@inproceedings{AdelmannHellmann2015, author = {Adelmann, Benedikt and Hellmann, Ralf}, title = {Comparative study of experimentally determined and simulated mechanical strength of fiber laser cut alumina substrates}, series = {Proceedings of the 7th International Congress on Laser Advanced Materials Processing}, volume = {2015}, booktitle = {Proceedings of the 7th International Congress on Laser Advanced Materials Processing}, number = {15}, pages = {1 -- 9}, year = {2015}, subject = {Laserschneiden}, language = {en} } @article{AdelmannRothHellmann2015, author = {Adelmann, Benedikt and Roth, Gian-Luca and Hellmann, Ralf}, title = {Back side ablation of SiC diodes using a q-switched NIR laser}, series = {Journal of Laser Micro/Nanoengineering}, volume = {10}, journal = {Journal of Laser Micro/Nanoengineering}, pages = {190 -- 194}, year = {2015}, subject = {Halbleiterdiode}, language = {en} } @article{AdelmannHellmann2015, author = {Adelmann, Benedikt and Hellmann, Ralf}, title = {Rapid micro hole laser drilling in ceramic substrates using single mode fiber laser}, series = {Journal of Materials Processing Technology}, volume = {2015}, journal = {Journal of Materials Processing Technology}, number = {221}, pages = {80 -- 86}, year = {2015}, subject = {Laserbohren}, language = {en} } @article{PreuschAdelmannHellmann2014, author = {Preusch, Florian and Adelmann, Benedikt and Hellmann, Ralf}, title = {Micromachining of AlN and Al2O3 using fiber laser}, series = {Micromachines}, volume = {2014}, journal = {Micromachines}, number = {5}, pages = {1051 -- 1060}, year = {2014}, subject = {Faserlaser}, language = {en} } @article{AdelmannHellmann2014, author = {Adelmann, Benedikt and Hellmann, Ralf}, title = {Investigation on flexural strength changes of alumina caused by cutting using fiber laser}, series = {Journal of Laser Micro/Nanoengineering}, volume = {9}, journal = {Journal of Laser Micro/Nanoengineering}, pages = {153 -- 160}, year = {2014}, subject = {Laserschneiden}, language = {en} } @inproceedings{AdelmannNeumeierSchleieretal.2015, author = {Adelmann, Benedikt and Neumeier, Benedikt and Schleier, Max and Wilmann, Eugen and Hellmann, Ralf}, title = {Optical cutting tear detection system for industrial fiber laser based cutting machines}, series = {Lasers in Manufacturing Conference 2015}, booktitle = {Lasers in Manufacturing Conference 2015}, year = {2015}, subject = {Laserschneiden}, language = {en} } @article{AdelmannSchleierNeumeieretal.2015, author = {Adelmann, Benedikt and Schleier, Max and Neumeier, Benedikt and Wilmann, Eugen and Hellmann, Ralf}, title = {Optical cutting interruption sensor for fiber lasers}, series = {Applied Sciences}, volume = {2015}, journal = {Applied Sciences}, number = {5/3}, pages = {544 -- 554}, year = {2015}, subject = {Laserschneiden}, language = {en} } @article{AdelmannNgoHellmann2015, author = {Adelmann, Benedikt and Ngo, Chan and Hellmann, Ralf}, title = {High aspect ratio cutting of metals using water jet guided laser}, series = {The International Journal of Advanced Manufacturing Technology}, volume = {2015}, journal = {The International Journal of Advanced Manufacturing Technology}, number = {80}, pages = {2053 -- 2060}, year = {2015}, subject = {Laserschneiden}, language = {en} } @article{RothEsenHellmann2017, author = {Roth, Gian-Luca and Esen, Cemal and Hellmann, Ralf}, title = {Femtosecond laser direct generation of 3D-microfluidic channels inside bulk PMMA}, series = {Optics Express}, volume = {25}, journal = {Optics Express}, number = {15}, pages = {18442 -- 18450}, year = {2017}, abstract = {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.}, subject = {Femtosekundenlaser}, language = {en} } @article{RothRungHellmann2016, author = {Roth, Gian-Luca and Rung, Stefan and Hellmann, Ralf}, title = {Welding of transparent polymers using femtosecond laser}, series = {Applied Physics A}, volume = {122}, journal = {Applied Physics A}, number = {2}, pages = {86 -- 90}, year = {2016}, abstract = {Based on nonlinear absorption, we report on laser welding of cycloolefin copolymers without any additional absorption layer employing infrared femtosecond laser. To the best of our knowledge, this is the first report of ultrashort laser welding of this material class, revealing a remarkable high processing speed of 20 mm/s in a single pass mode. Using a 1028 nm laser having a pulse duration of 220 fs at a repetition rate of 571 kHz leads to a welding seam width between 38 and 137 μm, depending on the applied laser average power. The welded joint is characterized by a maximum shear strength of 40 MPa. The experimental results are compared to those reported for femtosecond laser welding of PMMA and to those published for using a Thulium fiber laser.