chapter 2, Modeling Optical Scintillation

Excerpt

2.1 Introduction

An optical wave propagating through the atmosphere will experience irradiance (intensity) fluctuations, or scintillation, even over relatively short propagation paths. Scintillation is caused almost exclusively by small temperature variations in the atmosphere, resulting in index of refraction fluctuations (i.e., optical turbulence). Theoretical and experimental studies of irradiance fluctuations generally center around the scintillation index (normalized variance of irradiance fluctuations) defined by

where the quantity I denotes irradiance of the optical wave and the angle brackets < > denote an ensemble average or, equivalently, a long-time average. In weak fluctuation regimes [defined as those regimes for which the scintillation index (1) is less than unity], derived expressions for the scintillation index show that it is proportional to the Rytov variance for a plane wave where Cn2 (m^−2/3) is the index of refraction structure parameter, k = 2π/λ is the optical wave number, λ(m) is wavelength, and L(m) is the propagation path length between transmitter and receiver. The Rytov variance represents the scintillation index of an unbounded plane wave in weak fluctuations based on a Kolmogorov spectrum [Eq. (6) in Chapter 1], but is otherwise considered a measure of optical turbulence strength when extended to strong fluctuation regimes by increasing either Cn2 or the path length L, or both. It is known that the scintillation index increases with increasing values of the Rytov variance (2) until it reaches a maximum value greater than unity in the regime characterized by random focusing, so called because the focusing caused by large-scale inhomogeneities achieves its strongest effect. With increasing path length or inhomogeneity strength, the focusing effect is diminished by multiple self interference, and the peak fluctuations slowly begin to decrease, saturating at a level for which the scintillation index approaches unity from above.

©2001 Society of Photo-Optical Instrumentation Engineers