Fig. 1. Structural diagram of few-mode fiber communication system

Fig. 2. Schematic diagram of MPLC mode division multiplexer based on phase plate^[13]

Fig. 3. Photo of a MPLC mode-division multiplexer supporting 45 HG modes^[15]

Fig. 4. Photo of a transmissive MPLC mode-division multiplexer^[16]

Fig. 5. Scheme of OAM demultiplexer based on ODNN^[17]

Fig. 6. Mode-division multiplexer supporting 1035 HG modes^[21]. (a) Schematic diagram of the mode multiplexer supporting 1035 HG modes; (b) design diagram of the SLM phase plane

Fig. 7. Block diagram of FBG-based pattern multiplexer/demultiplexer^[23]

Fig. 8. Structural diagram of MSC-based all-fiber 6-mode multiplexer/demultiplexer^[25]. (a) All-fiber 6-mode multiplexer (b) all-fiber 6-mode demultiplexer

Fig. 9. Schematic diagram of DMSC-based mode-division multiplexer/demultiplexer^[26]

Fig. 10. IL curves for different modes of the 5-mode MSC^[29]

Fig. 11. Technical comparison between NMSPL and MSPL

Fig. 12. Structure of a 6-mode mixing photonic lantern^[39]

Fig. 13. Structure of an optical waveguide-based mode-division multiplexer/demultiplexer. (a) Asymmetric directional coupler^[41]; (b) multimode interferometer^[42]; (c) asymmetric Y-branch structure^[43]; (d) micro-ring resonator^[44]; (e) grating-assisted inverse coupler^[45]

Fig. 14. Inverse design frame of neural network^[62]

Fig. 15. Index profile of modes in strong coupling fibers^[67]

Fig. 16. Photograph of a few-mode OTDR device^[87]

Fig. 17. Experimental measurement results of impairment parameters in six-mode fiber^[87]

Fig. 18. Experimental measurement results of DMGD in six-mode fiber^[87]

Fig. 19. LP₀₁ fault detection results^[87]

Fig. 20. Diagram of the few-mode erbium-doped fiber amplifier

Fig. 21. Diagrams of the few-mode erbium-doped fiber with different doping materials^[93]. (a) Ring doping; (b) two-layer doping; (c) uniform doping

Fig. 22. Experimental setup for the four-mode erbium-doped fiber amplifier based on the bidirectional pumping^[98]

Fig. 23. Experimental setup for the few-mode erbium-ytterbium co-doped fiber amplifier based on the cladding pumping^[102]

Fig. 24. Diagrams of few-mode erbium-doped fibers with different refractive index distributions. (a) Ring refractive index distribution with the ring-core erbium-doped^[103]; (b) trench refractive index distribution^[104]; (c) single-trench ring refractive index distribution^[105]; (d) ring refractive index distribution with the cladding erbium-doped near the edge of the outer ring^[106]

Fig. 25. Experimental setup for the few-mode erbium-doped fiber amplifier with low differential modal gain^[110]

Fig. 26. DRA and LRA amplified transmission signal

Fig. 27. Diagrams of Raman fiber amplifier. (a) Diagram of the single-mode Raman fiber amplifier; (b) diagram of the few-mode Raman fiber amplifier

Fig. 28. Experimental setup of transmission system based on distributed few-mode Raman fiber amplifier^[111]

Fig. 29. Experimental setup of the second-order Raman amplified WDM/MDM transmission system^[117]

Fig. 30. Reverse design of neural networks based on machine learning^[118]

Fig. 31. AE architecture diagram for designing Raman gain profile in few-mode fibers^[119]

Fig. 32. Diagram of the few-mode hybrid Raman amplifier^[120]. (a) Experimental setup of the few-mode hybrid Raman amplifier; (b) mode intensity distributions of the four modes before entering the few-mode fiber

Fig. 33. Comparison of the computational complexities between TD-LMS and FD-LMS algorithms under different number of modes^[126]

Fig. 34. Comparison of the computational complexities among various MMA algorithms under different number of modes (N is the number of modes, and L is the number of taps)^[136]

Fig. 35. Diagram of multi-channel blind decinvolution signal equalization algorithm^[123]

Fig. 36. Comparison of the computational complexity between ICCFD-ICA and FD-ICA under different number of modes^[123]

Fig. 37. Solutions to increase transmission distance. (a) Schematic diagram of cyclic mode permutation transmission strategy^[140]; (b) unreplicated successive interference cancellation scheme^[140]; (c) cyclic mode-group permutation^[10]

Fig. 38. 30 km and 1045 km few-mode fiber transmission system structure diagrams^[142]

Fig. 39. 38-core few-mode fiber communication system with a transmission rate of 22.9 Pbit/s^[143]