TY - JOUR A1 - Adelmann, Benedikt A1 - Hellmann, Ralf T1 - Process optimization of laser fusion cutting of multilayer stacks of electrical sheets JF - International Journal of Advanced Manufacturing Technology KW - Laserschneiden Y1 - 2013 VL - 68 SP - 2693 EP - 2701 ER - TY - CHAP A1 - Hasenstab, Stefan A1 - Adelmann, Benedikt A1 - Hellmann, Ralf T1 - Laser Cutting of Alumina and Aluminum Nitride with a 500 W Single Mode Fiber Laser T2 - XXVII. microCAD International Scientific Conference KW - Laserschneiden KW - Aluminiumoxide Y1 - 2013 ER - TY - JOUR A1 - Adelmann, Benedikt A1 - Hürner, Andreas A1 - Schlegel, Tobias A1 - Bauer, Anton A1 - Frey, Lothar A1 - Hellmann, Ralf T1 - Laser Alloying Nickel on 4H-Silicon Carbide Substrate to Generate Ohmic Contacts JF - Journal of Laser Micro/Nanoengineering KW - Ohm-Kontakt KW - Nickellegierung Y1 - 2013 VL - 8 IS - 1 SP - 97 EP - 101 ER - TY - JOUR A1 - Hürner, Andreas A1 - Schlegel, Tobias A1 - Adelmann, Benedikt A1 - Mitlehner, Heinz A1 - Hellmann, Ralf A1 - Bauer, Anton A1 - Frey, Lothar T1 - Alloying of Ohmic Contacts to n-type 4H-SiC via Laser Irradiation JF - Materials Science Forum KW - Ohm-Kontakt KW - Legierung KW - Laser Y1 - 2013 VL - 740-742 SP - 773 EP - 776 ER - TY - CHAP A1 - Kalmár, László A1 - Hellmann, Ralf A1 - Régert, Tamás A1 - Varga, Zoltan T1 - Numerical simulation of heat transfer procedure in high power LED modules T2 - Proceedings of the 12th technical and scientific meeting KW - Lumineszenzdiode KW - Wärmeübertragung KW - Simulation Y1 - 2011 SP - 18 EP - 20 ER - TY - JOUR A1 - Belle, Stefan A1 - Scheurich, Steffen A1 - Hellmann, Ralf A1 - So, Sik A1 - Sparrow, Ian A1 - Emmerson, Greg T1 - Refractive index sensing for online monitoring water and ethanol contents in biofuels JF - Sensors KW - Biokraftstoff Y1 - 2014 U6 - https://doi.org/10.1117/12.853849 SN - 1424-8220 IS - 7726 ER - TY - CHAP A1 - Glaab, Johannes A1 - Hellmann, Ralf T1 - Small-scale low-through UV-C LED disinfection device, in waste processing and recycling T2 - Proceedings of the XXIV international Scientific conference Miskolc, Ungarn KW - Lumineszenzdiode KW - Desinfektionsapparat Y1 - 2014 ER - TY - JOUR A1 - Gockel, Tilo A1 - Engel, Michael A1 - Hellmann, Ralf T1 - 3D Laserscanning mit Linienlaser und Smart Camera – Systemauslegung mit Rasterlinsenoptik und Scheimpflugentzerrung JF - Elektor Sonderheft LEDs 3 KW - Laserscanner Y1 - 2011 IS - 9 SP - 79 EP - 84 ER - TY - JOUR A1 - Schneider, Johannes A1 - Bochtler, Ulrich A1 - Hellmann, Ralf A1 - Kaloudis, Michael A1 - Gockel, Tilo T1 - Effiziente Entwärmung eines Hochleistungs-UV-LED-Moduls JF - Elektor, Sonderheft LEDs 3 KW - Lumineszenzdiode Y1 - 2011 VL - 2011 IS - 9 SP - 12 EP - 17 ER - TY - CHAP A1 - Heilmann, Eva-Maria A1 - Hellmann, Ralf T1 - Water jet guided laser cutting using a green disc laser T2 - Proceeding XXV microCAD, International scientific conference, 2011, Section K: Material science/Material processing technologies KW - Wasserstrahlschneiden KW - Laserbearbeitung Y1 - 2014 ER - TY - JOUR A1 - Adelmann, Benedikt A1 - Hellmann, Ralf T1 - Investigation on Flexural Strength during Fiber Laser Cutting of Alumina JF - Physics Procedia KW - Faserlaser KW - Biegefestigkeit Y1 - 2013 VL - 41 SP - 405 EP - 407 ER - TY - JOUR A1 - Rung, Stefan A1 - Christiansen, Anne A1 - Hellmann, Ralf T1 - Influence of film thickness on laser ablation threshold of transparent conducting oxide thin-films JF - Applied Surface Science KW - Laserablation KW - Dünnschichttechnik Y1 - 2014 VL - 305 SP - 347 EP - 351 ER - TY - CHAP A1 - Rung, Stefan A1 - Bischoff, Christian A1 - Jäger, Erwin A1 - Umhofer, Udo A1 - Hellmann, Ralf T1 - Laser thin film ablation with multiple beams and tailored beam profiles T2 - Proceedings of SPIE 8967, Laser Applications in Microelectronic and Optoelectronic Manufacturing (LAMOM) XIX KW - Thin film KW - Ablation KW - Top-Hat KW - Parallel Processing KW - Beam-Splitter KW - Laserablation KW - Dünnschichttechnik Y1 - 2014 VL - 8967 ER - TY - JOUR A1 - Rung, Stefan A1 - Rexhepi, Mal A1 - Bischoff, Christian A1 - Hellmann, Ralf T1 - Laserscribing of thin films using top-hat laser beam profiles JF - Journal of Laser Micro/Nanoengineering KW - Laser scribing KW - laser beam shaping KW - Top-Hat KW - Dünne Schicht KW - Laser Y1 - 2013 VL - 8 IS - 3 SP - 309 EP - 314 ER - TY - CHAP A1 - Rung, Stefan A1 - Rexhepi, Mal A1 - Hellmann, Ralf T1 - Beam shaping for enhanced through-put of laser ablation of transparent conductive films T2 - Proceedings of the XXVII MicroCAD International Scientific Conference KW - Laserablation Y1 - 2013 VL - 2013 ER - TY - JOUR A1 - Bischoff, Christian A1 - Rung, Stefan A1 - Hellmann, Ralf T1 - Efficient micromachining of thin film layers with nanosecond lasers – beam shaping optics improve process quality and enhance throughput JF - Laser Technik Journal KW - Laser KW - Nanosekundenbereich KW - Dünne Schicht Y1 - 2012 VL - 9 IS - 5 SP - 15 EP - 20 ER - TY - CHAP A1 - Rosenberger, Markus A1 - Belle, Stefan A1 - Hellmann, Ralf T1 - Detection of unlabeled DNA hybridization with a planar Bragg Grating Sensor T2 - XXV International MicroCAD Conference – Sensing Technologies, Miskolc (Hungary) KW - Bragg-Reflektor Y1 - 2014 IS - Section R SP - 67 EP - 72 ER - TY - JOUR A1 - Amrhein, Frank A1 - Kehrer, Martin A1 - Schreck, Timo A1 - Bochtler, Ulrich A1 - Hellmann, Ralf A1 - Kaloudis, Michael T1 - UV-LED-Bestrahlung von Vergussmaterial: Schnelle Aushärtung auch im Schatten JF - adhäsion KLEBEN & DICHTEN KW - Lumineszenzdiode Y1 - 2011 IS - 5 SP - 38 EP - 41 ER - TY - JOUR A1 - Risch, Anna