Laser space link systems

In the space sector, optics can be considered to be in a position to compete with radio-frequency links in several fields of application.

Firstly, the immense need for high bandwidth (remote-sensing space missions: observation or high-resolution spectral imaging, the arrival of multimedia, the development of portable telephone links, high-definition digital television, etc.) will be met by networks of satellites in low earth orbit (LEO), communicating with each other via very high bandwidth optical links (in excess of several gigabits per second) and with the ground via radio frequency.

Other exchanges may also be necessary between low earth orbit (LEO) and geostationary earth orbiting (GEO). This is the case for the Silex link (LEO) onboard the Spot 4 satellite (launched in 1998), which is to communicate with the future Artemis satellite (GEO) scheduled for launch in 2001.

Finally, the high directivity of the laser beam (high antenna gain) will be put to good use for long-distance links: probes orbiting planets in the solar system and linked to the Earth (or a geostationary satellite acting as a relay). NASA (Jet Propulsion Laboratory) is carrying out major studies in this field.

Other applications are also conceivable: intrasatellite links, transmission of clock signals (synchronization), precise positioning of interferometer arms, highly localized direct communications with the ground. In all these cases, the key is to use a laser beam that is very well stabilized (in angular position or frequency).

Current radio frequencies (K _u or K _a , from 11 to 30 GHz) will pose problems for these increasingly high throughput missions: to ensure sufficient gain, antenna diameters will have to reach very large dimensions, resulting in a large footprint, difficulties in setting up on a satellite and dynamic disturbances that will lead to stabilization faults when maneuvering these antennas.

Optical frequencies are therefore used, corresponding to wavelengths of the order of 1 µm. This is because the light beam emitted by the laser and collimated by an antenna is all the more directional the higher the optical frequency (or the shorter the wavelength). The diameter d of the zone illuminated by a laser beam, at wavelength λ, collimated by a lens of diameter D, at distance L, and corresponding to the first diffraction lobe, is given by the relationship d = 2.44 λL/D (simplified case of a uniformly illuminated pupil).

In figure 1 , the satellite...