The output coupler usually has reflectance in the range 40% to 70%. In order to optimize power and spatial profile of the laser beam, the output coupler and rear mirror have optical surfaces with well-defined radii of curvature. As the output coupler transmits a high-power laser beam, it is made of ZnSe in order to minimize absorption and thermal lensing. The rear mirror usually has reflectance 99.5% which means that the transmitted laser beam has low power. However, as absorption in the coating is higher than in the output coupler, it is more important to use a substrate material with high thermal conductivity. Therefore, Germanium or Gallium Arsenide are used in most lasers.

The output coupler usually has reflectance in the range 40% to 70%. In order to optimize power and spatial profile of the laser beam, the output coupler and rear mirror have optical surfaces with well-defined radii of curvature. As the output coupler transmits a high-power laser beam, it is made of ZnSe in order to minimize absorption and thermal lensing. The rear mirror usually has reflectance 99.5% which means that the transmitted laser beam has low power. However, as absorption in the coating is higher than in the output coupler, it is more important to use a substrate material with high thermal conductivity. Therefore, Germanium or Gallium Arsenide are used in most lasers.

Folding mirrors are quite similar to turning mirrors. However, durability of the coating is more critical because the folding mirrors are exposed to the gas discharge and the extremely high laser power density inside the cavity. Therefore, the coating must have best-possible environmental resistance and low absorption in order to minimize thermal distortion.

Windows for CO2 lasers are used either for protecting sensitive and/or expensive optics, or for separating areas with different gas pressures. In all applications, high transmittance and low absorption are needed in order to minimize distortion of the laser beam. Therefore, such windows usually consist of ZnSe substrates with AR-coatings on both surfaces (same as focussing lenses).