VELOCITY-MEASUREMENTS OF 2 MOVING DIFFUSERS USING A TEMPORAL CORRELATION LENGTH OF DOUBLY-SCATTERED SPECKLE

Dynamic properties of doubly-scattered laser speckles produced by a cascade of two moving diffusers are investigated theoretically and experimentally. Space–time correlation functions for both single and double scattering cases are derived in terms of the roughness and the moving velocities of the diffusers, the spacing between them, the spot size and the wavefront-curvature radius of an illuminating laser beam, and the position of a detecting plane. Time-correlation length τc. defined by the 1/e correlation time is then evaluated to measure the velocities of moving diffusers. It is found that, if both the diffusers move simultaneously, the reciprocal 1/τc of the time-correlation length is no longer proportional to the velocity of each diffuser. Nevertheless, the relative velocity of the two diffusers is shown to be proportional to 1/τc, when they are illuminated at the beam waist with a suitable spot size and when the speckle fluctuations are detected in the far field. The root-mean-square velocity shows linear dependence on 1/τc under broad-beam illumination and near-field detection. An experimental study has been conducted to verify the theoretical results. Good agreement is obtained between theoretical and experimental results. © 1990 Taylor & Francis Ltd.