Fig. 1. Experimental setup of monolithic fiber amplifier system (MO: master oscillator; PA, pre-amplifier; CO, collimator).

Fig. 2. (a) Simulated spectra and (b) experimental spectra of phase-modulated triple-frequency seed laser.

Fig. 3. (a) Strain distribution along the active fiber. (b) SBS gain spectra of strained and unstrained fiber.

Fig. 4. Backward power as a function of the output power.

Fig. 5. Overlap of the SBS gain spectra (a) without the modulation frequency; (b) with the modulation frequency of 1 GHz; (c) with the modulation frequency of 2 GHz [the short dashed line in (b) represents envelope of the total SBS gain].

Fig. 6. Backward power as a function of the output power with different modulated frequencies.

Fig. 7. (a) Output power and backscattering power versus pump power. (b) Emission spectra of backward scattering. (c) Emission spectra of forward output laser. (d) FPI scanning spectra of triple frequency at the maximum output power.