Increasing input power dynamic range of SOA by shifting thetransparent wavelength of tunable optical filter

Gain-saturation-induced self-phase modulation (SPM) leading to pulse distortion in a semiconductor optical amplifier (SOA) is overcome by shifting a tunable optical filter (TOF). A recovered or broadened pulse can be obtained after filtering the amplified pulse in the SOA even if the short pulse is only 2-3 ps long. The input power dynamic range (IPDR) can be strongly increased by shifting the TOF and the direction of the shifted transparent wavelength is different for 10 Gb/s return-to-zero (RZ) or nonreturn-to-zero (NRZ) signals. The transparent wavelength of the TOF should be shifted to a longer wavelength for RZ signals and to a shorter for NRZ signals. 80-Gb/s optical time division multiplexing (OTDM) signal amplification in the SOA is demonstrated for the first time. We also demonstrate that a large IPDR for the 80-Gb/s OTDM signal can be obtained by shifting the TOF     

Polarisation mode dispersion tolerance of 10 Gbit/s NRZ and RZoptical signals

The authors have evaluated the performance of an optical receiver with polarisation mode dispersion (PMD) for non-return-to-zero (NRZ) and return-to-zero (RZ) signals at 10 Gbit/s, and found that the RZ format is more tolerant to PMD than is the NRZ format     

Optically preamplified receiver performance due to VSB filtering for 40-Gb/s optical signals modulated with various formats

Optically preamplified receiver performance according to the vestigial sideband (VSB) filtering has been numerically investigated for 40-Gb/s optical signals modulated with nonreturn-to-zero, duobinary nonreturn-to-zero (NRZ), return-to-zero (RZ), carrier-suppressed RZ, and duobinary carrier-suppressed RZ formats. The VSB filtering enables the spectral widths of NRZ, duobinary NRZ, and RZ signals to be reduced without severe power penalties at the receiver. On the other hand, carrier-suppressed RZ and duobinary carrier-suppressed RZ signals have no large advantages over VSB filtering because of the characteristics of their signals. Our results suggest that RZ signals are the most suitable modulation format for VSB filtering, without considering the filter loss, because of the tolerance of the intersymbol interference and a large spectral width. However, duobinary NRZ signals are the most suitable modulation format for VSB filtering, considering the filter loss, because of their narrow spectral width.     

Simultaneous all-optical demultiplexing and regeneration based onself-phase and cross-phase modulation in a dispersion shifted fiber

Simultaneous demultiplexing and regeneration of 40-Gb/s optical time division multiplexed (OTDM) signal based on self-phase and cross-phase modulation in a dispersion shifted fiber is numerically and experimentally investigated. The optimal walkoff time between the control pulse and OTDM signal is obtained by numerical simulation. Our experiment also shows that it is an effective method for realizing simultaneous demultiplexing and regeneration when used in the middle of a system or in the receiver with a proper walkoff time     

Timing-jitter-free demultiplexing of an all-optical signal processor based on the intensity-dependent self-wavelength conversion of Raman solitons

An all-optical signal processor based on the self-wavelength conversion of Raman solitons is described. The processor consists of an erbium-doped fiber amplifier (EDFA), a highly nonlinear fiber, and an optical bandpass filter. The timing-jitter-free demultiplexing of the processor was demonstrated by an 80-km transmission experiment. Error-free bit error rate (BER) performance was achieved. Transmitted fourfold optical time-division multiplexing (OTDM) signals with a large timing jitter of about 16 ps for an OTDM signal interval of 25 ps were demultiplexed to 9.95-Gb/s signals without signal degradation.     

Characterization of the Dynamic Absorption of Electroabsorption Modulators With Application to OTDM Demultiplexing

In this paper, a technique for characterizing the dynamic absorption of electroabsorption modulators (EAMs) under high-frequency sinusoidal modulation is described. By using an optical sampling oscilloscope (OSO), the nonlinear response of a modulator to 10- and 40-GHz modulation is accurately measured. The results for the dynamic absorption are used in a measurement-based model to calculate optical gating windows for the demultiplexing of a 160-Gb/s optical time-division multiplexed (OTDM) signal to its 10- and 40-Gb/s tributary signals. Good agreement is achieved between the calculated and measured gating windows.      

