TY - CHAP A1 - Schwarz, Simon A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - CO2 laser polishing of femtosecond shaped micro-optical components T2 - 3rd Conference on Laser Polishing LaP 2018 KW - Kohlendioxidlaser Y1 - 2018 ER - TY - CHAP A1 - Schwarz, Simon A1 - Rung, Stefan A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Axicon fabrication with ultrashort and CO2 laser T2 - LPM2018 The 19th International Symposium on Laser Precision Microfabrication KW - Ultrakurzzeitlaser KW - Kohlendioxidlaser Y1 - 2018 ER - TY - CHAP A1 - Schwarz, Simon A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Homogeneous low spatial frequency LIPSS on dielectrics generated by beam-shaped femtosecond pulsed laser irradiation T2 - LPM2018 The 19th International Symposium on Laser Precision Microfabrication KW - Femtosekundenlaser Y1 - 2018 ER - TY - CHAP A1 - Schwarz, Simon A1 - Rung, Stefan A1 - Hellmann, Ralf T1 - High quality one-dimensional low spatial frequency LIPSS on sapphire generated by Top-Hat beam shaped ultrashort-pulsed laser T2 - MultiScience - XXXI. microCAD International Multidisciplinary Scientific Conference KW - Ultrakurzzeitlaser Y1 - 2017 ER - TY - CHAP A1 - Schwarz, Simon A1 - Rung, Stefan A1 - Hellmann, Ralf T1 - Simulation of process parameters towards the generation of low spatial frequency LIPSS on dielectric materials T2 - MultiScience - XXX. microCAD International Multidisciplinary Scientific Conference KW - Laserbestrahlung KW - Quarzglas Y1 - 2016 ER - TY - CHAP A1 - Dudutis, Juozas A1 - Pipiras, Jokūbas A1 - Schwarz, Simon A1 - Rung, Stefan A1 - Hellmann, Ralf A1 - Gečys, Paulius T1 - Bessel beam formation with all laser fabricated axicon - case of glass dicing applications T2 - COLA 2019, 15th International Conference on Laser Ablation KW - Laserbestrahlung Y1 - 2019 ER - TY - CHAP A1 - Dudutis, Juozas A1 - Pipiras, Jokūbas A1 - Schwarz, Simon A1 - Rung, Stefan A1 - Hellmann, Ralf A1 - Račiukaitis, Gediminas A1 - Gečys, Paulius T1 - Glass cutting by oblate-tip axicon-generated Bessel-Gaussian beams T2 - Open Readings, 63rd International Conference for Students of Physics and Natural Sciences KW - Laserschneiden KW - Glas Y1 - 2019 ER - TY - JOUR A1 - Lutz, Christian A1 - Roth, Gian-Luca A1 - Rung, Stefan A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Efficient Ultrashort Pulsed Laser Processing by Dynamic Spatial Light Modulator Beam Shaping for Industrial Use JF - Journal of Laser Micro/Nanoengineering N2 - We report on the effect of different transversal beam shapes on the efficiency of ablation processes and the resulting surface characteristics. A possibility to efficiently apply ultrashort pulsed lasers with high average power is beam shaping. By using a cooled reflective based liquid crystal spatial light modulator to generate different spot distributions, it is possible to spatially allocate the available power to avoid excessive high fluences. In our experiments, we determine the optimal fluence to ablate the maximum volume per watt to be in the range of 0.2-0.4 J/cm². Based on this fluence, we increase the number of spots from one to a maximum of twenty to ablate steel in a multilayer scanning-based process. In this context, we examine the influence of different separation distances between the spots on the ablation efficiency and roughness. Subsequently to these investigations, we develop an efficient roughing process with higher ablation rates and a nearly constant roughness. KW - Ultrakurzzeitlaser KW - Lichtmodulator Y1 - 2021 U6 - https://doi.org/10.2961/jlmn.2021.01.2011 VL - 16 IS - 1 SP - 1 EP - 6 ER - TY - JOUR A1 - Roth, Gian-Luca A1 - Kefer, Stefan A1 - Hessler, Steffen A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Integration of Microfluidic and Photonic Components within Transparent Cyclic Olefin Copolymers by Using fs Laser JF - Journal of Laser Micro/Nanoengineering N2 - In this study, we report on the integration of microfluidic channels and optical components by focusing femtosecond laser radiation inside transparent cyclic olefin copolymer (COC) bulk material. An internal localized material modification is triggered based on nonlinear absorption of the laser radiation's high intensities inside the focal volume. The size and shape of the three-dimensional internal modification are controlled by using an adaptive beam shaping setup. The irradiated areas show a positive refractive index shift and can be used as Type I internal optical waveguides. Furthermore, precise control of the spatial pulse-to-pulse distance in combination with a suitable beam profile enables the integration of functional photonic elements, e.g., Bragg gratings, into the waveguide. Thus, it also enables the generation of integrated photonic sensors. In addition, internal fs laser-induced modifications are characterized by a lower thermal stability as compared to the pristine polymer material. By performing a post-annealing process step, internal hollow microstructures are created by gaseous degradation of the exposed areas. Circular microchannels can be generated in a deliberately chosen layout by employing motorized 3D stages. In comparison to etching-based fabrication methods the proposed technology facilitates unlimited channel lengths, as it omits restrictions arising from an etching selectivity and duration. Altogether, this contribution paves the way towards the fabrication of internal three-dimensional optofluidic devices, equipped with a photonic sensor. In contrast, microfluidic and photonic structures are both created by femtosecond laser direct writing inside of transparent polymers. KW - Femtosekundenlaser Y1 - 2021 U6 - https://doi.org/10.2961/jlmn.2021.01.2009 VL - 16 IS - 1 SP - 1 EP - 6 ER - TY - JOUR A1 - Kefer, Stefan A1 - Zettl, Julian A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Femtosecond Laser-Based Micromachining of Rotational-Symmetric Sapphire Workpieces JF - Materials N2 - Sapphire is a robust and wear-resistant material. However, efficient and high-quality micromachining is still a challenge. This contribution demonstrates and discusses two novels, previously unreported approaches for femtosecond laser-based micromachining of rotational-symmetric sapphire workpieces, whereas both methods are in principal hybrids of laser scanning and laser turning or laser lathe. The first process, a combination of a sequential linear hatch pattern in parallel to the workpiece’s main axis with a defined incremental workpiece rotation, enables the fabrication of sapphire fibers with diameters of 50 µm over a length of 4.5 mm. Furthermore, sapphire specimens with a diameter of 25 µm over a length of 2 mm can be fabricated whereas an arithmetical mean height, i.e., Sa parameter, of 281 nm is achieved. The second process combines a constant workpiece feed and orthogonal scanning with incremental workpiece rotation. With this approach, workpiece length limitations of the first process are overcome and sapphire fibers with an average diameter of 90 µm over a length of 20 cm are manufactured. Again, the sapphire specimen exhibits a comparable surface roughness with an average Sa value of 249 nm over 20 cm. Based on the obtained results, the proposed manufacturing method paves an innovative and flexible, all laser-based way towards the fabrication or microstructuring of sapphire optical devices, and thus, a promising alternative to chemical processes. KW - sapphire KW - femtosecond laser KW - micromachining KW - laser turning KW - ablation threshold KW - Femtosekundenlaser KW - Saphir Y1 - 2022 UR - https://www.mdpi.com/1996-1944/15/18/6233 U6 - https://doi.org/10.3390/ ma15186233 VL - 15 IS - 18 SP - 6233 EP - 6233 ER - TY - JOUR A1 - Zettl, Julian A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Fundamental Considerations and Analysis of the Energy Distribution in Laser Turning with Ultrashort Laser Pulses JF - mdpi Micromachines N2 - This article discusses the process of the laser turning of rotational symmetric, cylindrical components using ultrashort laser pulses with respect to the geometrical conditions and the resulting energy distribution during the laser turning process. As a result, process predictions and potential process optimizations are feasible. Particular attention is drawn to the laser spot formation on the cylindrical surface of the work piece in conjunction with the positioning of the laser beam relative to the rotation axis of the specimen. Based on fundamental calculations and experimental results, an optimum processing strategy is discussed, whereat the use of a trepanning optic in the laser turning process and the forming of a particular surface structure is additionally being issued. KW - ultrashort pulse laser KW - laser turning KW - tangential processing KW - trepanning optic KW - Ultrakurzzeitlaser Y1 - 2023 UR - https://www.mdpi.com/2072-666X/14/10/1838 U6 - https://doi.org/10.3390/mi14101838 VL - 14 IS - 10 SP - 1 EP - 15 ER - TY - JOUR A1 - Lutz, Christian A1 - Helm, Jonas A1 - Tschirpke, Katrin A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Drilling Sequence Optimization Using Evolutionary Algorithms to Reduce Heat Accumulation for Femtosecond Laser Drilling with Multi-Spot Beam Profiles JF - MDPI Materials N2 - We report on laser drilling borehole arrays using ultrashort pulsed lasers with a particular focus on reducing the inadvertent heat accumulation across the workpiece by optimizing the drilling sequence. For the optimization, evolutionary algorithms are used and their results are verified by thermal simulation using Comsol and experimentally evaluated using a thermal imaging camera. To enhance process efficiency in terms of boreholes drilled per second, multi-spot approaches are employed using a spatial light modulator. However, as higher temperatures occur across the workpiece when using simultaneous multi-spot drilling as compared to a single-spot process, a subtle spatial distribution and sequence of the multi-spot approach has to be selected in order to limit the resulting local heat input over the processing time. Different optimization approaches based on evolutionary algorithms aid to select those drilling sequences which allow for the combination of a high efficiency of multi-spot profiles, a low-generated process temperature and a high-component quality. In particular, using a 4 x 4 laser spot array allows for the drilling of 40,000 boreholes in less than 76 s (526 boreholes/s) with a reduced temperature increase by about 35%, as compared to a single spot process when employing an optimized drilling sequence. KW - USP laser processing KW - Laser drilling KW - Optimization of heat accumulation KW - Evolutionary Algorithms KW - Thermal simualtions KW - Laserbohren KW - Ultrakurzzeitlaser Y1 - 2023 UR - https://www.mdpi.com/1996-1944/16/17/5775 U6 - https://doi.org/10.3390/ma16175775 VL - 2023 IS - 16/5775 SP - 1 EP - 12 ER - TY - JOUR A1 - Sommer, David A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Static and Dynamic Mechanical Behaviour of Hybrid-PBF-LB/M-Built and Hot Isostatic Pressed Lattice Structures JF - Mechanical Properties of Polymeric, Metallic, and Composite Materials N2 - We report on a comprehensive study of the mechanical properties of maraging steel body-centred cubic lattice structures fabricated by a hybrid additive manufacturing technology that combines laser powder bed fusion with in situ high-speed milling. As the mechanical properties of additive manufactured components are inferior to, e.g., cast components, surface modifications can improve the mechanical behaviour. Different hybrid additive manufacturing technologies have been designed using additive and subtractive processes, improving process quality. Following this, mechanical testing is performed with respect to static tensile properties and dynamic stress, hardness, and porosity, comparing specimens manufactured by laser powder bed fusion only to those manufactured by the hybrid approach. In addition, the influence of different heat-treatment techniques on the mechanical behaviour of the lattice structures is investigated, namely solution and aging treatment as well as hot isostatic pressing. Thus, the influence of the superior surface quality due to the hybrid approach is evaluated, leading to, e.g., an offset of about 14–16% for the static testing of HIP lattice structures. Furthermore, the dynamic load behaviour can be improved with a finished surface, heading to a shift of the different zones of fatigue behaviour in the testing of hybrid-built specimens. KW - Hybrid additive manufacturing KW - Lattice structures KW - Hot isostatic pressing KW - Fatigue behaviour KW - Isostatisches Heißpressen KW - Selektives Laserschmelzen Y1 - 2023 