Common-path FSOC terminal enabled by a terraced MCF and gradient-descent laser nutation algorithm

Free-space optical communication (FSOC) terminals require rapid and accurate beam alignment. Conventional alignment schemes typically rely on beam splitting and dedicated sensing paths, which increase optical loss and system complexity. In this paper, we propose a common-path FSOC terminal enabled by a terraced multi-core fiber (MCF) and a gradient-descent laser nutation (GDLN) algorithm for fine alignment. The performance and operational field of view (FOV) of the proposed system were experimentally validated on a collimated-light tunnel platform, achieving a fine-alignment FOV of up to ±3.33 mrad. An outdoor experiment was further conducted between two buildings separated by 860 m. A six-axis motion platform applied external disturbances with an amplitude of 3° to the terminal at 4-hour intervals over a duration of 24 h. The system automatically realigned to restore the link within 4 s. Furthermore, real-time single-wavelength 10 Gbps IMDD transmission was demonstrated with a BER of 1.242×10^-11.