Wave thermalization sets high-power limitation to spatial beam self-cleaning in multimode optical fibers

We experimentally reveal a so-far unnoticed high-power limit to spatial beam self-cleaning in graded-index nonlinear multimode optical fibers. As the optical pulse power is progressively increased, we observed that the initial Kerr-induced beam clean-up and associated improvement of the spatial beam quality are eventually spoiled. Based on a holographic mode decomposition of the output field, we show that this beam degradation can be described in a thermodynamic approach to wave propagation as a manifestation of "high-temperature" thermalization, which depletes the fundamental mode in favor of a highly multimodal power distribution.