UR - https://www.mdpi.com/1996-1944/16/9/3556 U6 - https://doi.org/https://doi.org/10.3390/ma16093556 VL - 16 IS - 9 SP - 1 EP - 14 ER - TY - JOUR A1 - Bischoff, Kay A1 - Mücke, Dominik A1 - Roth, Gian-Luca A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - UV-Femtosecond-Laser Structuring of Cyclic Olefin Copolymer JF - Polymers N2 - We report on the laser ablation of cyclic olefin copolymer using an amplified ultrashort pulsed laser in the ultraviolet spectral range. In addition to a high ablation depth per laser-structured layer up to 74 μm at a fluence of 22 J cm−2, an excellent mean roughness 𝑅𝑎 of laser-patterned surfaces down to 0.5 μm is demonstrated. Furthermore, with increasing fluence, increasing ablation efficiencies up to 2.5 mm3 W−1 min−1 are determined. Regarding the quality of the ablation, we observed steep ablation flanks and low debris formation, though for fluences above 10.5 J cm−2 the formation of troughs was observed, being attributed to multiple reflections on the ablation flanks. For comparison, laser ablation was performed under identical conditions with an infrared laser wavelength. The results highlight that UV ablation exhibits significant advantages in terms of ablation efficiency, surface roughness and quality. Moreover, our results show that a larger UV focus spot accelerates the ablation process with comparable quality, paving the way for high-power UV ultrashort pulsed lasers towards an efficient and qualitative tool for the laser machining of cyclic olefin copolymer. The production of complex microfluidics further underlines the suitability of this type of laser. KW - Femtosekundenlaser Y1 - 2022 U6 - https://doi.org/https://doi.org/10.3390/polym14142962 VL - 14 IS - 14 SP - 2962 EP - 2962 ER - TY - JOUR A1 - Bischoff, Kay A1 - Kefer, Stefan A1 - Wienke, Alexander A1 - Overmeyer, Ludger A1 - Kaierle, Stefan A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Integration of Bragg gratings in aerosol-jetted polymer optical waveguides for strain monitoring capabilities JF - Optics Letters N2 - We demonstrate and discuss the integration of Bragg gratings in aerosol-jetted polymer optical waveguides, produced in the optical assembly and connection technology for component-integrated bus systems (OPTAVER) process. By using a femtosecond laser and adaptive beam shaping, an elliptical focal voxel generates different types of single pulse modification by nonlinear absorption in the waveguide material, which are arranged periodically to form Bragg gratings. Integration of a single grating structure or, alternatively, an array of Bragg grating structures in the multimode waveguide yields a pronounced reflection signal with typical multimodal properties, i.e., a number of reflection peaks with non-Gaussian shapes. However, the main wavelength of reflection, located around 1555 nm, is evaluable by means of an appropriate smoothing algorithm. When loaded by mechanical bending, a pronounced Bragg wavelength shift of this reflected peak up to 160 pm is detected. This demonstrates that the additively manufactured waveguides can be used not only for signal transmission but also as a sensor. KW - Bragg-Reflektor KW - Lichtwellenleiter Y1 - 2023 U6 - https://doi.org/https://doi.org/10.1364/OL.481801 VL - 48 IS - 7 SP - 1778 EP - 1781 ER - TY - JOUR A1 - Bischoff, Kay A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Preparation of Dispersed Copper(II) Oxide Nanosuspensions as Precursor for Femtosecond Reductive Laser Sintering by High-Energy Ball Milling JF - Nanomaterials N2 - This contribution demonstrates and discusses the preparation of finely dispersed copper(II) oxide nanosuspensions as precursors for reductive laser sintering (RLS). Since the presence of agglomerates interferes with the various RLS sub-processes, fine dispersion is required, and oversized particles must be identified by a measurement methodology. Aside from the established method of scanning electron microscopy for imaging individual dried particles, this work applies the holistic and statistically more significant laser diffraction in combination with dynamic image analysis in wet dispersion. In addition to direct ultrasonic homogenization, high-energy ball milling is introduced for RLS, to produce stable nanosuspensions with a high fine fraction, and, above all, the absence of oversize particles. Whereas ultrasonic dispersion stagnates at particle sizes between 500 nm and 20 μm, even after 8 h, milled suspension contains a high proportion of finest particles with diameters below 100 nm, no agglomerates larger than 1 μm and a trimodal particle size distribution with the median at 50 nm already, after 100 min of milling. The precursor layers produced by doctor blade coating are examined for their quality by laser scanning microscopy. The surface roughness of such a dry film can be reduced from 1.26 μm to 88 nm by milling. Finally, the novel precursor is used for femtosecond RLS, to produce homogeneous, high-quality copper layers with a sheet resistance of 0.28 Ω/sq and a copper mass concentration of 94.2%. KW - Laserstrahlsintern KW - Femtosekundenlaser Y1 - 2023 U6 - https://doi.org/https://doi.org/10.3390/nano13192693 VL - 13 IS - 19 SP - 2693 EP - 2693 ER - TY - CHAP A1 - Bischoff, Kay A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Acceleration of Femtosecond Reductive Laser Sintering of Copper(II) Oxide for Conductive Copper Patterns on Cyclic Olefin Copolymers T2 - International Congress on Applications of Lasers & Electro-Optics (ICALEO) 2023 N2 - We report on the acceleration of femtosecond reductive laser sintering of copper (II) oxide on the surface of transparent cyclic olefin copolymers (COC) by varying the focus size to generate metallic copper (Cu) layers. By using different focusing conditions, different process speeds are achieved, and the structural accuracies are controlled. The formation of Cu lines is observed for different focus sizes at various scan speeds, which is attributed to laser exposure duration and chemical reaction time. The formed lines are categorized and measured by optical microscopy and provide the basis for two-dimensional Cu surfaces. By hatching, high quality conductive films are produced with an increased focus diameter of up to 400 µm at a scan speed of 400 mm/s and a hatch distance of 250 µm resulting in a multiple increased process speed by a factor of thirteen compared to previous research. Using 4-tip measurement , a low sheet resistance of 0.165 Ω/sq was validated on these Cu films, which have an atomic Cu content of 89.5%. KW - Femtosekundenlaser Y1 - 2023 ER - TY - CHAP A1 - Bischoff, Kay A1 - Mücke, Dominik A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - UV-Femtosecond-Laser Structuring of Silicon Carbide T2 - International Congress on Applications of Lasers & Electro-Optics (ICALEO) 2023 N2 - We report on ultraviolet ultrashort pulsed laser processing of silicon carbide. Laser ablated cavities are evaluated with respect to their ablation rates, surface roughness and overall quality by varying the laser pulse fluence. Using this type of laser, high edge quality and surface roughness down to 107 nm can be achieved with an ablation efficiency of up to 0.228 mm 3 W −1 min −1. Functional surfaces are produced by generating laser induced periodic surface structures. The comparison with infrared wavelength underlines the advantages of the ultraviolet wavelength for laser micro machining this material. Significant differences with respect to the measured ablation depths and roughness as well as generated micro-and nanostructures appear. While infrared ablation is dominated by a chipping mechanism above a critical fluence of 9.93 Jcm −2 , higher ablation rates are observed with strong quality losses at the same time. In comparison to the infrared emission wavelength, in general , a significantly higher processing quality is achieved with the ultraviolet emission wavelength. In addition, the influence of spot size and repetition rate in UV processing is investigated. By increasing these parameters, a process acceleration without quality losses is enabled. The generation of a sophisticated microstructure exemplifies the advantages of processing silicon carbide with the UV laser reported here. KW - Femtosekundenlaser KW - Siliciumcarbid Y1 - 2023 ER - TY - JOUR A1 - Gvozdovskyy, Igor A1 - Bratova, Daria A1 - Kazantseva, Z. A1 - Malyuta, S. A1 - Lytvyn, Petro A1 - Schwarz, Simon A1 - Hellmann, Ralf T1 - Light-controllable hybrid aligning layer based on LIPSS on sapphire surface and PVCN-F film JF - Journal of Molecular Liquids N2 - The creation of aligning layers for the uniform orientation of liquid crystals is significant for both research and the application of liquid crystals. For all applications, the creation of aligning layers possessing controllable characteristics such as azimuthal and polar anchoring energies, easy-axis of director alignment and pretilt angle, in the same way as it is achieved by using photoaligning layers processed by light, is very important. Here, aligning properties of hybrid aligning layers created on the basis of sapphire surfaces additionally coated by photoaligning layer of PVCN-F are studied. These hybrid layers possess the properties of the nano-structured sapphire layer and the photosensitive PVCN-F layer, and complement each other. The irradiation time dependence of the azimuthal anchoring energy of the hybrid layers is studied. By using certain experimental conditions during irradiation of hybrid layers, e.g., polarization of light and irradiation time, a minimum value of the azimuthal anchoring energy, close to zero, was obtained. Atomic force microscope studies of the irradiated hybrid layers were also carried out. It was found that the behavior of the contact angle of nematic droplets placed on treated sapphire surfaces are in good agreement with properties of hybrid aligning layers and parameters of structuring surface obtained from AFM images. KW - Flüssigkristall KW - Saphir KW - Kristallfläche Y1 - 2023 U6 - https://doi.org/10.1016/j.molliq.2023.122623 VL - 387 IS - keine SP - 122623 EP - 122623 ER - TY - JOUR A1 - Bischoff, Kay A1 - Mücke, Dominik A1 - Schubert, Andreas A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Rheological Investigation of Highly Filled Copper(II) Oxide Nanosuspensions to Optimize Precursor Particle Content in Reductive Laser-Sintering JF - Liquids N2 - In this article, the particle concentration of finely dispersed copper(II) oxide nanosuspensions as precursors for reductive laser sintering (RLS) is optimized on the basis of rheological investigations. For this metallization process, a smooth, homogeneous and defect-free precursor layer is a prerequisite for adherent and reproducible copper structures. The knowledge of the rheological properties of an ink is crucial for the selection of a suitable coating technology as well as for the adjustment of the ink formulation. Different dilutions of the nanosuspension were examined for their rheological behavior by recording flow curves. A strong shear thinning behavior was found and the viscosity decreases exponentially with increasing dilution. The viscoelastic behavior was investigated by a simulated doctor blade coating process using three-interval thixotropy tests. An overshoot in viscosity is observed, which decreases with increasing thinning of the precursor. As a comparison to these results, doctor blade coating of planar glass and polymer substrates was performed to prepare precursor layers for reductive laser sintering. Surface morphology measurements of the resulting coatings using laser scanning microscopy and rheological tests show that homogeneous precursor layers with constant thickness can be produced at a particle–solvent ratio of 1.33. A too-high particle content results in an irregular coating layer with deep grooves and a peak-to-valley height Sz of up to 7.8 μm. Precise dilution control allows the fabrication of smooth surfaces with a Sz down to 1.5 μm. KW - Laserstrahlsintern Y1 - 2024 U6 - https://doi.org/http://dx.doi.org/10.3390/liquids4020019 SN - 2673-8015 VL - 4 IS - 2 SP - 382 EP - 392 PB - MDPI AG ER - TY - JOUR A1 - Bischoff, Kay A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Femtosecond reductive Laser Sintering under multiple focus conditions for rapid production of conductive copper layers JF - Procedia CIRP KW - Femtosekundenlaser KW - Laserstrahlsintern Y1 - 2024 U6 - https://doi.org/http://dx.doi.org/10.1016/j.procir.2024.08.189 SN - 2212-8271 VL - 124 SP - 629 EP - 633 PB - Elsevier BV ER - TY - JOUR A1 - Sommer, David A1 - Stockfleet, Henry A1 - Hellmann, Ralf T1 - Optimization of Mechanical Properties and Evaluation