Optical Communications, 2nd Edition

9

THE ATMOSPHERIC OPTICAL CHANNEL

In our earlier discussion of the optical space communication system, it was assumed, for the most part, that operation occurred over the free-space channel. The source field due to the modulated laser was considered a plane wavefield having beamwidths and power levels set by the transmitter parameters. The received field was then considered a spatially coherent field over the beamfront area, with a power level reduced by the free-space loss over the propagation distance.

When the space link involves transmission through the atmosphere, additional effects may have to be considered. In particular, the atmosphere produces additional space losses as well as possible beam distortion. System designers for space–atmospheric links must therefore be aware of these effects to properly assign power budgets, perform link analyses, and produce efficient system designs. In this chapter, we summarize some of the recent studies on optical channels and show how these results can be used to modify earlier analysis.