Tbit/s line-rate satellite feeder links enabled by coherent modulation and full-adaptive optics

Free-space optical(FSO)communication technologies constitute a solution to cope with the bandwidth demand of future satellite-ground networks.They may overcome the RF bottleneck and attain data rates in the order of Tbit/s with only a handful of ground stations.Here,we demonstrate single-carrier Tbit/s line-rate transmission over a free-space channel of 53.42 km between the Jungfraujoch mountain top(3700 m)in the Swiss Alps and the Zimmerwald Observatory(895 m)near the city of Bern,achieving net-rates of up to 0.94 Tbit/s.With this scenario a satellite-ground feeder link is mimicked under turbulent conditions.Despite adverse conditions high throughput was achieved by employing a full adaptive optics system to correct the distorted wavefront of the channel and by using polarization-multiplexed high-order complex modulation formats.It was found that adaptive optics does not distort the reception of coherent modulation formats.Also,we introduce constellation modulation–a new four-dimensional BPSK(4D-BPSK)modulation format as a technique to transmit high data rates under lowest SNR.This way we show 53 km FSO transmission of 13.3 Gbit/s and 210 Gbit/s with as little as 4.3 and 7.8 photons per bit,respectively,at a bit-error ratio of 1∙10−3.The experiments show that advanced coherent modulation coding in combination with full adaptive optical filtering are proper means to make next-generation Tbit/s satellite communications practical.