Submitted to the 22nd Annual Meeting of IEEE Photonics Society,session “Optical Interconnects and Processing Systems,”
Oct. 4-8, 2009, Belek-Antalya, Turkey.

Ultrafast distortion-free semiconductor-based all-optical gate that contains
optical-spectrum’s phase-and-amplitude synthesizer
Takehiro Nishida1, Satoshi Shinada2 , Naoya Wada2 , Takeru Yamaji1, and Yoshiyasu Ueno1
1

Univ. of Electro-Communications (UEC), Dept. of Electronic Engineering,
1-5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japan
Phone: +81-42-443-5207, E-mail: nishida@ultrafast.ee.uec.ac.jp
2National Institute of Information and Communications Technology (NICT),
4-2-1 Nukui-Kitamachi, Koganei, Tokyo 184-8795, Japan
Abstract: We have experimentally verified that our original all-optical-gate scheme removes waveform distortion that is otherwise strongly induced by such active-material’s second-slowest relaxation component.
1.

Introduction
All-optical wavelength converters based on semiconductor
optical amplifiers (SOAs) are promising devices for futurehigh-capacity optical time division multiplexing-wavelength
division multiplexing (OTDM-WDM) networks, due to theirultrahigh response, low power consumption, and integration
potential 1 . 320-Gb/s wavelength conversion with the delayed-interference (DI) scheme using a detuned optical bandpass filter (OBF) at the SOA output has recently been demonstrated 2 , and 40-Gb/s wavelength conversion has beenachieved by a pulse reformatting optical filter which independently controls both the intensity and phase of spectrum3.
For ultrahigh-speed signal processing above 320 GHz withon-off keying (OOK), pulse width shorter than 1 ps is necessary. It has been reported that under ultra-short control pulses
the gated output waveforms with the DI scheme is distorted
since the SOA gain recovery has second-slowest andfirst-slowest components (Fig. 1) due to carrier temperaturerelaxation and electron-hole recombination respectively 4,5 .
(Further, other ultrafast optical materials tend to contain morethan one time constant, for instance, quantum-dot switches6,
intersubband transition semiconductors7.) Therefore, Ueno etal. has numerically demonstrated a particular optical-spectrum-synthesizer (OSS) design with the expanded DIscheme (Fig. 2), by which the gated waveform distortion has
successfully been removed8.
In this work, we have experimentally verified that the newgate scheme with one of standard SOA’s removes the other-Fig. 1. Measured (solid) and calculated (dashed) SOA gain recoveries of a standard SOA (which differs from the SOA in Sections 2 and3), after it amplified a 1.25-GHz, 2.1-ps, 8-fJ input pulse. The firstand second-slowest recovery time constants were 54 ps and 6.5 ps,respectively.

Fig. 2. The new scheme of our all-optical gate which contains an
optical spectrum’s phase-and-amplitude synthesizer8.

wise very strong waveform distortion. Assuming bit-rates
faster than 160-Gb/s in next research steps, 2.0-ps, 40-GHz
input pulses were used in this work.
2.

Experimental setup
Our experimental setup is shown in Fig. 3. A mode-locked
laser diode (MLLD) generated 2.0-ps 1550-nm optical clock
pulses at a repetition rate of 10-GHz. The 10-GHz pulses train
was multiplexed up to 40-GHz through OTDM-MUX. These40-GHz clock pulses were injected into an SOA module together with 1545-nm cw light generated by a tunable laser-diode. The injection current for the SOA was set to 100
mA. The fiber-to-fiber gain of the SOA was +12 dB. The
first- and second-slowest recovery time constants were measured, respectively, to be 30 and 5 ps. The average powers ofclock pulses and of cw light at the input of SOA module were+2.2 dBm (41 fJ) and –4.9 dBm, respectively. At the outputof SOA, the cross-gain modulated (XGM) andcross-phase-modulated (XPM) cw light inside SOA was se-Fig. 3. Experimental setups of the two alternative all-optical gateschemes.
(a) the conventional delayed-interference (DI) gate.
(b) the optical-spectrum-synthesizer (OSS) gate in Fig. 2.
SOA: semiconductor optical amplifier, OBF: optical band-pass filter,
Q: quarter-wave plate, P: polarizer, OSO: optical sampling oscilloscope, OSA: optical spectrum analyzer, VBS: variable bandwidth
spectrum shaper.

