Nonperturbing measurements of spatially distributed underwater acoustic fields using a scanning laser Doppler vibrometer

Abstract

Localized changes in the density of water induced by the presence of an acoustic field cause perturbations in the localized refractive index. This relationship has given rise to a number of nonperturbing optical metrology techniques for recording measurement parameters from underwater acoustic fields. A method that has been recently developed involves the use of a Laser Doppler Vibrometer ͑LDV͒ targeted at a fixed, nonvibrating, plate through an underwater acoustic field. Measurements of the rate of change of optical pathlength along a line section enable the identification of the temporal and frequency characteristics of the acoustic wave front. This approach has been extended through the use of a scanning LDV, which facilitates the measurement of a range of spatially distributed parameters. A mathematical model is presented that relates the distribution of pressure amplitude and phase in a planar wave front with the rate of change of optical pathlength measured by the LDV along a specifically orientated laser line section. Measurements of a 1 MHz acoustic tone burst generated by a focused transducer are described and the results presented. Graphical depictions of the acoustic power and phase distribution recorded by the LDV are shown, together with images representing time history during the acoustic wave propagation.

Optical imaging of propagating Mach cones in water using refracto-vibrometry

“…When an ultrasonic wave front passed through the measurement beam, the vibrometer converted the resulting time-varying phase shifts into a time-varying voltage. The refracto-vibrometry technique is most sensitive to density variations in regions where the measurement beam is perpendicular to the direction of travel of a wave front; 4 this yields a single slice of the Mach cone. By sequentially measuring a large number of scan points, a full-field dataset of propagating wave fronts was obtained.…”

“…3 In the current study, full-field videos of Mach cones in water were obtained using an optical technique called refracto-vibrometry. [4][5][6][7] Short ultrasonic impulses were incident on a 12.7 mm diameter cylinder submerged in a water tank. These impulses launched longitudinal and shear waves through the cylinder, resulting in the formation of Mach cones in the surrounding water.…”

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

“…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.…”

Optically visualized sound field reconstruction using Kirchhoff–Helmholtz equation

“…The optical measurement of a sound field, originally developed in the area of ultrasonic and underwater acoustics [1,2], has received much attention since its extension to an audible sound field [3][4][5][6][7]. However, poor signal-to-noise ratio (SNR) due to the smallness of the quantity measured by optical methods is preventing its practical use in the audible range.…”