Saturation of optical scintillation by strong turbulence*

The diffraction theory of optical scintillations has so far failed to describe the propagation of light over paths where the integrated amount of refractive-index turbulence is sufficient to cause saturation of the scintillations. We present a simple, physically based elaboration of the first-order perturbation theory and compare it with observations. Our theory reproduces in detail the observed saturation curve and the observed spatial covariance of the scintillations. In particular, we show why the fine-scale structure of scintillations persists deep into the saturation regime.