Common-Path Heterodyne Interferometers

Common-path heterodyne interferometers (COPHIs) have been proposed for measuring small variations in the heights of surfaces. A COPHI could be used, for example, to measure the deviation of the surface of a mirror or other optical component from nominal flatness or nominal sphericity. Like phase-shifting interferometers that have been used previously for the same purpose, a COPHI would generate an optical-phase map equivalent to a topographical map of the surface under test. The advantages of the COPHI over a phase-shifting interferometer would be (1) greater resolution in phase and thus in surface height and (2) shorter measurement time.