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 ef fect, when an LDA system is used to measure sound waves. Theoretical and ex perimental results will be discussed. A theoretical expression will be deve loped for the movement of the fringes in terms of the angle of the sound wa ve, the wavenumber and the distance of propagation of the laser beams. Resu lts will show that as the wavenumber increases, the angle at which the maxi mum amplitude of the fringe movement occurs tends away from pi/2 to the las er 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. Crown Copyright (C) 1999 Pub lished by Elsevier Science Ltd. All rights reserved.