Also known as front mirrors, output couplers are designed to reflect a portion of the beam back into the laser resonator for continuous amplification while transmitting a portion of the beam to the outside for use. Therefore, the substrate material must be trans-missive at the required wavelength of 10.6 microns. Germanium and Gallium Arsenide substrates are commonly used for low to medium powered systems. The more expensive Zinc Selenide material is required for higher-powered lasers because of its lower absorption at 10.6 microns.
Rear mirrors are designed to reflect all or nearly all of a laser beam back through the laser gas mixture for amplification. The inside surface is given a highly reflective (99-100%) coating. In the 100% reflective case, silicon can be used as the substrate material and the outside surface does not need polishing or coating. Some rear mirrors, however, are designed to transmit a small (0.5-1.0%) percentage of the beam to a power detector for real-time beam monitoring. These mirrors must have a tranmissive substrate and the outside surface usually has an antireflective coating.
Fold mirrors are used to lengthen the resonator beam path to increase power without physically lengthening the entire system. They are usually made from silicon substrates and have highly reflective coatings. In high-power systems, copper substrates are used because of copper’s superior thermal conductivity.