Portuguese Optical Ground Station

This work focuses on optimizing the coupling between free-space optical beams and optical fibers within the framework of the Portuguese Optical Ground Station (OGS) with Quantum Key Distribution (QKD) capabilities. The main objective is to maximize the coupling efficiency of light from space into optical fibers, which is essential for improving the performance and reliability of optical communication systems. A theoretical model based on the overlap integral between the incident optical field and the fiber fundamental mode is developed to evaluate the impact of optical aberrations, telescope geometries, and atmospheric turbulence on coupling efficiency. Different scenarios are investigated, including unobstructed pupils, centrally obstructed pupils, and Gaussian beam propagation, with Zemax OpticStudio used to optimize mode matching conditions. The influence of atmospheric turbulence is modelled using Zernike polynomials and Kolmogorov statistics, enabling realistic propagation simulations. Finally, the report includes an experimental component focused on the characterization of hollow-core photonic crystal fibers (HC-PCF) and the practical validation of coupling efficiency, supporting the development of robust free-space-to-fiber optical interfaces.