Ground-to-Ground Optical Communications Demonstration

A bidirectional horizontal-path optical link was demonstrated between Strawberry Peak (SP), Lake Arrowhead, California, and the JPL Table Mountain Facility (TMF), Wrightwood, California, during June and November of 1998. The 0.6-m telescope at TMF was used to broadcast a 4-beam 780-nm beacon to SP. The JPL-patented Optical Communications Demonstrator (OCD) at SP received the beacon, performed fine tracking to compensate for the atmosphere-induced beacon motion and retransmitted a 844-nm communications laser beam modulated at 40 to 500 Mb/s back to TMF. Characteristics of the horizontal-path atmospheric channel as well as performance of the optical communications link were evaluated. The normalized variance of the irradiance fluctuations or scintillation index σ I at either end was determined. At TMF where a single 844-nm beam was received by a 0.6-m aperture, the measured σ I covered a wide range from 0.07 to 1.08. A single 780nm beam σ I measured at SP using a 0.09-m aperture yielded values ranging from 0.66 to 1.03, while a combination of four beams reduced the scintillation index due to incoherent averaging to 0.22 to 0.40. This reduction reduced the dynamic range of the fluctuations from 17 to 21 dB to 13 to 14 dB as compared with the OCD tracking sensor dynamic range of 10 dB. Predictions of these values also were made based on existing theories and are compared. Generally speaking, the theoretical bounds were reasonable. Discussions on the probability density function (PDF) of the intensity fluctuations are presented and compared with the measurements made. The lognormal PDF was found to agree for the weak scintillation regime as expected. The present measurements support evidence presented by earlier measurements made using the same horizontal path, which suggests that the aperture averaging effect is better than theoretically predicted. The temporal power spectral densities (PSDs) of the irradiance fluctuations show nominal agreement with theoretically predicted behavior. The focal-spot sizes of the 844-nm and 780-nm beams received at either end are reported and compared with atmospheric “seeing” predicted by theory. The limit in which the theories agree with measurements is inconsistent with certain assumptions that are made; removing these assumptions renders significant differences between theory and measurement. The measured peak-to-peak beam wander at TMF was 4 m as compared with the theoretical prediction of 0.8 m. The received optical power at each end validates the uncertainty