Wavefront shaping for optimized many-mode Kerr beam self-cleaning in graded-index multimode fiber

Abstract

We report experimental results, showing that the Kerr beam self-cleaning of many low-order modes in a graded-index multimode fiber can be controlled thanks to optimized wavefront shaping of the coherent excitation beam. Adaptive profiling of the transverse input phase was utilized for channeling the launched power towards a specific low-order fiber mode, by exploiting nonlinear coupling among all guided modes. Experiments were carried out with 7 ps pulses at 1064 nm injected in a five meters long multimode fiber operating in the normal dispersion regime. Optimized Kerr beam selfcleaning of five different LP modes is reported, with a power threshold that increases with the mode order.

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

“…It is important to stress, however, that there exist particular conditions of beam injection into the MMF that can excite only few modes of the fiber, as it has been recently reported in Refs. [67,90]. Consider for instance the case where a Gaussian beam with a radius comparable to that of the fundamental mode is injected at perfect normal incidence and exactly at the center of the MMF.…”

“…We note that such specific initial conditions are commented by the authors of Refs. [67,90], who state that beam-cleaning on the LP 11 mode requires "a lengthy and difficult to exactly reproduce manual procedure, i.e., with a tricky adjustment of tilted launching conditions".…”

“…When many modes are excited through generic initial conditions in the presence of a significant amount of structural disorder (L d L nl , z), the system exhibits a well developed discrete turbulence regime that is accurately described by the discrete kinetic equation derived here. On the other hand, for specific initial conditions characterized by the excitation of a small number of modes, then the discrete structure of the resonance manifold of MMFs can freeze the thermalization and thus the condensation processes within the short fiber lengths considered in the experiments (5-8m in [67,90]). The derived kinetic equations should not be relevant to describe this regime of few-mode interaction over small propagation lengths.…”

“…In particular, the impact of disorder may be considered as perturbative in this regime where the system can exhibit a partially phase-sensitive coherent interaction, which may probably be described by the tools developed to study finite size effects in turbulence, such as discrete and mesoscopic wave turbulence and the associated clusters of resonant mode interactions [7,[55][56][57][58][59][60][61][62][63][64][65][66]. Such a partially coherent few-mode interaction regime can be mastered by a fine tuning of the transverse profile of the laser excitation [67,90]. In the recent experiments [90] beam cleaning of many low-order modes has been reported owing to an ingenious feedback-induced adaptive profiling of the transverse wavefront phase of the coherent beam excitation (also see [91]).…”

Controlling spatiotemporal nonlinearities in multimode fibers with deep neural networks

“…Deep neural networks (DNNs) proved useful for classification/ reconstruction of the information sent to km-long MMFs solely from the intensity measurement at the output of the fibers 19,20 . For the nonlinear propagation regime, only adaptive algorithms have been brought forth and been shown to be successful in harnessing the entangled spatiotemporal nonlinearities such as Kerr beam self-cleaning of loworder modes 21 and optimization of the intensity of targeted spectral peaks generated by Raman scattering or four-wave In this article, we report the results of our studies on the effect of initial excitation condition of the GRIN MMFs to nonlinear spatiotemporal propagation by employing machine learning approaches. Specifically, we achieved control over multimodal nonlinear frequency conversion dynamics and demonstrated that spatiotemporal nonlinear pulse propagation can be learned by DNNs.…”