Abstract

The transverse field profile in oscillators with nonliinear media is determined by the interplay between the nonlinearity and the resonator geometry. In typical laser resonators the resulting field is a superposition of low order Hermite-Gaussian modes. However, there are some oscillator geometries, such as the self-imaging ring resonator[1],which do not lead to a well defined transverse mode. Since any optical ray is imaged back onto itself after one cavity round trip, there is no preferred optical axis. Diffraction effects will lead inevitably to high losses for rays which propagate far from the optical axis, however, there still exist a large number of possible modes, all with very similar losses. In this paper we show how to actively define the optical axis in such a self-imaging resonator by placing saturable photorefractive gain and loss nonlinearities in conjugate resonator planes. The transverse profile maximizes the oscillating power by saturating the photorefractive gain as little as possible, and saturating the photorefractive loss as much as possible. The resulting transverse mode turns out to be highly localized, but with an arbitrary axis of propagation inside the resonator.

© 1992 Optical Society of America