Fig. 1. Air-core ring few mode fiber. (a) Cross-sectional refractive index distribution of ACF; (b) effective refractive index difference between high order modes supported by ACF

Fig. 2. Structural diagram of vector MSC

Fig. 3. Effective refractive index of fundamental mode in SMFs with different core radii and matching between fundamental mode in SMF and each high order mode in ACF at 1550 nm wavelength

Fig. 4. Power conversion between fundamental mode HE11P and high order mode TM₀₁ and transmission spectra. (a) Conversion between two modes in vector MSC along z-direction at 1550 nm; (b) conversion between two modes versus wavelength at z＝L_c

Fig. 5. Power coupling efficiency and coupling length under each condition. (a) Power coupling efficiency versus wavelength and core distance; (b) coupling length versus wavelength and core distance; (c) mode power conversion at point A

Fig. 6. Bandwidth of coupler at each core distance. (a) Bandwidth versus core distance; (b) 3 dB bandwidth versus core distance

Fig. 7. Crosstalk at each core distance. (a) 10 μm; (b) 12 μm