Reason for speckles emergence in imaging velocity interferometer system for any reflector

When we detect the velocity of free surface using imaging velocity interferometer system for any reflector (VISAR) in the shock wave propagation, speckles severely disturb the interference fringe so that it is hard to get the velocity accurately. In this paper, we theoretically analyze two possible factors in terms of optical path that may introduce speckles: roughness of the plat of target and multimode fiber. Then we design three illumination ways to verify these two factors separately in experiment. The result of offline experiment shows that multimode fiber is the main reason for the emergence of speckles and this has instructive value for depressing speckles.