Fig. 1. Laser modes supported by 25/400 μm ytterbium doped fiber and their corresponding intensity and amplitude distributions

Fig. 2. Bending loss of ytterbium doped fiber at different bending radii in different modes

Fig. 3. Spot shape of output laser with different mode components in 25/400 μm ytterbium-doped fiber. (a) Combination one; (b) combination two; (c) combination three; (d) combination four

Fig. 4. Structure diagram of 5 kW annular fiber laser oscillator

Fig. 5. Forward pumping output characteristics of fiber laser oscillator. (a) Output power curve and corresponding light-light conversion efficiency; at maximum output power, (b) spectral characteristics of laser, (c) time-domain characteristic curve of laser and corresponding Fourier transform curve

Fig. 6. Backpump output characteristics of fiber laser oscillator. (a) Output power curve and corresponding light-light conversion efficiency; at maximum output power, (b) spectral characteristics of laser, (c) time-domain characteristic curve of laser and corresponding Fourier transform curve

Fig. 7. Bidirectional pumping output characteristics of fiber laser oscillator. (a) Output power curve and corresponding light-light conversion efficiency; at maximum output power, (b) spectral characteristics of laser, (c) time-domain characteristic curve of laser and corresponding Fourier transform curve

Fig. 8. Laser beam quality under different output powers, illustration shows shape of light spot at focus under different powers

Fig. 9. Measured results of fiber laser oscillator under full power operation. (a) 1 h output power data curve; (b) thermal image of optical fiber devices