Influence of a nonlinear active medium on the structure of natural oscillation modes of a Fabry–Perot interferometer

The results are given of a parametric calculation of the mode structure of a circular Fabry–Perot resonator of a solid-state laser. It is shown that an increase in the equivalent Fresnel number of the resonator (when its value is sufficiently large) alters the mode field distribution in some cases so that it resembles the modes of increasing order occupying the main part of the resonator, whereas in other cases the mode field is concentrated in a decreasing region near the resonator axis. The existence of several different solutions for the same mode is demonstrated. In the case of resonators with an equivalent Fresnel number much greater than unity the distributions of the intensity are close to geometric in the main volume of the resonator and estimates are given of the angular divergence of the output radiation.