Integrated Low-Jitter 400-MHz Femtosecond Waveguide
Laser
The MIT Faculty has made this article openly available. Please share
how this access benefits you. Your story matters.
Citation
Hyunil Byun et al. “Integrated Low-Jitter 400-MHz Femtosecond
Waveguide Laser.” Photonics Technology Letters, IEEE 21.12
(2009): 763-765. © 2009 IEEE
As Published
http://dx.doi.org/10.1109/lpt.2009.2017505
Publisher
Institute of Electrical and Electronics Engineers
Version
Final published version
Accessed
Wed May 25 21:49:51 EDT 2016
Citable Link
http://hdl.handle.net/1721.1/52360
Terms of Use
Article is made available in accordance with the publisher's policy
and may be subject to US copyright law. Please refer to the
publisher's site for terms of use.
Detailed Terms
IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 21, NO. 12, JUNE 15, 2009
763
Integrated Low-Jitter 400-MHz Femtosecond
Waveguide Laser
Hyunil Byun, Dominik Pudo, Sergey Frolov, Amir Hanjani, Joseph Shmulovich, Erich P. Ippen, and
Franz X. Kärtner
Abstract—An integrated passively mode-locked waveguide laser
generating 440-fs pulses at 394-MHz repetition rate is demonstrated on an 18 mm 44 mm silica waveguide chip. The laser
self-starts, and uses a saturable Bragg reflector for mode-locking.
Index Terms—Erbium waveguide, femtosecond pulse, high repetition rate, integrated laser, low jitter, soliton mode-locking.
I. INTRODUCTION
IGH repetition rate, low jitter sources of femtosecond
laser pulses are necessary for a variety of applications
including optical arbitrary waveform generation, frequency
metrology [1], and ultrafast sampling [2]. Passive mode-locking
enables low jitter femtosecond pulses while alleviating the need
for an external microwave oscillator. In the past, both polarization additive-pulse mode-locking and/or saturable Bragg
reflector (SBR) mode-locking [3], [4] have been used with
fiber and waveguide lasers, the latter leading to more compact
cavities and fewer components. Recent experimental demonstrations of high repetition rate fiber lasers based on the SBR
approach achieved repetition rates as high as 3 GHz [5] and
timing jitter as low as 20 fs in the frequency range [1 kHz,
10 MHz] [6]. Nevertheless, integrated versions of such lasers
have the additional benefits of being mass-producible with
reduced footprint, and increased stability. Early attempts at
passively mode locking with an erbium-doped waveguide
(EDW) as a gain medium and an SBR as a mode-locking
element were limited to lower repetition rate and picosecond
pulse operation [7], [8]. Femtosecond operation of such lasers
[9] was only achieved by employing nonlinear polarization
rotation mode-locking in an extended cavity, which is difficult
to integrate. In addition, in all of these cases the cavity utilized
free-space optics between the SBR and the EDW.
Monolithically integrated mode-locked semiconductor lasers
have achieved femtosecond pulses at repetition rates of tens of
gigahertz [10], [11]. However, due to the fast gain dynamics
H
Manuscript received January 19, 2009; revised March 02, 2009. First published March 27, 2009; current version published May 22, 2009. This work was
supported in part by the Defence Advanced Research Projects Agency (DARPA)
under Contract HR0011-05-C-0155 and Contract W911NF-04-1-0431, and in
part by the Air Force Office of Scientific Research (AFOSR) under Contract
FA9550-07-1-0014.
H. Byun, D. Pudo, E. P. Ippen, and F. X. Kärtner are with the Department of
Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA 02139 USA (e-mail: hibyun@mit.edu; dpudo@mit.
edu; ippen@mit.edu; kaertner@mit.edu).
S. Frolov, A. Hanjani, and J. Shmulovich are with CyOptics, Inc., South Plainfield, NJ 07080 USA (e-mail: sfrolov@cyoptics.com; ahanjani@cyoptics.com;
jshmulovich@cyoptics.com).
Color versions of one or more of the figures in this letter are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/LPT.2009.2017505
Fig. 1. (a) Integrated laser layout. (b) Snapshot of the laser setup. Inset of (a)
depicts the SBR’s reflection and dispersion spectra.
of the semiconductor gain medium, the measured timing jitter
of integrated femtosecond laser diodes is typically hundreds of
femtoseconds to a picosecond [12], [13]. Actively mode-locked
laser diodes have shown subfemtosecond timing jitter, but they
too rely on high-quality external oscillators and sophisticated
control circuitry [14], [15]. As a result, there is a clear need for a
source which would combine all the aforementioned characteristics: integration, high repetition rate, low jitter and self-driven
operation, as well as compatibility with mass-production technologies.
