Eight-mode ring-core few-mode fiber using cross-arranged different-material-filling side holes

We propose an eight-spatial-mode ring-core few-mode fiber (RC-FMF), utilizing the cross-arranged different-material-filling side holes (CA DMFSH) for effective index difference improvement. Two GeO₂-doped-silica side holes and two air-filling side holes are arranged orthogonally around the ring core, which have directionally different effects on the refractive index and the mode field distribution in the RC-FMF. The results indicate that the effective index difference (Δn) between adjacent spatial modes is larger than 1.96×10^-4, and the Δn between adjacent non-degenerated modes can be above 1.01×10^-3 at the same time. Bend-resistant performance and low nonlinearity are achieved in the designed RC-FMF. Broadband performances ranging from 1510 to 1630 nm are also analyzed. The CA-DMFSH-assisted structure shows great potential for enlarging the effective index difference, and the proposed fiber targets applications in the short-reach space-division multiplexing optical networking while eliminating the complex multi-input/multi-output digital signal processing.