@inproceedings{BischoffMueckeEsenetal.2023, author = {Bischoff, Kay and M{\"u}cke, Dominik and Esen, Cemal and Hellmann, Ralf}, title = {UV-Femtosecond-Laser Structuring of Silicon Carbide}, series = {International Congress on Applications of Lasers \& Electro-Optics (ICALEO) 2023}, booktitle = {International Congress on Applications of Lasers \& Electro-Optics (ICALEO) 2023}, year = {2023}, abstract = {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.}, subject = {Femtosekundenlaser}, language = {en} } @inproceedings{SommerHornungEsenetal.2024, author = {Sommer, David and Hornung, Simon and Esen, Cemal and Hellmann, Ralf}, title = {Optimization of mechanical properties of additive manufactured IN 718 parts combining LPBF and in-situ high-speed milling}, series = {Laser 3D Manufacturing XI}, booktitle = {Laser 3D Manufacturing XI}, editor = {Gu, Bo and Chen, Hongqiang}, publisher = {SPIE}, doi = {http://dx.doi.org/10.1117/12.3000952}, year = {2024}, subject = {Hochgeschwindigkeitsfr{\"a}sen}, language = {en} } @inproceedings{SommerSafiEsenetal.2024, author = {Sommer, David and Safi, Abdulrahman and Esen, Cemal and Hellmann, Ralf}, title = {Additive manufacturing of Nickel-based superalloy: optimization of surface roughness using integrated high-speed milling}, series = {Laser 3D Manufacturing XI}, booktitle = {Laser 3D Manufacturing XI}, editor = {Gu, Bo and Chen, Hongqiang}, publisher = {SPIE}, doi = {http://dx.doi.org/10.1117/12.3000972}, year = {2024}, subject = {Hochgeschwindigkeitsfr{\"a}sen}, language = {en} } @inproceedings{LutzSommerhuberKettneretal.2024, author = {Lutz, Christian and Sommerhuber, Ryan and Kettner, Matthias and Esen, Cemal and Hellmann, Ralf}, title = {Towards process control by detecting acoustic emissions during ultrashort pulsed laser ablation of multilayer materials}, series = {Laser-based Micro- and Nanoprocessing XVIII}, booktitle = {Laser-based Micro- and Nanoprocessing XVIII}, editor = {Kling, Rainer and Pfleging, Wilhelm and Sugioka, Koji}, publisher = {SPIE}, doi = {10.1117/12.3000954}, year = {2024}, abstract = {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.}, subject = {Ultrakurzzeitlaser}, language = {en} } @inproceedings{LutzHelmEsenetal.2024, author = {Lutz, Christian and Helm, Jonas and Esen, Cemal and Hellmann, Ralf}, title = {Computational optimization of borehole sequences for the reduction of heat accumulation in drilling processes using ultrashort pulse lasers}, series = {Laser-based Micro- and Nanoprocessing XVIII}, booktitle = {Laser-based Micro- and Nanoprocessing XVIII}, editor = {Kling, Rainer and Pfleging, Wilhelm and Sugioka, Koji}, publisher = {SPIE}, doi = {10.1117/12.3000930}, year = {2024}, abstract = {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.}, subject = {Ultrakurzzeitlaser}, language = {en} } @inproceedings{LutzSchwarzRungetal.2021, author = {Lutz, Christian and Schwarz, Simon and Rung, Stefan and Marx, Jan and Esen, Cemal and Hellmann, Ralf}, title = {Optical system for multi Bessel beam high power ultrashort pulsed laser processing using a spatial light modulator}, series = {Lasers in Manufacturing - LiM 2021}, booktitle = {Lasers in Manufacturing - LiM 2021}, year = {2021}, abstract = {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.}, subject = {Ultrakurzzeitlaser}, language = {en} } @inproceedings{KeferRosenbergerHessleretal.2020, author = {Kefer, Stefan and Rosenberger, Manuel and Hessler, Steffen and Girschikofsky, Maiko and Belle, Stefan and Roth, Gian-Luca and Schmauss, Bernhard and Hellmann, Ralf}, title = {Fabrication and Applications of Polymer Planar Bragg Grating Sensors based on Cyclic Olefin Copolymers}, series = {2019 Photonics \& Electromagnetics Research Symposium - Fall (PIERS - Fall)}, booktitle = {2019 Photonics \& Electromagnetics Research Symposium - Fall (PIERS - Fall)}, publisher = {Institute of Electrical and Electronics Engineers}, isbn = {978-1-7281-5304-9}, doi = {10.1109/PIERS-Fall48861.2019.9021801}, pages = {647 -- 655}, year = {2020}, abstract = {This contribution reviews recent advancements, current research and possible applications of polymer planar Bragg grating (PPBG) sensors with a focus on the utilization of cyclic olefin copolymer (COC) substrates. COC-PPBGs can be handled and fabricated efficiently by employing a single writing