non-PM 25/400 µ m Yb-doped fiber from Nufern. The output of the amplifier was terminated with a cladding mode stripper and an endcap. With an optimal PRBS pattern set at a clock rate of 4.4 GHz, we demonstrated 1 kilowatt of power with a slope efficiency of 84% and an ASE content of <3%. Beam quality measurements at 1 kilowatt provided M2 values of 1.09-1.17 with no sign of modal instability.

9728-2, Session 1

10-kW peak power femtosecond pulses from a mode-locked fiber ring laser emitting at 2.8

µ

m

Simon S . Duval, Michel Olivier, Vincent Fortin, Martin Bernier, Michel Piché, Réal Vallée, Ctr . d’Optique, Photonique et Laser (Canada) The direct generation of high-peak-power femtosecond pulses from a compact and efficient source in the mid-infrared spectral region (2-20 µ m) is of great interest for ultrabroad and ultrasensitive spectroscopy in the molecular fingerprint region as well as for medical and defense applications. Recently, we demonstrated the first mid-infrared femtosecond fiber laser operating at a wavelength above the transparency limit of silica fibers. This mode-locked fiber ring laser emitting at 2.8 µ m is based on an erbium doped double-clad fluoride fiber. Mode locking is achieved via nonlinear polarization evolution through the use of a quarter waveplate, a half waveplate and an optical isolator that also acts as a polarizer. End caps are spliced on both ends of the fluoride fiber to prevent fiber tip degradation and ensure long term mode-locked operation. We introduce here an improved cavity design based on numerical simulations. By adjusting the output coupling and the erbium-doped fiber length, 294-fs pulses with a peak power of 10.1 kW are generated at a repetition rate of 96.5 MHz. Since water vapor has an important absorption band around 2.8 µ m, the impact of the intracavity atmospheric absorption on the laser dynamics is also discussed. Numerical simulations, which agree well with the experimental results, suggest that the generation of 150-fs pulses with peak powers above 20 kW should be feasible without intracavity dispersion compensation.

9728-4, Session 1

Kilowatt-level narrow-linewidth monolithic fiber amplifier based on laser gain competition

Nader A . Naderi, Iyad Dajani, Angel Flores, Air Force Research Lab . (United States) Recent advances in power scaling of narrow-linewidth monolithic fiber amplifiers have made them suitable candidates for many applications including coherent beam combination for directed energy platforms. One major obstacle to power scaling of narrow-linewidth fiber amplifiers, however, is the onset of stimulated Brillouin scattering (SBS) which is the lowest threshold nonlinear process for such amplifiers. Over the past decade, several effective SBS mitigation techniques including the application of phase modulation and thermal gradients have been demonstrated. Another novel approach is based on the laser gain competition technique which relies on seeding the amplifier with two laser sources; one broadband while the other is narrow-linewidth possessing a longer wavelength. The SBS suppressing mechanism behind this technique relies on reduction of the effective length of the amplifier due to a sudden rise in the narrow-linewidth signal at the output end of the fiber. One key advantage of laser gain competition approach is its compatibility with other SBS mitigation techniques such as phase modulation and thermal gradient. In this study, we investigate power scaling of a two-tone Yb-doped monolithic fiber amplifier seeded with 1064 nm narrow-linewidth light along with spectrally broader 1034 nm light. For further SBS suppression, pseudo random bit sequence (PRBS) phase modulation was applied. A factor of 13 dB in SBS threshold enhancement was realized at a PRBS clock rate of 2.5 GHz; leading to 1 kW of output power at 1064 nm with 81% optical

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efficiency. The beam quality was near the diffraction limit with no sign of modal instability.

9728-5, Session 1

Conference 9728: Fiber Lasers XIII: Technology, Systems, and Applications 2 kW single-mode fiber laser with 20-m long delivery fiber and high SRS suppression

Yasuhiro Mashiko, Huy K . Nguyen, Masahiro Kashiwagi, Tomoharu Kitabayashi, Kensuke Shima, Daiichiro Tanaka, Fujikura Ltd . (Japan) available wavelengths. High power operation of such amplifiers with narrow linewidth seed signals is frequently limited by non-linear effects and higher order mode instabilities of output beam due to an interaction between the fundamental mode and higher order modes. In this paper we report a development of fiber amplifier with >1.5kW output power and M2-value of the beam <1.2 in broad wavelength band from 1030nm to 1070nm for 15GHz linewidth and up to 3m output delivery cable. The fiber amplifier with direct laser diode pumping in the ruggedized compact modular package has 40% wall-plug efficiency. The maximum output power of the broadband amplifier at longer wavelength edge of the band was limited by mode instability effects. The spectral dependence of higher order mode instability threshold and quantum defect heating are described and discussed.

A 2 kw single-mode fiber laser with a 20-m long delivery fiber and high back reflection resistance has been demonstrated. An Yb-doped fiber with large core size and differential modal gain is used to realize high SRS suppression and single-mode operation simultaneously. A length of a delivery fiber is 20 m that gives flexibility to the design of processing systems. An output power of 2 kW is achieved at a pump power of 2.86 kW. A slope efficiency is 70%. The Stokes light power by SRS is more than 50 dB below the laser light power at the output power of 2 kW even with a 20-m delivery fiber. Nearly diffraction-limited beam quality is also confirmed (M2 = 1.2). An output power of 3 kW is believed to be achieved by increasing pump power. The back reflection resistance properties of the fabricated single-mode fiber laser are evaluated numerically by SRS gain calculated from measured laser output spectra and fiber characteristics. Acceptable power of coupling light back into the delivery fiber is 500 W which is high enough for processing of highly reflective materials. The output power fluctuation caused by SRS and back reflection in materials processing will be well suppressed. Our high power single-mode fiber lasers can provide high quality and stable processing of highly reflective materials.

9728-8, Session 2

Passively cooled 405 W ytterbium fibre laser utilising a novel metal coated active fibre

Jae M . Daniel, Defence Science and Technology Group (Australia) and Aether Photonics Ltd . (Australia); Nikita Simakov, Defence Science and Technology Group (Australia) and Univ . of Southampton (United Kingdom); Alexander V . Hemming, Defence Science and Technology Group (Australia); W . A . Clarkson, Univ . of Southampton (United Kingdom); John Haub, Defence Science and Technology Group (Australia)

9728-6, Session 1

Kilowatt narrowline Yb fiber amplifier pumped by ultrahigh brightness pump

Charles X . Yu, Oleg Shatrovoy, Tso Yee Fan, MIT Lincoln Lab . (United States) We present results from a Yb fiber amplifier pumped by an ultrahigh brightness pump. The pump outputs 2 kW in a 200 GHz at 1050 W.

µ m, 0.2 NA multi-mode fiber. The pump utilizes wavelength-beam combining technology from Teradiode Inc. Two gain fibers have been utilized. Both fibers have MFD of 17 ?m. The high brightness enables relatively short gain fibers, 3-4 meters, while maintaining excellent pump absorption of 19 dB. The maximum fiber amplifier output power is 1350 W, limited by multimode instability, with 90% O-O efficiency and diffraction-limited BQ. The fiber amplifier linewidth is 3

9728-7, Session 2

>1.5kW narrow linewidth CW diffraction limited fiber amplifier with 40nm bandwidth

Roman Yagodkin, Nikolai Platonov, Alexander Yusim, Valentin P . Gapontsev, IPG Photonics Corp . (United States) Fiber laser output power scaling with near diffraction limited beam to 100s of kilowatts through spectral beam combining requires kW level fiber amplifiers with wide gain bandwidth and low nonlinearity for narrow linewidth seed amplification. Yb-doped fiber amplifier with diffraction limited beam and direct laser diode pumping is a remarkable source of CW IR radiation distinguished by high efficiency and broad range of We present a novel metal coated triple clad active fibre design, utilising an all glass inner cladding structure and aluminium outer coating. This metal coated active fibre enables a number of benefits to high power laser design, such as increase robustness and extended operating temperature range. As a demonstration of the advantages of this design a passively cooled ytterbium fibre laser is presented. Consisting of a 20 m length of active fibre coiled into a planar arrangement and mounted onto a high emissivity heatsink. Up to 405 W of output power was achieved without the need for active water or forced air cooling. The slope efficiency of this source was 74 % and maximum outer heat sink temperature was ~140°C. This arrangement allowed for significant weight and size savings to be achieved with the active fibre laser head weighing less than 100 g. We will discuss the design choices and trade-offs of metal coated active fibre on high power fibre laser systems as well as the prospects for further power scaling to the kW level.

9728-9, Session 2

2 kW narrow-linewidth monolithic continuous wave fiber laser with near diffraction-limited beam quality

Yang Xu, Wei Shi, Tianjin Univ . (China); Qiang Fang, HFB Photonics, Inc . (China) We demonstrate a monolithic continuous wave (CW) fiber laser source at 1070.25 nm, producing 2 kW laser power with very narrow spectral linewidth (<0.3 nm) and near diffraction-limited beam quality (M2 =~1.3). The laser consists of a CW fiber laser oscillator and two double cladding (DC) fiber amplifier in the master oscillator-power amplifier (MOPA) configuration. The master ocillator is a distributed Bragg reflected (DBR) fiber laser, producing ~ 6 W laser power with <0.1nm spectral linewidth. The two double cladding fiber amplifiers were developed to enhance the laser power up to ~200W and ~2050 W, respectively. The optical to optical efficiency of the main amplifier reach 84.8%. Under the full power output, the 3-dB linewidth and 20-dB linewidth of the laser emission spectrum was 0.28 nm and 1.2 nm respectively. The all-fiber construction of the 2kW near diffraction-limited laser source allows compact size, no maintenance and robust operation and thus enables various practical applications in industrial material processing

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Conference 9728: Fiber Lasers XIII: Technology, Systems, and Applications

and especially in the laser combining due to the pretty narrow spectral linewidth.

9728-10, Session 3

Low-threshold mode instability in Yb3+-doped few-mode fiber amplifiers: influence of a backward reflection

(Invited Paper)

Oleg L . Antipov, Institute of Applied Physics of the RAS (Russian Federation); Maxim S . Kuznetsov, Institute of Applied Physics of the RAS (Russian Federation) and N .I . Lobachevsky State Univ . of Nizhni Novgorod (Russian Federation); Valentin A . Tyrtyshnyy, IRE-Polus Co . (Russian Federation); Dmitry Alekseev, IRE-Polus Co . (Russian Federation) and Moscow Institute of Physics and Technology (Russian Federation); Oleg I . Vershinin, IRE Polus Co (Russian Federation) Mode instability (MI) in Yb-doped fiber amplifiers attracted great interest in the last years. This effect was referred to nonlinear transformation of power from the intense fundamental mode LP01 to the initially-weak higher-order modes. The MI was observed both in large mode area fibers with large core diameter and in few-mode fibers with a relatively small core. Two mechanisms of formation of refractive index gratings caused by population inversion (due to polarizability difference of the excited and unexcited Yb3+ ions) and temperature distributions induced by the modes interference field were discussed as the main reason for the MI. This paper is devoted to the detailed experimental investigation and theoretical modeling of the MI effect in Yb-doped fibers with core diameter of 8-10 um. The influence of a backward propagating wave (due to a backward reflection or scattering) on the MI threshold and a temporal behavior of the output power was investigated. The MI threshold was registered at 1-100 Watts pump power. The threshold was found to decrease dramatically in the presence of a backward reflection; an increase of the signal bandwidth or input power resulted in increase of the MI threshold. Numerical simulation revealed self-consistent growth of the higher-order mode and traveling electronic index grating accompanying the population grating induced by the mode interference field (due to different polarizability of the excited and unexcited Yb3+ ions).

9728-11, Session 3

Optimizing the mode instability threshold of high-power fiber laser systems

Cesar Jauregui-Misas, Hans-Jürgen Otto, Friedrich Schiller-Univ . Jena (Germany); Sven Breitkopf, Friedrich Schiller-Univ Jena (Germany); Jens Limpert, Friedrich Schiller-Univ . Jena (Germany); Andreas Tünnermann, Fraunhofer-Institut für Angewandte Optik und Feinmechanik (Germany) The average output power of Yb-doped fiber laser systems is currently limited by the onset of transverse mode instabilities. Besides, it has been recently shown that the transverse mode instability threshold can be significantly reduced by the presence of photodarkening in the fiber. Therefore, reducing the photodarkening level of the core material composition is the most straightforward way to increase the output average power of fiber laser systems but, unfortunately, this is not always easy or possible. In this paper we present guidelines to optimize the output average power of fiber laser systems affected by transverse mode instabilities and photodarkening. The study is focused not only on power amplifiers but also, for the first time to the best of our knowledge, on double-pass amplifiers and fiber oscillators. The guidelines derived from the simulations do not involve changes in the composition of the active material (except for its doping concentration), but they can still lead to a significant increase of the transverse mode instability threshold. The dependence of this parameter on the active ion concentration, the core conformation, the pump configuration (co-, counter- or bi-propagating pump) and wavelength or the mirror reflectivities in a fiber oscillator among others will be studied and discussed.

9728-12, Session 3

A comparison of mode instability in Yb- and Tm-doped fiber amplifiers

Arlee V . Smith, Jesse J . Smith, AS-Photonics, LLC (United States) Previously we developed a model of mode instability in Yb-doped fiber amplifiers based on the process of stimulated thermal Rayleigh scattering. Here we extend the model to treat Tm-doped fiber amplifiers pumped at 790 nm and including a strong cross relaxation process to populate the upper laser level. The quantum defect heating in such Tm fiber is approximately four times greater than in Yb fiber. At first glance this suggests much lower mode instability thresholds for Tm fibers. However, strong population saturation in Tm counteracts the greater heating, and leads to higher thresholds in Tm than in Yb fibers.

We model Tm operating at 2040 nm and Yb fibers operating at 1060 nm. Other properties are as much alike as possible to achieve a direct comparison of mode instability in the two fibers. This comparison uses 4 meter long 25/400 ?m core/cladding diameter fiber with equal V numbers (5.92) and with the doping levels adjusted for equal pump absorption. The doping levels were (NYb = 1.05E26/m3) and (NTm = 6.0E26/m3). At this doping level the cross relaxation in Tm increased the power efficiency from 39% to 62%. The fibers were co- and counter-pumped at 976 nm (Yb) and 790 nm (Tm), both seeded with 6 W input signal. For co-pumped amplifiers, signal output power at threshold was found to be 20% higher in Tm than Yb; for counter-pumped 45% higher.

9728-13, Session 3

Theoretical analysis of modal instability in high power core-pumped Raman amplifiers

Shadi A . Naderi, Ball Aerospace & Technologies Corp . (United States); Iyad Dajani, Jacob Grosek, Timothy Madden, Air Force Research Lab . (United States) It is well-known that the onset of the modal instability (MI) is a limiting factor for achieving higher powers in Yb-doped fiber amplifiers with good beam quality. In this paper, we present simulation results for MI in an alternate gain medium; a core-pumped Raman fiber amplifier (RFA). Raman amplification in silica is broad and tunable and thus Raman fiber lasers can offer access to wavelengths that are inaccessible via rare-earth-doped fiber lasers. Furthermore, since the core of a Raman fiber does not contain rare earth elements, photodarkening is not present and fabrication is relatively easy. A system of equations is derived to represent the transfer of energy in core-pumped RFAs and the simulation results are presented. Similar to Yb doped fiber amplifiers, heat generated in a Raman fiber amplifier primarily derives from the quantum defect, and time-dependent oscillations in the temperature ultimately cause the fundamental mode to transfer energy to higher-order transverse modes; thus degrading the beam quality of the amplifier output. The modal interference in the pump can also lead to a travelling index of refraction grating. Consequently, our model allows for propagating gratings that may be written by either pump or signal. Using this model, dependence of MI threshold on the Raman gain in a core pumped RFA is discussed. The gain tailored design is shown to be effective only if the outer region has significantly lower Raman gain. Finally, the effect

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Conference 9728: Fiber Lasers XIII: Technology, Systems, and Applications

of cross interactions on MI due to the presence of two gratings is discussed in detail. Bigot, Lab . de Physique des Lasers, Atomes et Molécules (France)

9728-14, Session 3

Average power limit of fiber-laser systems with nearly diffraction-limited beam quality

Hans-Jürgen Otto, Cesar Jauregui, Friedrich-Schiller Univ . Jena (Germany); Jens Limpert, Friedrich-Schiller Univ . Jena (Germany) and Helmholtz-Institute Jena (Germany); Andreas Tünnermann, Friedrich-Schiller-Univ . Jena (Germany) and Helmholtz-Institute Jena (Germany) and Fraunhofer-Institut für Angewandte Optik und Feinmechanik (Germany) The maximum average power that can be emitted from an ytterbium-based fiber-laser system is estimated. The analysis takes into account all the effects known so far that may limit the average power including transverse mode instabilities and photodarkening. Hereby, the recent finding that transverse mode instabilities depend on the average heat load is exploited. The results of this analysis show that there are three main limiting effects: stimulated Raman scattering, the brightness of the pump laser and transverse mode instabilities. Moreover, it seems feasible, disregarding possible practical constrains, that the average output power can be significantly increased to 50kW and beyond when following the proposed optimizations.

9728-16, Session 4

Accurate modeling of rod-type photonic crystal fiber amplifiers

Benjamin G . Ward, U .S . Air Force Academy (United States) Large mode area fiber is a well-known solution to prevent the appearance of nonlinear effects that impede the rise of the optical power in fiber lasers. Solid Core Photonic Bandgap Fibers (SC-PBGF) have shown over the last few years their potential to give access to large effective mode area. They are easy to splice and can be doped with rare-earth ions, their spectral properties enabling, for example, spectral filtering for high-power laser. We report on the design and the fabrication of a new all-solid Bragg fiber based on the pixelization and heterostructuration of its cladding. Pixelization is a discretization of the high index rings that compose the cladding to get true photonic bandgap guidance and heterostructuration consists in removing some of these “pixels” so as to increase the losses of higher order modes (HOM) by “sieve” effect. The cladding is made of only two high index rings. The thickness of the low index ring as well as the heterostructuration (its symmetry and the number of removed pixels) have been chosen to maximize the HOM confinement losses (above 10 dB/m) while keeping the fundamental mode losses low (below 0.1 dB/m). Single mode behavior has been confirmed experimentally even for short fiber length kept straight. The proposed geometry allows to have access to different MFD from 48 µ m to 60 µ m at 1 µ m wavelength by drawing the same stack with different core radius. As a result, a record MFD of 60 µ m is obtained in the case of SC-PBGF.

9728-18, Session 4

High-power fiber laser based on a non filamented-core fully-aperiodic large pitch fiber

Aurélien Benoit, Romain Dauliat, Dia Darwich, Raphaël Jamier, XLIM Institut de Recherche (France); Stephan Grimm, Jens Kobelke, Kay Schuster, Leibniz-Institut für Photonische Technologien e .V . (Germany); Philippe Roy, XLIM Institut de Recherche (France) Rod-type photonic crystal fiber amplifiers show tremendous potential to enable high peak and average output powers by virtue of their large mode field area and strong mode discrimination properties. In such fibers, employing a large-pitch air hole lattice causes higher order transverse modes to become delocalized thus reducing their overlap with the amplifying core of the fiber, and minimizing the gain they experience. Due to the extremely low effective numerical aperture of the core, the delocalization mechanism is very sensitive to thermal lensing in high average power amplifier configurations exhibiting significant heat loads. In these situations, the thermal lensing may distort the fundamental mode enough to influence the resulting heating distribution thus affecting simultaneously several key amplifier parameters such as local population inversions, mode field areas, and effective intermodal intensity overlap. These parameters affect both the fundamental mode output characteristics as well as power transfer to higher order transverse modes caused by stimulated thermal Rayleigh scattering. This paper describes a method of accurately modeling these amplifiers taking into account a converged solution to the thermo-optic feedback loop. This method also accounts for the possibility of asymmetric doping profiles and directly treats higher order mode STRS gain competition along the entire length of the amplifier. Example applications are described. This approach enables further fiber design optimization for higher yet peak and average power outputs.

Since the double-clad fiber architectures development, fiber-based laser have witnessed an impressive power scaling. The extracted power rising has been accompanied by the development of Very Large Mode Area (VLMA) fiber designs allowing overcome some key hurdles like the non-linear process or photo-darkening. However, due to the very large core size of fiber architectures, a new phenomenon, referring to modal instabilities, has been evidenced recently like the current limitation which hampers any further power increase in the field of fiber laser sources without a dramatic degradation of the emitted beam quality. In order to push away the appearance power threshold of this limitation, new aperiodic cladding microstructurations have been proposed to improve the higher-order modes (HOM) rejection out of the gain region and then to optimize the amplification of the sole fundamental mode. These aperiodic microstructures have proved recently their potential to enhance an efficient HOM delocalization enabling singlemode confinement in the core region with passive VLMA fibers.

In this communication we report on the first high power emission demonstration obtained using a solid non-filamented core fully-aperiodic large pitch fiber manufactured by the REPUSIL method based on the sintering and vitrification of micrometric doped silica powders. Using a simple laser cavity, an average output power of 233 W was achieved with an available pump power of 400 W for the first time in such a fiber. The preliminary M2 measurements have shown an excellent beam quality with values less than 1.4.

9728-17, Session 4

Ultra large mode area pixelated Bragg fiber

Jean-Paul Yehouessi, Géraud Bouwmans, Olivier Vanvincq, Andy Cassez, Rémi Habert, Yves Quiquempois, Laurent

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9728-20, Session 5

Conference 9728: Fiber Lasers XIII: Technology, Systems, and Applications

9728-19, Session 4

85

µ

m core rod fiber amplifier delivering 350 W/m

Mette M . Johansen, Technical Univ . of Denmark (Denmark); Mattia Michieletto, Technical Univ . of Denmark (Denmark) and NKT Photonics A/S (Denmark); Torben Kristensen, Thomas T . Alkeskjold, NKT Photonics A/S (Denmark); Jesper Lægsgaard, Technical Univ . of Denmark (Denmark) Ytterbium-doped silica based fiber lasers and amplifiers can generate megawatts of peak power and hundreds of watts in average power using direct amplification. Achievements of higher extracted output power have led to observations of transverse mode instabilities (TMIs) above a system and fiber dependent average power threshold. TMI significantly degrades beam quality and constitutes an impediment to future power scaling. High power operations of rod fiber amplifiers cause the TMI threshold to degrade probably related to photodarkening and increased core absorption.

In this work we test an improved version of the DMF rod fiber with an 85 µ m core and a 260 µ m pump air cladding in a high power setup. The improved DMF rod fiber delivers 350W of stable extracted output limited by the available pump power. To our knowledge, this is the highest achieved stable average output power from a 1m rod fiber amplifier, and an increase of 25% compared with the first generation DMF rod fiber. The slope efficiency was measured to 70%, and a maximum of 71% optical to optical efficiency was obtained. The first generation DMF rod fiber was operated in the leaky regime, whereas the improved DMF rod fiber is operated in the guiding regime resulting in higher core confinement and improved beam quality. The improved DMF rod fiber is tested several times allowing the fiber to degrade due to photodarkening, and also causing the TMI threshold to degrade from >360W to ~290W.

Semiconductor optical fibers for nonlinear applications

(Invited Paper)

Anna C . Peacock, Li Shen, Fariza H . H . Suhailin, Univ . of Southampton (United Kingdom); Natasha Vukovic, Univ of Southampton (United Kingdom); Noel Healy, Univ . of Southampton (United Kingdom)

9728-21, Session 5

Advances in ultrafast mid-IR fiber lasers

(Invited Paper)

Darren D . Hudson, The Univ . of Sydney (Australia) Ultrashort pulse generation in mid-Infrared fiber lasers operating near 3 fiber lasers in this exciting wavelength range. µ m has become a hot topic of research. In this talk, we cover the rise of ultrafast Using a saturable absorber mirror in a Er:ZBLAN fiber laser, we demonstrated the first stably mode-locked 3 sensitive broadband trace gas detection.

µ m class fiber laser in 2014, which generated 6 ps pulses with a peak power of > 0.5 kW. More recently, we employed nonlinear polarization rotation based mode-locking in this system. Using a mid-IR Frequency Resolved Optical Gating technique, we measured 500 fs pulses, with a peak power of > 6 kW. The leap to sub ps operation of these lasers systems opens the door for a wide range of exciting applications and experiments including mid-IR supercontinuum generation, mid-IR fiber based frequency combs, human tissue surgery, and Here, we review the progress from initial Q-switched laser systems generating 100 ns pulses with < 0.1 kW peak power all the way to the sub ps, multi-kW level mode-locked systems. The potential for these systems to be a driver for mid-IR fiber laser frequency comb sources is explored in detail, including calculations of supercontinuum and coherence. Finally, a view towards the future of ultrashort pulse generation in these systems is presented, with a particular emphasis on creating sub-100 fs gain bandwidth limited pulses directly from the oscillator.

9728-22, Session 5

Mid-infrared supercontinuum generation up to 4.6

µ

m using step-index indium fluoride fiber pumped by a femtosecond fiber laser near 2

µ

m

(Invited Paper)

Reza Salem, Thorlabs Quantum Electronics (United States); Zhuo Jiang, Dongfeng Liu, Robert M . Pafchek, Paul Foy, Mohammed Saad, Doug Jenkins, Alex E . Cable, Peter Fendel, Thorlabs Inc . (United States) The nascent field of semiconductor optical fibers is attracting increased interest as a means to exploit the optoelectronic functionality of the semiconductor materials directly within the fiber geometry [1]. Compared to their planar counterparts, this new class of waveguide retains many of the advantageous properties of the fiber platforms such as robustness, flexibility, cylindrical symmetry, and long waveguide lengths. Furthermore, owing to the large Kerr nonlinearity of the materials, fibers with semiconductors embedded in the core are also ideal for the development of integrated nonlinear optical devices. In this paper we review our progress in characterizing the transmission properties of the semiconductor fibers and demonstrate their use in nonlinear applications. Particular focus will be placed on fibers with a hydrogenated amorphous silicon (a-Si:H) core as this material offers several favorable properties such as high nonlinear coefficients, low fabrication costs, and low transmission losses. Exploiting these features, we have demonstrated an array of nonlinear processes in the fibers including ultrafast optical switching, modulation and broadband continuum generation. Although many of the first generation semiconductor fibers have been fabricated with relatively large micron sized cores, by scaling these down towards the nanoscale dimensions used on-chip the nonlinear effects will be enhanced, paving the way for the development of low power, high speed all-fiber systems.

[1] A.C. Peacock, J.R. Sparks, and N. Healy, “Semiconductor optical fibres: progress and opportunities,” Laser & Photonics Reviews 8, 53?72 (2014).

We report mid-infrared supercontinuum (SC) generation in a dispersion engineered step-index indium fluoride fiber pumped by a femtosecond fiber laser around 2 platform.

µ m. The SC spans 1.8 octaves from 1.25 dispersion wavelength close to 1.9 µ µ m to 4.6 through core size and numerical aperture adjustments to have a zero to be small and anomalous at the pump wavelength and to maintain a SC generation experiments based on the numerical modelling results. µ m with an average output power of 270 mW. The pump source is an all-fiber femtosecond laser that generates sub-100-fs pulses at 50 MHz repetition rate with 570 mW average power. The SC-generation fiber is engineered m. The fiber dispersion is calculated small absolute value (<7 ps/nm.km) over the mid-infrared region from 2 um to 4.5 um. Two fiber lengths of 30 cm and 55 cm are selected for the The measured spectra and the numerical modelling results are presented showing good agreement for both lengths. The femtosecond pumping regime is a key requirement for generating a coherent SC. We show by modelling that the SC is coherent for a pump with the same pulse width and energy as our fiber laser and added quantum-limited noise. The results are promising for the realization of coherent and high-repetition-rate SC sources, two conditions that are critical for spectroscopy applications using FTIR spectrometers. Additionally, the entire SC system is built using optical fibers with similar core diameters, which enables integration into a compact

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9728-25, Session 6

Conference 9728: Fiber Lasers XIII: Technology, Systems, and Applications

9728-24, Session 5

Average power scaling of ultrashort-pulse Tm-based fiber laser systems

Christian Gaida, Martin Gebhardt, Fabian Stutzki, Hans Jürgen Otto, Cesar Jauregui, Jens Limpert, Friedrich Schiller-Univ . Jena (Germany); Andreas Tünnermann, Fraunhofer-Institut für Angewandte Optik und Feinmechanik (Germany) We analyze the average power scaling capabilities of Tm-based fiber lasers in the context of transverse mode instabilities. Based on very fundamental scaling laws for the guiding properties of optical fibers, a four times higher critical thermal load for the operation at 2 µ m wavelength in comparison to 1 µ m systems can be expected prior to the onset of transverse mode instabilities. While for Yb-based systems critical thermal loads of 30-35 W/m have been demonstrated, an average thermal load of more than 120 W/m can be expected for Tm-based systems. In order to validate this hypothesis a high average power experiment was realized with a 1.2 m long Tm-doped large-pitch fiber as main amplifier, which was bi-directionally pumped. A pump-limited average output power of 248 W with a slope efficiency of more than 40% has been achieved. Heating of mirrors, coatings and the ambient atmosphere have been mitigated to maintain a Gaussian-like well confined beam profile. A detailed simulation of the thermal conditions in this fiber amplifier reveals an average thermal load of more than 100 W/m, which is more than three times higher than the critical value for state-of the-art near-diffraction limited systems operating around 1 µ m. Based on the current understanding of transverse mode instabilities in combination with the experimental results presented herein, the expected average power limits for various Tm-doped fibers can be predicted. Strategies for the optimization of Tm-based fiber laser systems, with respect to the fiber length, cross-relaxation and excited-state pumping, will be discussed.

Discretely tunable Tm-doped fiber-based MOPA using FBG arrays as spectral filters

Tobias Tiess, Saher Junaid, Martin Becker, Manfred Rothhardt, Leibniz-Institut für Photonische Technologien e .V . (Germany); Hartmut Bartelt, Leibniz-Institut für Photonische Technologien e .V . (Germany) and Abbe School of Photonics (Germany); Matthias L . Jäger, Leibniz Institut für Photonische Technologien e .V . (Germany)

9728-26, Session 6

Single-mode spectral beam combining of high power Tm-doped fiber lasers with WDM cascades

Saniye Sinem Yilmaz, Laser Zentrum Hannover e .V . (Germany) and Bilkent Univ . (Turkey); Christoph Ottenhues, Thomas Theeg, Laser Zentrum Hannover e .V . (Germany); Samir Lamrini, Karsten Scholle, Michael Schaefer, Peter Fuhrberg, LISA Laser Products OHG (Germany); Hakan Sayinc, Laser Zentrum Hannover e .V . (Germany); Fatih Ömer Ilday, Bilkent Univ . (Turkey); Jörg Neumann, Laser Zentrum Hannover e .V . (Germany); Ludger Overmeyer, Institut für Transport- und Automatisierungstechnik (Germany); Dietmar Kracht, Laser Zentrum Hannover e .V . (Germany) Thulium doped fiber lasers are being shown to be suitable for addressing a range of applications in both pulsed and CW mode of operation at this wavelength range. Over the last decade, the power scaling of 2 technology, which must be further explored and developed.

µ m sources has been demonstrated by 790 nm diode pumped thulium doped fiber lasers. These lasers have achieved kW power levels, as well as high power narrow line-width demonstrations of more than 600 W. Active and passive fibers supporting higher order modes were employed in such sources, leading to the degradation of the beam quality. A promising alternative approach is spectral beam combining of laser sources by using WDMs (wavelength division multiplexer) developed from truly single mode fiber for power scaling while simultaneously maintaining the excellent beam quality. Since many applications increasingly require high power thulium-doped fiber lasers with distinct wavelengths, spectral beam combination is a key We demonstrate an in-house-made WDM cascade for spatially combining signals and scaling up the laser power available from a truly single mode fiber. The experimental setup contains four identical continuous wave oscillators each emitting narrow line-width signals at three distinct wavelengths, which are 1920 nm, 1950 nm, 1977 nm and 2030 nm. These lasers are combined by using in-house-made WDMs and we achieved output power of more than 38 W with a combining efficiency of 80 %. Future work will include investigations on power scaling of the used individual laser sources and explore dense spectral combination with WDM cascades.

Over the past years, Thulium-(Tm-) doped fiber lasers in the 2 µ m region have gained lots of interest due to many potential applications in material processing and biophotonics. Based on the broad gain regions spanning from 1800nm to 2100nm, they offer the perfect basis to implement broadly tunable and user-friendly light sources like they are increasingly demanded in spectroscopic applications. Recently, a novel tuning mechanism based on a FBG array as versatile spectral filter has been reported combining a unique spectral freedom for customized tuning ranges and ultrabroad bandwidths with a fiber-integrated setup in order to maintain the advantages of the waveguide geometry. In this work, we demonstrate such a dispersion-tuned and pulsed fiber laser in the Tm domain around 1950nm using a modulator and a discrete FBG array to control the emission wavelength. In order to comply with the demands of potential applications in biophotonics, for the first time, this tuning concept is realized in a polarization-maintaining (PM) configuration ensuring linearly polarized output. While a simple FBG array is employed containing 5 gratings inscribed in PM fiber, we also discuss the possibility to implement FBGs inscribed in standard single mode fiber. Based on a tuning range of 58nm, the emission characteristics of the system are investigated showing pulse durations of around 15ns and a good spectral signal contrast. In order to highlight the prospect for tunable high-power operation, we have also implemented a single-pass amplification stage. In such a MOPA configuration, the average power is scaled to more than 25W.

9728-27, Session 6

Spatially resolved measurement of the core temperature in a high-power thulium fiber system

Till Walbaum, Matthias Heinzig, Fraunhofer-Institut für Angewandte Optik und Feinmechanik (Germany); Franz Beier, Fraunhofer-Institut für Angewandte Optik und Feinmechanik (Germany) and Institute of Applied Physics, Friedrich-Schiller-University Jena (Germany); Andreas Liem, Thomas Schreiber, Ramona Eberhardt, Fraunhofer Institut für Angewandte Optik und Feinmechanik (Germany); Andreas Tünnermann, Fraunhofer-Institut für Angewandte Optik und Feinmechanik (Germany) and Institute of Applied Physics, Friedrich-Schiller-University Jena (Germany) We present measurements of the temperature increase inside the active fiber of a thulium fiber amplifier during high power operation. At a pump power of over 100 W at 793 nm, we measure the core temperature

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Conference 9728: Fiber Lasers XIII: Technology, Systems, and Applications

distribution along the first section of a large mode area (LMA) highly thulium doped active fiber by use of an optical backscatter reflectometer. A mode field adaptor is used to maintain single mode operation in the LMA fiber, which is ensured by beam quality measurement. An increase in temperature of over 100 K can be observed in spite of conductive cooling, located at the pumped fiber end and jeopardizing the fiber coating. The recoated splice can be clearly identified as the hottest fiber region. This allows us to estimate the maximum thermally acceptable pump power for this amplifier.

We also observe that the temperature can be decreased by increasing the seed power, which is in agreement with theoretical predictions on the increase of cross relaxation efficiency by depletion of the upper laser level. This underlines the role of power scaling of the respective seed power of a thulium amplifier stage as a means of thermal management.

of producing high power pulses typically use free-space propagation or non-PM fibers but suffer from obvious environmental limitations. In an effort to increase the output power delivered by all-fiber Tm oscillators, high repetition rate cavities (as high as 611MHz) have been used. But such systems, which cannot use pulse-picking devices, are not suitable for many application requiring few MHz repetition rates. In this contribution we present an all integrated all-PM laser system with the potential to deliver femtosecond pulses after external recompression. It is, to the best of our knowledge, the first net-normal cavity all-PM Tm fiber oscillator. Our source has a very good (few percent) short and long term stability. By design it delivers long pulses which allow for direct amplification in a multi-stage system. This all-fiber all-integrated MOPA system producing 60ps, 6W average power, pulses at 20MHz is an ideal seed for supercontinuum generation. The current system is limited by the available pump diodes. The booster stage has an efficiency of 40%, which is 10% higher than similar amplification stages already published.

We will present at the conference the various laser and MOPA architectures we have developed together with their performances and stability.

9728-28, Session 6

Wavelength agile holmium-doped fiber laser

Nikita Simakov, Defence Science and Technology Group (Australia) and Univ . of Southampton (United Kingdom); Jae M . O . Daniel, Defence Science and Technology Group (Australia); Jon D . Ward, Gooch & Housego plc (United Kingdom); W . Andrew Clarkson, Univ . of Southampton (United Kingdom); Alexander V . Hemming, John Haub, Defence Science and Technology Group (Australia)

9728-30, Session 6

Comparison of in-band pumped Tm:fiber and Ho:fiber

Alex M . Sincore, Lawrence Shah, CREOL, The College of Optics and Photonics, Univ . of Central Florida (United States); Vadim Smirnov, OptiGrate Corp . (United States); Martin C . Richardson, CREOL, The College of Optics and Photonics, Univ . of Central Florida (United States) Sources in the 2.1?m wavelength region are required for numerous applications including remote sensing, LIDAR, free-space optical communications and for pumping mid-infrared optical parametric oscillators. In some instances it is desirable to be able to tune the wavelength of the 2.1?m source and such tuning can be achieved by mechanically rotating an etalon or diffraction grating. These mechanical components, while suitable for swept wavelength sources, are typically less reliable when rapid wavelength switching is desired due to the inertia associated with the rotation of the mechanical arrangement. Intra-cavity, all fiber etalons have less inertia, however these components are limited in peak power handling and sensitive to external temperature fluctuations. All of these elements also suffer from the problem that switching to another position involves a transition where the wavelength selective element is resonant at unwanted wavelengths.

Recently, acousto-optic tunable filters (AOTFs) have attracted substantial attention as a wavelength selective element in the 2 sources.

µ m range. The AOTF used in this report enables an electronically-controlled and inertia-free wavelength selective filter with a response FWHM of 1 nm. We incorporated this filter as a wavelength selective element in a holmium-doped fiber laser cavity and demonstrated a digitally-controlled wavelength agile source operating in the atmospheric transmission window around 2.1

µ m. We will discuss the operation in various regimes such as wavelength sweeping, switching and tuning. Finally we will discuss the power scaling of such sources and potential utility in generating wavelength agile mid-infrared High brightness, high power 2-micron laser sources are necessary for directed energy, spectroscopic sensing, medical applications, and generating mid-IR light via nonlinear processes. In the last ten years, such applications have increasingly relied upon lasers based on silica fiber doped with thulium (1.7 – 2.1 ?m) and holmium (2.05 – 2.15 ?m). In general, Tm:fiber lasers have been pumped using commercially available high power/brightness diodes at 790nm and utilize cross-relaxation processes to achieve optical to-optical efficiencies of 55-65%. However this still generates sizeable thermal distortion at high powers, thus alternative higher efficiency high power 2 ?m fiber laser architectures are still necessary. Both thulium and holmium doped silica fiber lasers can be “in-band” pumped with Tm:fiber lasers operating at ~1.90-1.95 ?m. Similar to ytterbium doped fiber, in band pumping has enabled >90% optical-to-optical efficiency in Tm:fiber lasers. Holmium, on the other hand, generally enables longer wavelength operation with low quantum defect; however, efficiencies <70% are typical in silica fibers. While not entirely understood, it is likely due to non-radiative processes such as phonon interactions or residual OH- absorption. This work utilizes an identical experimental setup to characterize the performance of Tm:fiber and Ho:fiber oscillators/amplifiers under in-band pumping, directly comparing polarization maintaining thulium-doped step-index fiber (PM Tm:SIF), non-PM Ho:SIF, and PM Tm:PCF (photonic crystal fiber). The pump and seed sources are wavelength tunable, allowing a wide parameter space for characterization.

9728-29, Session 6

All-PM fiber, net normal cavity, Tm-doped fiber laser

Claude Aguergaray, Patrick Bowen, ALPhANOV (France) As mid-infrared laser technology improves, the interest to develop high power stable, integrated, reliable fiber laser systems in the 2?m range arises. So far all-fiber passively mode-locked sources have been demonstrated in the solitonic regime with PM or non-PM fiber and in the stretched pulsed regimes with non-PM fibers. Indeed, net normal cavity lasers capable

9728-31, Session 7

Nanoparticle doping for improved Er doped fiber lasers

(Invited Paper)

E . Joseph Friebele, Colin C . Baker, Charles G . Askins, U .S . Naval Research Lab . (United States); John R . Peele, Sotera Defense Solutions, Inc . (United States); Barbara A . Marcheschi, Woohong R . Kim, Jas S . Sanghera, U .S . Naval Research Lab . (United States); Jun Zhang, Radha K . Pattnaik, Larry D . Merkle, Mark Dubinskii, U .S . Army

44 SPIE Photonics West 2016 · www.spie.org/pw

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Research Lab . (United States) Erbium-doped fiber lasers (EDFLs) are attractive for directed energy weapons applications because they operate in a wavelength region that is both eye-safer and a window of high atmospheric transmission. Fabrication of Er-doped fibers (EDFs) is typically done by solution doping where salts of Er and Al (added to reduce Er clustering) are dissolved in a solvent and introduced into porous silica “soot” that is subsequently dried and consolidated to form the fiber core. However, there is no mechanism for controlling the atomic environment of the Er, and in spite of the presence of Al, clustering still occurs. For cladding pumped high energy laser applications it is necessary to have a relatively high Er core absorption because of the relatively low Er absorption cross section and the areal dilution of the pump intensity in the large pump cladding. High Er concentrations result in the deleterious concentration quenching and cross-relaxation upconversion, which shorten the fluorescence lifetime and decrease laser efficiency. Nanoparticle (NP) doping is an alternate technique for making EDFs where the local atomic environment of the Er ions is established chemically during NP synthesis prior to doping. The Er ions are surrounded by a cage of aluminum and oxygen ions, thereby substantially reducing ion-ion energy exchange and its deleterious effects on laser performance. We report the fabrication and measurements of both core and cladding pumped NP doped EDFs. The optical-to-optical slope efficiency in a resonantly-core pumped MOPA configuration was 80.4%, which compares well with the record efficiency measured on a commercial solution-doped fiber. NP doping has the added benefit of incorporating much less Al in the fiber, e.g., the Al:Er ratio in the NP-doped fiber is only 7 compared to 150 in a comparable solution-doped fiber, but the Er lifetimes are the same.

9728-32, Session 7

Conference 9728: Fiber Lasers XIII: Technology, Systems, and Applications Bismuth-doped fibers and fiber lasers for a new spectral range of 1600-1800 nm

(Invited Paper)

Evgeny M . Dianov, Fiber Optics Research Ctr . of the Russian Academy of Sciences (Russian Federation)

9728-33, Session 7

Optical amplifiers and lasers based on tapered fiber geometry for power and energy scaling with low signal distortion

(Invited Paper)

Valery N . Filippov, Tampere Univ . of Technology (Finland); Yuri K . Chamorovskii, Konstantin M . Golant, Institute of Radio Engineering and Electronics (Russian Federation); Andrei O . Vorotynskii, Oleg G . Okhotnikov, Tampere Univ . of Technology (Finland) Amplifiers and lasers employed tapered double-clad fiber (T-DCF) as a gain medium exhibit numerous unique advantages compared with regular active fibers. Primarily, T-DCF with large modal size and core diameter at taper end up to 200 µ m could generate nearly ideal beam quality with low non-linear phase shift (B-integral) and is capable of high energy storage. The clad diameter of T-DCF up to 1.6 mm allows pumping by inexpensive powerful low brightness laser diode bars. The axial variation of taper diameter is accompanied by several attractive mechanisms which assist signal propagation. The variation of transverse size of the fiber results in higher pump absorption due to improved clad mode mixing in T-DCF. Tapering of gain fiber also causes an efficient ASE suppression for amplification of low duty cycle pulses. Axially non-uniform geometry of taper increases significantly the SBS threshold for nanosecond pulses amplification. We have found recently that the threshold of the modal instability of T-DCF amplifier increases significantly due to small difference between propagation constants of interfering modes.

We have experimentally demonstrated several amplifiers and lasers using ytterbium T-DCF gain fiber. Nanosecond actively Q-switched laser produced 1.6 mJ pulse energy has been used as an efficient pumping source for supercontinuum light generation. The picosecond all-fiber tapered MOPA system with record 300 µ J out-of-fiber pulse energy will be described in details.

For the NIR spectral region from 1150 to 1800 nm, including the ranges from 1250 to 1500 nm and 1600 to 1800 nm where efficient rare-earth fiber lasers are absent, bismuth-doped optical fibers are promising active materials. The last two spectral ranges are of great interest for some applications, in particular for optical fiber communication. Earlier, we developed Bi-doped fiber lasers and optical amplifiers operating in the first of these spectral ranges. Here, we report new results on the development of bismuth-doped optical fibers and fiber lasers for a spectral range of 1600-1800 nm. A single-mode bismuth-doped germanosilicate fiber (Bi-GSF) containing near 50 mol.% of GeO2 and less than 0.1 mol.% of Bi was drawn from a preform fabricated by the MCVD technique. Intensive luminescence with a maximum at 1700 nm was observed under excitation at 460, 950 and ~1600 nm. The decay of this luminescence under pumping at 975 nm has a single exponential shape with the lifetime 500 µ s. The unsaturable losses of the Bi-GSF at 1568 nm were 0.3 dB/m. The family of Bismuth-doped fiber lasers was realized using the Bi-GSF lengths 15-20 m and an Er-Yb fiber laser operating at 1568 nm as a pump source. The gain bandwidth of the Bi-GSF was 150 nm and this made it possible to obtain CW laser generation in a range of 1625-1775 nm. The output power of the laser at 1700 nm exceeded the watt-level with the slope efficiency 30%. Thus, now bismuth-doped fiber lasers cover the spectral region 1150-1800 nm.

9728-34, Session 8

Highly efficient Yb-free Er-La-Al doped ultra-low Na large mode area single-trench fiber laser

Deepak Jain, Shaiful Alam, Yongmin Jung, Pranabesh Barua, Martin M . N . Velazquez, Jayanta K . Sahu, Univ . of Southampton (United Kingdom) We demonstrate a 60 1,820 µ m2 to 1,960 µ µ m core diameter single-trench Yb free Er-La-Al doped fiber having 0.038 ultra-low-NA, fabricated using conventional MCVD process in conjunction with solution doping process. Numerical simulations ensure an effective single mode operation, the effective area varies from m2 for different thicknesses of trenches and resonant rings at 25cm bend radius (including bend-induced distortion). This fiber design can be fabricated with conventional fabrication approaches, which can dramatically reduce the fabrication cost, hence suitable for mass production. Moreover, all solid structure ensures easy cleaving and splicing. Experimental measurements demonstrate a robust effective single mode operation. Furthermore, this fiber in 4%-4% laser cavity shows a record efficiency of 46% with respect to absorbed pump power at 975nm. Our study addresses two key issues of power scaling at 1550nm using Er doped fibers. First, we develop a simple fiber design for mode area scaling at 1550nm, which can be easily fabricated using conventional approaches. Second we optimize the composition of Er-La-Al during solution doping process to avoid concentration quenching of Er. This optimization leads to high slope efficiency. Further, this fiber owing to its large effective area can be used in MOPA configuration for peak power scaling.

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Conference 9728: Fiber Lasers XIII: Technology, Systems, and Applications

9728-35, Session 8

All-fiber arbitrary pulse form generation and amplification in Tm3+ doped optical fibers

Mateusz Wysmolek, Hakan Sayinc, Laser Zentrum Hannover e .V . (Germany); Samir Lamrini, Peter Fuhrberg, LISA Laser Products OHG (Germany); Kristian Lauritsen, Dietmar Klemme, PicoQuant GmbH (Germany); Uwe Morgner, Laser Zentrum Hannover eV (Germany) and Leibniz Univ . Hannover (Germany); Jörg Neumann, Dietmar Kracht, Laser Zentrum Hannover e .V . (Germany) High energy nanosecond pulsed laser systems are finding numerous applications in material processing, nonlinear frequency conversion, free space data transmission etc. Arbitrary pulse forming allows for pulse generation tailored to the application or compensation of energy saturation in an amplifier leading to pulse deformation. In our approach we are using a directly modulated semiconductor laser diode emitting at 1950 nm with a PC controlled laser diode driver. The pulse shape is pre-defined by high precision software and can be changed on the fly making an external arbitrary pulse form generator unnecessary. Furthermore, the same software allows for the control of the pulse repetition rate and the generation of pulse bursts making it a promising candidate for material processing. Due to the low output power, the use of 3 fiber amplifier stages is necessary to achieve adequate energies for material processing. With the presented MOPA system we were able to achieve 30 kW of peak power with clean 10 ns pulses free from spectral and temporal deteriorations. The system is a robust and fully alignment free monolithic fiber setup. An unique feature is a computer controlled seed module with advanced control of pulse shape and repetition rate with burst mode option. To the best of our knowledge this is a first presentation of arbitrary pulse shaping and amplification to 100s µJ pulse energies in Tm3+ doped fibers.

9728-37, Session 8

All-fiber, 793nm cladding-pumped Tm-doped fiber laser with >90 nm of continuous tuning range

Robert A . Stegeman, Eric D . Park, Q-Peak, Inc . (United States) Tunable Tm-doped fiber laser sources are an attractive solution for chemical detection, medical lasers, and in conjunction with amplifiers, seeding of nonlinear optical processes. Tunability for Tm-doped fiber lasers traditionally comes from an external grating to provide cavity feedback, which has inherent instabilities associated with free-space coupling into and out of a small core optical fiber. This work demonstrates that an all-fiber solution, connected only with the most basic fusion arc splicer, can be made to provide a robust, reliable, and simple tunable 2-um tunable fiber laser source. The cavity consists of a 793-nm multi-mode laser diode, which is combined with the pass-through signal in a (2+1):1 fused fiber combiner. A piece of 10/130 double clad thulium-doped fiber follows as the gain medium. An isolator follows the gain fiber to prevent feedback and force a single propagation direction in the ring cavity. A tunable Fabry Perot filter follows the isolator which allows wavelength selection based on the finesse and free-spectral range. A 90/10 fused fiber tap allows 10% of the laser power to exit, while 90% is spliced to the input port of the fused fiber pump/signal combiner.

While operating in CW mode, the laser cavity demonstrates 90-nm of continuous tuning from 1970 – 2060 nm, with optical bandwidths of less than 0.05 nm with greater than 60 dB OSNR. The 90/10 output coupler allows several mW of average power to be emitted.

9728-36, Session 8

Chirped pulse amplification of a dissipative soliton thulium-doped fiber laser

Fangzhou Tan, Hongxing Shi, Peng Wang, Jiang Liu, Pu Wang, Beijing Univ . of Technology (China)

9728-38, Session 8

Comparison of high power large mode area and single mode 1908nm Tm-doped fiber lasers

Benjamin R . Johnson, Daniel Creeden, Julia Limongelli, Herman Pretorius, Jon F . Blanchard, Scott D . Setzler, BAE Systems (United States) We demonstrate on chirped pulse amplification of a dissipative soliton thulium-doped fiber laser. It consists of an all-fiber seed laser, a fiber-based stretcher, two-stage fiber amplifier and free space grating compressor. The oscillator works in the normal dispersion regime and delivers up-chirped pulses with output power of 3 mW at repetition rate of 29.3 MHz. The spectrum of the seed laser is located at 1938 nm with a 10 dB bandwidth of 50 nm. The output pulses is then stretched in ~50 m normal dispersion fiber to 72 ps pulse duration. A single- mode single-clad fiber amplifier is used to amplify the pulse energy to 1.78 nJ. A polarization controller is used after the pre-amplifier to change the polarization of the pulses after passing through a polarization maintaining (PM) isolator. In the main amplifier, a single-mode double-clad polarization maintaining Tm-doped fiber amplifier is used to boost the pulse energy to 229 nJ corresponding to an average power of 6.72 W, with a slope efficiency of 32.7 %. The amplified up-chirped pulses could be dechirped to a duration of 130 fs with energy of 153 nJ.

Most high power Tm-doped fiber lasers operate in the longer wavelength region of the emission spectrum, near 2050nm, where there are minimal re-absorption losses. The 1908nm region in Tm-doped fiber has a high emission cross-section, but it also has significant ground-state absorption, resulting in a higher overall laser threshold and reduced optical efficiency compared to operation at longer wavelengths within the thulium emission band. This three-level operation makes oscillator design critical to laser performance. Operating in the short-wavelength region of the thulium emission band is important for pumping of solid-state and fiber lasers. In this paper, we compare large mode area (LMA) and single-mode (SM) fiber geometries for use in high power 1908nm fiber lasers. With a simple end pumped geometry, we have generated 100W of 1908nm power with LMA fiber at 40% optical efficiency and 117W at 52.2% optical efficiency with a single-mode fiber. We have also power scaled both designs to the >200W power level, showing the capability for further scaling of power in this short wavelength region with high efficiency. In all cases, the fiber lasers are monotlithic, with no free-space coupling.

9728-39, Session 8

2?m single frequency fiber laser based on thulium-doped silica fiber

Shijie Fu, Tianjin Univ . (China); Wei Shi, Tianjin Univ .

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Conference 9728: Fiber Lasers XIII: Technology, Systems, and Applications

(China) and Tianjin Institute of Modern Laser & Optics Technology (China); Jichao Lin, Qiang Fang, Tianjin Institute of Modern Laser & Optics Technology (China); Quan Sheng, Haiwei Zhang, Tianjin Univ . (China); Jinwei Wen, Tianjin Institute of Laser Technology (China); Jianquan Yao, Tianjin Univ . (China) and Tianjin Institute of Modern Laser & Optics Technology (China) wavelength soliton can be seen. The pulse width of single-wavelength soliton at 1863 nm is measured to be 3.11 ps and the repetition rate is 2.6 MHz. The tunability is based on nonlinear polarization evolution (NPE) technique. The NPE effect induces wavelength-dependent loss in the cavity to effectively alleviate mode competition and enables the multi-wavelength tunable mode locking. The wavelength tuning capabilities can be realized by controlling the polarization in the fiber ring cavity. The system provides a simple and compact solution to tunable multi-wavelength mode locking in fiber lasers. Such tunable laser has potential applications in optical signal processing and communication. Single frequency fiber laser in 2 ?m regime has attracted intense interest recently due to the great performance in terms of low intensity noise, narrow linewidth and compactness for the applications ranging from coherent LIDAR, high-resolution spectroscopy, to nonlinear frequency conversion. The developed heavily Tm-doped germinate and silicate fibers have facilitated 2 ?m single frequency fiber lasers operating with higher output power and efficiency. However, there are still some limitations on mechanical strength and the compatibility between the active specialty fiber and passive silica fiber in the laser cavity. Fusion splicing between the highly-doped soft glass fiber and silica fiber with low-loss can be well done now, which is still challenging and requires careful handling. In this letter, we report a monolithic, DBR single frequency fiber laser at 1950 nm using a commercial Tm-doped silica fiber.

The maximum output power of the single longitudinal mode laser was 18 mW and slope efficiency versus the launched pump power was 11%. Single frequency operation was confirmed by the scanning Fabry-Perot interferometer and neither mode hopping nor mode competition was observed as the pump power increased. The linewidth was measured to increase from 37 to 99 kHz when the pump power rised from 135 to 235 mW. The RIN spectra measured at different pump power showed that it was dominated by a peak at the relaxation oscillation frequency of around 500 kHz and then decreased monotonically towards higher frequencies with the RIN level of around -150 dB/Hz, which approached the shot noise limit.

9728-15, Session PTue

Power scalability in rectangular core fiber

Nan Xia, Seongwoo Yoo, Xuan Wu, Huizi Li, Nanyang Technological Univ . (Singapore) Recently, Severe modal degradation was observed in high power, rare earth doped fiber lasers and amplifiers when the pump power was above a power threshold. Many numerical models have been proposed to illustrate the cause of such mode instability, and a good method to suppress the mode instability is of highly important to high power operation area. In this paper, we utilize the beam propagation method (BPM) to numerically simulate the mode instability in a rectangular core fiber with circular cladding shape for the first time. We compare the mode instability between the rectangular core fiber and the conventional circular core fiber. Even in the circular cladding, the rectangular core can dissipate heat more efficiently than the circular core, leading to reduced mode instability. The suppression is more apparent in the high aspect ratio (AR.) rectangular core fiber as the core-cladding boundary gets closer to the edge of the cladding. Utilization of such fiber is a potential way to suppress the modal degradation in high power lasers or amplifiers.

9728-40, Session 8

Widely tunable multi-wavelength Tm doped mode-locked fiber laser

Zhiyu Yan, Nanyang Technological University (Singapore) and Singapore Institute of Manufacturing Technology (Singapore); Xiaohui Li, Nanyang Technological Univ . (Singapore); Biao Sun, Jiaqi Lou, Nanyang Technological Univ . (Singapore) and Singapore Institute of Manufacturing Technology (Singapore); Perry Ping SHUM, Nanyang Technological University (Singapore); Xia YU, Singapore Institute of Manufacturing Technology (Singapore); Ying ZHANG, SIMTech, Agency for Science, Technology and Research (Singapore); Qi Jie Wang, Nanyang Technological Univ . (Singapore)

9728-23, Session PTue

Compact visible through mid-IR source based on a DFB diode, fiber amplifiers, PPLN and BIBO crystals

Igor V . Melnikov, National Research Univ . of Electronic Technology (Russian Federation) and Moscow Institute of Physics and Technology (Russian Federation) and Univ of Illinois at Urbana-Champaign (United States); Nikolai Balakleisky, National Research Univ of Electronic Technology (Russian Federation); Andrey A . Machnev, National Research Univ . of Electronic Technology (Russian Federation); J . Gary Eden, Univ . of Illinois at Urbana Champaign (United States) We propose and demonstrate a tunable dual- and tri- wavelength ultra-fast Tm-doped fiber laser for the first time, and the wavelength tuning range is more than 50 nm, the widest to the best of our knowledge.

The fiber laser is mode-locked by nonlinear polarization evolution (NPE) technique. The setup consists of 1.5-m Tm-doped fiber as the gain medium, two polarization controllers and one isolator to induce the NPE effect, two 793 nm laser diodes with the maximum power of 170 and 200 mW as the pump source, one coupler as the output port, and 70-m single-mode fiber. By increasing the pump power to 370 mW and either rotating or squeezing the PCs, dual- and tri-wavelength soliton appears. By slightly rotating or squeezing the PCs, multi-wavelength mode locking can be tuned from 1863 to 1915 nm. The fiber laser operates at soliton regime because of the typical Kelly sidebands. The separation between the two wavelengths remains around 10 nm, which is corresponded to the modulation period of cavity transmission. When decreasing the pump power to 310 mW, single We report a compact and robust source capable of generating diffraction limited light ranged from visible- to mid- IR (0.634-?m /1.5 – 1.7-?m / 3.4 – 3.2-?m), using a repetition–switchable single-pass PPLN and BIBO OPO architecture and, a narrow line semiconductor laser. This type of laser which integrates gain chips with a length of fiber that has a Bragg grating (BG) written in its core, has been proven to be a dense and certain source of short picoseconds to nanosecond pulses with peak power sufficient for effective parametric wavelength-tuning applications. The system is based on a narrow-line 1.064 µ m 15-ns MOFA-type system as a pump source, a cw tunable fiber seed laser (1.5 – 1.6 µ m), followed by a set of PPLN/BIBO-based OPGs generating pulsed 1.5 – 1.7 µ m by DFG of 1.064 µ m, and 0.634 µ m by SFG of both 1.5- µ m and 1.064 µ m, correspondingly. The laser system delivers nearly diffraction-limited beams both at the visible- and mid-IR tunable wavelengths. The pulse-to-pulse energy instability does not exceed 0.1 %. Optical elements of the scheme

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Conference 9728: Fiber Lasers XIII: Technology, Systems, and Applications

can be easily incorporated into the housing with footprint of 500x500x160 mm3. A variety of the undertaken measures (e.g. optimization of PPLN/BIBO lengths; temperature tuning; spectrum narrowing using the seeder; dimensions minimization, etc.) and technical treatment of the resultant device parameters make it tempting to claim this source to be a universal tool for many applications such as high-resolution spectroscopy, photo medicine, environmental control, etc.

9728-78, Session PTue

A new single-mode LMA optical fiber based on an anti-resonance in the cladding

Avidan Sharabi, Uzziel Sheintop, Shlomo Y . Goldin, Jerusalem College of Technology (Israel)

9728-76, Session PTue

All-fiber widely tunable thulium laser

Gary Stevens, Thomas H . Legg, Gooch & Housego (Torquay) Ltd . (United Kingdom) We present results from an ‘all-fibre’ thulium laser system that can be tuned to any wavelength between 1710 – 2110 nm, without using any moving mechanical parts. An Acousto-Optic Tunable Filter (AOTF) is used as the tuning element, which allows for the wavelength to be tuned in ~ 20 lasing action across the wavelength range. µ s. Core-pumped and cladding pumped thulium fibres are used to allow for We use in-house fabricated fused fibre couplers and combiners that have a flattened coupling response with wavelength to allow for the system to be built in ‘all fibre’ design. These couplers have a coupling response that only varies by +/- 10% over the 400 nm operating range. The laser can output powers between 1-5 mW over 1710 – 2110nm and has a linewidth of <0.2 nm. An Acousto-optic modulator is used as a switch on the output of the laser to switch the signal between core-pumped and cladding pumped amplifier stages. This allows for the output signals to be amplified to ~1W levels.

A new single-mode large-mode-area optical fiber is proposed. The primary part of the cladding is a thin layer with high refractive index. The layer possesses an array of holes drawn in the propagation direction. The array is periodic in the azimuthal direction. The core may be hollow. The light confinement is achieved via a transmission anti-resonance. Namely, the array of holes allows coupling between an optical mode inside the primary cladding layer and the light both in the core and in the outer space. The light then sees two channels to penetrate the cladding: direct transmission and holes-assisted transmission. A distractive interference between these channels is achieved at an appropriate combination of fiber parameters. Computer simulations of the fiber were performed using COMSOL. The open boundary was simulated using a perfectly matched layer and the attenuation constants of different modes were determined via the imaginary parts of their propagation constants. A fiber holding a single TE01 mode inside a core of 50 µ m radius for the vacuum wavelength 1.55 µ m was designed. The attenuation constant of the TE01 mode was found to be 5.8e-06 dB/cm] while the other modes had attenuation of at least 4 orders of magnitude larger. These results show the possibility of the realization of a single-mode LMA fiber as long as 7500 meters. The required fabrication tolerances were calculated and the fabrication seemed to be feasible.

9728-77, Session PTue

UV curable low refractive index clad coatings for high power fiber laser applications

Jung Hyun Oh, Selee Chang, Sangsoo Oh, Luvantix ADM Co ., Ltd . (Korea, Republic of); Suk-Youn Y . Suh, Luvantix SSCP (United States); Ilkwon Oh, Eunje Sung, Luvantix ADM Co ., Ltd . (Korea, Republic of)

9728-79, Session PTue

Characterization of chirally-coupled core (3C) fibers fabricated with direct nanoparticle deposition (DND)

Changgeng Ye, Joona J . Koponen, Ossi Kimmelma, Ville Aallos, nLIGHT Corp ., Lohja (Finland); Timothy S . McComb, Tyson L . Lowder, nLIGHT Corp . (United States) The thermal and optical stabilities of optical fiber coatings still remain active areas of research, especially for high-powered fiber laser applications. The yellowing and mechanical failure of the polymeric fiber coatings at high temperatures have been the limiting factors in the development of high-powered lasers. We have developed new UV-curable fluorinated polysiloxane coatings, which permit higher operational temperatures up to 180C or higher. Fluorinated polymethylsiloxane (PMS), acrylate PMS, and polydimethylsiloxane are copolymerized to form UV-curable low refractive index oligomers. These oligomers have a refractive index from 1.34 to 1.40. We have observed no visible changes to the optical properties when the cured samples were subjected to 200C for 1,000 hours. The oligomers also have excellent heat resistance. The TGA data revealed less than 5% weight loss at around 300C. We tested our UV-curable fluorinated siloxane high temperature (HT) series coatings against our standard fluorinated urethane LAP (Luvantix Advanced Process) series coatings. The HT series coatings showed a lower Tg of -40C compared to a Tg of 70C for the LAP series coatings, 80% reduction in yellowness at 250C, and a significantly higher degradation temperature. Fibers coated with the new HT series coatings exhibit thermal and optical robustness at well above 200C.

We report detailed characterization results of Yb-doped Chirally-Coupled Core (3CTM) fibers fabricated with nLIGHT’s proprietary Direct Nanoparticle Deposition (DND) technique. Owing to the DND technique and the strict process control, the doping and refractive index profiles and geometrical tolerances are well managed. Two types of 3CTM fibers with core/ clad geometries of 33/250 based on 55/400 µ µ m and 55/400 peak power >1MW with no sign of SRS.

µ m and another 25/250 µ m conventional large-mode-area (LMA) fiber are measured and the results are compared in terms of mode field diameter (MFD), modal content, transmission spectrum, etc. The near-field intensity profile is measured by imaging the fiber end onto a CCD camera. The MFD is calculated from the near-field intensity profile. The modal content of the fibers is measured by Spectrally and Spatially resolved imaging (S2 imaging). The results indicate the 3CTM structure can efficiently filter out all HOMs, making the 3CTM fiber effectively single-moded. The transmission spectra of 3CTM fibers are measured. The feasibility of stimulated Raman scattering (SRS) suppression with 3CTM fibers is demonstrated. A picosecond fiber amplifier is built m 3C fiber, showing robust single-mode operation with

48 SPIE Photonics West 2016 · www.spie.org/pw

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Conference 9728: Fiber Lasers XIII: Technology, Systems, and Applications

9728-80, Session PTue

Inner cladding influence on large mode area photonic crystal fiber properties under severe heat load

Enrico Coscelli, Federica Poli, Univ . degli Studi di Parma (Italy); Romain Dauliat, Univ . de Limoges (France) and Leibniz Institute of Photonic Technology (Germany) and XLIM Institut de Recherche (France); Dia Darwich, Univ . de Limoges (France) and XLIM Institut de Recherche (France) and CNRS (France); Annamaria Cucinotta, Stefano Selleri, Univ . degli Studi di Parma (Italy); Kay Schuster, Leibniz Institut für Photonische Technologien e .V . (Germany); Aurélien Benoit, Raphael Jamier, Philippe Roy, XLIM Institut de Recherche (France) to achieve all these requirements we have developed and realized a new fiber design. This fiber is the first polarization maintaining single-mode fiber delivering a flat top intensity. A high quality flat mode was obtained at 1.05

µ m through the use of a well-tailored index profile and single-mode behavior was verified by shifting the injection and using the S? imaging. Moreover, boron Stress Applying Parts (SAPs) including in the cladding led to a birefringence of 0.6x10-4 and a measured PER better than 20dB even for a long fiber length (~20 m). Alongside the fabrication, we developed a simulation code, using Comsol Multiphysics®, to take into account the stress dependency induced by the SAPs. Further modeling allows us to present an effectively single-mode fiber design, delivering a top-hat mode profile and exhibiting a polarizing behavior.

9728-82, Session PTue

1

µ

m mode-locked fiber laser with tungsten disulphide absorber

Yanrong Song, Heyang Guoyu, Kexuan Li, Zhiyuan Dou, Beijing Univ . of Technology (China) In the last years, the advantages provided by Yb-doped double-cladding Photonic Crystal Fibers (PCFs), that is very large mode area, strong pump absorption, efficient conversion and robust single-mode regime, have driven a significant development of high power fiber lasers. Currently, thermal effects represent the limit for further power scaling of fiber lasers, since they negatively affect the single-mode behavior of Large Mode Area (LMA) PCFs. In fact, beyond a certain average power threshold, unwanted energy transfer from the Fundamental Mode (FM) to the Higher-Order Modes (HOMs), originally weakly guided, occurs, causing a significant beam quality degradation. In this work, the influence of the size and the air-filling fraction of the inner cladding on the first HOM confinement in Yb-doped LMA PCFs under different heat load values has been investigated with a full-vector modal solver based on the finite element method, used also to solve the steady-state heat equation. In particular, the air-cladding inner dimension and the air-hole diameter in Symmetry-Free PCFs and Large Pitch Fibers have been modified in order to study which conditions facilitate the coupling between HOM and cladding modes, thus improving the delocalization of the former and making the fiber single-mode behavior more robust. Simulation results have shown that such coupling can be successfully exploited to suppress the HOMs when significant heat load is applied.

Abstract: Some potential applications based on passively mode-locked fiber lasers have been investigated. Recently, more effort has been concentrated on discovering and investigating new and high performance saturable absorbers(SAs) such as topological insulators (TIs), transition mental dichalcogenides (TMDs) which include molybdenum disulfide (MoS2), tungsten disulfide (WS2) and their analogue (tungsten diselenide, WSe2). Here we demonstrated an all-normal-dispersion Yb-doped mode-locked fiber laser based on tungsten disulphide saturable absorbers (WS2-SA). The WS2-SA were made by mixing WS2 nanosheets solution with polyvinyl alcohol (PVA), and then evaporated on a substrate. The WS2 nanosheets in the dispersion were observed by an atomic force microscope (AFM). The sizes of the WS2 nanosheets were about 150 to 300 nm and the thickness was about 4.6 nm. The modulation depth of the WS2 film was 1.78% and the saturable optical intensity was 410 MW/cm2, which were measured by a power-dependent absorption system. When the WS2 film was inserted into an Yb-doped fiber laser, the mode-locked pulses were obtained at the wavelength of 1030nm. The pulse width of 2.5 ns and a repetition rate of 2.84 MHz were reached. As the pump power increased to 350 mW, the maximum output power was ~8 mW. To the best of our knowledge, this is the first time to realize mode-locked pulses based on WS2-SA at 1?m waveband.

9728-81, Session PTue

Top hat single-mode polarization maintaining fiber and polarizing numerical design

Pierre Gouriou, Lab . de Physique des Lasers, Atomes et Molécules (France) and CEA (France); Florent Scol, Commissariat à l’Énergie Atomique (France); Benoit Sevigny, Constance Valentin, Yves Quiquempois, Laurent Bigot, Rémi Habert, Andy Cassez, Olivier Vanvincq, Lab . de Physique des Lasers, Atomes et Molécules (France); Emmanuel Hugonnot, Commissariat à l’Énergie Atomique (France); Géraud Bouwmans, Lab . de Physique des Lasers, Atomes et Molécules (France) Compactness, long term stability and no free-space alignment are important advantages of fiber lasers over bulky systems. These fiber lasers have also demonstrated their capability to deliver high-power pulses and are thus suitable for numerous applications. Nevertheless the intensity profile delivered usually has a Gaussian-like shape, which most of the time is sufficient, but it could be interesting, for many applications (laser biological tissues interactions, heat treatment, industrial laser processing or for seeding large-scale laser facilities like Laser MegaJoule) to obtain a homogeneous intensity profile at the fiber laser output. Moreover several of these applications required a linearly polarized output beam. In order

9728-83, Session PTue

5.5 W monolitic single-mode fiber laser and amplifier operating near 976 nm

Svetlana S . Aleshkina, Mikhail E . Likhachev, Fiber Optics Research Ctr . of the Russian Academy of Sciences (Russian Federation); Denis S . Lipatov, Institute of Chemistry of High-Purity Substances of the Russian Academy of Sciences (Russian Federation) and N .I . Lobachevsky State Univ . of Nizhni Novgorod (Russian Federation); Oleg I . Medvedkov, Konstantin K . Bobkov, Mikhail M . Bubnov, Fiber Optics Research Ctr . of the Russian Academy of Sciences (Russian Federation); Alexei N . Guryanov, Institute of Chemistry of High-Purity Substances of the Russian Academy of Sciences (Russian Federation) In the present work we demonstrate a novel fiber design for efficient generation and amplification of laser irradiation near the wavelength of 0.98 µ m. The main feature of the fiber design is an extremely low NA W-type refractive index profile (single-mode at core diameter of 28 µ m) that allowed us to achieve core-to-clad diameters ratio of about 0.31. The

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9728-84, Session PTue 9728-85, Session PTue

Conference 9728: Fiber Lasers XIII: Technology, Systems, and Applications

photodarkening-free silica-based glass matrix was used for fabrication of the Yb-doped core. Square-shaped fibers with dimensions of 28/80x80 µ m, and 40/115x115 µ m coated by low-index polymer (NA=0.46) were fabricated for the laser and amplifier, correspondingly. The fiber with dimension of 28/80x80 were 10 W and 25%, correspondingly. µ m was tested in a simplest all-fiber laser scheme cladding pumped by standard MM laser diode (30W at 915 nm). Output power of 5.5 W at 977 nm (limited by available pump power) was achieved in this configuration. Lasing threshold and slope efficiency The fiber with dimension of 40/115x115 slope efficiency was about 15%.

µ m was used for building an all-fiber amplifier scheme. The fabricated fiber was spliced with a standard pump and-signal combiner, seeded by standard semiconductor laser diode with power of 100 mW at 976nm and pumped by two multimode diodes with net power of 60 W. The maximum output power of 5.7 W was achieved. The

Experimental investigation of pedestal suppression in a figure-eight fiber laser by including a polarization asymmetrical NOLM

Migel A . Bello-Jimenez, Erika Hernández-Escobar, Instituto de Investigación en Comunicación Òptica (Mexico); Evgeny A . Kuzin, Baldemar Ibarra-Escamilla, Manuel Durán-Sánchez, Instituto Nacional de Astrofísica, Óptica y Electrónica (Mexico); Antonio Diez Cremades, Jose L . Cruz, Miguel V . Andrés, Univ . de València (Spain) A polarization asymmetrical nonlinear optical loop mirror (NOLM) is investigated to perform pedestal-free optical pulses in a figure-eight laser (F8L). The NOLM is composed of a symmetrical coupler, with output ports connected to 220 m of twisted fiber, and a quarter wave retarder (QWR) located asymmetrically into the loop. In the low-intensity regime the NOLM shows a periodical dependence on the QWR angle, with a minimal and maximal transmission equal to 0 and 0.5, respectively. The power-dependent transmission is investigated for each circular polarization component at the NOLM output, considering a right circular input polarization, and the dependence on the QWR angle. The results demonstrate that in the low-power regime the NOLM operates as a half wave plate and the output polarization is orthogonal to the input one. However, at higher power level the polarization component parallel to the input appears, with a transmission that always begins from zero at low power, allowing the rejection of low-intensity components. This property results very attractive to perform modelocked operation in a F8L. The low-intensity component, with polarization orthogonal to the input, can be implemented to initiate the modelocking process, whereas the parallel component can be associated to high-intensity optical pulses with zero transmission for low-intensity compoments. This configuration allows the generation of optical pulses with peak power close to the maximum peak power value obtainable from the average output power. Experimental results demonstrate that by employing this configuration we can obtain a contrast between the peak and continuous background higher that 30 dB.

Single-frequency Raman fiber amplifier emitting 11

µ

applications j 150 W peak-power at 1645 nm for remote methane sensing

Philippe Benoit, Nicolas Cézard, Anne Durécu, ONERA (France); Arnaud Mussot, Lab . de Physique des Lasers, Atomes et Molécules (France) and IRCICA (France); Alexandre Kudlinski, Univ . des Sciences et Technologies de Lille (France) and IRCICA (France); Guillaume Canat, ONERA (France) Remote methane concentration measurement using a Differential Absorption Lidar system can be performed using a single-frequency pulsed laser source at 1645.55 nm. This wavelength cannot be efficiently amplified in conventional Erbium Doped Fiber Amplifier as the gain band stops around 1620 nm. We report on a single-frequency polarization-maintaining pulsed amplifier at 1645 nm relying on stimulated Raman scattering (SRS) in highly nonlinear silica fibers (HNLF). Considering that SRS converts pump photons to photons frequency-downshifted by 13.2 THz with a gain bandwidth of 2 THz, a 1545 nm pump can efficiently amplify a 1645 nm seed laser. The drawback of using a HNLF is that the single-frequency signal will also experience stimulated Brillouin scattering (SBS) through its amplification.

This issue has been partially solved by designing a two-stage amplification setup minimizing SBS. In the first stage, a 20 m piece of HNLF has been used so that the effective length of the amplified signal stays under SBS threshold. In the second stage, we used a 2.5 m piece of HNLF and high pump peak-power to significantly reduce the effective length, allowing more amplification.

We report on generation of single-frequency 11 pulse repetition frequency.

µ J energy pulses at 1645 nm corresponding to 150 W peak-power and 80 ns pulse duration at 20 kHz

9728-86, Session PTue

Pulsed interferometric phase measurement for coherent beam combining

Jeremy Le Dortz, Thales Research & Technology (France); Marie Antier, Thales Optronique S .A .S . (France); Jérôme Bourderionnet, Christian Larat, Eric Lallier, Thales Research & Technology (France); Louis Daniault, Severine Bellanger, Lab . pour l’Utilisation des Lasers Intenses (France) and Ecole Polytechnique (France); Christophe Simon-Boisson, Thales Optronique S .A .S . (France); Jean-Christophe F . Chanteloup, Ecole Polytechnique (France) and Lab . pour l’Utilisation des Lasers Intenses (France); Arnaud Brignon, Thales Research & Technology (France) Coherent beam combining (CBC) of fiber amplifiers provides an attractive mean of reaching high peak and high average powers. Active CBC techniques involve phase detection and compensation of the phase variations of each amplifier. Interferometric phase measurement has proven to be particularly well suited to phase-lock a large number of fibers in continuous regime. In this presentation, we demonstrate for the first time the phase locking of three fibers in femtosecond pulse regime with this technique. A master oscillator generates pulses of 300fs (chirped at 200ps). The beam is split into four channels. Phase locking is implemented on three fibers with respect to the fourth, acting as a phase reference. Prior to phase locking, the optical path differences are adjusted. Interferograms for the three fibers are recorded at 1kHz with a camera. A dedicated algorithm is developed to measure both the phase and the delay between the fibers. The delay and phase shift are thus calculated collectively from a single image and piezo electric fiber stretchers are controlled in order to ensure compensation of time-varying phase and delay variations. The measured phase shift errors between the fibers are below ?/80rms when the servo-loop is operating. Phase noise power spectral densities of a fiber amplifier operating in the short pulse regime will be also presented in opened and closed loop to assess the bandwidth of the control loop and the capability of our locking technique to perform CBC of fiber amplifiers operating in the short pulse regime.

