Techniques to Mitigate the Effects of Atmospheric Turbulence on Free Space Optical Communication Links

Free space optical communication links are an attractive technology for broadband communications when fiber optic links are unavailable or simply not feasible. Atmospheric turbulence, aerosols, and molecular absorption all affect the propagation of optical waves in the atmosphere. Since atmospheric turbulence is the major source of errors on free space optical communication links, this dissertation investigates two techniques to reduce the impact of atmospheric turbulence on such links. These two techniques are aperture averaging and the incorporation of nonimaging optical elements into optical receiver systems. Aperture averaging is the process by which atmospheric turbulence-induced intensity fluctuations are averaged across a receiver aperture of sufficient size. We investigate the behavior of aperture averaging in weak and strong turbulence conditions by comparing experimental data with available models for plane and spherical wave propagation. New expressions for the aperture averaging factor in weak turbulence are given. In strong turbulence conditions, aperture averaging is analyzed with special attention to the various wavenumber spectrum models. This is the first report of experimental strong fluctuation aperture averaging data acquired in non-saturated conditions. Nonimaging optical elements are particularly useful for the mitigation of atmospheric turbulence-induced beam wander in the focal plane of a free space optical communication receiver. Experimental results of the bit error ratio enhancement due to the incorporation of a nonimaging optical element, specifically a compound parabolic concentrator, are presented. Two link ranges were tested, a 1.7 km link at the University of Maryland experiencing weak turbulence, and a 32.4 km link at the Naval Research Laboratory's Chesapeake Bay Detachment experiencing saturated, strong turbulence. These results are the first reported experimental test of a nonimaging optical element integrated into an outdoor free space optical communications system