Optical diffraction tomography applied to airborne ultrasound

Abstract

This paper describes the application of a light diffraction measurement method for airborne ultrasound. The method is based on Raman—Nath diffraction of a light beam combined with tomographic algorithms. Light intensity measurement data on acoustically diffracted light are used to characterize airborne ultrasound. The light intensity measurement data contain information on the integration of sound pressure along a specific light path. This integration allows the reconstruction of ultrasonic pressure in the cross-section of an ultrasonic beam. If the simplifying assumption of plane phase wavefronts is accepted the method gives absolute values for acoustic pressure. Reconstructions of phase surfaces are possible but are not discussed in this paper. The tomographic reconstruction of the measurement plane is done with an Algebraic Reconstruction Technique (ART) combined with nearest neighbour averaging. Three transducers were measured; results with two non-focussed air transducers with frequencies 200 kHz, 410 kHz and a focussed 1 MHz transducer are presented. Beam profiles, three-dimensional pressure maps and the optically detected acoustic ‘pulse-shape’ with tone-burst excitation are presented. The optical method offers good spatial resolution, bandwidth at least up to 1 MHz, and acoustic near-field measurements.