An experimental study of atmospheric optical transmission

This paper reports measurements made on a 23-mile, experimental, atmospheric, optical-transmission link for possible use as a standby substitute for microwave radio when the radio suffers severe multipath or obstruction fading. To allow comparison of transmission on a microwave and on an optical path, we used two parallel systems. One, a microwave system at 11 GHz, allowed frequency-selective fading to be measured, and the other, an optical system at 6328Å, allowed amplitude changes of the received optical signal to be obtained. The measured clear-air loss on the optical path is 27 dB. This measurement is made up of 17 dB of atmospheric scattering and 10 dB due to the receiving antennas intercepting only 10 percent of the beam at the receiver. The signal-to-noise ratio, calculated using measured background sky-noise and measured received power, is about 60 dB for a 100-MHz band. The beam diameter was measured to be 32 feet where the signal is down 20 dB. On the single occasion when frequency-selective microwave fading was observed, there was no fading of the optical signal. We find that it is necessary to control the transmitter elevation angle with a servo error signal from the receiver; the azimuth angle needs only occasional manual correction. The optical beam can be automatically reacquired after severe atmospheric attenuation, and that scintillation is usually several decibels, and occasionally as much as 10 dB.