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Perfect mirrors from a coating designer's point of view

Most quarterwave-stack laser reflectors and IR blocking filters have the capability of reflecting nearly 100 percent of the light over an extended wavelength range at all angles of incidence. These so-called 'perfect mirrors' have been around for many years, but the omnidirectional property has been recognized only recently. This paper reviews the properties of omnidirectional reflectors and points out an interesting feature related to the optimum design that may not have been anticipated from the point of view of an optical-coating designer. It is also shown that the wavelength region of omnidirectional performance of a single quarterwave-stack reflector can be extended by using standard thin-film design methods. For example, four adjacent quarterwave-stack reflectors can provide close to 100 percent reflectance at all angle over the complete visible spectrum. Important features of the designs that limit their usefulness as omnidirectional band-pass or edge filters are presented.