TY - JOUR A1 - Adelmann, Benedikt A1 - Hellmann, Ralf T1 - A study of SiC decomposition under laser irradiation JF - Applied Physics A KW - Laserbestrahlung KW - Faserlaser Y1 - 2017 VL - 2017 IS - 123 SP - 454 EP - 452 ER - TY - JOUR A1 - Adelmann, Benedikt A1 - Hellmann, Ralf T1 - Numerical and experimental study on flexural strength of laser microstructured alumina JF - Advances in Ceramic Science and Engineering KW - Faserlaser KW - Aluminiumoxide Y1 - 2016 VL - 2016 IS - 5 SP - 28 EP - 34 ER - TY - JOUR A1 - Adelmann, Benedikt A1 - Hellmann, Ralf T1 - SiC absorption of near-infrared laser radiation at high temperatures JF - Applied Physics A KW - Laserbestrahlung KW - NIR Y1 - 2016 VL - 2016 IS - 122 SP - 1 EP - 7 ER - TY - CHAP A1 - Adelmann, Benedikt A1 - Hellmann, Ralf T1 - Comparative study of experimentally determined and simulated mechanical strength of fiber laser cut alumina substrates T2 - Proceedings of the 7th International Congress on Laser Advanced Materials Processing KW - Laserschneiden KW - Faserlaser KW - Aluminiumoxide Y1 - 2015 VL - 2015 IS - 15 SP - 1 EP - 9 ER - TY - JOUR A1 - Adelmann, Benedikt A1 - Roth, Gian-Luca A1 - Hellmann, Ralf T1 - Back side ablation of SiC diodes using a q-switched NIR laser JF - Journal of Laser Micro/Nanoengineering KW - Halbleiterdiode KW - NIR Y1 - 2015 VL - 10 SP - 190 EP - 194 ER - TY - JOUR A1 - Adelmann, Benedikt A1 - Hellmann, Ralf T1 - Rapid micro hole laser drilling in ceramic substrates using single mode fiber laser JF - Journal of Materials Processing Technology KW - Laserbohren KW - Faserlaser Y1 - 2015 VL - 2015 IS - 221 SP - 80 EP - 86 ER - TY - JOUR A1 - Preusch, Florian A1 - Adelmann, Benedikt A1 - Hellmann, Ralf T1 - Micromachining of AlN and Al2O3 using fiber laser JF - Micromachines KW - Faserlaser KW - NIR KW - Mikrobearbeitung Y1 - 2014 VL - 2014 IS - 5 SP - 1051 EP - 1060 ER - TY - JOUR A1 - Adelmann, Benedikt A1 - Hellmann, Ralf T1 - Investigation on flexural strength changes of alumina caused by cutting using fiber laser JF - Journal of Laser Micro/Nanoengineering KW - Laserschneiden KW - Faserlaser Y1 - 2014 VL - 9 SP - 153 EP - 160 ER - TY - CHAP A1 - Adelmann, Benedikt A1 - Neumeier, Benedikt A1 - Schleier, Max A1 - Wilmann, Eugen A1 - Hellmann, Ralf T1 - Optical cutting tear detection system for industrial fiber laser based cutting machines T2 - Lasers in Manufacturing Conference 2015 KW - Laserschneiden KW - Faserlaser Y1 - 2015 ER - TY - JOUR A1 - Adelmann, Benedikt A1 - Schleier, Max A1 - Neumeier, Benedikt A1 - Wilmann, Eugen A1 - Hellmann, Ralf T1 - Optical cutting interruption sensor for fiber lasers JF - Applied Sciences KW - Laserschneiden KW - Faserlaser Y1 - 2015 VL - 2015 IS - 5/3 SP - 544 EP - 554 ER - TY - JOUR A1 - Adelmann, Benedikt A1 - Ngo, Chan A1 - Hellmann, Ralf T1 - High aspect ratio cutting of metals using water jet guided laser JF - The International Journal of Advanced Manufacturing Technology KW - Laserschneiden KW - Wasserstrahlschneiden Y1 - 2015 VL - 2015 IS - 80 SP - 2053 EP - 2060 ER - TY - JOUR A1 - Roth, Gian-Luca A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Femtosecond laser direct generation of 3D-microfluidic channels inside bulk PMMA JF - Optics Express N2 - We report on laser direct generation of 3D-microchannels for microfluidic applications inside PMMA bulk material by focused femtosecond pulses. Inner lying channels with cross sectional areas from 100 µm2 to 4400 µm2 are directly created in the volume of a PMMA substrate. Using the presented process, the channel length is fundamentally unlimited. Here we demonstrate a channel length of 6 meters inside a substrate with dimensions of 20 × 20 × 1.1 mm. The formation of the micro channels is based on nonlinear absorption around the focal volume that triggers a material modification. The modified volume can be selectively opened to form the channel by a subsequent annealing process. The cross section of the channel is strongly influenced by the energy distribution and illumination around the focal volume determined by the optical setup and