PROCEEDINGS ARTICLE | April 26, 1999
Proc. SPIE. 3615, Free-Space Laser Communication Technologies XI
KEYWORDS: Telescopes, Mirrors, Avalanche photodetectors, Surface plasmons, Clocks, Sensors, Space telescopes, Optical communications, Charge-coupled devices, Atmospheric optics
Mountain-top to mountain-top optical link experiments have been initiated at JPL, in order to perform a systems level evaluation of optical communications. Progress made so far is reported. The NASA, JPL developed optical communications demonstrator (OCD) is used to transmit a laser signal from Strawberry Peak (SP), located in the San Bernadino mountains of California. This laser beam is received by a 0.6 m aperture telescope at JPL's Table Mountain Facility (TMF), located in Wrightwood, California. The optical link is bi-directional with the TMF telescope transmitting a continuous 4-wave (cw) 780 nm beacon and the OCD sending back an 840 nm, 100 - 500 Mbps pseudo noise (PN) modulated, laser beam. The optical link path is at an average altitude of 2 Km above sea level, covers a range of 46.8 Km and provides an atmospheric channel equivalent to approximately 4 air masses. Average received power measured at either end fall well within the uncertainties predicted by link analysis. The reduction in normalized intensity variance ((sigma) I2) for the 4- beam beacon, compared to each individual beam, at SP, was from approximately 0.68 to 0.22. With some allowance for intra-beam mis-alignment, this is consistent with incoherent averaging. The (sigma) I2 measured at TMF approximately 0.43 plus or minus 0.22 exceeded the expected aperture averaged value of less than 0.1, probably because of beam wander. The focused spot sizes of approximately 162 plus or minus 6 micrometer at the TMF Coude and approximately 64 plus or minus 3 micrometer on the OCD compare to the predicted size range of 52 - 172 micrometer and 57 - 93 micrometer, respectively. This is consistent with 4 - 5 arcsec of atmospheric 'seeing.' The preliminary evaluation of OCD's fine tracking indicates that the uncompensated tracking error is approximately 3.3 (mu) rad compared to approximately 1.7 (mu) rad observed in the laboratory. Fine tracking performance was intermittent, primarily due to beacon fades on the OCD tracking sensor. The best bit error rates observed while tracking worked were 1E - 5 to 1E - 6.