A1 - Hellmann, Ralf T1 - Laser Scribing of Gallium Doped Zinc Oxide Thin Films using Picosecond Laser JF - Applied Surface Science KW - Zinkoxid KW - Dünne Schicht KW - Laser Y1 - 2014 VL - 258 IS - 5 SP - 1849 EP - 1853 ER - TY - JOUR A1 - Hellmann, Ralf A1 - Moorhouse, Colin T1 - Laser patterning of transparent electrode films, from solar panels through high-end display JF - Industrial Laser Solutions Magazine KW - Sonnenkollektor KW - Laser Y1 - 2014 IS - 11 ER - TY - JOUR A1 - Heilmann, Eva-Maria A1 - Hellmann, Ralf A1 - Seidl, Albrecht T1 - Water Jet Guided Laser Cutting of Silicon Thin Films Using 515nm Disc Laser JF - Journal of Laser Micro/Nanoengineering KW - Laserschneiden KW - Dünne Schicht Y1 - 2014 VL - 6 IS - 2 SP - 168 ER - TY - JOUR A1 - Adelmann, Benedikt A1 - Hellmann, Ralf T1 - Fast Laser Cutting Optimization Algorithm JF - Physics Procedia 12-1, Elsevier Properties S.A KW - Laserschneiden KW - Optimierung Y1 - 2011 VL - 12 IS - Part A SP - 591 EP - 598 ER - TY - JOUR A1 - Risch, Anna A1 - Hellmann, Ralf T1 - Picosecond Laser Patterning of ITO Thin Films JF - Physics Procedia 12-1, Elsevier Properties S.A KW - Laserbearbeitung KW - Dünne Schicht Y1 - 2014 IS - 12 SP - 133 EP - 140 ER - TY - CHAP A1 - Heilmann, Eva-Maria A1 - Hellmann, Ralf T1 - High Speed Fibre Laser Material processing of Stainless Steel Tapes Under Tension T2 - XXV International MicroCAD Conference - Material Processing Technologies, Miskolc (Hungary), 21-25 KW - Lasertechnologie Y1 - 2011 ER - TY - CHAP A1 - Adelmann, Benedikt A1 - Hellmann, Ralf T1 - Different modulation schemes in fibre laser cutting T2 - Proceeding XXV microCAD, International scientific conference, 2011, Section K: Material science/Material processing technologies KW - Laserschneiden KW - Faserlaser Y1 - 2014 SP - 1 EP - 6 ER - TY - CHAP A1 - Adelmann, Benedikt A1 - Hellmann, Ralf T1 - DOE based optimization of fibre laser cutting of metals T2 - Proceeding XXV microCAD, International scientific conference, 2011, Section K: Material science/Material processing technologies KW - Laserschneiden KW - Faserlaser Y1 - 2014 SP - 7 EP - 12 ER - TY - CHAP A1 - Risch, Anna A1 - Hellmann, Ralf T1 - Laser induced modification of GZO thin film T2 - Proceeding XXV microCAD, International scientific conference, 2011, Section K: Material science/Material processing technologies KW - Laser KW - Dünne Schicht Y1 - 2014 SP - 69 EP - 74 ER - TY - CHAP A1 - Risch, Anna A1 - Hellmann, Ralf T1 - ITO thin film laser patterning by picosecond laser T2 - Proceeding XXV microCAD, International scientific conference, 2011, Section K: Material science/Material processing technologies KW - Laserbearbeitung Y1 - 2014 SP - 63 EP - 68 ER - TY - CHAP A1 - Rosenberger, Manuel A1 - Belle, Stefan A1 - Hellmann, Ralf T1 - Optical interrogation of chemical and temperature bragg grating sensors using tunable fiber coupled bandpass filter T2 - Proceeding XXV microCAD, International scientific conference, 2011, Section H KW - Bragg-Reflektor Y1 - 2014 SP - 25 EP - 30 ER - TY - CHAP A1 - Belle, Stefan A1 - Rosenberger, Manuel A1 - Hellmann, Ralf T1 - Optical fibre sensing in the production process and storage of biofuels T2 - Proceeding XXV microCAD, International scientific conference, 2011, Section A KW - Biokraftstoff Y1 - 2014 SP - 21 EP - 26 ER - TY - JOUR A1 - Belle, Stefan A1 - Rosenberger, Manuel A1 - Hellmann, Ralf T1 - Evanescent wave planar bragg grating sensor for real-time detection of unlabelled biomolecules JF - Optics Express KW - Bragg-Reflektor KW - Biomolekül Y1 - 2014 ER - TY - JOUR A1 - Girschikofsky, Maiko A1 - Rosenberger, Manuel A1 - Belle, Stefan A1 - Brutschy, Malte A1 - Waldvogel, Siegfried A1 - Hellmann, Ralf T1 - Optical planar bragg grating sensor for real-time detection of benzene, toluene und xylene in solvent vapour JF - Sensors & Actuators, B - Chemical KW - Bragg-Reflektor Y1 - 2014 VL - 171/172 IS - 8/9 SP - 338 EP - 342 ER - TY - JOUR A1 - Girschikofsky, Maiko A1 - Rosenberger, Manuel A1 - Belle, Stefan A1 - Brutschy, Malte A1 - Waldvogel, Siegfried A1 - Hellmann, Ralf T1 - Highly sensitive detection of naphthalene in solvent vapor using a functionalized PBG refractive index sensor JF - Sensors KW - Bragg-Reflektor KW - Sensor Y1 - 2014 U6 - https://doi.org/10.3390/s120202018 VL - 12 IS - 2 SP - 2018 EP - 2025 ER - TY - CHAP A1 - Rosenberger, Manuel A1 - Belle, Stefan A1 - Hellmann, Ralf T1 - Detection of biochemical reaction and DNA hybridization using a planar bragg grating sensor T2 - Proceeding SPIE, Vol. 8073, article id: 80730C KW - Bragg-Reflektor KW - Sensor Y1 - 2014 U6 - https://doi.org/10.1117/12.886785 IS - 8073 ER - TY - JOUR A1 - Risch, Anna A1 - Christiansen, Anne A1 - Hellmann, Ralf A1 - Gockel, Tilo T1 - Laserstrukturierung vonTCOs zur Herstellung organischer Leuchtdioden JF - Elektor, Sonderheft LEDs 3 KW - Transparent-leitendes Oxid KW - Lumineszenzdiode KW - Laserbearbeitung Y1 - 2011 IS - 9 SP - 39 EP - 41 ER - TY - JOUR A1 - Amrhein, Frank A1 - Kehrer, Martin A1 - Schreck, Timo A1 - Bochtler, Ulrich A1 - Hellmann, Ralf A1 - Kaloudis, Michael T1 - More efficient curing of UV sensitive mounting materials JF - Practical metallography KW - Aushärtung Y1 - 2014 VL - 48 IS - 7 SP - 356 EP - 364 ER - TY - CHAP A1 - Risch, Anna A1 - Hühnlein, Kristof A1 - Knöll, Jürgen A1 - Hellmann, Ralf T1 - Laserscribing of transparent conductive oxides - a comparative study using nanosecond and picosecond pulsed solid state lasers T2 - Proceedings of the 20st International Scientific Conference, Mittweida, 2009 KW - Festkörperlaser Y1 - 2014 ER - TY - CHAP A1 - Kaloumenos, Mareiki A1 - Amrhein, Frank A1 - Glaab, Johannes A1 - Sauer, Michael A1 - Hellmann, Ralf A1 - Kaloudis, Michael T1 - Development of a UV-Light Source with Highest Power Density Based on Light Emitting Diode Technology for Applications in Production Engineering, in Production Engineering and manufacturing Systems T2 - Proceedings of the XXIV International Scientific Conference Miskolc, Ungarn KW - Lumineszenzdiode Y1 - 2010 ER - TY - JOUR A1 - Hellmann, Ralf A1 - Glaab, Johannes A1 - Gockel, Tilo T1 - Wasserentkeimung mit UV-Leuchtdioden JF - Elektor Sonderheft, LEDs 2 KW - Wasseraufbereitung KW - Lumineszenzdiode Y1 - 2010 VL - 2010 IS - 2 SP - 40 EP - 45 PB - Elektor-Verlag CY - Aachen ER - TY - JOUR A1 - Bochtler, Ulrich A1 - Hellmann, Ralf A1 - Kaloudis, Michael A1 - Amrhein, Frank A1 - Glaab, Johannes A1 - Kaloumenos, Mareiki A1 - Sauer, Michael A1 - Gockel, Tilo T1 - LUVLED - UV-Leistungslichtquelle auf Leuchtdiodenbasis JF - Elektor Sonderheft, LEDs 2 KW - Lumineszenzdiode Y1 - 2010 VL - 2011 IS - 2 SP - 57 EP - 63 PB - Elektor-Verlag CY - Aachen ER - TY - JOUR A1 - Amrhein, Frank A1 - Bochtler, Ulrich A1 - Hellmann, Ralf A1 - Kaloudis, Michael A1 - Kaloumenos, Mareiki T1 - Vergleich der thermischen Eigenschaften unterschiedlicher insulated metal substrat-Leiterplatten JF - PLUS, Leiterplattentechnik KW - Gedruckte Schaltung KW - Trägersubstanz Y1 - 2010 IS - 8 SP - 1758 PB - Hanser Verlag CY - München ER - TY - JOUR A1 - Heilmann, Eva-Maria A1 - Kowalski, Thomas A1 - Hellmann, Ralf A1 - Teigelkötter, Johannes T1 - Water Jet Guided Laser Cutting of High Temperature Superconductors JF - Journal of Laser Micro/Nanoengineering KW - laser KW - Laserschneiden Y1 - 2013 U6 - https://doi.org/10.2961/jlmn.2013.01.0014 VL - 8 IS - 1 SP - 70 EP - 74 ER - TY - JOUR A1 - Hellmann, Ralf A1 - Gockel, Tilo T1 - Optische Charakterisierung von Leuchtdioden JF - Zeitschrift ELEKTOR, Sonderheft LED-Special 4 – Leuchtdioden in Theorie und Praxis, 2012 KW - Leuchtdioden KW - Lumineszenzdiode Y1 - 2012 IS - 9 SP - 40 EP - 46 ER - TY - CHAP A1 - Kalmár, László A1 - Hellmann, Ralf A1 - Régert, Tamás A1 - Varga, Zoltan T1 - Numerical investigation of unsteady heat transport procedure in high power LED module T2 - GEP KW - LED module KW - Lumineszenzdiode Y1 - 2012 VL - 2012 IS - 1 SP - 43 EP - 46 ER - TY - JOUR A1 - Schneider, Johannes A1 - Bochtler, Ulrich A1 - Hellmann, Ralf A1 - Kaloudis, Michael A1 - Gockel, Tilo T1 - LUVLED III – Optimierung eines Kühlkonzeptes zur Aktivluftkühlung eines Hochleistungs-UV-LED-Moduls JF - Zeitschrift ELEKTOR, Sonderheft LED-Special 4 – Leuchtdioden in Theorie und Praxis, 2012 KW - UV-LED Modul KW - Lumineszenzdiode Y1 - 2012 IS - 9 SP - 20 EP - 29 ER - TY - CHAP A1 - Hühnlein, Kristof A1 - Tschirpke, Katrin A1 - Hellmann, Ralf T1 - Optimisation of laser cutting using design of experiment - A comparative study of cutting thin ceramic layers using a fibre laser and a CO2 laser T2 - Physics procedia T2 - 6th international conference on laser assisted net shape engineering KW - Laserschneiden Y1 - 2014 SN - 1875-3892 VL - 5 IS - Part B SP - 243 EP - 252 ER - TY - JOUR A1 - Risch, Anna A1 - Hühnlein, Kristof A1 - Knöll, Jürgen A1 - Hellmann, Ralf T1 - Picosecond and nanosecond pulsed laserscribing on transparent conductive indium tin oxide JF - Journal of Material processing technolgies JF - Proceedings of the XXIV international scientific conference Miskolc, Ungarn KW - Laserbearbeitung Y1 - 2014 SN - 0924-0136 ER - TY - JOUR A1 - Bochtler, Ulrich A1 - Hellmann, Ralf A1 - Kaloudis, Michael T1 - Neuartige UV-Lichtquelle auf Leuchtdiodenbasis, Packaging und Hybridschaltungen JF - PLUS KW - Lumineszenzdiode Y1 - 2010 IS - 2 PB - Leuze-Verlag ER - TY - CHAP A1 - Glaab, Johannes A1 - Hellmann, Ralf T1 - Germicidal potential of ultraviolet-LED emitting different wavelength for the inactivation of indigenous coliform bacteria T2 - Water research conference, Lissabon, Portugal KW - Lumineszenzdiode KW - Kolibakterium Y1 - 2014 ER - TY - CHAP A1 - Rosenberger, Manuel A1 - Belle, Stefan A1 - Hellmann, Ralf T1 - Optical interrogation of bragg grating sensors using tunable fibre based bandpass filter T2 - Proceedings of the 20st international science conference, Mittweida KW - Bragg-Reflektor KW - Sensor Y1 - 2014 ER - TY - CHAP A1 - Belle, Stefan A1 - Scheurich, Steffen A1 - Hellmann, Ralf T1 - Interrogation water content in organic solvents by planar bragg grating sensors T2 - Proceedings SENSOR KW - Bragg-Reflektor KW - Sensor Y1 - 2014 SN - 978-3-9810993-6-2 IS - Vol. II, P3.2 SP - 324 EP - 329 ER - TY - CHAP A1 - Scheurich, Steffen A1 - Belle, Stefan A1 - Hellmann, Ralf A1 - So, Sik A1 - Sparrow, Ian A1 - Emmerson, Greg T1 - Application of a silica-on-silicon planar optical waveguide. Bragg grating sensor for organic liquid compound detection T2 - SPIE Optics an optoelectronics conference, Proceedings of SPIE, Vol. 7356, Prag, CR KW - Bragg-Reflektor KW - Sensor Y1 - 2014 SN - 978-0819476302 IS - 7356 ER - TY - CHAP A1 - Scheurich, Steffen A1 - Belle, Stefan A1 - Hellmann, Ralf A1 - So, Sik A1 - Sparrow, Ian A1 - Emmerson, Greg T1 - Planar optical waveguide bragg grating chemical sensor for online process control T2 - 3rd integrated optics - sensors, sensing structures and methodes Korbielow, PL KW - Bragg-Reflektor KW - Sensor Y1 - 2014 ER - TY - JOUR A1 - Hock, Klaus A1 - Adelmann, Benedikt A1 - Hellmann, Ralf T1 - Comparative study of remote fiber laser and water-jet guided laser cutting of thin film metal sheets JF - Physics procedia KW - laser KW - Laserschneiden Y1 - 2012 VL - 39 SP - 225 