TY - JOUR A1 - Roth, Gian-Luca A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Circular microchannels inside bulk polymethylmethacrylate generated by femtosecond laser using slit beam shaping JF - Journal of Laser Applications N2 - The authors report on the laser generation of three-dimensional microchannel architectures with fundamentally unlimited channel length by focused 514 nm femtosecond laser pulses inside polymethylmethacrylate bulk material. The three-dimensional channel layout is implemented by moving the specimen using three-dimensional motorized stages, allowing freely chosen complex shaped channel architectures. Based on nonlinear absorption of high intensities around the focal volume, a material modification, including a refractive-index shift is triggered. After laser exposure, the channel is created by a gaseous degradation of exposed areas during an annealing process. Cross section and aspect ratio of thus generated microchannels are strongly influenced by the numerical aperture of the applied objective. To obtain a circular cross-sectional shape, high focusing objectives are necessary. In this report, the authors improve the existing setup by using an elliptical input beam, transforming the intensity distribution of the focal area in the propagation direction to a circular shape even for low and medium numerical aperture focusing objectives and therefore avoiding a small working distance as a limiting factor for processing depth. The elliptical input beam is obtained by insertion of a slit mask in front of the focusing objective. The ellipticity of the incident beam defines the aspect ratio of the generated microchannels. Furthermore, the size of internal microchannels is adjustable by varying laser and annealing process parameters. KW - Femtosekundenlaser Y1 - 2019 VL - 31 IS - 2 SP - 022603-1 EP - 022603-5 ER - TY - JOUR A1 - Roth, Gian-Luca A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Vertical Microchannels for Microfluidic Multilayer Interconnections in PMMA JF - Journal of Laser Micro/Nanoengineering N2 - We report on laser generation of microchannels by focused femtosecond laser pulses inside PMMA bulk material. This process enables direct fabrication of three dimensional microfluidic channel architectures with generally unlimited channel length. Based on nonlinear ultrashort pulse laser absorption of high intensities around the focal volume, a material modification including a refractive index shift is triggered. After laser exposure the channel is created by a gaseous degradation of exposed areas during an annealing process. The 3D channel layout can easily be realized by moving the specimen using 3D motorized stages, allowing freely chosen complex shaped channel architectures. In this report, the creation of inner lying vertical microchannels is examined. In addition to previously shown horizontal channel architectures vertical channels are necessary to fulfill the definition of an internal three-dimensional microfluidic structure and to create interconnections in multilevel lab on chip devices. The cross section and geometry of vertical channels are strongly influenced by laser parameters such as pulse duration, laser power and writing speed. The influence of these process parameters on the channels diameter is determined and parameters are optimized regarding a controllable, stable and reproducible process. The functionality of vertical microchannels as an interconnection between multilayer microfluidic devices is demonstrated using a Rhodamine B solution. KW - Polymethylmethacrylate KW - Femtosekundenlaser Y1 - 2018 VL - 13 IS - 3 SP - 155 EP - 159 ER - TY - CHAP A1 - Roth, Gian-Luca A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Internal micro structuring of transparent optical polymers by fs laser T2 - Laser-based Micro-and Nanoprocessing XIV 2020 N2 - Lab-on-chip systems are based on components to transport, mix, separate and analyse small volumes of different fluids. The consecutive integration of more complex functions into a single and compact chip demands on multilayer systems. As the classical production using a stacking and joining of single processed layers is elaborate and limited in terms of multilayer structures, an uprising trend to fabricate those devices is the internal, three dimensional processing of transparent substrates by using ultrashort laser pulses. In this study, we report on the generation of internal hollow architectures created by focused 514nm femtosecond laser pulses inside optical polymer bulk materials of different polymers. The three-dimensional channel layout is implemented by moving the sample using three-dimensional motorized stages, allowing arbitrary complex shaped internal channel architectures. Size and cross sectional shape of a single internal generated microchannel are determined by the intensity distribution of the focal voxel. In particular, we show a comprehensive parameter study to improve this laser process with respect to a higher processing speed and stability. KW - Femtosekundenlaser Y1 - 2020 U6 - https://doi.org/10.1117/12.2543487 VL - 11268 SP - 112681L PB - International Society for Optics and Photonics ER - TY - JOUR A1 - Roth, Gian-Luca A1 - Rung, Stefan A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Microchannels inside bulk PMMA generated by femtosecond laser using adaptive beam shaping JF - Optics Express N2 - In this