Finding anisotropic ellipse of turbulence fluctuations from beam intensity correlations
The presence of anisotropic turbulence statistics within the first meter from the ground surface has been known for decades from optical beam propagation experiments. We have carried over a number of measurements of a single lowest-order Gaussian He-Ne laser beam propagating close to the terrain. The experiments took place on the grounds of the University of Miami, FL over a grassy field in a variety of meteorological conditions and at a number of different heights from the ground. The spatial-temporal fluctuations of the intensity of a slightly divergent beam were recorded by a CCD camera and analyzed by the statistical moment calculations. The beam statistics were used for inferring the parameters of turbulent anisotropy, such as the ellipticity and the orientation of the refractive index anisotropy ellipse. Our results indicate that along the sub-kilometer link the two-point intensity correlation function of the beam manifests evident anisotropic features (anisotropic factors and ellipse angle). Our findings are of importance for designing and optimizing optical systems operating in the presence of anisotropic turbulence and can also be used as a simple technique for determining the parameters of anisotropic turbulence.