Concerning the equations governing nonlinear pulse propagation in randomly birefringent fibers

The multiple-scale expansion is used to derive averaged equations governing the slow evolution of a pulse in a randomly birefringent, non-polarization-preserving fiber. It is shown that, in the limit when the average beat length is much less than the correlation length of random variations of the fiber’s parameters, these equations are two nonlinearly coupled nonlinear Schrödinger equations, with the cross-coupling coefficient being, in general, different from 1. The effect of perturbations of the averaged equations that result from randomness of the fiber parameters is estimated.