Florian Moll, Alessia Scriminich, Christoph Gohle, Stefanie Häusler, Bettina Heim, Davide Orsucci, Johannes Prell, Javier Garcia Olmedo, Douglas Laidlaw
Quantum key distribution (QKD) enables secure communication even in presence of quantum computers. Satellite based systems enable worldwide key distribution and connection of terrestrial quantum networks. In future, QKD satellite constellations can be operated to service ground users that are connected using an optical ground station. These QKD ground stations are assumed to be deployed in two scenarios. One is the option as end-user ground station, in which the ground user is directly connected to the satellite via the ground station. The other option is the use as a provider ground station, in which the ground user is connected via an intermediary fiber line. The performance of the satellite to ground QKD link depends on, amongst other, the environmental and infrastructural impacts of the site where it is located. Unlike in astronomical sites, the QKD Optical Ground Station (OGS) are not positioned in selected optimal environment, especially considering atmospheric turbulence and extinction. The QKD OGS mainly has to cope with the given environment instead. Knowledge of the site’s meteorological and infrastructural environment is important to assess performance of a QKD OGS when deployed. This paper discusses the most relevant aspects of a QKD OGS site characterization. These are atmospheric turbulence, atmospheric extinction, and background light. As a concrete example, the DLR Optical Ground Station Oberpfaffenhofen nearby Munich is used. Existing data is summarized, reviewed and needs for further data acquisition defined.
Free-Space Optical Communication (FSOC) links between Earth-based Optical Ground Stations (OGSs) and satellites offer immense potential to securely and efficiently exchange vast amounts of information with worldwide coverage. However, atmospheric turbulence inhibits this potential by distorting laser beams, as they propagate through the atmosphere. Adaptive Optics (AO) systems are typically employed at the OGS to correct for these adverse effects and can increase the efficiency of laser light being coupled into an optical fibre for a downlink laser beam. Concurrently, the same AO system can be used to increase the coupling of laser light into an orbiting satellite by pre-distorting the uplink laser beam. In such a scenario, the downlink laser beam is used to measure the distortions that are applied by the atmosphere, and the conjugate of these distortions can then be applied to the uplink laser beam. The atmosphere then corrects the pre-distorted beam, resulting in a flat wavefront at the top of the atmosphere, as well as stable and efficient coupling of light into the satellite. This work showcases the successful experimental ground-to-satellite links in the spring of 2023 between DLR’s recently commissioned OGS and TESAT’s laser communications terminal (LCT-135)—i.e., part of the Technology Demonstration Payload No. 1 (TDP-1) on the geostationary satellite, Alphasat. Pre-distortion was successfully applied via an AO system testbed within the OGS, which resulted in extremely power efficient bi-directional tracking links with Alphasat. The findings of this work show that the application of pre-distortion AO not only improves the coupling of laser light at the satellite, but also reduces the scintillation experienced at the satellite, thus improving the robustness of the link.
Free Space Optical Communications (FSOC) links with satellites are limited by atmospheric turbulence in up and downlink. Adaptive Optics (AO) systems at the Optical Ground Station (OGS) can mitigate the adverse effects on the uplink by “predistorting” the transmitted laser beam such that its wavefront is corrected by the turbulence. The Point Ahead Angle (PAA) means that the downlink light is not a perfect wavefront reference for the AO system. GEOStar is a project created to demonstrate the feasibility of using a Laser Guide Star (LGS) in the direction of the uplink path to enable better predistortion of the transmitted beam. A novel setup uses a sub-pupil of the 1m diameter ESA-OGS to transmit the communications light to the satellite and a sub-pupil on the opposite side of the telescope aperture is used to launch the LGS. An LGS WFS observes the light from the LGS whilst a similar Near Infra-Red WFS observes downlink light from the satellite such that the measurements can be directly compared. A deformable mirror is used to predistort the uplink beam. The system is currently being integrated ready for shipment to Tenerife and measurements with the optical terminal TDP-1 on AlphaSat are scheduled for Q2 2024.
The optical turbulence profile is a key parameter in tomographic reconstruction. With interest in tomographic adaptive optics for the next generation of ELTs, turbulence profiling campaigns have produced large quantities of data for observing sites around the world. In order to be useful for Monte Carlo AO simulation, these large datasets must be reduced to a small number of profiles. There is commonly large variation in the structure of the turbulence, therefore statistics such as the median and interquartile range of each altitude bin become less representative as features in the profile are averaged out. Here we present the results of the use of a hierarchical clustering method to reduce the 2018A Stereo-SCIDAR dataset from ESO Paranal, consisting of over 10,000 turbulence profiles measured over 83 nights, to a small set of 18 that represent the most commonly observed profiles.
Six Laser Guide Stars (LGS) are included in the design of the European Extremely Large Telescope (ELT), with all of its current instruments taking advantage of them using Shack-Hartmann (SH) wavefront sensors (WFS). However, this implementation raises new issues related to the unprecedented elongation that results from the perspective effect combined to the thickness of the sodium layer. In order to investigate wavefront sensing with an elongated LGS on a SH WFS, we are taking advantage of the presence of the multi-object adaptive optics demonstrator CANARY on the William Herschel Telescope (WHT), in La Palma island, that was upgraded with a sodium LGS WFS for our experiment. The LGS is generated by ESO’s transportable Wendelstein LGS unit and the elongation is obtained by positioning the laser launch telescope 40 meters away from the WHT. With this experiment we are able to measure wavefronts using an elongated LGS WFS. In this paper, we present results obtained during the latest run of observations in September 2017. In these results is comprised an error breakdown of wavefront measurement on elongated LGS. The performances of several centroiding methods are compared thanks to this error breakdown. Additionally, we take advantage of varying observation conditions with respect to seeing and sodium profile to establish the robustness of the different centroiding methods. Finally, these performances are evaluated for different SH designs, to explore which compromises can be reached with respect to pixel scale and sub-aperture field of view.
To approach optimal performance advanced Adaptive Optics (AO) systems deployed on ground-based telescopes must have accurate knowledge of atmospheric turbulence as a function of altitude. Stereo-SCIDAR is a high-resolution stereoscopic instrument dedicated to this measure. Here, its profiles are directly compared to internal AO telemetry atmospheric profiling techniques for CANARY (Vidal et al. 20141), a Multi-Object AO (MOAO) pathfinder on the William Herschel Telescope (WHT), La Palma. In total twenty datasets are analysed across July and October of 2014. Levenberg-Marquardt fitting algorithms dubbed Direct Fitting and Learn 2 Step (L2S; Martin 20142) are used in the recovery of profile information via covariance matrices - respectively attaining average Pearson product-moment correlation coefficients with stereo-SCIDAR of 0.2 and 0.74. By excluding the measure of covariance between orthogonal Wavefront Sensor (WFS) slopes these results have revised values of 0.65 and 0.2. A data analysis technique that combines L2S and SLODAR is subsequently introduced that achieves a correlation coefficient of 0.76.
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