Nonlinear waves in multimode fibers

We overview recent advances in the field of nonlinear guided wave propagation in multimode fibers. It is only in recent years that the study of nonlinear optics in multimode fibers has experienced a revival of research interest. Nonlinear arrays of linearly coupled multicore fibers permit spatiotemporal reshaping and coherent combining of ultrashort optical pulses. Spatial-division multiplexing is an emerging technology for increasing the capacity of optical communication links, and the presence of nonlinear mode coupling requires a careful analysis. Multimode nonlinear fibers have a strong potential for the implementation of a new class of high-power fiber lasers. We describe experiments of Raman beam cleanup that permit to implement quasi-single-mode Raman fiber laser with multimode pumps. Next we discuss the recently discovered effect of Kerr beam self-cleaning, whereby a speckled signal at the output of a multimode fiber may evolve toward a bell-shaped transverse profile.