lected by a 5 nm bandwidth optical filter. The optical wavelength converter with the conventional DI scheme consistedof a SOA, an OBF, and a Mach-Zehnder Interferometer(MZI). The MZI contained a birefringent calcite crystal
whose relative delay time (Δt) was 5.0 ps. The optical phasebias between the fast component and the slow component wasadjusted by rotating the Q1 and P1 located after the calcite
In this work, we used a proto-type sample of the variablebandwidth spectrum shaper (VBS9, Optoquest Co., Ltd.), oneof the commercially-available, compact, high-resolution
OSSes. The maximum intensity-extinction ratio was +22 dB,
the maximum phase quantity was 2 π, and the spectral resolution was from 9.7 GHz to 11.8 GHz. We have tried to verify
the validity of the new gate scheme in Ref. 8, by controlling
the VBS basically according to this scheme’s recipe.
3.

Experimental results
Fig. 4 (a) and (b) show the 40-GHz gated output spectrum
and waveform of the MZI. The spectral envelope was close to
the calculated dashed curve of a transform-limited 3.6-ps hyperbolic secant pulse in Fig. 4 (a). The inconsistency of thered-spectrum components between the measured results andcalculated results (dotted) was outstanding compared withthat of the blue-spectrum components. At the present time, it
is not clear what causes this inconsistency. We observed themeasured waveform with strong distortion induced second-slowest relaxation components in Fig. 4 (b). The floor
level noise between pulses was part of the background noiselevel due to amplified spontaneous emission from an EDFAin front of the OSO. Particular shape of the measured waveforms with distortion in Fig. 4 (b) (i.e., there was the mainstrong pulse next to the satellite weak pulse) was approximately consistent with the calculated results.
Fig. 4 (c) and (d) show the 40-GHz gated output spectrum
and waveform of VBS with the intensity and phase profile
based on our original numerical model8. The spectral envelope approximately matched the calculated dashed curve for aFig. 5. Measured (solid) and calculated (dotted and dashed) optical
spectra (a) and waveforms (b) at the gate’s output after empiricallyadjusting the intensity of the first, second, and third red-spectrum
components from those in Fig. 4. (c) and (d).

transform-limited 3.3-ps sech2 pulse in Fig. 3 (c). We observed the satellite pulses between the main pulses of themeasured waveform in Fig. 4 (d).
The waveform and spectrum of the VBS output measured
after empirically adjusting the intensity of the first, second,
and third red-spectrum components from the case of Fig. 4 (c)and (d) are shown in Fig. 5. We observed the output waveform without distortion in Fig. 5 (a). The pulse duration andthe extinction ratio were estimated 5.0-ps and 10 dB, respectively. The spectral envelope approximately matched the calculated dashed curve for a transform-limited 4.4-ps pulse inFig. 5 (b), which suggests that the output pulses contains little
chirping, or we speculated that the pulse width was broadened
due to the limited resolution (< 800 fs) of the OSO we used.
4.

Conclusions
We experimentally verified that the strong waveform distortion induced by the second-slowest relaxation time constant of semiconductor materials in general is clearly removed
by our switching the all-optical gate scheme from the conventional one to the new one. We needed to slightly adjust both
real and imaginary parts of the spectral profile, but the magnitudes of these adjustments were relatively small. The reasonof this need is presently under study between our model andproto-type OSS. Even though relatively short 2-ps pulses withintentionally lower rate (40 GHz) without binary codes wereused in this experimental work, this new scheme will work
for more realistic 160-Gb/s pseudorandom data pulses, aswell, where the ratio of pulse’s width to its spacing ranges
from 0.15 to 0.20.
Acknowledgements
We thank Mr. Takeshi Makino and Mr. Hiroyuki Sumimoto ofNICT for technical supports. We also thank Mr. Sungchol Park andMr. Takuya Yoda of Optoquest, Co., Ltd. for valuable discussion andtechnical supports for the proto-type sample of VBS.

References

Fig. 4. Measured (solid) and calculated (dotted, dashed) optical spectra and waveforms at the output of all-optical gates with 2.0-ps,40-GHz, 41-fJ clock input pulses.
(a), (b): with the DI scheme (i.e., without spectrum synthesis).
(c), (d): with the OSS scheme (i.e., with theoretical designed synthesis scheme).

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