IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 13, NO. 10, OCTOBER 2001

1091

168-Gb/s All-Optical Wavelength Conversion With aSymmetric-Mach–Zehnder-Type Switch
Shigeru Nakamura, Yoshiyasu Ueno, and Kazuhito Tajima

Abstract—Error-free all-optical wavelength conversion at168 Gb/s, which is the highest repetition rate ever reported, hasbeen achieved by using a Symmetric-Mach–Zehnder (SMZ)-typeswitch. Low-power-penalty 84-Gb/s operation is also demonstrated. The push–pull switching mechanism of the SMZ switch
enables such ultrafast operation based on cross-phase modulation associated with the carrier depletion in a semiconductor
optical amplifier. The configuration of the Delayed-Interference
Signal-wavelength Converter, which is a simplified variant of theSMZ switch, is used in this experiment.
Index Terms—All-optical switch, interferometer, nonlinear optics, semiconductor optical amplifier, ultrafast, wavelength conversion.

I. INTRODUCTION

THE SCHEME of ultrafast optical networks based on
optical-time-division multiplexing (OTDM) technology
attracts much attention in terms of not only high capacity but
also high flexibility. In such networks, the expected bit rates
per wavelength channel are over 100 Gb/s, and thus various
signal processing at these ultrahigh bit rates will be done in
the optical domain. In achieving such ultrafast optical signal
processing, the Symmetric-Mach–Zehnder (SMZ) all-opticalswitch family, including the original SMZ switch [1], thePolarization-Discriminating SMZ (PD-SMZ) switch [2], andthe Delayed-Interference Signal-wavelength Converter (DISC)[3]–[5] is quite promising. These all-optical switches use
essentially the same mechanism to enable high-speed andhigh-efficiency switching. Although switching is based on a
refractive index change induced by a carrier density change ina semiconductor, the push–pull switching mechanism cancels
out the effect associated with slow relaxation of the induced
refractive index change. The applicability of the SMZ switch
family to ultrafast demultiplexing (DEMUX) has alreadybeen verified in various experiments. We have demonstrated
200-fs switching [6], showing the capability of over 1-Tb/s
DEMUX. Recently, error-free 168-Gb/s DEMUX with thehybrid-integrated SMZ (HI-SMZ) switch [7] has also been
demonstrated. However, to utilize the SMZ switches for awider range of applications such as logic, regeneration [8],or wavelength conversion [9], operation excited by higher
repetition data-modulated optical pulses is required on top ofManuscript received January 18, 2001; revised June 18, 2001. This work wassupported in part by the Femtosecond Technology Project under the management of the Femtosecond Technology Research Association supported by theNew Energy and Industrial Technology Development Organization.
The authors are with System Devices and Fundamental Research, NEC Corporation, 305-8501 Ibaraki, Japan (e-mail: s-nakamura@dy.jp.nec.com).
Publisher Item Identifier S 1041-1135(01)08074-0.

being ultrafast. To date, 100-Gb/s wavelength conversion [10]and 80-Gb/s pulse regeneration [11] have been shown by using
the push–pull switching mechanism of the SMZ all-opticalswitch incorporating semiconductor optical amplifiers (SOAs).
In such operation, the pattern effect due to high repetition
data-modulated signal pulses is suppressed by injecting unmodulated continuous-wave (CW) light or clock pulses into
SOAs at relatively high average power [4], [12]. Several
experiments [10] exhibiting ultrafast all-optical wavelength
conversion accompanied the conversion from return-to-zero(RZ) to nonreturn-to-zero (NRZ) and/or logic inversion.
Although such format conversion is also useful depending onapplications, unchanging pulse duration and noninverting logic
would be generally required.
Here, we report on the first error-free 168-Gb/s all-optical
wavelength conversion by using the DISC. Low-power-penalty
84-Gb/s operation is also demonstrated. The results confirm that
the SMZ-type switches can be driven by data-modulated signalpulses at a bit rate of 160 Gb/s, which is considered to be thebit rate for the first-generation OTDM system. The wavelength
converter operates without logic inversion and keeps the pulse
duration almost unchanged from input to output.
II. EXPERIMENTAL SETUP
The experimental setup for 168-Gb/s wavelength conversion
is shown schematically in Fig. 1. The DISC consists of a nonlinear waveguide and a delay line, as detailed in [3]. An SOA
module was used as the nonlinear waveguide. In the SOA, signal
pulses deplete carriers, and thus give cross-phase modulation toCW light at another wavelength. At the delay line, the two temporally displaced components of the phase-modulated CW lightinterfere, forming a pulse output. In this experiment, the delayline was composed of a calcite, a Babinet–Soleil phase shifter,
and a polarizer. The duration of the output pulse from the DISC
was determined by the birefringence of the calcite and was unrestricted by the slow relaxation of the induced nonlinear phaseshift.
The optical source for the signal pulses was an actively modelocked fiber laser (PriTel, Inc.) that generated 1.0-ps 1564-nm
pulses at a repetition rate of 10.5 GHz. The 168-Gb/s signal
pulses were generated by modulating the output of the fiber
laser with a pseudorandom bit sequence (PRBS) with a lengthand then passively multiplexing the modulated pulses.
of 2To obtain uniformly multiplexed signal pulses, we used twostage fiber-based delay lines and two-stage spatial delay lines,
where the intensities and intervals of multiplexed pulses wereadjustable. These 168-Gb/s signal pulses were injected into the1041–1135/01$10.00 © 2001 IEEE

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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 13, NO. 10, OCTOBER 2001

Fig. 1. Experimental setup. MLFL: Mode-locked fiber laser. MOD:Modulator. PPG: Pulse pattern generator. C: Calcite. BS: Babinet–Soleil phaseshifter. PL: Polarizer.

