The main barrier to acceptance of laser-based communications for free-space applications, according to engineers at ThermoTrex Corp. (San Diego, CA), is not the maturity of the technology but the lack of convincing demonstrations of its performance and reliability. To remedy this situation the company has completed several free-space-communications technology demonstrations during the past year. These tests include a long-range (42 km) mountaintop-to-mountaintop 2 Gbit/s communication link at an altitude of 7000 ft, and, in early 1995, a high-capacity videoconferencing link operating over a distance of 10 km between the San Diego Convention Center and the Naval Research and Development Laboratory (Point Loma, CA). More tests are planned for later this year.
The benefits of laser-based free-space communications as compared to conventional radio-frequency (RF) and microwave systems--especially in situations involving high-data-rate communications between satellites, aircraft, and ground stations--include higher bandwidth and more security from a lighter-weight, smaller package that consumes less power than equivalent RF systems. Although interference from cloud coverage could interrupt a data link, ThermoTrex engineers believe that, for high-flying aircraft, the probability of not completing a link due to cloud coverage is actually very small. And for downlinks to earth from aircraft or satellite transceivers, RF systems could provide a backup to overcome interference from clouds. According to project manager Scott Bloom, the RF systems can be configured to prevent data bottlenecks.
A laser-based free-space communications link involves a modulated laser beam being detected and demodulated at a point distant from the originating laser. Implementing such a link, however, means embracing several ad vanced technologies. The detector/receiver must find the laser beam in the first place, and, having found it, must remain locked onto it while both transmitter and receiver may be moving; furthermore, the detector must be able to reject a significant amount of background light in order to distinguish the signal from the noise. The Lasercom laser communications transceivers designed by ThermoTrex researchers have reportedly addressed many of these issues. The systems used in the technology demonstrations achieve data rates exceeding 1 Gbit/s using two polarization-multiplexed channels.
Laser transceiver
The Lasercom system communications lasers are four directly modulated 810-nm semiconductor devices; outputs from two of the lasers are left-circularly polarized while the other two laser outputs are right-circularly polarized, thus providing the two channels that are polarization multiplexed. The peak output power of each channel is 300 mW (150 mW per laser). Two other 852-nm wavelength-locked diode lasers provide a 50-mW beacon signal for tracking and signal acquisition. The system achieves a 1.25° acquisition field of view in all lighting conditions because a narrow-band atomic line filter is incorporated in front of the CCD tracking camera (see box on page 18 and Laser Focus World, Feb. 1994, p. 18). Relative orientation of the transceivers to each other is not a factor because all laser beams leaving the transceivers are circularly polarized.
The Schmidt-Cassegrain-design receiver telescope (see Laser Focus World, Feb. 1995, p. 66) collects light from both the 852-nm acquisition and 810-nm communications lasers, directing the 852-nm light through the atomic line filter to a 256 × 256-pixel CCD camera and the 810-nm light through demultiplexing optics onto low-noise avalanche photodiodes. The tracking camera operates at a frame readout rate of 200 Hz, which allows the transceiver`s gimbal to mechanically correct for small platform vibrations up to about 20 Hz.
Additional demonstrations of the Lasercom system are scheduled for later this year and early in 1996. Much of this work is sponsored by the Ballistic Missile Defense Organization. Tests will include longer-range terrestrial links as well as air-to-air and space-to-ground links. In 1997, plans call for placement of transceivers aboard US military satellites. Longer-term applications of the system transceivers to be launched might include linking a ring of satellites around the earth to provide additional high-bandwidth communications capacity for the "information superhighway."