Log-Amplitude Fluctuations of Laser Beam in a Turbulent Atmosphere

In this paper, the Rytov approximation is used to calculate the log-amplitude fluctuations of a laser beam in an atmosphere whose statistics are assumed to be homogeneous and isotropic and to obey the Kolmogorov law. Asymptotic expressions are obtained for the variance of the log amplitude in the near field of the transmitter aperture. It is found that the variance is not homogeneous across the beam cross section. Near the center of the beam, the variance is somewhat smaller than that of an infinite plane wave; it increases toward the edge of the beam to a value which is greater than that of an infinite plane wave. The increase is most pronounced in the near field of the inner scale l0 of the turbulence when the transmitter aperture size is comparable to l0.