Design of Holes-Assisted Step-Index 12-Mode Fiber With Greatly Improved Mode Spacing

We propose a holes-assisted few-mode fiber (HFMF) consisting of a GeO₂ doped SiO₂ core, a SiO₂ cladding, and three strategically positioned holes. The incorporation of these holes produces a significant contribution in separating the non-degenerated linearly polarized (LP) modes and degenerated spatial modes, thereby considerably improving the mode spacing. Numerical simulation results demonstrate that our designed HFMF can support 12 spatial modes with the min effective index difference between adjacent non-degenerated LP modes (Δn_eff) larger than 1.65 × 10⁻³ and the min effective index difference between adjacent degenerated spatial modes (δn_eff) above 2.64 × 10⁻⁴ at the wavelength of 1550 nm, breaking through the trade-off between mode spacing and the number of modes. Furthermore, the min Δn_eff and min δn_eff of exceeding 1.63 × 10⁻³ and 2.44 × 10⁻⁴ can be achieved over the whole C and L bands, respectively. The new designed fiber is specifically tailored for less MIMO mode division multiplexing (MDM), enabling significant capacity enhancement in optical communication systems.