Abstract

A common problem in laser systems is the requirement for path length matching of separate beam paths. A device is described which is easy to use, inexpensive, and capable of measuring equal path lengths to 0.5% of the coherence length. A mirror in one leg of an interferometer introduces a 2-μm path change at 100 Hz using a PZT and ramp generator. An rms voltmeter sends the rms voltage level from a detector placed at the output of the interferometer to an oscilloscope. If a translation stage attached to a mirror of the interferometer is moved at a known rate, the resultant oscilloscope trace represents the visibility vs optical path difference. The peak signal corresponds to the minimum path length difference. Using an argon laser with a 20-mm coherence length, the zero optical path length difference point was located to within 100 μm after an initial path difference of 50 mm. This was confirmed with a white light source. Experiments are discussed in which the coherence length of the source was gradually changed to obtain a variable yardstick.

© 1987 Optical Society of America