step procedure which features simultaneous generation of waveguide and Bragg grating structure within an injection molded substrate. The resulting photonic structures exhibit an attenuation as low as 1.2 dB cm-1 and a reflectivity up to 99 \%. The potential of COC-PPBGs for high-temperature applications is proven by demonstrating temperature measurements up to 160 °C. Moreover, it is possible to employ single PPBGs for multidimensional stress and strain sensing or even three-dimensional shape reconstruction. Due to their excellent properties, it is feasible to integrate COC-PPBGs into commercial-grade carbon fiber reinforced polymer workpieces for structural health monitoring. Furthermore, utilization of appropriate coatings enables functionalization of PPBGs for refractive index sensing and thus biochemical applications. Beside the fabrication and characterization of COC-based PPBGs, this contribution exemplifies and reviews such applications.}, subject = {Optischer Sensor}, language = {en} } @inproceedings{KeferZettlSchmaussetal.2023, author = {Kefer, Stefan and Zettl, Julian and Schmauss, Bernhard and Esen, Cemal and Hellmann, Ralf}, title = {High-Precision Micromachining of Sapphire Towards Optical Waveguiding Structures using Femtosecond Lasers}, series = {Laser-based Micro- and Nanoprocessing XVII}, booktitle = {Laser-based Micro- and Nanoprocessing XVII}, editor = {Kling, Rainer and Pfleging, Wilhelm and Watanabe, Akira}, isbn = {9781510659230}, doi = {10.1117/12.2648758}, pages = {3 -- 11}, year = {2023}, abstract = {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}, subject = {Femtosekundenlaser}, language = {en} } @inproceedings{KeferPapeGriesetal.2023, author = {Kefer, Stefan and Pape, Natalie and Gries, Nikola and Roth, Gian-Luca and Schmauss, Bernhard and Hellmann, Ralf}, title = {Towards Pressure Sensors Based on Polymer Planar Bragg Gratings}, series = {Organic Photonic Materials and Devices XXV}, booktitle = {Organic Photonic Materials and Devices XXV}, isbn = {9781510659414}, doi = {10.1117/12.2648723}, year = {2023}, abstract = {While Bragg grating-based optical devices have shown promising performances for pressure sensing applications, their sensitivity, especially in the low-pressure regime, is unsatisfying and needs to be optimized by elaborate designs, such as cantilevers or other extrinsic mechanical transducers. This contribution demonstrates and discusses a novel concept for optical pressure sensors based on polymer planar Bragg gratings. Waveguide and Bragg grating are fabricated underneath the surface of a temperature-stable and humidity-insensitive cyclic olefin copolymer substrate by means of a femtosecond laser. Based on the employed direct-writing procedure, in combination with adaptive, in-situ beam shaping with a spatial light modulator, writing depth, i.e., location of the photonic structures within the substrate, as well as Bragg grating periodicity and positioning can be deliberately chosen. Afterwards, the polymer substrate is post-processed with a high-precision micro mill, so a diaphragm comprising the integrated photonic structures is generated. The resulting diaphragm exhibits a thickness of 300 µm and a diameter of 10 mm. Finally, the optical sensor is packaged and sealed to form an air-filled gas pocket underneath the diaphragm. Deformations of the diaphragm by external pressure changes translate to strain variations along the waveguide axis and thus perturb the Bragg grating period. This leads to changes in the grating's wavelength of main reflection, which can be evaluated in order to quantify the relative external pressure. With this straightforward optical sensor concept, pressure sensitivities up to 39 pm kPa-1, within relative pressures ranges from 78 kPa to 372 kPa, are achieved.}, subject = {Bragg-Reflektor}, language = {en} } @inproceedings{KeferSchmaussHellmann2021, author = {Kefer, Stefan and Schmauss, Bernhard and Hellmann, Ralf}, title = {POLYMER PLANAR BRAGG GRATINGS BASED ON BULK CYCLIC OLEFIN COPOLYMERS: FABRICATION AND FUNCTIONALIZATION}, series = {Proceedings of Student Conference on Sensors, Systems and Measurement 2021}, booktitle = {Proceedings of Student Conference on Sensors, Systems and Measurement 2021}, isbn = {978-80-01-06822-9}, year = {2021}, subject = {Bragg-Reflektor}, language = {en} }