40-Gb/s OTDM to 4×10 Gb/s WDM conversion in monolithic InPMach-Zehnder interferometer module

Transformation of high bit-rate optical time-domain multiplexed (OTDM) signals into a multitude of lower bit-rate wavelength-division-multiplexed (WDM) channels is demonstrated by means of a single monolithically integrated indium phosphide Mach-Zehnder interferometer with semiconductor optical amplifiers in its arms. Full demultiplexing of 10-Gb/s OTDM signals into 4×10-Gb/s WDM channels is demonstrated. Bit-error-rate penalties are below 1.5 dB for polarization independent signal conversion throughout the 1.55-μm wavelength range     

Optical Signal-to-Noise Ratio Estimation Using Reference Asynchronous Histograms

This paper presents and experimentally demonstrates a novel method for the estimation of optical signal-to-noise ratio (OSNR) based on the comparison of an asynchronous histogram of the signal under analysis with a reference asynchronous histogram. The latter is acquired from the signal under analysis at a calibration stage. The proposed method allows the use of optical amplification to increase the sensitivity of the optical monitoring system (OMS) by a factor 20 dB, when using an erbium doped fiber preamplifier. In addition, the use of a semiconductor optical preamplifier, initially designed for nonlinear operation at 2.5 Gb/s is used in the OMS to preamplify 40-Gb/s signals, achieving a sensitivity gain of 10 dB. It will be experimentally demonstrated that the proposed method is applicable to 40-Gb/s nonreturn to zero (NRZ) signals arbitrarily degraded by group velocity dispersion (GVD). Furthermore, accurate monitoring of the OSNR of return-to-zero (RZ) signals will also be possible using a simple RZ-to-NRZ converter based on narrow-band optical filtering within the OMS. The proposed method also allows estimating of the GVD-induced OSNR penalty between the signal under analysis and the signal at the calibration stage.     

All-Optical Chromatic Dispersion Monitoring for Phase-Modulated Signals Utilizing Cross-Phase Modulation in a Highly Nonlinear Fiber

We propose and experimentally demonstrate an all-optical chromatic dispersion (CD) monitoring technique for phase-modulated signals utilizing the cross-phase-modulation effect between the input signal and the inserted continuous-wave probe. The probe's optical spectrum changes with the accumulated CD on the input signal, indicating that the optical power variations can be measured for monitoring. The experimental results show that this technique can monitor up to 120 ps/nm of CD for a 40-Gb/s return-to-zero differential phase-shift keying (RZ-DPSK) transmission system, with the maximum measured optical power increment of 16.5 dB. The applicability of this monitoring technique to higher bit-rate phase-modulated signals, such as 80-Gb/s RZ differential quadrature phase-shift keying and 80-Gb/s polarization-multiplexed RZ-DPSK, is also investigated via simulation.      

OTDM add-drop multiplexer based on time-frequency signal processing

A time-division add-drop multiplexer capable of high-extinction-ratio operation is presented both theoretically and experimentally. The approach used is based on time-to-frequency domain conversion of optical signals and relies upon the switching of linearly chirped optical pulses. By converting a 40-Gb/s optical time-division multiplexing (OTDM) signal to 4 /spl times/ 10-Gb/s wavelength-division multiplexing (WDM) channels and using fiber Bragg gratings for frequency-domain add-drop multiplexing, a timeslot suppression ratio in excess of 30 dB and error-free operation for the dropped, through, and added channels were achieved. A further stage of WDM-to-TDM signal conversion was used to map the resulting signal back into the time domain. Moreover, it is shown that it is straightforward to simultaneously operate on multiple channels by simply cascading gratings to make more complex filtering functions without the requirement for any further synchronization of the tributary channels.     

