Investigations on the active compensation of the focal shift in scanning systems using a temperature signal

For the purpose of realizing a fast and cost-efficient manipulation of the laser beam in production applications, such as welding, marking, and cutting, scanner systems in combination with F-Theta objectives are state-of-the-art. Owing to the absorption of the laser beam power and the resulting heat load acting on the optical system, a change of the focal plane (the so-called focal shift) occurs, significantly affecting the behavior during the application process. A linear correlation between the temperature on the optical surface of a standard F-Theta objective and the focal shift was determined whereby the coefficient of determination R² is higher than 0.99. Furthermore, two industrial welding applications were investigated using this standard objective, and the resulting temperature distribution along the optical surfaces was also investigated. The results show that a single measurement point appears to be sufficient to obtain a capable input signal for a compensation method. A compensation device was implemented and a sufficient reduction of the focal shift was realized for the examined time range.