Dramatic acceleration of wave condensation mediated by disorder in multimode fibers

TL;DR

This paper demonstrates that structural disorder in multimode fibers dramatically accelerates wave thermalization and condensation, explaining optical beam self-cleaning through a new kinetic theory and experimental validation.

Contribution

The authors derive a kinetic equation showing disorder-induced acceleration of wave condensation and experimentally observe this transition in multimode fibers.

Findings

Disorder accelerates thermalization by several orders of magnitude.

Wave condensation observed with up to 60% condensate fraction.

Experimental results confirm theoretical predictions.

Abstract

Classical nonlinear waves exhibit a phenomenon of condensation that results from the natural irreversible process of thermalization, in analogy with the quantum Bose-Einstein condensation. Wave condensation originates in the divergence of the thermodynamic equilibrium Rayleigh-Jeans distribution, which is responsible for the macroscopic population of the fundamental mode of the system. However, achieving complete thermalization and condensation of incoherent waves through nonlinear optical propagation is known to require prohibitive large interaction lengths. Here, we derive a discrete kinetic equation describing the nonequilibrium evolution of the random wave in the presence of a structural disorder of the medium. Our theory reveals that a weak disorder accelerates the rate of thermalization and condensation by several order of magnitudes. Such a counterintuitive dramatic acceleration of condensation can provide a natural explanation for the recently discovered phenomenon of optical beam self-cleaning. Our experiments in multimode optical fibers report the observation of the transition from an incoherent thermal distribution to wave condensation, with a condensate fraction of up to 60% in the fundamental mode of the waveguide trapping potential.