Theoretical analysis on polarization deviation and switch windowoptimization in nonlinear optical loop mirror demultiplexer

This paper investigates theoretically two of the dominant issues on a nonlinear optical loop mirror (NOLM) demultiplexer: the sensibility to the polarization deviation between signal and control pulses and the optimization of the switch window width. The complete nonlinear Schrodinger equations concerning the different states of polarization between signal and control lights are firstly established to study the impact of the polarization deviation on the demultiplexed signal. Considering simultaneously the channel crosstalk and the timing jitter between signal and control pulses, the switch window width of NOLM is optimized to achieve the best demultiplexing performance. The theoretical analysis shows that the polarization deviation has to be controlled less than 20° within the bit error rate (BER) of 10^-9 s. The optimal amount of the pulse walkoff is a little less than half of the slot width of the OTDM system     

Signal-to-noise ratio analysis of 100 Gb/s demultiplexing usingnonlinear optical loop mirror

This paper investigates experimentally and theoretically the signal-to-noise ratio (SNR) characteristics of 100 Gb/s all-optical demultiplexing using a nonlinear optical loop mirror (NOLM). The analysis takes into account two effects that degrade the SNR associated with NOLM demultiplexing. First is channel crosstalk originating from the leakage of nontarget channels. Second is the intensity fluctuations of demultiplexed signals caused by the combined effects of timing jitter and a profile of the switching window. Considering these two effects, power penalties associated with NOLM. Demultiplexing are theoretically evaluated using the conventional noise theory of an optical receiver followed by an optical preamplifier. Experimental results of bit error rate measurements for 100 Gb/s demultiplexing using three different NOLMs with different intrinsic crosstalk values, defined by signal transmittance in the absence of control pulses, show that the power penalties are in good agreement with the evaluation based upon our proposed analysis. It can be found from our investigation in demultiplexing from 100 to 10 Gb/s that intrinsic crosstalk of less than -25 dB, corresponding to a coupling ratio, K, of |K-0.5|⩽0.03, is required for the power penalty of less than 1 dB. The root-mean-square (rms) value of the relative timing jitter necessary for obtaining a sufficient timing tolerance width for combining control and signal pulses is determined     

Demultiplexing of 168-Gb/s data pulses with a hybrid-integratedsymmetric Mach-Zehnder all-optical switch

We have developed a hybrid-integrated symmetric Mach-Zehnder all-optical switch and evaluated the demultiplexing of 168-Gb/s data pulses at a repetition rate of 10 GHz with this switch. A compact, stable device was realized by assembling semiconductor optical amplifiers as nonlinear waveguides on a planar lightwave circuit in a self-aligned manner. A 6.0-ps switching window needed for 168-Gb/s demultiplexing was provided by the push-pull operation of the symmetric Mach-Zehnder all-optical switch. Demultiplexed signal light showed a high extinction ratio of better than 18 dB. Error-free demultiplexing with a bit error rate of 10^-11 was achieved     

Active vertical-coupler-based optical crosspoint switch matrix for optical packet-switching applications

Packet-switching characteristics are optimized across an integrated 4 /spl times/ 4 optical crosspoint switch matrix consisting of active vertical-coupler-based switch cells. Optical gain difference between the shortest and the longest paths less than 3 dB is demonstrated. Bit error rate (BER) and power penalty measurements during packet routing have also been carried out over the entire 4 /spl times/ 4 matrix. At a 10-Gb/s packet data rate, a less than 1-dB power penalty is observed across the switch matrix, and the possibility for error-free packet routing is demonstrated with no BER floor observed.     

