The subsurface damage and metal impurities have been the main laser damage precursors of fused
silica while subjected to high power laser irradiation. Light field enhancement and thermal
absorption were used to explain the appearance of damage pits while the laser energy is far
smaller than the energy that can reach the intrinsic threshold of fused silica. For fused silica optics
manufactured by magnetorheological finishing or advanced mitigation process, no scratch-related
damage site occurs can be found on the surface.
In this work, we implemented a photothermal absorption technique based on thermal lens method
to characterize the subsurface defects of fused silica optics. The pump beam is CW 532 nm
wavelength laser. The probe beam is a He-Ne laser. They are collinear and focused through the
same objective. When pump beam pass through the sample, optical absorption induces the local
temperature rise. The lowest absorptance that we can detect is about the order of magnitude of
0.01 ppm. When pump beam pass through the sample, optical absorption induces the local
temperature rise.
The photothermal absorption value of fused silica samples range from 0.5 to 10 ppm. The damage
densities of the samples were plotted. The damage threshold of samples at 8J/cm2 were gived to
show laser damage performance of fused silica.The results show that there is a strong correlation
between the thermal absorption and laser damage density. The photothermal absorption technique
can be used to predict and evaluate the laser damage performance of fused silica optics.
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