Single longitudinal mode Brillouin fiber laser with fiber ring and reflective saturable absorber

A single-longitudinal mode (SLM) Brillouin fiber laser is demonstrated, which leverages stimulated Brillouin scattering (SBS) in the fiber to achieve linewidth compression. The Brillouin ring cavity has a substantial length of 1 km, resulting in a longitudinal mode spacing that is significantly smaller than the width of the Brillouin gain spectrum. This leads to a multi-longitudinal mode (MLM) state for the output laser. To achieve SLM output, mode selection filters such as fiber ring and reflective saturable absorber (RSA) are introduced. The proposed laser has a threshold of ~28 mW, with an output linewidth compressed from 100 kHz to 20.21 kHz using delayed self-heterodyne interferometry (DSHI). The optical signal-to-noise ratio (OSNR) improved from 58.2 dB to 80.8 dB, and the maximum laser conversion efficiency reached 26%.