IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 13, NO. 6, JUNE 2001
629
Suppression of Amplitude Modulation of Pulses
Generated from a Subharmonically Mode-Locked
Semiconductor Laser
H. C. Bao, H. F. Liu, Y. J. Wen, and A. Nirmalathas
Abstract—A feedforward modulation technique is used to suppress amplitude modulation associated with 40-GHz pulses generated from a subharmonically mode-locked semiconductor laser.
It is shown that the amplitude modulation can be suppressed by
as much as 20 dB. The improvement of amplitude modulation is
shown for different subharmonic numbers.
Index Terms—Amplitude modulation, electric-absorption modulator, feedforward modulation technique, pulse generation, semiconductor lasers, subharmonic mode-locking.
I. INTRODUCTION
T
HE GENERATION of ultrashort pulses at very high repetition rates is important for high-speed optical communication systems [1], [2]. Passive mode locking of semiconductor lasers is a simple technique for generating short pulses
at high-frequencies [3]. However, the generated pulses exhibit a
high level of phase noise and are not synchronized to an external clock. Lower phase noise can be realized by using either the hybrid mode-locking or active mode-locking technique,
where an electrical signal corresponding to the laser’s cavity resonance frequency is injected. With these techniques, the upper
frequency is generally limited by the availability of high-frequency drive electronics. Recently, two new techniques for stabilizing passively mode-locked semiconductor lasers have been
developed where electrical or optical pulses with a repetition
rate corresponding to a subharmonic frequency of the laser’s
cavity resonance frequency are used as the injection signals
[4]–[9]. These techniques can reduce the phase noise of the
mode-locked pulses and alleviate the high-frequency constraints
placed on radio-frequency (RF) driving circuits. However, the
pulses generated by these techniques suffer from undesirable
amplitude modulation [5], which limits the application of such
high repetition pulses in optical communications. Recently we
proposed a feedforward modulation technique to suppress the
amplitude modulation associated with subharmonically modelocked pulses and demonstrated generation of 140-GHz pulse
train [9]. In this letter, we present detailed characterization of
the suppression of amplitude modulation using a 40-GHz subharmonically hybrid mode-locked semiconductor laser in both
time domain and frequency domain. We also investigated the effectiveness of this technique at different subharmonic numbers.
Manuscript received January 8, 2001; revised February 23, 2001.
The authors are with the Australian Photonics Cooperative Research Centre,
Photonics Research Laboratory, Department of Electric and Electronic Engineering, The University of Melbourne, Parkville, VIC 3010, Australia.
Publisher Item Identifier S 1041-1135(01)04566-9.
Fig. 1.
Experimental setup for suppression of amplitude modulation.
II. EXPERIMENTAL SETUP
A schematic diagram of the experimental setup is shown in
Fig. 1. A multisection monolithic semiconductor laser with a
saturable absorption (SA), a gain, a phase control (PC) and a
distributed Bragg reflector (DBR) section was used in this study.
When the gain section was biased at a current higher than 80 mA
and the absorber was reversely biased at 0.3 V, the laser generated passively mode-locked pulses at its resonant frequency
of 39.2 GHz with a pulsewidth of 6.5 ps (measured with an autocorrelator). The time bandwidth product was 0.45 which was
close to the transform-limited value. The RF signal from the
synthesizer was divided into two paths. One path provided the
injection signal for driving the laser SA section to synchronize
the mode-locked pulses by the subharmonic mode locking technique. The signal of the other path was applied to an electroabsorption (EA) modulator for further modulation of the modelocked pulses and reduction of the amplitude modulation existing in the pulses. The phase difference between the two RF
paths was adjusted by a phase shifter. The amplitude modulation
in the pulse output in both time domain and frequency domain
was characterized by a streak camera with a resolution of 10 ps
and an RF spectrum analyzer HP 8564E, respectively.
III. EXPERIMENTAL RESULTS
Subharmonic mode locking was realized by modulating the
saturable absorber section at a subharmonic of the laser’s res. As the power of the electrical signal
onance frequency
was increased the phase noise of mode-locked pulses was reduced, however, the amplitude modulation was also increased
1041–1135/01$10.00 © 2001 IEEE
Authorized licensed use limited to: SWINBURNE UNIV OF TECHNOLOGY. Downloaded on June 22, 2009 at 21:29 from IEEE Xplore. Restrictions apply.
630
IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 13, NO. 6, JUNE 2001
Fig. 2. The fourth subharmonic mode-locked pulses without feedforward modulation technique. (a) Streak camera trace of the mode-locked pulse train. (b) RF
spectrum of mode-locked pulse train.
Fig. 3. The fourth subharmonic mode-locked pulses with feedforward modulation technique. (a) Streak camera trace of the mode-locked pulse train. (b) The RF
spectrum of the mode-locked pulse train.
[5]. When the phase noise of the signal generated by fourth sub4) decreased from 57.33 dBc/Hz
harmonic mode locking (
at 10-kHz offset frequency to 90 dBc/Hz at 10-kHz offset frequency by increasing the power of the electrical signal, the amplitude modulation of pulses became significant, which can be
clearly seen from the streak camera trace shown in Fig. 2(a). The
RF spectrum of the mode-locked pulses is shown in Fig. 2(b).
If we define the amount of amplitude modulation as the ratio
to that at
of spectral intensity at modulation frequency
laser resonant frequency , then the amplitude modulation of
the fourth subharmonic was 9 dB.
