Dr. Helawae Friew Kelemu Profile

Dr. Helawae Friew Kelemu

at Deutsches Zentrum für Luft- und Raumfahrt e.v

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Publications (4)

Proceedings Article | 12 March 2024 Poster + Paper

GEOStar: demonstration of laser guide star adaptive optics for free space optical communications

Andrew Reeves, Nancy Ageorges, Dirk Kampf, Dailos Guerra Ramos, Mohamad Abdo, Ilija Hristovski, Helawae Friew Kelemu, Douglas Laidlaw, Zoran Sodnik, Ramon Mata Calvo, Bruno Femenía-Castellá

Proceedings Volume 12877, 128771S (2024) https://doi.org/10.1117/12.3002594

KEYWORDS: Wavefront sensors, Near infrared, Adaptive optics, Satellites, Beam path, Design, Sensors, Cameras, Telescopes, Sodium

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Proceedings Article | 26 October 2022 Presentation + Paper

Investigation of advanced control for adaptive optics in free-space optical communication

Helawae Friew Kelemu, Andrew Reeves, Ramon Mata Calvo, Wolfgang Drewelow, Torsten Jeinsch

Proceedings Volume 12266, 122660B (2022) https://doi.org/10.1117/12.2636270

KEYWORDS: Single mode fibers, Modeling and simulation, Turbulence, Adaptive optics, Free space optics, Scintillation, Autoregressive models, Atmospheric propagation

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Proceedings Article | 9 March 2020 Presentation + Paper

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

Ramon Mata Calvo, David Allioux, Andrew Reeves, Antonin Billaud, Juraj Poliak, Olivier Pinel, Helawae Friew Kelemu, Guillaume Labroille, Mathias Richerzhagen

Proceedings Volume 11272, 112720Q (2020) https://doi.org/10.1117/12.2546076

KEYWORDS: Adaptive optics, Scintillation, Single mode fibers, Satellites, Turbulence, Telecommunications, Fiber couplers, Satellite communications, Telescopes, Atmospheric turbulence

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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.

Proceedings Article | 4 March 2019 Paper

Optical technologies for very high throughput satellite communications

Ramon Mata Calvo, Juraj Poliak, Janis Surof, Andrew Reeves, Mathias Richerzhagen, Helawae Friew Kelemu, Ricardo Barrios, Carlos Carrizo, Raphael Wolf, Fabian Rein, Annika Dochhan, Karen Saucke, Wolfram Luetke

Proceedings Volume 10910, 109100W (2019) https://doi.org/10.1117/12.2513819

KEYWORDS: Satellites, Adaptive optics, Satellite communications, Telecommunications, Scintillation, Optical communications, Receivers, Turbulence, Modulation, Atmospheric optics, Optical networks, Laser guide stars, Homodyne detection, Digital signal processing

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