Technique for measuring laser beam propagation direction fluctuations

A technique has been developed for the measurement of a component of the position of a small target within a laser beam and used to measure atmospherically induced beam-pointing fluctuations. A modulation is impressed on the beam in a manner such that the modulation phase varies with position across the beam, so the relative modulation phase of the signal returned from a remote retroreflector is indicative of the position of the retroreflector within the beam. The far-field modulation phase distribution is shown to be relatively unaffected by turbulence, and results are given for a series of experimental measurements of beam-pointing fluctuations.