  In an optical experiment, we report a wave turbulence regime that, starting
with weakly nonlinear waves with randomized phases, shows an inverse cascade of
photons towards the lowest wavenumbers. We show that the cascade is induced by
a six-wave resonant interaction process and is characterized by increasing
nonlinearity. At low wavenumbers the nonlinearity becomes strong and leads to
modulational instability developing into solitons, whose number is decreasing
further along the beam.