Impact of extinction ratio and switching efficiency of nonlinearoptical loop mirror demultiplexer on error rate performance

A theory is evaluated that allows the calculation of the worst-case input power penalty of an optical demultiplexer using a nonlinear optical loop mirror (NOLM) as high-speed switch. Due to finite NOLM extinction ratio and switching efficiency reduction there exists a strong dependence of the penalty on the bitrate of the drop channel, which is investigated for different 100 Gbit/s demultiplexer configurations. Assuming a NOLM of 2 km loop fiber, a 20-dB extinction ratio, and a switching pulse EDFA of 14.5 dBm, demultiplexing with a penalty of less than 7 dB should be possible for drop rates between 2.5 GHz and 11.1 GHz     

Optical demultiplexing at 6 Gb/s using a semiconductor laser amplifier as an optical gate

A semiconductor laser amplifier (SLA) has been employed successfully for optical demultiplexing in two-channel optical time division multiplexed system experiments at 6 and 2 Gb/s. Demultiplexing of 6-Gb/s (2-Gb/s) signals was demonstrated with a power penalty of 1.6 dB (3.0 dB) at bit error rates of 10/sup -9/. It is also shown that a reduction of the generated amplified spontaneous emission can be obtained by optical gating/demultiplexing for systems incorporating inline amplifiers. A 0.5-dB improvement in sensitivity was achieved as a result of using an SLA for demultiplexing from 2.0 to 1.0 Gb/s in a system with one inline Er/sup 3+/-doped fiber amplifier.<>     

Effects of group velocity dispersion on self/cross phase modulationin a nonlinear Sagnac interferometer switch

Time-resolved numerical analysis of a nonlinear Sagnac interferometer switch (NSIS) reveals that the combined effects of group velocity dispersion (GVD), self phase modulation, cross phase modulation, and pump-probe walk-off seriously degrade switching performance when the soliton number N of the pump pulse is under 5. This means that the peak power of short pump pulses cannot be reduced to less than the critical value at N>5 to prevent the effect of GVD. This restriction is more severe for pump pulses in the anomalous dispersion region than for those in the normal dispersion region because of higher-order soliton compression. System designs for time-division demultiplexers that use NSISs and picosecond pump pulses generated by a laser-diode coupled to erbium-doped fiber amplifiers are discussed. It is found that 1:32 demultiplexing from 160 to 5 Gb/s and 1:8 demultiplexing from 80 to 10 Gb/s with a switching contrast of more than 60 are possible using diode-laser-pumped 1- and 2-ps pump pulses, respectively     

Semiconductor monolithic wavelength selective router using agrating switch integrated with a directional coupler

A semiconductor monolithic wavelength selective router is proposed. An electrically tunable grating switch is monolithically integrated with a directional coupler. The effect of sidelobe on the demultiplexing characteristics of the router is investigated, and it is shown that the sidelobes degrade the wavelength crosstalk and introduce a power penalty for demultiplexing characteristics. A phase-synchronized sampled grating is introduced to suppress the sidelobes, and wavelength router switching is observed in this structure. The extinction ratio is over 10 dB at a current injection of 25 mA. Using this structure, power-penalty-free wavelength selective routing at a 2.5 Gb/s wavelength-division-multiplexed NRZ random pattern is demonstrated     

Fiber four-wave mixing demultiplexing with inherent parametricamplification

We present an all-optical time-division demultiplexer with 22 dB conversion efficiency, using four-wave mixing (FWM) at 1550 nm in a single-mode dispersion-shifted fiber. Error-free demultiplexing of 20 Gb/s data to 10 Gb/s is obtained, with 1.4 dB power penalty at BER=10 ^-9. We also derive theoretical expressions for the conversion efficiency, the maximum bit rate that is possible to demur and the shape of the switching window. In particular we emphasize the importance of phase-matching in order to achieve a flat switching window. Furthermore the pump must remain undepleted to avoid a detrimental reduction of the extinction ratio due to instantaneous parametric gain saturation     

Interferometric crosstalk reduction by phase scrambling

Interferometric crosstalk, arising from the detection of undesired signals at the same nominal wavelength, may introduce large power penalties and bit-error rate (BER) floor significantly restricting the scalability of optical networks. In this paper, interferometric crosstalk reduction in optical wavelength-division multiplexing (WDM) networks by phase scrambling is theoretically and experimentally investigated. Enhancement of 7- and 5-dB tolerance toward crosstalk is measured in a 2.5-Gb/s transmission link of 100 km and 200 km of SSMF, respectively. This result proves the feasibility of optical networking in the local area network/metropolitan area network (LAN/MAN) domain while tolerating the relatively high crosstalk levels of present integrated optical switching and cross-connect technology. Experiment is in good agreement with theory. Recommendations on the use of phase scrambling to reduce crosstalk in WDM systems are given     

