High average laser power very often causes thermal beam distortion inside the transmitting optical components. This effect is especially pronounced in contaminated optics. It results in a significant decrease of the beam quality and a decrease of the effective focal length, also referred to as focus shift. Both effects directly modify the laser spot size on the work piece which is very critical for most laser material processing applications. Moreover 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 quantitatively determine the focus shift. To this end a dedicated “reference process” is defined, which takes benefit of the beam diameter dependence of the deep-penetration welding threshold. After a defined time of preheating to thermally load the optics, a weld trace of a few centimeters is generated in a sample applying a laser power ramp. The position of the transition from heat-conduction welding to deep-penetration welding, which is easily noticeable by visual inspection of the seam width on the workpiece surface, is measured with a simple caliper. We show that with this information of four different welds with suitably varying laser parameters the focus shift can easily be calculated with appropriate accuracy.