Design of Variable Reflectivity Output Coupler for High Power CO2 Lasers

In order to increase the laser power of CO2 lasers the active volume has to be enlarged. For CO2 lasers there are two possibilities: to increase either the resonator length or the tube diameter. A larger tube diameter means a higher Fresnel number and the laser will be most likely operated with an unstable resonator configuration, which extracts a beam with higher beam quality than a multimode stable resonator. Increasing the resonator length while the diameter remains constant leads to a smaller Fresnel number and a stable resonator configuration becomes reasonable. However, in both cases diffraction effects will be observed due to the limiting apertures, which affect the beam quality especially for high laser powers. To reduce these effects so called variable reflectivtiy mirrors (VRM) have been introduced (1) and investigated especially for solid state lasers (2). So far most investigations have been carried out on unstable VRM-resonators.