Self-filtering resonator of a cw CO2 laser

An investigation was made of a self-filtering unstable resonator in a cw CO₂ laser. The divergence of the output beam was close to the diffraction limit. An approximate comparison with a stable resonator of the same length indicated that the use of a self-filtering unstable resonator in cw electric-discharge CO₂ lasers with an output power in the kilowatt range made it possible to increase severalfold the active volume occupied by the lowest mode without significant deterioration of the beam quality or of the energy efficiency of the resonator.