See discussions, stats, and author profiles for this publication at: http://www.researchgate.net/publication/258798821 All-solid-state continuous-wave yellow laser at 561 nm under in-band pumping ARTICLE in JOURNAL OF THE OPTICAL SOCIETY OF AMERICA B · JANUARY 2013 Impact Factor: 1.97 · DOI: 10.1364/JOSAB.30.000095 CITATIONS READS 5 20 5 AUTHORS, INCLUDING: Jing Gao Xianjin, Eric, Dai Chinese Academy of Sciences University of Florida 45 PUBLICATIONS 577 CITATIONS 11 PUBLICATIONS 17 CITATIONS SEE PROFILE SEE PROFILE Haixuan Sun Xiaodong Wu Chinese Academy of Sciences Suzhou Institute of Biomedical Engineerin… 7 PUBLICATIONS 26 CITATIONS 11 PUBLICATIONS 35 CITATIONS SEE PROFILE SEE PROFILE Available from: Jing Gao Retrieved on: 08 November 2015 Gao et al. Vol. 30, No. 1 / January 2013 / J. Opt. Soc. Am. B 95 All-solid-state continuous-wave yellow laser at 561 nm under in-band pumping Jing Gao,* Xianjin Dai, Long Zhang, Haixuan Sun, and Xiaodong Wu Suzhou Institute of Biomedical Engineering and Technology, Chinese Academy of Sciences, Suzhou, Jiangsu 215163, China *Corresponding author: [email protected] Received October 17, 2012; revised November 5, 2012; accepted November 12, 2012; posted November 12, 2012 (Doc. ID 178246); published December 6, 2012 We demonstrate a continuous-wave (cw) yellow laser at 561 nm generation by intracavity frequency doubling of a direct in-band pumped Nd:YAG laser on the R1 → Y 6 transition. By using an inserted solid glass etalon and a special coating design, the 1123 nm single line oscillation is realized. With an absorbed pump power of ∼21.8 W, the maximum output power of ∼2.3 W is achieved, with an optical-to-optical efficiency of ∼10.6%. This is, to the best of our knowledge, the highest cw output power at 561 nm of a diode-pumped frequency doubled Nd:YAG laser. © 2012 Optical Society of America OCIS codes: 140.3580, 140.3530, 140.3480, 140.3515. 1. INTRODUCTION Yellow lasers around 560 nm are highly interesting for their potential in biomedicine and biotechnology [1–4]. These sources have higher absorption in hemoglobin, better penetration through the nuclear sclerotic cataracts, lower absorption in lutein of the macula lutea, less heat dispersion in the neurosensory retina, and less discomfort to the patient. Thus, this type of laser plays an important role in ophthalmology [5–7]. Especially, the 561 nm emission can excite phycoerythrin and its tandems and red fluorescent proteins, such as DsRed and dTomato; as a result, this laser has become a standard for confocal microscopy and flow cytometry and is being used for laser Doppler velocimetry [8,9]. Frequency doubling of the R1 → Y 6 transition at 1123 nm in the Nd:YAG gain medium has been proven to be an efficient way of generating a 561 nm yellow laser. Nd:YAG single crystal, the most widely used laser gain medium until now, has become the most promising candidate due to its excellent combination of mechanical, thermo-optical, and spectral properties. The first laser diode (LD) pumped 561 nm laser with 0.5 mW continuous-wave (cw) output was reported by Guo et al. in 2004, employing a diode-end-pumped structure with an 808 nm LD and an extracavity frequency doubled scheme with a periodically poled lithium niobate (PPLN) crystal, respectively . In 2008, Räikkönen et al. demonstrated a diode-end-pumped passively Q-switched Nd:YAG laser, and its extracavity frequency doubling into the yellow range by using a potassium titanyl arsenate (KTA) crystal. The pulsed repetition rate was 12 kHz, and the average power was 55 mW . In 2010, Yao et al. presented a 561 nm laser with 1.2 W cw output power by intracavity frequency doubling of a diodeend-pumped Nd:YAG laser with an 808 nm LD and a lithium triborate (LBO) crystal, resulting in an optical-to-optical efficiency (ηo-o ) of 13.3% and M 2 ∼ 1.67 . Georges et al. demonstrated a