Single-polarization single-frequency Brillouin fiber laser that emits almost 5 W of power at 1 µm

We demonstrate a high-power single-polarization single-frequency 1064 nm Brillouin fiber laser (BFL) that is constructed with polarization-maintaining germanium-doped fiber with a core/cladding diameter of 20/400 µm. A maximum output power of 4.9 W is achieved with a slope efficiency of 68% and an optical signal-to-noise ratio of 65 dB. To the best of our knowledge, this is the highest power output from a single-frequency fiber laser. The polarization extinction ratio is over 18.7 dB and the BFL output presents a good transverse mode. The BFL shows a significant reduction (10-15 dB) in both the relative intensity noise and frequency noise of the pump source, while the estimated linewidth is 170 kHz with a measurement time of 2 ms at the maximum output power. It is believed that the high power output in combination with the decreased relative intensity and frequency noise renders the proposed BFL an important candidate for applications in optical sensing and high-purity microwave signal synthesis.