Abstract
A method to optically measure surface acoustic displacements simultaneously over an array of detection points is described. Optical phase gratings are used to create a detection array of laser beams that is directed to the specimen. The scattered beams from the detection array are then collected and combined with a single reference beam in a photorefractive crystal in a multiplexed two-wave mixing configuration. The output beams of the resulting multiplexed photorefractive interferometer are imaged onto either a single photodetector (matched-filter scheme) or to separate photodetector elements (phased-array scheme). The optical matched filter requires less electronic overhead. By using different object beam footprints, it is possible to configure optical matched-filter receivers that selectively detect SAWs (broadband, narrow-band, chirped, etc.) with enhanced sensitivity. The responses of equispaced and chirped array receivers to propagating SAWs are discussed. The more general phased-array configuration, although requiring greater electronic overhead, provides time-domain signals from each element for postprocessing. This system can therefore be synthetically focused by applying appropriate phase shifts between the detected signals to create images of the acoustic scattering region. Both these multiplexed optical detection schemes, with their high spatial resolution and subnanometer displacement sensitivities, have applications in nondestructive testing and materials characterization.
Published Version
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