Optimal feedback in efficient ring double-cavity optical parametric oscillators

It is shown that the use of two feedback circuits with matched transfer constants and optimal phase incursions in a nondegenerate optical parametric oscillator (OPO) makes it possible to localise the extremes of intensity distributions of interacting waves on the output face of a nonlinear crystal, which provides maximum possible conversion efficiency of pump energy. The optimisation procedure in this case is rather flexible because it is reduced to ambiguous matching of the period and shift of the extremes of exact analytic solutions of the corresponding problem in the form of cnoidal waves with respect to the nonlinear crystal position. Unlike the single-cavity OPO scheme, both these parameters can substantially exceed the nonlinear crystal length and even tend to infinity, which corresponds to solitary soliton-like solutions.