Abstract

When a laser beam impinges on the edge of a razor blade, it produces on an observation screen a bright line perpendicular to the edge. According to Takenaka and Fukumitsu¹ the intensity along this line varies as 1/x² for relatively large x (where x is the distance from the geometrical shadow boundary). The precise mathematical relation of this intensity distribution is a consequence of diffraction theory. We illuminated a small mirror fixed on a tuning fork with this bright line (produced by a He-Ne laser) and we detected a part of the reflected light power with a small photodiode. The angular displacement of the tuning fork was thus monitored by means of the light intensity variations produced at the detector. It was possible to detect very small vibrations of the tuning fork and also to measure the angular displacement by evaluating the ratio of the ac component of the signal to the dc component. The latter corresponds to the x value of the detector position at the chosen operation point when the tuning fork is at rest. Thus light intensity measurements of a diffracted laser beam constitute an efficient and simple method to monitor small angular displacements. This new method could advantageously replace the more elaborate ones based on interferometry when a moderate precision is required. Moreover the time resolution of this diffraction method is limited only by the response time of the detector since there are no moving mechanical parts in the system.

© 1985 Optical Society of America