Statistical mechanics of beam self-cleaning in GRIN multimode optical fibers

Abstract

Since its first demonstration in graded-index multimode fibers, spatial beam self-cleaning has attracted a growing research interest. It allows for the propagation of beams with a bell-shaped spatial profile, thus enabling the use of multimode fibers for several applications, from biomedical imaging to high-power beam delivery. So far, beam self-cleaning has been experimentally studied under several different experimental conditions. Whereas it has been theoretically described as the irreversible energy transfer from high-order modes towards the fundamental mode, in analogy with a beam condensation mechanism. Here, we provide a comprehensive theoretical description of beam self-cleaning, by means of a semi-classical statistical mechanics model of wave thermalization. This approach is confirmed by an extensive experimental characterization, based on a holographic mode decomposition technique, employing laser pulses with temporal durations ranging from femtoseconds up to nanoseconds. An excellent agreement between theory and experiments is found, which demonstrates that beam self-cleaning can be fully described in terms of the basic conservation laws of statistical mechanics.

Multimode optical fiber beam-by-beam cleanup

“…However, one may appreciate that, in the spite of the different wavelengths and pulse durations of the two laser sources, the threshold input peak powers remain relatively similar in both two experiments. This is consistent with previous observations that beam self-cleaning is a purely spatial effect while group velocity dispersion remains negligible [31].…”

“…Because of its all-in-fiber nature, beam-by-beam cleaning can be naturally incorporated in state-of-the-art MMF-based technologies, such as space division multiplexed networks and high-power fiber lasers. Finally, since the beam self-cleaning effect has been described as result of wave thermalization [31], [33], our results may pave the way for new developments in the physics of multimode wave systems.…”

“…However, the occurrence of beam self-cleaning does not have a precise power threshold, so that suddenly it appears when a certain input power and coupling condition is reached. To the contrary, it is a phenomenon that leads a beam towards its state of equilibrium (bell-shape), which is gradually reached upon its propagation [31].…”

Thermal control of the topological edge flow in nonlinear photonic lattices

“…Quite recently, optical thermalization effects have been observed for the first time in optical multimode fibers 50 , 51 and time-synthetic lattices 52 . In principle, photonic topological lattices can be inscribed in silica glass 4 , 9 , 53 or can be implemented in time-synthetic dimensions.…”

Interplay of thermalization and strong disorder: Wave turbulence theory, numerical simulations, and experiments in multimode optical fibers

“…This phenomenon received a recent renewed interest with the discovery of spatial beam cleaning in multimode optical fibers (MMFs) [19][20][21]. Along this line, RJ thermalization and light condensation in MMFs have been discussed [22][23][24][25][26][27][28] and recently observed experimentally [29][30][31][32][33].…”