Experimental Verification of Anisotropic Optical Turbulence on Building Rooftops

A laser beam propagating over the atmosphere is negatively affected by absorption, scattering, and atmospheric turbulence. Scattering and absorption cause a reduction in the optical power, while the turbulence results in optical intensity fluctuations. Classically, the statistics of turbulence are assumed to be locally isotropic, which means the turbulence looks statistically the same in all directions perpendicular to the laser beam propagation direction. This assumption was taken merely for the sake of mathematical simplicity, which consequently excludes some hidden effects of the atmosphere. Previous studies predicted that the atmospheric turbulence is anisotropic, in which the turbulence statistics vary with different directions perpendicular to the laser beam propagation direction. Recent experiments have confirmed the existence of both isotropic and anisotropic turbulence near the ground. However, anisotropy is believed to usually be present at high altitude and more evident for large turbulence cells. This motivates our study in this paper to verify the anisotropic optical turbulence at higher altitude by setting up experiments on building rooftops. Our experimental results confirm the evidence of anisotropy via measurements of average intensity and scintillation index of a propagating Gaussian beam. Furthermore, a mathematical model is also developed for reproducing the measured anisotropic behaviors.