Detection of low intensity sound with laser Doppler anemometry

Abstract

Laser Doppler anemometry (LDA) sound measurement systems have traditionally been designed to achieve point measurements of a sound field so they keep the laser path lengths through the water short to minimize the signal contribution from the acousto-optic effect and use relatively large beam separation angles to minimize the interference fringe volume. While the traditional LDA approaches provide reasonable sensitivity to the Doppler shifts from microparticles oscillating in a low frequency sound field, at high frequencies the sound measurement sensitivity depends on the optical path length difference (OPD). OPD increases due to the acousto-optic effect as the sound pressure is integrated over the entire path length. An LDA system using a small beam separation angle and long laser light paths through the water is shown to be capable of detecting low intensity sound pressure signals. Theoretical and experimental results are presented.