# Experimental Study on the Damage of Optical Materials by out of Band Composite Laser
## 3 Experimental Study of Composite Laser Damaged Flat Window
### Long-Pulse Laser Damage Experiment  
A long-pulse laser damage flat window experiment was performed. The absorption rates of a 1.03 \(\upmu\)m laser for flat windows were measured. The measurement results are shown in Table 5. The absorption rate of the Si window for lasers in the 1.03 \(\upmu\)m band was higher than that of the 1.06 \(\upmu\)m band, and the absorption rate of the Ge window for lasers in the 1.03 \(\upmu\)m band was slightly lower than the 1.06 \(\upmu\)m band.  
We performed a S-on-1 single-pulse experiment, loaded five pulses, and observed the experimental results, as shown in Table 6 below.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Single-Pulse Energy (J)** & **Silicon Window Damage Effect** & **Germanium Window Damage Effect** \\ \hline
3 & N/A & N/A \\
6 & N/A & N/A \\
11 & N/A & Optical Coating damage \\
17 & N/A & Optical Coating damage \\
23 & N/A & Optical Coating damage \\
29 & Optical Coating damage & Substrate damage \\
36 & Optical Coating damage & Substrate damage \\
44 & Substrate damage & Substrate damage \\
51 & Substrate damage & Substrate damage \\
80 & Substrate damage & Substrate damage \\ \hline \hline \end{tabular}
\end{table}
Table 6: Experimental results of the long-pulse laser.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Laser Power**} & \multicolumn{3}{c}{**Silicon Window**} & \multicolumn{3}{c}{**Germanium Window**} \\ \cline{2-5}  & **Peak Temperature** & **Damage Effect** & **Peak Temperature** & **Damage Effect** \\  & **(\({}^{\circ}\)C)** & & **(\({}^{\circ}\)C)** & **Damage Effect** \\ \hline
350 & 40 & N/A & 70 & N/A \\
525 & 120 & N/A & 123 & N/A \\
700 & 250 & N/A & 179 & N/A \\
875 & 369 & N/A & 269 & N/A \\
1050 & 558 & N/A & 374 & N/A \\
1225 & 660 & Optical Coating damage & 544 & Optical Coating damage \\
1440 & 740 & Optical Coating damage & 948 & Substrate damage \\
1575 & 750 & Substrate damage & & \\ \hline \hline \end{tabular}
\end{table}
Table 4: Experimental results of the continuous laser.  
\begin{table}
\begin{tabular}{c c c} \hline \hline  & **Silicon Broadband Window** & **Germanium Broadband Window** \\ \hline Incident laser power & 6 W & 6 W \\ Reflected laser power & 3.5 W & 4.3 W \\ Absorption rate & 41.7\% & 28.3\% \\ \hline \hline \end{tabular}
\end{table}
Table 5: Absorption rate of the 1.03 \(\upmu\)m laser by a flat window.  
According to Table 6, when the laser energy reached 29 J, the Si window had coating damage, and at the same time, the plasma flash produced by the breakdown of electrons appeared in the irradiation area [28]. With the increase of energy, the damage of the optic film became more serious. The damage phenomenon is shown in Figures 5 and 6. When the energy reached 44 J, substrate damage occurred. When the energy reached 80 J, the first pulse caused substrate damage. When the laser energy reached 11 J, coating damage occurred in the Ge window. The coating damage became more serious at 17-23 J, and when the energy reached 29 J, the first pulse caused damage to the substrate. The long-pulse laser and continuous laser irradiation on windows were mainly caused by surface thermal fusion damage, and plasma absorption was the main cause of damage.