Design and Analysis of a Transmitter Optical System for Low Earth Orbit Satellite Quantum Key Distribution

This study investigates the design and performance of a transmitter optical system optimized for Low-Earth-Orbit (LEO) satellite-based Free-Space Quantum Key Distribution(FS-QKD). The architecture integrates a 120 mm aperture telescope, a beam expander, and an optical-axis alignment module, complemented by a pointing control unit to maintain link stability. Performance was evaluated across 660 nm, 810 nm, and 1,550 nm wavelengths. Link budget analysis for a 10 km channel with a 200 mm receiver aperture demonstrated that a 660 nm beacon laser achieves a diffraction loss of 4.71 dB and a received power of 15.5 dB, with the beam expanding to approximately 440 mm. Notably, the integration of a Fast Steering Mirror(FSM) enhanced the average Signal-to-Noise Ratio(SNR) by nearly ninefold. These results validate the system's efficacy in mitigating pointing errors and optimizing optical coupling for robust inter-satellite quantum communication.