# Experimental Study on the Damage of Optical Materials by out of Band Composite Laser
## 3 Experimental Study of Composite Laser Damaged Flat Window
### Continuous Laser Damage Flat Window Experiment  
The continuous laser was used to verify the damage effect of the out-of-band laser on the flat window. Due to thermal stress deformation caused by laser irradiation, the spot of collimated light was shifted. The thermal deformation state of the flat window under continuous laser loading was observed with a scattering detection method--that shown in Figure 2a is before loading, that shown in Figure 2b is the spot shift caused by thermal distortion during loading, and that shown in Figure 2c is the spot shift before the lens body exploded.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Continuous Laser** & **Long-Pulse Laser** & **Short-Pulse Laser** \\ \hline Wavelength & 1.06 μm & 1.03 μm & 1.06 μm \\ Maximum power/energy & 2000 W & 100 J & 250 mJ \\ Pulse width & N/A & 400 μs & 10 ns \\ Gain medium & Nd:YAG & Yb:YAG & Nd:YAG \\ \hline \hline \end{tabular}
\end{table}
Table 1: Laser parameters.  
Figure 1: Laser damage effect experimental platform.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Density** & **Specific Heat** & **Thermal** & **Coefficient of** \\  & **(g/cm\({}^{3}\))** & **Capacity** & **Conductivity** & **Linear Expansion** \\  & **(J/(kg-k))** & **(W/(m-k))** & **(I/C)** & **Coating Range** \\ \hline Germanium single crystal & 5.33 & 310 & 59.8 & \(5.4\times 10^{-6}\) & 3–5 μm \\ Silicon single crystal & 2.33 & 712 & 159 & \(2.55\times 10^{-6}\) & 2–6 μm \\ \hline \hline \end{tabular}
\end{table}
Table 2: Physical properties of substrate.  
By comparing the experimental results, it could be seen that the film color of the germanium window changed from 600 to 660 \({}^{\circ}\)C, and ripples were generated (Figure 3). At 660 \({}^{\circ}\)C, the film began to melt, and optical coating damage occurred. At 700 \({}^{\circ}\)C, substrate damage began. When the laser power reached 1500 W, the window body exploded. The film damage of the silicon window began to occur at 500 \({}^{\circ}\)C (Figure 4), and the damage of the film was similar to that of the germanium window mirror. When the laser power reached 1440 W, the substrate of the silicon window exploded at 950 \({}^{\circ}\)C.  
When the laser was irradiated to the flat window, due to the asymmetrical distribution of the high-power laser beam and non-uniform absorption, the temperature gradient and thermal stress in the material were caused. The larger temperature gradient and thermoelastic stress first appeared in the irradiated area, which made the window material generate thermal distortion. When the thermal stress gradually accumulated and exceeded the strength limit of the material, the irradiated area first burst, eventually causing macro damage to the window [26; 27]. It can be seen from Table 4 that the damage thresholds of the NIR lasers out of band on the Ge and Si windows were close. The reason for the body damage was that the substrate absorption rate increased after the optical coating melted  
Figure 4: Damage of germanium broadband window (200\(\times\)).  
Figure 3: Damage of silicon broadband window (200\(\times\)).  
Figure 2: Changes in scattered light spots caused by thermal distortion. (**a**) Irradiation begins (**b**) Start to deform (**c**) Body explode.  
and the temperature rose became faster, which caused substrate bursting. The damage of the optical film system was mainly caused by melting. It could be seen that the film system disappeared and the substrate was severely ablated when observed in the central area of irradiation. This was because when continuous laser light irradiated the surface of the window mirror, the thermal stability of the substrate material was better than that of the film. The thermal stress when the substrate was heated to 550-600 \({}^{\circ}\)C was not enough to cause the body damage of the window, but the optical film had begun to melt and be destroyed.