Actively cooled ZnSe output coupler for high-power CO2 lasers

In CO2-laser engineering, combining high output power with a good laser beam quality is a very challenging design problem. Due to residual absorption in the output coupler (O.C.), thermal aberrations are added to the transmitted beam which result in thermal focussing of the window and in a decreased beam quality. Using thermal finite element analysis in combination with the new concept of embedded Gaussian beam, the influence of the cooling geometry on beam quality can be studied in detail. In particular, the case is considered where the usual conductive cooling is complemented by convective cooling through a gas film blown over the outer surface of the output coupler. To confine the gas jet over this surface a diamond machined piece of ZnSe is used. We present experimentally obtained values of the laser beam quality on a 2.5 kW and a 4 kW resonator, provided with this actively cooled output coupler.