Fig. 1. Random-distributed-feedback Raman fiber laser experimental setup. (a) Simplified RRFLs with LD direct pumping. (b) The output spectrum of the pump. (c) The reflection spectrum of the HR FBG. (d) The output spectrum of signal light. MM-LDs, multimode laser diodes; HR FBG, high-reflection fiber Bragg grating; GRIN, graded index; RDFB, random distributed feedback.

Fig. 2. Output signal spectrum and power of RRFLs. (a) Output spectrum at different signal optical power levels. (b) Evolution characteristics of signal light power with injected pump light power. (c) The optical-to-optical conversion efficiency of signal light.

Fig. 3. The output 3 dB linewidth of the signal laser at different output power levels.

Fig. 4. Beam quality factor M² (left-hand axis) and the corresponding brightness enhancement (right-hand axis) at different signal light power levels.

Fig. 5. (a) Beam quality of pump light and (b) beam quality of signal light at maximum power.

Fig. 6. (a) The temporal stability characteristics of the RRFL output signal light. (a) The normalized time-domain measurement result of the RRFL signal output at various power levels. (b) The normalized standard deviation (NSTD) of the output signal light intensity.