Jpn. J. Appl. Phys. Vol. 39 (2000) pp. L 806–L 808
Part 2, No. 8A, 1 August 2000
c°2000
The Japan Society of Applied Physics

Ultrafast 168 GHz 1.5 ps 1 fJ Symmetric-Mach-Zehnder-TypeAll-Optical Semiconductor SwitchYoshiyasu U ENO ∗ , Shigeru NAKAMURA and Kazuhito TAJIMA
Photonic and Wireless Devices Research Laboratories, NEC Corporation, 34 Miyukigaoka, Tsukuba, Ibaraki 305-8501, Japan(Received May 30, 2000; accepted for publication June 12, 2000)

We report on the ultrahigh-repetition all-optical switching of a semiconductor-optical-amplifier-based symmetric-MachZehnder-type switch. Record high 168 GHz, 1.5-ps-wide, 10-dB-extinction switch windows were observed. This repetition
rate is ten times higher than the semiconductor optical amplifier’s cutoff frequency. The control optical pulse energy used wasa record low 1 fJ (average power = 170 µW). Clearly isolated 168 GHz components were also observed in the output pulses’
optical spectrum, which was in contrast to the continuum-like spectrum of the input pulses.
KEYWORDS: repetition, ultrafast, all-optical, symmetric, Mach-Zehnder, semiconductor optical amplifier, switch window, extinction

Ultrafast all-optical semiconductor switches (including
demultiplexers, wavelength converters, regenerators, logicgates, and soliton-preserving modulators) have been intensively studied in an effort to increase the time-divisionmultiplexing (TDM) bit rate of each wavelength-divisionmultiplexed (WDM) channel from 10 to 40 Gbps to 160 Gbps
and even higher (possibly more than 1 Tbps in the future).
Such high-speed TDM rates will not only increase a network’s throughput but also dramatically improve its flexibility. Among these ultrafast switches, the symmetric-MachZehnder (SMZ)-type switch,1, 2) composed of semiconductor
waveguides and an interferometer, has shown a 200 fs switch
response time using relatively low-power 3 pJ 130 fs opticalcontrol pulses.3) The switch worked as an all-optical demultiplexer; it switched one pulse out of a 1.5 THz optical pulse
sequence. In principle, the control pulse energy requirement
can be reduced to less than 100 fJ by operating the semiconductor switch in an optical amplification mode incorporating
semiconductor optical amplifiers (SOAs), instead of in an absorption mode as in ref. 3. The response time of SOA-based
SMZ switches is 0.67–1 ps.4, 5) This response time suggeststhat the SOA-based switch will still be able to demultiplex
Tbps-class optical signals. Demultiplexing of 80 to 168 Gbps
signals has been reported using SOA-based SMZ switches.6, 7)
The SMZ switch can also function as an all-optical
wavelength converter,8, 9) regenerator,10, 11) logic gate,12) andsoliton-preserving modulator.13) For these functions, in contrast to demultiplexing, the switch is required to be not only
ultrafast in response time but in repetition rate as well. Thisis because an SMZ switch used in such applications must
operate at a repetition rate which equals the input signal
rate (160 GHz or higher). The repetition rates of the SMZtype switches reported previously, however, were limited to100 GHz,12) in which a 5 ps switch window was all-opticallyformed by 100 fJ control pulses.
In this work we report on a demonstration of 168-GHzrepetition regular switching. This repetition rate is ten times
higher than the carrier cutoff frequency of the SOA usedin the experiment. The switch all-optically opened 1.5 psswitch windows that have a high extinction. The control
pulse energy used in our experiment was a record low 1 fJ
(average power = 170 µW).
Figure 1 shows our experimental setup. We used a simpli∗ E-mail address: y-ueno@cb.jp.nec.com

