Nonlinear Interference Noise in Raman-amplified WDM Systems

Raman amplification is a solution for broadband wavelength-division multiplexed (WDM) systems that require fine and reconfigurable gain tuning. However, as the fiber nonlinearity ultimately limits the performance of fiber-optic communication systems, accurate models of the nonlinear interference affecting signal propagation in the presence of multiple Raman pumps are critical for system design. In this study, we propose an extension of the time-domain nonlinear interference noise (NLIN) model for analyzing Raman-amplified polarization multiplexed WDM links. Our proposed model enables the evaluation of the NLIN power and inter-channel power transfer induced by stimulated Raman scattering (SRS) in systems with Raman-gain equalization. It also accounts for the evolution of the average signal power along the link, which varies from channel to channel. To compute the noise contributions efficiently, we employ a method that enables us to apply the model in three relevant pumping scenarios: co-propagating, counter-propagating, and bidirectional pumping. Using the model, we evaluate the differences in NLIN power over all the channels. Additionally, we can estimate the amplified stimulated emission power, which can be used to optimize the optical signal-to-noise ratio with respect to the input signal and pump powers.