## 3 Results and Discussion  
Conditions Required for the Selective Generation of Stokes Orders with Maximum Conversion Efficiency  
The characteristics of a pulse-burst Raman laser significantly depend on the characteristics of the pumping radiation, and the spectral reflectance of the output mirrors. The optimum values of the pump subpulse power density, \(\mathbf{p_{532}}\), and the reflection coefficients, \(\mathbf{R_{\lambda}}\), of the output mirrors at the wavelengths of the first, second, and third Stokes order were determined during operation in SP mode, and used for operation in BP mode.  
To obtain the high Raman laser energy \(\mathbf{{{\rm{\bf{E}}}_{RAM}}}\), and the energy efficiency \(\mathbf{\eta}\) (\(\mathbf{\eta}=\mathbf{{{\rm{\bf{E}}}_{RAM}}}/\mathbf{{{\rm{\bf{E}}}_{532}}}\), \(\mathbf{{{\rm{\bf{E}}}_{532}}}\) is the energy of the 532 nm pump radiation), we produced an optimization of the Raman laser output mirrors for \(\mathbf{p_{532}}\geq 100\) MW/cm\({}^{2}\). The maximum level of \(\mathbf{p_{532}}\) was always restricted by the appearance of the higher (unwanted) Stokes order. Table 2 indicates the values of \(\mathbf{p_{532}}\) (max), measured when the higher Raman order intensity reached 5% of the intensity of the required order (an estimation was made using the spectral line intensity ratio), and \(\mathbf{\eta}\) for three combinations of mirror reflection coefficients, which were used in the experiment to optimize the generation of the second Stokes order (589 nm).  
Similar results were obtained for the first (559 nm) and the third (621 nm) Stokes orders:  
* For 559 nm: \(\mathbf{R_{559}}=2\%\), \(\mathbf{R_{589}}<1\%\), \(\mathbf{p_{532}}\) (max) \(\sim\)200 MW/cm\({}^{2}\), \(\mathbf{\eta}=63\%\);
* For 621 nm: \(\mathbf{R_{559}}=99.5\%\), \(\mathbf{R_{589}}=99.2\%\), \(\mathbf{R_{621}}=1.8\%\), \(\mathbf{R_{654}}=1.9\%\), \(\mathbf{p_{532}}\) (max) \(\sim\)300 MW/cm\({}^{2}\), \(\mathbf{\eta}=50\%\).  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Mirror Reflection Coefficients, \(\mathbf{R_{\lambda}}\)** & \(\mathbf{p_{532}}\) **(max)** & \(\mathbf{\eta}\) **\%** \\ \hline
1 & \(\mathbf{R_{559}}=99.5\%\), \(\mathbf{R_{589}}=1.5\%\), \(\mathbf{R_{621}}=0.6\%\) & \(\sim\)200 MW/cm\({}^{2}\) & 58 \\ \hline
2 & \(\mathbf{R_{559}}=99.5\%\), \(\mathbf{R_{589}}=10.5\%\), \(\mathbf{R_{621}}=0.35\%\) & \(\sim\)260 MW/cm\({}^{2}\) & 60 \\ \hline
3 & \(\mathbf{R_{559}}=99.5\%\), \(\mathbf{R_{589}}<0.3\%\), \(\mathbf{R_{621}}<0.1\%\) & \(\sim\)320 MW/cm\({}^{2}\) & 62 \\ \hline \hline \end{tabular}
\end{table}
Table 2: The optimum parameters for the selective generation of the second Stokes order.  
Figure 2: The optical scheme of the Raman laser. 1, the KGW crystal; 2, the pump mirror; 3, the output mirror.  
One can see that the best results were obtained using output mirrors with characteristics similar to long-pass filters with a cut-off wavelength that corresponds to the wavelength of the required Raman order. The reflection coefficient at the required wavelength does not significantly affect the characteristics of the Raman laser. However, the reflection coefficient at the wavelength of the next Stokes component should be minimal, to increase the threshold of the unwanted order. All these results are a consequence of the large Raman gain in our experimental conditions. A simple calculation shows that, using a double-pass pumping scheme with a 4.1 \(\times\) 4.1 mm pumping spot, and 50 mm length crystal (the KGW steady-state gain is 11.8 cm/GW [6]) in SP mode, we can obtain a gain increment G \(>\) 25, allowing the efficient conversion of pump radiation into Stokes radiation [17], even in a cavity-less configuration, at **E\({}_{\textbf{532}}\)**\(\sim\) 0.2 J, which is two times less than the maximum energy of our 532 nm source.