Integrated 2 GHz femtosecond laser based on a planar Erdoped lightwave circuit
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OSA / CLEO/QELS 2010
a565_1.pdf
CFE5.pdf
Integrated 2 GHz femtosecond laser based on
a planar Er-doped lightwave circuit
Hyunil Byun1, Dominik Pudo1, Sergey Frolov2, Amir Hanjani2, Joseph Shmulovich2,
Erich P. Ippen1, and Franz X. Kärtner1
1
Department of Electrical Engineering and Computer Science, and Research Laboratory of Electronics,
Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA
Tel: +1-617-253-8302, Fax: +1-617-253-9611, Email: hibyun@mit.edu
2
CyOptics, 600 Corporate Court, South Plainfield, NJ 07080, USA
Abstract: An integrated passively mode-locked 2-GHz waveguide laser generating 285-fs pulses is
demonstrated. It is based on a 500-MHz repetition rate laser integrated together with a pulse
interleaver on a 45×50 mm silica waveguide chip.
©2010 Optical Society of America
OCIS codes: (140.3280) Laser amplifiers, (140.3500) Lasers, erbium, (320.7090) Ultrafast lasers
1. Introduction
High repetition rate 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 (P-APM) [3] and/or saturable Bragg reflector (SBR) mode-locking [4,5]
have been used with fiber and waveguide lasers, with the latter leading to more compact cavities and fewer required
components. Recent experimental demonstrations of high repetition rate fiber lasers based on SBR mode-locking
achieved fundamental repetition rates as high as 3 GHz [6] and a timing jitter as low as 20 fs in the frequency range
[1 kHz, 10 MHz] [7]. Nevertheless, integrated versions of such lasers have the additional benefits of being massproducible with reduced footprint, and increased stability. Femtosecond operation of such lasers [8] was achieved
recently using planar silica waveguide technology without free-space optics between the SBR and the erbium-doped
waveguide (EDW), producing 440-fs pulses at a repetition rate of 394 MHz. In this paper, based on the same
waveguide technology, we demonstrate an integrated chip comprising a 500-MHz femtosecond laser and an
interleaver multiplying the repetition rate to 2 GHz. The 500-MHz laser generates 285-fs pulses (inferred by optical
spectrum and soliton characteristics) with an average output power of 0.4 mW for a pump power of 175 mW. The
pulse repetition rate is multiplied to 2 GHz using a two-stage interleaver achieving 15 dB sideband suppression.
2. Experimental results and discussion
The experimental setup is depicted in Fig. 1. The waveguide chip contains three sub-components; a laser cavity to
generate femtosecond pulses at a fundamental repetition rate of 500 MHz, a pulse interleaver to multiply the
repetition rate to 2 GHz, and an amplifier. The laser cavity consists of a 5.0-cm-long erbium doped waveguide with a
group-velocity dispersion of 60 fs2/mm. A 14.8 cm section of phosphorous-doped (P-doped) silica waveguide with a
dispersion of -24 fs2/mm is used to obtain a net anomalous intracavity dispersion and enable soliton mode-locking
[9]. 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 SBR is a commercial unit (Batop) with 14% modulation depth, a 2 ps recovery time, and a
saturation fluence of 25 J/cm2. Pump power is provided by an external 980 nm laser diode coupled into the
waveguide chip. The laser was operated with 175 mW of cavity-coupled pump power; the intracavity signal power
was measured to be 4 mW, corresponding to a 10 pJ intracavity pulse energy. The laser output is split by a 3-dB
coupler , to feed the interleaver and a separate output port.
a)
500 MHz laser SBR
14.8 cm
passive
5.0cm
gain
loop
10% out
b)
pump
50:50
SBR
500 MHz output
Er- WG (laser)
100
2000
90
Er- WG (amp)
50:50
50:50
50:50
0
80
-2000
1520 1540 1560 1580
Wavelength (nm)
70
Pump and
output
15 cm
2 GHz output
1cm
Fig. 1. 500 MHz / 2 GHz waveguide laser: a) chip schematic with dispersion and reflectance data of the SBR and b) experimental setup.
978-1-55752-890-2/10/$26.00 ©2010 IEEE
OSA / CLEO/QELS 2010
a565_1.pdf
CFE5.pdf
Each stage of the interleaver splits the input beam into two paths, delays one of them by the corresponding half
period of the input pulse train, and combines the original pulse train with its delayed version, such that the repetition
rate is doubled. Two stages of the pulse interleaver convert the repetition rate from 500 MHz to 2 GHz. The
amplifier recovers the optical power lost through pulse interleaving and further increases the output power of the
overall integrated system.
