Application and assessment of laser Doppler velocimetry for underwater acoustic measurements

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

“…8,35,36 A second method uses a laser Doppler vibrometer to detect and measure the velocity of a pellicle or transducer surface. 9,33,37 These two interferometric methods involve phase and frequency modulation. As noted by Dewhurst and Shan,38 phase is sensitive to displacement and frequency is sensitive to velocity.…”

“…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.…”

“…9,33,39 A number of effects associated with pellicles in the form of a circular membrane have been noted and addressed in the literature. These include non-flatness of the pellicle, 34 frequency-dependent acoustic transmission, 8,31 excitation of Lamb waves, 31 and, generally, motion-following.…”

“…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.…”

“…However, it has been observed in practice that high-frequency ultrasonic signals often reach full amplitude within about two cycles. 9,33 Some of the observed build-up in amplitude is due to the delay in arrival of acoustic waves emanating from parts on the transducer surface away from that nearest to the measurement point.…”