Nonlinear loop mirror-based all-optical signal processing in fiber-optic communications

All-optical data processing is expected to play a major role in future optical communications. The fiber nonlinear optical loop mirror (NOLM) is a valuable tool in optical signal processing applications. This paper presents an overview of our recent advances in developing NOLM-based all-optical processing techniques for application in fiber-optic communications. The use of in-line NOLMs as a general technique for all-optical passive 2R (reamplification, reshaping) regeneration of return-to-zero (RZ) on-off keyed signals in both high-speed, ultralong-distance transmission systems and terrestrial photonic networks is reviewed. In this context, a theoretical model enabling the description of the stable propagation of carrier pulses with periodic all-optical self-regeneration in fiber systems with in-line deployment of nonlinear optical devices is presented. A novel, simple pulse processing scheme using nonlinear broadening in normal dispersion fiber and loop mirror intensity filtering is described, and its employment is demonstrated as an optical decision element at a RZ receiver as well as an in-line device to realize a transmission technique of periodic all-optical RZ-nonreturn-to-zero-like format conversion. The important issue of phase-preserving regeneration of phase-encoded signals is also addressed by presenting a new design of NOLM based on distributed Raman amplification in the loop fiber. © 2008 Elsevier Inc. All rights reserved

Detection and processing of phase modulated optical signals at 40 Gbit/s and beyond

This thesis addresses demodulation in direct detection systems and signal processing of high speed phase modulated signals in future all-optical wavelength division multiplexing (WDM) communication systems where differential phase shift keying (DPSK) or differential quadrature phase shift keying (DQPSK) are used to transport information. All-optical network functionalities -such as optical labeling, wavelength conversion and signal regeneration- are experimentally investigated. Direct detection of phase modulated signals requires phase-to-intensity modulation conversion in a demodulator at the receiver side. This is typically implemented in a one bit delay Mach-Zehnder interferometer (MZI). Two alternative ways of performing phase-to-intensity modulation conversion are presented. Successful demodulation of DPSK signals up to 40 Gbit/s is demonstrated using the proposed two devices. Optical labeling has been proposed as an efficient way to implement packet routing and forwarding functionalities in future IP-over-WDM networks. An in-band subcarrier multiplexing (SCM) labeled signal using 40 Gbit/s DSPK payload and 25 Mbit/s non return-to-zero(NRZ) SCM label, is successfully transmitted over 80 km post-compensated non-zero dispersion shifted fiber (NZDSF) span. Using orthogonal labeling, an amplitude shift keying (ASK)/DPSK labeled signal using 40 Gbit/s return-to-zero (RZ) payload and 2.5 Gbit/s DPSK label, is generated. WDM transmission and label swapping are demonstrated for such a signal. In future all-optical WDM networks, wavelength conversion is an essential functionality to provide wavelength flexibility and avoid wavelength blocking. Using a 50 m long highly nonlinear photonic crystal fiber (HNL-PCF), with a simple four-wave mixing (FWM) scheme, wavelength conversion of single channel and multi-channel high-speed DPSK signals is presented. Wavelength conversion of an 80 Gbit/s RZ-DPSK-ASK signal generated by combining different modulation formats is also reported. Amplitude distortion accumulated over transmission spans will eventually be converted into nonlinear phase noise, and consequently degrade the performance of systems making use of RZ-DPSK format. All-optical signal regeneration avoiding O-E-O conversion is desired to improve signal quality in ultra long-haul transmission systems. Proof-of-principle numerical simulation results are provided, that suggest the amplitude regeneration capability based on FWM in a highly nonlinear fiber (HNLF). The first reported experimental demonstration of amplitude equalization of 40 Gbit/s RZ-DPSK signals using a 500 m long HNLF is presented. Using four possible phase levels to carry the information, DQPSK allows generation of high-speed optical signals at bit rate that is twice the operating speed of the electronics involved. Generation of an 80 Gbit/s DQPSK signal is demonstrated using 40 Gbit/s equipment. The first demonstration of wavelength conversion of such a high-speed signal is implemented using FWM in a 1 km long HNLF. No indication of error floor is observed. Using polarization multiplexing and combination of DQPSK with ASK and RZ pulse carving at a symbol rate of 40 Gbaud, a 240 Gbit/s RZ-DQPSK-ASK signal is generated and transmitted over 50 km fiber span with no power penalty. In summary, we show that direct detection and all-optical signal processing -including optical labeling, wavelength conversion and signal regeneration- that already have been studied intensively for signals using conventional on-off keying (OOK) format, can also be successfully implemented for high-speed phase modulated signals. The results obtained in this work are believed to enhance the feasibility of phase modulation in future ultra-high speed spectrally efficient optical communication systems