}, subject = {Femtosekundenlaser}, language = {en} } @article{RothRungHellmann2017, author = {Roth, Gian-Luca and Rung, Stefan and Hellmann, Ralf}, title = {Ultrashort pulse laser micro-welding of cyclo-olefin copolymers}, series = {Optics and Lasers in Engineering}, volume = {93}, journal = {Optics and Lasers in Engineering}, pages = {178 -- 181}, year = {2017}, abstract = {We report on the joining of transparent thermoplastic polymers using infrared femtosecond laser pulses. Due to nonlinear absorption, the developed micro-welding process for cyclo-olefin copolymers does not require any intermediate absorbing layers or any surface pre-processing of the welding partners. In view of an optimized and stable micro-welding process, the influence of the welding speed and focal position on both, the quality and shear force strength are investigated. We highlight that welding seam widths of down to 65 µm are feasible for welding speeds of up to 75 mm/s. However, a variation of the welding speed affects the required focal position for a successful joining process. The shear force strength of the welding seam is determined to 37 MPa, which corresponds to 64\% of the shear strength of the bulk material and is not affected by the welding speed.}, subject = {Laserschweißen}, language = {en} } @article{RungBokanSchwarzetal.2020, author = {Rung, Stefan and Bokan, Kevin and Schwarz, Simon and Esen, Cemal and Hellmann, Ralf}, title = {Influence of laser wavelength on the modification of friction between 100Cr6 steel and polytetrafluoroethylene by femtosecond laser-induced periodic surface structures}, series = {Journal of Laser Applications}, volume = {32}, journal = {Journal of Laser Applications}, number = {3}, doi = {10.2351/7.0000036}, pages = {032017-1 -- 032017-6}, year = {2020}, subject = {Femtosekundenlaser}, language = {en} } @article{RothHesslerKeferetal.2020, author = {Roth, Gian-Luca and Hessler, Steffen and Kefer, Stefan and Girschikofsky, Maiko and Esen, Cemal and Hellmann, Ralf}, title = {Femtosecond laser inscription of waveguides and Bragg gratings in transparent cyclic olefin copolymers}, series = {Optics Express}, volume = {28}, journal = {Optics Express}, number = {12}, doi = {10.1364/OE.388364}, pages = {18077 -- 18084}, year = {2020}, abstract = {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.}, subject = {Femtosekundenlaser}, language = {en} } @article{RothHaubnerKeferetal.2020, author = {Roth, Gian-Luca and Haubner, Julian and Kefer, Stefan and Esen, Cemal and Hellmann, Ralf}, title = {Fs-laser based hybrid micromachining for polymer micro-opto electrical systems}, series = {Optics and Lasers in Engineering}, volume = {137}, journal = {Optics and Lasers in Engineering}, number = {10362}, doi = {10.1016/j.optlaseng.2020.106362}, pages = {1 -- 8}, year = {2020}, abstract = {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.}, subject = {Femtosekundenlaser}, language = {en} } @article{KeferSauerHessleretal.2020, author = {Kefer, Stefan and Sauer, Theresia and Hessler, Steffen and Kaloudis, Michael and Hellmann, Ralf}, title = {Microstructure-Based Fiber-to-Chip Coupling of Polymer Planar Bragg Gratings for Harsh Environment Applications}, series = {Sensors (Basel)}, volume = {20}, journal = {Sensors (Basel)}, number = {19}, doi = {10.3390/s20195452}, pages = {5452 -- 5452}, year = {2020}, abstract = {This article proposes and demonstrates a robust microstructure-based fiber-to-chip coupling scheme for planar Bragg grating devices. A polymer planar Bragg grating substrate is manufactured and microstructured by means of a micromilling process, while the respective photonic structures are generated by employing a sophisticated