Acousto-optic phase modulation of a laser Doppler vibrometer signal resulting from the radiated pressure produced by a submerged cylindrical shell: Numerical predictions and experimental results.

Abstract

It has been proposed that large stand-off laser Doppler vibrometer (LDV) measurements have significant errors resulting from index of refraction modulation due to the acoustic pressure distribution along the laser light paths between the structure and the probe [Y. H. Berthelot et al., J. Acoust. Soc. Am. 97, 3347(A) (1995)]. A high fidelity numerical model has been used to predict the pressure distribution and shell motions for a submerged finite ribbed cylinder under point excitation. Generally speaking, these predictions show low levels of contamination even for large stand-off probe distances. In addition, an experimental study has been conducted at NRL using a newly integrated three-axis vibrometer and near-field pressure probe where shell motions and near-field pressure distributions were measured for the case of a submerged ribbed cylinder under point excitation. The experiments and the associated numerical modeling both indicate low levels of acousto-optic contamination of the shell velocity signals. These findings contrast previously reported predictions of the level of acoustically induced error for large stand-off in-water LDV designs. [Work supported by ONR.]