In the cavity of a CO2 laser, CO2 molecules are excited by a gas discharge. This excitation energy is fed into a laser beam if it has sufficient intensity. For building up this intensity, mirrors are placed at both ends of the discharge such that the laser beam is reflected back and forth many times. Such an arrangement is called a laser cavity. In real cavities, both mirrors have some transmittance: one of them is the output coupler where the transmitted beam constitutes the useable laser beam; the other one is the rear mirror where the transmitted beam has very low intensity and is used for controlling purposes. - For building up a laser cavity with output laser power of several kW, the total length of the discharge needs to be several meters. Covering this distance with one discharge is very problematic. Therefore, it is split up into several discharges working in line. In order to make the mechanical setup as compact as possible, the path of the laser beam within the laser cavity is "folded" several times by using suitable mirrors which are called total reflectors
Output coupler and End mirror
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.
Total Reflectors are quite similar to turning mirrors. However, durability of the coating is more critical because the total reflectors 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.