Kerr beam self-cleaning on the LP₁₁ mode in graded-index multimode fibers

Abstract

We report the experimental observation of Kerr beam self-cleaning in a graded-index multimode fiber, leading to output beam profiles different from a bell shape, close to the LP01 mode. For specific coupling conditions, nonlinear coupling among the guided modes can reshape the output speckle pattern generated by a pulsed beam into the low order LP11 mode. This was observed in a few meters long multimode fiber with 750 ps pulses at 1064 nm in the normal dispersion regime. The power threshold for LP11 mode self-cleaning was about three times larger than that required for Kerr nonlinear selfcleaning into the LP01 mode.Multimode optical fibers (MMFs) are currently extensively revisited for communication applications, and because they provide a convenient experimental platform for the investigation of complex space-time nonlinear dynamics. Various nonlinear propagation phenomena have been theoretically predicted to occur in multimode fibers since the eighties [1-5], but it is not until recently that some of them were actually observed. Consider, for instance, multimode optical solitons [6] and geometric parametric instability (GPI) [7]. On the other hand, experiments on nonlinear propagation in multimode fibers have recently revealed an unexpected effect that was named Kerr beam self-cleaning. It consists in the reshaping, at high powers, of the speckled output intensity pattern into a bell-shaped beam close to the fundamental LP01 mode of a graded index (GRIN) MMF. Such nonlinear beam evolution was observed at power levels below the threshold for frequency conversion, as well as below the self-focusing threshold, with subnanosecond to femtosecond pulses propagating in the normal dispersion regime [7][8][9][10]. It is generally admitted today that Kerr self-cleaning results from a complex nonlinear coupling, or fourwave mixing (FWM) interaction among a large population of guided modes. Namely, the combination of spatial self-induced periodic imaging and Kerr nonlinearity creates a periodic longitudinal modulation of the refractive index of the fiber core. This permits quasi-phase matching and energy exchange between guided modes by means of FWM [11]. The nonlinear energy exchange between the fundamental mode and the high-order modes (HOMs) exhibits a nonreciprocal behavior, driven by self-phase modulation [8,12]. In these conditions, all the energy transferred in the fundamental mode remains definitively trapped, which explains the robust nature of the self-cleaning process. Besides that model, alternative concepts based on instability of the HOMs [10], or on modified wave turbulence theory [4] have been introduced, which could also explain the unconventional spatial dynamics observed in MMFs.In this Letter, we report the experimental demonstration of Kerr beam self-cleaning in favor of the LP11 mode of a gradient index (GRIN) MMF. This is quite unexpected, since all previous works on Kerr beam self-cleaning, based on either numerical investigations or experiments, reported self-cleaning of the output field into a smo...

Refractive index profile tailoring of multimode optical fibers for the spatial and spectral shaping of parametric sidebands

“…Note that similar oddparity beam shapes were also experimentally observed in a more systematic study carried out with the ideal GRIN fibers in Ref. [31], and also numerically predicted in Ref. [27], in a different context of multimode solitons.…”

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

“…Tailoring of the input yielded Kerr self-cleaning on the LP11 structure, as expected (see Fig.4). This result is similar to that reported in [24], except that in that reference the self-cleaning of the LP11 mode was obtained with a lengthy and difficult to exactly reproduce manual procedure, i.e., with a tricky adjustment of tilted launching conditions. The occurrence of a similar shape and orientation of the intensity pattern both in the near field and far field attests that the phase distribution of the output field is in fact the one expected for an LP11 eigenmode (see the Appendix for details).…”

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

“…This is due to a significant reduction of the number of resonances and their corresponding efficiencies, which leads to an effective freezing of the process of thermalization and condensation. The same argument also explains why condensation is not observed in the recent experiments of beam cleaning under specific injection conditions [67], a feature that will be discussed in relation with the impact of a perturbation on the dispersion relations.…”