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Nonlinear Optical Signal Processing in Optical Packet Switching Systems
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Nonlinear Optical Signal Processing in Optical Packet Switching Systems Nonlinear Optical Signal Processing in Optical Packet Switching Systems Document Transcript

  • 978 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 18, NO. 2, MARCH/APRIL 2012 Nonlinear Optical Signal Processing in Optical Packet Switching Systems Junya Kurumida, Member, IEEE, and S. J. Ben Yoo, Fellow, IEEE (Invited Paper) Abstract—This paper discusses nonlinear optical signal process- ing employed in optical packet switching systems. Nonlinear optical signal processing provides optical label (header) recognition, opti- cal switching, wavelength conversion, and time buffering with typi- cally higher capacity, lower latency, and lower power consumption than electronic counterparts. In order to provide diverse signal processing functions, large-scale integration of nonlinear optical signal processing devices is essential. We discuss possible future directions in optical packet switching involving nonlinear optical signal processing of optical packets with advanced data and label modulation formats. Index Terms—Internet, nonlinear optics, optical burst switch- ing, optical circuit switching, optical flow switching, optical packet switching, optical-label switching, photonic switching. I. INTRODUCTION THE FUTURE Internet expects to demand protocol-agile and high capacity networking in support of a vari- ety of applications including 3-D multimedia entertainment, telemedicine, and cloud computing. The Internet Protocol (IP) supports all applications on any physical layer platforms, fol- lowing the hour-glass model [1]. While the generality and het- erogeneity of the ‘‘Internet hourglass” are the critical strengths of the Internet that made it so successful, new multimedia, data services, and cloud computing are driving the needs for high performance, high utilization, and secure networks. A unified networking platform in support of voice, data, and multimedia applications are attractive especially on a high-capacity opti- cal layer. Since IP uses packets as the unit of transport and switching, the capability to switch and transport packets with low latency and high throughput can enhance the performance of the IP network. As IP packets are most naturally accommo- dated in the form of packet switching, optical packet switching may be the technology best suited for the future Internet provid- ing both high-capacity and high-utilization, aggregation [2] of such packets at the edge into bursts, flows, or circuits can fur- ther improve energy efficiency and reduce the complexity of the control plane. In particular, optical-label switching (OLS) [3] Manuscript received February 19, 2011; accepted April 4, 2011. Date of publication July 29, 2011; date of current version March 2, 2012. J. Kurumida is with the National Institute of Advanced Industrial Scienece and Technology, Tsukuba, Ibaraki, 305-8568 Japan (e-mail: j.kurumida@aist.go.jp). S. J. B. Yoo is with the Department of Electrical and Computer Engineering, University of California, Davis, CA 95616 USA (e-mail: sbyoo@ucdavis.edu). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JSTQE.2011.2143390 Fig. 1. Optical packet switching architecture with (a) synchronous and fixed- length packet forwarding, and (b) asynchronous and variable-length packet forwarding. can utilize a unified control plane in support of optical packet switching (OPS), optical burst switching (OBS), optical flow switching (OFS), and optical circuit switching (OCS) [4]–[6]. The unified control plane [7], [8] of OLS also supports appli- cations’ differing degrees [9] of quality of service (QoS), class of service (CoS), and type of service (ToS) requirements by mapping them onto the optical labels [4]–[6]. The minimum requirement of the OLS system is to support OPS with low latency, high throughput, and high scalability. Fig. 1 illustrates a simplified schematic of an OPS system for (a) synchronous and fixed-length packet forwarding, and (b) asynchronous and variable-length packet forwarding. In both cases, the OPS system includes a switch controller, an input controller, and an output controller in the control plane, and an input interface, a switch fabric, and an output interface in the data plane. In the synchronous OPS, the input interface must syn- chronize the packets, whereas in the latter, no synchronization is necessary. The input interface in both cases extracts the head- ers; the input control detects the headers and sends the header content information to the switch control where forwarding ta- ble lookup, contention resolution, arbitration, and forwarding decision take place. The switch controller then sends the new switching state command to the switching fabric and also sends new header information to the output control so that the switch- ing fabric will switch and the output interface can conduct 1077-260X/$26.00 © 2011 IEEE
