Acoustic pressure measurement by acousto-optic tomography

Measurements of acoustic fields in liquids are generally made by means of hydrophones, which limits are clearly known: spatial resolution limited by the active element size and need for periodic calibration when one wants absolute measurements. This work describes how the acousto-optic effect can be used for precise ultrasonic beam mapping. The laser beam of an optical interferometer is launched in the liquid in a direction perpendicular to the ultrasonic beam, and is reflected back by a mirror. Since the optical index is modified by the ultrasonic pressure, the receiving photodiode will see a phase change proportional to the line integral of the acoustical pressure along the light path, i.e. across the ultrasonic beam. By moving the laser beam, one can get a set of projections of the acoustical map at different angles. Then a classical filtered backprojection algorithm is used to perform the tomographic reconstruction and recover the acoustical field values in absolute values of pressure if ones knows the piezo-optic coefficient of the liquid. Experiments have been made in water using a Thales SH 140 interferometer and a HIFU transducer. Optical beam deflection gives an upper limit for high intensities. In our experiment, a maximum pressure integral of 3.35 MPa.mm was measured. With reasonable averaging, a sensitivity of σ_v=3.53 μrd/√Hz can be obtained leading to a measurement dynamic range of at least 87.5 dB. The spatial resolution is linked to the laser beam size (a few tens of microns) and the measurement bandwidth is 40 MHz, so it is easy to follow the growing of ultrasonic harmonics