Rear mirrors, typically GaAs, Ge, or ZnSe, are partial reflectors with a very high reflection-to-transmission ratio (99.0% to 99.7%), and are key optical components in laser resonators or laser cavities. Rear mirrors, like output couplers, are a part of the lasing process. Thus, high reflectivity is desired. The rear mirrors slight transmission is used in conjunction with power meters to test for laser resonator output power.
When laser resonator designs require rear mirrors to be total reflectors, Si, Cu, or Mo substrates are used, the latter being typically uncoated.