Payload-envelope detection and label-detection integrated photonic circuit for asynchronous variable-length optical-packet switching with 40-gb/s RZ payloads and 10-gb/s NRZ labels

A photonic integrated circuit that performs 40-Gb/s payload-envelope detection (PED) and 10-Gb/s label detection for asynchronous variable-length optical-packet switching is demonstrated. The circuit consists of an InP photonic integrated device combined with electronic GaAs and InP devices on a carrier. Asynchronous variable-length optical packets with 40-Gb/s return-to-zero (RZ) payloads and 10-Gb/s non-RZ (NRZ) labels are processed by the circuit. The circuit outputs a PED electrical signal that represents the temporal location of the payload and a 10-Gb/s electrical signal representing the optical label. The optical label is detected error free. The PED signal has a rise/fall time of 3-ns and 150-ps jitter. The PED signal was also used to erase and rewrite the optical labels error free.     

Design and Performance of an All-Optical Wavelength Converter Based on a Semiconductor Optical Amplifier and Delay Interferometer

The design and performance of an all-optical wavelength converter based on a semiconductor optical amplifier (SOA) and a delay interferometer (DI) are presented for 10-Gb/s return-to-zero (RZ) transmission systems with pulsewidths of 25-45 ps. The performance of the wavelength converter is investigated in terms of the operating conditions for the SOA and DI, and the properties of the input signal. The regenerative characteristics of the wavelength converter are also examined for input signals degraded by amplified spontaneous emission noise and residual dispersion. With proper design, the SOA-DI structure provides a high-performance all-optical wavelength converter for 10-Gb/s RZ transmission with practical pulsewidths.     

Performance evaluation of prechirped RZ and CS-RZ formats inhigh-speed transmission systems with dispersion management

This paper describes the transmission performance of prechirped return-to-zero (RZ) and prechirped carrier-suppressed return-to-zero (CS-RZ) signals over a periodically dispersion-compensated transmission line. We analyze the transmission characteristics of both formats, taking account the transmitter configuration expected, in which pulse chirping is generated by using both a phase modulator and a linear dispersion compensating device. We also discuss the dependence of the transmission characteristics on phase modulation, pre- and postcompensating dispersion, and receiver optical and electrical filter widths. We show that, in single-channel transmission, phase modulation effectively reduces the intrachannel nonlinear interaction and improves the transmission performance. Next, we discuss the transmission characteristics of chirped RZ and chirped CS-RZ signals in dense wavelength division multiplexed (DWDM) signal transmission. In 100-GHz spaced 40-Gb/s-per-channel systems, it is shown that the phase modulation must be carefully optimized in order to minimize the linear crosstalk and waveform distortion induced by the intra- and interchannel nonlinear interaction in the transmission fiber     

OTDM transmitter using WDM-TDM conversion with an electroabsorptionwavelength converter

An all-optical multiplexing technique using wavelength division multiplexing (WDM)-time division multiplexing (TDM) conversion with an electroabsorption wavelength converter has been proposed and demonstrated. The effectiveness of this WDM-TDM conversion technique for various pulsewidth settings was experimentally investigated. The fluctuation of the signal performance, which was inevitably caused by the coherent crosstalk between adjacent pulses in the conventional optical time division multiplexing (OTDM) technique, were successfully suppressed, even in the case of wide pulse duration. High Q-factor performance has been maintained for a wide range of duty ration from 36% to 74%. By introducing this technique to the optical time division multiplexer, a highly stable and high-quality 40-Gb/s optical signal can be effectively produced without generating the short pulse or setting two tributaries at orthogonal polarization states, and without introducing high-speed electronics for signal multiplexing. The WDM-TDM conversion with an electroabsorption wavelength converter was extended to 60-Gb/s operation by using three 20-Gb/s tributaries. A clear eye opening was confirmed for a waveform after the WDM-TDM conversion of the 60-Gb/s signal     

All-Optical RZ-DPSK WDM to RZ-DQPSK Phase Multiplexing Using Four-Wave Mixing in Highly Nonlinear Fiber

We propose and experimentally demonstrate an all-optical phase multiplexing scheme using four-wave mixing in a highly nonlinear fiber. Two 10-Gb/s pi/2-shifted return-to-zero (RZ) differential phase-shift-keying (DPSK) wavelength-division-multiplexing (WDM) signals are successfully phase-multiplexed into a 20-Gb/s RZ differential quadrature phase-shift-keying signal with a negative 1.6-dB power penalty. With more input DPSK WDM signals, the proposed scheme can be applied to obtain a multilevel phase-shift-keying signal with increased capacity and enhanced spectral efficiency.     