EP - 231 ER - TY - CHAP A1 - Kalmár, László A1 - Hellmann, Ralf A1 - Régert, Tamás A1 - Varga, Zoltan T1 - Numerical investigation of heat transport procedure in high power LED modules T2 - Proceedings of the XXVI International MicroCAD Conference, Miskolc (Hungary) KW - heat transport procedure KW - Lumineszenzdiode KW - Wärmeübertragung Y1 - 2012 SP - 43 EP - 46 ER - TY - CHAP A1 - Bischoff, Christian A1 - Jäger, Erwin A1 - Rung, Stefan A1 - Hellmann, Ralf T1 - Diffractive beam shaping optics for efficient micromachining T2 - Proceedings of the LASYS (Internationale Fachmesse für Laser-Materialbearbeitung) – SLT (Stuttgarter Lasertage) 2012 KW - micromachining KW - Laserbearbeitung Y1 - 2012 ER - TY - CHAP A1 - Koller, Georg A1 - Hessler, Steffen A1 - Rosenberger, Manuel A1 - Belle, Stefan A1 - Hellmann, Ralf T1 - Micro-structured optical waveguide fabrication in PMMA using low frequency reactive ion etching T2 - Proceedings of the XXVI International MicroCAD Conference – Sensing Technologies, Miskolc (Hungary) KW - waveguide fabrication KW - Wellenleiter Y1 - 2012 ER - TY - CHAP A1 - Girschikofsky, Maiko A1 - Rosenberger, Manuel A1 - Belle, Stefan A1 - Brutschy, Malte A1 - Waldvogel, Siegfried A1 - Hellmann, Ralf T1 - α-Cyclodextrin functionalized planar Bragg grating sensor for the detection of small arene traces in solvamt vapour T2 - 14th international meeting on chemical sensors (IMCS 2012) KW - sensor detection KW - Bragg-Reflektor Y1 - 2012 SN - 978-3-9813484-2-2 U6 - https://doi.org/10.5162/IMCS2012/7.5.1 SP - 646 EP - 648 ER - TY - CHAP A1 - Rung, Stefan A1 - Hellmann, Ralf T1 - Laser scribing of metallized transparent conductive oxide – A comparative study using top hat and Gaussian beam profiles T2 - Proceedings of the XXVI International MicroCAD Conference – Material Processing Technologies KW - laser scribing KW - Laserbearbeitung KW - Laserritzen Y1 - 2012 ER - TY - CHAP A1 - Girschikofsky, Maiko A1 - Rosenberger, Manuel A1 - Belle, Stefan A1 - Hellmann, Ralf T1 - Surface modifications of planar bragg grating refractive index sensors for gas detection T2 - Proceedings of the XXVI International MicroCAD Conference – Sensing Technologies, Miskolc (Hungary) KW - surface modification KW - Bragg-Reflektor Y1 - 2012 ER - TY - JOUR A1 - Preusch, Florian A1 - Rung, Stefan A1 - Hellmann, Ralf T1 - Influence of Polishing Orientation on the Generation of LIPSS on Stainless Steel JF - Journal of Laser Micro/Nanoengineering KW - Laserbearbeitung KW - Nicht rostender Stahl Y1 - 2016 U6 - https://doi.org/10.2961/jlmn.2016.01.0025 VL - 11 SP - 137 EP - 142 ER - TY - JOUR A1 - Rung, Stefan A1 - Barth, Johannes A1 - Hellmann, Ralf T1 - Characterization of Laser Beam Shaping Optics Based on Their Ablation Geometry of Thin Films JF - Micromachines N2 - Thin film ablation with pulsed nanosecond lasers can benefit from the use of beam shaping optics to transform the Gaussian beam profile with a circular footprint into a Top-Hat beam profile with a rectangular footprint. In general, the quality of the transformed beam profile depends strongly on the beam alignment of the entire laser system. In particular, the adjustment of the beam shaping element is of upmost importance. For an appropriate alignment of the beam shaper, it is generally necessary to observe the intensity distribution near the focal position of the applied focusing optics. Systems with a low numerical aperture (NA) can commonly be qualified by means of laser beam profilers, such as a charge-coupled device (CCD) camera. However, laser systems for micromachining typically employ focus lenses with a high NA, which generate focal spot sizes of only several microns in diameter. This turns out to be a challenge for common beam profiling measurement systems and complicates the adjustment of the beam shaper strongly. In this contribution, we evaluate the quality of a Top-Hat beam profiling element and its alignment in the working area based on the ablated geometry of single pulse ablation of thin transparent conductive oxides. To determine the best achievable adjustment, we develop a quality index for rectangular laser ablation spots and investigate the influences of different alignment parameters, which can affect the intensity distribution of a Top-Hat laser beam profile. KW - Dünne Schicht KW - Laserablation Y1 - 2014 VL - 5 IS - 4 SP - 943 EP - 953 ER - TY - CHAP A1 - Jäger, Erwin A1 - Rädel, Ulrich A1 - Bischoff, Christian A1 - Umhofer, Udo A1 - Rung, Stefan A1 - Hellmann, Ralf T1 - Laser micromachining – parallel processing with multiple top hat beams T2 - 8th International Conference on Photonic Technologies LANE 2014 KW - Laserbearbeitung Y1 - 2014 ER - TY - JOUR A1 - Sommer, David A1 - Götzendorfer, Babette A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Design Rules for Hybrid Additive Manufacturing Combining Selective Laser Melting and Micromilling JF - Design and Post Processing for Metal Additive Manufacturing N2 - We report on a comprehensive study to evaluate fundamental properties of a hybrid manufacturing approach, combining selective laser melting and high speed milling, and to characterize typical geometrical features and conclude on a catalogue of design rules. As for any additive manufacturing approach, the understanding of the machine properties and the process behaviour as well as such a selection guide is of upmost importance to foster the implementation of new machining concepts and support design engineers. Geometrical accuracy between digitally designed and physically realized parts made of maraging steel and dimensional limits are