Fig. 1. Comparison of thermal deposition and temperature distribution of fibers with different absorption coefficients. (a) Variation of thermal deposition distribution caused by fibers with different absorption coefficients; (b) variation of temperature gradient distribution of fibers with different absorption coefficients

Fig. 2. Comparison of refractive index distribution of fibers with different absorption coefficients and effect of fiber core numerical aperture on mode instability threshold. (a) Effect of absorption coefficient on thermal-induced refractive index; (b) effect of NA of fiber core on mode instability threshold[19]

Fig. 3. Structural diagram of all-fiber high-power fiber laser amplifier with narrow linewidth

Fig. 4. Time domain signal diagrams at different output laser powers. (a) Data map within 200-1200 W; (b) scope chart at 1300 W

Fig. 5. Frequency domain signal diagrams at different output laser powers. (a) 10 W; (b) 100 W; (c) 800 W; (d) 1000 W; (e) 1200 W: (f) 1300 W

Fig. 6. Characteristics of pattern and beam quality of output laser. (a) Patterns of beams at different laser powers; (b) variation in beam quality factor with laser power

Fig. 7. Time domain and frequency domain signal distribution test results of fiber B under different laser powers. (a) Time domainsignal distributions at different output laser powers; (b) frequency domain signal distributions at different output laser powers

Fig. 8. Laser spot and beam quality characteristics of fiber B vary with output laser power. (a) Relationship between output laser power and pump power and variation in spot pattern with laser power; (b) variation in beam quality factor with laser power