Sensitivity of reflection transducers

This article discusses the sensitivity of new high-speed optical transducers for measuring acoustic transients propagating in water. These devices are based upon the measurement of reflectance at a solid–water interface using a probe beam, perturbations in reflected intensity being brought about by the changes in pressure from an acoustic transient traversing the boundary. It is found that the sensitivity of such transducers is highly dependent not only upon the angle of incidence of the probe beam used to measure reflectance, but also the divergence of the probe beam and the acoustic properties of the transducer material. This results in a trade off between obtaining high spatial resolution and high sensitivity with such a transducer, and higher sensitivities give rise to a more nonlinear response. The fabrication and calibration of a robust prototype transducer made out of perspex is discussed and initial experimental results measuring acoustic transients associated with laser cavitation are shown. High spatial resolution and sensitivity were achieved by focusing the probe beam onto the interface at an angle which was slightly smaller than the critical angle. Finally, a transducer with yet higher sensitivity fabricated out of glass is demonstrated, allowing the study of smaller amplitude rarefaction waves.