Alternative passive fiber coupling system based on multi-plane light conversion for satellite-to-ground communications

Global coverage of internet access is essential for digitalization in society, becoming a disruptive technology in industry, education or political participation for example. Satellite communications is a complementary approach to the terrestrial fiber network, which can provide world-wide coverage with few satellites in geostationary orbit or with low-earth-orbit constellations. Optical wavelengths offer multiple THz of available spectrum that can be used to connect satellites to the ground network with high-throughput links, solving the radiofrequency bandwidth bottleneck, without regulations. Cloud covereage and atmospheric turbulence are the main challenge in guaranteeing the same availability as in terrestrial fiber-based systems. While the former can be addressed by site diversity, for the latter, other mitigation strategies are required. Adaptive optics is a common approach to correct for atmospheric phase distortions and ensure stable fiber coupling. However, this approach requires a relatively complex active setup and therefore a collaboration between DLR Institute of Communications and Navigation and Cailabs has been formed to investigate alternative passive solutions for low-complexity ground stations. Coupling into multimode fibers does not require adaptive optics due to the large fiber core, however the coupled signal is distributed into multiple fiber-modes and is therefore incompatible with standard telecommunications components. Cailabs Multi-Plane Light Conversion (MPLC) technology overcomes this issue, selectively demultiplexing the fibermodes into single-mode fibers. Here, DLR's adaptive optics system and the MPLC technology in a turbulence-relevant environment for GEO communications are compared, investigating the advantages of the MPLC approach for compensating strong turbulence. This paper presents an overview of the measurement setup and analyzes the single-mode fibers outputs of the spatial demultiplexer and the measured phase-distortions from a wavefront sensor.