Design and Characterization of Ring-Assisted Few-Mode Fibers for Weakly Coupled Mode-Division Multiplexing Transmission

We report on the design and characterization of ring-assisted (RA) few-mode fibers with improved minimum mode effective index difference MinΔ<italic>n</italic>_eff. By introducing a high-index RA structure whose position coincides with the trough of mode power distribution of LP₀₂ mode, the effective index of the LP ₂₁ mode can be largely elevated while that of LP₀₂ mode remains almost unaffected. The dependences of both Δ<italic>n</italic>_eff and effective mode area <italic>A</italic>_eff on different ring parameters have been investigated. We fabricated RA 4-mode fibers with a minimum Δ<italic>n </italic>_eff (1.8 × 10⁻³) which is 2.2 times larger than that of step-index (SI) 4-mode fiber (0.8 × 10⁻³) without obvious degradation on other characteristics. The fabricated RA 4-mode fiber has a low mode-average loss and a low mode dependent loss of 0.23 and 0.02 dB/km at 1550-nm wavelength, respectively, which are comparable with that of an SI 4-mode fiber fabricated for reference purposes. The distributed inter-mode crosstalk of the fibers has been measured using swept-wavelength interferometry with a winding tension of 0.5 N. We observed similar distributed inter-mode crosstalk between the RA-FMF and the SI-FMF with low tension and the results show that no additional deformation was introduced by the high-index ring. As a result, the fabricated RA-FMF with a larger MinΔ<italic>n</italic>_eff is expected to be more robust than SI-FMF in real applications. Moreover, we adopted this method for the design of 7-mode fibers and successfully obtained a minimum Δ<italic>n</italic>_eff as high as 1.7 × 10⁻³ and an effective area larger than 100 <italic>μ</italic>m².