The refractive index n1 of a transparent layer of quarter-wave optical thickness coating a transparent substrate of refractive index n2 can be chosen to produce half-wave retardation (HWR) in reflection and no change of polarization in refraction at any angle of incidence ø. The function n1(ø, n2), and the associated polarization-independent reflectance of the film-substrate system R(ø, n2) are determined. Such a coated surface can be used as a beam splitter with excellent characteristics (e.g., split fractions that do not depend on source polarization, a split beam whose polarization is identical to that of the incident beam and operation over a wide range of incidence angles). A concrete example of a coated Ge-slab beam splitter for 10.6-µm radiation at ø = 45° is given. The beam-splitter face of the slab is coated with the HWR layer, and the exit face is coated with a double layer that produces total refraction without change of polarization. Such a beam splitter is tolerant to film-thickness errors and is reasonably achromatic over a small (e.g., 10–11-µm) wavelength range. When used in a Michelson interferometer this beam splitter renders its operation totally independent of source polarization.
Journal of the Optical Society of America A
R. M. A. Azzam, "Thin-film beam splitter that reflects light as a half-wave retarder and transmits it without change of polarization: application to a Michelson interferometer," J. Opt. Soc. Am. A 3, 1803-1808 (1986)