Spatial beam cleanup by pure Kerr processes in multimode fibers

Recent experiments with pulse propagation in multimode graded-index fibers have shown a nonlinear improvement in beam quality, even in situations where dissipative processes such as Raman scattering play no significant role. In this Letter, numerical simulations of beam cleanup by third-order Kerr nonlinearities in a multimode fiber are used to demonstrate that in the absence of dissipative processes beam cleanup is crucially dependent on spectral/temporal disorder and does not occur in a continuous-wave model. This finding is in accordance with fundamental considerations on entropy. (C) 2018 Optical Society of America