Robustness investigation of an in-situ absorption measurement system for laser processing

The laser-material interaction Katrin Wudy for laser-material processes, such as laser welding, where several phase changes occur, still needs to be fully understood. The high energy input in the material in a short time and small space renders the laser-material interaction a complex and highly dynamic mechanism. This interaction and the resulting radiation absorption govern the process behavior during manufacturing and, in turn, the quality of parts. In situ monitoring of the laser radiation absorption can be facilitated to evaluate the process. The absorption can be determined using an integrating sphere. To employ such a measuring system reliably without intense calibration effort for high throughput experiments, the robustness of the measuring signal toward changes to the system is crucial. This work evaluates said robustness by a series of experiments for which the signal of the integrating sphere diode and the back-reflection sensor of the laser are considered. The measuring system was altered by varying the electronic gain and the distance between the sphere and the specimen. Multiple experiments with varying laser powers were conducted for every configuration of the measuring system. Additionally, the stability of the reflective properties of the coated inner sphere was evaluated by analyzing the change in the measurements over time. The experimental results show the robustness of the integrating sphere signal trend toward the changes in the measurement parameters and degradation of the inner sphere coating. These are properties of a robust measuring system, which has excellent use in high throughput experiments for fundamental research in laser-material interaction.