  We investigate the temporal photonic analogue of the dam-break phenomenon for
shallow water by exploiting a fiber optics setup. We clearly observe the decay
of the step-like input (photonic dam) into a pair of oppositely propagating
rarefaction wave and dispersive shock wave. Our results show evidence for a
critical transition of the dispersive shock into a self-cavitating state. The
detailed observation of the cavitating state dynamics allows for a fully
quantitative test of the Whitham modulation theory applied to the universal
defocusing nonlinear Schroedinger equation.
