Stability of the caustic in planospherical resonators with an internal lens

theoretical analysis is made of planospherical resonators with an internal lens. A proposal is made to use dimensionless parameters which make it possible to represent the stability diagram of the investigated resonators in a generalized form. A derivation is given of the equation for an ellipse in the stability diagram defining resonators with a stable caustic. It is shown that the range of permissible changes in the optical power of a thermal lens is inversely proportional to the square of the fundamental-mode radius in the internal lens plane. The analysis made makes it possible to design solid-state laser resonators insensitive to changes in the optical power of a lens induced thermally in the active element.