The effect of atmospheric optical turbulence on laser communications systems: Part 1, theory

Laser Communication (lasercom) systems whose beam paths traverse the Earth’s atmosphere, in whole or in part, are subject to the effects of optical turbulence: the random variation in the index of refraction due to small temperature variations. These effects include beam tilt, wander, and spreading, as well as scintillation: the variation in intensity across the receiver’s aperture plane. These effects can result in fades on the order of milli-seconds or longer, and so are important considerations in the design of lasercom links and systems. Stochastic analysis of optical turbulence has been developed since the 1940’s, but formal courses are still relatively rare, particularly courses for the professional lasercom engineer. Much of the literature has been devoted to the effects on imaging and directed energy systems, where analysis of mean effects often suffices. This differs considerably from lasercom, where low probability events, leading to fades, are important. The intent of this paper is to provide a concise but reasonably thorough tutorial on the mathematical theory regarding this subject, presenting the background necessary to analyze the effects of turbulence on lasercom systems. Measures for the correlation of phase and intensity across the receiver’s aperture will be developed, as well as the evolution in time. Analytic equations are only valid for mild turbulence, and so wave-optics simulation, a method for predicting the effects of moderate and strong turbulence, will also be discussed. In Part 2, we will consider specific lasercom geometries and architectures designed to mitigate fades due to turbulence.