CMG5.pdf
a2691_1.pdf

40-GHz mode-locked pulse generation
with a new scheme of SOA-based pulse generators
Rei Suzuki, Takashi Ohira, Jun Sakaguchi, and Yoshiyasu Ueno
Univ. of Electro-Communications, Dept. of Electronic Engineering, 1-5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japansuzuki@ultrafast.ee.uec.ac.jp

Abstract: We have successfully demonstrated high-quality, 40-GHz, 4.5-ps, 1550-nm, passively-mode-locked
pulse generation, with our original SOA-based pulse-generator scheme.
©2006 Optical Society of America
OCIS codes: (250.5980) Semiconductor optical amplifiers; (320.7080) Ultrafast devices; (060.2330) Fiber optics communications;(190.5970) Semiconductor nonlinear optics

1. Introduction
High-quality short-pulse optical sources with high-repetition rates are key elements for realizing
ultrahigh-speed optical-communication systems that flexibly combine ultrafast TDM and WDM technologies. In
addition to conventional mode-locked laser diodes (MLLD’s), semiconductor-optical- amplifier (SOA) -basedmode-locked ring lasers have recently been proposed because of their possible advantages in the long-term
stability, mass-production, and large-scale integration in the near future [1, 2]. In contrast to the MLLD’s,
however, the mode-locking mechanism that determines the width of the output pulses of those SOA-based lasers
has not been clearly studied, to the authors’ knowledge. On the other hand, the present authors have proposed a
new scheme of SOA-based mode-locked-pulse source that contains a delayed-interference signal-wavelength
converter (DISC [3,4]), and have successfully demonstrated mode-locked 10-GHz, 5-ps pulse generations [5, 6].
It has been shown that the output-pulse’s width (5 ps) in this scheme is strongly determined by an interference
delay time (Δt= 5 ps) inside the DISC [5, 6].
In this work, we demonstrate 40-GHz pulse generation, for the first time to the authors’ knowledge.
2. Experimental set up
Our experimental set up is shown in Fig. 1. The continuous-wave (cw) light at 1549.965nm was injected into
the ring cavity at 0dBm. We adjusted the cw wavelength to the top of the transmission spectrum of the 40-GHz
etalon inside the cavity. The DISC in this pulse-generation-scheme converts the polarization of the re-circulating
pulses from TE to TM (or vice versa) [5]. The DISC in this work consisted of a commercial SOA (Avanex 1901,drive current = 150 mA) and a MZI. The polarization direction of the cw light at the SOA input facet wascarefully aligned to either the TE or TM axis of the SOA, for preventing unexpected nonlinear polarization
rotations. In the SOA, the phase of the cw light is cross-phase-modulated by the round-trip input pulses. In thebirefringent calcite crystal in the asymmetric MZI, then, the phase-modulated cw light is split into fast and slow
components, between which an optical delay time Δt is given. At the end of the MZI, the two-split cw
components interfere with each other, and consequently Δt-long pulses are newly generated from the input cw inprinciple. The polarizer was carefully adjusted to strongly remove the orthogonally polarized old input pulses,
while passing through the newly generated pulses. As a result, unwanted cw lasing of the ring cavity wasstrongly suppressed.
The EDFA inside the ring cavity was temporarily inserted in our present ring cavity, for compensating for ourrelatively high fiber-coupling losses (and the relatively low transmittance of the DISC).

3. Experimental results
Fig. 2 shows the autocorrelation trace of the 40-GHz pulse trains, when we used a 5.0-ps calcite crystal. Theextinction ratio (15.9 dB) of the trace has indicated the high extinction ratio (i.e., high quality) of the generated
40-GHz pulses. The envelope of the autocorrelation trace was in better agreement with a sech2 fit than gaussian
fit. Assuming a sech2 pulse shape, the pulse width was estimated to be 4.5 ps, which corresponded well with theDISC-MZI’s delay time Δt =5.0 ps (the time-bandwidth product was 0.59). Fig. 3 (a) shows the power of thesecond-blue spectral component as a function of the pulse loop gain (which was defined in Ref. 6). The
measured results with the 40-GHz etalon in Fig. 3 has matched well with the pulse-generation mechanism that-1-CMG5.pdf
a2691_1.pdf

