Fig. 1. Diagram of effective refractive index matching principle^[6]

Fig. 2. Planar waveguide mode manipulation^[6]

Fig. 3. Fiber optic mode manipulation. (a) Long period fiber grating^[8]; (b) mode selective coupler^[9]; (c) photon lantern^[10]

Fig. 4. Free space mode manipulation^[15]

Fig. 5. Structure diagram of planar waveguide mode (de)multiplexing system^[18,20]

Fig. 6. Mode (de)multiplexing structure diagram based on mode selection coupler^[30]

Fig. 7. Mode (de)multiplexing technique based on photonic lanterns. (a) Structure diagram of mode (de)multiplexing^[33-34]; (b) profile of fabricated photonic lanterns^[36]

Fig. 8. Free-space-based mode (de)multiplexing techniques. (a) Mode obtained through phase plate conversion^[39]; (b) experimental structure diagram of multi-plane light transformation^[42]

Fig. 9. (De)Multiplexing of large quantity patterns^[46]

Fig. 10. Prototype of self-developed fiber optic comprehensive parameter 3D tester

Fig. 11. Fiber test and mode evolution^[54]. (a) Fiber microscopic observation and 3D refractive index reconstruction; (b) mode evolution of fiber fusion zone

Fig. 12. Mode (de)multiplexing technology. (a) Mode conversion based on multi-plane light conversion technology; (b) design of mode selector based on fiber coupling

Fig. 13. Active fiber gain control. (a) Channel coupling and gain control of active fiber and passive fiber^[59]; (b) 1050 km multi-dimensional multiplexing gain equalization transmission system

Fig. 14. Fiber optic mode measurement technology. (a) Mode decomposition results after radial denoising^[61]; (b) mode decomposition and reconstruction^[63]