Characterization of the modal distribution from linear and nonlinear mode coupling in multimode fibers

We introduce a new modal decomposition method which permits to analyse, with unprecedented accuracy, the mode power distribution from long spans of multimode optical fibers. We show that a Bose-Einstein distribution permits to describe the steady-state distribution both in the linear and in the nonlinear regime, up to soliton generation. The distribution results from the interplay of strong linear disorder caused by fiber imperfections, and nonlinear four-wave mixing.