The effects of laser power induced thermal lensing are difficult to monitor and may lead to weld or cutting defects. Considering the ongoing increase in power of laser sources and improvements in terms of beam quality, focal shifts require intensified attention and thus the necessity of technical countermeasures. This paper depicts the origins of thermal lensing by demonstrating the influence of the laser power density on the optical characteristics of welding heads. In the past, considerable investigations were made to monitor the positioning of the focal spot, which are introduced throughout the paper. A review of auto-focus techniques summarizes the efforts taken to control the positioning of the welding head.
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