Observation of Geometric Parametric Instability Induced by the Periodic Spatial Self-Imaging of Multimode Waves

Abstract

Spatiotemporal mode coupling in highly multimode physical systems permits new routes for exploring complex instabilities and forming coherent wave structures. We present here the first experimental demonstration of multiple geometric parametric instability sidebands, generated in the frequency domain through resonant space-time coupling, owing to the natural periodic spatial self-imaging of a multimode quasi-continuous-wave beam in a standard graded-index multimode fiber. The input beam was launched in the fiber by means of an amplified microchip laser emitting sub-ns pulses at 1064 nm. The experimentally observed frequency spacing among sidebands agrees well with analytical predictions and numerical simulations. The first-order peaks are located at the considerably large detuning of 123.5 THz from the pump. These results open the remarkable possibility to convert a near-infrared laser directly into a broad spectral range spanning visible and infrared wavelengths, by means of a single resonant parametric nonlinear effect occurring in the normal dispersion regime. As further evidence of our strong space-time coupling regime, we observed the striking effect that all of the different sideband peaks were carried by a well-defined and stable bell-shaped spatial profile.

Spatial beam self-cleaning in multimode fibres

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Multimode optical fibres are enjoying a renewed attention, boosted by the urgent need to overcome the current capacity crunch of single-mode fibre systems and by recent advances in multimode complex nonlinear optics [1][2][3][4][5][6][7][8][9][10][11][12][13]. In this work, we demonstrate that standard multimode fibres can be used as ultrafast all-optical tool for transverse beam manipulation of high power laser pulses.

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“…In fibres with parabolic index profile, the propagation constants of the modes take equally spaced values, so that coherent mode beating induces a periodic local intensity oscillation along the fibre, which the Kerr effect translates into a periodic longitudinal modulation of the refractive index [4,10,12]. With a simpler two-mode excitation, such type of self-induced or dynamic long-period Bragg grating was previously exploited in [25] for demonstrating light-controlled mode conversion in a GRIN MMF.…”

Efficient simulation of multimodal nonlinear propagation in step-index fibers

“…As the number of guided modes increases, one must eventually expect the numerical complexity of the method to scale as OMN P. One can improve this to ON P by abandoning the modal expansion entirely [9], evaluating the linear dispersion operator in real space by finite-differences, or possibly 2D FFTs (in which case one has OPN log N scaling). In this case, however, the dispersion operator is approximated, and P must be chosen large enough to obtain a satisfactory accuracy.…”

“…Interactions between different guided modes can lead to phase-matched four-wave mixing over wide frequency ranges and with pump wavelengths in the normal-dispersion regime of the fibers [4][5][6]. Pumping large-core multimode fibers with a parabolic index variation to minimize walkoff between guided modes has been shown to enable complex intermodal nonlinear interactions, in particular enabling short-wavelength dispersive-wave generation from soliton collapse and spatiotemporal soliton oscillations at elevated pulse energies [7,8], and sideband formation from geometric parametric instabilities [9]. Also, in the emerging low-loss hollow-core fibers guiding by antiresonant effects, intermodal nonlinearities can be important for the highly interesting frequency-conversion processes enabled by these waveguides [10].…”

Spatial and spectral nonlinear shaping of multimode waves

“…We demonstrate a new way to simultaneously control the frequency and modal content of multimode beams. Namely, we experimentally observed that the natural periodic self-imaging of multimode waves leads, via spatiotemporal modulation instability, to the generation of a series of spatially coherent sidebands with exceptionally large frequency shifts [6].…”