50 SPIE Photonics West 2016 · www.spie.org/pw

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Conference 9728: Fiber Lasers XIII: Technology, Systems, and Applications

9728-87, Session PTue

Statistical model of the energy transfer in Er3+:Yb3+-codoped glasses

Michael Steinke, Jörg Neumann, Dietmar Kracht, Peter Wessels, Laser Zentrum Hannover e .V . (Germany) A novel statistical model for the energy transfer process in Er3+:Yb3+ codoped glasses, e.g. fibers, is presented. The model is based on an existing stochastic model for the homogeneous up-conversion in Er3+-doped glasses and the most important advantage is the correct modeling of the energy transfer as a dipole-dipole interaction. Thus, the model avoids the unreasonable but common modeling of the Er3+-to-Yb3+ energy transfer as a fixed rate which is simply multiplied with the upper Yb3+ state population and the ground state Er3+ population. Exemplary results obtained with the novel statistical model behave reasonable, i.e. the population densities of the Yb3+ and Er3+ upper states depend on the individual parameters of the model as expected. Therefore, the presented model provides good prospects to study optimization strategies of Er3+:Yb3+-codoped fibers. Furthermore, the model can be further developed and expanded in the future, e.g. by including energy migration amongst the individual Yb3+-ions and Er3+-ions.

Major advantages over traditional methods for these applications can be achieved with dual-comb technique. For example, dual-comb spectroscopy provides orders of magnitude improvement in acquisition speed over standard Fourier-transform spectroscopy while still preserving the high resolution capability. Wider adoption of the technique has, however, been hindered by the need for complex and expensive ultrafast laser systems. Here, we present a simple and robust dual-comb technique that employs a free-running bidirectionally mode-locked fiber laser operating at telecommunication wavelength. Two femtosecond frequency combs (with a small difference in repetition rates) are generated from a single laser cavity to ensure mutual coherent properties and common noise cancellation. We demonstrate real-time absorption spectroscopy measurements without the need for complex servo locking or adaptive sampling with accurate frequency referencing and relatively high signal-to-noise ratio. The compact and all-fiber implementation scheme makes this technique a promising tool for practical, outside-of-laboratory applications.

9728-90, Session PTue

Passive mode locking through nonlinear coupling with different dual-core fiber length

Xiaohui Fang, Beijing Univ . of Technology (China)

9728-88, Session PTue

Experimental study of a linear cavity dual wavelength Er/Yb double clad fiber laser operating in self-Q-switch, self-pulsing and CW

Manuel Durán-Sánchez, Instituto Nacional de Astrofísica, Óptica y Electrónica (Mexico) and Cátedras CONACyT (Mexico); Ricardo I . Álvarez-Tamayo, Univ . Tecnológica de Puebla (Mexico); Olivier J . M . Pottiez, Ctr . de Investigaciones en Óptica, A .C . (Mexico); Berenice Posada Ramírez, Baldemar Ibarra-Escamilla, Evgeny A . Kuzin, Instituto Nacional de Astrofísica, Óptica y Electrónica (Mexico); Antonio Barcelata-Pinzón, Univ . Tecnológica de Puebla (Mexico) The effect of fiber length on ultrafast pulse formation in active dual-core fiber laser is investigated numerically, which has been ignored in previews studies. The simulation results show that stable self-starting mode-locked operation can be realized with a dual-core fiber length shorter than the linear coupling length. A filter is necessary to stabilize mode-locking operation when the fiber length is longer than the linear coupling length. And mode-locking operation can not be self-started from noise with a fiber length equal to the linear coupling length.

9728-91, Session PTue

Characterization technique for long optical fiber cavities based on beating spectrum of multi-longitudinal mode fiber laser and beating spectrum in the RF domain

George A . Adib, Yasser M . Sabry, Diaa A . Khalil, Ain Shams Univ . (Egypt) We present experimental results of a proposed dual wavelength fiber laser with Er/Yb double clad fiber. With pump power variations the laser to 1 W Self-Q-switched pulses are obtained. With a pump power range from 1 W to 2 W, self-pulsing operation is observed. With pump power above 2 W CW operation is achieved. The linear cavity laser is based in the use of two fiber Bragg gratings for wavelength selection and a Sagnac interferometer for cavity losses adjustment to obtain dual-wavelength operation. Power efficiency is around 36%. In self-Q-switch operation maximal repetition rate is 60kHz with pulse duration in micro seconds range. With pump power of 10 W, the maximal average output power of 3.6 W is obtained for CW operation.

9728-89, Session PTue

Dual-comb spectroscopy with a free running bidirectionally mode-locked fiber laser

Khanh Q . Kieu, Seyed Soroush Mehravar, Robert A . Norwood, Nasser N . Peyghambarian, College of Optical Sciences, The Univ . of Arizona (United States) Dual-comb technique has enabled exciting applications in high resolution optical spectroscopy, precision distance measurements, and 3D imaging. Optical fiber cavities are basic building blocks in different applications including fiber laser, fiber-laser frequency combs,swept laser source, environmental and rotation sensors. For many applications, the cavity length is needed to be relatively long- in the order of meters or kilometers- to achieve a fine 3-dB bandwidth and free spectral range stability. The characterization of long fiber cavities is, thus, essential to design these systems and predict their practical performance.The conventional techniques for optical cavity characterization are not suitable for long fiber cavities due to the cavities’ small free spectral ranges and due to the wavelength instability (length variations) caused by the environmental effects.In this work, we present a novel technique to characterize long fiber cavities using multi-longitudinal mode fiber laser source and RF spectrum analyzer. The output of the laser is used as an input to the cavity under test. The output of the cavity is fed to a square-law optical detector and its electrical output is recorded on the RF spectrum analyzer. Then, the RF spectrum is used to obtain the response of the cavity. The method has been applied experimentally to characterize ring cavities with lengths of 6 m and 2.4 km. The fiber laser source is formed in a ring configuration, where the fiber laser cavity length is chosen to be 15 km to ensure the free spectral range is much smaller than the free spectral range of the characterized passive fiber cavities. The results are compared to theoretical predictions with very good agreement.

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Conference 9728: Fiber Lasers XIII: Technology, Systems, and Applications

9728-92, Session PTue

Modelocked, Q-switched, or Q-switched modelocked operation of a fiber oscillator by adjusting the mode-field area within the graphene oxide saturable absorber

Manuel Ryser, Alexander M . Heidt, Thomas Feurer, Valerio Romano, Univ . Bern (Switzerland) We have demonstrated a fiber ring oscillator in modelocking, Q-switching and simultaneous Q- switched modelocking operation mode at constant pump power level. The different oscillation states of the cavity were reached by varying the mode-field area within the graphene oxide saturable absorber (GO-SA) and not as commonly done by adjusting the optical pump power. All fibers and fiber coupled elements used in our setup are polarization maintaining. Thus, the only available mode-locking mechanism in this fiber ring oscillator is the saturable aborption at the GO-SA and mode-locking based on nonlinear polarization rotation can be excluded.

Widely used methods for producing ultrashort pulses in laser cavities are Q- switching and modelocking. Q-switching allows the generation of milli-joule pulses with microsecond to nanosecond duration and hertz to many kilohertz repetition rates. Mode-locking produces typically pico joule to nano-joule pulses with femto- to picosecond pulse durations and megahertz to gigahertz repetition rates. Furthermore, a laser cavity can also operate simultaneously in Q-switched and modelocking regime to produce a pulse train at MHz with a modulation envelope at kHz repetition rate. The Q-switched modelocked pulses are modulated in amplitude the most intense pulses can reach energies several times higher than would be possible with pure modelocking.

With the approach presented here various operation modes can be achieved at the same pump power level, which gives great flexibility in generating ultrashort optical pulses over a broad range of durations, repetition rates and energies from the same laser cavity.

9728-93, Session PTue

Switchable dual-pulse-shape mode-locked figure-eight all-PM fibre master oscillator with 0.5 W-level average output

Sergey M . Kobtsev, Aleksey V . Ivanenko, Yurii Fedotov, Sergey V . Smirnov, Novosibirsk State Univ . (Russian Federation); Artur Golubtsov, Sergey Khripunov, Novosibirsk State Univ (Russian Federation) For the first time a method for switching between generation of single- and double-scale pulses has been demonstrated in a mode-locked figure eight NALM-based all-PM-fibre Yb master oscillator by adjustment of two pumps power. Introduction into a F8 configuration of a non-linear amplifying loop mirror with two active media not only ensured relatively high average output power of the master oscillator (> 0.5 W at 22-MHz repetition rate), but also allowed switching laser operation from one pulse type (single-scale coherent pulses with duration of < 10 ps) to another - femtosecond clusters with envelope width of 16 ps and sub-pulse duration < 200 fs. Implementation of an all-PM-fibre configuration dispensed with the requirement of polarisation controllers (they were used in the previous configuration of this master oscillator) and to trigger mode-locked operation by selection of power levels of the two pump sources. The suggested optical layout features output via two ports. A larger portion of the radiation exited the master oscillator through port 1 at the average output power of 600 mW for single-scale pulses and 525 mW for femtosecond clusters (double-scale pulses). The average power of radiation exiting through port 2 amounted to 140 and 280 mW correspondingly. The proposed method of switching between generation of single-scale and double-scale pulses by adjustment of relative levels of two pump sources provides considerably faster and more reproducible regime switching in comparison to switching by adjustment of polarisation controllers.

9728-94, Session PTue

Broadband wavelength tuning of hybrid femtosecond Er/Tm fiber laser system in microstructured suspended-core tellurite fiber

Maksim Y . Koptev, Institute of Applied Physics of the RAS (Russian Federation); Elena A . Anashkina, Institute of Applied Physics of the RAS (Russian Federation) and N .I . Lobachevsky State Univ . of Nizhni Novgorod (Russian Federation); Alexey V . Andrianov, Institute of Applied Physics of the RAS (Russian Federation); Vitaly V . Dorofeev, Institute of Chemistry of High-Purity Substances of the Russian Academy of Sciences (Russian Federation); Alexey F . Kosolapov, Fiber Optics Research Ctr . of the Russian Academy of Sciences (Russian Federation); Sergey V . Muravyev, Institute of Applied Physics of the RAS (Russian Federation); Arkady V . Kim, Institute of Applied Physics of the RAS (Russian Federation) and N .I . Lobachevsky State Univ . of Nizhni Novgorod (Russian Federation) We propose a widely tunable in the 1.6-2.6 ?m range femtosecond fiber laser source, generating high-quality sech-shaped pulses with the duration of the order of 100 fs. The source contains an all-fiber hybrid Er/ Tm femtosecond laser system that generates 2 nJ 150 fs pulses in Erbium channel and 4 nJ 125 fs pulses in Thulium channel. The laser was coupled to a piece of suspended-core microstructured TeO2-WO3-La2O3 glass fiber with 3.2 ?m core diameter and ZDW of approximately 1.5 ?m. We experimentally obtained tunable high-quality Raman solitons up to 2.25 and 2.6 ?m with the pump at 1.6 and 2 ?m, respectively. Their spectral widths correspond to the Fourier transform-limited duration of about 100 fs. We have also made theoretical studies of self-frequency soliton shifting in the tellurite fiber with carefully measured and calculated parameters, based on the one-way wave equation dealing with the full electric field of light. Our numerical model is in a very good agreement with the experiment and also shows Raman soliton self-frequency shift in the range well beyond 3 ?m for increased pump energy. So, the demonstrated turnkey fiber-based laser source can be used for applications requiring high-quality ultrabroadband femtosecond optical pulses.

9728-95, Session PTue

Tunable pulse width and multi-megawatt peak-power pulses from a nonlinearly compressed monolithic fiber MOPA system

Ryutarou Yamashita, Kazuo Maeda, Goro Watanabe, Kazuyoku Tei, Shigeru Yamaguchi, Tokai Univ . (Japan); Jun Enokidani, Shin Sumida, OPT-i Co ., Ltd (Japan) We report on tunable pulse width and energetic pulse generation from a nonlinearly compressed monolithic fiber MOPA system. The master seed source employs a Mach-Zehnder intensity modulator (MZIM). This seed source has operational flexibility with respect to pulse width, 90 ps to 2 ns and repetition rate, 200 kHz to 2 MHz. The seed pulses are amplified by a monolithic three-stage amplifier system based on polarization maintain Yb-doped fibers. The maximum output power was 32 W at the shortest pulse condition, the pulse width of 90 ps and repetition rate of 2 MHz. A spectral width after amplification was broadened to 0.65 nm at RMS width. Both of ASE and SRS are not observed in the spectrum. After amplification, we also demonstrated pulse compression with a small piece of chirped volume-bragg-grating (CVBG) which has the dispersion rate of 81 ps/nm. As a result of pulse compression, the shortest pulse width was reduced from

52 SPIE Photonics West 2016 · www.spie.org/pw

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90 ps to 4 ps, which brought an increase of the peak power up to 2.9 MW. The autocorrelation traces are well matched with the Gaussian function. The compressed pulses are clean with very little structure in there wings. We can expand the operation range of the monolithic-fiber MOPA system in pulse width, 4 ps to 2 ns.

9728-96, Session PTue

Conference 9728: Fiber Lasers XIII: Technology, Systems, and Applications Fabrication and investigation of active composite fibers with phosphate core and silica cladding

Sergey L . Semjonov, Olga N . Egorova, Oleg I . Medvedkov, Maxim S . Astapovich, Andrey G . Okhrimchuk, Evgeny M . Dianov, Fiber Optics Research Ctr . of the Russian Academy of Sciences (Russian Federation); Boris I . Denker, Boris I . Galagan, A . M . Prokhorov General Physics Institute of the Russian Academy of Sciences (Russian Federation); Sergey E . Sverchkov, A . M . Prokhorov General Physics Institute (Russian Federation) 25, 16302 (2007)]. Using numerical simulation the solitons collision and merge of solitons in dispersion oscillating fiber are considered. Rogue wave arises as result of the inelastic collision of optical solitons. Inelastic collisions are forced by periodical variation of the fiber dispersion. Due to inelastic interactions, the solitons merge together and give birth to a new soliton whose amplitude is significantly greater than the amplitude of the other satellite solitons. After the merging process, the dynamics of the output pulse is determined mainly by a single high-amplitude soliton called as rogue wave. The parameters of the rogue wave depends on the modulation period of the fiber diameter, repetition rate of the host laser. The generation of single high-amplitude soliton can be actively controlled via chirp of input pulses. The generation of the rogue waves both in active and passive dispersion oscillating fibers is considered.

9728-98, Session PTue

Broadband optical amplification with water-free hexagonal double-clad Bi doped silica fiber

Soichi Kobayashi, Mikoto Takahashi, Mizuki Ohara, Ikki Kondo, Chitose Institute of Science and Technology (Japan); Yusuke Fujii, Photonic Science Technology, Inc . (Japan) Phosphate glass is a unique host for lanthanide ion doping: high concentrations of rare earth (RE) ions can be incorporated into phosphate glass, the phosphate glass host provides efficient and irreversible energy transfer from Yb3+ ions to Er3+. Additionally, Yb-doped phosphate glass is highly resistant against photodarkening even at high Yb3+ concentration. High doping levels of phosphate fibers make it possible to obtain high gain and high output power per unit length, thus reducing the length of active fibers in comparison to silica based fibers.

A substantial drawback of the use of phosphate glasses as a material for fiber fabrication in comparison with silica is their low stability; exposure to air moisture causes degradation of the phosphate fibers over time. Moreover, due to the sharp difference in their physical properties, splicing of phosphate and silica fibers is difficult. In this presentation, we describe composite optical fibers with rare-earth co-doped phosphate-glass core in a silica-glass cladding. High RE-ion concentrations in the phosphate core of the composite fiber allow fiber length reduction in comparison with silica fibers. The silica cladding provides high mechanical strength and protects the phosphate core from air moisture while making it easier to splice with silica fibers. Both fabrication of fibers and their optical properties will be discussed. The lasing efficiency of the composite fibers was found to be high in both cases - cladding and core pumping. 1.3 - 1.55 ?m optical amplifiers for the long distance up-stream and down-stream networks for a future increase of fiber access network in telecommunications are attractive. A bismuth-doped silica glass has a potential of the broadband spectrum as laser and amplifier applications at 1.3 -1.55 ?m. The bismuth-doped fiber lasers and amplifiers were discussed by the Dianov group fabricating by the MOCVD method. In this report optical amplification characteristics at 1.3 - 1.55 ?m are presented with the water free hexagonal double-clad bismuth-doped silica fiber (HDC-BDF) made by the VAD method. The bismuth and aluminum ions were vapor– ?phase doped into the silicon and germanium oxide. The concentration of Bi oxide in the core glass was measured as 0.8 wt% by EDX spectroscopy. Refractive index difference between the core and the first cladding in the preform was measured as 0.5 % by the optical fiber preform analyzer. The hexagonal preform was drawn into the fiber where the core and the first cladding diameters were measured as 7 ?m and 100 ?m between flat surfaces, respectively. The relative refractive index difference between the silica first-cladding and the polymer first-cladding is 3.6%. Pumping into the HDC-BDF was performed by using the 15 degree tilt-polished fiber from the hexagonal surface with the multimode fiber pigtail of the pumping LD. In the case of the hexagonal fiber it is easy to realize a perfect splicing with the single-mode fiber. 1 dB/m amplification in 1310 nm was measured with 4 m long HDC-BDF where SNR is over 35 dB with -40 dBm input signal.

9728-97, Session PTue

Controlled generation of optical rogue waves in dispersion oscillating fiber

Alexey Sysoliatin, Konstantin Gochelashvili, A . M . Prokhorov General Physics Institute of the Russian Academy of Sciences (Russian Federation); Andrey I . Konyukhov, N .G . Chernyshevsky Saratov State Univ . (Russian Federation); Leonid A . Melnikov, Saratov State Technical Univ . (Russian Federation); Mikhail Y . Salganskii, Institute of Chemistry of High-Purity Substances of the Russian Academy of Sciences (Russian Federation) Generation of high-intensity rogue waves from optical turbulence or breathers is considered usually as statistically-rare process [J.M. Dudley et al. Nature Photonics 8, 755 (2014)]. We propose a new approach for the generation of separated optical rogue waves “on demand”. The proposed scheme utilizes dispersion oscillating fiber. Variation of the fiber dispersion can be made during its drawing process [A.A. Sysoliatin et al. Opt. Expr.

9728-99, Session PTue

A diode drive mechanism for always resonant pumping with laser diodes without wavelength locking

S . Arun, Ctr . for Nano Science and Engineering (CeNSE) (India) and Indian Institute of Science (India); Balaswamy Velpula, Indian Institute of Science (India) and Ctr . for Nano Science and Engineering (India); Great Chayran, Indian Institute of Science (India); P . Vanitha, Abhishek Kumar, V . R . Supradeepa, Indian Institute of Science (India) and Ctr . for Nano Science and Engineering (India) An essential requirement in lasers and amplifiers utilizing a rare-earth doped gain medium is to have substantial match between the emission wavelengths of the pump laser diodes to the absorption band of the gain medium. This would enable the optimal balance of laser efficiency

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Conference 9728: Fiber Lasers XIII: Technology, Systems, and Applications

and nonlinearity. Further, this leads to improved system reliability - by minimizing issues arising from unabsorbed pump and cost - by minimizing the length of gain media. However, the emission wavelengths vary as a function of various parameters, primarily output power (drive current), temperature and variation in device parameters. The latter two factors can be easily overcome through the choice of proper heat-sink temperatures and by using diodes binned according to their emission wavelength. However, a fundamental limitation is the variation of emission wavelength as a function of output power (drive current). Currently, the alternative is to use wavelength locked laser diodes. This comes at the cost of reduced efficiency and/or enhanced price owing to additional components and packaging necessary in each module.

In this work, we demonstrate a novel drive scheme for standard laser diode modules without wavelength locking. This scheme enables “always resonant” pumping by the diodes. The deleterious effect of emission wavelength drift accompanying power tuning, is completely avoided. In this work, we demonstrate the drive mechanism and its performance in a fiber amplifier configuration. We anticipate this scheme to have significant impact in enabling a cost-effective solution which achieves an optimal balance of efficiency, nonlinearity and reliability in laser systems.

electrically tunable all-fiber graphene device with high efficiency based on strongly enhanced graphene-evanescent wave interaction. We successfully fabricated a graphene-based field effect transistor on a side-polished fiber mediated by an ion-liquid. Gate-variable electrical transport and related optical properties are simultaneously studied as a function of number of graphene sheet, which reveals that it exhibits non-resonant large optical transition change (> 90%) for applied voltage of less than 3 V. Taking the advantages of all-fiber device with high optical damage threshold, the proposed device is successfully integrated into all-fiber laser system as an electrically tunable in-line graphene saturable absorber. We observed that nonlinear saturable absorption properties of our all-fiber graphene device can be tuned as we adjust the applied electrical signal. This subsequently modifies the laser cavity condition, enabling gate-controlled pulsed fiber laser operation at various operation regimes including continuous wave mode-locking and Q-switching. Further possible applications of the proposed device for tunable nonlinear optic signal generation, fine control of passively mode-locked fiber laser, and broadband optoelectronic devices will be also discussed based on our all-fiber graphene devices with enhanced device efficiency.

9728-100, Session PTue

Understanding gain saturation: a pseudo intensity limiter in pulsed fiber amplifiers

Nishant K . Shekhar, Sourav Das Chowdhury, Ranjan Sen, Mrinmay Pal, Central Glass and Ceramic Research Institute (India)

9728-102, Session PTue

High power, high signal-to-noise ratio single-frequency 1 ?m Brillouin all-fiber laser

Jing Wang, Yubin Hou, Qian Zhang, Dongchen Jin, Ruoyu Sun, Hongxing Shi, Jiang Liu, Pu Wang, Beijing Univ . of Technology (China) In the process of development of high energy nanosecond pulsed fiber lasers, we confronted a phenomenon that manifested itself as an intensity limiter much like the non-linear scatterings, namely SBS and SRS. The phenomenon was coined ‘Gain Saturation’ in laser literature, notorious for its distorting ability of square shaped pulses. But its intensity limiting behavior was accentuated only when a 100ns width Gaussian pulse was unable to get amplified over 2 KW of peak power. Higher pumping only led to broadening of the pulse to 120ns without enhancement of its peak power. A numerical simulation was done so as to mimic the occurrence of gain saturation. Initially the numerical model predicted a higher intensity achievable than what was measured in the experiment. This mismatch of numerical prediction and experimental observation was later corrected by incorporating the effect of ‘Numerical Aperture’ of the fiber. The numerical simulation combined with experimental observation led to an empirical relation between fiber parameters, fiber material and the maximum intensity that could be achieved for a Gaussian pulse of constant width before gain saturation appeared. Based on the empirical relational, a fiber with required dimension was fabricated which later in a table top experiment attained the desired peak power combined with high energy. Simultaneously, a fluid analogy was also devised in order to explain in an alternate way the happening of ‘Gain Saturation’. The fluid analogy also helped in simplifying the complexities of “Gain Saturation’.

We have demonstrated a high power, high-optical signal-to-noise ratio (OSNR) single-frequency 1 ?m Brillouin all-fiber laser. The Brillouin fiber laser (BFL) consists of a continuous-wave Yb-doped single-frequency seed source, one-stage Yb-doped fiber amplifier (YDFA) pumped by fiber pigtailed multimode laser diode and a single-pass Brillouin ring cavity. The seed source is a homemade short-linear-cavity distributed Bragg reflector (DBR) single-frequency fiber laser with the output power of 35 mW and the linewidth of 20 kHz. Through one-stage YDFA, the seed laser is amplified to 2.615 W limited by the optical power of the isolator. The laser is then split into two unequal parts with 99/1 fiber coupler and the output from 99% coupler is served as the Brillouin pump (BP) with 2.588 W output power, which is injected into the single-pass Brillouin ring cavity. By optimizing the length of the cavity to 10m, stable single-longitudinal-mode operation is obtained with 2 kHz linewidth owing to the linewidth narrowing effect. The single-frequency BFL generates an average output power of 1.402 W at 2.588 W pump power with 70% output coupler. The laser does not show any phenomena of saturation, so more output power is expected if a higher power pump laser is launched. The slope efficiency is 79% considering the loss of the circulator. The OSNR of the BFL in the maximum output power is 77 dB which can be 27 dB better than the BP owing to the intensity and phase-noise reduction of the SBS process.

9728-101, Session PTue

Electrically tunable in-line graphene saturable absorber for pulsed fiber laser applications

Dong-Il Yeom, Ajou Univ . (Korea, Republic of) Active control of light in an optical fiber has been studied with great interest due to its compatibility with diverse fiber-optic communication and fiber laser systems. Although the optical absorption properties of graphene can be modified through the Fermi-level control in a graphene layer by applied electrical signal, realization of gate-controlled graphene device in an optical fiber platform remains highly challenging. In this presentation, we report an

9728-103, Session PTue

Yb-doped large mode area fibers with depressed clad and dopant confinement

Vincent Roy, Claude Paré, Pierre Laperle, Louis Desbiens, Yves Taillon, INO (Canada) Large mode area (LMA) fibers with depressed-index cladding layer and confinement of rare-earth dopants can provide effective suppression of high-order modes (HOMs). The latter favors preferential amplification of the fundamental mode through optimized differential in mode overlap with rare-earth dopants whereas the former results in increased differential bending losses as a result of the lower effective numerical aperture seen by HOMs. Both of these methods are shown herein to be quite effective at

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Conference 9728: Fiber Lasers XIII: Technology, Systems, and Applications

suppressing HOMs for fibers with large core diameters when implemented together. A polarization-maintaining Yb-doped double-clad fiber with 35/250 µ m core/clad diameter has been fabricated from conventional MCVD and solution doping process according to this design. The fiber which has an effective fundamental mode area close to 500 scalable over 1000 µ µ m2 yields near diffraction limited output with beam quality factor M2 close to 1.1 when tested as a power amplifier with a 1064 nm coherent seed light source. The exceptional beam quality is maintained for high power, well over 20dB of gain. Beam pointing measurements provide additional evidence for near single-mode behavior as the pointing fluctuations are shown to be negligible. Besides, the beam characteristics are further examined for coupling conditions that yield a significant fraction of energy into HOMs in order to show how effective is the suppression of these modes. We are confident for the effective mode area of the LMA fiber design proposed herein to be readily m2 and yet allow for reasonably good beam quality.

provided an error signal for active phase stabilization which was performed via Locking of Optical Coherence by Single-Detector Electronic-Frequency Tagging (LOCSET).After phase stabilization, the beams were coherently combined via the 1x5 DOE. A total combined output power of 5 kW was achieved with 82% combining efficiency. The intrinsic DOE splitter loss was 5%. Similarly, the losses due to non-ideal polarization extinction were 2.5%. Other losses include residual phase error control (1-2%), amplifier amplified spontaneous emission (ASE), and largely, fractional beam displacement and misalignment errors. Near diffraction-limited beam quality at 5 kW was attained with a measured M2 value of 1.06.

9728-106, Session PTue

Linewidth investigation of high-power single-frequency Tm-doped fiber amplifier

Haiwei Zhang, Wei Shi, Quan Sheng, Tianjin Univ . (China)

9728-104, Session PTue

Simultaneous mode and nonlinear frequency conversion of HOMs

Oleg Shatrovoy, Boston Univ . (United States); Siddharth Ramachandran, Lars Rishoj, The Boston Univ . Photonics Ctr . (United States) We present a simulated device that performs efficient (90%) second harmonic generation (SHG) of a truncated Bessel beam that mirrors higher order modes (HOM) of fibers, while simultaneously converting it to a Gaussian-like spatial profile. The simulations reveal that a 1064-nm 250-W truncated Bessel beam with 9 rings, mirroring the profile of an LP0,10 mode, under noncollinear phase matching conditions, converts into a 225-W beam with primarily a central spot. The central spot has 93% of its power within a 5mrad divergence angle and a 70% overlap with a perfect Gaussian beam. The simulated results reveal two attractive features – the feasibility of efficiently converting HOMs of fibers into Gaussian-like beams, and the ability to simultaneously perform frequency conversion, which may have applications in realizing high-power sources in non-traditional colors.

The split-step method was used to alternatingly apply the solutions of the linear and nonlinear parts of the differential equations describing the second-harmonic interaction with diffraction over a small propagation steps. Beam input power and phase mismatch for optimal output were chosen by performing a parameter sweep of those variables.

9728-105, Session PTue

Multi-kW coherent combining of fiber lasers seeded with pseudo random phase modulated light

Angel Flores, Air Force Research Lab . (United States); Thomas Ehrehreich, Roger H . Holten, Leidos, Inc . (United States); Iyad Dajani, Air Force Research Lab . (United States) Recently, we have shown that assuming an optimal pattern is chosen, pseudo-random bit sequence (PRBS) phase modulation provides superior SBS suppression to that provided by white noise source (WNS) for a given fiber length and signal linewidth. Furthermore, we successfully demonstrated coherent beam combining (CBC) of two 150W PRBS phase modulated fiber amplifiers. In this work, we demonstrate CBC of five path length-matched 1.2 kW fiber amplifiers with a diffractive optical element (DOE) and report a combined output power of 5 kW. Each non polarization-maintaining fiber amplifier provides approximately 1.2-1.3 kW of near diffraction-limited output power (M2~1.1). A low power sample of each laser was utilized for active polarization control. After polarization locking, polarization extinction ratios (PER) of 15-16 dB were measured for each amplifier. A low power sample of the combined beam after the DOE We analyse the linewidth characteristic of a high-power single-frequency Tm-doped fiber amplifier (TDFA) with the power up to 45 W for the first time. The Lorentzian line shape 3dB linewidth of the output signal is ~56 kHz at the maximum power. Moreover, the linewidth of the output laser with different power is measured via the delayed self-heterodyne method. The experimental results indicate that the Lorentzian line shape linewidth decreases firstly and then increases later with the increase of output signal power. It shows that the Lorentzian linewidth of high-power single-frequency TDFA increases with the increment of the amplifier gain factor. However, with the increase of pump power, the broadening of the laser linewidth is due to the increase of the in-band amplified stimulated emission, which can reduce the amplifier gain factor.

9728-107, Session PTue

2

µ

m ultrafast fiber laser modelocked by mechanically exfoliated Sb2Te3

Jan Tarka, Jakub Boguslawski, Maciej Kowalczyk, Grzegorz J . Sobon, Jaroslaw Z . Sotor, Wroclaw Univ . of Technology (Poland) We demonstrate the usage of a new saturable absorber material -antimony telluride (Sb2Te3) for efficient mode-locking of an Thulium-doped fiber laser. The Sb2Te3 layers were obtained by mechanical exfoliation and transferred onto the fiber ferrule. The all-fiber laser was capable of generating optical solitons with the full width at half maximum of 4.5 nm centered at 1945 nm, with 39.5 MHz repetition rate and more than 60 dB signal to noise ratio. The pulse energy of the generated 850 fs pulses was at the level of 30 pJ.

9728-109, Session PTue

Hollow core anti-resonant fibres with split cladding

Xiaosheng Huang, Seongwoo Yoo, Ken Tye Yong, Nanyang Technological Univ . (Singapore); Feng Luan, Shenzhen University (China); Wenliang Qi, Daryl Ho, Nanyang Technological Univ . (Singapore) An improved design for hollow-core inhibited coupling fibres (HC-ICFs) is presented. It consists of eliminating asymmetric part of the core-cladding boundary of a hollow inhibited coupling fibre and has a fibre structure with split cladding (SCF). The SCF has desirable properties such as a symmetric and discontinuous core-cladding boundary which help to reduce the structure deformation in fabrication process. In addition, the discontinuous core-cladding boundary can avoid the high loss introduced by

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9728-110, Session PTue

Conference 9728: Fiber Lasers XIII: Technology, Systems, and Applications

the touching areas between capillaries. The negative curvature also can be easily achieved in this fibre design even without the pressure control during fabrication process. Simulation results are presented to compare the confinement loss of : conventional hollow inhibited coupling fibre (HIF) and SCF. Overall the leakage losses of HIF and SCF are in the same level and both have the potential to have lower background loss than the telecom fibres. However, the structure like HIF is more likely to suffer a core shape deformation since its core-cladding boundary is asymmetric and continuous.

The relationship between confinement loss and tube pitch amplification coefficient, g, has been studied and a confinement loss lower than 0.01dB/ km can be achieved by controlling the pitch amplification coefficient. The fabricated SCF shows promising results with insignificant deformation, and the measured transmission is consistent with the simulation result.

Dependence of photodarkening under different wavelength pumping

Huizi Li, Seongwoo Yoo, Sidharthan Raghuraman, Daryl Ho, Xuan Wu, Tianye Huang, Nanyang Technological Univ . (Singapore) couplers. The novel wavelength swept laser source will offer high stable and narrow linewidth, which especially benefits for image depth and image stability for optical coherence tomography (OCT) and high sensitivity for optical sensor

9728-112, Session PTue

A passively mode locked thulium doped fiber laser using bismuth telluride deposited multimode interference

Minwan Jung, Korea Institute of Science and Technology (Korea, Republic of) and Univ of Seoul (Korea, Republic of); Junsu Lee, The Univ . of Seoul (Korea, Republic of); Wongeun Song, Advanced Photonics Research Institute (Korea, Republic of) and The Univ . of Seoul (Korea, Republic of); Ju Han Lee, The Univ . of Seoul (Korea, Republic of); Woojin Shin, Gwangju Institute of Science and Technology (Korea, Republic of) A photodarkening (PD) effect in ytterbium (Yb)-doped fiber is dependent on the number of excited ions in the upper level of Yb ion. However, the dependence of pumping wavelengths has been overlooked. In our work, we show that the PD is not only dependent of the population inversion level but also the pump wavelength. We use different wavelengths (918nm and 980nm) to core-pump a Yb:Al fiber to measure the PD. The inversion levels are kept at same values at both pumping wavelengths. The PD is assessed by temporal transmitted power decay at 620 nm as a probe beam, and transmission spectra change after the PD. The results reveal that under the same inversion level, 980 nm pumping can lead to more significant photodarkening effect than 918 nm counterpart. In addition, the 980 nm pumping wavelength induce stronger UV emission, which might suggest the UV emission is related to the creation of the PD. Our results indicates that using 918 nm pumping source would be helpful to reduce the PD.

9728-111, Session PTue

Stable and narrow-linewidth wavelength swept laser at 800nm based on acousto optic tunable filter

Gahee Han, Nam-Su Park, Chang-Hyun Park, Chang-Seok Kim, Pusan National Univ . (Korea, Republic of) We proposed a stable and narrow-linewidth wavelength swept laser source at the 800 nm region using an acousto-optic tunable filter (AOTF) and a passive fiber ring resonators (FRRs). In swept-source optical coherence tomography (SS-OCT), factors of wavelength swept laser such as linewidth and wavelength stability are dependent on the image depth and image stability. It is consists by a semiconductor optical amplifier (SOA), a fiber isolator, a fiber coupler, an AOTF and two passive FFRs. Conventional sweeping has been performed by mechanical filter such as fabry-perot tunable filter (FFP-TF) and galvanometer. However, mechanical movement complicates the device design and thus induces the limited stability and tuning speed. Recently, it has been known that AOTF is capable of yielding stable and fast performance. It offers a wide tuning range, high tuning speed and stable operation against vibration and temperature due to non mechanical. The narrow linewidth is realized by a passive fiber ring resonators (FRRs), which consists of two FRRs. Owing to the vernier effect, each lasing modes can be significantly suppressed to obtain the narrower linewidth. As a result, the single longitudinal mode can be successfully demonstrated. The FFRs, which corresponds to a longitudinal mode space of each length, are implemented by simply connecting ports of 3-dB fiber High peak power ~2 ?m pulse lasers are practically useful in numerous applications, such as eye-safe LIDAR, medicine, spectroscopy, remote sensing and mid-infrared (IR) generation. Especially, optical fiber lasers exhibit a range of advantages over their bulk optics-based counterparts in terms of beam quality, reliability, and environmental stability. The mode-locking technique is commonly used to obtain high peak power by generating short pulses in a laser cavity and the broad gain at ~2 ?m of Thulium doped fiber allows potentially ultra-short pulse operation in mode locked laser. The saturable absorbers such as semiconductor saturable absorber mirror (SESAM), polarization beam splitter (PBS), fiber loop mirror, carbon nanotube (CNT), graphene and topological insulator have been deployed passively mode-locked fiber lasers. The saturable absorber based on topological insulators has been attracting much attention due to its broad operation wavelength comparing to the other methods, and simplicity for forming the fiber based saturable absorption devices such as a sandwiching method, a D-shaped fiber, a tapered fiber. In this paper, we proposed all fiber saturable absorption devices that has spectral filtering property using a bismuth telluride and MMI interaction. By deploying the proposed MMI based saturable absorber, a wavelength fixed passively mode locked thulium doped fiber laser was demonstrated operating at a wavelength of 1958 nm. The optical bandwidth of ~4 nm is experimentally obtained at a repetition rate of 22.7MHz.

9728-113, Session PTue

Wavelength selective Tm doped all fiber laser using grating pair and null core fiber

Wongeun Song, Advanced Photonics Research Institute (Korea, Republic of) and The Univ . of Seoul (Korea, Republic of); Minwan Jung, Advanced Photonics Research Institute (Korea, Republic of) and Univ . of Seoul (Korea, Republic of); Daeyoung Kim, Advanced Photonics Research Institute (Korea, Republic of); Ju Han Lee, The Univ . of Seoul (Korea, Republic of); Bong-Ahn Yu, Woojin Shin, Advanced Photonics Research Institute (Korea, Republic of) We firstly proposed all-fiber band pass filter (BPF) operating at 2 um wavelength using concatenation of long period fiber grating (LPFG), null core silica fiber (NCSF) and long period fiber grating (LPFG). The first and the second LPFG have same resonance dip. When the light has met the first LPFG, according to phase-matching condition, the fundamental core mode couples to the cladding mode and the Rest of light except the first

56 SPIE Photonics West 2016 · www.spie.org/pw

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Conference 9728: Fiber Lasers XIII: Technology, Systems, and Applications

LPFG’s resonant wavelength continues to propagate through the core of the fiber and it experiences high transmission loss at NCSF section. Therefore, NCSF plays the role of core mode blocker in this structure. At the second grating, the cladding mode light is coupled to the core mode at the second LPFG that has same resonant wavelength of the first LPFG. After the light pass through the proposed LPEG-NCSF-LPFG structure, it shows band pass filtering characteristics at resonance wavelength of LPFG pair. The distance between the two LPFG centers including the NCSF segment was less than 70 mm and the total length of the device was less than 120 mm. The fabricated BPF has 4.2 dB insertion loss and less than 1 dB polarization dependence loss (PDL). The full width half maximum of transmission band is measured about 12.4 nm. The transmission of non-resonant signals were suppressed more than 6 dB. In order to evaluate the proposed device as a wavelength selective device, a wavelength selective Thulium doped fiber was demonstrated using the proposed device as a wavelength selector in the laser cavity. As the proposed device has a merit of all fiber device such as cost-effective, compactness and high power endurable, the proposed device could be used as an all fiber wavelength selective component in various applications at 2 um wavelength.

9728-114, Session PTue

Enhanced higher order mode delocalization through highly asymmetric rod-type VLMA fiber

Zeinab Sanjabi Eznaveh, J . E . Antonio Lopez, Gisela López Galmiche, James Anderson, Axel Schülzgen, Rodrigo Amezcua Correa, CREOL, The College of Optics and Photonics, Univ . of Central Florida (United States) We propose and experimentally investigate a novel design of a micro structured single mode (SM), very large mode area (VLMA) ytterbium doped rod type fiber amplifier featuring an enhanced higher-order mode (HOM) delocalization and efficient preferential gain in active fibers. The proposed fiber design consists of six high-index germanium-doped silica rods integrated asymmetrically in the cladding of the fiber structure in order to remove the reflection symmetry and therefore, weakening the overlap of the LP11-like modes with the fiber core. This innovative fiber design with a core diameter of 66?m enables effective SM operation close to diffraction limited beam quality M2 of 1.3 in a broad spectral range of 850-1600nm with mode field area (MFA) of 2560um2 at 1064nm. The enhanced features of this fiber implies the improved threshold like onset of modal instabilities in high power fiber amplifiers.

To prove the robust SM operation of the fiber under any launching condition, we translated the input beam along x from -30?m to +30?m attempting to excite any HOM and recorded a series of near field images at 1064nm wavelength. No HOM was observed which confirms the effective SM operation of the asymmetric rod fiber. To demonstrate highly effective HOM delocalization, the spatially and spectrally (S2) resolved mode imaging measurement was used to yield the mode images as a function of group delay. The peaks in the FT spectrum were corresponding to the cladding modes which particularly proves the scalability of the HOMs’ filtering capability of the proposed fiber design.

9728-116, Session PTue

Simple all-PM-fiber laser system seeded by an all-normal-dispersion oscillator mode locked with a nonlinear optical loop mirror

Jan Szczepanek, Tomasz M . Kardas, Univ . of Warsaw (Poland); Michal Nejbauer, Univ . of Warsaw (Poland) and Institute of Physical Chemistry (Poland); Czes?aw Radzewicz, Institute of Experimental Physics, Faculty of Physics, Univ . of Warsaw (Poland); Yuriy Stepanenko, Univ . of Warsaw (Poland) and Institute of Physical Chemistry (Poland) In this paper we report an all-PM-fiber laser amplifier system seeded by an all-normal-dispersion oscillator mode-locked with a Nonlinear Optical Loop Mirror (NOLM). The presented all-normal-dispersion cavity works in a dissipative soliton regime and delivers highly-chirped, high energy pulses above 2.5 nJ with full width at half maximum below 220 fs. After the all PM-fiber amplifying stage spliced directly to the output of the oscillator we received pulses with the energy above 42 nJ and time duration below 200 fs. The electrical field of optical pulses from the system was reconstructed using the SPIDER technique. The influence of nonlinear processes on the pulse temporal envelope was investigated.

9728-117, Session PTue

High average power harmonic mode locking of a Raman fiber laser based on nonlinear polarization evolution

Jun Liu, Chujun Zhao, Shenzhen Univ . (China); Yanxia Gao, Shenzhen University (China); Dianyuan Fan, Shenzhen Univ . (China) We experimentally demonstrate the operation of a stable harmonically mode-locked Raman fiber laser based on the nonlinear polarization rotation technique. A maximum average output power of up to 235 mW is achieved at the repetition rate of 466.2 MHz, corresponding to the 1695th order harmonic mode-locking operation. The temporal width of the mode-locked pulse train is 450 ps. The experimental results should shed some light on the design of wavelength versatile ultrashort lasers with high average output power.

9728-118, Session PTue

486nm blue laser operating at 500 kHz pulse repetition frequency

Daniel Creeden, Jon Blanchard, Herman Pretorius, Julia Limongelli, Scott D . Setzler, BAE Systems (United States)

9728-115, Session PTue

All-bismuth ultrafast fiber systems

Teppo Noronen, Oleg G . Okhotnikov, Tampere Univ . of Technology (Finland) No Abstract Available Compact, high power blue light in the 470-490nm region is difficult to generate due to the lack of laser sources which are easily convertible (through parametric processes) to those wavelengths. By using a pulsed Tm-doped fiber laser as a pump source for a 2-stage second harmonic generation (SHG) scheme, we have generated ~2W of 486.5nm light at 500kHz pulse repetition frequency (PRF). To our knowledge, this is the highest PRF and output power achieved in the blue region based on a frequency converted, monolithic fiber laser. This pump laser is a pulsed Tm-doped fiber laser/amplifier which generates 12W of 1946nm power at 500kHz PRF with diffraction-limited output from a purely single-mode fiber. The output from this laser is converted to 973nm through second harmonic generation (SHG). The 973nm is then converted to 486.5nm via another SHG stage. This architecture operates with very low peak power, which can

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Conference 9728: Fiber Lasers XIII: Technology, Systems, and Applications

be challenging from a nonlinear conversion standpoint. However, the low peak power enables the use of a single-mode monolithic fiber amplifier without undergoing nonlinear effects in the fiber. This also eliminates the need for novel fiber designs, large-mode area fiber, or free-space coupling to rod-type amplifiers, improving reliability and robustness of the laser source. Higher power and conversion efficiency are possible through the addition of Tm-doped fiber amplification stages as well as optimization of the nonlinear conversion process and nonlinear materials. In this paper, we discuss the laser layout, results, and challenges with generating blue light using a low peak power approach. the laser’s stability. In this paper, an all-fiber kW-level narrow bandwidth amplifier with concise profile is demonstrated. The seed of the amplifier is an oscillator built on a pair of narrow bandwidth fiber Bragg gratings (FBGs). The FBGs help to assure a narrow bandwidth output of the amplifier as 0.078 nm. The amplifier achieves near diffraction limited output of 823 W, which is the known highest power output for a narrow bandwidth fiber amplifier seeded by a FBG-based oscillator. And the opto-optic efficiency reaches 84.5%, which is rather high for a fiber amplifier, especially for a narrow bandwidth one. In the amplifier configuration, a newly designed cladding stripper with segment-based structure is brought in, realizing high efficiency and high power cladding light leakage. Besides, the mechanism of laser operation in a narrow bandwidth fiber amplifier is studied and simulated. Influences of seed and fiber parameters on SBS threshold in such amplifier are discussed as well, which provides guides in further power scaling of narrow bandwidth fiber lasers.

9728-119, Session PTue

Generation of broadband mid-infrared supercontinuum radiation in cascaded soft-glass fibers

Christian Kneis, Institut Franco-Allemand de Recherches de Saint-Louis (France) and Univ . Bordeaux 1 (France); Thierry Robin, Benoît Cadier, iXFiber SAS (France); Inka Manek-Hönninger, Univ . Bordeaux 1 (France); Marc Eichhorn, Christelle Kieleck, Institut Franco-Allemand de Recherches de Saint-Louis (France); Laurent Brilland, Selenoptics (France); Johann Troles, Celine Caillaud, Institut des Sciences Chimiques de Rennes, University of Rennes (France)

9728-121, Session PTue

Millijoule class, all-fibered front end nanosecond pulse, single frequency, with spatially coherent top-hat beam output used as seeder for high power laser: current status and future perspectives of industrial version

Jean-François Gleyze, Arnaud Perrin, Pierre Gouriou, Florent Scol, Commissariat à l’Énergie Atomique (France); Constance Valentin, Géraud Bouwmans, Lab . de Physique des Lasers, Atomes et Molécules (France); Emmanuel Hugonnot, Commissariat à l’Énergie Atomique (France) The generation of mid-infrared (mid-IR) radiation, ranging from 2 - 5 it can be used for, e.g. in free space optical communication systems, range finding and remote chemical sensing applications. It also plays (Tm3+)-doped double-clad fiber laser, emitting at 2 µ average output power performance. So far, 24 W of 2 µ fiber. The broadest SC spectrum from the ZBLAN fiber went up to 4.1 µ an increasing role in medicine, for instance in optical tissue ablation or wavelength range. In this research study, a ZrF4-BaF2-LaF3-AlF3-NaF m, is getting much attention in the recent years thanks to many applications optical coherence tomography, owing to the high water absorption in that (ZBLAN) fiber is pumped by a Q-switched mode-locked (QML) thulium m, to generate mid-IR supercontinuum (SC). Further spectral broadening is achieved by pumping a chalcogenide photonic crystal fiber (PCF) with the SC radiation from the ZBLAN fiber. Different ZBLAN fiber designs and chalcogenide materials are characterized and compared regarding their potential for broadband high m radiation in QML regime has been achieved with 5.1 W SC output power from the ZBLAN (GeAsSe) PCF fiber showed wavelength broadening up to 4.4 radiation from the ZBLAN fiber with a spectrum from 3.5 µ µ m to 3.9 chalcogenide fiber will be shown and further spectral broadening. µ µ m with m. First proof of principal experiments with a germanium-arsenide-selenide an output power level of 5 mW. The pump power has been 50 mW of SC m. The coupling efficiency of the ZBLAN radiation into the GeAsSe fiber has been around 30%. Power scaling of the SC generation in the ZBLAN and the

9728-120, Session PTue

High power narrow bandwidth fiber amplifier with a FBG-based seed

Jinping Hao, Hong Zhao, Dayong Zhang, Liming Zhang, Kun Zhang, North China Research Institute of Electro optics (China) Narrow bandwidth fiber laser has drawn increasing attention for its use in unique application areas such as gravitational wave detection, range finding, lidar, and coherent beam combination. However, power scaling of such laser is mainly prohibited by stimulated Brilliouin scattering (SBS) - one type of nonlinearity effects. Previous solutions to SBS limitation either call for additional control or increase the complexity of the laser, which could affect In large scale laser facility dedicated to laser-matter interaction including inertial confinement fusion, such as LMJ or NIF, high-energy main amplifier is injected by a laser source in which the beam parameters must be controlled. For many years, the CEA has developed nano-joule pulses all-fiber front end sources, based on the telecommunications fiber optics technologies. Thanks to these technologies, we have been able to precisely control temporal shaping and phase-modulated pulse. Nowadays, fiber lasers are able to deliver very high power beams and high energy pulses for industrial needs (laser marking, welding,…). This new fiber laser technology has a great potential to improve stability and versatility of high-energy system front-end.

Therefore, we have currently developed new nanosecond pulses fibered amplifiers able to increase output pulse energy up to the mJ level. These amplifiers are based on flexible fibers and not on rod type. This allows us to achieve a compact source.

To be compatible with main amplifier section injection, the Gaussian intensity profile of fibered system must be transformed into ‘top-hat’ profile. To reach this goal, we have recently developed an elegant and efficient solution based on a single-mode fiber which directly delivers a spatially coherent ‘top-hat’ beam.

In this conference, we will present last results of mJ-class top-hat all-fiber laser system, the results and the industrial prototype which can be used as a front-end of high-power lasers or as a seeder for other types of lasers. We will discuss the opportunity of such a system in LMJ architecture.

9728-122, Session PTue

2.9 GHz 780w narrow linewidth fiber laser

Hong Zhao, Nianjiang Chen, North China Research Institute of Electro-optics (China) Narrow linewidth fiber lasers are wide applied in coherent detection, gravitational wave detection, coherent beam combing and wavelength

58 SPIE Photonics West 2016 · www.spie.org/pw

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Conference 9728: Fiber Lasers XIII: Technology, Systems, and Applications

beam combining.But in HPNL fiber laser, the main limitation factor of power increasing is SBS. Through sinusoidal phase modulating single frequency laser, we can increase longitudinal mode number. And then SBS can be suppressed. So, the keystone of sinusoidal phase modulation is analyzed. The keystone of sinusoidal phase modulation is LiNbO3 crystal composing phase modulator, its refractive index changing under outward electric field influence and then input light wave modulated. The light that is inputted into phase modulator is called carrier frequency.The light spectrum broadening is the full width at half maximum (FWHM) of waveform envelope, which is composed with these sideband frequency spectrums. The frequency distribution of sinusoidal phase modulating single frequency laser is simulated. The electrical frequency is 150MHz. Phase modulating coefficient is. The laser is composed of seed source adding three stage amplifier. Wherein, single frequency laser diode is employed as the seed source with 50mW, 1064.34nm and less than 1MHz. The phase modulator is employed to broaden spectrum. Its electro optical bandwidth, half wave voltage, maximum modulating voltage peak to peak value and maximum endured optical power are 150MHz, 2.5V, 20V and 20dBm, respectively. High frequency signal generator is employed as sinusoidal electrical signals to trigger source. Its output frequency range and maximum output power are 65kHz to 8GHz and 16dBm, respectively. The radio frequency driver (RF driver) is used to amplify electrical signals. Its cut-off frequency, saturated output power and maximum gain are 20GHz, 26dBm and 30dB, respectively. In experiment, the signal generator output frequency is 150MHz. The phase modulating coefficient are , and , respectively.

In the first and second amplifier stage, Yb3+ doped double cladding fiber with core/cladding diameter of 10/130?m and length of 3m is employed as gain fiber (10/130 YDF). Its absorption coefficient is 3.9dB/m at 975nm. In the third amplifier stage, Yb3+ doped double cladding fiber with core/ cladding diameter of 25/400?m and length of 12m is employed as gain fiber (25/400 YDF). Its absorption coefficient is 1.6dB/m at 976nm.

To prevent feedback, an optical isolator (ISO) is connected between each stage. The second and third amplifier stage is connected by mode field adaptor (MFA). In order to eliminate the cladding light, the cladding light stripper is connected between the third and output end. In order to prevent the backward scattering light bashing the front stage device, a tap coupler is connected between the second and three amplifier stage. Its splitting ratio is 5/95. Its function is to monitor reverse SBS power (SBS monitor). When the reverse power growing nonlinearly, showing that SBS has reached threshold. In this moment, the amplifier stage power supply should be shut off quickly to prevent the front stage device being bashed.

can be no better than that of an individual element. This is known as side-by-side beam combining. Polarization beam combining is often used to combine two arrays of orthogonal polarization. This can increase the brightness a factor of 2 at best.

There are two approaches, wavelength beam combining (WBC) and coherent beam combining (CBC), to scaling the brightness by large amounts, in principle by as much as the number of elements. In WBC, the array elements operate at different wavelengths and a dispersive optical system is used to overlap the different wavelengths spatially. Typical dispersive optical systems use gratings, prisms, or wavelength-selective reflectors. This is equivalent to what is done in wavelength division multiplexing for optical communications. The differences here are that the goal is higher power, and, therefore, the efficiency is more important. In CBC, the beams are interferometrically combined, or phased. If the beams are phased properly, then constructive interference occurs and the power can be combined into a single beam.

This tutorial will cover the fundamentals of laser beam combining, including requirements on the array elements, basic scaling laws, and implementations. Examples from the literature will be used to show the progress being made.

9728-41, Session 9

High-power, high-brightness laser beam combining

(Invited Paper)

John D . Hybl, Darren A . Rand, MIT Lincoln Lab . (United States) There is continuing interest in increasing the power and improving the beam quality of laser sources for a variety of applications including materials processing, pumping, power transmission, and illumination. One approach is to continue to develop improved lasers with higher power and good beam quality. Another approach, particularly relevant to semiconductor and fiber lasers, is to beam combine large arrays of lasers. Beam combining has become increasing viable over the past decade as the community has developed a better understanding of the requirements imposed by beam combining, and various implementations have been successfully demonstrated in the laboratory. These implementations are beginning to see commercial application.

Key metrics for high-power arrays include the output power, the brightness, and the spectral width. To achieve high brightness, both high power and good beam quality are required. High-power diode arrays currently are composed of large numbers of emitters tiled side-by-side with the emitters being mutually incoherent with respect to each other. As the number of array elements increases, the beam quality decreases, and the brightness

9728-42, Session 9

Femtosecond fiber-CPA system employing coherent combination of a multicore fiber

Arno Klenke, Friedrich-Schiller-Univ . Jena (Germany) and Helmholtz Institute Jena (Germany); Michal Wojdyr, Michael Müller, Friedrich-Schiller-Univ . Jena (Germany); Marco Kienel, Friedrich-Schiller-Univ . Jena (Germany) and Helmholtz-Institute Jena (Germany); Jens Limpert, Andreas Tünnermann, Friedrich-Schiller-Univ . Jena (Germany) and Helmholtz-Institute Jena (Germany) and Fraunhofer-Institut für Angewandte Optik und Feinmechanik (Germany) Coherent combination of multiple amplifiers has become an established technology to overcome limitations of the average power, peak power and pulse energy of fiber-CPA systems. However, so far, these systems were built by duplicating standard single amplifier systems and adding the necessary components to realize the combination process. This limits the realistically achievable number of channels due to constraints of the footprint and component count. Therefore, the integration of multiple signal channels in a multicore fiber is of major interest.

In this contribution, we present coherent combination using an ytterbium doped multicore fiber as the main amplifier in a femtosecond fiber-CPA system. The fiber possesses four signal cores with a mode-field diameter of 36 µ m each and a shared pump cladding. The incident beam from the frontend is split into four parallel beams with a segmented-mirror splitter (SMS). This splitter consists of a high reflective mirror and a second mirror with zones of different reflectivities. The beams are coupled into the signal cores of the multicore fiber and after amplification, they are recombined into a single beam again using a second SMS. A LOCSET stabilization system is employed to optimize the path lengths for constructive interference at the output. After compression, the system can emit up to 120 the future.

µ J pulses with a duration of 250 fs and a drastically improved pulse quality compared to a single core fiber. Due to the compact setup of this amplifier, laser systems with a large number of coherently combined channels will be realizable in

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Conference 9728: Fiber Lasers XIII: Technology, Systems, and Applications

9728-43, Session 9

Self-compression to 24 MW peak power in a fused silica solid-core fiber using a high repetition rate thulium-based fiber laser system

Martin Gebhardt, Friedrich-Schiller-Univ . Jena (Germany) and Helmholtz Institute Jena (Germany); Christian Gaida, Fabian Stutzki, Friedrich-Schiller-Univ . Jena (Germany); Steffen Hädrich, Friedrich-Schiller-Univ . Jena (Germany) and Helmholtz Institute Jena (Germany); Cesar Jauregui, Friedrich-Schiller-Univ . Jena (Germany); Jens Limpert, Andreas Tünnermann, Friedrich-Schiller-Univ . Jena (Germany) and Helmholtz Institute Jena (Germany) and Fraunhofer-Institut für Angewandte Optik und Feinmechanik (Germany) In a proof-of-principle experiment of cascaded coherent pulse stacking, an amplified chirped-pulse burst mainly consisting of four equal amplitude pulses is coherently stacked into a single output pulse using two cascaded GTIs with a normalized pulse energy enhancement of 3.64 times. Input pulse burst is modulated using electro-optic modulators directly after a 122MHz mode-locked oscillator and a stretcher, and amplified into the microjoule millijoule range prior to stacking using an amplification chain with a final 55?m chirally-coupled-core (CCC) fiber amplifier.

Cascaded pulse stacking opens a path to extracting all stored >10mJ pulse energy in ultrashort pulses from a fiber CPA system with negligible nonlinear distortions.

9728-45, Session 9

Electro-optically controlled divided-pulse amplification

Michael Mueller, Marco Kienel, Michal Wojdyr, Arno Klenke, Jens Limpert, Andreas Tünnermann, Friedrich-Schiller Univ . Jena (Germany) Complementing ultrafast thulium-based fiber-laser systems with subsequent nonlinear pulse compression can enable unique laser parameters at around 2 µ m operation wavelength. Significant pulse shortening and peak power enhancement have been accomplished using a fused silica solid-core fiber. In this fiber a pulse peak power of 24 MW was achieved without catastrophic damage due to self-focusing. As compared to operation in the well-explored 1 µ m wavelength regime increasing the emission wavelength to 2 µ m is of twofold advantage for nonlinear compression in fused-silica solid-core fibers. This is because on the one hand the self-focusing limit scales quadratically with the wavelength. On the other hand the dispersion properties of fused silica allow for self-compression of ultrashort pulses beyond 1.3 generated 1 µ experiment around 2 µ increase in the critical power for self-focusing.

µ m wavelength, which leads to strong spectral broadening from very compact setups without the need of external compression. Using this technique we have J-pulses with 24 fs FWHM pulse duration (<5 optical cycles), 24 MW peak power at 24 W of average power. To the best of our knowledge, this is the highest average power obtained from any nonlinear compression m wavelength and, the first demonstration of peak powers beyond 20 MW within a fused-silica solid-core fiber. This result is of significant importance for the development of interesting driving sources for high-field physics applications. It further emphasizes that thulium-doped fiber-based chirped-pulse amplification systems may outperform their ytterbium-based counterparts in terms of peak power due to the fourfold Today, high-power femtosecond laser systems enable demanding industrial and scientific applications. The occurrence of nonlinear effects up to optically induced damage limit performance scaling, even after applying elaborate methods such as chirped-pulse amplification. Coherent beam combination and divided-pulse amplification are promising techniques to exceed the current limitations by temporally increasing the beam area and the pulse duration, respectively. Actively stabilized implementations achieved the highest combining efficiencies even for strong amplifier saturation and large nonlinear phase accumulation. However, the system complexity and the alignment sensitivity grow fast – asking for higher system integration. In this contribution, we present a spatiotemporal combining setup in a proof-of-principle experiment with an entirely fiber-coupled front-end. Unlike in previous experiments, where the temporal pulse division was achieved using free-space optical delay lines, the pulses are taken directly from the pulse train of the oscillator. Also the spatial division is totally fiber coupled. Thereby, the free-space paths and the alignment requirement are cut in half. However, the combination inevitably remains in free-space considering application in high-power lasers. For the combination of 4 pulses, a combining efficiency larger than 95% is demonstrated. The efficiency is largely independent of the combined pulse energy and the temporal pulse contrast is better than 20 dB. Potentially, this approach allows for self-optimization of the combination due to the many degrees of freedom accessible with the EOMs.

9728-44, Session 9

Cascaded coherent pulse stacking from fiber chirped-pulse amplifiers

Tong Zhou, John M . Ruppe, Cheng Zhu, John A . Nees, Univ . of Michigan (United States); Russell B . Wilcox, Lawrence Berkeley National Lab . (United States); Almantas Galvanauskas, Univ . of Michigan (United States)

9728-46, Session 10

Single frequency 1560nm Er:Yb fiber amplifier with 207W output power and 50.5% slope efficiency

Daniel Creeden, Herman Pretorius, Julia Limongelli, Scott D . Setzler, BAE Systems (United States) We are developing a new technique of cascaded coherent pulse stacking (CPS) for enhancing ultrafast pulse energies from fiber chirped-pulse amplification (CPA) systems by a factor of up to 100-1000 times. It is based on coherent temporal combining of a sequence of amplified chirped equal-amplitude input pulses into a single chirped output pulse using Gires Tournois interferometers (GTI). Cascaded CPS is accomplished with a burst of N equal-amplitude input pulses by storing all the energies of the first N-1 input pulses in the cavities through destructive interference at the partial reflector of the final GTI, followed by sequential extraction of the energy from each cavity through constructive interference of the N-th pulse with the intra-cavity pulses at the partial reflectors. The GTI roundtrip length can be folded using Herriott cells, which are essentially compactly folded optical delay lines and can greatly reduce the footprint of the GTIs.

High power fiber lasers/amplifiers in the 1550nm spectral region have not scaled as rapidly as Yb-, Tm-, or Ho-doped fibers. This is primarily due to the low gain of the erbium ion. To overcome the low pump absorption, Yb is typically added as a sensitizer. Although this helps the pump absorption, it also creates a problem with parasitic lasing of the Yb ions under strong pumping conditions, which generally limits output power. Other pump schemes have shown high efficiency through resonant pumping without the need for Yb as a sensitizer. Although this can enable higher power scaling due to a decrease in the thermal loading, resonant pumping methods require long fiber lengths due to pump bleaching, which limits the power scaling which can be achieved for single frequency output. By using an

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Conference 9728: Fiber Lasers XIII: Technology, Systems, and Applications

Er:Yb fiber and pumping in the minima of the Yb pump absorption at 940nm, we have been able to simultaneously generate high power, single frequency output at 1560nm while suppressing the 1-micron ASE and enabling higher efficiency compared to pumping at the absorption peak at 976nm. We have demonstrated single frequency amplification (540Hz linewidth) to 207W average output power with 49.3% optical efficiency (50.5% slope efficiency) in an LMA Er:Yb fiber. We believe this is the highest reported efficiency from a high power 9XXnm pumped Er:Yb-doped fiber amplifier. This is significantly more efficient that the best-reported efficiency for high power Er:Yb doped fibers, which, to-date, has been limited to ~41% slope efficiency.

9728-47, Session 10

Polarization maintaining, very-large-mode area, Er fiber amplifier for high energy pulses at 1572.3 nm

Jeffrey W . Nicholson, Anthony M . DeSantolo, Man F . Yan, Patrick W . Wisk, Brian J . Mangan, Gabe S . Puc, OFS Fitel LLC (United States); Anthony W . Yu, Mark A . Stephen, NASA Goddard Space Flight Ctr . (United States) Very-large-mode area (VLMA) Er-doped fiber amplifiers , core pumped by high-power 1480 nm, Raman fiber lasers, generate diffraction limited, high energy pulses at 1.5 micron wavelengths, and have applications in femtosecond fiber chirp-pulse amplifiers [ ] and high-energy soliton generation, for example. They have been demonstrated with core diameters greater than 50 microns and effective areas greater than 1100 square microns. In spite of the success of VLMA Er amplifiers, there have been few results on making polarization maintaining large-mode area Er-doped fibers. A 26 micron mode-field diameter (~ 530 square micron Aeff) polarization maintaining, Er-Yb doped photonic crystal fiber laser was previously demonstrated. In this work, we demonstrate for the first time, a polarization maintaining, Er-doped VLMA amplifier with greater than 1000 square micron effective area. We then use this amplifier to demonstrate high energy, one microsecond pulse amplification at 1572.3 nm. The CO2 absorption line centered at 1572.3 nm has been chosen due to confluence of several spectroscopic properties. It is relatively insensitive to temperature changes compared to other lines in the absorption band, free of absorption features from other atmospheric constituents, and has a convenient peak absorption amplitude that allows measurement of the full atmospheric column that optimizes SNR (i.e - it does not saturate, but is a large enough feature that it is easy to distinguish from background variations.). Single frequency, 1572.3 nm, 1 microsecond pulses at 7.2 kHz repetition frequency were amplified to 400 W peak power with a pulse energy of 368 microJoule. Polarization extinction ratio of the signal was better than 20 dB, and M2 = 1.1.

9728-48, Session 10

Power scaling of Er-doped LMA triple-clad fiber laser based on silicate glass

Jun Zhang, Youming Chen, Radha Pattnaik, Mark Dubinskii, U .S . Army Research Lab . (United States); Shibin Jiang, AdValue Photonics, Inc . (United States) A triple-clad silicate glass fiber with highly Er–doped core has been studied in a resonantly (in-band) pumped fiber laser configuration. Over 100 W laser output power at 1615.3 nm has been achieved with laser diode pumping at ~1530 nm and an optimized fiber length. To the best of our knowledge, it is the highest output power ever achieved from the fiber laser based on a silicate glass fiber. An efficiency of 55% (laser output versus absorbed pump power) was achieved in this first major power scaling experiment. 1. Introduction Efficient power scaling of eye-safe Yb-free Er-doped silica fiber lasers based on 4I13/2 ? 4I15/2 transitions of Er3+ ions is hindered by low Erbium concentration due to low solubility of Er and a trend of Er ions to clustering in silica glass fibers. A typical core pump absorption of Erbium doped LMA silica fiber at 1530 nm is ~ 60 dB/m which is ~20 times lower than that of Yb doped silica LMA fibers (1200 dB/m). Due to the low core absorption a much smaller clad-to-core ratio is required to retain reasonable clad absorption, which inevitably limits the total diode pump power that can be coupled in the cladding. Recent development of Er-doped silicate glass indicates that the novel silicate material is able to accept much higher Er concentration before clustering, which results in a 300-500 dB/m core pump absorption. Recently, 75% laser efficiency was demonstrated with resonant pumping from the 100 µ m clad diameter silicate glass fiber [1]. Presented here are the results of resonantly (in-band) diode-pumped fiber laser power scaling with a new LMA triple-clad silicate glass fiber. Some of the fiber parameters are presented in Table 1.

13 individual InGaAsP/InP based, volume Bragg grating (VBG)-narrowed (?? = 2 nm FWHM), 1530-nm fiber coupled laser diode modules were combined into a single fiber output using a 13x1 ITF pump combiner. The combiner output is a 250 µ m, 0.46 NA fiber matching the pump cladding of the Er-doped silicate fiber. Figure 1 indicates a simplified experimental setup. A pair of Semrock filters performs as a sharp edge dichroic wavelength separator with ~40 dB extinction ratio between the pump and laser wavelengths. A VBG or a broadband HR mirror are used to provide >99% laser feedback. VBG with the HR bandwidth of 1.5 nm at 1570 nm provides a laser wavelength selection, as needed. Broadband HR mirror provides a non-selective laser feedback. The pump end of the fiber is a straight cleave with 5.2% Fresnel reflection. The output signal is reflected by the second pump filter and analyzed. Figure 2 shows the laser performance of the fiber. A fiber lengths of 10, 7, and 3 meters were used for Figure 2 shows the laser performance of the fiber. A fiber lengths of 10, 7, and 3 meters were used for optimization of optical efficiency. Over 100 W of CW output is achieved with the fiber of 10 meter length. 55% efficiency was observed with the fiber length of ~7 meters. Laser output was characterized spectrally and spatially. We found the fiber output to be nearly single-mode. We will present the fiber laser performance with two types of mirrors, VBG and non-selective broad band mirror. We also measured a pretty high propagation loss of 0.3 dB/m, which happens to be a major contributor to optical efficiency being much lower than the quantum defect limited. Due to relatively high heat deposition level fiber laser operation required water cooling. Due to observable fiber micro defects at this early development stage fiber damage occurs at the pumped fiber tips during high power operation. We believe that much better efficiency and operation stability can be achieved with improved material quality.

In conclusion, we have demonstrated a scalability potential of a newly developed triple-clad silicate glass fiber with highly Er–doped core. With resonant diode pumping we achieved a laser output power of over 100 W at 1615.3 nm which is believed to be the highest output power achieved from fiber laser based on a silicate glass fiber.

References: 1. Qiang, ZX; Geng, JH; Luo, T; Zhang, J; Jiang, SB, “High-efficiency ytterbium-free erbium-doped all-glass double cladding silicate glass fiber for resonantly-pumped fiber lasers”, Appl. Opt. 53 (4), pp. 643-647 (2014).

9728-49, Session 11

Eye-safe ns pulses from a high-aspect ratio-core fiber amplifier

Fabio Di Teodoro, Raytheon Co . (United States); Friedrich P . Strohkendl, Raytheon Space and Airborne Systems (United States) High-peak-power, eye-safe ns-pulse fiber sources are well suited to remote sensing owing to good atmospheric transmission, spectral compatibility

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9728-50, Session 11

Conference 9728: Fiber Lasers XIII: Technology, Systems, and Applications

with mature photo-detectors, reliance on telecom component/materials, and usability in environments where glint can reach bystanders. To scale peak power with low nonlinear effects, fibers of larger cores are required, in which it is generally difficult to have at once good beam quality and support for tight coiling. To address this issue, we use the semi-guiding high-aspect-ratio-core (SHARC) fiber concept. SHARC fibers feature elongated rectangular cores. In the short dimension, the fiber core behave as those in single-mode or large-mode area fibers. Tight coiling is thus possible. In the orthogonal, elongated dimension, all modes are actually leaky, the fundamental transverse mode being favored by differential loss design. Confined rare-earth-doping further helps select the fundamental mode via the greater spatial overlap with the transverse doping profile.

We amplified 1ns, 30kHz-rep.-rate, 1560nm pulses in an Er-doped SHARC fiber having mode-field area of approximately 2000 sq. microns. The fiber (~5m long) was coiled at ~15cm diameter and in-band core-pumped by a 1470nm Raman fiber laser. In-band core-pumping provides low quantum defect heating and high pump absorption per unit length, which permit the use of short, lower-nonlinearity, fibers while keeping waste heat removal manageable. From the SHARC fiber, we obtained peak power ~ 100 kW with good spectral and spatial quality. Much higher peak power is anticipated in other SHARC fibers being developed characterized by mode areas > 20,000 sq. microns and same coiling diameters.

Temperature measurements in an ytterbium fiber amplifier up to the mode instability threshold

Franz Beier, Friedrich-Schiller-Univ . Jena (Germany); Matthias Heinzig, Till Walbaum, Nicoletta Haarlammert, Thomas Schreiber, Ramona Eberhardt, Fraunhofer-Institut für Angewandte Optik und Feinmechanik (Germany); Andreas Tünnermann, Friedrich-Schiller-Univ . Jena (Germany) and Fraunhofer-IOF (Germany)

9728-51, Session 11

Efficient ytterbium-doped phosphosilicate double-clad leakage-channel-fiber laser at 1008-1020 nm

Guancheng Gu, Clemson Univ . (United States); Zhengyong Liu, The Hong Kong Polytechnic Univ . (Hong Kong, China); Fanting Kong, Clemson Univ . (United States); Hwa-Yaw Tam, The Hong Kong Polytechnic Univ . (Hong Kong, China); Ramesh K . Shori, SPAWAR Systems Ctr . (United States); Liang Dong, Clemson Univ . (United States) Thermal management is critical for kw-level power lasers, where mode instability driven by quantum defect heating is a major challenge. Tandem pumping using 1018nm fiber lasers are used to enable both high brightness and low quantum defect. It is, however, difficult to realize efficient 1018nm YDFL. The best demonstration to date is limited by the use of both conventional aluminosilicate host and smaller core diameters. In these cases, higher inversion is required due to the aluminosilicate host and higher pump brightness is required due to the smaller core, which results in high signal brightness for the same output power. These factors lead to large pump power to exit fiber, resulting in poor efficiency. Phosphosilicate host, on the other hand, requires much lower inversions to reach the gain threshold at 1018nm. The combination of phosphosilicate host and large-core leakage channel fibers (LCF) is a perfect candidate for efficient 1018nm fiber laser. We report a highly efficient Yb-doped phosphosilicate LCF laser with a quantum defect of 4.1% using a ~50 420 µ inversions required. µ m-core diameter and ~420 µ m cladding diameter. The slope efficiency with respect to the launched pump power at 1018nm is 70%. The ASE suppression is >60dB. The large cladding of m demonstrates a combination of high efficiency, ~4% quantum defect and high-power low-brightness diode pumping. We have also studied the limits of operating ytterbium fiber lasers at shorter wavelengths and found the efficiency to fall off at shorter wavelengths due to the much higher The output power scalability of Ytterbium doped high-power fiber lasers is limited by several effects, e.g. mode instabilities and the thermal destruction of the rare earth doped fiber as well as of passive components. Since mode instabilities are a thermo-optical effect, the longitudinal thermal load and the resulting temperature distribution are of high interest to develop and substantiate the understanding of such limitations. We report on the measurement of the longitudinal temperature distribution in an amplifier fiber during high power operation. The measurement signal of an optical frequency domain reflectometer is coupled to an ytterbium doped amplifier fiber via a wave division multiplexer. The mode instability threshold of the fiber under investigation was determined to be 870?W. In a first experiment, the longitudinal temperature distribution was examined for different pump powers with a sub mm resolution. The results show even small temperature variations induced by slight changes of the environmental conditions along the fiber. With respect to the ytterbium ion interaction between 910 and 1150?nm, an OFDR laser source in the 1300?nm region was used. A clad light stripper prevents the passive optical components from the destruction by residual pump light. The longitudinal temperature distribution was determined for different pump powers up to a signal output power of 300?W. The mean temperature is increasing by an increased pump power and a maximum temperature increase of 10?K was measured. The qualitative progression of the longitudinal temperature distribution corresponds to the results of a rate equation simulation.

9728-52, Session 11

Extremely low NA Yb doped preforms (0.03) fabricated by MCVD

Vincent Petit, Richard P . Tumminelli, Coherent, Inc ., Salem (United States); John D . Minelly, Victor Khitrov, Coherent, Inc . (United States) We report the fabrication of extremely low NA preforms (<0.03), highly doped with Yb using a conventional MCVD system. Our lowest NA preform was drawn to a 50um core step-index double-clad fiber operating in a single mode regime (M2=1.04). The fiber had an MFD and an Aeff greater than 35um and 1000um2 respectively. In a fiber laser configuration, the efficiency was greater than 85% without any sign of photodarkening. To the best of our knowledge, by using our extremely low NA preforms we have demonstrated the largest MFD and Aeff to date for a single-mode step index double–clad fiber without involving any micro-structuration.

9728-53, Session 11

High energy, narrow linewidth 1572nm ErYb-fiber based MOPA for a multi aperture CO2 trace-gas laser space transmitter

Doruk Engin, Brian Mathason, Fibertek, Inc . (United States); Mark A . Stephen, Anthony W . Yu, NASA Goddard

62 SPIE Photonics West 2016 · www.spie.org/pw

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Space Flight Ctr . (United States); He Cao, Jean-Luc Fouron, Mark Storm, Fibertek, Inc . (United States) A high energy (~2.5mJ), high average power (~20W), near transform limited (<100MHz) and polarized 1572.3nm, erbium-doped fiber amplifier (EDFA) based transmitter is being developed for remote atmospheric CO2 sensing from space. Stimulated Brillouin Scattering (SBS) is a major obstacle for achieving required peak powers. A multi-aperture transmitter architecture utilizing cladding pumped erbium-yterbium (ErYb) fiber amplifiers is the most mature EDFA technology approach for achieving the high peak power requirements. Here, a cladding pumped polarization maintaining (PM) ErYb fiber based power amplifier optimized for high energy and high efficiency operation at 1572.3nm is presented. Using COTS PM ErYb fiber, the power amplifier, has been demonstrated to achieve 0.44kW peak power (440uJ pulse energy) and 3.3W average power with transform limited linewidth (<10MHz). It has been shown that the power amplifier can support a 50% increase in pulse energy when the linewidth is increased to 100MHz. Furthermore, a custom double clad ErYb LMA fiber has been designed and produced with slightly larger core and smaller clad sizes which is expected to support x2 the energies produced by the COTS LMA fibers. The fiber is fabricated with recently developed highly efficient ErYb core doping concentrations, including higher Er doping levels, which is expected to be especially favorable for efficient 1572nm amplification. Presentation will include measured performance of the novel ErYb fiber.

9728-54, Session 11

Conference 9728: Fiber Lasers XIII: Technology, Systems, and Applications 1018nm SM fiber laser with 230W CW power

Yaakov Glick, Yoav Sintov, Roey Zuitlin, Shaul Pearl, Revital Feldman, Zvi Horvitz, Noam Shafir, Soreq Nuclear Research Ctr . (Israel) We have developed a high power SM fiber laser at 1018nm, producing 230W CW, with an M2 ~ 1.1 and light to light efficiency of 75%. Interest in this wavelength has increased in the past few years due to its suitability for use as a pump fiber laser in tandem pumping configurations, mainly intended for obtaining high power fiber lasers. To the best of our knowledge this is the highest power described in the open literature from a SM fiber laser at this wavelength. One of the challenges of this type of laser is the low gain at this wavelength in comparison to that at ~1030nm, which may limit the power level achieved prior to the evolution of amplified spontaneous emission (ASE) at around 1030nm. As a result, effort must be implemented in order to inhibit lasing at this preferred wavelength range, which may, in turn, lead to damage to the laser due to the unstable nature of lasing resulting from sporadic reflections. We have employed careful simulations which take into account the various wavelength dependent parameters such as the fiber absorption, emission, saturation effects and the cavity mirror’s reflection, in addition to the fiber geometrical parameters (core/ clad diameters, length). The results of the simulations were calibrated to the experimental results obtained, and then used to design the reported laser. Parameters that were found to be vital in suppressing higher wavelength lasing were the fiber length and the extinction ratio between the reflections of the FBG reflectors at 1018nm vs. at 1030nm.

