Optimizing graded-index few-mode fiber for space division multiplexing

We investigate the graded-index few-mode fiber (GI-FMF) to realize a 4-LP-mode (i.e. LP₀₁, LP₁₁, LP₂₁, and LP₀₂) fiber for mode-division-multiplexed transmission. This study optimizes the GI-FMF for both, first, for large effective indices differences (Δn_eff), and second, for low differential mode delay (DMD) between any two LP modes, for different optimized parameters. Thus, it shows that GI-FMF is suitable for both weakly-coupled few-mode fiber (WC-FMF) as well as strongly-coupled few-mode fiber (SC-FMF) via adjusting the profile parameter (α), refractive index difference between core and cladding (n_co - n_clad), and core radius (a). We report the optimized parameters for WC-GI-FMF with large effective indices difference (Δn_eff) of 0.6 × 10^-3 and low |DMD| of 5.4 ns/km while the minimum effective mode area (Min.|A_eff|) is 80 µm² and bending loss (BL) of the highest order mode is 0.005 dB/turn (much lower than 10 dB/turn) at a 10 mm bend radius. Here, we could break down the degeneracy between LP₂₁ and LP₀₂ mode, which remains a challenging task in GI-FMF. To the best of our knowledge, this is the lowest DMD (5.4 ns/km) ever reported for such a weakly-coupled (Δn_eff = 0.6 × 10^-3) 4-LP-mode FMF. Similarly, we optimized the parameters for SC-GI-FMF with Δn_eff of 0.1 × 10^-3 and the lowest DMD of 0.9 ns/km while Min.|A_eff| is ≫ 100 µm² and BL of higher order mode is 6 dB/turn (< 10 dB/turn) at 10 mm bend radius. Further, we investigate narrow air trench-assisted SC-GI-FMF to reduce the DMD and achieve the lowest DMD of 16 ps/km for a 4-LP-mode GI-FMF with a minimum Δn_eff of 0.7 × 10^-5.