Browsing by Author "Graf, Thomas"

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Open Access
Adjustment of the geometries of the cutting front and the kerf by means of beam shaping to maximize the speed of laser cutting
(2023) Lind, Jannik; Hagenlocher, Christian; Weckenmann, Niklas; Blazquez-Sanchez, David; Weber, Rudolf; Graf, Thomas
The shape of the laser beam used for fusion cutting significantly influences the geometry of both the cutting front and the cutting kerf. The angle of the cutting front in turn impacts the local absorptivity, while the width of the kerf defines the amount of material, which has to be molten. The kerf’s geometry therefore determines the maximum possible cutting speed at which a successful cut is feasible with a given available laser power. The absorptivity, the width of the kerf, and the maximum possible cutting speed can be estimated from a simple model considering the conservation of energy and rough geometrical approximations. In order to verify the prediction of the model, the geometry of the cutting front and kerf resulting from different processing conditions was observed by means of online high-speed X-ray diagnostics. The geometry of the interaction zone was recorded with a framerate of 1000 Hz during fusion cutting of 10-mm-thick samples of stainless steel. Comparing the results obtained with different shapes of the laser beam, it was found that the absorptivity is increased when the beam’s longitudinal cross-section (parallel to the feed) is enlarged. Reducing the width of the beam in the transversal direction normal to the feed reduces the cross-sectional area of the cutting kerf. The findings show a good agreement with the geometric model which enabled the prediction of the absorptivity and the cross-sectional area of the cutting kerf and hence allows to reliably estimate the maximum cutting speed for different shapes of the laser beam, laser power, and sheet thicknesses.
Open Access
Amplification of radially polarized ultra-short pulsed radiation to average output powers exceeding 250 W in a compact single-stage Yb:YAG single-crystal fiber amplifier
(2020) Beirow, Frieder; Eckerle, Michael; Graf, Thomas; Ahmed, Marwan Abdou
A simple and compact single-stage Yb:YAG single-crystal fiber amplifier was setup to amplify 784 fs long seed pulses to an output energy of 6 μJ and an average output power of 290 W. The experimental results are verified by numerical models to estimate the limitations of the SCF technology with regards to beam quality and average output power.
Open Access
Analysis of laser-induced damage threshold of circular grating waveguide structures exposed to sub-picosecond laser radiation centered at a wavelength of 1030 nm
(2025) Didychenko, Denys; Li, Fangfang; Savchenko, Anton; Zideluns, Janis; Stehlik, Marek; Gallais, Laurent; Lumeau, Julien; Lemarchand, Fabien; Karvinen, Petri; Kuittinen, Markku; Pruss, Christof; Jamon, Damien; Graf, Thomas; Abdou Ahmed, Marwan
The laser-induced damage threshold of a grating waveguide output coupler (GWOC) exposed to laser radiation at a wavelength of 1030 nm and with a pulse duration of 500 fs was investigated. The GWOC is a combination of a sub-wavelength circular grating and a partial reflector based on a Nb2O5 and SiO2 multilayer sequence. It was designed to be used as an output coupler of a thin-disk laser cavity for the generation of beams with radial polarization. The results revealed a laser-induced damage threshold (LIDT) fluence of 0.36 J/cm² for single-pulse tests and 0.26 J/cm² for multiple-pulse conditions with up to 1000 shots. These threshold values are comparable to those of an unstructured output coupler with Nb2O5 and SiO2 coating layers, highlighting the minor influence of the grating on the LIDT.
Open Access
Analytical description of the criterion for the columnar-to-equiaxed transition during laser beam welding of aluminum alloys
(2021) Böhm, Constantin; Hagenlocher, Christian; Wagner, Jonas; Graf, Thomas; Weihe, Stefan
Open Access
Analytical model for the depth progress during laser micromachining of V-shaped grooves
(2022) Holder, Daniel; Weber, Rudolf; Graf, Thomas
An analytical model is presented that allows predicting the progress and the final depth obtained by laser micromachining of grooves in metals with ultrashort laser pulses. The model assumes that micromachined grooves feature a V-shaped geometry and that the fluence absorbed along the walls is distributed with a linear increase from the edge to the tip of the groove. The depth progress of the processed groove is recursively calculated based on the depth increments induced by successive scans of the laser beam along the groove. The experimental validation confirms the model and its assumptions for micromachining of grooves in a Ti-alloy with femtosecond pulses and different pulse energies, repetition rates, scanning speeds and number of scans.
