The effect of atmospheric optical turbulence on laser communication systems-Part 2, practice

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. In Part 1 we developed the stochastic theory necessary to analyze the effects of turbulence on lasercom links. In Part 2 we analyze several specific link geometries, predicting the effects and discussing methods of mitigating fades associated with those effects. Specifically, we will look at the requirements for tip-tilt systems necessary in many systems to correct received beam tilt, and to pre-correct pointing for a transmitted beam. We will then consider how adaptive optics may benefit certain lasercom systems. The concept of "beamlets" will be introduced, and the benefits of mitigating the effects of turbulence on ground to space uplinks will be analyzed.