analyzed by stripe line projection. In particular, we identify design rules for numerous basic geometric elements like walls, cylinders, angles, inclinations, overhangs, notches, inner and outer radii of spheres, chamfers in build direction, and holes of different shape, respectively, as being manufactured by the hybrid approach and compare them to sole selective laser melting. While the cutting tool defines the manufacturability of, e.g., edges and corners, the milling itself improves the surface roughness to Ra < 2 µm. Thus, the given advantages of this hybrid process, e.g., space-resolved and custom-designed roughness and the superior geometrical accuracy are evaluated. Finally, we exemplify the potential of this particular promising hybrid approach by demonstrating an injection mold with a conformal cooling for a charge socket for an electro mobile KW - hybrid additive manufacturing; high-speed milling; selective laser melting; construction rules KW - Selektives Laserschmelzen KW - Hochgeschwindigkeitsfräsen Y1 - 2021 UR - https://www.mdpi.com/1996-1944/14/19/5753 U6 - https://doi.org/https://doi.org/10.3390/ma14195753 VL - 14 IS - 19 SP - 1 EP - 23 ER - TY - JOUR A1 - Sommer, David A1 - Pape, Dominik A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Tool Wear and Milling Characteristics for Hybrid Additive Manufacturing Combining Laser Powder Bed Fusion and In Situ High-Speed Milling JF - Ultra-Precision Manufacturing Technology for Difficult-to-Machine Materials N2 - We report on milling and tool wear characteristics of hybrid additive manufacturing comprising laser powder bed fusion and in situ high-speed milling, a particular process in which the cutter mills inside the powder bed without any cooling lubricant being applicable. Flank wear is found to be the dominant wear characteristic with its temporal evolution over utilization period revealing the typical s-shaped dependence. The flank wear land width is measured by microscopy and correlated to the achievable surface roughness of milled 3D-printed parts, showing that for flank wear levels up to 100 μm a superior surface roughness below 3 μm is accessible for hybrid additive manufacturing. Further, based on this correlation recommended tool, life scenarios can be deduced. In addition, by optimizing the finishing tool start position and the number of afore-built layers, the milling process is improved with respect to the maximum millable angle for undercut surfaces of 3D-printed parts to 30° for the roughing process and to 40° for the entire machining process including finishing KW - hybrid additive manufacturing; high-speed milling; laser powder bed fusion; tool wear KW - Hochgeschwindigkeitsfräsen KW - Werkzeugverschleiß Y1 - 2022 UR - https://www.mdpi.com/1996-1944/15/3/1236 U6 - https://doi.org/https://doi.org/10.3390/ma15031236 VL - 15 IS - 3 SP - 1 EP - 13 ER - TY - JOUR A1 - Rung, Stefan A1 - Hellmann, Ralf T1 - Laser-Induced Period Surface Structures to Improve Solderability of Electrical Solder Pads JF - Applied Sciences KW - Laserstrahllöten Y1 - 2021 UR - https://www.mdpi.com/2076-3417/12/1/80/htm U6 - https://doi.org/10.3390/app12010080 VL - 2022 IS - 12/1 SP - 80 EP - 88 ER - TY - JOUR A1 - Rung, Stefan A1 - Häcker, Niklas A1 - Hellmann, Ralf T1 - Thermal imaging of high power ultrashort pulse laser ablation of alumina towards temperature optimized micro machining strategies JF - IOP Conference Series: Materials Science and Engineering KW - Ultrakurzzeitlaser KW - Aluminiumoxide Y1 - 2021 UR - https://iopscience.iop.org/article/10.1088/1757-899X/1135/1/012027 U6 - https://doi.org/0.1088/1757-899X/1135/1/012027 VL - 1135 IS - 012027 SP - 1 EP - 9 ER - TY - CHAP A1 - Schwarz, Simon A1 - Rung, Stefan A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Current results on laser-based production of complex optics in glass T2 - 10. Wetzlarer Herbsttagung "Moderne Optikfertigung", 29. - 30. September 2020, Wetzlar KW - Laserbearbeitung KW - Fertigungstechnik Y1 - 2020 ER - TY - CHAP A1 - Schwarz, Simon A1 - Rung, Stefan A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - UKP-Prozess mit anschließendem CO2-Laserpolieren zur Fertigung komplexer Optiken T2 - 10. Workshop Laserbearbeitung von Glaswerkstoffen KW - Linse KW - Array KW - Laserbearbeitung KW - Ultrakurzzeitlaser KW - Kohlendioxidlaser Y1 - 2020 ER - TY - GEN A1 - Schwarz, Simon A1 - Hellmann, Ralf T1 - Rapid laser fabrication of complex glass optics T2 - International Research Days N2 - Poster KW - Laserbearbeitung Y1 - 2021 ER - TY - JOUR A1 - Schleier, Max A1 - Adelmann, Benedikt A1 - Esen, Cemal A1 - Glatzel, Uwe A1 - Hellmann, Ralf T1 - Development and evaluation of an image processing algorithm for monitoring fiber laser fusion cutting by a high-speed camera JF - Journal of Laser Applications KW - Laser cutting KW - Image processing KW - Optical imaging KW - Image sensors KW - Laser applications KW - Faserlaser KW - Laserschneiden KW - Bildverarbeitung Y1 - 2021 UR - https://doi.org/10.2351/7.0000391 U6 - https://doi.org/10.2351/7.0000391 VL - 2021 IS - 33 SP - 032004 EP - 032004 ER - TY - JOUR A1 - Adelmann, Benedikt A1 - Hellmann, Ralf T1 - Simultaneous Burr and Cut Interruption Detection during Laser Cutting with Neural Networks JF - Sensors KW - laser cutting KW - quality monitoring KW - artificial neural network KW - burr formation KW - cut interruption KW - Laserschneiden KW - Neuronales Netz Y1 - 2021 UR - https://doi.org/10.3390/s21175831 U6 - https://doi.org/10.3390/s21175831 VL - 2021 IS - 21 SP - 5831 EP - 5831 ER - TY - JOUR A1 - Adelmann, Benedikt A1 - Schleier, Max