| Atmospheric temperature gradients and wind shear cause random fluctuations in the index of refraction. This phenomenon, called turbulence, distorts the wavefront of any propagating optical beam, resulting in intensity fading. Turbulence also destroys both temporal and spatial coherence of the beam as it propagates. This dissertation describes a novel technique to indirectly measure the lateral coherence of a beam that propagates in clear air turbulence. A constant intensity laser beam is transmitted towards two retro-reflectors separated by a distance d. The analysis of the normalized intensity probability density function (PDF) and intensity variance of the return light enables an indirect indication of the lateral coherence distance. Indeed, as d goes from less than to more than the beam lateral coherence distance, the PDF evolves from "U-shaped" to log-normal, the shape it would have if only one retro-reflector was illuminated. At the same time, the intensity variance reaches a steady minimum value equal to the sum of the normalized intensity variances of the two beams coming back from each retro-reflector.; A direct advantage of the loss of spatial coherence is the possible use of an array of multiple modulating retro-reflectors, implemented as multiple quantum well layers on the retro-reflector surfaces, in a folded path atmospheric optical communication system. This system enables the optical transmission of information from a remote point where no laser transmitter source is needed. The second part of this dissertation describes such a system and predicts its bit error rate performance. |
