Optical Turbulence Profiles in the Atmosphere

Abstract

Turbulence induces phase fluctuations on light waves traveling through the atmosphere. The main effect of those perturbations on imaging systems is to diminish the attainable angular resolution, whereas on free-space laser communications the turbulence drastically affects system performances. Adaptive Optical (AO) methods are aimed at reducing those cumbersome effects by correcting the phase disturbances introduced by atmospheric turbulence. The development of suchmethods would not have seen the light without research of the turbulent fluctuations of the refractive index of air, the so called Optical Turbulence (OT). It is necessary to study the statistical properties of the perturbed wavefront to design specific AO systems and to optimize their performances. Some of the useful parameters for this characterization and their impact on AO are the following: The Fried parameter r0 (Fried, 1966), which is inversely proportional to the width of the image of point-like source (called ”seeing”), leads to the determination of the spatial sampling of the wavefront for a given degree of correction (Rousset, 1994). r0 is the dominant parameter in the calculation of the phase fluctuation variance. The coherence time τ0 (Roddier, 1999), during which the wavefront remains practically unchanged, is needed to determine the temporal bandwidth of an AO system and the required brightness of the reference sources used to measure the wavefront. The isoplanatic angle θ0 (Fried, 1982), corresponding to the field of view over which the wavefront perturbations are correlated, determines the angular distance between the corrected and the reference objects, for a given degree of correction.