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Among the atmospheric properties that adversely affect laser propagation is air turbulence. One common optical parameter of air turbulence is the refractive index structure constant that quantifies the fluctuations in the refractive index caused by temperature fluctuations and hence air density fluctuations. There is a reason to believe, from theory and from sparse data that, when propagation occurs under widespread cloudy conditions, the refractive index structure constant is significantly reduced. Therefore the intensity of a propagating laser beam will not be degraded nearly as much as would be expected under clear or lightly scattered cloud conditions. New experimental data will be presented that support this hypothesis. The refractive index structure constant was measured for various cloud-cover conditions during daytime with additional factors present, such as changing crosswinds and precipitation. It was possible to observe the apparent pattern of the decrease of the refractive index structure constant by two orders of magnitude during the periods of increasing cloud-cover evaluated by the measurement of solar irradiance. The statistical correlation coefficient between the log of solar irradiance and the log of the refractive index structure constant was found to be around 0.9 (the closer it is to the maximum of 1.0, the stronger the correlation). The measurements were conducted with a commercial scintillometer/anemometer (1 m above ground, 500-m optical path length) in Northern Alabama in late spring and summer. The effect is believed to be due to the reduction of solar radiation, which caused the temperature gradient that initiated convection in the air. The results of this work can find their application in designing free space laser communication systems and military laser systems.
Distribution A. Approved for public release; distribution unlimited. 1 Sep 04
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