Focus-position sensing using laser speckle.
The laser speckle resulting from a beam focused in proximity to a nonspecular surface has been examined, and a relationship between the speckle velocity and the distance from focus of the nonspecular surface is derived. An empirical verification of the relationship was performed, and the ability to determine surface position with respect to focus was demonstrated to an accuracy within 0.02 of the Rayleigh limit with the surface substantial distances from focus.
McLaughlin
J L
JL