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
Avoiding the formation of pores during laser welding of copper hairpins by dynamic beam shaping
(2025) Reinheimer, Eveline Nicole; Haas, Michael; Zaiß, Felix; Omlor, Markus; Spurk, Christoph; Olowinsky, Alexander; Beckmann, Felix; Moosmann, Julian; Hagenlocher, Christian; Weber, Rudolf; Graf, Thomas
Any pores formed during laser welding of copper hairpins affect the structural integrity and the mechanical/electrical functionality of the joint. We report on investigations using synchrotron X-ray imaging techniques to observe the formation of the pores in the processing zone while welding. It was found that all pores are formed at the joint gap and that the small pores are distributed throughout the complete joint volume as a result of the melt flow induced by the movement of the laser beam. This insight has led to the development of a welding strategy that minimizes pore formation by avoiding the movement of the laser beam across the joint gap. This was achieved by rapid beam shaping based on coherent beam combining (CBC) technology.
Open Access
Bending of Lloyd’s mirror to eliminate the period chirp in the fabrication of diffraction gratings
(2024) Bienert, Florian; Röcker, Christoph; Graf, Thomas; Abdou Ahmed, Marwan
Open Access
The change of the absorptance at the transition from partial- to full-penetration laser welding
(2024) Wagner, Jonas; Hagenlocher, Christian; Weber, Rudolf; Graf, Thomas
Full-penetration laser welding processes are necessarily associated with significant changes of the geometrical properties of the keyhole at the beginning of the process when the keyhole expands all the way through the workpiece and finally pierces the bottom of the sheet. The impact that this transition has on the absorptance was investigated by means of X-ray imaging to determine the geometry of the keyhole and subsequent raytracing to calculate the distribution of the absorbed irradiance. The results show a significant drop of the overall absorptance when the bottom of the capillary opens through the rear side of the workpiece which in practice is noticed by an unstable behavior of the keyhole. Since the drop of the absorptance is less pronounced for smaller diameters of the keyhole, one may recommend the application of laser beams with small diameters at least during the initial phase until the keyhole is fully developed and reliably reaches through the bottom surface of the welded sheet.
Open Access
Data-driven prediction and uncertainty quantification of process parameters for directed energy deposition
(2023) Hermann, Florian; Michalowski, Andreas; Brünnette, Tim; Reimann, Peter; Vogt, Sabrina; Graf, Thomas
Laser-based directed energy deposition using metal powder (DED-LB/M) offers great potential for a flexible production mainly defined by software. To exploit this potential, knowledge of the process parameters required to achieve a specific track geometry is essential. Existing analytical, numerical, and machine-learning approaches, however, are not yet able to predict the process parameters in a satisfactory way. A trial-&-error approach is therefore usually applied to find the best process parameters. This paper presents a novel user-centric decision-making workflow, in which several combinations of process parameters that are most likely to yield the desired track geometry are proposed to the user. For this purpose, a Gaussian Process Regression (GPR) model, which has the advantage of including uncertainty quantification (UQ), was trained with experimental data to predict the geometry of single DED tracks based on the process parameters. The inherent UQ of the GPR together with the expert knowledge of the user can subsequently be leveraged for the inverse question of finding the best sets of process parameters by minimizing the expected squared deviation between target and actual track geometry. The GPR was trained and validated with a total of 379 cross sections of single tracks and the benefit of the workflow is demonstrated by two exemplary use cases.
Open Access
Design of convex-shaped transparent heat spreaders for symmetrical cooling of a thin-disk laser crystal by mechanical pressing
(2024) Wolter, Jan-Hinnerk; Voss, Andreas; Graf, Thomas; Abdou Ahmed, Marwan
We present the design for a double-sided cooling of a thin-disk laser crystal using transparent, convex-shaped heat spreaders. The thermal contact between laser crystal and heat spreaders is established without bonding by solely applying an axial compressive force. A model of the contact behavior taking into account the deformation of the surfaces of the heat spreaders due to bending under mechanical loading is presented. The scope and accuracy of the modeling is verified by numerical simulations using the finite-element method. Based on the modeling, an exemplary design of the heat spreader which has been used for the first experimental demonstration of this concept is carried out. Based on the model, an almost ideal scalability of the heat transfer coefficient is predicted.
