Fig. 1. (a) Schematic of the femtosecond laser phase scanning method. (b) Reflection spectra of HR-FBG and LR-FBG.

Fig. 2. Schematic of the high-power fiber oscillator.

Fig. 3. Output power versus pump power. Insets: beam profiles at the operation of 3005 W, 3992 W, 4528 W, and 5027 W.

Fig. 4. Laser spectra under different output power.

Fig. 5. Temperature of (a) HR-FBG and (b) LR-FBG at different operation power.

Fig. 6. Oscillator stability test: (a) variation of output power at the operation of ∼4030 W during 1 h, (b) variation of output power at the operation of 5027 W, and (c) variation of beam quality measured at ∼4030 W.