-40
-40
-50
-50
-40
-50
-60
-70
-80
-60
-70
intensity (dBm)
-30
intensity (dBm)
intensity (dBm)
-20
1530 1540 1550 1560 1570
wavelength (nm)
-90
-60
-70
-80
0.5
1
1.5
2
2.5
frequency (GHz)
-90
0.5
1
1.5
2
2.5
frequency (GHz)
Fig. 2. Measurement traces: a) optical spectrum, b) RF spectrum at the 500 MHz output port, and c) RF spectrum at the 2 GHz output port. Both
RF spectra are measured over 3 GHz span with 10 MHz resolution.
Figure 2 depicts the measurement results obtained so far. The 8.8 nm full-width half-maximum (FWHM) optical
bandwidth enables 285 fs duration transform-limited pulses. The laser typically operates at 1560 nm as in [8], but, at
that wavelength, the SBR is positively dispersive with +1200 fs2. This is sufficient positive dispersion to make the
total cavity dispersion positive and disable soliton mode-locking, because the integrated laser cavity by itself is
negatively dispersive with only -1100 fs2 per round trip. In order to achieve net negative dispersion, we slightly
adjusted the butt-coupling losses between the SBR and the waveguide chip which shifted the operating wavelength
to 1545 nm, where the SBR has negative dispersive of -1500 fs2. As a consequence, stable soliton mode-locking was
obtained although the increased cavtiy losses significantly reduced the available output power. In the future, positive
dispersion of the erbium doped waveguide can be reduced and the negative dispersion of the P-doped waveguide can
be increased by increasing the waveguide height. Then, the negative dispersion of the laser cavity itself could be as
large as -4000 fs2 per round trip so that even the positive dispersion of +1200 fs2 the SBR at 1560 nm would not
interfere with soliton formation.
The laser is self-starting. As the pump power is increased, the laser first operates in a mode-locked Q-switching
state before transitioning to a continuous-wave soliton mode-locked state at a pump power of 150 mW. The RF
spectrum at the 500-MHz output port indicates a stable pulse train at a repetition rate of frep=500.5 MHz. The first
and second stages of the pulse interleaver suppress the sidebands at the 1 and 2 GHz harmonic by more than 30 dB
and 15 dB, respectively. The incomplete suppression can be attributed to mismatched delay lengths and/or deviation
from ideal coupling ratios of 50:50. Simulations indicate that a coupling ratio deviation to 40:60 would account for
the measured amplitude of sidebands. This issue can be easily improved by adding tuning capabilities for both the
delay and the coupling ratio.
3. Conclusion
We demonstrated a 2 GHz optical pulse train from an integrated waveguide chip that consists of a mode-locked
femtosecond waveguide laser and a repetition rate multiplication device. The laser generates 285-fs pulses at 500
MHz, with the cavity being composed of a 5.0 cm section of erbium-doped alumino-silicate waveguide and a 14.8
cm section of P-doped silica waveguide for dispersion engineering. The repetition rate of 500 MHz is multiplied to 2
GHz with more than 15 dB suppression via integrated pulse interleavers.
4. References
[1] S. T. Cundiff, Nature, 450, pp. 1175-1176, 2007.
[2] J. Kim, M. Park, M. Perrott, and F. Kärtner, Opt. Express, 16, pp.16509-16515, 2008.
[3] H. A. Haus, E. P. Ippen and K. Tamura, IEEE J. Quant. Electron., 30, pp.200-208, 1994.
[4] H. A. Haus, J. Appl. Phys., 46, pp.3049-3058, 1975.
[5] R. Paschotta, U. Keller, Appl. Phys. B, 73, pp. 653-662, 2001.
[6] J. Chen, J. W. Sickler, H. Byun, E. P. Ippen, S. Jiang, and F. X. Kärtner, in Ultrafast Phenomena XIV: Proceedings of the 16th International
Conference (Italy, 2008), page 727-729.
[7] H. Byun, D. Pudo, J. Chen, E. P. Ippen, F. X. Kärtner, Opt. Lett., 33, pp. 2221-2223, 2008.
[8] H. Byun, A. Hanjani, S. Frolov, E. P. Ippen, D. Pudo, J. Shmulovich, F. X. Kärtner, IEEE Photon. Technol. Lett., 21, pp.763-765, 2009.
[9] F. X. Kärtner and U. Keller, Opt. Lett., 20, pp.16-18, 1995.