1 Introduction As a light source of fiber laser sensor, narrow linewidth fiber laser has the characteristics of interference to electromagnetic field, safety, small size and remote control. Currently, there are three schemes for obtaining single longitudinal mode narrow linewidth fiber lasers. 1) Non-coherent technology that eliminates spatial hole burning caused by standing wave effect by controlling the polarization state of the encountering light waves in the cavity; 2) adding unpumped doped fiber in the laser cavity to select the frequency and suppressing the saturated absorption of the mode-hopping Body; 3) short cavity fiber laser, including DFB fiber laser and short cavity DBR fiber laser. Comparing the three schemes, it is found that schemes 1 and 2 require the use of multiple polarization controllers, and most of them have annular cavity structures, which are difficult to control, low conversion efficiency, and extremely low output power; while scheme 3 has a simple structur