Fig. 1. Schematic diagram of FM-EDFA

Fig. 2. Typical structure of MC-EDFA with fiber-core pumping^[14]

Fig. 3. Typical structure of MC-EDFA with cladding-pumping^[15]

Fig. 4. MC-FM-EDFA with cladding-pumping^[16]

Fig. 5. Structure of few-mode erbium-doped fiber amplifier^[17]

Fig. 6. Few-mode erbium-doped fiber amplification system with SLM^[19]

Fig. 7. Two-mode fiber transmission system^[20]. (a) Structural diagram; (b) refractive index difference profile of erbium-doped fiber

Fig. 8. Refractive index profile and mode field distributions of ring core fiber^[22]

Fig. 9. Ring-core fibers with different structures. (a) Refractive index distribution and doping structure of ring-core fiber with trench^[25]; (b) cross section of ring core erbium-doped fiber^[26]

Fig. 10. Different doping profiles

Fig. 11. Doping profile designs. (a) Monolayer doping^[27]; (b) multilayer doping with the same doping concentration^[28]; (c) multilayer doping with different doping concentrations^[29]

Fig. 12. Different multi-core single-mode fibers. (a) 12-core erbium-ytterbium co-doped double-cladding fiber^[30]; (b) annular cladding-pumped multi-core erbium-doped fiber^[31]

Fig. 13. Schematic diagram of MC-EDFA with hybrid pumping^[34]. (a) Single-stage; (b) multi-stage

Fig. 14. Cross section and refractive index distribution of double-cladding six-core three-mode fiber^[16]

Fig. 15. Different MC-FM-EDF. (a) Ring-core seven-core fiber^[36]; (b) six-core fiber^[37]

Fig. 16. Nanopore-assisted double-cladding few-mode fiber^[38]

Fig. 17. Variation of gain with transmission distance for four modes. (a) Forward pumping; (b) backward pumping