26th European Conference on Optical Communication (ECOC 2000),
Sept. 4th-7th, 2000, Munich, Germany, Vol. 1, pp. 13-14.

168-Gb/s OTDM WAVELENGTH CONVERSIONUSING AN SMZ-TYPE ALL-OPTICAL SWITCH
Y. Ueno, S. Nakamura, H. Hatakeyama, T. Tamanuki, T. Sasaki, and K. Tajima
Optoelectronics and High Frequency Device Research Labs, NEC Corporation
34 Miyukigaoka, Tsukuba, Ibaraki 305-8501, Japan (y-ueno@cb.jp.nec.com)
Abstract:Wavelength conversion at an OTDM bit rate (168 Gb/s) using an SMZ-type all-optical switch is reported.
The pattern effect was suppressed to within ±0.6 dB. The output pulse width was 1.5 ps. Extremely-low-power input
pulses were used.
Introduction
Recently, semiconductor-based symmetric-MachZehnder (SMZ)-type all-optical switches (Fig. 1, /1-3/)
have been examined for possible use in optical-timedivision-multiplexed (OTDM) communication systems
because of their femtosecond-class ultrafast response times/4/. This response is caused by an ultrafast carrier-induced
nonlinear refractive-index change in the semiconductor.
Any effects associated with slow carrier recovery (around100 ps) are masked by the time-differential interference ofa phase-shifted lightwave. The time of light pulse
propagation through the semiconductor waveguide (2-10ps) does not limit the response, either. One application ofSMZ-type switches is an all-optical demultiplexer. Errorfree demultiplexing from 168-Gb/s pseudo-random signals
to 10.5-Gb/s signals has already been achieved /5/ (wherethe switch worked at a repetition rate of 10.5 GHz).
SMZ switches have potentials to play a broad rangeof roles, such as wavelength conversion, 2R regeneration,
3R regeneration, and logic gates, in OTDM systems. To
make these applications usable for 160-Gb/s systems,
however, the switches must operate at 160 Gb/s because
the switches for these applications are controlled by input
signal pulses, not divided-clock pulses (Table 1).
Moreover, the signal-pattern effect must be suppressed atthat ultrahigh frequency, by increasing either the input
clock-pulse intensity or the input continuous-wave (CW)
light intensity.
Another important factor in the switch function
design is synchronization. One design for wavelength
conversion and regeneration is to gate each λ2 input clock
pulse, being controlled by each λ1 input signal pulse. Thegate width should be narrow enough (4-5 ps for 160 Gb/s)
to distinguish each clock pulse. The other design is to gatea λ2 input CW light, controlled by each λ1 input signal.
The second way has the advantage of not requiring
synchronization. The gate width, however, needs to benarrower (< 2 ps) to form 160-Gb/s output pulses from theCW component. This gate width is comparable to thesecond slowest relaxation time of the semiconductor
carriers (carrier cooling time, 2-3 ps).
Previously-reported repetition frequencies of SMZtype switches are summarized in Table 1. The highest
frequency was 100 Gb/s, where the gate width was reported
to be 5 ps /9/. While the original SMZ switch (/1/, Fig.
1(a)) was used in /5, 6/, a polarization-descriminating SMZ
switch (/2/, sometimes called as a UNI) was used in /8, 9/.
A delayed-interference signal-wavelength converter (DISC/3/, Fig. 1(b)) was used in /7/. These switches share thesame switch mechanism.

This study demonstrates wavelength conversion at
168 Gb/s with using an SMZ-type switch, where 168-Gb/s,1.1-ps, 1-fJ quasi-random pulses control the switch.
Controlled by these pulses, the switch gates co-propagating
CW light and generates 1.5-ps pulses.
Table 1

controlledbysignal pulses

Switch gates CW

Switch gates pulses

Wavelength conv.

Wavelength conv.

