Optical Testing with the Digital Heterodyne Interferometer

In the digital heterodyne interferometer, the single frequency output of a laser is split, and each component is frequency shifted by separate Bragg cells. The difference of the frequency shifts can be 0 Hz for conventional "see the fringe" operation, or 1 MHz for accurate phase measurement. One of these components then has its polarization rotated by 90°, and the two are combined without loss by a polarization selective beam combiner cube. The composite beam is expanded to 2 cm, and injected into a polarization selective Twyman-Green interferometer. Each arm has a quarter-wave plate oriented such that the return radiation has its polarization rotated 90°, and the two beams then exit the interferometer. Thus, at the interference plane there exists light of orthogonal polarizations with one polarization having traveled the reference path and one the test path; and the polarizations have a 1 MHz frequency difference. A linear polarizer oriented at a 45° angle to these polarizations causes the beams to mix.