single-writing UV-exposure method. A stripped standard single mode fiber is inserted into the microstructure, which is filled with a UV-curable adhesive, and aligned with the integrated waveguide. After curing, final sensor assembly and thermal treatment, the proposed coupling scheme is capable of withstanding pressures up to 10 bar, at room temperature, and pressures up to 7.5 bar at an elevated temperature of 120 °C. Additionally, the coupling scheme is exceedingly robust towards tensile forces, limited only by the tensile strength of the employed single mode fiber. Due to its outstanding robustness, the coupling scheme enables the application of planar Bragg grating devices in harsh environments. This fact is underlined by integrating a microstructure-coupled photonic device into the center of a commercial-grade carbon fiber-reinforced polymer specimen. After its integration, the polymer-based Bragg grating sensor still exhibits a reflection peak with a dynamic range of 24 dB, and can thus be employed for sensing purposes.}, language = {en} } @article{SchwarzRothRungetal.2020, author = {Schwarz, Simon and Roth, Gian-Luca and Rung, Stefan and Esen, Cemal and Hellmann, Ralf}, title = {Fabrication and evaluation of negative axicons for ultrashort pulsed laser applications}, series = {Optics Express}, volume = {Optics Express}, journal = {Optics Express}, number = {28}, pages = {26207 -- 26217}, year = {2020}, subject = {Ultrakurzzeitlaser}, language = {en} } @inproceedings{SchwarzRungEsenetal.2020, author = {Schwarz, Simon and Rung, Stefan and Esen, Cemal and Hellmann, Ralf}, title = {Manufacturing of optical elements by non-contact laser processing}, series = {Seventh European Seminar on Precision Optics Manufacturing, Proceedings}, booktitle = {Seventh European Seminar on Precision Optics Manufacturing, Proceedings}, number = {11478}, year = {2020}, subject = {Laserbearbeitung}, language = {en} } @article{ZettlKlarEsenetal.2020, author = {Zettl, Julian and Klar, Maximilian and Esen, Cemal and Hellmann, Ralf}, title = {Generation of Rotationally Symmetric Micro Tools using Ultrashort Laser Pulses}, series = {Journal of Laser Micro/Nanoengeneering}, volume = {15}, journal = {Journal of Laser Micro/Nanoengeneering}, number = {2}, doi = {10.2961/jlmn.2020.02.2007}, pages = {118 -- 122}, year = {2020}, abstract = {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.}, subject = {Ultrakurzzeitlaser}, language = {en} } @article{SchleierAdelmannNeumeieretal.2017, author = {Schleier, Max and Adelmann, Benedikt and Neumeier, Benedikt and Hellmann, Ralf}, title = {Burr formation detector for fiber laser cutting based on a photodiode sensor system}, series = {Optics \& Laser Technology}, volume = {96}, journal = {Optics \& Laser Technology}, number = {-}, doi = {10.1016/j.optlastec.2017.04.027}, pages = {13 -- 17}, year = {2017}, subject = {Laserschneiden}, language = {en} } @article{SchleierAdelmannEsenetal.2017, author = {Schleier, Max and Adelmann, Benedikt and Esen, Cemal and Hellmann, Ralf}, title = {Cross-correlation-based algorithm for monitoring laser cutting with high-power fiber lasers}, series = {IEEE Sensors Journal}, volume = {18}, journal = {IEEE Sensors Journal}, number = {4}, doi = {10.1109/JSEN.2017.2783761}, pages = {1585 -- 1590}, year = {2017}, subject = {Laserschneiden}, language = {en} } @article{DudutisPipirasSchwarzetal.2020, author = {Dudutis, Juozas and Pipiras, Jokūbas and Schwarz, Simon and Rung, Stefan and Hellmann, Ralf and Račiukaitis, Gediminas and Gečys, Paulius}, title = {Laser-fabricated axicons challenging the conventional optics in glass processing applications}, series = {Optics Express}, volume = {28}, journal = {Optics Express}, number = {4}, pages = {5715 -- 5730}, year = {2020}, subject = {Laserbearbeitung}, language = {en} } @article{SchwarzRungHellmann2017, author = {Schwarz, Simon and Rung, Stefan and Hellmann, Ralf}, title = {Two-Dimensional Low Spatial Frequency Laser-Induced Periodic Surface Structuring of Sapphire}, series = {Journal of Laser Micro/Nanoengineering}, volume = {12}, journal = {Journal of Laser Micro/Nanoengineering}, number = {2}, pages = {67 -- 71}, year = {2017}, subject = {Femtosekundenlaser}, language = {en} }