The influence of a standing wave on laser Doppler signals

The effects of an acoustic standing wave on the laser beams of a laser-Dopp ler-anemometry set-up have been investigated. The resulting changes in refr active index give rise to oscillating fringes rather than stationary ones a s is generally assumed. The theory has been developed to produce the form o f the fringe movement due to this acousto-optical effect. A theoretical exp ression for the ratio of the apparent motion of the particles in the fringe s to the actual motion has been derived and compared with experimental resu lts for the case of an underwater laser-Doppler-anemometry system. It has b een shown that, over long propagation distances of the laser beams or for h igh sound frequencies, the acousto-optical effect is dominant over any part icle movement due to the acoustic wave. This means that the apparent motion of the particles seen by the detector may have a larger amplitude than exp ected and, although it will be easier to detect, the analysed signal need n ot accurately represent the region of the flow of interest.