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Spatio-temporal mode coupling in highly multimode physical systems permitsnew routes for exploring complex instabilities and forming coherent wavestructures. We present here the first experimental demonstration of multiplegeometric parametric instability sidebands, generated in the frequency domainthrough resonant space-time coupling, owing to the natural periodic spatialself-imaging of a multimode quasi-continuous-wave beam in a standardgraded-index multimode fiber. The input beam was launched in the fiber by meansof an amplified microchip laser emitting sub-nanosecond pulses at 1064 nm. Theexperimentally observed frequency spacing among sidebands agrees well withanalytical predictions and numerical simulations. The first order peaks arelocated at the considerably large detuning of 123.5 THz from the pump. Theseresults open the remarkable possibility to convert a near-infrared laserdirectly into a broad spectral range spanning visible and infrared wavelengths,by means of a single resonant parametric nonlinear effect occurring in thenormal dispersion regime. As a further evidence of our strong space-timecoupling regime, we observed the striking effect that all of the differentsideband peaks were carried by a well-defined and stable bell-shaped spatialprofile.

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