## 3 Experimental Results and Discussions
### Main Amplifier and Suppression of SRS effect
But in high peak power amplifier, suppression of the SRS process and pump light absorption efficiency also limit the min absorption coefficient. With Equations (4)-(6), to avert the SRS process, when operating the QCW pump, the thermal fracture pump power \(P_{av}\) depends on the pulse intensity and can be given by:  
Figure 4: Obtained spectrum of first Stokes wave of Raman light as high intensity laser passing through the Nd:YVO\({}_{4}\) crystal. \[IL\leq 1.55\;\mathrm{GW/cm} \tag{2}\]0  
Figure 5 shows the relationship between the maximum pump power and the maximum laser intensity at QCW pump duty cycle of 10% and 1 kHz level for Nd:YVO\({}_{4}\) amplifier, which is described as Equation (7). It can be seen that, the higher amplified intensity is the lower the maximum pump power will be. When the amplified intensity is 1 GW/cm\({}^{2}\), the maximum pump power approaches 60 W. But when the amplified intensity increases to 4 GW/cm\({}^{2}\), the maximum pump power decreases to 15 W. At high peak power amplifier, the diode-end-pumped power for bulk Nd:YVO\({}_{4}\) is limited to suppression of the SRS process and thermal fracture of bulk Nd:YVO\({}_{4}\) crystal. Therefore, the amplification ability of an end-pumped Nd:YVO\({}_{4}\) amplifier is limited to the laser intensity. In the main amplifier, the absorption efficiency of Nd:YVO\({}_{4}\) (0.2%, 1.5 cm) with absorption coefficient was 2.5 cm\({}^{-1}\) and could reach 95%. Based on Equation (6), the calculated maximum average pump power was about 60 W at 1 kHz with QCW pump duty cycle of 10%, so each sub-amplifier pumped by 50 mJ pulses can work safely. Table 1 shows parameters of the picosecond amplification stages. With an input energy of 4.5 mJ and a total of 300 mJ pump energy into the main amplifier, we achieved maximum output energy of 65.4 mJ, corresponding to an optical efficiency of 20.3%. The spot size in different stages were gradually increased to ensure the laser intensity was below 1 GW/cm\({}^{2}\) and reduce the Raman gain. At different amplified stages, \(I_{max}*L_{crystal}\) in each stage had the maximum value of 1.31 and below the SRS threshold of 1.55. In addition, catadioptric mirrors between the two amplifiers are anti-reflective coated to reduce the accumulation of Raman light. Via these measures, we achieved tens of mJ energy sub-ns pulse through a Nd:YVO\({}_{4}\) amplifier without the SRS process, as shown in \[IL\leq 1.55\;\mathrm{GW/cm} \tag{2}\]1 There is no Raman light at a wavelength of 1176 nm (first Stokes wave) and wavelength 1313 nm (second Stokes wave) in the maximum laser output. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Stage** & \(E_{pump}\) **[m]** & \(E_{out}\) **[m]** & \(D_{out}\) **[mm]** & \(\eta_{o-o}\) **[\%]** & \(I_{max}\) **[GW/cm\({}^{2}\)]** & \(L_{crystal}\) **[cm]** & \(I_{max}*L_{crystal}\) **[GW/cm]** \\ \hline Seed & - & 0.055 & - & - & - & - \\ Pre-amplifier & 28.6 & 4.5 & 2.0 & 16.7 & 0.32 & 1.0 & 0.32 \\ Main amplifier 1 & 100 & 22.8 & 2.6 & 18.3 & 0.72 & 1.5 & 1.08 \\ Main amplifier 2 & 100 & 44.9 & 3.4 & 22.1 & 0.82 & 1.5 & 1.23 \\ Main amplifier 3 & 100 & 65.4 & 4.0 & 20.5 & 0.87 & 1.5 & 1.31 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Parameters of the sub-ns amplification stages. Figure 5: The relationship between the maximum pump power and the maximum laser intensity in an end-pumped Nd:YVO\({}_{4}\) amplifier at a QCW pump duty cycle of 10%. Figure 7 shows the spectra of the oscillator and amplifying systems. This sub-nanosecond oscillator had several longitudinal mode. The main longitudinal mode got more amplification, and, after pulse amplification, spectrum bandwidth became narrower. Narrower spectrum caused by gain narrowing would increase pulse width. A change of pulse width after amplification was shown in Figure 8. The pulse from the oscillator and amplifying systems was measured by a 8 GHz analog oscilloscope and an InGaAs photodiode (UPD-70-IR2-P, ALPHALAS). The pulse width of the oscillator is 450 ps.