Abstract

A nonlinear cavity, filled with an isotropic nonlinear medium and driven by a linearly polarized coherent input field, has been the subject of intense study over the last decade. This system can show a number of nonequilibrium phase transitions, including optical bistability and transverse pattern formation.^1-4 In particular, a mean field model³ was found to give rise to hexagonal patterns, at least for a self-focusing medium. A self-defocusing medium has not been studied in great detail for the reason that the parameter regime in which transverse pattern formation occurs coincides with the bistable regime: The instability responsibility for pattern formation then simply serves to drive the system from the unstable lower branch to the stable upper branch. This conclusion, however, is based on the assumption that the internal field in the nonlinear cavity preserves its polarization state as that of the input field. Here we extend the mean field model to include polarization effects. This extension leads to a polarization instability that can occur without accompanying optical bistability for a self-defocusing medium. In this case we find that rolls dominate close to the instability threshold, while further from equilibrium we observe a variety of structures including dislocations, disclinations, Roman arches, and target patterns.

© 1994 IEEE