Fig. 1. Schematic diagrams of BOFL and UOFL

Fig. 2. Schematic diagram of the structure of OAIFL

Fig. 3. Schematic diagram of boundary conditions for BOFL^[56]

Fig. 4. Simulation results comparison of the BOFL and UOFL. (a) Output power; (b) N₂ proportion; (c) fiber core temperature;

Fig. 5. Schematic diagram of boundary conditions for OAIFL

Fig. 6. Comparison of simulation results between UOFL and OAIFL under bidirectional pump configuration. (a) Laser power distribution inside the fiber; (b) temperature distribution inside the fiber; (c) output spectra of YDF; (d) output spectra after passing 10 m GDF

Fig. 7. Schematic diagram of the 10 kW BOFL

Fig. 8. Experimental results of 10 kW BOFL. (a) Output power or efficiency versus pump power; (b) output spectra at different output powers; (c) output time-domain signal at both ends and the corresponding frequency-domain result at the highest output power; (d) beam quality evolution

Fig. 9. Schematic diagram of the 10 kW OAIFL

Fig. 10. Experimental results of the 10 kW OAIFL. (a) Variation of output power and efficiency with pump power; (b) spectra at different output powers; (c) time-domain and frequency-domain signals at the highest output power; (d) beam quality at the highest output power

Fig. 11. Structural diagram of OAI-BOFL based on ytterbium-doped fiber with a core/cladding diameter of 20/400 µm and core/cladding diameter of 25/400 µm

Fig. 12. Experimental results of 2×4 kW high beam quality OAI-BOFL. (a) Output power and efficiency variation curves; (b) spectra of A-end at different output powers; (c) spectra of B-end at different output powers; (d) time-domain signals and corresponding FFT spectra at the highest power; (e) beam quality curve at both ends and the spot shape at the waist of the beam at the highest output power