  Optical solitons in multimode fibers have been predicted 40 years ago and
extensively investigated theoretically. Transmission experiments in nonlinear
multimode fibers have gained renewed interest, motivated by their potential to
extend the capacity of long-distance transmission systems; only in the last few
years, new experiments have revealed unexpected properties of optical solitons
propagating in graded-index and step-index multimode fibers, partially
re-writing the existing theory. Here we provide an overview of the recent
experimental, numerical and theoretical studies which revealed those new
properties. It will be shown that multimode fiber solitons form with specific
pulse width and energy dependent on the wavelength, and that they naturally
evolve toward fundamental-mode Raman solitons. New soliton fission mechanisms,
governed by the modal dispersion, will be explained. Possible applications in
space-division multiplexed systems will be discussed. A recent thermodynamic
approach to soliton condensation will be described.
