Few-Mode Fibers for Mode-Division-Multiplexed Systems

We describe the design trade-offs that are at stake when optimizing few-mode fibers (FMFs) that support a high number (≥ 6) of LP modes. We particularly detail the design of 6-LPmode fibers that allow to multiply the capacity by a tenfold factor (two modes being spatially non-degenerate and four modes being two times spatially degenerate). For low-differential-mode-groupdelay (low-DMGD) FMFs adapted to strongly-coupled modedivision-multiplexed systems, trench-assisted graded-index-core profiles can be optimized to have Max|DMGD| <;10 ps/km and undesired leaky LP modes appropriately cut off, while all guided LP modes show good robustness (Bend Losses <;10 dB/turn at 10 mm bend radius). Such low-DMGD FMFs being sensitive to process variability, we show how fiber concatenations can efficiently compensate for this issue and that values <;25 ps/km can realistically be reached. For weakly-coupled FMFs adapted to weakly-coupled mode-division-multiplexed systems, step-index-core profiles can be optimized to have large effective index differences, Δn
_eff
, between the LP modes (Min|Δn
_eff
| >1.0 × 10
^-3
) to limit mode coupling and A
_eff
>~100 μm
²
to limit intra-mode non-linearity with good mode robustness. For such weakly-coupled FMFs, sensitivity to process variability is small and main characteristics do not significantly change when variations are within the manufacturing tolerances. We also briefly discuss experimental validations.