The CO2 laser mitigation method has been developed to mitigate the ultraviolet laser damage site on a fused silica surface. The mitigation process was monitored by an on-line white light scattering imaging system in order to ensure that the mitigation is successful. Additionally, a total internal reflection microscope was utilized to analyze the mitigation pit. By optimizing the laser mitigation parameters, the rough damage site can be replaced by a smooth Gaussian-shaped mitigation pit. The chemical composition of the damage sites and the CO2 laser mitigation pits was also measured with energy dispersive x-ray spectroscopy. It reveals that the oxygen deficiency center defect of the ultraviolet laser damage site is removed after CO2 laser mitigation, which helps us better understand the CO2 laser mitigation process.
The growth of laser induced damage on the surface of fused silica plays a major role in determining the operation fluence
and optics lifetime in high power laser system. In this paper, the damage growth characteristic of fused silica and possible
growth mechanisms were investigated. The morphology of damage site was measured by scanning electron microscopy
(SEM) and optical microscopy (OM). The finite difference time domain (FDTD) method was used to calculate the electric
field distribution around the damage site. Furthermore, energy dispersive spectrometers (EDS) micro-analysis technique,
x-ray photoelectron spectrometer (XPS) and Raman spectroscopy were applied to detect the chemical composition, point
defect and microstructure of damage site in order to explore the growth mechanism. It’s found that the growth threshold is greatly affected by the size of damage site, and the growth threshold of damage site is much lower than that of undamaged area. Theoretical calculation demonstrated that the rough damage site can strong modulate the distribution of electric field and result in the enhancement of local light field around the damage site. Results also showed that the oxygen defect was generated and the structure was changed after initial laser damage. Based on the above analysis, a mechanism of laser-induced damage growth on fused silica surface was proposed.
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