To suppress the change in polarization of the transmitted He-Ne or CO2 laser beam through a system consisting of four spherical mirrors, a system of four separate transparent thin films is designed. Each mirror is initially coated opaquely with silver, and the transparent film for preserving the polarization state is on top of the silver substrate. The thin film designs for the system are obtained using a computer optimization process, and their stability for random thickness errors and for the change in the angle of incidence are checked. The final design obtained for a He-Ne laser beam uses TiO2 film and that for a CO2 laser beam uses PbTe film.
Sung Hong Hahn,
Sang Soo Lee,
"Nearly Polarization Preserving Dielectric Thin Film Design For A Four-Spherical-Mirror System," Optical Engineering 28(12), 281306 (1 December 1989). https://doi.org/10.1117/12.7977148