Waveguide operation of a wide-aperture Ar+ laser

A ring structure of the profile of a laser beam, observed in a wide-aperture unstable resonator, appeared due to reflection of the intracavity beam from the discharge channel walls. The geometric-optics approximation was used to develop a model which was in agreement with the experimental results, including the divergence and the ring propagation angles. Detailed investigations were made of near-concentric and confocal unstable resonators. The eigenmodes corresponding to the formation of a closed path by the beam in one and two round trips through the resonator were recorded. In the waveguide regime the output power in the form of blue–green Ar⁺ lines was ~5 W.