diode-end-pumped single frequency 561 nm laser with an 808 nm LD and a potassium titanyl phosphate (KTP) crystal, giving a 0.5 W cw yellow output with ηo-o ∼ 0740-3224/13/010095-04$15.00/0 18% and M 2 ∼ 1.1 . In 2012, as high as 60 W of 561 nm laser average output was acquired by acousto-optical Q-switching and intracavity frequency doubling in an 808 nm diodeside-pump structure, with ηo-o ∼ 6.1% and M 2 ∼ 30 . In this paper, we demonstrate a compact, diode-endpumped 561 nm laser with ∼2.3 W cw output power by using an 885 nm LD and a diffusion bonded composite crystal. The optical-to-optical efficiency with respect to the absorbed pump power is ∼10.6%. To the best of our knowledge, the cw output power represents the highest level ever reported for diode-pumped frequency doubled Nd:YAG lasers at 561 nm. 2. THEORETICAL ANALYSIS Figure 1 presents the diagram of the typical energy levels in Nd:YAG crystal. To realize efficient yellow laser operation, the first step is to obtain the fundamental infrared 1123 nm transition (the red line). However, there are two factors should be considered. First, the stimulated-emission cross section (σ) of the 1123, 1064, 1318, 1338, and 946 nm transitions are determined as 3.0, 45.8, 8.7, 9.2, and 5.1 × 10−20 cm2 , respectively . In this case, the other transitions with higher gain should be suppressed to ensure the efficient oscillating of 1123 nm line. A special coating design is an effective solution for this problem [6,7]. Second, there are three lines in an extremely narrow spectral range (∼7 nm), i.e., the 1112, 1116, and 1123 nm lines. Considering that the σ for these lines are equivalent , it is very difficult to realize single line oscillating at 1123 nm simply by optical coating. To solve this question, a solid etalon is introduced into the resonator. When an etalon is inserted into the resonator, the round-trip phase change of the light beam can be given by  2π · 2nd · cos θ; (1) δ λ where n is the refractive index of the etalon, λ is the wavelength of the light, d is the thickness of the etalon, and θ is © 2013 Optical Society of America 96 J. Opt. Soc. Am. B / Vol. 30, No. 1 / January 2013 Gao et al. the 1112 and 1116 nm lines can be restrained due to the higher insertion losses. We note that, when θ 0.049 rad, the transmission for the 1123 nm line is about 100%, and moreover, the transmissions for the 1112 and 1116 nm lines are about 85% and 95%, respectively. Therefore, this angle can achieve low insertion loss at 1123 nm under the premise of suppressing the 1112 and 1116 nm lines. In terms of the pumping scheme, we believe that the in-band pumping concept is one of the most efficient schemes [14–16]. Seen from Fig. 1, the in-band pumping directly into the 4 F3∕2 level changes the quantum defect ratio from 0.72 for the pump at 809 nm to 0.788 for the pump at 885 nm in case of the 1123 nm oscillation. This induces a reduction of the fractional thermal load by ∼25% for the efficient laser emission at 1123 nm, from 0.28 to 0.21, which is beneficial to power scaling of the fundamental 1123 nm line and also the frequency doubled 561 nm laser . 3. EXPERIMENTAL SETUP Fig. 1. (Color online) Diagram of the energy levels of Nd:YAG with a description of the main pumping and transition lines. the incident angle. Therefore, the transmission can be expressed as −1 4r δ 2 T 1 · sin ; 2 1 − r2 (2) where r is the reflectivity of the etalon surface. Consequently, the transmission curves of the 1112, 1116, and 1123 nm lines versus the tilt angle of the etalon can be calculated and shown in Fig. 2. Typical parameters are listed as follows: n 1.46, d 0.13 mm, and r 0.04. As can be seen from Fig. 2, the three curves are clearly separated. This allows us to adjust the inserting losses at the three lines, which is important for the control of the corresponding net gain inside the cavity and the