Multiple pass laser interferometer

The multiple pass laser interferometer is an instrument made possible because of the extreme temporal coherence of gas lasers. The optical schematic is shown for a system with input and output plane waves which can be used to describe the basic principle and characteristics of this class of interferometers. A laser beam, adjusted for uniphase operation (TEM₀₀ mode), passes through a collimator which adjusts the beam to the required size. Energy in the beam passes through high reflectance mirror M1. Two telescope systems situated between mirrors M1 and M2, image the two mirrors in coincidence at the object plane (entrance pupils of the telescopes). Beam energy leaves the system through M2 where "read-out" optics image the surface of M1 for viewing.