of Fatigue Behavior of Selective Laser Sintered Polyamide-12 Components JF - Polymers N2 - In this paper, a comprehensive study of the mechanical properties of selective laser sintered polyamide components is presented, for various different process parameters as well as environmental testing conditions. For the optimization of the static and dynamic mechanical load behavior, different process parameters, e.g., laser power, scan speed, and build temperature, were varied, defining an optimal parameter combination. First, the influence of the different process parameters was tested, leading to a constant energy density for different combinations. Due to similarities in mechanical load behavior, the energy density was identified as a decisive factor, mostly independent of the input parameters. Thus, secondly, the energy density was varied by the different parameters, exhibiting large differences for all levels of fatigue behavior. An optimal parameter combination of 18 W for the laser power and a scan speed of 2666 mm/s was determined, as a higher energy density led to the best results in static and dynamic testing. According to this, the variation in build temperature was investigated, leading to improvements in tensile strength and fatigue strength at higher build temperatures. Furthermore, different ambient temperatures during testing were evaluated, as the temperature-dependent behavior of polymers is of high importance for industrial applications. An increased ambient temperature as well as active cooling during testing was examined, having a significant impact on the high cycle fatigue regime and on the endurance limit. KW - Selective Laser Sintering KW - Tensile Strength KW - Fatigue Behaviour KW - Laserstrahlsintern Y1 - 2024 U6 - https://doi.org/http://dx.doi.org/10.3390/polym16101366 SN - 2073-4360 VL - 16 IS - 10 PB - MDPI AG ER - TY - JOUR A1 - Sommer, David A1 - Hornung, Simon A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Surface roughness optimization of hybrid PBF-LB/M-built Inconel 718 using in situ high-speed milling JF - The International Journal of Advanced Manufacturing Technology N2 - AbstractWe report on the optimization of the surface roughness of hybrid additive manufactured Ni superalloys, combining a conventional laser powder bed fusion process with in situ high-speed milling. This remarkable hybrid approach has only recently been applied to different steel types and barely to Ni superalloys which opposite to steel appear to be challenging for milling processes, particularly within the powderbed of laser powder bed fusion. Different influencing factors on the surface roughness are varied in this study, following the Taguchi method. Their effect is evaluated with respect to the average surface roughness and the maximum surface roughness. The signal-to-noise ratio for the varied parameters infeed, z-pitch, feed rate, and spindle speed is calculated, determining their relevance on the surface roughness, and defining an optimal parameter combination. As the surface quality is optimized to $$\varvec{R_a=0.47\, \mu m}$$ R a = 0.47 μ m , the definition of the optimal parameter combination is of the highest relevance for the application of this novel manufacturing approach for Inconel. Using linear regression, the resulting surface roughness of these parameters is predicted, getting validated by the experimental evaluation. Due to a further analysis, including EDX analysis and a quantitative element analysis at different positions of the flank of the milling cutter, wear characteristics as well as the dissipation of the coating of the milling cutter are detected. The flank wear and the resulting breakage of the cutting edge are defined as the main reasons of a rising surface roughness. KW - Hybrid additive manufacturing KW - High-speed Milling KW - Taguchi-Method KW - Hochgeschwindigkeitsfräsen Y1 - 2024 U6 - https://doi.org/http://dx.doi.org/10.1007/s00170-024-13382-5 SN - 0268-3768 VL - 132 IS - 3-4 SP - 1741 EP - 1751 PB - Springer Science and Business Media LLC ER - TY - CHAP A1 - Sommer, David A1 - Hornung, Simon A1 - Esen, Cemal A1 - Hellmann, Ralf ED - Gu, Bo ED - Chen, Hongqiang T1 - Optimization of mechanical properties of additive manufactured IN 718 parts combining LPBF and in-situ high-speed milling T2 - Laser 3D Manufacturing XI KW - Hybrid additive manufacturing KW - High-speed Milling KW - Hot isostatic pressing KW - Fatigue behaviour KW - Hochgeschwindigkeitsfräsen Y1 - 2024 U6 - https://doi.org/http://dx.doi.org/10.1117/12.3000952 PB - SPIE ER - TY - CHAP A1 - Sommer, David A1 - Safi, Abdulrahman A1 - Esen, Cemal A1 - Hellmann, Ralf ED - Gu, Bo ED - Chen, Hongqiang T1 - Additive manufacturing of Nickel-based superalloy: optimization of surface roughness using integrated high-speed milling T2 - Laser 3D Manufacturing XI KW - Hochgeschwindigkeitsfräsen Y1 - 2024 U6 - https://doi.org/http://dx.doi.org/10.1117/12.3000972 PB - SPIE ER - TY - JOUR A1 - Schleier, Max A1 - Adelmann, Benedikt A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Image Processing Algorithm for In Situ Monitoring Fiber Laser Remote Cutting by a High-Speed Camera JF - Sensors N2 - We present an in situ process monitoring approach for remote fiber laser cutting, which is based on evaluating images from a high-speed camera. A specifically designed image processing algorithm allows the distinction between complete and incomplete cuts by analyzing spectral and geometric information of the melt pool from the captured images of the high-speed camera. The camera-based monitoring system itself is fit to a conventional laser deflection unit for use with high-power fiber lasers, with the optical detection path being coaxially aligned to the incident laser. Without external illumination, the radiation of the melt from the process zone is recorded in the visible spectral range from the top view and spatially and temporally resolved. The melt pool size and emitted sparks are evaluated in dependence of machining parameters such as feed rate, cycles, and focus position during cutting electrical sheets. KW - Laserschneiden KW - Faserlaser Y1 - 2022 U6 - https://doi.org/10.3390/s22082863 SN - 1424-8220 VL - 22 IS - 8 PB - MDPI AG ER - TY - JOUR A1 - Schleier, Max A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - High speed melt flow monitoring and development of an image processing algorithm for laser fusion cutting JF - Journal of Laser Applications N2 - This contribution presents high-speed camera monitoring of melt pool dynamics for steel during laser fusion cutting and compares the images with recordings in aluminum. The experiments are performed by a 4 kW multimode fiber laser with an emission wavelength of 1070 nm. To visualize the thermal radiation from the process zone during the cutting process, the kerf is captured at sample rates of up to 170 000 frames per second without external illumination with a spectral response between 400 and 700 nm, allowing measurements of the melt flow dynamics from geometric image features. The dependencies of the melt flow dynamics on laser processing parameters, such as feed rate, gas pressure, and laser power, can be evaluated. The monitoring system is placed both off-axis and mounted to a conventional cutting head, with the monitoring path aligned to the processing laser for a coaxial and lateral view of the cut kerf. The measured signal characteristics of the images captured from the melt pool are examined in the visible spectral range of the emitted thermal radiation from the process zone. Moreover, a specifically developed image processing algorithm is developed that process and analyze the captured images and extract geometric information for a measurement of the melt flow. KW - Laserschneiden KW - Laserschmelzen Y1 - 2022 U6 - https://doi.org/10.2351/7.0000785 SN - 1042-346X VL - 34 IS - 4 PB - Laser Institute of America ER - TY - JOUR A1 - Schleier, Max A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Evaluation of a Cut Interruption Algorithm for Laser Cutting Steel and Aluminum with a High-Speed Camera JF - Applied Sciences N2 - We report on a monitoring system based on a high-speed camera for fiber laser fusion cutting. The monitoring system is used without an external illumination retrofit on a conventional cutting head, with the optical path aligned coaxially to the incident laser, permitting a direct, spatially, and temporally resolved detection of the melt pool area in the cut kerf from the top view. The dependence of the melt pool area on laser processing parameters such as laser power and feed rate are thus evaluated for stainless steel, zinc-coated steel, and aluminum, respectively. The signal characteristics of the images captured from the melt pool are examined in the visible spectral range of the emitted secondary thermal radiation from the process zone. An ad hoc developed image processing algorithm analyzes the spectral and geometric information of the melt pool from high-speed camera images and distinguishes between complete and incomplete cuts. KW - Laserschneiden KW - Laserschmelzen Y1 - 2023 U6 - https://doi.org/10.3390/app13074557 SN - 2076-3417 VL - 13 IS - 7 PB - MDPI AG ER - TY - JOUR A1 - Schleier, Max A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Vision transformer based cut interruption detection and prediction of laser fusion cutting from monitored melt pool images JF - Journal of Laser Applications N2 - Incomplete cuts during laser fusion cutting result in a closed kerf, preventing the workpiece from detaching from the sheet and resulting in rework or rejection. We demonstrate the approach of a vision transformer, used for image classification, to detect cut interruption during laser fusion cutting in steel and aluminum. With events impending an incomplete cut in steel, we attempt to predict cut interruption before they even occur. To build a data set for training, cutting experiments are carried out with a 4 kW fiber laser, forcing incomplete cuts by varying the process parameters such as laser power and feed rate. The thermal radiation from the process zone during the cutting process is captured with a size of 256 × 256 px2 at sample rates of 20 × 103 fps. The kerf is recorded with a spectral sensitivity between 400 and 700 nm, without external illumination, which enables the melt to be observed in the range of the visual spectrum. The vision transformer model, which is used for image classification, splits the image into patches, linearly embedded with an added position embedding, and fed to a standard transformer encoder. For training the model, a set of images was labeled for the respective classes of a complete, incomplete, and impending incomplete cut. With the trained model, incomplete cuts in steel and aluminum can then be recognized and impending incomplete cuts in steel can be predicted in advance. KW - Laserschneiden KW - Laserschmelzen Y1 - 2025 U6 - https://doi.org/10.2351/7.0001611 SN - 1042-346X VL - 37 IS - 1 PB - Laser Institute of America ER - TY - CHAP A1 - Lutz, Christian A1 - Sommerhuber, Ryan A1 - Kettner, Matthias A1 - Esen, Cemal A1 - Hellmann, Ralf ED - Kling, Rainer ED - Pfleging, Wilhelm ED - Sugioka, Koji T1 - Towards process control by detecting acoustic emissions during ultrashort pulsed laser ablation of multilayer materials T2 - Laser-based Micro- and Nanoprocessing XVIII N2 - We report on process sensing using a membrane-free optical microphone to monitor the acoustic emission during ultrashort pulsed laser ablation of multilayer materials. The acoustic emission during ablation is used to detect material transitions, with the specific signatures allowing to create a reliable process control for identifying individual layers. The outstanding properties of membrane-free optical microphones in terms of high bandwidth and high temporal resolution are ideally qualified for characterizing an ultrashort pulsed laser process, with its properties and capabilities being presented in this contribution. In particular, for layer- and material-selective ablation of multilayer printed circuit board components, copper and polyimide layers are ablated and the material transition is detected by analyzing the acoustic signal at different frequency levels, which is a novelty in the field of ultrashort pulsed laser process sensing. The investigations show, that the optical microphone can be used to resolve both the scanning paths and ablated layers by means of interruptions in a time-resolved acoustic spectrogram. Furthermore, as a result of a higher ablation rate of polyimide compared to copper and thus the increase of the emitted acoustic energy, the material transition between copper and polyimide layers can clearly be detected. The