In this letter, we demonstrate a 394-MHz, self-starting,
passively mode-locked femtosecond laser based on planar
silica waveguide technology. The laser generates 440-fs pulses
with an average output power of 1.2 mW for a pump power of
400 mW and with timing jitter of 24 fs [10 kHz, 10 MHz].
II. EXPERIMENTAL SETUP
The experimental setup is depicted in Fig. 1. The laser cavity
consists of a 5-cm section of erbium (Er)-doped alumino–silicate waveguide with a group-velocity dispersion of 30 fs mm.
A 20-cm section of phosphorous (P)-doped silica waveguide
with a dispersion of 25 fs mm is used to obtain a net anomalous intracavity dispersion to enable soliton mode-locking [16].
The length of the Er-doped section is determined to generate the necessary intracavity power, while the length of silica
1041-1135/$25.00 © 2009 IEEE
Authorized licensed use limited to: MIT Libraries. Downloaded on November 23, 2009 at 17:02 from IEEE Xplore. Restrictions apply.
764
IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 21, NO. 12, JUNE 15, 2009
Fig. 2. Scanning electron microscope (SEM) image of the waveguide chip,
taken over the side surface to show the output waveguide.
Fig. 3. Measured gain spectrum of the Er-doped waveguide for three different
pump power levels.
waveguide is determined in order to achieve appropriate dispersion for soliton operation. Decreasing the average dispersion
shortens the pulsewidth, but too little dispersion can lead to instability due to the insufficient suppression of the continuum.
A loop mirror is used at one end to provide 10% output coupling, while the other end is butt-coupled to an external SBR.
The length of the loop mirror can be adjusted to minimize the
footprint of the waveguide cavity while maintaining a constant
effective silica section length, namely 20 cm. The SBR is a commercial unit with 14% modulation depth, a 2-ps recovery time
and a saturation fluence of 25 J cm . Pump power is provided
by an external 976-nm laser diode coupled to the waveguide
chip.
III. WAVEGUIDE FABRICATION AND CHARACTERISTICS
The fabrication of Er-doped films of aluminosilicate glass
was done by radio-frequency (RF) sputtering on oxidized Si
substrates. Waveguide cores were defined using conventional
lithography and etching, and finally a silica upper cladding was
deposited [17]. A similar process using P-doped glass was applied to fabricate the passive waveguide section. The Er-doped
section and the P-doped silica–glass section have effective mode
areas of 10 and 40 m , respectively, and are connected through
a mode convertor with loss less than 0.15 dB. The P-doped section is tapered to be wider near the chip edge to minimize the
coupling loss to the single-mode fiber, as shown in Fig. 2.
The Er-doped waveguides were characterized using a separate chip with straight Er-doped waveguides. Fig. 3 shows
the measured gain spectrum for three different power levels
from the 976-nm pump diode and confirms that the peak gain
at 1532 nm is 1.8 dB/cm. The measured pump absorption at
976 nm was 0.4 dB/cm.
Fig. 4. Measurement traces at 394 MHz: (a) normalized optical spectrum
before and after amplification (dashed line); (b) RF spectrum (3-GHz span,
10-MHz resolution); (c) RF spectrum (100-kHz span, 500-Hz resolution);
(d) 10-s persistence trace; (e) background free autocorrelation trace; and
(f) single sideband (SSB) phase noise of the first harmonic of the laser with
instrument noise limit (short-dashed line) and integrated root-mean-square
(rms) timing jitter (long-dashed line).
IV. EXPERIMENTAL RESULTS
The laser was operated with 400 mW of pump power at
976 nm; the intracavity signal power was measured to be
12 mW, corresponding to a 30-pJ intracavity pulse energy.
The output pulses with an average power of 1.2 mW are then
amplified to 18 mW using an Er-doped fiber amplifier (980-nm
pump, 350 mA), detected using a 10-GHz photodiode, and
measured with a 500-MHz sampling scope and a signal source
analyzer (Agilent E5052).
