Fig. 1. The general layout of the MOPA system: SESAM, semiconductor saturable absorber mirror; LD, laser diode at 976 nm; WDM, 980/1060 wavelength division multiplexer; YSF, ytterbium-doped single-mode fiber; FBG, fiber Bragg grating; ISO, isolator; BP, bandpass filter; LMA, large mode-field area; CPS, cladding pump stripper; XLMA, extra-large mode area; DM, dichroic mirror.

Fig. 2. Measurement results of the seed: (a) spectral width in the logarithmic scale (with a resolution of 0.02 nm); (inset) snapshot of pulse train; (b) trace of intensity autocorrelation; (c) spectrum of pre-amplified signal: after and (inset) before the BPF2.

Fig. 3. (a) Experimentally observed spectral evolution with different pumping currents; (b) typical spectral evolution of an unchirped pulse under SPM; (c) signal bandwidth of seed and after amplification; (d) autocorrelation trace of the ultimate pulse.

Fig. 4. (a) Raman content versus pump current of the first booster amplifier; (inset) global spectrum under 6 A pump measured at ISO4; (b) output power versus pump of the second booster amplifier; (c) spectrum at 1220 W output measured at the end of the XLMA.

Fig. 5. Output stability of the system. (a) With a full collimated beam captured by a charge-coupled device (CCD); (inset) sampling spot selected by pinhole; (b) Fourier transform of the temporal trace; (c) power stability within 1 hour.