Cover glass influence on high power fiber laser induced focus shift

The condition of the optics of a processing head is an influencing factor on the focus stability at laser beam welding with high brightness laser beam sources. The small percentage of laser light absorbed in the optical elements of a processing head leads to a thermal focus shift. The amount of the resulting shift of the focal plane depends on the condition of the optical elements since imperfections like contaminations and surface defects can lead to an increasing absorption of laser light. Especially the cover glass which protects the focusing lens from environmental influences and contaminations from the welding process is subject to significant condition changes. In this paper a multimode 8 kW fiber laser was used to investigate the influence of the cover glass on the focal plane stability of a laser welding head.