A Laser Link Operating at One Gigabit/Sec

Abstract

This paper describes the design of a 1 Gbps digital telemetry/ communication system achieving a probability of bit error of 10⁻⁶. The system is based on the use of near future (approximately 2 years) components and is suitable for use in satellite- satellite, ground-satellite, and aircraft-satellite links. Both the advantages and problems associated with the use of laser links are cited, and the techniques used in this system that make use of the advantages and overcome the problems are identified. Small packages and apertures are used to achieve high ERP (116 dBW), narrow beams with small apertures permit use of low prime power (170 watts total for laser), enormously greater link privacy and reduced susceptibility to counter measures is obtained, direct detection eliminates doppler search and tracking, and the small package size gives satellite systems multiple package possibilities. To obtain these advantages very narrow (5 p radian) operational beams must be acquired and tracked, point-ahead offset for relative terminal motions must be implemented, effects of atmospheric disturbances must be minimized if the atmosphere is in the link, and any repeater configurations must demodulate/remodulate because of lack of any laser system capability for frequency translation and amplification. The following unique features of laser systems as opposed to microwave cussed: requirements for acquisition and tracking point-ahead requirements, unique modulation types hand circular polarization, unique types of noise and background shot noise, elimination of doppler direct detection, ability of the receiving system on its receiving aperture, unique atmospheric propagation disturbances, and possibility of signal coding to remove the signal spectrum from the vicinity of the noise spectrum.