An interferometer is described in which the two interfering light beams follow the same geometrical path; they differ in their direction of polarization. The interferometer is used for the measurement of a difference in the optical path between the lengths of two etalons I and II, each bounded by reflecting or semireflecting surfaces. By using plane polarized light, in each etalon a quarter wavelength plate at 45° with the plane of polarization, and an analyzer in the 90° position, interference will result between a beam (12) reflected by the top semireflecting mirror of I and the opaque one at the bottom of II, and a beam (21) reflected by the bottom mirror of I and the top one of II. Beams of other combinations of reflections are extinguished.
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