Measurement of focused ultrasonic fields using a scanning laser vibrometer

Abstract

With the development of optical techniques, scanning laser vibrometers have been applied successfully in measuring particle velocities and distributions in ultrasonic fields. In this paper, to develop the optical interferometry in measuring focused fields with small amplitude, the "effective" refractive index used for plane waves and extended for spherical waves is presented, the piezo-optic effect as a function of the incident angle of the laser beam is simulated, and the ultrasonic field produced by a concave spherical transducer is calculated numerically around its focal region. To verify the feasibility of the optical method in detecting focused ultrasonic fields, a measurement system was set up that utilized both a scanning laser vibrometer and a membrane hydrophone. Measurements were made in different zones of a focusing transducer, and good results were acquired from the optical interferometry in regions where acoustic waves travel in plane form or spherical form. The data obtained from the optical method are used to reconstruct acoustic fields, and it is found that the focal plane, the maximum pressure, and the beamwidth of the transducer can be forecasted accurately.

Ultrasonic radiation from wedges of cubic profile: Experimental results

“…See Refs. [10,11] for examples of laser Doppler vibrometers being used to image sound fields. This method extracts an instantaneous magnitude and phase, averaged over the entire path of the laser's emitted and reflected light, and thus can only obtain a 2D projection of the 3D sound field it scans.…”

Acousto-optic effect compensation for optical determination of the normal velocity distribution associated with acoustic transducer radiation

“…Measurement of transducer vibration by a heterodyne laser interferometer in the form of a scanning laser Doppler vibrometer is demonstrably feasible, 9,10,30,33,42 although with the caveat of correctly compensating for the acoustooptic effect. Given suitable optical reflectivity, the vibration of a transmitting transducer can be measured directly.…”

“…10 The field is not so well behaved, in general, especially in the so-called nearfield. 44 Recognition of the spatial complexity of the nearfield led Sapozhnikov et al 19,20 to conclude that the normal velocity on the transducer, or another surface, such as that of a pellicle, cannot be determined in general.…”

“…9,10,42 It has also been modeled for crossbeam measurement of the acoustic field in a plane parallel with that of the radiating transducer 9,39,43 or crosswise to the beam axis in the case of a focusing transducer. 10 None of these methods addresses the case of oblique incidence, which is being considered here, in addition to nearfield effects.…”

“…Applications include acousto-optic modulators (AOMs), 3 e.g., Bragg cells; 2,4 anemometry in air and water; 5,6 calibration of acoustical devices used in medicine 7 apropos of radiation dosage and power delivery, e.g., for diagnostics, treatment by acoustically induced hyperthermia, and lithotripsy; [8][9][10] Schlieren visualization of ultrasonic fields, 11,12 e.g., radiation or scattering of sound; 13,14 and vibrometry to detect and quantify mechanical vibrations, e.g., for materials characterization, 15 non-destructive testing and evaluation, 16,17 and visualization of patterns of surface vibration; 6,18 among other things. There is some question about the influence of the acousto-optic effect when the aim is direct optical measurement of (i) vibrations of a radiating transducer surface, or (ii) acoustically induced vibrations in a very thin, optically reflective, acoustically transparent membrane, called a pellicle, suspended in the acoustic field of the radiating transducer.…”