Real-Time Measurements of Beam Self-Cleaning Dynamics

The rich nonlinear dynamics of propagation in multimode graded-index (GRIN) fibers has recently gained significant attention, with particular interest in understanding the mechanisms of “beam self-cleaning” - the preferential energy transfer from higher-order towards lower order modes induced by the Kerr effect [1], [2]. Such self-cleaning has been the subject of many studies and has been shown to be associated with a Rayleigh-Jeans distribution of modal amplitudes [2], [3]. However, all previously conducted experiments have relied on pulse averaging and assumed negligible spectral broadening during propagation. Here, we report the first real-time measurements of the full spatial field from a GRIN fiber in the nonlinear regime using off-axis holography with a tunable reference bandwidth allowing us to characterize the field associated with specific spectral components. We show for the first time that the spatial field distribution at the pump wavelength is in fact never “clean” but rather that self-cleaning is a manifestation of the spectral broadening induced by self-phase modulation (SPM) and corresponding superposition of newly generated spectral components.