A novel self-synchronization scheme is proposed that utilizes both cross gain modulation (XGM) and phase modulation, including self-phase modulation (SPM) and cross-phase modulation (XPM), in a semiconductor optical amplifier (SOA). The scheme consists of an SOA and a delay line interferometer. It is simple, integrable, and does not require any special marker pulses. With a proper design of the SOA parameters and interferometer delay time, a synchronization clock with an intensity contrast ratio of more than 20 dB can be obtained from return-to-zero (RZ) pseudorandom bit sequences (PRBSs) at different operation speeds.
The error probability of differential quadrature phase-shift keying (DQPSK) systems is analyzed considering both self- and cross-phase modulation (SPM and XPM)-induced nonlinear phase noise. XPM-induced nonlinear phase noise is treated here as Gaussian distributed phase noise. In DQPSK systems where group velocity dispersion (GVD) is perfectly compensated span after span, the influence of XPM-induced nonlinear phase noise on the error bribability performance can be dominant compared with that of the SPM-induced nonlinear phase noise. The 10-Gb/s systems, whose walkoff length is larger than that of the 40 Gb/s systems, are more sensitive to XPM-induced nonlinear phase noise.