Pattern formation in 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 neglected only phase patterns are formed, and the intensity distribution is homogeneous. Walk-off changes the instability from absolute to convective for some parameter range. In the absolutely unstable regime it induces for each polarization component of light a competition between two phase stripe patterns (traveling waves) of different wavelength. Phase stripe patterns at each of the wavelengths are equally likely to be selected, and, after a transient regime of coexistence, one of them takes over. In the convectively unstable regime the existence of intensity patterns sustained by noise is shown. The patterns arise from the interference between traveling waves that are generated by the dynamical amplification of noise.