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Thermal lensing is a well-known but undesired effect in high power laser optics for welding, 3D-printing and other technologies. Stability and performance of laser processing depend on the possibility to control and minimize the thermo-optical effects induced by non-uniform (gradient) heating due to absorption of laser energy in optical elements: paraxial focus shift and thermally induced aberration, which lead to a change in size and intensity profile of the focal spot. Analysis of primary physical effects: geometrical deformation of optical surfaces and the material transformation into a gradient refractive medium, allows the quantitative estimation of the wavefront beam distortion leading to focus shift and aberration. It also allows formulating an optimal relationship between the physical properties of optical materials to reduce the change in the wavefront through mutual compensation of thermo-optical effects induced by the thermal expansion and the refractive index change – athermalization condition. Athermal optics exhibit minimized thermal focus shift and aberration even when absorbing laser energy in the bulk material and coatings, by contamination or scratches. Considering physical characteristics the Temperature Coefficient of the Optical Pathlength and ThermoOptical Ratio allows determining the optimal materials for optics: athermal crystalline Quartz and specialty glasses, Sapphire with high thermal conductivity. Weak birefringence of Quartz and Sapphire doesn’t prevent their successive use in laser optics. The comparison of the theoretical analysis and experimental validation results of optics made of Fused Silica, N-BK7, crystalline Quartz and Sapphire confirm the theoretical method for reducing the thermal focus shift and effectiveness of the suggested approach.
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