Proceedings Paper • newInfrared thin film polarization preserving reflectors
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The design of ideal infrared thin-film polarization preserving reflectors requires the equalization of the p and s polarization reflectances and zero differential phase shift between them. Depending on system design requirements for absolute reflectivity, either non-absorbing substrates such as zinc selcnide, or metallic films such as silver or gold are commonly utilized. In addition, a few dielectric layers are deposited onto the substrate for reflection enhancement and phase correction. This paper will investigate the design of enhancement layers with refractive indices n1 and n2, onto various substrates for a single wavelength and in most cases, 45 degrees angle of incidence. Indices of actual film materials such as thorium fluoride, germanium and zinc sulfide will be utilized to demonstrate actual design performance. Also, an equation is presented that is used to predict the differential phase shift sensitivity to wavelength centering of a quarter wave stack. In this case, the film indices determine the incidence angle sensitivity. Next, the incidence angle sensitivity of some designs is investigated. Some designs act as polarization preserving reflectors from normal incidence to nearly 80 degrees angle of incidence. A brief summary of conventional enhanced metal coatings is presented, along with design methods. Some applications of polarization preserving reflectors are described, especially for C02 lasers.