Abstract

In optical parametric oscillators (OPO’s) the phase-matching condition among the pump and the signal/idler is reached by exploiting the crystal birefringence. Quite often, this implies that beams transversally walk off each other because of the misalignement of the Poynting vectors. The walk-off can affect the transverse pattern formation causing the pattern to drift and turning the instability to be convective. In the convectively unstable regime noise can be amplified and gives rise to macroscopic patterns [1]. Here, we demonstrate that the presence of the walk-off also affects the process of pattern selection. The term breaks the rotational symmetry and stripes orthogonal to the direction of the walk-off are preferentially selected. The wave vector of the selected stripes depends on the walk-off too. This effect is enhanced at small signal detunings, both negative and positive; there, the value of the selected wave-vector can be sensibly different from the predictions obtained neglecting the walk-off. In particular, the analysis indicates that a structure can be also observed for positive detunings, where the zero wavenumber (no pattern) is selected when neglecting walk-off effects. This is confirmed by the numerical solutions of the governing equations which model a realistic device where pumping is restricted to a portion of space of large enough size. The observed patterns consist of a sequence of dark stripes (i.e. stripes of phase defects) as shown in figures 1 and 2.

© 1998 IEEE