process design. The 3D channel layout can easily be realized by moving the specimen using 3D motorized stage, allowing freely chosen complex shaped channel architectures. Within a comprehensive parameter study, varying laser power, number of multi-passes, writing speed and writing depths, we identify an optimized process in terms of attainable channel height, width and aspect ratio, as well as process stability and reproducibility. The proof of concept for an application in three dimensional microfluidic systems is provided by florescence microscopy using a dye rhodamine B solution in isopropanol. KW - Femtosekundenlaser Y1 - 2017 VL - 25 IS - 15 SP - 18442 EP - 18450 ER - TY - JOUR A1 - Roth, Gian-Luca A1 - Rung, Stefan A1 - Hellmann, Ralf T1 - Welding of transparent polymers using femtosecond laser JF - Applied Physics A N2 - Based on nonlinear absorption, we report on laser welding of cycloolefin copolymers without any additional absorption layer employing infrared femtosecond laser. To the best of our knowledge, this is the first report of ultrashort laser welding of this material class, revealing a remarkable high processing speed of 20 mm/s in a single pass mode. Using a 1028 nm laser having a pulse duration of 220 fs at a repetition rate of 571 kHz leads to a welding seam width between 38 and 137 μm, depending on the applied laser average power. The welded joint is characterized by a maximum shear strength of 40 MPa. The experimental results are compared to those reported for femtosecond laser welding of PMMA and to those published for using a Thulium fiber laser. KW - Femtosekundenlaser KW - Laserschweißen Y1 - 2016 VL - 122 IS - 2 SP - 86 EP - 90 ER - TY - JOUR A1 - Roth, Gian-Luca A1 - Rung, Stefan A1 - Hellmann, Ralf T1 - Ultrashort pulse laser micro-welding of cyclo-olefin copolymers JF - Optics and Lasers in Engineering N2 - We report on the joining of transparent thermoplastic polymers using infrared femtosecond laser pulses. Due to nonlinear absorption, the developed micro-welding process for cyclo-olefin copolymers does not require any intermediate absorbing layers or any surface pre-processing of the welding partners. In view of an optimized and stable micro-welding process, the influence of the welding speed and focal position on both, the quality and shear force strength are investigated. We highlight that welding seam widths of down to 65 µm are feasible for welding speeds of up to 75 mm/s. However, a variation of the welding speed affects the required focal position for a successful joining process. The shear force strength of the welding seam is determined to 37 MPa, which corresponds to 64% of the shear strength of the bulk material and is not affected by the welding speed. KW - Laserschweißen KW - Femtosekundenlaser Y1 - 2017 VL - 93 SP - 178 EP - 181 ER - TY - JOUR A1 - Rung, Stefan A1 - Bokan, Kevin A1 - Schwarz, Simon A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Influence of laser wavelength on the modification of friction between 100Cr6 steel and polytetrafluoroethylene by femtosecond laser-induced periodic surface structures JF - Journal of Laser Applications KW - Femtosekundenlaser Y1 - 2020 U6 - https://doi.org/10.2351/7.0000036 VL - 32 IS - 3 SP - 032017-1 EP - 032017-6 ER - TY - JOUR A1 - Roth, Gian-Luca A1 - Hessler, Steffen A1 - Kefer, Stefan A1 - Girschikofsky, Maiko A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Femtosecond laser inscription of waveguides and Bragg gratings in transparent cyclic olefin copolymers JF - Optics Express N2 - We report on a femtosecond laser based fabrication technique that enables simultaneous single-step generation of optical waveguides and Bragg gratings inside bulk cyclic olefin copolymers. Due to the nonlinear absorption of focused and spatially modulated laser radiation with a wavelength of 514 nm and a pulse duration of 450 fs, a modification concluding a refractive index shift increase inside the substrate can be achieved. A sophisticated characterization of the generated waveguides by means of an elaborate cut-back method reveals