Observation of geometric parametric instability induced by the periodic spatial self-imaging of multimode waves

TL;DR

This paper demonstrates the first experimental observation of geometric parametric instability sidebands in multimode fibers, showing potential for broad spectral conversion via resonant space-time coupling in nonlinear optical systems.

Contribution

It introduces the experimental demonstration of geometric parametric instability sidebands caused by natural spatial self-imaging in multimode fibers, supported by analytical and numerical validation.

Findings

Observed large frequency detuning of 123.5 THz from pump

Sidebands carried by stable bell-shaped spatial profiles

Potential for broad spectral conversion from near-infrared to visible and infrared

Abstract

Spatio-temporal 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-nanosecond 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 a 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.