detection of this process event can be used for process control. KW - Ultrakurzzeitlaser Y1 - 2024 U6 - https://doi.org/10.1117/12.3000954 PB - SPIE ER - TY - CHAP A1 - Lutz, Christian A1 - Helm, Jonas A1 - Esen, Cemal A1 - Hellmann, Ralf ED - Kling, Rainer ED - Pfleging, Wilhelm ED - Sugioka, Koji T1 - Computational optimization of borehole sequences for the reduction of heat accumulation in drilling processes using ultrashort pulse lasers T2 - Laser-based Micro- and Nanoprocessing XVIII N2 - We report on laser drilling of borehole arrays using a high-power ultrashort pulse laser with particular focus on reducing heat accumulation in the workpiece by optimizing the drilling sequence, particularly for highly efficient multi-spot drilling. Different optimization approaches are chosen to improve the drilling sequence, also comparing a simplex algorithm and an evolutionary algorithm. From a laser application point of view, we also compare drilling sequences using a single spot and up to 16-fold multi-spots generated by a spatial light modulator, as to accelerate the drilling process in terms of the number of drilled holes per second. To evaluate the temperatures generated during drilling of up to 40,000 holes in less than 76 seconds in stainless steel foil, temperatures are measured by a thermal imaging camera and subsequently compared to a COMSOL-based simulation for all optimized drilling sequences. With respect to an average temperature of 706 °C without optimization, a reduction by 252 °C, i.e., a reduction by nearly 36 % based on the Celsius scale, is achieved using a 4 × 4 beam splitter and an optimized drilling sequence with a drilling rate of 526 holes per second. In addition, using a 2 × 2 beam splitter, a temperature reduction of up to 40.5 % is achieved for a drilling process with a rate of 129 holes per second using an optimized drilling sequence. KW - Ultrakurzzeitlaser Y1 - 2024 U6 - https://doi.org/10.1117/12.3000930 PB - SPIE ER - TY - JOUR A1 - Marx, Jan A1 - Seefeldt, Malte A1 - Haske, Damian A1 - Lutz, Christian A1 - Hellmann, Ralf A1 - Esen, Cemal A1 - Ostendorf, Andreas T1 - Holographic freeform micro hole processing using Bessel beams JF - Optics & Laser Technology N2 - Bessel beams are advantageous tools for micro drilling applications due to their diffraction-free properties, their long focal length, and their small focal spot diameter. However, their application in industry is limited because they are challenging to integrate into flexible beam shaping systems. This work is intended to overcome limitations in flexibility by presenting a new approach paving the way to high-aspect ratio freeform holes. Therefore, a spatial light modulator was integrated into an axicon-based setup to merge diffraction-free ablation with the flexibility of holographic spatial beam shaping. A dynamic change of the phase hologram displayed on the spatial light modulator during the process allows for the generation of arbitrary ablation geometries. In contrast to other holographic approaches, the diffraction-free nature of the Bessel beam allows ablation over an extended focal depth without any focus control needed. The applicability of the approach was demonstrated by processing arbitrary hole shapes, such as ellipses, squares, or star-shapes. The hole sizes of the freeform holes range from 4 μm to 80 μm. Furthermore, up-scaling of the process is demonstrated by using the setup for generation of multi-Bessel spots. KW - Mikrobearbeitung KW - Laserbohren Y1 - 2025 U6 - https://doi.org/10.1016/j.optlastec.2024.112287 SN - 0030-3992 VL - 183 PB - Elsevier BV ER - TY - JOUR A1 - Lutz, Christian A1 - Jung, Marcel A1 - Tschirpke, Katrin A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Optimization of Heat Accumulation during Femtosecond Laser Drilling Borehole Matrices by Using a Simplex Algorithm JF - Materials N2 - We report on an optimization study of percussion drilling thin metal sheets employing a high repetition rate, high power femtosecond laser with respect to the resulting heat accumulation. A specified simplex algorithm was employed to optimize the spatial drilling sequence, whereas a simplified thermal simulation using COMSOL was validated by comparing its results to the temperature measurements using an infrared camera. Optimization for drilling borehole matrices was aspired with respect to the generated temperature across the processed specimen, while the drilling strategy was altered in its spatial drilling sequence and by using multi-spot approaches generated by a spatial light modulator. As a result, we found that an optimization strategy based on limited consecutive holes in a Moore neighborhood led to reduced temperatures and the shortest process times. KW - Femtosekundenlaser Y1 - 2022 U6 - https://doi.org/10.3390/ma15144829 SN - 1996-1944 VL - 15 IS - 14 PB - MDPI AG ER - TY - CHAP A1 - Lutz, Christian A1 - Schwarz, Simon A1 - Rung, Stefan A1 - Marx, Jan A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Optical system for multi Bessel beam high power ultrashort pulsed laser processing using a spatial light modulator T2 - Lasers in Manufacturing - LiM 2021 N2 - We report on an optical setup for multi Bessel beam processing combining a refractive axicon and a spatial light modulator. Based on their particular beam profile, Bessel beams exhibit various advantages over conventional Gaussian beams for ultrashort pulsed laser processing. Especially for micromachining of transparent materials, applications such as micro-hole drilling or the generation of voids benefit from the increased focal length of the applied Bessel beam. In addition, on account of the significantly increased average output power of industrial ultrashort pulsed lasers over the last years, there is a high demand on multi spot applications for using the available laser power in efficient production processes. Our optical concept combines the dynamic possibilities of beam splitting using spatial light modulator with the benefits of Bessel beams facilitating multi Bessel beam processing. KW - Ultrakurzzeitlaser KW - Lichtmodulator Y1 - 2021 ER - TY - JOUR A1 - Kefer, Stefan A1 - Sauer, Theresia A1 - Hessler, Steffen A1 - Kaloudis, Michael A1 - Schmauss, Bernhard A1 - Hellmann, Ralf T1 - Robust Polymer Planar Bragg Grating Sensors Embedded in Commercial-Grade Composites JF - Polymers N2 - This contribution demonstrates the functionality of polymer planar Bragg grating (PPBG) sensors integrated into commercial-grade carbon fiber reinforced polymer (CFRP) components. Multiple CFRP specimens are generated by curing a stack of pre-impregnated fibers inside of a heated mechanical press, exposing the polymer sensor to a pressure of 7 bar and a temperature of 120 °C for 2 h. After integration, the sensor still exhibits a strong and evaluable signal. Subsequent flexural experiments reveal a linear response of the integrated sensor’s Bragg wavelength to the CFRP specimen’s maximum deflection. Additional findings demonstrate that the embedded PPBG can be used to detect plastic deformations of a CFRP workpiece, whereas a linear correlation of plastic deformation to the resulting Bragg signal offset is determined. A plausibility check of the obtained results is delivered by a comparison of three-point flexural experiments on bulk CFRP workpieces, without integrated sensors and additional specimens featuring external optical sensors affixed to their surface. It is found that PPBGs based on cyclic olefin copolymers are able to overcome the temperature-related limitations of traditional polymer-based optical sensors and can thus be directly integrated into commercial-grade composites during production. KW - Bragg grating sensors KW - carbon fiber reinforced polymers KW - cyclic olefin copolymers KW - Bragg-Reflektor KW - Optischer Sensor Y1 - 2020 UR - https://www.mdpi.com/2073-4360/12/3/715/htm U6 - https://doi.org/10.3390/polym12030715 VL - 12 IS - 3 SP - 715 EP - 715 ER - TY - CHAP A1 - Kefer, Stefan A1 - Rosenberger, Manuel A1 - Hessler, Steffen A1 - Girschikofsky, Maiko A1 - Belle, Stefan A1 - Roth, Gian-Luca A1 - Schmauss, Bernhard A1 - Hellmann, Ralf T1 - Fabrication and Applications of Polymer Planar Bragg Grating Sensors based on Cyclic Olefin Copolymers T2 - 2019 Photonics & Electromagnetics Research Symposium - Fall (PIERS - Fall) N2 - This contribution reviews recent advancements, current research and possible applications of polymer planar Bragg grating (PPBG) sensors with a focus on the utilization of cyclic olefin copolymer (COC) substrates. COC-PPBGs can be handled and fabricated efficiently by employing a single writing step procedure which features simultaneous generation of waveguide and Bragg grating structure within an injection molded substrate. The resulting photonic structures exhibit an attenuation as low as 1.2 dB cm-1 and a reflectivity up to 99 %. The potential of COC-PPBGs for high-temperature applications is proven by demonstrating temperature measurements up to 160 °C. Moreover, it is possible to employ single PPBGs for multidimensional stress and strain sensing or even three-dimensional shape reconstruction. Due to their excellent properties, it is feasible to integrate COC-PPBGs into commercial-grade carbon fiber reinforced polymer workpieces for structural health monitoring. Furthermore, utilization of appropriate coatings enables functionalization of PPBGs for refractive index sensing and thus biochemical applications. Beside the fabrication and characterization of COC-based PPBGs, this contribution exemplifies and reviews such applications. KW - Bragg gratings , Gratings , Optical waveguides , Temperature sensors , Substrates KW - Optischer Sensor KW - Bragg-Reflektor Y1 - 2020 UR - https://ieeexplore.ieee.org/document/9021801 SN - 978-1-7281-5304-9 U6 - https://doi.org/10.1109/PIERS-Fall48861.2019.9021801 SP - 647 EP - 655 PB - Institute of Electrical and Electronics Engineers ER - TY - JOUR A1 - Kefer, Stefan A1 - Dai, Jixiang A1 - Yang, Minghong A1 - Schmauss, Bernhard A1 - Hellmann, Ralf T1 - Hypersensitive H2 sensor based on polymer planar Bragg gratings coated with Pt-loaded WO3-SiO2 JF - Optics Letters N2 - This letter demonstrates a novel hydrogen sensor based on a polymer planar Bragg grating coated with Pt-loaded WO3-SiO2. The reflected Bragg signal shows a distinct peak splitting correlated to substrate anisotropies originating from the injection molding process. Especially at low H2 concentrations, both sensing peaks exhibit an outstanding response to the heat generated by the exothermic reaction between hydrogen molecules and coating. Thereby, a hydrogen volume ratio of 50 ppm leads to a Bragg wavelength shift of -37 pm, which yields an outstandingly low detection limit of only 5 ppm H2 in air. Thus, functionalized polymer planar Bragg gratings are eminently suitable for H2 leak detection applications. KW - Bragg Grating Sensors KW - Hydrogen Detection KW - Cyclic Olefin Copolymers KW - Gassensor KW - Wasserstoff KW - Bragg-Reflektor Y1 - 2020 UR - https://www.osapublishing.org/ol/abstract.cfm?uri=ol-45-13-3601 U6 - https://doi.org/https://doi.org/10.1364/OL.395341 VL - 45 IS - 13 SP - 3601 EP - 3604 ER - TY - GEN A1 - Kefer, Stefan A1 - Dai, Jixiang A1 - Yang, Minghong A1 - Schmauss, Bernhard A1 - Hellmann, Ralf T1 - Hypersensitive H2 sensor based on polymer planar Bragg gratings coated with Pt-loaded WO3–SiO2: erratum N2 - We present an erratum to our Letter [Opt. Lett. 45, 3601 (2020)]. Labeling errors in two figures and an incorrect sentence are revised. The corrections have no influence on the conclusions of the original Letter. N2 - Erratum KW - Bragg Grating, Hydrogen Detection, Functionalization KW - Gassensor KW - Wasserstoff KW - Bragg-Reflektor Y1 - 2020 UR - https://www.osapublishing.org/ol/abstract.cfm?uri=ol-45-16-4498 U6 - https://doi.org/https://doi.org/10.1364/OL.401819 VL - 45 IS - 16 SP - 4498 EP - 4498 ER - TY - JOUR A1 - Kefer, Stefan A1 - Bischoff, Kay A1 - Roth, Gian-Luca A1 - Haubner, Julian A1 - Schmauss, Bernhard A1 - Hellmann, Ralf T1 - Tunable Bulk Polymer Planar Bragg Gratings Electrified via Femtosecond Laser Reductive Sintering of CuO Nanoparticles JF - Advanced Optical Materials N2 - This contribution demonstrates and discusses electrically tunable polymer planar Bragg gratings based on bulk cyclic olefin copolymers. A lithographic single-writing-step method and femtosecond laser reductive sintering of copper(II) oxide nanoparticles are subsequently employed in order to generate buried photonic structures and copper conducting paths on top of the polymer substrate. This way, the necessary number of process steps for fabricating a planar polymer-based electro-optical device is greatly