An Experimental Study of Atmospheric Optical Transmission

We report system parameters measured on a 23-mile, atmospheric, optical-transmission path. The object of the investigation is to determine whether a modulated optical signal transmitted through the atmosphere could be used as a stand-by substitute for a microwave radio link when transmission over the radio path is impaired by clearair multipath or obstruction fading. 607 So far, we have learned that: (i) The control of the transmitter elevation angle can be accomplished with an error signal from the receiver. This compensates for atmospheric refractive index gradient changes, which change the curvature of the optical beam. (ii) There is very little need for horizontal beam correction. (iii) The transmitter can be scanned up and down when severe path loss has attenuated the signal below the detection level so as to reestablish the link after the path loss is reduced. (iv) The clear-air path loss is 27 dB. Of this, 17 dB is due to scattering and 10 dB is due to the receiver intercepting only 10 percent of the beam. (u) The received beam is 20 feet in diameter where the received signal is 10 dB below its value at the beam center and 32 feet in diameter where the signal is down 20 dB. (vi) The calculated clear-air peak-signal-to-average-noise ratio, including the background light, is between 57 and 63 dB, using a 20-mW laser. It has been determined 1 that rain and fog can cause as much as 0.1dB/ft optical-transmission loss, and, at such times, optical transmission on a 20- to 30-mile path is very difficult.