Statistics of Intra-channel Four-Wave Mixing induced phase noise in coherent RZ-DQPSK transmission systems

摘要

Digital signal processing (DSP) based coherent detection attracts extensive studies recently. DSP technology simplifies coherent detection and brings it more advantages and channel impairments can be compensated electrically. The lumped dispersion compensation scheme makes the fiber link simple, flexible and potentially cost effective. However, such pseudo-linear long haul transmission systems are greatly degraded by intra-channel nonlinearities at high bit rates. The optical pulses are highly dispersed and overlap strongly during propagation, resulting serious intrachannel four-wave mixing (IFWM). IFWM introduces pattern dependent nonlinear phase shifts to the signals. It has been pointed out that IFWM induced phase noise are correlated from symbol to symbol. The autocorrelation function has been studied in detail. Here we extend the result to fiber link with single mode fiber (SMF) for transmission only. We studied the statistics of IFWM induced phase noise in Return to Zero-Differetnitally coded Quadrature Phase Shift Keying (RZ-DQPSK) transmission systems with identical fiber spans and SMF fiber spans numerically. The results show that the phase noise correlation of neighbouring bits is reduced for the latter one but the correlation length is increased with fiber span number and chromatic dispersion value.