Optical and Mechanical Design of a Phase-Induced Amplitude Apodizer for Efficient Fiber Coupling in Optical Communication Ground Stations AMOS SA

Abstract

Ground-to-space optical communication systems require injecting laser light into a single-mode fiber with maximum efficiency. This efficiency is fundamentally limited by the amplitude mismatch between the diffracted focal field and the Gaussian fiber mode, a mismatch that affects any finite aperture and is further exacerbated by a central obscuration. This represents a fundamental ceiling that adaptive optics alone cannot overcome. For the SALTO telescope used in the MOCA project (ESA/ARTES), with an obstruction ratio of 0.36, this limit stands at approximately 60%, reduced to 50% by the current injection optics. This thesis presents the optical and mechanical design of a Phase-Induced Amplitude Apodizer (PIAA) intended to break this amplitude ceiling. The PIAA uses a pair of mirrors to geometrically redistribute the beam intensity with near-unity throughput, reshaping the input pupil into a profile matched to the fiber mode. A Python-based geometric model derives the radial remapping law and mirror sag profiles from flux conservation, yielding a predicted coupling efficiency of 97.8%. The design is then validated in Ansys Zemax OpticStudio, where a systematic focal spot broadening by a factor of 1.21 is identified, attributed to diffraction at the finite annular exit pupil, and corrected by rescaling the target mode field radius. A compact mechanical assembly is designed in Autodesk Fusion using standard Thorlabs components, and a full tolerance and sensitivity analysis demonstrates that active compensation restores near-nominal coupling across the full range of expected fabrication errors. The final validated design achieves a coupling efficiency of 94.6%, representing a gain of 35 percentage points over the unapodized baseline.