Online focus shift measurement in high power fiber laser welding

The occurrence of focus shift in high power fiber laser welding has been reported. The small percentage of laser light absorbed in the optical elements of the welding head leads to a thermal focus shift. Contaminations on the optical surfaces are enhancing the absorption and thus the focus shift. Thus, one of the main influencing factors on focus shift is the condition of the optics of the head. The topic of this paper is the experimental characterization of focus shift behavior, and the development of a novel method to measure the shift of the focal plane during processing. This method allows for considering the shift caused by every optical element within the path of the beam. The online focus shift measurement is the missing part for a closed loop focus shift control during high power fiber laser welding.