KURUMIDA AND YOO: NONLINEAR OPTICAL SIGNAL PROCESSING IN OPTICAL PACKET SWITCHING SYSTEMS 979
header replacement (and possibly signal regeneration). While
the asynchronous, variable-length packet switching relieves the
need for packet synchronization or segmentation processes, the
contention probability is higher in this case, so that more ef-
fective contention resolution schemes are necessary. While the
conventional electronic packet switching used electronic signal
processing in both control and data planes, future OPS sys-
tems can utilize all-optical processing for many such processes
to exploit high-speed, parallelism, and potentially low power
consumption compared to electronic counterparts. In particular,
nonlinear optical (NLO) signal processing can exploit nonlinear
transfer functions essential for achieving packet switching. This
paper reviews and compares the NLO signal processing in OPS
II. SIGNAL PROCESSING NEEDS IN OPTICAL PACKET
As discussed earlier, optical signal processing can reside in
the control plane and the data plane of OPS systems. The control
plane of OPS systems included label (header) processing such as
optical label (header) extraction, lookup, rewriting (reinsertion).
The data plane of OPS systems included optical signal regen-
eration and optical switching in time, space, and wavelength
A. Optical Signal Processing in the Control Plane
Optical labels (headers) should be easily detached from and
attached to the optical data payload in OPS systems using op-
tical techniques in order to avoid high-speed electronic pro-
cessing in the data plane. Hence, the format of the optical la-
bel (header) has important implications for the OPS system
architecture. Once extracted, the label can be all-optically or
electronically recognized and the new label can be generated
based on optical or electronic label processing. Various label
encoding schemes based on serial or parallel processing meth-
ods have been demonstrated . Examples of parallel optical
labeling methods are: subcarrier multiplexing –, wave-
length multiplexing , orthogonal modulations , and op-
tical code based optical labels , , . Serial optical
labeling methods utilize time domain multiplexed (TDM) la-
bels . Koch et al.,  showed high-speed payload envelope
detection and time domain switching utilizing nonlinear optical
transfer functions of lasers and cross-gain modulation (XGM)
semiconductor optical ampliﬁers (SOAs). While optoelectronic
types (O/E-E/O) of label processors have been demonstrated
using serial-to-parallel conversion techniques at 100 Gb/s ,
the advantages of all-optical label processing can potentially
achieve lower power, higher bit rates, and lower latency for
label detection. The main disadvantage of the all-optical label
detection is the scalability in the number of labels it can detect
and process compared to the optoelectronic counterparts sup-
ported by DRAMs that can support larger than 8 GByte label
space. The current research direction is mostly in all-optical
label processing combining a hybrid-conﬁguration for future
photonic routers , . This section discusses signal pro-
Fig. 2. Block diagram of an optoelectronic packet-switching system using
time-to-wavelength conversion (courtesy of ).
cessing methods and the processing speed for OPS systems from
linear and nonlinear all-optical signal processing.
1) Optical Logic Gates in the Control Plane: One of the
most straightforward methods for optical labels is bit serial pro-
cessing combined with electronic logic functions. While serial
label processing requires relatively strict control of the timing
between the label and the payload, it can still support opti-
cal transparency for the data payload. Typically, hybrid opti-
cal signal processing including O/E header processing is uti-
lized. Since the large signal integrated (LSI) circuits do not
support clock speed beyond 10 GHz, they typically employ se-
rializer/deserializer (SERDES) for label recognition functions
while paying the penalty in latency, which strictly depends on
synchronization time between clock and signal.
On the other hand, all-optical label processing methods for
bit serial processing methods can achieve low power and high
bit-rate label processing, for example, by using multistage
switches . Photonic integration circuits (PICs) can greatly
reduce latency and power, and support high-speed serial label
An example of hybrid optical label signal processing is to
combine optical and electronic processing method including all-
optical serial-to-parallel conversion by the time-to-wavelength
mapping method. Fig. 2 shows a schematic diagram of such a
system by Teimoori et al. . A combination of the time-to-
wavelength all-optical processor and the electronics (ADC and
FPGA) forms a label processor within the optoelectronic packet
switching system. In this example, the time-to-wavelength con-
verter converts the time domain label to a parallel bit stream,
which will be detected by the photo detectors (PDs) and the low
voltage differential signaling (LVDS) circuits. There are several
types of label recognition circuits based on optical logic gate
functions in the optoelectronic packet-switch , , .