a maximum attenuation of 3.2 dB/cm. Additionally, a Mach-Zehnder interferometer is used to examine the waveguide’s refractive index profile. The integrated Bragg grating structures exhibit reflectivities up to 95 % and a spectral full width at half maximum of 288 pm, at a Bragg wavelength of 1582 nm, whereas the grating period can be deliberately chosen by adapting the fabrication parameters. Thus, due to its increased flexibility and the resulting dispensability of cost-intensive phase masks, this method constitutes an especially promising fabrication process for polymer Bragg gratings inside of bulk materials. KW - Femtosekundenlaser KW - Wellenleiter KW - Bragg-Reflektor Y1 - 2020 U6 - https://doi.org/10.1364/OE.388364 VL - 28 IS - 12 SP - 18077 EP - 18084 ER - TY - JOUR A1 - Roth, Gian-Luca A1 - Haubner, Julian A1 - Kefer, Stefan A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Fs-laser based hybrid micromachining for polymer micro-opto electrical systems JF - Optics and Lasers in Engineering N2 - We report on femtosecond laser direct writing of electrically conductive copper structures on transparent cyclic olefin copolymer based planar optical chips. The process is based on a laser-induced reduction of CuO nanoparticles dissolved in a water-soluble resin. Thus generated conductive copper structures are characterized with respect to their chemical composition and electrical resistivity. In addition, the application of ultrashort laser pulses enables a hybrid micromachining approach comprising ablation of polymer substrates and the fabrication of copper patterns in a single laser processing setup. A possible electro-optical application of this approach is demonstrated by employing the copper structures as an electro-thermal microheater in combination with a polymer planar optical Bragg grating sensor. This, in turn, highlights the potential of generating copious micro-opto-electro-mechanical polymer structures for numerous sensing applications ranging from Lab-on-Chip to environmental applications using the presented hybrid micromachining approach. KW - Femtosekundenlaser KW - Bragg-Reflektor Y1 - 2020 U6 - https://doi.org/10.1016/j.optlaseng.2020.106362 VL - 137 IS - 10362 SP - 1 EP - 8 ER - TY - JOUR A1 - Kefer, Stefan A1 - Sauer, Theresia A1 - Hessler, Steffen A1 - Kaloudis, Michael A1 - Hellmann, Ralf T1 - Microstructure-Based Fiber-to-Chip Coupling of Polymer Planar Bragg Gratings for Harsh Environment Applications JF - Sensors (Basel) N2 - This article proposes and demonstrates a robust microstructure-based fiber-to-chip coupling scheme for planar Bragg grating devices. A polymer planar Bragg grating substrate is manufactured and microstructured by means of a micromilling process, while the respective photonic structures are generated by employing a sophisticated single-writing UV-exposure method. A stripped standard single mode fiber is inserted into the microstructure, which is filled with a UV-curable adhesive, and aligned with the integrated waveguide. After curing, final sensor assembly and thermal treatment, the proposed coupling scheme is capable of withstanding pressures up to 10 bar, at room temperature, and pressures up to 7.5 bar at an elevated temperature of 120 °C. Additionally, the coupling scheme is exceedingly robust towards tensile forces, limited only by the tensile strength of the employed single mode fiber. Due to its outstanding robustness, the coupling scheme enables the application of planar Bragg grating devices in harsh environments. This fact is underlined by integrating a microstructure-coupled photonic device into the center of a commercial-grade carbon fiber-reinforced polymer specimen. After its integration, the polymer-based Bragg grating sensor still exhibits a reflection peak with a dynamic range of 24 dB, and can thus be employed for sensing purposes. KW - Bragg grating sensors KW - cyclic olefin copolymers KW - fiber-to-chip coupling KW - microstructures KW - micromilling Y1 - 2020 UR - https://www.mdpi.com/1424-8220/20/19/5452/htm U6 - https://doi.org/10.3390/s20195452 