Fig. 1. Raman fiber amplifier experimental setup. (a) Implementation of the amplifier; (b) cross-section through the input fiber bundles and the fusion point of the pump and signal combiner (PSC). BQA, beam quality analyzer; OPM, optical power meter; OSA, optical spectrum analyzer.

Fig. 2. The power output characteristics of the RFA based on GRIN fiber, including total power output, residual pump power at 1080 nm, signal power at 1130 nm and second-order Raman power.

Fig. 3. The spectrum of a Raman amplifier for various signal laser power outputs.

Fig. 4. The beam quality parameter M² of the output laser with varying output signal laser power, including the amplified signal laser, the launched pump laser, and the residual pump laser.

Fig. 5. (a) The BE value and (b) the beam shape at the focal spot in the BQA, including the seed and pump lasers before the PSC; and the seed laser, the signal laser at maximum power, the launched pump laser at maximum power and the residual pump laser at maximum power after the PSC.