Fig. 1. Refractive index profile of improved fiber structure

Fig. 2. Effective refractive index versus wavelength

Fig. 3. Relationship between effective refractive index difference and wavelength

Fig. 4. Light intensity and phase diagram of the vortex beam. (a) OAM11 mode intensity map; (b) OAM21 mode intensity map; (c) OAM31 mode intensity map; (e) OAM11 mode phase diagram; (f) OAM21 Mode phase diagram; (g) OAM31 mode phase diagram; (h) LP11 mode field distribution; (i) LP21 mode field distribution; (j) LP31 mode field distribution

Fig. 5. Accommodate the relationship between OAM order and radius. (a) OAM order and r1; (b) OAM order and ring radius of low refractive index layer Δr

Fig. 6. Relationship between accommodate OAM order and refractive index n2 of low refractive index layer