reduced. The response of a fully electrified grating structure follows temperature changes, induced by the copper conducting path, with sensitivities up to −31 pm K−1. Dilatometric measurements show that the specimen's behavior is correlated to the situationally reduced thermal expansion of the bulk polymer substrate. In consequence, the tuning response of the photonic platform follows a second order polynomial, whereas a direct current of 30 mA, which correlates to a power consumption of 18.3 mW, leads to a local temperature increase and a residual Bragg wavelength shift of 19.6 K and −547 pm, respectively. Moreover, the outstanding flexibility of the proposed fabrication concept is underlined by demonstrating alternative conducting path geometries, whereas one of the additional designs is adapted to control the spectral width of the Bragg grating's reflection peak. KW - Bragg gratings KW - CuO nanoparticles KW - cyclic olefin copolymers KW - electro-optic tuning KW - integrated photonics KW - Femtosekundenlaser KW - Bragg-Reflektor Y1 - 2021 UR - https://onlinelibrary.wiley.com/doi/10.1002/adom.202002203 U6 - https://doi.org/https://doi.org/10.1002/adom.202002203 VL - 9 IS - 13 SP - 2002203 EP - 2002203 ER - TY - CHAP A1 - Kefer, Stefan A1 - Zettl, Julian A1 - Schmauss, Bernhard A1 - Esen, Cemal A1 - Hellmann, Ralf ED - Kling, Rainer ED - Pfleging, Wilhelm ED - Watanabe, Akira T1 - High-Precision Micromachining of Sapphire Towards Optical Waveguiding Structures using Femtosecond Lasers T2 - Laser-based Micro- and Nanoprocessing XVII N2 - While sapphire is one of the most durable materials, its properties entail that high-precision machining, especially in the sub-millimeter regime, is still challenging. This contribution demonstrates and discusses novel femtosecond laser-based micromachining approaches for the fabrication of rotational-symmetric sapphire workpieces, specifically the generation of optical fibers by means of laser lathe of sapphire rods and the practical realization of windmill fibers. In addition, volume refractive index modification in planar sapphire substrates is presented to induce photonic crystal waveguides. The micromachined structures are comprehensively examined with respect to geometric fidelity, surface roughness, refractive index modification, and potential optical waveguiding properties. All micromachining approaches are done by means of frequency-doubled or frequency-tripled femtosecond laser radiation. Different laser optical setups including laser scanning head, spatial beam profilers including a spatial light modulator and axial rotatory movement of the specimen are employed for micro structuring and in-depth refractive index modifications. In particular for laser lathe, a sophisticated scanning pattern, in combination with an incremental axial rotatory movement of the specimen, allows for the precise diameter reduction of sapphire rods with 250 µm diameter to fibers with outer diameters of 25 µm. By supporting the workpiece with a V-groove fixture, multi-mode fibers with lengths up to 20 cm can be processed with an average surface roughness of 250 nm. Additionally, an adapted ablation scanning sequence enables the first practical demonstration of sapphire windmill fibers. Furthermore, using a spatial light modulator allows for the adaption of the laser propagation properties as to enable volume refractive index modifications with free-form arrangement. Hexagonal patterns of refractive index modifications surrounding a pristine waveguide core are fabricated and single-mode waveguiding at 1550 nm is verified. Finally, the possibility of integrating Bragg gratings into this photonic waveguide type is demonstrated KW - sapphire KW - femtosecond laser KW - micromachining KW - laser lathe KW - rod-to-fiber KW - Femtosekundenlaser KW - Saphir Y1 - 2023 SN - 9781510659230 U6 - https://doi.org/10.1117/12.2648758 SP - 3 EP - 11 ER - TY - JOUR A1 - Kefer, Stefan A1 - Pape, Dominik A1 - Roth, Gian-Luca A1 - Hessler, Steffen A1 - Schmauss, Bernhard A1 - Hellmann, Ralf T1 - Micromilling-assisted fabrication of monolithic polymer ridge-type waveguides with integrated photonic sensing structures JF - Optical Materials Express N2 - This study demonstrates and discusses a novel approach for the fabrication and rapid prototyping of monolithic photonic platforms comprising a ridge-type waveguide with integrated sensing structures. First, the bulk injection-molded cyclic olefin copolymer substrates are micromilled in order to define the physical extension of the ridge structure. Cross-sections down to 30 × 30 µm2, exhibiting a mean surface roughness of 300 nm, are achieved with this process. Subsequently, UV radiation is used to modify the ridge structure’s refractive index, which leads to the formation of an optical waveguide. By employing a phase mask, it is possible to equip the photonic platform with a Bragg grating suitable for temperature measurements with a sensitivity of −5.1 pm K-1. Furthermore, an integrated Fabry-Pérot cavity, generated during the micromilling step as well, enables refractive index measurements with sensitivities up to 1154 nm RIU-1. KW - Cyclic Olefin Copolymers KW - Micromilling KW - Bragg Grating KW - Fabry-Pérot Etalon KW - Bragg-Reflektor KW - Optischer Sensor Y1 - 2021 UR - https://www.osapublishing.org/ome/fulltext.cfm?uri=ome-11-8-2389&id=453071 U6 - https://doi.org/https://doi.org/10.1364/OME.425778 VL - 11 IS - 8 SP - 2389 EP - 2400 ER - TY - GEN A1 - Kefer, Stefan A1 - Bischoff, Kay A1 - Roth, Gian-Luca A1 - Haubner, Julian A1 - Schmauss, Bernhard A1 - Hellmann, Ralf T1 - Tunable Bulk Polymer Planar Bragg Gratings Electrified via Femtosecond Laser Reductive Sintering of CuO Nanoparticles (Advanced Optical Materials 13/2021) N2 - This cover image outlines the fabrication method of a polymer planar Bragg grating electrified via femtosecond laser reductive sintering of CuO nanoparticles (see article number 2002203 by Stefan Kefer and co-workers). Based on this sophisticated methodology, bulk cyclic olefin copolymer substrates can be equipped with integrated photonic structures comprising a waveguide as well as a Bragg grating. Its reflective characteristics can be efficiently tuned by means of the subsequently generated Cu conducting path, whereas the applied femtosecond laser process enables an almost limitless degree of freedom towards conducting path geometries. N2 - Cover zum zugehörigen Artikel KW - Femtosekundenlaser KW - Bragg-Reflektor Y1 - 2021 UR - https://onlinelibrary.wiley.com/doi/10.1002/adom.202170048 U6 - https://doi.org/https://doi.org/10.1002/adom.202170048 ER - TY - CHAP A1 - Kefer, Stefan A1 - Roth, Frederick A1 - Kaloudis, Michael A1 - Schmauss, Bernhard A1 - Hellmann, Ralf T1 - Monitoring of Composite Bicycle Components using Polymer Planar Bragg Gratings T2 - SMSI 2021 - Sensors and Instrumentation N2 - This study demonstrates mobile load monitoring of a composite bicycle component using an application- customized polymer planar Bragg grating sensor, evaluated by a mobile interrogation unit. After a referencing procedure, the mechanical load of a seat post is monitored while cycling through a test track. KW - Bragg Grating KW - Composites KW - Cyclic Olefin Copolymers KW - Micromilling KW - Load Monitoring KW - Bragg-Reflektor KW - Verbundwerkstoff KW - Fahrrad Y1 - 2021 UR - https://www.ama-science.org/proceedings/details/3992 SN - 978-3-9819376-4-0 U6 - https://doi.org/10.5162/SMSI2021/B9.2 SP - 161 EP - 162 PB - AMA Service GmbH CY - Wunstorf, Germany ER - TY - JOUR A1 - Kefer, Stefan A1 - Limbach, Tobias A1 - Pape, Natalie A1 - Klamt, Kathrin A1 - Schmauss, Bernhard A1 - Hellmann, Ralf T1 - Birefringence in Injection-Molded Cyclic Olefin Copolymer Substrates and Its Impact on Integrated Photonic Structures JF - Polymers N2 - This contribution quantifies the birefringence within injection-molded cyclic olefin copolymer plates and discusses its impact on the mechanical properties of the plates. It also focuses on the impact of birefringence on integrated waveguides and Bragg gratings and provides fabrication guidelines for such structures. The anisotropy in all three dimensions of the workpiece is examined by means of polarimetry and a prism coupler. It is found that the birefringence is inhomogenously distributed within the workpieces, whereas the maximum birefringence not only varies locally, but also depends on the observation direction. Overall, a maximum birefringence of 10 × 10−4 is found at the plate’s surface near the injection gate. The anisotropy then reduces exponentially towards the center of the workpiece and saturates at 1.8 × 10−4, in a depth of 0.4 mm. Thus, the birefringence strongly affects near-surface photonic structures. It is found that, depending on their orientation and the local birefringence of the substrate, waveguides and Bragg gratings fabricated with comparable parameters behave completely differently in terms of polarization-dependent optical attenuation, cross-sectional intensity distribution and Bragg reflection signal. For example, the support of the TM mode can vary between total loss and an optical attenuation of 0.9 dB × cm−1. In consequence, this study underlines the importance of quantifying the birefringent state of an injection-molded cyclic olefin copolymer workpiece if it is supposed to serve as a substrate for integrated photonic structures. The study furthermore demonstrates that birefringence effects can be omitted by burying the photonic structures deeper into the volume of the thermoplastic. KW - cyclic olefin copolymer KW - birefringence KW - integrated photonics KW - Bragg grating KW - injection molding KW - Bragg-Reflektor KW - Wellenleiter KW - Doppelbrechung Y1 - 2024 UR - https://www.mdpi.com/2073-4360/16/2/168 U6 - https://doi.org/10.3390/polym16020168 VL - 16 IS - 2 SP - 168 EP - 168 ER - TY - CHAP A1 - Kefer, Stefan A1 - Pape, Natalie A1 - Gries, Nikola A1 - Roth, Gian-Luca A1 - Schmauss, Bernhard A1 - Hellmann, Ralf T1 - Towards Pressure Sensors Based on Polymer Planar Bragg Gratings T2 - Organic Photonic Materials and Devices XXV N2 - While Bragg grating-based optical devices have shown promising performances for pressure sensing applications, their sensitivity, especially in the low-pressure regime, is unsatisfying and needs to be optimized by elaborate designs, such as cantilevers or other extrinsic mechanical transducers. This contribution demonstrates and discusses a novel concept for optical pressure sensors based on polymer planar Bragg gratings. Waveguide and Bragg grating are fabricated underneath the surface of a temperature-stable and humidity-insensitive cyclic olefin copolymer substrate by means of a femtosecond laser. Based on the employed direct-writing procedure, in combination with adaptive, in-situ beam shaping with a spatial light modulator, writing depth, i.e., location of the photonic structures within the substrate, as well as Bragg grating periodicity and positioning can be deliberately chosen. Afterwards, the polymer substrate is post-processed with a high-precision micro mill, so a diaphragm comprising the integrated photonic structures is generated. The resulting diaphragm exhibits a thickness of 300 µm and a diameter of 10 mm. Finally, the optical sensor is packaged and sealed to form an air-filled gas pocket underneath the diaphragm. Deformations of the diaphragm by external pressure changes translate to strain variations along the waveguide axis and thus perturb the Bragg grating period. This leads to changes in the grating’s wavelength of main reflection, which can be evaluated in order to quantify the relative external pressure. With this straightforward optical sensor concept, pressure sensitivities up to 39 pm kPa-1, within relative pressures ranges from 78 kPa to 372 kPa, are achieved. KW - Bragg grating KW - pressure sensor KW - integrated photonics KW - cyclic olefin copolymers KW - femtosecond laser KW - Bragg-Reflektor KW - Drucksensor Y1 - 2023 SN - 9781510659414 U6 - https://doi.org/10.1117/12.2648723 ER - TY - JOUR A1 - Voigt, Jorrit A1 - Bock, Thomas A1 - Hilpert, Uwe A1 - Hellmann, Ralf A1 - Möckel, Michael T1 - Increased relative density and characteristic melt pool Signals at the edge in PBF-LB/M JF - Additive Manufacturing N2 - Limited process control can cause metallurgical defect formation and inhomogeneous relative density in laser powder bed fusion manufactured parts. This study shows that process monitoring, based on optical melt-pool signal analysis is capable of tracing relative density variations: Unsupervised machine learning, applied to cluster multiple-slice monitoring data, reveals characteristic patterns in this noisy time-series signal, which