9728-55, Session 11

Monolithic high peak-power coherent Doppler lidar system

Leonid V . Kotov, Fiber Optics Research Ctr . of the Russian Academy of Sciences (Russian Federation) and Moscow Institute of Physics and Technology (Russian Federation); Albert Töws, Alfred Kurtz, Fachhochschule Köln (Germany); Konstantin K . Bobkov, Fiber Optics Research Ctr . of the Russian Academy of Sciences (Russian Federation); Svetlana S Aleshkina, Fiber Optics Research Ctr (Russian Federation); Mikhail M . Bubnov, Fiber Optics Research Ctr . of the Russian Academy of Sciences (Russian Federation); Denis S . Lipatov, Institute of Chemistry of High-Purity Substances of the Russian Academy of Sciences (Russian Federation) and N .I . Lobachevsky State Univ . of Nizhni Novgorod (Russian Federation); Alexei N . Guryanov, Institute of Chemistry of High-Purity Substances of the Russian Academy of Sciences (Russian Federation); Mikhail E . Likhachev, Fiber Optics Research Ctr . of the Russian Academy of Sciences (Russian Federation) High-peak power monolithically build large mode area (LMA) polarization maintaining (PM) Er-doped fiber amplifier for Doppler wind lidar application is demonstrated. We developed novel double-clad single-mode PM LMA Er-doped fiber as well as LMA PM passive components for light delivering and collection of backscattered Doppler signal. The active fiber core was based on P2O5-Al2O3-SiO2 glass matrix and had diameter of 36 birefringence in LMA core and keep cladding size of 130 telecom-grade amplifiers µ m. µ m. This glass matrix allows one to improve pump conversion efficiency of the fiber and keep core-to-cladding refractive index difference low enough for single-mode operation regime. Despite of large core size the fiber has relatively low bend sensitivity and was winded with 10 cm diameter without significant bending loss. Optimization of stress rods size compound and position in active and passive fibers was done to provide high enough We built a cladding-pumped at 980 nm co-propagating amplifier based on 5 m of developed active fiber and LMA passive components at the output. Polarization extinction ratio at the output of the system exceeded 18 dB. M2 of the amplifier output beam was measured to be less than 1.1. Amplifier was tested under 800 ns single-frequency seed pulses at 1556 nm and its stimulated Brillouin scattering threshold was found to be as high as 100 W. Additionally, wind speed measurements were performed using the amplifier as the last stage in lidar system. The developed lidar system provides three times increase of measurement range compare to one based on standard

9728-56, Session 12

Energetic tunable ultrafast sources using soliton shifting in HOMs

Lars Rishoj, Gautam Prabhakar, Siddharth Ramachandran, The Boston Univ . Photonics Ctr . (United States) The effect of soliton self-frequency shift (SSFS) in optical fibers, i.e. intra pulse energy transfer to longer wavelengths via Raman scattering, has often been exploited for realizing wavelength tunable femtosecond fiber sources. One potentially attractive application of such tunable lasers is multiphoton microscopy, which requires energetic ultrafast pulses around 1300 nm. However, as solitons require anomalous dispersion (D > 0) it is only possible in conventional single mode fibers at wavelengths longer than 1300 nm, due to the material dispersion of silica. This constraint is lifted by highly confining the fundamental mode, as is commonly realized with photonic crystal fibers (PCFs). However, the small effective areas, typically 2-5 ?m2, limit obtainable pulse energies to sub-nJ levels. By utilizing stable higher order LP0,m modes, it is possible to lift this constraint as anomalous dispersion and large area are simultaneously achievable. In this work, we demonstrate continuous soliton shifting from a commercial 1030-nm fs laser up to 1314 nm, with output pulse energies of 12.5 nJ and widths of 75 fs using a 610 ?m2 mode-area LP0,9 mode in a multimode fiber. Significantly, this demonstration, of a fiber based tunable laser solution, is comparable to or exceeds energies obtainable from high-repetition-rate free-space commercial systems that provide sub-10-nJ energies at 1300 nm for multiphoton imaging applications.

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Conference 9728: Fiber Lasers XIII: Technology, Systems, and Applications

9728-57, Session 12

Investigation of a 10 MHz, non-steady state cavity for pulse energy enhancement of ultrafast fiber lasers

Sven Breitkopf, Stefano Wunderlich, Friedrich-Schiller Univ . Jena (Germany) and Abbe School of Photonics (Germany); Tino Eidam, Active Fiber Systems GmbH (Germany); Evgeny Shestaev, Thomas Gottschall, Friedrich-Schiller-Univ . Jena (Germany) and Abbe School of Photonics (Germany); Henning Carstens, Max Planck-Institut für Quantenoptik (Germany) and Ludwig Maximilians-Univ . München (Germany); Simon Holzberger, Ludwig-Maximilians-Univ . München (Germany); Ioachim Pupeza, Max-Planck-Institut für Quantenoptik (Germany); Jens Limpert, Friedrich-Schiller-Univ . Jena (Germany) and Active Fiber Systems GmbH (Germany) and Helmholtz Institute Jena (Germany); Andreas Tünnermann, Friedrich Schiller-Univ . Jena (Germany) and Helmholtz Institute Jena (Germany) and Fraunhofer-Institut für Angewandte Optik und Feinmechanik (Germany) Passive steady-state enhancement cavities have been a subject of studies for several decades. Even though this technique is well known and the improvement of the pulse energy of high-repetition-rate lasers is something many applications ask for, the benefit of extracting the enhanced fs-pulses from such a resonator for applications outside of the cavity has not yet been fully exploited. We present a passive 30-m long enhancement cavity that supports a steady-state enhancement of 198, which is the highest enhancement that has ever been reached in such a long cavity. Furthermore, we demonstrate the extraction of a short burst with a total energy of 53.6 µ J employing an acousto-optic modulator (AOM) as a switching device. The cavity was seeded with pulses of 1.49 has never been shown before.

µ J energy at 10 MHz repetition rate. The individual output coupled pulses showed an energy enhancement of up to 8.5 while the whole burst contained the entire energy of 36 input pulses. The extraction of stretched fs pulses from such a long enhancement cavity the pulse to be amplified. After amplification in independent fiber amplifiers, the spectral parts are coherently combined to reconstruct an amplified spectrum that can be larger than in standard linear F-CPA configurations. The experimental set-up starts with a mode-locked oscillator which is spectrally broadened to tens of nanometers and spectrally separated, through a dichroïc mirror, in two parts which are amplified in two different Yb-doped fiber photonic-crystal fiber. The outputs of these amplifiers are then spectrally recombined and sent in a compressor. Coherent combining is ensured through active stabilization of the relative optical phase based on frequency tagging (LOCSET technique). In this configuration we are able to generate 10 µ J, 97 fs pulses at 1 MHz corresponding to an average power of 10 W.

9728-59, Session 12

Multi-mJ bursts of green light obtained by frequency doubling the output of a fiber based MOPA

Eitan E . Rowen, Nir Shalev, Eran Tal, Jacob Lasri, Eran Inbar, Spectra-Physics Tel-Aviv (Israel) Recently, fiber lasers have been incorporated in many industrial applications, due to their advantages in beam quality, wall-plug efficiency and low maintenance. Moreover, green and UV fiber-based lasers have been introduced, and have displayed the same advantages. One major drawback, compared to solid-state lasers, is the relatively low pulse energy that is extractable from a fiber amplifier. In this report, we demonstrate a fiber based laser that generates bursts of >4mJ at a wavelength of 532nm in a pulse train of 70-300 pulses. The output of an Yb-doped fiber amplifier chain is doubled in a single pass through an LBO crystal with efficiency of above 65%. This high energy is obtained by temporal control of both the seed diode, and pump diodes. The seed-diode generates a burst of pulses with a pulse width of 5-15ns at a frequency of 2-8MHz. Each burst is amplified to a peak power that allows efficient SHG in a chain of amplifiers, with pump diodes that are driven in pulses synchronized to the burst.

Such a solution may have many industrial and other applications, where fiber-based solutions have many advantages, but suffer a disadvantage of relatively low pulse energy.

9728-58, Session 12

10

µ

J, ultrashort sub-100 fs FCPA synthesizer

Florent Guichard, Marc Hanna, Lab . Charles Fabry (France); Ronic Chiche, Lab . de l’Accélérateur Linéaire (France); Yoann Zaouter, Amplitude Systèmes (France); Fabian Zomer, Lab . de l’Accélérateur Linéaire (France); Franck Morin, Clemens Hönninger, Eric P . Mottay, Amplitude Systèmes (France); Patrick Georges, Lab . Charles Fabry (France) Fiber chirped-pulse amplifiers (F-CPA) are now widely recognized as a powerful tool for producing high average power and energetic pulses. However the linear character of chirped-pulse amplification implies that gain narrowing limits the available pulse duration at the output of such systems to several hundreds of femtoseconds, typically around 200-300 fs. In the past few years, several schemes based on spectral coherent combining scheme have shown a strong potential to overcome such limitation. In this contribution we demonstrate the generation of ultrashort sub-100 fs pulse together with an output and compressed energy of 10 µ J. High output energy is achieved by incorporating a spectral coherent combining scheme into a large stretching ratio (1 ns) architecture, thus avoiding detrimental nonlinear effects. Femtosecond pulses emitted by an ultrashort oscillator are separated into several beams, each carrying a different spectral slice of

9728-60, Session 12

High-power picosecond pulse delivery through hollow core photonic band gap fibers

Mattia Michieletto, NKT Photonics A/S (Denmark) and DTU Fotonik (Denmark); Mette M . Johansen, DTU Fotonik (Denmark); Jens K . Lyngsø, NKT Photonics A/S (Denmark); Jesper Lægsgaard, Ole Bang, DTU Fotonik (Denmark); Thomas T . Alkeskjold, NKT Photonics A/S (Denmark) The increasing demand of fibers able to delivery higher and higher average and peak power pulsed laser light, delivered undistorted and through several meters is pushing towards the development and exploitation of hollow core fibers. Hollow core fibers represent indeed the ideal candidate for beam delivery due to the minimal overlap between the fundamental mode and the silica, and therefore extremely low nonlinearities and high damage thresholds. In this work we demonstrated robust and bend insensitive fiber delivery of high power pulsed laser with diffraction limited beam quality for two different kind of hollow core photonic band gap fibers.. The light source for this experiment consists of ytterbium-doped double clad fiber aeroGAIN-ROD-PM85 in a high power amplifier setup. It provided 22ps pulses with a maximum average power of 95W, 40MHz repetition rate

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at 1032nm (~2.4?J pulse energy), with M2 <1.3. We determined the facet damage threshold for a 7-cells hollow core photonic bandgap fiber and showed up to 59W average power output for a 5 meters fiber. The damage threshold for a 19-cell hollow core photonic bandgap fiber exceeded the maximum power provided by the light source and up to 76W average output power was demonstrated for a 1m fiber. In both cases no special attention was needed to mitigate bend sensitivity. The fibers were coiled on 8 centimeters radius spools and even lower bending radii were present. In addition stimulated rotational Raman scattering arising from nitrogen molecules was measured through a 42m long 19 cell hollow core fiber.

9728-61, Session 12

Conference 9728: Fiber Lasers XIII: Technology, Systems, and Applications Self-tuning fiber lasers

Steven Brunton, J . Nathan Kutz, Univ . of Washington (United States); Xing Fu, Nokia (United States) Advanced methods in data science are driving the characterization and control of nonlinear dynamical systems in optics. In this work, we investigate the use of machine learning, sparsity methods and adaptive control to develop a self-tuning fiber laser, which automatically learns and adapts to maintain high-energy ultrashort pulses. We investigate the mode-locked laser that uses nonlinear polarization rotation (NPR) to achieve saturable absorption using wave plates and a polarizer. The optical components may be mechanically actuated with servomotors to tune the system and avoid the multi-pulsing instability. In particular, a two-stage procedure is introduced consisting of a machine learning algorithm to recognize different dynamical regimes, followed by an adaptive control algorithm to reject disturbances and track optimal solutions despite stochastically varying system parameters. The machine learning algorithm, called sparse representation for classification, comes from machine vision and is typically used for image recognition. The adaptive control algorithm is extremum seeking control, which has been applied to a wide range of systems in engineering; extremum-seeking is beneficial because of rigorous stability guarantees and ease of implementation. Together, these algorithms result in continuous high-performance operation, despite significant disturbances to the system, including large stochastic fluctuations of the birefringence.

9728-62, Session 13

Rod-type photonic crystal fibers: Only the performance counts

(Invited Paper)

Jens Limpert, Friedrich-Schiller-Univ . Jena (Germany) Rod-type photonic crystal fibers currently hold all the performance records of pulsed fiber laser systems with diffraction-limited beam quality. Furthermore, these fibers constitute the ideal building blocks for the next generation high performance ultrashort fiber laser systems based on parallelization. In this talk, it will be discussed how giving away 10 years ago one of the most popular characteristics of an optical fiber, namely its flexible nature, has led to an enormous performance evolution of pulsed, and in particular femtosecond, fiber lasers. Additionally, new rod-type fiber designs and the operation in alternative wavelength regions will be presented.

9728-64, Session 13

The rod type fiber adventure from lab to market

(Invited Paper)

François Salin, MoriaLase / EOLITE Systems (France) Since their invention in 2003 rod type fibers have found their way to applications in the industrial market. Their ability to provide high power short pulses with good beam quality makes them a strong competitor to classical bulk Nd:vanadate systems. I will review some of the historical success and failures of this technology with applications from Extreme UV lithography to solar cells.

9728-65, Session 14

2

µ

m all-fiber dissipative soliton master oscillator power amplifier

Dmitry Gaponov, NOVAE (France); Laure Lavoute, NOVAE (France); Sébastien Février, XLIM Institut de Recherche (France); Ammar A . Hideur, CORIA (France); Nicolas Ducros, NOVAE (France) During the last decade, a growing interest to high power ultrafast sources at 2 micron has emerged. This relatively new wavelength falls into the absorption band of some important organic materials and attracts a particular attention for material processing applications. However a number of physical and technological issues limit the development of powerful thulium-based ultrafast lasers. Nevertheless, a record level of 200 MW peak power was recently demonstrated. However, state-of-the-art systems are still at the stage of laboratory development and could hardly be integrated for industrial use. In this work, we present our significant progress towards compact ultrafast high-power all-fibered lasers emitting in the 2 stage. µ m wavelength range. An all fiber integrated oscillator based on the dissipative soliton concept delivers highly energetic and positively chirped pulses at a high repetition rate. Pulses from this oscillator are then directly amplified in a classical master oscillator power amplifier scheme without the need for any pre-amplifier The seed oscillator emits 48 ps pulses at 1940 nm with 50 mW of average power at 10 MHz repetition rate. Direct amplification provides 2 W of average power and 120 kW of peak power after compression. By increasing further the threshold for optical nonlinearities in the amplifier by means of a fiber stretcher, the laser system delivers 4.3 W before compression and 220 kW of peak power for 900 fs recompressed pulses. To the best of our knowledge, this is the first demonstration of direct amplification of dissipative solitons in the 2 micron wavelength range.

9728-66, Session 14

High power single-frequency erbium ytterbium co-doped all-fiber laser in MOPA

Xiaolei Bai, Wei Shi, Quan Sheng, Tianjin Univ . (China)

9728-63, Session 13

Industrializing the world’s most sophisticated active optical fibers

(Invited Paper)

Thomas Tanggaard Alkeskjold, NKT Photonics A/S (Denmark); Jes Broeng, DTU Fotonik (Denmark) No Abstract Available High power single-frequency lasers in 1.5-?m eye-safe region with narrow spectrum linewidth are of great importance in applications of high resolution spectroscopy and coherent Doppler LIDAR. MOPA systems based on erbium: ytterbium (Er: Yb) co-doped fiber (EYDF) could achieve higher output power, narrower linewidth and better stability compared with other coherent sources for this region such as optical parametric oscillators and Raman laser and therefore have being the most promising method. In 2004, C. Alegria et al. demonstrated a single-frequency fiber MOPA with 83 W output power at 1552nm with linewidth of 13 kHz. Then in 2005, Y. Jeong et al. achieved a output power of 151 W. However, the MOPAs are still needed to be all-fiberized and the linewidth evolution during the amplification

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Conference 9728: Fiber Lasers XIII: Technology, Systems, and Applications

has never been investigated. In this work, we demonstrate an all-fiber single-frequency EYDF MOPA system operating at 1550 nm with 56.4 W of maximum output power and a slope efficiency of 37.0%. The linewidth measurement based on delayed self-heterodyne interferometric technique shows the linewidth increases continuously with the pump power and a linewidth of 4.2 kHz is observed with the maximum output power of 56.4 W, which is the narrowest linewidth with over 50 W output power in this wavelength region ever reported.

9728-68, Session 14

High power, picosecond green laser based on a frequency-doubled, all-fiber, narrow bandwidth, linearly polarized, Yb-doped fiber laser

Wenyan Tian, Yelena Isyanova, Robert A . Stegeman, Ye Huang, Q-Peak, Inc . (United States); Logan R . Chieffo, Peter F . Moulton, Q-Peak Inc . (United States)

9728-67, Session 14

Mode-locked fiber laser with cascaded generation of coherent Raman dissipative solitons

Denis S . Kharenko, Institute of Automation and Electrometry (Russian Federation) and Novosibirsk State Univ . (Russian Federation); Anastasia E . Bednyakova, Novosibirsk State Univ . (Russian Federation) and Institute of Computational Technologies (Russian Federation); Evgenii V . Podivilov, Institute of Automation and Electrometry (Russian Federation) and Novosibirsk State Univ . (Russian Federation); Mikhail P . Fedoruk, Novosibirsk State Univ . (Russian Federation) and Institute of Computational Technologies (Russian Federation); Alexander A . Apolonskiy, Ludwig-Maximilians-Univ . München (Germany) and Institute of Automation and Electrometry (Russian Federation); Sergey A . Babin, Institute of Automation and Electrometry (Russian Federation) and Novosibirsk State Univ . (Russian Federation) High power picosecond green lasers are very attractive for high precision material processing and micromachining applications. Their unique features enable precision cutting and patterning of sensitive materials used in many advanced devices without the requirement for a post-process cleaning. Although nanosecond green lasers based on intra-cavity frequency-doubled Q-switched Nd-doped lasers have been one of the most popular green lasers and widely used in industrial laser-machining, they are not suitable for high precision material processing and micromachining due to relatively long pulse width and multi-mode beam quality. We report the development of an all-fiber, near 70-KW peak power, 16-ps pulse-width, narrow bandwidth, linearly polarized, 1064-nm fiber laser suitable for high power picosecond green laser generation. Our 1064-nm fiber laser delivers average power up to 110 W in a narrow bandwidth with a minimal nonlinear distortion at a repetition of 100-MHz. We will report the development of high power picosecond green lasers at 532 nm based on single-pass frequency-doubling of our all-fiber based 1064-nm fiber laser in the nonlinear material LBO. Using a 15-mm long LBO crystal, we have generated 30 W of picosecond green laser average power with a conversion efficiency of 41% from a 73-W, 16-ps 100-MHz fiber laser. To our best knowledge, this is the highest average-power picosecond green laser generated from all-fiber, narrow bandwidth, linearly polarized, picosecond Yb-doped fiber laser Generation of highly-chirped dissipative solitons is a powerful technique to obtain high-energy femtosecond pulses in mode-locked lasers based on fiber or other solid-state active media. Energy scaling of the most powerful Yb-doped fiber laser by means of cavity lengthening using PM fibers is known to be limited by the stimulated Raman scattering (SRS), converting the excess pulse energy to the red-shifted Stokes wave (Raman pulse). Re-injection of the Raman pulse into the laser cavity with proper timing can form so-called Raman dissipative soliton (RDS), as we have shown recently. Here we report on the first experimental demonstration of the cascaded generation of the higher-order RDS in similar scheme providing synchronous pumping of consecutive Stokes orders in the common cavity of Yb fiber laser by means of several intra-cavity feedback loops optimized for each order. The generated pulses (conventional dissipative soliton at 1020 nm, 1st and 2nd order RDSs at 1065 nm at 1115 nm, correspondingly) are shown to be linearly-chirped and compressible to 200-300 fs. Moreover, they appear to be mutually coherent that has been confirmed by efficient coherent combining exhibiting <40 fs interference fringes within the combined pulse envelope. The noise level grows with increasing order, but it can be reduced by a stronger spectral filtering, as shown by numerical simulation. This approach opens the door towards cascaded generation of coherent dissipative solitons in a broad spectral range (so-called dissipative soliton comb) that can improve areas such as frequency comb generation, pulse synthesis, biomedical imaging and also emerge new applications.

9728-69, Session 14

Advances in CO2 laser fabrication for high power fibre laser devices

Keiron Boyd, Simon M . Rees, Defence Science and Technology Group (Australia); Nikita Simakov, Defence Science and Technology Group (Australia) and Univ . of Southampton (United Kingdom); Jae M . O . Daniel, Defence Science and Technology Group (Australia); Robert Swain, Eric W . Mies, Sub-Micron Engineering (United States); Alexander V . Hemming, Defence Science and Technology Group (Australia); W . Andrew Clarkson, Univ . of Southampton (United Kingdom); John Haub, Defence Science and Technology Group (Australia) CO2 laser processing facilitates contamination free, rapid, precise and reproducible fabrication of devices for high power fibre laser applications. We present advances in CO2 laser fabrication techniques for cleaving of complex optical fibre designs and fabrication of cladding light strippers. Our end-face processing techniques utilize a 9.6 ?m CO2 laser to produce flat, smooth and symmetric fibre end-face profiles with no rounding or melting at the edges of the fibre, achieving fibre cleave angles of less than 0.06° and with a reproducibility better than 0.03° - this is an order of magnitude smaller than previously demonstrated CO2 processing approaches. To the best of our knowledge this is also the first demonstration of a CO2 process that has generated a fibre end-face topography substantially smaller than a typical mechanical cleave. We also investigate the challenges associated with controlled CO2 laser ablation of fibres with multiple layers of doped and un-doped glass as well as non-circular designs.

The efficient removal of excess cladding light at high optical power without degradation of core mode propagation or localized heating is of

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9728-70, Session 15

Conference 9728: Fiber Lasers XIII: Technology, Systems, and Applications

significant interest in the production and operation of compact, robust, high power fibre lasers and amplifiers. We present and characterize a CO2 laser technique for the fabrication of compact all glass cladding strippers with losses of up to 20dB/cm, tensile strength above the force required to mechanically cleave the fibre and core propagation losses of less than 0.016 dB/cm.

New generation of high average power industry grade ultrafast ytterbium fiber lasers

Alexander Yusim, Igor Samartsev, Oleg Shkurikhin, IPG Photonics Corp . (United States); Daniil V . Myasnikov, IRE Polus Co . (Russian Federation); Andrey Bordenyuk, Nikolai Platonov, Vijay Kancharla, Valentin P . Gapontsev, IPG Photonics Corp . (United States)

9728-72, Session 15

Novel all-polarization-maintaining femtosecond Yb-fiber laser for in-vivo nonlinear microscopy in zebrafish larvae

Aart Verhoef, Medizinische Univ . Wien (Austria) and Technische Universtität Wien (Austria); Lingxiao Zhu, Univ . Wien (Austria) and Technische Univ . Wien (Austria); Marco Andreana, Medizinische Univ . Wien (Austria); Martin Distel, St . Anna Kinderkrebsforschung e .V . (Austria); Stine Møller Israelsen, Karsten Rottwitt, Technical Univ . of Denmark (Denmark); Wolfgang Kautek, Univ . Wien (Austria); Andrius Baltuska, Technische Univ . Wien (Austria); Alma del Carmen Fernandez Gonzalez, Medizinische Univ . Wien (Austria) and Technische Univ . Wien (Austria); Wolfgang Drexler, Angelika Unterhuber, Medizinische Univ . Wien (Austria) Ultrafast fiber lasers are widely used for medical and industrial applications due to their high efficiency, compact size, good beam quality, and excellent reliability. Pulse energy up to 10 microjoules are required for medical application such as cataract surgery and corrective eye surgery. Less than 100 microjoules is required for micromachining applications. Sufficiently short pulses less than 20 ps are used to minimize the heat affected zone and kerf through a process triggered by multiphoton absorption. Cutting speed is a critical parameter for industrial applications such as glass and sapphire cutting. High average power improves the speed of cutting these materials. Increasing the pulse repetition while keeping tight control of the pulse energy achieves the necessary cut quality at high speed. Fiber laser configuration is highly scalable with regards to average output power and is a good fit for high speed glass and sapphire cutting. We report on industrial grade picosecond and femtosecond pulse Yb fiber lasers with >100 microjoule pulse energy and 100s of Watts of average power for improved laser machining speed of sapphire and glass. The highly efficient laser with >25% wall plug efficiency in a compact 3U rackmountable configuration has >2m fiber delivery cable. Customer reconfigurable features such as controllable repetition rate, fine pulse duration control, burst mode operation and adjustable pulse energy permit the customer to tailor the laser to their application.

We developed a semiconductor saturable absorber mirror modelocked all PM femtosecond fiber laser where for the first time a PM higher-order-mode fiber was used for intracavity dispersion compensation. The ring cavity of the oscillator has a repetition rate of 12 MHz, the total intracavity cavity dispersion is close to zero. The oscillator delivers 0.45 nJ pulses that can be externally recompressed down to 97 fs, which to the best of our knowledge are the shortest pulses generated from an all-PM Yb-fiber oscillator. The observed 1 Hz linewidth, absence of any sidebands, and 80 dB signal/ background ratio of the output pulse train measured with an RF spectrum analyzer with 1 Hz resolution bandwidth illustrate the excellent stability of the oscillator. To allow for stronger signals and faster data collection for nonlinear microscopy, we amplify the pulses in a PM single-mode Yb-fiber amplifier to ~20 nJ after stretching them with 100 m of PM980. Using a pair of grisms we recompress the pulses to ~150 fs duration.

With this source we have performed in-vivo nonlinear microscopy in zebrafish larvae, where we have simultaneously measured backscattered second harmonic and back-propagating fluorescence from two-photon excited red-fluorescent protein. To avoid optical damage to the larvae, the incident energy was limited to 5 nJ. Strong second harmonic signal can be observed from the collagen fibers in the tail finn, as well as from the muscles in a zebrafish tail. Fluorescence from macrofages labeled with mCherry, that can be used to identify cancer, is also observed.

9728-71, Session 15

Mechanical reliability of double clad fibers in typical deployment conditions

Michael Walornyj, Jaroslaw Abramczyk, Kanishka Tankala, Nils Jacobson, Nufern (United States)

9728-73, Session 15

Efficient pump combiner’s for fiber lasers and amplifiers

Lalitkumar Bansal, Andrea Rosales-Garcia, OFS Fitel LLC (United States); V . R . Supradeepa, Indian Institute of Science (India); Thierry F . Taunay, Clifford Headley III, OFS Fitel LLC (United States) Large mode area (LMA) double clad fibers (DCF) are widely used in high power laser applications. Optical reliability of DCFs has been studied and a predictive model for determining the fiber lifetime has been reported. While tensile strengths and n-values have been reported for DC fibers, mechanical reliability of fibers used in high power laser applications has yet to be studied. In these applications, large diameter LMA fibers are often tightly coiled to suppress higher order modes to achieve SM operation as well as to minimize form factor. The mechanical stresses resulting from bending large diameter fibers can significantly exceed those seen in the telecom industry. The mechanical reliability of telecom fibers has been thoroughly studied and models for predicting fiber lifetime for typical stresses experienced in deployment conditions are available. An analysis of the factors affecting the lifetime of DCF fibers in typical deployment conditions using such models has not been conducted. This paper, for the first time, will provide a comprehensive analysis all the factors impacting lifetime of LMA fibers. Furthermore, it highlights the choices manufacturers and users of DCF can make to enhance the longevity of the fibers. Nufern’s LMA-YDF-20/400 used in kW class fiber lasers is used as a case study to illustrate the parameters that impact mechanical service lifetime of the fiber. The power threshold for nonlinear effects in fiber lasers and amplifiers is inversely proportional to the fiber length. The required gain fiber length is in turn proportional to the cladding area of the gain fiber. In order to minimize nonlinearities, the smallest possible cladding diameter is desirable. On the other hand the amount of pump power that can be coupled into the gain fiber is proportional to its cladding area, and a large cladding is preferable. In an all fiber source configuration, pump light can be injected into the gain fiber by using a tapered fiber bundle [1]. This consists of multiple pump fibers that are closely packed together, tapered down and spliced to an output fiber. The output fiber is typically the same diameter as the gain fiber. As the bundle is tapered, the NA of the light that can propagate through it increases with the taper ratio. For low loss, the brightness condition, shown in Eq. 1 should be satisfied.

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Conference 9728: Fiber Lasers XIII: Technology, Systems, and Applications

D_o^2 ?NA?_o^2 ≥ ?nD?_in^2 ?NA?_in^2 (1) Where Do/Din is the diameter of the output/input fiber, NAo/NAin is the numerical aperture of the output/input fiber, and n is the number of fibers in the bundle. The pump combiner design thus depends on the diameter and the NA of the diode pigtail fiber from a given diode manufacturer. Combiners based on a single bundle with up to 19 pump arms are typically available with pump transmission of 95%. When a larger number of pump ports is required, a so-called tree architecture shown in Fig. 1 offers a superior approach. It consists of a number of pump combiners which are spliced to the pump arms of a 2nd stage pump-signal combiner. This architecture has the additional advantage of simplifying the design for PM performance.

A typical, industry-standard diode [2][3] uses 105/125 ?m (core/cladding) diameter pigtail fiber with light coupled into 0.15 NA. Based on Eq. (1) and using such diodes, typical commercially available 7x1 pump combiners designed to couple into 220/240 ?m diameter output fiber with 0.22 NA achieve pump transmission efficiency of about 90%. To improve overall combiner performance, we have demonstrated a 1st stage pump combiner in which 7 diodes are combined into a 200/220 um 0.22 NA output fiber, with an average transmission of 99%, as seen in Fig. 2a. This improvement in pump combiner performance leads to a significantly improved high power tree pump combiner system. Figure 2b shows the transmission through the tree architecture using four such 7x1 multimode combiners. We have tested the optimized combiner system up to 1.3kW with 95% pump transmission efficiency and a thermal slope efficiency of 0.02 C/W, when heat sunk to a cooling plate at 25?C, the hottest spot was on the splice point which is 10 mm away from the fiber coating which is most prone to thermal damage. The (6+1)x1 PM single mode combiner has a 15/330 ?m output fiber with a signal loss of 0.2dB and a PER of 20dB. The overall pump transmission through the tree architecture is increase by 6% compared to trees based on commercially available combiners, offering a significant improvement in both efficiency and management of waste heat. The key characteristic of the combiner system is summarized in Table 1. An additional advantage of the improved pump combiner is that the output pigtail fiber now has 330 ?m cladding diameter as opposed to 400um, offering a significant improvement in pump brightness. This will translate into an approximately 50% increase in nonlinear thresholds for gain fibers with the same fiber core properties, due to the reduction in fiber length. With this device improved system efficiency, fewer diodes are needed, offering cost savings, and there is less burden on thermal management. In summary, we demonstrate a (42+1)x1 PM cascaded combiner system, with a 330um output cladding diameter, that has a 6% improvement in pump transmission compared to a standard 400 ?m combiner system. In addition, with this smaller cladding, and a correspondingly shorter fiber length the threshold for nonlinearities will increase. The combiner system has a high pump efficiency of 95%, driven by high efficiency 99% pump combiners. The pump and signal combiner has passed a high power handling test of 1.3 kW with thermal slope of 0.02 C/W, limited by available diode power. beam profile with a specific angular divergence.

Here, a novel specialty fiber, made to satisfy the standard beam delivery (BD) cable requirements and designed to tailor mode up-conversion in a low maintenance and cost-effective single fiber device is reported.

Using numerical simulations based on spatial mode overlap calculations between the SM launch and the BD fiber, the design is optimized in order to achieve the required beam profile and beam divergence. According to the modeling predictions, a specialty fiber with 100 um core and 360 um cladding was fabricated and characterized with less than 2 dB/km attenuation in the 1 um wavelength range. Flat-top beam profile with a 3.8 BPP was demonstrated out of this fiber when coupling light from a SM 20 um core, 0.06 NA and 400 um cladding fiber laser.

Numerical and experimental results will be detailed for 50, 100 and 200 um core diameter fibers delivering flat-top beam profiles with BPP values around 2, 4 and 8 mm x mrad respectively. Moreover, results of high power testing of the specialty fiber beam delivery cable at 2 kW laser power will be presented.

9728-75, Session 15

Compact frequency-quadrupled pulsed 1030nm fiber laser

Chris McIntosh, Lew Goldberg, Brian J . Cole, Alan D . Hays, U .S . Army RDECOM CERDEC NVESD (United States) A compact 1030nm fiber laser for ultraviolet generation at 257.5nm is presented. The laser employs a 35cm long polarization-maintaining 20um core (0.07 NA), 130um clad (0.45 NA) ytterbium fiber. In order to ensure lasing in the fundamental mode, the fiber was coiled into a 30mm diameter. One end of the fiber was flat-cleaved to serve as the output coupler, while the other end of the fiber was angle-cleaved. The light from a 975nm grating stabilized pump (0.1 NA, 105um clad diameter) was injected at a 15° launch angle using a 1:1 relay lens into the flat-cleave end of the fiber. A reflective long-pass filter was placed at the other fiber end to recycle the pump and a Cr:YAG saturable absorber (19% unsaturated transmission) was placed at the waist of a second 1:1 relay lens for passive Q-switching. A volume Bragg grating (VBG) was employed as the mirror. The fundamental 1030nm output was 2.0W average (in a 125uJ pulse train) for 10W launched pump power. A 15mm length of lithium triborate (LBO) was chosen for second harmonic generation and a 7mm length of beta-barium borate (BBO) was used for the fourth harmonic generation. The second harmonic generation (SHG) conversion efficiency was 38% (770mW) and the fourth harmonic generation (FHG) efficiency was 26% (200mW). The laser’s spectral linewidth was <0.1nm FWHM at 10W pump power, which should avoid broadening of Raman excitations when used for explosives spectroscopy.

9728-74, Session 15

Next generation of specialty beam delivery fibers delivering flat-top beams with controlled BPP for high-power CW and pulsed laser systems

Clémence Jollivet, Kevin F . Farley, Michael Conroy, Jaroslaw Abramczyk, Nufern (United States); Steffen Belke, Frank Becker, ROFIN-SINAR Laser GmbH (Germany); Kanishka Tankala, Nufern (United States) Most kW power level fiber laser systems operate in single-mode (SM) regime, preferred for its stable output performances with diffraction limited beam profile. However, several applications, including laser-assisted material processing, require kW-level beams with uniform intensity distribution such as flat-top profiles and controlled beam divergence. Therefore, there is a need for all-fiber devices able to up-convert the input SM into a uniform

68 SPIE Photonics West 2016 · www.spie.org/pw

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Conference 9729: High Energy/Average Power Lasers and Intense Beam Applications IX

Monday - Tuesday 15–16 February 2016 Part of Proceedings of SPIE Vol . 9729 High Energy/Average Power Lasers and Intense Beam Applications VIII 9729-1, Session 1

Recent advances in flowing diode pumped alkali lasers (FDPAL)

Greg A . Pitz, Air Force Research Lab . (United States) Scaling of Diode Pumped Alkali Lasers (DPALs) has been demonstrated at the Air Force Research Laboratory (AFRL) at high power with excellent optical:optical slope efficiencies. Previously, a rubidium (Rb) dual flow – Master Oscillator Power Amplifier (MOPA) configuration and a rubidium cesium (Cs) Multi-Alkali Multi-Line (MAML) laser have been demonstrated. New potassium (K) diode modules are anticipated to yield output power scaling to significantly greater than previously demonstrated values. Due to the small spin-orbit splitting between the 2P states of K as compared to Rb and Cs, collisional relaxation can be achieved using noble gases in place of hydrocarbons. This will help mitigate the substantial challenge of alkali contamination of the optical windows caused by photochemical breakdown.

9729-2, Session 1

Operation of static and flowing Cs DPAL with different buffer gas mixtures

Randall Knize, Boris V . Zhdanov, Matthew Rotondaro, Michael Shaffer, U .S . Air Force Academy (United States) This talk presents the results of an experimental study of a Cs DPAL operating in pulsed and continuous wave (CW) modes with different buffer gases. We explored such buffer gases as Ethane (C2H6), Methane (CH4) and mixtures of these hydrocarbons with noble gases He and Ar. In these experiments we varied two parameters: buffer gas composition and the total buffer gas pressure to explore how these factors affect lasing efficiency of a Cs DPAL in pulsed and CW modes of operation. Using a pulsed pump source, we observed effects of DPAL output power degradation in time from its starting maximum value at the beginning of the pump pulse to its CW level. We studied how the characteristic time of power degradation and power drop level depends on buffer gas content and pump power. Also we explored the possibility of mitigating of the power degradation effect by flowing the gain medium. transonic (M ~ 0.9) DPALs, taking into account fluid dynamics and kinetic processes in the lasing medium is reported. The performance of these lasers is compared with that of supersonic (M ~ 2.7 for Cs and M ~ 2.4 for K) DPALs. The power achieved in the supersonic and transonic K DPALs is higher than for the subsonic version by ~ 84% and ~ 27%, respectively, showing an advantage of the supersonic device over the transonic and subsonic ones. Comparison between end- and transverse-pumping by rectangular beams of the same cross section shows that in terms of brightness and beam quality, end-pumping geometry is preferred, although the output power is not affected by the pump geometry.

9729-4, Session 1

Power scaling a wavelength-narrowed diode laser system for pumping alkali vapors

F . W . Hersman, The Univ . of New Hampshire (United States) and Xemed LLC (United States); J . H . Distelbrink, J . Ketel, J . Wilson, D . W . Watt, Xemed LLC (United States) Diode lasers have high electrical-to-optical efficiency, but the spectral range of their output is too broad to be fully absorbed on alkali vapors. One method for locking the output wavelength and reducing the linewidth is to feed light from a wavelength selective optical cavity back into the emitters. Ten years ago our team developed a stepped-mirror that allowed a single external cavity to lock the wavelength of a stack of diode array bars by equalizing path lengths between each emitter and the grating. Here we report a laser architecture that combines one such step-mirror external cavity with an array of power dividers, each of which sends a portion of this feedback power to a separate diode array bar stack. We have assembled a system with four beam splitters that distribute feedback power to five stacks of ten bars each. Cylindrical lenses in the external cavity allow the magnifications of the fast and slow axes to be separately optimized. Fast axis magnification was chosen at M=20, allowing spectral linewidth on single emitters as low as 30pm. Alignment of the diode elements, splitters, step mirror, and other external cavity elements will determine overall system linewidth and efficiency. We report data from our 3kW prototype and discuss its scalability to several tens of kilowatts.

9729-3, Session 1

Modeling of static and flowing gas diode pumped alkali lasers

(Invited Paper)

Boris D . Barmashenko, Ilya Auslender, Eyal Yacoby, Karol Waichman, Oren Sadot, Salman Rosenwaks, Ben-Gurion Univ . of the Negev (Israel) Modeling of static and flowing gas subsonic, transonic and supersonic Cs and K diode pumped alkali lasers is reported. A simple optical model of K DPAL applied to the highly efficient static, pulsed K DPAL [Zhdanov et al, Optics Express 22, 17266 (2014)], shows good agreement between the calculated and measured dependence of the laser power on the incident pump power. The model reproduces the observed threshold pump power, 22 W, which is much higher than that predicted by standard models of the DPAL. The reason for the large values of the threshold power is that the volume occupied by the excited K atoms contributing to the spontaneous emission is much larger than the volumes of the pump and laser beams in the laser cell, resulting in large energy losses due to the spontaneous emission. 3D CFD modeling of subsonic (Mach number M ~ 0.2) and

9729-5, Session 1

Alkali D2 line laser optically pumped by two color free-free absorption

Andrey E . Mironov, James G . Eden, Univ . of Illinois at Urbana-Champaign (United States); William Goldshlag, University of Illinois at Urbana-Champaign (United States) Excimer-pumped alkali lasers (XPALs) represent pumping configuration for alkali-atomic lasers that serves as an alternative to classical diode pumped alkali lasers (DPALs). The population inversion in XPALs is achieved by the photoassociation of ground state alkali-rare gas molecules via the blue satellite of the alkali D2 line. The original XPAL pumping scheme provides a broad pump acceptance spectrum (~ 2 nm) and high quantum efficiency (>95%). However, due to the low absorption cross section normally associated with an alkali-rare gas blue satellite, the original XPAL scheme generally suffers from poor utilization of the pump energy.

We report here a novel pumping scheme for XPALs in which the alkali-noble gas mixtures are pumped with two colors, resulting in greater efficiency and an order of magnitude increase in the pump absorption coefficient. Our

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Conference 9729: High Energy/Average Power Lasers and Intense Beam Applications IX

experiments show that synchronous pumping of alkali-noble gas mixtures results in an increase in the overall laser efficiency by at least a factor of 1.7. Also, the effective quantum efficiency of the system is above 100% due to the extraction of 102 cm^-1 of thermal energy per emitted photon. This pumping scheme appears to be applicable to a broad range of alkali-rare gas mixtures. The spectroscopy and kinetics of this new family of lasers will be discussed.

9729-8, Session 1

Deactivation and reaction of excited states of Rb in collisions with H2, CH4 and C2H6

Valeriy N . Azyazov, Aleksei P . Torbin, Samara State Aerospace Univ . (Russian Federation); Alexander M . Mebel, Florida International Univ . (United States); Sean Bresler, Michael C . Heaven, Emory Univ . (United States)

9729-6, Session 1

Wave optics simulation of diode pumped alkali laser (DPAL)

(Invited Paper)

Masamori Endo, Tokai Univ . (Japan); Ryuji Nagaoka, Hiroki Nagaoka, Toru Nagai, Fumio Wani, Kawasaki Heavy Industries, Ltd . (Japan) A numerical simulation code of diode pumped alkali laser (DPAL) is developed. We have been observing the output power rollover in our Cs DPAL, when Cs partial pressure increases, despite the pump power absorption is better. We expect that a detailed simulation code, considering spatial mode overlap and spectral divergence of the pump light could identify the reason of the problem. The code employs the Fresnel-Kirchhoff diffraction integral for both laser mode and pump light propagations between the resonator mirrors. A three-dimensional atomic-photon rate equations are calculated simultaneously to determine the three-dimensional local gain. Spectral divergence of the pump source is represented by a series of (typically 21) pump lights with different wavelengths, occupying the same volume. Stimulated emission and absorption are calculated for these lines with different cross sections determined by the pressure broadening of the Cs atom. The developed simulation is tested for various operational condition of our small-scale (6.5W) DPAL apparatus. Excellent agreement has been seen in most of the tests. Especially, the agreement of the output power dependence on the focal position of the pump light proves the adequacy of the model. Finally, the roll over the output power has been correctly reproduced. It has been found that the main channel of the pump power drain is the spontaneous emission from the upper level of lasing transition, due to the fairly short (35 ns) life time.

Optically pumped alkali vapor lasers commonly use methane (CH4) or ethane (C2H6) as the agent to induce energy transfer between the optically pumped level (n2P3/2) and the upper laser level (n2P1/2). A complication with this scheme is that the alkali metal eventually reacts with the hydrocarbons, yielding particulate carbon and metal hydrides as contaminants in the gain medium. The reactions of ground state alkali metals with methane and ethane are endothermic, but excitation to the first excited 2P state is sufficient to make the reactions slightly exothermic. This is experimentally verified for the related reaction M(2P)+H2 ? MH + H, and thermodynamic data predict that the reactions of M(2P) with methane and ethane will also be exothermic. We have used laser pump-probe methods to examine the reactions of Rb(n2P) with H2, CH4, and C2H6 for states with n=5, 6, and 7. The H2 reactions were examined to validate the LIF method used for RbH detection. For methane and ethane, pump-probe measurements that examined the Rb(5s) ground state recovery kinetics indicated loss due to reaction following pulsed excitation to the n2P states. Surprisingly, the RbH product was not detected. The results from high level ab initio calculations are being used to study the reactive interactions between Rb, methane and ethane.

9729-9, Session 1

Myths, legends and facts; from SDI to tactical battlefield lasers: Reflections of a ‘star warrior’

(United States)

(Invited Paper)

James A . Horkovich, Directed Energy Professional Society

9729-7, Session 1

Measurement of the total ionization rate in an operating Cs DPAL

Michael Shaffer, Boris V . Zhdanov, Matthew Rotondaro, Randall Knize, U .S . Air Force Academy (United States) This talk presents the results of experiments on the direct measurement of the ionization rates in an operating continuous wave static Cs DPAL. Ionization occurs due several possible mechanisms including multiphoton processes. In these experiments, stainless steel electrodes were mounted inside the lasing alkali cell and biased by the external voltage source. The collected currents, estimated to be in the nanoampere range, were measured by a PicoAmmeter. We designed and manufactured an alkali vapor cell with AR coated windows and mounted the electrodes inside the cell such that they are parallel to the optical axis of the laser cavity and to the direction of the pump and lasing beams (longitudinal pumping design). The alkali cell was filled with 1 g of metallic Cs and Methane buffer gas at 600 Torr. The cell was temperature controlled and the vapor density was 1.5 x 1013 cm-3. In these experiments, we have collected data for total ionization current as a function of applied bias voltage across the electrodes for different values of incident resonant pump power in the range 3W – 20W with a maximum output laser power about 10W. These values of ionization currents were converted into total ionization rates and compared to values calculated in previous publications. This talk presents a history of missile defense and the “Star Wars” program and its’ evolution to today’s tactical battlefield laser systems, marking the 30th anniversary of President Ronald Reagan’s “Star Wars” speech. Since Archimedes’ “Burning Glass” at the siege of Syracuse 212 B.C. through the development of the LASER man has been fascinated with the idea of using directed energy weapons. But nothing did more to focus this effort than the threat posed by Mutually Assured Destruction. Under Reagan’s “Star Wars” plan years and billions of dollars were invested in making high energy laser systems a reality. This presentation discusses the fundamentals of laser physics and traces the development of these systems in the USA and USSR from the Gas Dynamic LASER laboratory in the 1960s and the USAF Airborne Laser Laboratory of 1981 through the SDI era and up to today. In reflecting on the effort invested in developing this technology, this interdisciplinary talk addresses the role that this technology played in changing the geopolitical state of the cold war and continues to play in international defense efforts today. Note: This talk takes 45 – 50 minutes in its complete form. It is most suited as a plenary or historical talk,

9729-10, Session 2

Laser excitation dynamics of argon metastables generated in atmospheric pressure flows by microwave frequency microplasma arrays

Wilson T . Rawlins, Kristin L . Galbally-Kinney, Steven J . Davis, Physical Sciences Inc . (United States); Alan R .

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9729-11, Session 2

Conference 9729: High Energy/Average Power Lasers and Intense Beam Applications IX

Hoskinson, Jeffrey A . Hopwood, Tufts Univ . (United States) The optically pumped rare-gas metastable laser is a chemically inert analogue to diode-pumped alkali (DPAL) and alkali-exciplex (XPAL) laser systems. Scaling of these devices requires efficient generation of electronically excited metastable atoms in a continuous-wave electric discharge in flowing gas mixtures at elevated pressure. This paper describes on-going investigations of the use of linear microwave micro-discharge arrays to generate metastable rare-gas atoms at atmospheric pressure in optical pump-and-probe experiments for laser development. Each array consists of a set of microstrip transmission line resonators with a small (25-100 micron) gap to ground in which the microplasma is ignited. Power requirements to ignite and sustain the plasma at 1 atm are low, <30 W. We report on the micro-discharge excitation and optical pumping dynamics of argon metastables, Ar (4s, 1s5) (Paschen notation), generated in flowing mixtures of Ar and He from 20 Torr to 1 atm. Ar(1s5) concentrations, optically pumped gain, and absorption linewidths are determined by diode laser absorption; Ar(4p) population distributions are determined by laser induced fluorescence excitation spectroscopy. We observe optically pumped excitation of the 1s5 ? 2p9 transition at 811.5 nm and the corresponding optical gain and lasing action on the 2p10 ? 1s5 transition at 912.3 nm, where the 2p10 state is populated by collisional energy transfer from 2p9. We will present systematic observations of parametric behavior with pressure, gas flow conditions, and micro-discharge geometry and power. We will discuss considerations and expectations for scaling the optically pumped micro discharge to higher output powers.

Demonstration of a diode-pumped CW advanced noble gas laser (ANGL)

Greg A . Pitz, Air Force Research Lab . (United States)

9729-13, Session 2

Measurement of pressure broadening of the Kr 811.3 nm absorption line using a diode laser

Pavel A . Mikheyev, Samara State Aerospace Univ . (Russian Federation); Alexander K . Churnyshov, Nikolay I . Ufimtsev, P .N . Lebedev Physical Institute (Russian Federation); Anna R Ghildina, Samara State Aerospace University (Russian Federation); Valeriy N . Azyazov, Samara State Aerospace Univ . (Russian Federation); Michael C . Heaven, Emory Univ . (United States) Optically pumped all-rare-gas laser (OPRGL) with unique properties comparable to a diode pumped alkali laser (DPAL) was recently proposed. To study this promising laser system it is necessary to have reliable diagnostics for the active medium. Pressure broadening coefficients, for both self- and foreign-gas collision partners, are needed for spectroscopic measurements of number densities and temperatures in rare gas discharge plasma. However, a literature search shows that pressure broadening coefficients for rare gas lines for mixtures that are of interest for the OPRGL are surprisingly hard to find. Diode laser absorption spectroscopy was employed for measurements of pressure broadening coefficients for krypton in an RF discharge. A MQW-diode laser (L808P030, Thorlabs) with a short external cavity was used as the source of probe radiation. The natural isotopic distribution of Kr was taken into account, and an appropriate fit function was constructed to facilitate determination of the pressure broadening coefficients. The broadening coefficients for the Kr 811.3 nm line atto 300 K were: ?Kr Kr = (2.4 ± 0.2) ?10 10 s 1cm3 for self-broadening and ?Kr He = (3.1 ± 0.2) ?10. 10 s 1cm3 for broadening by helium.

The Advanced Noble Gas Laser (ANGL) uses a mild RF discharge to excite argon (Ar) atoms from the ground state (s0) to the metastable s5 state, allowing Ar to be optically pumped (s5 – p9) as a quasi-three level laser similar to the Diode Pumped Alkali Laser (DPAL). Helium (He) collisional mixing from the p9 to the p10 state allows lasing from p10 to s5 at 912 nm. Previously, 2 mW of average output power has been achieved at the Air Force Research Laboratory (AFRL) using a 811 nm pulsed Titanium:Sapphire (Ti:Sapph) laser to pump a plasma discharge tube with a 1-10% Ar:He mixture. Recently, number density and gain diagnostics have been conducted in a parallel-plate discharge configuration. This architecture was also used in demonstrating the world’s first continuous wave (cw) diode pumped ANGL.

9729-14, Session 2

Narrow spectral width laser diode for metastable argon atoms pumping

Jun Gao, Wuhan National Lab . for Optoelectronics (China); Bin Li, Xinbing Wang, Huazhong Univ . of Science and Technology (China); Duluo Zuo, Wuhan National Lab . for Optoelectronics (China) and Huazhong Univ . of Science and Technology (China)

9729-12, Session 2

Pulsed discharge production Ar* metastables

Jiande Han, Michael C . Heaven, Emory Univ . (United States); William F . Bailey, Daniel J . Emmons, Glen P . Perram, Air Force Institute of Technology (United States) The production of relatively high densities of Ar* metastables (>10e12 cm-3) in Ar/He mixtures, at total pressures close to 1 atm, is essential for the efficient operation of an optically pumped Ar* laser. We have used emission spectroscopy and diode laser absorption measurements to observe the production and decay of Ar* in a parallel plate pulsed discharge. With discharge pulses of 1 ?s duration we find that metastable production is dominated by processes occurring within the first 100 ns of the gas break-down. Application of multiple, closely spaced discharge pulses yields insights concerning conditions that favor metastable production. This information has been combined with time-resolved measurements of voltage and current. The experimental results and preliminary model of the discharge kinetics will be presented.

The optically pumped metastable rare gas laser (OPRGL) has attracted more and more attention for its potential to obtain high power laser with good beam quality and atmospheric transmittance. Among all of the rare gases, argon is the cheapest one and the OPRGL using it will be easily conducive to large-scale applications. As the absorption line is an atomic one, the first step for the high power OPRGL of argon is to realize a high power pump source with narrow spectral width emitting around 811.53 nm.

Diode laser pump source for OPRGL of argon was realized by employing a complex external cavity coupled with volume Bragg grating (VBG). A commercial available c-mount LD with rated power of 6 W was used. The LD were studied in both the free running mode and VBG external cavity. For the external cavity laser diode, the output beam of LD was collimated by an aspherical lens, and the temperatures of VBG and LD were controlled separately by thermal-electrical-coolers (TEC). A high resolution spectrometer with resolution limit 13 pm near 812 nm was applied to measure the spectra of the output laser beam. Maximum output power of 3.9 W with FWHM less than 25 pm and peak wavelength located around 811.53 nm was obtained. The peak wavelength could be tuned more than 300 pm by precise control of the VBG temperature. These results will benefit the further research on OPRGL of argon.

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9729-15, Session 3

Conference 9729: High Energy/Average Power Lasers and Intense Beam Applications IX

Jensen, General Atomics Aeronautical Systems, Inc . (United States)

Optical pumping of the oxygen-iodine laser medium

Marsel V . Zagidullin, P .N . Lebedev Physical Institute (Russian Federation); Mikhail S . Malyshev, Samara State Aerospace Univ . (Russian Federation); Valeriy N . Azyazov, Samara State Aerospace Univ . (Russian Federation); Michael C . Heaven, Emory Univ . (United States) The kinetics of the processes in an O2 – I2 – He – H2O gas flow that is irradiated simultaneously by light at wavelengths near 500 nm and 1315 nm, is considered. Radiation at 500 nm is used to photodissociate about 1% of iodine molecules. The radiation at 1315 nm excites atomic iodine to the 2P1/2 state. Singlet oxygen molecules are produced via the energy exchange process I(2P1/2)+O2(3?) ? I(2P3/2) + O2(1?), while I(2P1/2)+O2(1?) energy pooling produces oxygen molecules in 1? state. I(2P1/2) and O2(1?) then accelerate the dissociation of I2. A kinetic analysis was performed for a gas flow at 70 torr pressure, 300 K temperature, 60 W/cm2 500 nm light intensity and 5 kW/cm2 IR irradiation. It is shown that this scheme produces an oxygen – iodine medium with a high degree of iodine dissociation and a relative content of singlet oxygen O2(a1?) exceeding 10 %. Having formed a supersonic gas flow with a temperature ~100 K from this medium, one can reach a small-signal gain of about 10–2 cm–1 on the 2P1/2 – 2P3/2 transition in iodine atoms. The specific power per unit flow cross section in the oxygen – iodine laser with this active medium may reach ~100 W cm–2 at an optical efficiency of 60%. If H2O is excluded, which is an active O2(1?) quencher, it is possible to achieve an O2(1?) fraction of up to 70% and inversion of the oxygen b1?=>X3? transition.

We report on investigation of novel 2 µ m thulium (Tm)–based laser accelerator driver (LAD) offering efficient generation of high-energy pulses with high-peak power at high repletion rate, high efficiency, and with near diffraction-limited beam quality (BQ). Laser accelerators of nuclear particles by ultrashortpulse laser-generated plasmas offers much reduced size and cost compared to conventional accelerators of the same energy, which would drastically cut the cost of high-energy particle research on collider based facilities and advanced light sources, thus replacing the traditional mammoth-size and costly accelerator research facilities with room-size systems [1].

LAD operating at 2 µ m wavelength offers ponderomotive force 4x that of 1 µ m and 6x that of the traditional 0.8 ?m LAD. In addition, the Tm bandwidth of nearly 400 nm offers >15% tunability and generation of ultrashort pulses down to <30 fs. The “2-for-1” pump architecture of Tm ion enables >20% wall-plug efficiency.

This work presents the relative performance of several Tm-doped materials and LAD configurations.

Comparisons to traditional 0.8 µ m Ti-sapphire and 1 Energy Grant Number DE- SC0013762.

µ m Yb lasers are also shown. Experimental data on gain uniformity and thermo-optical distortions are presented. This work was in-part supported by the US Department of 1. Proc. of Workshop on Laser Technology for Accelerators, Summary Report, US Department of Energy, January 23-25, 2013

9729-16, Session 3

Oxygen assisted iodine atoms production in an RF discharge

Pavel A . Mikheyev, Samara State Aerospace Univ . (Russian Federation); Nikolay I . Ufimtsev, P .N . Lebedev Physical Institute (Russian Federation); Andrey V . Demyanov, Troitsk Institute for Innovation and Fusion Research (Russian Federation); Igor V . Kochetov, Valeriy N . Azyazov, P .N . Lebedev Physical Institute (Russian Federation); Anatoly P . Napartovich, Troitsk Institute for Innovation and Fusion Research (Russian Federation); Michael C . Heaven, Emory Univ . (United States) Experiments and modeling of CH3I dissociation in the plasma of a 40 MHz RF discharge were performed. A discharge chamber of an original design, consisting of quartz tubes between two planar electrodes, produced iodine atoms at number densities up to 2?1016 cm 3. Contamination of the walls of the tubes did not impair the discharge stability, providing a good iodine production rate. Addition of oxygen into the Ar:CH3I mixture resulted in a substantial increase in CH3I dissociation efficiency. At a discharge power 200 W, complete CH3I dissociation in Ar:CH3I:O2 mixture was observed. The fraction of discharge power spent on iodine atom production at 0.17 mmol/s CH3I flow rate was 16%. Modeling showed satisfactory agreement with the experiments.

9729-19, Session 4

Effect of laser power on the microstructural behaviour and strength of modified laser deposited Ti6Al4V+Cu alloy for medical application

Mutiu F . Erinosho, Esther T . Akinlabi, Univ . of Johannesburg (South Africa) The excellent biocompatibility property of Grade 5 titanium alloy has made its desirability largely increasing in the field of biomedical. The titanium alloy (Ti6Al4V) was modified with the addition of 3 weight percent (wt %) copper via a laser deposition process using the Ytterbium fiber laser with a wavelength of 1.047 µ m. Therefore, this paper presents the effect of laser power on the microstructural behaviour and strength of the modified Ti6Al4V+Cu alloy. The laser powers were varied between 600 W and 1600 W respectively while all other parameters such as the scanning speed, powder flow rates and gas flow rates were kept constant. The melt pool and width of the deposited alloy increases as the laser power was increased. The ?-lamella was observed to be finer at low laser power, and towards the fusion zone, Widmanstettan structures were fused and become smaller; and showing an evidence of ?-martensite phases. The strength of the modified alloy was derived from the hardness values. The strength was observed to increase initially to a point as the laser power increases and afterwards decreased as the laser power was further increased. The improved Ti6Al4V+Cu alloy can be anticipated for biomedical application.

9729-18, Session 4

Wide-bandwidth Tm-based amplifier for laser acceleration driver

Drew A . Copeland, John Vetrovec, Amardeep S . Litt, Aqwest, LLC (United States); Joseph M . Fukumoto, Steven

9729-20, Session 4

High pulse energy 1123nm laser of Nd:GdLuAG mixed garnet medium

Yang Liu, Zhaojun Liu, Sasa Zhang, Jinbao Xia, Yanmin Zhang, Chen Guan, Shandong Univ . (China) Diode-pumped CW and passively Q-switched lasers of Nd:GdLuAG mixed garnet at 1123 nm were demonstrated. The maximum average output power of CW operation was 4.13 W. For Q-switched operation, the average output

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9729-21, Session PTue

Conference 9729: High Energy/Average Power Lasers and Intense Beam Applications IX

power was 800 mW, the corresponding single pulse energy was 133.8 µ J. The Nd:GdLuAG laser emitting at 1123 nm was obtained for the first time to the best of our knowledge, which proves that the Nd:GdLuAG mixed garnet has a better ability of energy storage than Nd:YAG in 1123 nm oscillation.

Lidar for monitoring methane emission in Siberian permafrost

Alexsandr S . Grishkanich, Aleksandr Zhevlakov, Sergey Kascheev, ITMO Univ . (Russian Federation); Igor Sidorov, Univ . of Eastern Finland (Finland); Valentin Elizarov, ITMO Univ . (Russian Federation); Andrey Mak, National Research Univ ITMO (Russian Federation)

9729-22, Session PTue

The control of CO2 lasing temporal characteristics by modulated self-injected irradiation

Vadim V . Kiyko, A . M . Prokhorov General Physics Institute of the Russian Academy of Sciences (Russian Federation) and ITMO Univ . (Russian Federation); Danil Mikhaylov, A . M . Prokhorov General Physics Institute of the Russian Academy of Sciences (Russian Federation) Over the past 10 years the rate of temperature in the Siberia increases almost twice higher than the average rate of warming of the planet. Identifying methane anomalies responsible for the temperature increase, by hiking trails in the Arctic requires great human labor. It is necessary to use lidar methods for search and identification of methane from permafrost. Necessary to create a Raman lidar for monitoring of emissions of methane hydrate from the permafrost. Hyperspectral resolution would resolve the isotope shifts in the Stokes spectra, thereby to determine the isotopic composition of methane ratio C14/C12 CH4 carbon emissions and identify the source for study (permafrost or oil deposits) Isotopic composition of the methane (concentration of 13C, 14C and D) can provide information concerning the methane origin and formation time. Analysis of the concentration of radioactive isotope 14C allows discerning methane fractions with biological and abiological origins. Isotope 14C appears in the atmosphere due to the cosmic rays. The interaction of nitrogen 14N nuclei with neutrons from the cosmic rays transforms them into the 14C isotope nuclei. Half life of the 14C isotope is 5 730 years, which is significantly exceeds lifetime of absolute majority of Earth life forms. Later on radioactive carbon nuclei emit electron and once again become a stable isotopes 14N. Carbon isotope 14C interacts with the atmospheric oxygen by forming molecules of carbon dioxide 14CO2. Later these molecules along with the ones of the stable carbon isotope 12CO2 are included into the processes of the living organic matter formation trough the photosynthesis. Automated airborne lidar will allow estimation of the methane emissions intensity, as well as evaluation of qualitative and quantitative parameters of of the methane anomalies.

A modified theoretical model of CO2 laser supplemend with self injection of laser output irradiation to unstable cavity is presented. It is based on classical one-dimensional six-temperature model. The model is supplemented with terms that consider influence from self-injected irradiation by external optical system that selects and returns portion of output irradiation. The self-injection influence on static and dynamic parameters of laser system was studied. Also, the dynamic of lasing with temporal modulated self-injection was studied.

It is shown that returning even small part of the output irradiation and its amplification in an active medium radically changes dynamic parameters of laser. It is demonstrated that with temporal modulation of self-injection it is possible to obtain the lasing mode similar to Q-switch by output parameters (like as pulse shape and duration). Despite similar output parameters, dynamical processes in laser cavity in Q-switched mode and temporal modulated self-injected mode are completely different.

A significant influence on the output characteristics of the laser can be obtained by controlling power an order of magnitude less than the output power of the laser. For example, the numerical simulation showed that with self-injecting power near 5% of laser output power could be obtained the pulse-periodical lasing mode with pulse duration near 300 ns, pulse repetitive rate 20 kHz and peak-to-average power relation near 20. The numerical results have been experimentally verified. During the experiments the lasing mode was changed from CW to pulse-periodical with parameters close to mentioned above with temporal modulated self-injection only.

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Conference 9730: Components and Packaging for Laser Systems II

Tuesday - Thursday 16–18 February 2016 Part of Proceedings of SPIE Vol . 9730 Components and Packaging for Laser Systems II 9730-1, Session 1

Integrated disruptive components for 2

µ

m fibre lasers (ISLA): project overview and passive component development

(Invited Paper)

Gary Stevens, Tom Legg, Gooch & Housego (Torquay) Ltd . (United Kingdom); Peter Shardlow, ORC, University of Southampton (United Kingdom) In this presentation, an overview of the EU FP7 project ISLA (Integrated disruptive componentS for 2 µ m fibre Lasers) is given. The aim of ISLA was to develop a set of “building block” components and a “tool-kit” of processes to define an integrated modular common platform for two micron fibre lasers consisting of compatible and self-consistent active and passive fibres, fused fibre couplers and combiners, fibre-coupled isolators, modulators and high power pump laser diodes. We also present results from our work on developing passive components for 2 µ m fibre lasers. This includes high power pump combiners that have been tested up to 0.5 kW and combiners for in-band pumping of holmium lasers. Couplers for use as splitters, power monitors and wavelength division multiplexers have also been demonstrated. Wide-band couplers, with a coupling ratio that only varies ± 12% over 400 nm, have also been developed to exploit the wide tuning range possible with thulium fibre lasers.

Research into different isolator materials was also conducted to find materials with large Verdet constants to be used in 2 > 23 dB was achieved.

µ m isolators. Fibre coupled isolators were then manufactured using a selection of these materials. Isolators that had insertion losses of < 1 dB and isolation of > 35 dB were demonstrated using PM and non-PM fibres. In the PM isolators, PER

9730-2, Session 1

Acousto-optic devices for operation with 2

µ

m fibre lasers

Jon D . Ward, Gooch & Housego PLC (United Kingdom); Gary Stevens, Gooch & Housego (Torquay) Ltd . (United Kingdom); Peter C . Shardlow, Univ . of Southampton (United Kingdom) Fibre lasers operating in the 2 µ m region are of increasing interest for a range of applications, including laser machining and biomedical systems. Fibre laser manufacturers were able to call upon enabling technologies used by the telecoms industry when developing lasers at 1 µ m & 1·5 µ m, but at longer wavelengths, for example 2 µ m, many such components are either unavailable or immature.

We report on recent developments of Acousto-Optic Modulators/ Frequency-Shifters and Tunable Filters that are specifically optimised for use with fibre systems operating at or around 2 parameter when used in a fibre system.

µ m. AO devices are interesting due their ability to conserve spatial-coherence making them appropriate for use with single-mode optical fibres. We describe how choice of interaction medium is an important consideration, particularly affecting the drive power and the polarisation behaviour of the device – the latter being an important We also describe two designs of AO Tunable Filter intended for laser tuning. Both designs have been demonstrated intracavity in 2 µ m fibre lasers. The first gives exceptionally narrow resolution (??/?<0·1%). The second design is of a novel type of AOTF where a matched pair of AOTFs is configured to give a substantially net zero frequency-shift with very little or no loss of pointing stability, any minor deviations in manufacture being self compensated. Furthermore, small controlled frequency-shifts (up to about 10kHz) may be introduced with little or no detriment to the alignment of the system.

9730-3, Session 1

Broadband saturable absorber mirrors

Kangpeng Wang, Aidan Baker-Murray, Werner J . Blau, Trinity College Dublin (Ireland) Intracavity saturable absorbers are required for passive modelocking. By attaching the saturable absorber to one of the laser resonator mirrors, a saturable absorber mirror (SAM) device is formed.

Saturable absorbers used in the past were typically organic dyes, which suffer from short lifetimes, toxicity, and complicated handling, limiting their application to mostly laboratory systems. Alternative solid-state saturable absorbers include crystals such as Cr:YAG, which typically operate for only limited ranges of wavelengths, recovery times and saturation levels. Semiconductor saturable absorber mirrors (SESAMs) currently dominate the market. However, these have a narrow tuning range (tens of nanometres), limited product lifetime and require complex fabrication and packaging. Novel nonlinear materials with broadband absorption are therefore required for wideband tuneable operation and for new upcoming wavelength bands.

Graphene is currently at the centre of a significant global research effort. Its electronic and mechanical properties are ideal for micromechanical systems, thin-film transistors, and transparent and conductive composites and electrodes. Here, we exploit the optoelectronic properties of graphene for the ultrafast laser market. Pauli blocking following intense illumination results in saturable absorption, independent of wavelength. This effect may be used to passively modelock almost any laser in the visible, near- and mid-infrared region of the electromagnetic spectrum, paving the way to graphene-based photonics.

As part of the FP7 project ISLA, Graphene SAMs were fabricated by a simple wet deposition process onto a metal mirror substrate. Their intensity response was tested at various infrared wavelengths in a so-called i-scan set-up. The results obtained were compared directly with a commercially obtained SESAM from BATOP. The figure below clearly shows saturable absorption with a response that is at least comparable, if not better than the SESAM. Particularly the rapidly expanding 2 µ m wavelength range has huge commercial potential as it is of special importance for applications, such as gas detection, including long-range LIDAR applications, free-space optical communication, medical diagnostics, laser surgery, optical pumping of longer wavelength solid-state lasers, material processing and security applications. Hence, the broad-band applicability of our Graphene SAM represents a major market advantage with significant commercial prospects.

9730-4, Session 1

Thulium-doped silica fibers optimized for high lasing efficiency

Peter C . Shardlow, Deepak Jain, Richard Parker, Jayanta K . Sahu, W . Andrew Clarkson, Univ . of Southampton (United Kingdom) Two-micron lasers are of great interest for a range of applications, from spectroscopy to polymer machining and laser surgery, as well as being an important stepping stone for wavelength generation further into the

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Conference 9730: Components and Packaging for Laser Systems II

mid infra-red band via nonlinear frequency conversion. Tm-doped silica fibre lasers are especially attractive as they are capable of operating at wavelengths from below 1700nm to more than 2100nm, and can be pumped by commercially available high power laser diodes operating around ~793nm. Moreover, by optimising the dopant concentration within the Tm fibre core a beneficial two-for-one cross-relaxation process can be exploited allowing efficiencies far-above the quantum limit for 793nm pumped Tm fibre lasers. Indeed, efforts to optimise the core composition to enhance this process have been the subject of many studies, but as yet the best slope efficiencies reported for high power cladding-pumped Tm doped silica fibre lasers remain well below the theoretical maximum (~80%). Here we report on our recent Tm fibre development work targeted at improving the two for-one cross-relaxation efficiency through optimising core composition and doping profiles. Using a fibre with a Tm concentration of 3.6 wt.% yielded a slope efficiency with respect to absorbed pump power of ~70% and with power levels up to 100W, limited by available pump power. Further studies indicate that efficiencies close to the theoretical limit of ~80% should be achievable with the appropriate core design.

Feedback Quantum Cascade Laser (DFB-QCL). The geometrical design and polarization properties have been defined using numerical simulations. A non-symmetric chalcogenide MOF has been realized by taking into account the theoretical results. Finally, the fiber properties have been evaluated using a DFB-QCL emitting at 7.4 µ m and the polarization maintaining at the output of the chalcogenide fiber has been demonstrated . The combination between a DFB-QCL with such non-conventional fiber has led to the development of the first single-mode polarized fibered Mid IR laser emitting at 7.4

µ m.

9730-7, Session 2

Mid-IR fused fiber couplers

Gary Stevens, Toby Woodbridge, Gooch & Housego (Torquay) Ltd . (United Kingdom)

9730-5, Session 2

Volume Bragg gratings for 2-micron laser systems

Vadim Smirnov, OptiGrate Corp . (United States); Alex Sincore, Joshua Bradford, Lawrence Shah, Martin Richardson, CREOL, The College of Optics and Photonics, Univ . of Central Florida (United States); Oleksiy Mokhun, Alexei L . Glebov, OptiGrate Corp . (United States); Leonid Glebov, CREOL, The College of Optics and Photonics, Univ . of Central Florida (United States) and OptiGrate Corp . (United States) Volume Bragg Gratings (VBGs) in photo-thermo-refractive glass have outstanding properties in 2-micron spectral region providing extremely high spectral and angular selectivity, diffraction efficiency up to 99.99%, controlled chirp dispersion, wavelength multiplexing capability, etc. The unique VBG properties enable their use in 2-micron lasers as output couplers, end-mirrors, spectral and angular selectors, stretchers and compressors of ultra-short pulses, spectral beam combiners, notch, bandpass, and spatial filters. This paper reviews recent VBG technology developments as well as various results on VBG applications that can lead to major improvements of fiber, solid-state, and diode laser system performance in 2-micron spectral range.

9730-6, Session 2

Polarization maintaining chalcogenide fiber for mid-IR quantum cascade laser pigtailing

Céline Caillaud, Univ . de Rennes 1 (France); Clément Gilles, mirSense (France); Laurent Brilland, PERFOS (France) and SelenOptics (France); Laurent Provino, PERFOS (France); Mathieu Carras, Mickael Brun, mirSense (France); David Méchin, PERFOS (France); Johann Troles, Univ . de Rennes 1 (France) and SelenOptics (France) The mid-infrared molecular fingerprint region has gained great interest in the last decade thanks to development of on-chip semiconductor lasers. For integrated-optic devices and optical sensors based on interferometric techniques, the polarization state of the transmitted signal must be kept constant. In this context, a low-loss single-mode chalcogenide microstructured optical fiber (MOF) which presents a polarization maintaining has been achieved in order to be connected to a Distributed We present results from our recent efforts on developing single-mode fused couplers in ZBLAN fibre. We have developed a custom fusion workstation for working with lower melting temperature fibres, such as ZBLAN and chalcogenide fibres. Our workstation uses a precisely controlled electrical heater designed to operate at temperatures between 100 – 250oC as our heat source. The heated region of the fibers was also placed in an inert atmosphere to avoid the formation of microcrystal inclusions during fusion. We firstly developed a process for pulling adiabatic tapers in 6/125 ?m ZBLAN fibre. The tapers were measured actively during manufacture using a 2000 nm source. The process was automated so that the heater temperature and motor speed automatically adjusted to pull the taper at constant tension. This process was then further developed so that we could fuse and draw two parallel 6/125 ?m ZBLAN fibres, forming a single-mode coupler. Low ratio couplers (1-10%) that could be used as power monitors were manufactured that had an excess loss of 0.76 dB. We have also manufactured 50/50 splitters and wavelength division multiplexers (WDMs). However, the excess loss of these devices was typically 2 - 4 dB. The increased losses were due to localised necking and surface defects forming as the tapers were pulled further to achieve a greater coupling ratio.

Initial experiments with chalcogenide fibre have shown that our process can be readily adapted for chalcogenide fibres and we have so far demonstrated adiabatic tapering and fusion of a variety of chalcogenide fibres.

9730-8, Session 2

Thermal management of quantum cascade lasers in an individually addressable monolithic array architecture

Leo J . Missaggia, Christine A . Wang, Michael K . Connors, Brian G . Saar, Antonio Sanchez-Rubio, Kevin J . Creedon, George W . Turner, William Herzog, MIT Lincoln Lab . (United States) There are a number of military and commercial applications for high power laser systems in the mid to long infrared wavelength range. By virtue of their demonstrated watt level high power performance and wavelength diversity, quantum cascade (QC) laser and amplifier devices are an excellent choice of emitter for those applications. In order to realize the power levels of interest, beam combining of arrays of these emitters would be required and as a result, array technology must be developed. With this in mind, packaging and thermal management strategies were developed to facilitate the demonstration of a monolithic QCL array operating under CW conditions. Thermal models were constructed and simulations performed to determine the effect of gain region temperature rise on parameters such as array element ridge width, pitch and cavity length. The results of the simulations were considered in determining an appropriate QCL array configuration. State-of-the-art micro-impingement cooling along with an

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Conference 9730: Components and Packaging for Laser Systems II

electrical distribution scheme comprised of AlN multi-layer technology were integrated into the design. The design of the module allows for individual electrical addressability of the array elements, a method of phase control demonstrated previously for coherent beam combining of diode arrays, along with access to both front and rear facets. Hence, both laser and single pass amplifier arrays can be accommodated. A module was realized containing a 5 mm cavity length monolithic QCL array comprised of 7 elements on 450 ?m pitch. An output power of 3.16 W was demonstrated under CW conditions at an emission wavelength of 9 ?m. Blue-emitting external cavity laser diode.

[1] Wenzel, H., Fricke, J., Klehr, A., Knauer, A., Erbert, G., “High-power 980nm DFB RW lasers with a narrow vertical far field,” Photon. Technol. Lett. 18, 737–739 (2006).

[2] Klehr, A., Bugge, F., Erbert, G., Fricke, J., Knauer, A., Ressel, P., Wenzel, H., Trekle, G., “High power broad-area 808nm DFB lasers for pumping solid state lasers,” Proc. SPIE 6133, 61330 (2006).

9730-9, Session 3

High power diode laser array development using completely indium free packaging technology with narrow spectrum

Dong Hou, Jingwei Wang, Lijun Gao, Dong Lu, Xingsheng Liu, Xiaoning Li, Xi’an Focuslight Technologies Co ., Ltd . (China)

9730-11, Session 3

Optical feedback module for stabilizing the optical power of laser-diode modules without thermoelectric coolers

John Downing, USL Technologies LLC (United States) The application of high power diode lasers is keep increasing in many fields, including scientific research, aerospace, display and medical applications etc.

The horizontal array of high power diode laser stack is widely used in pumping applications in industry. The spectrum with narrow FWHMand accurate central wavelength of the diode laser stack is required for high absorption efficiency of the pumping crystal in pumping applications. However, the control of spectrum uniformity of the multiple diode laser stacks is difficult due to the wavelength shift before and after bonding on the heatsink. Thus the precision control of the spectrum is critical in high power diode laser stack development. Soft solder of indium is used to bond laser diode bar onto copper heatsink traditionally, however, the repeated on-off current-cycling in laser operations can cause mechanical stress, which leads to various failures, such as material cracking, migration and thermal fatigue. In this work, a sophisticated high power and high performance horizontal array (30bars module) of conduction cooled diode laser have been developed. Uniform wavelength with narrow FWHM and accurate central wavelength is obtained through the numerical simulation and wavelength screening of unit stacks of diode laser. The wavelength value of FWHM and FW 90% energy of 30bars module achieves 2.69nm and 4.39nm in working condition. AuSn bonding technology and CTE-matched submount are applied to acquire high reliability and high output power (6000W in total). The hard solder bonding technology and spectrum control technology are promising approaches to achieve superior laser performance in industry.

Operating wavelength and beam quality, photodiode responsivity and spatial uniformity, and the properties of beam-forming optics vary systematically with temperature in laser-diode systems. Consequently, temperature is the dominant environmental factor affecting their stability. The net effect of temperature on optical power stability can be as large as 5,000 ppm/oC. Traditionally, temperature effects have been suppressed by operating laser diodes at constant temperature with thermoelectric coolers (TECs). While this method is a virtual industry standard in applications such as spectroscopy that require constant wavelength, more than half of the electrical power for system operation can be consumed by the TEC. This paper reports the development of an optical feedback module for stabilizing the output optical power of laser diodes without TECs. It complements prior work on electronic and optical methods for stabilizing VCSEL power. The optical feedback module comprises a weakly polarizing beamsplitter, a monitor photodiode characterized by low temperature sensitivity (< 10 ppm/oC), highly uniform responsivity (< 100 ppm/mm), and sub 1%-per year drift configured in an APC circuit. Since the module samples the output laser beam, it provides true closed-loop control that is immune to errors arising from differential aging of the front- and back-facet optical properties. Tests with an inexpensive (<$15), single-mode, 5 mW, red laser diode, which mode hops demonstrate that a system temperature coefficient of 19 ppm/oC can be obtained. This represents a 35-fold improvement in stability over what can be achieved with the internal monitor photodiode. Measurement systems that require ultra-stable power and high wall-plug efficiency are ideal candidates for the technology so long as they can tolerate wavelength variations in the range from 1.5 nm (blue and green bands) to 15 nm (red and NIR bands) over a 50o C range.

