Fig. 1. Schematics of modes in each MG for gradient-type refractive index multimode fiber and radial first-order ring-core fiber. (a) Intensity of LP modes in each MG for GI-MMF; (b) intensity of LP modes in each MG for RCF; (c) intensity and phase of OAM modes in each MG for RCF[18]

Fig. 2. (a) RI distribution and (b) cross-sectional diagram of measured GIRCF; calculated (c) effective refractive index distributions and (d) group velocity delays in each MG at C band [31]

Fig. 3. (a) Cross-section of quad-mode fiber; (b)-(e) mode field intensity distributions of supported modes; (f) corresponding dispersion curves; (g) experimental system for OAM mode generation and phase distribution verification [48]

Fig. 4. (a) Transmission spectrum of LPFG with mode coupling between LP01 and LP11 modes; (b) intensity profiles and interference patterns of generated LP11 mode and first-order OAM mode [47]

Fig. 5. Period and transmission spectrum of LPFG with four-mode fiber mode coupling between fundamental mode and LP21 mode [48]. (a) Period; (b) transmission spectrum

Fig. 6. Intensity profiles of mode fields and interference patterns of reference Gaussian beams[48]

Fig. 7. Block diagram of RCF-based OAM MDM digital coherent transceiver system[31]

Fig. 8. Diagram of few-mode-fiber-based vector mode division multiplexing system[69]

Fig. 9. Experiment results of OAM mode detection based on one-dimensional GCPPG (insets depicting locations and directions of dark lines) [77]

Fig. 10. Experimental results of high-order vortex beams measured by hyperbolic gradient grating [80]