HORIZONTAL LINE-OF-SIGHT TURBULENCE OVER NEAR-GROUND PATHS AND IMPLICATIONS FOR ADAPTIVE OPTICS CORRECTIONS IN LASER COMMUNICATIONS

Atmospheric turbulence over long horizontal paths perturbs phase and c an also cause severe intensity scintillation in the pupil of an optica l communications receiver, which limits the data rate over which inten sity-based modulation schemes can operate. The feasibility of using lo w-order adaptive optics by applying phase-only corrections over horizo ntal propagation paths is investigated. A Shack-Hartmann wave-front se nsor was built and data were gathered on paths 1 m above ground and be tween a 1- and 2.5-km range. Both intensity fluctuations and optical p ath fluctuation statistics were gathered within a single frame, and th e wave-front reconstructor was modified to allow for scintillated data . The temporal power spectral density for various Zernike polynomial m odes was used to determine the effects of the expected corrections by adaptive optics. The slopes of the inertial subrange of turbulence wer e found to be less than predicted by Kolmogorov theory with an infinit e outer scale, and the distribution of variance explained by increasin g order was also found to be different. Statistical analysis of these data in the l-km range indicates that at communications wavelengths of 1.3 mu m, a significant improvement in transmitted beam quality could be expected most of the time, to a performance of 10% Strehl ratio or better. (C) 1998 Optical Society of America.