A novel analysis of the impact of parametric gain on WDM systems

In most practical fiber systems, the received ASE noise is a non-white random process, and may present a correlation between the in-phase and quadrature components, with respect to the optical carrier. This is due to such nonlinear effects as Parametric Gain (PG), which is induced by the interaction of fiber Kerr nonlinearity and quadratic dispersion. In WDM systems, the optical signals propagating along the fiber act as a set of pumps and generate spectral regions where small signals experience PG. In long-haul WDM optically amplified links, where signals propagate together with ASE noise, added by in-line EDFAs, optical power can be transferred from signals to noise, thus significantly modifying the ASE spectral shape at the receiver and potentially degrading the system performance. In this paper, we show the results obtained analyzing the performance of a realistic long-haul optical WDM system, using a method based on a properly chosen Karhunen-Loeve expansion of the input ASE noise. Our method is an extension of the technique originally introduced for the study of radar non-coherent detection. The analysis has been tested through comparisons with simulation results on several system scenarios, obtaining analytical and simulated BER plots differing for less than 0.05 dB