Fig. 1. Experimental setup of the Raman fiber amplifier. The inset in the top shows the refractive index distribution and the illustration at the bottom of the figure shows the output beam profiles of the pump light after coupling through combiners with different output fibers.

Fig. 2. The maximum achievable output power at different fiber lengths with corresponding output spectrum of the Raman fiber amplifier pumped by a lower-brightness laser. (a) Output spectral power under maximum power at various fiber lengths. (b) The evolution of output power and efficiency. (c) The physical process behind the constant second-order Raman threshold under low-brightness pumping.

Fig. 3. The maximum achievable output power at different fiber lengths with corresponding output spectrum of the Raman fiber amplifier pumped by a lower-brightness laser. (a) Output spectral power under maximum power at various fiber lengths. (b) The evolution of output power and efficiency.

Fig. 4. The output power evolution of the Raman amplifier under various fiber lengths when pumped by lasers with different brightness. (a) Result of lower-brightness pumping. (b) Result of higher-brightness pumping.

Fig. 5. The output power and spectral evolution of the Raman fiber amplifier with extra seed. (a) The evolution of output power. (b) Output spectra at 500 W and 4 kW. (c) The evolution of output spectra.