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LDV: Laser Doppler Velocimeter

Particle velocity and turbulence measurements

Theory of Operation

In LDV, two coherent laser beams are crossed to form a small measurement probe volume. Fringes are formed in the probe volume due to the interference between the two coherent beams. A particle entering the measurement probe volume scatters the incident light beams in all directions and the scattered light is detected by a photodetector placed in particular direction (forward-scatter or backscatter). Since the particle is moving with respect to the incident laser beam and the photodetector, the scattered light is shifted in frequency with respect to the incident beam due to the well-known Doppler effect. Furthermore, because of the angular differences between the two laser beams that are simultaneously incident on the particle, the light scattered from each towards the photodetector experience different Doppler shits. Since the incident laser beams are coherent, the scattered light from the two incident beams interfere, and the photodetector provides a signal that is temporally modulated at the Doppler difference frequency – the Doppler burst. The Doppler difference frequency can be directly and linearly related to a velocity component of the moving particle. Additional pairs of crossed beams can be used to measure the second and third components of particle velocity.

Fringes in the measurement volume

Relation of Doppler frequency to particle velocity