Widely tunable ultra-narrow linewidth single-longitudinal-mode Brillouin fiber laser with low threshold

A widely tunable ultra-narrow linewidth single-longitudinal-mode Brillouin fiber laser (SLM-BFL) with low threshold is proposed and experimentally demonstrated by using a 205 m long high-nonlinear fiber (HNLF) as Brillouin gain media. The single-longitudinal-mode (SLM) operation is guaranteed by cascaded filters composed of a fiber ring Fabry-Perot filter and a self-induced narrow filter based on a 4 m unpumped erbium-doped fiber. Owing to the long ring cavity, the BFL has a low threshold of 12.8 mW and ultra-narrow linewidth of 0.29 kHz. The BFL can be tuned from 1532 to 1577 nm. The optical signal-to-noise ratio of the fiber laser is higher than 70 dB during the 45 nm tune range.