Observation of pattern formation in optical parametric oscillators

We have observed transverse pattern formation in a near-confocal, type II, triply resonant optical parametric oscillator (OPO) operating close to freq uency degeneracy for the signal and idler modes. In the region of observati on of these patterns, the OPO oscillation threshold increases. Complex ring patterns and latticelike structures are observed. These patterns are due t o the nonlinear coupling of many transverse modes (on the order of 25). The transverse distribution of the light is different in the signal, idler, an d transmitted pump modes.