Fig. 4 depicts the measurement results. The 5-s persistence
trace shows excellent signal stability, while the RF spectrum in
(c) indicates a sidemode suppression ratio of 80 dB. The 8.4-nm
full-width at half-maximum (FWHM) optical bandwidth before amplification implies 300-fs-duration transform-limited
pulses. After amplification, the optical bandwidth decreases
to 7.4 nm, corresponding to 340 fs. The autocorrelation measurement yielded a pulse duration of 440 fs. The difference is
attributed to incomplete compensation of the chirp added by
the Er-doped fiber. The laser is self-starting; and, as the pump
power is increased, the laser first operates in a mode-locked
-switching state before transitioning to a continuous-wave
soliton mode-locked state at a pump power of 160 mW. Pump
powers much higher than 400 mW resulted in pulse breakup as
the intracavity energy was too high to sustain a single soliton
for the given round-trip dispersion. Fig. 4(f) shows the phase
noise of the first harmonic (394 MHz) of the laser. The timing
jitter integrated from 20 MHz progressively down to 1 kHz
is also shown. The timing jitter integrated from 10 kHz to
Authorized licensed use limited to: MIT Libraries. Downloaded on November 23, 2009 at 17:02 from IEEE Xplore. Restrictions apply.
BYUN et al.: INTEGRATED LOW-JITTER 400-MHz FEMTOSECOND WAVEGUIDE LASER
TABLE I
LASER PARAMETERS
765
The laser provides good stability in a simple and potentially
scalable design. No polarization control or active stabilization
is required, and the laser is self-starting. Higher repetition rates
can be achieved by further reducing the cavity length while optimizing the pumping scheme.
REFERENCES
TABLE II
EXPERIMENTAL AND SIMULATED RESULTS. (*TRANSFORM-LIMITED
PULSEWIDTH CALCULATED FROM THE MEASURED OPTICAL
SPECTRUM FWHM)
20 MHz is 24 fs, which is close to the noise sensitivity limit of
the Agilent E5052 signal analyzer. We expect that the timing
jitter can be further reduced to the few-femtosecond or even
subfemtosecond range [18].
V. NUMERICAL SIMULATION
The experimentally observed pulse characteristics can be
compared with theoretical predictions based on the split-step
Fourier method, using our waveguide and SBR parameters. The
second-order dispersion, the Kerr effect, and gain filtering with
parabolic gain shape are considered in the simulation. The SBR
is modeled using the rate equation for a slow saturable absorber
[16]. Table I summarizes the values used in the simulation,
while Table II compares the predicted and experimental laser
characteristics.
Good agreement, within 3%, between the experimental and
predicted spectral characteristics is obtained. To remove the effect of uncompensated chirp, the comparison is based on the
transform-limited pulsewidth both for the simulated and measured results. Some discrepancy may be attributed to the nonuniform dispersion profile of the SBR, as its dispersion can change
as much as 1000 fs within the optical bandwidth of the laser
output (inset of Fig. 1).
VI. SUMMARY
We demonstrated a stable, passively mode-locked integrated
femtosecond waveguide laser with an SBR as a mode-locking
element. The laser generates 440-fs pulses at 394 MHz, with
the cavity being composed of a 5-cm section of Er-doped alumino–silicate waveguide and a 20-cm section of P-doped silica
waveguide for dispersion engineering. Such a design provides
a compact and robust integrated low-jitter femtosecond source
without the need for driving electronics for active modulation.
[1] S. T. Cundiff, “Metrology: New generation of combs,” Nature, vol.
450, pp. 1175–1176, 2007.
[2] A. Bartels, R. Cerna, C. Kistner, A. Thoma, F. Hudert, C. Janke, and
T. Dekorsy, “Ultrafast time-domain spectroscopy based on high-speed
asynchronous optical sampling,” Rev. Sci. Instrum., vol. 78, pp.
035107–8, 2007.
[3] H. A. Haus, “Theory of mode locking with a fast saturable absorber,”
J. Appl. Phys., vol. 46, pp. 3049–3058, 1975.
[4] R. Paschotta and U. Keller, “Passive mode locking with slow saturable
absorbers,” Appl. Phys. B, Lasers Opt., vol. 73, pp. 653–662, 2001.
[5] J. Chen, J. W. Sickler, H. Byun, E. P. Ippen, S. Jiang, and F. X.
Kaertner, “Fundamentally mode-locked 3 GHz femtosecond erbium
fiber laser,” in Proc. 16th Int. Conf. Ultrafast Phenomena XIV, Stresa,
Lago Maggiore, Italy, 2008.
