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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 …