The optical logic gates combined with serial to parallel con-
version can support rapid decision processes based on the label
An example of such optical logic gates includes cascaded
semiconductor optical ampliﬁer- Mach-Zehnder interferome-
ters (SOA-MZI) . By exploiting the nonlinear transfer func-
tion of the SOA-MZI, one can construct an optical XOR logic
980 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 18, NO. 2, MARCH/APRIL 2012
gate. It is then possible to construct a programmable logic gate
consisting of many such XOR logic gates for label detection
and processing. While the level of photonic integration avail-
able today for such SOA-MZIs is still far below that of the elec-
tronic logic gates, the optical logic gates can conduct high-speed
(>10 Gb/s) label processing for several byte labels.
Another example of optical devices with nonlinear optical
transfer functions to support can support logic functions is an
ultrafast nonlinear interferometer (UNI) , . UNI is a
single-arm interferometer, and can be realized a stable high-
speed logic processor. Because pump and signal path in the
nonlinear material has an identical path, unstable refractive in-
dex changes of the nonlinear device can be eliminated. For
making several bits label recognition, number of UNI logic gate
is required . Again, photonic integration of the UNI circuits
is a must for UNI to be useful in the control plane of OPS
2) Optical Label Pattern Recognition in the Control Plane:
Optical labels using optical code-division-multiple-access
(O-CDMA) techniques support all-optical label encoding, de-
coding, and conversion –. O-CDMA research investi-
gated optical codes applied in various combinations of wave-
length and time domains . Further, O-CDMA can utilize
encoding in the phase or the amplitude domains, and spectral
phase encoding provides greater cardinality compared to the am-
plitude counterpart , . On the other hand, decoding re-
quires nonlinear processing either by using optical or electronic
methods since decoded optical codes will exhibit differing peak
power levels with identical integrated energy . Fig. 3 shows
the conceptual setup for a correlation technique based on ﬁber
Bragg gratings (FBGs) and the nonlinear peak detection using
photodiodes (PDs). Here, the time domain wavelength label en-
coded by three different colors is split into three separate label
detection circuits each including an FBG, a PD, and an opti-
cal circulator. The label detection circuit with the correct color
sequence encoded on the FBG will yield the high peak power
level at the PD. Then the label detection circuit will activate
the corresponding optical gate switch to perform optical packet
switching based on the label content. Instead of using PDs and
electronic circuits, all-optical detection using nonlinear opti-
cal circuits can overcome the latency associated with electronic
signal processing. As an example, parallel optical correlation
in the time domain using an optical code is distinctive way to
realize ultrafast label processing  at the expense of requiring
multiple O-CDMA decoder optical circuits in parallel. Wada et
al.  describes 160-Gb/s OTDM data payload on eight wave-
lengths with optical code-division multiple-access (O-CDMA)
labels detected in FBG-based label. Further, Furukawa et al. 
introduced multiple optical code label processing using arrayed
waveguide grating (AWG) based multiencoder and 10 GEther-
net to 80 Gb/s packet converter. Fig. 4 shows the architecture
and the optical code encoder/decoder.
B. Optical Switching in the Data Plane
Optical switching functions in OPS systems are enabled by
the optical switch fabric and the control interface to the control
Fig. 3. Conceptual setup for a correlation technique based on ﬁber Bragg
Fig. 4. (a) Architecture of OPS system. (b) Multiple optical label processing
(courtesy of ).
unit. In this section, we will consider switching in the space do-
main and consider wavelength conversion and time buffering in
later sections. In particular, a combination of tunable wavelength
conversion and spectral deﬂection can provide space switching
(and wavelength switching). Since optical packet signals are
considered to be in very short durations with typical packet sizes
in the 10 ns∼1 μs range, OPS systems require rapid switching
typically in the nanosecond timescale. While all-optical switch-
ing technologies typically offer high data rate, scalability, and
low latency switching, their characteristics are strongly depen-
dent on the control plane. The control interfaces and the data
plane architecture strongly affect the system performance. The
following shows several examples of switches using linear or
nonlinear optical transfer functions.
