Measurement of Acoustic Surface Wave Propagation Characteristics by Reflected Light

This Letter reports on a novel method of recording acoustic surface wave patterns by reflected light. In this method the surface perturbations due to the sound wave act as a moving reflection grating, causing the reflected light to exhibit a phase corrugation which is a replica of the sound wave and moves in synchronism with it. At a certain distance from the grating this phase corrugation changes into a corresponding amplitude corrugation. A fixed grating located at this point, followed by a photomultiplier, serves to extract an electrical signal at the original sound frequency. This signal of which the amplitude is proportional to the surface perturbation is fed to an X-Y recorder. By slowly moving the substrate on which the surface wave travels, a recording is made of the surface wave pattern in any desired cross section. Examples are given of 8 MHz standing and traveling surface wave patterns on a ferroelectric ceramic substrate.