Differential plane mirror interferometer

Claim 1

A differential plane mirror interferometric system comprising: a pair of plane mirrors separable by a variable optical path length; source means for emitting an input beam comprising two stabilized orthogonally polarized optical frequencies having a frequency difference, said source means further comprising means for providing an electrical reference signal corresponding to said frequency difference between said two stabilized optical frequencies; tilted parallel plate means optically coupled to said input beam for converting said input beam into two separated parallel orthogonally polarized beams; means optically disposed in the path of one of said two separated parallel orthogonally polarized beams for converting said two separated parallel orthogonally polarized beams into two separated parallel beams having the same polarization; means optically coupled to said two separated parallel same polarized beams for causing one of said two separated parallel same polarized beams to be reflected twice by one of said pair of plane mirrors and the other of said two separated parallel same polarized beams to be reflected by the other of said pair of plane mirrors to produce two parallel output beams having the same polarization; means optically disposed in the path of one of said two separated same polarized parallel output beams for converting said two separated same polarized parallel output beams into two separated orthogonally polarized parallel output beams; means optically coupled to said two separated parallel orthogonally polarized output beams for converting said two separated parallel orthogonally polarized output beams into a single output beam having a pair of orthogonally polarized frequency components, with a phase difference therebetween being directly proportional to said variable optical path length between said pair of plane mirrors; means optically coupled to said single output beam for mixing said orthogonally polarized components thereof and producing an electrical measurement signal therefrom; and means operatively connected to said electrical measurement signal and said electrical reference signal for indicating a difference in phase between said electrical reference signal and said electrical measurement signal, said indicated phase difference being proportional to said variable optical path length between said pair of plane mirrors; whereby an optical configuration extremely insensitive to measurement error and misalignment is provided for said interferometric system, said tilted parallel plate means comprising a first set of regions of reflection, antireflection and polarization coatings, said means for converting said two separated parallel orthogonally polarized output beams into said single output beams comprising said first set of regions and coatings on said tilted parallel plate means, said tilted parallel plate means further comprising a second set of regions of reflection, antireflection and polarization coatings, said second set of regions and coatings comprising said input beam converting means.