Fig. 1. Four-pass amplification optical layout.

Fig. 2. (a) Gain coefficient (logarithm of the gain factor) vs. the pump power with and without the filter for N31 and NAP2. (b) Peak value (PV) of wavefront distortion vs. the pump power with and without the filter.

Fig. 3. Gain distribution over the cross section of the rod.

Fig. 4. (a) Wavefront distribution. (b) Wavefront distribution in horizontal and vertical lines. (c) Wavefront distribution of the high-order distortion in horizontal and vertical lines.

Fig. 5. Principle of the thermally induced birefringence compensation.

Fig. 6. Intensity distribution of the near field (a) without and (b) with pre-compensation.

Fig. 7. (a) Intensity distribution in the far field. (b) Energy concentration rate curve of the focal spot. The dashed line is for a square 6-order-super-Gaussian beam, and the solid line is the experimental data.

Fig. 8. Pulse energy versus time in 30 min for the input and output pulse energies.