Controllable lasing characteristics in Brillouin random fiber lasers by tailoring gain and random feedback fibers

Brillouin random fiber lasers (BRFLs), the combination of stimulated Brillouin scattering (SBS) gain and random distributed feedback, offer narrow linewidth lasing with simplicity and flexibility. However, BRFLs may not gain broad acceptance unless the fundamental lasing mechanisms governing their operation are fully understood and the lasing properties are effectively manipulated. Here, we demonstrate the control of lasing characteristics in BRFLs by tailoring the SBS gain fiber and the scattering pattern of the random feedback fiber. Experimental results show that BRFLs with 5 cm random fiber gratings (RFGs) feedback exhibit lower intensity fluctuation, longer coherence time, and more stable frequency compared to those with 6 km Rayleigh scattering fiber (RSF) feedback thanks to more correlated phase, lower mode density, and weaker dependence on external variations of RFGs. The low-randomness RFG feedback further improves the coherence time and intensity fluctuation attributed to the small period variation of sub-gratings. Moreover, the BRFL based on the high gain fiber and the strong scattering RFG feedback with low loss achieves high lasing efficiency and low threshold. The frequency jitter, intensity noise, and coherence time are also improved by reducing the gain fiber from 20 km to 1 km due to decreased mode hopping from mode competition. These results clarify the impact of gain and random feedback fibers on BRFL performance, offering insights for optimizing complex laser design for applications requiring high frequency stability and long coherence time.