1) Linear Optical Switching Methods:
a) Multimode-Interference (MMI) Switches: Optical
switch fabric architectures often involve switching units of
size 1 × 2 or 2 × 2. Takeda et al.  describe all optical
switches and ﬂip-ﬂops based on a multimode-interference
(MMI) bistable laser diode (BLD) with distributed Bragg
reﬂectors (DBRs). Fig. 5 shows the proposed device archi-
tecture. The structure works as a single ﬂip-ﬂop (FF) logic
element. This type of device has wavelength tuning function
KURUMIDA AND YOO: NONLINEAR OPTICAL SIGNAL PROCESSING IN OPTICAL PACKET SWITCHING SYSTEMS 981
Fig. 5. Multimode-interference bistable laser diode with distributed Bragg
Fig. 6. Schematic of 1×5 InP–InGaAsP optical phased-array switch .
by the band-ﬁlling effect and the free-carrier plasma effect
with carrier injections to the DBRs. A large-scale switch can
incorporate them in a PIC by monolithic integration. Based
on this structure, the dynamic FF operation of the device was
investigated. An experimental result shows fast switching time
280 and 228 ps rising and falling times, respectively. The
switching speed has an enough capability for OPS system, and
it has compatibility to the waveguide devices.
b) Waveguide Switches: A semiconductor waveguide
type optical switch has an advantage of high-speed switch-
ing. Sato et al.  proposed an X-shaped optical waveguide
switch which consists of GaAs-based double-heterostructure
ridge waveguides. It operates at 40 Gb/s and achieves less
than 1.4 ns switching time by electronic triggering. The struc-
ture of the switch facilitates the fabrication of the number of
switches on the single wafer. Tanemura et al.  proposed and
demonstrated an integrated optical phased-array switch. Fig. 6
shows schematic of 1 × 5 InP-InGaAsP optical phased-array
switch . Wideband operation in C-band (1520–1580 nm)
and fast switching response below 10 ns are reported. The pro-
posed switch has been utilized for OPS system, 160-Gb/s optical
time-domain-multiplexed packets were dynamically switched to
16 outputs with power penalties of 0.7 dB  implying possible
applications in high-throughput OPS systems.
Fig. 7. Conﬁguration of multiformat OPS .
2) Nonlinear Optical Switching Methods:
a) Arrayed Waveguide Grating Router (AWGR) and
Tunable Wavelength Converters: A combination of tunable
wavelength conversion and wavelength routing devices can pro-
vide space and wavelength switching functions. A typical con-
ﬁguration is to use AWGR together with a tunable wavelength
converter at the input stage and a ﬁxed wavelength converter
at the output stage. However, all-optical wavelength converter
based on an SOA-MZI can support digital transparency. There-
fore, OPS routers with optical-label switching technology have
an opportunity to transparently support multiple data rates. On
the other hand, wavelength conversion technologies based on
parametric nonlinear optical processes can support any data for-
mat and protocols. Kurumida et al.  reported multiformat
OPS using four-wave-mixing (FWM). Fig. 7 shows the diagram
of an OPS router for multiformat switching. It consists of func-
tion blocks, called label extractor (LE) and label rewriter (LR)
for subcarrier multiplexing (SCM) techniques. The key func-
tion blocks for the switching router, are the tunable wavelength
converter (T-WC) and the ﬁxed wavelength converter (F-WC).
