Fig. 1. Power evolution of YDFL with high beam quality^[36]

Fig. 2. Variation of bending loss and mode area of fundamental mode with bending radius in 48 μm core diameter and NA=0.065 fibers. (a) Bending loss; (b) mode area

Fig. 3. Bending loss of LP_11o mode varied with fiber core diameter and bending diameter

Fig. 4. Influence of 1080 nm return signal laser on 1018 nm oscillator^[51]. (a) Experimental setup; (b) experimental and simulation results of 1018 nm oscillator power changing with the injected 1080 nm return laser; (c) simulation results of 1018 nm oscillator power changing with the injection of 1080 nm return light under different YDF lengths; (d) simulation results of 1018 nm oscillator power changing with the reflectivity of OC-FBG under different return 1080 nm power

Fig. 5. Backward tandem-pumped amplifier based on 25/250 μm double-clad YDF^[51]. (a) Experimental setup; (b) output power; (c) output spectra under different power; (d) output spectra of forward and backward tandem-pumped amplifiers at 3.94 kW

Fig. 6. Bi-directional tandem-pumped amplifier based on 30/250 μm double-clad YDF^[52]. (a) Experimental setup; (b) output power and efficiency curves; (c) output spectra under different power; (d) beam quality and beam profile under different power; (e) output time trace and spectra at 6.22 kW

Fig. 7. Experimental results of 10 kW fiber laser^[54]. (a) Output power versus pump power; (b) spectra versus output power; (c) beam quality versus output power; (d) power fluctuation and its Fourier spectra (inset)

Fig. 8. Mode field distribution under different bending radii ( white dotted line is the core range )

Fig. 9. When bending radius of the amplifier is 0.15 m and 0.3 m, output laser mode purity varied with relative doping radius under different mode purity seed injection^［55］. (a) 0.15 m; (b) 0.3 m

Fig. 10. Output laser mode purity of the amplifier varies with the core diameter and doping ratio^[55]

Fig. 11. Forward tandem-pumped amplifier based on 40/250 μm CYDF^[62]. (a) Power and efficiency curves; (b) output spectra under different power; (c) beam quality of CYDF-based amplifier under different power; (d) beam quality of YDF-based amplifier under different power

Fig. 12. Experimental results of forward tandem-pumped amplifier based on 40/250 μm CYDF^[63]. (a) Refractive index distribution of fiber core; (b) cross-section photo of the fiber; (c) output power and optical-to-optical efficiency at different pump power (inset: power meter at maximum output power); (d) output spectra at different power

Fig. 13. Experimental results of bi-directional tandem-pumped amplifiers based on 40/250 μm CYDF^[64]. (a) Laser structure; (b) output power versus pump power; (c) output spectra under different power; (d) output time trace and spectrum at maximum power; (e) beam quality at maximum power

Fig. 14. Experimental results of backward tandem-pumped amplifier based on 40/250 μm CYDF^[65]. (a) Output power versus pump power; (b) spectra under different power; (c) beam quality at different power

Fig. 15. Experimental results of backward tandem-pumped amplifiers based on 30/250-48/400 μm STYDF. (a) TYDF longitudinal section; (b) spectra under different power; (c) output power versus pump power; (d) beam quality of TYDF-based amplifier under different power; (e) beam quality of YDF-based amplifier under different power

Fig. 16. Structure of YDF with F-doped inner cladding^[108]. (a) Cross section of inner cladding; (b) 2D refractive index of inner cladding

Fig. 17. Schematic of 10 kW single mode fiber laser