ABSTRACT
Interferometers for the detection of ultrasonic movements of surfaces may be divided into two distinct types. In the first type, light scattered or reflected from a surface is made to interfere with a reference beam, thus giving a measure of optical phase and hence instantaneous surface displacement. The second type of interferometer is designed as a high-resolution optical spectrometer to detect changes in the frequency of the scattered or reflected light. The use of laser light sources has led to an enormous increase in the performance and ease of use of this type of interferometer. Laser light is characterized by its high intensity, collimation and monochromaticity and it is only by the use of lasers that the high signal to noise ratios and submicrosecond response times necessary for the detection of ultrasound can be achieved. Non-laser light is invariably derived from an extended source and must be severely collimated to obtain a sufficiently parallel beam.