160-gb/s OTDM signal source with 3R function utilizing ultrafast mode-locked laser diodes and modified NOLM

High-quality 160-Gb/s optical time-division-multiplexing (OTDM) signal source with 3R functions was demonstrated using ultrafast mode-locked laser diodes (MLLDs) and a nonlinear optical fiber loop mirror (NOLM) modified with inline and external polarizers and an inline optical phase-bias compensator. Pulse quality for each OTDM channel was successfully improved and equalized owing to the clock extraction at the true data bit rate using a 160-GHz MLLD and also to the improvement of the switching performance of the NOLM.     

On-the-Fly All-Optical Contention Resolution for NRZ and RZ Data Formats Using Packet Envelope Detection and Integrated Optical Switches

We present a packet-by-packet contention resolution scheme that combines packet detection, optical space switching, and wavelength conversion performed in the optical domain by integrated optical switches. The packet detection circuit provides the control signals required to deflect and wavelength-convert the contending packets so that all the packets are forwarded to the same output without any collision or packet droppings. We demonstrate the compatibility of the scheme with both nonreturn-to-zero (NRZ) and return-to-zero (RZ) modulation formats by recording error-free operation for 10-Gb/s NRZ and 40-Gb/s RZ packet-mode traffic     

Optimum filter bandwidths for optically preamplified NRZ receivers

We present a comprehensive treatment of optically preamplified direct detection receivers for non-return-to-zero (NRZ) and return-to-zero (RZ) on/off keying modulation, taking into account the influence of different (N)RZ optical pulse shapes, specified at the receiver input, and filter transfer functions; optical Fabry-Perot filters (FPFs) and Bragg gratings as well as electrical fifth-order Bessel and first-order RC low-pass filters are considered. We determine optimum optical and electrical filter bandwidths and analyze the impact of bandwidth deviations on receiver sensitivity. Optimum receiver performance relies on a balance between noise and intersymbol interference (ISI) for NRZ transmission, while for RZ reception detection noise has to be traded against filter-induced signal energy rejection. Both for NRZ and 33% duty cycle RZ, optical filter bandwidths of around twice the data rate are found to be optimum. Receivers using RZ coding are shown to closely approach the quantum limit, and thus to outperform NRZ-based systems by several decibels. We further analyze the impact of important degrading effects on receiver sensitivity and optimum receiver bandwidths, including receiver noise, finite extinction ratio, chirp, and optical carrier frequency (or optical filter center frequency) fluctuations     

Ultrahigh-Speed OTDM-Transmission Technology

This paper reviews ultrahigh-speed data transmission in optical fibers based on optical time division multiplexing (OTDM) transmission technology. Optical signal processing in the transmitter and receiver as well as the requirements on ultrahigh-speed data transmission over a fiber link are discussed. Finally, results of several OTDM-transmission experiments, including 160-Gb/s transmission over 4320 km, 1.28-Tb/s transmission over 240 km, and 2.56-Tb/s transmission over 160-km fiber link, are described     

Low insertion loss and low dispersion penalty InGaAsP quantum-well high-speed electroabsorption modulator for 40-Gb/s very-short-reach, long-reach, and long-haul applications

We present a metal-organic-chemical-vapor-deposition-grown low-optical-insertion-loss InGaAsP/InP multiple-quantum-well electroabsorption modulator (EAM), suitable for both nonreturn-to-zero (NRZ) and return-to-zero (RZ) applications. The EAM exhibits a dynamic (RF) extinction ratio of 11.5 dB at 1550 nm for 3 Vp-p drive under 40-Gb/s modulation. The optical insertion loss of the modulator in the on-state is -5.2 dB at 1550 nm. In addition, the EAM also exhibits a 3-dB small-signal response (S21) of greater than 38 GHz, allowing it to be used in both 40-Gb/s NRZ and 10-Gb/s RZ applications. The dispersion penalty at 40 Gb/s is measured to be 1.2 dB over /spl plusmn/40 ps/nm of chromatic dispersion. Finally, we demonstrate 40-Gb/s transmission performance over 85 km and 700 km.