Objective Traditional single-wavelength fiber lasers make it challenging to meet the increasing capacity demands for modern optical fiber communication systems. Employing multiple single-wavelength fiber lasers as light sources by wavelength division multiplexing technology is bound to increase system complexity and costs. Additionally, there are potential applications in multi-dimensional information fiber sensing for multi-wavelength fiber lasers. Therefore, multiwavelength fiber lasers with stable performance have been widely studied and can be adopted to expand communication systems and meet the needs of multi-dimensional information fiber sensing. However, the problems for stable operation of multi-wavelength erbium-doped fiber laser (EDFL) are as follows. At room temperature, due to the homogeneous broadening of erbium-doped fiber, it is easy to cause mode competition, which reduces the stability of multi-wavelength fiber lasers with narrow wavelength intervals. Based on the dual-wavelength linear cavity EDFL, we select a simple linear cavity fiber laser structure, optimize the reflectivity and center wavelength of fiber Bragg grating (FBG), and realize a stable dual-wavelength laser output by the polarization hole burning (PHB) effect. Compared with the existing PHB schemes, the laser features a simple and compact structure, low cost, and good stability. We hope that our study will help realize dual-wavelength linear cavity fiber lasers with excellent performance at room temperature. Methods We study the influence of the structure of dual-wavelength linear cavity EDFL and FBG parameters (reflectivity and center wavelength) on the output performance of a dual-wavelength laser. Firstly, the output power and dual-wavelength laser spectra of 3 dB fiber loop mirror ( FLM) and high reflectivity-FBG (HR-FBG) as the total reflector respectively, and low reflectivity-FBG (LR-FBG) as the output mirror are compared. Secondly, based on the double fiber Bragg gratings (DFBGs) as a cavity mirror, two HR-FBGs with the same reflectivity are adopted as the total reflector. The output power, dual-wavelength laser spectra, and power stability with the same reflectivity and different reflectivities are compared when two LR-FBGs are utilized as the output mirror. Finally, based on the first two groups of experiments, DFBGs are leveraged to constitute the cavity, and the reflectivities of the two HR-FBGs and the two LR-FBGs are equal respectively. The output power and dual-wavelength laser spectra of DFBGs with different center wavelength intervals (Delta lambda of 4, 8, and 12 nm) are compared. Additionally, the long-term laser stability is analyzed, which includes the temporal stability of spectra, center wavelength changes, power fluctuations, and 3 dB bandwidth stability. Results and Discussions Firstly, the contrast experiment is carried out based on dual-wavelength linear cavity EDFL with FLM and HR-FBG structures. The results show that the slope efficiency of the two EDFL structures is basically equal. The optical signal-to-noise ratio ( OSNR) based on the HR-FBG structure is still higher than that based on the FLM structure (Fig. 7), which indicates that the laser output performance of the linear cavity EDFL based on the HR-FBG structure is better. Secondly, in the dual-wavelength linear cavity EDFL based on HR-FBG structure, the influence of the same and different reflectivities of LR-FBG on the EDFL output performance is studied. The contrast experiment shows that the slope efficiency and OSNR of the two LR-FBGs with the same reflectivity are higher than those with different reflectivities, and the output power is more stable (Fig. 8). Finally, we study the effect of varying center wavelength intervals of DFBGs on the EDFL output performance. The contrast experiment shows that as the center wavelength interval of DFBGs gradually increases, the slope efficiency of the dual-wavelength linear cavity EDFL gradually decreases, and the OSNR of the two wavelength lasers gradually rises (Fig. 9). By adjusting the polarization controller (PC), the dual-wavelength laser output spectra of EDFL with three wavelength intervals will not hop with time. Constantly, the larger center wavelength interval leads to smaller center wavelength changes (Fig. 10), and smaller output power fluctuations and 3 dB bandwidth (Fig. 11). Conclusions We realize a dual-wavelength linear cavity EDFL based on DFBGs with a simple structure and output a stable dual-wavelength laser at room temperature by the PHB effect, with the output performance analyzed. The results show that the output performance of the HR-FBG structure is better than that of the FLM structure. When the reflectivities of the two LR-FBGs adopted as the output mirror is the same, the output performance is better than that under different reflectivities. Additionally, as the center wavelength interval of DBFGs gradually increases, the OSNR of EDFL gradually improves, and the dual-wavelength laser output gradually stabilizes, but its slope efficiency will decrease due to the gain characteristics of EDF at different wavelengths. Finally, we realize that the stable results in dualwavelength linear cavity EDFL with the OSNR of 1550 nm and 1562 nm are about 50. 24 dB and 51. 19 dB. The center wavelength fluctuations are less than 0. 030 nm and 0. 035 nm, and the power fluctuations are less than 0. 061 mW and 0. 059 mW, with 3 dB bandwidth of similar to 0. 146 nm and similar to 0. 144 nm respectively. The output results are better in the dualwavelength linear cavity.
