Phase fluctuations of a laser beam propagating through a turbulent atmosphere are studied as a function of the distance in a cross section of the beam. An interferometer is described that allows simultaneous measurements of phase fluctuations for pairs of rays that travel in the beam at different distances between each other. Experiments with this method have been performed in an urban center at a distance of 3.5 km and 0.5 km from the source (a helium-neon laser). The results of these measurements are described, and the behavior of the mean square fluctuation of the phase difference is studied. The values of the structure constant Cn are calculated.
M. Bertolotti, M. Carnevale, L. Muzii, and D. Sette, "Interferometric Study of Phase Fluctuations of a Laser Beam Through the Turbulent Atmosphere," Appl. Opt. 7, 2246-2251 (1968)