Architecture for air-turbulence-compensated dual-wavelength heterodyne interferometer

摘要

An apparatus for measuring the change in position of a stage mirror with reference to a reference mirror when the stage mirror moves between first and second positions. The apparatus includes a light source for generating first and second coincident light beams, the first light beam having a wavelength &lgr;.sub.1 and the second light beam having a wavelength &lgr;.sub.2 where &lgr;.sub.1 =M&lgr;.sub.2. The first light beam includes two orthogonally polarized components differing in frequency by a first beat frequency, F.sub.ref (&lgr;. sub.1), and the second light beam includes two orthogonally polarized components differing in frequency by a second beat frequency, F.sub.ref (&lgr;. sub. 2), where F.sub.ref (&lgr;.sub.2)=M F.sub.ref (&lgr;. sub.1), and M is an integer greater than 1. A polarization dependent beam splitter directs one of the orthogonally polarized components of each of the light beams to the reference mirror and the other of the orthogonally polarized components of each of the light beams to the stage mirror. The polarization dependent beam splitter also recombines the orthogonally polarized components after the orthogonally polarized components have been reflected by either the reference mirror or the stage mirror. The light intensities of the recombined light beams are measured in first and second detectors. The outputs of these detectors are combined to provide an optical path measurement and a correction term that corrects for the density of air along the measurement path, and hence, corrects for any turbulence in the air.