COHERENT LIGHT PROPAGATION THROUGH A TURBULENT ATMOSPHERE: MEASUREMENTS OF THE OPTICAL FILTER FUNCTION
ARMY BALLISTIC RESEARCH LAB ABERDEEN PROVING GROUND MD
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Two He-Ne lasers operating at 6328 A have been utilized to make simultaneous measurements of the effects of scintillation over homogeneous optical paths of 650 and 1300 m to study the transfer of coherent radiation through a turbulent medium. At the path terminus, multiple sampling of each laser beam was effected using a photo-optical technique which records a 61-cm cross section of an optical beam. Concurrent with the optical data, wind-speed and direction recordings were made at multiple points along the optical path in order to estimate the homogeneity of meteorological conditions. Near the path terminus, measurements of wind shear and temperature lapse were taken. In addition, high-speed thermometry techniques were utilized to compute one- dimensional temperature spectra as well as the thermal structure coefficient CT. Data were gathered during temperature-lapse, neutral, and inversion conditions. Log-irradiance scans derived from the optical data were used to compute log- irradiance power spectra, variance, and other statistical quantities. Using these optical and meteorological data, optical filter functions were then calculated for spatial frequencies above 87 cm and are used to compare with current theories. The saturation of the log-irradiance data is again observed, and the isotropy of the irradiance fluctuations is examined.