Observation of stability of Gaussian beams and off-axis beam-cleaning in graded-index media

TL;DR

This paper demonstrates that Gaussian beams in large graded-index media remain stable under nonlinear effects, and complex beams can self-clean into single-lobed profiles, with experimental and numerical validation.

Contribution

It reveals the stability of Gaussian beams and the self-cleaning of complex beams in large GRIN waveguides, extending understanding beyond small fiber systems.

Findings

Gaussian beams remain nearly invariant under nonlinear effects

Spatially-complex beams can self-clean into single-lobed profiles

Numerical simulations confirm experimental observations

Abstract

While nonlinear effects in graded-index (GRIN) multimode fibers have been studied extensively, little is known about nonlinear effects in larger GRIN waveguides, where the number of modes approaches infinity and modal dispersion becomes negligible. Here we show that Gaussian beams remain nearly invariant even with large nonlinear phase accumulation and on- or off-axis trajectories in GRIN rods. In addition, spatially-complex beams can undergo self-cleaning to single-lobed profiles for both on- and off-axis trajectories. Numerical simulations exhibit the features observed in experiments, and a general interpretation of these results that makes connection to beam-cleaning phenomena observed in GRIN fibers is proposed.