The laser feedback interferometer

Abstract

A simple laser interferometer is described. It consists of a He-Ne laser, a mirror which provides optical feedback, and a photodetector. The principle enables interferometry to be performed over path lengths of up to 50 m at either 0.6328 μm or 3.39 μm. Alignment of the instrument and detection of fringes is in both cases performed at 0.6328 μm. The interferometer may be used to measure length and also velocities up to at least 80 cm sec-1 and will produce fringes when only 1% of the output is fed back. The effect can be explained on the basis of Fabry-Pérot theory and a simple experiment to confirm this is described.