Selection of longitudinal and transverse modes in a wide-aperture laser with a multimirror resonator

It is shown that a four-mirror resonator composed of main and auxiliary partial resonators, of plane–plane and unstable configurations respectively, can simultaneously provide a small angular beam divergence and highly coherent laser radiation. Single-frequency radiation was obtained experimentally from a pulsed CO₂ laser having an angular divergence close to the diffraction limit.