Optical space communications systems

In an increasingly connected world, communication requirements have grown exponentially. The dominance of optical communications in the terrestrial domain in recent decades is explained by their high data transmission capacities over long distances. Their increasing adoption in the space domain has been driven by the combined need to increase data rates while reducing system size and cost. Optical space communications are already proving their worth for inter-satellite links, notably with the advent of low-earth orbit constellations (Space Development Agency (SDA), Starlink) and for data repatriation via a geosynchronous satellite (European Data Relay Satellite System, EDRS). Development efforts are now concentrating on increasing data rates and creating links that pass through the atmosphere, adding a further level of complexity.

Apart from the promise of very high data rates, optical space telecommunication systems offer a number of additional advantages over radio frequency (RF) systems:

• there are no regulations governing the use of these frequencies. In fact, they are so large $\left(\sim \mathrm{\hspace{0.17em}}190\mathrm{\hspace{0.17em}}\mathrm{THz}\mathrm{\hspace{0.17em}}\mathrm{ce}\mathrm{\hspace{0.17em}}\mathrm{qui}\mathrm{\hspace{0.17em}}\mathrm{équivaut}\mathrm{\hspace{0.17em}}\mathrm{à}\mathrm{\hspace{0.17em}}1\mathrm{\hspace{0.17em}}550\mathrm{\hspace{0.17em}}\mathrm{nm}\right)$ and beams so narrow $\left(\sim \mathrm{\hspace{0.17em}}10\mathrm{\hspace{0.17em}\mu }\mathrm{rad}\right)$ that the risk of interference with another system is negligible;

• Their discretion makes them difficult to spy on and scramble, which is of great interest to the armed forces;

• they are more compact than radio-frequency systems (at equivalent data rates);

• they reduce electromagnetic compatibility problems on spacecraft.

On the other hand, these systems still suffer from a few hard points:

• Spatialization of terrestrial components is not always easy because of the advanced electronics used;

• they operate almost exclusively on point-to-point links;

• they are extremely sensitive to cloud cover, which...