Bidirectional 20-Gbit/s OOK Link with a 4-Telescope Array and Incoherent Signal Combining

We present experimental results on a bidirectional link between a telescope array and a single-telescope antenna. Each telescope is coupled to a single-mode fiber. A fine tracking mechanism equips each telescope. The array telescopes provide diversity channels with uncorrelated noises, in particular uncorrelated turbulence fluctuations and ASE noise. We apply signal combining with incoherent detection. The array is meant to be on ground whereas the single telescope terminal would typically be above the atmosphere. In the downlink, the 1545-nm beam is received by each array telescope, optically amplified, filtered, photodetected and digitized. In the uplink, The ITU grid channels 26, 27, 28 and 29 (100 GHz spacing) are emitted. BER is measured at 20 Gbit/s in both link directions over a 400m free-space test range in Berlin. We discuss the scalability of the telescope array for up- and down-links.