Open Access
Design, fabrication, and characterization of dual-wavelength inhibited-coupling guiding hollow-core fibers
(2025) Chen, Bowen; Kühlthau, Tim; Kleem, Götz; Graf, Thomas; Abdou Ahmed, Marwan
The present paper reports on the design, fabrication, and characterization of an 8-tube inhibited-coupling guiding hollow-core photonic crystal fiber (IC-HCPCF) capable of guiding both the beam emitted from an Yb:YAG laser at the fundamental wavelength of λ=1030nmand its second harmonic at λ=515nm. By controlling the strut thickness of the glass capillaries to approximately 362nm, the transmission of laser radiation at both wavelengths was possible with low losses. Optimizing the outer diameter of the glass capillaries mitigates the bending-induced increase of the confinement loss at the wavelength of 515nmwithout compromising the optical performance of the fiber at the wavelength of 1030nm. Experimental results confirm the near to diffraction-limited beam quality M2<1.15of the laser beams exiting the fiber at both operational wavelengths. Operating in the first transmission band at the wavelength of 1030nm, the calculated chromatic dispersion is 1.02ps/(nm∙km), despite a diameter of the hollow core of 40μm. At the wavelength of 515nmthis value amounts to 0.62ps/(nm∙km). The measured losses are 27.5±0.3dB/kmat the wavelength of 515nmand 25.7±0.7dB/kmat the wavelength of 1030nm, which is comparable to the loss of state-of-the-art IC-HCPCFs with tubular cladding structures. The measured bending-induced increase of the confinement losses confirms the potential of the proposed approach for flexible, low-loss guiding of ultrashort laser pulses at the two wavelengths using a single fiber. This gained flexibility can significantly enhance the options for wavelength selection in laser material processing applications.
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
Detrimental effects of period-chirped gratings in pulse compressors
(2023) Bienert, Florian; Röcker, Christoph; Dietrich, Tom; Graf, Thomas; Abdou Ahmed, Marwan
Open Access
Dry metal forming using volatile lubricants injected into the forming tool through flow-optimized, laser-drilled microholes
(2020) Henn, Manuel; Reichardt, Gerd; Weber, Rudolf; Graf, Thomas; Liewald, Mathias
A novel tribologic system was developed in which volatile lubricants (carbon dioxide-CO2 or nitrogen-N2) were used as a substitute for mineral oil-based lubricants in deep drawing processes. This process allows an intermediate medium to be introduced into the tool contact surfaces under high pressure by flow-optimized, laser-drilled microholes. This eliminates the need for subsequent cost-intensive cleaning processes as volatile lubricants evaporate while expanding to ambient pressure without leaving any residue. This article gives an overview of the current findings to enable and characterize the novel tribologic system. The areas of microhole laser drilling by ultrashort pulsed laser radiation, characterization of the novel tribologic system and realization of the system using a prototype tool will be described.
Open Access
Efficient delivery of sub-ps laser pulses with pulse energies exceeding 100 µJ at average powers of up to 95 W through a hollow-core fiber
(2025) Kühlthau, Tim; Chen, Bowen; Graf, Thomas; Abdou Ahmed, Marwan
We report on the efficient delivery of sub-picosecond laser pulses with high peak and high average powers through a home-made 7.5 m long tubular inhibited-coupling guiding hollow-core photonic-crystal fiber (IC-HCPCF). The experiments were performed using an ultrafast laser generating pulses with durations between 430 fs and 560 fs at a central wavelength of 1030 nm. Pulses with an energy of either 263 µJ or 150 µJ were coupled into the fiber at average powers of either 48 W or 95 W, respectively. In both cases, the transmittance through the fiber was measured to exceed 85% without detectable distortion of the temporal pulse shapes or damage to the fiber.
Open Access
Enhancing heat transfer at low temperatures by laser functionalization of the inner surface of metal pipes
(2024) Holder, Daniel; Peter, Alexander; Kirsch, Marc; Cáceres, Sergio; Weber, Rudolf; Onuseit, Volkher; Kulenovic, Rudi; Starflinger, Jörg; Graf, Thomas
The latent heat transfer during vapour condensation in the condenser section of passive heat transport devices such as the two-phase closed thermosiphon is limited by film condensation. Dropwise condensation provides an increase of the heat transfer coefficient by up to one order of magnitude and can be achieved with a water-repellant surface. The inner surface of pipes made from stainless steel was functionalized by laser surface texturing with ultrashort laser pulses and subsequent storage in a liquid containing long-chained hydrocarbons. The pipes were separated into half-pipes by wire eroding to enable laser texturing of the inner surface, and were then joined by electron beam welding after laser texturing. As a result, superhydrophobic and water-repellent surfaces with a contact angle of 153° were obtained on the inner surface of the pipes with a length of up to 1 m. The functionalized pipes were used in the condenser section of a two-phase closed thermosiphon to demonstrate a heat transfer rate of 0.92 kW at 45 °C, which is approximately three times the heat transfer rate of 0.31 kW of a smooth reference pipe.
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
Experimental analysis on CPA-free thin-disk multipass amplifiers operated in a helium-rich atmosphere
(2022) Bienert, Florian; Loescher, André; Röcker, Christoph; Graf, Thomas; Abou Ahmed, Marwan
Es wird der Einfluss von Helium als atmosphärisches Gas in Scheibenlaser-multipass-Ultrakurzpulsverstärkern untersucht.
Open Access
Feasibility assessment of parallelized helical drilling
(2023) Brinkmeier, David; Onuseit, Volkher; Graf, Thomas