Phase Variations in Atmospheric Optical Propagation

Temperature structure in the atmosphere, transported by the wind across a laser beam, produces time variations in the optical path length. Using a He-Ne laser (0.6328 μm) on a 70-m propagation path, we measured the optical phase variations at four different spacings, ρ≤30 cm. Simultaneously, a midpath measurement of wind velocity and temperature structure parameter, CT2, provided the necessary meteorological measurements to compare the observed phase structure function with Tatarski’s theoretical curve. We obtained excellent agreement between theory and experiment. Direct measurements of the outer scale of turbulence, taken continuously over a 24-h period at a height of 1.6 m, indicated an average outer scale of 1.3 m with diurnal variations of ±20%. The frequency spectrum of the received phase difference at each of the four spacings is plotted and its implications for the data-sampling rate are examined. The curves obtained exhibit excellent agreement with the predicted spherical-wave phase-difference frequency spectrum.