consequential efficient single 1123 nm line oscillation. By tuning the etalon to a proper angle, at which the transmission at 1123 nm is close to the peak value of the solid blue line, Fig. 2. (Color online) Transmission curves for 1112, 1116, and 1123 nm lines varied with the tilt angle of the etalon. The schematic diagram of the experimental setup is shown in Fig. 3. A fiber coupled laser diode array [LDA, (DILAS, Inc.)] was employed as the pump source, which delivers a maximum output power of 40 W at 885 nm from the end of a fiber with a core diameter of 400 μm and a NA of 0.22. The FWHM linewidth of ∼1.4 nm is favorable to achieve good absorption efficiency for the pump light due to an excellent overlap between the absorption spectra of the gain medium and the diode laser’s emission spectra. The pump radiation was injected into the gain medium through two coupling lenses with a waist spot diameter of ∼400 μm. A diffusion bonded YAG/ Nd:YAG crystal, which consists of an 8 mm long, 1.0 at. % Nd-doped part and a 4 mm long undoped end cap with a cross section of 3 mm × 3 mm, was used as the gain medium. Both surfaces of the crystal were well polished and coated for high transmission (HT) at 1123 nm (T > 99.8%) and the pump wavelength (T > 99%). When the pump wavelength was tuned to match the absorption peak of the laser crystal, an average absorption coefficient of ∼1.1 cm−1 was obtained at the maximum pump level. The frequency doublers were antireflection (AR) coated at both 1123 and 561 nm to reduce the cavity loss. A thermal-electric cooler with a stability of 0.1 °C was used to control the temperature of both the laser crystal and the frequency doubler. The laser output power was tested by a power sensor L50(300)A and recorded by a power meter VEGA (Ophir, Inc.). A thin solid fused silica etalon was inserted inside the laser cavity to enforce the laser operation on the 1123 nm line. The etalon had a thickness d 0.13 mm and a refractive index n 1.46. A V-type cavity insensitive to the thermal lens was designed to favor the frequency doubling. The geometric lengths of arm Fig. 3. (Color online) Schematic diagram of the experimental setup of the 561 nm laser. Gao et al. Vol. 30, No. 1 / January 2013 / J. Opt. Soc. Am. B L1 and L2 were selected to be 70 and 35 mm, respectively. Mirrors M2 and M3 have radii of curvature of 50 and 400 mm, respectively. With the approximation of a thin lens in the middle of the laser crystal, the mode radii in the laser crystal and the LBO were calculated by an ABCD matrix formalism with M 2 ∼ 2 and determined as ∼180 μm and ∼80 μm, respectively . To suppress the more efficient transitions including 1064, 1318, 1338, 946 nm lines in the laser crystal and achieve efficient yellow laser output, a special coating for the cavity mirrors was accomplished. M1 was coated for high reflection (HR) (R > 99.8%) at 1123 nm, HT at 1064 nm (T > 85%) and 946 nm (T > 95%), and AR at 885 nm (T > 95%). M2 was coated for HR (R > 99.8%) at 1123 nm, and HT at 561 nm (T > 85%). M3 was coated for HR at 1123 and 561 nm (R > 99.8%) and HT at 1318 and 1338 nm (T > 90%). 4. RESULTS AND DISCUSSION With respect to the high damage threshold, the small walk-off angle, and the wide spectral and angular acceptance bandwidths with a modest effective nonlinear coefficient, LBO seems to be the most promising candidate for the frequency doubler of the high-power, high-brightness yellow laser [6,7]. Therefore, two different LBO crystals cut for type-I critical matching were employed as the frequency doublers. One is cut at θ 90° and φ 8.3° with dimensions of 3 mm × 3 mm × 10 mm; the other is cut at θ 90° and φ 7.5° with dimensions of 3 mm × 3 mm × 15 mm. The dependence of the yellow output power on the absorbed pump power is shown in Fig. 4. The threshold of the yellow laser was measured about 2.5 W. At an absorbed pump