the ring cavity starts generating mode-locked pulses when the pulse loop gain approximately exceeds 0 dB [6].
When we exchanged the 5.0-ps calcite crystal in DISC to a 2.2-ps calcite crystal, we observed 40-GHz
mode-locked pulses, again (Fig. 4). The pulse width in this case was estimated to be 2.0ps, and its
time-bandwidth product was 0.53. Figure 4 (b) shows nonlinear phase shifts in the SOA, which weresystematically measured with the experimental method that was described in Ref. 4. The measured phase shift
under the 5-ps condition in Fig. 4(a) was close to that under the 2-ps condition in Fig. 2(a).
4. Conclusion
In this work, we successfully generated high-extinction, nearly-transform-limited 40-GHz, 4.5-ps, 1550-nm
pulses from the DISC-loop-type pulse generator. We also generated 40-GHz, 2-ps pulses. Those pulse widths
have matched well with the interference delay time, Δt of the DISC polarization converter inside the ring cavity.
The origin of the relatively poor extinction ratio in case of the 2-ps pulse generation is under study.
ACKNOWLEDGMENTS

The authors would like to thank Optoquest Co., Ltd. for providing the polarization controllers.
Reference[1] K. Vlachos, et al, IEEE J. Quantum Electronics, 10 (2003) 147.
[2] L. Schares, et al.,J. Lightwave Technol., 22 (2004) 859.
[3] Y. Ueno et al, IEEE Photonics Technol Lett, 10 (1998) 346.
[4] Y. Ueno et al., JOSA B19 (2002) 2573.
[5] Y. Ueno et al., Appl. Phys. Lett. 79 (2001) 2520.

MZISOAQ P BPF

QP

90

calciteQH

etalonΔf = 40GHz

0.5

0.0-37.5 -25 -12.5-10(a)-20-20

SHG Intensity (a. u.)

Intensity (dBm)0(b)-40-60-80(a)1.00.5

0.0-37.5 -25 -12.5-300-40-50-60-100+10Pulse loop gain, G loop (dB)-200 +12.5 +25 +37.5

Delay (ps)1553 1554 1555 1556 1557

Intensity (dBm)

Component power (dBm)

Fig.1. Experimental set up.
SOA : semiconductor optical amplifier,
MZI : Mach-Zehnder interfero meter, BPF : band pass filter,
P : polarizer, H : half wave plate, Q : quarter wave p late.
00(b)-5-10-15-20-15 -10 -50+5 +10 +15Delay (ps)Fig.2. Autocorrelat ion trace of 40-GHz, 4.5-ps pulse train.
Solid curves : measured results,
Dashed curve : calcu lated sech 2 curve,
Dotted curve: calculated Gaussian curve.

EDFADFB-LD

0 +12.5 +25 +37.5+5

Delay (ps)delay

90 H Q BPF
10(a)
1.0

Phase shift [rad/π]10

SHG Intensity (dB)DISC

SHG Intensity (a. u.)[6] R. Suzuki et al., IQEC and CLEO-PR 2005, CFM1-4, Tokyo, July 15, 2005.

Wavelength (nm)10.5(b)0.1
0.05

0.01+1+5+10+50

SOA input pulse enegry (fJ)

Fig.4. Measured results of 40-GHz, 2.0-ps pulse train.
(a) Autocorrelat ion trace. SOA input pulse enegry = 14 fJ.
(b) Nonlinear phase shifts (solid curve), in co mparison withthose of 40-GHz, 4.5-ps pulse train.

(c)-40-60-801553 1554 1555 1556 1557

Wavelength (nm)Fig.3. Th reshold condition for pulse generation.
(a) The dependence of the second blue component power on pulse loop gain.
(b,c) Optical spectra with a pulse loop gain of +11.5 d B (b ), +3.9 dB (c).
Dashed lines are the cw wavelength (1554.925 n m).

-2-