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45-km horizontal path optical link demonstration

Observations made during a mountain-top-to-mountain-top horizontal optical link demonstration are described. The optical link spans a range of 46 Km at an average altitude of 2 Km above sea level. A multibeam beacon comprised of eight laser beams emerging from four multimode fiber coupled lasers (780 nm) is launched through a 0.6 m diameter telescope located at the JPL Table Mountain Facility (TMF) in Wrightwood, California. The multibeam beacon is received at Strawberry Peak located in the San Bernardino Mountains of California. The NASA, JPL developed optical communications demonstrator (OCD) receives the beacon, senses the atmospheric turbulence induced motion and using an upgraded fine steering loop actively points a communications laser beam (852 nm, 400 Mbps on-off key modulated, PN7 pseudo random bit sequence) to TMF. The eight-beam beacon allowed a four-fold reduction in normalized irradiance or scintillation index. This in turn was sufficient to eliminate beacon fades sensed by the OCD and enable performance evaluation of the fine steering loop. The residual tracking error was determined to be +/- 1.1 to +/- 1.7 (mu) rad compared to a model prediction of +/- 3.4 (mu) rad. The best link performance observed showed average bit error rates (BER) of 1E-5 over long durations (30 seconds); however, instantaneous BERs of at least 0.8E-6 over durations of 2 ms were observed. The paper also discusses results pertaining to atmospheric effects, link analysis, and overall performance.