Fig. 1. Schematic diagram of optical structure of fiber laser oscillator

Fig. 2. Refractive index profile of Yb³⁺- doped fiber

Fig. 3. Output characteristics of 976 nm LD forward-pumped fiber laser. (a) Output power and corresponding light-to-light conversion efficiency; (b) laser spectrum at output power of 1.36 kW; (c) time domain signals at P1 and P2 recorded by PD; (d) Fourier spectrum

Fig. 4. Output characteristics of forward hybrid-pumped fiber laser. (a) Output power and corresponding light-to-light conversion efficiency;(b) spectrum at 2.12 kW output power; (c) time domain signal and corresponding Fourier spectrum recorded by PD when output power is 2.12 kW

Fig. 5. Output characteristics of bidirectional hybrid-pumped fiber laser. (a) Output power and corresponding light-to-light conversion efficiency; (b) spectra at QBH with core diameter of 34 μm under various output powers; (c) time domain signal and Fourier spectrum recorded by PD when output power is 7.06 kW; (d) beam quality at output power of 7.06 kW

Fig. 6. Collected output voltage versus time during full-power aging test of fiber laser oscillator

Fig. 7. Laser oscillator stability test results. (a) Output power of laser oscillator during stability test; (b) surface state of red copper under high-reflection resistance test when laser output power is 7 kW