SPATIOTEMPORAL INSTABILITIES AND THRESHOLD CONDITION IN A BROAD-AREA OPTICAL PARAMETRIC OSCILLATOR

The role of diffraction on the threshold condition for signal generati on in a broad-area, doubly-resonant optical parametric oscillator (OPO ) operating close to degeneracy is analytically investigated starting from a three-dimensional model of the OPO equations. The instability d omain for signal generation, as derived in terms of general cavity par ameters, is shown to be composed by two families of multiballoon (or m ulticonical) regions, which correspond to the two discrete sets of lon gitudinal modes for the OPO. Depending on the cavity detuning paramete r, parametric oscillation is preferred to occur on either one of the t wo mode families, and may be associated to an off-axis emission. Due t o the conservation of the photon momentum in the nonlinear down-conver sion process, a discrimination among the different instability balloon s exists, which explains the asymmetric behavior of threshold curves v ersus cavity length tuning and makes unlikely the occurrence of multic onical emission in the OPO. (C) 1998 Elsevier Science B.V. All rights reserved.