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Author: search.filters.author.Graf, ThomasSubject: search.filters.subject.620

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Now showing 1 - 10 of 28
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    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).
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    Estimation of the depth limit for percussion drilling with picosecond laser pulses
    (2018) Förster, Daniel J.; Weber, Rudolf; Holder, Daniel; Graf, Thomas
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    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.
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    Heat accumulation during pulsed laser materials processing
    (2014) Weber, Rudolf; Graf, Thomas; Berger, Peter; Onuseit, Volkher; Wiedenmann, Margit; Freitag, Chistian; Feuer, Anne
    Laser materials processing with ultra-short pulses allows very precise and high quality results with a minimum extent of the thermally affected zone. However, with increasing average laser power and repetition rates the so-called heat accumulation effect becomes a considerable issue.
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    Intra-cavity wavelength multiplexing of high-brightness thin-disk laser beams
    (2022) Boubekraoui, Ayoub; Beirow, Frieder; Graf, Thomas; Abdou Ahmed, Marwan
    We report on the first demonstration of an intra-cavity spectral beam combining of two fundamental-mode laser beams generated by a dual Yb:YAG thin-disk resonator. The two thin-disk lasers (TDLs) were operated at the two slightly different wavelengths of 1028 nm and 1032 nm. A resonant diffraction grating waveguide structure was used as common spectral stabilizer and combiner. An average power exceeding 200 W with close to diffraction-limited beam quality (M2<1.3) was obtained with the presented approach.
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    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.
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    High-quality percussion drilling with ultrashort laser pulses
    (2021) Feuer, Anne; Weber, Rudolf; Feuer, R.; Brinkmeier, David; Graf, Thomas
    The influence of the laser fluence on the quality of percussion-drilled holes was investigated both experimentally and by an analytical model. The study reveals that the edge quality of the drilled microholes depends on the laser fluence reaching the rear exit of the hole and changes with the number of pulses applied after breakthrough. The minimum fluence that must reach the hole’s exit in order to obtain high-quality microholes in stainless steel was experimentally found to be 2.8 times the ablation threshold.
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    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.
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    Modelling of natural convection in thin-disk lasers
    (2020) Dietrich, Tom; Röcker, Christoph; Graf, Thomas; Abdou Ahmed, Marwan
    In this paper, we present a FEM-model that can be used to investigate the effects of thermally induced natural convection at the thin-disk laser crystal. Based on this simulation, we calculated the distribution of the refractive index of the ambient gas for the case of air and helium. By evaluating the optical path difference of a beam at normal incidence, the angular tilt (gas wedge) in the plane of the direction of convection as well as the spherical contribution (gas lens) was calculated for a set of different pump spot geometries and temperatures of the pumped area on the surface of the laser disk. Equations were derived that allow to simply calculate the tilt angle and the focal length of the gas lens for different temperatures of the disk and pump spot diameters for air as ambient medium.