9730-10, Session 3

Blue-emitting external cavity laser diode

Hong Man Na, Korea Univ . (Korea, Republic of)

9730-12, Session 4

Teradiode’s high brightness semiconductor lasers

(Invited Paper)

Robin K . Huang, TeraDiode, Inc . (United States) Blue-emitting laser diode is required many industrial fields such as medical application, spectroscopy, and measurement techniques. Enhancement of output power with narrow bandwidth is currently one of the important factors for evaluating the laser diode specifications. Although the direct emitting blue laser diode is available for the high output power, it has a relatively wide spectral bandwidth and a lack of wavelength stabilization with injection current. The appending of internal grating is one of the possible ways to the enhancement [1.2], but it is still technically challenging due to the difficulties of processing in a blue laser diode. In order to improve both the narrow bandwidth and the high output power simultaneously, one of the most promising methods is to use an external cavity.

In this paper, a blue emitting External cavity laser diode utilizing AR coated broad-area solitary blue laser diode was demonstrated. The high efficient output power, narrow spectral bandwidth, wavelength-locking and wavelength-tuning ability with varying injected currents were investigated in TeraDiode is manufacturing multi-kW-class ultra-high brightness fiber coupled direct diode lasers for industrial applications. A fiber-coupled direct diode laser with a power level of 4,680 W from a 100 µ m core diameter, <0.08 numerical aperture (NA) output fiber at a single center wavelength was demonstrated. Our TeraBlade industrial platform achieves world record brightness levels for direct diode lasers. The fiber-coupled output corresponds to a Beam Parameter Product (BPP) of 3.5 mm-mrad and is the lowest BPP multi-kW-class direct diode laser yet reported. This laser is suitable for industrial materials processing applications, including sheet metal cutting and welding. This 4-kW fiber-coupled direct diode laser has comparable brightness to that of industrial fiber lasers and CO2 lasers, and is over 10x brighter than state-of-the-art direct diode lasers.

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9730-14, Session 4

Conference 9730: Components and Packaging for Laser Systems II

9730-13, Session 4

Packaging of hard solder 500W QCW diode laser array

Xiaoning Li, Jingwei Wang, Xi’an Focuslight Technologies Co ., Ltd . (China); Dong Hou, Xi’an Focuslight Technologies Inc . (China); Xingsheng Liu, Xi’an Focuslight Technologies Co ., Ltd . (China) With the increasing applications of high power semiconductor lasers in industry, advanced manufacturing, aerospace, medical systems, display, entertainment, etc., there is a strong demanding of diode lasers with high power and high reliability. Currently, the typical peak output power of commercially available diode laser is only 150-300W per bar for quasi continuous wave (QCW) used. The performance of diode laser can be significantly affected by the packaging structure and packaging process. In this work, a novel micro channel cooler (MCC) for stack array laser with good heat dissipation capacity and high reliability is presented. Numerical simulations on the effects of different MCC structure on the thermal behavior and thermal stress behavior are conducted and analyzed. Based on this new MCC packaging structure, a high power hard solder laser array of 500W has been fabricated. A series of techniques such as spectrum control and smile control have been applied to achieve a narrow spectrum and high beam quality. The performances of the laser array are characterized. A narrow spectrum of 3.17 nm and an excellent smile effect are obtained. The lifetime of the laser array is more than 1.38*10^9 shots and still ongoing.

Fully automated hybrid diode laser assembly using high precision active alignment

Gunnar Böttger, Daniel Weber, Fraunhofer-Institut für Zuverlässigkeit und Mikrointegration (Germany); Friedemann Scholz, Eagleyard Photonics GmbH (Germany); Henning Schröder, Martin Schneider Ramelow, Fraunhofer-Institut für Zuverlässigkeit und Mikrointegration (Germany); Klaus-Dieter Lang, Technische Univ . Berlin (Germany) Fraunhofer IZM, Technische Universität Berlin and eagleyard Photonics will present various implementations of current micro-optical assemblies for high quality free space laser beam forming and efficient fiber coupling.

The laser modules shown are optimized for fast and automated assembly in small form factor packages via state-of-the-art active alignment machinery, using alignment and joining processes that have been developed and established in various industrial research projects. Operational wavelengths and optical powers ranging from 600 to 1600 nm and from 1 mW to several W respecitvely will be addressed, for application in high-resolution laser spectroscopy and sensing or laser-medical treatment.

Today, the precision of micro-optics assembly is mostly limited by the accuracy of the bonding process - and in the case of adhesive bonding by the prediction and compensation of adhesive shrinkage during curing. In this contribution, we present a novel approach to address adhesive bonding based on hybrid control system theory. In hybrid control, dynamic systems are described as “plants” which produce discrete and/or continuous outputs from given discrete and/or continuous inputs, thus yielding a hybrid state space description of the system. The task of hybrid controllers is to observe the plant and to generate a discrete and/or continuous input sequence that guides or holds the plant in a desired target state region while avoiding invalid or unwanted intermediate states. Our approach is based on a series of experiments carried out in order to analyze, define and decouple the dependencies of adhesive shrinkage on multiple parameters, such as application geometries, fixture forces and UV intensities. As some of the dependencies describe continuous effects (e.g. shrinkage from UV intensity) and other dependencies describe discrete state transitions (e.g. fixture removal during curing), the resulting model of the overall bonding process is a hybrid dynamic system in the general case. For this plant model, we then design and evaluate hybrid controllers with the potential to optimize process control for a selection of assembly steps, thus improving the repeatability of related production steps like beam-shaping optics, mounting of turning mirrors for fiber coupling or building resonators evaluating power, mode characteristics and beam shape.

9730-16, Session 4

980nm diode laser pump modules operating at high temperature

Jenna Campbell, Freedom Photonics, LLC (United States); Tadej Semenic, Teledyne Scientific Co . (United States); Paul O . Leisher, Rose-Hulman Institute of Technology (United States); Avijit Bhunia, Teledyne Scientific Co . (United States); Milan Mashanovitch, Daniel Renner, Freedom Photonics, LLC (United States) Existing thermal management technologies for diode laser pumps place a significant load on the size, weight and power consumption of High Power Solid State and Fiber Laser systems, thus making it impossible for these laser systems to be used in many important practical applications. This problem is being addressed by the team formed by Freedom Photonics and Teledyne Scientific through the development of novel high power laser chip array architectures that can operate at temperatures higher than 50 degrees Celsius with high efficiency and also the development of an advanced thermal management system for efficient heat extraction from the laser chip array. This paper will present experimental results for the optical, electrical and thermal characteristics of 980 nm diode laser pump modules operating effectively with liquid coolant at a temperature of 50 degrees Celsius, showing a very small change in performance as the operating temperature increases from 20 to 50 degrees Celsius. These pump modules can achieve output power of many Watts per array lasing element with an operating Wall-Plug-Efficiency (WPE) of 50% and higher, at elevated temperatures. The paper will also discuss the technical approach that has enabled this high level of pump module performance and opportunities for further improvement.

9730-15, Session 4

Approaching improved adhesive bonding repeatability

Christian Schlette, Institute for Man-Machine Interaction (Germany); Tobias Müller, Daniel Zontar, Fraunhofer Institut für Produktionstechnologie IPT (Germany); Jürgen Rossmann, Institute for Man-Machine Interaction (Germany); Christian Brecher, Fraunhofer-Institut für Produktionstechnologie IPT (Germany)

9730-43, Session PTue

Efficient ultrafast fiber laser using chirped fiber Bragg grating and chirped volume Bragg grating stretcher/compressor configuration

Andrejus Michailovas, EKSPLA uab (Lithuania); Saulius Frankinas, Nerijus Rusteika, EKSPLA uab (Lithuania) and Ctr . for Physical Sciences and Technology (Lithuania);

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9730-44, Session PTue

Conference 9730: Components and Packaging for Laser Systems II

Vadim Smirnov, Ruslan Vasilyeu, Alexei L . Glebov, OptiGrate Corp . (United States) The power scaling of ultrafast fiber laser is restricted due to nonlinear pulse distortion in fiber. Hence sufficient pulse stretching is necessary to reduce pulse peak power and nonlinear effects. To achieve high pulse energy (~10 µ J) stretched pulse duration is typically in the range of 100ps – 1ns and the physical size of the matched diffraction grating compressor is large. An alternative technology for large-sized diffraction grating compressor is pulse compression based on large aperture chirped volume Bragg grating (CVBG). In the typical schemes CVBG is used as a pulse stretcher and a compressor, however this configuration requires additional dispersion compensation to eliminate the residual normal dispersion of the fiber. As the dispersion of CVBG cannot be easily tuned, compensation of the dispersion in the system is cumbersome and requires cutting back some part of the fiber to obtain shortest pulse.

In this work we present a novel chirped pulse amplification (CPA) configuration consisting of chirped fiber Bragg grating (CFBG) with tunable dispersion as pulse stretcher and CVBG as pulse compressor. Both CFBG and CVBG are written with linear chirp profile so that total dispersion of the system, including the fiber, is close to zero. In addition dispersion profile of CFBG can be fine tuned by inducing thermal gradient along the grating so that an optimization of the duration of the output pulses can be performed. We demonstrate that using this stretcher/ compressor configuration pulses can be stretched up to 300 ps and compressed to sub-1ps without residual pedestal.

Radiation pressure based laser power measurements using photonic microcantilevers

Lewis G . Carpenter, Christopher Holmes, Peter A . Cooper, James C . Gates, Corin B . E . Gawith, Peter G . R . Smith, Univ . of Southampton (United Kingdom)

9730-45, Session PTue

Machine platform and software environment for rapid optics assembly process development

Sebastian Sauer, Sebastian Haag, Tobias Müller, Daniel Zontar, Christian Brecher, Fraunhofer-Institut für Produktionstechnologie IPT (Germany) The assembly of optical components for laser systems is proprietary knowledge and typically done by well-trained personnel in clean room environment as it has major impact on the overall laser performance. Rising numbers of laser systems drives laser production to industrial level automation solutions allowing for high volumes by simultaneously ensuring stable quality, lots of variants and low cost. Therefore, an easy programmable, expandable and reconfigurable machine with intuitive and flexible software environment for process configuration is required.

Fraunhofer IPT demonstrated the automated ultra-precise alignment of optical systems in previous publications with an approach to the fully automated assembly. With that expertise on optical assembly processes, Fraunhofer IPT made the next step towards industrializing the production of optical systems. For these purposes Fraunhofer IPT integrated its micromanipulator technology into an automatic assembly machine and focused on modularizing both mechanical and software components. The platform war designed and approved to enable laser companies with low expertise and resources in mechanical engineering and industrial control to transfer manual assembly processes to automated or semi automated processes. Alongside with reduced production costs, higher yields and reduced cycle times company internal process knowledge about measurement setups or alignment strategies as well as bonding remains proprietary. Crucial to enable rapid automation of complex processes by non automation experts is an intuitional software user interface for control development. Furthermore, the laser experts must be capable of integrating well-established laboratory equipment such as measurement systems into the control environment of the industrial production process. We demonstrate 300 mW level laser power measurements utilising radiation pressure induced microcantilever deflection. Our approach incorporates a channel waveguide within a microcantilever, representing a low cost, fully integrated optomechanical power measurement. This approach has the benefit of fast response times (kHz) in comparison to thermal detectors and we are able to operate our devices over a wider wavelength range (i.e. limited by the reflectivity of gold) than traditional semiconductor detectors. Our microcantilever devices consist of a reflecting mirror fabricated on a microcantilever; embedded within the cantilever is a channel waveguide with a series of Bragg gratings that can measure cantilever deflection and temperature simultaneously. The ability to interrogate cantilever deflection and temperature allows for the disentanglement of radiation pressure and absorbed light. Cantilever deflection and device temperature can be measured simultaneously by monitoring shifts in Bragg grating central wavelengths by monitoring Bragg gratings in areas of high strain within the cantilever and within bulk areas of the same chip, respectively. In previous demonstrations we have shown mass and temperature sensitivities of 1 µ g and 0.03 °C respectively. We will present our latest results on the use of these microcantilevers for high sensitivity radiation pressure based power measurements. We are currently investigating the suitability of these devices for measuring beyond watt level laser sources and will discuss the challenges associated with this.

9730-46, Session PTue

Sub-nanosecond passively Q-switched monolithic green laser

Lihe Zheng, Takunori Taira, Institute for Molecular Science (Japan) A sub-nanosecond passively Q-switched monolithic green laser with laser-head size of 35 x 35 x 35 mm3 was developed for laser processing on organic superconducting transistor with a flexible substrate. A composite YAG/Nd:YAG/Cr4+:YAG/YAG crystal was used for generating fundamental wavelength at 1064 nm. The gain medium was a [100]-cut Nd:YAG crystal. [110]-cut Cr4+:YAG crystal with initial transmission of 30% was used as saturable absorber. Undoped YAG was used as heat sink. The output coupler with transmission of 50% was coated on the end surface of Cr4+:YAG crystal. The stable passively Q-switched 1064 nm laser was operated without additional cooling. A fiber-coupled 806.6 nm, qCW laser diode with output power of 30 W and pump pulse duration of 245 energy of 111 µ JST (Japan Science and Technical Agency) for this work.

µ s was used. The pump beam was focused into 1.1 mm spot with the help of coupling lenses. A LBO crystal (? = 90°,?= 11.4°) with size of 5? 5 ? 10 mm3 lead to output J, pulse duration of 560.8 ps and peak power of 198kW at 532 nm. A cartridge heater was used to maintain the LBO crystal at fixed temperature of 29.8 ? for second harmonic generation. Further experiments will focus on the angle dependence between [100]-cut Nd:YAG and [110]-cut Cr:YAG for higher output energy. We acknowledge the support of SENTAN,

78 SPIE Photonics West 2016 · www.spie.org/pw

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Conference 9730: Components and Packaging for Laser Systems II

9730-47, Session PTue

Optical attachment for spatial transformation of excimer laser beam

Alexsandr S . Grishkanich, Alexsandr Zhevlakov, ITMO Univ . (Russian Federation); Egor Gavrilov, Lasers and Optical Systems, J .S .C . (Russian Federation) and Institute for Laser Physics (Russian Federation); Sergey Kascheev, ITMO Univ . (Russian Federation); Veli Kujanpää, VTT Industrial Systems (Finland); Timo Savinainen, VTT Industrial systems (Finland) and VTT Industrial Systems (Finland) approach for true X/Y scanning at line speeds of 200 meters per second. Utilizing an innovative approach which incorporates both high speed/ high precision polygon mirrors working in concert with galvanometers, an integrated scan head which accommodates almost all wavelengths at speeds up to 200 meters per second is now possible.

9730-17, Session 5

Beam shaping concepts for kW-class CW and QCW diode lasers

(Invited Paper)

Andreas Unger, Willhelm Fassbender, Holger Müntz, Bernd Köhler, Jens Biesenbach, DILAS Diodenlaser GmbH (Germany) The divergence value difference in two directions of discharge excimer laser beam reduces the energy concentration in plane of DUV radiation focusing as well as in the image localization plane. The goal of this work was the shaping a laser beam with enhanced homogeneity of intensity distribution in transverse section and the same divergence in two orthogonal directions with respect to the beam axis. An optical attachment designed and manufactured is intended for the DUV laser beam divergence equalization required in the various material micromachining and technologies of surface radiation modification including IC manufacture. KrF laser beam with orthogonal section of 3 x 20 mm and divergence of 2 x 5 mrad has been respectively conversed to one with a square area of 20 x 20 mm and divergence of 5 x 5 mrad.

9730-48, Session PTue

Polarization and wavelength insensitive optical feedback control systems for stabilizing CO2 lasers

Mohamed Amine Jebali, AFL (United States) Power scaling of multi-kilowatts fiber laser has been driving the development of glass and fiber processing technology. Designed for large diameter fibers, this technology is used for the fabrication of fiber-based components such as end-pump and side pump combiners, large diameter encaps ,ball lenses for collimators and focusers… The use of 10.6um CO2 lasers as heating element has proven uncomparable flexibility, process control and repeatability when compared to conventional heating methods. This low maintenance technology provides an accurate, adjustable and uniform heating area by absorption of fused silica of the 10.6m laser radiation. However, commercially available CO2 lasers often experience some power, polarization and mode instability, that becomes important at 20W output power and higher. In the present paper we present a polarization and wavelength insensitive optical feedback control system for stabilizing commercially available CO2 lasers. Less than 1% power fluctuation was achieved at different laser power levels, ranging from as 5 to 40W.

9730-49, Session PTue

High speed printing with polygon scan heads

Glenn Stutz, Lincoln Laser Co . (United States) With the recent advent of pulse on demand (PoD) Femtosecond and Picosecond high speed lasers, industry applications as widespread as medical, industrial and manufacturing are now seeking to take advantages of the economies of scale that these technologies offer. Applications include cataract surgery, high speed printing on plastic packaging, OCT, fine line ablation and cutting and high speed manufacturing processing to name a few.

This presentation provides an overview of the leading edge scan head technology which allows users to implement an integrated total solution In modern diode lasers beam shaping of the highly asymmetric laser beam, which exits the front facet of the semiconductor laser material, is a crucial step towards cost efficient high brightness laser modules which in turn can be further combined towards kW-class diode lasers and can be efficiently fiber coupled. In order to scale up the power of a single laser module in an economic way, high fill factor laser bars are employed. The increased power density from such a laser bar requires improved cooling technologies. On the other hand the increased fill factor of the bar makes advanced beam shaping necessary to be able to achieve small focal spot sizes and couple the laser module efficiently into optical fibers. Finally, to be able to mass produce the laser modules, it is desirable to design the module in a way that allows automated packaging and optics alignment.

In this talk, the beam shaping concepts developed at DILAS for high fill factor bars are presented. Starting from optical simulation and choice of optical elements the laser modules incorporating these bars are presented. The concepts developed enable very compact laser modules of up to 2kW of power at a single wavelength with beam qualities of less than 40mm x mrad. Optionally these modules can be wavelength stabilized via external feedback. The packaging technology developed enables the automated alignment of the optics and cooling is DI-water free. Based on the same concepts very compact free space and fiber coupled QCW packages are presented as well.

9730-18, Session 5

Packaging of wavelength stabilized 976nm 100W 105

µ

m 0.15 NA fiber coupled diode lasers

Xiaochen Jiang, Rui Liu, Yanyan Gao, Tujia Zhang, Xiaoguang He, Jing Zhu, Qiang Zhang, Thomas C . Yang, Cuipeng Zhang, BWT Beijing Ltd . (China) Fiber coupled diode lasers are widely used in many fields now especially as pumps in fiber laser systems. In many fiber laser applications, high brightness pumps are essential to achieve high brightness fiber lasers. Furthermore, 976nm wavelength absorption band is narrow with Yb3+ doped fiber lasers which is more challenging for controlling wavelength stabilized in diode laser modules. This study designed and implemented commercial available high brightness and narrow wavelength width lasers to be able to use in previous mentioned applications. Base on multiple single emitters using spatial and polarization beam combining as well as fiber coupling techniques, we report a wavelength stabilized, 105?m NA 0.15 fiber coupled diode laser package with 100W of optical output power at 976 nm, which are 14 emitters inside each multiple single emitter module. The emitting aperture of the combined lasers output are designed and optimized for coupling light into a 105?m core NA 0.15 fiber. The spectral width is roughly 0.5 nm (FWHM) and the wavelength shift per °C < 0.02nm. The output spectrum is narrowed and wavelength is stabilized using Volume Bragg gratings (VBGs). The high brightness package has an electrical to optical efficiency better than 45% and power enclosure more than 90% within NA 0.12.

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Conference 9730: Components and Packaging for Laser Systems II

A reliability study has been included on this kind of hermetic package. Mechanical shock, vibration and accelerated aging tests show that the package is reliability and the MTTF is calculated to be more than 100k hours at 25?.

power out of a 400 devices.

µ m fiber and 0.18 NA beam, compared to the current generation elementTM products, while also improving the reliability by an estimated 20%. Additionally, we will report on the improvements in VBG locking, where we have increased the wavelength stabilized range from 6 A for our current generation devices, to a 15 A range for next generation

9730-19, Session 5

Ultrashort pulse written volume-Bragg gratings in fused silica for external stabilization of diode lasers with ultra-low spectral-drift

Daniel Richter, Christian Voigtländer, Jens U . Thomas, Ria G . Krämer, Friedrich-Schiller-Univ . Jena (Germany); Hagen Zimer, TRUMPF Laser GmbH (Germany); Andreas Tünnermann, Fraunhofer-Institut für Angewandte Optik und Feinmechanik (Germany) and Friedrich-Schiller-Univ . Jena (Germany); Stefan Nolte, Friedrich-Schiller-Univ . Jena (Germany) and Fraunhofer-Institut für Angewandte Optik und Feinmechanik (Germany) We report on reducing the power dependent drift of externally stabilized diode lasers emitting around 969 nm by employing ultrashort pulse written Volume-Bragg-Gratings (VBGs) in fused silica. Spectrally stabilized sources are of special interest for a large variety of applications. One example is the use of high power laser diodes for optical pumping of lasers with a very narrow peak of the absorption cross section. Robust external stabilization of the laser diode source can be obtained using a VBG. Common VBGs are typically written with UV light and post processed into special photo thermal-refractive materials. However, these devices suffer from a small residual absorption, which typically leads to a shift of about 200 pm for an optical load change of 50W passing a 10mm2 facet of the grating.

Here we report on applying ultrashort laser pulses and the phase mask scanning technique to directly inscribe VBGs in fused silica. The gratings realized exhibit comparable grating geometries and reflectivities as the conventional VBGs. However, the residual drift of only 20 pm which is one order of magnitude lower and the best value obtained so far to the best of our knowledge. We will present detailed information about the inscription process as well as the grating details and we will outline pathways for even further improvement.

9730-20, Session 5

Performance improvements to wavelength stabilized high power 885nm diode laser modules

David M . Hemenway, Ling Bao, Manoj Kanskar, Mark DeVito, Wolfram Urbanek, Mike P . Grimshaw, Kevin Bruce, David Dawson, Robert Martinsen, nLIGHT Corp . (United States); Paul O . Leisher, Rose-Hulman Institute of Technology (United States)

9730-21, Session 6

Integration and performance of graphene modulators in Er fiber frequency combs

(Invited Paper)

Martin E . Fermann, IMRA America, Inc . (United States) No Abstract Available

9730-22, Session 6

Efficient crystalline fiber lasers and broadband emissions

(Invited Paper)

Shih-Chang Wang, Teng-Yi Yang, Dong-Yo Jheng, Chun Yang Hsu, Tzu-Te Yang, National Taiwan Univ . (Taiwan); Pinghui S . Yeh, National Taiwan Univ . of Science and Technology (Taiwan); Sheng-Lung L . Huang, National Taiwan Univ . (Taiwan) Heat dissipation limits the high power performance of various solid state laser sources. Drawing solid-state gain media into crystalline fibers significantly increases the surface area (for heat removal) to gain volume (for power generation) ratio. The core diameter can be as small as 3 ?m to tens of microns with high crystallinity. With waveguide structure using glass or ceramic claddings, the mode quality is ensured and the propagation loss is reduced. Various widely used solid-state laser crystals, such as Ti:sapphire and YAGs doped with Ce3+, Yb3+, Cr4+ ions, have been demonstrated as tunable crystalline fiber lasers (> 150-nm tuning range), high slope efficiency (> 75%), and high-brightness broadband light emitters. The clad-pump scheme used in high power silica fiber lasers have also been applied on crystalline fiber light sources for power scaling and efficient laser diode coupling. The recent advancement on the codrawing laser-heated pedestal growth technique on broadband emissions have shown superiority on the application for optical coherence tomography where broadband, high brightness, CW, and Gaussian spectral shape are preferred for in vivo and real-time diagnosis with cellular resolution.

The challenges on the development of glass/ceramic-clad sapphire and YAG fibers, including core diameter uniformity, dopant segregation, residual strain, post-growth thermal treatment, and the thermal expansion coefficient mismatch between the crystalline core and glass/ceramic clad, will be addressed in the presentation. The present status on the development of single-mode crystalline fibers using high index glass or ceramic as the fiber clad will also be discussed.

There is an increasing demand for high power diode lasers packages with stabilized wavelengths in the range of 878 nm to 888 nm for DPSS laser pumping applications. In this paper we present nLIGHT’s most recent development of wavelength-stabilized high power, single emitter laser diode packages, elementTM , for DPSS laser pumps. We will report on how we have scaled single emitter power from 10 W per emitter with our prior generation of 200 µ m wide and 3.8 mm long devices to 13 W per emitter for next generation of 5 mm cavity length device for 200 µ m and 0.22 NA fiber products. Furthermore, we will also discuss results for 15 W per emitter devices for 400 um and 0.22 NA fiber products. The improvement in power at the chip-on-submount level results in approximately 50% increase in

9730-23, Session 6

Modeling and optimization of multimode fused fiber combiners

Yu Liu, Hao Yu, Politecnico di Torino (Italy); Alessio Califano, OPI Photonics s .r .l . (Italy); Andrea Braglia, Politecnico di Torino (Italy) and OPI Photonics (Italy); Guido Perrone, Politecnico di Torino (Italy) Fused fiber combiners are essential components for the realization of alignment free, compact and robust all-fiber systems for communications,

80 SPIE Photonics West 2016 · www.spie.org/pw

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Conference 9730: Components and Packaging for Laser Systems II

industrial material processing, medicine, sensing, etc. In particular, in high power applications, combiners are used in fiber and direct diode lasers to scale up the power by merging the output of several lower power sources, and this poses stringent requirements on the coupling efficiency and on the acceptable beam quality degradation. This, not only to avoid wasting power, but also to prevent overheating for reliability issues. Moreover, combiners used in different systems, have peculiar requirements and thus necessitate proper tailoring of the design and fabrication procedures. The usual path to optimize combiner performance is by trial-and-error methods, but this is costly and time-consuming; furthermore, care must be taken in their characterizations since measurements are usually affected by working conditions (e.g. the laser diodes). To help overcoming these drawbacks, we have developed a procedure to predict the efficiency of combiners before manufacturing, therefore allowing the fabrication of high performance devices with reduced experimental optimization costs. The proposed approach is based on statistical analysis applied to ray tracing simulations that takes into account fiber types, geometries, glass material properties and required beam quality. Comparison between predictions and experimental realizations of 7-to-1 and 19-to-1 combiners are given, analyzing the impact of taper length, taper ratio, confining tube material and diode numerical aperture, in both of single- and double-clad delivery fibers. L . Dallas, Avo Photonics, Inc . (United States) Tunable Diode Laser Absorption Spectroscopy (TDLAS) has become an industrial-accepted analytical technique for accurate and precise measurement of specific gas components in complex gas mixtures, often laden with liquids or solid particulates. Examples include measuring the moisture content of natural gas within pipelines or in the airflow of heat treatment furnaces, and oxygen content in hydrocarbon vapors such as oil, liquid fuel, and natural gas storage tanks. A key virtue if TDLAS is that the analyte need not contact electrical components; only the low-power laser beam needs to probe the sample. Thus, the technique is suitable for use in potentially explosive hazardous area installations. Particularly useful is the backscatter configuration, wherein the laser beam projects from a transceiver into the sampling area and scatters from a non-cooperative surface within the area. The portion of the scattered light returned to the transceiver contains information from which analyte concentration is deduced.

Emerging applications for this technology, e.g., measuring oxygen in vehicle or aircraft fuel tanks, demand high-volume manufacturing of low-cost, compact and lightweight transceivers. This paper describes the design of such a hermetically-sealed package, occupying a volume less than 1 in3 and weighing less than 0.05 lb. Estimated costs for this critical system component are less than $1000 per unit when produced in lots of 500, approximately a 5x reduction compared to the current state-of-the-art.

9730-24, Session 6

Key optical components for spaceborne lasers

Jens Löhring, Matthias Winzen, Heinrich Faidel, Jörn Miesner, Heinz-Dieter Plum, Jürgen Klein, Oliver Fitzau, Martin Giesberts, Wolfgang Brandenburg, Anja Seidel, Natascha Schwanen, Dana Risters, Hans-Dieter Hoffmann, Fraunhofer-Institut für Lasertechnik (Germany) Spaceborne lidar systems have a large potential to become powerful instruments in the field of atmospheric research. Obviously, they have to be in operation for some years without any maintenance like readjusting. Furthermore, they have to withstand strong temperature cycles typically in the range of -30 to +50 °C as well as mechanical shocks and vibrations, especially during launch. Additionally, the avoidance of any organic material inside the laser box is required, particularly in UV lasers.

For atmospheric research pulses of about several 10 mJ at repetition rates of several 10 Hz are required in many cases. Those parameters are typically addressed by DPSSL that comprise components like: laser crystals, nonlinear crystals in pockels cells, faraday isolators and frequency converters, passive fibers, diode lasers and of course a lot of mirrors and lenses. In particular, some components have strong requirements regarding their tilt stability that is often in the range of <10 µ rad.

In most of the cases components and packages that are used for industrial lasers do not fulfil all those requirements. Thus, the packaging of all these key components has been developed to meet those requirements only making use of metal and ceramics beside the optical component itself. All joints between the optical component and the laser baseplate are soldered or screwed. No clamps or adhesives are used. Most of the critical properties like tilting after temperature cycling have been proven in several tests. Currently, these components are used to build up first prototypes for spaceborne systems.

9730-25, Session 7

Cost-effective manufacturing of compact TDLAS sensors for hazardous area applications

(Invited Paper)

Michael B . Frish, Mathew C . Laderer, Clinton J . Smith, Physical Sciences Inc . (United States); Ryan Ehid, Joseph

9730-26, Session 7

Multi-100W class, fully integrated, monolithic ytterbium-doped photonic crystal fiber amplifier module

(Invited Paper)

Johan Boullet, ALPhANOV (France); Germain Guiraud, Lab . Photonique, Numérique et Nanosciences (France) and Azur Light Systems (France); Giorgio Santarelli, Lab . Photonique, Numérique et Nanosciences (France); Cyril Vincont, Simon Salort, Christophe Pierre, ALPhANOV (France) Extreme core dimensions of active photonic crystal fiber (PCF) has increased by one order of magnitude the capabilities of ultrafast fiber lasers: hundred watts of average power, multi-gigawatt of peak power are now delivered by fiber based systems.

Anyway, intrinsic characteristic of PCF make development of fully monolithic architecture extremely challenging: as air holes ensure the guiding properties of the fiber, fundamental glass processing operations such as cleaving or splicing while conserving extreme wave-guiding properties of the active medium remain tricky points to address.

PCF amplifiers are then traditionally used in free space configurations, consequently losing the strength of the fiber technology, i.e its high potential of integration in fully monolithic systems.

We have developped a fully monolithic PCF amplifier module based on the largest core flexible single-moded amplifying fiber on the market, the DC 200/40-PZ-Yb of NKT-photonics. The developed special combiner allows us to couple 6 pumps of 50 W and 5 W of signal at 1064 nm in the PCF fiber. We produced 210 W of average power at 1064 nm in both single frequency and pulsed (100 ps/100 kW) regime, which is the highest power ever delivered by a monolithic PCF amplifier. The stainless steel output patchcord of the module is ended by high power SMA connector including mJ-class fused silica endcaps. PCF assembly is integrated in a thermally optimized, rugged module and was free runned for more than 25 days at > 100W average power with an excellent peak to peak power stability < 1%.

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Conference 9730: Components and Packaging for Laser Systems II

9730-27, Session 7

Optical packaging for downhole photometry

(Invited Paper)

Jess V . Ford, Weatherford International Ltd . (United States); Tom Haslett, Avo Photonics, Inc . (United States)

9730-29, Session 8

Hybrid high power fs lasers

(Invited Paper)

Bojan Resan, Lumentum (Switzerland) and Univ . of Applied Sciences and Arts Northwestern (Switzerland) No Abstract Available Black Gold, Texas Tea, fossil fuel, these are just a few euphemisms for the crude oil, a key commodity driving the economies of the world. As the quest for oil production continues worldwide, more and more emphasis is placed on the need to characterize the crude oil in situ before expensive production equipment is built at the well site. This requires the development of sensor technology that withstands the very harsh environments of the wellbore. One area of active development is termed downhole fluid analysis, which is the implementation of optical spectroscopy and fluorimetry in the wellbore during exploration and development activities. This requires that the entire optical system, light sources, photodetectors, routing optics and control electronics, be transported to the measurement location either in a logging while drilling tool string or conveyed using wireline. This paper examines how a 20 channel photometric analyzer was packaged to address the thermal (up to 177°C), pressure (up to 20 kpsi) and vibration (up to 4.5 G) requirements dictated by the application. Specific emphasis will be placed on the optical component packaging and then the integration required for the entire assembly to meet the operational requirements. Data will be presented illustrating operation of the system at the elevated temperatures required.

9730-28, Session 7

Model-based adhesive shrinkage compensation for increased bonding repeatability

Tobias Müller, Sebastian Haag, Daniel Zontar, Sebastian Sauer, Christian Brecher, Fraunhofer-Institut für Produktionstechnologie IPT (Germany); Christian Schlette, Jürgen Rossmann, Institute for Man-Machine Interaction (Germany)

9730-30, Session 8

Small structures, big impact! High performance diffraction gratings for laser applications

Frank Fuchs, Gitterwerk GmbH (Germany) Diffraction gratings are essential components of many modern pico- and femtosecond laser systems, with the most prominent application in the framework of chirped pulse amplification (CPA). Over the last years these laser systems have been introduced very successfully into many industrial applications, especially related to materials processing of various kind. Driven by the demands of the market the employed laser sources are under constant development and optimization. In this context the optimization of the grating components is of pronounced interest due to their direct impact on the laser performance. While the geometrical aspects of gratings are well understood and easily calculated, the diffraction performance has to be modeled carefully in close feedback to the manufacturing. Sadly this leaves the user out of the loop, and without good design rules for the compressor layout. Even worse textbook-knowledge about diffraction gratings mostly neglects the advances in grating design and manufacturing of the last decade(s). In this contribution we provide a user oriented overview about the possibilities of modern grating design and manufacturing for high line density gratings. Both transmissive and reflective devices are discussed at the example of typical applications. We derive and provide design rules as a guidance for the grating user.

The assembly process of optical components consists of two phases – the alignment and the bonding phase. In the alignment phase, the limitation of the precision is given by the measurement equipment and the manipulation technology applied. Today’s micromanipulators in combination with beam imaging setups allow for an alignment in the range of far below 100nm. However, precisely aligned optics need to be fixated in their position. State of the art in optics bonding for laser systems is adhesive bonding with UV-curing adhesives. Adhesive bonding is a multi-factorial process and subject to statistical process deviations. Thus, process repeatability is decided and limited in the bonding phase. .As matters of fact, UV-curing adhesives inherit shrinkage effects during their curing process, making offsets for shrinkage compensation mandatory. Enhancing the process control of the adhesive bonding process is the major goal of the activities described in this paper. To improve the precision of shrinkage compensation a dynamic shrinkage prediction is envisioned by Fraunhofer IPT. Intense research activities are being practiced to gather a deeper understanding of the parameters influencing adhesive shrinkage behavior. These effects are of various nature – obviously being the raw adhesive material itself as well as its condition, the bonding geometry, environmental parameters like surrounding temperature and of course process parameters such as curing properties. Understanding the major parameters and linking them in a model-based shrinkage-prediction environment is the basis for improved process control. Results are being deployed by Fraunhofer in prototyping, as well as volume production solutions for laser systems.

9730-31, Session 8

Integrated pulse stretchers for high-energy CPA and OPCPA systems

Lawrence Shah, Nathan Bodnar, Joshua Bradford, Benjamin Webb, Jess Lane, Michael Chini, Martin Richardson, Univ . of Central Florida (United States) Pulse duration and dispersion control are critical in high-energy chirped pulse amplification (CPA) systems. Practically, pulse compressor design is determined by the center wavelength and bandwidth of the system, the output beam size, and the chirped pulse duration. As such, the pulse stretcher must be designed to compensate the dispersion of the compressor as well as that of any optics in the amplifier path. This work will describe efforts to design and incorporate an integrated Bragg grating stretcher into a Ti:Sapphire system designed to produce 100 fs pulses with 5 TW peak power after compression. The compress is a conventional Treacy design consisting of two 1800 l/mm diffraction gratings providing -10 ps2 group delay dispersion (GDD) and +1 ps3 third order dispersion. In this application, the integrated Bragg grating must provide positive GDD and negative third order dispersion. Similar pulse stretching techniques have been used in relatively low energy fiber and solid-state systems, but to our knowledge this is first example of this technique in a TW class system. This work will also discuss the use of similar integrated Bragg gratings to stretch the pulse duration of pump lasers for optical parametric CPA (OPCPA). In each case, packaging the stretcher as an integrated device greatly reduces system footprint and alignment complexity and highlights further opportunities for integration in high-energy ultrashort pulse lasers.

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9730-32, Session 8

Conference 9730: Components and Packaging for Laser Systems II Efficient chirped Bragg gratings for stretching and compression of high power ultra short laser pulses at 800-2500 nm

Vadim Smirnov, Eugeniu Rotari, Ruslan Vasilyeu, Oleg Smolski, Alexei L . Glebov, Leonid Glebov, OptiGrate Corp . (United States)

9730-35, Session 9

Engineering of freeform refractive and reflective slow axis collimation solutions

Martin Forrer, Hansruedi Moser, Hans Forrer, Dzelal Kura, FISBA AG (Switzerland) Stretchers and compressors based on volume chirped Bragg gratings (CBGs) in photo thermo refractive (PTR) glass allowed design of compact Ultra Short Pulse Laser (USPL) systems with high pulse energy and average power. The recent advances in CBG fabrication technologies enabled CBGs with spectral bandwidth exceeding 40 nm and diffraction efficiencies as high as 90%. The improvements in CBG bandwidth and efficiency broaden substantially their application range for USPL systems in the whole spectral region from 800 to 2500 nm. This paper will overview recent progress in the technology and applications of CBGs in PTR glass. The engineering standard for slow axis collimation today is a spherical plano convex collimator lens. With increasing quality and assembly requirements, freeform optics are promoted for specific advantages. With the industrial requirement for scaling up to high quantities, the industrial production means have to be qualified for scalability and repeatability With the technology of precision glass molding a versatile production tool allows for high quality optical solutions. The results presented include optical design comparison of spherical to aspherical refractive and reflective slow axis collimation on a standard diode laser single emitter situation and the achievable component production quality.

9730-33, Session 8

Broadband 7 microns OPCPA pumped by a 2 microns picosecond Ho:YLF CPA system

Christophe Simon-Boisson, Olivier Chalus, Thales Optronique S .A .S . (France); Daniel Sanchez, Michael Hemmer, Matthias Baudisch, Seth Cousin, ICFO - Institut de Ciències Fotòniques (Spain); Jens Biegert, ICFO - Institut de Ciències Fotòniques (Spain) and Institució Catalana de Recerca i Estudis Avançats (Spain); Kevin Zawilski, Peter G . Schunemann, BAE Systems (United States); Vadim Smirnov, OptiGrate Corp . (United States); Heinar Hoogland, Ronald Holzwarth, Menlo Systems GmbH (Germany) Here we report an OPCPA system generating high energy pulses centered at 7 ?m with a spectrum supporting a < 90 fs pulse duration, corresponding to sub-4 optical cycles. The OPCPA consists of three OPA stages which are pumped by an optically synchronized home built Ho:YLF picosecond CPA system operating at 2 ?m wavelength, both seeded by beams generated by the same multicolor fiber oscillator which deliver 3 outputs, one from the initial Er fiber oscillator and 2 outputs around 2 ?m after generation of a supercontinuum from the oscillator in a nonlinear fiber, 1 broadband to be mixed through DFG in a CSP crystal with output 1 to produce the MWIR seed of the OPCPA and 1 narrowband to seed the 2 ?m pump laser. This pump laser is a CPA using Chirped Volume Bragg Gratings for both stretcher and compressor. Active medium is Holmium doped YLF pumped by a 100 W cw 1.9 ?m Thulium fiber laser. A room temperature regenerative amplifier using a RTP Pockels cell delivers above 4 mJ whereas a cryogenically cooled amplifier has reached 40 mJ then compressed to 35 mJ with 11 ps pulse duration. To date, we have generated up to 550 ?J of energy around 7 ?m using only 12 mJ of our pump split into the 3 ZGP based OPCPA stages. The pump laser currently delivers up to 35 mJ at 100 Hz, implying straightforward scalability of the OPCPA to the mJ level in the near future

9730-36, Session 9

Wavefront distortion of laser beams corrected with volume Bragg gratings in photothermorefractive glass

Fan Gao, Xiaojie Sun, Jing Hu, Xiang Zhang, Xiao Yuan, Soochow Univ . (China) The uniform near-field distribution and focusing characteristics of laser beams, which is closely connected with spatial frequencies and wavefront distortion of beams, are very important for high energy and power laser applications, such as laser processing and laser fusion, etc. The deformable mirror and spatial filter are used to compensate for the wavefront distortion and filter out the high frequencies, respectively. However, the medium and high distortions in laser beams are difficult to correct with the deformable mirror and spatial filter. The transmitting volume Bragg gratings (TBGs) has been used for angular filtering to clean the amplitude modulations of laser beams, and may be used to compensate for wavefront distortion.

In this paper, the wavefront characteristics of filtered beams through the TBG are discussed. The wavefront distortion corrected with TBG is demonstrated with a 1053 nm laser beam. The TBG used in the experiment has the angular selectivity of 1.3 mrad and period of 2?m. The wavefront distortion distribution of the source beam is modulated with a hard-edge aperture. The results show that the PV value and RMS value of the filtered beam are reduced from 0.293 ? to 0.169 ? and 0.054 ? to 0.032 ?, and the Strehl ratio of filtered beam is increased from 0.91 to 0.98. The focusing property of the output beam is improved, and the encircled energy of the beam is improved from 95% in 31.2 ?rad to 95% in 29.4 ?rad, which may be a simple and effective approach for high energy and power laser applications.

9730-37, Session 9

Customised low-angle refractive diffusers for high power laser applications

Matthew O . Currie, Paul Blair, Roy McBride, Eoin Murphy, PowerPhotonic Ltd . (United Kingdom)

9730-34, Session 9

Fiber laser beam combining for power scaling

(Invited Paper)

Eric C . Honea, Robert Afzal, Matthias Savage-Leuchs, Dan Hu, Craig Robin, Lockheed Martin Laser and Sensor Systems (United States) We describe the design and fabrication of smooth and continuous customized refractive diffusers for the beamshaping of high power lasers, specifically lasers in which a low diffusion angle is required. Traditional diffusers often have randomised surfaces with strong high frequency peaks in their spatial frequency content. However, these diffusers are not suitable for use in high power laser systems where diffraction angles can approach required diffusion angles. Diffraction angles which exceed diffusion angles present as overall system loss, which in high power laser systems may be unacceptable as they may cause component damage or catastrophic failure.

to be uploaded later.

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9730-38, Session 9

Conference 9730: Components and Packaging for Laser Systems II

We show that precise definition of the spatial frequency content of a surface over a lower frequency range allows for the maximisation of diffuser efficiency and performance, even at diffusion angles of only a few milliradians or less. Additionally, the image plane intensity distribution can be extensively shaped in order to obtain favourable processing conditions for numerous systems. By introducing a novel implementation of an Iterative Fourier Transform (IFT) algorithm, we present a method to design and subsequently fabricate highly customised beamshaping diffusers that retain maximum efficiency even when used at low diffusion angles.

Advanced centering of mounted optics

Christian Wenzel, Innolite GmbH (Germany) monolithic fiber spools; detachable and repeatable fiber collimators; low loss fiber-to-fiber coupling schemes; and high power fiber-coupled isolators.

9730-40, Session 10

Laser surface texturization for high power cladding light stripper

Claude Aguergaray, Catherine Lesaux, Léo Lebrun, Marc Faucon, Rainer Kling, Christophe Pierre, Johan Boullet, ALPhANOV (France) Camera objectives or laser focusing units consist of complex lens systems with multiple lenses. The optical performance of such complex lens systems is depended on the correct positioning of lenses in the system. Deviations in location or angle within the system directly affect the achievable image quality. To optimize the achievable performance of lens systems, these errors can be corrected by machining the mount of the lens with respect to the optical axis. The Innolite GmbH and Opto Alignment Technology have developed a novel machine for such center turning operation. A confocal laser reflection measurement system determines the absolute position of the optical axis with reference to the spindle axis. As a strong advantage compared to autocollimator measurements the utilized Opto-Alignment Technology is capable of performing centration and tilt measurements without changing objectives on any radius surface from 2 mm to infinity and lens diameters from 1 mm to 300 mm, including cylinder, aspheric, and parabolic surfaces. In addition, it performs significantly better on coated lenses.

The optical axis is skewed and offset in reference to the spindle axis as determined by the measurement. Using the information about the mount and all reference surfaces, a machine program for an untrue turning process is calculated from this data in a fully automated manner. Since the optical axis is not collinear with the spindle axis, the diamond tool compensates for these linear and tilt deviations with small correction movements. This results in a simple machine setup where the control system works as an electronic alignment chuck. Remaining eccentricity of <1 µ m and angular errors of < 5 sec are typical alignment results.

9730-39, Session 10

Packaging of fiber lasers and components for use in harsh environments

(Invited Paper)

Daniel Creeden, Benjamin R . Johnson, Casey W . Jones, Charles R . Ibach, Michael L . Lemons, Peter A . Budni, James P . Zona, Adam J . Marcinuk, Chris L . Willis, James Sweeney, Scott D . Setzler, BAE Systems (United States) High power continuous and pulsed fiber lasers and amplifiers have become more prevalent in laser systems over the last ten years. In fielding such systems, strong environmental and operational factors drive the packaging of the components. These include large operational temperature ranges, non-standard wavelengths of operation, strong vibration, and lack of water cooling. Typical commercial fiber components are not designed to survive these types of environments. Based on these constraints, we have had to develop and test a wide range of customized fiber-based components and systems to survive in these conditions. In this paper, we discuss some of those designs and detail the testing performed on those systems and components. This includes the use of commercial off-the-shelf (COTS) components, modified to survive extended temperature ranges, as well as customized components designed specifically for performance in harsh environments. Some of these custom components include: ruggedized/ High average power fiber-laser require today the development of a technology capable of transporting and managing such intense beams. Despite a conjoint improvement of their architectures, coupling light from one fiber to the next remain particularly challenging at high average power due to light leaking into the cladding that eventually destroys the fiber. Several techniques have demonstrated good performances to capture and dissipate this undesirable light. All of them aim at depositing a guiding material on the surface of the fiber that forces the light out. Amongst them, the deposition of silica particles and surface etching with hydrofluoric acid have shown the best performances. But both their implementation is difficult due to a complicated process to deposit the silica micro-particles or to the toxic chemicals needed. We propose here a simple technique based on laser surface texturization to create efficient, high-power, light-strippers. Using a UV laser, we directly micro-machine the surface of the fiber. We have stripped 98% of the light coupled in the cladding, up to several tens of Watts, and reduced the NA from 0.3 to 0.06. Thus demonstrating the superior ability of our technique to evacuate light with smaller NA. The process described herein can be used on any type of fiber, it is more efficient, very simple, hazard-free and perfectly repeatable. We will present the various depths and geometries we have tested together with our high power (>200W) tests. We will also present light-strippers machined with a 1030nm laser, alongside structural and mechanical analyses of the samples.

9730-41, Session 10

High power coatings for line beam laser optics of up to 2-meter in length

Mathias Mende, Jürgen Kohlhaas, Wolfgang Ebert, LASEROPTIK GmbH (Germany) Laser material processing plays an important role in the fabrication of the crucial parts for state-of-the-art smartphones and tablets. With industrial line beam systems a line shaped beam with a length above one meter and an average power of several thousand watts can be realized. To ensure excellent long axis beam homogeneity, demanding specifications regarding the substrate surface form tolerances and the coating uniformity have to be achieved for each line beam optic. In addition, a high laser damage threshold and a low defect density are required for the coatings. In order to meet these requirements, the MAXIMA ion beam sputtering machine was developed and built by LASEROPTIK.

This contribution describes the functional principle of MAXIMA deposition machine, which adapts the IBS technology with its highest coating quality to the field of large area deposition. Furthermore, recent developments regarding the process control by optical broadband monitoring are discussed. Finally experimental results on different thin film characteristics as for example the coating uniformity for plane and curved surfaces, the microstructure of multilayers and the laser damage resistance are presented.

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Conference 9730: Components and Packaging for Laser Systems II

9730-42, Session 10

Environmental durability of antireflective surface structures on silica windows for high energy lasers

Lynda E . Busse, Jesse A . Frantz, U .S . Naval Research Lab . (United States); Menelaos K . Poutous, Rajendra Joshi, The Univ . of North Carolina at Charlotte (United States); Ishwar D . Aggarwal, Sotera Defense Solutions, Inc . (United States); Jasbinder S . Sanghera, U .S . Naval Research Lab . (United States) High transmission as well as environmental durability of silica glass exit apertures are critical for their practical use in kW-level, high power laser systems. In actual land, sea and marine environments, these windows can, for example, be vulnerable to rain and sand erosion as well as salt/ fog conditions. These exit aperture windows have traditionally had anti reflective (AR) coatings applied to reduce surface reflections, but these coatings can suffer delamination and damage under adverse environmental conditions. An alternative approach is to directly etch anti-reflective surface structures (ARSS) on the windows, with no extraneous materials on the surface. We have demonstrated high transmission (> 99.9%T) and very high laser damage thresholds (100 J/cm2) at 1.06 µ m for silica windows with ARSS. In this paper we will present results for MILSPEC durability tests on silica windows both with and without ARSS that were conducted at a government facility, involving rain and sand erosion as well as salt fog testing. Results will be reported using a variety of test conditions, such as variable impact speeds and angles of incidence. In light of these results, we will discuss their utility as exit apertures in high energy laser systems.

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Conference 9731: Nonlinear Frequency Generation and Conversion: Materials, Devices, and Applications XV

Monday - Wednesday 15–17 February 2016 Part of Proceedings of SPIE Vol . 9731 Nonlinear Frequency Generation and Conversion: Materials, Devices, and Applications XV 9731-1, Session 1

Self-stabilized 3-5 OPO

µ

(Invited Paper)

m frequency comb based on frequency-divide-by-two GaAs

Kevin F . Lee, Christian Mohr, Jie Jiang, IMRA America, Inc . (United States); Peter G . Schunemann, BAE Systems (United States); Konstantin L . Vodopyanov, CREOL, The College of Optics and Photonics, Univ . of Central Florida (United States); Martin E Fermann, IMRA America, Inc . (United States) Degenerate optical parametric oscillators (OPOs) divide optical frequencies by two with only modest pumping. This makes them promising mid-infrared frequency comb sources. Our measurements show that our degenerate OPO preserves the frequency comb stability of the pump to sub-Hz levels. However, degenerate OPOs are often overlooked due to their interferometric cavity stabilization requirements. We find that the stability requirements of our system are actually much simpler, because thermal feedback results in self-stabilization. When the OPO is oscillating, absorption of the intracavity field increases the crystal temperature, and subsequently the effective cavity length, which is fortunately the right direction to stabilize degenerate oscillation in our system. Our OPO is based on an orientation-patterned GaAs crystal, pumped by a stabilized 2 W, 418 MHz, optically-referenced Tm frequency comb, generating a broadband, mid-infrared frequency comb centered at 4 comb.

µ m. We have observed continuous OPO oscillation for almost an hour without cavity length feedback. These measurements show that a degenerate OPO can serve as a simple device to downconvert a frequency applications including trace molecular detection and chemical sensing with massively parallel spectral data acquisition.

9731-3, Session 2

Miniature multioctave light source based on a monolithic microcavity

(Invited Paper)

Wei Liang, Anatoliy A . Savchenkov, OEwaves, Inc . (United States); James F . McMillan, Columbia Univ . (United States); Zhenda Xie, Univ . of California, Los Angeles (United States); Jan Burkhart, Columbia Univ . (United States); Vladimir S . Ilchenko, OEwaves, Inc . (United States); Chee Wei Wong, Univ . of California, Los Angeles (United States); Andrey B . Matsko, Lute Maleki, OEwaves, Inc . (United States) We have demonstrated generation of multi-octave CW optical spectrum in a micro-resonator optical parametric oscillator (OPO) pumped with milliwatt level CW light. The generated harmonics span from UV to near-IR. The generation is facilitated by linear interaction among different mode families in the resonator that phase matches the nonlinear frequency conversion. By selecting proper experimental conditions the system can efficiently generate different combinations of spectral harmonics separated by hundreds of THz. Because of the significant frequency difference between the OPO harmonics, it is possible to separate them efficiently using a diffraction grating. The system can be heterogeneously integrated as a chip scale device of very low volume and power consumption. It can be potentially used to generate quantum correlated photon pairs, or used as an ultra narrow linewidth source to seed high power laser at various visible wavelengths.

9731-2, Session 1

Octave-wide frequency comb centered at 4

µ

m based on a subharmonic OPO with Hz-level relative linewidth

Viktor O . Smolski, Jia Xu, CREOL, The College of Optics and Photonics, Univ . of Central Florida (United States); Peter G . Schunemann, BAE Systems (United States); Konstantin L . Vodopyanov, CREOL, The College of Optics and Photonics, Univ . of Central Florida (United States) We study coherence properties of a more-than-octave-wide (2.5-7.5 µ m) mid-IR frequency comb based on a 2 µ m Tm-fiber-laser-pumped degenerate (subharmonic) optical parametric oscillator (OPO) that uses orientation-patterned gallium arsenide (OP-GaAs) as gain element. By varying intracavity dispersion, we observed a ‘phase’ transition from a single-comb state (at exactly OPO degeneracy) to a two-comb state (near degenerate operation), characterized by two spectrally overlapping combs (signal and idler) with distinct carrier-envelope offset frequencies. We achieve this by generating a supercontinuum (SC) from the mode-locked Tm laser that spans most of the near-IR range, and observing RF beats between the SC and parasitic sum-frequency light (pump + OPO) that also falls into the near-IR. We found RF linewidth to be <15 Hz (a resolution of our spectrum analyzer), which proves that coherence of the pump laser comb is preserved to a high degree in a subharmonic OPO. Transition to a two-comb state was characterized by a symmetric splitting of the RF peak. Low pump threshold (down to 7 mW), high (73 mW) average power and high (up to 90%) pump depletion make this comb source very attractive for numerous

9731-4, Session 2

Generation of ultra-low-noise optical parametric combs

(Invited Paper)

Stojan Radic, Univ . of California, San Diego (United States) In contrast to conventional methods for frequency generation that rely on intense, pulsed source seeding of nonlinear process, new class of parametric techniques now allow for low-power generation over wide spectral ranges. To generate new optical frequency, it is necessary to induce efficient three- or four-photon process in selected physical platform. The efficiency of such process is defined by a product of interacting intensity, material nonlinearity and interaction length. A construction of long, lossless and dispersionless nonlinear waveguide can, at least in principle, alleviate the need to seed the frequency generation using a peak-intense source such as mode-locked laser. Indeed, if parametric mixer can be efficiently seeded by stabilized continuous-wave laser, then it can be expected that newly generated frequencies would be accompanied by qualitatively lower noise level. Recognizing this challenge, we describe the principle, construction and performance of CW-seeded frequency combs. We show that new device can generate power-equalized combs over hundreds of nanometers, while being seeded by telecom-grade (Watt-scale), rather than kWatt-scale pulsed sources. The ability to replicate CW source emission in traveling wave parametric mixer is identified as a basic advantage over conventional, resonance-based frequency generators. More importantly, this technique feature was recently leveraged towards the construction of low-noise, frequency-tunable combs that cannot be generated, even in principle,

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bay any comparable technique. The implications new technology will be illustrated with sensing and communication applications.

9731-5, Session 3

Conference 9731: Nonlinear Frequency Generation and Conversion: Materials, Devices, and Applications XV New generation of highly efficient ultrahigh power visible and UV fiber lasers

(Keynote Presentation)

Alexey Avdokhin, Valentin P . Gapontsev, IPG Photonics Corp . (United States) components: an Yb-fiber oscillator, a pulse stretcher, amplifiers and a pulse compressor. The output power of 2 mW from the oscillator was amplified up to 23 W through two Yb-doped fiber based pre-amplifiers and an Yb doped photonic crystal fiber based power amplifier. The pulse duration after compression was as short as 200 fs, and an intracavity pulse energy as high as 100 ?J was achieved. The resulting odd-order harmonics radiations extended down to 30 nm, and the 7th harmonic radiation centered at 149 nm had an average output power of up to 0.5 mW. The 7th harmonic radiation had a brilliance of 1.9?10^16(photons/s/mm^2/mrad^2/0.1%BW), which is comparable to a synchrotron radiation at the same wavelength. A computer controlled phase-lock system was able to maintain the intracavity phase-locking for more than 24 hours.

Remarkably successful commercial deployment of high power fiber laser technology has revolutionized many areas of laser applications by providing significantly more economical, efficient and compact solutions than the ones offered by alternative laser platforms. However, since most of the commercial fiber lasers operate in the infrared part of the spectrum, applications that required high power visible or UV radiation, for a long time still had to continue relying on gas lasers or solid state lasers with frequency conversion. Even though the process of frequency conversion of light generated by fiber lasers has been successfully employed for many years, due to physical limitations of the active fiber medium generation of high average power radiation in visible and UV spectral ranges has been a very challenging task. Only over the last years, with IPG launch of a program on development of commercial high-power fiber-based visible and UV laser sources, it became feasible to overcome many of the existing limitations and demonstrate possibility of generating light with high and ultrahigh average power levels in visible and UV spectral ranges. Inherent versatility of fiber lasers’ design has allowed adapting key laser parameters to a broad range of requirements, while maintaining all the other benefits of fiber approach – very high efficiency, compactness and low cost. In this presentation, we will report on our progress in satisfying these real-world requirements and discuss various solutions which are developed or under development for different existing and emerging applications. The high-power fiber laser platforms which we will overview include highly-stable high-power single-frequency green lasers for demanding scientific markets, as well as high-lumen broadband laser sources of inherently de-speckled green and red light for cinema projection and entertainment applications. We will also discuss high average power QCW green and UV lasers for cutting, welding and annealing, as well as high peak power pulsed green and UV lasers for micromachining and laser surface cleaning; single-mode lasers for high precision applications, as well as ultrahigh power multimode lasers for material processing; lasers with other colors, i.e. orange, for medical and other emerging applications; and a lot more.

9731-7, Session 3

High efficiency fourth-harmonic generation from nanosecond fiber master oscillator power amplifier

Xiaodong Mu, Paul Steinvurzel, Todd S . Rose, William T . Lotshaw, Steven M . Beck, James H . Clemmons, The Aerospace Corp . (United States) Frequency up-conversion of fiber lasers is an attractive method for generating compact and efficient deep ultraviolet (DUV) radiation, which has a variety of applications in Lidar, spectroscopy, lithography, and material processing. Here, we demonstrate high efficiency DUV conversion to 266 nm through frequency quadrupling of a 1064-nm Yb-doped fiber master oscillator power amplifier (MOPA). The MOPA consists of a 1064-nm diode seeder (master oscillator) and a 3-stage fiber amplifier containing conventional double-clad polarization-maintaining Yb-doped fibers. The average output power, spectral bandwidth, pulse width and repetition rate are 1.31 W, 40 pm (10.6 GHz), 1.42 ns and 20 kHz, respectively. The second harmonic is generated in two 15-mm long critically phase-matched LBO crystals with walk-off compensating alignment. We achieve an output power of 0.92 W at 532 nm and a second harmonic generation (SHG) efficiency of 74%. By comparison, the maximum SHG efficiency in a single LBO crystal is measured to be only ~59%. The fourth-harmonic generation (FHG) is achieved in a 10-mm long BBO crystal. For an input power of 0.86 W, we generated 0.40 W at 266 nm, corresponding to a conversion efficiency of 47%. The overall FHG efficiency from 1064 nm to 266 nm is 30%. To the best of our knowledge, this is the highest conversion efficiency reported in a single-pass FHG. We found that the pumping ratio of the 3 stages of the fiber amplifier is essential for reaching such a high FHG efficiency.

9731-6, Session 3

High power high-harmonics generation using a 30-m enhancement cavity

(Invited Paper)

Shuntaro Tani, Akira Ozawa, Zhigang Zhao, Makoto Kuwata-Gonokami, Yohei Kobayashi, The Univ . of Tokyo (Japan) High harmonic generation (HHG) in an enhancement cavity provides high-repetition-rate coherent light sources in the vacuum ultraviolet (vuv) to soft x-ray region, with key applications in photoemission spectroscopy and diffractometry. One major drawback of HHG with high repetition rates, typically 100 MHz, was the relatively low conversion efficiency from the fundamental pulses to the higher order harmonics ones due to the smaller pulse energies. To achieve sufficiently high output power, we developed a 30-m long enhancement cavity system driven by a 10-MHz Yb-doped fiber laser system. The lowered repetition rate increased the pulse energies and realized the high-conversion efficiency of 10^?6 while keeping an adequate repetition rate for spectroscopic applications.

The seed laser for the external cavity consisted of the following

9731-8, Session 3

Comparison of yellow light emitting micro integrated laser modules with different geometries of the crystals for second harmonic generation

Julian Hofmann, Nils Werner, David Feise, Alexander Sahm, Roland Bege, Bernd Eppich, Gunnar Blume, Katrin Paschke, Ferdinand-Braun-Institut (Germany) Compact and efficient laser sources around 560 nm are demanded for several medical applications as well as confocal microscopy and atom spectroscopy. Until today, this wavelength region cannot be reached by direct emitting semiconductor-based laser sources with sufficient efficiency. Therefore, the most promising approach for compact and efficient laser sources are lasers emitting around 1120 nm combined with second harmonic generation.

As the different applications for laser light at 560 nm have very different requirements on e.g. the power, the spectral linewidth and the beam quality of the emitted light, we have developed three micro integrated laser

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modules to fulfill the demands of each application. The modules for the low power applications (power < 100 mW) consist only of a laser with an internal distributed Bragg reflector [1], a grinlens and a ridge-waveguide crystal for second harmonic generation. This most simple setup is easy to align and low cost but limited in power and linewidth. For high power applications (power > 200 mW) we boost the power of the laser with a tapered amplifier in a master-oscillator power-amplifier configuration [2] and use either bulk or planar-waveguide crystals for second harmonic generation. This - especially for the planar-waveguide crystal - much more complex setup offers higher power and better linewidth as we use an optical isolator to shield the laser from back-reflections here.

At the conference we will present a theoretical analysis of the expected performance of the different setups and compare the results with the results of our experimental analysis.

[1] Paschke, K.; Wenzel, H.; Fiebig, C.; Blume, G.; Bugge, F.; Fricke, J. and Erbert, G., “High Brightness, Narrow Bandwidth DBR Diode Lasers at 1120 nm”, IEEE Photonics Technology Letters., vol. 25, , pp. 1951-1954 (2013) [2] Spießberger, S.; Schiemangk, M.; Sahm, A.; Wicht, A.; Wenzel, H.; Peters, A.; Erbert, G. and Tränkle, G., “Micro-integrated 1 Watt semiconductor laser system with a linewidth of 3.6kHz”,Optics Express, vol. 19, , pp. 7077-7083 (2011)

9731-9, Session 3

Conference 9731: Nonlinear Frequency Generation and Conversion: Materials, Devices, and Applications XV Compact deep UV laser system at 222.5 nm by single-pass frequency doubling of high-power GaN diode laser emission

Norman Ruhnke, André Müller, Bernd Eppich, Reiner Güther, Martin Maiwald, Bernd Sumpf, Götz Erbert, Günther Tränkle, Ferdinand-Braun-Institut (Germany) Photonics - Mid-Infrared Lasers (United States); Viktor Smolski, IPG Photonics Mid Infrared Lasers (United States); Sergey B . Mirov, IPG Photonics - Mid-Infrared Lasers (United States) and The Univ . of Alabama at Birmingham (United States); Valentin P . Gapontsev, IPG Photonics Corp . (United States) Cr2+ doped polycrystalline ZnS and ZnSe possess a unique blend of physical, spectroscopic, and technological parameters. Four-level energy structure, broad vibronic emission bands, absence of excited-state absorption, and close to 100% quantum efficiency of fluorescence enable room-temperature mid-IR lasers with power in excess of 50 W and a broad tuning range (1.9 – 3.3 at 2.4 µ m.

µ µ m). Kerr-lens mode-locked polycrystalline Cr2+:ZnS lasers with up to 2 W average power and <29 fs pulse duration m central wavelength were recently demonstrated. Polycrystalline Cr2+:ZnS/ZnSe consist of a multitude of microscopic single-crystal grains. The broad distribution of grain sizes and orientations results in so-called random quasi-phase-matching (RQPM). The distinctive features of RQPM are a linear dependence of the conversion yield with length of the medium and ultra-wide bandwidth. Combination of RQPM in polycrystals with high nonlinearity of II-VI semiconductors opens several avenues for efficient frequency conversion of few-cycle pulses directly in the gain medium of the mode-locked laser. We demonstrate that significant fraction of mid-IR laser emission can be converted to spectrally broad (up to 22 THz, FWHM) second harmonic pulses (SHG). The SHG power (up to 250 mW) is high enough for real-world applications. We also observe generation of higher optical harmonics as well as sum frequency mixing between mid-IR pulses and a cw pump beam. Furthermore, the MW-level optical power inside the resonator of mode-locked laser allows us to consider efficient down conversion in polycrystalline Cr2+:ZnS/ZnSe, which may lead to ultrafast oscillators with exceptionally broad spectral coverage spanning from 2 to 6 Deep ultraviolet (DUV) lasers emitting below 300 nm are of great interest for many applications, for instance in medical diagnostics or for detecting biological agents. Established DUV lasers, e.g. gas lasers or frequency quadrupled solid-state lasers, are relatively bulky and have high power consumptions. A compact and reliable laser diode based system emitting in the DUV could help to address applications in environments where a portable and robust light source with low power consumption is needed. In this work, a compact DUV laser system based on single-pass frequency doubling of high-power GaN diode laser emission is presented. A commercially available high-power GaN laser diode from OSRAM Opto Semiconductors serves as a pump source. The laser diode is spectrally stabilized in an external cavity diode laser (ECDL) setup in Littrow configuration. The ECDL system reaches a maximum optical output power of 700 mW, maintaining narrowband emission below 60 pm (FWHM) at 445 nm over the entire operating range. By direct single-pass frequency doubling in a BBO crystal with a length of 7.5 mm a maximum DUV output power of 16 ?W at a wavelength of 222.5 nm is generated. The presented concept enables compact and efficient diode laser based light sources emitting in the DUV spectral range that are potentially suitable for field applications where small footprint and low power consumption is essential.

9731-10, Session 4

Mid-IR Kerr-lens mode-locked polycrystalline Cr:ZnS and Cr:ZnSe lasers with intracavity frequency conversion via random quasi-phase-matching

(Invited Paper)

Sergey Vasilyev, Igor S . Moskalev, Mikhail S . Mirov, IPG

9731-11, Session 4

Broadband electrical control of second harmonic generation in bilayer MoS2 by inversion symmetry breaking

Julian Klein, Walter Schottky Institut (Germany) and Technische Univ . München (Germany); Jakob Wierzbowski, Armin Regler, Jonathan Becker, Walter Schottky Institut (Germany); Florian Heimbach, Technische Univ . München (Germany); Michael Kaniber, Walter Schottky Institut (Germany); Kai Müller, Stanford Univ . (United States) and Walter Schottky Institut (Germany); Jonathan J . Finley, Walter Schottky Institut (Germany) The emergence of truly two-dimensional materials like graphene augmented the range of commercially available material systems for future utilization in sophisticated electronic and optoelectronic device architecture. Especially, the semiconducting transition metal dichalcogenides (SMTDs) MoS2, MoSe2, WS2 and WSe2 play a key role in bridging the gap among bandgap-less graphene and insulating hBN. The outstanding properties of STMDs are based on a direct bandgap in the monolayer limit with an optical transition in the visible. The intrinsic electronic and optical signature in few-layer crystals is strongly driven by crystallographic symmetry properties. The inherently broken inversion symmetry in monolayer crystals manifests their recently reported advantage for valleytronic applications.

Here, we present intense tunability of second-harmonic generation in naturally inversion symmetric 2H stacked bilayer MoS2. Control of the nonlinear conversion efficiency is achieved by altering the crystal’s inversion symmetry via immense externally applied perpendicular electric fields. This is established in a novel lithographically designed Si(n)-SiO2-STMD-Al2O3 metal micro-capacitor device with optical access. We excite bilayer MoS2 crystals with ultrashort pulses of ~ 70 fs within a spectral window of 840 – 1000 nm (1.24 – 1.47 eV). We observe a significant tunability throughout

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the probed region with a ~ 60 fold conversion amplification at its optimum. We attribute the spectral sensitivity of the electrically controlled second harmonic generation to the material intrinsic second-harmonic response. Our results demonstrate the potential of efficient electrically driven broadband frequency doubling by external control of the symmetry properties of 2H bilayer MoS2.

9731-12, Session 4

Conference 9731: Nonlinear Frequency Generation and Conversion: Materials, Devices, and Applications XV Improved grating propagation during HVPE growth of orientation-patterned gallium arsenide and gallium phosphide

Peter G . Schunemann, Daniel J . Magarrell, Lee Mohnkern, Leonard A . Pomeranz, BAE Systems (United States) Quasi-phase-matched semiconductors represent the future of mid-IR nonlinear optical materials, and orientation-patterned GaAs (OP-GaAs) is the first successful incarnation of this technology. GaAs offers very high nonlinearity (d14=94 pm/V) and thermal conductivity (55W/mK), and when grown from the vapor phase exhibits extremely low 2-?m absorption losses (< 0.01 cm-1). The application of OP-GaAs has been limited by the availability of samples with sufficient thickness, grating quality, and transparency for practical device applications. The primary obstacle to aperture scaling OP-GaAs has been the tapering of domain wall boundaries: instead of parallel vertical propagation of both polarities, it is observed that domains nucleating on the original (etched) substrate orientation grow gradually wider at the expense of the adjacent domains growing on the inverted layer. As a result the duty cycle of the MBE template gradually shifts away from the optimal 50:50 (degrading conversion efficiency) during HVPE growth until ultimately the inverted domains are annihilated. Previous work has shown that the domain spreading angle can be minimized by growth at lower temperatures (680°C), but we have achieved better material quality and OPO performance for growth closer to 710°C at the expense of worse domain spreading. In parallel work on orientation patterned gallium phosphide (OP-GaP) we observed near-perfect domain propagation at elevated growth temperatures (795°C) and V/III ratios > 6. In this work we examined the effect of V/III ratio, growth rate, and growth temperature on domain propagation in OP-GaAs and OP-GaP. Maximum thickness and resulting OPO/DFG performance will be reported.

9731-13, Session 4

Frequency conversion efficiency in free standing periodically oriented gallium nitride

Christopher G . Brown, Univ . Research Foundation (United States); Steven R . Bowman, Jennifer K . Hite, Jaime A . Freitas, Francis J . Kub, Charles R . Eddy Jr ., Igor Vurgaftman, Jerry R . Meyer, U .S . Naval Research Lab . (United States); Jacob H . Leach, Kevin Udwary, Kyma Technologies, Inc . (United States) Gallium Nitride’s (GaN) material properties of broadband transparency, high thermal conductivity, and wide-band gap make it a promising candidate for high power frequency conversion devices. GaN possesses a nonlinear susceptibility similar in magnitude to lithium niobate, due to strong internal polarization, but conventional phase matching is prevented due to GaN’s weak birefringence. In order to obtain efficient nonlinear optic frequency conversion, patterned inversion growth has been developed to induce quasi-phase matching (QPM). We have fabricated and tested periodically oriented gallium nitride (PO-GaN) devices in order to obtain QPM frequency conversion. We report recent measurements of second harmonic generation, free carrier absorption, bulk scattering losses, and absorption due to unintentionally doped impurities.

9731-14, Session 5

Homo and heteroepitaxial growth and study of orientation-patterned GaP for nonlinear frequency conversion devices

Vladimr L . Tassev, Rita D . Peterson, Shivashankar Vangala, Michael Snure, Martin Kimani, Air Force Research Lab . (United States) Frequency conversion in orientation-patterned (OP) materials is a leading approach for generating tunable mid- and long-wave coherent infrared radiation for a wide variety of applications. The strong interest led to intensive investigation of a number of nonlinear optical materials. GaP is an especially promising material, due to its unique properties—high nonlinear susceptibility, low two-photon absorption in the convenient pumping range 1–1.7 µ µ m and high thermal conductivity. Growth of OPGaP has encountered several challenges, such as relatively low quality of commercially available GaP wafers, and uncontrollable parasitic nucleation that reduces the growth rate and the layer quality, especially during longer runs. Here we describe an original approach for producing thick, high-quality OPGaP with excellent domain fidelity via a one-step HVPE homo or heteroepitaxial growth process with growth rate of about 100 µ m/h. Two-inch wafer bonded OPGaP templates and templates in which polarity alternation was achieved in a MBE assisted procedure were used. AFM, SEM, XRD, EDS and TEM showed smooth surface morphology and high crystalline quality, while optical characterization confirmed substantial reduction of the 2-4 m absorption typical for all n-type GaP samples, low 2PA (??

favor the growth near the interface are also provided. ≤ 0.1 cm/GW, compare with 15-16 cm/GW for GaAs) and low optical losses. Important crystallographic considerations on how to avoid the appearance of (111)p facet that could overgrow the pattern, and why alternating the polarity may

9731-15, Session 5

Linear and nonlinear optical properties of GaAs and GaP grown using hydride vapor phase epitaxy

Shekhar Guha, Air Force Research Lab . (United States); Jean Wei, Joel M . Murray, Jacob O . Barnes, Air Force Research Lab . (United States) and UES, Inc . (United States); Peter G . Schunemann, BAE Systems (United States) Frequency doubling, tripling and quadrupling of carbon dioxide lasers operating in the 9.2 to 10.8 ?m wavelength range provide the means to generate tunable radiation in the 4.6 to 5.4 ?m, 3.1 to 3.6 ?m and 2.3 to 2.7 ?m wavelength ranges, respectively. Orientation patterned GaAs and GaP are promising nonlinear optical materials for these processes because of their high nonlinearity and low absorption at the relevant wavelengths. In order to calculate the expected conversion efficiencies, one must have an accurate knowledge of the material d coefficient for these interactions at each wavelength. Along with the dispersion of the refractive indices, it is also important to know the wavelength and temperature dependence of the absorption coefficient of the materials.

In the infrared spectral range, neither the wavelength dependence of the d coefficient value nor the temperature and wavelength dependence of the absorption coefficients of GaAs and GaP are readily available. We have therefore undertaken a detailed study to determine these values. The absorption coefficients of crystals grown by the hydride vapor phase epitaxy (HVPE) process have been measured over a temperature range of 77 K to 295 K for the 1 to 20 tunable in the 2 to 10 µ µ m wavelength range and compared with materials grown from melt and slow cooling. Using a picosecond laser m wavelength range, the values of the d coefficient were measured using HVPE-grown single crystals.

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9731-16, Session 5

Determination of the type II phase transition region in random relaxor ferroelectrics using Cherenkov second harmonic microscopy

Mousa Ayoub, Hannes Futterlieb, Jörg Imbrock, Cornelia Denz, Westfälische Wilhelms-Univ . Münster (Germany) Ferroelectrics are pivotal materials in devices for sensor applications, data storage, and selective optical parametric processes. Methods for the accurate characterization of these materials regarding the dynamics of their growth and switching are critical in the exploration and understanding of the outstanding properties for technological developments. This complex dynamics has been well studied by past work. However, how the phase transition of relaxor ferroelectrics in fact takes place is still under active debate. The combination of the measurements of the characteristics, achieved until now with, the visualization in the volume is still an open requirement. In this contribution we demonstrate the first three-dimensional monitoring of the evolution of the spontaneous polarization (Ps) during transition from the para- to ferroelectric phase and vice versa. This allows us to determine the Curie temperature range more accurately, what is required for relaxor ferroelectrics. The monitoring is based on a key sensitive feature to the domain wall. This is optically provided by Cherenkov-type second harmonic microscopy. Here we identify chronologically the dimension of the evolution process. The microscopic measurements are combined with second-harmonic measurements in far field. The medium studied here, is a relaxor random strontium barium niobate (SBN), in which Ps is represented by 3D needle-like objects (several hundreds of micrometers in length). The quantitative value of Ps is recorded simultaneously by measuring the poling current. Our results pave the way for deeper understanding of the ferroelectric nature for more accurate modeling of this complex behavior that is fundamental for all applications of ferroelectrics.

9731-17, Session 5

Conference 9731: Nonlinear Frequency Generation and Conversion: Materials, Devices, and Applications XV Optical limiting properties of carbon disulfide at 2.05 ?m wavelength

Lars G . Holmen, Magnus W . Haakestad, Norwegian Defence Research Establishment (Norway)

9731-46, Session 5

Nonlinear refraction dynamics of solvents and gases

(Keynote Presentation)

Eric W . Van Stryland, Peng Zhao, Trenton Ensley, Matthew C . Reichert, David J . Hagan, CREOL, The College of Optics and Photonics, Univ . of Central Florida (United States) We use our recently developed femtosecond excitation-probe beam deflection (BD) method to measure the temporal dynamics of nonlinear refraction in various liquids and gases for different polarization combinations. This allows us to determine the temporal response function of these materials. In turn, this allows us to predict the outcome of other experiments, e.g. Z-scan, as a function of the pulsewidth used. For gases, the coherent revivals reveal the IR rotational spectrum including changes in line-width with rotational quantum number. Comparisons can also be made between the second hyperpolarizability in liquid and gas phase.

9731-18, Session 6

High-power mid-infrared high repetition rate supercontinuum source based on a chalcogenide step-index fiber

Stefan Kedenburg, Tobias R . J . Steinle, Florian Mörz, Andy Steinmann, Harald Giessen, Univ . Stuttgart (Germany) We demonstrate a tunable and robust femtosecond supercontinuum source with a maximum output power of 550 mW and a maximum spectral width of up to 2.0 ?m which can cover the mid-infrared region from 2.3 ?m up to 4.9 ?m by tuning the pump wavelength. As light source we use a synchronously pumped fiber-feedback OPO and a subsequent OPA which delivers femtosecond, Watt level idler pulses tunable between 2.5 ?m and 4.1 ?m. These pulses are launched into As2S3 chalcogenide step-index fibers with core diameters of 7 and 9 ?m. The spectral behavior of the supercontinuum is investigated by changing the pump wavelength, core diameter, fiber length, and pump power. Self-phase modulation is identified as the main broadening mechanism in the normal dispersion regime. This source promises to be an excellent laboratory tool for infrared spectroscopy owing to its high brilliance as demonstrated for the CS2-absorption bands around 3.5 ?m.

