Abstract

Recent studies on wave turbulence revealed that a purely classical system of random waves can exhibit a process of condensation, whose thermodynamic properties are analogous to those of Bose-Einstein condensation [1-3]. Classical wave condensation finds its origin in the natural thermalization toward the Rayleigh-Jeans equilibrium distribution, whose divergence is responsible for the macroscopic occupation of the fundamental mode of the system. The experimental study of condensation in a conservative (cavity-less) configuration constitutes a major challenge, because of the prohibitive large propagation lengths required to achieve thermalization. In contrast with this commonly accepted opinion, a remarkable phenomenon of spatial beam self-cleaning has been recently discovered in graded-index multimode optical fibers (MMFs) [4-7]. This phenomenon is due to a purely conservative Kerr nonlinearity [7] and its underlying mechanism still remains debated.

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