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 MinΔneff. By introducing a high-index RA structure whose position coincides with the trough of mode power distribution of LP02mode, the effective index of the LP21mode can be largely elevated while that of LP02mode remains almost unaffected. The dependences of both Δneffand effective mode area Aeffon different ring parameters have been investigated. We fabricated RA 4-mode fibers with a minimum Δneff(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 MinΔneffis 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 Δneffas high as 1.7 × 10-3and an effective area larger than 100 μm2.