A1 - Hellmann, Ralf T1 - Laser Cut Interruption Detection from Small Images by Using Convolutional Neural Network JF - Sensors KW - laser cutting KW - remote sensing KW - convolutional neural network KW - cut interruption KW - image processing KW - Laserschneiden KW - Neuronales Netz Y1 - 2021 UR - https://doi.org/10.3390/s21020655 U6 - https://doi.org/10.3390/s21020655 VL - 2021 IS - 21(2) SP - 655 EP - 655 ER - TY - JOUR A1 - Zettl, Julian A1 - Rung, Stefan A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Tangential Laser Turning of Fused Silica Using Ultra-short Laser Pulses JF - Journal of Laser Micro/Nanoengineering N2 - Laser turning with ultra-short laser pulses provides the possibility of manufacturing rotationally symmetric geometries, even of hard to machine materials as fused silica, down to a micrometer scale. In this laser micro machining approach, the laser beam is oriented perpendicularly to the rotational axis and tangentially to the material, whereas the target geometry is realized by moving the constantly rotating specimen according to the specified geometry under the focused laser spot. Due to the fric-tion-less laser ablation it is possible to realize a turning process in fused silica without the use of a contacting tool and therefore without causing mechanical tensions in the work piece. The processing head focuses the laser to a calculated spot size of 18 μm in diameter and the applied processing gas pressure assists in removing the ablated material. In this study, femto-second laser turning of fused silica with a diameter of 3.7 mm is investigated. The influence of the applied pulse energy is stated and the use of a trepanning optic during the turning process is characterized. Furthermore, compre-hensive parameter studies in order to optimize roughness and to find the maximum achievable abla-tion rate are shown. Limited by the mechanical speed of the rotational axis, the highest feasible abla-tion rate is 19.5 mm³/min. A roughness of Ra 0.65 μm are demonstrated. KW - Ultra-short Laser Pulses KW - Laser Turning KW - Fused Silica KW - Ablation Rate KW - Process Efficiency KW - Laserbearbeitung KW - Mikrobearbeitung KW - Quarzglas KW - Ultrakurzer Lichtimpuls Y1 - 2021 U6 - https://doi.org/10.2961/jlmn.2021.01.2008 VL - 16 IS - 1 SP - 42 EP - 46 ER - TY - JOUR A1 - Zettl, Julian A1 - Klar, Maximilian A1 - Rung, Stefan A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Laser turning with ultrashort laser pulses JF - Journal of Manufacturing Processes N2 - We report on a comprehensive micromachining study of rotationally symmetric parts using femtosecond laser. A laser turning process with tangential impingement of the laser radiation complemented by a trepanning optics is implemented as to accomplish a true laser lathe micromachining approach. With the objective of optimized ablation rate and reduced surface roughness, the influence of pulse energy, feed rate, trepanning diameter and angular beam incidence, respectively, is investigated for processing stellite rods. We find the smallest feasible feature sizes of 8.5 μm in diameter and smoothest surfaces with an arithmetic average of the roughness profile as low as 0.18 μm. The surface roughness, however, appears to be limited by the occurrence of laser induced periodic surface structures provoked by the femtosecond laser radiation. Furthermore, the variation of the fluence in accordance to the incidence on a curved surface is discussed and the heat input into the material is examined through a longitudinal cross section. KW - Ultrashort laser pulses KW - Turning KW - Ablation rate KW - Tangential processing KW - Trepanning optic KW - Ultrakurzer Lichtimpuls KW - Laserbearbeitung KW - Mikrobearbeitung KW - Femtosekundenlaser Y1 - 2021 U6 - https://doi.org/10.1016/j.jmapro.2021.06.025 VL - 2021 IS - 68 SP - 1562 EP - 1568 ER - TY - JOUR A1 - Götzendorfer, Babette A1 - Mohr, Thomas A1 - Hellmann, Ralf T1 - Hybrid Approaches for Selective Laser Sintering by Building on Dissimilar Materials JF - Materials KW - Laserstrahlsintern Y1 - 2020 U6 - https://doi.org/doi.org/10.3390/ma13225285 VL - 2020 IS - 13 (22) SP - 5285 EP - 5296 ER - TY - JOUR A1 - Götzendorfer, Babette A1 - Kirchgässner, Hannah A1 - Hellmann, Ralf T1 - Tunable, Anisotropic Permeability and Spatial Flow of SLM Manufactured Structures JF - Materials KW - Permeabilität KW - Selektives Laserschmelzen Y1 - 2021 U6 - https://doi.org/doi.org/10.3390/ma14185205 VL - 2021 IS - 14 SP - 5205 EP - 5220 ER - TY - JOUR A1 - Schwarz, Simon A1 - Rung, Stefan A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Enhanced ablation efficiency using GHz bursts in micromachining fused silica JF - Optics Letters KW - Fräsen KW - Mikrobearbeitung KW - Quarzglas KW - Femtosekundenlaser Y1 - 2021 VL - 46 IS - 2 SP - 282 EP - 285 ER - TY - JOUR A1 - Schwarz, Simon A1 - Götzendorfer, Babette A1 - Rung, Stefan A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Compact Beam Homogenizer Module with Laser-Fabricated Lens-Arrays JF - applied sciences KW - Femtosekundenlaser KW - Linse KW - Array Y1 - 2021 VL - 2021 IS - 11 SP - 1018 EP - 1018 ER - TY - JOUR A1 - Schwarz, Simon A1 - Rung, Stefan A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Ultrashort pulsed laser backside ablation of fused silica JF - Optics Express KW - Femtosekundenlaser KW - Quarzglas KW - Mikrobearbeitung Y1 - 2021 VL - 29 IS - 15 SP - 23477 EP - 23486 ER - TY - CHAP A1 - Sauer, Theresia A1 - Kefer, Stefan A1 - Ruppert, Wolfgang A1 - Hellmann, Ralf A1 - Kaloudis, Michael T1 - Integration of Bragg grating sensors in components