Scalable WDM phase regeneration in a single phase-sensitive amplifier through optical time lenses

Scalable solutions for data regeneration of multiple parallel channels are elusive. Here the authors report a scalable wavelength-division multiplexing technique for phase regeneration and demonstrate the highest reported number of regenerated wavelength-division multiplexed channels in a single phase regenerator

Optical code-division multiple access system and optical signal processing

This thesis presents our recent researches on the development of coding devices, the investigation of security and the design of systems in the optical cod-division multiple access (OCDMA) systems. Besides, the techniques of nonlinear signal processing used in the OCDMA systems fire our imagination, thus some researches on all-optical signal processing are carried out and also summarized in this thesis. Two fiber Bragg grating (FBG) based coding devices are proposed. The first coding device is a superstructured FBG (SSFBG) using ±π/2-phase shifts instead of conventional 0/π-phase shifts. The ±π/2-phase-shifted SSFBG en/decoders can not only conceal optical codes well in the encoded signals but also realize the reutilization of available codes by hybrid use with conventional 0/π-phase-shifted SSFBG en/decoders. The second FBG based coding device is synthesized by layer-peeling method, which can be used for simultaneous optical code recognition and chromatic dispersion compensation. Then, two eavesdropping schemes, one-bit delay interference detection and differential detection, are demonstrated to reveal the security vulnerability of differential phase-shift keying (DPSK) and code-shift keying (CSK) OCDMA systems. To address the security issue as well as increase the transmission capacity, an orthogonal modulation format based on DPSK and CSK is introduced into the OCDMA systems. A 2 bit/symbol 10 Gsymbol/s transmission system using the orthogonal modulation format is achieved. The security of the system can be partially guaranteed. Furthermore, a fully-asynchronous gigabit-symmetric OCDMA passive optical network (PON) is proposed, in which a self-clocked time gate is employed for signal regeneration. A remodulation scheme is used in the PON, which let downstream and upstream share the same optical carrier, allowing optical network units source-free. An error-free 4-user 10 Gbit/s/user duplex transmission over 50 km distance is reazlied. A versatile waveform generation scheme is then studied. A theoretical model is established and a waveform prediction algorithm is summarized. In the demonstration, various waveforms are generated including short pulse, trapezoidal, triangular and sawtooth waveforms and doublet pulse. ii In addition, an all-optical simultaneous half-addition and half-subtraction scheme is achieved at an operating rate of 10 GHz by using only two semiconductor optical amplifiers (SOA) without any assist light. Lastly, two modulation format conversion schemes are demonstrated. The first conversion is from NRZ-OOK to PSK-Manchester coding format using a SOA based Mach-Zehnder interferometer. The second conversion is from RZ-DQPSK to RZ-OOK by employing a supercontinuum based optical thresholder

