Validity of the geometric approximation in determination of the field coherence function in an unstable resonator with a randomly inhomogeneous medium

The geometric approximation is used to derive an expression for the coherence function of the lowest mode in an unstable resonator with bounded mirrors filled with a randomly inhomogeneous medium. The limits of validity of this approximation are indicated. For mirrors having a Gaussian reflection coefficient a study is made of the influence of aperture limitation of optical beams on the angular spectrum of the field in the resonator.