Measurements of airborne ultrasound with optical diffraction tomography

An optical measurement method for airborne ultrasound based on Raman-Nath diffraction of a light beam is described. Tomographic algorithms have been combined with light intensity measurements to allow the calculation of ultrasonic pressure in the cross-section of an ultrasonic beam. The method theoretically gives absolute values for acoustic pressure and enables the study of acoustic waveforms without inserting a probe into the acoustic field. The reconstruction of the measurement plane is done with algebraic reconstruction techniques (ARTs) combined with nearest neighbor averaging. Compared with pulse-echo methods the optical method offers increased spatial resolution, increased bandwidth and the possibility of measurements very close to the transducer surface. Results from measurements on two nonfocused air transducers with frequencies of 200 kHz, 410 kHz as well as a focused 1 MHz transducer are presented. Beam profiles and 3D pressure maps with a spatial resolution better than 0.5 mm are presented. The acoustic pulse-shape with tone-burst excitation is also illustrated