Fig. 1. The basic system setup of the NFA.

Fig. 2. The power distribution of pump, signal, and Raman waves in forward and backward propagating directions. It can be found that Raman Stokes wave (1120 nm) has arisen in both directions, and the power is 642 W and 387 W for forward and backward directions, respectively.

Fig. 3. (a) The power distribution of the pump, signal, and first-order Stokes waves; (b) the forward and (c) backward output spectra of the NFA. The seed is consisted of 200 W 1070 nm laser and 10 W 1120 nm laser, respectively.

Fig. 4. The dependence of backward output power on the power of 1120 nm seed laser. Insets are the output spectra in backward direction at different power.

Fig. 5. The center temperature of the core for NFA and conventional amplifier in (a) forward pumping scheme and (b) bi-direction pumping scheme. In the calculation, all the parameters of the bi-direction pumping scheme are the same as of the forward pumping scheme, excepting the pump power is divided equally into two directions.

Fig. 6. The experimental setup of the NFA^[34].

Fig. 7. Theoretical prediction and experimental results of NFA.

Fig. 8. Experimental result of 1.5 kW NFA^[36].

Fig. 9. The experimental setup of the tandem-pumped NFA.

Fig. 10. The output property of the tandem-pumped NFA: (a) output power; (b) spectrum at full power.

Fig. 11. The measured 1090 nm laser power in the case of only 1090 nm seed with power of 56 W.

Fig. 12. The power distribution of the NFA example proposed in Section 3.1 calculated by the FWM model.