Multiphysikalische Simulation und Kompensation thermooptischer Effekte in Optiken für Laseranwendungen

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

Utilizing The Transparency Of Semiconductors Via Backside Machining With A Nanosecond 2 Μm Tm:Fiber Laser

Semiconductors such as Si and GaAs are transparent to infrared laser radiation with wavelengths \u3e1.2 μm. Focusing laser light at the back surface of a semiconductor wafer enables a novel processing regime that utilizes this transparency. However, in previous experiments with ultrashort laser pulses we have found that nonlinear absorption makes it impossible to achieve sufficient optical intensity to induce material modification far below the front surface. Using a recently developed Tm:fiber laser system producing pulses as short as 7 ns with peak powers exceeding 100 kW, we have demonstrated it is possible to ablate the backside surface of 500-600 μm thick Si and GaAs wafers. We studied laser-induced morphology changes at front and back surfaces of wafers and obtained modification thresholds for multipulse irradiation and surface processing in trenches. A significantly higher back surface modification threshold in Si compared to front surface is possibly attributed to nonlinear absorption and light propagation effects. This unique processing regime has the potential to enable novel applications such as semiconductor welding for microelectronics, photovoltaic, and consumer electronics. © 2014 SPIE

Theoretical and experimental analysis of scan angle-depending pulse front tilt in optical systems for laser scanners

For realising fast and highly dynamical laser-based material processing, scanner systems are already utilised for many different industrial applications. Furthermore, ultra-short pulsed (<1 ps) laser sources provide possibilities of processing most different materials with highest accuracy. Owing to the large spectral bandwidth of ultra-short laser pulses, dispersion in optical components becomes relevant. The dispersion in optical systems for laser scanners may lead to scan angle-depending pulse properties as, for example, pulse front tilt. The investigation of these effects is not state of the art today but absolutely necessary to exploit the full potential of laser scanners for ultra-short pulse applications. By means of an exemplary focusing lens, the simulation and experimental analysis of scan angle-depending pulse front tilt is presented for the first time

Post-Processing Of 3D-Printed Parts Using Femtosecond And Picosecond Laser Radiation

Additive manufacturing, also known as 3D-printing, is a near-net shape manufacturing approach, delivering part geometry that can be considerably affected by various process conditions, heat-induced distortions, solidified melt droplets, partially fused powders, and surface modifications induced by the manufacturing tool motion and processing strategy. High-repetition rate femtosecond and picosecond laser radiation was utilized to improve surface quality of metal parts manufactured by laser additive techniques. Different laser scanning approaches were utilized to increase the ablation efficiency and to reduce the surface roughness while preserving the initial part geometry. We studied post-processing of 3D-shaped parts made of Nickel- and Titanium-base alloys by utilizing Selective Laser Melting (SLM) and Laser Metal Deposition (LMD) as additive manufacturing techniques. Process parameters such as the pulse energy, the number of layers and their spatial separation were varied. Surface processing in several layers was necessary to remove the excessive material, such as individual powder particles, and to reduce the average surface roughness from asdeposited 22-45 μm to a few microns. Due to the ultrafast laser-processing regime and the small heat-affected zone induced in materials, this novel integrated manufacturing approach can be used to post-process parts made of thermally and mechanically sensitive materials, and to attain complex designed shapes with micrometer precision. © 2014 SPIE

Femtosecond Laser Post-Processing of Metal Parts Produced by Laser Additive Manufacturing

High-repetition rate femtosecond laser radiation was utilized to improve surface quality of metal parts manufactured by laser additive techniques. This novel approach can be used to postprocess parts made of heat-sensitive materials, and to attain the designed net shape with micrometer precision

Femtosecond Laser Post-Processing Of Metal Parts Produced By Laser Additive Manufacturing

High-repetition rate femtosecond laser radiation was utilized to improve surface quality of metal parts manufactured by laser additive techniques. This novel approach can be used to postprocess parts made of heat-sensitive materials, and to attain the designed net shape with micrometer precision. © Owned by the authors, published by EDP Sciences, 2013

Ultrafast Laser-Based Post-Processing Of Parts Produced By Additive Manufacturing

Laser Additive Manufacturing (LAM) is a near-net shape manufacturing approach, meaning that the resulting part geometry can be considerably affected by heat-induced distortions, solidified melt droplets, partially fused powders, and surface modifications induced by the laser tool motion and processing strategy. High-repetition rate femtosecond laser radiation was utilized to improve surface quality of metal parts manufactured by laser additive techniques. Different laser scanning approaches were utilized to increase the ablation efficiency and to improve the surface finish. Processing of 3D-shaped parts made of Ni- and Ti-base superalloys resulted in the reduction of the average surface roughness to a few microns. This approach can be used to post-process parts made of thermally and mechanically sensitive materials, and to attain complex designed shapes with micrometer precision

Trans-Wafer Processing Of Semiconductors With Nanosecond Mid-Ir Laser Radiation

In recent years, a major push was made for the use of novel laser sources in the processing of semiconductors and other materials used in photovoltaic and IC applications. In addition to a large number of highly automated laser processes already adopted by the industry, more laser-based processing approaches are being developed to improve performance and reduce manufacturing costs. Common semiconductors are transparent in the infrared spectral region. Therefore laser sources operating at mid-IR wavelengths can be successfully utilized to induce material modifications in semiconductor wafers even beyond the laser-incident surface. We present our initial studies of this processing regime utilizing a self-developed nanosecond-pulsed thulium fiber laser emitting at the wavelength 2 μm. Our experimental approach confirmed that morphology changes could be induced not only at the front (laser-incident) surface of the wafer, but also independently at the back surface. We investigated the influence of process parameters, such as the incident pulse energy, duration and focusing conditions, on the induced surface morphology. In addition, we studied experimental routes to a number of potential applications of this processing regime, such as the PV cell edge isolation and the wafer scribing