Ultranarrow Linewidth Brillouin Fiber Laser

Brillouin fiber laser with ultranarrow linewidth of 30 Hz is demonstrated for the first time without phase locking loop, the laser operates single longitudinal mode, while mode selection is achieved by a Fabry-Perot interferometer and a section of coreless fiber with Farday rotation mirror through optical phase conjugation, the Brillouin gain is provided by a 25-km fiber. The output linewidth is a factor of 100 reductions comparing with pump laser. Moreover, the laser exhibits an ultralow-frequency noise (<;15 Hz/Hz
^1/2
at 10 Hz). The proposed laser configuration provides an ideal tool to achieve linewidth reduction of any laser from kilohertz level to subhundred hertz with lower frequency noise.