made of carbon fiber reinforced polymers T2 - Tagungsband der 20. GMA/ITG-Fachtagung Sensoren und Messsysteme 2019, 25.06.2019 - 26.06.2019, Nürnberg, Deutschland N2 - This contribution discusses the integration of polymer planar Bragg grating sensors (PPBG) into carbon fiber reinforced polymer (CFRP) components. For the first time, it is shown that PPBGs based on cyclic olefin copolymers can be integrated into commercial-grade composites, thereby withstanding the demanding production processes. Pre-impregnated fibers are stacked and partially modified to form a sensor pocket. Afterwards, the CFRP specimen containing the optical sensor is cured in a heated mechanical press for 2 hours at a pressure of 7 bar and a temperature of 120 °C. A subsequent evalutaion of the sensor signal shows a Bragg wavelength shift of 1236 pm and a decline in signal amplitude of -2 dB. Three-point flexural tests of the cured sample reveal a linear behavior of the sensor signal towards external loads. The determined sensitivity in dependence of the CFRP specimen's maximum central deflection is -112 pm/mm, while correlation to the applied force results in a sensitivity of -5 pm/N. KW - Bragg grating sensors KW - carbon fiber reinforced polymers KW - cyclic olefin copolymers KW - structural health monitoring KW - Bragg-Reflektor Y1 - 2019 SN - 978-3-9819376-0-2 U6 - https://doi.org/10.5162/sensoren2019/P3.12 SP - 806 EP - 810 ER - TY - JOUR A1 - Rung, Stefan A1 - Bokan, Kevin A1 - Kleinwort, Frederick A1 - Schwarz, Simon A1 - Simon, Peter A1 - Klein-Wiele, Jan-Hendrik A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Possibilities of Dry and Lubricated Friction Modification Enabled by Different Ultrashort Laser-Based Surface Structuring Methods JF - Lubricants N2 - In this contribution we report on the possibilities of dry and lubricated friction modification introduced by different laser surface texturing methods. We compare the potential of Laser-Induced Periodic Surface Structures and Laser Beam Interference Ablation on 100Cr6 steel in a linear reciprocating ball-on-disc configuration using 100Cr6 steel and tungsten carbide balls with load forces between 50mN and 1000 mN. For dry friction, we find a possibility to reduce the coefficient of friction and we observe a pronounced direction dependency for surfaces fabricated by Laser Beam Interference Ablation. Furthermore, Laser-Induced Periodic Surface Structures result in a load-dependent friction reduction for lubricated linear reciprocating movements. This work helps to identify the modification behaviour of laser generated micro structures with feature sizes of approximately 1μm and reveals new possibilities for surface engineering KW - laser-induced periodic surface structures KW - laser beam interference ablation KW - dry friction KW - lubricated friction KW - laser surface texturing KW - Laser KW - Oberflächenstruktur Y1 - 2019 U6 - https://doi.org/10.3390/lubricants7050043 VL - 7 IS - 43 SP - 1 EP - 13 ER - TY - JOUR A1 - Rung, Stefan A1 - Bokan, Kevin A1 - Rutsch, Kevin A1 - Schwarz, Simon A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Laser Induced Periodic Surface Structures on 100Cr6 Steel for Modification of Friction Demonstrated with Stribeck Test JF - Journal of Laser Micro/Nanoengineering N2 - In this contribution, we report on surface functionalization by introducing laser induced periodic surface structures on 100Cr6 bearing steel to modify complex tribological properties. The advanced approach of this study is the measurement of the coefficient of friction by performing a ball-on-disk Stribeck test on laser structured surfaces with polytetrafluoroethylene balls in a lubricant environment. The Stribeck test reveals the modified friction behavior using translation speeds up to 106 mm/min and load forces between 100 mN and 5000 mN. Our results show increased stiction and coefficient of friction for laser structured surfaces in the regime of boundary lubrication which is attributed to the laser induced surface asperities. Decreased coefficient of friction is observed in the regime of mixed and hydrodynamic lubrication, i.e. for velocities higher than 1000 mm/min, for surfaces covered by laser induced periodic surface structures with low spatial frequency. KW - Laser KW - Oberflächenstruktur Y1 - 2018 U6 - https://doi.org/10.2961/jlmn.2018.03.0004 VL - 13 IS - 3 SP - 160 EP - 165 ER - TY - JOUR A1 - Schwarz, Simon A1 - Rung, Stefan A1 - Hellmann, Ralf T1 - Generation of laser-induced periodic surface structures on transparent material-fused silica JF - Applied Physics Letters KW - Laser KW - Oberflächenstruktur KW - Siliziumdioxid KW - Quarzglas Y1 - 2016 VL - 108 IS - 18 SP - 181607 EP - 181607 ER - TY - JOUR A1 - Schwarz, Simon A1 - Rung, Stefan A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Fabrication of a high-quality axicon by femtosecond laser ablation and CO2 laser polishing for quasi-Bessel beam generation JF - Optics Express KW - Femtosekundenlaser Y1 - 2018 VL - 26 IS - 18 SP - 23287 EP - 23294 ER - TY - JOUR A1 - Schwarz, Simon A1 - Rung, Stefan A1 - Hellmann, Ralf T1 - One-dimensional low spatial frequency LIPSS with rotating orientation on fused silica JF - Applied Surface Science KW - Quarzglas KW - Laserbestrahlung Y1 - 2017 VL - 2017 IS - 411 SP - 113 EP - 116 ER - TY - JOUR A1 - Schwarz, Simon A1 - Hellmann, Ralf T1 - Fabrication of Cylindrical Lenses by Combining Ultrashort Pulsed Laser and CO2 Laser JF - Journal of Laser Micro/Nanoengineering KW - Zylinderlinse Y1 - 2017 VL - 12 IS - 2 SP - 76 EP - 79 ER - TY - JOUR A1 - Schwarz, Simon