Precision angle measurement for FSOC alignment enabled by a multi-core fiber.

Free-space optical communication terminals require compact architectures and high-precision alignment schemes. However, conventional incident-angle sensing usually relies on beam splitting and dedicated position sensors, increasing system complexity and terminal size. Moreover, power-feedback alignment typically provides a limited field of view (FOV) and slow convergence. We propose a direct beam incident-angle measurement method based on a multi-core fiber (MCF) for a common-path FSOC terminal. By exploiting the coupling-power characteristics of a seven-core fiber, the incident angle is directly inferred without any auxiliary position-sensing branch. We establish an incident-angle estimation model and further improve its accuracy through thin-plate-spline (TPS) calibration, achieving high-precision angle measurement over ±840 µrad with a mean error of 21.72 µrad and a root-mean-square error (RMSE) of 23.74 µrad. A 550 m outdoor experiment further demonstrates stable beam alignment and 10.3125 Gb/s IMDD transmission.