Quantitative imaging of transient acoustic fields by optical heterodyne interferometry

We have built an optical imaging system of acoustic fields transmitted in water by piezoelectric transducers. This system includes a thin reflective membrane immersed in front of the transducer and a broadband compact heterodyne interferometer. The motion of the membrane, induced by the acoustic pressure, modulates the phase of a reflected optical wave. A coherent demodulation provides the absolute mechanical displacement. To record the acoustic field, the transducer is moved parallel to the membrane. Experiments carried out with focused or planar transducers show the capability of this system to map acoustic field displacements as low as 1 A, with a high resolution both in time (10 ns) and spatial (20 mm) domains. As an example of applications, the acoustic field was plotted in the focal plane of a 25-MHz focused transducer having a beam waist of 0.25 mm. In the case of planar transducers, B-scans of the transmitted acoustic field, clearly show the occurrence of plane waves and edge waves. Surface motions of piezocomposite transducers have also been investigated