Coherence of the radiation field at the exit from an unstable resonator filled with a weakly scattering medium

The approximation of the random phase screen model is used to solve analytically the problem of the coherence function of an optical field in an unstable resonator filled with a weakly scattering medium. The range of validity of the solution is determined. It is found that the coherence function of an optical field in an unstable resonator represents a multicomponent mode and the number of components is closely related to the efficiency of "regularization" of the field by the resonator. An approximate expression is derived for the energy losses of an optical beam per pass through the resonator.