Pattern formation in the presence of walk-off for a type II optical parametric oscillator

We show the relevance of walk-off effects in pattern formation in a type II optical parametric oscillator at frequency degeneracy. With walk-off negle cted only phase patterns are formed, and the intensity distribution is homo geneous. Walk-off changes the instability from absolute to convective for s ome parameter range. In the absolutely unstable regime it induces for each polarization component of light a competition between two phase stripe patt erns (traveling waves) of different wavelength. Phase stripe patterns at ea ch of the wavelengths are equally likely to be selected, and, after a trans ient regime of coexistence, one of them takes over. In the convectively uns table regime the existence of intensity patterns sustained by noise is show n. The patterns arise from the interference between traveling waves that ar e generated by the dynamical amplification of noise. (C) 1999 Optical Socie ty of America [S0740-3224(99)00209-X].