A dual differential interferometer for measurements of broadband surface acoustic waves

Abstract

A simple dual interferometer which uses two pairs of orthogonally polarized optical beams to measure both the amplitude and direction of propagation of broadband ultrasonic surface waves is described. Each pair of focused laser probe beams is used in a separate wideband differential interferometer to independently detect the component of surface wave motion along one direction of the surface. By combining the two output signals corresponding to both components, the two-dimensional surface profile and its variation as a function of time may be determined. Although the system has an optically adjustable -3db acoustic bandwidth of more than two decades (eg. 30kHz to 3MHz for acoustic emission measurements) and may detect peak displacements in the sub-Angstrom range, it is designed to be insensitive to low-frequency specimen translation. Potential applications in nondestructive evaluation are discussed.