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 MinΔn eff . By introducing a high-index RA structure whose position coincides with the trough of mode power distribution of LP 02 mode, the effective index of the LP 21 mode can be largely elevated while that of LP 02 mode remains almost unaffected. The dependences of both Δn eff and effective mode area A eff on different ring parameters have been investigated. We fabricated RA 4-mode fibers with a minimum Δ neff (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 MinΔn 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 Δn eff as high as 1.7 × 10 -3 and an effective area larger than 100 μm 2 .