Coherent combining of self-cleaned multimode beams

Abstract

A low intensity light beam emerges from a graded-index, highly multimode optical fibre with a speckled shape, while at higher intensity the Kerr nonlinearity may induce a spontaneous spatial self-cleaning of the beam. Here, we reveal that we can generate two self-cleaned beams with a mutual coherence large enough to produce a clear stable fringe pattern at the output of a nonlinear interferometer. The two beams are pumped by the same input laser, yet are self-cleaned into independent multimode fibres. We thus prove that the self-cleaning mechanism preserves the beams’ mutual coherence via a noise-free parametric process. While directly related to the initial pump coherence, the emergence of nonlinear spatial coherence is achieved without additional noise, even for self-cleaning obtained on different modes, and in spite of the fibre structural disorder originating from intrinsic imperfections or external perturbations. Our discovery may impact theoretical approaches on wave condensation, and open new opportunities for coherent beam combining.

Energy and wave-action flows underlying Rayleigh-Jeans thermalization of optical waves propagating in a multimode fiber (a)

“…2(c). Note that the properties of phase coherence of a condensed cleaned beam have been recently studied experimentally [48].…”

Latest experimental advances in nonlinear multimode fiber optics

“…Specifically, we demonstrated that the Kerr-induced self-cleaning mechanism preserves the beams' mutual coherence via a noise-free parametric process. Indeed, we have shown that it is possible to generate two independently self-cleaned beams with a mutual coherence high enough to produce a clear stable fringe pattern at the output of a nonlinear interferometer [5]. Next, we will discuss the possibility to induce single-mode short-wave/mid-infrared supercontinuum based on the recently observed space-time effect, the so-called geometric parametric instability in graded-index MMFs.…”

Spatial Beam Evolution in Nonlinear Multimode Fibers

“…Spatial beam self-cleaning has been reported in both the normal and in the anomalous dispersion regimes, e.g., at 1.064µm and 1.5µm, thus opening the way for possible applications in laser manufacturing and life science [4]. In addition, the spatial self-cleaning process can generate coherent beams, which opens up new perspectives for the generation of high powers based on coherent beam combining methods [5].…”