[6] H. Byun, D. Pudo, J. Chen, E. P. Ippen, and F. X. Kärtner, “High-repetition-rate, 491 MHz, femtosecond fiber laser with low timing jitter,”
Opt. Lett., vol. 33, pp. 2221–2223, 2008.
[7] E. R. Thoen, E. M. Koontz, D. J. Jones, D. Barbier, F. X. Kärtner, E.
P. Ippen, and L. A. Kolodziejski, “Erbium–ytterbium waveguide laser
mode-locked with a semiconductor saturable absorber mirror,” IEEE
Photon. Technol. Lett., vol. 12, no. 2, pp. 149–151, Feb. 2000.
[8] J. B. Schlager, B. E. Callicoatt, R. P. Mirin, N. A. Sanford, D. J. Jones,
and J. Ye, “Passively mode-locked glass waveguide laser with 14-fs
timing jitter,” Opt. Lett., vol. 28, pp. 2411–2413, 2003.
[9] D. J. Jones, S. Namiki, D. Barbier, E. P. Ippen, and H. A. Haus, “116-fs
soliton source based on an Er–Yb codoped waveguide amplifier,” IEEE
Photon. Technol. Lett., vol. 10, no. 5, pp. 666–668, May 1998.
[10] D. J. Derickson, R. J. Helkey, A. Mar, J. R. Karin, J. G. Wasserbauer,
and J. E. Bowers, “Short pulse generation using multisegment modelocked semiconductor lasers,” IEEE J. Quantum Electron., vol. 28, no.
10, pp. 2186–2202, Oct. 1992.
[11] C. T. A. Brown, M. A. Cataluna, A. A. Lagatsky, E. U. Rafailov, M.
B. Agate, C. G. Leburn, and W. Sibbett, “Compact laser-diode-based
femtosecond sources,” New J. Phys., vol. 6, p. 175, 2004.
[12] J. P. Tourrenc, A. Akrout, K. Merghem, A. Martinez, F. Lelarge, A.
Shen, G. H. Duan, and A. Ramdane, “Experimental investigation of
the timing jitter in self-pulsating quantum-dash lasers operating at 1.55
m,” Opt. Express, vol. 16, pp. 17706–17713, 2008.
[13] M. G. Thompson, A. R. Rae, R. V. Penty, I. H. White, D. Larson, K.
Yvind, J. Hvam, A. R. Kovsh, S. Mikhrin, D. A. Livshits, and I. Krestnikov, “Monolithic hybrid and passive mode-locked 40 GHz quantum
dot laser diodes,” in Proc. ECOC, Cannes, France, 2006.
[14] S. Gee, S. Ozharar, F. Quinlan, J. J. Plant, P. W. Juodawlkis, and P.
J. Delfyett, “Self-stabilization of an actively mode-locked semiconductor-based fiber-ring laser for ultralow jitter,” IEEE Photon. Technol.
Lett., vol. 19, no. 7, pp. 498–500, Apr. 1, 2007.
[15] F. Quinlan, C. Williams, S. Ozharar, S. Gee, and P. J. Delfyett, “Selfstabilization of the optical frequencies and the pulse repetition rate in a
coupled optoelectronic oscillator,” J. Lightw. Technol., vol. 26, no. 15,
pp. 2571–2577, Aug. 1, 2008.
[16] F. X. Kärtner and U. Keller, “Stabilization of solitonlike pulses with a
slow saturable absorber,” Opt. Lett., vol. 20, pp. 16–18, 1995.
[17] J. Shmulovich, A. J. Bruce, G. Lenz, P. B. Hansen, T. N. Nielsen, D.
J. Muehlner, G. A. Bogert, I. Brener, E. J. Laskowski, A. Paunescu, I.
Ryazansky, D. C. Jacobson, and A. E. White, “Integrated planar waveguide amplifier with 15 dB net gain at 1550 nm,” in Proc. OFC/IOOC,
San Diego, CA, 1999.
[18] J. Kim, J. Chen, J. Cox, and F. X. Kärtner, “Attosecond-resolution
timing jitter characterization of free-running mode-locked lasers,” Opt.
Lett., vol. 32, pp. 3519–3521, 2007.
Authorized licensed use limited to: MIT Libraries. Downloaded on November 23, 2009 at 17:02 from IEEE Xplore. Restrictions apply.