Fig. 1. Schematic diagram of the structure of a fiber laser amplifier based on a cylindrical water-cooled column (Seed: seed laser; MFA: mode field adapter; CLS: cladding light stripper; End cap: fiber end cap)

Fig. 2. Output spectrum and beam quality. (a) Output spectrum of the seed laser; (b) beam quality of the laser output from the MFA

Fig. 3. Main results of the lasers for different bending diameters. (a) Output power and corresponding slope efficiency at different pump powers; (b) typical output spectra for different bending diameters

Fig. 4. Performance parameters of the output laser for different bending diameters. (a) Variation of the beam quality of the output laser with the output power; (b) time-domain signals of the output laser before and after the occurrence of TMI detected by the photodetector; (c) FFT results corresponding to the time-domain signals in Fig. (b); (d) calculated σ by FFT results of time-domain signals as a function of the output power; (e) relative brightness of the output laser near the TMI threshold

Fig. 5. Schematic diagram of the laser after pump wavelength optimization

Fig. 6. Experimental results after optimizing the pump wavelength and fiber length. (a) Comparison of TMI thresholds before and after optimization; (b) calculated σ using FFT results of time-domain signals as a function of the output power; (c) output spectrum at maximum power (7100 W); (d) beam quality and relative brightness at maximum output power; (e) relative brightness of the output laser near the TMI threshold under different bending diameters when the LD with a central wavelength of 981 nm is pumped