Fig. 1. Schematic diagram of the spiral phase and intensity distribution of OAM with three topological charges (-1, 0, +1)[10]

Fig. 2. Cross-section and main parameters of ring fiber[16]. (a) Schematic diagram of ring fiber; (b) refractive index profile of ring fiber and mode intensity profile of LP11; (c) effective refractive index for the three vector modes of the scalar LP11

Fig. 3. New type of air core ring fiber[18]. (a) Refractive index profile of ring fiber; (b) effective refractive index of LPl1 mode

Fig. 4. Fiber of MOMRF[21]. (a) Cross-section and geometrical structure of MOMRF; (b) refractive index profile of a single ring

Fig. 5. Improved MOMRF[22]. (a) Cross-section of improved MOMRF; (b) refractive index profile of a single ring; (c) intensity and phase distributions of HE31 related OAM mode in a single ring

Fig. 6. Geometrical structure and refractive index distribution of optical fiber

Fig. 7. Cross-section and main parameters of C-PCF[26]. (a) Structure diagram of C-PCF; (b) intensity distributions of the vortex modes (TE01, HE21, EH11, HE31, EH21, HE41); (c) normalized intensity distribution in high-index ring region with different order modes

Fig. 8. Cross-section and main parameters of C-PCF[28]. (a) Structure diagram of novel C-PCF; (b) normalized intensity distribution in high-index ring region with different order modes

Fig. 9. Schematic diagram of wavelength tunable microstructured optical fiber cross section

Fig. 10. Block diagram of OAM multiplexing system