power of ∼21.8 W, the maximum yellow laser outputs of ∼2.3 W with the 10 mm long LBO and ∼1.8 W with the 15 mm long LBO were achieved, yielding optical-to-optical efficiencies versus the absorbed pump power of ∼10.6% and 8.3%, respectively. To the best of our knowledge, the 2.3 W cw yellow output represents the highest level ever reported in a diode-pumped frequency doubled Nd:YAG laser at 561 nm. The yellow output power P SHG at 561 nm as a function of the absorbed pump power P abs was evaluated based on the plane-wave model introduced by Smith . With this approach, the yellow output power can be written as [18–20] P SHG πω21 2 L Is − k i 8k Is 1∕2 2 Li 2 k 4 2K c P abs − Li ; k Is Is 97 Fig. 4. (Color online) CW output power of 561 nm laser versus the absorbed pump power with different frequency doublers. The solid blue line represents the modeling with loss Li ∼ 0.014. The inset is the lasing spectrum of the 561 nm laser at the maximum output. medium and ω2 in the LBO nonlinear crystal have been determined in Section 3. Based on the Findlay–Clay analysis , the pump coupling coefficient K c and the resonator loss Li were determined as ∼0.0079 W−1 and ∼0.014, respectively, when using the 10 mm long LBO as the frequency doubler. From Fig. 4, it can be seen that the agreement between the experimental result (the red line, squares) and the model (the solid blue line) is good. A fiber spectrometer (HR4000, Ocean Optics, Inc.) with a resolution of ∼0.24 nm was used to monitor the laser spectrum. The inset of Fig. 4 presents the optical spectrum of the frequency doubled laser at the highest output power. It is noticed that, thanks to the special coating design and the inserted etalon, only one single laser line at 561 nm can be observed. The transverse spatial profile and the beam propagation factors of the yellow laser beam were also measured with (3) where I s is the saturation intensity of the 1123 nm transition in the gain medium. The parameter k is the effective nonlinearity of the LBO crystal: k 4π 2 ω21 d2eff l2c Z 0 2 3 β; λ2ω ω2 n (4) where Z 0 377 Ω is the vacuum impedance, lc denotes the LBO crystal length, n ∼ 1.6 is the refractive index, deff 0.84 pm∕V is the nonlinear coefficient of LBO, and β is a factor that accounts for the phase mismatch between the fundamental wavelength λω and the second harmonic wavelength in the second pass of LBO. The laser beam radius ω1 in the gain Fig. 5. (Color online) Beam quality measurements of the 561 nm laser under the maximum output. The inset is its three-dimensional intensity profile. 98 J. Opt. Soc. Am. B / Vol. 30, No. 1 / January 2013 Fig. 6. (Color online) Stability of the yellow laser power at the maximum output level. a M 2 -200s-FW (Ophir–Spiricon, Inc.) at the maximum output power (see Fig. 5). The beam propagation factors M 2 were found to be about 2.0 and 1.7 in the X and Y directions, respectively. Considering that the Nd:YAG single crystal is an optically isotropic material, we assume that the astigmatism was mainly ascribed to the folded angle in the cavity and the thermally induced birefringence in the gain medium . Using the StarLab2.40 software from Ophir, Inc., We tested the short-term power stability of the 561 nm laser at the maximum output power. The power fluctuation value was calculated according to the standard deviation divided by the mean value of the yellow output. Figure 6 depicts the power fluctuation of the yellow laser, which was determined to be ∼5.6% within the given 20 min. 5. CONCLUSION In summary, we have reported a compact diode-end-pumped frequency doubled Nd:YAG/LBO laser at 561 nm. By employing an 885 nm LD and a diffusion bonded Nd:YAG crystal, efficient power scaling was achieved. With an absorbed pump power of ∼21.8 W, we obtained up to ∼2.3 W of cw output power at 561 nm. 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