VL - 20 IS - 19 SP - 5452 EP - 5452 ER - TY - JOUR A1 - Schwarz, Simon A1 - Roth, Gian-Luca A1 - Rung, Stefan A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Fabrication and evaluation of negative axicons for ultrashort pulsed laser applications JF - Optics Express KW - Ultrakurzzeitlaser Y1 - 2020 VL - Optics Express IS - 28 SP - 26207 EP - 26217 ER - TY - CHAP A1 - Schwarz, Simon A1 - Rung, Stefan A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Manufacturing of optical elements by non-contact laser processing T2 - Seventh European Seminar on Precision Optics Manufacturing, Proceedings KW - Laserbearbeitung KW - Optisches Bauelement Y1 - 2020 IS - 11478 ER - TY - JOUR A1 - Zettl, Julian A1 - Klar, Maximilian A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Generation of Rotationally Symmetric Micro Tools using Ultrashort Laser Pulses JF - Journal of Laser Micro/Nanoengeneering N2 - We report on the fabrication of rotationally symmetrical geometries with ultrashort laser pulses impinging the constant rotating workpiece tangentially. This particular micro machin-ing of geometry is referred to as laser turning, upon which the laser beam is, in general, orient-ed perpendicularly to the rotational axis and tangentially to the work piece. The target geome-try is realized by moving the constantly rotating specimen according to the specified geometry along the focused laser spot. In contrast to mechanical turning, laser turning induces a mini-mized amount of lateral forces through friction that might deteriorate the geometry. In this study, laser turning is examined for stellite as a function of pulse energy and pulse length in the range between 240 fs and 10 ps. Shorter pulse lengths and higher pulse energies increase the achievable ablation rate, while in the pulse length regime of below 1 ps lower roughness is achieved. With a maximum ablation rate of 1.27 mm³/min and a minimum roughness of 0.17 µm, laser turning combines both, fast processing and high surface quality. KW - Ultrakurzzeitlaser Y1 - 2020 U6 - https://doi.org/10.2961/jlmn.2020.02.2007 VL - 15 IS - 2 SP - 118 EP - 122 ER - TY - JOUR A1 - Schleier, Max A1 - Adelmann, Benedikt A1 - Neumeier, Benedikt A1 - Hellmann, Ralf T1 - Burr formation detector for fiber laser cutting based on a photodiode sensor system JF - Optics & Laser Technology KW - Unique sensor; Detection of burr formation; Signal characteristics associated to melt bath dynamics; KW - Laserschneiden KW - Faserlaser Y1 - 2017 U6 - https://doi.org/10.1016/j.optlastec.2017.04.027 VL - 96 IS - - SP - 13 EP - 17 ER - TY - JOUR A1 - Schleier, Max A1 - Adelmann, Benedikt A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Cross-correlation-based algorithm for monitoring laser cutting with high-power fiber lasers JF - IEEE Sensors Journal KW - Detection algorithm, laser cutting, online monitoring KW - Laserschneiden KW - Hochleistungslaser KW - Faserlaser Y1 - 2017 U6 - https://doi.org/10.1109/JSEN.2017.2783761 VL - 18 IS - 4 SP - 1585 EP - 1590 ER - TY - JOUR A1 - Dudutis, Juozas A1 - Pipiras, Jokūbas A1 - Schwarz, Simon A1 - Rung, Stefan A1 - Hellmann, Ralf A1 - Račiukaitis, Gediminas A1 - Gečys, Paulius T1 - Laser-fabricated axicons challenging the conventional optics in glass processing applications JF - Optics Express KW - Laserbearbeitung KW - Quarzglas Y1 - 2020 VL - 28 IS - 4 SP - 5715 EP - 5730 ER - TY - JOUR A1 - Schwarz, Simon A1 - Rung, Stefan A1 - Hellmann, Ralf T1 - Two-Dimensional Low Spatial Frequency Laser-Induced Periodic Surface Structuring of Sapphire JF - Journal of Laser Micro/Nanoengineering KW - Femtosekundenlaser Y1 - 2017 VL - 12 IS - 2 SP - 67 EP - 71 ER - TY - CHAP A1 - Schwarz, Simon A1 - Rung, Stefan A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - All laser-based fabrication of optical elements T2 - Proceedings of SPIE 11270, Frontiers in Ultrafast Optics: Biomedical, Scientific, and Industrial Applications XX KW - Ultrakurzzeitlaser KW - Kohlendioxidlaser KW - Femtosekundenlaser Y1 - 2020 ER - 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 -