Several types of infrared sensors are based on sensitive focal plane arrays. In such sensors, the intensity will typically increase by a factor 10^7 at the focal plane, compared to the intensity of the incoming radiation. Such arrays are thus vulnerable when illuminated with high-intensity laser pulses. One solution for protecting the array against such pulses is to use an optical limiter, which is a passive device that blocks radiation with high intensity and transmits radiation with low intensity. We here present results where carbon disulfide (CS2) has been tested as an optical limiting material when illuminated with 25 ns pulses with up to several hundred mJ energy at 2.05 µ m wavelength. The laser had a beam quality M^2=1.5, and the beam was focused onto a CS2-cell using a lens with an effective f-number of 10 and a focal length of 50 mm. The light emerging from the CS2 cell was refocused onto an aperture, with a field of view of 1.4 mrad. Pulse energies of up to 150 mJ were incident on the cell, while at most 0.6 mJ was transmitted through the aperture, due to dielectric breakdown and beam filamentation in the CS2 cell at high pulse energies. In addition, the nonlinear index of refraction of CS2 at 2.05 performed at 2 ?m wavelength using CS2.

µ m wavelength was measured using the z-scan technique, yielding a value of (2.8±1.0)•10^(-18) m^2/W. To our knowledge, these are the first optical limiting experiments

9731-19, Session 6

Experimental study of supercontinuum generation in an amplifier based on an Yb3+ doped nonlinear photonic crystal fiber

Tobias Baselt, Christopher Taudt, Fraunhofer IWS Dresden (Germany) and Westsächsische Hochschule Zwickau (Germany) and TU Dresden (Germany); Bryan Nelsen, Westsächsische Hochschule Zwickau (Germany); Andrés Fabián Lasagni, Fraunhofer IWS Dresden (Germany) and TU Dresden (Germany); Peter Hartmann, Westsächsische Hochschule Zwickau (Germany) and Fraunhofer IWS Dresden (Germany) The use of supercontinuum light sources in different optical measurement methods, like microscopy or optical coherence tomography, has increased significantly compared to classical wideband light sources. The development of various optical measurement techniques benefits from the high brightness and bandwidth, as well as the spatial coherence of these sources. For some applications, only a portion of the broad spectral range

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Conference 9731: Nonlinear Frequency Generation and Conversion: Materials, Devices, and Applications XV

can be used. Therefore, an increase of the spectral power density in limited spectral regions would provide a clear advantage over spectral filtering. This study describes a method to increase the spectral power density of supercontinuum sources by amplifying the excitation wavelength inside a nonlinear photonic crystal fiber (PCF). An ytterbium doped photonic crystal fiber was manufactured by a sol-gel process and used in a fiber amplifier setup as the nonlinear fiber medium. In order to characterize the fiber’s optimum operational characteristics, group-velocity dispersion (GVD) measurements were performed on the fiber during the amplification process. For this purpose, a notch-pass mirror was used to launch the radiation of a stabilized laser diode at 976 nm into the fiber sample for pumping. The performance of the fiber was compared with a conventional PCF. Finally, the system as a whole was characterized in reference to common solid state-laser-based photonic supercontinuum light sources. An improvement of the power density up to five times was observed between 1030 nm to 1350 nm wavelengths.

sensitive detection system. Due to the high reflectivity of the cavity mirrors, the beam experiences a long effective optical path length resulting in large absorption values and allowing to achieve detection at very low concentrations. If the mirrors reflectivity is high over a large spectral region, the technique allows the simultaneous detection of gases that exhibits absorption lines in different wavelength ranges. A supercontinuum source which possesses high brightness over a very large bandwidth and which is perfectly spatially coherent is therefore ideal to exploit the full potential of the method. Here, using a compact supercontinuum source we demonstrate for the first time multi-components gas detection of acetylene and methane with high sensitivity in the mid-infrared, over a bandwidth extending from 3000 to 3500 nm. These results are significant not only because they illustrate the potential of incoherent supercontinuum sources for spectroscopy in the mid-infrared but also because they represent the largest continuous detection window reported so far.

9731-20, Session 6

All-normal dispersion supercontinuum generation in the near-infrared by Raman conversion in standard optical fiber

Christophe Louot, Erwan Capitaine, Badr M . Shalaby, Katarzyna Krupa, Alessandro Tonello, Dominique Pagnoux, Claire Lefort, Philippe Leproux, Vincent Couderc, XLIM Institut de Recherche (France) Coherent Raman spectroscopy methods like coherent anti-Stokes Raman scattering (CARS) or stimulated Raman scattering (SRS) can be implemented by using a monochromatic pump wave and a broadband Stokes wave. In the case of 1064 nm pumping, a Stokes spectrum extending up to 1600 nm is required in order to probe chemical bonds from the fingerprint area to the CH stretching region. This broadband spectrum has to be particularly flat, with the highest possible spectral power density. Usually, such spectrum is obtained by means of supercontinuum generation in a solid-core photonic crystal fiber, which is operated in the anomalous dispersion regime. Here we demonstrate all-normal dispersion supercontinuum generation in the 1080-1600 nm range by propagating sub-nanosecond pulses in a high numerical aperture standard optical fiber (Corning HI 980, with zero dispersion wavelength at 1600 nm). The extreme saturation of the Raman gain provides a flat spectrum in the considered range, making this broadband source particularly suitable for coherent Raman spectroscopy. The unusual regime of supercontinuum generation, i.e. Raman gain saturation regime, is highlighted. It is investigated through a complete numerical and experimental spectrotemporal study, and the corresponding results are compared with those obtained from a photonic crystal fiber. Finally the possibility of operating spectrometer-free time resolved coherent Raman spectroscopy is introduced.

9731-21, Session 6

Multi-component gas detection in the mid infrared with supercontinuum

Caroline Amiot, Piotr Ryczkowski, Antti Aalto, Juha Toivonen, Goëry Genty, Tampere Univ . of Technology (Finland) The measurement of gas concentration is paramount in many industrial applications ranging from emission control to chemical reaction optimization. Many gases of interest possess very strong absorption lines in the mid-infrared; in fact stronger than in any of other spectral regions. Incoherent broadband cavity enhanced absorption spectroscopy is a simple and robust method for the detection of gases with high sensitivity. In this technique a broadband source is coupled to a high-finesse confocal cavity filled with gas and the transmitted light is analyzed with a wavelength-

9731-35, Session PTue

Novel efficient high power parametric THz source based on QPM nonlinear crystal fiber

Pengxiang Liu, Wei Shi, Degang Xu, Tianjin Univ . (China); Nasser N . Peyghambarian, The Univ . of Arizona (United States) We proposed a novel GaAs-based crystal fiber configuration for efficient THz difference frequency generation (DFG), which combines the single mode THz fiber and the quasi-phase-matching for DFG THz generation. Calculations were performed on the characteristics of energy conversion and output beam focusing. Theoretical results indicated that the proposed THz crystal fiber structure can provide high power and high brightness THz radiation. The output power, spectral power density and brightness are also analyzed, based on the calculation of the dynamic of energy conversion and the characteristics of the THz beam focusing. High output power (average ~1 W) and excellent focusing characteristic allow us to achieve THz generation with high brightness (100 MW/(sr•cm2)), a promising value for many applications.

9731-36, Session PTue

Walk-off free 266 nm generation of freely triggerable 60 ps pulses in periodically poled LBGO

Thomas Schoenau, Dietmar Klemme, Romano Haertel, Kristian Lauritsen, Rainer Erdmann, PicoQuant GmbH (Germany) Laser pulses from diode based laser systems in the UV range are of great interest in the fields of microscopy and spectroscopy. Providing UV wavelength pulses at variable repetition frequencies or on demand requires nonlinear frequency conversion in single pass arrangement with a crystal featuring a high effective nonlinear coefficient. In the past, such devices based on borate materials (e.g. BBO, CLBO) suffer intrinsically from walk-off due to the phase-matching condition. As a result, extensive efforts had to be made to obtain a clean transverse mode without distortion at the expense of induced power loss.

Our approach presented here utilized the newly available PP-LBGO device with 2nd-order quasi-phase matching (QPM) to obtain a walk-off free and undistorted fundamental transverse mode. We investigated and compared the conversion efficiency of PP-LBGO with a classic beta-BBO crystal and measured the beam quality. The pump laser is based on a 1064 nm distributed feedback (DFB) gain-switched seed laser diode and two-stage fiber amplifier. After second harmonic generation to 532 nm the light is focused into the PP-LBGO for single pass generation of 266 nm. Since the

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PP-LBGO is non-hygroscopic, the material is easy to handle and a long life time is expected and under investigation.

The target power level of 1 mW average power at 80 MHz is sufficient for a wide range of applications in the life sciences, such as fluorescence spectroscopy and confocal microscopy.

9731-37, Session PTue

Conference 9731: Nonlinear Frequency Generation and Conversion: Materials, Devices, and Applications XV Pulsed 266 nm laser based on fiber laser source for sensing application

Junji Hirohashi, Yasuhiro Tomihari, Satoshi Makio, Yasunori Furukawa, Oxide Corp . (Japan); Marc Le Flohic, Keopsys SA (France) Forth harmonic (266 nm) laser was demonstrated based on pulsed fiber laser combined with frequency convertor for sensing application. Fundamental 1064 nm laser was fiber based laser with?repetition rate of 50 kHz, peak power of 25 kW, pulse width of 1 ns and average power of 1 W. The light was delivered by polarization maintain large mode area fiber and connected to the frequency convertor. In the frequency convertor, second harmonic 532 nm was generated by PP-Mg:SLT with more than 50% of conversion efficiency without walk-off and then forth harmonic 266 nm was generated by BBO with 30% of conversion efficiency. Both frequency conversion devices were operated at 40 degree-C. The 266 nm with average power of more than 150 mW was confirmed with stable operation. Since the output properties of relatively high repetition rate and high peak power, it is suitable for sensing application from the point of better signal noise ratio and measurement speed. In addition, since the combination of this fiber laser head and frequency convertor with simple single pass configuration, it is possible to realize affordable laser module for easy to integrate into the portable sensing system such as LIDAR systems. The shorter wavelength such as 266 nm could be attractive to characterize much smaller particle sensing comparing to the visible or IR sensing systems.

9731-38, Session PTue

Stimulated polariton scattering in KTA crystal and its application in tunable stokes laser generation

Jie Zang, Zhenhua Cong, Xiaohan Chen, Xingyu Zhang, Zengguang Qin, Zhaojun Liu, Jianren Lu, Shandong Univ . (China); Shiqi Jiang, Shandong Univ . (China); Qiang Fu, Dong Wu, Shandong Univ . (China) The stimulated polariton scattering (SPS) is a nonlinear process in which the second- and third-order nonlinear effects are involved. The essential condition for SPS in a crystal is the existence of one or more intense transverse optical A1 modes which are both infrared- and Raman-active. In the process of the SPS, three waves including the pumping wave, the generated Stokes wave and the polariton wave interact in the overlapped beam area. The momentum conservation and energy conservation must be satisfied simultaneously. The SPS can be used to generate tunable terahertz wave and tunable Stokes laser emission near the pumping wavelength. SPS can occur in a few crystals. Potassium titanyl arsenate (KTiOAsO4, KTA) is one of them. This paper presents the tunable Stokes laser characteristics based on the SPS in KTA crystal. The pumping source is a 1064.2 nm Q-switched laser. The pulse energy, pulse width, repetition rate and beam size are 125 mJ, 10 ns, 10 Hz and 3.5 mm, respectively. The tenability is realized by adjusting the angle between the pumping beam and the Stokes laser cavity axis. When the angle is changed from 1.875° to 6.500°, five tunable ranges from 1077.9 to 1079.0 nm, from 1080.1 to 1080.8 nm, from 1082.8 to 1083.6 nm, from 1085.5 to 1085.8 nm, from 1086.8 to 1088.4 nm are obtained. The maximum pulse energy is 24.7 mJ obtained at the wavelength of 1078.6 nm.

9731-39, Session PTue

Polarization study of a supercontinuum light source for different wavelengths through a photonic crystal fiber

Julian M . Estudillo-Ayala, Jose D . Filoteo-Razo, Juan Carlos Hernández-Garcia, Univ . de Guanajuato (Mexico); Jesus Pablo Lauterio-Cruz, Ctr . de Investigaciones en Óptica, A .C . (Mexico); Daniel Jáuregui-Vázquez, Univ . de Guanajuato (Mexico); Baldemar Ibarra-Escamilla, Instituto Nacional de Astrofísica, Óptica y Electrónica (Mexico); Oliver J . M . Pottiez, Ctr . de Investigaciones en Óptica, A .C . (Mexico); Roberto Rojas-Laguna, Univ . de Guanajuato (Mexico); Evgeny A . Kuzin, Instituto Nacional de Astrofísica, Óptica y Electrónica (Mexico) In this work we show the changes of polarization at different wavelengths in the end of a photonic crystal fiber (PCF) by means bandpass filters in a supercontinuum light source. A linear and circular polarization was introduced in a piece of PCF, showing the changes of the polarization for each wavelength of each one of the filters from 450 to 700nm. We used a microchip laser as pumping source with wavelength of 532nm and short pulses least than 1 ns with repetition rate of 9KHz. We obtained a continuous spectrum in the visible spectral region. We show a comparison of the polarization state at the fiber input with respect to polarization state in the fiber output for different wavelengths by rotating the axes of the PCF.

9731-40, Session PTue

2 nm continuously tunable 488nm micro integrated diode-laser-based SHG light source for Raman spectroscopy

Marcel Braune, Martin Maiwald, Bernd Sumpf, Günther Tränkle, Ferdinand-Braun-Institut (Germany) Raman spectroscopy in the visible spectral range is of great interest due to resonant enhancement of signals in organic samples. Nevertheless, fluorescence and background signals can mask these lines. Shifted Excitation Raman Difference Spectroscopy is a potential tool to overcome this distortion. To apply this method, a dual wavelength light source is necessary. The distance between these two wavelengths should be approximately the spectral width of the bands under study. In this work, a micro-integrated SHG light source emitting at 488 nm with a continuous tuning range up to 2 nm (83 cm-1) and without moving parts is presented. With this feature the SERDS distance between the excitation wavelengths can be adjusted according to the target.

The pump source, a DFB laser emitting at 976 nm, and a PPLN waveguide crystal are directly mounted on a micro-Peltier-element. Due to the comparable temperature tuning of laser (66 pm/K) and crystal (72 pm/K), by changing the common temperature from 15°C to 72°C and slight adjustment of laser current (4 pm/mA), the above mentioned tuning range is achieved. An approximately constant 488 nm output power of about 27 mW was measured over the whole spectral range. At lower temperatures, the output power reaches 30 mW. With increasing temperature due to the decrease of the pump power, the SHG power deteriorates with -0.1 mW/K. Moreover, the concept delivers a suppression of the amplified spontaneous emission by the SHG crystal. The use of bandpass-filters is therefore not necessary. A wider SERDS distance becomes easily feasible.

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Conference 9731: Nonlinear Frequency Generation and Conversion: Materials, Devices, and Applications XV

9731-41, Session PTue

Phase-matching properties of GaS0.4Se0.6 for type-2 DFG in the 100.4 1030.6

µ

m range

Kiyoshi Kato, Chitose Institute of Science and Technology (Japan); Valentin P . Petrov, Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie (Germany); Nobuhiro Umemura, Chitose Institute of Science and Technology (Japan) Although we reported the Sellmeier equations for GaSxSe1-x (x=0, 0.09, 0.40, and 1.0) that provide excellent reproduction of the phase-matching conditions for harmonic generation of a CO2 laser at 10.5910

µ m [Kato and Mikami, Appl. Opt., 53, 2177(2014)], the birefringence of GaSe and GaS at 1THz given by these Sellmeier equations are about 0.38 and 0.34 smaller than the experimental values of Zhang et al. [J. Appl. Spectroscopy, 77, 850(2011)] and Molloy et al. [CrystEngComm., 16, 1995(2014)], respectively.

In order to construct the accurate Sellmeier equations for GaSxSe1-x in the THz range, we measured the phase-matching angles for type-2 DFG between a Nd:YAG laser and BBO/OPO in a c-cut 4.7-mm-thick GaS0.4Se0.6 crystal at 100.4-1030.6

µ m. These data were used to refine the Sellmeier equations for GaSxSe1-x in the THz range.

By using these Sellmeier equations for GaSxSe1- x (0?x?0.40), we calculated the phase-matching conditions for the near-IR to THz frequency conversion achieved by Nazarov et al. [Appl. Phys. Lett., 99, 081105(2011) and 100, 136104(2012)]. For GaS0.29Se0.71 pumped by a Ti:Al2O3 laser at 0.797

value of ?ext=8º.

µ m, our index formula gives the type-1 phase-matching angle of ?ext=7.86º for generating the 1.8THz radiation, which agrees well with their experimental While, for the same crystal pumped at 0.790

?)|=0.12.

µ m, our formula gives the eee-type phase-matching angle of??ext=45.0º for generating the 1THz radiation and the refractive index mismatch of |ne, gr(?, ?)-ne(?, ?)|=0.1096, which agree with their experimental values of ?ext=45º and |ne,gr(?, ?)-ne(?, Thus, the utility of our Sellmeier equations for GaSxSe1-x is demonstrated.

9731-42, Session PTue

Studying an advanced regime of the non collinear two-phonon light scattering for applications to the optical spectrum analysis

Alexandre S . Shcherbakov, Adan O . Arellanes, Instituto Nacional de Astrofísica, Óptica y Electrónica (Mexico) Principally new features of the non-collinear two-phonon light scattering governed by elastic waves of finite amplitude in birefringent bulk crystals are detected and observed. The main goals of our investigations are to reveal novel important details inherent in the nonlinearity of this effect and to study properties of similar parametric nonlinearity both theoretically and experimentally in wide-aperture crystals with moderate linear acoustic attenuation. An additional degree of freedom represented by the dispersive birefringence factor, which can be distinguished within this nonlinear phenomenon, is characterized. This physical degree of freedom gives us a one-of-a-kind opportunity to apply the strongly non-linear two-phonon light scattering in practice for the first time. The local unit-level maxima in the distribution of light scattered into the second order appear periodically as the acoustic power density grows. It makes possible to identify a few transfer function profiles peculiar to these maxima in the isolated planes of angular-frequency mismatches. These maxima give us an opportunity to choose the desirable profile for the transfer function at the fixed angle of incidence for the incoming light beam with a wide spectrum .The needed theoretical analysis is developed and proof-of-principle experiments, performed with a specially designed wide-aperture acousto-optical cell made of the calomel (?-Hg2Cl2) crystal, are presented. The obtained spectral resolution ~0.235 Å at 405 nm (i.e. the resolving power ~17,200) can be compared with the most advanced acousto-optical spectrometers for space/airborne operations. Evidently, our results with the calomel-based acousto-optical cell look like the best we can mention at the moment.

9731-43, Session PTue

Thermal study of second harmonic generation in periodically poled crystals

Alphonse L . Rasoloniaina, Rodolphe Collin, Christelle Pareige, Ecole Nationale Supérieure des Sciences Appliquées et de Technologie (France); Stéphane Balac, Univ . de Rennes 1 (France); Thierry Chartier, Ecole Nationale Supérieure des Sciences Appliquées et de Technologie (France); Pascal Besnard, CNRS-Fonctions Optiques pour les Technologeis de l’information (France); Alain Mugnier, David Pureur, Quantel Group (France) High-power green lasers present a great interest for many applications such as DNA sequencing, ophthalmology, biotechnology or laser Doppler velocimetry. Second-harmonic generation (SHG) based on quasi-phase matching (QPM) technique presents some advantages such as compactness, high conversion efficiency and high beam quality. In QPM technique, a precise control of crystal temperature is required to obtain the optimum conversion. However, for high powers, thermal effects due to fundamental harmonic (1064 nm) absorption, second harmonic (532 nm) absorption and green induced infrared absorption (GRIIRA) become non-negligible. Temperature gradient takes place and leads to a local variation of the index of refraction. The phase matching becomes temperature dependent and this limits the efficiency. In this case, an active temperature control of the crystal is needed to compensate this phenomenon. In this work, we propose both theoretical and experimental studies of continuous-wave second-harmonic generation in a periodically-poled congruent lithium niobate crystal doped with a magnesium oxide crystal (MgO:PPLN). The temperature distribution in the crystal is obtained by solving: (i) the propagation equations for the fundamental harmonic and second harmonic by a split-step Fourier method and (ii) heat transfer equation by finite elements method where the longitudinal and transverse variations of temperature are taken into account. We present a comparison between theoretical simulations and experimental results. A good agreement with the experiment is observed in term of temperature acceptance or temperature control. Second harmonic generation up to 2.5 W of output power is achieved in 25 mm long crystal.

9731-44, Session PTue

Third-harmonic generation in metallo dielectric stacks

Han Li, Joseph W . Haus, Partha P . Banerjee, Univ . of Dayton (United States) We examine the problem of third-harmonic generation (THG) in a thin film stack fabricated from multiple layers of metal and dielectric thin films, also called a metallodielectric [1]. We apply the transfer matrix method extended to cover obliquely incident fields [2] to the problem of third-harmonic generation. Due to interference and evanescent field penetration through the metal layers the transmission is much higher than for a single thick metal film.

Specifically, we examine THG using silver layers sandwiched between a relatively high index dielectric (around a value of 2), such as tantalum pentoxide or zinc oxide. The THG simulations are an extension of the method developed in Ref. [2] where second-harmonic generation was examined. In the metallodielectric case we use realistic dielectric materials

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data and a complex third-order nonlinear coefficient of the metal. The reflected and transmitted third-harmonic efficiencies are optimized by varying each layer thickness within experimentally reasonable limits and by changing the angle of incidence of the pump wave. Further THG metallodielectrics using metals such as copper and gold, as well as selected dielectric materials, will be examined using the same methodology. The comparison between the various metallodielectrics will be made with recommendations for experimental studies.

9731-45, Session PTue

Conference 9731: Nonlinear Frequency Generation and Conversion: Materials, Devices, and Applications XV Surface states of silicon crystalline films detected by nonlinear optical laser spectroscopy

Dmitry E Milovzorov, Fluens Technology Group Ltd . (Russian Federation) nonlinear processes at lower densities that compromise energy coupling or cause energy deposition in undesirable locations. Dominant processes are scattering on ion-acoustic waves (Stimulated Brillouin Scattering), scattering on electron-plasma waves (Stimulated Raman Scattering), or two-plasmon decay. These are complex instabilities that require advanced computer simulations even for approximate predictions of the problems being caused by them. We will discuss laser-plasma instabilities (LPI), measurement techniques such as near beam imaging, and methods to minimize these instabilities using beam smoothing techniques. Measurements for various levels of LPI will illustrate the progress made at Sandia in order to advance the efforts in magneto-inertial fusion.

Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

Resonant second-harmonic generation by nanocrystalline silicon films was studied by using optical spectroscopy for a different polarization schemes for incident laser and output radiation. The morphology of films is different for any of them: rectangular crystals in the film with average nanocrystals size 9.7 nm and irregular for the film with nanocrystal size 16.1 nm. There is a differences in resonant spectral peaks positions: 3.22 eV and 3.31 eV for the silicon film with nanocrystal size 9.7 nm; and 3.2 eV and 3,26 eV for the silicon film with nanocrystal size 16.1 nm. SHG spectra show the resonance sharp peaks which is related to the defect or surface states in band gap of silicon. From the rigid surface the optical nonlinear response on second harmonic can be described as stochastic value that can be estimated as nonlinrar function of fundamental laser intensity multiplied on coefficient on nonlinear transformation of radiation and correlation function between added field fractions generated by various parts of rigid films.

9731-23, Session 7

Efficient cascaded generation of narrowband linearly-polarized radiation in random Raman fiber laser

Sergey A . Babin, Ekaterina A . Zlobina, Sergey I . Kablukov, Evgeniy V . Podivilov, Institute of Automation and Electrometry (Russian Federation)

9731-22, Session 7

Delivering kilojoules of pre-heat to fusion targets in Sandia’s Z-Machine: Or why do we care about nonlinearities in laser-plasma interactions?

(Keynote Presentation)

Matthias Geissel, Adam J . Harvey-Thomson, Thomas J . Awe, Sandia National Labs . (United States); Michael E . Campbell, Lab . for Laser Energetics (United States); Matthew R . Gomez, Eric Harding, Christopher Jennings, Mark W . Kimmel, Patrick F . Knapp, Sandia National Labs . (United States); Sean M . Lewis, The Univ . of Texas at Austin (United States); Kyle Peterson, Marius Schollmeier, Adam B . Sefkow, Jonathon E . Shores, Daniel B . Sinars, Stephen A . Slutz, Ian C . Smith, Christopher S . Speas, Roger A . Vesey, John L . Porter, Sandia National Labs . (United States) Random Raman lasers attract now a great deal of attention as they operate in non-active turbid or transparent scattering media. In the last case, single mode fibers with feedback via Rayleigh backscattering are used to generate high-quality directed laser beam with relatively narrow modeless spectrum. However, generation in such random Raman fiber lasers (RRFLs) is limited in polarization properties: the light is usually depolarized or has unstable polarization, even for the first Stokes wave. Here we demonstrate a linearly-polarized cascaded random Raman lasing in a PM fiber with polarized pumping. Quantum efficiency of converting input pump radiation (1.05?m) into the 1st (1.11?m), 2nd (1.17?m) and 3rd order (1.23?m) Stokes waves amounts to 79%, 83%, and 77%, respectively. Taking that the passive losses at propagation of pump radiation in the fiber are ~15%, almost all pump photons are converted into the generated Stokes wave, regardless of the order. Herewith, polarization extinction ratio (PER) is as high as >22 dB for all the waves at powers up to ~10 W. The laser bandwidth grows with increasing Stokes order, but it is almost independent on the generated power varying in the range of 0.8-1.3nm, 1.4-2.3nm and 2.4-3.3nm for the consecutive orders, respectively. At that, the generated spectrum remains to be sufficiently narrower than the Raman gain profile. An analytical model has been developed describing well the generated power and spectrum for all components of the cascaded RRFL. The unique features of such source with a potential of broad-range tuning offer new opportunities in applications.

Sandia National Laboratories pursues a novel concept of inertial confinement fusion that includes a pre-magnetization of the fuel. The ‘Magnetized Liner Inertial Fusion’ concept heats up deuterium untilizing the Z-Beamlet laser while exposed to a magnetic field and subsequently implodes fuel using the ‘Z’ pulsed power facility. While the interaction of lasers with matter is well understood for processes at kilowatt power levels, the pre-heat process in MagLIF reaches terawatt powers and intensities beyond 10^14 W/cm?, which are subject to complex and largely nonlinear phenomena.

A thin solid density window covers the laser entrance hole to confine the fuel initially, and a pre-pulse decompresses it below the critical plasma density, enabling propagation into the fuel. However, there are

9731-24, Session 7

Temporal characterization of a multi wavelength hybrid Brillouin-erbium fiber laser

Victor L . Lambin-Iezzi, Ecole Polytechnique de Montréal (Canada); Thomas F . S . Büttner, Ctr . for Ultrahigh bandwidth Devices for Optical Systems (Australia); Amirhossein Tehranchi, Sébastien Loranger, Ecole Polytechnique de Montréal (Canada); Irina V . Kabakova, Benjamin J . Eggleton, The Univ . of Sydney (Australia); Raman Kashyap, Ecole Polytechnique de Montréal (Canada)

94 SPIE Photonics West 2016 · www.spie.org/pw

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9731-25, Session 7

Conference 9731: Nonlinear Frequency Generation and Conversion: Materials, Devices, and Applications XV

We have investigated hybrid Multi-Wavelength Brillouin and erbium fiber lasers (MWBEFL) which are particularly interesting because of their simplicity, robustness, their only need for components, low power threshold, high tunability and their ability to generate a large number of Brillouin shifted frequencies. Their basic principle is to combine the narrow nonlinear gain offered by SBS in an undoped fiber with the broadband linear gain from an erbium doped fiber to enable the cascaded process of SBS. These lasers represent a convenient way for generating multi-wavelengths with comb-like optical spectra that have a frequency channel spacing equal to the Brillouin frequency shift ~10 GHz or multiple of it. Numerous configurations for MWBEFLs have been suggested over the years, but their characterization has been limited to optical and radio-frequency spectral measurements. Here, we provide a detailed temporal characterization of several MWBEFL configurations by measuring the optical power of the single frequency channels with high temporal resolution. It is found that the power in each channel is highly unstable due to the excitation of several cavity modes. We also provide real-time measurements for a configuration that was reported to emit phase-locked picosecond pulse trains, concluded from autocorrelation measurements [1]. The real-time measurements reveal a high degree of instability without the formation of stable pulse trains.

References: [1] S. Loranger, V. L. Iezzi, and R. Kashyap, “Demonstration of an ultra-high frequency picosecond pulse generator using an SBS frequency comb and self phase-locking,” Opt. Express, vol. 20, pp. 19455-19462, 2012.

Compact silica-fiber Brillouin laser with highly damped intensity-noise

Schadrac Fresnel, Stéphane Trebaol, Yohann Léguillon, Christelle Pareige, Pascal Besnard, Ecole Nationale Supérieure des Sciences Appliquées et de Technologie (France); Sophie LaRochelle, Univ . Laval (Canada) Compact and low-cost coherent sources are needed to improve system performances of sensors and telecommunication systems, e.g. in coherent optical communication links and microwave photonic applications. Brillouin fiber lasers (BFLs) have been attracting a lot of interest lately due to their very narrow linewidth and very low relative intensity noise (RIN) and frequency noise (FN). Indeed, the first Stokes component (S1) generated by a pump can have more than 6 dB RIN-reduction compared to the pump. Furthermore, its FN can be reduced by 10-20 dB, depending on the pump characteristics and on the packaging of the Brillouin laser. We have already shown an 8 dB reduction (above the theoretical 6 dB-limit) in S1-RIN using a compact chalcogenide BFL with two Stokes orders. This can be explained by the fact that, above the second order threshold, the lasing S1 component is saturated leading to a compression of its intensity noise. In this paper we examine robust and compact silica BFLs with improved noise properties. Since single frequency operation is mandatory to achieve coherent sources, the Free Spectral Range of the laser cavity has to be higher than the Brillouin gain-bandwidth or, in other words, a short cavity length has to be employed (< 20 m). We designed a 16.66 m-long polarization-maintaining silica-fiber cavity (FSR = 12 MHz) with a 10 dBm threshold for S1, 15 dBm for the second-order component S2, and 18 dBm for S3. Operating above the S2 threshold, we show a severe damping of the S1 RIN of 20 dB, while maintaining a FN reduction of 10 dB, compared to that of the pump.

laser is demonstrated. Average output power level of 1.25 W is generated at the first Stokes wavelength of 2602 nm for 9.7 W incident pump power. Output pulse widths as short as 19 ns was measured at a repetition rate of 6.67 kHz. Near-diffraction limited beam quality is observed (M2=1.25). This simplified Raman laser configuration can harness the high average power levels offered by Thulium- and Holmium-doped solid-state and fiber lasers to generate fixed-wavelength and tunable output at 2.3-2.8 um.

9731-27, Session 8

Designing non-trivial QPM spectral shapes in Titanium in-diffused PPLN

(Invited Paper)

Alexander V . Sergienko, Boston Univ . (United States) Integrated optics based on lithium niobate offers the versatility of multi functional devices connected by channel waveguides on a single substrate. The enabling technology is the diffusion of lithographically defined titanium stripes into lithium niobate resulting in the formation of low loss channel waveguides. Combined with quasi-phase matching induced by electric field poling this leads to non-linear frequency convertion of exceptional efficiency. The distinct advantage of Ti in-diffused waveguides is that both Type I and Type II phase matching processes are allowed by a suitable choice of the poling period. Moreover, the low optical background facilitates the quantum frequency conversion of polarization-encoded photonic qubits with high fidelity. The careful design of poling period together with the waveguide geometry allows controlling the transverse mode structure of the qubits. Additional flexibility in the poling profile design permits the tailoring of phase matching spectrum. A quasi-rectangular flat-top or Gaussian spectrum as broad as 200 nm is designed by chirping and apodization of the poling period. We discuss applications of such devices to quantum frequency conversion of temporal qdits, which has been proposed for higher dimensional encoding of quantum information, and mid-infrared chip-scale frequency combs generation by frequency down conversion of commercially available Er combs at 1550 nm. Additional constraints imposed on quasi-phase matching would allow control over spectral phase as well. We discuss extended phase matching conditions in the context of generating frequency-correlated photon pairs by spontaneous parametric down conversion.

9731-28, Session 8

Broadband wavelength control for optical parametric oscillation in radially-poled whispering gallery resonators

Sarah-Katharina Meisenheimer, Univ . of Freiburg (Germany) and Fraunhofer-Institut für Physikalische Messtechnik (Germany); Josef U . Fürst, Univ . of Freiburg (Germany); Annelie Schiller, University of Freiburg (Germany); Karsten Buse, Fraunhofer-Institut für Physikalische Messtechnik (Germany) and Univ . of Freiburg (Germany); Ingo Breunig, Univ . of Freiburg (Germany)

9731-26, Session 7

Mid-infrared, external cavity BaWO4 Raman laser at 2602 nm with 1.25-W output power

Onur Kuzucu, ASELSAN Inc . (Turkey) An external cavity BaWO4 Raman source pumped by a Q-switched Ho:YAG Optical parametric oscillators (OPOs) provide tunable continuous-wave coherent light for, e.g., infrared spectroscopy. In such current OPO-based systems, typically light of a pump laser with a wavelength of about 1 µ m is down-converted by a periodically-poled lithium niobate crystal placed inside a mirror cavity. Whispering gallery resonators (WGRs) are promising cavities for miniaturizing the current OPO setup. However, identification of the whispering gallery modes and controlled tuning of wavelengths over a wide spectral range are essential and still challenging for employing OPOs in WGRs for infrared spectroscopy. In this work, we demonstrate that identification of the whispering gallery modes of pump, signal, and idler light can be achieved by comparing experimental data to simulations

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Conference 9731: Nonlinear Frequency Generation and Conversion: Materials, Devices, and Applications XV

for both, the pump spectrum and the tuning behavior of the OPOs. For controlled tuning of the output wavelengths over a wide infrared spectral range (1.7–2.5 µ m), a whispering gallery pump mode with selected radial and polar mode numbers is addressed. The pump wavelength and these mode numbers are kept fixed while increasing the temperature of the crystal in well-defined steps, going from one longitudinal mode to the next. This provides wider and more reliable tuning than it has been achieved before. Experimental data showing mode identification, improved tuning and the applicability to spectroscopy are presented for a calligraphically poled WGR with 2 mm diameter and 28 µ m domain period length, being suitable for quasi phase matching of 1.04 µ m pump light. of average power at 2039 nm by using a reflective Volume Bragg Grating (VBG). Devices such as piezo-controlled etalons can provide rapidly tunable, narrow-linewidth power from this system.

9731-31, Session 9

Mid-infrared nonlinear upconversion imaging and sensing

(Invited Paper)

Christian Pedersen, Peter Tidemand-Lichtenberg, Technical Univ . of Denmark (Denmark)

9731-29, Session 8

Broadly tunable OPGaAs OPO pumped by Cr:ZnSe laser

Rita D . Peterson, Gary Cook, Air Force Research Lab . (United States) Coherent sources that are broadly and continuously tunable in the mid- and longwave infrared are of interest for a variety of scientific, commercial, and military applications. The advantages in an OPO of quasi-phasematched materials like orientation-patterned gallium arsenide (OPGaAs) come at the cost of the angle tuning possible in birefringent nonlinear crystals. Temperature tuning is limited by the material’s dn/dT value, and lacks speed and stability. A better alternative is to tune the OPO by tuning the pump laser. Here we report an OPGaAs OPO pumped by a gain-switched Cr:ZnSe laser which was continuously tuned by an intracavity etalon. The etalon also narrowed the output linewidth to 2 nm. The Cr:ZnSe laser operated at a repetition rate of 500 Hz with a 25 ns pulsewidth. The pump was focused to a spot size (1/e^2) of 100 with a period of 97 µ over a range of almost 4.5 µ µ m at the center of a simple linear resonator formed by two 5-cm ROC mirrors. The OPGaAs crystal was 14 mm long, m, and was mounted with no active cooling. Tuning the pump laser over a range of 90 nm (2385-2475 nm) produced OPO output m (3500-7450 nm). OPO tuning was ultimately limited by coatings on the crystals and the resonator mirrors, as the Cr:ZnSe laser is capable of much broader tuning as a pump source. A maximum slope efficiency of 21% was obtained, with a pulse energy threshold of 93 µ J.

9731-30, Session 8

High-repetition rate, picosecond-pulse, tunable, mid-IR PPLN OPG source

Yelena Isyanova, Wenyan Tian, Q-Peak, Inc . (United States); Peter F . Moulton, MIT Lincoln Lab . (United States) Mid-wavelength infrared (MWIR) and long-wavelength infrared (LWIR) electromagnetic radiation spanning the 3-8 aspect hereof is that the 3-15 The 3-15 µ µ µ m and 8-15 “fingerprint” region. A second aspect relates to the 3-5 µ µ m range, respectively, is important for a range of spectroscopic applications. A first m range covers the fundamental absorption band of nearly all gasses AND bear specific vibrational chemical signature of molecules; a key to new spectroscopy, chemical sensing and identification. m wavelength region is therefore also a part of the so called m and 8-13 µ m atmospheric “transmission windows” enabling applications such as environmental gas monitoring (e.g. CH4, NOx, SOx and CO2), thermal (heat) radiation and remote sensing (stand of detection). Other examples are food analysis and cancer research. The exploitation of MWIR and LWIR therefor holds great promises for science, society and industry. A major challenge for exploitation of this highly preferred optical mid IR band has been severe technological challenges related to low-noise detection and imaging. We suggest nonlinear upconversion, using diode pumped solid state laser technology, followed by visible light detection, to demonstrate low noise detection and mid-IR imaging for a range of applications, even at room temperature. Theoretical and technological aspects of upconversion for detection and imaging will be presented as well as relevant mid-IR applications for gas sensing and analysis of complex molecules.

9731-32, Session 9

Multi-octave IR pulses generation in DAST crystal

Carlo Vicario, Paul Scherrer Institut (Switzerland); Gunnar Arisholm, Norwegian Defence Research Establishment (Norway); Christoph P . Hauri, Paul Scherrer Institut (Switzerland) and Ecole Polytechnique Fédérale de Lausanne (Switzerland); Balazs Monoszlai, Paul Scherrer Institut (Hungary) and Univ . of Pécs (Hungary) We report here on the performance of a narrow-line, mid-IR source based on a PPLN-crystal optical parametric generator (OPG). The PPLN crystal was pumped by a pulsed, 20-MHz-rate, 1064-nm Yb:fiber-based source operating with 20-psec pulses and average output power of 20 W. Prior to achieving the parametric generation, we built an optical parametric amplifier (OPA). We used a 2051-nm diode with output power of 0.8 mW as a seed source for the OPA. The 2-cm long PPLN crystal with the 31.1 ?m poling period was placed in a high-temperature oven at 137 deg. C. We achieved the OPA and OPG threshold at 17 W and 18 W of pump power, respectively. Since the average output power of the OPG was low, we replaced the 2-cm crystal with the 5-cm one with the same grating period. With this crystal, the OPG threshold was observed at 5.2 W of pump power. Seeding was not necessary and did not affect the performance of the OPG. The maximum output power measured was 0.55 W. The OPG produced a broad spectrum between 2027 nm and 2239 nm. Since we were interested in producing a narrow, <1 nm linewidth output, we placed a band-pass filter after the OPG that allowed us to select a 30 nm bandwidth output. We achieved further line reduction (0.7 nm) and 4.5 mW Infrared pulses with large spectral width and wavelength extending from 1.2 to 3.5 µ µ m are generated by multiple nonlinear processes in thin organic crystal DAST (4-N, N-dimethylamino-4’-N’-methylstilbazolium tosylate). In the experiment, the crystal is pumped by an optical parametric amplifier delivering few mJ, 65 femtosecond pulses at a central wavelength of 1.5 m. A net redshift of the input wavelengths and a spectral width up to 1.5-octave are achieved when the DAST is pumped at fluence larger than 10 mJ/cm 2. The compact experimental setup provides spectrally broadened pulse with energy up to 700 ?J and conversion efficiency of 25 %. Scaling the pulse energy up to several mJ is feasible with a larger DAST crystal and higher pump energy at constant pump fluence. The results are qualitatively reproduced by numerical simulations, which provide an explanation to the spectral broadening process and accounts for the asymmetric broadening towards longer wavelength. The presented effect is ascribed to cascaded second order processes mediated by the exceptionally large effective ?2 nonlinearity of DAST, but the shape of the spectrum indicates that a delayed ?3 process may also be involved. Further studies to investigate the temporal compression of the multi-octave spectra presented here are ongoing.

96 SPIE Photonics West 2016 · www.spie.org/pw

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Conference 9731: Nonlinear Frequency Generation and Conversion: Materials, Devices, and Applications XV

9731-33, Session 9

Simultaneous fully phase-matched sum frequency and second harmonic generation of a high-power CW fiber laser using an aperiodically poled LN

Ameneh Bostani, Amirhossein Tehranchi, Raman Kashyap, Ecole Polytechnique de Montréal (Canada) Generation of broadband spectra centered at short wavelengths around 775 nm has applications in biomedicine and spectroscopy. Up-conversion techniques such as second harmonic and sum frequency generation exploiting lasers in the communication band provide coherent radiation in short wavelengths. Nonlinear materials such as periodically poled lithium niobate (PPLN) have been extensively researched for this purpose due to several advantages . However, PPLN cannot work for simultaneous SHG and SFG due to its narrow bandwidth for a reasonable efficiency. Also, as the effective period of PPLN changes by angular rotation and temperature, fine alignment and controlled temperature are required to produce the maximum efficiency for a designed wavelength. However, aperiodically poled nonlinear materials in the form of chirped and step-chirped (SC) gratings are proposed for broadband up-conversion to solve these problems . Moreover, high-power CW fiber lasers with a broader bandwidth compared to monochromic sources are available at many wavelengths including the C-band. Therefore, one can benefit from larger conversion bandwidth of SHG and SFG besides temperature-insensitivity of SC-PPLN and simultaneously up-convert the wavelengths of available fiber lasers. Here, we use a high-power fiber laser, an amplified narrowband laser and an SC-PPLN to simultaneously generate SF and SH using the entire spectrum of the fiber laser. The input spectrum depicts the high-power (25 W) laser and the narrowband laser operating around 1550 nm and 1555 nm, respectively. The output spectrum illustrates the simultaneous broadband SHG of the high-power laser around 775 nm and broadband SFG around 776.5 nm

9731-34, Session 9

Evidence of Anderson localization effects in random Raman lasing

Brett H . Hokr, Texas A&M Univ . (United States); Marlan O . Scully, Texas A&M Univ . (United States) and Baylor Univ . (United States) and Princeton Univ . (United States); Vladislav V . Yakovlev, Texas A&M Univ . (United States) Anderson localization, also known as strong localization, is the absence of diffusion in turbid media resulting from wave interferences. The effect was originally predicted for electron motion, and is widely known to exist in systems of less than 3 dimensions. However, Anderson localization with photons in 3 dimensional systems remains an elusive and controversial topic. Random Raman lasing offers the unique combination of large gain and virtually zero absorption. The lack of absorption makes long path length, localized modes possible and the presence of gain offsets what little absorption is present, and preferentially amplifies localized modes due to their large Q factors compared with typical low Q modes present in complex media. Random Raman lasers exhibit several experimentally measured properties that diverge from classical, particle-like, diffusion. First, the temporal width of the emission being 1 to a few nanoseconds in duration when it is pumped with a 50 ps laser is a full order of magnitude longer than is predicted by Monte Carlo simulations. Second, as the sample thickness is increased from 1 mm to 10 mm, the lasing threshold is decreased by 20%. Monte Carlo simulations and a coupled diffusion equation model both predict only a negligible change as the sample is increased from about 1000 transport mean free paths to 10,000. We will highlight these clear divergences from particle-like behavior, which can only be explained by the presence of high-Q localized modes consistent with Anderson localization.

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Conference 9732: Real-time Measurements, Rogue Events, and Emerging Applications

Monday - Tuesday 15–16 February 2016 Part of Proceedings of SPIE Vol . 9732 Real-time Measurements, Rogue Events, and Emerging Applications 9732-1, Session 1

Reviving analog computing with optics

Daniel R . Solli, Bahram Jalali, Univ . of California, Los Angeles (United States) Analog computing machines replicate the dynamics of an inaccessible problem with a tractable, user-controlled system. These devices have been employed in the form of geared mechanisms for astronomical computations since antiquity, while more modern examples include mechanical systems for gun-fire control and general-purpose calculation machines based on analog electronics. The rise of digital machines and advanced algorithms has rendered these analog machines largely obsolete. Nevertheless, computation time and accuracy remain limiting factors for simulations of complex dynamics, which are known for their dramatic sensitivities to initial conditions and diverse solution spaces. We discuss the concept of optical computing not as a replacement for digital machines but as an analog hardware accelerator. Optical co-processors offer a means to address computational challenges in nonlinear dynamics. Specifically, rapidly evolving optical systems coupled with advanced real-time measurement techniques can be harnessed to generate massive amounts of complex data (aka big data) that would take an electronic digital computer far longer to simulate (by many orders of magnitude). Previous interest in optical computing has been directed at mirroring digital electronics, an effort challenged by the lack of an efficient, compact switch. In contrast, optical hardware accelerators are a move in a different direction, which is already meeting with success.

9732-2, Session 1

Resolving the buildup of mode-locking with real-time spectroscopy at 90 MHz

(Invited Paper)

Georg Herink, Claus Ropers, Georg-August-Univ . Göttingen (Germany); Bahram Jalali, Univ . of California, Los Angeles (United States); Daniel R . Solli, Univ . of California, Los Angeles (United States) and Georg-August Univ . Göttingen (Germany) Passive mode-locking forms the basis of today’s ultrafast science and technology. Mode locking is a complex process, and once established, it is remarkably robust and stable. In contrast, the transition to mode locking is highly stochastic and remains difficult to study experimentally despite the technological maturity of modern sources. Conventional forms of measurement are generally not suitable for capturing such rapid, non repetitive phenomena. Moreover, the timescales involved span many orders of magnitude from a single cavity roundtrip of nanosecond duration up to many thousands of roundtrips (milliseconds). Here, we present real-time spectroscopy of the transition to the mode-locked state in a Kerr-lens mode-locked titanium-sapphire (20-fs) oscillator over 900,000 consecutive periods. These experiments are enabled by the mapping of spectral information to the time domain via the dispersive Fourier transform, utilizing group-velocity dispersion and a real-time oscilloscope. We resolve the dynamics over the entire buildup, observing characteristic features on various timescales. The onset of mode-locking begins from a random mixture of quasi-continuous waves, milliseconds before the actual establishment of a femtosecond pulse. Yet, the most rapid spectral broadening, along with marked wavelength shifts, occurs within a few hundred roundtrips. In addition, we observe transient mode beatings that can be employed as a time-resolved probe of the dynamic intracavity nonlinearity. These results lend new insights into the mode-locking transition and illustrate the utility of real-time spectroscopy as a diagnostic tool for ultrashort sources.

9732-3, Session 1

Dynamics of soliton explosions in ultrafast fibre lasers

Miro Erkintalo, Antoine F . J . Runge, Neil G . R . Broderick, The Univ . of Auckland (New Zealand) A soliton explosion is a dramatic effect, whereby a pulse circulating in a mode-locked laser undergoes a sudden structural collapse, yet remarkably recovers its original shape within a few roundtrips. Our group recently reported the first observation of such explosions in a fibre laser. Here, we expand on our initial work, reporting a detailed numerical and experimental study of the dynamics and characteristics of soliton explosions.

Our experiment is based on a passively mode-locked Yb-doped fibre laser, in which explosions occur close to the boundary between stable and noise-like operation. To capture the events, we use the dispersive Fourier transformation to record, in real time, the pulse-to-pulse spectra of the laser. We explore a variety of operating conditions by systematically adjusting the laser pump power and its cavity length. We also use a realistic model based on a set of generalized nonlinear Schrodinger equations to simulate the explosion dynamics.

We find that the explosion dynamics are strongly influenced by the operating conditions. As a general trend, the frequency of the events increases as the laser’s parameters move closer towards the boundary of unstable operation. In fact, when sufficiently close to that boundary, “explosions” can even become more frequent than ordinary pulses. Moreover, our simulations reveal that complex features in the spectral and temporal profiles of the explosion events can be explained in terms of a multi-pulsing instability. Our results provide new insights into the nature of soliton explosions, elucidating the processes that underpin the destabilization of passively mode-locked lasers.

9732-4, Session 1

Nonlinear time series analysis: towards an effective forecast of rogue waves

(Invited Paper)

Günter Steinmeyer, Simon Birkholz, Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie (Germany); Carsten Brée, Weierstrass-Institut für Angewandte Analysis und Stochastik (Germany); Ayhan Demircan, Leibniz Univ . Hannover (Germany) The discovery of optical rogue waves initiated a new direction of research, aiming at finding common patterns and similar mechanisms in the emergence of extreme-value statistics across a variety of completely different physical systems. While the ubiquity of the rogue waves is certainly interesting all by itself, the usefulness of this universal concept has to be ultimately gauged by resulting advance in understanding and predicting ocean rogue waves. To this end we performed an analysis of ocean rogue wave records and time series of optical rogue waves. Our analysis is based on nonlinear time series analysis, in particular the Grassberger Procaccia algorithm (GPA), and clearly indicates that rogue wave dynamics may emerge due to two different scenarios. Only in case of a quantum mechanical origin, the resulting rogue waves are completely unpredictable and appear without a warning. However, if the underlying physics is

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Conference 9732: Real-time Measurements, Rogue Events, and Emerging Applications

ultimately turbulence, as in the ocean, rogue waves bear a surprising amount of predictability. Moreover, our analysis reveals that nearly all aspects of ocean rogue waves can be explained by linear interference of a small but variable number of elementary waves. Investigating the resulting probability density functions in this situation, it appears striking that rogue waves can only form when at least 10 waves of equal amplitude interfere. At larger numbers, the formation of rogue waves grows rapidly and exceeds all unconditional global estimates by factors of ten and more. Finally, GPA offers a unique opportunity for estimating this characteristic number. Analyzing the 20 minute record of the original Draupner event recorded on New Year’s Day 1995, GPA indicates a characteristic number of waves exceeding 12, which is significantly larger than recorded in any previous analysis. This clearly indicates that GPA analysis of wave dynamics can be effectively exploited for a forecast of rogue wave prone storms. effects in optics first observed in the spatial domain have been demonstrated, including for example all-optical magnification of ultrafast data rates by a thousand fold, real-time detection of single-shot spectra at hundreds of MHz speed, or temporal cloaking. Here, exploiting duality between light propagation in space and time, we demonstrate the temporal analog of ghost imaging. We use a conventional fast detector that does not see the temporal ‘object’ to be characterized, and a slow integrating ‘bucket’ detector that does see the object but without resolving its temporal structure. Our experiments achieve temporal resolution at the picosecond level and show how temporal ghost imaging allows to overcome temporal distortions from a direct measurement induced by e.g. dispersion or attenuation. The approach is scalable, can be integrated on-chip and offers great promise for dynamic imaging of ultrafast waveforms.

9732-5, Session 1

Extreme events in coupled nanolasers

Mathias Marconi, Philippe Hamel, Fabrice Raineri, Ariel Levenson, Alejandro M . Giacomotti, Lab . de Photonique et de Nanostructures (France)

9732-7, Session 1

Lighting up microscopy with random Raman lasing

Brett H . Hokr, Texas A&M Univ . (United States); Marlan O . Scully, Texas A&M Univ . (United States) and Baylor Univ . (United States) and Princeton Univ . (United States); Vladislav V . Yakovlev, Texas A&M Univ . (United States) Intense research is currently devoted to extreme events –such as rogue waves– in photonic devices such as supercontinuum optical fibers [1], mode-locked lasers and more recently integrated photonic resonators [2]. In general, high pumping levels (either for passive or active devices) are required in order to reach heavy tail statistics in the output intensity. In this work we show that extreme event statistics can emerge from nonlinear semiconductor coupled cavities with energy levels as low as few hundreds of emitted photons. For this, we use a nanophotonic device given by two coupled semiconductor photonic crystal nanolasers. This system has been recently shown to undergo spontaneous breaking of the mirror symmetry through a pitchfork bifurcation of the stable lasing (antibonding) state of the compound cavity system [3]. Here we study the intensity fluctuations of the second, unstable (bonding) mode. We observe heavy tail statistics in the form of deviations from exponential (Rayleigh) distributions. We show that the emission of this dark mode exhibits large fluctuations with mean output energies of only few hundreds of emitted photons. This opens a new avenue for the study of large deviations in the statistics of “few photon” states. [1] Dudley et al., Nature Photonics, 8(10), 755-764 (2004) [2] Liu et al, Nature Physics 11, 358–363 (2015) [3] Ph. Hamel et al., Nature Photonics 9, 311 (2015) Wide-field microscopy, where full images are obtained simultaneously, is limited by the power available from speckle-free light sources. Currently, the vast majority of wide-field microscopes use either mercury arc lamps, or LED’s as the illumination source. The power available from these sources limits wide-field fluorescent microscopy to tens of microseconds temporal resolution. Lasers, while capable of producing high power and short pulses, have high spatial coherence. This leads to the formation of laser speckle that makes such sources unsuitable for wide-field imaging applications. Random Raman lasers offer the best of both worlds by producing laser like intensities, short, nanosecond-scale, pulses, and low spatial coherence, speckle-free, output. These qualities combine to make random Raman lasers 4 orders of magnitude brighter than traditional wide-field microscopy light sources. Furthermore, the unique properties of random Raman lasers make possible the entirely new possibilities of wide-field fluorescence lifetime imaging or wide-field Raman microscopy. We will introduce the relevant physics that give rise to the unique properties of random Raman lasing, and demonstrate early proof of principle results demonstrating random Raman lasing emission being used as an imaging light source. Finally, we will discuss future directions and elucidate the benefits of using random Raman lasers as a wide-field microscopy light source.

9732-6, Session 1

Temporal ghost imaging

Piotr Ryczkowski, Margaux Barbier, Tampere Univ . of Technology (Finland); Ari Friberg, Univ . of Eastern Finland (Finland); John M . Dudley, Univ . de Franche-Comté (France); Goëry Genty, Tampere Univ . of Technology (Finland) Ghost imaging is a technique for constructing an image of an object without actually seeing this object. It is based on structured illumination where the correlation between light transmitted through an object and the spatially resolved intensity pattern of the incident light allows to reconstruct a ghost image of the original object. A central aspect to ghost imaging is the mutual spatial correlation of the two beams, which may be quantum or classical. Various light sources can be used including spatially-entangled photon sources, spatially incoherent classical light sources, or structured light fields programmed by a spatial light modulator.

Exploiting space-time duality, the temporal analog of many propagation

9732-8, Session 2

Measurement of the complete temporal intensity and phase of supercontinuum

(Invited Paper)

Tsz Chun Wong, Michelle Rhodes, Rick Trebino, Georgia Institute of Technology (United States) The challenges facing single-shot measurement of supercontinuum are many. Typical supercontinuum generated from photonic-crystal fiber is several ps long and has well over 100 nm of spectral bandwidth, easily achieving a time-bandwidth product (TBP) of ~100 or more. As a result, such supercontinuum pulses are extremely complex, with fine structure in both the temporal and the spectral domains. To further complicate the task, due to the small cores of photonic-crystal fiber, a supercontinuum pulse is quite weak, having at most ~20 nJ of energy. Since this energy is spread over a few ps, the intensity of these pulses is also quite low. While addressing one (or occasionally two) of these requirements at a time is now routine, addressing them all simultaneously in order to measure supercontinuum on a single shot has not yet been accomplished, despite an ever-increasing need to do so.

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Here, we report a general technique that succeeds in this endeavor. In order to deal with the low energy of the supercontinuum pulse, we use a cross-correlation frequency resolved optical gating (XFROG) setup with a high-energy (regeneratively amplified) reference pulse. This setup provides much more signal and hence sensitivity than a self-referenced nonlinear measurement. Even better, XFROG has been demonstrated to retrieve extremely complicated pulse shapes very reliably[29]. We crossed the pulses at an angle, mapping relative delay onto transverse position, using the so-called transverse geometrical smearing in order to achieve single-shot measurement (and which also avoids any distortions due to the smearing effect). We also chose a polarization-gating (PG) geometry, which has essentially infinite bandwidth and so nicely solves the spectral-width problem immediately. Unfortunately, PG is a third-order nonlinear process and therefore, even with a high-intensity reference pulse, still requires a relatively long (>1mm) nonlinear medium for adequate signal strength. This ordinarily proves highly problematic due to so-called longitudinal geometrical smearing effects. We solved this problem by imparting significant pulse-front tilt onto the reference pulse and simultaneously using a particular beam-crossing angle that precisely cancels out the geometrical smearing. This conveniently also increased the delay range, increasing the maximum measurable pulse length.

With this technique, we were able to measure complex supercontinua on a single shot for the first time. We will discuss this technique and these measurements, as well as other issues important to measuring such pulses.

9732-9, Session 2

Conference 9732: Real-time Measurements, Rogue Events, and Emerging Applications Roguescope: Real-time high-throughput spectroscopy at 100 million frames per second

Mohammad H . Asghari, Univ . of California, Los Angeles (United States); Paul Trinh, Time Photonics Inc . (United States); Bahram Jalali, Univ . of California, Los Angeles (United States) The real-time measurement of fast non-repetitive events is arguably the most challenging problem in the field of instrumentation and measurement. These instruments are needed for investigating rapid transient phenomena such as chemical reactions, fast physical phenomena, phase transitions, protein dynamics in living cells and impairments in data networks. Optical spectrometers are the basic instrument for performing sensing and detection in chemistry, physics and biology applications. Unfortunately, the scan rate of a spectrometer is often too long compared with the timescale of the physical processes of interest. In terms of conventional optical spectroscopy, this temporal mismatch means that the instrument is too slow to perform real-time single-shot spectroscopic measurements. Single-shot measurement tools such as frequency-resolved optical gating (FROG) and spectral phase interferometry for direct electric-field reconstruction (SPIDER) are, although powerful, therefore unable to perform pulse resolved spectral measurements in real time. Roguescope is a commercially available single-shot optical spectrometer with a frame rate of up to 100 Million frames per second, at least one thousand times faster than the next fastest spectrometer. The Roguescope real-time capability is enabled by photonic time-stretch implemented by Time-Stretch Dispersive Fourier Transform. The Roguescope can capture large data sets to reveal rare events with meaningful accuracy. Applications include optical rouge waves, laser transients, chemical reactions, and nonlinear dynamics. RogueScope is an essential tool for measurements of fast stochastic processes such as laser transients, rare events and outliers in optical systems. RogueScope is ideal for capturing non-Gaussian statistics that are signatures of complex dynamics.

9732-10, Session 2

Towards pattern generation and chaotic series prediction with photonic reservoir computers

Piotr Antonik, Michiel Hermans, François Duport, Marc Hälterman, Serge Massar, Univ . Libre de Bruxelles (Belgium) Reservoir Computing is a bio-inspired computing paradigm for processing time dependent signals that is particularly well suited for analog implementations. Our team has demonstrated several photonic reservoir computer with performance comparable to digital algorithms on a series of benchmark tasks such as channel equalisation and speech recognition. Recently, we showed that our opto-electronic reservoir computer could be trained online with a simple gradient descent algorithm programmed on an FPGA chip. This setup makes it in principle possible to feed the output signal back into the reservoir, and thus highly enrich the dynamics of the system. This will allow to tackle in hardware complex prediction tasks, such as pattern generation, chaotic and financial series prediction, that have so far only been studied in digital implementations. Here we report simulation results of our opto-electronic setup with FPGA chip and output feedback applied to pattern generation and Mackey-Glass chaotic series prediction. The simulations take into account the major aspects of our experimental setup. We find that pattern generation can be easily implemented on the current setup with very good results. The Mackey-Glass series prediction task is more complex and requires a large reservoir and more elaborate training algorithm. With these adjustments promising result are obtained, and we now know what improvements are needed to match previously reported numerical results. These simulation results will serve as basis of comparison for experiments we will carry out in the coming months.

9732-11, Session 2

Real-time characterization of spectral coherence of ultrafast laser based on optical time-stretch

Yiqing Xu, Xiaoming Wei, Zhibo Ren, Kenneth K . Y . Wong, Kevin Tsia, The Univ . of Hong Kong (Hong Kong, China) Nonlinearly generated broadband ultrafast laser have been increasingly utilized in many applications. However, traditional techniques of characterizing these sources lack the ability to observe the instantaneous features and transitory behaviours of both amplitude and phase. With the advent of the optical time stretch techniques, the instantaneous shot to-shot spectral intensity can be directly measured continuously at an unprecedentedly high speed. Meanwhile, the information of the real-time phase variation, which is carried by the frequency-time mapped spectral signal has yet been fully explored. We present a technique of experimentally measuring the spectral coherence dynamics of broadband pulsed sources. Our method relies on a delayed Young’s type interferometer combined with optical time-stretch. We perform the proof-of-principle demonstrations of spectral coherence dynamic measurement on two sources: a supercontinuum source and a fiber ring buffered cavity source, both with a repetition rate of MHz. By employing the optical time stretch with a dispersive fiber, we directly map the spectral interference fringes of the delayed neighbouring pulses and obtain a sufficiently large ensemble of spectral interferograms with a real-time oscilloscope (80Gb/s sampling rate). This enables us to directly quantify the spectral coherence dynamics of the ultrafast sources with a temporal resolution down to microseconds. Having the ensemble of single shot interferograms, we also further calculate the cross spectral coherence correlation matrices of these ultrafast sources. We anticipate that our technique provides a general approach for experimentally evaluating the spectral coherence dynamics of ultrafast laser generated by the nonlinear processes e.g., modulation instability, supercontinuum generation, and Kerr resonator.

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9732-12, Session 2

Dissipative temporal solitons in an excitable micropillar laser with integrated saturable absorber and delayed feedback

Sylvain Barbay, Félix Lelièvre, Ali Golestani, Foued Selmi, Rémy Braive, Grégoire Beaudoin, Isabelle Sagnes, Lab . de Photonique et de Nanostructures (France) A micropillar laser with integrated saturable absorber and delayed feedback is shown experimentally and theoretically to sustain controllable trains of dissipative temporal solitons controlled by adequate optical perturbations in the excitable regime. We show that the pulse train can be started or resynchronized (retiming) with a single perturbation and that the system can store a large variety of temporal pulse patterns. We discuss the relevance of the various timescales at stake for such functionalities. Besides its interest as a compact source of controllable pulses that can be arranged if needed in arrays, this system has also interesting potential for neuromimetic processing of information since it can be considered as a leaky-integrate-and-fire (LIF) optical neuron with delayed feedback. The LIF neuron model is a widespread model in neuroscience and in neurocomputing models and as such it is at the foundation of neural networks, a promising avenue for alternative computation systems.

9732-13, Session 3

Conference 9732: Real-time Measurements, Rogue Events, and Emerging Applications Periodic wave-trains, supercontinuum generation and the formation of rogue waves in an optical fiber cavity

(Invited Paper)

Majid Taki, Zheng Liu, Saliya Coulibaly, Univ . des Sciences et Technologies de Lille (France); François Léo, Univ . Libre de Bruxelles (Belgium)

9732-14, Session 3

Convective Nozaki-Bekki holes in a long laser

Svetlana Slepneva, Tyndall National Institute (Ireland); Ben O’Shaughnessy, Cork Institute of Technology (Ireland) and Tydall National Institute (Ireland); Stephen P . Hegarty, Cork Institute of Technology (Ireland) and Tyndall National Institute (Ireland); Bryan Kelleher, Univ . College Cork (Ireland); Sergio Rica, Univ . Adolfo Ibáñez (Chile); Guillaume Huyet, Cork Institute of Technology (Ireland) and Tyndall National Institute (Ireland) We investigate both theoretically and experimentally the properties of a 20km long unidirectional fiber ring cavity laser that includes a semiconductor optical amplifier and a tunable Fabry-Perot filter. The laser displays turbulent dynamics when the filter transmission is set at a wavelength longer than 1320nm since the laser operates in a self-focusing regime. At lower wavelengths the laser displays multistability between cw and turbulent regimes. We investigate the stability of these solutions both numerically and experimentally by controlling the value of detuning, which is the frequency mismatch between the filter driving frequency and the cavity roundtrip frequency. For the decreasing detuning, the cw solution becomes unstable leading to the appearance of turbulent emission. For the increasing detuning, a subcritical bifurcation occurs, leading to the stabilisation of another cw solution via a turbulent regime. To further investigate the dynamics of this regime, we drive the filter in resonance with the laser round trip frequency. This regime, referred to as Fourier Domain Mode-Locking (FDML), is commonly used to design high-frequency swept sources since the entire sweep is stored in the cavity. It is also worthwhile to notice the analogy with a one dimensional system with a spatially varying detuning. In such a case, we observe the formation of cw and turbulent domains and near the subcritical bifurcation point, we observe the continuous formation of convective turbulent domains that grow and merge into one turbulent region. A detailed analysis shows that these domains emerge from the cw solution by first creating Nozaki-Bekki holes. We study analytically, numerically and experimentally the spontaneous formation of dissipative periodic solutions in a coherently driven passive optical fiber cavity under Raman effect and third-order dispersion. It is shown that the latter, that breaks the time-reversal symmetry, affects the bifurcation nature, at onset of the instability, leading to a transition from a sub- to a super-critical bifurcation. The analytical description of this bifurcation demonstrates an original dependence of the nonlinear saturation term upon the third-order dispersion. A striking feature is the existence of an optimal nonlinear resonance leading the maximum asymmetry in the spectrum. Our theory reveals the key role of third-order dispersion on the asymmetry in the spectrum of the dissipative structures, explains early observations, and the predictions are in excellent agreement with our experimental findings.

In the strongly nonlinear regime, Eckhaus instabilities characterize the secondary instabilities where stable propagating dissipative wave-trains are generated with a definite frequency and a constant group velocity. Their stability range is enlarged in the presence of the third-order dispersion, which determines completely their group velocity.

An increasing in the injected pump value leads to transitions from periodic wave trains to chaotic regimes with a continuous spectrum and the appearance of rogue waves in the form of abnormal high amplitudes. We have analytically characterized this transition and determined the transition curves from regular to chaotic regimes where supercontinuum and rogue waves may occur.

9732-15, Session 3

Levy statistics and rare events in random laser emission

Ravitej Uppu, Tata Institute of Fundamental Research (India) and Univ . Twente (Netherlands); Sushil A . Mujumdar, Tata Institute of Fundamental Research (India) Random lasers are intriguing complex systems that have proven to be a rich testbed of unique statistical studies. Based on the interplay of optical gain and light diffusion [1], the inherent randomness in these systems manifests several fluctuations, including the realization of non-Gaussian heavy-tailed intensity distributions. One puzzling phenomenon observed from these apparently ‘dirty’ systems is the genesis of first-order temporally coherent peaks in the emission spectrum. The high intensities of these peaks constitute the outliers in an otherwise well-behaved distribution. The origin of coherent emission in these systems has recently been modeled using a novel platform of exponentially tempered Lévy sums [2]. The underlying Lévy variables correspond to the amplified spontaneous emission photons that diffuse through multiple scattering. The exponentially-tempered Levy sum model was shown to accurately capture the strong intensity fluctuations. These strong fluctuations in the sums signify strong fluctuations of the extreme values in the summands, i.e. the Lévy variables. Here, we explore the importance of these rare extreme summands in determining the fluctuations in the Lévy sums. Further, we investigate the effect of tempering on the rarity of these extreme summands to explain the Lévy to Gaussian crossover in the statistics of sums. Careful experimental analysis along with detailed photon transport

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Conference 9732: Real-time Measurements, Rogue Events, and Emerging Applications

Monte Carlo simulations signify the importance of rare extreme summands in determining the statistical nature of the intensity fluctuations in random laser emission.

[1] D.S. Wiersma, Nat. Phys., 4, 359 (2008) [2] R. Uppu, and S. Mujumdar, Phys. Rev. Lett., 114, 183903 (2015) Finally, we will discuss how the spectral dynamics, i.e. instantaneous spectrum at each cavity round-trip, could be measured in fiber lasers generating irregular train of pulses of quasi-CW radiaton via combination of heterodyning and intensity spatio-temporal measurement concept. Using this method, we will reveal the existence quasi-stationary localizes lasing modes in the radiation of random fibre laser.

9732-16, Session 3

Spatiotemporal chaos induces extreme events in a spatially extended microcavity laser

Sylvain Barbay, Lab . de Photonique et de Nanostructures (France); Saliya Coulibaly, Lab . de Physique des Lasers, Atomes et Molécules (France); Foued Selmi, Lab . de Photonique et de Nanostructures (France); Marcel G . Clerc, Univ . de Chile (Chile) Extreme and rare events are ubiquitous in nature. They are characterized by rare and high amplitude excursions of a given variable characterizing a physical system with respect to its long time average. The study of extreme events and extreme waves in optics has been primarily motivated by the analogy with rogue waves in hydrodynamics whose formation mechanism is still not well understood but include ingredients such as spatial instabilities, nonlinearities and noise. Here we consider a spatially extended microcavity laser with integrated saturable absorber in the self-pulsing regime. This system has two advantages: first, its typical timescales are fast enough to allow large recordings in a small amount of time and hence to allow for accurate statistics. Second and even more important, it does not display irregular or aperiodic dynamics and hence extreme events without spatial coupling. Hence we can really study the role of spatial coupling in the emergence of extreme events. By recording the dynamics simultaneously in two different spatial points we are able to study whether the extreme events occur through a mechanism of coherent structure collision, as already reported in different systems. With the help of a mathematical model, linear stability and numerical analysis of the dynamics we unveil the dynamical origin of the extreme events found in the occurrence of spatiotremporal chaos rather than in the merging dynamics of coherent structures. The understanding of the formation mechanism of these extreme phenomena is an important step to devise strategies to control them.

9732-18, Session 4

Caustics, rogue waves and an optical sea

Amaury Mathis, Pierre-Ambroise Lacourt, Luc Froehly, Shanti Toenger, FEMTO-ST (France); Frédéric Dias, Univ . College Dublin (Ireland); Goery Genty, Tampere Univ . of Technology (Finland); John M . Dudley, FEMTO-ST (France) Rogue waves are statistically rare events with extreme amplitude or intensity which have been observed in numerous physical systems. In optics particularly, the study of rogue waves has provided new insights into the dynamics of noise-induced soliton propagation during supercontinuum generation. Most studies have focused on how nonlinearity can lead to rogue-wave behavior, but we report here an experimental demonstration of optical rogue waves in a purely linear system which yields a 2D random spatial “sea” of coherent radiation. The experiments are based on free space propagation of a random spatial phase applied via spatial light modulator. By varying the nature of the applied random phase pattern, we are able to see the development of random caustics in the propagating field that yield large amplitude events that satisfy the “significant wave height” criteria of rogue waves. To validate this analogy, we performed optical phase retrieval techniques on the propagating field to extract the amplitude from the intensity distribution – amplitude, not intensity, being relevant in an oceanic context. Additional results show the clear relationship between the width of the spatial spectrum and the emergence of rogue wave events. Indeed, slightly non-Gaussian spectra with high-frequency content but no strong focusing were tested, significantly modifying the statistics of events. These results from optics confirm previous studies of the possible role of caustics in ocean wave dynamics, and provide further evidence that purely linear mechanisms cannot be discounted when studying rogue wave generation mechanisms.

9732-17, Session 4

Spatio-temporal intensity dynamics of passively mode-locked fiber laser

(Invited Paper)

Dmitry V . Churkin, Aston Univ . (United Kingdom); Srikanth Sugavanam, Aston University (United Kingdom) Usually lasers are thought to be operated in some temporal regime. In our work we experimentally show that a laser could be operated in a distinct, albeit complex and dynamic spatio-temporal regime. Different spatio-temporal regimes could have different periodicity properties over different scales, i.e. different spatio-temporal patterns could emerge. We will demonstrate the methodology of real-time measurements of intensity spatio-temporal dynamics on an example of passively mode-locked fibre laser generating noise-like pulses of stochastic filling. Different spatio temporal generation regimes varying by their stochasticity and periodicity properties are experimentally found.

Further, we propose an experimental tool of ACF evolution mapping to reveal the constituents of thought-to-be stochastic radiation and will use this tool to directly detect various types of dark and bright localised structures, including spatio-temporal rogue waves and shock waves, generating within though-to-be stochastic temporal dynamics.

9732-19, Session 4

Slow deterministic vector rogue waves

Sergey V . Sergeyev, Chengbo Mou, Stanislav Kolpakov, Vladimir Kalashnikov, Sergei K . Turitsyn, Aston Univ . (United Kingdom) Rogue wave (RWs) as a concept has been initially introduced in oceanography to describe giant waves with amplitudes that are much larger than an average and so, despite low emergence probability, resulting in destructive impact in nature and society. The main obstacles in modelling and predicting RWs is in the scarcity of events and inability to perform full scale experiments to cause rogue waves to appear in real-world scenarios such as financial markets, power grids, oceans, human brains etc. In this context, mode-locked lasers are perfect test bed systems to study RWs origin and techniques of mitigation with a potential to apply results in numerous disciplines – social sciences, natural sciences and technology and engineering. Here for the first time we demonstrate experimentally and theoretically a new mechanism of deterministic vector RWs emergence in erbium-doped fiber lasers mode-locked with carbon nanotubes. Unlike fast RW dynamics in lasers with pulse-to-pulse power variation, we have found slow vector rogue waves which have the states of polarization and power variations at the time scale of hundreds round trips. We showed that tuning the birefringence and anisotropy in cavity enables random switching between two orthogonal states of polarization that is similar to the stimulated by periodic barrier modulation transitions of the Brownian particle between two potential wells, i.e. Stochastic Resonance. Tailoring

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anisotropy in the cavity leads to asymmetry in the escape probabilities from the potential wells that under certain conditions results in rare switching events with the output power distribution satisfying criteria of RWs.

9732-20, Session 4

Conference 9732: Real-time Measurements, Rogue Events, and Emerging Applications Optical isolator based on topological insulator nano-particles

Moti Fridman, Shir Shahal, Vlada Artel, Doron Naveh, Bar Ilan Univ . (Israel) integrable turbulence. Numerical simulations of 1D-NLSE with stochastic initial conditions reproduce quantitatively the experiments. Our numerical investigations suggest that the statistical features experimentally observed rely on the stochastic generation of coherent analytic solutions of 1D-NLSE.

Pierre Walczak, Stéphane Randoux, and Pierre Suret, Optical Rogue Waves in Integrable Turbulence, Phys. Rev. Lett. 114, 143903 (2015)

9732-22, Session 4

A route to rogue wave generation

Saliya Coulibaly, Zheng Liu, Majid Taki, Univ . des Sciences et Technologies de Lille (France) Optical isolators which serve as one-way optical channels are a crucial component in numerous applications [1]. To obtain optical isolation, the time reversal symmetry must be broken. This braking of time reversal symmetry is usually done via Faraday rotator with constant magnetic field [2] and was also demonstrated with nonlinear interaction of traveling pump waves [3] and with circular polarized light manipulations [4]. We present, optical isolator based on the interaction between light and topological isolators. Our optical isolator is free from external magnetic field and is based on spontaneous time reversal symmetry braking by the topological insulators. The time reversal symmetry braking is obtained through the interaction of the traveling light in tapered fiber with the topological electrons in the surface and in the bulk of Sb2Te3 nano-particles. The interaction of light with the topological electrons rotates the input polarization similar to the Faraday rotator but with no external magnetic field.

Our technique can lead to novel type of non-reciprocal fiber devices and can be implemented in on-chip photonic waveguides. The full experimental and theoretical details of our optical isolator will be presented.

References: [1] D. Jalas, et. al. “What is – and what is not – an optical isolator” Nat. Photon. 7, 579 (2013) [2] K. Shiraishi, et. al., Appl. Opt. 23, 1103 (1984) [3] H. Ramezani, et. al. Phys. Rev. A 82, 043803 (2010) [4] J. W. Goodman “Introduction to Fourier optics” (2005) The frequency combs have emerged as a powerful tool for high-resolution metrology. If in the beginning, ultrafast mode-locked lasers were the main sources of frequency combs, there are now new ways to generate them. By new ways, we especially refer to those based on the use of continuously pumped optical cavity containing a Kerr-type medium–Kerr frequency comb. That is, the Kerr frequency combs belong to the category of optical dissipative structures since they correspond to the spectra associated to the cavity solitons or the periodic solutions and so far, expected to exhibit complex dynamics. In this study, we focused on the dynamics of the periodic solutions that appear in Kerr cavities. More specifically, we study the route to the chaos that flows out from these structures. What we propose here is to consider the route to chaos by means of dynamical systems theory and chaos tools in extended systems. For this purpose, we consider the Lugiato-Lefever model which describes the dynamics of intra-cavity field in Kerr cavities. An instability of the basic solution of this model gives rise to a tunable periodic state with a comb-like spectrum. After providing an analytical description of this comb, its stability analysis have been carried out by means of computation the Lyapunov spectrum allowing to characterize the transition from stable comb to chaotic regime in which rogue waves have been observe and statistically characterized.

