Detection of the transient motion of a scattering surface by two-wave mixing in a photorefractive crystal

Optical detection of the transient surface motion of objects is an area of far reaching practical importance. Applications include the vibration monitoring of many engineering structures (power plants, aircrafts, engines of various kinds...) and more recently the detection of ultrasound produced by pulse laser excitation. This technique, now in active development in our laboratory and elsewhere, extends ultrasonic inspection of materials to conditions otherwise difficult or impossible (complex geometries, hot products on a production line...). Various optical systems have been devised for detecting surface motion and have been recently reviewed1. We are reporting here the novel use of two wave mixing in a photorefractive crystal for detecting such a motion and in particular ultrasonic displacement produced by pulsed laser excitation. This approach has the merit of being applicable to a rough surface by integrating over many speckles and of having a very broad frequency detection bandwidth. The use of two-wave mixing for coherent homodyne detection of arbitrary wavefronts was previously reported and applied to the detection and amplification of an intensity modulated wave2. In the case of interest here of the detection of surface motion, the signal wave which has been scattered off the surface is instead phase modulated, and the scheme previously reported cannot be directly used since the transmitted signal wave and the diffracted pump wave, which acts as local oscillator, are in phase. Linear and sensitive detection of phase shifts requires instead the interfering light fields to be in quadrature. Before describing an optical configuration which satisfies this requirement, we analyze the effect of a small phase shift applied to the signal wave in a two-wave mixing configuration.