ABSTRACT
Substrates for precision mirrors must be dimensionally stable over time, have low or near zero thermal expansion over the service temperature range, have high thermal diffusivity to minimize thermal gradients and associated distortion, be capable of achieving a very smooth surface finish and of accepting a reflective coating which meets the optical requirements. A high thermal diffusivity reduces transient temperature gradients in the mirror substrate and the consequent distortions due to nonuniform thermal expansion. Any specific application will involve a trade-off analysis among the various substrate material candidates. Thermal conductivities of metals tend to decline and those of fused silica and silica-based glasses to rise as the temperature goes up. Mirror substrates may be coated with metals, dielectric multilayers, or metals with nonmetallic overcoats depending on the specific requirements. When mirrors in laser systems become excessively hot due to energy absorption, then the application of cooling becomes necessary.