  • 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 systems. II. SIGNAL PROCESSING NEEDS IN OPTICAL PACKET SWITCHING SYSTEMS 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 domains. A. Optical Signal Processing in the Control Plane (Label Processing) 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 [10]. Examples of parallel optical labeling methods are: subcarrier multiplexing [11]–[15], wave- length multiplexing [16], orthogonal modulations [17], and op- tical code based optical labels [10], [18], [19]. Serial optical labeling methods utilize time domain multiplexed (TDM) la- bels [20]. Koch et al., [21] 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 [22], 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 [23], [24]. This section discusses signal pro- Fig. 2. Block diagram of an optoelectronic packet-switching system using time-to-wavelength conversion (courtesy of [26]). 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 [25]. Photonic integration circuits (PICs) can greatly reduce latency and power, and support high-speed serial label processing. 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. [26]. 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 [18], [26], [27]. The optical logic gates combined with serial to parallel con- version can support rapid decision processes based on the label content. An example of such optical logic gates includes cascaded semiconductor optical amplifier- Mach-Zehnder interferome- ters (SOA-MZI) [28]. 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) [29], [30]. 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 [27]. Again, photonic integration of the UNI circuits is a must for UNI to be useful in the control plane of OPS systems. 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 [31]–[33]. O-CDMA research investi- gated optical codes applied in various combinations of wave- length and time domains [34]. 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 [35], [36]. 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 [37]. 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 [38] at the expense of requiring multiple O-CDMA decoder optical circuits in parallel. Wada et al. [38] 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. [39] 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 gratings. Fig. 4. (a) Architecture of OPS system. (b) Multiple optical label processing (courtesy of [39]). 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. [40] 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 reflectors [40]. Fig. 6. Schematic of 1×5 InP–InGaAsP optical phased-array switch [42]. 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. [41] 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. [42] proposed and demonstrated an integrated optical phased-array switch. Fig. 6 shows schematic of 1 × 5 InP-InGaAsP optical phased-array switch [42]. 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 [43] implying possible applications in high-throughput OPS systems. Fig. 7. Configuration of multiformat OPS [44]. 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. [44] 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 [45]. 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) [30] 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 [46]. 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 characteristics.
  • 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 [47]. 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) [48]–[50]. 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 [51]–[53]. 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 devices. 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 [54]. Using pump and probe lights, wavelength conver- sion function is accomplished. SOA-MZIs provide 2R optical regeneration capability [55]. 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 [56]. 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 [60]. 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 [57]. 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 [58], [59], 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. [60] 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 [61]. On the other hand, integration schemes for delay lines are advancing. A 100 ns delay optical buffer has been re- alized using silica waveguides [62]. And a compact coiled fiber delay with a phased array switch has been demonstrated [63].
  • 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 [64]. 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 [64]. 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 [65]. 2) Nonlinear Optical Time Buffers: Slow-light delay lines have limited capacity and delay-bandwidth product [66]. Basi- cally, very slow group velocity always has very low bandwidth or throughput. There are many applications involving microring resonators [67]. However, control method of the delay needs to be investigated. 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 [66]. The main challenge lies in buffering a reasonable number of “bits.” Typical demon- strations of slow light buffering have been for less than 100 bits. 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 [65]. 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) [68] has demonstrated its capability of creating high-speed data (1.2 Tb/s) [69] 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 [68]. Recently, He et al. [70] 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 labels. 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 [71]. 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 formats. 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 is 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 or receiver. 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 wavelength range. 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. 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Okamoto, and S. J. B. Yoo, “Modulation-format transparent optical arbitrary wave- form generation based optical-label switching transmitter with all-optical label extraction using FBG,” in Proc. IEEE Lasers & Electro-Opt. Soc. Annu. Conf., 2009, Belek-Antalya, Turkey, pp. 575–576. [71] X. W. Yi, R. X. Yu, J. Kurumida, and S. J. B. Yoo, “A theoretical and experimental study on modulation-format-independent wavelength con- version,” J. Lightw. Technol., vol. 28, no. 4, pp. 587–595, Feb. 2010. 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).