  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.
