Multiphysikalische Simulation und Kompensation thermooptischer Effekte in Optiken für Laseranwendungen; 1. Auflage

Dissertation, RWTH Aachen University, 2019; Aachen : Apprimus Verlag, Ergebnisse aus der Lasertechnik 1 Online-Ressource (ix, 128 Seiten) : Illustrationen, Diagramme (2019). = Dissertation, RWTH Aachen University, 2019
The universal use of the tool laser requires application-specific optical systems for beam guiding and shaping. These optics absorb a small proportion (typically < 0.1 %) of laser radiation causing an inhomogeneous heating of the optical components. Their optical properties change since the refractive index depends on the local temperature and thermal deformation occurs. The primary effect is a focus shift. For designing thermally stable optical systems, a multi-physical simulation is needed. In this thesis, thermo-optical effects are analyzed in three areas of application, namely Selective Laser Melting (SLM), the usage of plastic optics, and optics for High Harmonic Generation (HHG). For this purpose, simulations and experiments are performed, and approaches for compensation strategies are discussed. When performing additive manufacturing by SLM, also known as Laser Powder Bed Fusion (LPBF), thermo-optical effects occur quite frequently. This is particularly true if the protective window, which separates the optical unit and the processing chamber, gets contaminated by smoke and splashes emerging during the process. Furthermore, there are no commercial devices available for measuring the transient, angle-dependent focus shift of a SLM unit. To close this gap, transient simulations are performed, and thermo-optical effects are analyzed systematically. The focus shift is simulated for different exposure and scanning strategies, and the results are compared. The lateral focus shift usually varies across the working plane and can induce a shrinkage as well as a displacement of the manufactured structures. Up to now, plastic optics have not been utilized for focusing laser radiation because of significant thermo-optical effects. Since thermoplastics feature similar optical and mechanical properties, a compensation of thermally-induced effects cannot be obtained by combining different plastics. In this thesis, the following approach is taken: The thermo-optical effects are compensated for by designing a lens shape that is adapted to the operating point, i. e. laser power, beam diameter, and beam profile. Simulations predict that the thermally-induced focus shift of a PMMA-lens at 10 W laser power can be fully offset by applying this approach, and that a diffraction-limited focusing can be achieved. Today, the time-averaged output power of ultrashort pulse laser sources exceeds 100 W. Due to the large bandwidth, optics exclusively made of fused silica cannot be utilized, because dispersion would lead to an increased pulse duration. In order to make the optics both achromatic and athermal, a novel simulation approach is developed. For this purpose, a thermomechanical finite element analysis and an optical analysis, which is based on ray-optical and wave-optical methods, are coupled. Hence, the pulse shape in the focal region can be computed both in the spatial domain and in the time domain. By applying this novel simulation approach, an achromatic doublet made of calcium fluoride and fused silica is designed and analyzed. This doublet is intended to be used in the field of High Harmonic Generation.
Published by Apprimus Verlag, Aachen