Influence of birefringence- and core ellipticity-induced mode coupling on the gain statistics of forward-pumped Raman amplified few-mode fiber links

The equations describing light propagation in a few-mode fiber for space-division multiplexing are derived under the presence of linear mode coupling and both Kerr- and Raman-induced nonlinearity. By considering physical models of stress birefringence and core ellipticity, the effect of such fiber imperfections on the gain of a forward-pumped Raman-amplified link is assessed through numerical simulations. The average gain and the variation of signal power at the output of the amplified fiber span is numerically evaluated for different levels of coupling strength in fibers supporting 2 and 4 groups of LP modes, identifying three main propagation regimes and assessing the effect of coupling between different groups of degenerate modes.