EFFECTS OF THE REFRACTIVE-INDEX SPECTRAL MODEL ON THE IRRADIANCE VARIANCE OF A GAUSSIAN-BEAM

The normalized irradiance variance of a Gaussian beam in the weak-fluctuation regime is examined numerically with the use of various spectral models for refractive index fluctuations. The Kolmogorov, Tatarskii, and modified von Karman spectral models are chosen as conventional models, while the Hill numerical spectral model and the Andrews analytic approximation to the Hill model are selected to feature the characteristic bump at high wave numbers. The latter two models are known to predict higher scintillation levels than conventional spectral models when the Fresnel zone and the inner scale are of comparable size. Outer scale effects appear minimal near the centerline of the beam but can reduce off-axis scintillation significantly. Inner scale effects are prominent on axis as well as off axis, although they sometimes tend to diminish near the diffractive beam edge. Analytic approximations are developed for the irradiance variance based on the Kolmogorov, modified von Karman, and Andrews spectral models. These analytic expressions are generally in excellent agreement with numerical results.