Tech Briefs

Simple Laser Communications Terminal for Downlink From Earth Orbit at Rates Exceeding 10 Gb/s

Implementation of this technology will surpass the spectrum-allocation and bandwidth limitations of current RF systems.

A compact, low-cost laser communications transceiver was prototyped for downlinking data at 10 Gb/s from Earth-orbiting spacecraft. The design can be implemented using flight-grade parts. With emphasis on simplicity, compactness, and light weight of the flight transceiver, the reduced-complexity design and development approach involves:

Laser Communications Terminal consists of the optical head on a 2-axis gimbal (left), and an electronics/laser box (right)." class="caption" align="right">1. A high-bandwidth coarse wavelength division multiplexed (CWDM) (4×2.5 or 10-Gb/s data-rate) downlink transmitter. To simplify the system, emphasis is on the downlink. Optical uplink data rate is modest (due to existing and adequate RF uplink capability).

2. Highly simplified and compact 5-cmdiameter clear aperture optics assembly is configured to single transmit and receive aperture laser signals. About 2 W of 4-channel multiplexed (1,540 to 1,555 nm) optically amplified laser power is coupled to the optical assembly through a fiber optic cable. It contains a highly compact, precision-pointing capability two-axis gimbal assembly to coarse point the optics assembly. A fast steering mirror, built into the optical path of the optical assembly, is used to remove residual pointing disturbances from the gimbal. Acquisition, pointing, and tracking are assisted by a beacon laser transmitted from the ground and received by the optical assembly, which will allow transmission of a laser beam.

3. Shifting the link burden to the ground by relying on direct detection optical receivers retrofitted to 1-mdiameter ground telescopes.

4. Favored mass and volume reduction over power-consumption reduction. The two major variables that are available include laser transmit power at either end of the link, and telescope aperture diameter at each end of the link. Increased laser power is traded for smaller-aperture diameters.

5. Use of commercially available spacequalified or qualifiable components with traceability to flight qualification (i.e., a flight-qualified version is commercially available). An example is use of Telecordia-qualified fiber optic communication components including active components (lasers, amplifiers, photodetectors) that, except for vacuum and radiation, meet most of the qualifications required for space.

6. Use of CWDM technique at the flight transmitter for operation at four channels (each at 2.5 Gb/s or a total of 10 Gb/s data rate). Applying this technique allows utilization of larger active area photodetectors at the ground station. This minimizes atmospheric scintillation/turbulence in duced losses on the received beam at the ground terminal.

7. Use of forward-error-correction and deep-interleaver codes to minimize atmospheric turbulence effects on the downlink beam.

Target mass and power consumption for the flight data transmitter system is less than 10 kg and approximately 60 W for the 400-km orbit (900-km slant range), and 12 kg and 120 W for the 2,000-km orbit (6,000- km slant range). The higher mass and power for the latter are the result of employing a higher-power laser only.

This work was done by Joseph M. Kovalik, Hamid Hemmati, Abhijit Biswas, and William T. Roberts of Caltech for NASA's Jet Propulsion Laboratory. NPO-48413

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Simple Laser Communications Terminal for Downlink From Earth Orbit at Rates Exceeding 10 Gb/s (reference NPO-48413) is currently available for download from the TSP library.

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