fied SMZ switch: a delayed-interference signal-wavelength
converter (DISC).9, 14) The DISC’s switch mechanism is identical to that of the SMZ switch. The DISC is composed ofthree parts: an SOA, a passive Mach-Zehnder (MZ) interferometer, and a band-pass filter. In the DISC switch configuration, each input λ1 pulse controls the switch and all-opticallyforms a switch window with respect to the copropagating λ2continuous-wave (CW) light. The width of the switch window
is determined by the MZ delay time 1t (between the two-split
λ2 components). The transmittance of the switch window isdetermined by the gain and the nonlinear phase shift given tothe λ2 light. Each switch window gates the λ2 light and consequently forms a λ2 pulse at the DISC output. The band-passfilter allows these λ2 pulses to pass but blocks the amplified
λ1 control pulses.
The part of the SOA that induced the ultrafast nonlinear
phase shift was composed of two cascaded bulk-InGaAsP
active-layer SOAs. We used two SOAs in order to increase
the nonlinear phase shift at 168 GHz. Each SOA had an unsaturated gain of 700 and a carrier lifetime of 60 ps. The MZinterferometer was composed of bulky optics (a bi-refringent
calcite crystal, a Babinet-Soleil phase retarder, a quarter-wave
plate, and polarizers). The two ends of the MZ interferometer were lens-coupled to pigtails of the second SOA moduleand the band pass filter module, respectively (note: the entire DISC structure can be integrated on a silica-based planarlightwave circuit5, 7) and consequently be made polarization
insensitive, for practical application). The length of the calcite crystal determined the MZ delay time, which was measured to be 1.2 ps by observing the interferometer fringe spacing (16.8 nm). The phase retarder was calibrated and thenused to optimize the interferometer’s phase bias.14)

Fig. 1. Experimental Setup: 168 GHz 1.0 ps 1545 nm control pulses modulate 1560 nm CW light through all-optical SMZ switch (DISC configuration). SOA (nonlinear element) consists of two cascaded SOA modules.

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Y. U ENO et al.

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The 168 GHz, 1545 nm, 1.0 ps input pulses were generated
by the 16-times multiplexing of the 10.5 GHz pulses from anactively-mode-locked fiber ring laser and preamplified by an
Er-doped fiber amplifier (EDFA). The 1560 nm CW light wasgenerated by an external-cavity laser diode. The switch output was detected by a streak camera and an autocorrelator
after being amplified by another EDFA. We confirmed thatneither EDFA broadened the 1.0 ps pulses.
Figure 2(a) shows the 168 GHz 1545 nm control pulses thatwere input to the switch, as detected by the streak camera.
The nonuniformity of the peak heights originated from the16-times multiplexing. Figure 2(b) shows the 1560 nm component at the SOA output, where the input pulse energy (coupled to the SOA) was 10 fJ and the input CW power was
80 µW. The 1560 nm component indicated cross-gain modulation (XGM) induced by the control pulses through the carrier density oscillation. The extinction ratio of the XGM wassmall, as expected, because the carrier lifetime was 10 times
the control-pulse spacing so that the carrier density recovered
only partially between the control pulse arrivals. The solid
curve in Fig. 2(c) shows the optical spectrum at the SOAoutput. This spectrum shows the amplified 1545 nm pulses,the cross-phase-modulated (XPM) 1560 nm components, thebroadly spread amplified spontaneous emission (ASE) of theSOAs, and the EDFA’s ASE peak at 1536 nm. The dashedcurve in Fig. 2(c) shows the SOA output spectrum with the10 fJ input pulses but without the input CW. The dottedcurve shows the SOA output spectrum without any input light.
Comparisons between these spectra indicate that only the input control pulses strongly depleted the carrier density (CWlight passed through the SOA as a probe). The measured average gain (G t ) with respect to the 1560 nm CW was saturated
to 75 by the 10 fJ control pulses. The G t recovered partiallyto 280 when the control-pulse energy was reduced to 1 fJ.
The shape of the XPM spectrum with 168-GHz-spaced
components at around 1560 nm suggests the magnitude of thenonlinear phase shift given to the 1560 nm CW component.
By accurately reproducing the XPM spectrum shape using ournumerical simulation14) in which the 168 GHz carrier oscilla-tion was taken into account, we estimated the nonlinear phaseshift induced by the 1 fJ control pulses to be 0.31π . Whenthe pulse energy was increased to 10 fJ, the nonlinear phaseincreased only slightly to 0.42π because of the above mentioned carrier density depletion.
Figure 3(a) shows the 168 GHz 1560 nm switch output
pulses controlled by the 10 fJ pulses. The CW input power
was 80 µW. An extinction ratio of 10 dB was obtained after optimizing the MZ interferometer phase bias to 1.14π(with monitoring the 1560 nm XPM spectrum shape14) ). No
1545 nm component leakage was detected, as indicated by
the switch output without the CW input (dashed curve). The
background noise (dashed curve) originated from the ASE of
the EDFAs and the SOAs. The switch output peak intensity
at the band-pass filter output was measured to be 40 µW.
When the control-pulse energy was decreased to 1 fJ andthe phase bias was optimized to be 1.09π , the switch output
peak intensity increased to 55 µW, as shown in Fig. 3(b). The
reason for this peak-intensity increase is that the increase in
the gain contributed more than the decrease in the nonlinear
phase shift. If the coupling losses between optical components are neglected, the intrinsic switch transmittance is 48.
This transmittance coincides approximately with interference
factor sin2 (18NL /2) (18NL : nonlinear phase shift) multiplied by gain G t , as expected. An extinction ratio of 10 dB
was obtained again. The width of the switch output pulses
observed using an autocorrrelator was 1.5 ps.
It should be noted that the switch repetition time (6.0 ps) inFigs. 3(a) and 3(b) was ten times shorter than the carrier lifetime (determined by carrier population relaxation) of 60 ps.
In addition, the switch width was close to the carrier coolingtime of approximately 2 ps.4) Our results revealed that neither
the population relaxation nor the carrier cooling deteriorates
aspects of the SMZ switch performance such as the switch
window shape or its extinction ratio.
The switch in this work was controlled by continuous
pulses only (Fig. 3). This type of operation is sufficient forsoliton-preserving modulators. In contrast, the switch forwavelength conversion and regeneration is controlled by digitally encoded signal pulses. This type of operation could
cause a pattern induced effect. It is well known that the pattern induced effect can be suppressed by increasing the CW
input power (holding-beam technique8, 15) ) because the CW
light effectively accelerates the carrier recovery.
Finally, Fig. 4 shows the dramatic change in the optical
spectral quality of the pulses we observed in our experiments.
The spectrum of the 168 GHz 1.0 ps input pulses shown inFig. 4(a) did not indicate any 168 GHz structures. Because

