Product Code: ICA11_1802

Reference Process for Determination of Thermal Focus Shift
Authors:
Axel Hess, Institut fur Strachlwekzeuge (IFSW); Stuttgart Germany
Rudolf Weber, Institut fur Strachlwekzeuge (IFSW); Stuttgart Germany
Thomas Graf, Institut fur Strachlwekzeuge (IFSW); Stuttgart Germany
Marcel Schaefer, Manz AG; Stuttgart Germany
Christiane Thiel, Institut fur Strachlwekzeuge (IFSW); Stuttgart Germany
Presented at ICALEO 2011

High average laser power very often causes thermal beam distortion inside the transmitted optical components. This effect is very pronounced in contaminated optics. It results in a significant decrease of the beam quality and a decrease of the effective focal length, also known as focus shift. Both directly modify the laser spot size on the work piece which is very critical for most laser processes. Actually both, beam quality and focus shift measurements are time consuming and require expensive equipment, which furthermore involves additional thermally loaded optics.
This paper describes a novel method which uses the welding process itself to determine quantitatively the focus shift. This reference process takes benefit of the beam diameter dependence of the deep penetration welding threshold. After a defined preheating time to thermally load the optics, a weld trace of a few centimeters is generated in a prepared sample applying a laser power ramp. The position of the deep penetration transition, which is easily noticeable by visual inspection, is measured with a simple ruler. The focus shift can be calculated from four such welds with appropriate different laser parameters.