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Laser Beam Products has developed a line of 90° phase
retarding mirrors. They are designed for CO2
lasers. They are tested with a high power industrial CO2
laser, which is what most applications use. The wavelength of
this is nominally 10.6um.
They give a 90° phase shift (+/- 2 degrees) when used at 45 degrees incidence to the incoming beam and at 45 degrees incidence to the polarization plane. Typically reflectivity is 98.5%+. The balance is absorption. The base mirror material is copper.
These mirrors will convert a linearly polarized beam into a circularly polarized beam. Reflecting this circularly polarized beam off of another 90° phase retarding mirror will convert the beam back into a linearly polarized beam. In this case the final beam would be the opposite to the first, so if the first beam were "S polarization," the final beam would be "P polarization".
Because these are used with metal cutting lasers which are high power, standard phase shift mirrors are large in diameter.