Fig. 1. Schematic diagram of experimental setup (TPSR represents tunable pulse stretcher, PM-YDF represents polarization-maintaining Yb-doped fiber, DC-YDF represents double-clad Yb-doped fiber, ISO represents isolator, AOM represents acousto-optic modulator, PCF represents photonic crystal fiber, HWP represents half-wave plate, QWP represents quarter-wave plate, PBS represents polarizing beam-splitter, PZM represents piezo-driven mirror, TFP represents thin film polarizer, LW represents laser window, PM represents power meter, and SHG-FROG represents second harmonic generation frequency resolved optical gating)

Fig. 2. Output power as a function of pump power of each channel. (a) Channel 1; (b) channel 2; (c) channel 3; (d) channel 4

Fig. 3. Power stability and phase stability of combined beams. (a) Power stability; phase error of (b) channels 1 and 2, (c) channels 3 and 4, and (d) channels 1-4 after starting phase stabilization system

Fig. 4. Spectrum of combined beam under full power amplification

Fig. 5. Beam profiles of the individual channels and after coherent combination of all channels at full power amplification

Fig. 6. Measurement of pulse characteristics after dispersion compensation by grating only. (a) Measured FROG trace; (b) retrieved FROG trace; (c) measured spectrum (dashed line), retrieved spectrum (solid line), and phase (dotdash line); (d) retrieved temporal envelope (solid line), temporal phase (dot dash line), and Fourier transform-limited temporal envelope (dashed line)

Fig. 7. Measurement of pulse characteristics after accurate high-order dispersion compensation using TPSR. (a) Measured FROG trace; (b) retrieved FROG trace; (c) measured spectrum (dashed line), retrieved spectrum (solid line), and phase (dot dash line); (d) retrieved temporal envelope (solid line), temporal phase (dot dash line), and Fourier transform-limited temporal envelope (dashed line)

Fig. 8. M² after compression of the combined beam under full power amplification (inset is profile of focus beam)