Fig. 1. Experimental setup of the ultra-low noise 2 μm single-frequency PM fiber laser

Fig. 2. RIN of 1 550 nm SFL with and without feedback control loop

Fig. 3. RIN (a) and frequency noise (b) of 2 μm SFFL pumped with and without feedback

Fig. 4. RIN (a) and frequency noise (b) of the thulium-doped polarization-maintaining fiber amplifier pumped with and without feedback

Fig. 5. Frequency response to PZT modulation (a) and frequency drift of 2 μm SFFL in 1 h (b)

Fig. 6. Linewidth of 2 μm single-frequency laser at 5.2 W output (a), power stability within 1 h (b) and spectra of 2 μm single-frequency laser before and after amplification (c)