Abstract: In recent years, space division multiplexing (SDM) technology has been extensively utilized to enhance the capacity of optical fiber communications, with SDM fibers serving as the key enabling foundation. Among them, weakly coupled few-mode optical fibers reduce inter-mode coupling, effectively suppress mode crosstalk, and significantly lower the complexity and cost of the transmission system. In this paper, different structural types of weakly coupled few-mode fibers, and the work undertaken by our research team on center-assisted ring-core fibers are presented. Center-assisted ring-core fibers achieve the design objective of significantly separating spatial mode degeneracy while keeping polarization degeneracy unseparated under the conventional core-cladding refractive index difference level of communication fibers, and exhibit good tolerance to fabrication errors. Furthermore, design rules for controlling the separation of spatial mode degeneracy in center-assisted ring-core fibers are identified, unveiling the principle of regulating specific spatial mode separations by altering the symmetry of the fiber’s refractive index profile. The results provide valuable guidance for the design of similar fiber structures.

Key words: space division multiplexing (SDM), weakly coupled few-mode fiber, center-assisted ring-core fiber, degenerate mode