A differential fibre-optic device has been developed for noncontact detection of laser-generated ultrasound. Using a 5mW He-Ne laser as the source, laser beam delivery on to the sample of the surface utilised a singlemode fibre and lens combination. When photoacoustic ultrasonic transients propagated across the surface, the incident spot was perturbed, with backscattered light monitored by a fibre-optic bundle incorporating differential photodetection. Symmetric and asymmetric Lamb wave modes have been observed in a 50 µm aluminium sheet, as have longitudinal and Rayleigh waves in a 30 mm-thick sample.
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Williams et al. (1995) studied this question.
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