The usage of CubeSat platforms has seen a significant increase over the last decade. CubeSats are compact and cost-efficient. With it the need for free-space optical communication (FSOC) between satellites and optical ground stations did increase as well. Achieving a good ratio between the size and the performance of the optical payload is still a challenge. We propose a FSOC terminal to be implemented on a 16U CubeSat platform. A combination of a coarse pointing assembly (CPA) based on a dual Risley prism scanner in front of an all-metal freeform telescope, and a fine pointing assembly (FPA) with a fast-steering mirror (FSM) were developed. The Risley prisms have a smaller mechanical envelope compared to classical gimbal-based mirror mounts or periscopes but can still provide a suitable range of beam deflection. The use of such a Risley prism scanner has been a research topic in aerospace for quite some time. Especially the challenging driving and controlling of the nonlinear beam pointing behavior presents a challenge. To keep the payload on the CubeSat feasibly small, only a microcontroller with limited calculation power is used. Therefore, we propose a combined control scheme for the CPA and FPA based on simplified calculations and the use of classical digital control theory. Coarse and fine pointing are controlled in a closed loop pointing simultaneously.
In recent years, quantum key distribution (QKD) has seen the first proof-of-concept demonstrations from space. Next on the agenda towards a full-blown global quantum internet is to address the more practical aspects, such as efficiency, flexibility, and accessibility of QKD services. One of the main challenges that remains to be solved in this regard is to enable operation in the presence of daylight noise. Here, we present a complete framework for modelling daylight QKD from an orbiting satellite. We include the effects of atmospheric turbulence and adaptive optics correction at the receivers. We consider single- and multi-mode fibre coupling as a means of spatial filtering, for which we derived simple formulas for estimating coupling efficiencies of signal as well as noise. Using our framework, we identify the most critical system parameters for daylight operation and discuss the choice of signal wavelength and detection technology. Finally, we provide simulation results for various parameter combinations in a hypothetical daylight QKD between Berlin and Munich via a satellite in a low Earth orbit. The results show a clear advantage of 800 nm signal wavelength over 1550 nm with the currently available technology. Moreover, we show the relevance of single-mode fibre coupling and the importance of detectors with low timing jitters. We anticipate our work will provide valuable insight and tools to aid the future feasibility studies of daylight QKD in dual-downlink configurations. Additional presentation content can be accessed on the supplemental content page.
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