Browsing by Author "Förster, Daniel J."

Now showing 1 - 6 of 6

Open Access
Analytical model for the depth progress of percussion drilling with ultrashort laser pulses
(2021) Holder, Daniel; Weber, Rudolf; Graf, Thomas; Onuseit, Volkher; Brinkmeier, David; Förster, Daniel J.; Feuer, Anne
A simplified analytical model is presented that predicts the depth progress during and the final hole depth obtained by laser percussion drilling in metals with ultrashort laser pulses. The model is based on the assumption that drilled microholes exhibit a conical shape and that the absorbed fluence linearly increases with the depth of the hole. The depth progress is calculated recursively based on the depth changes induced by the successive pulses. The experimental validation confirms the model and its assumptions for percussion drilling in stainless steel with picosecond pulses and different pulse energies.
Open Access
Determination of the thermally induced focal shift of processing optics for ultrafast lasers with average powers of up to 525 W
(2018) Faas, Sebastian; Förster, Daniel J.; Weber, Rudolf; Graf, Thomas
The continuous increase of the average laser power of ultrafast lasers is a challenge with respect to the thermal load of the processing optics. The power which is absorbed in an optical element leads to a temperature increase, temperature gradients, changing refractive index and shape, and finally causes distortions of the transmitted beam. In a first-order approximation this results in a change of the focal position, which may lead to an uncontrolled change of the laser machining process. The present study reports on investigations on the focal shift induced in thin plano-convex lenses by a high-power ultra-short pulsed laser with an average laser power of up to 525 W. The focal shift was determined for lenses made of different materials (N-BK7, fused silica) and with different coatings (un-coated, broadband coating, specific wavelength coating).
Open Access
Estimation of the depth limit for percussion drilling with picosecond laser pulses
(2018) Förster, Daniel J.; Weber, Rudolf; Holder, Daniel; Graf, Thomas
Open Access
Process limits for percussion drilling of stainless steel with ultrashort laser pulses at high average powers
(2022) Brinkmeier, David; Holder, Daniel; Loescher, André; Röcker, Christoph; Förster, Daniel J.; Onuseit, Volkher; Weber, Rudolf; Abdou Ahmed, Marwan; Graf, Thomas
The availability of commercial ultrafast lasers reaching into the kW power level offers promising potential for high-volume manufacturing applications. Exploiting the available average power is challenging due to process limits imposed by particle shielding, ambient atmosphere breakdown, and heat accumulation effects. We experimentally confirm the validity of a simple thermal model, which can be used for the estimation of a critical heat accumulation threshold for percussion drilling of AISI 304 steel. The limits are summarized in a processing map, which provides selection criteria for process parameters and suitable lasers. The results emphasize the need for process parallelization.
Open Access
Review on experimental and theoretical investigations of ultra-short pulsed laser ablation of metals with burst pulses
(2021) Förster, Daniel J.; Jäggi, Beat; Michalowski, Andreas; Neuenschwander, Beat
Laser processing with ultra-short double pulses has gained attraction since the beginning of the 2000s. In the last decade, pulse bursts consisting of multiple pulses with a delay of several 10 ns and less found their way into the area of micromachining of metals, opening up completely new process regimes and allowing an increase in the structuring rates and surface quality of machined samples. Several physical effects such as shielding or re-deposition of material have led to a new understanding of the related machining strategies and processing regimes. Results of both experimental and numerical investigations are placed into context for different time scales during laser processing. This review is dedicated to the fundamental physical phenomena taking place during burst processing and their respective effects on machining results of metals in the ultra-short pulse regime for delays ranging from several 100 fs to several microseconds. Furthermore, technical applications based on these effects are reviewed.
Open Access
Validation of the software package IMD for molecular dynamics simulations of laser induced ablation for micro propulsion
(2013) Förster, Daniel J.
Das Konzept für einen Mikroantrieb auf Basis von Laserablation (MICROLAS) des Instituts für Technische Physik (ITP) des DLR Stuttgart stellt eine mögliche Alternative zu bestehenden Konzepten im nN - μN-Bereich dar. Es basiert auf Laserablation, also dem Materialabtrag bei Bestrahlung eines Targets mit einem gepulsten Laser. Die Möglichkeit zur genauen Regelung der eingetragenen Energie durch den Laser sowie dessen Repetitionsrate sollte zu regelbarem Materialabtrag führen, der in einem weiten Bereich von genau einstellbaren Impulsbits resultieren kann. Die Genauigkeit, technische Machbarkeit sowie Reproduzierbarkeit wird und wurde in einer Reihe von Studien am ITP geprüft. In der vorliegenden Arbeit wird das Programm IMD des Instituts für Theoretische und Angewandte Physik (ITAP) der Universität Stuttgart zur Simulation des Materialabtrages diskutiert. Durch Skripte werden aus den Daten des Programms wichtige Parameter extrahiert und für ein weiteres Programm bereitgestellt, das die zeitliche Entwicklung des Ablationsjets simulieren soll (PICLas Code). Die Ergebnisse der durchgeführten Simulationen werden mit solchen eines hydro-dynamischen Simulationsprogramms (VLL) sowie mit experimentellen Daten verglichen.