These blocks must provide format independent conversion for
multiformat packets. For more wavelength controllability, dual-
pump FWM wavelength conversion based packet switching sys-
tem was also demonstrated . A wavelength control method
would be an important issue including a suppression method of
the pump wavelength. Wavelength conversion techniques will
be discussed in the next section.
b) Ultrafast Nonlinear Interferometer (UNI) Gates: Ul-
trafast nonlinear interferometers (UNI)  can be used as an
all-optical high-speed gate for OPS systems. UNI provides sev-
eral picosecond switching window by utilizing optically induced
gain and index nonlinearities in a semiconductor ampliﬁer. As
mentioned in Section II-2, this type of device can have a stable
logic function, however, conﬁguration requires a relatively strict
control of the timing for gate open/close control. This charac-
teristic is suitable for OTDM technologies, but a gate switching
method is applicable to label/payload (L/P) separation function
in OPS systems. An all-optical L/P separation method using
UNI AND gate with clock recover has been demonstrated .
The switching technique is able to be in principle extendable
to higher rates (>100 Gb/s) with appropriate clock recovery
sub-systems. On the other hand, photonic integration of UNI
gates to a large fabric can face challenges due to polarization
982 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 18, NO. 2, MARCH/APRIL 2012
C. Wavelength Conversion
Wavelength conversion allows ﬂexible signal wavelength
assignments in the network . All-optical wavelength con-
version technologies have a potential advantage over the opto-
electric counterpart in realizing relatively lower packaging costs
and dimensions. This section discusses wavelength conversion
techniques for OPS system.
1) Four-Wave Mixing (FWM): Nonlinear optical wave-
mixing supports a transparent condition in optical networks
preserveing phase and amplitude information simultaneously.
This feature performs format independent wavelength conver-
sion. Conversion efﬁciency of the wavelength in the ﬁber is the
most critical issues for the system designs; therefore, highly
nonlinear ﬁbers (HNLFs) with improvements in the conversion
efﬁciency have been pursued even for relatively short (<100 m)
ﬁber lengths. The switching control requires pump wavelength
tuning based on high-speed tunable lasers or laser diode arrays.
2) Difference-Frequency-Generation (DFG): A second or-
der nonlinear optical interactions between a pump wave and a
signal wave can yield another wave by difference-frequency-
generation (DFG) –. It is capable of chirp-reversal and
multiwavelength conversion. Although FWM can achieve sim-
ilar multichannel conversion, DFG is free from satellite sig-
nals which appear in FWM techniques. While DFG is a vi-
able technique for OPS, it has not been demonstrated for OPS
switching functions but used in label processing –.
While both FWM and DFG require phasematching, DFG re-
quire phasematching between the signal wavelength band and
the pump wavelength band that differ by a factor of two, hence
faces more challenges in fabrication. Quasiphase matched struc-
tures have been demonstrated in both LiNbO3 and AlGaAs
3) Cross-Phase Modulation (XPM): The intensity depen-
dence of the refractive index change makes nonlinear phe-
nomenon known as cross-phase modulation (XPM). It can oc-
cur when two or more optical signals are injected into a device.
Optical data signals traveling through a nonlinear material ex-
periences phase changes by a control light. The XPM effect is
utilized in an interferometer conﬁguration. One of the examples
of the switch is SOA-MZI, which is an all-optical switch using
carrier dynamics of the SOA. Basically, this type of devices
uses the nonlinear refractive index change induced in semicon-
ductors . Using pump and probe lights, wavelength conver-
sion function is accomplished. SOA-MZIs provide 2R optical
regeneration capability . On the other hand, there is a non-
interferometric ultrafast switch based on HNLF. A simple 2 × 2
all-optical switch based on signal polarization rotation induced
in HNLF by XPM has been proposed .
4) Cross-Gain Modulation (XGM): The gain in a SOA satu-
rates as the optical power level increases. This characteristic can
modulate the ampliﬁer gain with an input optical signal. A CW
light at the desired output wavelength is modulated by the gain
variation, so after the SOA it carries the same information as the
intensity modulated input signal. For high-speed data signals,
semiconductors are required relatively higher current values for
rapid carrier recovery.
Fig. 8. System architecture of the time-slot-interchange system .