9732-21, Session 4

Optical rogue waves in integrable turbulence

Pierre Suret, Lab . de Physique des Lasers, Atomes et Molécules (France); Pierre Walczak, Stephane Randoux, Univ . des Sciences et Technologies de Lille (France) and Lab . de Physique des Lasers, Atomes et Molécules (France)

9732-23, Session 5

FDML lasers with MHz wavelength sweep repetition rates for fastest real-time OCT, spectroscopy, and sensing

(Invited Paper)

Wolfgang Draxinger, Wolfgang Wieser, Optores GmbH (Germany); Jan Philip Kolb, Tom Pfeiffer, Matthias Eibl, Univ . zu Lübeck (Germany); Thomas Klein, Optores GmbH (Germany); Sebastian N . Karpf, LMU München (Germany); Robert A . Huber, Univ . zu Lübeck (Germany) and Ludwig Maximilians-Univ . München (Germany) We report optical experiments allowing to investigate integrable turbulence in the focusing regime of the one dimensional nonlinear Schrodinger equation (1D-NLSE). In analogy with broad spectrum excitation of one dimensional water-tank, we launch random initial waves in a single mode optical fiber. We have developed an original setup which allows the precise measurement of statistics of random light rapidly fluctuating with time. Inspired by the time-resolved fluorescence upconversion experiments and by the optical sampling (OS) oscilloscope, the principle of our method is based on asynchronous OS. Using our apparatus, we measure precisely the probability density function (PDF) of optical power of the partially coherent waves rapidly fluctuating with time. The typical time scale of the resolution is 250fs. In the experiments performed in the focusing regime, the PDF of optical power fluctuations is found to evolve from the normal law to a strong heavy-tailed distribution, thus revealing the formation of rogue waves in Fourier Domain Mode Locked (FDML) lasers are narrowband, rapidly wavelength swept light sources that can generate more than 100nm sweep range at 1000-1550nm center wavelength. At linewidths of a few pm, several million wavelength sweeps per second are possible. The main application of FDML lasers is optical coherence tomography (OCT), where they can achieve about 10x higher speed compared to most other approaches. OCT is a recent microscopic 3D imaging technique and FDML enables imaging speeds of several ten volumes per second. The corresponding voxel rate of about 2 Gigavoxel/s can be sustained and recently live processing of the data on a consumer grade graphics processing unit was demonstrated. The live processing will be important for OCT applications like future OCT aided 4D surgical microscopes. Additionally, fast real-time OCT without live processing will have many more applications - especially in situations where patient motion, turbulent flow dynamics or a maximum total imaging time make non-realtime “sampling-type” approaches unacceptable. Besides the presentation of our own progress on evaluating emerging applications of ultra-high speed real-time OCT, the talk will review work and possibilities of FDML lasers for fast real-time spectroscopy and sensing applications.

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Conference 9732: Real-time Measurements, Rogue Events, and Emerging Applications

9732-24, Session 5

Single-shot high-resolution fiber-based phase-diversity photodetection of optical pulses

Christophe Dorrer, Leon Waxer, Adam Kalb, Elizabeth Hill, Jake Bromage, Univ . of Rochester (United States) with transform limited spectra we can determine suitable initial coefficients and the fastest optimization method [1-4]. The algorithm takes advantage of MATLAB built-in tools for constrained and non-constrained optimization. For complex synthetic pulses the algorithm converges in less than a minute with errors less than 0.1%. For measured pulses the error is limited by the uncertainty of nonlinear spectral measurements by different spectrometers which can be as large as 1%, therefore errors around or less than 1% for measured pulses are acceptable. Inclusion of third harmonic enables us to choose a better starting point and decreases the number of local minima which drastically reduces the convergence times and error. To eliminate time reversal ambiguity we can place a known material in the beam path and reduce the number of global minima to one. Temporally characterizing optical pulses is an important task when building, optimizing, and using optical sources. Direct photodetection with high bandwidth photodiodes and real-time oscilloscopes is only adequate for optical pulses longer than ~10 ps; diagnostics based on indirect strategies are required to characterize femtosecond and sub-10-ps coherent sources.1 Most of these diagnostics are based on nonlinear optics and can be difficult to implement for the single-shot characterization of non-repetitive events. A temporal diagnostic based on phase diversity has been demonstrated in the context of picosecond high-energy laser systems, where single-shot pulse measurements are required for system safety and interpretation of experimental results.2 A plurality of ancillary optical pulses obtained by adding known amounts of chromatic dispersion to the pulse under test are directly measured by photodetection and algorithms are used to accurately reconstruct the input pulse shape from the measured instantaneous power of the ancillary pulses versus time and optical spectrum. This high-sensitivity (~30-pJ) diagnostic is based on a pulse replicator composed of fiber splitters and delay fibers, making it possible to operate with fiber sources and free-space sources after fiber coupling, and with pulses significantly shorter than the photodetection impulse response. Experimental data obtained with various oscilloscopes having bandwidth in the 45- to 70 GHz range will be presented to demonstrate accurate operation at pulse durations in the 1- to 100- ps range. Simulations of the diagnostic and pulse reconstruction algorithm will be presented.

This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE NA0001944, the University of Rochester, and the New York State Energy Research and Development Authority. The support of DOE does not constitute an endorsement by DOE of the views expressed in this article.

1. Walmsley, I. A. and Dorrer, C., “Characterization of ultrashort electromagnetic pulses,” Adv. Opt. Photon. 1, 308–437 (2009).

2. Dorrer, C., Waxer, L., Kalb, A., Hill, E. M. and Bromage, J., “Single-shot characterization of optical pulses below the resolution limit by phase diversified photodetection,” [Conference on Lasers and Electro-Optics 2015], OSA Technical Digest (online), Optical Society of America, Washington, DC, Paper JTh5C.5 (2015).

9732-26, Session 5

Unstable multipulsing can be invisible to some ultrashort pulse measurement techniques

Michelle Rhodes, Zhe Guang, Rick Trebino, Georgia Institute of Technology (United States) Multiple pulsing is a feature of most mode-locked ultrafast laser systems at very high pump powers, and slight variations in the pump power around certain regimes can cause sinusoidally-varying or even chaotic separations among pulses. While this behavior should be absent from well-engineered and well-aligned lasers, it is nevertheless still possible, especially if the system is poorly maintained. Although we have previously studied unstable trains of very noisy pulses, they are significantly different from simple satellite pulses. The impact of unstable multipulsing on modern pulse measurement methods is still unknown.

While spectral shearing interferometry for direct electric field reconstruction (SPIDER) has been shown to be incapable of seeing fluctuating complex pulses, analytical models have suggested that small variations in separation between double-pulses could theoretically be visible in a SPIDER trace and in the resulting measured field, even if the relative phase of the two pulses is allowed to vary. However, we have now performed simulations and find that allowing only the relative phase of a satellite pulse to vary causes the satellite to wash out of the SPIDER measurement completely, even if the separation between pulses is stable. Allowing the separation between pulses (or other pulse parameters) to vary also does not improve matters: SPIDER continues to measure only a single pulse, a phenomenon known as the coherent artifact.

On the other hand, although techniques like FROG and autocorrelation cannot accurately determine the precise properties of satellite pulses, they do succeed in seeing them.

9732-25, Session 5

Spectral phase interrogation using nonlinear spectra (SPINS)

Aram Gragossian, The Univ . of New Mexico (United States); Brook A . Jilek, Sandia National Labs . (United States); Mansoor Sheik-Bahae, The Univ . of New Mexico (United States)

9732-27, Session 5

Modelling of noise-like pulses generated in mode-locked fibre lasers

Sergey Smirnov, Sergey M . Kobtsev, Novosibirsk State Univ . (Russian Federation) There are many techniques for calculating the spectral phase of ultrashort pulses. They all use multiple expensive components and are difficult to build and expensive to buy. We present and algorithm for short pulse characterization buy using one spectrometer and three spectra. Spectral Phase Interrogation using Nonlinear Spectra (SPINS) takes advantage of information embedded in fundamental, second harmonic (SH) and third harmonic (TH) spectra to retrieve the spectral phase and calculate pulse duration. SPINS generates trial SH and TH from the measured fundamental and an initial guess for the spectral phase. The spectral phase is approximated by 6th order Taylor expansion around central frequency. These coefficients are adjusted until the difference between measured and trial SH and TH is minimized. By comparing measured nonlinear spectra Noise-like pulses (NLP) generated in certain passively mode-locked regimes of fibre lasers [1] hold a substantial promise in various applications because they can carry relatively high energy, are not sensitive to the propagation medium dispersion, and can be non-linearly transformed with comparatively high efficiency [2, 3]. However, the structure of these pulses is still not completely understood, and neither is the statistics of temporal&spectral NLP components. The present paper for the first time proposes and studies a relatively simple NLP model that matches the experimental data well and suggests that there is a correlation between phases of neighbouring spectral components of NLP. Comparison of a relatively basic model of “random pulses” with the results of NLP modelling in mode-locked fibre lasers based on coupled non-linear Schrödinger equations (NLSE) demonstrates that

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Conference 9732: Real-time Measurements, Rogue Events, and Emerging Applications

it is possible to use this proposed model for highly efficient simulations of promising NLP applications, such as material processing, non-linear frequency conversion, microscopy, and others. The present work explains the proposed NLP model and compares temporal distributions, intensity autocorrelation function of NLP, statistics of sub-pulse duration, and figures of mode cross-correlations predicted by new model and by NLSE. Further discussed is the possible application of the proposed model in studies of transitional lasing regimes (intermediate between generation of coherent pulses and NLP).

1. S. Smirnov, et al. Optics Express 20(24), 27447 (2012).

2. S. Smirnov, et al. Optics Express 22(1), 1058 (2014).

3. S. Kobtsev, et al. Optics Express 22(17), 20770 (2014).

9732-28, Session 5

Development of on-line laser power monitoring system

Chien-Fang Ding, Meng-Shiou Lee, Kuan-Ming Li, National Taiwan Univ . (Taiwan) Since their invention, lasers have been applied in many fields, including material processing, communications, metrology biomedical engineering, and defense. Laser power is an important parameter in laser material processing, for example, laser cutting and laser drilling. Because laser power is influenced by ambient temperature, laser power status is monitored using laser power meters to ensure effective material processing. Current laser power meters have long response times. Consequently, they cannot measure laser power accurately within a short time. To achieve simultaneous laser power monitoring and effective material processing, we developed a complementary metal-oxide-semiconductor (CMOS) camera–based online laser power monitoring system. The CMOS camera captures images of an incident laser beam after it is split and attenuated by a beam splitter and a neutral density filter, respectively. By comparing the average brightness of the beam spots and the measurement results of the laser power meter, we can estimate laser power. In the continuous measurement mode, the average measurement error of the developed laser power monitoring system was approximately 3%, and it responded at least 3.6 s quicker than conventional thermopile power meters do. In the trigger measurement mode, which allows for synchronization of the CMOS camera with intermittent laser output, the average measurement error was lower than 3%, and the quickest response time was 20 ms.

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Conference 9733: High-Power Diode Laser Technology and Applications XIV

Monday - Tuesday 15–16 February 2016 Part of Proceedings of SPIE Vol . 9733 High-Power Diode Laser Technology and Applications XIV 9733-1, Session 1

Reliability of high power laser diodes with external optical feedback

Dennis Bonsendorf, Stephan Schneider, Jens Meinschien, LIMO Lissotschenko Mikrooptik GmbH (Germany); Jens W Tomm, Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie (Germany) Direct diode laser systems gain importance in the fields of material processing and solid-state laser pumping. With increased output power, also the influence of strong optical feedback has to be considered. Uncontrolled optical feedback is known for its spectral and power fluctuation effects, as well as potential emitter damage. We found that even intended feedback by use of volume Bragg gratings (VBG) for spectral stabilization may result in emitter lifetime reduction. To provide stable and reliable laser systems design, guidelines and maximum feedback ratings have to be found. We present a model to estimate the optical feedback power coupled back into the laser diode waveguide. It includes several origins of optical feedback and wide range of optical elements. The failure threshold of InGaAs and AlGaAs bars has been determined not only at standard operation mode but at various working points. The influence of several feedback levels to laser diode lifetime is investigated up to 4000h. The analysis of the semiconductor leads to a better understanding of the degradation process by defect spread. Facet microscopy, LBIC- and electroluminescence measurements deliver detailed information about semiconductor defects before and after aging tests. Laser diode protection systems can monitor optical feedback. With this improved understanding, the emergency shutdown threshold can be set low enough to ensure laser diode reliability and high enough to provide better machine usability avoiding false alarms.

9733-2, Session 1

Rapid stress-testing vs. long-term aging: a case study of 980-nm emitting single spatial mode lasers

Jens Wolfgang W . Tomm, Martin Hempel, Max-Born Institut für Nichtlineare Optik und Kurzzeitspektroskopie (Germany); David Venables, Victor Rossin, Erik Zucker, Lumentum (United States); Thomas Elsaesser, Max-Born Institut für Nichtlineare Optik und Kurzzeitspektroskopie (Germany) Single-spatial mode lasers emitting at 980-nm are exposed to two types of lifetime testing, namely rapid stress-testing and long-term aging. We find that both activate the same sudden degradation mechanism, namely internal catastrophic optical damage at 0.6-1.0 mm behind the front facet. In the case of ultra-high power operation, we show that the mechanism that initializes this effect is a lateral widening of the optical mode, resulting in increased absorption outside the waveguide. Defects formed during long term aging may eventually lead to a comparable mode distortion and finally to device failure. Single-pulse stress-testing allows for activation several degradation mechanisms in a device one after the other, and allows for distinguishing between gradual aging and defects that are independent of the aging status. Although the results presented are monitored with a rather complex setup, it turns out that monitoring the devices along their laser-axis with cameras provides rich information on gradual and sudden degradation effects. We expect that after once the acting mechanisms are understood, stress-tests can be performed with simple and cheap setups, consisting of a pulse generator and a standard CCD-camera only. Although we are convinced that accelerated long-term aging tests will remain the main method for lifetime predictions of diode lasers, stress-tests could pave the way towards more time-efficient testing, e.g., for comparison of different technology variants in development. While this report focuses on the behavior of a specific device type, both setup and methodology are transferable to other types of diode lasers.

9733-3, Session 1

Reliability, failure modes, and degradation mechanisms in high power single- and multi-mode InGaAs-AlGaAs strained quantum well lasers

Yongkun Sin, Nathan Presser, Zachary Lingley, Miles Brodie, Adam Bushmaker, Brendan Foran, Steven C . Moss, The Aerospace Corp . (United States) High-power single- and multi-mode InGaAs-AlGaAs strained QW lasers are critical components for telecommunications and potential space satellite communications systems. However, little has been reported on failure modes of state-of-the-art SM InGaAs-AlGaAs strained QW lasers although it is crucial to understand failure modes and underlying degradation mechanisms in developing these lasers that meet lifetime requirements for space satellite systems. Our present study addresses these issues by performing long-term life-tests followed by FMA and physics-of-failure investigation. We performed long-term accelerated life-tests on state-of-the-art SM and MM InGaAs-AlGaAs strained QW lasers. Our life-tests have accumulated over 20,000 test hours for SM lasers and over 35,000 test hours for MM lasers. FMA was performed on failed SM lasers using EBIC. This technique allowed us to identify failure types by observing dark line defects. All the SM failures we studied showed catastrophic and sudden degradation and all of these failures were bulk failures. Our group previously reported that bulk failure or COBD is the dominant failure mode of MM InGaAs-AlGaAs strained QW lasers and this is the first report demonstrating that the dominant failure mode of SM and MM InGaAs-AlGaAs strained QW lasers is the bulk failure. Since degradation mechanisms responsible for COBD are still not well understood, we also employed FIB and high-resolution TEM to further study dark line defects and dislocations in pre- and post-aged SM and MM lasers. In addition, the photocurrent spectra were measured from localized areas to study the effects of COBD on changes in absorption characteristics in MM lasers.

9733-4, Session 1

Reliability study on high power 638-nm triple emitter broad area laser diode

Tetsuya Yagi, Kyosuke Kuramoto, Kaoru Kadoiwa, Ryuta Wakamatsu, Motoharu Miyashita, Mitsubishi Electric Corp . (Japan) The digital cinema projector based on laser light source started its installation to the theaters, and the laser diode (LD) backlight LCD already appeared in the consumer market. So far, the next may be a laser based projector with one spatial light modulator (SLM), which uses the laser light source under the pulse condition for time division color expression. We already developed the 638 nm triple emitter broad area (BA) LD assembled

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Conference 9733: High-Power Diode Laser Technology and Applications XIV

on 9.0-mm TO package for this application, which outputs peak power of 5.5 W under pulse operation with duty cycle (Dc) of 30%, 25 C. In this paper, the latest reliability study on this LD will be described in detail. Main cause of the degradation of 638-nm BA-LD is catastrophic optical mirror degradation (COMD). It was reported that the output power, at which the COMD occurs, (Pcod) decreases as the aging time goes by. When Pcod reaches to the operation power, the failure due to COMD occurs. The Pcod behaviors on the several aging conditions were studied. It was revealed that the mean time to failure (MTTF) of the LD was proportional to the -4.3 power of the output power. The long term aging under the over-drive condition was also performed. The emitter failed individually and the failed emitter didn’t affect the other emitters in the same chip. By using them, the MTTF of this LD was estimated exceeding 20,000 hours under the output of 2.5 W, Dc of 30%.

levels over 15 W and 20 % conversion efficiency from 20% FF bars with 2 mm cavity length on a conductively cooled platform. Life testing of the 1470 nm lasers bars over 14,000 hours under constant current mode has shown no significant degradation.

9733-7, Session 2

A fiber-coupled 9xx module with tap water cooling

David A . Schleuning, Athanasios N . Chryssis, Geunmin Ryu, Guoli Liu, Heiko Winhold, Li Fan, Zuntu Xu, Tawee Tanbun Ek, Sami Lehkonen, Bruno Acklin, Coherent, Inc . (United States)

9733-5, Session 1

Sequential description of the catastrophic optical damage of high power laser diodes

Jorge Souto, Jose Luis Pura, Alfredo Torres, Juan Jimenéz, Univ . de Valladolid (Spain); Mauro A . Bettiati, Francois J . Laruelle, 3SP Technologies S .A .S . (France) The catastrophic optical degradation (COD) of high power laser diodes is described on the bases of the cathodoluminescence (CL) analysis of the degraded areas, and a thermomechanical model accounting for the mechanism leading the degradation process. The COD is frequently observed at the front mirror; although, under very high power conditions (pulsed current operation) internal COD can also occur. CL is a very suitable experimental technique to reveal the main features of degradation in laser diodes; the defects generated by the degradation in the laser cavity are observed by CL imaging; simultaneously spectrally resolved CL permits to monitor the spectral changes induced by the degradation. The COD is observed as a very local process, associated with the formation of networks of extended defects and subsequent propagation as a result of the local enhancement of the temperature. In view of this observation, one can set-up a thermomechanical model describing the thermal runaway leading to the COD. The thermal stresses generated by the local heating are the leading forces driving the plastic deformation of the laser structure; the generation and propagation of networks of dislocations, mostly localized in the QW layer, are observed in the CL images. Important physical parameters of the laser structure involved in this process are the thermal conductivity and the mechanical strength. We demonstrate that the evolution of these parameters during laser operation accounts for the sudden temperature increase in local regions of the laser cavity.

9733-6, Session 1

Improved long wavelength 14xx and 19xx nm InGaAsp/InP lasers

Tawee Tanbun-Ek, Rajiv Pathak, Zuntu Xu, Heiko Winhold, Serguei Kim, Fei Zhou, Arne-Heike Meissner-Schenk, Michael Peter, Geunmin Ryu, David A . Schleuning, Coherent, Inc . (United States) We report on our progress developing long wavelength high power laser diodes based on the InGaAsP/InP alloy system emitting in the range from 1400 to 2010 nm. Output power levels exceeding 50 Watts CW and 40% conversion efficiency were obtained at 1470 nm wavelength from 20% fill factor (FF) bars with 2 mm cavity length mounted on water cooled plates. Using these stackable plates we built a water cooled stack with 8 bars, successfully demonstrating 400 W at 1470 nm with good reliability. In all cases the maximum conversion efficiency was greater than 40% and the maximum power achievable was limited by thermal rollover. For lasers emitting in the range from 1930 to 2010 nm we achieved output power Many direct diode and fiber laser pumping applications require a multi-kW laser diode module. A new 9xx nm laser diode chip has been developed with improved efficiency, brightness, and reliability. A water-cooled packaging architecture utilizing hard solder has been developed using an electrically isolated cooler which provides low thermal resistance (<0.25 oK/W). The package has been studied in accelerated CW conditions as well as hard-pulse on-off cycling mode operation. The electrical isolation enables the use of “tap water” to cool the laser diodes. A fiber-coupled module converts the asymmetric fast and slow axis beams into an optical fiber with over 2kW of power in a single wavelength, single polarization configuration. The module has passed a number of passive environmental qualification tests. Extensive accelerated multi-cell lifetests using packaged bars, water cooled stacks, and optical modules have been performed with over ten million device hours of test.

9733-8, Session 2

High-brightness laser diode module over 300W with 100µm/Na 0.22 fiber

Yohei Kasai, Shinichi Sakamoto, Yukihiko Takahashi, Ken Katagiri, Fujikura Ltd . (Japan); Yuji Yamagata, OPTOENERGY Inc . (Japan); Akira Sakamoto, Daiichiro Tanaka, Fujikura Ltd . (Japan) High-brightness fiber-coupled laser diode module is one of the most important devices for high-power fiber laser systems. Fibers with 100 um core diameter are widely used for these modules because their cladding diameters are the same as fibers used in telecommunication market and they are easy to handle. The power up to only 150 W is commercially available with 100 um fibers, and 200 um core size is needed for higher power modules. We have overcome this limitation and achieved optical output power over 300 W with 100 um / NA 0.22 fiber by integrating the several tens of optimally-designed single emitter laser diodes in a newly designed package. We employed unique design such as Asymmetric Decoupled Confinement Heterostructure (ADCH) and the stripe width to increase the durability for the catastrophic optical damage.

The beams from each laser diode are aligned precisely by optimally designed lenses and mirrors, especially pitch of stacked beam along the fast axes and the current dependence of the divergence angle of the slow axes. In addition, polarization multiplex technique is used to double the brightness.

High-heat conducting materials are used as base materials of the module package to reduce the thermal resistance of the module. The junction temperature of the laser diode is suppressed and high-power and high reliability laser diode modules are obtained. As a result, over 300 W power are achieved without thermal rollover at 16 A.

The high-brightness modules have a great advantage for high power fiber lasers such as 10 kW and beyond.

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Conference 9733: High-Power Diode Laser Technology and Applications XIV

9733-9, Session 2

Multi-kW high-brightness fiber coupled diode laser based on two dimensional stacked tailored diode bars

Andreas Bayer, Andreas Unger, Bernd Köhler, Matthias Küster, Sascha Dürsch, Heiko Kissel, Jens Biesenbach, DILAS Diodenlaser GmbH (Germany) The demand for high brightness fiber coupled diode laser devices in the multi kW power region is mainly driven by industrial applications for materials processing, like metal welding, which requires a beam quality better than 30 mm x mrad. Reliability, modularity, and cost effectiveness are key factors for success in the market. We have developed a scalable and modular diode laser architecture that fulfills these requirements through use of a simple beam shaping concept based on two dimensional stacking of tailored diode bars mounted on specially designed, tap water cooled heatsinks.

For a single wavelength, up to eight of these building blocks, implying a total of 32 tailored bars, can be stacked into a submodule, polarization multiplexed, and coupled into a 400 µ m, 0.12NA fiber. Output power of such a submodule is more than 1 kW with an electro-optical efficiency above 50%. Due to the simple beam shaping concept, the majority of the optical alignment processes inside the submodule are fully automated to guarantee constant quality, reliability, and a cost-efficient production process.

Scalability into the multi kW region is realized by wavelength combining of replaceable submodules in the spectral range from 900 – 1100 nm. We present results of a laser source based on this architecture with an output power of more than 4 kW and a beam quality of 25 mm x mrad.

As applications for diode lasers in the fields of solid-state pumping, fiber laser pumping, and materials processing continue to demand higher power and brightness, there is a strong market for highly efficient, lightweight diode pump modules. Many of these applications, such as airborne or field transportable systems, require a high degree of mobility, while others may simply benefit from lower module size where space is at a premium. We present an update on several lightweight diode pump modules with power output in the range of 600W to almost 1kW, weight-to-power ratio (WPR) from 0.9 to 0.7 kg/kW, and electro-optical efficiencies well in excess of 50%.

DILAS has previously reported on the IS46, a successful pumping product with a 300W output from a ~300g module and many hundred modules in the field. Here we present an updated version of the IS46 providing ~350W output power at the same weight. Also presented are updated results from the previously reported IS53 module, which now shows 600W of output, and data from a new prototype capable of producing nearly 1kW from a 225um, 0.22NA fiber. The basic technologies that allow such compact systems will be discussed, with special emphasis on the engineering challenges of high efficiency, lightweight system design. In particular, primary drivers that allow high optical efficiency, the management of waste heat, and the design of the associated laser cooling architecture will be reviewed.

9733-12, Session 3

Reduced-mode (REM) diodes enable high brightness fiber-coupled modules

Manoj Kanskar, Ling Bao, Zhigang Chen, David Dawson, Mark DeVito, Weimin Dong, Mike P . Grimshaw, Xinguo Guan, Marty Hemenway, Keith Kennedy, Rob Martinsen, Wolfram Urbanek, Shiguo Zhang, nLIGHT Corp . (United States)

9733-10, Session 3

Advances in high-power 9XXnm laser diodes for pumping fiber lasers

Jay Skidmore, Matthew Peters, Victor Rossin, James Guo, Yan Xiao, Jane Cheng, Allen Shieh, Raman Srinivasan, Jaspreet Singh, Cailin Wei, Richard Duesterberg, James J Morehead, Erik Zucker, Lumentum (United States) Laser diodes with high brightness and low cost are critical for pumping kW-class fiber lasers. The ST-series multi-emitter 9XXnm fiber-coupled pump platform was previously designed to balance reliable output power with suitable brightness (i.e., 140W ex-fiber, 105um core, with > 95% power into 0.15NA). The goal of the present study is to reduce pump count by increasing the diode power level, while maintaining a similar brightness level of the ST pump. In this way, we can reduce the total pump cost (in terms of $/W) at the system level. Laser diodes are spatially multiplexed into a 135um diameter core fiber. The package architecture leverages the same optical train and mechanical structure that was qualified previously. As compared to ST form factor, the package is extended by < 1”, whereas the height, width and bolt pattern remain unchanged for backward compatibility. With the latest-generation laser diode pump platform, we have achieved 250W CW power at 22A at ~ 30C case temperature. The power conversion efficiency is 55% (peak) that drops to 47% at 22A with little thermal roll over. Greater than 95% of the light is collected at < 0.15NA at 16A drive current.

There is an increasing demand for high-power, high-brightness diode lasers at 885 nm, 915 nm and 976 nm for applications such as fiber laser pumping, materials processing, solid-state laser pumping, and consumer electronics manufacturing. The kilowatt CW fiber laser pumping (915 nm for industrial fiber lasers & 976 nm for directed energy fiber amplifiers) particularly requires the diode lasers to have both high power and high brightness in order to achieve high-performance. This paper presents our continued progress in the development of high power and high brightness fiber-coupled product platform, element, to address these applications. Recent improvements in fiber-coupled power has been enabled by significant advances in the slow-axis brightness of broad area lasers. We have demonstrated slow-axis brightness as high as 4.3 W/mm-mrd resulting from reducing the number of allowed modes in the slow-axis direction. Further improvement is underway. This new generation of reduced-mode diodes (REM-diodes) have half the slow-axis divergence compared to regular BALs at the same operating powers. As a result, we have achieved >160 watts from a 2?6 element module in the 9xx nm spectral range out of a 105 µ m/0.15 NA beam. We have also extended the REM concept to geometrically multiplexing multiple of these devices to scale up power at higher BPP. We will present performance and reliability data on these devices. Further slow-axis brilliance improvement is underway and we will report the latest findings.

9733-11, Session 3

Lightweight diode laser pump modules push to a new class of higher power and lower weight

David A . Irwin, DILAS Diode Laser, Inc . (United States)

9733-13, Session 3

DPAL pump system exceeding 3kW at 766nm and 30 GHz bandwidth

Tobias P . Koenning, Dan McCormick, David A . Irwin, Dean Stapleton, Tina Guiney, Steve G . Patterson, DILAS Diode Laser, Inc . (United States) Due to their low quantum defect, diode pumped alkali metal vapor lasers (DPALs) offer the promise of scalability to very high average power

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Conference 9733: High-Power Diode Laser Technology and Applications XIV

levels while maintaining excellent beam quality. Research on DPALs has progressed to ever increasing power levels across multiple gain media species over the last years, necessitating pump power in the kW range. Each material requires a specific pump wavelength: near 852 nm for cesium, 780 nm for rubidium, 766 nm for potassium, and 670 nm for lithium atoms. The shorter pump wavelength below 800nm are outside the typical wavelength range for pump diodes developed for diode pumped solid state lasers (DPSS). The biggest challenge in pumping these materials efficiently is the need for maintaining the narrow gain media absorption band of approximately 0.01 nm while greatly increasing power. Typical high power diode lasers achieve spectral widths around 3nm (FWHM) in the near infrared spectrum, but optical gratings may be used internal or external to the cavity to reduce the spectral width. Recently, experimental results have shown yet narrower line widths ranging from picometers at very low power levels to sub-100 picometers for water cooled stacks around 1kW of output power.

The focus of this work is the development of a fiber-based pump system for potassium DPAL. The individual tasks are the development of high power 766nm chip material, a fiber-coupled module as a building block, and a scalable system design to address power requirements from hundreds of watts to tens of kilowatts. Results for a 3kW system achieving ~30GHz bandwidth at 766nm will be shown. Approaches for power-scaling and size reduction will be discussed.

(Germany) We present our building block system concept, which consists of 500 W building blocks, which will be stacked via variations of combining methods to multi kW systems, while enabled easy field maintenance. The key subassembly is a structure consisting of single emitters, automatically aligned individual fast axis collimation lenses and a monolithic slow axis collimation and redirection device. By varying the diode wavelengths they can later be combined by various techniques and therefore power scaled. The diodes on a subassembly are vertically stacked, can be wavelength stabilized and lead to an output power of well above 100 W with BPP of <3.5 x 5 mm*mrad (fast x slow axis). These base modules now can be further power scaled by combinations of polarization and wavelength multiplexing, which results in building blocks with > 500 W optical output power within a 100 quality.

µ m fiber with 0.15 NA or by combination of several building blocks up to multi kW systems without change in beam In addition the built-in electronics is monitoring, controlling and diagnosing each individual wavelength channel via, further more due to the 48 V architecture and only 12 A per channel, the laser systems are capable of short µ s pulses up to cw.

The combination of this intelligent electronics with industrial interfaces as TCP/IP, which allows remote control and –diagnostic, and the building block concept enables easy and sustainable field maintenance.

9733-14, Session 4

Spectral beam combining of multi-single emitters

Baohua Wang, Weirong Guo, Zhijie Guo, Dan Xu, Jing Zhu, Qiang Zhang, Thomas C . Yang, Xiaohua Chen, BWT Beijing Ltd . (China) Spectral beam combination expands the output power while keeps the beam quality of the combined beam almost the same as that of a single emitter. Spectral beam combination has been successfully achieved for high power fiber lasers, diode laser arrays and diode laser stacks. We have recently achieved the spectral beam combination of multiple single emitter diode lasers. Spatial beam combination and beam transformation are employed before beams from 25 single emitter diode lasers can be spectrally combined. An average output power about 220W, a spectral bandwidth less than 9 nm (95% energy), a beam quality similar to that of a single emitter and electro-optical conversion efficiency over 46% are achieved. In this paper, Rigorous Coupled Wave analysis is used to numerically evaluate the influence of emitter width, emitter pitch and focal length of transformation lens on diffraction efficiency of the grating and spectral bandwidth. To assess the chance of catastrophic optical mirror damage (COMD), the optical power in the internal cavity of a free running emitter and the optical power in the grating external cavity of a wavelength locked emitter are theoretically analyzed. Advantages and disadvantages of spectral beam combination are concluded.

9733-16, Session 4

Compact 35

µ

m fiber coupled diode laser module based on dense wavelength division multiplexing of NBA mini-bars

Ulrich Witte, Martin Traub, Angelo Di Meo, David Rubel, Marcus Hamann, Stefan Hengesbach, Hans-Dieter Hoffmann, Fraunhofer-Institut für Lasertechnik (Germany) We present a compact, modular and cross talk free approach for dense wavelength division multiplexing of high power diode lasers based on ultra steep dielectric filters. The filters are characterized in a newly developed characterization stage and have a band edge steepness of approximately 0.7 nm. One submodule consists of two mini bars that are geometrically stacked in the fast axis (FA). The collimated beams of the individual emitters are subsequently spectrally combined by use of dielectric filters. For fiber coupling, a rotationally symmetric focusing lens is utilized. The mini bars consist of 5 narrow stripe broad area (NBA) emitters with a beam parameter product in the range of 2 mm mrad and a wavelength spacing of 2.5 nm between 2 adjacent emitters. The near field with of the emitters is 30 µ m up to 40 µ m. Internally stabilized DFB-NBA laser diodes are compared with AR-coated and externally stabilized NBA laser diodes. For external stabilization one partially reflective mirror is used to build an external self locking cavity. Experimental results for fiber coupling (35 µ m core diameter, NA < 0.18) of internally and externally stabilized diode lasers are presented. Additionally, optical losses are analyzed and alternative optical designs to overcome the current limitations of the setup are discussed.

9733-15, Session 4

Building block diode laser concept for high brightness laser output in the kW range and its applications

Andreas Grohe, Fabio Ferrario, Haro Fritsche, Thomas Hagen, Holger Kern, Ralf Koch, Bastian Kruschke, Axel Reich, Dennis Sanftleben, Ronny Steger, Till Wallendorf, Wolfgang Gries, DirectPhotonics Industries GmbH

9733-17, Session 4

High-power operation of coherently coupled tapered laser diodes in an external cavity

Guillaume Schimmel, Ioana Doyen, Sylvie Janicot, Marc Hanna, Patrick Georges, Gaëlle Lucas-Leclin, Lab . Charles Fabry (France); Jonathan Decker, Paul Crump, Goetz Erbert, Ferdinand-Braun-Institut (Germany); Simeon Kaunga-Nyirenda, Daniel Moss, Steve Bull, Eric Larkins,

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Conference 9733: High-Power Diode Laser Technology and Applications XIV

The Univ . of Nottingham (United Kingdom) Coherent Beam Combining (CBC) consists in superposing several beams that are coherent with each other, thereby generating a single high-power laser beam with excellent spectral and spatial properties. We investigate a new CBC architecture using a common laser external cavity on the back side of the emitters for phase locking, while coherent beam superposition of phase-locked beams is realized on the front side. This technique leads to a separation of the phase-locking stage – which takes place in the common external cavity on the rear side of the lasers - and the beam combining stage – which is achieved on the front side outside the cavity. In this contribution, this technique is applied to high-brightness tapered devices demonstrating the potential of extended-cavity phase-locking arrangements for high-power operation.

The rear-side common resonator used for phase-locking of the two emitters is a passive external cavity designed as a Michelson interferometer. The phase-locking range and the resistance of the external cavity to perturbations are investigated. On the front side the combining efficiency is above 80% and results in an output power of 6.5 W in a nearly diffraction limited beam (M?(4?) ≤ 1.2). The corresponding electrical-to-optical efficiency is above 20%. The combined power is stabilized with an automatic adjustment of the driving currents.

9733-18, Session 4

Wavelength locking of single emitters and multi-emitter modules: Simulation and experiments

Dan Yanson, Noam Rappaport, Ophir Peleg, Yuri Berk, Nir Dahan, Genady Klumel, Ilya Baskin, Moshe Levy, Yoram Karni, SCD SemiConductor Devices (Israel) Wavelength-stabilized high-brightness single emitters are commonly used in fiber-coupled laser diode modules for pumping Yb-doped lasers at 976 nm, and Nd-doped ones at 808 nm. We investigate the spectral behavior of single emitters under wavelength-selective feedback from a volume Bragg (or volume hologram) grating (VBG) in a multi-emitter module. By integrating a full VBG model as a multi-layer thin film structure with commercial raytracing software, we simulated wavelength locking conditions as a function of beam divergence and angular alignment tolerances. Good correlation between the simulated VBG feedback strength and experimentally measured locking ranges, in both angle and laser temperature, is demonstrated.

The challenges of assembling multi-emitter modules based on beam stacked optical architectures are specifically addressed, where a common VBG is placed to wavelength-lock all emitters together. The wavelength locking conditions must be achieved simultaneously with a high fiber coupling efficiency for each emitter in the module. It is shown that even a minor angular misorientation between fast and slow-axis collimating optics can have a dramatic effect on the spectral and power performance of the module.

We report on the progress towards development of a wavelength-stabilized fiber laser pump module, which uses a VBG to provide wavelength-selective optical feedback in the collimated portion of the beam. Powered by our purpose-developed high-brightness single emitters, the module reaches 45 W output from a 105 µ nm are also presented.

m core, 0.15 NA fiber with an 0.3 nm linewidth at a wavelength of 976 nm. Preliminary wavelength-locking experiments at 808

9733-19, Session 4

Simultaneous frequency stabilization and high-power dense wavelength division multiplexing (HP-DWDM) using an external cavity based on volume Bragg gratings (VBGs)

Stefan Hengesbach, Sarah Klein, Carlo Holly, Ulrich Witte, Martin Traub, Hans-Dieter Hoffmann, Fraunhofer-Institut für Lasertechnik (Germany) Multiplexing technologies enable the development of high-brightness diode lasers for direct industrial applications. We present a compact High-Power Dense Wavelength Division Multiplexer (HP-DWDM) with an average channel spacing of 1.7 (1.5) nm and a subsequent external cavity mirror to provide feedback for frequency stabilization and multiplexing in one step. The „self-locking“ multiplexing unit consists of four Volume Bragg Gratings (VBGs) with 99% diffraction efficiency and seven dielectric mirrors to overlay the radiation of five input channels with an adjustable channel spacing of 1-2 nm. In detail, we focus on the analysis of the overall optical efficiency and the change of the beam parameter product and the spectral width. The performance is demonstrated using five 100 microns wide multimode 9xx single emitters with M2 ≤ 17. Because of the feedback the lateral (multimodal) spacial and angular intensity distribution changes strongly and the beam parameter product decreases by a factor of 1.2 to 1.9. Thereby the angular intensity distribution is more affected than the near field intensity distribution. The spectral width per emitter decreases to 3-200 pm (FWHM) depending on the injection current and the reflectance of the feedback mirror (0.75%, 1%, 2%, 4% or 6%). Thus, the laser bandwidth is much smaller than the wavelength selectivity of the Volume Bragg Grating due to gain narrowing and an optimized feedback strength. The overall optical multiplexing efficiency ranges between 77% and 86%. With some modifications (e.g. enhanced AR coatings) we expect 90-95%.

9733-20, Session 5

940nm QCW diode laser bars with 70% efficiency at 1 kW output power at 203K: analysis of remaining limits and path to higher efficiency and power at 200K and 300K

(Invited Paper)

Carlo F . Frevert, Frank Bugge, Steffen Knigge, Arnim Ginolas, Götz Erbert, Paul Crump, Ferdinand-Braun-Institut (Germany) Both high-energy-class laser facilities and commercial high-energy pulsed laser sources require reliable optical pumps with the highest pulse power and electro-optical efficiency. Although commercial quasi-continuous wave (QCW) diode laser bars reach output powers of 300…500W further improvements are urgently sought to lower the cost per Watt, improve system performance and reduce overall system complexity. Diode laser bars operating at temperatures of around 200K show significant advances in performance, and are particularly attractive in systems that use cryogenically cooled solid state lasers. We present the latest results on 940nm, passively cooled, 4mm long QCW diode bars which operate under pulse conditions of 200?s, 10Hz at an output power of 1kW with efficiency of 70% at 203K: a two-fold increase in power compared to 300K, without compromising efficiency. We discuss how custom low-temperature design of the vertical layers can mitigate the limiting factors such as series resistance while sustaining high power levels. We then focus on the remaining obstacles to higher efficiency and power, and use a detailed study of multiple vertical structures to demonstrate that the properties of the active region are a major performance limit. Specifically, one key limit to series resistance is transport in the layers around the active region (resistance is a

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9733-21, Session 5 9733-22, Session 5

Conference 9733: High-Power Diode Laser Technology and Applications XIV

clear function of well depth) and the differential internal efficiency is closely correlated to the transparency current density. Tailoring the barriers around the active region and reducing transparency current density thus promise bars with increased performance at temperatures of 200K as well as 300K.

High duty-cycle, high-efficiency QCW stacks for medical applications

Alex Kindsvater, Matthias Schroeder, Ekkehard A . Werner, Sebastian Seidel, Martin Woelz, Valentin Loyo, JENOPTIK Laser GmbH (Germany) High duty-cycle, high-efficiency QCW stacks are ideally suited for medical and cosmetic applications. The 8-bar stack with a bar-to-bar pitch of 1,7mm emits 808 or 940nm wavelength with 65 to 95 Joule depending on pulse conditions at 25°C water temperature. The value is limited by thermal effects and can be increased by choosing the lower limit of the cooling specification. This stack is specified for up to 130 A at 50ms, 15% d.c., down to 45 A at 400ms, 55% d.c. The reliability for long pulse operation is targeted to reach up to 450 hours of total life time. That is equivalent to 15 Mshots at pulse durations less or equal 100ms or 4 Mshots at 400ms. The stack heat sink is cooled by filtered tap water at 15 – 25°C with flow rates of up to 0.9 ±0.1 l/min at inlet pressures up to 1.7 ±0.1 bar. Efficiency, thermal impedance and wavelength plots are presented in the paper.

980nm semiconductor lasers and spot size converter monolithically integrated technology research

Wentao Guo, Manqing Tan, Institute of Semiconductors (China) output powers Popt > 10W with efficiency > 60%. However, their application is limited due to poor in-plane beam parameter product BPPlat=0.25?FF95%?w95% (FF95% and w95% are emission angle and aperture, 95% power content). We present progress in improving BPPlat by choosing the proper vertical designs. First, we show that large vertical asymmetry is beneficial, with extreme-double-asymmetric (EDAS) waveguide designs delivering a ~1 mm?mrad reduction in the BPP ground level, compared to a reference (more symmetric) design. Possible reasons include a reduced number of lateral modes, a modified thermal lens profile or an improvement in the beam quality of the individual modes. Measured thermal profiles show no clear evidence for a difference in the lateral thermal lens when EDAS designs are used and 2-D waveguide simulation based on the profiles shows no significant difference in confinement factor of higher order modes. Therefore, the improved BPPlat is most likely due to an improvement in the lateral mode quality. Second, we test the benefit of low modal gain factor Gg0, predicted to improve BPPlat via a suppression of filamentation. EDAS-based lasers with single and double quantum well active regions were compared, with 2.5x reduced Gg0, for 2.2x reduced filament gain. However, no difference is seen in measured BPPlat, giving evidence that filamentary processes are no longer a limit. In contrast, devices with lower Gg0 demonstrate an up to twofold reduced near field modulation depth, potentially enabling higher facet loads and increased device facet reliability.

9733-24, Session 5

Optical mode engineering and high power density per facet length (>8.4 kW/cm) in tilted wave laser diodes

Nikolay Ledentsov, Vitaly A . Shchukin, VI Systems GmbH (Germany); Mikhail V . Maximov, Nikita Y . Gordeev, Nikolay A . Kaluzhniy, Sergey A . Mintairov, Alexey S . Payusov, Yuri M . Shernyakov, Ioffe Physical-Technical Institute (Russian Federation) The 980nm semiconductor lasers are the important pump sources of the Erbium-Doped Fiber Amplifier (EDFA) and Erbium-Doped Superfluorescent Fiber Source (ED-SFS). They have important significance to the high speed, large capacity, long distance all-optical communication systems and the high-precision fiber optic gyroscope (FOG). The asymmetric twin waveguide (ATG) technology was used to prepare the 980nm FP cavity semiconductor lasers and the spot size converter integrated devices. The SSC-LD equivalent resonator model has been established to do the theoretical calculation of the coupling coefficient and gain coefficient. The design of the device structure has been optimized by researching the influencing mechanism of tapered waveguide’s shape and the epitaxial layer’s structure parameters on the output characteristics of the devices. The technical scheme of SSC-LD’s preparation has been studied on using ATG technology. Finally, the sample with the center wavelength of 980nm±3nm, less than 10° for vertical far field divergence angle has been made in the experiment.

9733-23, Session 5

Assessing the influence of the vertical design on the lateral beam quality of high power broad area diode lasers

Martin Winterfeldt, Steffen Knigge, André Maaßdorf, Ferdinand-Braun-Institut (Germany); Martin Hempel, Jens W . Tomm, Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie (Germany); Götz Erbert, Paul Crump, Ferdinand-Braun-Institut (Germany) GaAs-based high-power broad-area diode lasers deliver NIR optical Thick waveguide laser diodes allow a reduced impact of the catastrophic optical mirror damage and thermal “smile” effects in laser chips and bars. Tilted Wave Lasers (TWLs) [1] based on optically coupled thin active and thick passive waveguides may offer an ultimate solution, in which the thickness of the passive waveguide can vary from ~10–35 um realized by an epitaxial layer [2] and up to ~100–150 um [2] if a transparent substrate is employed as the passive waveguide. Two vertical lobe emission has an advantage for the application in coherent laser stacks, based on external glass plate reflector. We show that by proper engineering of the waveguide one can realize high performance TWL diodes at different tilt angles of the lobes (e.g., +/–27 degrees or +/–9 degrees) to the junction plane. The TWL with the lobes at +/–9 degrees allows above 95% of the power to be emitted within a vertical angle below 25 degrees, which is important for laser stack applications also using conventional optical coupling schemes. A single lobe emission at zero angle can be realized. A high differential efficiency ~90% and a current–source limited pulsed power >42W for as–cleaved TWL device at a cavity length of 1.5 mm and a stripe width of 50 um is shown. Thus the power per facet length in a laser bar in excess of 8.4 kW/cm can be reached.

The work was supported by ZIM Program of BMWi (Project COLIBRI). [1] N. N. Ledentsov, et al., Electron. Lett. 47, 1339–1340 (2011) [2] N. N. Ledentsov, et al., Proc. SPIE 8965, 89650Q (2014)

9733-25, Session 5

Single-mode tapered DBR lasers emitting 400 mW at 1550 nm

Jukka Viheriälä, Joel Salmi, Tampere Univ . of Technology (Finland); Jaakko M . Mäkelä, Univ . of Turku (Finland);

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Conference 9733: High-Power Diode Laser Technology and Applications XIV

Antti T . Aho, Heikki A . Virtanen, Tomi Leinonen, Mihail M . Dumitrescu, Mircea Guina, Tampere Univ . of Technology (Finland) Lecomte, Olivier Parillaud, Michel Krakowski, III-V Lab . (France) We report the development and the performance of 1550 nm tapered DBR laser diodes. These devices are particularly attractive for applications requiring eye-safe high power single-mode operation, such as LIDAR and range-finding. The DBR was fabricated using a regrowth-free process based on surface gratings and InP/AlGaInAs QWs. A CW output power as high as 400 mW in single-mode operation was achieved at room-temperature. At maximum output power the SMSR was 45 dB, proving the excellent behavior of the surface gratings. The ridge waveguide and DBR surface grating were defined using soft-UV NIL, while the patterning of metals and dielectrics was done using regular UV-lithography. This “mix-and-match” processing technique combines the best from both lithography methods. To our knowledge this is the first demonstration of DBR lasers fabricated using soft-UV-NIL on wafer-scale. The processed chips consisted of a 1 mm long DBR-grating section, a 1 mm long straight ridge waveguide section and a tapered section with 6 degrees full opening angle. The length of the tapered section has been varied between 0.5 mm and 1.85 mm to study the power scalability. For characterization the chips were p-side-down mounted on an AlN170 submount containing multiple electrically isolated AuSn-pads, which enabled driving each laser section separately. By increasing the length of tapered section from 0.5 mm to 1.85 mm the maximum CW power at room temperature could be scaled from 125 mW to 400 mW. The design guidelines and power scaling capabilities will be discussed.

9733-26, Session 6

High-power single emitters and low fill factor bars emitting at 808 nm

Agnieszka Pietrzak, Ralf Hülsewede, Martin Zorn, JENOPTIK Diode Lab GmbH (Germany); Jens Meusel, JENOPTIK Laser GmbH (Germany); Jürgen Sebastian, JENOPTIK Diode Lab GmbH (Germany) Single emitters emitting at a wavelength around 808 nm are highly desired as pump sources of low power solid state lasers which are widely utilized e.g., in micro-marking, metrology-instrumentation, and for build compact NIR-light sources. For this wavelength region different quantum well material can be used resulting in different polarization directions. We present the latest development of high-power single emitters having emitter apertures of 95 µ m, 100 µ m and 200 µ m based on TM-polarized material. Furthermore, different cavity length of 2 mm and 4 mm are investigated. Short-cavity devices were characterized up to powers over 4 W with a wall plug efficiency above 60%. Long-cavity devices were characterized up to their maximum optical output power of 15 W from 100 µ m wide emitters and 22 W from 200 µ m wide emitters with 4 mm cavity length mounted on passively cooled heat-sinks. The operation point of these devices is 8 W and 12 W, respectively with wall-plug efficiencies of ~55%. Furthermore first results on 5-emitters bars are presented operating at 40 W.

9733-27, Session 6

Thermal investigation on high power dfb broad area lasers at 975 nm, with 60% efficiency

Roberto Mostallino, Michel Garcia, III-V Lab . (France); Yannick Deshayes, Univ . Bordeaux 1 (France); Alexandre Larrue, Yannick Robert, Eric Vinet, III-V Lab . (France); Laurent Bechou, Univ . Bordeaux 1 (France); Michel The request of high power diode lasers in the range of 910-980nm is growing. This kind of device has many applications, such as fiber laser pumping, material processing, solid-state laser pumping, defense and medical/dental applications. The key role of this device lies in the wall plug efficiency (?e ) of converting electrical power into optical power. The high value of ?e allows high power level and reduces the level of current heating in lasers. The need of wavelength thermal stabilization is more evident in the case of multimode 975nm diode lasers used for pumping the Yb, Er and Yb/Er co-doped solid-state laser, which has a narrow absorption line close to 975nm. Narrow spectral width operation (<1 nm), combined with thermal stabilization (0.07 nm/°C), is achievable in the high power diode laser using an optical feedback provided by a uniform distributed feedback grating (DFB), introduced via etching and re-growth techniques into the semiconductor layers. We present the laser structure (Al-free in the active region) and the process techniques necessary to define the gratings into the structure. We will draw attention onto our performance in term of spectral response (SMSR>45dB with a width <1nm), the internal parameters (low internal losses ~1 1/cm, dP/dI~1 W/A, Rs~40m?), the measurement of thermal resistance (Rth ) (for the first time with high power laser diode at 975nm) the T3ster equipment, and the thermal simulation in order to understand the heat distributions and the eventually bottleneck.

9733-28, Session 6

Advances in 808nm high power diode laser bars and single emitters

John M . Morales, Sami Lehkonen, David A . Schleuning, Bruno Acklin, Coherent, Inc . (United States) Key applications for 780-830nm high power diode lasers include the pumping of various gas, solid state, and fiber laser media; medical applications including hair removal; direct diode materials processing; and computer-to-plate (CtP) printing. Many of these applications require high brightness fiber coupled beam delivery, requiring high brightness optical output at the bar and chip level. Many require multiple bars per system, with aggregate powers on the order of kWs, placing a premium on high wallplug efficiency. This paper presents Coherent’s recent advances in the production of high brightness, high efficiency bars and chips at 780-830nm. Results are presented for bars and single emitters of various geometries. Performance data is presented demonstrating peak wallplug efficiencies of 64% in CW mode. Reliability data is presented demonstrating >>20k hrs reliability for products including 60W 18% fill factor and 80W 28% fill factor conduction cooled bars.

9733-29, Session 6

Characterization of high performance silicon-based VMJ PV cells for laser power transmission applications

Mico Perales, MH GoPower Co ., Ltd . (United States); Mei huan Yang, John Wu, Talan Hsu, MH GoPower Co ., Ltd . (Taiwan); Wei-sheng Chao, MH GoPower Co ., Ltd . (Taiwan) and MH GoPower Co ., Ltd . (Taiwan); Kun-hsien Chen, MH GoPower Co ., Ltd . (Taiwan) and MH GoPower Co ., Ltd (Taiwan); Terry Zahuranec, MH GoPower Co ., Ltd . (United States) Continuing improvements in the cost and power of laser diodes have been critical in launching the emerging fields of power over fiber (PoF), and laser power beaming. Laser power is transmitted either over fiber (for PoF), or through free space (power beaming), and is converted to electricity by photovoltaic cells designed to efficiently convert the laser light. MH

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GoPower’s (MHGP) VMJ PV cell, designed for high intensity photovoltaic applications, is fueling the emergence of this market, by enabling unparalleled photovoltaic receiver flexibility in voltage, cell size, and power output. Our research examines the use of the VMJ cell in these laser power transmission applications. We first fully characterize the performance of the VMJ cell under various laser conditions, including multiple near IR wavelengths, multiple light intensities ranging from mW to tens of watts per cm2, and different beam uniformities. We also investigate the impact of the physical dimensions (length, width, and height) of the VMJ cell on its performance. We then delve deeper into VMJ cell performance within the power over fiber application, as we examine the effectiveness of the VMJ cell in various receiver packages that deliver target voltage, uniformity, intensity, and power levels. By designing and characterizing multiple receivers, we illustrate techniques for packaging the VMJ cell for achieving optimal performance and target voltages, as well as minimizing package size, for power over fiber applications.

9733-30, Session 7

Conference 9733: High-Power Diode Laser Technology and Applications XIV Integrated high power VCSEL systems

Holger Moench, Ralf Conrads, Stephan Gronenborn, Philips Technologie GmbH (Germany); Xi Gu, Philips Lighting B .V . (Netherlands); Michael Miller, Philips GmbH U-L-M Photonics (Germany); Pavel Pekarski, Jens Pollmann Retsch, Philips Research (Germany); Armand Pruijmboom, Philips Lighting B .V . (Netherlands); Ulrich Weichmann, Philips Technologie GmbH (Germany) Fraunhofer-Institut für Produktionstechnologie IPT (Germany) In this paper we present an industrial grade assembly solution for micro optics utilized in high power diode laser stacks.

With ever-rising demand for HPDL-stacks higher degrees of automation are becoming economically viable solutions. Proven assembly technology and concepts can be transferred from close industrial domains as a starting point but need to be extended for a successful application in laser industry. Quality demands of micro optics in high-power environments cannot be satisfied with geometric specification alone; hence alignment tasks of optical components often focus on the optimization of product functions instead. The integration of these domain specific active alignment algorithms and measurement setups in classic industrial automation environments are main challenges when moving towards highly automated solutions. Stack assembly is additionally demanding as measurement equipment and current stack level need to be synchronized during the assembly. Furthermore laser bars are individually contacted to reduce heat progression and excessive laser radiation.

We will present a detailed analysis of integration challenges and viable approaches in regard to software, compact measurement equipment for active alignment and unbroken process observation for optimized shrinkage compensation based on calculated quality metrics. Furthermore we analyze the process capability and cycle times of an automated FAC assembly for stacks.

High power VCSEL system technology extends to proper coolers, bonding technology and integrated optics i.e. far beyond the VCSEL chip itself. The paper discusses high performance components and processes dedicated to the needs of VCSEL array chips in high power systems. New approaches help to eliminate components and process steps and make the system more robust and easier to manufacture.

Densely packed high power systems rely on perfectly matched micro channel coolers. New cooler concepts with integrated electrical and mechanical interfaces have been investigated and offer advantages for high power system design.

The bonding process of chips on sub-mounts and coolers has been studied extensively and for a variety of solder materials. High quality of the interfaces as well as good reliability under normal operation and thermal cycling have been realized. A viable alternative to soldering is sintering with a silver paste. The very positive results which have been achieved with rather different technologies indicate the robustness of the VCSEL chips and the suitability for high power systems.

Beam shaping micro-optics can be integrated on the VCSEL chip in a wafer scale process by replication of lenses in a polymer layer. The performance of VCSEL arrays with integrated collimation lenses has been positively evaluated and the integrated chips are fully compatible with all further assembly steps.

The integrated high power systems make the application even easier and more robust. New examples in laser material processing and pumping of solid state lasers will be presented.

9733-31, Session 7

Industrial grade assembly solution for micro optics utilized in high power diode laser stacks

Daniel Zontar, Fraunhofer-Institut für Produktionstechnologie IPT (Germany); Harald Vogt, MA micro automation GmbH (Germany); Sebastian Haag, Tobias Müller, Sebastian Sauer, Christian Brecher,

9733-32, Session PTue

Spatial mode phase locking in a laser diode bar

Shir Shahal, Hamootal Duadi, Moti Fridman, Bar-Ilan Univ . (Israel) High efficiency, compact size and low price are some of the reasons for the wide spreading of high-power laser diode bars. However, laser diode bars have low beam quality due to their multiple emitters. One way to overcome this drawback is to obtain phase locking of the different emitters of the diode bar, which increases the beam quality and enables a smaller focused beam. Phase locking of diode lasers was previously demonstrated by Talbot effect [1], and off axis coupling [2]. However, these coupling methods work only for periodic emitters. We demonstrate passive phase locking technique of laser diode bars which is robust, stable and works with non-periodic diode bar emitters. We built a degenerate diode laser cavity of 906 nm wavelength and placed two CCD cameras for measuring the near field and far field intensity distributions. The degenerate cavity contains a diode bar with antireflection coating, a 4-f system and an output coupler. In order to phase lock the emitters of the diode bar, we insert a varying aperture into the focal plane of the 4-f system [3]. We measured the output beam quality and power as a function of the aperture width. Our results show that the beam quality can be improved by a factor of 2.7 with reduction of less than 30% of the output power. These results can lead to novel high power diode lasers with increased beam quality. The full experimental details and results will be presented.

References: 1. V. V. Apollono, S. I. Derzhavin, V. A. Filonenko et al. “High power laser diode array phase-locking” SPIE, 3889,134 (2000).

2. Z. Su, Q. Lou, J. Dong, J. Zhou and R. Wei, “Beam quality improvement of laser diode array by using off-axis external cavity” Opt. Express, 15, 11776 (2007). 3. N. Davidson, M. Nixon, E. Ronen, M. Fridman, and A. Friesem, “Phase locking large arrays of lasers” in Conference on Lasers and Electro-Optics 2012, OSA Technical Digest (online) (Optical Society of America, 2012), paper CTu3N.7.

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9733-33, Session PTue

Holographic 3D display based on laser recording and reconstruction

Hongyue Gao, JIcheng Liu, Shanghai Univ . (China) Our work focuses on static and dynamic holographic 3D display bases on laser recording and reconstruction. Laser plays an important role in holography, and provides wider color than LED display. In this paper we will present some work on 3D hologram print and holographic 3D video display in materials, which aims at future 3D pictures and 3D displayers, because holography is a true 3D technique. Computer generated holograms, holographic storage materials, and hologram print system in holographic print study will be introduced. Furthermore, real time dynamic holographic 3D display in materials, which may be developed in holographic 3D televisions or holographic 3D projectors in future, will also be presented. More important, this is a very typical laser application because developments of holographic 3D displayers depend on development of lasers. We hope that more and more lase experts can pay attention to holographic 3D display by this presentation, and the holographic display can come into our lives as soon as possible, since holographic 3D display, as a true 3D display, has huge application area.

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Conference 9734: Vertical External Cavity Surface Emitting Lasers (VECSELs) VI

Monday - Tuesday 15–16 February 2016 Part of Proceedings of SPIE Vol . 9734 Vertical External Cavity Surface Emitting Lasers (VECSELs) VI 9734-1, Session 1

How we developed OP-VECSELs at Micracor

States)

(Invited Paper)

Mark E . Kuznetsov, AXSUN Technologies Inc . (United Semiconductors have been used as laser gain medium since the earliest days of lasers. Current injection of carriers across a diode p-n junction has been the preferred way to pump such lasers, leading to the ubiquitous diode lasers used across a wide range of applications. Optical pumping of semiconductor lasers has also been used through the years, but mostly as a research tool on the path to the ultimately electrically pumped version. In the mid 1990s, a group led by Aram Mooradian in a small startup company Micracor demonstrated Optically Pumped Semiconductor Lasers (OPSL) with remarkable properties that led to their rapid development and wide use in research and industrial applications. Using vertical cavity configuration emitting light normal to the plane of the wafer, and applying an external mirror to stabilize a large area fundamental transverse mode, optical pumping allowed operation of watt-class lasers with excellent beam quality and wide wavelength coverage enabled by the bandgap-engineered semiconductor media. These Vertical External Cavity Surface Emitting Lasers (VECSEL) proved to be so versatile, that arrayed electrically-pumped version of these lasers, NECSEL, was developed in another startup company, Novalux, led again by Aram Mooradian. In this presentation I will describe our early work on OPSLs at Micracor, where we took the dreams of a new remarkable laser and found a path to make them a reality. We also remember Aram Mooradian as a person and scientist and his foundational contributions to the field of VECSEL lasers.

9734-2, Session 1

Evolution of the Novalux extended cavity surface-emitting laser (NECSEL)

(Invited Paper)

John G . McInerney, Univ . College Cork (Ireland) The NECSEL is a VECSEL based on an electrically pumped semiconductor gain element with integrated Bragg gratings and a single-ended external compound cavity for mode selection and coherence enhancement. Initially designed for 980 nm operation, it produced multi-watt quasi-CW outputs with M2 < 1.2. A hybrid integrated version produced ~0.1 W CW in a TO style can. Mode-locked versions of the full NECSEL produced ~1 ps pulses transform limited with ~0.1 kW peak powers. Subsequent designs used intracavity frequency doubling, initially using bulk nonlinear crystals for blue emission, later migrating to quasi-phasematched PPLN arrays as these became available.

9734-3, Session 1

Continuous-wave VECSELs: Review of recent progress

(Invited Paper)

Mircea Guina, Tampere Univ . of Technology (Finland) The development of continuous-wave VECSELs has reached a maturity level that enables deployment of a wide range of applications in spectroscopy, pumping and cooling of solid-state media, biophotonics, or laser projection, just to mention a few. During the past years, the most diverse and intense developments have aimed at reaching new wavelengths by deploying emerging semiconductor materials and related technologies for gain mirror fabrication. To this end we could mention the development of strain compensated GaInAs/GaAs gain mirrors, development of GaInNAsSb/ GaAs-based gain mirrors with wavelength coverage between 1.1 µ m and 1.5 µ m, development of InAs/GaAs QD-based mirrors, use of wafer bonding technology, development of GaInAsSb/GaSb gain mirrors with emission beyond 2 µ m, or the emergence of InGaN/GaN-based VECSELs with emission at blue wavelengths. On the other hand, intracavity frequency doubling has enabled attaining power levels above 20W for wavelengths covering green, yellow, and orange bands. Major developments have been also made concerning generation of UV wavelengths as well as of THz radiation. Moreover, cavity stabilization techniques combined with wavelength selective elements, have enabled attaining high-power outputs with a narrow-linewidth, and eventually continuously tuneable, which are instrumental features for spectroscopic applications. The paper is focused on reviewing the major progress in continuous-wave VECSELs. Emphasis is put on advances concerning power scaling and related thermal management techniques, and wavelength coverage. The progress will be discussed in connection with established and emerging applications that have fostered the tremendous progress experienced by VECSEL technology during the last decade.

9734-4, Session 1

Ultrafast vertical external cavity surface emitting lasers (VECSELs)

(Invited Paper)

Ursula Keller, ETH Zürich (Switzerland) Aram Mooradian made pioneering contributions for a novel type of laser that has bridged the gap between semiconductor lasers and solid state lasers. The high-power optically pumped vertical-external-cavity surface-emitting laser (VECSEL) combines the best of both worlds: the semiconductor gain medium allows for flexible choice of emission wavelength via bandgap engineering and good pump efficiency is obtained from fairly low-cost, low-beam-quality high-power diode laser bars into a near-diffraction-limited output beam. The VECSEL is part of the family of VCSELs which are the most frequently manufactured semiconductor lasers today. In contrast to a VCSEL a VECSEL has an external cavity and is either optically or electrically pumped. VECSELs also have been passively modelocked using a semiconductor saturable absorber mirror (SESAM). The MIXSEL (i.e. modelocked integrated external-cavity surface emitting laser) combines the gain of VECSELs with the saturable absorber of a (SESAM) in a single semiconductor device. Hence, self-starting and stable passive modelocking is obtained in a simple straight cavity formed by the semiconductor chip and a curved output coupler. The VECSEL and MIXSEL are part of the family of ultrafast semiconductor disk lasers which rapidly advanced over the last decades. The strong interest from industry for inexpensive, compact and reliable ultrafast laser sources in the picosecond and femtosecond domain has driven this technology towards first commercial products. Frequency metrology and biomedical applications would benefit from sub-200 femtosecond pulse durations with peak powers in the kilowatt range. This talk is a tribute to Aram Moradian and will give an overview of the recent advances.

9734-5, Session 2

Gigahertz dual-comb modelocked diode pumped semiconductor and solid-state lasers

(Invited Paper)

Sandro M . Link, Mario Mangold, Matthias Golling,

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Conference 9734: Vertical External Cavity Surface Emitting Lasers (VECSELs) VI

Alexander Klenner, Ursula Keller, ETH Zürich (Switzerland) One passively modelocked laser can simultaneously generate two gigahertz, orthogonally polarized and collinear frequency combs with slightly different but adjustable pulse repetition rates. This is successfully demonstrated for both, a semiconductor disk laser (SDL), also referred to as vertical external cavity surface emitting laser (VECSEL) and a solid-state Nd:YAG laser, each pumped with the same multimode diode array pump laser. This laser uses an intracavity birefringent crystal, which splits the one cavity beam into two spatially separated and orthogonally polarized beams. The SDL is a modelocked integrated external-cavity surface emitting laser (MIXSEL) that combines the saturable absorber of a SESAM with the gain of a VECSEL in a single semiconductor structure and enables dual-output operation in a simple linear straight cavity. The demonstration of dual-polarized output with the Nd:YAG laser extends this technique to ion-doped solid-state laser technologies. Such compact and cost-efficient dual-comb sources are interesting for spectroscopic applications. The microwave frequency comb, generated by the optical beat of the two orthogonal pulse trains, can be used to down-convert the terahertz optical frequencies into the electronically accessible microwave regime. The comb is detected simply by superimposing the two beams in the same polarization on a photodetector. Using a second birefringent crystal of same length but with the optical axis rotated by 90° results in two pulse trains with the same pulse repetition frequency, allowing to directly detect and characterize the relative carrier envelope offset (CEO) frequency dynamics. The stabilization and noise characterization of the dual comb laser will be presented.

Andrew P . Turnbull, Univ . of Southampton (United Kingdom); Markus Polanik, Univ . Ulm (Germany); Edward A . Shaw, Theo Chen Sverre, Univ . of Southampton (United Kingdom); Peter Unger, Univ . Ulm (Germany); Anne C . Tropper, Univ . of Southampton (United Kingdom) We present a VECSEL based on a gain sample design which utilizes only a single layer dielectric Al2O3-coating for dispersion management. The calculated GDD, over a 30-nm range around the lasing wavelength of 1035 nm, was -38 ± 33 fs2. The gain structure generated in combination with a surface-recombination SESAM pulse durations down to 193 fs with an average power of 400 mW at 1.6 GHz with an incident pump power of 30 W, setting a new peak power record for sub-200 fs mode-locked VECSELs. The obtained sech2-pulses were not transform-limited and had a time bandwidth product of up to 0.64. A subsequent FROG analysis of the pulse train revealed significant higher-order chirp characteristics. In order to investigate where the higher-order dispersion components arise from pulse trains generated by various VECSELs, with similar gain chip layer designs that have only small differences, have been investigated using the FROG analysis. An optimized gain chip design for higher-order dispersion management could reduce the time bandwidth product and thus further push the pulse duration towards the 100-fs mark, at which point VECSELs become an interesting source for applications such as direct GHz frequency comb generation.

9734-6, Session 2

Recent progress in high-power ultrafast MIXSELs

Cesare G . E . Alfieri, Dominik Waldburger, Sandro M . Link, Emilio Gini, Matthias Golling, Bauke W . Tilma, Mario Mangold, Ursula Keller, ETH Zürich (Switzerland) A modelocked integrated external cavity surface emitting laser (MIXSEL) is the most simple and compact type of ultrafast semiconductor disk laser, as it is operated in a straight linear cavity. High-power ultrafast MIXSELs offer highest repetition rate flexibility together with high performance in terms of pulse duration, average and peak power and are thus ideal candidates for applications in the fields of metrology and imaging.

Here, we present the first sub-300-fs MIXSEL, fabricated by a regrowth scheme in MOVPE and MBE. The structure includes a lower field enhancement in the gain and an improved dispersion. We obtained 235 mW of average output power at a wavelength of 1044 nm and a repetition rate of 3.35 GHz. A pulse duration of 253 fs was retrieved from frequency resolved optical gating measurements. The resulting 240-W peak power is the highest from a MIXSEL thus far. At 10 GHz repetition rate, we obtained a pulse duration of 279 fs and 310 mW of average output power.

In addition, we developed MOVPE-grown saturable absorbers with fast recovery dynamics that were subsequently implemented in the first entirely MOVPE-grown MIXSEL. This MIXSEL delivered 1.56 W of average output power with 15.4 ps pulses at a repetition rate of 3.66 GHz. In a different configuration pulses as short as 2.6 ps with 301 mW of average output power at 5.84 GHz repetition rate were obtained. MOVPE-growth not only reduces growth complexity and losses introduced by regrowth of a MIXSEL, but also makes the technology more accessible for the optoelectronics industry.

9734-8, Session 2

Pulse shortening of an ultrafast VECSEL

Dominik Waldburger, Cesare G . E . Alfieri, Sandro M . Link, Emilio Gini, Matthias Golling, Mario Mangold, Bauke W . Tilma, Ursula Keller, ETH Zürich (Switzerland) Ultrafast, optically pumped, vertical external-cavity surface-emitting lasers (VECSELs) are excellent sources for industrial and scientific applications that benefit from compact semiconductor based high-performance lasers with gigahertz pulse repetition rates and excellent beam quality. Applications such as self-referenced frequency combs and multi-photon imaging could benefit from sub 150-fs pulse durations combined with high pulse peak power.

Here, we present our recent progress in sub-150-fs semiconductor saturable absorber mirror (SESAM) modelocked VECSELs. In order to obtain the short pulse durations, we utilized precise dispersion engineering and an active region designed for broadband gain. Furthermore, a low and flat average field enhancement of the active InGaAs quantum wells of ≈ 0.5 (normalized to 4 outside the structure) ensures high gain saturation fluences. In combination with a SESAM with fast recovery dynamics, 128-fs pulses with 303 W of pulse peak power were obtained. The average output power was 80 mW with a pulse repetition rate of 1.8 GHz at a central wavelength of 1033 nm. In a similar configuration 147-fs pulses with 328 W of pulse peak power have been achieved. Both laser configurations operated in a fundamental transverse mode with an M2 value of <1.05 in both orthogonal directions. Those results mark the shortest pulse durations of any ultrafast VECSELs in the >50-mW average power regime.

With full characterization of the gain and saturable absorber parameters, we can identify current limitations of our ultrafast VECSELs and present guidelines to maintain the short pulse durations while scaling the technology to even higher pulse peak power.

9734-7, Session 2

High-order dispersion in sub-200-fs pulsed VECSELs

Christopher R . Head, Univ . of Southampton (United Kingdom); Alexander Hein, Univ . of Ulm (Germany);

116 SPIE Photonics West 2016 · www.spie.org/pw

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Conference 9734: Vertical External Cavity Surface Emitting Lasers (VECSELs) VI

9734-9, Session 2

Advances in low-repetition-rate modelocked semiconductor disk lasers in multi-pass cavity geometries

(Invited Paper)

Loyd J . McKnight, Peter J . Schlosser, Alexander A . Lagatsky, Martin D . Dawson, John-Mark Hopkins, Fraunhofer Ctr . for Applied Photonics (United Kingdom) Modelocked semiconductor disk lasers (SDLs) offer a low-cost, compact and versatile solution for nonlinear microscopy, however, the desirable repetition rate of 80-100 MHz is hard to achieve without the short carrier lifetime in the semiconductor gain chip causing multiple pulsing effects or a drop in efficiency. We have developed a number of novel multi-pass cavities that solve this problem by passing the intracavity beam through the optically pumped gain region multiple times per round trip of the cavity. This has the effect of reducing the time between intracavity pulses hitting the gain region whilst ensuring a clean and efficient low repetition rate output. Using 8 passes of the gain region per round trip we achieved a repetition rate of 86 MHz offering output powers above 120 mW and pulse durations of 2.2 ps. This represents a peak power greater than 500 W whilst maintaining gain and semiconductor saturable absorber mirror (SESAM) chips at room temperature. A number of cavities will be presented including those with repetition rates of 112 MHz and 230 MHz with similar power levels and pulse durations. For this work we designed antiresonant SDL gain wafers at a wavelength of ~955 nm, capable of efficiently exciting green fluorescent protein in a two-photon process, and a surface-recombination SESAM that offers a fast relaxation time. Details of the novel cavities and the gain and SESAM structures will be presented including a comprehensive characterization of the clean modelocked operating regime.

9734-10, Session 3

Single frequency 2-3 micron VECSELs

(Invited Paper)

Marcel Rattunde, Peter Holl, Steffen Adler, Fraunhofer Institut für Angewandte Festkörperphysik (Germany); Sebastian Kaspar, AIM INFRAROT-MODULE GmbH (Germany); Andreas Bächle, Elke Diwo, Rolf Aidam, Wolfgang Bronner, Joachim Wagner, Fraunhofer-Institut für Angewandte Festkörperphysik (Germany) Narrow-linewidth, tunable laser sources in the 2 – 3 µ m wavelength range are of special interest for a range of applications such as gas analysis, remote sensing, LIDAR, optical free-space communication, laser seeding as well as for fundamental research like quantum optical experiments. Vertical external cavity surface emitting lasers (VECSEL) based on the (AlGaIn)(AsSb) materials system are ideally suited to serve these needs. High-efficient, high-power gain mirrors are available [1,2] and by including wavelength-selective elements in the laser cavity, narrow-linewidth single frequency emission and wavelength tunability can be achieved [3]. Without active stabilization, the emission-linewidth of the VECSEL can be well below 100 kHz and by scaling the pump- and mode-spot to larger diameters, over 2W (3W) single-frequency emission could be achieved at 2.0 µ m and 20°C (3°C) heatsink temperature [4]. In this presentation recent advances concerning these single-frequency sources will be reviewed with special emphasis on the different mounting and heat extraction technologies for the VECSEL chip and their influence on the spectral properties and the VECSEL efficiency. Further on, alternative approaches for wavelength control (such as self-seeding) will be presented and discussed, together with a requirement analysis for the different applications.

References [1] P. Holl et al, IEEE J Sel Top Quantum Electron. 21 (6), (2015).

[2] J. Paajaste et al., J. Cryst. Growth 311, 1917 (2009) [3] O. Ochotnikov, Ed., “Semiconductor Disk Laser”, Wiley-VCH, 2010 [4] S.Kaspar et al., IEEE JSTQE 19, 1501908 (2013)

9734-11, Session 3

Industrial integration of high coherence tunable single frequency semiconductor lasers based on VECSEL technology for scientific instrumentation in NIR and MIR

Stephane Denet, Innoptics SAS (France); Baptiste Chomet, Univ . Montpellier 2 (France); Vincent Lecocq, Laurence Ferrières, Innoptics SAS (France); Mikhaël Myara, Laurent Cerutti, Univ . Montpellier 2 (France); Isabelle Sagnes, Lab . de Photonique et de Nanostructures (France); Arnaud Garnache, Univ . Montpellier 2 (France) Applications such as spectroscopy, lidar, or velocimetry require highly coherent tunable laser sources. The Vertical External Cavity Surface Emitting Laser (VECSEL) III-V semiconductor technology is a good candidate, with promising lab results already demonstrated. Inside a research group gathering expertise in chip design and epitaxy, laser cavity design, compact and robust laser packaging and noise measurements, we are currently developing a family of VECSEL-based modules, in the 0.8-1.1 µ m (GaAs-based) and 2-2.5 µ m (Sb-based) spectral windows, with future developments at 1.5 µ m.

We are presenting here tunable single frequency modules at 1 µ m and 2.3 µ m. Featuring a ? VCSEL chip being part of a sub-mm laser cavity, the modules also comprise a compact system for optical pumping, means for laser temperature monitoring, and a monitoring photodiode.

At 1 µ m we demonstrate high power (> 60mW), high coherence, low divergence diffraction limited TEM00 beam, high spectral purity (SMSR >55dB) and linear polarization (50dB PER), while at 2.3 µ m broadband continuous tuning (> 7nm) and overall tunability of 80 nm are achieved. Those performances can be reached thanks to a high finesse cavity associated with ideal homogeneous QW gain behavior. In addition, the design without any intracavity element offers a robust single frequency regime with a good long term wavelength stability.

Ongoing developments intend to enhance the main specifications for each application (tunability at 2.3 µ m and low noise at 1 µ m). Dedicated control electronics are also under development in order to improve laser stability.

9734-12, Session 3

Dual-frequency VECSELs for microwave photonics applications

(Invited Paper)

Fabien Bretenaker, Lab . Aimé Cotton (France) and Ecole Normale Supérieure de Cachan (France) Optical generation of tunable high-purity radio-frequency (RF) signals is essential for the future optoelectronic communication systems such as broadband mobile systems [1], satellite networks [2], long-range transmission of high purity radio-frequency (RF) references, etc. One way to generate such low noise optically carried RF signals is to build two-frequency lasers. In this case, the beatnote between the two optical frequencies provides the desired RF signal. This approach has been applied in the case of solid-state lasers [3,4]. But, in this case, the excess noise induced by the laser relaxation oscillation noise is detrimental to the spectral purity of the carried RF signal. However, recently, optically pumped Vertical External Cavity Surface Emitting Lasers (VECSELs) have been shown to obey the class-A regime, i. e., to exhibit no relaxation oscillations. These

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Conference 9734: Vertical External Cavity Surface Emitting Lasers (VECSELs) VI

lasers are thus capable of emitting a single-frequency with an ultralow intensity noise [5-7]. Our aim here is to show how such VECSELs can be made to oscillate in dual-frequency regime [8,9], and to report our recent efforts to elucidate the physical mechanisms that give rise to the phase noise observed in the generated RF signal [10,11,12].

1. D. C. Ni, H. R. Fetterman, and W. Chew, IEEE Trans. Microwave Theory Technol. 38, 608 (1990).

2. U. Gliese, E. L. Christensen, and K. E. Stubkjaer, J. Lightwave Technol. 9, 779 (1991).

3. M. Alouini, B. Benazet, M. Vallet, M. Brunel, P. Di Bin, F. Bretenaker, A. Le Floch, and P. Thony, IEEE Photon. Tech. Lett. 13, 367 (2001).

4. G. Pillet, L. Morvan, M. Brunel, F. Bretenaker, D. Dolfi, M. Vallet, J.-P. Huignard, and A. Le Floch, J. Lightwave Technol. 26, 2764 (2008).

5. G. Baili, M. Alouini, D. Dolfi, F. Bretenaker, I. Sagnes, and A. Garnache, Opt. Lett. 32, 650 (2007).

6. G. Baili, F. Bretenaker, M. Alouini, L. Morvan, D. Dolfi, and I. Sagnes, J. Lightwave Technol. 26, 952 (2008).

7. G. Baili, L. Morvan, G. Pillet, S. Bouchoule, Z. Zhao, J.-L. Oudar, L. Ménager, S. Formont, F. Van Dijck, M. Faugeron, M. Alouini, F. Bretenaker, and D. Dolfi, J. Lightwave Technol 32, 3489 (2014).

