Laser communications in space
Lasers in space had a more ominous meaning back in the Reagan era, but hopefully that tarnished image of our optically coherent friend is behind us. Communication has now been demonstrated between two satellites separated by a distance of 5000 km, using lasers no larger than a matchbox and weighing only 130 g.
Germany's Tesat-Spacecom established communication between the German satellite TerraSAR-X and the US satellite NFIRE, with a data transfer rate of 5.5 Gbit per second, which is comparable with 400 DVDs per hour. This is approximately 100 times greater bandwidth than can be achieved with conventional radio communications.
The development of this technology has many obvious benefits, both for ground to earth orbit communications, as well as inter-satellite links. When there is only a brief window of opportunity to transmit data, such as when a satellite passes within site of a ground station, it is important to transfer the data rapidly and without error. This is of particular importance for earth observation satellites that may need to transmit large image files.
The diode pumped laser modules were primarily developed by the Fraunhofer Institute for Laser Technology in Germany. They were made a small as possible, a prerequisite for any satellite payload due to the high launch costs. They also had to be able to survive some inhospitable conditions, including the acceleration and vibrational effects of the launch and the harsh effects of the sun in space .