5) Electro-Absorption Modulator (EAM): Optical nonlin-
earities induced by an EAM have a faster response compare
with an SOA type of device. High-speed wavelength conversion
at 100 Gb/s was demonstrated by a delayed interferometer type
EAM . The absorption recovery time can be less than 10 ps
and the required pulse peak power would be less than 100 mW
in such devices.
D. Optical Buffering
While optical buffering is not an absolutely necessary func-
tion in OPS systems when alternative switching methods in
wavelength or time domains are available to provide contention
resolution and arbitration, optical buffering can provide signif-
icant reductions in packet-loss rates rising from packet con-
tention. In synchronous OPS systems, buffers are required
for synchronization of incoming packets in addition to con-
tention resolution. There are two major conﬁgurations of the
buffers which are called recirculate-loop type and travelling-
wave type , , but a way to get the delay is switch using a
discrete set of ﬁxed delay lines. On the other hand, material reso-
nance type buffers are attractive. We discuss about the buffering
function and the capability for optical switching system from
linear/nonlinear all-optical approaches.
1) Linear Optical Time Buffers:
a) Fiber Delay Lines: Pan et al.  have experimentally
demonstrated the applicability of the wavelength-routed optical
buffer. Fig. 8 shows the buffer architecture, which is called the
time-slot-interchange system. There are two wavelength switch-
ing stage which are called tunable wavelength converter (TWC)
and ﬁxed wavelength converter (FWC). The ﬁrst stage of the
switching, the input packet that is required contention resolu-
tion, is switched to appropriate length of the ﬁber. After this,
FWC can convert to required wavelength for the output. If the
system needs to have higher resolution of the delay, number of
the AWGR port and number of wavelengths are required in the
system. As one more example, a time buffer (TB) has been im-
plemented with an array of ﬁber delay lines in a load-balancing
switching system. Under the 2 × 2 conﬁguration of the system,
the delay lines were fully shared between the two input/and out-
put paths . On the other hand, integration schemes for delay
lines are advancing. A 100 ns delay optical buffer has been re-
alized using silica waveguides . And a compact coiled ﬁber
delay with a phased array switch has been demonstrated .
KURUMIDA AND YOO: NONLINEAR OPTICAL SIGNAL PROCESSING IN OPTICAL PACKET SWITCHING SYSTEMS 983
Fig. 9. Block diagram of the optical packet buffer based on conver-
sion/dispersion technique .
b) Dispersive, Slow Light Devices: A highly dispersive
material has a characteristic of the wavelength dependent delay
that utilizes the chromatic dispersion. This characteristic can be
utilized as an optical buffer. 40 Gb/s optical packet buffer based
on continuously tunable conversion/dispersion technique has
been reported . Fig. 9 shows a conceptual block diagram of
the buffer. In this example, the buffer consists of two functional
paths. In the upper path, the relative delay for the selected packet
signals is adjusted by three wavelength conversion stages for the
conversion/dispersion technique. The lower path is utilized for
the packet detection and the vacant slot control.
On the other hand, slowlight variable optical buffers offer
more ﬂexibility and controllability than ﬁxed-length buffering
approaches. Cascaded stages of tunable silica microresonator
ring works as a variable optical buffer. Fig. 10 shows OPS fabric
architecture with variable optical delays. The conﬁguration of
the buffer is shown in Fig. 10(c). Continuous delays up to 357 ps
with 40 GHz bandwidth for a 32-ring structure and continuous
delays up to 2483 ps for an 8-ring structure variable optical
buffer on the desired wavelength were demonstrated .
