Optimum beam size for laser beam propagating through atmospheric turbulence

A previous study concluded that the optimum beam diameter for a laser beam propagating through atmospheric turbulence is of the order of the coherence scale. It is shown that the optimum size is critically dependent on beam wander and pointing accuracy, and can be much smaller. Consequently, the beam size maximising mean intensity may not coincide with the size minimising fluctuations, and a compromise may be necessary. © IEE: 1999

Adaptive beam-size control scheme for ground-to-satellite optical communications

Atmospheric turbulence severely degrades the performance of ground-to-satellite optical links. Employment of adaptive optics to enhance ground-to-space optical communication systems has recently been considered and possible benefits have been shown. Uplink scintillation reduction using multiple transmitters is also being considered. What appears to be currently missing in such work is the realization that transmitter beam size is a crucial design parameter and its optimum value changes continuously according to changing turbulence conditions along the propagation path. We emphasize this point and propose a configuration in which the uplink transmitter beam size is controlled in real time in response to measured turbulence parameters to maximize mean intensity and minimize fluctuations on the satellite receiver. The full analytical evaluation is not tractable, but semianalytic simulations can be run to explore the improvement for different scenarios and site conditions. Some preliminary simulation results are presented and the difficulties hindering the achievement of more meaningful results through simulation are discussed. Controlling the beam size, especially by a factor of 2 or so, in relatively long time scales should not be a problem. The technology is sufficient to design an experiment to prove its feasibility