1.3-$mu$m Amplifier-Free All-Optical 2R Regenerator Using Two-Mode Injection-Locked Distributed Feedback Laser Diode

This study presents a novel and cost-effective 1.3-mum all-optical 2R regenerator based on a two-mode injection-locked distributed feedback laser diode. The proposed 2R regenerator, with 14.13-dB small signal gain, has achieved an amplifier-free 10-Gb/s straight line transmission over 60 km while keeping the power penalty less than 0.84 dB at bit-error rate (BER) =10^-9. In addition, properties like BER degradation, output extinction ratio, gain, and data-rate transparency are also experimentally investigated     

Optical time-division multiplexing for very high bit-ratetransmission

Optical time-division multiplexing (OTDM) extends and expands the well-known techniques of electrical time-division multiplexing into the optical domain. In OTDM, optical data streams are constructed by time-multiplexing a number of lower-bit-rate optical streams. Opportunities for very high-speed transmission and switching are created by removing limitations set by the restricted bandwidth of electronics and by capitalizing on the inherent high-speed characteristics of optical devices. An overview of recent work in optical time-division multiplexing and demultiplexing is presented. Design considerations affecting system architecture are described. Emphasis on the factors that limit system performance, such as crosstalk between multiplexed channels. Examples of very high bit-rate optical time-division multiplexed system experiments using short pulses from mode-locked semiconductor lasers and high-speed Ti:LiNbO₃ waveguide switch/modulators are presented     

All optical signal regularizing/regeneration using a nonlinearfiber Sagnac interferometer switch with signal-clock walk-off

All-optical signal regularizing/regeneration using a nonlinear fiber Sagnac interferometer switch (NSIS) that employs signal-clock walk-off is investigated. The NSIS realizes all-optical signal regeneration, including timing and amplitude regularizing, by switching clock pulses with amplified input signals using a walk-off-induced, wide, square switching window and intensity-dependent transmittance of the device. First, characteristics (in both the temporal and spectral domains) of the all-optical signal regeneration achieved with the NSIS are investigated theoretically and experimentally. They certify that if clock pulses are within the square switching window obtained with signal-clock walk-off, the clock pulses can be modulated according to the data that the input signals carry and retain their temporal and spectral profiles. This means that if clock pulses can be prepared that meet the system requirements, the NSIS can convert input signals that may not satisfy system requirements into high-quality output signals. Limitations on the switching contrast due to the cross-phase modulation of counterpropagating reference pulses is also discussed. Second, two possible applications of NSIS-based all-optical signal regularizing/regeneration, 1) an all-optical multiplexer with an optical clock and 2) an all-optical regenerative repeater, are discussed. Preliminary experiments with ~10-ps pulses at bit rates of ~5 Gb/s that use locally prepared optical clock pulses, show that the NSIS provides an error-free regeneration function with a certain tolerance for pulse-period irregularity if a proper optical clock is obtained     

A Hybrid OTDM Scheme With Enhanced Demultiplexing Performance

We propose a novel hybrid optical time-division multiplexing (OTDM) approach, which contains hybrid modulation formats of return-to-zero on-off keying and return-to-zero differential-phase-shift keying, and investigate its demultiplexing performance. Compared with conventional OTDM with homogenous modulation format, the target demultiplexed channel in a hybrid OTDM signal suffers from much less degradation due to the possible crosstalk from the adjacent channels. We experimentally demonstrate 84.88- to 10.61-Gb/s hybrid OTDM demultiplexing and achieve a relatively wide switching window, which cannot be realized by using the conventional OTDM. Moreover, experimental results at 42.44 Gb/s show a much larger tolerance against timing misalignment in demultiplexing, which further validates the improved demultiplexing performance by using the hybrid OTDM scheme.     

