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

TLDR

Free‑space laser communication systems suffer performance degradation when heavy fog or smoke obscures the line of sight. The authors propose that an optimized system with narrower beam divergence and active tracking could yield significantly better performance. They measured the bit‑error rate of a 570 Mbit/s lasercom system over a 2.4 km folded path using an 810‑nm laser with 186 mW peak power and a deliberately degraded 830 µrad beam divergence to assess transmission effects. When transmission dropped to 0.25 %, BER reached 10⁻⁷, but improved to below 10⁻¹⁰ at transmissions above 2.5 %; performance was about 10 dB below theoretical predictions due to scintillation, multiple scattering, and absorption, yet these results were obtained without error‑correction or active tracking.

Abstract

Free-space laser communication (lasercom) systems are subject to performance degradation when heavy fog or smoke obscures the line of sight. The bit-error rate (BER) of a high-bandwidth (570 Mbits/s) lasercom system was correlated 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 calculated, with the discrepancy attributed to scintillation, multiple scattering, and absorption. Peak power of the 810-nm communications laser was 186 mW, and the beam divergence was purposely degraded to 830 µrad. These results were achieved without the use of error correction schemes or active tracking. An optimized system with narrower beam divergence and active tracking could be expected to yield significantly better performance.