Durable, Non-Changing Metal-Dielectric And All-Dielectric Mirror Coatings

Many aerospace applications of optics require fully or partially reflecting mirrors with stable optical properties and long term durability. Two effects which are detrimental to the performance of reflective optical systems can commonly occur with conventional mirror coatings: (a) decrease of reflectance over an extended period of use due to environmental contamination or to change in the chemical composition of the reflecting layer caused by oxidation or corrosion, and (b) the spectral shift of the reflectance characteristic upon exposure to variable humidity and/or temperature. The latter is particularly common with standard electron beam deposited multilayer thin films, and becomes critical if the mirror has a dichroic or polarizing function which usually requires long term stability of the color-separating or polarizing edge of the spectral response. The reactive low voltage ion plating process produces very dense, smooth and hard coatings with long term stability of the spectral characteristics and are not affected by exposure to variable humidity, temperature, or vacuum. This paper summarizes previous results obtained with protected aluminum mirrors, polarizing beam splitters, and high reflectors for laser applications, and presents some new results obtained with durable non changing coatings