STUDIES OF THE EFFECTS OF ATMOSPHERIC TURBULENCE ON FREE SPACE OPTICAL COMMUNICATIONS.

Abstract

Even after several decades of study, inconsistencies remain in the application of atmospheric turbulence theories to experimental systems, and the demonstration of acceptable correlations with experimental results. This dissertation shows a flexible empirical approach for improving link performance through image analysis of intensity scintillation patterns coupled with frame aperture averaging on a free space optical (FSO) communication link. Aperture averaging is the effect of the receiver size on the power variance seen at the receiver. A receiver must be large enough to collect sufficient power and reduce scintillation effects at a given range, but must also be of practical size. An imaging system for measuring the effects of atmospheric turbulence and obscuration on FSO links will be presented. Weak and intermediate turbulence results will be shown for an 863 meter link at the University of Maryland. Atmospheric turbulence has a significant impact on the quality of a laser beam propagating through the atmosphere over long distances. Turbulence causes intensity scintillation and beam wander from propagation through turbulent eddies of varying sizes and refractive index. This can severely impair the operation of target designation and FSO communications systems. A new geometrical model to assess the effects of turbulence on laser beam propagation in such applications will be presented. The atmosphere along the laser beam propagation path is modeled as a spatial distribution of spherical bubbles with refractive index discontinuity statistically distributed according to various models. For each statistical representation of the atmosphere, the path of rays is analyzed using geometrical optics. These Monte Carlo techniques can assess beam wander, phase shifts and aperture averaging effects at the receiver. An effective Cn2 can be determined by correlating beam wander behavior with the path length. In addition, efficient computational techniques have been developed for various correlation functions that are important in assessing the effects of turbulence. The Monte Carlo simulations are compared with the predictions of wave theory. This is the first report to present weak and intermediate turbulence results using an efficient imaging technique. It is also the first report to geometrically simulate aperture averaging.