Decibels versus dollars: long-range atmospheric optical communications on a tight budget

Christopher Long, Michael Groth, Clinton Turner, Olga Korotkova
2008 Atmospheric Propagation of Electromagnetic Waves II  
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more » ... hs, the correction of minor typographical and grammatical errors, and very slight differences in formatting. ABSTRACT 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. H An optical range record using tone-modulated Morse code was set by KY7B/7, WA7CJO and WA7LYI, on 8 June, 1991 between Arizona mountaintops 248 km apart using 15 mW HeCd lasers on 442 nm (violet light). The receiver used a photomultiplier behind a Fresnel lens of about 40 cm by 40 cm aperture. This record, and the 1963 range record of 189 km for one-way speech, was finally broken on 3 October, 2007 by Clinton Turner KA7OEI, Ron Jones K7RJ, Gordon Smith K7HFV, and Brett Sutherland N7KG in Utah, operating over a distance of 278 km -a feat that also included two-way Morse communications. I The use of red light preferred as atmospheric transmission losses are lower at the longer, visible wavelengths while the use of visible rather than infrared light understandably simplifies construction and alignment. Additionally, silicon photodiodes have better sensitivity in the red end of the visible spectrum than at the shorter visible wavelengths.
doi:10.1117/12.768227 fatcat:uifxmhpzhvffdlriqa7xq7brsq