Fig. 1. Experimental setup of high power tandem-pumped, narrow-bandwidth and all-fiberized fiber amplifier.

Fig. 2. (a) The measured spectral distribution of the phase modulated laser; (b) the trend of backward power ratio ( $R_{bs}$ ) along with power scaling.

Fig. 3. (a) The change of $R_{bs}$ along with output power; (b) the spectral bandwidth as a function of output power (inset: the frequency offset distributions at different output powers).

Fig. 4. (a) The power scaling process of the MOPA system along with increase of pump power; (b) the increase of $R_{bs}$ (%) as a function of the output power.

Fig. 5. (a) The change of spectral bandwidth with the output power (inset: the frequency offset at several different output powers); (b) the spectrum from 1000 to 1200 nm at 3940 W.

Fig. 6. (a) The normalized intensity signals collected by PD; (b) the corresponding Fourier spectral distributions; (c) the measured $M^{2}$ factor along with power enhancement; (d) the typical far-field beam profiles at different output powers.