Noise Suppression Mechanisms in Regenerators Based on XGC in an SOA With Subsequent Optical Filtering

A theoretical model is presented to analyze the noise suppression characteristics in cross-gain-compression (XGC)-based regenerators, where an optical bandpass filter (BPF) is cascaded after a semiconductor optical amplifier (SOA). Analytical expressions of variance are obtained for in-phase and quadrature noises after SOA and after filter. The result shows that the physical mechanism behind XGC-based noise suppression is that gain change in the SOA is out of phase with the input power fluctuation, which leads to a negative cross-correlation term in the expression of in-phase noise variance at SOA output. On the other hand, nonlinear phase noise introduced by refractive index change in the SOA will contributes to quadrature noise, which is proportional to the square of the linewidth enhancement factor ~ αN². It is proposed that a slightly blue-shifted optical BPF can convert phase/quadrature noise-related frequency chirp into amplitude fluctuation that can cancel a part of the amplitude fluctuation after SOA, thus lead to further reduction of in-phase noise. This mechanism can be used to alleviate the negative effect of SOA-induced nonlinear phase noise and enhance the overall noise suppression performance of XGC-based regenerators, which is experimentally and numerically demonstrated at 10 Gbit/s.