Single-mode laser operation in a wide-aperture unstable cavity with a semitransparent output coupler and intracavity astigmatism

The feasibility and conditions of attaining single-mode operation of a CO₂ laser with Fresnel numbers of the order of 10 and considerable intracavity astigmatism were investigated experimentally employing an unstable cavity with a homogenous output coupler. The condition for single-mode laser output was an instability of the cavity in at least one of the symmetry planes imposed by astigmatism.