yCombining efficiency promotion of an all-fiber coherent combination system with special coupling-ratio fiber couplers for spatial diversity antennas in FSOC

The combination of numerous signals for a spatial diversity free-space optical communication (FSOC) framework can be actualized based on fiber-optic devices or digital signal processing. The optical combination process is normally achieved in binary tree structure to cascade multiple signals, so the number of input laser beams is subject to binary condition. In this article, we coherently combined an arbitrary number of laser beams by using a modified scheme based on the fiber couplers with specific coupling ratios. The theoretical research of three fibers combination is expressed and the practical combining efficiency of 98.3% is obtained in the experiment, which surpasses the theoretical greatest combining efficiency of customarily using 3-dB couplers. For a particular spatial diversity antenna that consists of 19 apertures, we designed its optical combination module based on specific fiber couplers. In the condition of phase differences among 19 input beams are compensated with SPGD algorithm, the average output power rises 25-40 times in the closed loop; the experimental combining efficiency reaches 56.5%, which is very close to the transmission efficiency of 59.3% where the combined module serves as a laser splitter without phase compensation. It means that overlooking the devices' insert losses, the coherent combining efficiency reaches 95.3%. This study shows that optical combination provides an effective alternative for the digital signals combining process in spatial diversity FSOC.