Linewidth-Narrowed, Linear-Polarized Single-Frequency Thulium-Doped Fiber Laser Based on Stimulated Brillouin Scattering Effect

A 2-μm linear-polarized hybrid Brillouin-Thulium fiber laser (BTFL) was experimentally investigated for linewidth narrowing. The low threshold for the BTFL in terms of Brillouin pump of ~200 mW, benefits from the gain of active fiber in the ring cavity for both Brillouin pump and generated Stokes light. More than 205 mW single-frequency Stokes laser was obtained with linewidth reduction ratio of ~8 times, from 36 to 4.6 kHz. Linear-polarized output was achieved with a polarization extinction ratio of 23.8 dB.