Modal phase-locking in multimode nonlinear optical fibers

Spatial beam self-cleaning, a manifestation of the Kerr effect in graded-index multimode fibers, involves a nonlinear transfer of power among modes, which leads to robust bell-shaped output beams. The resulting mode power distribution can be described by statistical mechanics arguments. Although the spatial coherence of the output beam was experimentally demonstrated, there is no direct study of modal phase evolutions. Based on a holographic mode decomposition method, we reveal that nonlinear spatial phase-locking occurs between the fundamental and its neighboring low-order modes, in agreement with theoretical predictions. As such, our results dispel the current belief that the spatial beam self-cleaning effect is the mere result of a wave thermalization process. (c) 2023 Optica Publishing Group

Automated summary

Spatial beam self-cleaning in graded-index multimode fibers, arising from the Kerr effect, involves nonlinear power transfer among modes to produce robust bell-shaped output beams. Although spatial coherence of the output beam has been experimentally demonstrated, direct study of modal phase evolutions has been lacking. Using a holographic mode decomposition method, our findings reveal nonlinear spatial phase-locking between the fundamental mode and neighboring low-order modes, confirming theoretical predictions and challenging the current belief of wave thermalization as the sole cause of the spatial beam self-cleaning effect.