Theory of saturation of optical scintillation by strong turbulence for arbitrary refractive-index spectra

Abstract

There is strong evidence that the amplitude of a light wave propagating through turbulence becomes Rayleigh distributed (i.e., the irradiance becomes exponentially distributed) in the limit of strong turbulence, which implies that the log-amplitude variance tends to π2/24. We find that the theory by Clifford et al. [ J. Opt. Soc. Am.64, 148– 154 ( 1974)] for saturation of scintillation by strong refractive turbulence can be made to obey this limit for power-law refractive-index spectra. However, for a nonzero inner scale of turbulence (no matter how small), the theory predicts that log-amplitude variance tends to zero in the limit of strong turbulence. A generalization of the theory is derived that obeys the π2/24 limit for arbitrary refractive-index spectra, a nonzero inner scale being a particular case. The new theory has no arbitrary parameters. Both old and new modulation transfer functions have different behavior for nonzero inner scale at both very large and very small spatial wave numbers when compared with the case of zero inner scale. This differing behavior affects the log-amplitude variance even if the Fresnel-zone size is much greater than the inner scale, provided that the lateral coherence length of phase is less than the inner scale. This differing behavior also applies at all spatial wave numbers if the Rytov variance is strongly affected by the inner scale. For strong (but finite) turbulence strength, the predicted log-amplitude variance is larger for a smaller ratio of Fresnel-zone size to inner scale, which is in quantitative agreement with observations.