VISAR: displacement-mode data reduction

Abstract

A Velocity Interferometer System for Any Reflector (VISAR) is a laboratory tool that measures high velocities by continuously measuring the Doppler shift of laser light reflected from a moving surface. It produces lower output frequencies than a displacement interferometer in which Doppler-shifted laser light from a moving target is mixed with unshifted laser light. To obtain lower frequencies a VISAR employs a wide-angle Michelson interferometer with a time delay in one leg. Undelayed and delayed light rays are thus mixed to detect the relatively small difference between two Doppler shifts produced by accelerating motion at two slightly different velocities. In most VISAR data reduction programs the velocity is assumed to be proportional to the interferometer fringe count at any instant. This yields velocity details that are inaccurate over the interferometer delay time. In the examples of this paper the signal time resolution was shorter than the interferometer delay. The subject of this paper is a data reduction method that uses the displacement information in suitable VISAR signals to recover velocity features that occur during the interferometer delay. 1.