Unstable resonator with a semitransparent exit mirror for a fast-flow CO2 laser

A theoretical and experimental investigation was made of an unstable resonator having a semitransparent exit mirror and a magnification close to unity with Fresnel numbers of 5–15 and active medium gains of 0.5–2.4. Numerical modeling was employed in order to find a condition for minimum diffraction losses. Under the conditions of an electric-discharge fast-flow kilowatt CO₂ laser, the radiation divergence was improved by roughly a factor of two relative to that for a telescopic resonator without an appreciable reduction in the mode volume and output power.