Significance   Ultra-narrow-linewidth (~ kHz) lasers have attracted much research interest because of their wide applications in optical communications, fiber sensors, and so on. The linewidth of lasers affects the performance of the systems, such as the communication length, the minimum detection signal, and the measurement accuracy. Brillouin fiber lasers (BFLs), based on stimulated Brillouin scattering (SBS) in fibers, present Hz-scale ultra-narrow linewidth due to their intrinsic linewidth-narrowing effect. With its development in the past several decades, the compact Brillouin/erbium fiber laser (BEFL) becomes the frontier of the research on BFLs. Unlike the erbium-doped fiber lasers or laser diodes, the compact BEFL presents Hz-scale linewidth without complicated feedback loop or extremely precise isolation from the temperature and vibration variations. Besides the advantage of ultra-narrow linewidth, the BEFL simultaneously presents very stable central frequency and stable fast tuning. These outstanding performances make the compact BEFL a very ideal laser source for many applications, especially for phase-generation-carrier (PGC) interferometric fiber sensors.

  Progress  The development of narrow-linewidth BFLs went through three stages, i.e. the early-days BFLs, the traditional BEFLs, and the compact BEFLs. The BFLs were introduced according to the three development stages. In the early days, the BFL was based on SBS in a single-mode fiber resonator (Fig.1-4). The Brillouin pump (BP) was injected in the cavity and generates SBS in the single-mode fibers. The BFL needed high pump threshold or critical pump coupled resonator due to the small Brillouin coefficient. The Brillouin/erbium fiber laser (BEFL) was then proposed to overcome the need of a pump couple resonator by introducing an erbium-doped fiber amplifier in the resonator (Fig.5). The BEFL presents low threshold and high output power (Fig.6(b)), but it needed over 100-m single-mode fibers as the Brillouin gain medium. Long cavity causes mode hopping easily. Many studies were carried out to establish single-longitudinal BEFL, such as multi-resonance-cavity BEFL (Fig.7), short-cavity BEFL based on high-nonlinearity special fibers (Fig.8), single-mode BEFL based on Brillouin pump preamplification (Fig.9). Short-cavity, low-threshold BEFLs were desirable. Until 2012, the compact BEFL was proposed based on a length of erbium-doped fiber (EDF) providing both the Brillouin gain and linear gain (Fig.10). It presented short cavity and low threshold. In 2013, an all-polarization-maintained ultra-short-ring-cavity compact BEFL was reported (Fig.11). The mechanism, characteristics, and applications of the compact BEFL were studied in the following 10 years. A series of progress has been achieved on the studies of compact BEFL. This kind of fiber laser showed 3-Hz ultra-narrow linewdith (Fig.16), stable central frequency, and stable fast tuning (Fig.26). The phase noise of the BEFL is lower than the state-of-the-art commercial laser diodes (Fig.18). The outstanding performance of the compact BEFL leads to many important potential applications, such as in high-accuracy interferometric fiber sensors (Fig.27-28) and Brillouin distributed fiber sensors (Fig.29-30).

  Conclusions and Prospects  The optical communication and sensing systems are in great need of high-performance ultra-narrow-linewidth lasers. The BFLs, based on SBS in fibers, present Hz-scale ultra-narrow linewidth. The BFLs have already been developed to the stage of the compact BEFLs, which present ultra-narrow linewidth, stable central frequency, and stable fast tuning simultaneously. Compared with the state-of-the-art narrow-linewidth external-cavity laser diodes, the compact BEFL presents even lower phase noise. The applications in interferometric fiber sensors and distributed fiber sensors are validated for the compact BEFL. The advantages of the applications of the compact BEFLs are verified. The compact BEFL has a fully independent intellectual property, it has great significance for the localization of producing many important optoelectric information systems. The research aims to provide some reference for the study and applications of narrow-linewidth lasers in the future. It is expected that the compact BEFL will be modularized so that it could be widely used in more applications.