A1 - Rung, Stefan A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Influence of pulse duration on high-precision manufacturing of 3D geometries JF - Journal of Laser Micro/Nanoengineering KW - Dreidimensionale Geometrie Y1 - 2018 VL - 13 IS - 3 SP - 292 EP - 295 ER - TY - JOUR A1 - Schwarz, Simon A1 - Rung, Stefan A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Homogeneous low spatial frequency LIPSS on dielectric materials generated by beam-shaped femtosecond pulsed laser irradiation JF - Journal of Laser Micro/Nanoengineering KW - Femtosekundenlaser Y1 - 2018 VL - 13 IS - 2 SP - 90 EP - 94 ER - TY - JOUR A1 - Schwarz, Simon A1 - Rung, Stefan A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Rapid fabrication of precise glass axicon arrays by an all laser-based manufacturing technology JF - Journal of Laser Applications KW - Femtosekundenlaser Y1 - 2020 VL - 32 IS - 1 SP - 012001 EP - 012001 ER - TY - JOUR A1 - Schwarz, Simon A1 - Rung, Stefan A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Surface Plasmon Polariton Triggered Generation of 1D-Low Spatial Frequency LIPSS on Fused Silica JF - Applied Sciences KW - Femtosekundenlaser KW - Quarzglas Y1 - 2018 VL - 8 IS - 9 SP - 1624 EP - 1624 ER - TY - JOUR A1 - Trautmann, Anika A1 - Roth, Gian-Luca A1 - Nujiqi, Benedikt A1 - Walter, Thomas A1 - Hellmann, Ralf T1 - Towards a versatile point-of-care system combining femtosecond laser generated microfluidic channels and direct laser written microneedle arrays JF - Microsystems & Nanoengineering N2 - Microneedle-based microfluidic systems have a great potential to become well-accepted medical devices for simple, accurate, and painless drug delivery and lab-on-a-chip diagnostics. In this work, we report on a novel hybrid approach combining femtosecond direct laser written microneedles with femtosecond laser generated microfluidic channels providing an important step towards versatile medical point-of-care systems. Hollow microneedle arrays are fabricated by a laser system designed for two-photon polymerization applications. Compression tests of two different types of truncated cone-shaped microneedle arrays prepared from OrmoComp® give information about the microneedle mechanical strength, and the results are compared to skin insertion forces. Three-dimensional microchannels are directly created inside PMMA bulk material by an ultrashort pulse laser system with vertical channels having adjustable cross-sectional areas, which allow attaching of microneedles to the microfluidic system. A comprehensive parameter study varying pulse duration and repetition rate is performed on two-photon polymerization to identify an optimal laser power range for fabricating microneedles using the same pulse duration and repetition rate as for microchannels. This addresses the advantage of a single laser system process that overcomes complex fabrication methods. A proof of concept flow test with a rhodamine B dye solution in distilled water demonstrates that the combination of microneedles and microchannels qualifies for microfluidic injection and extraction applications. KW - Femtosekundenlaser Y1 - 2019 VL - 5 IS - 6 SP - 1 EP - 9 ER - TY - JOUR A1 - Roth, Gian-Luca A1 - Esen, Cemal A1 - Hellmann, Ralf T1 - Control of femtosecond laser generated microfluidic channels inside poly (methyl methacrylate) JF - Journal of Laser Applications N2 - In this study, the authors report on femtosecond laser direct generation of microchannels inside poly(methyl methacrylate) with variable dimensions ranging from a channel height of 20 to 350 μm and an aspect ratio down to 1.2. Focused ultrashort laser pulses are used to trigger a material modification in the focal area which can be selectively opened by a subsequent annealing process. A 3D microchannel architecture can easily be realized by moving the specimen using motorized stages, allowing freely chosen complex shaped channel layouts. While the laser wavelength is identified as one of the main parameters determining process effectiveness, another core of this study is the variation of channel dimensions by the numerical aperture of the applied objective to focus the laser. The authors find that both size and shape of the modified region and the resulting microchannel can be controlled by altering the numerical aperture. KW - Femtosekundenlaser KW - Polymethylmethacrylate Y1 - 2018 VL - 30 IS - 3 SP - 032016-1 EP - 032016-6 ER - TY - JOUR A1 - Roth, Gian-Luca A1 - Adelmann, Benedikt A1 - Hellmann, Ralf T1 - Cutting and Drilling of SiC Semiconductor by Fiber Laser JF - Journal of Laser Micro/Nanoengineering N2 - We present an experimental study on fiber laser micromachining of 4H-SiC semiconductor. In particular, we optimize the process time for percussion drilling of 440 µm and 330 µm thick wafers with a minimum drilling time of, eg, 7.5 ms being achieved for 330 µm thick wafers. The drilled holes are characterized by a low taper, a high circularity and entry diameters of about 60 µm. For laser cutting, effective cutting speeds of 2 mm/s and 0.63 mm/s are obtained for 330 µm and 440 µm wafer thickness, respectively, with the cut kerf width being 65 µm. Both processes are associated with a good quality that is characterized by no chipping and cracks. The capability of the fiber laser based micromachining process is highlighted by cutting complex micrometer scale inner and outer contours into partially metallized SiC wafers with high quality. KW - Laserschneiden KW - Faserlaser Y1 - 2015 VL - 10 IS - 3 SP - 279 EP - 283 ER - 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 -