Dynamic speckles, which carry information about beam parameters of a diffuse object, are produced by a moving diffuse object under illumination of a Gaussian beam. In this paper, we consider that the diffuse object moves in a plane with constant velocity and discuss the statistical properties of dynamic speckles for estimating the variation of focusing spot size. The space–time statistical properties of dynamic speckle have been revealed by analyzing the space–time cross-correlation function of speckle intensity fluctuations detected at two points in the receiving plane. We discuss the influence of the distance between two point detectors on the detection results by simulation analyses, and the theoretical analysis results are verified by experiment. This method, which applies feedback of the dynamic speckle fields for estimating the variation of focusing spot size, will help a laser focusing system optimize focusing performance.
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