Fig. 1. (a) Schematic of the narrow-linewidth single-frequency Er³⁺-doped fiber laser based on a whispering gallery mode (WGM) micro-resonator; (b) Photograph of the WGM resonator^[10]

Fig. 2. Schematic diagram of Yb³⁺-doped crystal-derived silica fiber fabrication using the preform with a YAG crystal core and a silica cladding, where the insets are the optical images of the (1) side view and (2) cross-sectional view of the crystal-derived silica fiber^[19]

Fig. 3. Output power as a function of pump power of the Watt-level 1050 nm single-frequency Yb³⁺-doped phosphate fiber laser; Inset: Experimental setup of the single-frequency fiber laser^[23]

Fig. 4. (a) Experimental setup of 1.7 µm single-frequency fiber laser based on Tm³⁺-doped fiber as the saturable absorber; (b) Output power of the single-frequency laser with respect to the pump power^[24]

Fig. 5. Experimental setup of 200 Hz ultra-narrow linewidth Er³⁺-doped single-frequency fiber laser^[25]

Fig. 6. (a) Experimental setup of the 2 µm narrow-linewidth single-frequency Brillouin-Thulium-based fiber laser; (b) Curves of single-frequency laser output power; Inset: Measured Stokes linewidth based on delayed self-heterodyne method^[26]

Fig. 7. (a) Experimental setup of the low-noise single-frequency fiber laser based on booster optical amplifier (BOA) and self-injection locking (SIL) techniques; (b) Comparison on laser noise of the single-frequency laser under free-running status, and noise control status with BOA and SIL techniques^[31]

Fig. 8. Power evolution of single-frequency fiber amplifiers in all-fiber configurations

Fig. 9. (a) The core radius of the tapered fiber along with the fiber length; (b) Laser output power and backward power of the main amplifier stage with tapered gain fiber^[48]

Fig. 10. (a) Experimental setup of the single-frequency all-fiber amplifier based on hybrid pump of the cascaded Yb³⁺-doped fibers; (b) Laser output power of the main amplifier stage under singular 976 nm pump, and 976 nm and 915 nm hybrid pump; (c) Measured beam quality of the single-frequency amplifier at the output power of 435 W^[49]