## 2 Experimental Setup  
The experimental setup is shown in Figure 1. First, a 980 nm semiconductor laser is used as a pump laser source for Erbium-doped fiber (EDF). Wavelength selection is performed by using a fiber Bragg grating (FBG) with a central wavelength of 1551.95 nm and a reflectivity of 98.34% on port 2 of optical circulator 1. To accumulate sufficient feedback of backscattered Rayleigh light and increase the threshold value of Brillouin scattering, single-mode optical fiber is added to not only stimulate the next cycle of backscattered Rayleigh light under a Faraday rotating mirror (FRM), but also adjust the light polarization state. This also controls the RBS feedback and suppresses the side mode through a variable optical attenuator (VOA). This optical structure utilizing backscattering can be seen as an infinite number of cascaded scattering cross-sections. The scattered light is accumulated as weak feedback to compress the linewidth. Backscattered light enters the main cavity from port 3 of optical circulator (OC) 2. Therefore, the signal light and backscattered Rayleigh light share the same light path and simplify the resonator structure. In addition, adjusting the VOA can effectively and efficiently make the resonant cavity become a SLM lasingoutput. Finally, an optical power meter and optical spectrum analyzer (OSA) can be used to monitor the output of the 90/10 fiber ratio coupler.  
The laser linewidth is measured using the fiber delayed self-heterodyne detection method [20; 21; 22], as shown in Figure 2. This method is simple in structure and can meet the parameter requirements measured in this experiment. The laser output launches to a 50:50 fiber coupler accompanied by a VOA to adjust the laser power at each router. The upper router uses an acousto-optic modulator to induce a frequency shift of 80 MHz, while the lower branch uses 75 Km SMF as a delay line. Two lights enter a 50:50 fiber ratio splitter. Finally, the beat signal is detected by a photodetector, and its linewidth is displayed on an electrical spectrum analyzer. Since the required fiber delay line must be longer than the laser coherence time, the time delay between two lights is enough to destroy the coherence between two lasers. It can be considered that two incoherent lasers with central closing wavelengths create a beat frequency. Double the laser linewidth can be observed on a high-resolution electron spectroscopy analyzer. Therefore, the full width at half maximum (FWHM) shown on the spectrum analyzer should be divided by two to obtain the correct laser line width.  
Figure 1: The structure of the experimental setup. WDM: wavelength division multiplexing device; EDF: Erbium-doped fiber; FBG: fiber Bragg grating; OC 1, OC 2: optical circulator; PC: polarization controller; SMF: single-mode fiber; VOA: variable optical attenuator; FRM: Faraday rotating mirror; OSA: optical spectrum analyzer.  
Figure 2: Experiment setup for measuring laser linewidth. VOA: variable optical attenuator; AOM: acousto-optic modulator; PD: photoelectric detector; ESA: electrical spectrum analyzer.  
Under the scheme of linewidth measurement in Figure 2, the signal light is accurately displaced at 80 MHz through the Acousto-optic modulator (AOM). It enables the stable output of the laser power to a particular range, but the laser drifts slightly due to external factors. There is a mode jump phenomenon. The average power intensity is taken during linewidth measurement, and it is found that external factors directly affect the line width value. Therefore, it is necessary to minimize environmental factors, such as vibration and temperature changes.