This undesirable amplitude modulation can be reduced by
using an EA modulator to further modulate the mode-locked
pulse output at the same frequency as that of the injected signal
. The subharmonically mode-locked pulses contain an
. The
RF signal component at the injection frequency
component provided by the EA modulator can be adjusted to
be antiphase ( ) to that component with the help of a phase
shifter since the two components are provided by the same RF
synthesizer. Therefore, the two components will cancel each
other. Fig. 3(b) shows the measured RF spectrum of the output
after the pulses passing through the EA modulator. Compared
was reduced by 26 dB.
with Fig. 2(b), the component at
Therefore, after the subharmonically mode-locked pulses
passing through the EA modulator the amplitude modulation
was decreased to 35 dB. The suppression of amplitude modulation can also be seen from the pulse waveform measurement
in the time domain shown in Fig. 3(a). In addition, the other
,
shown in Fig. 3(b) were also
two components at
suppressed 9 and 20 dB respectively. On the other hand, the
intensity at remained nearly unchanged. Therefore, this technique effectively suppresses the RF power of the subharmonic
components that contribute to the amplitude modulation, but
does not affect the RF power of the component at , which is
very desirable for the generation of pulses with low amplitude
modulation.
The improvement of amplitude modulation suppression was
3
investigated for different subharmonic numbers, from
7. Because the laser has better frequency response at
to
lower frequencies, the amplitude modulation of subharmonically mode-locked pulse outputs was larger at higher subharmonic numbers. The open circles in Fig. 4 show the results of
measured amplitude modulation of the laser when the RF power
injected into the SA section was fixed at 23.3 dBm. As can
be seen, the level of amplitude modulation was increased from
20 dB at
3 dB to 7 dB at
7. However, after the
output passing through the EA modulator, an improvement of
over 20 dB in amplitude modulation was achieved at all subharmonic numbers as shown by the open squares in Fig. 4. In
particular, under fifth subharmonic mode locking the amplitude
Authorized licensed use limited to: SWINBURNE UNIV OF TECHNOLOGY. Downloaded on June 22, 2009 at 21:29 from IEEE Xplore. Restrictions apply.
BAO et al.: SUPPRESSION OF AMPLITUDE MODULATION OF PULSES
631
mode-locking, when used in conjunction with the feedforward
modulation technique, can generate high repetition rate pulses
with clear reduced amplitude modulation characteristics.
REFERENCES
[1] K. Y. Lau, “Narrow-band modulation of semiconductor lasers at
millimeter wave frequencies ( 100 GHz) by mode locking,” IEEE J.
Quantum Electron., vol. 26, pp. 250–261, Feb. 1990.
[2] Y. K. Chen and M. C. Wu, “Monolithic colliding-pulse mode-locked
quantum well laser,” IEEE J. Quantum Electron., vol. 28, pp.
2176–2185, Oct. 1992.
[3] S. Arahira, Y. Matsui, and Y. Ogawa, “Mode-locking at very high repetition rates more than terahertz in passively mode-locked distributedBragg-reflector laser diodes,” IEEE J. Quantum Electron., vol. 32, pp.
1211–1224, July 1996.
[4] T. Hoshida, H. F. Liu, M. R. H. Daza, M. Tsuchiya, T. Kamiya, and Y.
Ogawa, “Generation of 33 GHz stable pulse trains by subharmonic electrical modulation of a monolithic passively mode-locked semiconductor
laser,” Electron. Lett., vol. 32, pp. 572–573, 1996.
[5] A. T. Hoshida, H. F. Liu, M. Tsuchiya, Y. Ogawa, and T. Kamiya, “Extremely low-amplitude modulation in a subharmonically hybrid modelocked monolithic semiconductor laser,” IEEE Photon. Technol. Lett.,
vol. 8, pp. 1160–1162, Sept. 1996.
[6] X. Wang, H. Yokoyama, and T. Shimizu, “Synchronized harmonic frequency mode-locking with laser diodes through optical pulse train injection,” IEEE Photon. Technol. Lett., vol. 8, pp. 617–619, May 1996.
[7] A. Nirmalathas, H. F. Liu, Z. Ahmed, D. Novak, and Y. Ogawa, “Subharmonic synchronous and hybrid mode-locking of a monolithic DBR
laser operating at millimeter-wave frequencies,” IEEE Photon. Technol.
Lett., vol. 9, pp. 434–436, Apr. 1997.
[8] S. Arahira, S. Kutsuzawa, Y. Matsui, D. Kunimatsu, and Y. Ogawa,
“Generation of synchronized subterahertz optical pulse trains by repetition-frequency multiplication of a subharmonic synchronous modelocked semiconductor laser diode using fiber dispersion,” IEEE Photon.
Technol. Lett., vol. 10, pp. 209–211, Feb. 1998.
[9] Y. J. Wen, H. F. Liu, D. Novak, and A. Nirmalthas, “Generation of
140 GHz pulses by subharmonic synchronous mode-locking of a FabryPerot semiconductor laser with suppressed amplitude modulation,” in
Proc. IEEE LEOS 2000 Annu. Meeting, Puerto Rico, Nov. 2000, Postdeadline paper PD 1.3.
>
Fig. 4. Amplitude modulation of subharmonic mode-locked pulses with and
without modulation by EA modulator. : Without modulation by EA modulator.
: With modulation by EA modulator.
modulation was reduced by as large as 30 dB. Under seventh
subharmonic mode locking, although the amplitude modulation
was as large as 7 dB, it was suppressed to 16 dB. The effective
suppression of the amplitude modulation by using the feedforward modulation technique was significant for all subharmonic
numbers.
IV. CONCLUSION
The amplitude modulation of subharmonically mode-locked
pulses has been greatly reduced by utilizing the feedforward
modulation technique. For mode-locked pulses at subharmonic
3 dB to
7, the amplitude modulanumbers from
tion has been suppressed by as much as 20 dB. Subharmonic
Authorized licensed use limited to: SWINBURNE UNIV OF TECHNOLOGY. Downloaded on June 22, 2009 at 21:29 from IEEE Xplore. Restrictions apply.