Fused Quartz Uv Laser Initial Damage Morphology Studies

The design of high power UV laser systems is limited to a large extent by the laser initiated damage (LID) performance of transmissive fused silica optical components. How to improve their ability to withstand intense laser pulses is one of the main concerns for builders of large laser systems.In the present paper, at first we discussed the physical mechanism associated with intrinsic and defect-related damage in fused silica, as well as the physical phenomena to damage growth. Using the CCD online damage detection system, we got several types of laser initialed silica damage morphology. With the assumption that laser energy deposition process only through thermal diffusion, we performed simulations of the thermal distribution in spherical CeO₂ defects and the surrounding fused silica. At the end of this paper, we statistical investigated the different morphology between various polishing processes. We also observed fused silica surface/subsurface mechanical characteristic by nanoindentation tests. Furthermore we discussed the impact on samples' surface/subsurface with different polishing process.The main received conclusions and experience will be summarized as below:[1]. We observed several types of damage morphology on the fused silica samples manufactured by means of pitch polishing,polyurethane pad polishing,magnetorheological finishing and HF etch. Two types damage sites are observed on the input surface of fused silica: micro-pits and star-like crack with the diameter of 0.8～2.5μm and 1.0～5.5μm, respectively. Correspondingly, there are three damage types on the output surface namely micro-pits,shell-like damage and crater with the diameter of 0.48-1.33μm,4～20μm and 12～30μm, respectively.[2]. The statistically ratio of diameter to depth for the input surface and output surface of fused silica are 15 and 9 respectively. Two distinct morphologies of submicron damage were revealed by AFM: single hole and double hole. The formation of single hole is relational with melting and vaporization, while double hole should take into account both pure thermal (melting and vaporization) and thermo-mechanical (fracture-generation). We also performed simulations of the thermal distribution in spherical CeO₂ defects and the surrounding fused silica. The consequence exhibit that radius of thermal diffusion is about 200nm, comparable with the size of wavelength and micro-pits. The damage morphology develops from micro-pits to crater with fluence increase. Meanwhile the damage mechanism also changes from thermal conduction to mechanical effect.[3]. Contrasted the damage morphology with various polishing process, we discovered the consistency in damage progress. We inspected the surface hardness and reduced modulus of fused silica samples manufactured by means of MRF and pad/pitch polishing with Triboindenter nanomechanical test system. The result exhibited that the redeposited hydrated layer is about 200nm with a little fluctuation among different polishing processes. Ascribed to special material removal mechanism, the sample deal with MRF showed 50% hardness decrease to the bulk value. So there is a soft hydrated layer on MRF polished fused silica surface which may have higher fracture toughness to resist the formation of subsurface cracks during polishing process.