Contents I. Introduction
Brillouin random fiber lasers (BRFLs), as a new breed of highly coherent laser source, have potential applications in coherent optical communications and interferometric fiber sensing, owing to their superior characteristics of frequency noise suppression contributed by narrowband Brillouin photon-phonon interactions as well as randomly distributed feedback [1]–[3]. Typically, Brillouin scattering in long-span standard single mode fibers (SSMFs) is activated as narrowband gain while intrinsic Rayleigh scattering along SSMFs is required for sufficient distributed feedback for random fiber lasers [4]–[6]. Considering both of the Brillouin gain and Rayleigh backscattering coefficients, kilometer-long conventional SSMFs are basically required to accumulate sufficient gain and feedback strength, which however leads to a bulky laser configuration. In this case, such BRFLs are usually exposed to ambient perturbations, e.g. instantaneous temperature drifts and mechanical vibrations, which hence inevitably imposes remarkable laser noise including both of the laser intensity and fluctuations. Instead of intrinsic Rayleigh scattering, a random fiber grating (RFG) with highly enhanced distributed feedback along much reduced physical fiber length was alternatively proposed [7]–[10]. In 2019, Zhou Z et al. demonstrated a distributed feedback BRFL using polarization-maintaining RFG and proved theoretically and experimentally that it is insensitive to temperature and vibration with intensity noise and frequency noise suppression characteristics [9]. In 2024, Wang H et al. realized a singlefrequency BRFL with mode-hop-free operation and an ultrahigh frequency stability of 0.48 MHz for 120 s by using a 200 m long strongly scattering disordered grating [10]. On the other hand, a long-span gain fiber in BRFLs is basically required to accumulate sufficient Brillouin gain to surpass the loss for the random lasing emission, which however deserve further optimization in terms of laser compactness as well as laser noise reduction.
