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The power available in many industrial high-power 1 µm laser-cutting systems now exceeds 10 kW. To take advantage of that power, systems must be fast and nimble in order to accurately trace toolpaths at high speed. These power levels drive focusing head designs to be compact and lightweight. The best way to achieve this, in general, is with an all-transmissive design consisting of lenses and windows. The industry-standard material for lenses is fused silica, which has extremely low absorption. However, should the lens become contaminated, the poor thermal conductivity of fused silica traps much of the absorbed power inside the optic, raising its temperature and index of refraction, which causes the focus of the system to shift back toward the focusing head. At some point, the amount of focus shift becomes unsuitable for material processing and the focusing head must be replaced or refurbished with clean optics. Sapphire is an alternative lens material that until recently was only used in windows due to optical fabrication challenges in the polishing of aspheric surfaces. At Coherent Corp., we have overcome many of those challenges and currently offer sapphire aspheres. The key advantage of sapphire over fused silica is its relatively high thermal conductivity. This enables any absorbed power to quickly reach the edge of a sapphire optic where water-cooled mounts can remove the heat and keep the lens temperature low. Accounting for all the property differences between the two materials, sapphire should result in less focus shift than a comparable fused silica optic with the same absorbed power. The purpose of this study is to measure and compare the focus shift properties of the two materials in a controlled setting at high and low levels of absorbed power. Sapphire was also tested for laser-induced-damage threshold (LIDT) by Spica Technologies Inc.
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