can be co-registered with geometrical positions in the build part. For cylindrical 15–5 PH stainless steel specimens, manufactured under constant process parameters and post-analyzed by µ-computer tomography, correlations between such patterns and an increased local relative density at the edge have been observed. Finite element method (FEM) modeling of thermal histories at exemplary positions close to the edge suggest pre-heating effects caused by neighboring laser scan trajectories as possible reasons for the increased melt pool intensity at the edge. KW - Maschinelles Lernen KW - Schmelze Y1 - 2022 UR - https://www.sciencedirect.com/science/article/pii/S2214860422001993?via%3Dihub U6 - https://doi.org/https://doi.org/10.1016/j.addma.2022.102798 VL - 2022 IS - 57/102798 SP - * EP - * ER - TY - CHAP A1 - Kefer, Stefan A1 - Schmauss, Bernhard A1 - Hellmann, Ralf T1 - POLYMER PLANAR BRAGG GRATINGS BASED ON BULK CYCLIC OLEFIN COPOLYMERS: FABRICATION AND FUNCTIONALIZATION T2 - Proceedings of Student Conference on Sensors, Systems and Measurement 2021 KW - Bragg-Reflektor Y1 - 2021 UR - https://scssm.fel.cvut.cz/wp-content/uploads/2021/07/Proceedings-SCSSM-2021.pdf SN - 978-80-01-06822-9 ER - TY - CHAP A1 - Kefer, Stefan A1 - Pape, Natalie A1 - Schmauss, Bernhard A1 - Hellmann, Ralf T1 - Fabrication of Lattice-Like Waveguides in Planar Cyclic Olefin Copolymers T2 - 28th International Conference on Optical Fiber Sensors N2 - This work demonstrates the femtosecond laser-based fabrication of lattice-like waveguides in planar cyclic olefin copolymers. An overview of the fabrication process is provided and waveguiding in the C-band is proven via optical near-field analysis. KW - Wellenleiter KW - Femtosekundenlaser Y1 - 2023 SN - 978-1-957171-30-2 U6 - https://doi.org/10.1364/OFS.2023.Th6.69 ER - TY - CHAP A1 - Kefer, Stefan A1 - Schmauss, Bernhard A1 - Hellmann, Ralf T1 - Sapphire-Based Planar Bragg Grating Devices T2 - 28th International Conference on Optical Fiber Sensors N2 - This study reports on the long-term stability and the high-temperature capability of sapphire-based photonic crystal waveguides with integrated Bragg gratings. Furthermore, their Bragg grating reflectivity as well as their temperature sensitivity is quantified. KW - Bragg-Reflektor KW - Wellenleiter Y1 - 2023 SN - 978-1-957171-30-2 U6 - https://doi.org/10.1364/OFS.2023.W2.4 ER - TY - JOUR A1 - Yang, Yongting A1 - Bischoff, Kay A1 - Mücke, Dominik A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - UV-ultrashort pulsed laser ablation of fused silica JF - Journal of Laser Applications N2 - The authors report on ultraviolet ultrashort pulsed laser ablation of fused silica and compare the achievable micromachining results to those obtained by using the fundamental emission wavelength in infrared. Ablation in ultraviolet reveals a stable efficiency for increasing fluences, whereas using an infrared beam exhibits a decreasing trend of the ablation efficiency at higher and increasing fluences. In addition, a significant improvement in the surface quality is found by using an ultraviolet wavelength in a fluence range up to 20 J/cm2 compared to infrared, e.g., revealing an Ra of down to 0.45 μm on using the ultraviolet wavelength compared to Ra = 0.56 μm on using infrared at fluences up 15 J/cm2. Moreover, taking advantage of the high available pulse energy, the authors compare the achievable ablation efficiency and surface roughness using a conventionally focused ultraviolet beam and a defocused ultraviolet beam, finding that the defocused ultraviolet beam possesses a processing quality comparable to that of the focused beam. Finally, the authors exemplify the potential of ultraviolet ultrashort pulsed laser ablation by using a Tesla mixer for microfluidic integration of fused silica. KW - Laserablation KW - Quarzglas KW - Laserbestrahlung Y1 - 2024 U6 - https://doi.org/http://dx.doi.org/10.2351/7.0001197 SN - 1042-346X VL - 36 IS - 1 PB - Laser Institute of America ER - TY - JOUR A1 - Lutz, Christian A1 - Schwarz, Simon A1 - Marx, Jan A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Multi-Bessel Beams Generated by an Axicon and a Spatial Light Modulator for Drilling Applications JF - MDPI Photonics N2 - We report on an optical setup to generate multi-Bessel beam profiles combining a refractive axicon and a spatial light modulator. Based on their particular beam profile, Bessel beams offer advantageous properties for micro drilling processes and internal volume processing, especially for transparent materials. In addition, the laser power of industrial, ultrashort pulsed lasers has increased significantly over the last few years, offering the possibility for highly efficient processes using multi-spot profiles. Our optical concept combines the dynamic possibilities of beam splitting using a spatial light modulator with the benefits of Bessel beams, which facilitates multi-Bessel beam processing. Beside the simulation and experimental evaluation of the generated multi-Bessel beams, we exemplify the applicability of the developed module for the perforation of thin metal foils by micro drilling. KW - Bessel beam KW - Axicon KW - Optical System KW - Ultrashort pulsed laser KW - Material ablation KW - Bessel-Bündel KW - Ultrakurzzeitlaser Y1 - 2023 UR - https://www.mdpi.com/2304-6732/10/4/413 U6 - https://doi.org/10.3390/photonics10040413 VL - 2023 IS - 10/413 SP - 1 EP - 9 ER - TY - CHAP A1 - Marx, Jan A1 - Esen, Cemal A1 - Lutz, Christian A1 - Hellmann, Ralf A1 - Ostendorf, Andreas T1 - Holographic tuning of physical axicons N2 - Axicon generated Bessel beams are a popular tool for high aspect ratio precision laser drilling. Spot diameter and working distance are given by the geometric parameters of the axicon and the wavelength used. Thus, it is difficult to manipulate the beam shape of a Bessel beam for a given setup. Spatial light modulators (SLMs) overcome limitations in flexibility. However, due to the limited phase shift of SLMs, only Bessel beams with flat cone angles and large focal length can be generated. In this contribution, an approach for generating Bessel beams with a shorter, but tunable focal length is presented. A physical axicon was combined with an SLM. A holographic image of a negative axicon is put on the SLM to generate a ring beam, which is focused by a subsequent physical axicon to get a small focal diameter. Thus, different sized high aspect ratio micro holes can be drilled without using any moving components. KW - Bessel beam KW - Micro drilling KW - Axicon KW - Spatial light modulator KW - Beam shaping KW - Bessel-Bündel KW - Axicon KW - Strahlformung, Laser Y1 - 2023 VL - 2023 IS - 154 SP - 1 EP - 9 PB - LiM 2023 Proceedings ER - TY - JOUR A1 - Kefer, Stefan A1 - Pape, Natalie A1 - Bischoff, Kay A1 - Schmauss, Bernhard A1 - Hellmann, Ralf T1 - Lattice-Like Waveguides With Integrated Bragg Gratings in Planar Cyclic Olefin Copolymers JF - Journal of Lightwave Technology N2 - This contribution demonstrates femtosecond laser direct writing of lattice-like waveguides in planar cyclic olefin copolymer substrates. Based on numerical simulation and experimental near-field analysis, stable single-mode waveguiding around wavelengths of 1550 nm is demonstrated. The waveguiding mechanism is based on a hexagonal array of laser-induced, positive refractive index modification lines. Thus, the lateral extension of the guided mode can be adapted by varying the fabrication parameters and, in consequence, the resulting cross-sectional arrangement of the refractive index perturbations. With an optical attenuationof 2.2 dB·cm-1 around 1550 nm, the fabricated waveguides are well-suited for on-chip integrated photonic devices. Moreover, the waveguides can also be equipped with Bragg gratings to enable the application of the photonic platform as a sensing device. Dependingon their length, the Bragg grating structures exhibit reflectivities ofup to 99% and spectral widths down to 0.3 nm. The flexibility of the fabrication process and the sensing capabilities of the lattice-like waveguides with integrated Bragg gratings are underlined by an exemplary application study demonstrating a relative pressure sensor. For that, a photonic platform is micromilled to generate a 300 µm thick diaphragm and a reference pressure chamber. The strain introduced to the diaphragm by external pressure changes can then be quantified by the integrated photonic structures.This way, absolute pressure sensitivities of up to 38 pm·kPa-1 can be achieved in a relative pressure range from −60 to 100 kPa. The newly-developed lattice-like waveguides with integrated Bragggratings are therefore well-suited for the realization of novel and adaptable photonic devices and sensors. KW - Bragg grating KW - cyclic olefin copolymers KW - femtosecond laser KW - integrated photonics KW - lattice-like waveguide KW - pressure sensor KW - Wellenleiter KW - Bragg-Reflektor Y1 - 2024 U6 - https://doi.org/10.1109/JLT.2023.3328323 SN - 0733-8724 VL - 42 IS - 18 SP - 6302 EP - 6311 PB - Institute of Electrical and Electronics Engineers (IEEE) ER - TY - JOUR A1 - Marx, Jan A1 - Lutz, Christian A1 - Hellmann, Ralf A1 - Esen, Cemal T1 - Holographic multi-spot generation for ultra-short pulse Bessel beam processing of stainless steel JF - Procedia CIRP KW - Ultrakurzzeitlaser Y1 - 2022 U6 - https://doi.org/10.1016/j.procir.2022.08.003 SN - 2212-8271 VL - 111 SP - 648 EP - 652 PB - Elsevier BV ER - TY - CHAP A1 - Lutz, Christian A1 - Schwarz, Simon A1 - Marx, Jan A1 - Esen, Cemal A1 - Hellmann, Ralf ED - Kling, Rainer ED - Pfleging, Wilhelm ED - Sugioka, Koji T1 - Combination of an axicon and a spatial light modulator for the generation of multiBessel beams for microdrilling using ultrashort pulsed laser T2 - Laser-based Micro- and Nanoprocessing XVIII N2 - Due to their special beam profile, Bessel beams offer advantageous properties for micro-drilling and internal volume processing, especially for transparent materials. In addition, the laser power of industrial ultrashort pulsed lasers hasnincreased significantly in recent years, offering the possibility of highly efficient processes using multi-spot profiles. We report on optical simulations and an experimental optical setup for generating multi-Bessel beams by combining a refractive axicon and a spatial light modulator. This setup offers a new possibility of multi-Bessel beam generation by functional division of the Bessel beam generation by the axicon and the possibility of separation flexible beam splitting by a spatial light modulator. The beam profiles generated with this setup are analyzed and compared in this study using optical simulations and experiments. In addition to the analysis of the Bessel beams in the machining plane, they are also analyzed in the propagation direction by means of intensity distributions measured by a camera in small-step z positions. Furthermore, the experimental applicability of the module-based beam shaping system is demonstrated by integrating into an existing laser system. KW - Ultrakurzzeitlaser Y1 - 2024 U6 - https://doi.org/10.1117/12.3000946 PB - SPIE ER - TY - JOUR A1 - Sommer, David A1 - Hornung, Simon A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Design Guide for Hybrid-Additive Manufacturing of Inconel 718 Combining PBF-LB/M and In Situ High-Speed Milling JF - Journal of Manufacturing and Materials Processing N2 - As the correlation between design rules and process limitations is of the upmost importance for the full exploitation of any manufacturing technology, we report a design guide for hybrid-additive manufacturing of Inconel 718. Basic limitations need to be evaluated for this particular hybrid approach that combines laser powder bed fusion (PBF-LB/M) and in situ high-speed milling. Fundamental geometric limitations are examined with regard to the minimum feasible wall thickness, cylinders, overhanging structures, and chamfers. Furthermore, geometrical restrictions due to the integrated three-axis milling process with respect to inclinations, inner angles, notches, and boreholes are investigated. From these findings, we derive design guidelines for a reliable build process using this hybrid manufacturing. Additionally, a design guideline for the hybrid-additive manufacturing approach is presented, depicting a step-to-step guide for the adjustment of constructions. To demonstrate this, a powder nozzle for a direct energy deposition (DED-LB/M) process is redesigned following the previously defined guidelines. This redesign encompasses analysis of the existing component and identification of problematic areas such as flat angles, leading to a new construction that is suitable for a hybrid-additive manufacturing