Differential displacement measuring interferometer

A linear displacement interferometer system for accurately measuring changes in linear displacement for determining the flatness of or angle between a pair of mirrored surfaces, said interferometer having an optical axis and an axis of symmetry and comprising:

• a source of a frequency stabilized input beam having two linear orthogonally polarized components;
• means optically associated with said input beam for spatially separating said input beam into a reference beam and a test beam, said reference beam and said test beam each having an associated wavelength and direction and being parallel and orthogonally polarized;
• means located in one of said spatially separated beams for rotating the polarization of the beam incident thereto parallel to the polarization state of the other of said spatially separated beams;
• means for modifying the polarization and/or direction of said separated beams, said modifying means having an entrance face and an exit face;
• wedge prism means disposed about said interferometer axis of symmetry for refracting said test beam at a predetermined deviation angle to said optical axis for forming a spot on each of said pair of mirrored surfaces, said interferometer axis of symmetry being normal to and intersecting the midpoint of said entrance and exit faces;
• and
• stage mirror means and reference mirror means disposed along said interferometer axis of symmetry where said refracted beam crosses said interferometer axis of symmetry, said stage mirror means and said reference mirror means each comprising one of said pair of mirrored surfaces, said refracting means and said modifying means being disposed between said mirrored surfaces, said sport formed by said refracting means comprising a single area approximately the size of said test beam;
• whereby any ambiguity concerning the refracted beams is eliminated.