1. Technical Field
The invention relates to optical data transmission and it is particularly suitable for use in data links between satellites (particularly between a satellite placed in a low orbit and a geostationary satellite).
2. Prior Art
Optical data transfer using frequency multiplexing has been known for long. An example may be found in U.S. Pat. No. 3,501,640 (Harris).
In optical links between satellites it has already been proposed to use laser diodes as light sources. Laser diodes are small size and reliable, have a high field and a considerable life duration. Among laser diodes, gallium arsenide diodes are of particular interest: monomode diodes capable of supplying a power up to about 50 mW continuously are available in the wavelength range between 810 and 850 nm, and efficient sensors are available in the wavelength range between 400 and 1100 nm.
The unavailability of laser diodes having a power higher than 50 mW and the need for transmitting high digital flow rates (typically 500 Mbits/s) or the advantage of transmitting over several independent channels make it however necessary to use several laser diodes simultaneously for transmission on a same beam. Different power summation and multiplexing techniques have already been proposed. They are summed up in an article by Begley et al, "Selection of laser diode beam combining techniques for free space communication" Proceedings of SPIE, Volume 616, 21-22 January 1986, Los Agneles, pp. 276-280. The proposed approaches have serious drawbacks. In particular, most of them use dichroic mirrors which receive light beams under an oblique incidence for multiplexing or demultiplexing (the second operation being more difficult to achieve, for it must provide sufficient separation between adjacent channels). The dichroic mirrors fail to reach that result perfectly and require that there is an appreciable wavelength difference between adajacent channels.
It has also been proposed to use the very high polarization of light emitted by laser diodes and to use summation of the beams coming from several diodes along a single output path rather than frequency multiplexing. This solution however has not proven satisfactory, probably because the known arrangements make use of components which receive light under oblique incidence for bringing all beams along the same output axis.