Fig. 1. Structure diagram of all-fiber high-power narrow-linewidth fiber laser amplifier

Fig. 2. Effect of reducing thermo-optic coefficient on mode instability threshold power

Fig. 3. Mechanism of doping elements in fiber core. (a) Distribution of the Yb ion in quartz glass when molar ratio of phosphorus to aluminum is less than 1; (b) distribution of the Yb ion in quartz glass when molar ratio of phosphorus to aluminum is larger than 1; (c) schematic of fluorine doping mechanism

Fig. 4. Output characteristics of the laser. (a) Experimentally measured laser power and slope efficiency; (b) beam quality and far-field spot at 2.2 kW; (c) forward and backward emission spectrum of 2.2 kW output beam