8. G. Baili, L. Morvan, M. Alouini, D. Dolfi, F. Bretenaker, I. Sagnes, and A. Garnache, Opt. Lett. 34, 3421 (2009).

9. F.A. Camargo, J. Barrientos, G. Baili, L. Morvan, D. Dolfi, D. Holleville, S. Guerandel, I. Sagnes, P. Georges, and G. Lucas-Leclin, Opt. Lett. 34, 3421 (2009).

10. S. De, V. Pal, A. El Amili, G. Pillet, G. Baili, M. Alouini, I. Sagnes, R. Ghosh, and F. Bretenaker, Opt. Expr. 21, 2538 (2013).

11. S. De, A. El Amili, I. Fsaifes, G. Pillet, G. Baili, F. Goldfarb, M. Alouini, I. Sagnes, and F. Bretenaker, J. Lightwave Technol. 32, 1307 (2014).

12. S. De, G. Baili, S. Bouchoule, M. Alouini, and F. Bretenaker, Phys. Rev. A 91, 053828 (2015).

emitting gain chip. The gain chip had ten 7-nm-thick GaInAs quantum wells, with an indium mole fraction of 35%. The accumulation of net strain was eliminated by using GaAsP strain compensation layers. An intra-cavity birefringent filter and an etalon were used to achieve single frequency operation and wavelength tuning. The laser line-width was measured to be less than 140 kHz using a heterodyne measurement with a narrow linewidth fiber laser. The laser was stabilized against long term drifts using a frequency offset lock between the VECSEL and the fiber laser (which was stabilized to a Doppler free transition in molecular Iodine) or a direct lock to an Iodine transition.

Second harmonic generation in a waveguide doubler produced up to 350mW at 559nm which was subsequently frequency doubled in an external build up cavity to produce up to 22mW in the UV at 280nm. The system was used to Doppler-cool a single 25 Mg ion in an RF Paul trap on the S1/2 |3,3>->P3/2 |4,4> transition. The slightly saturated and residually Doppler-broadened line width of this transition was measured to be 45MHz. The laser was also tuned to 1121nm to perform high resolution spectroscopy of the S1/2 |2,2>->P1/2 |3,3> transition. This work demonstrates the versatility of VECSELs in high precision spectroscopy of trapped atoms and provides a scalable laser source for the development of ion trap quantum information processors.

9734-14, Session 4

Generation of new spatial and temporal coherent light states using III-V semiconductor laser technology: VORTEX, continuum, dual frequency for THz

(Invited Paper)

Arnaud Garnache, Mohamed Seghilani, Romain Paquet, Mohamed Sallhai, Baptiste Chomet, Mikhael Myara, Stephane Blin, Univ . Montpellier 2 (France); Isabelle Sagnes, Gregoire Beaudoin, Luc Legratiet, LPN CNRS (France); Philippe Lalanne, LP2N IOGS (France)

9734-13, Session 3

Laser cooling of trapped ions using a frequency quadrupled VECSEL

Shaun C . Burd, National Institute of Standards and Technology (United States) and Univ . of Colorado at Boulder (United States); Tomi Leinonen, Jussi-Pekka Penttinen, Optoelectronics Research Ctr . (Finland); David T . Allcock, National Institute of Standards and Technology (United States); Raghavendra Srinivas, National Institute of Standards and Technology (United States) and Univ . of Colorado at Boulder (United States); Daniel H . Slichter, Andrew C . Wilson, Dietrich Leibfried, National Institute of Standards and Technology (United States); Mircea Guina, Optoelectronics Research Ctr . (Finland); David J . Wineland, National Institute of Standards and Technology (United States) and Univ . of Colorado at Boulder (United States) The recent advances in the field of quantum information processing have paved the way for high performance computing and high precision measurements of physical parameters. These developments rely on laser cooling of atoms and ions using narrow-linewidth, resonantly tuned lasers. Vertical-external-cavity surface-emitting laser (VECSEL) system device geometry enables the combination of single-frequency operation and good beam quality with the wavelength versatility of semiconductors, which are advantageous features for these applications. Here we report on laser cooling and high precision spectroscopy of trapped Magnesium ions using a frequency quadrupled VECSEL. Light at 1118nm was generated using an I-cavity VECSEL with a bottom Since years, the VeCSEL concept is pointed out as a technology of choice for beyond-state-of-the-art laser light sources, demonstrating wavelength flexibility, high power, high spatial and temporal coherence, linear polarization state, CW or ultra-short pulsed operation, compactness and functionalities. The targeted coherent state is typically the gaussian TEM00, single frequency, linearly polarized light state. In this work, we take advantage of the VeCSEL technology for generation of new kinds of coherent states, thanks to insertion of intracavity functions based on flat photonics. These new coherent states target many applications including optical tweezers, telecommunications, fundamental physics, sensors… A first part of this work aims at demonstrating new spatial domain coherent state. We developed a semiconductor flat photonic technology for intracavity transverse phase and intensity control. Intensity control is obtained thanks to sub-wavelength metallic masks. For phase control, we developed an ultra-low loss metamaterial technology. This technological development permitted generation and control of high coherence single or dual high order Laguerre-Gauss mode, VORTEX or Hermite-Gauss mode, preserving the spatial and temporal coherence properties of usual TEM00 VeCSELs. It paves the way for the generation of new coherent beams and functionalities: wavelength filtering, dual frequency for THz source. In a second part, we explore new time domain coherent state: owing to a high gain semiconductor chip design and insertion of an intracavity acousto-optic frequency-shifter, we demonstrated and studied the first semiconductor based low noise broadband modeless laser, with a 300 GHz band and a coherence length of 4km.

118 SPIE Photonics West 2016 · www.spie.org/pw

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9734-15, Session 4

Conference 9734: Vertical External Cavity Surface Emitting Lasers (VECSELs) VI Terahertz quantum cascade VECSEL

Luyao Xu, Christopher A . Curwen, Philip W . C . Hon, Tatsuo Itoh, Benjamin S . Williams, Univ . of California, Los Angeles (United States) Vertical-external-cavity surface-emitting lasers (VECSELs) have been successfully used in the visible and near-infrared to achieve high output power with excellent Gaussian beam quality. However, the concept of VECSEL has been impossible to implement for quantum-cascade (QC) lasers, since the optical gain is based on intersubband transitions of electrons, which only interact with the electric field polarized perpendicular to the quantum wells plane according to the “intersubband selection rule”. In this work we demonstrated a terahertz (THz) quantum cascade external cavity laser, which is also the first VECSEL in the terahertz regime. The enabling component for the QC-VECSEL is an amplifying metasurface reflector composed of a sparse array of metallic sub-cavities, which allows the normally incident radiation to interact with the electrically pumped QC gain medium. The QC-VECSEL is implemented in a plano-plano Fabry-Perot cavity, with a wire-grid polarizer as its output coupler. A beam of 4.3° ? 5.1° FWHM divergence and a near-Gaussian profile is observed with an output power > 5 mW. THz QC-lasers have often been characterized by poor beam quality due to their sub-wavelength metallic waveguides. The symmetry and circularity of the beam are further improved by placing an aperture into the cavity. Our work on THz QC-VECSEL initiates a new approach towards achieving scalable output power in combination with a diffraction-limited beam pattern for THz QC-lasers. Furthermore, the intra-cavity access of the VECSEL gives tremendous versatility to QC-lasers, such as the ability for mode engineering.

9734-16, Session 4

1.2?m emitting VECSEL based on type-II aligned QWs

Christoph Möller, Christian Berger, Christian Fuchs, Philipps-Univ . Marburg (Germany); Antje Ruiz Perez, NAsP III/V GmbH (Germany); Stephan W . Koch, Philipps-Univ . Marburg (Germany); Jörg Hader, Jerome V . Moloney, Nonlinear Control Strategies (United States); Wolfgang Stolz, Philipps-Univ . Marburg (Germany) and NAsP III/V GmbH (Germany)

9734-17, Session 4

Widely tunable DBR-free semiconductor disk laser

Zhou Yang, Alexander R . Albrecht, The Univ . of New Mexico (United States); Jeffrey G . Cederberg, Sandia National Labs . (United States); Shawn Hackett, The Univ . of New Mexico (United States) and Air Force Research Lab . (United States); Mansoor Sheik-Bahae, The Univ . of New Mexico (United States) Semiconductor disk lasers (SDLs) have gained lots of attention due to their wavelength flexibility and high output powers. With the external cavity configuration, they are superior to edge-emitting semiconductor lasers for applications requiring intra-cavity optical elements, like non-linear crystals for frequency conversion or saturable absorbers for mode-locking. Theoretically, SDL is power scalable but the low thermal conductivity semiconductor DBR impedes the heat removal process. For DBR-free SDLs, a multi-quantum-well (MQW) active region is grown on a substrate with a sacrificial layer. Then the MQW is lifted off and van der Waals bonded onto transparent high thermal conductivity substrates, such as diamond. Without a need for lattice-matched semiconductor distributed Bragg reflectors (DBRs), these SDLs can be realized in more material systems and cover a wider wavelength range. Also, the tuning range would no longer be limited by the high-reflection bandwidth of the DBR, but instead by the broader high reflection band of the external dielectric mirrors. Based on thermal analysis, the DBR-free geometry with diamond heatspreaders on both sides of the active region has advantages in thermal management over traditional SDLs. For an InGaAs MQW sample bonded to one single-crystalline CVD diamond, preliminary experiments show a record 78 nm tuning range centered at 1150nm. The CW output power at 1150 nm was 2.5 W. For another active region, 4 W output power (limited by the available pump power) is collected at 1040 nm. Currently efforts are underway to improve laser performance and power scaling by mitigating intracavity losses.

9734-18, Session 5

Resonant measurements of nonlinear lensing in a VECSEL gain sample

(Invited Paper)

Adrian H . Quarterman, Univ . of Dundee (United Kingdom); Edward E . Shaw, Univ . of Southampton (United Kingdom); Keith G . Wilcox, Univ . of Dundee (United Kingdom) Since the invention of VECSELs, their great spectral coverage has been demonstrated and emission wavelengths in the range from UV to almost MIR have been achieved. However, in the infrared regime the laser performance is affected by Auger losses which become significant at large quantum defects. In order to reduce the Auger losses and to develop more efficient devices in the IR, type-II aligned QWs have been suggested as alternative gain medium for semiconductor lasers.

We present the first room temperature VECSEL containing type-II aligned quantum wells arranged as resonant periodic gain. The quantum wells consist of (GaIn)As/Ga(AsSb)/(GaIn)As heterostructures. The structure was grown bottom-up on GaAs substrate and flip-chip bonded onto a diamond heat spreader. The device, pumped at 808 nm, emits >1 W of cw output power at an emission wavelength of 1.2 ?m. A detailed study of the device is performed in order to investigate the potential of such novel type-II gain media for future applications. These investigations include the determination of the power and temperature dependent shift rates. The gain temperatures at laser threshold and at maximum output power are determined. Furthermore, edge photoluminescence and reflectivity measurements are performed in order to accurately determine the detuning. These experimental results are compared with fully microscopic simulations.

In recent years there have been several reports describing self-modelocking (SML) in vertical-external-cavity surface-emitting lasers (VECSELs). Some of these reports have suggested that the behaviour that has been observed results from nonlinear lensing in the VECSEL gain sample in a manner analogous to Kerr lens modelocking in solid state lasers. However to date none of the groups that have reported SML in VECSELs have performed measurements to ascertain whether nonlinear lensing occurs in the gain sample. Measurements of nonlinear lenses in VECSEL gain samples are therefore of value not only in order to understand the behaviour observed in the reports of SML, but also as a potentially crucial design tool for any mode-locked VECSEL, regardless of the modelocking method used.

In a previous publication [1] we described measurements which demonstrated that a defocussing nonlinear lens was present in an unpumped VECSEL gain sample, and that the inverse focal length of the lens increased with pump power, to the point of becoming a focussing lens for sufficiently high pump powers. Unfortunately, by necessity this measurement was performed using a probe laser which was not resonant with the quantum wells in the sample, meaning that the values measured may well be different from those experienced under operating conditions in a VECSEL. This paper will describe a more complete characterisation of VECSEL gain sample nonlinear lensing with a probe laser whose wavelength

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is resonant with the gain sample quantum wells.

[1] Quarterman et al. Applied Physics Letters, 106, 011105 (2015).

9734-19, Session 5

Ultrafast characterization of semiconductor gain and absorber devices for mode-locked VECSELs

Caleb Baker, Maik Scheller, Hwang-Jye Yang, College of Optical Sciences, The Univ . of Arizona (United States); Stephan W . Koch, College of Optical Sciences, The Univ . of Arizona (United States) and Philipps-Univ . Marburg (Germany); Ronald J . Jones, Jerome V . Moloney, College of Optical Sciences, The Univ . of Arizona (United States); Antje Ruiz Perez, Wolfgang Stolz, Philipps-Univ . Marburg (Germany); Sadhvikas Addamane, Ganesh Balakrishnan, The Univ . of New Mexico (United States) As Vertical External-Cavity Surface-Emitting Lasers (VECSELs) become increasingly interesting candidates among the ultrafast class of mode locked sources, better understanding of their carrier dynamics on ultrafast timescales is necessary. To this end, we present a comprehensive characterization of semiconductor gain and absorber devices utilizing novel measurement techniques to supplement traditional pump and probe measurements. For our studies, we utilize a mode-locked, amplified, ytterbium fiber laser emitting 12nJ pulses with a bandwidth centered at 1040nm and a pulse duration of 120fs as probe laser source. The pulses are spectrally broadened in a single-mode fiber to cover the wavelength range between 940nm to 1100nm. The resulting pulses are compressed using a pulse shaper to a pulse duration of 20fs. The pulse shaper is also used to spectrally resolve measurements by creating 100fs pulses with varying center wavelengths to interact with the samples.

Time resolution in the few femtosecond range results from traditional pump and probe measurements performed on VECSEL and SESAM samples with different designs. In-situ characterizations based on an asynchronous optical sampling technique of VECSEL samples mode-locked in the sub-500fs regime reveal additional long time recoveries of the gain present in real lasing condition.

Our results indicate that multiple carrier relaxation mechanisms exists ranging in timescale from hundreds of femtoseconds to pico- and nano second regimes which affect the stability of mode-locking states as well as the pulse duration. Our study can contribute to development design strategies to optimize the gain chips and absorbers for further performance improvements.

mode-locking regimes, thought to arise from a non-linear lens in the VECSEL gain structure.

Recently, the first measurement of the non-linear lens in a gain chip showed that the power and sign of the lens depends sensitively on the pumping conditions. The photon energy of the probe laser used in these measurements was less than the band gap energy of the quantum wells. Third-order refractive indices in semiconductors, however, are known to vary rapidly with the photon energy around the band gap. Here we present reflection z-scan measurements that probe the quantum well structure slightly above the band gap, in resonance with the typical operating wavelength of the laser. The strength of the non-linear lens is experimentally determined at pump densities and with pulse fluences approaching values typical for SESAM mode-locked VECSELs.

9734-21, Session 5

Self-mode-locked vertical-external-cavity surface-emitting laser

(Invited Paper)

Arash Rahimi-Iman, Mahmoud Gaafar, Christoph Möller, Max Vaupel, Fan Zhang, Dalia Al-Nakdali, Philipps-Univ . Marburg (Germany); Ksenia A . Fedorova, Aston Univ . (United Kingdom); Wolfgang Stolz, Philipps-Univ . Marburg (Germany) and NAsP III/V GmbH (Germany); Edik U . Rafailov, Aston Univ . (United Kingdom); Martin Koch, Philipps-Univ . Marburg (Germany) Within the last 15 years, a new class of pulsed lasers emerged which proved capable of generating ultrashort pulses. Semiconductor disk lasers, also referred to as vertical-external-cavity surface-emitting lasers (VECSELs), have become promising sources of fs-pulsed laser light which indeed can serve many applications, such as ultrafast spectroscopy, metrology, multiphoton microscopy, or material processing. Moreover, the external cavity design of such lasers allow for versatility, compactness and beam quality – aspects which are all beneficial to the employment of VECSELs. However, with respect to a practical use, also robustness, flexibility and cost efficiency play a role, which can be addressed via an approach referred to as saturable-absorber-free mode-locking, or self-mode-locking (SML).

Much work has been performed in the field of saturable-absorber-based pulsed VECSELs by the community to provide ever shorter pulses using resonator-integrated or even chip-integrated semiconductor saturable absorber mirrors (SESAMs), which have demonstrated an excellent performance of pulsed VECSELs with pulse durations in the range of 100 fs to picoseconds. Nevertheless, SML VECSELs can circumvent limitations naturally imposed on the device’s performance by SESAMs, which are costly and have to be individually designed for a certain operation wavelength. Here, we highlight recent achievements in the field of self-mode-locking of VECSELs with quantum-well as well as quantum-dot based gain media. Furthermore, assuming a nonlinear lensing effect to play a significant role for SML operation, we have studied the influence of a few VECSEL parameters on SML, such as power densities and cavity geometries, in the context of recent considerations of a nonlinear refractive index.

9734-20, Session 5

Reflection z-scan measurements of the non-linear lens in VECSEL gain structures

Edward A . Shaw, Univ . of Southampton (United Kingdom); Adrian Quarterman, Univ . of Dundee (United Kingdom); Andrew P . Turnbull, Theo Chen Sverre, Christopher R . Head, Anne C . Tropper, Univ . of Southampton (United Kingdom); Keith G . Wilcox, Univ . of Dundee (United Kingdom) Recent advances in thermal management of VECSEL gain structures using ‘flip-chip’ bonding have enabled the use of much higher pump densities than were previously possible. This has led to the achievement of record high output powers for both continuous-wave and mode-locked VECSELs. High pump densities have been associated with the observation of self-

9734-22, Session 6

Narrow linewidth visible/UV semiconductor disk lasers for quantum technologies

(Invited Paper)

David Paboeuf, Brynmor E . Jones, Julio M . Rodríguez García, Peter J . Schlosser, Univ . of Strathclyde (United Kingdom); Dariusz Swierad, Joshua Hughes, Ole Kock, Lyndsie Smith, Kai Bongs, Yeshpal Singh, The Univ . of Birmingham (United Kingdom); Stefano Origlia, Stephan Schiller, Heinrich-Heine-Univ . Düsseldorf (Germany);

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Conference 9734: Vertical External Cavity Surface Emitting Lasers (VECSELs) VI

Jennifer E . Hastie, Univ . of Strathclyde (United Kingdom) Optically-pumped semiconductor disk lasers (SDLs) have been shown over the past decade to possess all the qualities required for quantum technology applications. Indeed the very specific dynamics of these lasers make them perfect candidates to achieve low intensity noise and sub-Hz linewidth. Moreover, bandgap engineering and nonlinear conversion allow very broad spectral coverage from the ultraviolet to the mid-infrared, while the external cavity provides an excellent beam quality.

At the University of Strathclyde, we have been working for several years on GaInP based SDLs emitting in the 670-690 nm spectral band. Powers up to 1 W and single frequency emission have been achieved with these structures. By locking to a reference cavity, a relative frequency noise of 5 kHz (over 1 s) has been achieved, demonstrating the suitability of these sources for high coherence emission. This high coherence can be transferred to wavelengths in the UV by means of non-linear conversion. Such high coherence UV sources could find applications in UV photolithography or atmospheric spectroscopy. We recently started to develop laser sources suitable for quantum technologies, in particular the laser cooling of Strontium atoms within optical clocks. A power above 100 mW and relative frequency stability of 5 kHz has been achieved. This narrow linewidth laser at 689 nm has been moved to the University of Birmingham and used to trap millions of Sr atoms cooling down to ~170?K. This establishes the narrow linewidth and high spectral purity of the laser. Results and characterisation carried out will be provided.

9734-23, Session 6

InGaAs-QW VECSEL emitting >1300-nm via intracavity Raman conversion

Daniele C . Parrotta, Riccardo Casula, Univ . of Strathclyde (United Kingdom); Jussi-Pekka Penttinen, Tomi Leinonen, Tampere Univ . of Technology (Finland); Alan J . Kemp, Univ . of Strathclyde (United Kingdom); Mircea Guina, Tampere Univ . of Technology (Finland); Jennifer E . Hastie, Univ . of Strathclyde (United Kingdom) We report intracavity Raman conversion of a long-wavelength InGaAs-QW VECSEL to ~1320nm, the longest wavelength ever achieved by a VECSEL pumped Raman laser. The set-up consisted of a VECSEL capable of >17W at 1180nm and tunable from 1141-1203nm and 30mm-long KGd(WO4)2 (KGW) Raman crystal in a coupled-cavity Raman resonator. The Raman cavity was separated from the VECSEL resonator by a tilted dichroic mirror, which steers the Raman beam to an output coupler external to the VECSEL. The spectral emission of the VECSEL, and consequently of the Raman laser, is set by a 4mm-thick quartz birefringent filter in the VECSEL cavity. The KGW Raman laser is capable of emitting 2.5W at 1315nm, with M^2=2.5 and >4% diode-to-Stokes conversion efficiency. The Raman laser emission is tunable from 1295-1340nm, limited by the free spectral range of the birefringent filter. Spectral broadening of the fundamental emission is observed during Raman conversion. At the maximum output power the VECSEL, the total linewidth of the output spectrum is ~0.7nm FWHM. As a consequence, the Raman laser emission is also relatively broad (~0.9nm FWHM). Narrow (<0.2nm FWHM) emission is obtained by inserting an additional 100?m etalon within the VECSEL cavity. With this configuration the Raman threshold is achieved at lower fundamental intracavity power, suggesting an enhanced effective Raman gain, but the maximum output power of the Raman laser is 1.8W. Further characterization (power transfers, emission spectra, estimation of the effective Raman gain and thermal lensing power) will be reported.

9734-25, Session 6

A 1.5-W frequency doubled semiconductor disk laser tunable over 40 nm at around 745 nm

Esa J . Saarinen, Jari Lyytikäinen, Sanna Ranta, Antti Rantamäki, Antti Saarela, Tampere Univ . of Technology (Finland); Alexei Sirbu, Vladimir Iakovlev, Eli Kapon, Ecole Polytechnique Fédérale de Lausanne (Switzerland); Oleg G . Okhotnikov, Tampere Univ . of Technology (Finland) 1.5 W of output power was obtained in the challenging wavelength range between 700 and 800 nm by frequency doubling a wafer-fused 1.49-?m semiconductor disk laser (SDL). The SDL was pumped with low-cost 980 nm diodes. The demonstrated scheme for accessing wavelengths in the vicinity of 750 nm is a viable alternative to bulky and expensive Ti:Sapphire and Alexandrite lasers that require pumping with visible light. Moreover, the performance of diodes emitting directly at around 750 nm is limited by the lack of appropriate compound semiconductors. The gain element of the SDL comprised an InP-based resonant periodic gain section and a GaAs based distributed Bragg reflector assembled in contact with a transparent intracavity diamond heat spreader. A 5-mm long bismuth borate crystal was used for doubling the frequency in a Z-shaped cavity where the gain element was placed as a folding mirror. A total optical-to-optical efficiency of 8.3 % was achieved. The SDL was tunable from 720 to 764 nm with an intracavity birefringent plate. Full width at half maximum (FWHM) of the tuning curve was about 30 nm, centered at 745 nm. The beam quality parameter M2 of the laser remained below 1.5 at all power levels. The laser is attractive for biomedical applications such as photodynamic therapy and fluorescence imaging that benefit from the low absorption of light in tissue in this spectral range.

9734-26, Session 7

Advances in optically pumped semiconductor lasers for blue emission under frequency doubling

States)

(Invited Paper)

Yanbo Bai, Jeffrey A . Wisdom, John P . Charles, Patrick Hyland, Christian Scholz, Zuntu Xu, Yong Lin, Eli S . Weiss, Juan L . Chilla, Arnaud Y . Lepert, Coherent, Inc . (United Optically pumped semiconductor lasers (OPSL) offer the advantage of excellent beam quality, wavelength agility, and high power scaling capability. In this talk we will present our recent progress of high power 920 nm OPSLs frequency doubled to 460nm for lightshow applications. Fundamental challenges and mitigations are revealed through electrical, optical, thermal, and mechanical modeling. Results also include beam quality enhancement in addressing the competition from diode lasers.

9734-27, Session 7

Over 10 Watt, collinear blue and green vertical external cavity surface emitting laser

Michal L . Lukowski, Chris Hessenius, Jason T . Meyer, Mahmoud Fallahi, College of Optical Sciences, The Univ . of Arizona (United States) We report on the development and demonstration of a high power, two color, collinear, blue and green vertical external cavity surface emitting laser (VECSEL). Two different InGaAs/GaAs VECSEL chips designed to operate

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Conference 9734: Vertical External Cavity Surface Emitting Lasers (VECSELs) VI

around 970 nm and 1070 nm are used to make two separate V-folded laser cavities. Each cavity contains a birefringent filter and a lithium triborate (LBO) nonlinear crystal oriented such the outputs from two cavities are orthogonal with respect to each other. Two LBO crystals are angle cut to phase match to 970 nm and 1070 nm fundamental wavelengths and provide blue and green outputs via second harmonic generation (SHG), respectively. These two separate beams are combined in a polarizing beam splitter (PBS) to get a single beam which contains both colors. Thanks to high intracavity circulating power in a VECSEL, over 5 Watts of blue and over 5 Watts of green output power is generated through nonlinear conversion. Overall, a 10 Watt beam made of orthogonally polarized, collinear blue and green wavelengths is presented. This beam can be a great basis for a high power, white laser source, if red output is added in a similar manner.

9734-29, Session 7

Gain chip design, power scaling and intra cavity frequency doubling with LBO of optically pumped red-emitting AlGaInP VECSELs

Hermann Kahle, Cherry M N . Mateo, Uwe Brauch, Roman Bek, Univ . Stuttgart (Germany); Thomas Schwarzbäck, TRUMPF Laser Systems for Semiconductor Manufacturing GmbH (Germany); Michael Jetter, Thomas Graf, Peter Michler, Univ . Stuttgart (Germany)

9734-28, Session 7

Optimization of 2.5

µ

m VECSEL: influence of QW strain

Peter Holl, Marcel Rattunde, Steffen Adler, Andreas Bächle, Elke Diwo-Emmer, Rolf Aidam, Wolfgang Bronner, Joachim Wagner, Fraunhofer-Institut für Angewandte Festkörperphysik (Germany) VECSEL have attracted considerable interest in recent years due to their capability of simultaneously delivering high output powers and a high quality output beam. High-performance VECSELs have been realized at wavelengths ranging from the UV to the near-to-mid IR [1,2].

Using the (AlGaIn)(AsSb) material system, VECSEL covering the 2 – 3 was achieved so far at emission wavelengths around 2.0 µ µ m range can be realized. The best laser performance of GaSb-based VECSELs m with over 30% slope efficiency, a low threshold pump power density of 1.1 kW/cm2 at 20°C heatsink temperature and concomitant a high output power exceeding 7 W in CW operation (depending on the mounting technology) [3]. These parameters were degrading significantly for longer wavelength devices emitting around 2.5 µ m [4] and 2.8 µ m [5]. But for applications like the generation of MWIR light (3-8 µ m) by pumping ZGP-OPOs, high-power VECSELs around 2.5 peak at around 2.9 µ µ m are required to suppress absorption losses, while for medical treatment, high-power operation near the water absorption m is desirable.

We will present results of our ongoing research strand for further optimization of the semiconductor heterostructure design of longer wavelength GaSb-based VECSELs. By using highly strained QWs with a lattice mismatch of 4.3% we were able to realize a 2.5 µ µ m emitting VECSEL with very high slope efficiency above 30%, corresponding to an external quantum efficiency exceeding 50%, and a low threshold power density of 0.8 kW/cm2. These values are as good as those for the best performing 2.0 m VECSELs. With a front SiC heatspreader and operated in a standard linear cavity, over 7 W of CW output power were achieved for this 2.5 µ m emitting VECSEL structure when operated at 20°C.

We will compare laser structures with different emission wavelengths and discuss the role of the QW strain on laser performance.

References [1] O. Ochotnikov, Ed., “Semiconductor Disk Laser”, Wiley-VCH, 2010 [2] M. Rahim et al. Appl. Phys. Lett. 95, 241107 (2009) [3] P. Holl et al, IEEE J Sel Top Quantum Electron. 21 (6), (2015).

[4] Paajaste et al., J Cryst Growth 323, (2011) [5] Rösener et al., Opt. Lett., 36, (2011) The wide range of applications in biophotonics, television or projectors, spectroscopy and lithography made the vertical external cavity surface emitting lasers (VECSELs) an important category of power scalable lasers. The possibility of bandgap engeneering, inserting frequency selective and converting elements into the open laser cavity and laser emission in the fundamental Gaussian mode leads to ongoing growth of the area of applications for tuneable laser sources. We present an AlGaInP-VECSEL system with a multi quantum well structure consisting of compressively strained GaInP quantum wells in an Al x Ga 1-x InP separate confinement heterostructure with an emission wavelength around 665 nm. The VECSEL chip with its n-? cavity is pumped by a 532 nm Nd:YAG laser under an angle of 50° normal to the surface. In comparison, a gain chip design for high absorption values at pump wavelengths around 640 nm with the use of quantum dot layers as active material is also presented. This leads due to the much better quantum efficiency also to better thermal conditions which again enables power scaling in a defined range. Frequency doubling is now realized with an anti-reflection coated lithium borate crystal, while a birefringent filter, placed inside the laser cavity under Brewster’s angle, is used for frequency tuning.

9734-30, Session 8

Influence of kinetic hole filling on the stability of mode-locked semiconductor disk lasers

(Invited Paper)

Jerome V . Moloney, Isak Kilen, Jörg Hader, College of Optical Sciences, The Univ . of Arizona (United States); Stephan W . Koch, Philipps-Univ . Marburg (Germany) It has been understood for some time that the influence of many-body microscopic carrier-carrier scattering should strongly influence ultrashort optical pulse formation in semiconductor lasers. Our recent simulations show strong departures of electrons and holes from quasi-equilibrium Fermi distributions during short pulse generation in semiconductor disk laser mode-locking. The pulse burns a kinetic hole in the inversion distribution whose depth dictates whether stable single pulses or pulse breakup into multiple pulses will occur. Kinetic hole filling from a nearby unsaturated carrier reservoir can create individual pulses lying outside the net gain window. The filling of this kinetic hole can be qualitatively captured with a dual rate model although the final recovery of the distribution to quasi equilibrium Fermi distributions depends on carrier momentum dependent carrier scattering rates.

In this talk, I will discuss how such nonlinear carrier dynamics affects both gain recovery in the semiconductor disk laser chip and in the saturable absorber mirror (SESAM). We will contrast pulse dynamics in a simple linear cavity and a more commonly utilized V-cavity. Kinetic hole filling when important, feeds carriers into the spectral window supporting the original pulse and leads to an overall amplification of the circulating pulse. Detailed simulations have been carried out varying both saturable and outcoupling losses. Simulation results will be compared with experimental pump probe studies of a variety of VECSEL chips and saturable absorber mirror materials. A goal of the study is to see if it is feasible to generate ultrashort, high-energy pulses in a semiconductor disk laser.

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9734-32, Session 8

Conference 9734: Vertical External Cavity Surface Emitting Lasers (VECSELs) VI

9734-31, Session 8

Mode-locked AlGaInP-VECSEL for the red and UV spectral range

(Invited Paper)

Michael Jetter, Roman Bek, Hermann Kahle, Quynh Duong Ederer, Ana Cutuk, Univ . Stuttgart (Germany); Thomas Schwarzbäck, Univ . Stuttgart (Germany) and TRUMPF Lasersystems for Semiconductor Manufacturing GmbH (Germany); Maria Ana Cataluna, Univ . of Dundee (United Kingdom); Peter Michler, Univ . Stuttgart (Germany) Vertical external-cavity surface-emitting lasers (VECSELs) are an important category of power-scalable semiconductor lasers either in continuous wave or pulsed mode. Especially, by introducing a semiconductor saturable absorber mirror (SESAM) into the cavity ultrafast laser sources can be provided. Most research has been done so far in the infrared spectral range from 830 nm to 1.5 µ m, only recently mode-locked VECSEL were realized in the visible and UV spectrum.

We report on our recent advances in passive mode locking of an AlGaInP VECSEL emitting at between 650 nm and 665 nm. Both the gain structure and the SESAM are fabricated by metal-organic vapor-phase epitaxy and include a Bragg mirror consisting of 55 ?/4 pairs of Al$_{0.45}$GaAs/ AlAs. The active region and the SESAM contain either GaInP quantum wells or InP quantum dots in an AlGaInP active region in a resonant or anti-resonant design, depending on the divers laser set-ups. We use a v-shaped cavity with a concave output coupler, serving as a folding mirror to tightly focus onto the absorber, with a repetition frequency of ~ 800 MHz. Depending on the used combinations of gain and SESAM chips, FWHM puls width durations between 250 fs and a few picoseconds were achieved. Furthermore, we have placed a nonlinear crystal in front of the SESAM to achieve pulsed UV operation around 325 nm with a pulse duration of 1.2 ps. Next to the laser results, detailed characterization of the used structures will be presented.

Colliding pulse mode-locked VECSEL

Declan Marah, Univ . College Cork (Ireland); Alexandre Laurain, College of Optical Sciences, The Univ . of Arizona (United States); Wolfgang Stolz, Stephan Koch, Antje Ruiz Perez, Philipps-Univ . Marburg (Germany); John McInerney, Univ . College Cork (Ireland); Jerome Moloney, College of Optical Sciences, The Univ . of Arizona (United States)

9734-33, Session 8

Advances in commercial, mode-locked vertical external cavity surface emitting lasers

(Invited Paper)

Nils Hempler, Walter Lubeigt, Bartlomiej Bialkowski, Craig J . Hamilton, Gareth T . Maker, Graeme P . A . Malcolm, M Squared Lasers Ltd . (United Kingdom) This paper will describe the current state-of-the-art in commercially available, mode-locked Vertical External Cavity Surface Emitting Lasers (VECSEL). Based on indium gallium arsenide quantum well gain structures, our systems operate with a fixed wavelength in the region between 900 nm – 1060 nm and with Watt-level output powers, low repetition rates of 200 MHz and <500fs durations. Crucially, the paper will discuss power scaling and pulse shortening approaches. In this context, we will discuss aspects of the intracavity heatspreader technology and how this influences the power performance of the systems. Furthermore, we will provide a discussion of how extra-cavity pulse compression techniques can be used to achieve short pulse operation without the need for careful dispersion balance inside the resonator. Full details on the resonator and system design will be given alongside a discussion on how these affect the system’s output characteristics. In addition, the development challenges that have been overcome to bring this promising technology to market will be discussed. These include: thermal management challenges, electronic control system development and robust mechanical design requirements.

9734-24, Session PTue

Vertical external cavity surface emitting lasers for sodium Guidestar applications and improvement of current Guidestar systems

Shawn Hackett, Air Force Research Lab . (United States); Alexander R . Albrecht, Zhou Yang, The Univ . of New Mexico (United States); Jeffrey G . Cederberg, Sandia National Labs . (United States); Mansoor Sheik-Bahae, The Univ . of New Mexico (United States) An efficient optically pumped vertical external cavity surface emitting laser (VECSEL) operating at 1178nm is demonstrated. The output of this laser is frequency doubled to 589nm with intra-cavity frequency doubling with an LBO crystal for use as a source for sodium guidestar applications. This represents a novel solution for excitation of the sodium layer of the atmosphere for adaptive optics systems for use with large aperture telescopes. Simulated VECSEL returns from the sodium layer for VECSEL excitation with the Starfire Optical Range’s 3.5m telescope are shown to greatly enhance the utility of current sodium guidestar systems. Recent development of high power femtosecond pulse modelocked VECSEL with gigahertz pulse repetition rates sparked an increased interest from the scientific community due to the broad field of applications for such sources, such as frequency metrology, high-speed optical communication systems or high-resolution optical sampling. To the best of our knowledge, we report for the first time a colliding pulse modelocked VECSEL, where the VECSEL gain medium and a semiconductor saturable absorber (SESAM) are placed inside a ring cavity. This cavity geometry provides both a practical and an efficient way to get optimum performance from a modelocked laser system. The two counter propagating pulses in our ring cavity synchronize in the SESAM because the minimum energy is lost when they saturate the absorption together. This stronger saturation of the absorber increases the stability of the modelocking and reduces the overall losses of the laser for a given intra-cavity fluence, leading to a lower modelocking threshold. This also allows the generation of fundamental modelocking at a relatively low repetition rate (< GHz) with a higher output power compared to conventional VECSEL cavity. We obtained a total output power of 2.2W with an excellent beam quality, a pulse repetition rate of 1GHz and a pulse duration ranging from 1ps to 3ps. The emitted spectrum was centered at 1007nm with a FWHM of 3.1nm, suggesting that shorter pulses can be obtained with adequate dispersion compensation. The laser characteristics and the influence of the relevant parameters on the pulse duration and stability are studied in detail.

9734-34, Session PTue

Tunable repetition rate VECSEL for resonant acoustic-excitation of nanostructures

Theo Chen Sverre, Christopher R . Head, Andy Turnbull, Edward Shaw, Anne Tropper, Otto Muskens, Univ . of Southampton (United Kingdom) It has been occasionally reported that the signal observed in a pump-probe vibrational spectroscopy experiment shows resonant enhancement when successive pump pulses arrive in phase with the vibration mode. The pulse repetition frequency (PRF) of a conventional femtosecond laser is likely,

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Conference 9734: Vertical External Cavity Surface Emitting Lasers (VECSELs) VI

however, to be only 100 MHz, corresponding to a high-order sub-harmonic of the GHz regime resonances exhibited by nanoscale structures. A laser for which the PRF is high enough to excite a mechanical oscillator on every cycle will drive oscillation with amplitude enhanced by at least Q/ π relative to that created by a single pulse. At such a high PRF, it will be necessary to reduce the pulse energy, since heating effects will limit the average power tolerated by the structure under test. The amplitude of vibration induced by the resonant PRF laser will therefore be multiplied by a factor T0*Q/(T π a more highly excited dynamical regime than conventional excitation by to study nanomechanics in a highly anharmonic regime. Here we report structures will be described.

) relative to that induced by a laser with long interval T between pulses: T0 is the vibration period. For lightly damped resonances with T0 ~ 1 ns and Q-factor ~ 50 or more, periodic impact excitation starts to be able to access isolated pulses. Detection sensitivity is enhanced, and it becomes possible preliminary characterisation of selected nanostructures that promise to be accessible to studies of this kind. The performance of a tunable PRF mode locked VECSEL designed for resonant periodic impact excitation of these limited output beams and a tuning range of tens of nanometers. Moreover, VECSELs enable efficient intra-cavity frequency-doubling, reaching the visible and near-ultra- violet range, where there are interesting atomic transitions. Compared to external frequency doubling schemes, an intra cavity doubled VECSEL set-up is simpler and can be scaled to higher output powers at the second harmonic. We report the development of an intra-cavity frequency doubled VECSEL emitting at 570 nm for photo-ionization of Mg atoms. The laser employed a cavity with V- geometry with a gain chip at the end of one cavity arm. The gain chip had a bottom-emitting design with ten GaInAs quantum wells of 7 nm thickness, which were strain-compensated by GaAsP. Inside the cavity, a critically-phase-matched LBO crystal was used for second harmonic generation and a birefringent filter and an etalon for wavelength selection. We demonstrated over 2W of single-frequency output power at 570.6 nm. The laser was used to perform Doppler-free modulation transfer spectroscopy of molecular Iodine to generate dispersion shaped error signals suitable for top-of-fringe laser frequency stabilization. Locking to these ~20MHz wide features allows the laser to be stabilized against long term frequency drifts. This work progresses towards externally frequency doubling the VECSEL’s output to the UV for performing photo-ionization experiments on Mg.

9734-35, Session PTue

Efficiency and power scaling of in-well and multi-pass pumped AlGaInP-VECSELs

Cherry M . N . Mateo, Uwe Brauch, Hermann Kahle, Roman Bek, Univ . Stuttgart (Germany); Thomas Schwarzbäck, Univ . Stuttgart (Germany) and TRUMPF Laser Systems for Semiconductor Manufacturing GmbH (Germany); Michael Jetter, Marwan Abdou Ahmed, Peter Michler, Thomas Graf, Univ . Stuttgart (Germany)

9734-37, Session PTue

Multi-photon imaging with high peak power VECSELs

Shamil Mirkhanov, Adrian H . Quarterman, Conor J . C . P . Smyth, Samuel Swift, Keith G . Wilcox, Univ . of Dundee (United Kingdom) The vertical external cavity surface-emitting laser (VECSEL) offers multi Watt continuous wave output powers in fundamental TEM00 mode, diffraction-limited beam quality and wavelength outputs spanning from UV to mid IR either in the fundamental wavelength or in combination of frequency mixing, making it a versatile type of laser source. While multi Watt level output powers in the fundamental wavelength have been demonstrated in the 1 ?m region, above 1 Watt of output powers have been demonstrated in the fundamental wavelength operation of VECSELs in the red spectral range. Such lasers, which are based on the AlGaInP material system, are limited with the pump lasers in the green spectral region and operating temperatures below 0°C. The pumping scheme presented in this contribution demonstrates the enhanced quantum efficiency of an AlGaInP-based VECSEL, increased operating temperature of up to 50°C and increased output power. The critical role of optimizing the sample design both for the pump and laser wavelengths, pump spot size, and the number of pump light passes are experimentally investigated.

9734-36, Session PTue

Single-Frequency 570 nm VECSEL for photo-ionization of magnesium

Shaun Burd, National Institute of Standards and Technology (United States) and Univ . of Colorado at Boulder (United States); Tomi Leinonen, Jussi-Pekka Penttinen, Optoelectronics Research Ctr . (Finland); Dietrich Leibfried, Andrew C . Wilson, National Institute of Standards and Technology (United States); Mircea Guina, Optoelectronics Research Ctr . (Finland) GaInAs-based vertical-external-cavity surface-emitting lasers (VECSELs) exhibit advantageous properties for applications in photo-ionization and high precision spectroscopy of several species of atomic ions. Besides offering a broad wavelength coverage between 920 nm and 1.2 µ m, the VECSEL geometry allows single-frequency emission with close to diffraction Multiphoton imaging (MPI) of biological samples using VECSELs was first demonstrated in 2011 by R. Aviles-Espinosa et.al [1]. In the initial report of MPI using a VECSEL, the laser had a peak power of ~350 W, and a pulse duration of 1.5 ps at a repetition rate of ~500 MHz. Much development has been put into reducing the repetition rate of mode-locked VECSELs that leads to lower phototoxicity and better image quality. However, gigahertz repetition rate VECSELs can now produce few-100-fs pulses at multi-watt power levels and >4 kW peak power. Here we report MPI using a kilowatt peak power, gigahertz repetition rate femtosecond VECSEL. The VECSEL we use typically produces 400 fs pulses with an average power of 1.5 W at 1010 nm. It consists of a 10 quantum well gain structure, which is flip-chip bonded onto a thermal diamond and pumped with up to 35 W of 808 nm pump light. The VECSEL was mode-locked using a surface recombination SESAM. The microscope used is a commercially available Nikon TE 2000-U universal microscope with the Radiance 2100 MP as a scanning system. We use Convollaria and C.elegans samples in our experiments since they are relatively stable, and allow repeatable benchmarking of the performance of our VECSEL laser source against commercial laser systems. We also characterize image quality from the VECSEL MPI systems at 1 GHz and 3 GHz and study how image quality varies with laser parameters such as pulse duration, pulse energy and peak power.

[1] R. Aviles-Espinosa et. al. Biomedical Optics Express, 2, 739-747 (2011).

9734-38, Session PTue

Thermal management of VECSELs by front surface direct liquid cooling

Conor J . C . P . Smyth, Adrian H . Quarterman, Shamil Mirkhanov, Keith G . Wilcox, Univ . of Dundee (United Kingdom) Efficient thermal management is vital for VECSELs as it affects the output power and several aspects of the performance of a device. Presently there exist two distinct methods of effective thermal management which

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9734-39, Session PTue

Conference 9734: Vertical External Cavity Surface Emitting Lasers (VECSELs) VI

both possess their merits and disadvantages. Substrate removal of the VECSEL gain chip has proved a successful method in devices emitting at a wavelength near 1 µ m. However for other wavelengths the substrate removal technique has proved less effective primarily due to the thermal impedance of the distributed Bragg reflectors. The second method of thermal management involves use of crystalline heat spreaders bonded to the gain chip surface. Although this is an effective thermal management scheme the disadvantages are additional loss and etalon effects that filter the gain spectrum, making modelocking more difficult and normally resulting in multiple peaks in the spectrum. There are disadvantages associated with both methods connected to crystal cost and sample processing. It is for these reasons that the surface liquid cooling thermal management method has been investigated in this project. Direct liquid cooling involves flowing a temperature controlled liquid over the samples surface. In this project COMSOL was used to model surface liquid cooling of a VECSEL sample in order to investigate and compare its predicted performance with current standard thermal management techniques. Based on modelling, experiments are carried out in order to evaluate the performance of the technique. Conclusions are drawn regarding the future potential of surface liquid cooled VECSEL systems as a low cost alternative to existing techniques.

A serially-connected two-chip VECSEL for dual-wavelength emission with high intracavity power

Fan Zhang, Mahmoud Gaafar, Christoph Möller, Philipps Univ . Marburg (Germany); Wolfgang Stolz, Philipps-Univ . Marburg (Germany) and NAsP III/V GmbH (Germany); Martin Koch, Arash Rahimi-Iman, Philipps-Univ . Marburg (Germany)

9734-40, Session PTue

Hybrid metal-semiconductor mirror for high power VECSEL

Alexandre Laurain, Kokou Gbele, College of Optical Sciences, The Univ . of Arizona (United States); Jorg Hader, University of Arizona (United States); Wolfgang Stolz, Philipps-Univ . Marburg (Germany); Stephan Koch, Antje Ruiz Perez, Philipps-Universität Marburg (Germany); Jerome Moloney, College of Optical Sciences, The Univ . of Arizona (United States) In a typical VECSEL, the relatively low gain requires a highly reflective mirror (>99.8%) attached to the gain medium, and is usually realized with a semiconductor Distributed Bragg Reflector (DBR). However, to reach the reflectivity needed, one has to use lattice matched semiconductor materials with high contrast refractive index, which can be challenging especially with InP based materials. Even with GaAs based material, one has to use 24 pairs of GaAs/AlAs to reach a reflectivity of 99.8%. This relatively thick DBR stack has a poor thermal conductivity and is a major limitation for high power diamond bonded VECSELs. Here, we demonstrate the realization of a low thermal impedance hybrid metal-semiconductor mirror VECSEL. We used only 14 pairs of AlGaAs/AlAs, transparent at the pump wavelength of 808nm, and we used a patterned mask to deposit pure gold on areas of the chip to be pumped, and Ti/Au on some other area to circumvent the poor adhesion of gold on GaAs. The addition of this metallic mirror requires the DBR region to be finished with an optical phase matching layer. A higher gain is observed on area metallized with pure gold and an output power of 4W at 1050nm was obtained with an rms fluctuation < 1% over 1 hour of operation. No lasing was obtained on areas metallized with Ti/ Au, showing the effectiveness of the metallic Au mirror and validating the bonding quality. Laser characteristics are studied in details and compared to simulations and chip processing will be discussed.

Vertical-external-cavity surface-emitting lasers (VECSELs) have been proven to be an excellent platform for the realization of high-power multi-mode or single-frequency continuous-wave operation, as well as mode-locked emission. Moreover, because of the emission-wavelength flexibility and the open cavity, this kind of laser is especially attractive to applications requiring intracavity frequency conversion. For instance, in order to develop a powerful continuous-wave terahertz source based on the difference frequency generation in a VECSEL, several schemes have been realized to produce dual-wavelength emission. However, the separation of the two wavelengths hardly exceeds a few nanometers, being limited by the gain bandwidth of a single VECSEL chip, explaining the approach to go for multi chip cavities. Here, we demonstrate a flexible and compact cavity design, which serially connects two different VECSEL chips in one cavity. Dual-wavelength emission with a wavelength separation of 10 nm and over 600 W intracavity power has been generated. Since the two wavelengths exhibit the same polarization, intracavity type-I second harmonic generation and sum frequency generation have been performed in a LiNbO3 crystal. This design also shows the potential to offer dual-wavelength emission with other desirable wavelength separations, by employing different chip combinations and filters. Furthermore, we investigate the dependence between the emission wavelengths and intracavity incidence angles on the VECSEL chips, in order to extend the range of accessible wavelengths and thus splittings with this cavity design.

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Conference 9735: Laser Applications in Microelectronic and Optoelectronic Manufacturing (LAMOM) XXI

Monday - Thursday 15–18 February 2016 Part of Proceedings of SPIE Vol . 9735 Laser Applications in Microelectronic and Optoelectronic Manufacturing (LAMOM) XXI 9735-1, Session 1

CIGS P3 scribes using ultra-short laser pulses and thermal annealing

Gabor Matthäus, Klaus Bergner, Friedrich-Schiller-Univ . Jena (Germany); Mawuli Ametowobla, Andreas Letsch, Robert Bosch GmbH (Germany); Andreas Tünnermann, Stefan Nolte, Friedrich-Schiller-Univ . Jena (Germany) Thin-film photovoltaic panels are based on individual solar cells which are monolithically interconnected in series. These interconnections are commonly realized via mechanical micro scribing. In order to reduce the “dead area” represented by the interconnection zone and hence to increase the net solar output, tightly packed microscopic scribes have to be realized. For this reason, laser micro-machining gained increasing interest in recent years. However, regarding CIGS thin-film modules, the third structuring stage P3, which is responsible for isolating successive cells, in general suffers from decreased shunt resistance after laser processing.

Here, we demonstrate high-impedance CIGS P3 scribes using ultra short laser pulses and thermal annealing. During laser processing, we applied various ultra-short pulse laser systems, which delivered different pulse lengths (0.5 - 230 ps) at different wavelengths (388 - 1030 nm) at comparable pulse energies up to several microjoulses. We obtained a wide processing window which allowed a reliable structuring at very high speeds (?10 m/s). Immediately after laser processing, the samples revealed a significant drop in cell performance compared to mechanically processed reference cells, however, after thermal treatment, a permanent performance increase was achieved which outperformed the mechanically structured reference cells. In order to study the underlying annealing effect, we simulated the J-V characteristics using an equivalent circuit model yielding excellent agreement with our experimental results. We show, that the main drawback during ultra-short pulse processing is based on induced defect states at high densities. These defect states can be significantly reduced within minutes by thermal annealing at moderate temperatures.

9735-2, Session 1

Selective structuring of multi-layer functional thin films using a laser-induced shockwave delamination process

Martin Ehrhardt, Pierre Lorenz, Lukas Bayer, Leibniz Institut für Oberflächenmodifizierung e .V . (Germany); Carlos Molpeceres, Univ . Politécnica de Madrid (Spain); Carlos Antonio Herrera Ramirez, Abengoa Solar Espana SA (Spain); Klaus-Peter Zimmer, Leibniz-Institut für Oberflächenmodifizierung e .V . (Germany) The laser-assisted microstructuring of thin films, especially for electronic applications without damaging the layers or the substrates, is a challenge for laser micromachining techniques. The P3 scribing of copper indium gallium selenide (CIGS) solar cells on different carrier foil was studied using shock-wave-induced film delamination patterning (SWIFD). The delamination process is induced by a shock wave, generated by the laser ablation of the rear side of the carrier foil. The morphology and size of the resultant thin-film structures were studied by scanning electron microscopy (SEM) dependent on the laser parameters. Furthermore, the composition after the laser treatment was analyzed by energy-dispersive X-ray spectroscopy (EDX). To demonstrate the practical functionality of the SWIFD process the solar cells were electrically characterized after the structuring process. Furthermore, to improve the physical understanding of the process, the delamination process was studied by shadowgraph experiments. The process was simulated using the finite element method, and the simulation results were compared with the experimental ones.

9735-3, Session 1

High throughput laser scribing of Cu(In,Ga)Se2 thin-film solar cells

Andreas Burn, Christian Heger, Berner Fachhochschule Technik und Informatik (Switzerland); Stephan Bücheler, Shiro Nishiwaki, EMPA (Switzerland); David Bremaud, Roger Ziltener, Flisom AG (Switzerland); Lukas Krainer, Gabriel J . Spuehler, Onefive GmbH (Switzerland); Valerio Romano, Berner Fachhochschule Technik und Informatik (Switzerland) Solar cells based on Cu(In,Ga)Se2 absorbers show the highest efficiencies among all thin-film technologies. Their potential is underlined by efficiency world records of 21.7 percent, demonstrated on cell level (ZSW) and above 16 percent for an entire module (TSCM, Manz). In industrial production of CIGS solar modules, laser scribing is used for the monolithic interconnection of cells. However, different than in other fields of laser applications, laser scribing never improves solar cell performance but reduces active area, increases series resistance and decreases parallel resistance of the module which all reduce its performance. In the past years we conducted a comprehensive study on laser sources and parameters for selective ablation of photovoltaic thin-films. Various aspects have been analyzed and processes were validated in functional mini-modules. We have demonstrated 16.6 percent efficiency on an all-laser patterned, grid-less 8-cell mini-module. In this module we demonstrated low dead-zone interconnects of <70 scribing processes. µ m width and a high throughput P3 lift-off scribing process, which selectively removes only the front contact of the cell stack. In agreement with results from other groups we reached the highest module performances with a P2 process involving direct ablation of the CIGS layer using picosecond laser pulses. However, necessary high pulse-to-pulse overlap and moderate repetition frequency typically limit the process speed to <200 mm/s which is not compatible with industrial mass production. In our recent study we optimized P2 scribing processes to overcome this throughput limitation and reach process speeds in excess of 1 m/s. We will present results of our validation experiments using this set of optimized

9735-4, Session 1

Dual-beam laser thermal processing of silicon photovoltaic materials

Brian J . Simonds, National Institute of Standards and Technology (United States); Anthony Teal, Tian Zhang, The Univ . of New South Wales (Australia); Joshua A . Hadler, National Institute of Standards and Technology (United States); Zibo Zhou, Sergey Varlamov, Ivan Perez-Wurfl, The

126 SPIE Photonics West 2016 · www.spie.org/pw

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Conference 9735: Laser Applications in Microelectronic and Optoelectronic Manufacturing (LAMOM) XXI

Univ . of New South Wales (Australia) Laser processing for photovoltaics (PV) has traditionally been used for very small dimension features where focused beams are rastered to create lines for edge isolation, scribing, and selective emitter formation and holes for drilling in wrap through style cells. Larger area laser applications are under investigation, for instance in liquid-phase crystallization of thin film Si on glass, however these rely on additional isothermal heat input. We have developed an all-laser processing technique using two industrially-relevant continuous-wave fiber lasers operating at 1070 nm that is capable of both substrate heating with a large defocused beam and material processing with second scanned beam, suitable for a variety of PV processes. We have demonstrated this technique for rapid crystallization of thin film (~10 ?m) silicon on glass, which is a low cost alternative to wafer-based PV. We have also applied this technique to wafer silicon to control dopant diffusion at the surface region where the focused line beam rapidly melts the substrate that then regrows epitaxially. Finite element simulations have been used to model the melt depth. This readily scalable process is carried out in a matter of tens of seconds for an area around 10 cm2 using only about 1 kW of total power. In this talk, we will discuss our results with both c-Si and thin film silicon.

9735-5, Session 2

Tailored femtosecond Bessel beams for high-throughput, taper-free through silicon vias (TSVs) fabrication

(Invited Paper)

Fei He, Shanghai Institute of Optics and Fine Mechanics (China) and RIKEN Ctr . for Advanced Photonics (Japan); Junjie Yu, Wei Chu, Zhaohui Wang, Yuanxin Tan, Ya Cheng, Shanghai Institute of Optics and Fine Mechanics (China); Koji Sugioka, RIKEN Ctr, for Advanced Photonics (Japan) Laser writing for selective plating of conductive lines for electronics has several great advantages, compared to conventional printed circuit board technology. Firstly, this method is faster and cheaper for prototyping. Secondly, material consumption is reduced, because it works selectively. But the biggest merit of this method is potentiality to produce molded interconnect device, enabling to create electronics in 3D structure, thus saving space, materials and cost of production.

One of the basic techniques of laser writing for selective plating on plastics is Laser Direct Structuring (LDS). In LDS method special fillers are mixed in polymer matrix. These fillers are activated during laser writing process and in the next processing step scanned area can be selectively plated by metals. Thus LDS is a 2-step process. There are some commercial materials available on the market; however, mostly they are based on expensive fillers, usually palladium. In this work we have investigated new material: polypropylene with carbon based additives. It was tested for LDS approach, using picosecond and nanosecond pulses. Different laser processing parameters (laser energy, scanning speed, number of scans, pulse durations, wavelength and overlapping of scanned lines) were applied in order to determine the optimal regime of activation. We have measured sheet resistance of selectively plated samples using different laser processing parameters and the same chemical bath conditions. The activation process was also investigated using Raman spectroscopy analysis. Selectivity tests showed high plating resolution. The narrowest width of plated line was less than 23 ?m. In Raman experiments two spectra of treated and untreated plastic surfaces were compared. Finally, our material was applied for prototype of electronic circuit board on a 2D surface.

9735-7, Session 2

Laser applications in advanced chip packaging

Dirk Mueller, Coherent, Inc . (United States); Andrew Held, Coherent Inc . (United States); Rainer Pätzel, Coherent GmbH (Germany); Dave Clark, Coherent, Inc . (Germany); Joris F . P . van Nunen, Coherent GmbH (Germany) For higher-density integration and acceleration of operating speed in Si ICs, 3D integration of wafers and/or dies is essential. Fabrication of current 3D ICs relies on 3D assembly which electrically connects stacked chips to form a single circuit. A key technology for the 3D assembly is TSVs which are vertical electrical connections passing completely through silicon chips to electrically connect vertically assembled Si ICs. Typical TSVs have wide features, with diameters of a range from several microns to 50 ?m and depths up to 500 ?m with aspect ratios up to 15 depending on the application and integration scheme. In this work, we present high throughput, taper-free TSVs fabrication (i.e., 2 ?m-dia. and 5 ?m-dia. vias in 50 ?m-thick and 100 ?m-thick substrates respectively) using femtosecond Bessel beams operated at different wavelengths from 400 nm to 2.4 ?m. In particular, the propagation characteristics of femtosecond laser Bessel beams during TSVs fabrication are theoretically and experimentally investigated. Furthermore, special phase filters are designed to suppress the damages induced by the side-lobes of Bessel beams for high-quality TSVs fabrication. Our technique can be potentially used for 3D assembly in manufacture of 3D silicon integrated circuits.

We will provide a detailed look at the most recent technology advancements and emerging laser applications used in the IC packaging industry. While drilling HDI with CO2 is well established, the mobile device industry’s push for miniaturization is forcing features to be further reduced in size while reducing their manufacturing cost. To this end carbon-monoxide lasers have recently become commercially available and are operating around 5 down to 25 5 µ µ µ m wavelength. Their shorter wavelength allows for a smaller focus and hence drilling hole diameters m whilst keeping the complexity and cost similar to CO2 lasers. On a separate front, the cost of ownership for Excimer lasers has recently been reduced significantly, which has made this class of lasers attractive for structuring redistribution layers of IC substrates with feature sizes down to m. We can now span the gamut from 100 µ m to 5 µ m or µ -via drilling. In addition to demonstrating these new processes, we compare throughput and performance with the more established use of CO2 and ns-UV lasers.

9735-6, Session 2

Laser-induced selective copper plating of polypropylene surface

Karolis Ratautas, Mindaugas Gedvilas, Ina Stankevi?ien?, Aldona Jagminien?, Eugenijus Norkus, Ctr . for Physical Sciences and Technology (Lithuania); Nello Li Pira, Ctr . Ricerche Fiat S .C .p .A . (Italy); Stefano Sinopoli, U . Emanuele, Bioage Srl (Italy); Gediminas Ra?iukaitis, Ctr . for Physical Sciences and Technology (Lithuania)

9735-8, Session 2

Laser processing of metal nanowire for flexible and stretchable electronics

Seung Hwan Ko, Hyunmin Cho, Dongkwan Kim, Seoul National Univ . (Korea, Republic of) Nanowire percolation network has various application as an alternative to ITO transparent conductor. Laser processing for selecive laser nanowelding was developed for low temperature nanowire annealing process on plastic substrate and used for flexible and stretchable electronics fabrication.

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Conference 9735: Laser Applications in Microelectronic and Optoelectronic Manufacturing (LAMOM) XXI

9735-9, Session 3

3D photonic and opto-fluidic devices

(Invited Paper)

Ajoy K . Kar, Heriot-Watt Univ . (United Kingdom) in microfluidic environments such as 3D electro-orientation of cell motions, dielectrophoretic manipulation of microparticles and electro-rotation in microscale spaces.

Focussed ultrashort laser pulses can modify the local refractive index of certain materials, significant research has been expended into using ultrafast lasers to fabricate integrated optical devices. Integrated optical waveguides – the optical analogue of wires – can be simply fabricated by translating the sample in the path of such short optical pulse trains, which effectively amounts to writing the desired optical circuit in a controlled way in that sample. This direct-write approach offers several key benefits over conventional fabrication techniques. It neither requires use of expensive clean room facilities, nor involves complex film deposition and subsequent etching processes. This technology can also yield 3D structures, unachievable through conventional techniques.

The structural changes induced by the deposited optical energy may manifest themselves in a variety of ways e.g. refractive index and etch-rate changes. Such changes in the properties of the material can in turn be used to create a variety of new and exciting integrated photonic and biophotonic devices. Using the highly localized refractive index modification to directly inscribe optical waveguides in various dielectric materials, we have demonstrated a number of active devices e.g. waveguide amplifiers and mode-locked lasers. Utilizing selective etching we are able to demonstrate cell separator and imaging flow cytometer. In my talk I will present how the ultrafast laser inscription technology can be used to develop active and passive photonics components. I will also describe how to exploit new materials e.g. laser and highly nonlinear materials, such that several all-optical devices could be monolithically integrated on the same substrate in the form of optical integrated circuits for biophotonic applications.

9735-10, Session 3

Femtosecond laser fabricated electrofluidic devices in glass for 3D manipulation of biological samples

Jian Xu, Katsumi Midorikawa, Koji Sugioka, RIKEN (Japan)

9735-11, Session 3

Photo-direct machining of polydimethylsiloxane using laser plasma EUV sources

Tetsuya Makimura, Hikari Urai, Univ . of Tsukuba (Japan); Daisuke Nakamura, Akihiko Takahashi, Kyushu Univ . (Japan); Hiroyuki Niino, National Institute of Advanced Industrial Science and Technology (Japan); Tatsuo Okada, Kyushu Univ . (Japan) Polydimethylsiloxane (PDMS) is a material used for micro total analysis systems / lab-on-chips due to its flexibility, chemical / thermo-dynamic stability, bio-compatibility and moldability. For further development, it is inevitable to develop a technique to fabricate three dimensional structures in micrometer-scale at high aspect ratio. We have investigated a technique for micromachining of PDMS by means of photo direct machining using laser plasma EUV radiations. The EUV radiations around 10 nm, 13.5 nm, and 11 nm were generated by irradiation of Ta, Sn, and Xe targets, respectively, with Nd:YAG (10 ns) and pulsed TEA CO2 laser (50 ns) light. The generated EUV radiations were focused on PDMS surfaces at power densities up to 10^8 W/cm2, using an ellipsoidal mirror. Applying the technique, we demonstrated microfabrication of a through hole with a diameter of 1 micrometer in a PDMS sheet. We measured ablation depth using the different EUV sources and found that ablation depth is governed by power density of EUV light on PDMS surfaces.The surfaces are ablated at rates up to 200 nm/shot. From the threshold, the accumulated EUV energy is estimated to be comparable to that to decompose a PDMS surface into small fragments. X-ray photoelectron spectroscopy has revealed that there is no chemical modification induced by the EUV irradiation. All these properties are suitable for micromachining of PDMS elastomers at high aspect ratios.

9735-12, Session 3

Laser-assisted morphing of complex three dimensional objects

Yves Bellouard, Jakub Drs, Ecole Polytechnique Fédérale de Lausanne (Switzerland) Femtosecond (fs) laser microfabrication allows localized modification in the interior of glass to control physical, chemical and optical properties. Such modification in three-dimensional (3D) space relies on the intrinsic feature of nonlinear absorption of the fs laser by glass, enabling fabrication of 3D microfluidics and optofluidics for the lab-on-a-chip applications. Integration of microelectric components into such lab-on-a-chip devices will further enhance functionalities of themselves. Previously we proposed a technique based on hybrid fs laser microfabrication to realize electrofluidics in which microelectric components are integrated into 3D glass microfluidic structures. 3D microchannels with smooth internal walls were first prepared in photosensitive glass by fs laser-assisted chemical wet etching process combined with post-annealing. Then, space-selective metallization of microchannels was achieved by fs-laser direct-write ablation using the same laser followed by electroless metal plating.

In this paper, we demonstrate further progress of this technique in terms of controllability and flexibility in 3D electrode patterning to fabricate high performance electrofluidic devices. Introduction of water to the ablation site during the laser direct writing realizes debris-free microstructuring of internal walls of glass channels with high controllability and flexibility of laser modification in 3D space. Compared with conventional fabrication methods, electrodes with 3D designable geometries can be facilely prepared at any positions in closed microchannels, which offers the ability of producing 3D controllable electric fields in the channels. Influence of laser processing parameters and electroless plating process on the performance of fabricated devices are discussed. The fabricated electrofluidic devices are applied to demonstrate multifunctional manipulation of biological samples Morphing refers to the smooth transition from a specific shape into another one, in which the initial and final shapes can be significantly different. A typical illustration is to turn a cube into a sphere by continuous change of shape curvatures.

Here, we demonstrate a process of laser-induced morphing, driven by surface tension and thermally-controlled viscosity. As a proof-of-concept, we turn 3D glass structures fabricated by a femtosecond laser into other shapes by locally heating up the structure with a feedback-controlled CO2 laser. We further show that this laser morphing process can be accurately modeled and predicted.

9735-13, Session 4

Laser induced forward transfer: a novel tool for printing sensors and characterizing surface wetting properties

Ioanna Zergioti, National Technical Univ . of Athens (Greece)

128 SPIE Photonics West 2016 · www.spie.org/pw

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Conference 9735: Laser Applications in Microelectronic and Optoelectronic Manufacturing (LAMOM) XXI

During the last decade, laser based techniques have received significant attention as direct, cost-effective and high resolution printing/patterning techniques applied in a variety of applications including sensors, cell printing and microelectronic devices. The versatility that Laser Induced Forward Transfer (LIFT) offers the capability of its implementation for the fabrication of a variety of sensing devices, i.e. resistive, amperometric, transistor-based optical sensing, and on different substrates. Up to date, the LIFT technique has been applied for the fabrication of several types of biosensors with high spatial resolution in a direct approach. In this work, examples of chemical and biosensors fabricated based on the LIFT technique will be presented. One of the key advantages of LIFT is that can be used as an advanced process for functionalizing the sensors surface, with biomaterials, without the need for intermediate layers, or chemical functionalization techniques, that often require treatment with toxic solvents and hazardous chemicals. The unique characteristic of creating high speed liquid jets, that LIFT technique offers, leads to the physical adsorption of the biomaterials on the sensors’ surfaces. In addition, following these results, we have introduced Laser Induced Forward Transfer of liquids method, to characterize the wetting properties of liquid repellent surfaces. With this method, laser pulses are creating liquid jets with very high velocity up to 260 m/s and consequent liquid impact pressures up to 37 MPa. This unique characteristic, along with the capacity to print liquids of various viscosities, makes this technique an ideal candidate to characterize surfaces that withstand high liquid impact pressures without losing their liquid repelling properties, as well as to outline their possible applications. Wissenschaften München (Germany); Matthias Domke, FH Vorarlberg (Austria) Selective and precise structuring of materials with ultrashort laser pulses is based on the electronic absorption of laser energy. Heat generation and diffusion in the electronic system, transfer of energy from the electronic to the lattice system, and the subsequent heat generation and diffusion in the lattice system are the consequence. Finally, at or slightly above the ablation threshold phase transitions and pressure waves are leading to material removal by laser ablation.

Ultrafast time-resolved experiments and numerical simulation are used to generate a deeper understanding of the various kinds of laser ablation, e.g. direct and confined laser ablation for different ultrashort pulse durations.

In this work we present a state of the art overview of the mechanisms of confined laser ablation and the pulse duration dependence of ablation efficiency.

9735-16, Session 5

Laser energy deposition at the surface of dielectrics exposed to single 15-fs laser pulse

Corinne Pasquier, Marc L . Sentis, Olivier P . Utéza, Nicolas Sanner, Lasers, Plasmas et Procédés Photoniques (France) and Aix-Marseille Univ . (France)

9735-14, Session 4

Green microfabrication for flexible electronics by laser direct synthesis and pattering technology

Ming-Tsang Lee, National Chung Hsing Univ . (Taiwan) In this study, the photo-thermo-fluidic transport phenomena in a novel laser direct synthesis and patterning (LDSP) technology that is applied to rapidly fabricate flexible conductors on a polymer substrate (polyimide film) is investigated. In the manufacturing process, a focused continuous wave green laser beam was directed using a galvanometer system to locally and selectively heat the transparent and particle-free reactive silver ink by the absorption of laser energy from the polyimide substrate. Silver patterns with good electrical conductivity are obtained. The LDSP technology can be operated directly at atmospheric pressure and room temperature. It does not require the use of vacuum chamber, oven and photo-mask, etc. Therefore, this novel technology offers a new approach to the cost-effective and green fabrication for flexible electronics. The effects of laser power, scanning speed, number of scans and ink concentration on the properties of the resulted silver patterns are investigated both experimentally and theoretically. For the theoretical study, numerical simulations of the transient heat transfer analysis in the microscale heating and chemical reaction spot on the polyimide surface is carried out. Important considerations to the LDSP process on the photo-thermo-fluidic transfer multiphysics are discussed based on the comparison of simulated results and the experimental measurements.

9735-15, Session 5

Towards a more complete understanding of laser ablation with ultrashort pulses: Mechanisms of confined laser ablation and pulse duration dependence of laser ablation efficiency

(Invited Paper)

Heinz P . Huber, Jan Winter, Juergen Sotrop, Regina Moser, Stephan Rapp, Rudolph Reiel, Hochschule für Angewandte We present investigations of interaction of a femtosecond laser (down to 15 fs pulse at 800nm) in the single-shot laser regime with the surface of two dielectric materials. We consider fused silica and sapphire, which have different electronic recombination time, when they are exposed to ultrashort pulses.

The first part of our work is to determine the damage threshold (2.0± 0.22 J/cm? for fused silica and for sapphire 2.66±0.24 J/cm?) and the ablation threshold [1] (respectively 2.1±0.17 J/cm? and 2.8±0.23 J/cm?). Our experimental results confirm that if the pulse duration is short then the two thresholds are close [2]. Secondly, the evolution of the measured transmission, reflexion and inferred absorption signals is studied as a function of fluence. We separate our investigation depending on the incident energy: below ablation threshold, between ablation threshold and the energy below the critical energy in air (for which appear nonlinear propagation effects [3], like Kerr effects or air ionization), and high energy. The study is completed by a spectral balance in transmission and in reflexion, to identify the absorbed wavelengths and to reveal the spectrum evolution according to the laser incident energy. A comparative study on the dynamics of energy deposition between a 15-fs pulse and longer pulse durations (30 fs [4] and 450 fs [5]) will be conducted to highlight the advantage of ultra-short pulses, in the context of micromachining applications.

[1] N. Sanner, O. Utéza, B. Bussiere, G. Coustillier, A. Leray, T. Itina, M. Sentis, Appl. Phys. A 94, (2009) 889–897.

[2] B. Chimier, O. Utéza, N. Sanner, M. Sentis, T. Itina, P. Lassonde, F. Légaré, J.C. Kieffer, Phys. Rev. B 84 (2011) 094104.

[3] C. Pasquier, P. Blandin, R. Clady, N. Sanner, M. Sentis, O. Utéza , Yu Li, Shen Yan long, Opt. Commun. 355 (2015) 230–238.

[4] M. Lebugle, N. Sanner, N. Varkentina, M. Sentis, and O. Utéza, J. Appl. Phys. 116 (2014) 063105.

[5] N. Varkentina, N. Sanner, M. Lebugle, M. Sentis and O. Utéza , J. Appl. Phys.114 (2013) 173105.

9735-17, Session 5

Laser damage experiments at 10 fs in air

Olivier P . Utéza, Corinne Pasquier, Raphaël Clady, Nicolas

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Sanner, Marc L . Sentis, Lasers, Plasmas et Procédés Photoniques (France) A critical issue for safe development and operation of emerging laser infrastructures of Petawatt class, combining hundreds of J and sub-15 femtosecond pulses, is the problem of damage of the optical components ensuring the delivery of the ultrahigh intensity beam to the target. The damage evaluation shall be ideally conducted in vacuum to provide the exact conditions of operation for the optical components and materials under test. Indeed, the laser-induced damage threshold (LIDT) is expected to vary depending on the ambiance (air/vacuum) that may affect the energy exchange between the surrounding gas and the material surface. For instance, considering metals, smaller ablation thresholds have been measured in air compared to vacuum [2]. Nevertheless, performing damage tests in air is of interest for laser manufacturers and optics suppliers, since it enables rapid feedback and comparative assessments for optimization of optical components. However, due to the short duration of the incident pulses and their high sensitivity to nonlinear effects, the preservation of the optical beam properties till the sample located at the focus of a focusing element may be questionable [3].

In this work, we thus address the difficulties implied by the use and manipulation of ultrashort laser pulses in the regime of damage of optical materials. In particular, we determine the exact laser conditions in which the threshold of modification of the material is evaluated. Secondly, to demonstrate the test-bench capabilities, we show the reliable measurement of the laser-induced damage threshold of a material (fused silica) extensively used in photonics.

[1] E. G. Gamaly, N. R. Madsen, M. Duering, A. V. Rode, V. Z. Kolev, and B. Luther-Davies, Phys. Rev. B 71 (2005) 174405. [2] C. Pasquier, P. Blandin, R. Clady, N. Sanner, M. Sentis, O. Utéza , Yu Li, Shen Yan long, Opt. Commun. 355 (2015) 230–238.

9735-18, Session 5

Conference 9735: Laser Applications in Microelectronic and Optoelectronic Manufacturing (LAMOM) XXI Laser ablation of borosilicate glass with high power shaped UV nanosecond laser pulses

Philipp von Witzendorff, Andrea Bordin, Oliver Suttmann, Laser Zentrum Hannover e .V . (Germany); Rajesh S . Patel, James M . Bovatsek, Spectra-Physics® (United States); Ludger Overmeyer, Laser Zentrum Hannover e .V . (Germany) The application of thin borosilicate glass as interposer material requires methods for separation and drilling of this material. Laser processing with short and ultra-short laser pulses have proven to enable high quality cuts by either direct ablation or internal glass modification and cleavage. A recently developed high power UV nanosecond laser source allows for pulse shaping of individual laser pulses. Thus, the pulse duration, pulse bursts and the repetition rate can be set individually at a maximum output power of up to 60 W. This opens a completely new process window which could not be entered with conventional Q-switched pulsed laser sources. In this study, the novel pulsed UV laser system was used to study the laser ablation process on 400 µ m thin borosilicate glass at different pulse durations ranging from 2–10 ns and a pulse burst with two 10 ns laser pulses with a separation of 10 ns. Single line scan experiments were performed to correlate the process parameters and the laser pulse shape with the ablation depth and cutting edge chipping. Increasing the pulse duration within the single pulse experiments from 2 ns to longer pulse durations led to a moderate increase in ablation depth and a significant increase in chipping. The highest material removal was achieved with the 2x10 ns pulse burst. Experimental data also suggest that chipping could be reduced while maintaining a high ablation depth by selecting an adequate pulse overlap. We also demonstrate that real-time combination of different pulse patterns during drilling a thin borosilicate glass produced holes with low overall chipping at a high throughput rate.

9735-19, Session 6

Ab initio molecular dynamics simulations of femtosecond-laser-induced anti-Peierls transition in antimony

(Invited Paper)

Eeuwe S . Zijlstra, Tobias Zier, Bernd Bauerhenne, Univ . Kassel (Germany); Sergej Krylow, University of Kassel (Germany); Martin E . Garcia, Univ . Kassel (Germany) Intense femtosecond-laser pulses are able to induce ultrafast nonthermal melting of different materials along pathways that are inaccessible under thermodynamic conditions. In order to investigate the nonthermal motion of atoms upon femtosecond laser excitation we performed ab-initio molecular dynamics simulations on laser-excited potential energy surfaces using our code CHIVES (Code for Highly excited Valence Electron Systems). We found surprising results. Our simulations show that for silicon irradiated below the melting threshold room-temperature phonons become thermally squeezed, and that the presence of squeezed thermal phonons announces complete atomic disordering as a function of fluence, and effect this effect which general and not material specific [1]. Furthermore, by studying irradiated silicon for fluences above the melting threshold, we found that the atoms move successively superdiffusively and fractionally diffusively before becoming diffusive. We found fractional atomic diffusion, not reported so far in materials, during more than 800 fs [2]. In this talk I will show, that combining the effects mentioned above, control of the nonthermal melting process by two pump pulses is possible. Moreover, simulations of the behavior of thin silicon films upon intense femtosecond laser irradiation show a remarkable effect: nonthermal melting does not start at the surface but in the middle of the film. The various signatures of nonthermal melting and the differences to thermal melting will be discussed. In order to extend our studies for the study of nucleation phenomena, we developed an analytical interatomic potential for laser-excited silicon, which depends on the electronic temperature. Effects like bond softening in the presence of hot electrons are taken into account. With the help of this potential we were able to perform large-scale simulations and study nucleation dynamics during nonthermal melting. Finally, it will be shown in this talk that the combination of accurate time-resolved optical measurements with ab-initio calculation of atomic displacements from the knowledge of the reflectivity has clear advantages with respect to time-resolved crystallography and allows a detailed visualization and control of two-dimensional atomic motion in solids [3].

