Effects of fog on the bit-error rate of a free-space laser communication system

Free-space laser communication (lasercom) systems are subject to performanc e degradation when heavy fog or smoke obscures the line of sight. The bit-e rror rate (BER) of a high-bandwidth (570 Mbits/s) lasercom system was corre lated with the atmospheric transmission over a folded path of 2.4 km. BER's of 10(-7) were observed when the atmospheric transmission was as low as 0. 25%, whereas BER's of less than 10(-10) were observed when the transmission was above 2.5%. System performance was approximately 10 dB less than calcu lated, with the discrepancy attributed to scintillation, multiple scatterin g, and absorption. Peak power of the 810-nm communications laser was 186 mW , and the beam divergence was purposely degraded to 830 mu rad. These resul ts were achieved without the use of error correction schemes or active trac king. An optimized system with narrower beam divergence and active tracking could be expected to yield significantly better performance. (C) 1999 Opti cal Society of America.