Fig. 1. The dependence of the thresholds of SF on the beam quality.

Fig. 2. The SRS thresholds in experiments and prediction.

Fig. 3. The SPM-induced spectrum broadening at high peak power intensity: (a) the measured output pulse shape of the PCF fiber amplifier stage, and the predicted output pulse shape based on laser rate equations at different output pulse energy; (b) at pulse energy of 0.595 mJ, the RMS bandwidth of the output spectrum as a function of nonlinear coefficient; (c) the output spectrum at different peak power of output pulses; (d) the measured and calculated broadening factors as a function of peak power, and compared with the calculated results in small-signal model.

Fig. 4. The experimental setup for cascaded fiber amplifiers in multimode operation.

Fig. 5. The feasibility of 50 mJ pulse energy and 5 MW peak power delivered by the multistage fiber amplifier system.

Fig. 6. The experimental setup for cascaded fiber amplifiers in single-mode operation.

Fig. 7. The output pulse energy as a function of pump energy and the beam quality of the output pulses.

Fig. 8. End face of the large pitch fiber used and its several major modes.