Jongchoel Won

Recent advances in quantum computing pose a potential threat to conventional public-key crypto systems such as RSA(Rivest-Shamir-Adleman), motivating the development of quantum key distribution systems. In free-space quantum key distribution, fiber-coupling efficiency is a critical factor because optical loss directly affects the secret key rate and ultimately limits the achievable transmission distance. Therefore, minimizing coupling loss through stable beam tracking is essential, particularly in free-space QKD platforms involving satellites, drones, and mobile terminals[1,2]. In fine beam-tracking systems, beacon signals for the fast-steering mirror control loop can be detected using either quadrant detectors or CMOS cameras. Quadrant detectors offer fast response times and are suitable for real-time feedback control, but their tracking performance can be strongly affected by the spatial profile, size, and symmetry of the incident beam[3]. In contrast, CMOS cameras provide direct beam-profile information and are less sensitive to beam-shape distortion, but they generally suffer from slower response times. In this work, we investigate a fine-tracking configuration in which a beacon laser is transmitted through a central aperture in an off-axis parabolic mirror. As a result, the reflected beam has a centrally obscured, non-ideal Gaussian profile, which may degrade the tracking accuracy of a quadrant-detector-based system. We compare the performance of quadrant-detector- and CMOS-camera-based feedback schemes for fast steering mirror control under non-ideal beam-detection conditions. Furthermore, we analyze the influence of beam size and beam-profile distortion on tracking performance and discuss optimization strategies for stable fiber coupling in free-space QKD systems.