Broadcast-and-select photonic ATM switch with frequency divisionmultiplexed output buffers

A photonic ATM switch has been developed with frequency division multiplexed (FDM) output buffers. The switch has a broadcast-and-select network architecture using fixed-frequency-channel transmitters and a passive star configuration. Although it has a simple structure, it can provide either broadcast or multicast switching. The output buffers, which resolve cell contentions, are comprised of fiber delay lines that can easily handle signal speed of over 10 Gb/s. Experimental switching of two-multiplexed 10 Gb/s cells with a 2.8-dB power penalty demonstrated high-speed switching     

Speed limit of all-optical gate switches using cascaded second-order nonlinear effect in quasi-phase-matched LiNbO/sub 3/ devices

In all-optical gate switches that employ the cascaded second-order nonlinear effect in quasi-phase-matched (QPM) LiNbO/sub 3/ devices, walkoff between the fundamental and second harmonic pulses is very large. The authors experimentally show that crosstalk of the switch induced by such walkoff limits the switching speed, but that the switching speed can significantly be enhanced by walkoff compensation. Using a 20-mm-long QPM LiNbO/sub 3/ waveguide device, the authors switch one of twin pulses separated by 6.25 ps without crosstalk, showing the possibility of switching a 160-Gb/s signal.     

Highly stable optical add/drop multiplexer using polarization beam splitters and fiber Bragg gratings

We demonstrate a novel wavelength-division add/drop multiplexer employing fiber Bragg gratings and polarization beam splitters. The multiplexer is easy to fabricate without any special technique such as UV trimming, and yet shows very stable performance with less than 0.3-dB crosstalk power penalty in a 0.8-nm-spaced, 2.5-Gb/s-per-channel wavelength-division multiplexing (WDM) transmission system.     

40-GHz dual-mode-locked widely tunable sampled-grating DBR laser

Generation of 40-GHz alternate-phase pulses is demonstrated using a dual-mode-locked sampled-grating distributed Bragg reflector laser. More than 10-dB extinction and >20-dB sidemode suppression ratio is measured over the >40-nm tuning range of the laser. Based on captured phase noise spectra, the timing jitter is estimated in the 0.35-0.41-ps range. The demonstrated dual-mode laser would form an attractive basis for an integrated 40-Gb/s return-to-zero transmitter.     

A chopped quantum-well polarization-independent interferometricswitch at 1.53 μm

We have theoretically designed and realized a phase shifter for a low-loss Mach-Zehnder interferometric switch. The phase shifter is based on 0.85% tensile strained InGaAs-InP chopped quantum-well material. We realized a Mach-Zehnder interferometric switch with polarization-independent switching voltages as low as 3.3±0.05 V at 1525 nm for a switch with a 4-mm-long phase shifting section. The wavelength sensitivity of the switch is 0.036 V/nm for TE and 0.053 V/nm for TM polarization. Calculations of the electro-refraction in the -0.85% strained chopped quantum-well (QW) material based on the 4×4 Luttinger-Kohn Hamiltonian show that the electro-refraction due to the quantum-confined Stark effect (QCSE) for TM polarization is equal to the sum of the mutually comparable QCSE electro-refraction and the Pockels effect for TE polarization in waveguides along the [11¯0] axis. Our first-principle model for calculating the electro refraction is an accurate design tool for predicting device performance in complicated layer structures. The shortest possible phase shifter with a <-25 dB crosstalk penalty due to electro-absorption unbalance can be as short as 2.2 mm. This compact switch is predicted to have a 6-V switching voltage and a 15-nm window for polarization-independent switching with a <-25-dB crosstalk penalty. With a slight increase of the strain, this chopped QW material can be used for polarization independent switching around 1550 nm     

All-optical 80-gb/s time-division demultiplexing using polarization-insensitive cross-phase modulation in photonic crystal fiber

We describe an all-optical 80-Gb/s time-division demultiplexer, which utilizes cross-phase modulation in a commercial photonic crystal fiber. Compared to back-to-back 10-Gb/s measurements, the demultiplexer achieves better than a 2.5-dB power penalty for all eight channels. More importantly, we demonstrate a novel scheme for polarization-insensitive operation, which uses only the birefringence of the fiber itself and proper alignment of the clock pulse polarization. Using this technique, the polarization sensitivity of the converted power is found to be less than 0.4 dB, allowing for error-free demultiplexing even while the data polarization state is scrambled.