2) Nonlinear Optical Time Buffers: Slow-light delay lines
have limited capacity and delay-bandwidth product . Basi-
cally, very slow group velocity always has very low bandwidth
or throughput. There are many applications involving microring
resonators . However, control method of the delay needs to
Electromagnetically induced transparency (EIT) and pho-
tonic crystals (PCs) have a capability for making a variable
optical buffering system. However, even if an ideal slow-light
buffer and an EIT device can make compact physical size, they
have loss-limited buffering capacity . The main challenge
lies in buffering a reasonable number of “bits.” Typical demon-
strations of slow light buffering have been for less than 100
III. FUTURE OPTICAL PACKET SWITCHING SYSTEMS
A. High Spectral Efﬁciency Optical Packets and Optical
Arbitrary Waveform Generation
Future optical packet switching systems expect to include
packets with extremely high spectral efﬁciency. Then it be-
Fig. 10. OPS fabric architecture with variable optical delays for (a) input
queuing and (b) output queuing. (c) An all-pass ﬁlter structure of a vari-
able optical buffer, and (d) simulated transmission spectra of single ring with
100-GHz free-spectral range (FSR) for different power coupling coefﬁcients
Fig. 11. 1.2 Tb/s data generated on a 400 GHz comb bandwidth (3 b/s-Hz).
(a) Optical comb spectrum with amplitude and phase control. (b) Generated
16 QAM data stream measured in the time domain.
comes necessary to generate and process optical packets gen-
erated by advanced modulation format. Optical arbitrary wave-
form generation (OAWG)  has demonstrated its capability
of creating high-speed data (1.2 Tb/s)  with high spectral ef-
ﬁciency (3b/s-Hz) using advanced modulation formats (QPSK,
16 QAM, etc). OAWG generates arbitrary waveforms through
the line-by-line intensity and phase modulations of each individ-
ual spectral line across the entire bandwidth of a coherent optical
frequency comb . Recently, He et al.  demonstrated gen-
eration and detection of optical label switching packets using
OAWG techniques. Fig. 11 shows 1.2 Tb/s data generated on a
400 GHz comb bandwidth (3 b/s-Hz): (a) Optical comb spec-
trum with amplitude and phase control. (b) Generated 16 QAM
data stream measured in the time domain. Fig. 12 illustrates
the label and the payload spectrum. Subsequent label extraction
showed error-free performance for the all-optically extracted
B. All-Optical IQ Wavelength Converters
1) I-Q Wavelength Conversion: As we discussed in Sec-
tion II, wavelength conversion is a key technology to real-
ize scalable photonics networks especially for OPS networks.
984 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 18, NO. 2, MARCH/APRIL 2012
Fig. 12. Label extraction results (target intensity: Blue “x” or blue dashed
line, measured intensity: Blue stem or blue solid line; target phase: Red “x” or
red dashed line; measured phase: Red circle or red solid line). NRZ-OOK: (a)
Packet spectrum. (b) Label spectrum.
Fig. 13. Schematics of single polarization I-Q wavelength conversion by using
an optical hybrid and I-Q wavelength converter.
Modern optical networks adopting advanced modulation for-
mats including differential phase-shift keying (DPSK) and dif-
ferential quadrature phase-shift keying (DQPSK) will beneﬁt
from modulation format transparent all-optical wavelength con-
version. While FWM and DFG support the format transparency,
they failed to provide signal regeneration for advanced modula-
tion formats. We proposed a multiformat wavelength conversion
technique with potential signal regeneration capability, based on
the optical cross modulation in SOAs in interferometric conﬁgu-
ration . In this proposal, two identical MZI-type wavelength
converters in a nested structure with a 90◦
optical hybrid allow
independent wavelength conversion of in-phase (I) and quadra-
ture (Q) components before I-Q combination. Fig. 13 shows the
generalized schematics of single polarization case.