approach. KW - Hochgeschwindigkeitsfräsen KW - Rapid Prototyping, Fertigung Y1 - 2025 U6 - https://doi.org/http://dx.doi.org/10.3390/jmmp9030088 SN - 2504-4494 VL - 9 IS - 3 PB - MDPI AG ER - TY - JOUR A1 - Sommer, David A1 - Truetsch, Ben A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Fractographic Analysis and Fatigue Behavior of Additively Manufactured Ni‐Superalloy Components with Post Processing Heat Treatment and Hot Isostatic Pressing JF - Advanced Engineering Materials N2 - A report is made on a study of mechanical properties and fractographic characteristics of laser powder bed fusion (PBF‐LB/M)‐built Inconel 718, performing heat treatments and hot‐isostatic pressing. For this, tensile components are heat‐treated by different processes, as namely stress relief (SR), SR and double aging (SR + DA), and hot‐isostatic pressing are conducted. For the mechanical testing, the ultimate tensile strength (UTS) as well as the fatigue behavior are evaluated, examining differences in maximum load behavior, elongation, and the different regimes of fatigue. As changes in material structure can be observed, the sole SR leads to a diminished UTS, while the combination of SR + DA develops an UTS of Rm = 1277 MPa. Within the fatigue behavior, the HIP shows a very balanced material structure with an increased high cycle and very high cycle regime, as the texture gets homogenized during the heat treatment. The metallographic analysis can quantify the material changes, as the density and the hardness are improved by virtue of the heat treatments. Furthermore, the fractographic analysis shows the differences in fracture behavior, arising due to the microstructural changes, as crack initiation points, crack propagation, and forced fractures can be categorized by scanning electron microscopy. KW - Rapid Prototyping, Fertigung KW - Isostatisches Heißpressen KW - Selektives Laserschmelzen Y1 - 2025 U6 - https://doi.org/http://dx.doi.org/10.1002/adem.202500078 SN - 1438-1656 VL - 27 IS - 12 PB - Wiley ER - TY - JOUR A1 - Flachs, Dennis A1 - Emmerich, Florian A1 - Roth, Gian-Luca A1 - Hellmann, Ralf A1 - Thielemann, Christiane T1 - Laser-bonding of FEP/FEP interfaces for a flexiblemanufacturing process of ferroelectrets JF - Journal of Physics: Conference Series N2 - This paper presents an optimized laser-bonding process for piezoelectric energy-harvesters based on thin fluorinated-ethylene-propylene (FEP) foils, using an ultra-short-pulse(USP) laser. Due to the minimized thermal stress in the material during bonding, achieved bypulse durations of few picoseconds, we created seams down to 40μm width without generatingholes in the 12.5μm thick FEP-foils. Using a galvanometer scanning system allowed for fastbonding-speed up to several centimeters per second, making the process also suitable for largestructures and areas. The achieved bond strength of the seams under influence of shearingstress was examined using tensile testing, which showed a sufficient strength of about 25 % of the maximum strength of an unbonded, single layer of FEP. KW - Ultrakurzzeitlaser KW - Piezoelektrizität Y1 - 2019 U6 - https://doi.org/10.1088/1742-6596/1407/1/012107 VL - 1407 IS - 012107 SP - 1 EP - 5 ER - TY - JOUR A1 - Häfner, Tom A1 - Bischoff, Kay A1 - Helm, Jonas A1 - Kunz, Tim A1 - Rung, Stefan A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Picosecond laser microvia drilling of ABF material using MHz burst mode JF - Materials Research Express N2 - We report on a comprehensive study of laser percussion microvia drilling of Ajinomoto build-up film (ABF) material using an ultrashort pulsed laser in MHz burst mode. After laser processing, microvia drilling quality is being evaluated by the fabricated diameter and taper using laser scanning microscopy and metallography. The influences of the incubation effect, heat accumulation and shielding effects as a result of pulse to pulse interactions are being discussed on the ablation threshold, penetration depth and laser microvia drilling quality. We find that an increasing heat accumulation in MHz burst mode processing is responsible for the void formation and delamination of the insulating ABF layer. Therefore, the parameter clearance is introduced to evaluate these effects on the microvia sidewalls. For a comparable clearance, applying 2 intra-burst pulses achieves an average reduced taper of down to 19.5% compared to single pulse mode. At the same time, a reduced laser drilling time of 16.7% per microvia highlights the enormous potential of the MHz burst mode for laser drilling of ABF material in printed circuit board fabrication. KW - Ultrakurzzeitlaser KW - Pikosekundenbereich Y1 - 2023 VL - 10 IS - 9 SP - 096301 EP - 096311 ER - TY - CHAP A1 - Franz, Daniel A1 - Mücke, Dominik A1 - Kunz, Tim A1 - Rung, Stefan A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - UV picosecond laser drilling of ABF film material for printed circuit boards using laser burst mode and beam shaping T2 - International Congress on Applications of Lasers & Electro-Optics (ICALEO) 2023 N2 - We report on an experimental study of laser microvia percussion drilling of Ajinomoto build-up film (ABF) material using an ultrashort pulsed laser in the ultraviolet region. In order to optimize the drilling quality in terms of the fabricated taper and microvia diameter, the MHz burst mode is used in combination with both a Gaussian and a top-hat laser intensity distribution. To evaluate the influence of the top-hat beam shape on defocusing, microvias were drilled at different z-positions in the range of 200 μm evenly distributed around the focal plane. The drilling quality was evaluated by laser scanning microscopy and metallography. When using a top-hat beam shape, a reduced taper of down to 26.8 % on average is achieved compared to a Gaussian beam. For the application of 3 intra-burst pulses, we find an improvement of the taper by 11.6% compared to the single pulse mode due to the increasing heat accumulation. Using a top-hat beam shape in combination with the MHz burst mode, an 18.2 % reduction in laser drilling time for a microvia with a similar taper is remarkable, demonstrating the high potential for scaling throughput in electronics manufacturing. KW - Pikosekundenbereich KW - Laserbohren Y1 - 2023 ER - TY - JOUR A1 - Franz, Daniel A1 - Yang, Yongting A1 - Michel, Luis A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Evaluation of an ultrashort pulsed laser robot system for flexible and large-area micromachining JF - Journal of Laser Applications N2 - We report for the first time on the realization, characterization and application of an ultrashort pulsed laser robot system for flexible and large-area 2D and 3D laser micromachining with 6 articulated axes. To characterize the dynamic positioning of the laser beam during and after axes movement, CMOS image sensors were integrated into the beam path. A method introduced for the alignment of the optical axes allows a reduction of the deviations in laser beam positioning to less than 141.8 ± 92.9 μm within a 110° rotation range of axis 4. In addition, a high laser beam positioning repeatability of less than 102.2 μm is demonstrated over a total period of 14 h for a movement of axis 5 within a range of 0° to 90°. Initial laser cutting, laser structuring and laser marking applications on automotive dashboards and glass substrates are presented for flexible and large area 2D and 3D manufacturing. By applying a special laser cutting strategy for processing AF 32 eco thin glass, high cutting quality is achieved with a taper of up to 96.3% without the generation of cracks, demonstrating the innovative potential of the high-precision laser robot system. Nonetheless, different identified inherent influences of each axis 1–5 during robot axis movement demand for an innovative beam stabilization concept to achieve high precision in laser beam positioning. KW - Ultrakurzzeitlaser KW - Industrieroboter Y1 - 2023 N1 - Proceedings of the International Congress of Applications of Lasers & Electro-Optics (ICALEO 2023) VL - 35 IS - 4 SP - 042057 EP - 042065 ER - TY - CHAP A1 - Franz, Daniel A1 - Roth, Gian-Luca A1 - Rung, Stefan A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Superimposed beam deflection using acousto-optical deflectors in combination with a galvanometer scanner T2 - Lasers in Manufacturing - LiM 2021 KW - Galvanometer Y1 - 2021 ER - TY - JOUR A1 - Franz, Daniel A1 - Schuster, David A1 - Schwarz, Simon A1 - Rung, Stefan A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Fabrication and analysis of through-glass vias for glass-based electronic packaging using an ultrashort pulsed laser JF - Optics and Lasers in Engineering KW - Ultrakurzzeitlaser KW - Laserbohren Y1 - 2025 U6 - https://doi.org/10.1016/j.optlaseng.2025.109106 SN - 0143-8166 VL - 193 PB - Elsevier BV ER - TY - JOUR A1 - Franz, Daniel A1 - Yang, Yongting A1 - Esen, Cemal A1 - Hellmann, Ralf ED - Esen, Cemal T1 - Flexible 2D Laser Cutting of Ultra-Thin Glass Using an Ultrashort Pulsed Laser Robotic System JF - Journal of Laser Micro/Nanoengineering KW - Laserschneiden KW - Ultrakurzzeitlaser Y1 - 2026 U6 - https://doi.org/10.2961/jlmn.2026.01.2004 SN - 1880-0688 VL - 21 IS - 1 PB - Japan Laser Processing Society ER - TY - CHAP A1 - Franz, Daniel A1 - Schuster, David A1 - Schwarz, Simon A1 - Rung, Stefan A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Ultrashort pulsed laser drilling of through-glass vias for glass core packaging T2 - Proceedings of LAMP2025 – the 9th International Congress on Laser Advanced Materials Processing, Ise-City, Japan, June 2025 KW - Ultrakurzzeitlaser KW - Laserbohren Y1 - 2025 ER - TY - CHAP A1 - Sommer, David A1 - Truetsch, Ben A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Microstructural Fracture Behaviour of PBF-LB/M Inconel 718 Components within different HIP Processes T2 - Lasers in Manufacturing, München, Juni 2025 N2 - We report on a study of different hot isostatic pressing (HIP) cycles, improving the mechanical properties of additively manufactured Inconel 718 components. For this, PBF-LB/M built components are post-processed by different HIP sequences, as gas pressure and processing time are varied, leading to differences in microstructure and material characteristics. Static and dynamic mechanical testing are performed, evaluating the changes in mechanical properties with particular focus on the ultimate tensile strength and endurance limit. Furthermore, metallographic analysis is employed to investigate the achieved density and microhardness. Microstructural analysis, showing the grain boundaries, is used to identify generated phases and precipitations of the material matrix. Moreover, the fracture behaviour is classified by grain deformation during mechanical testing. As the HIP leads to microstructural changes of Inconel 718 components, mechanical properties can be improved significantly, enhancing the ultimate tensile strength and simultaneously the endurance limit. KW - Isostatisches Heißpressen Y1 - 2025 ER - TY - CHAP A1 - Sommer, David A1 - Peters, Maximilian A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Influence of in-situ high-speed milling within a hybrid additive manufacturing approach on the fatigue behaviour of Inconel 718 lattice structures T2 - Lasers in Manufacturing, München, Juni 2025 N2 - As lattice structures in various designs are used in additive manufacturing for lightweight components, the mechanical characterisation and fracture behaviour is of upmost importance for their industrial application. In this study, the fatigue behaviour of Inconel 718 lattice structures is evaluated, comparing sole PBF-LB/M to a hybrid additive manufacturing process combining PBF-LB/M with in-situ high-speed milling. At first, the static and dynamic mechanical load behaviour of different packing densities is analysed, determining the compressive strength and the endurance limit. Secondly, hybrid additive manufactured components are compared to PBF-LB/M built parts with respect to these mechanical properties, revealing improved compressive properties and modified regimes of fatigue. In addition, differences in fracture behaviour are qualified by fractographic and surface analysis. Overall, it can be summarized that the mechanical load characteristics, especially the fatigue behaviour, are improved for hybrid additively manufactured components with a superior surface quality of Ra < 1 µm. KW - Hochgeschwindigkeitsfräsen Y1 - 2025 ER - TY - CHAP A1 - Bischoff, Kay A1 - Kefer, Stefan A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Towards entirely ultrashort pulsed laser based process chain to manufacture polymer lab-on-chip systems with