Few-mode fibers, capable of transmitting a limited number of modes, have been successfully applied in spatial division multiplexing in optical fiber communication systems. The effective separation of modes in few-mode fibers can usually be measured by the mode effective refractive index difference. To achieve effective mode separation, an eccentric elliptical and concentric circle dual low-refractive index region assisted ring-core few-mode fiber structure is proposed in this paper. The proposed fiber effectively eliminates the spatial degeneracy of the LP_mn mode group. The effects of the dimensions and position of the eccentric elliptical low-refractive index region, as well as the size of the concentric circular low-refractive index region, on the effective refractive index differences between adjacent spatial modes were systematically investigated. The underlying mode-control mechanism was elucidated, and the wavelength-dependent characteristics, confinement losses, and fabrication tolerances of the supported spatial modes were analyzed.
The results show that the fiber exhibits the best overall performance when ls=4.3µm, b=1.7µm, and r₄=2.6µm. Over the wavelength range of 1530-1610 nm, the proposed fiber supports nine guided spatial modes. The effective refractive index differences between adjacent spatial modes remain above 3.16×10^-4, reaching a maximum of 7.84×10^-4. Except for the highest-order mode, the confinement losses of the other eight modes remain below 0.503 dB/km. At a wavelength of 1550 nm, the effective refractive index differences between adjacent spatial modes exceed 3.28×10^-4, with a maximum value of 7.33×10^-4. Except for the highest-order mode, the confinement losses of the other eight spatial modes are all lower than 0.0242 dB/km. Throughout the target wavelength band, the effective refractive index differences between the orthogonal polarization states of each spatial mode remain on the order of 10^-7-10^-6, indicating negligible polarization mode splitting. The proposed fiber therefore lifts the degeneracy among spatial modes while largely preserving polarization degeneracy.
This fiber structure broadens the available transmission bandwidth, effectively suppresses intermodal crosstalk, and improves the utilization efficiency of the supported modes, thereby providing a promising alternative for increasing the transmission capacity of space-division-multiplexed communication systems. Especially in short distance and high-capacity optical interconnect scenarios, it can better leverage its advantage of effectively separating spatial modes without separating polarization modes.