It consists of a 90◦
optical hybrid and an I-Q wavelength
converter. Without loss of generality, a single polarization can
be written as
Si(t) = (Ii + jQ) · exp(jωit) (1)
where the subscript i stands for input, and is the angular fre-
quency of the optical carrier. The phase noise term is dropped
for simplicity. I and Q carry the transmitted information and
can represent any modulation format. A local laser input to the
optical hybrid can be simpliﬁed as
l(t) = exp(jωit + jϕl) (2)
where ϕl is the random laser phase and is a function of t. The
polarization states of the incoming signal and the local laser are
aligned by a polarization controller. I-Q wavelength converter
is composed of a pair of nested MZI with nonlinear devices
Fig. 14. Experimental setup of wavelength conversion for multiple modulation
and an outside MZI. The two inner MZIs are biased at the null
point and the outside one biased at π/2. In Fig. 13, each nonlinear
optical device provides optical cross-modulation and is assumed
to only respond to the intensity of the input optical signal and
have a proper low-pass function to ﬁlter out the unwanted high-
frequency components after the optical hybrid. The probe laser
lwc(t) = exp(jωwc t + jϕwc). (3)
After combining upper side arm and lower side arm of the
MZI, the converted signal can be derived and written as
Swc(t) = (Ii + jQi) · exp(jϕl) · lwc(t). (4)
Thus, the incoming optical signal is successfully converted to
another wavelength with the phase noise term that can be con-
sidered as a wider linewidth laser source at either the transmitter
2) Performance of the I-Q Wavelength Conversion: We have
evaluated I-Q wavelength converter based on 20 Gb/s RZ-DPSK
and 5 Gb/s NRZ-DQPSK signals. Fig. 14 shows the experi-
mental setup of wavelength conversion for multiple modulation
formats. The structure of the I-Q wavelength converter is mag-
niﬁed in the upper side of the block. It is including an integrated
SOA-MZI pair and two external fused ﬁber couplers as an ex-
ternal interferometer. The transmitter includes a laser source at
1545 nm and three modulators, which are conﬁgured to gen-
erate different modulation formats. PRBS 27
is used for the
BER measurement of DQPSK format. The local laser is tuned
to the same wavelength as the transmitter and its polarization is
aligned with the transmitted signal by a polarization controller.
After the optical hybrid, the optical power levels of the local
laser and the transmitted signals are −6 dBm and −11 dBm
for RZ formats, and −9 dBm and −14 dBm for NRZ formats,
respectively. The narrow linewidth (100 kHz) of the local laser
helps to reduce the phase noise in the converted signal as illus-
trated in (4). We drove the four SOAs at 425, 450, 470, and 492
mA during the experiments for optimal performance. The opti-
cal power (5 dBm) of the probe laser at 1540 nm saturates the
KURUMIDA AND YOO: NONLINEAR OPTICAL SIGNAL PROCESSING IN OPTICAL PACKET SWITCHING SYSTEMS 985
Fig. 15. Input dynamic range and wavelength range for I-Q wavelength con-
verter using 10 Gb/s RZ-DPSK signal. (a) Input dynamic range. (b) Input
SOAs, thus provides a wide linear range for the optical cross-
modulation. The converted signal has relatively low power level
(−8 dBm), mainly because we bias the inner SOA-MZIs at their
null points with OSNRs around 35 dB. One more SOA and ﬁlter
was used to suppress the intensity noise generated by the wave-
length converter, which is not necessary if we can ﬁnd a better
way to stabilize the device. For testing the input signal dynamic
range and wavelength range, we used 10 Gb/s RZ-DPSK format
and take the BER measurements at different input wavelengths
and different input power levels (measured right after the BPF
following the EDFA).
This scheme shows at least a 6 dB input dynamic range
[Fig. 15(a)], partially due to the presence of the local laser.
And it supports input signals over a wavelength range of 6 nm
for 10 Gb/s RZ-DPSK [Fig. 15(b)].
IV. SUMMARY AND FUTURE DIRECTIONS
Future optical networks demand high capacity and agile
switching technologies that effectively handle IP trafﬁc. Optical
packet switching handles IP packets with high-utilization rate,
especially when incorporated in optical label switching systems.
