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 (Delta n(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 (alpha), refractive index difference between core and cladding (n(co) - n(clad)), and core radius (alpha). We report the optimized parameters for WC-GI-FMF with large effective indices difference (Delta n(eff)) of 0.6 x 10(-3) and low vertical bar DMD vertical bar of 5.4 ns/km while the minimum effective mode area (Min.vertical bar A(eff)vertical bar) is 80 mu m(2) 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 (Delta n(eff) = 0.6 x 10(-3)) 4-LP-mode FMF. Similarly, we optimized the parameters for SC-GI-FMF with Delta n(eff) of 0.1 x 10(-3) and the lowest DMD of 0.9 ns/km while Min.vertical bar A(eff)vertical bar is >> 100 mu m(2) 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 Delta n(eff) of 0.7 x 10(-5). (C) 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement