Abstract

Optical parametric oscillators (OPOs) are devices of growing importance for several applications [1]. In the degenerate configuration OPOs permit to enhance the conversion between the pump (2ω₀, SH) and the subharmonic or fundamental (ω₀, FF) fields, both being close to two resonance modes of the cavity. However, cw-operating degenerate OPOs undergo different instabilities, such as bistability and self-pulsing. These instabilities, predicted in the framework of the dispersionless mean field model, give rise to hysteresis and chaotic self-pulsing, respectively [2-4]. In this communication we are aimed at showing that this scenario changes drastically as long as dispersion is included in the mean-field description. In particular dispersion allows for nonlinear phase-matching of nondegenerate processes, which results into a novel instability of the OPO steady-states, namely the build-up of sideband pairs equally detuned (say, by ±Ω) from both the FF and SH carriers. This novel mechanism is analogous to modulational instability recently predicted for cavityless parametric amplification [5]. In the OPO, however, the dissipative nature of the problem favours the stable formation of periodic patterns (i.e., pulse trains) at a repetition rate Ω. The stability analysis reveals that these periodic trains are stable at power much larger than the threshold for the competing chaotic self-pulsing, and hence constitute an intrinsic mechanism of stabilization of the OPO. Moreover the formation of periodic trains via modulational instability turns out to be a preferential mechanism even when the OPO is bistable, or, unexpectedly, even when it operates below the cw threshold for conversion to the subharmonic.

© 1996 Optical Society of America