(168G, this work)

3R(80G /8/)(40G /6, 7/)

Logic gates

2R(100G /9/)

(not reported yet)controlled

Demultiplexerby(168G-10.5G /5/)

clock pulses

signal pulse

SOACW

SOA(a)
outputfilter

signal pulse(∆t-delayed)signal pulse
CWSOA

phase shifterfilter(b)
output(∆t-delayed)

Figure 1: SMZ-type all-optical switches(a): SMZ switch (configured for wavelength conversion)(b): delayed-interference signal-wavelength converter(DISC)The switch structure and the experimental setup
The DISC structure in Fig. 1(b) was used forwavelength conversion in this study. Two cascadedpolarization-insensitive SOA modules were used for theSOA part of the converter. To concentrate on the highrepetition all-optical performance of the SOA, we built apassive Mach-Zehnder interferometer (MZI) using spatialbeam optics (i.e. a birefringent calcite crystal, a BabinetSoleil phase shifter, beam splitters, and a polarizer /10/). It
should be noted that the DISC can be made polarization-

Among all the 16-bit-word patterns, the ‘11110000
00000000’ pattern in Fig. 2(a) should induce nearly thelargest pattern effect. As shown in Fig. 2(a), the patterneffect was suppressed to within ±0.6 dB. (When the CW
input was weak, we observed pattern effects of over ±3dB). Futhermore, the output pulse heights in Figs. 2(a) and2(b) matched that in Fig. 2(c).
Auto-Correlation Signal ( V )insensitive by using a planar-lightwave-circuit MZI, in amanner similar to an integrated SMZ /5/. The MZI delay
time that determines the all-optical gate width was set to1.2 ps. The MZI phase bias was optimized with a phaseshifter /10/.
168-Gb/s quasi-random pulses were formed bymanually multiplexing 10.5-GHz 1.1-ps 1545-nm pulses.
The 10.5-GHz pulses were generated by a mode-locked
fiber ring laser. The 1560-nm CW input light wasgenerated by an external-cavity semiconductor laser. Eachof the input lights was amplified by an Er-doped fiber
amplifier (EDFA). The DISC output was also amplified by
an EDFA and detected by a streak camera (resolution= 1.11.2 ps) and an auto-correlator. The output power level was
carefully calibrated at the filter output. The total couplingloss from the SOA chip through the filter output was 12dB.

0.30.20.10.0-30-20-100 +10 +20 +30
Delay ( ps )Figure 3: Auto-correlation trace of output pulses(a)100(b)

10050
Time ( ps )

(c)

10050
Time ( ps )100100050
Time ( ps )100050Time ( ps )100050Time ( ps )100

2010010020020100

200

Intensity ( a.u. )50Time ( ps )Intensity ( µW )Intensity ( a.u. )0Intensity ( µW )20

Output Pulses, 1560 nm

Intensity ( µW )Intensity ( a.u. )Input Pulses, 1545 nm

20Finally, we evaluated the output pulse width with anauto-correlator. The width of the three patterns of output
pulses was 1.5 ps. A typical auto-correlation trace for the‘1111000000000000’ output pulses is shown in Fig. 3.
In conclusion, we have demonstrated wavelength
conversion using an SMZ-type all-optical switch at anOTDM bit rate (168-Gb/s), using quasi-random input
pulses. The signal-pattern-induced effect was suppressed towithin ±0.6 dB. The output pulse width determined by theall-optical gating width was measured to be 1.5 ps(comparable to the carrier cooling time). The input signal
power was much lower (1 fJ/pulse, average= 84 µW) thanthat previously reported at 100 Gb/s. Part of this work wasperformed under the management of the Femtosecond
Technology Association supported by the New Energy andIndustrial Technology Development Organization.

10References

0Figure 2: 168-Gb/s wavelength conversion

Experimental results
Several patterns of 168-Gb/s pulses were input to theDISC, as shown in Fig. 2. The optimum input pulse energy
was 1 fJ (100 times smaller than that reported in /9/), whichcorresponds to an average power of 84 µW at 168 Gb/s.
The optimum input CW power to suppress the patterneffect was 300 µW. The optimum MZI phase bias was
1.017π /10/. These operating conditions were carefully kept
unchanged throughout the measurements.
Figure 2 shows a typical set of results. When 168Gb/s repeating ‘1111000000000000’ 1545-nm pulses
controlled the DISC, the DISC gated the 1560-nm CW
light and formed 168-Gb/s ‘1111000000000000’ 1560-nm
pulses, as shown in Fig. 2(a). The wavelength was
successfully converted from 1545 nm to 1560 nm. Similar
168-Gb/s ‘1100110000000000’ pulses were converted in asimilar manner (Fig. 2(b)). The 168-Gb/s continuous pulses
were converted as shown in Fig. 2(c). The peak height nonuniformity of the continuous input pulses was caused byour 1:8 multiplexer, used only for results in Fig. 2(c).

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