Fig. 1. Schematic of the novel cavity.

Fig. 2. Schematic of the narrow-linewidth MOPA fiber amplifier with a bi-directional pump structure.

Fig. 3. The input and output powers and their conversion efficiency at different pumping ratios.

Fig. 4. Results of output spectra at different pumping ratios: (a) 1:1.9, (b) 1:3.3 and (c) 1:6.4.

Fig. 5. The evolution of 20 dB linewidth versus the output power for different cases.

Fig. 6. The evolution of ratio of SRS and beam quality versus output power: (a) case 1; (b) case 2; (c) case 3 (inset: beam spot at 5005 W).

Fig. 7. Temporal characteristics at the final output power in the three cases: (a) signals of the PD and (b) results of FFT.

Fig. 8. Diagram of the coiling shape for gain fiber.

Fig. 9. (a) The variation curves of the output laser power and the optical-to-optical efficiency with the pump power. (b) The output spectrum at 6 kW. (c) The evolution of beam quality with output power. (d) The signal of the PD at the maximum output power and the FFT result.

Fig. 10. The results of laser brightness enhancement.