Reflective device for polarization rotation.

It is shown that a three-mirror device with ideal mirror reflectivities may be used in place of a halfwave plate to rotate the plane of polarization of a plane polarized beam by any angle with no loss of polarization purity. For coated mirrors, with more general reflectivities, specifications are set on the allowed reflectivity ratio and phase shift difference for the s and p components in order that polarization purity be preserved to a specified amount.