Patent ID: 6014216
Filing Date: 2000-01-11
Classification: G01B

Abstract:
A apparatus for measuring the change in position of a stage mirror with reference to a reference mirror when said stage mirror moves between first and second positions, said apparatus comprising:a light source for generating first and second coincident light beams, said first light beam having a wavelength .lambda..sub.1 and said second light beam having a wavelength .lambda..sub.2 where .lambda..sub.1 =M.lambda..sub.2, said first light beam comprising two orthogonally polarized components differing in frequency by a first beat frequency, F.sub.ref (.lambda..sub.1) and said second light beam comprising two orthogonally polarized components differing in frequency by a second beat frequency, F.sub.ref (.lambda..sub.2), where F.sub.ref (.lambda..sub.2)=M F.sub.ref (.lambda..sub.1), M being an integer greater than 1;a polarization dependent beam splitter for directing one of said orthogonally polarized components of each of said light beams to said reference mirror and the other of said orthogonally polarized components of each of said light beams to said stage mirror and for recombining said orthogonally polarized components after said orthogonally polarized components have been reflected by either said reference mirror or said stage mirror;a first detector for detecting the intensity of light in said first light beam after said orthogonally polarized components of said first light beam have been recombined by said polarization dependent beam splitter, said first detector generating a first detector signal having a magnitude equal to the light intensity at said first detector, said first detector signal oscillating at an instantaneous frequency of F.sub.1 (t);a second detector for detecting the intensity of light in said second light beam after said orthogonally polarized components of said second light beam have been recombined by said polarization dependent beam splitter, said second detector generating a second detector signal having a magnitude equal to the light intensity at said second detector, said second detector signal oscillating at an instantaneous frequency of F.sub.2 (t);a reference signal generator for generating a reference signal that oscillates at said second beat frequency;an optical path measurement circuit for measuring the difference in the number of oscillations of said second detector signal and said reference signal generator during the period in which said stage mirror moves from said first position to said second position and for generating an optical path signal indicative of said difference; anda correction term circuit for measuring the number of oscillations in a signal comprising MF.sub.1 (t)-F.sub.2 (t) during the period in which said stage mirror moves from said first position to said second position and for generating a correction signal indicative of said measured number of oscillations.