Energy and wave-action flows underlying Rayleigh-Jeans thermalization of optical waves propagating in a multimode fiber

TL;DR

This paper investigates how optical waves in multimode fibers undergo thermalization and condensation driven by energy and wave-action flows, supported by experimental and theoretical analysis of the wave dynamics.

Contribution

It demonstrates the energy and wave-action flow mechanisms underlying Rayleigh-Jeans thermalization in multimode optical fibers, including experimental evidence and stability analysis.

Findings

Light condensation driven by energy flow to higher-order modes

Bidirectional wave-action redistribution between fundamental and higher modes

Possibility of inverted flows leading to negative temperature equilibrium

Abstract

The wave turbulence theory predicts that a conservative system of nonlinear waves can exhibit a process of condensation, which originates in the singularity of the Rayleigh-Jeans equilibrium distribution of classical waves. Considering light propagation in a multimode fiber, we show that light condensation is driven by an energy flow toward the higher-order modes, and a bi-directional redistribution of the wave-action (or power) to the fundamental mode and to higher-order modes. The analysis of the near-field intensity distribution provides experimental evidence of this mechanism. The kinetic equation also shows that the wave-action and energy flows can be inverted through a thermalization toward a negative temperature equilibrium state, in which the high-order modes are more populated than low-order modes. In addition, a Bogoliubov stability analysis reveals that the condensate state is stable.