electrical, photonic, and microfluidic functionalities T2 - The 5th Smart Laser Processing Conference (SLPC) KW - Ultrakurzzeitlaser Y1 - 2024 UR - https://confit.atlas.jp/guide/event/opic2024/subject/SLPCp-22/date?cryptoId=&lo=fin&eventCode=opic2024 ER - TY - JOUR A1 - Sommer, David A1 - Hornung, Simon A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - In situ high-speed milling and thermal post-processing for the optimization of fatigue behaviour of additive-manufactured IN718 components JF - Journal of Materials Research N2 - We report on the optimization of tensile properties and fatigue behaviour of Laser Powder Bed Fusion (PBF-LB/M)-built IN718 components employing an innovative hybrid additive-subtractive technology and various heat treatments. The hybrid approach comprises PBF-LB/M with in situ high-speed milling. Double Ageing, solution and ageing treatment, as well as Hot Isostatic Pressing are used to improve ultimate tensile strength and endurance limit. As a result, the advantages of the hybrid-machined components are clearly demonstrated by their improved mechanical properties. These enhanced properties are associated to changes in microstructure, which are identified as precipitations of IN718. Furthermore, crack initiation and crack growth for high cycle numbers as well as sudden fractures are determined. KW - Hochgeschwindigkeitsfräsen Y1 - 2025 U6 - https://doi.org/https://doi.org/10.1557/s43578-025-01656-x SN - 0884-2914 VL - 40 IS - 16 SP - 2388 EP - 2401 PB - Springer Science and Business Media LLC ER - TY - JOUR A1 - Sommer, David A1 - Truetsch, Ben A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Effect of Hot Isostatic Pressing and Sequenced Heat Treatment on the Mechanical Properties of Hybrid Additive Manufactured Inconel 718 Components JF - Journal of Manufacturing and Materials Processing N2 - We report on the effect of hot isostatic pressing combined with solution and ageing treatment in different sequences on the mechanical properties of Inconel 718 specimens, which in turn have been fabricated by a hybrid additive manufacturing approach. The latter combines conventional laser powder bed fusion and in-situ high speed milling, yielding superior surface quality as being quantified by Ra about 1 μm. In a comparative study between hybrid additively manufactured parts and those built without milling, we find that, in general, any combination of heat treatment leads to a higher ultimate tensile strength and an improved endurance limit, while, however, hot isostatic pressing affects these figures of merit most. In addition, metallographic analysis reveals increased density and hardness for hot isostatic pressed parts due to precipitation hardening. These improvements of the mechanical properties are found to be even more pronounced when the printed parts are manufactured by the hybrid additive approach, i.e., for parts with improved surface conditions. KW - Isostatisches Heißpressen Y1 - 2025 U6 - https://doi.org/https://doi.org/10.3390/jmmp9110378 SN - 2504-4494 VL - 9 IS - 11 PB - MDPI AG ER - TY - JOUR A1 - Sommer, David A1 - Peters, Maximilian A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Enhancing mechanical properties of double-periodic lattice structures using hybrid-additive manufacturing JF - The International Journal of Advanced Manufacturing Technology N2 - We report on a study of the mechanical properties of hybrid-additive manufactured Inconel 718 double-periodic lattice structures. For this, a hybrid approach, combining Laser Powder Bed Fusion (PBF-LB/M) and in-situ high-speed milling, is employed. Within this technique, the PBF-LB/M process is interrupted after several layers, as the milling process starts, enabling a machining of inlaying structures. KW - Hochgeschwindigkeitsfräsen Y1 - 2026 U6 - https://doi.org/https://doi.org/10.1007/s00170-025-17373-y SN - 0268-3768 VL - 142 IS - 7-8 SP - 4065 EP - 4071 PB - Springer Science and Business Media LLC ER - TY - JOUR A1 - Sommer, David A1 - Hornung, Simon A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Design Guide for Hybrid-Additive Manufacturing of Inconel 718 Combining PBF-LB/M and In Situ High-Speed Milling JF - Journal of Manufacturing and Materials Processing N2 - As the correlation between design rules and process limitations is of the upmost importance for the full exploitation of any manufacturing technology, we report a design guide for hybrid-additive manufacturing of Inconel 718. Basic limitations need to be evaluated for this particular hybrid approach that combines laser powder bed fusion (PBF-LB/M) and in situ high-speed milling. Fundamental geometric limitations are examined with regard to the minimum feasible wall thickness, cylinders, overhanging structures, and chamfers. Furthermore, geometrical restrictions due to the integrated three-axis milling process with respect to inclinations, inner angles, notches, and boreholes are investigated. From these findings, we derive design guidelines for a reliable build process using this hybrid manufacturing. Additionally, a design guideline for the hybrid-additive manufacturing approach is presented, depicting a step-to-step guide for the adjustment of constructions. To demonstrate this, a powder nozzle for a direct energy deposition (DED-LB/M) process is redesigned following the previously defined guidelines. This redesign encompasses analysis of the existing component and identification of problematic areas such as flat angles, leading to a new construction that is suitable for a hybrid-additive manufacturing approach. KW - Hochgeschwindigkeitsfräsen Y1 - 2025 U6 - https://doi.org/https://doi.org/10.3390/jmmp9030088 SN - 2504-4494 VL - 9 IS - 3 PB - MDPI AG ER - TY - JOUR A1 - Sommer, David A1 - Hellmann, Ralf T1 - On the Impact of Process Parameters and Ambient Temperature on Tensile Strength and Fatigue Behaviour of SLS-Built Polyamide-12 JF - Key Engineering Materials N2 - We report on a comprehensive study of tensile strength and fatigue behaviour depending on process parameters and ambient temperature. For this, Polyamide-12 components are fabricated using Selective Laser Sintering. Firstly, different process parameters like, e.g, scan-speed, laser power, and applied energy density are varied. Secondly, the ambient temperature during testing is varied, evaluating the impact of decreased, respectively increased test temperatures on the characteristics of the Polyamide-12 components. For all components, the static and dynamic mechanical load behaviour is investigated, quantifying the changes in the Ultimate Tensile Strength (UTS) and the endurance limit. As the applied energy density transpires as a decisive parameter, a variation leads to significant changes in UTS and endurance limit, whereas an adjustment of scan-speed and laser power at a constant energy density do not affect the mechanical properties. Finally, the ambient temperature during testing is evaluated, demonstrating different ambient application conditions. The impact of an active cooling of the component as well as increased temperatures on the mechanical behaviour is tested, providing fundamental findings on the operating life of Polyamide-12 components. KW - Selektives Laserstrahlsintern Y1 - 2025 U6 - https://doi.org/https://doi.org/10.4028/p-F0XrgQ SN - 1662-9795 VL - 1019 SP - 31 EP - 36 PB - Trans Tech Publications, Ltd. ER - TY - JOUR A1 - Bischoff, Kay A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Femtosecond reductive Laser Sintering under multiple focus conditions for rapid production of conductive copper layers JF - Procedia CIRP N2 - Reductive laser sintering is an advantageous alternative to conventional precious metal- or mask-based processes for the electrification of polymers and has been demonstrated in a variety of sensor applications. Ultrashort pulsed lasers promise precise control of the energy applied during sintering and ablation. Besides fine tracks, microelectronic circuits include wide contact pads, which leads to time-consuming processing with small laser spot sizes. The use of larger focal diameters, which results in wider lines and thus a higher metallization rate, is accompanied by a loss of structure resolution. We show the influence of focusing on the femtosecond reductive laser sintering of copper(II) oxide on cyclic olefin copolymers. While small focus diameters down to 23 µm provide high accuracy, increasing the laser power and the focus diameter up to 400 µm significantly accelerates the process, first by enlarging the line width and second by increasing the scanning speed due to the exposure time. Two-dimensional copper electrodes for 23 µm and 200 µm focal diameters were generated by hatching individual lines and compared with respect to their morphology. The enlarged spot can be used for high-speed generation of electrodes with a surface conductivity of 117 mΩ/sq. Hybrid processes with production under combined focusing conditions are presented for the first time. KW - Laserstrahlsintern KW - Ultrakurzzeitlaser Y1 - 2024 U6 - https://doi.org/10.1016/j.procir.2024.08.189 SN - 2212-8271 VL - 124 IS - 124 SP - 629 EP - 633 PB - Elsevier BV ER - TY - JOUR A1 - Sommer, David A1 - Truetsch, Ben A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Fractographic Analysis and Fatigue Behavior of Additively Manufactured Ni‐Superalloy Components with Post Processing Heat Treatment and Hot Isostatic Pressing JF - Advanced Engineering Materials N2 - A report is made on a study of mechanical properties and fractographic characteristics of laser powder bed fusion (PBF‐LB/M)‐built Inconel 718, performing heat treatments and hot‐isostatic pressing. For this, tensile components are heat‐treated by different processes, as namely stress relief (SR), SR and double aging (SR + DA), and hot‐isostatic pressing are conducted. For the mechanical testing, the ultimate tensile strength (UTS) as well as the fatigue behavior are evaluated, examining differences in maximum load behavior, elongation, and the different regimes of fatigue. As changes in material structure can be observed, the sole SR leads to a diminished UTS, while the combination of SR + DA develops an UTS of R m  = 1277 MPa. Within the fatigue behavior, the HIP shows a very balanced material structure with an increased high cycle and very high cycle regime, as the texture gets homogenized during the heat treatment. The metallographic analysis can quantify the material changes, as the density and the hardness are improved by virtue of the heat treatments. Furthermore, the fractographic analysis shows the differences in fracture behavior, arising due to the microstructural changes, as crack initiation points, crack propagation, and forced fractures can be categorized by scanning electron microscopy. KW - Nickellegierung KW - Superlegierung Y1 - 2025 U6 - https://doi.org/https://doi.org/10.1002/adem.202500078 SN - 1438-1656 VL - 27 IS - 12 PB - Wiley ER - TY - JOUR A1 - Sommer, David A1 - Hornung, Simon A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Evaluation of the Impact of Heat Treatment Processes and Surface Conditions on the Mechanical Properties of PBF-LB/M-Built Inconel 718 Components JF - Materials Science Forum N2 - A study of the mechanical properties of hybrid additive manufactured IN718 components is presented, optimising mechanical properties due to an in-situ high-speed milling and different heat treatment processes. At first, the impact of different heat treatment processes is investigated, as the changes in microstructure during the process lead to different mechanical properties. Static and dynamic mechanical load behaviour is tested, quantifying microstructural changes by means of the Ultimate Tensile Strength (UTS) and the endurance limit. Furthermore, sole PBF-LB/M- and hybrid built components are compared, investigating the effect of a surface finish on the static and dynamic load behaviour, as superficial cracks and melting errors diminish the UTS and the endurance limit of PBF-LB/M-built components. Within these experiments, a change of fatigue behaviour for the heat-treatedstates can be observed, compared to the as-built state of the PBF-LB/M, as the development of different phases during heat treatment leads to an improvement of the endurance limit for, e.g., solution and ageing treated components. Additionally, the improvement of the surface quality to Ra = 2 µm leads to a significant increase of the dynamic mechanical load behaviour of hybrid-built components, as superficial cracks and surface defects are reduced. KW - Hochgeschwindigkeitsfräsen Y1 - 2025 U6 - https://doi.org/https://doi.org/10.4028/p-9gR9cI SN - 1662-9752 VL - 1155 SP - 11 EP - 17 PB - Trans Tech Publications, Ltd. ER -