Fig. 1. Schematic of all-fiber amplifier with bi-directional pumping configuration

Fig. 2. Output power curves of all-fiber amplifier with single-directional pumping configuration before suppression of TMI. (a) Output power for forward pumping and detailed figure when output power drops; (b) output power for backward pumping and frequency domain signal at threshold

Fig. 3. Output power and spectra of all-fiber amplifier with single-directional pumping configuration after suppression of TMI. (a) Output power and spectrum at 1390 W for forward pumping; (b) output power and spectrum at 1810 W for backward pumping

Fig. 4. Output process of 3 kW near single-mode laser. (a) Spectra before optimization under different output powers; (b) spectra after optimization under different output powers; (c) beam spots and beam qualities at 1, 2, 3 kW; (d) output power of idle pumping port; (e) output power versus pump power; (f) optical-to-optical efficiency

Fig. 5. Test results of amplifier for 10 h of operation. (a) Output power curve; (b) power fluctuation curve

Fig. 6. Output results at different moments in 10 h of continuous test. (a) Spectra; (b) beam qualities;(c) time domain distributions; (d) frequency domain distributions