Measurement of large ultrasonic displacements with an optical probe

A simple method suitable for extracting large mechanical displacements from the phase modulation of an optical beam reflected from a moving surface is presented. This phase is obtained from a digital processing of the output signals of an analog quadrature demodulation circuit. In the MHz range, transient displacements larger than the optical wavelength have been measured with a standard heterodyne interferometer. Performances and limitations of this technique are discussed