Fig. 1. Refractive index distribution of homemade 20 μm/400 μm Yb-doped fiber

Fig. 2. Structural diagram of main oscillator power amplifier based on homemade 20 μm/400 μm Yb-doped fiber

Fig. 3. Output laser characteristics. (a) Output power and slope efficiency; (b) beam quality versus output laser power; (c) laser spectra under different powers; (d) time domain and frequency domain characteristics of laser at highest power

Fig. 4. Power output stability test result of laser system at 4 kW