Laser Doppler anemometry and the acousto-optic effect

This paper deals with the effect of phase variations along the paths of the laser beams due to refractive index changes caused by the acousto-optic effect, when an LDA system is used to measure sound waves. Theoretical and experimental results will be discussed. A theoretical expression will be developed for the movement of the fringes in terms of the angle of the sound wave, the wavenumber and the distance of propagation of the laser beams. Results will show that as the wavenumber increases, the angle at which the maximum amplitude of the fringe movement occurs tends away from pI/2 to the laser beam bisector, and towards the angle of the laser beams. It will also be shown that the phase modulations detected experimentally are in reasonable agreement with those predicted theoretically.