Patterns, space-time chaos and topological defects in nonlinear optics

By increasing the Fresnel number F of a ring cavity with photorefractive gain, we show the transition between a low F regime, where few modes compete with a regular (periodic alternation) or irregular (chaotic itinerancy) time patterns, and a high F regime, where many modes oscillate simultaneously, giving rise to chaotic domains spatially correlated over a length much shorter than the wavefront size (spatiotemral chaos). Investigating the phase of the two dimensional optical field we show that, for increasing F, it is characterized by an increasing number of topological defects. We show that the defect pattern evolves in time according to nonlinear dynamics and that the mean separation of the defects is closely related to the spatial correlation length of the field. Finally we present a theoretical model which contains the relevant physical aspects of the experiment, and which gives numerical results in qualitative agreement with the experiment. © 1992.