The Turbulence Channel
In Chapter 4 we examined the guided optical channel or fiber link. In this chapter we consider the turbulent atmosphere as an unguided optical channel. It is well known that turbulence-induced random fluctuations in the atmosphere’s temperature generate corresponding random irregularities in the index of refraction. Upon passing through these irregularities, the wavefronts associated with an optical beam become distorted, the magnitude of the distortions depending on the strength of the turbulence and the length of the atmospheric optical path. Among the effects which are attributable to wavefront distortion and which can seriously degrade the performance of an optical communication system are (1) spreading of the beam beyond that normally caused by diffraction, (2) scintillation of the received intensity, (3) a decrease in the spatial and temporal coherence, and (4) wander of the beam from position to position. Quantification of these effects requires a theoretical understanding of the relationship between the properties of the medium and the transmitted optical radiation.
KeywordsConvection Covariance Coherence Autocorrelation Refraction
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