Optical heterodyne communications experiments at 10.6 µ

A 10.6-μ optical heterodyne communication system that is in operation over a 30-km (18-mile) path is described. Television signals have been transmitted over the beam by the use of optical FM achieved with internal electrooptic phase modulation of the transmitter oscillator. A frequency deviation of 3 MHz is thus obtained for a driving voltage of 1 kV compared with the fundamental frequency noise of the system of about 1 kHz. For clear-day operation, average carrier-to-noise ratios at the receiver approach 60 dB, and demodulated video signal-to-noise ratios average 50 dB. Optical propagation experiments reveal amplitude scintillation noise in excess of 20 dB, represented by fluctuations of +10 dB and -10 dB from the average carrier level. Other observations show that the 10.6-μ beam has a remarkable ability to penetrate fog. Data related to scintillation, visibility, temperature, humidity, and optical absorption are presented.