Influence of walkoff on pattern formation in nondegenerate optical parametric oscillators - art. no. 016604

Convective and absolute nature of instabilities in nondegenerate optical pa rametric oscillators with large transverse section, for negative detunings and in the presence of walkoff, is examined. The asymptotic response of the signal and idler fields to a transverse localized two-dimensional perturba tion is evaluated. The presence of walkoff breaks the rotational symmetry i n the transverse plane, and the system, at the absolute instability thresho ld, selects traveling waves propagating in the walkoff direction among an i nfinity of unstable spatiotemporal modes. We show that in optical parametri c oscillators (OPO's) with negative detunings, contrary to the case of posi tive detunings, the walkoff shrinks the region of convective instabilities, and even may suppress the convective/absolute transition. Hence, in a cert ain range of parameters, signal field envelopes in the form of wave packets of zero group velocity are found where the instability is absolute at the onset, although the walkoff is present. We also show that nonlinear pattern selection is ruled by the cross-coupling terms appearing in the asymmetric coupled Ginzburg-Landau equations derived near-threshold of the signal and idler generation. The numerical solutions of the original OPO equations co nfirm the analytical predictions for the values of the instability threshol ds and the corresponding selected patterns.