Three decades of experiment by the authors have shown that the combination of high intensity light emitting diodes,
silicon photodiodes, and large aperture moulded Fresnel lens collimators of moderate focal length provide effective and
economical long distance atmospheric optical communications. While the use of larger transmitter and receiver lenses
increases optical flux at the detector, their greatest advantage is in dramatically reducing the depth of the scintillation or
rapid signal fading. This is caused by differential phase distortion, beam steering, focusing/defocusing by air turbulence
cells along the transmission path, and the effects of local coherence. It has been observed that scintillation effects
diminish rapidly when the transmitter and receiver apertures are larger than the central diffraction peak of the distant
aperture, or about 30-cm diameter for red light over a 160-km path.
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