Citation
Abstract
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 C2 at either I end was determined. At TMF where a single 844-nm beam was received by a 0.6-m aperture, the measured C2 covered a wide range from 0.07 to 1.08. A single 780I nm beam C2 measured at SP using a 0.09-m aperture yielded values ranging from I 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 efiect 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 difierences 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 predicted by link analysis. The temporal beacon spot motion on the OCD focal-plane array, a charge-coupled device (CCD), caused by atmosphere-induced phase tilt was used to determine the PSD. These PSDs were used together with the OCD tracking model to predict the uncompensated error and were found to be in agreement with measured values ranging from 4.8 to 9.3 µrad. The beacon motion PSDs also showed a partial correlation (0.67) with the intensity fluctuations sensed by the OCD focal-plane array. The best bit-error rates measured at TMF were 1 × 10−5 and were limited mainly by beacon fades, though beam wander of the OCD beam also may have played a role.
Keywords
Details
- Volume
- 42-141
- Published
- May 15, 2000
- Pages
- 1–31
- File Size
- 1.0 MB