Ultra-low noise 2 μm high power single-frequency fiber laser for next generation gravitational-wave detector (Invited)

An acoustic optical modulator (AOM) laser power feedback loop was used to suppress the intensity noise of the pump laser, and the intensity noise reduction of more than 5 dB (@f=1 Hz-50 kHz) was obtained. The relative intensity noise of the 2 μm single-frequency fiber laser obtained 3-15 dB suppression (@f=1 Hz-50 kHz), and the intensity noise level was close to the detector limit (@f=40-400 Hz). Meanwhile, its frequency noise is also suppressed by 3-8.4 dB. After the two-stage Thulium-doped polarization-preserving fiber amplifier, the output power of the 2 μm single-frequency laser was increased to about 5.2 W with almost no significant increase in frequency noise, and the frequency noise levels were all at 100 Hz/\sqrt \rmHz (f>13 Hz). The frequency response was 45 MHz/V, the frequency drift was 41.4 MHz@1 h, the power fluctuation was less than 0.4%@1 h, and the linewidth was less than 5 kHz. This kind of ultra-low noise 2 μm single-frequency fiber laser will be a candidate laser source for the next generation of gravitational wave detectors.