Dispersive Hydrodynamic Dam-Break Flow in Optical Fibers

We show that optical ¯bers are an ideal vessel to observe and characterize phenomena where the light turns out to behave as a photon fluid, exhibiting behaviors that are reminiscent of typical dispersive hydrodynamic °ows. When the ¯bers operate in the strongly nonlinear regime along with normal dispersion, a distinctive trait of such °ows, is the formation of shock waves which are regularized into strongly oscillating expanding wavetrains, so-called dispersive shock waves. Such dispersive shocks are clearly observed either in four-wave mixing pumped by cw lasers, or with dark or bright pulses, being enhanced by a non-zero pedestal in the latter case. A particularly interesting case, on which we report henceforth in detail, is the evolution of an injected abrupt jump in optical power. Such problem is the photonic analogue of the well known one-dimensional dam breaking problem of hydrodynamics, where the optical power and chirp play the role of water height and horizontal velocity, whose evolution is studied after the rupture of a dam with di®erent upstream and downstream levels of still water. By using a full ¯ber optics platform we have been able to clearly observe the optical °ow, which is characterized, in this case, by the decay of the steplike 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 featuring a point of zero intensity along the envelope. This transition can be predicted by modulation theory based on applying Whitham averaging to the universal defocusing nonlinear SchrÄodinger equation. This provides an accurate analytical description of the rarefaction-shock pair in the picosecond regime of our experiment. The detailed observation of the cavitating state dynamics allows, for the ¯rst time (to the best of our knowledge), for a fully quantitative experimental test of the validity of modulation theory for any dispersive nonlinear model.