DISC together with unmodulated 1547-nm CW light generated
by an external-cavity semiconductor laser. The injection current
for the SOA was set to 250 mA. The average powers of the signalpulses and of the CW light at the input of the SOA module were
10 and 16 dBm, respectively. The signal pulse energy coupled tothe SOA was estimated to be 240 fJ. The signal pulse duration at
the input of the DISC was 1.8 ps. The time delay provided by thebirefringence of the calcite was set to 2.0 ps, which determinedthe duration of the output pulse. Thus, the pulse duration waskept almost unchanged before and after wavelength conversion.
Also note that logic was not inverted in this wavelength conversion. The 168-Gb/s output of the DISC was demultiplexed to10.5 Gb/s by the HI-SMZ switch [7] to measure eye diagramsand bit error rates (BERs).

Fig. 2. (a) Streak camera trace for DISC output. Inset is a trace measured at ahigher time resolution. (b) Auto-correlater trace for DISC output. The full-widthat half-maximum of the trace is 2.9 ps, indicating a duration of 1.9 ps when thesech pulse shape is assumed.

Fig. 3. Eye diagram for DISC output after DEMUX.

III. RESULTS AND DISCUSSION
Fig. 2(a) shows the 168-Gb/s output waveform of the DISC
measured by a streak camera. The trace was recorded as the accumulation of many PRBS pulses, and thus was observed asa regular pulse sequence. The trace indicates good uniformity
for all 16 multiplexed channels. From a trace measured with ahigher time resolution ( 1.3 ps), shown in the inset of Fig. 2(a),the extinction ratio was estimated to be more than 10 dB. An
output pulse duration of 1.9 ps was confirmed by the autocorrelation measurement as shown in Fig. 2(b).
Fig. 3 is the eye diagram for the 168-Gb/s output of theDISC measured after DEMUX. A clear eye opening has beenachieved. Fig. 4(a) is the result of BER measurement, showing
error-free 168-Gb/s wavelength conversion. All 16 multiplexedchannels showed similar BER performance. For comparison,a BER measurement result for error-free 84-Gb/s operationis also shown in Fig. 4(b). (In 84-Gb/s operation, a pattern
was used to drive the 10.5-Gb/s modulator
length of 2Fig. 4. (a) BER measurement results for 168 Gb/s.
10.5-Gb/s baseline.
MUX DEMUX at 168 Gb/s. MUX wavelength conversion DEMUX
at 168 Gb/s. (b) BER measurement results for 84 Gb/s.
10.5-Gb/s baseline.
MUX wavelength conversion DEMUX at 84 Gb/s.

+++++

before MUX.) The received power was measured for 10.5-Gb/ssignals fed into the preamplifier erbium doped fiber amplifier(EDFA) of the detection system. The power penalty measured
from the 10.5-Gb/s baseline includes the effects of MUX
and DEMUX. For 84-Gb/s operation, the power penalty forNAKAMURA et al.: 168-Gb/s ALL-OPTICAL WAVELENGTH CONVERSION WITH SMZ-TYPE SWITCHMUX and DEMUX was estimated to be about 2.5 dB from aseparate experiment [7]. Thus, the power penalty attributed to84-Gb/s wavelength conversion was as low as 2 dB at a BER of. The power penalty for 168-Gb/s wavelength conversion
increased to about 6 dB. These power penalties for wavelength
conversion are mainly due to a residual pattern effect inducedin the SOA by the data-modulated signal pulses. Because of thepattern effect, we did not reach error-free operation with longer
PRBS lengths. The mechanism of suppressing the pattern effectby using the relatively high average power of the CW lightis interpreted by the effective reduction in the carrier lifetimeof the SOA [4], [12]. For higher repetition, effective carrier
lifetime should be decreased by increasing the power of theCW light. However, the intense CW light consumes the gain ofthe SOA, and thus reduces the refractive index change inducedby the signal pulses. Thus, minimizing the pattern effect andmaximizing the refractive index change should be balanced ata high repetition rate. We believe that this tradeoff is overcome
by developing SOAs in which higher current injection isenabled. Although the observed performance depended on thepolarization of the input signal pulses at present, the operation
can be optimized to be polarization-independent in principle.
IV. CONCLUSION
We have achieved error-free all-optical wavelength conversion at 168 Gb/s, which is the highest repetition rate everreported, with the Delayed Interference Signal-wavelength
Converter. Low-power-penalty 84-Gb/s operation has also
been achieved. We believe that the present operation is more
complete than earlier 100-Gb/s-level experiments because thepulse duration is kept nearly unchanged, logic is noninverted,and high extinction ratio output is obtained. The results confirm
that the Symmetric-Mach–Zehnder-type all-optical switches
can be driven by data-modulated 160-Gb/s signal pulses andare applicable to various optical signal processing in future
OTDM systems.

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