An improved approach for process monitoring in laser material processing

Process monitoring is used in many different laser material processes due to the demand for reliable and stable processes. Among different methods, on-axis process monitoring offers multiple advantages. To observe a laser material process it is unavoidable to choose a wavelength for observation that is different to the one used for material processing, otherwise the light of the processing laser would outshine the picture of the process. By choosing a different wavelength, lateral chromatic aberration occurs in not chromatically corrected optical systems with optical scanning units and f-Theta lenses. These aberrations lead to a truncated image of the process on the camera or the pyrometer, respectively. This is the reason for adulterated measurements and non-satisfying images of the process. A new approach for solving the problem of field dependent lateral chromatic aberration in process monitoring is presented. Therefore, the scanner-based optical system is reproduced in a simulation environment, to predict the occurring lateral chromatic aberrations. In addition, a second deflecting system is integrated into the system. By using simulation, a predictive control is designed that uses the additional deflecting system to introduce reverse lateral deviations in order to compensate the lateral effect of chromatic aberration. This paper illustrates the concept and the implementation of the predictive control, which is used to eliminate lateral chromatic aberrations in process monitoring, the simulation on which the system is based the optical system as well as the control concept.