Abstract

Compared to their single-mode counterparts, multimode fibers have traditionally been excluded from any short-pulse applications because of their intrinsic intermode dispersion, already limiting data rates to MSa/s on rather short distances. Recent experimental observations of beam self-cleaning [1] have dramatically changed our view on multimode fibers, promising the concentration of large pulse energies in the fundamental transverse mode of such a fiber and, possibly, even data rates that could previously only be demonstrated in single-mode fibers, yet with much higher signal-to-noise ratios. Even more recently, a thermodynamic explanation for these findings emerged, suggesting a thermalization process of the modal contents that eventually converges towards a Rayleigh-Jeans (RJ) distribution [2,3]. Here, I will outline that the RJ distribution appears only as one limiting case in a far more general scenario. Moreover, the RJ law requires a number of restrictive assumptions that do not seem to be generally fulfilled. In particular, the generalized thermodynamic description is not limited to closed or conservative systems. Consequently, an entropy decrease with nonlinear propagation does not automatically lead to a breach of the second law of thermodynamics, which emerged as the moot point of previous attempted thermodynamic descriptions.

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