DPSK regeneration: phase and amplitude noise suppression based on Kerr medium

The scope of this thesis is to identify and propose new schemes for Deferential Phase Shift Keying (DPSK) regeneration. DPSK modulation format presents increased robustness against ASE noise, which makes it strong candidate for use in long haul transmission systems. To achieve reasonable DPSK regeneration suppression of both the amplitude and phase noise is required. Three types of all-optical regenerators that make use of a Kerr medium, which can be a highly nonlinear fiber are analyzed. The first scheme is based on a modified nonlinear optical loop mirror (NOLM), with a subsequent addition of a bidirectional attenuator (DANOLM). The bidirectional attenuator allows to counterbalance the generation of the phase noise generated by the Gordon Mollenauer effect inside the Kerr medium. The second type of optical regenerator is based on the Self Phase Modulation (SPM) effect and offset filtering. Finally a novel scheme derived from the concepts of two former setups is presented and compared to the previous proposed. The operational conditions for optimum noise rejection are identified for each one of them. Through numerical simulations and detailed benchmarink we identify that our proposal outperforms all the schemes that have been presented previously in literature

All-optical Regeneration For Phase-shift Keyed Optical Communication Systems

All-optical signal processing techniques for phase-shift keyed (PSK) systems were developed theoretically and demonstrated experimentally. Nonlinear optical effects in fibers, in particular four-wave mixing (FWM) that occurs via the ultra-fast Kerr nonlinearity, offer a flexible framework within which numerous signal processing functions can be accomplished. This research has focused on the regenerative capabilities of various FWM configurations in the context of processing PSK signals. Phase-preserving amplitude regeneration, phase regeneration, and phase-regenerative wavelength conversion are analyzed and demonstrated experimentally. The single-pump phase-conjugation process was used to regenerate RZ-DPSK pulse amplitudes with different input noise distributions, and the impact on output phase characteristics was studied. Experiments revealed a limited range over which amplitude noise could effectively be suppressed without introduction of phase noise, particularly for signals with intensity pattern effects. Phase regeneration requires use of phase-sensitive amplification (PSA), which occurs in nonlinear interferometers when the pump and signal frequencies are degenerate (NI-PSA), or in fiber directly through single-stage (degenerate) or cascaded (non-degenerate) FWM processes. A PSA based on a Sagnac interferometer provided the first experimental demonstration of DPSK phase and amplitude regeneration. The phase-regenerative capabilities of the NI-PSA are limited in practice by intrinsic noise conversion (amplitude to phase noise) and to a lesser extent by the requirement to modulate the pump wave to suppress stimulated Brillouin scattering (SBS). These limitations are relaxed in novel materials with higher SBS thresholds and nonlinearities. Degenerate FWM provides PSA in a traveling-wave configuration that intrinsically suppresses the noise conversion affecting the NI-PSA, while providing stronger phase-matched gain. Experiments confirmed superior phase-regenerative behavior to the NI-PSA with simultaneous reduction of amplitude noise for NRZ-DPSK signals. Phase-regenerative wavelength conversion (PR-WC) provides the regenerative properties of PSA at a new wavelength, and was proposed and demonstrated for the first time in this research. The parallel implementation of two FWM processes, phase-conjugation and frequency conversion, provides two idlers which exhibit interesting and useful regenerative properties. These were investigated theoretically and experimentally. Ideal phase-regenerative behavior is predicted when the contributing FWM processes are equally phase-matched, which can be maintained over any interaction length or wavelength shift provided the pump powers are properly adjusted. Depleted-pump regime PR-WC provides simultaneous phase and amplitude regeneration. Experiments confirmed regenerative behavior for wavelength shifts of the idlers up to 5 nm. Two techniques for phase regeneration of 4-level PSK signals were developed and evaluated. The first is based on parallel operation of PSAs suitable for processing 2-level PSK signals, where phase projection and regeneration are combined to recover the input data. Analysis of this scheme outlined the conditions required for effective phase regeneration and for practical implementation using known PSAs. A novel process based on FWM (parallel phase-conjugation followed by PSA) was developed and analyzed, and demonstrated using numerical simulations. These studies provide a basis for further work in this area