# Experimental Study on the Damage of Optical Materials by out of Band Composite Laser
## 3 Experimental Study of Composite Laser Damaged Flat Window
### Composite Laser Damage Experiment  
According to the results of the pulse laser damage to the window, although the short-pulse laser had a higher peak power, it did less damage to the window and had difficulty damaging the substrate. Therefore, this paper used a combination of a long-pulse and a CW laser to perform the damage experiment. The trigger time of the long-pulsed laser was controlled by digital delay pulse generator. When the current temperature of the thermal imager reached a preset value, the CW laser was turned off and the pulsed laser was triggered at the same time. The results of the composite laser experiment are shown in Tables 8 and 9 below.  
The damage phenomenon is shown in Figures 9 and 10. It could be seen from the experimental results of the Ge window that the composite laser could cause slight damage to the film system at the laser energy of 2 J and 145 \({}^{\circ}\)C. The results of the Si window experiments showed that the composite laser started to damage the optic film system at 17 J. When the pulsed laser energy is less than 17J, the continuous laser pre-heating temperature needs to exceed 300 \({}^{\circ}\)C to cause mild film damage,  
Figure 8: Damage phenomenon of the Ge window with short -pulse laser irradiation.  
\begin{table}
\begin{tabular}{c c c} \hline \hline \begin{tabular}{c} **Single-Pulse Energy (J)** \\ \end{tabular} & \
\begin{tabular}{c} **Continuous Laser Heating** \\ **Temperature** \\ \end{tabular} &
\begin{tabular}{c} **Germanium Window Damage** \\ **Effect** \\ \end{tabular} \\ \hline
2 & 100 & N/A \\
2 & 145 & Optical Coating damage \\
3 & 80 & N/A \\
3 & 100 & Optical Coating damage \\
3 & 160 & Optical Coating damage \\
3 & 220 & Substrate damage \\ \hline \hline \end{tabular}
\end{table}
Table 8: Damage phenomenon of the Ge window by the composite laser.  
Figure 7: Damage phenomenon of the Si window with short-pulse laser irradiation.  
which was consistent with the long-pulse laser damage mechanism discussed above. Increasing the continuous laser preheating temperature while maintaining the pulse laser energy at 17 J led to more serious optic film damage, and the substrate damage was caused when the preheating temperature reached 260 \({}^{\circ}\)C.  
Table 10 summarizes the damage threshold data for different lasers. The giant-pulse laser waveform used in this experiment was of the spike and square mixed waveform. The energy of the spike pulse as about three-to-four times that of the square pulse. After the flat window was pre-heated by continuous light, the spike pulse part first acted on the window body. Its high peak energy was absorbed by the optical film and substrate. The optical film was instantly melted under the double temperature rise effect, and the residual energy was directly absorbed by the substrate and eventually caused damage to the substrate.  
Figure 10: Damage phenomenon of the Si window by the composite laser.  
\begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Single-Pulse Energy (J)**} & **Continuous Laser Heating** & **Germanium Window Damage** \\  & **Temperature** & **Effect** \\ \hline
11 & 100 & N/A \\
11 & 200 & N/A \\
11 & 310 & Optical Coating damage \\
17 & 100 & Optical Coating damage \\
17 & 120 & Optical Coating damage \\
17 & 168 & Optical Coating damage \\
17 & 210 & Optical Coating damage \\
17 & 266 & Substrate damage \\ \hline \hline \end{tabular}
\end{table}
Table 9: Damage phenomenon of the Si window by the composite laser.  
Figure 9: Damage phenomenon of the Ge window by the composite laser.