Nonlinear optical signal processing is essential in OPS systems
by providing label processing and switching in time, space, and
wavelength domains. Nonlinear optical signal processing typ-
ically provides higher speed, lower latency, and lower power
consumption compared to electronic counterparts. The utility
of nonlinear optical signal processing will be greatly enhanced
when large-scale photonic integration is realized to provide di-
verse signal processing functions. Future optical networks ex-
pect to employ optical label switching systems with OPS ca-
pabilities in support of advanced modulation formats with high
spectral efﬁciency. Optical label processing with OAWG us-
ing diverse modulation formats and I-Q wavelength conversion
with multiformat optical regeneration has been recently demon-
strated. Future OPS systems expect to nonlinear optical signal
processing applied to advanced modulation format data with
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KURUMIDA AND YOO: NONLINEAR OPTICAL SIGNAL PROCESSING IN OPTICAL PACKET SWITCHING SYSTEMS 987
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Junya Kurumida (M’07) received the B.Engr. de-
gree in communication egineering and the M.Engr.
degree in electronics and applied physics from Osaka
Electro-Communication University, Osaka, Japan, in
1996 and 1998, respectively, and the Dr.Engr. degree
in information technology from the Tokyo Institute
of Technology, Yokohama, Japan, in 2007.
From 1998 to 2004, he worked at Fujitsu Ltd.,
Kawasaki, Japan, where he was engaged in research
on and the development of dense wavelength-division
multiplexing optical-transmission systems. He is cur-
rently with the National Institute of Advanced Industrial Science and Technol-
ogy, Tsukuba, Japan. He is also collaborating with the Department of Electrical
and Computer Engineering, at the University of California, Davis.
S. J. Ben Yoo (S’82–M’84–SM’97–F’07) received
the B.S. degree in electrical engineering with distinc-
tion, the M.S. degree in electrical engineering, and the
Ph.D. degree in electrical engineering with minor in
physics, all from Stanford University, Stanford, CA,
in 1984, 1986, and 1991, respectively.
He currently serves as Professor of electrical engi-
neering at the University of California at Davis (UC
Davis) and Director of UC Davis CITRIS (Center
for Information Technology Research in the Inter-
est of Society). His research at UC Davis includes
all-optical switching devices, systems, and networking technologies for the
future generation Internet and computing systems. In particular, he is conduct-
ing research on architectures, systems integration, and network experiments
related to all-optical label switching routers, Tb/s optical arbitrary waveform
generation, and optical interconnect technologies. Prior to joining UC Davis
in 1999, he was a Senior Research Scientist at Bell Communications Research
(Bellcore), leading technical efforts in optical networking research and systems
integration. His research activities at Bellcore included optical-label switch-
ing for the next-generation Internet, power transients in reconﬁgurable opti-
cal networks, wavelength interchanging cross-connects, wavelength converters,
vertical-cavity lasers, and high-speed modulators. He also participated in the ad-
vanced technology demonstration network/multiwavelength optical networking
(ATD/MONET) systems integration, the OC-192 synchronous optical network
(SONET) ring studies, and a number of standardization activities which led to
documentations of Generic Requirements, GR-2918-CORE (1999), GR-2918-
ILR (1999), GR-1377-CORE (1995), and GR-1377-ILR (1995) on dense WDM
and OC-192 systems. Prior to joining Bellcore in 1991, he conducted research
at Stanford University on nonlinear optical processes in quantum wells, a four-
wave-mixing study of relaxation mechanisms, and ultrafast diffusion-driven
photodetectors. During this period, he also conducted research on lifetime mea-
surements of intersubband transitions and on nonlinear optical storage mecha-
nisms at Bell Laboratories and IBM Research Laboratories, respectively.
Prof. Yoo is a Fellow of the IEEE Lasers & Electro-Optics Society (LEOS),
a Fellow of the Optical Society of America (OSA), and a Member of Tau Beta
Pi. He is a recipient of the DARPA Award for Sustained Excellence in 1997, the
Bellcore CEO Award in 1998, and the Mid-Career Research Faculty Award (UC
Davis) in 2004. He is General Co-Chair for Photonics in Switching Conference
2007 and 2010. He also served as an Associate Editor for the IEEE PHOTON-
ICS TECHNOLOGY LETTERS, Guest Editor for the IEEE/OSA JOURNAL
OF LIGHTWAVE TECHNOLOGY (2005), and the IEEE JOURNAL OF SPE-
CIAL TOPICS IN QUANTUM ELECTRONICS (2007).