We consider the communication channel given by a fiber optical transmission line. We develop a method to perturbatively calculate the information capacity of a nonlinear channel, given the corresponding evolution equation. Using this technique, we compute the decrease of the channel capacity to the leading order in the perturbative parameter for fiber optics communication systems.
Evgenii E. Narimanov and Partha Mitra, "The Channel Capacity of a Fiber Optics Communication System: Perturbation Theory," J. Lightwave Technol. 20, 530- (2002)