IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 13, NO. 10, OCTOBER 2001 1091
168-Gb/s All-Optical Wavelength Conversion With a
Symmetric-Mach–Zehnder-Type Switch
Shigeru Nakamura, Yoshiyasu Ueno, and Kazuhito Tajima
Abstract—Error-free all-optical wavelength conversion at
168 Gb/s, which is the highest repetition rate ever reported, has
been achieved by using a Symmetric-Mach–Zehnder (SMZ)-type
switch. Low-power-penalty 84-Gb/s operation is also demon-
strated. The push–pull switching mechanism of the SMZ switch
enables such ultrafast operation based on cross-phase modula-
tion 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 the
SMZ switch, is used in this experiment.
Index Terms—All-optical switch, interferometer, nonlinear op-
tics, semiconductor optical amplifier,ultrafast,wavelengthconver-
sion.
I. INTRODUCTION
T
HE 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-optical
switch family, including the original SMZ switch [1], the
Polarization-Discriminating SMZ (PD-SMZ) switch [2], and
the Delayed-Interference Signal-wavelength Converter (DISC)
[3]–[5] is quite promising. These all-optical switches use
essentially the same mechanism to enable high-speed and
high-efficiency switching. Although switching is based on a
refractive index change induced by a carrier density change in
a 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 already
been 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 the
hybrid-integrated SMZ (HI-SMZ) switch [7] has also been
demonstrated. However, to utilize the SMZ switches for a
wider 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 of
Manuscript received January 18, 2001; revised June 18, 2001. This work was
supported in part by the Femtosecond Technology Project under the manage-
ment of the Femtosecond Technology Research Association supported by the
New Energy and Industrial Technology Development Organization.
The authors are with System Devices and Fundamental Research, NEC Cor-
poration, 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-optical
switch incorporating semiconductor optical amplifiers (SOAs).
In such operation, the pattern effect due to high repetition
data-modulated signal pulses is suppressed by injecting un-
modulated 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 on
applications, 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 signal
pulses at a bit rate of 160 Gb/s, which is considered to be the
bit 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. E
XPERIMENTAL SETUP
The experimental setup for 168-Gb/s wavelength conversion
is shown schematically in Fig. 1. The DISC consists of a non-
linear waveguide and a delay line, as detailed in [3]. An SOA
module was usedas the nonlinear waveguide.In the SOA, signal
pulses deplete carriers, and thus give cross-phase modulation to
CW light at another wavelength. At the delay line, the two tem-
porally displaced components of the phase-modulated CW light
interfere, forming a pulse output. In this experiment, the delay
line 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 un-
restricted by the slow relaxation of the induced nonlinear phase
shift.
The optical source for the signal pulses was an activelymode-
locked 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 length
of 2
and then passively multiplexing the modulated pulses.
To obtain uniformly multiplexed signal pulses, we used two-
stage fiber-based delay lines and two-stage spatial delay lines,
where the intensities and intervals of multiplexed pulses were
adjustable. These 168-Gb/s signal pulses were injected into the
1041–1135/01$10.00 © 2001 IEEE
1092 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 phase
shifter. PL: Polarizer.
DISC together with unmodulated 1547-nm CW light generated
by an external-cavity semiconductor laser. The injection current
for the SOAwassetto 250 mA. Theaveragepowersofthesignal
pulses and of the CW light at the input of the SOA module were
10 and 16 dBm, respectively. The signal pulse energy coupled to
the SOA was estimated to be 240 fJ. The signal pulse duration at
the input of theDISC was 1.8 ps. The time delay provided by the
birefringence of the calcite was set to 2.0 ps, which determined
the duration of the output pulse. Thus, the pulse duration was
kept almost unchanged before and after wavelength conversion.
Also note that logic was not inverted in this wavelength conver-
sion. The 168-Gb/s output of the DISC was demultiplexed to
10.5 Gb/s by the HI-SMZ switch [7] to measure eye diagrams
and bit error rates (BERs).
III. R
ESULTS 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 ac-
cumulation of many PRBS pulses, and thus was observed as
a regular pulse sequence. The trace indicates good uniformity
for all 16 multiplexed channels. From a trace measured with a
higher 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 autocorre-
lation measurement as shown in Fig. 2(b).
Fig. 3 is the eye diagram for the 168-Gb/s output of the
DISC measured after DEMUX. A clear eye opening has been
achieved. Fig. 4(a) is the result of BER measurement, showing
error-free 168-Gb/s wavelength conversion. All 16 multiplexed
channels showed similar BER performance. For comparison,
a BER measurement result for error-free 84-Gb/s operation
is also shown in Fig. 4(b). (In 84-Gb/s operation, a pattern
length of 2
was used to drive the 10.5-Gb/s modulator
Fig. 2. (a) Streak camera trace for DISC output. Inset is a trace measured at a
higher timeresolution. (b) Auto-correlater trace for DISC output. The full-width
at half-maximum of the trace is 2.9 ps, indicating a duration of 1.9 ps when the
sech
pulse shape is assumed.
Fig. 3. Eye diagram for DISC output after DEMUX.
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/s
signals 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 SWITCH 1093
MUX and DEMUX was estimated to be about 2.5 dB from a
separate experiment [7]. Thus, the power penalty attributed to
84-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 induced
in the SOA by the data-modulated signal pulses. Because of the
pattern effect, we did not reach error-free operation with longer
PRBS lengths. The mechanism of suppressing the pattern effect
by using the relatively high average power of the CW light
is interpreted by the effective reduction in the carrier lifetime
of the SOA [4], [12]. For higher repetition, effective carrier
lifetime should be decreased by increasing the power of the
CW light. However, the intense CW light consumes the gain of
the SOA, and thus reduces the refractive index change induced
by the signal pulses. Thus, minimizing the pattern effect and
maximizing the refractive index change should be balanced at
a high repetition rate. We believe that this tradeoff is overcome
by developing SOAs in which higher current injection is
enabled. Although the observed performance depended on the
polarization of the input signal pulses at present, the operation
can be optimized to be polarization-independent in principle.
IV. C
ONCLUSION
We have achieved error-free all-optical wavelength con-
version at 168 Gb/s, which is the highest repetition rate ever
reported, 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 the
pulse 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 and
are applicable to various optical signal processing in future
OTDM systems.
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