A design concept for a proposed airborne or spaceborne free-space optical-communication terminal provides for simultaneous reception of signals from multiple other optical-communication terminals aboard aircraft or spacecraft that carry scientific instruments and fly at lower altitudes. The concept reflects the need for rapid acquisition and tracking of the signals coming from the lower-altitude terminals as they move across the field of view.
As shown in the upper part of the figure, the optical train of the terminal would include a telescope aimed at the scene below via a gimballed flat mirror, which would be used to scan the field of view over a wide angular range. The lower part of the figure schematically depicts some of the optical and electronic channels used in the reception of data signals from, and the transmission of a beacon signal to, the lower terminals. This scheme is based on an architecture that provides for imaging of a small portion of the transmitted beam on a focal-plane array of photodetectors. Equipped with fast-read-out circuitry, the focal-plane array would be used in simultaneous acquisition and tracking.
The design concept includes an operational scenario in which each lower terminal would be assigned a unique uplink wavelength for its transmitted laser beam, which would serve as both its uplink communication beam and its beacon. An optical link would be initiated by a lower terminal, which would transmit a wide beam up to the higher terminal. The lower terminal would then await an acknowledgement of acquisition of its signal by the higher terminal before proceeding with a "handshake" and subsequent communications.
In the higher terminal, the uplinked beams from the lower terminals would be split between a data and a tracking channel, most of the beam power going to the data channel. In the tracking channel, the beams would then pass with minimal attenuation through a dichroic beam splitter and onto two electronically actuated beam- steering mirrors, which would reflect the beams onto a diffraction grating that would separate the beams by wave-length. The beams would then impinge on separate spots on the focal-plane array of photodetectors.
The downlink beam would be reflected by a fast-steering mirror, which would be driven to correct for vibrations measured by inertial sensors. The downlink beam would then be reflected out through the telescope by use of a mirror that would be partially (<1 percent) transmissive. The small part of the beacon beam transmitted through the mirror would impinge on a retroreflector and thereby be sent back to the focal-plane array to provide information on the pointing direction of the downlink beam.
The uplink from each lower terminal would be validated by means of the downlink beacon. To make this possible, the fast-steering mirror would also be made to rapidly scan the downlink beam across the locations of the lower terminals as indicated by the locations of their beam spots on the focal-plane array.
In the data channel,the uplink signals would impinge on a diffraction grating, then each beam would be focused on data-detector-and-beam-stabilization unit (denoted in the figure as di for the ith beam). Each di would include a quadrant detector and a fine-steering mirror acting together as parts of a servo loop to maintain strength of the uplink signal on a data detector.
This work was done by Keith Wilson of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Electronics/Computers category.
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Refer to NPO-30621, volume and number of this NASA Tech Briefs issue, and the page number.