Abstract:

Acousto-optical measurements inside transparent media is a well-known noninvasive method for acoustic field probing. The laser interferometric detection technique provides a noncontact, local, and quantitative measurement of pressure or dilatation fields in transparent media [X. Jia, Ch. Mattei, and G. Quentin, J. Appl. Phys. 77, 5528--5537 (1995)]. The optical phase shift of a probe beam passing through an ultrasonic beam is detected using a Mach--Zender interferometer and is related to the amplitude and phase of the pressure field. Since the optical phase shift integrates the pressure field over the optical beam pathlength, this technique assumes a constant acoustic field along the laser beam path. In order to obtain a complete mapping of the acoustic field, as required in many applications, e.g., transducer characterization, we propose the use of tomographic techniques associated with the interferometric detection which permits a reconstruction of the acoustic field in the acoustic beam cross section from a set of projection data. The time variation of the pressure field in a cross section as well as the amplitude and phase spectra can be displayed. The mapping of the near pressure field (1 mm) of an ultrasonic transducer is achieved using this technique and other examples of field characterization are presented. [sup a)]Permanent address: G.P.S., Univ. Paris 7, CNRS Ura No. 17, 2 place Jussieu, 75251 Paris Cedex 05, France.