Wave condensation with weak disorder versus beam self-cleaning in multimode fibers

Abstract

Classical nonlinear random waves can exhibit a process of condensation. It originates in the singularity of the Rayleigh-Jeans equilibrium distribution and it is characterized by the macroscopic population of the fundamental mode of the system. Several recent experiments revealed a phenomenon of spatial beam cleaning of an optical field that propagates through a graded-index multimode optical fiber (MMF). Our aim in this article is to provide physical insight into the mechanism underlying optical beam self-cleaning through the analysis of wave condensation in the presence of structural disorder inherent to MMFs. We consider experiments of beam cleaning where long pulses are injected in the and populate many modes of a 10-20 m MMF, for which the dominant contribution of disorder originates from polarization random fluctuations (weak disorder). On the basis of the wave turbulence theory, we derive nonequilibrium kinetic equations describing the random waves in a regime where disorder dominates nonlinear effects. The theory reveals that the presence of a conservative weak disorder introduces an effective dissipation in the system, which is shown to inhibit wave condensation in the usual continuous wave turbulence approach. On the other hand, the experiments of beam cleaning are described by a discrete wave turbulence approach, where the effective dissipation induced by disorder modifies the regularization of wave resonances, which leads to an acceleration of condensation that can explain the effect of beam self-cleaning. The simulations are in quantitative agreement with the theory. The analysis also reveals that the effect of beam cleaning is characterized by a repolarization as a natural consequence of the condensation process. In addition, the discrete wave turbulence approach explains why optical beam self-cleaning has not been observed in step-index multimode fibers.

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References in corpus (18)

• Quantum fluids of light
• Statistical mechanics and dynamics of solvable models with long-range interactions
• Bose-Einstein condensation of photons in an optical microcavity
• Thermodynamic theory of highly multimoded nonlinear optical systems
• Nonlinear propagation in multi-mode fibers in the strong coupling regime
• Soliton trapping in multimode fibers with random mode coupling
• Condensation in disordered lasers: theory, 3D+1 simulations and experiments
• Dramatic acceleration of wave condensation mediated by disorder in multimode fibers
• Fast and accurate modelling of nonlinear pulse propagation in graded-index multimode fibers
• Prethermalization of atoms due to photon-mediated long-range interactions
• Dissipative dynamics of superfluid vortices at non-zero temperatures
• Wavefront shaping for optimized many-mode Kerr beam self-cleaning in graded-index multimode fiber
• Discrete Wave Turbulence
• Resonant interactions of nonlinear water waves in a finite basin
• Light focusing in the Anderson Regime
• Robust energy transfer mechanism via precession resonance in nonlinear turbulent wave systems
• Statistical physics of nonlinear wave interaction
• On the origins of spectral broadening of incoherent waves:Catastrophic process of coherence degradation