CO2laser communication systems for near-earth space applications

Projects of the growth of earth-sensing systems for the latter half of the 1980's show a data transmission requirement of 300 Mbit/s and above. Mission constraints and objectives lead to the conclusion that the most efficient technique to return the data from a sensing satellite to a ground station is through a geosynchronous data relay satellite. Of the two links that are involved (sensing satellite to relay satellite and relay satellite to ground), a laser system is most attractive for the space-to-space link. A five-year program was conducted which has shown the basic technical feasibility of accomplishing this link with a CO
2
laser system operating at a wavelength near 10 µm. The system uses a mercury-cadmium-telluride infrared mixer cooled to approximately 100 K by a radiative cooler. The laser local oscillator and coupling-modulated laser transmitter use conductively cooled beryllium oxide construction. The optical antenna used for transmission and reception has a gain of nominally 92 dB with a 3-dB beamwidth of 80 µr. Tracking jitter is less than ten µr and signal acquisition occurs in less than 1 min. The receiver subsystem has a sensitivity of 10
^-19
W/Hz, accommodates a 300-Mbit/s data rate, and can track Doppler frequency variations over a ±700-MHz range.