ABSTRACT
The surfaces of bare optical materials rarely have the desired reflectance/transmittance characteristics so they have to be coated. Reflective surfaces inside a laser resonator act both as a transmission loss and as a source of extraneous modes. These considerations are very important for low-gain media and for high-refractive-index substrates. Coatings can be anti-reflectance coatings to reduce/eliminate the surface loss, anti-reflectance coatings to eliminate constructive interference losses, controlled reflectance/transmittance characteristics, band-pass and rejection filters, or total reflectors. The deposition process and conditions have to be chosen with care so that the design of the deposited coating can be reproduced exactly and the substrate/coating stress/strain can be minimized. Electron-beam deposition has been widely used to evaporate the highly refractory materials that cannot be deposited using thermal techniques. Plasma deposition is particularly useful for depositing hard carbon and silicon coatings. Rugate technology is a very promising and flexible design technique providing the deposition process can give the desired uniformity over the substrate area.
