Threshold conditions for oscillation with the aid of shifted and unshifted dynamic gratings

An analysis is made of the threshold conditions for parametric oscillation as a result of frequency degenerate pumping in a semiopen linear resonator. It is shown that the threshold is highly sensitive to an unbalance of the pump wave intensities; in the case of media with a local nonlinear response the optimal parametric oscillation conditions correspond to exact equality of the pump wave intensities, whereas in the case of a nonlocal response the equality of pump wave intensities prevents parametric oscillation altogether. The calculated results are confirmed by experiments on parametric oscillators made of cadmium telluride (local response) and of barium sodium niobate (nonlocal response).