1) E. S. Zijlstra, A. Kalitsov, T. Zier and M. E. Garcia, Squeezed Thermal Phonons Precurse Nonthermal Melting of Silicon as a Function of Fluence, Physical Review X 3, 011005 (2013). 2) E. S. Zijlstra, A. Kalitsov, T. Zier and M. E. Garcia, Fractional Diffusion in Silicon, Advanced Materials 25, 5605-5608 (2013). 3) H. Katsuki, J.C. Delagnes, K. Hosaka, K. Ishioka, H. Chiba, E.S. Zijlstra, M.E. Garcia, H. Takahashi, K. Watanabe, M. Kitajima, Y. Matsumoto, K.G. Nakamura and K. Ohmori, All-optical control and visualization of ultrafast two-dimensional atomic motions in a single crystal of bismuth, Nature Communications 4, 2801 (2013).

9735-20, Session 6

Ultrafast laser-induced complex refractive index changes in metals measured by pump-probe ellipsometry

Stephan Rapp, Albert Althammer, Max Bung, Heinz P . Huber, Hochschule für Angewandte Wissenschaften München (Germany) Ultrashort pulsed lasers are gaining increasing significance in industrial

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Conference 9735: Laser Applications in Microelectronic and Optoelectronic Manufacturing (LAMOM) XXI

applications. They offer the advantages of precise and efficient material processing. Thus, intensive research is needed to obtain a fundamental understanding of the physical laser-material interaction. Data showing the ultrafast changes of the optical material properties in response to ultrashort laser pulse processing are still missing. Such data could improve the understanding of ultrafast laser induced phase transitions and their temporal occurrence. Additionally, they could quantify the material absorption change during the pulse impact and possibly explain ablation efficiency changes observed for different pulse lengths. Moreover, these data are important to verify simulated results. In this work, a unique pump-probe ellipsometry microscope is presented to investigate the transient absorption. A laser at a wavelength of 1053 nm and a pulse duration of 650 fs (FWHM) is used as pump source, frequency doubled pulses are used for illumination. The polarization state of the reflected probe-pulse is analysed than to obtain the ellipsometric angles psi and delta and the real and the imaginary part of the complex refractive index. Ellipsometric data acquisition and processing are explained in detail. The setup allows the time-resolved determination of the complex refractive index, describing also the material absorption, with a sub-ps temporal resolution. Measurement series on molybdenum samples are presented. They show the change of the real part (n) as well as the imaginary part (k) of the complex refractive index.

9735-22, Session 7

Ultrashort-pulse laser processing of transparent materials: Insight from numerical and semi-analytical models

(Invited Paper)

Nadezhda M . Bulgakova, HiLASE Ctr . (Czech Republic) and Institute of Thermophysics (Russian Federation); Vladimir P . Zhukov, Institute of Computational Technologies (Russian Federation) and Novosibirsk State Technical Univ . (Russian Federation); Yuri P . Meshcheryakov, Institute of Hydrodynamics (Russian Federation); Tomá? Mocek, HiLASE Ctr . (Czech Republic)

9735-21, Session 6

Numerical study of the influence of picosecond laser spot size on ablated depth and threshold fluence of metal

Yiming Zhang, Berner Fachhochschule Technik und Informatik (Switzerland) and Univ . Bern (Switzerland); Benjamin Lauer, Beat Neuenschwander, Valerio Romano, Berner Fachhochschule Technik und Informatik (Switzerland) Picosecond laser systems have been widely used in industrial microprocessing applications since they are a cost-effective tool to achieve high throughput. To better understand the ablation process , firstly the dependence of the ablation depth and the threshold fluence on the laser spot size were measured utilizing a 10ps pulsed laser system . Further, a 2D axisymmetric model was established to demonstrate the mechanism of the phenomenon. Three sets of the spot radii, namely 15.5?m, 31.5?m and 49.6?m, with equal laser peak fluence ranging from 0.2J/cm2 to 10J/cm2 were applied on copper. It was found that the laser ablation depth in the centre increases with decreasing spot size at identical peak fluence while the ablation threshold is proportional to the spot size. A 2D axisymmetric thermomechanical model was developed to qualitatively illustrate the mechanism of this phenomenon. The equivalent thermal stress, based on von Mises criterion, and the compressive stress were indicated to play a crucial role. Their impact on the ablation depth and threshold fluence is based on the lattice temperature gradient along the surface of the material, leading to spallation and possible mechanical property modifications within the range of lower laser peak fluences. The deviation of the experimental results from the simulation in the case of higher laser peak fluences might derive from the difference between the dynamic process of the experiment and the static approach of the simulation model. This also demonstrates that such a thermomechanical model is better suited for cold ablation with ultrashort pulsed laser because of the lack of the theory describing the stress wave propagation in liquid state. The interaction of ultrashort laser pulses with transparent optical materials has proven to be a powerful technique of modification of material properties for technological applications based on 3D photonic structures. The physics behind laser-induced modification phenomenon is extremely rich and still far from complete understanding. In this work, the models will be reported which have been developed to describe the processes inside bulk glass induced by ultrashort laser pulses with following material evolution up to microsecond time scale. The most sophisticated model to the date consists of two parts. The first part solves Maxwell’s equations supplemented by the rate and hydrodynamics equations for generated free electrons. The model resolves spatiotemporal dynamics of free-electron population. Its output in the form of the absorbed energy map serves as the initial condition for thermoelastoplastic model, which yields the final material redistribution. The simulations performed for a wide range of irradiation conditions have allowed to clarify the timescales at which modification occurs after single laser pulses. We demonstrate the ability of the model to quantitatively predict the levels of laser-induced material heating and deformations. The results of simulations of pump-probe experiments have revealed development of defect-mediated screening of laser light. For the linearly polarized laser beam, the effect of light absorption asymmetry is demonstrated. Finally, a new model will be reported for multi-pulse irradiation regimes that takes into account laser beam scanning. Being simplified, this model, however, enables gaining insight into the roles of defects and heat accumulation.

9735-23, Session 7

Fundamental investigations of ultrashort pulse micromachining of different types of crystalline lithium niobate

Mareike Stolze, Thomas Herrmann, Johannes A . L’huillier, Photonik-Zentrum Kaiserslautern e .V . (Germany) In recent years, LiNbO3 have been produced considerable interests in a wide field of applications in modern photonics and in surface acoustic wave technologies. Up to date several techniques have been established for the realization of smooth ridge structures in such crystal material, like the processing by ultrashort-pulse laser micromachining. However, not only the fabrication of smooth ridges is interesting even the understanding of the interaction mechanism between ultrashort laser pulse and ferroelectric material is important. The challenge by micromachining LiNbO3 is its outstanding property of pyroelectricity. By inducing a large temperature gradient caused by strong absorption of laser radiation optical damage by cracks could be arise. However, for material removal high excitation intensities in the order of TW/cm? are needed to overcome the bonding force of the solid, which results in a local rise of temperature of several 1000 K within a few microseconds. Thus, thermally driven processes influence significantly the ablation process especially by applying high repetition pulse trains. We studied the ablation behaviour of congruent, stoichiometric and

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chemically reduced LiNbO3 by using ultrashort laser pulses. The influence of crystal orientation as well as of wavelength and pulse duration on the laser induced damages will be presented. Based on the experimental results we propose a first model that addresses the charge transport by high intensity excitation with ultra-short laser pulses. Further, we are taking the advantage of processing smooth ridges with high-repetition UV picosecond laser-pulses and a low-loss ridge waveguide in congruent LiNbO3 will be demonstrated.

9735-24, Session 8

Ultrafast laser processing of transparent materials supported by in-situ diagnostics

Malte Kumkar, Myriam Kaiser, Jonas Kleiner, TRUMPF Laser- und Systemtechnik GmbH (Germany); Daniel Grossmann, TRUMPF Laser- und Systemtechnik GmbH (Germany) and RWTH Aachen Univ . (Germany); Daniel Flamm, TRUMPF Laser- und Systemtechnik GmbH (Germany); Klaus Bergner, Stefan Nolte, Friedrich-Schiller Univ . Jena (Germany) femtosecond lasers, laser filamentation opens the door to an even wider application area. To achieve a basically zero-gap cutting or perforation line, those laser-generated filaments are placed close to each other by a relative movement of the work piece and/or the processing head at typical speeds of 100–1000mm/s, depending on the material thickness and the desired cut geometry. Suitable ultrafast lasers for the filamentation process have been developed, built up as a MOPA chain to allow for very high repetition rates and the burst mode option, which provides pulse packages with nanosecond separation and programmable power slopes. The presentation will give an overview on the fundamentals of laser filamentation by comparing numerical simulation data with experimental results and will describe the influence of the most important parameters of ultra short pulsed lasers on the formation of filaments. Examples for successful industrial applications will be presented to demonstrate the huge potential of that technology.

9735-43, Session PTue

Formation of periodic micro/nanostructure onto silicone rubber surface by ArF excimer laser

Wisnu Setyo Pambudi, Masayuki Okoshi, National Defense Academy (Japan); Tsugito Yamashita, Kanto Gakuin Univ . (Japan) We report on developing industrial NIR ultrafast laser processing of transparent materials based on nonlinear absorption. The targeted applications are industrial front and rear side ablation, drilling and inscription of modification for cleaving and selective laser etching of glass and sapphire in sheet geometry. We applied pump probe technology and in situ stress birefringence imaging for fundamental studies on the influence of energy and duration (100 fs – 20 ps), temporal and spatial spacing, focusing and beam shaping of the laser pulses. By pump probe technique we are able to visualize differences of spatio temporal build up of absorption, self focusing, shock wave generation for standard single spot, multi spot and beam shaped focusing. Incubation effects and disturbance of beam propagation due to modifications or ablation can be observed. In-situ imaging of stress birefringence gained insight in transient build up of stress with and without translation. The results achieved so far demonstrate that transient stress has to be taken into account in scaling the laser machining throughput of brittle materials. Furthermore it points out that transient stress birefringence is a good indicator for accumulation effects, supporting tailored processing strategies.

We correlate the findings from in-situ diagnostics with ex-situ analysis. Some application results will be shown for display cutting, for drilling by selective etching of laser induced modifications or ablation.

9735-25, Session 8

Laser filamentation of glass and other transparent, brittle materials: fundamentals and applications

Roland M . Mayerhofer, Rofin-Baasel Lasertechnik GmbH & Co . KG (Germany); Abbas S . Hosseini, ROFIN-Sinar, Inc . (United States) Glass and other brittle, transparent materials offer unique properties, that will fuel a continuously growing use in consumer electronics, medical devices, integrated circuits, architectural, automotive, aerospace and a variety of other interesting market segments. Due to low absorption in the visible and infrared spectrum and a typically low thermal-shock resistance, laser processes on glass have always been challenging. Beside the successfully established fusion cutting technology using pulsed fiber lasers especially for sapphire and ceramics, as well as the option to perform an ablation cut through thin brittle materials by pico- or Periodic micro/nanostructuring of silicone rubber surface was induced by the irradiation of 193 nm ArF excimer laser. The ArF laser was focused on the surface of silicone rubber by each microsphere made of silica glass of 2.5 micron diameter, which covered the entire surface of the silicone. The single pulse fluence and irradiation time of the ArF laser were approximately 10 mJ/cm2 and 1 min, respectively. The pulse repetition rate was 1 Hz. The silicone rubber surface underneath each microsphere selectively swelled due to the photodissociation of Si-O bonds of silicone into the lower molecules during the ArF laser irradiation. From the observations by an optical microscope and atomic force microscope, the formed micro/nanostructures were also found to be periodical: they were formed at regular intervals of approximately 2.5 micron. Contact angle of water was measured to see hydrophobicity of the samples; it showed approximately 150 degrees on the micro/nanostructured silicone, indicating a clear super-hydrophobic property.

9735-44, Session PTue

SiO2-glass drilling by short-pulse CO2 laser with controllable pulse-tail energy

Kazuyuki Uno, Takuya Yamamoto, Miyu Watanabe, Tetsuya Akitsu, Univ . of Yamanashi (Japan); Takahisa Jitsuno, Osaka Univ . (Japan) Transparent dielectric materials including SiO2 glass absorb infrared light efficiently. We developed a short-pulse CO2 laser (9.2 – 11.4 ?m) pumped by a longitudinally excitation scheme. The laser pulse waveform of our CO2 laser has a spike pulse with a pulse width of about 100 ns and an energy of several mJ, and a pulse tail with a controllable energy (about 0 – 50 mJ). The spike pulse gives ablation process and the pulse tail gives heat process. Therefore, in this work, we investigated SiO2-glass drilling characteristics depended on the pulse-tail energy, the fluence, and the number of shots. In the longitudinally excited CO2 laser, the laser tube consisted of a 45-cm long alumina ceramic pipe with an inner diameter of 9 mm, a ZnSe output coupler (R85%), and a high-reflection mirror with (r20 m). The laser pulse waveform was controlled by an excitation circuit, an input energy and medium gas. Several types of laser pulse waveforms (for example, the same spike-pulse energy of 1 mJ and the different pulse-tail energy of 7 mJ, 12 mJ

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Conference 9735: Laser Applications in Microelectronic and Optoelectronic Manufacturing (LAMOM) XXI

or 21 mJ) were focused with a ZnSe lens with a focal length of 5 cm.

Higher pulse-tail energy produced the deeper ablation depth at the same spike-pulse energy or the same fluence. Moreover, in a SiO2 glass with a thickness of 300 ?m, drilling a through hole with the diameter of 88 ?m (back side 56 ?m) was demonstrated. We try efficiently drilling small through holes by our short-pulse CO2 laser with controllable pulse-tail energy.

CO2 laser fusion splicer can produce results within the breaking strength range of pristine (un-stripped, un-spliced) fiber. Improvements in fiber preparation methods (especially fiber stripping and cleaning) can enable better exploitation of the high strength splicing benefits of laser fusion splicing.

9735-45, Session PTue

Graphical fiber shaping control interface

Eric T . Basso, Yasuyuki Ninomiya, AFL (United States) Modern demand for miniaturization has driven the development of increasingly compact, reliable, and low-cost fiber-based components for use in a variety of applications in the medical, sensing, and telecom fields. The availability of these increasingly complex components is credited to glass processing technology that enables engineers to develop repeatable fabrication processes for glass processing machines. These machines offer great flexibility, capable of controlling the position, motion, imaging, and heating of optical fibers, but their complexity often makes development lengthy and difficult. This is especially true of more advanced components, such as lensed fibers, tapered ball lenses and multi-tapered fibers, requiring long hours of development for engineers and specially trained operators for fabrication that often needs multiple passes and operator assistance.

In this paper, we detail an improved graphical user interface for defining single-pass novel shaping techniques. It is a C# program capable of creating shaping sequences through a mixer-like interface, directly manipulating execution timelines, motor properties and logic blocks in one easy to read window. This allows users who can “talk through” a process to easily translate concept to instructions. This window works in tandem with a customizable data collection module and video feed, allowing users to collect relevant data and fine-tune processes with one program. This approach seeks to offer unique modularity and debugging capability to researchers and operators during the process development and production phases, as well as easy translation of any conceivable machine instructions to a controllable, repeatable command stack.

9735-47, Session PTue

Femtosecond laser processing of transparent materials for assembly-free fabrication of photonic microsensors

Lei Yuan, Clemson Univ . (United States); Jie Huang, Missouri Univ . of Science and Technology (United States); Jie Liu, Yang Song, Qi Zhang, Jincheng Lei, Hai Xiao, Clemson Univ . (United States) Research and development in photonic micro/nano devices and structures have experienced a significant growth in recent years, fueled by their broad applications as sensors for in situ measurement of a wide variety of physical, chemical and biological quantities. Recent advancement in ultrafast and ultraintense pulsed laser technology has opened a new window of opportunity for one-step fabrication of micro- and even nano-scale 3D structures in various solid materials. When used for fabrication, fs lasers have many unique advantages such as negligible cracks, minimal heat affected-zone, low recast, and high precision. These advantages enable the unique opportunity to fabricate integrated sensors with unprecedented performance, enhanced functionalities and improved robustness. In this paper, we summarize our recent research progresses on the understanding, design, fabrication, characterization of various photonic sensors for energy, defense, environmental, biomedical and industry applications. Femtosecond laser processing/ablation of various glass materials (fused silica, doped silica, sapphire, etc.) will be discussed towards the goal of one-step fabrication of novel photonic sensors and new enabling photonic devices. A number of new photonic devices and sensors will be presented.

9735-46, Session PTue

Benefits of CO2 laser heating for high reliability fiber splicing

Usman B . Nasir, Douglas M . Duke, AFL (United States); Elli Saravanos, Corning Cable Systems LLC (United States) The use of a CO2 laser as a heat source became commercially available for optical fiber splicing and component fabrication only in recent years. In addition to long-term trouble-free and low-maintenance heat source operation, laser fusion splicing offers unique benefits for fabrication of high power optical components, as well as for splice strength. CO2 laser heating is different from other heating methods in that the power from the CO2 laser beam is efficiently absorbed by the outer layer of the glass, which in turn conducts the energy inwards. In this case, there is no consumable heating element such as electrodes or resistive filaments that may leave contaminants or deposits on the glass surface. The CO2 absorptive heating can also be very well controlled, with minimal vaporization and re-deposition of the glass itself. The CO2 laser beam can be guided and shaped for processing certain fiber types and combinations that do not process well by other methods. CO2 laser enables radiative heating in addition of absorption, permitting glass processing techniques that are not supported by alternative heating methods.

Heating by a CO2 laser results in a contamination-free glass surface, with little surface damage or irregularity, hence contributing to remarkable physical (splice) strength. Splice strength data in Figure-1 shows that the

9735-48, Session PTue

Tribological properties of femtosecond laser-induced periodic surface structures on metals

Jörn Bonse, Robert Koter, Manfred Hartelt, Dirk Spaltmann, Simone Pentzien, Bundesanstalt für Materialforschung und -prüfung (Germany); Sandra Höhm, Arkadi Rosenfeld, Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie (Germany); Jörg Krüger, Bundesanstalt für Materialforschung und -prüfung (Germany) Laser-induced periodic surface structures (LIPSS, ripples) were generated on steel and titanium surfaces upon irradiation with multiple linear polarized femtosecond laser pulses (pulse duration 30 fs, central wavelength 790 nm). The experimental conditions (laser fluence, spatial spot overlap) were optimized in a sample-scanning geometry for the processing of large surface areas covered homogeneously by the nanostructures. The irradiated surface regions were subjected to optical microscopy (OM), white light interference microscopy (WLIM) and scanning electron microscopy (SEM) revealing sub-wavelength spatial periods. The nanostructured surfaces were tribologically tested under reciprocal sliding conditions against a sphere of hardened 100Cr6 steel at 1 Hz using paraffin oil and engine oil as lubricants. After 1000 sliding cycles at a load of 1.0 N, the corresponding wear tracks were characterized by OM and SEM. For specific conditions the laser-generated nanostructures endured the tribological treatment.

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Simultaneously, a significant reduction of the friction coefficient was observed in the laser-irradiated (LIPSS-covered) areas when compared to the non-irradiated surface, indicating the potential benefit of laser surface structuring for tribological applications.

9735-49, Session PTue

Femtosecond laser ablation of silica based glasses and the role of the dissociation energy

Moritz Grehn, Technische Univ . Berlin (Germany); Thomas Seuthe, Fraunhofer-IKTS CMD (Germany); Michael Höfner, Nils Griga, Technische Univ . Berlin (Germany); Alexandre Mermillod-Blondin, Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie (Germany); Markus Eberstein, Fraunhofer-IKTS CMD (Germany); Jörn Bonse, Bundesanstalt für Materialforschung und -prüfung (Germany) The role of glass composition on its fs-laser ablation threshold fluence is of ongoing discussion. In this study, silica based binary and ternary glasses are investigated with respect to their ablation fluence threshold upon the irradiation of 120 fs, 800 nm laser pulses. With the alteration of the stoichiometry of the glasses, the influence of the composition to their fs- laser ablation threshold is investigated. The absorbed energy density at the ablation threshold is estimated by a multiphoton absorption model. This energy density is compared to a calculated energy density which is needed to decompose the glass into its atomic constituents. Within the experimental accuracy, both quantities are in good agreement. Hence, it is concluded that the laser ablation fluence threshold is driven by two antagonistic parameters: the binding energies of the molecule orbitals in the glass compound which opposes the excitation energy provided by the absorbed fraction of the laser pulse. If the excitation of the material overcomes the binding forces the material decomposes and ablation becomes manifest. The model used for the quantity of absorbed energy density allows also for an assessment of the surface topography. Therefore, crater topographies are compared to the model and are found to be in good agreement with the crater profiles obtained by atomic force microscopic measurements.

9735-50, Session PTue

High-efficiency bispectral laser source for EUV lithography

Aleksandr P . Zhevlakov, ITMO Univ . (Russian Federation); Ruben P . Seisyan, Ioffe Physical-Technical Institute (Russian Federation); Viktor Bespalov, Valentin Elizarov, National Research Univ ITMO (Russian Federation); Alexsandr S . Grishkanich, Sergey V . Kascheev, ITMO Univ . (Russian Federation) The power consumption in the two-pulse bispectral primary source could be substantially decreased by replacing the SRS converters from 1.06 µ m into 10.6 µ m wavelength as the preamplifier cascades in ??2 laser channel at the same efficiency radiation of EUV source.

The creation of high volume manufacturing lithography facilities with the technological standard of 10-20 nm is related to the implementation of resist exposure modes with pulse repetition rate of 100 kHz. One of the promising schemes supporting such mode is based on the extreme ultraviolet (EUV) radiation emitted from plasma formed by illumination of target with a focused laser beam. The main power pulse comes from the ??2 laser with a certain time delay, thereby increasing the conversion efficiency.

The effect of a gigantic image contrast transfer in the nonlinear inorganic resist observed under illumination by the high-intensity radiation provides a high resolution of the integrated circuit (IC) topology elements. It contains the master oscillator SSL, stimulated Raman scattering (SRS) conversion cascades and the ??2 amplifier.

In this scheme SSL serves for preliminary target heating and plasma initiation and as a pump source for the series of SRS converters. Radiation of the second Stokes component with the wavelength of 10.6 it promising for the creation of LPP EUV sources.

µ m from the final SRS converter is injected into the ??2 amplifier as an input. Note that in this case the SRS converters do not require a power supply. They are pumped solely by the low power SSL radiation with wavelength of 1.06 µ m and the first Stokes component obtained from the conversion of this radiation. Several rotated mirrors are set for adjustment of the time delay between the preliminary and main pulses in the final SRS cell containing hydrogen by pressure of 60 bar. Low power consumption of the proposed scheme makes

9735-26, Session 9

Observation of pressure waves due to ultra-short laser ablation in thin molybdenum films by reflective and transmissive pump-probe microscopy

Rudolf Reiel, Christoph Aichele, Stephan Rapp, Jürgen Sotrop, Heinz P . Huber, Hochschule für Angewandte Wissenschaften München (Germany) Selective laser structuring of thin molybdenum layers from the glass substrate side using ultra-short pulses can exhibit up to a factor of 10 higher ablation efficiencies compared to direct ablation. Observations suggest that ultrafast heating followed by thermal expansion in the metal generates an propagating pressure wave leading to bulging of the metal. In this study time resolved measurements of the transmittance and reflectivity by a transmission enhanced pump-probe microscopy setup was used to monitor the ablation process of a 10 nm to 50 nm thick Mo layers on a glass substrate. The generated transient signal is superimposed by oscillating features, which can be discriminated to interference caused by moving interfaces with different refractive indexes. The sources have been identified as a sound wave in glass (5880 m/s), a shock wave in air (1000-3000 m/s) and the motion of the metal layer (60-200 m/s). Taking energy conservation into account the lack of light caused by scattering and absorption can be calculated with the obtained transmission and reflection data. It is shown that this light extinction is rising with growing fluences above the ablation threshold. Additionally, it has been noticed that under the precondition of reaching the vaporization threshold in the center of the laser pulse, the size of the removed metal layer equals the area where the melting threshold is reached.

9735-27, Session 9

Investigative analysis: Experimental data compared to simulation of confined laser ablation for silicon dioxide layers on silicon substrates

Regina Moser, Jürgen Sotrop, Heinz P . Huber, Hochschule für Angewandte Wissenschaften München (Germany); Gerd Marowsky, Laser-Lab . Göttingen e .V . (Germany) Confined laser ablation by ultra-short laser pulses is an innovative technique for precise micromachining. It has recently been shown that the energy of the laser pulse is confined between an absorbing and transparent layer, which enables high precision and energy efficient processes. This study compares experimental data of confined laser ablation with numerical simulation. The laser pulse is transmitted through a transparent silicon

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Conference 9735: Laser Applications in Microelectronic and Optoelectronic Manufacturing (LAMOM) XXI

dioxide (SiO2) layer and absorbed by silicon (Si) substrate. In this confined two layer system, ablation of the SiO2 occurs. The energy is partially absorbed and reflected by the electronic-system, which is simulated with the Drude-model for semiconductors. The electron-photon coupling interaction and subsequent heat transfer by the lattice-system initiated by the ultra-short laser pulse is simulated with a two-temperature model. The simulation results agree with previous pump-probe measurements that reflection is time-dependent. The results also show a gas-liquid mixture at the SiO2/Si interface, with a different reflection index. The volume of the gas-liquid mixture and the thermo-mechanical bulging have good agreements with the SiO2 bulging well known from accepted pump-probe measurements. These important observations suggest that this novel numerical simulation can be utilized to predict and analyze the ablation behavior of similar two layer systems.

technology represents the method of choice for the deposition of highly uniform functional thin-films from solution onto arbitrary substrate materials on large areas. Direct writing by means of selective patterning with ultrashort pulse laser ablation will be utilized to generate the desired patterns on previously deposited thin-films. Special techniques of direct and confined laser ablation will be applied to achieve selective structuring of the films, without damaging adjacent layers, with a lateral resolution in the ?m-region. Either selective contact opening (for conductive interconnections) or isolating lines can be realized. Both functions are important for the different device architectures. Ultrafast pump-probe microscopy with a temporal resolution of about 1 ps is used to show fundamental aspects of the laser ablation process.

9735-28, Session 9

Quantized blistering of transparent films with femtosecond laser interference

Stephen Ho, Prasoon Jha, Peter R . Herman, Univ . of Toronto (Canada) Femtosecond laser interactions inside thin transparent dielectric films of refractive index, nfilm, with tight focusing presents strong nonlinear interactions that can be confined at the Fabry-Perot fringe maxima to generate thin nanoscale plasma disks of 20 to 45 nm thickness separated on half-wavelength, ?/2nfilm. This thin-film interference drives nano-disk explosions which in turn enable nano-cleaving of subwavelength internal cavities at single or multiple periodic depths at low laser exposure. Higher laser exposure will otherwise lead to a controlled digital ejection at fractional film depths with quantized-depth thickness defined by the laser wavelength.

In our previous work, we have already developed and demonstrated quantized ejection and formation of nanocavities in SiNx film with laser wavelengths of 522 nm 800 nm and 1044 nm. The present paper extends this high-resolution nano-cleaving to inside thin films for SiOx to create nanoblisters of various sizes that open sub-wavelength nanocavities with thin membranes at single or multiple periodic depths. This opens a new lab-in-film direction to form nano-fluidic channels for submicron particle detection and sensing in a SiOx thin film that has the advantageous properties of chemical inertness, optically transparent, high wettability, and low autofluorescence. Further, this thin film nanostructuring opens the opportunity of integrating numerous functionalities onto devices which already employed thin films, including CMOS microelectronics and photonics, photovoltaics, LED, lab-on-chips, and MEMS. The process window is extended to short laser wavelength of 343 nm, and further examined over the limits of laser pulse duration and coherence for producing fractional film ejections, and open and closed nanoblisters.

9735-30, Session 10

Laser-induced periodic surface structures (ripples): Dynamics, control, and applications

(Invited Paper)

Jörn Bonse, Bundesanstalt für Materialforschung und -prüfung (Germany); Sandra Höhm, Arkadi Rosenfeld, Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie (Germany); Jörg Krüger, Bundesanstalt für Materialforschung und -prüfung (Germany) During the past few years significantly increasing research activities in the field of laser-induced periodic surface structures (LIPSS, ripples) have been reported since the generation of LIPSS in a single-step process provides a simple way of surface nanostructuring towards a control of optical, mechanical or chemical surface properties. In this contribution the current research state in this field is reviewed. The formation of LIPSS upon irradiation of metals, semiconductors and dielectrics by multiple linearly polarized Ti:sapphire fs-laser pulses (duration: 30-150 fs) is studied experimentally and theoretically. Different types of LIPSS with periods even below 100 nm can be generated. Their dynamics and formation mechanisms are analyzed and identified in ultrafast optical experiments (time-resolved diffraction & polarization controlled double pulse experiments at different wavelengths). Complementing theoretical calculations of the laser-induced carrier dynamics address transient changes of the optical properties of the irradiated materials and reveal the importance of surface plasmon polaritons in the early stage of LIPSS formation. Various applications of these nanostructures are outlined. Their beneficial effect on the tribological performance is demonstrated for metals.

9735-29, Session 9

Ultra-short pulse laser patterning of fully printed flexible devices based on carbon nanotubes thin-films

Juergen Sotrop, Heinz P . Huber, Hochschule für Angewandte Wissenschaften München (Germany)

9735-31, Session 10

Sub-diffraction limit nanostructures induced by femtosecond laser direct writing

Xiaolong He, Purdue Univ . (United States) and Harbin Institute of Technology (China); Woongsik Nam, Xianfan Xu, Purdue Univ . (United States) Random carbon nanotube (CNT) networks have raised a continuously increasing interest among a broad and multidisciplinary community of researchers throughout the last decade. The remarkable and concurrently diverse properties of such networks have rendered them suitable for a wide range of applications in science and engineering. Among the most promising applications are transparent conductive electrodes, thin-film transistors and circuits, mechanical and chemical sensors. To produce such novel devices, two key technologies are combined: Scalable spray For the past decades, advances in optical techniques and processing have achieved a rapid reduction in the feature size of nanostructures. In particular, optical lithography has been the primary tool to create fine-scale patterns required for nano-fabrication. However, the diffraction nature of light is a barrier for achieving nanometre feature and resolution in conventional optical lithography, and various ways have been attempted to achieve sub diffraction limited patters e.g. [1]. Direct writing of laser-induced periodic surface structures (LIPSS) has been an area of ongoing examination for over a decade as a flexible and low-cost method to create nanostructures with periods significantly smaller than the laser wavelength. LIPSS can be easily formed on nearby all kinds of materials, including metals, semiconductors,

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Conference 9735: Laser Applications in Microelectronic and Optoelectronic Manufacturing (LAMOM) XXI

and insulators[2-4], but the quality and the random nature of LIPSS are still the remained obstacle for some applications. In this work, we demonstrate a laser-direct-writing technique to exploit LIPSS far below the diffraction limit. The technique has the capability to create a single line which is unlike other LIPSS techniques where only multiple, periodic line structures can be formed. In addition, the approach is low-cost, maskless, and highly flexible. The formation of LIPSS is carefully controlled on SU-8 photoresist. Effects of exposure power, polarization direction and scan speed are observed respectively. Nanostructure less than 50 nm feature size is obtained and characterized using scanning electron microscopy and atomic force microscopy, as shown in Fig. 1. We expect that our approach could be a promising alternative for high-resolution lithography with its simplicity and flexibility.

of graphene from step and terrace regions using 4 ° off-cut 4H-SiC(0001) substrates.

The KrF excimer laser with a wavelength of 248 nm was used as a local heating source on the SiC surfaces. Pulse duration of the laser was 55 ns, and repetition rate was 1000 Hz. Si atoms are preferentially sublimated from the SiC surface by the laser irradiation, and graphene growth is induced by the rearrangement of surplus carbon on the SiC surface. The graphene growth was performed in ambient Ar of 500 Pa.

C-AFM images obtained by simultaneous measurements of the surface morphology and local current revealed that graphene formation starts from step region on the SiC(0001) surface at the irradiation number of 3,000 shots, and the graphene formation regions expand to terrace region at 5,000 shots. From results of HR-TEM measurements, the number of graphene layers increases with increasing the laser irradiation number, and 5-7 layers graphene is formed at 20,000 shots.

9735-32, Session 10

Ultrafast laser ablation of transparent materials

Lara Bauer, TRUMPF Laser GmbH (Germany) and Univ . Stuttgart (Germany); Simone Russ, TRUMPF Laser GmbH (Germany); Myriam Kaiser, Malte Kumkar, Birgit Faisst, TRUMPF Laser- und Systemtechnik GmbH (Germany); Rudolf Weber, Thomas Graf, Univ . Stuttgart (Germany)

9735-34, Session 11

Role of enhanced laser field in laser processing of nanomaterials

Tao Zhang, Seungkuk Kuk, Stony Brook Univ . (United States); Eunpa Kim, Costas P . Grigoropoulos, Univ . of California, Berkeley (United States); David J . Hwang, Stony Brook Univ . (United States) Glass has recently found increasing importance in industrial markets, especially in the rapidly growing sector of display or cover glass applications for consumer electronics. However, cutting of brittle materials is a demanding process since high quality standards are required. Cutting with ultrafast lasers has great potential for processing brittle materials efficiently without time consuming post-processing operations. However, the interaction of ultra-short laser pulses with transparent materials is not yet understood well enough to determine how to simultaneously increase the edge quality, the efficiency of the process and the productivity.

The present work investigates the influence of the pulse duration and the temporal spacing between pulses on the ablation of aluminosilicate glass by comparing the results obtained with laser pulses with durations of 400 fs, 900 fs and 6 ps. For 1 and 0.4 ps the ablation threshold fluences are lower than for 6 ps. For 0.4 ps pulses we found an increased ablation threshold fluence compared to the ablation threshold fluence of 1 ps pulses. We found that surface modifications occur already at fluences below the single pulse ablation threshold and that laser-induced periodic surface structures (LIPSS) emerge as a result of those surface modifications. Scanning electron micrographs of LIPSS generated with 0.4 ps laser exhibit a more periodic and less coarse structure as compared to structures generated with 6 ps. Furthermore we will report on the influence of temporal spacing of the pulses on occurrence of LIPSS and the impact on the quality of the cutting edge. Lasers have proven to be unique tools for a highly selective processing of nanomaterials system on the basis of the enhanced laser field, maintaining other sensitive portion in the system untouched. However, in many practical applications, a wide interspacing distribution among nanomaterials and nonlinear laser absorption properties of the nanomaterials in the highly excited nanomaterials states, frequently lead to rather adverse effects in terms of controlled nanomaterials processing. In this study, we will take a few laser nanomaterials processing examples mainly based on the nanowires system including the sprayed metallic nanowires for transparent electrode applications and selective semiconductor nanowires growth from the metallic nanocatalysts, and discuss on the role of the enhanced laser field via the combined theoretical and experimental investigations. Specific aims of properly utilizing the enhanced laser fields are to achieve improved electrical conductance for practical transparent electrode applications, and to facilitate directed growth of semiconductor nanowires at designated sample locations, respectively.

9735-35, Session 11

Ultrafast laser mixing of metals as a route to create stable nanocrystalline materials

Keegan J . Schrider, Ben R . Torralva, Steven M . Yalisove, Univ . of Michigan (United States)

9735-33, Session 11

Observation of graphene growth on SiC(0001) surfaces induced by KrF excimer laser irradiation

Masakazu Hattori, Hiroshi Ikenoue, Daisuke Nakamura, Tatsuo Okada, Kyushu Univ . (Japan) We have proposed a novel direct growth method of graphene on SiC (0001) surfaces by KrF excimer laser irradiation and we observed growth processes of the graphene by conductive-AFM (C-AFM) measurements. In our previous reports, we conclude that double layers graphene can be formed by laser irradiation of 1.2 J/cm2 and 5000 shots, and grain size of the graphene is about several tens nm. In the C-AFM images, the formation region of graphene can be observed as current spots because graphene has low conductively. In this report, we investigate local growth processes Nano crystalline metals possess extraordinary mechanical properties including increased wear resistance and strength, but they are often unstable and exhibit grain coarsening at relatively low temperatures so their desirable properties quickly degrade. It has been shown that electrodeposited nanocrystalline Ni-W alloys are stabilized by the weak segregation of W to Ni grain boundaries [1]. This talk will describe our work to create similar, stable nanocrystalline alloys by mixing metals via ultrafast laser irradiation. We will present Transmission Electron Microscopy demonstrating the intermixing of sputtered multilayer films of Ni and W, and showing the microstructure of the resulting Ni-W alloy. If time permits we will address how the weight percent of Ni and W, the thickness of Ni and W layers, and laser fluence effect the microstructure.

[1] Detor, AJ. and Schuh, CA., “Tailoring and patterning the grain size of nanocrystalline alloys.” Acta Mater. 55, 371–379 (2007).

136 SPIE Photonics West 2016 · www.spie.org/pw

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Conference 9735: Laser Applications in Microelectronic and Optoelectronic Manufacturing (LAMOM) XXI

9735-36, Session 11

Fabrication and bandgap engineering of doped ZnO microspheres by simple laser ablation in air

Daisuke Nakamura, Tetsuya Shimogaki, Toshinobu Tanaka, Fumiaki Nagasaki, Yuki Fujiwara, Mitsuhiro Higashihata, Tatsuo Okada, Kyushu Univ . (Japan) To guarantee the required excellent accuracy even at high scanning speeds a new interferometry based encoder technology was used, that provides a high quality signal for closed-loop control of the galvo scanner position. Low inertia encoder design enables a very dynamic scanner system which can be driven to very high line speeds by a specially adapted control solution. We will present results with marking speeds up to 25 m/s using a f = 100 mm objective obtained with a new scanning system and scanner tuning maintaining a precision of about 5 µ m. Further it will be shown that, especially for short line lengths, the machining time can be minimized by choosing the proper speed which has not to be the maximum one .

ZnO one of the promising candidates for ultraviolet (UV) emitting devices, such as UV light emitting diodes and UV lasers. In addition, ZnO nano/ microstructures have attracted a considerable attention as building blocks because of their high crystalline quality and unique structures. Among of them, symmetrical ZnO nano/microcrystals can serve as good resonance cavities without additional mirrors due to the high refractive index of ZnO. In our study, we have succeeded in synthesizing ZnO nano/microspheres by a simple atmospheric laser ablation method, and demonstrated UV WGM lasing from the spheres. In this synthesizing method, a rapid melting and solidifying process may effective for doping impurities into ZnO. In this study, we synthesized doped ZnO spherical crystals using MgO contained targets, and blue-shift of UV WGM lasing peaks from the Mg-doped ZnO microsphere was observed. In this paper, structural and optical properties of the spherical ZnO microcrystals are reported.

9735-37, Session 12

Nanotexturation inspired by nature

(Invited Paper)

Rainer Kling, Marc Faucon, Girolamo Mincuzzi, ALPhANOV (France) Nanometric topographies on surfaces provide specific functions in nature like the well-known lotus effect. To reproduce these topographies with laser technologies is a big challenge but represents a key enabling technology for industrial commercialisation. High average power ultrafast lasers are a crucial element to obtain high throughput fabrication. In this presentation the creation mechanisms of LIPSS are introduced. Potentials and limitations to master the nanoscale will be discussed with examples of technical applications of surface functions covering decoration, tribology and antireflection.

9735-38, Session 12

Time-optimized laser micro machining by using a new high dynamic and high precision galvo scanner

Beat Jaeggi, Beat Neuenschwander, Markus Zimmermann, Berner Fachhochschule Technik und Informatik (Switzerland); Markus Zecherle, Ernst Wilhelm Boeckler, SCANLAB AG (Germany) High accuracy, quality and throughput are key factors in laser micro machining. To obtain these goals the ablation process, the machining strategy and the scanning device have to be optimized. The precision is influenced by the accuracy of the galvo scanner and can further be enhanced by synchronizing the movement of the mirrors with the laser pulse train. To maintain a high machining quality i.e. minimum surface roughness, the pulse to pulse distance has also to be optimized. Highest ablation efficiency is obtained by choosing the proper laser peak fluence together with highest specific removal rate. The throughput can now be enhanced by simultaneously increasing the average power, the repetition rate as well as the scanning speed to preserve the fluence and the pulse to pulse distance. Therefore a high scanning speed is of essential importance.

9735-39, Session 12

Studies on laser material processing with nanosecond & sub-nanosecond and picosecond & sub-picosecond pulses

Jie Zhang, Sha Tao, Brian Wang, Jay Zhao, Advanced Optowave Corp (United States) From an academic point of view, laser material processing with short & ultrafast pulses, including nanosecond (ns), picosecond (ps) and femtosecond (fs), has been well-studied and well-understood. There are very clear processing solutions, such as using what type laser for what type materials to achieve the best quality; however, for the applications within manufacturing, it is not so simple because of their impact on throughput & cost. Quality is no longer the most important request. Instead, throughput and cost have become more critical. Good processing solutions have to meet both quality and throughput targets. According to laser market value, the shorter the laser pulse width, the higher the laser price is. Thus, it is important to provide the most suitable and affordable solution to users by choosing the right laser sources (wavelength, pulse width) based on the properties of the material to be processed. In this paper, processing of several industry-favored key materials of metal, semiconductor, glass, ceramic and polymer have been systematically studied using different lasers including ns, sub-ns, ps and sub-ps pulses. The quality and speed are based on the industry user’s requirements. The studies include both theoretical modeling and experiments. In simulations, 1D model was employed to simulate ns and sub-ns laser ablation and the two-temperature model is employed to simulate ps, the sub-ps laser ablation process to understand the underlying physical mechanism, and predict the ablation rates. The model was verified by a classic laser ablation experiment and then applied to other materials. In the experiments, a full scope of laser ablation-based cutting, drilling, and patterning of different materials with different pulse widths were carried out. Our studies show that with optimized processing conditions, appropriate beam delivery system and theright laser source, it is feasible to achieve good quality and high throughput by using affordable lasers.

9735-40, Session 13

Large area micro-/nano-structuring using direct laser interference patterning

(Invited Paper)

Andres F . Lasagni, Fraunhofer IWS Dresden (Germany) and TU Dresden (Germany); Tim Kunze, Matthias Bieda, Fraunhofer IWS Dresden (Germany); Denise Günther, TU Dresden (Germany) and Fraunhofer IWS Dresden (Germany); Anne Gärtner, Technische Universität Dresden (Germany); Valentin Lang, TU Dresden (Germany) and Fraunhofer IWS Dresden (Germany); Andreas Rank, TU Dresden (Germany); Teja Roch, Fraunhofer IWS Dresden (Germany) and TU Dresden (Germany)

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Conference 9735: Laser Applications in Microelectronic and Optoelectronic Manufacturing (LAMOM) XXI

Smart surfaces are a source of innovation in the 21st Century. Potential aplications can be found in a wide range of fields where improved optical, mechanical or biological properties can enhance the functions of products. In the last years, laser based fabrication methods have significantly improved their potential to functionalize surfaces based on the strong development of ultrashort pulsed laser systems as well as due to the higher available laser power. Laser surface processing excels over mechanical, chemical, and electric discharge texturing because it allows localized modifications with a large degree of control over the shape and size of the features that are formed. Furthermore, a greater range of sizes can be produced. In addition, it is generally cheaper than e-beam texturing and more flexible, because it does not require vacuum. Various textures can be accurately produced by controlling processing parameters such as beam intensity, spatial and temporal profile, wavelength, and processing environment (background gas or liquid). On the other hand, the fabrication of surface patterns with feature sizes of 1 to 10 µ m requires significant efforts to assure high quality and homogeneity over large areas. In this study, the fabrication of spatially ordered structures using Direct Laser Interference Patterning (DLIP) is demonstrated. Different application examples, including the processing of 2D and 3D surfaces are introduced and applied to improve the surface properties of polymers, metals and coatings. Initial investigations of large area structuring of stamps for 2D and roll-to-roll (R2R) processing are also discussed.

9735-42, Session 13

Laser-assisted manufacturing of thermal energy devices

Tao Zhang, Mahder Tewolde, Stony Brook Univ . (United States); Ki-Hoon Kim, Dong-Min Seo, Stony Brook Univ (United States); Jon P . Longtin, David J . Hwang, Stony Brook Univ . (United States) In this study, we will present recent progress in the laser-assisted manufacturing of thermal energy devices that require suppressed thermal transport characteristics yet maintaining other functionalities such as electronic transport or mechanical strength. Examples of such devices to be demonstrated include thermoelectric generator or insulating materials. To this end, it will be shown that a hybrid additive and subtractive manufacturing approaches are significantly facilitated by unique processing capabilities of lasers under both thermal and non-thermal mechanisms, in a highly controllable fashion. On the basis of thermal, electrical, mechanical and morphological characterization of the laser-manufactured devices in conjunction with theoretical consideration, strategies for the optimal thermal energy device fabrication will be discussed.

9735-41, Session 13

Power scaling into the 100W regime for surface structuring of metals with ultra short laser pulses

Beat Neuenschwander, Markus Zimmermann, Beat Jaeggi, Berner Fachhochschule Technik und Informatik (Switzerland) Beside high precision and high surface quality also power scaling represents a key factor for successful transfer of ultra-short pulsed laser micromachining to industrial applications. To maintain efficiency and quality machining should be done at the optimum fluence and a pulse to pulse distance of about half of the spot radius. In former experiments power scaling was demonstrated for different steel grades with a polygon line scanner up to an average power of 40 W with a spot radius of about 30 µ m at a repetition rate up to 8 MHz. Due to the higher threshold fluence this set up could not be used for copper and scaling was demonstrated with a galvo scanner and a spot radius of 16 a repetition rate of 1 MHz.

µ m only. Hence the maximum marking speed of about 10 m/s limited the average power to about 10 W at Higher average power becomes accessible with a new laser system having twice the average power and the existing polygon line scanner or with a new galvo scanner, offering higher speeds, and the existing laser system. With these systems the power scaling experiments on steel and copper will be continued and other materials will be investigated as well to check if heat accumulation and/or shielding effects take place and will influence the removal rate and the machining quality.

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Conference 9736: Laser-based Micro- and Nanoprocessing X

Tuesday - Thursday 16–18 February 2016 Part of Proceedings of SPIE Vol . 9736 Laser-based Micro- and Nanoprocessing X 9736-1, Session 1

Laser assisted glass-glass micro welding

(Invited Paper)

Ville Hevonkorpi, Heidi Lundén, Antti Määttänen, Primoceler, Inc . (Finland) The use of glass in semiconductor industry has been growing the past years and the use is estimated to grow considerably during the coming years. For efficient manufacturing, especially when using ultra-thin wafers, novel bonding technologies are needed. In this paper, a laser assisted additive free glass-glass welding technology is presented. Furthermore, the use of laser assisted welding in temporary bonding and in manufacturing of hermetic packages for optical sensors is investigated. The reliability of the weld and the robustness of the process is verified by moisture resistance and temperature cycling testing. A large quantity, one hundred samples, was selected to the moisture resistance testing to define the repeatability of the welding process. Two glass types used commonly in manufacturing consumer products and medical devices were selected. These glass types are Borofloat 33 and D263T.

Glass-glass welding proved to be a reliable bonding process offering a non-outgassing, room temperature bonding. In addition, it was verified that the weld is hermetic having a good resistance to thermal cycling. No changes in the welding seam were discovered during moisture resistance or temperature cycling tests.

9736-3, Session 1

New trends in laser micromachining

Frank Gaebler, Coherent (Deutschland) GmbH (Germany); Joris van Nunen, Andrew Held, Coherent Inc (United States) Ultrafast laser machining is gaining traction as it offers very precise processing of materials with low thermal impact. Large-scale industrial ultrafast laser applications show that the market can be divided in various sub segments. One set of applications demand low power, compact footprint and are extremely sensitive to the laser price whilst still demanding 10ps or shorter laser pulses. A second set of applications are very power hungry and only become economically feasible for large scale deployments at power levels in the 100W class. There is a growing demand for applications requiring fs-laser pulses. In our presentation we would like to describe these sub segments by using selected applications from the automotive and electronics industry e.g. drilling of gas/diesel injection nozzles, dicing of LED substrates and structuring of battery foils for xEV vehicles. We close the presentation with an outlook to micromachining applications e.g. µ via hole drilling and foil processing with unique new CO lasers emitting 5 µ m laser wavelength.

9736-2, Session 1

Femtosecond laser induced local compositional changes in glass for photonics applications

(Invited Paper)

Javier Solis, Instituto de Óptica “Daza de Valdés” (Spain) Although compositional changes induced by fs-laser irradiation of glass have been studied by more than ten years, only recently it has been shown that these changes can be efficiently used to modify the local refractive index of glasses with a large degree of controllability. We have shown that fs-laser irradiation at high repetition rate in phosphate glasses with lanthanum and potassium oxides acting as glass modifiers leads to local element redistribution, and the formation of efficient passive and active waveguides with a refractive index contrast >10E(-2). The local refractive index changes, in this case, are due to the cross migration of La and K species, La being the element controlling the local refractive index of the structures. A similar mechanism has been also observed in a different glass family (tellurites). The feasibility of inducing the controlled migration of ions by fs-laser irradiation opens up new prospects for fabricating efficient integrated optical devices inside glasses by direct femtosecond laser writing, overcoming the refractive index contrast limitations associated to more conventional glass modification mechanisms. We will review our results regarding fs-laser induced ion migration for photonics applications with special emphasis in the control that can be exerted over their characteristics (size, refractive index change and local composition) as well on the performance of the so generated structures as passive and active waveguides and lasers. Results regarding the use of in-situ plasma emission imaging during the writing process will also be presented in order to discuss the origin of the ion migration phenomenon.

9736-4, Session 1

Laser joining of metal-glass nanocomposite and glass

Amin Abdolvand, Univ . of Dundee (United Kingdom) Direct joining techniques of glass with a focused ultra-short (femtosecond or picosecond) pulsed laser beam have recently been reported. In these studies, by focusing the ultra-short laser pulses at the interface between the glass substrates, the nonlinear absorption of the beam has been exploited and glass joint strength of 14.9 MPa was reported. However, there are a number of shortcomings associated with this technique. The process requires a lens objective of high numerical aperture, typically in the range of 0.4 - 0.65. This leads to a poor working distance and welding depth, and ultimately restricts the welding efficiency and the processing speed. It also imposes strict requirements on the surface quality of the work pieces - currently within ?/4. The above constitutes a serious challenge for widespread adoption of this technique by industry. We present rapid and strong joining of clear glass to glass containing randomly distributed embedded spherical silver nanoparticles upon nanosecond pulsed laser irradiation at 532 nm. The embedded metallic nanoparticle were (30 nm in diameter) were embedded in a thin surface layer the nano-composite. A joint strength of 12.5 MPa was achieved for a very low laser fluence (< 0.2 J/cm2) and scanning speed of 10 mm/s. The bonding mechanism is discussed in terms of absorption of the laser energy by nanoparticles and the transfer of the accumulated localised heat to the surrounding glass leading to the local melting and formation of a strong bond.

9736-5, Session 1

Laser induced permanent and peculiar shape transformation of embedded metallic nanoparticles in glass

Amin Abdolvand, William A . Gillespie, Mateusz A . Tyrk, Svetlana A . Zolotovskaya, Univ . of Dundee (United Kingdom)

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9736-6, Session 2

Conference 9736: Laser-based Micro- and Nanoprocessing X

Dielectrics containing metal nanoparticles comprise a promising class of materials for many applications in optoelectronics. These nanocomposites are of interest due to their unique linear and nonlinear optical properties. These properties are dominated by the strong surface plasmon resonances of the metal nanoparticles. The spectral position and shape of the SPRs can be designed within a wide spectral range throughout the visible and near-infrared spectra by choice of the metal and the dielectric matrix, plus manipulation of nanoparticle size, shape, and spatial distribution.

Radially and azimuthally polarized picosecond (~10 ps) pulsed laser irradiation at 532 nm wavelength led to the permanent reshaping of spherical silver nanoparticles (of 40 nm) embedded in a thin layer of soda lime glass. We demonstrate formation of the Ag nano-ellipsoids and analyze their orientation within the irradiated areas as a result of the polarization state of the incident beam. The fabricated nanostructures are characterized using Second Harmonic Generation from the elliptical nanoparticles for determination of the shape and position of nanoparticles within the modified areas.

The presented reshaping method adds a versatile technique to structural manipulations of metal-glass nanocomposites at large scale. It paves the way for nanoparticle shape modification in terms of future new experiments that could be performed with different laser polarizations in order to achieve more complicated nanoparticle structures. This method can lead to nano engineering of novel optical materials and indeed sophisticated storage of information in this class of nanomaterials.

Ultrafast laser direct micro-/nano fabrication: Towards 4D optical printing

(Invited Paper)

Mangirdas Malinauskas, Sima Rek?tyt?, Albertas ?ukauskas, Simas Butkus, Vilnius Univ . (Lithuania); Saulius Juodkazis, Swinburne Univ . of Technology (Australia) [6] M. Malinauskas et al., Micromachines, 5(4), 839 (2014).

[7] J. Maciulaitis et al., Biofabrication, 7, 015015 (2015).

9736-7, Session 2

Rapid fabrication of microdevices using laser direct writing and replica moulding technique

Arkadiusz J . Antonczak, Bogusz D . Stepak, Krzysztof M . Abramski, Wroclaw Univ . of Technology (Poland) This paper presents a method that enables fast and low-cost fabrication of microchannels with oval cross-section. The procedure is based on formation of a concave meniscus at the interface between an initially cured PDMS and a polymeric mould fabricated using excimer laser. In this technique, the mould is not filled with uncured PDMS. The replica is formed by expanding gas trapped within the structures of the mould during thermal curing. A second shaping factor is connected with surface phenomena at the interface between the mould, gas and partially cured PDMS. The final shape of the meniscus is determined when the PDMS reaches the high cure extent. The microchannels with oval cross-section are obtained by using a completely cured PDMS replica as a mould in an analogical second fabrication step. As a result an all-in-PDMS chip can be produced. The cross-section of channels can be controlled by changing the curing conditions. We investigated the influence of the initial PDMS curing time and pressure during final curing on the geometry of the created microchannels. The fabricated microstructures are characterized by constant depth and high quality of the surface.

9736-8, Session 2

Waveguides and nonlinear refractive index in chalcogenide glass containing Ag2S nanocrystals

Juliana M . P . Almeida, Emerson C . Barbano, Lino Misoguti, Univ . de São Paulo (Brazil); Craig B . Arnold, Princeton Univ . (United States); Cleber R . Mendonça, Univ . de São Paulo (Brazil) Today ultrafast laser based micro- and nano-processing technologies enable the most advanced material manufacturing options. It is proved in terms of spatial resolution, fabrication throughput, choice of materials, and 3D structuring capability. Tightly focused femtosecond pulses empower determination of light-matter interaction mechanisms, thus creating a potential to choose the type of material modification within ultra-confined 3D space. Selectivity is the key performance indicator of femtosecond direct laser writing opening the prospect to tune the physical (mechanical stiffness, refractive index), chemical (reactivity), biological (biocompatibility, resorption) material properties. This can be employed for 4D optical printing, where besides the 3 space coordinates, the material deposition (or modification of its properties) can be distinctively controlled. Here we report on recent advances in fs-DLW based material processing [1,2], specifically: - Study of lithography mechanisms, mainly the influence of light’s polarization to spatial resolution [3]; - Tuning the refractive index in 3D lithography towards GRIN free-form microoptics [4]; - Reversible deformations of composite material polymeric microstructures in fluids [5]; - Light filament assisted ablation processing of fused filament fabrication manufactured 3D objects [6]; - Pre-clinical study of SZ2080 microporous scaffolds for cartilage tissue engineering [7].

[1] M. Malinauskas et al., Phys. Rep. 533(1), 1 (2013).

[2] M. Malinauskas et al., Light: Sci. Appl. to be published, (2015).

[3] T. Jonavicius et al., submitted.

[4] A. Zukauskas et al., under review.

[5] S. Rekstyte et al., in preparation. The large interest in nonlinear optical materials has been motivated by their potential use in the fabrication of all-optical devices. Arsenic trisulfide is a chalcogenide glass that has received special attention due to its high refractive index and transparency over MIR region. Moreover, As2S3 presents a variety of photosensible properties, including photocrystallization and photodarkening, which potentiates its applications in infrared technologies [1]. Recently, we have developed a single-step synthesis of silver sulfide nanocrystals (Ag2S-NCs) in As2S3 [2]. Ag2S is also a semiconductor, but when synthetized at the nanometer scale can be used as NIR emitters, sensitizers for solar cells and substrates for SERS. In this work, we have encompassed the features of As2S3 and Ag2S-NCs in a waveguide, produced by femtosecond laser micromachining. This technique enables to write complex geometry, which is an important step towards integrated systems. At first, thin films were produced on glass substrates. Then, femtosecond laser micromachining (800 nm, 50 fs, 5 MHz) was employed to obtain the threshold energy for inducing modification, being 0.19 ± 0.02 nJ. Adjusting the fabrication conditions we have been able to write slab waveguides. In addition, the nonlinear optical properties of pure As2S3 and Ag2S-NCs doped waveguides have been investigated at the ultra-short pulses regime. Because the material is thin (~ 500 nm), a recently developed method, called nonlinear ellipse rotation, was applied to obtain the nonlinear index of refraction [3]. The nonlinear refractive index of the sample was determined to be two orders of magnitude higher than fused silica.

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Conference 9736: Laser-based Micro- and Nanoprocessing X

References: [1] A. Zakery, and S. R. Elliott, “Optical properties and applications of chalcogenide glasses: a review,” Journal of Non-Crystalline Solids 330, 1-12 (2003).

[2] J. M. P. Almeida, C. Lu, C. R. Mendonça, and C. B. Arnold, “Single-step synthesis of silver sulfide nanocrystals in arsenic trisulfide,” Optical Materials Express 5, 1815-1821 (2015).

[3] M. L. Miguez, E. C. Barbano, S. C. Zilio, and L. Misoguti, “Accurate measurement of nonlinear ellipse rotation using a phase-sensitive method,” Optics Express 22, 25530-25538 (2014).

damage by local heating is reduced to well below a few microns. In view of these outstanding characteristics of lasers, it is very surprising that sample preparation for microstructure diagnostics did not derive the advantages from laser micro-machining. microPREP, is a new laser-micromachining tool developed by 3D-Micromac capable of making fast, clean, and efficient laser ablation available for the preparation of samples for transmission electron microscopy (TEM). The workflow follows a three-stage approach. First, a supporting basic structure is cut from the feedstock. Second, the supported structure is laser-thinned down to a few microns and third, the supported and laser-thinned structure is thinned using a focused ion beam or an ion broad beam.

The examples presented in this paper support this approach and show it is ready to be applied in different areas of microstructure diagnostics and has very high potential for failure diagnostics.

9736-9, Session 2

Ultrafast graphene and carbon nanotube film patterning by picoseconds laser pulses

Ivan I . Bobrinetskiy, AIMEN - Asociación de Investigación Metalúrgica del Noroeste (Spain); Alexey V Emelianov, National Research University of Electronic Technology (Russian Federation); Nerea Otero, Pablo Romero, AIMEN - Asociación de Investigación Metalúrgica del Noroeste (Spain) Carbon nanomaterials are the most promising objects for advanced electronic applications, due to its extraordinary chemical and physical properties. Nonetheless, after more than two decades of intensive research, the application of carbon-based nanostructures in real electronic and optoelectronic devices is still a big challenge due to lack of scalable integration in microelectronic manufacturing. The laser processing can be the most attractive tools for graphene device manufacturing because of a huge variety of results of graphene lattice and pulsed laser irradiation interaction: functionalization, oxidation, reduction, etching and ablation, growth, etc. with resolution down to the nanoscale. The idea of totally mask-less processing of graphene and carbon nanotube films by laser pulses is attractive to reduce costs, improve flexibility, and reduce alignment operations, by producing fully functional devices in single direct-write operations. In this paper, a picoseconds laser with a wavelength of 515 nm and pulse width of 30 ps is used to pattern carbon nanostructures in two ways: ablation and chemical functionalization. The light absorption leads to thermal ablation of graphene and carbon nanotube film under the fluence 60-90 J/cm2 with scanning speed up to 2 m/s. Just below the ablation energy the two-photon absorption leads to adding functional groups to carbon lattice changing optical properties of graphene. This opens the way for both geometrical configuration of carbon based nanostructures and physical and chemical properties alteration in one process by direct laser writing methods.

9736-11, Session 3

Formation of copper micropatterns by laser direct writing using copper nanoparticle ink

Akira Watanabe, Jinguang Cai, Tohoku Univ . (Japan); Gang Qin, Lidan Fan, Henan Polytechnic Univ . (China) Recent progress in printed electronics is based on the development of various kinds of nanomaterials in the past decade. Many types of silver nanoparticle inks have been proposed for printing electric wires. Because of the strong risk of the silver migration in an electronic circuit, copper wiring materials are indispensable for recent printed electronics. However, one of the problems in the application of the copper nanoparticles as a printing material is the rapid surface oxidation in air. In this work, we report the laser direct writing using a copper nanoparticle ink to prepare a conductive copper micropattern. The rapid laser sintering process reduced the oxidation of the copper nanoparticle surface and enabled the formation of a conductive micropattern at room temperature in air. A high spatial resolution is also advantage of the laser direct writing method. Recently, the fabrication process of a copper grid structure is under intense investigation toward indium tin oxide (ITO)-free devises. Transparent conductive electrode is one of the essential components in printable electronics devices. A low-cost, indium-free, and crack-tolerant electrode is of great interest in the printed electronics in the field of touch screen displays. In the manufacturing processes, the patterning of metal grid structure is responsible for more than 50% of touch screen production cost. The laser direct writing using a copper nanoparticle ink has a possibility to develop an efficient patterning process for a copper grid structure. Various kinds of copper grid structures were prepared and then the conductivities and transparency were discussed.

9736-10, Session 3

microPREP: a new laser tool for high volume sample preparation

(Invited Paper)

Uwe Wagner, ED Micromac AG (Germany); Tino Petsch, 3D-Micromac AG (Germany); Michael Krause, Thomas Höche, Fraunhofer Institute for Mechanics of Materials (Germany) Over the past fifty year, lasers have found many, often groundbreaking applications in science and technology. The most important features of lasers are that photons are inherently free of contamination, extremely high energy densities can be focused in very small areas and the laser beam can be precisely positioned using deflection mirrors. By reducing pulse lengths from a few nanoseconds down to the picosecond or femtosecond range, material ablation is becoming increasingly “athermal”, i.e. structure

9736-12, Session 3

Multiwave hybrid laser processing of micrometer scale features for flexible electronic circuits

Joseph T . Hillman, Y . Sukhman, D . Miller, M . Oropeza, C . Risser, Universal Laser Systems, Inc . (United States) Laser processing is a versatile method for creating micrometer scale features for flexible electronic circuits. Selective laser ablation has been used to create vias through polyimide insulating layers to make electrical contact with underlying copper traces. The 10.6 µ m wavelength light from a CO2 laser is absorbed efficiently by the polyimide, but is reflected by the copper surface making the via formation process completely selective. The process is enhanced by combining the CO2 laser beam with a 1.062 process.

µ m laser beam from an Yb-doped fiber laser. This multiwave hybrid laser beam provides a cleaner copper surface, while maintaining the selectivity of the via formation

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Multiwave laser processing also serves as an efficient tool for rapid prototyping. Conductive materials such as silver or carbon can be selectively ablated from an insulating substrate substrate like PET using a 1.062 it to vaporize. However the polymer substrate is transparent to this the substrate. Switching to a 9.3 Finally a high power 10.6 system.

µ µ µ m wavelength. This wavelength is absorbed by the conductive layer causing wavelength, allowing the conductor to be patterned with no damage to m CO2 laser enables efficient marking of the PET substrate due to the strong absorption line at this wavelength. m CO2 laser is used to cut the finished circuit to the desired shape. In this way, three essential prototyping steps that each require a different laser wavelength can be completed on a single laser

9736-14, Session 3

Conference 9736: Laser-based Micro- and Nanoprocessing X Zero degree contour cutting below 100

µ

m feature size with femtosecond laser

Klaus Stolberg, Susanna Friedel, JENOPTIK Laser GmbH (Germany) methods: laser ablation, plasma etching, chemical oxidation, and thermal rescaling of polymers are compared in terms of suitability for surface enhanced Raman sensing/spectroscopy (SERS). As thickness of gold is increasing from tens to hundreds of nanometers an additional surface nano gap/groove pattern develops and increases SERS intensity. Benchmarking of different substrates was carried out by measureing SERS using a self assembled layer of thiophenol on the surface of gold. Mechanisms of SERS enhancement by the electromagnetic and chemical factors are discussed together with strategies to achieve higher SERS sensitivity and selectivity in sensor applications. Randomness of the surface pattern is shown to be an important parameter for the additional 2-4 factor in SERS enhancement. In the case of transparent SERS samples with nano-islands of gold at coverage close to the threshold of gold film percolation threshold, an additional enhancement proportional to the refractive index square, n^2 is demonstrated for illumination from the substrate side (n is the refractive index of the substrate). It is shown that dark field imaging of SERS substrates clearly shows the optimum wavelength for excitation of Raman scattering. Microfeatures in mechanical parts can be generated by variety of “traditional” technologies like EDM, chemical etching or micromilling. All of them have limitations with respect of feature size, productivity or even chemical reactivity. Since a lot of years there are efforts to use the advantages of laser processing: small feature size can be reached by laser spot size in the µ m range; productivity can be enhanced by laser power scaling or beam shaping and lack of mechanical contact prevents introduction of foreign materials or stress in the process. Furtheron the laser processability does not depend on mechanical, chemical, electrical behavior like other technologies do. Unfortunately cw and pulsed lasers down to ns pulse duration show undesired thermal effects. We demonstrate steep wall angles by the application of a trepanning optics and together with a 10 W femtosecond laser laser cutting, drilling and engraving of 100 to 500 µ m thick metals like Cu-alloy, stainless steel, titanium and tantalum can be achieved in a non-thermal melt-free process. Particularly for thin materials, this is advantageous compared to traditional laser fusion cutting, because in this thermal process heat accumulation usually causes mechanical distortion or edge melting as well as material bending caused by pressure of the processing gas.

The combination of beam deflection in trepanning optics and sample motion allowed us to work in a special “laser milling mode” with rotating beam. Zero degree taper angle as well as positive or negative tapers can be achieved. We compare the trepanning results with the results of fine cutting optics. We also demonstrate contour cutting of sophisticated materials like ceramics and polymers.

9736-16, Session 4

Laser heating surface finishing for adapting the nucleate boiling heat transfer

Nerea Otero, Pablo M . Romero, Paula Rico, AIMEN - Asociación de Investigación Metalúrgica del Noroeste (Spain); Jose M . Saiz-Jabardo, Pablo Fariñas, Univ . da Coruña (Spain) Nucleate boiling is a heat transfer mechanism which is characterized by high heat transfer rates, adequate for applications where high power density is present. Though exhaustively investigated during the last decades, the physical mechanisms have not yet being adequately defined. One of the most important aspects is the effect of the heating surface microstructure on the rate of heat transfer and its interaction with the boiling liquid.

The present experimental research has been focused in the development by laser technologies of surface microstructures for developing heat transfer enhanced surfaces. Different laser sources were selected, to modify the surface structure of stainless steel, copper and brass. With the main aim of understanding the effect of the surface finishing on the nucleate boiling, the generated surfaces have been analyzed with confocal microscopy, contact angle and surface tension measurements, in order to relate the surface finishing and its effect on the nucleate boiling heat transfer. Additionally, the effect of the laser treatment on the surface microstructure has been studied by scanning electron microscopy with energy dispersive X ray spectroscopy.

9736-13, Session 4

Surface functionalization with femtosecond lasers

(Invited Paper)

Chunlei Guo, Univ . of Rochester (United States) No Abstract Available

9736-15, Session 4

Nanotextured surfaces for surface enhanced Raman spectroscopy and sensors

(Invited Paper)

Saulius Juodkazis, Swinburne Univ . of Technology (Australia) Properties of the gold-coated nanotextured surfaces prepared by different

9736-17, Session 4

Single shot ultrafast laser ablation of single layer CVD graphene

Abel Gil Villalba, Chen Xie, Roland Salut, Luca Fufaro, Remo Giust, Maxime Jacquot, Pierre-Ambroise Lacourt, John M . Dudley, François Courvoisier, FEMTO-ST (France) Graphene is a major material for next-generation electronics and optoelectronics. Fast patterning technologies are needed, that can operate in the sub-micron regime and over areas of square-meter scale. Ultrafast laser ablation is ideally suited since the process is single step and contact free.

We investigate the single shot ablation of single layer-CVD graphene by ultrafast lasers. We have developed a novel technique to characterize the fluence for ablation threshold that is valid at nanometric scale and allows for investigating the potential variations with crater diameter. We use a 3-wave interference beam pattern that generates approx. 40 craters simultaneously. The numerical correlation of SEM image of the crater

142 SPIE Photonics West 2016 · www.spie.org/pw

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Conference 9736: Laser-based Micro- and Nanoprocessing X

pattern and of the fluence pattern provides insights in the ablation process. We extract ablation threshold fluence for pulse durations between 120 fs and 3 ps. We observe a constant threshold for ablation for all scales but we measured a strong decrease of the ablation probability for crater diameters below 1 µ m. Yet the decrease of the probability of the presence of defects can explain this drop, we highlight the importance of free-electron diffusion in graphene. Indeed, graphene has extremely high electron mobility: the diffusion of free-electrons competes with the confinement of laser energy deposition that is necessary for ablation at nanometric scale.

9736-51, Session PTue

System design of programmable 4f phase modulation techniques for rapid intensity shaping: A conceptual comparison

Matthias Roth, TU Dresden (Germany); Jörg Heber, Fraunhofer-Institut für Photonische Mikrosysteme (Germany); Klaus Janschek, TU Dresden (Germany)

9736-19, Session 4

Nanosecond pulsed laser generation of holographic structures on metals

Krystian L . Wlodarczyk, Heriot-Watt Univ . (United Kingdom); Marcus Ardron, Nick J . Weston, Renishaw plc (United Kingdom); Duncan P . Hand, Heriot-Watt Univ . (United Kingdom) We present a laser-based process for the generation of phase holographic structures directly onto the surface of metals. This process uses 35ns laser pulses of wavelength 355nm to generate optically-smooth surface deformations on a metal. The laser-induced surface deformations (LISDs) are produced by either localised laser melting or the combination of melting and evaporation. The geometry (shape and dimension) of the LISDs depends on the laser processing parameters, such as pulse energy and the number of laser pulses in a spot, as well as on the chemical composition of a metal and the spatial intensity distribution of the laser beam used in the process. In this paper, we will explain the mechanism of the LISDs formation on various metals, such as 304-grade stainless steel, 99% pure nickel and nickel-chromium Inconel® alloys. In addition, we provide information about the design and fabrication process of holographic structures, including issues we are currently facing. Finally, it will be demonstrated that the laser-generated holographic structures can be used as robust marks for identification and traceability of high value metal components and products.

A generic optical 4f-setup, consisting of two lenses and two modulators placed in the consecutive focal planes, forms the basis for several beam shaping approaches for intensity modulation. The primary interest of this study lies in light-efficient generation of patterns or multiple spots in the image plane by means of pure phase modulating elements. 4f approaches share the property that due to the one-to-one relationships between output intensity and input phase, no online calculation is needed. The resulting low computational complexity is offering a major advantage of 4f-methods compared to the widely used Fourier Holography and makes them attractive candidates for real-time applications. Increasing availability of fast phase modulators, e.g. on MEMS basis, demands for an evaluation of the performance of these concepts. Our aim is to assess the applicability of these 4f methods to high-power applications. We present a system level trade-off for several variants of 4f intensity shaping by phase modulation. It includes micro mirror based phase manipulation combined with amplitude masking in the Fourier plane, Generalized Phase Contrast, a generalization of Zernike’s phase contrast microscopy relying on pure phase modulation in both image and Fourier plane, and phase-only correlation, a technique originating from signal processing likewise employing two programmable phase filters. This paper compares these concepts using a generic setup and derives figures of merit for energy efficiency, pattern homogeneity, pattern image quality, maximum output intensity and flexibility with respect to the displayable pattern. Numerical simulations illustrate our findings.

9736-50, Session PTue

Random lasing of microporous surface based on Cr2+:ZnSe crystal

Xianheng Yang, Guoying Feng, Shouhuan Zhou, Sichuan Univ . (China) A random lasing emission based on microporous surface of Cr2+:ZnSe crystal was demonstrated. The microporous surface was prepared by femtosecond pulsed laser ablation in high vacuum (below 5?10-4 Pa). The scanning electron microscope results show that there are a mass of micropores with an average size of ~13 ?m and smaller ones with ~1.2 ?m on the surface of Cr2+:ZnSe crystal. The adjacent micropore spacing of the smaller micropores ranges from 1 ?m to 5 ?m. The X-ray diffraction patterns reveal that the crystallization of Cr2+:ZnSe changes little after pulsed laser ablation. Under 1750 nm excitation of Nd:YAG (355 nm) pumped optical parametric oscillator, a random lasing emission with center wavelength of 2350 nm and laser-like threshold of 0.3 mJ/pulse is observed. The emission lifetime of 2350 nm laser reduces from 800 ns to 30 ns as the pump energy increases above threshold.

9736-52, Session PTue

Direct formation of 100 nm-sized structure by laser-induced forward transfer (LIFT) using femtosecond laser beam

Takahiro Nakamura, Koki Omachi, Shinichi Sato, Tohoku Univ . (Japan) Direct writing techniques have become a worldwide topic due to the capability of direct formation of desired 2D and 3D structures of various materials. However, there are still difficulties to form fine structures, because the minimum pixel size is limited to micrometer order. Laser-induced forward transfer (LIFT) can be an alternative route for the fabrication of submicron-scale structures. The size of transferred fraction of the carrier film by LIFT can be decreased to sub-micron by controlling laser energy and focal spot size of applied laser beam on the carrier film. In the present study, we demonstrated the direct formation of submicron structures by LIFT in different laser fluence conditions. A metal Cr film, which was formed on a transparent cover glass substrate with the thickness of 30 nm as a carrier, was placed directly on a Si single crystal substrate. Single shot of femtosecond laser pulse (lambda = 800 nm, 100 fs) was focused by an objective lens (?100, NA=0.8) and irradiated on the Cr carrier film from the backside of the transparent glass substrate. Although multiple deposition of Cr particles with wide size distribution were observed on the Si substrate surface for the laser energy higher than 5.5 nJ, spherical shaped Cr particles with the size of 100 nm were transferred at the exact position of the substrate on demand with the laser energy of 5.0 nJ. Effect of the intensity distribution at the cross section of introduced laser beam on the LIFT was also examined.

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Conference 9736: Laser-based Micro- and Nanoprocessing X

9736-53, Session PTue

Heat transfer analysis of two wavelengths laser microprocessing inside glass

Aoi Matsumoto, Takayuki Tamaki, Shinichi Enoki, Nara National College of Technology (Japan); Keisuke Yahata, Mitsuboshi Diamond Industrial Co . ,LTD . (Japan); Etsuji Ohmura, Osaka Univ . (Japan) Two wavelengths laser microprocessing system has been developed, where femtosecond laser and CO2 laser beam are irradiated at the same time and wide-area processing can be obtained. The target of this research is to establish a heat transfer analysis model for this system.

First we simulate the heating phenomenon inside BK7 glass (OHARA, S-BSL7, 30?5?0.67 mm) with either femtosecond or CO2 laser alone by the finite element method, using ANSYS mechanical APDL.

The femtosecond laser has a wavelength of 1.064 µ m and a repetition frequency of 1 MHz.

Because we consider the heating phenomenon that continues for several seconds, the femtosecond laser could be assumed as continuous wave source.

When the femtosecond laser is assumed to be absorbed in a cone-shaped region, the temperature distribution got closer to experimental results compared to the cases of rod- or cylinder- shaped absorption region.

Next we simulate the heating phenomenon with a CO2 laser whose wavelength is 10.6 µ m.

The beam size of CO2 laser is determined from the experimental results and the heating with a CO2 laser is modeled as surface heat source.

By the analysis results, the CO2 laser beam is considered to be heat source.

9736-54, Session PTue

Online process monitoring at quasi simultaneous laser transmission welding using a 3D-scanner with integrated pyrometer

Anton Schmailzl, Sebastian Steger, Michael Dostalek, Stefan Hierl, Ostbayerische Technische Hochschule Regensburg (Germany) Quasi-simultaneous laser transmission welding is a well-known joining process for thermoplastics and mainly used in automotive- and medical industry. For process control usually the so called set-path monitoring is used, where the weld is specified as “good” if the welding time is inside a defined confidence interval. However the detection of small-sized remaining gaps or thermal damaged zones is not possible. The analyzation of the weld seam temperature during welding offers the possibility to overcome this problem. In this approach a 3D-scanner is used instead of a scanner with flat-field optic. In spite of using a pyrometer at a certain spectrum beside the laser wavelength no color-corrected optic is needed in order to provide that laser- and detection-spot are coaxial. Experimental studies on polyethylene t-joints have shown that the signal-noise ratio is adequate, despite using a long working distance and a small optical aperture. The effects on temperature are studied for defects like gaps in the joining zone. Therefore gaps with different geometries are milled into the samples. In case of producing housings for electronic-parts the effect of a bonding wire between the joining partners is also investigated. Both defects can be identified by a local temperature deviation even at a feed rate of several meters per second. Furthermore strategies for signal-processing are demonstrated. By this, a decision can be made whether defects are remaining or not. Consequently an online detection of local defects is possible, so that a dynamic process control is feasible.

9736-55, Session PTue

Pattern transfer, self-organized surface nanostructuring, and nanodrilling of dielectrics using nanosecond laser irradiation

Pierre Lorenz, Leibniz-Institut für Oberflächenmodifizierung e .V . (Germany); Joachim Zajadazc, Leibniz-Institut für Oberflächenmodifizierung eV (Germany); Martin Ehrhardt, Lukas Bayer, Klaus-Peter Zimmer, Leibniz-Institut für Oberflächenmodifizierung e .V . (Germany) Nanostructures have a widespread field of applications. The structuring of different dielectrics (fused silica, sapphire, and diamond) was studied, assisted by a nanosecond laser-induced self-organised molten molybdenum layer deformation process. At low laser fluence the irradiation of thin metal layers on dielectric surfaces results in a melting and nanostructuring process of the metal layer and partially of the dielectric surface. The resultant structures are dependent on the substrate and the laser parameters. Furthermore, a subsequent high laser fluence treatment of the metal nanostructures results in different features: (i) pattern transfer, (ii) self organized surface nanostructuring, and (iii) nanodrilling. (i) Pattern Transfer: The irradiation of the pre-structured metal layer with high laser fluences allows the transfer of the lateral geometry of the metal nanostructures into the dielectric surface. (ii) Self-organized surface nanostructuring: The multi-pulse irradiation of the metal layer/dielectric system with moderate laser fluences results in a self-organized nanostructuring of the dielectric surface. (iii) Nanodrilling: The multi-pulse low laser fluence irradiation of the metal layer results in the formation of metal droplets and a further high fluence irradiation of the laser-generated metal droplets results in a stepwise evaporation of the metal and in a partial evaporation of the dielectrics and, finally, in the