TY - JOUR

T1 - Wave turbulence, thermalization and multimode locking in optical fibers

AU - Ferraro, M.

AU - Baudin, K.

AU - Gervaziev, M.

AU - Fusaro, A.

AU - Picozzi, A.

AU - Garnier, J.

AU - Millot, G.

AU - Kharenko, D.

AU - Podivilov, E.

AU - Babin, S.

AU - Mangini, F.

AU - Wabnitz, S.

N1 - This work was supported by : the EU - Next Generation EU under the Italian National Recovery and Resilience Plan (NRRP), Italy, Mission 4, CUP B53C22004050001, partnership on “Telecommunications of the Future” (PE00000001 - program “RESTART”), the European Innovation Council Pathfinder Open MULTISCOPE (101185664), the Italian Ministerial grant PRIN2022 “SAFE”, Italy (2022ESAC3K), Sapienza University of Rome Seed PNR, Italy (SP12218480C7D1E9), the Italian Ministry of Health under the Ricerca Finalizzata 2021 program, Italy (Project Code RF-2021-12373094), the Russian Science Foundation, Russia (21-72-30024-П), the Centre national de la recherche scientifique (CNRS), Conseil régional de Bourgogne Franche-Comté, iXCore Research Fondation, Agence Nationale de la Recherche, France (ANR-23-CE30-0021, ANR-19-CE46-0007, ANR-15-IDEX-0003, ANR-21-ESRE-0040). Some of the calculations were carried out using HPC resources from DNUM CCUB (Centre de Calcul, Université de Bourgogne).

PY - 2025/11

Y1 - 2025/11

N2 - We present a comprehensive overview of recent advances in theory and experiments on complex light propagation phenomena in nonlinear multimode fibers. On the basis of the wave turbulence theory, we derive kinetic equations describing the out-of-equilibrium process of optical thermalization toward the Rayleigh-Jeans (RJ) equilibrium distribution. Our theory explains the effect of beam self-cleaning (BSC) in graded-index (GRIN) fibers, whereby a speckled beam transforms into a bell-shaped beam at the fiber output. We theoretically explore the role of random refractive index fluctuations along the fiber, and show how these imperfections can assist the observation of BSC in a practical experimental setting. This conclusion is supported by the derivation of wave turbulence kinetic equations that account for the presence of a time-dependent disorder (random mode coupling). The kinetic theory reveals that a weak disorder accelerates the rate of RJ thermalization and condensation. On the other hand, although strong disorder is expected to suppress wave condensation, the kinetic equation reveals that an out-of-equilibrium process of condensation and RJ thermalization can still occur. The kinetic equations are validated by numerical simulations of the nonlinear Schrodinger equation. We outline a series of recent experiments, which permit to confirm the statistical mechanics approach for describing beam propagation and thermalization. For example, we highlight the demonstration of entropy growth, and point out that there are inherent limits to peak-power scaling in multimode fiber lasers. We conclude by pointing out the experimental observation that BSC is accompanied by an effect of modal phase-locking. From the one hand this explains the observed preservation of the spatial coherence of the beam, but also it points to the need of extending current descriptions in future research.

AB - We present a comprehensive overview of recent advances in theory and experiments on complex light propagation phenomena in nonlinear multimode fibers. On the basis of the wave turbulence theory, we derive kinetic equations describing the out-of-equilibrium process of optical thermalization toward the Rayleigh-Jeans (RJ) equilibrium distribution. Our theory explains the effect of beam self-cleaning (BSC) in graded-index (GRIN) fibers, whereby a speckled beam transforms into a bell-shaped beam at the fiber output. We theoretically explore the role of random refractive index fluctuations along the fiber, and show how these imperfections can assist the observation of BSC in a practical experimental setting. This conclusion is supported by the derivation of wave turbulence kinetic equations that account for the presence of a time-dependent disorder (random mode coupling). The kinetic theory reveals that a weak disorder accelerates the rate of RJ thermalization and condensation. On the other hand, although strong disorder is expected to suppress wave condensation, the kinetic equation reveals that an out-of-equilibrium process of condensation and RJ thermalization can still occur. The kinetic equations are validated by numerical simulations of the nonlinear Schrodinger equation. We outline a series of recent experiments, which permit to confirm the statistical mechanics approach for describing beam propagation and thermalization. For example, we highlight the demonstration of entropy growth, and point out that there are inherent limits to peak-power scaling in multimode fiber lasers. We conclude by pointing out the experimental observation that BSC is accompanied by an effect of modal phase-locking. From the one hand this explains the observed preservation of the spatial coherence of the beam, but also it points to the need of extending current descriptions in future research.

UR - http://arxiv.org/abs/2505.11299

UR - https://www.mendeley.com/catalogue/4680a07a-32e3-317a-a240-91fe61f5f928/

UR - https://www.scopus.com/record/display.uri?eid=2-s2.0-105008228893&origin=inward&txGid=24dc6397b802e14546d7d5ea56d7079a

U2 - 10.1016/j.physd.2025.134758

DO - 10.1016/j.physd.2025.134758

M3 - Article

JO - Physica D: Nonlinear Phenomena

JF - Physica D: Nonlinear Phenomena

SN - 0167-2789

M1 - 134758

ER -