Fig. 1. Mode groups in 4-mode fiber and their relationships^[21]

Fig. 2. Schematic of mode selective coupler

Fig. 3. Simulation results of mode selective coupler. (a) Phase-matching curves; (b)(c) energy coupling diagrams; (d)(e) evolution of mode field distribution

Fig. 4. Experimental results of mode selective coupler. (a) Optical vortex beams; (b) vector beams

Fig. 5. Simulation and experimental results of 1064 nm mode conversion. (a) Evolution of mode field distribution; (b) vector beams; (c) optical vortex beams^[36]

Fig. 6. Schematic of acoustically-induced few-mode fiber grating. (a) Schematic of experimental setup; (b) beat length of LP_11a mode and LP_11b mode, and relationship between grating period and applied frequency; (c) dual-resonant transmission spectrum^[41]

Fig. 7. Experimental setup for ultrafast vector optical field generation based on mode selective coupler^[29]

Fig. 8. Experimental results of femtosecond vector beams. (a) Output spectrum of output 1; (b) output spectrum of output 2; (c) pulse train; (d) relationship between output power and pump power^[31]

Fig. 9. Mode fields of femtosecond vector beams. (a) Mode field of TM₀₁ mode; (b) mode field of TE₀₁ mode^[31]

Fig. 10. Experimental results of ultrafast optical vortex beams. (a) Output spectrum of LP₁₁ coupler; (b) output spectrum of LP₂₁ coupler; (c) pulse train; (d) relationship between output power and pump power^[29]

Fig. 11. Mode fields of ultrafast optical vortex beams. (a) LP₁₁ mode; (b) OAM_±1 mode; (c)(d) spiral interferograms of LP₁₁; (e) LP₂₁ mode; (f) OAM_±2 mode; (g)(h) spiral interferograms of LP

Fig. 12. Experimental setup for 1 μm ultrafast HOM pulse generation^[36]

Fig. 13. Experimental results of 1 μm ultrafast HOM pulses. Output spectra (left), pulse trains (middle), and slope efficiencies (right) of (a)-(c) LP₁₁ mode, (d)-(f) LP₀₂ mode, and (g)-(i) LP₂₁ mode^[36]

Fig. 14. Experimental setup for ultrafast HOM laser generation based on AIFG^[59]

Fig. 15. Experimental results of ultrafast HOM laser generation based on AIFG. Output spectra (left), pulse trains (middle), and slope efficiencies (right) of (a)-(c) LP_11a mode and (d)-(f) LP_11b mode^[59]