Towards optical data highways through the atmosphere

Abstract

The European Data Relay System (EDRS) is operational, optically transferring data from currently four LEO Earth observation satellites to the geo-stationary EDRS-A spacecraft at 1.8 Gbps. The demand has increased to extend these point-to-point optical links towards a full optical network in space and enable high data rate links between space assets and between ground and space. This article presents the ESA developments towards high data rate optical free space feeder links. The performance of an optical link from a ground station to a geostationary relay spacecraft experiences major limitations by atmospheric turbulence. To overcome this limitation, a free-space optical link experiment over 13 km is being set up. It shall assess the gain in irradiance and corresponding reduction of the scintillation index by pre-distortion of the optical “uplink” beam based on the measured wave-front disturbances of the “downlink” beam using an adaptive optics system. A second experiment will answer the question if the isoplanatic angle covers the point ahead angle in a ground to GEO link. This was/will be done by correlation measurements on double stars separated between 3.6 and 4.1 arcsec in varying elevation angles and atmospheric turbulence conditions. A third experiment shall address the potential gain and limitations of the implementation of Wavelength Division Multiplexing (WDM) into optical inter-satellite links. WDM being a standard technique to increase the data handling capacity of fibre networks by injecting multiple data streams into one single fibre using only one set of transmit and receive optics.