Optical Free-Space Communications

The topic of optical free-space communications evolved as early as the development of the laser in the 1960s. After the invention of optical fiber its application was mainly expected in space, where it shows its main advantages such as low weigh and small size terminal, large bandwidth and low power consumption.

The Institute´s work in optical free-space communications was initiated during the initial phases of the early satellite network projects Iridium and Globalstar. The focus was on space transmission with the best possible efficiencies. The Institute has set a world record in sensitivity with 20 photons per bit for un-coded coherent modulation in 1990. It contributed to the development of the DLR laser communications terminal (LCT). 

In more recent times, the Institute has started to study the possibility of optical transmission through the atmosphere. The influence of atmospheric turbulences on the phase coherence was studied. Simple diversity algorithms for mitigating the effect were developed. They were tested in an experimental setup between the Wallberg and Oberpfaffenhofen at a distance of 60 km.

Currently, the Institute is developing an Optical Ground Station (OGS) for broadband optical communications and a more thorough study of atmospheric-turbulences. The ground station is designed for a wide range of applications including satellite and aeronautical links (aircrafts, High Altitude Platforms…). The capability of the OGS was tested in a major trial with a stratospheric balloon providing 1.25 Gbps with 100 mW transmit power over a distance of 60 km with a reasonably low bit error rate in 2004. In 2006 the OGS performed a successful downlink experiment with the Japanese LEO satellite KIODO. Further trials with the German DLR-LCT onboard TerraSAR-X are planned for autumn 2007.

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