Fig. 2. Optical components before and after cascaded SOAs (a) 168 GHz
1545 nm control pulses, (b) 1560 nm component at SOA output, (c) optical
spectra at SOA output.

Fig. 3. All-optically switched 1560 nm component at switch output
168 GHz control pulse energies were (a) 10 fJ (average power = 1.7 mW)
and (b) 1 fJ (170 µW).

L 808Y. U ENO et al.

Jpn. J. Appl. Phys. Vol. 39 (2000) Pt. 2, No. 8Aof these types of switches for use in a variety of ultrafast
signal-processing applications (wavelength converters, regeneraters, logic gates, and soliton-preserving modulators) atover 160 Gbps. Furthermore, this switch regenerates the optical spectral quality of pulses, a useful feature for all-opticalcross-connects and transmission of ultrahigh-speed opticalsignals.
Acknowledgements
Fig. 4. Comparison of optical spectrum qualities of 168 GHz pulses (a)1545 nm control pulses, and (b) 1560 nm output pulses (dashed curveshows calculated spectrum of transform-limited 1.2 ps pulses).

there was already spectral instability of the input pulses at
the 10.5 GHz mode-locked laser output, we attributed thecontinuum-like spectrum to the fluctuations of the lasing optical frequency (to an order of 110 GHz = 10.08 nm). In
contrast, the spectrum in Fig. 4(b) of the 168 GHz 1.5 ps
switch output pulses in Fig. 3(b) was composed of clearly
isolated 168-GHz-spaced components. We observed similar
dramatic changes in the spectral quality at switch repetitionsof 10.5 GHz and 42 GHz. These results indicate that the spectral quality of the input pulses was ‘regenerated’ by the SMZ
switch. We attribute this regenerative property to the incoherent nature of the switch mechanism (in contrast to four-wavemixing), as the optical spectrum quality of the switch output was determined by that of the CW input light. Although
the control pulse determines the switch window through thecarrier-density change, only the pulse’s intensity profile affects the carrier-density change. Because the carrier-densitychange occurs due to the incoherent band-filling effect andsince both the nonlinear refractive-index change and the gain
are relatively insensitive to the wavelength (in the range of
one order of 168 GHz), the spectral quality of the control
pulses does not affect that of the switch output pulses.
In summary, the repetition rate limit of an SMZ-type alloptical semiconductor switch was pushed up to 168 GHz.
The switch window width was 1.5 ps and the control
pulse energy was 1 fJ. These results, combined with thewell-established holding-beam technique for suppressing thesignal-pattern-induced effects, support the realistic potential

We thank Takemasa Tamanuki and Hiroshi Hatakeyama
for discussions on SOA structures and performance. Part ofthis work was performed under the management of the Femtosecond Technology Research Association supported by theNew Energy and Industrial Technology Development Organization.

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