contribution, we report on the generation of internal microchannels with basically unlimited channel length inside of PMMA bulk material by femtosecond laser. A precisely controllable and stable circular channel cross section is obtained by using a spatial light modulator to compensate the writing depth depending spherical aberration. Furthermore, the generation of a rotatable elliptical input beam by adaptive optics ensures a fitting of the beam shaping to the writing direction. In this study, we report on both, the effect of the ellipticity of the input beam and the effect of a correction of the spherical aberration on the circularity of the resulting internal microchannels. Moreover, we demonstrate the application of this writing technique by creating microfluidic testing structures inside of a transparent standard polymer. KW - Femtosekundenlaser KW - Polymethylmethacrylate Y1 - 2020 VL - 28 IS - 4 SP - 5801 EP - 5811 ER - TY - JOUR A1 - Roth, Gian-Luca A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - A New Approach to Seal Polymer Microfluidic Devices Using Ultrashort Laser Pulses JF - Journal of Laser Micro/Nanoengineering N2 - We report on the joining of two different transparent thermoplastic polymers using infrared femtosecond laser pulses. Solely being based on nonlinear absorption, the developed micro-welding process for cyclo-olefin copolymers and polycarbonate does not require any intermediate absorbing layers or any surface pre-processing of the welding partners. In addition, it circumvents any heat damages at the outer surfaces of the welding partners. Femtosecond laser welding of cyclo-olefin copolymers is improved by increasing the numerical aperture of the focusing setup resulting in a wider and more homogenous welding cross-section. Welding results are classified by shear strength and fracture modes are analyzed using scanning electron microscopy. Furthermore, we demonstrate, to the best of our knowledge, first results of ultrashort pulse laser welding of transparent polycarbonate, one of the most used transparent polymers. In view of an optimized and stable micro-welding process, we study the influence of laser power and focal position on welding results. The application of ultrashort pulse laser welding for Lab-on-chip fabrication technologies is demonstrated by sealing an ultrashort pulse laser ablated microfluidic device based on a transparent polymer substrate. KW - Femtosekundenlaser KW - Laserschweißen Y1 - 2019 VL - 14 IS - 1 SP - 49 EP - 53 ER - TY - CHAP A1 - Roth, Gian-Luca A1 - Wolf, Bastian A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Generation of internal 3D microfluidic architectures in polymers by fs laser processing T2 - Lasers in Manufacturing 2019 N2 - Microfluidic lab on chip systems require components to transport, mix, separate and analyse small volumes of different fluids. In this study, we report on the laser generation of internal hollow architectures created by focused 514 nm femtosecond laser pulses inside PMMA bulk material. Size and cross-sectional shape of a single internal generated microchannel are determined by the intensity distribution inside the focal voxel and can be controlled either by the numerical aperture of the focusing objective or by laser beam shaping. As both approaches are practically limited with respect to the realizable cross-sectional shapes, we present a process based on an internal hatching to expand the achievable channel cross-sections and thereby enable the possibility to create complex 3D shaped internal structures. This process is applied to create fully internal functional microfluidic elements such as mixers which are part of most polymer lab-on-chip systems. KW - Femtosekundenlaser Y1 - 2019 ER - TY - JOUR A1 - Roth, Gian-Luca A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Laser beam microwelding of transparent plastics JF - Joining Plastics N2 - The laser beam welding of transparent plastic components is still a challenge according to the state of the art. A new approach for the joining of two transparent polymeric components using ultrashort laser beam pulses is presented in this report. In this respect, the high peak pulse intensity associated with these lasers permits non-linear absorption processes and thus the production of a locally well-defined heat source in the interior even of a transparent material or at the boundary layer between two transparent substrates. Welding results for transparent plastics of the cycloolefin copolymer and polycarbonate types are shown in this study. In addition to the investigation into the influences of fundamental process parameters on the weld geometry, the leak tightness is demonstrated in this study using a microfluidic system. KW - Laserschweißen Y1 - 2018 VL - 12 IS - 3/4 SP - 2 EP - 8 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 - 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 - 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 - 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 - 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 - 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 - 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 - 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 - 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 - 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 - 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 -