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Free-space laser communication technologies. Conference

Design and implementation of a mobile optical ground station for air-to-ground FSOC-links

Peter Krause, Marcel Rothe, Ingo Gebhart, Matthias Goy, Conor Waldron, Martin Landmann, Fabian Steinlechner

Published Mar 5, 2026• 1 citation
Publisher

Abstract

We present QuBUS, a mobile optical ground station designed for free-space optical communication (FSOC) and quantum key distribution (QKD). Built into a transportable lab container, QuBUS includes key subsystems such as an optical antenna with uplink lasers, a freeform telescope, acquisition and tracking hardware, relay optics, and an adaptive optics system for single-mode fiber coupling. The station is being prepared for an air-to-ground quantum communication experiment involving an aircraft-based transmitter. Initial 7-km horizontal link tests validate core functionalities, demonstrating QuBUS’s potential as a compact and deployable platform for future secure quantum communication networks.

DOI
10.1117/12.3079664

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We present QuBUS, a mobile optical ground station designed for free-space optical communication (FSOC) and quantum key distribution (QKD). Built into a transportable lab container, QuBUS includes key subsystems such as an optical antenna with uplink lasers, a freeform telescope, acquisition and tracking hardware, relay optics, and an adaptive optics system for single-mode fiber coupling. The station is being prepared for an air-to-ground quantum communication experiment involving an aircraft-based transmitter. Initial 7-km horizontal link tests validate core functionalities, demonstrating QuBUS’s potential as a compact and deployable platform for future secure quantum communication networks.
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A worldwide growing interest in fast and secure data communications pushes technology development along two lines. While fast communications can be realized using laser communications in fiber and free-space, inherently secure communications can be achieved using quantum key distribution (QKD). By combining both technologies in a single device, many synergies can be exploited, therefore reducing size, weight and power of future systems. In recent experiments we demonstrated quantum communications over large distances as well as between an aircraft and a ground station which proved the feasibility of QKD between moving partners. Satellites thus may be used as trusted nodes in combination with QKD receiver stations on ground, thereby enabling fast and secure communications on a global scale. We discuss the previous experiment with emphasis on necessary developments to be done and corresponding ongoing research work of German Aerospace Center (DLR) and Ludwig Maximilians University Munich (LMU). DLR is performing research on satellite and ground terminals for the high-rate laser communication component, which are enabling technologies for the QKD link. We describe the concept and hardware of three generations of OSIRIS (Optical High Speed Infrared Link System) laser communication terminals for low Earth orbiting satellites. The first type applies laser beam pointing solely based on classical satellite control, the second uses an optical feedback to the satellite bus and the third, currently being in design phase, comprises of a special coarse pointing assembly to control beam direction independent of satellite orientation. Ongoing work also targets optical terminals for CubeSats. A further increase of beam pointing accuracy can be achieved with a fine pointing assembly. Two ground stations will be available for future testing, an advanced stationary ground station and a transportable ground station. In parallel the LMU QKD source size will be reduced by more than an order of magnitude thereby simplifying its integration into future free-space optical communication links with CubeSats.
We report on the design and development of a quantum backend for an optical ground station for space-based quantum communication and science experiments. The quantum backend will enable the Optical Communication Telescope Laboratory (OCTL) to establish links with quantum satellites in the future. We aim to test this quantum enabled ground station with upcoming satellite Quantum Key Distribution (QKD) missions. We present measurements of the ground station properties that are relevant for future quantum links. Specifically, we discuss the polarization disturbance imposed by the optical communication telescope and present mitigation strategies in the form of polarization control systems. In addition to the optical design, we also present an end-to-end QKD model that is used to guide the development.
Quantum Key Distribution (QKD), either fiber based or free-space, allows for provably secure key distribution solely based on the laws of quantum mechanics. Feasibility of QKD systems in aircraft-ground links was demonstrated with a successful key exchange. Experiment flights were undertaken during night time at the site of the German Aerospace Center (DLR) Oberpfaffenhofen, Germany. The aircraft was a Dornier 228 equipped with a laser communication terminal, originally designed for optical data downlinks with intensity modulation and direct detection. The counter terminal on ground was an optical ground station with a 40 cm Cassegrain type receiver telescope. Alice and Bob, as the transmitter and receiver systems usually are called in QKD, were integrated in the flight and ground terminals, respectively. A second laser source with 1550 nm wavelength was used to transmit a 100 MHz signal for synchronization of the two partners. The so called BB84 protocol, here implemented with faint polarization encoded pulses at 850nm wavelength, was applied as key generation scheme. Within two flights, measurements of the QKD and communication channel could be obtained with link distance of 20 km. After link acquisition, the tracking systems in the aircraft and on ground were able to keep lock of the narrow QKD beam. Emphasis of this paper is put on presentation of the link technology, i.e. link design and modifications of the communication terminals. First analysis of link attenuation, performance of the QKD system and scintillation of the sync signal is also addressed.