@inproceedings{KeferSchmaussHellmann2023, author = {Kefer, Stefan and Schmauss, Bernhard and Hellmann, Ralf}, title = {Sapphire-Based Planar Bragg Grating Devices}, series = {28th International Conference on Optical Fiber Sensors}, booktitle = {28th International Conference on Optical Fiber Sensors}, isbn = {978-1-957171-30-2}, doi = {10.1364/OFS.2023.W2.4}, year = {2023}, abstract = {This study reports on the long-term stability and the high-temperature capability of sapphire-based photonic crystal waveguides with integrated Bragg gratings. Furthermore, their Bragg grating reflectivity as well as their temperature sensitivity is quantified.}, subject = {Bragg-Reflektor}, language = {en} } @article{KeferLimbachPapeetal.2024, author = {Kefer, Stefan and Limbach, Tobias and Pape, Natalie and Klamt, Kathrin and Schmauss, Bernhard and Hellmann, Ralf}, title = {Birefringence in Injection-Molded Cyclic Olefin Copolymer Substrates and Its Impact on Integrated Photonic Structures}, series = {Polymers}, volume = {16}, journal = {Polymers}, number = {2}, doi = {10.3390/polym16020168}, pages = {168 -- 168}, year = {2024}, abstract = {This contribution quantifies the birefringence within injection-molded cyclic olefin copolymer plates and discusses its impact on the mechanical properties of the plates. It also focuses on the impact of birefringence on integrated waveguides and Bragg gratings and provides fabrication guidelines for such structures. The anisotropy in all three dimensions of the workpiece is examined by means of polarimetry and a prism coupler. It is found that the birefringence is inhomogenously distributed within the workpieces, whereas the maximum birefringence not only varies locally, but also depends on the observation direction. Overall, a maximum birefringence of 10 × 10-4 is found at the plate's surface near the injection gate. The anisotropy then reduces exponentially towards the center of the workpiece and saturates at 1.8 × 10-4, in a depth of 0.4 mm. Thus, the birefringence strongly affects near-surface photonic structures. It is found that, depending on their orientation and the local birefringence of the substrate, waveguides and Bragg gratings fabricated with comparable parameters behave completely differently in terms of polarization-dependent optical attenuation, cross-sectional intensity distribution and Bragg reflection signal. For example, the support of the TM mode can vary between total loss and an optical attenuation of 0.9 dB × cm-1. In consequence, this study underlines the importance of quantifying the birefringent state of an injection-molded cyclic olefin copolymer workpiece if it is supposed to serve as a substrate for integrated photonic structures. The study furthermore demonstrates that birefringence effects can be omitted by burying the photonic structures deeper into the volume of the thermoplastic.}, subject = {Bragg-Reflektor}, 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} } @article{KeferPapeRothetal.2021, author = {Kefer, Stefan and Pape, Dominik and Roth, Gian-Luca and Hessler, Steffen and Schmauss, Bernhard and Hellmann, Ralf}, title = {Micromilling-assisted fabrication of monolithic polymer ridge-type waveguides with integrated photonic sensing structures}, series = {Optical Materials Express}, volume = {11}, journal = {Optical Materials Express}, number = {8}, doi = {https://doi.org/10.1364/OME.425778}, pages = {2389 -- 2400}, year = {2021}, abstract = {This study demonstrates and discusses a novel approach for the fabrication and rapid prototyping of monolithic photonic platforms comprising a ridge-type waveguide with integrated sensing structures. First, the bulk injection-molded cyclic olefin copolymer substrates are micromilled in order to define the physical extension of the ridge structure. Cross-sections down to 30 × 30 µm2, exhibiting a mean surface roughness of 300 nm, are achieved with this process. Subsequently, UV radiation is used to modify the ridge structure's refractive index, which leads to the formation of an optical waveguide. By employing a phase mask, it is possible to equip the photonic platform with a Bragg grating suitable for temperature measurements with a sensitivity of -5.1 pm K-1. Furthermore, an integrated Fabry-P{\´e}rot cavity, generated during the micromilling step as well, enables refractive index measurements with sensitivities up to 1154 nm RIU-1.}, subject = {Bragg-Reflektor}, language = {en} } @article{KeferBischoffRothetal.2021, author = {Kefer, Stefan and Bischoff, Kay and Roth, Gian-Luca and Haubner, Julian and Schmauss, Bernhard and Hellmann, Ralf}, title = {Tunable Bulk Polymer Planar Bragg Gratings Electrified via Femtosecond Laser Reductive Sintering of CuO Nanoparticles}, series = {Advanced Optical Materials}, volume = {9}, journal = {Advanced Optical Materials}, number = {13}, doi = {https://doi.org/10.1002/adom.202002203}, pages = {2002203 -- 2002203}, year = {2021}, abstract = {This contribution demonstrates and discusses electrically tunable polymer planar Bragg gratings based on bulk cyclic olefin copolymers. A lithographic single-writing-step method and femtosecond laser reductive sintering of copper(II) oxide nanoparticles are subsequently employed in order to generate buried photonic structures and copper conducting paths on top of the polymer substrate. This way, the necessary number of process steps for fabricating a planar polymer-based electro-optical device is greatly reduced. The response of a fully electrified grating structure follows temperature changes, induced by the copper conducting path, with sensitivities up to -31 pm K-1. Dilatometric measurements show that the specimen's behavior is correlated to the situationally reduced thermal expansion of the bulk polymer substrate. In consequence, the tuning response of the photonic platform follows a second order polynomial, whereas a direct current of 30 mA, which correlates to a power consumption of 18.3 mW, leads to a local temperature increase and a residual Bragg wavelength shift of 19.6 K and -547 pm, respectively. Moreover, the outstanding flexibility of the proposed fabrication concept is underlined by demonstrating alternative conducting path geometries, whereas one of the additional designs is adapted to control the spectral width of the Bragg grating's reflection peak.}, subject = {Femtosekundenlaser}, 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} } @article{KeferSauerHessleretal.2020, author = {Kefer, Stefan and Sauer, Theresia and Hessler, Steffen and Kaloudis, Michael and Schmauß, Bernhard and Hellmann, Ralf}, title = {Robust Polymer Planar Bragg Grating Sensors Embedded in Commercial-Grade Composites}, series = {Polymers}, volume = {12}, journal = {Polymers}, number = {3}, doi = {10.3390/polym12030715}, pages = {715 -- 715}, year = {2020}, abstract = {This contribution demonstrates the functionality of polymer planar Bragg grating (PPBG) sensors integrated into commercial-grade carbon fiber reinforced polymer (CFRP) components. Multiple CFRP specimens are generated by curing a stack of pre-impregnated fibers inside of a heated mechanical press, exposing the polymer sensor to a pressure of 7 bar and a temperature of 120 °C for 2 h. After integration, the sensor still exhibits a strong and evaluable signal. Subsequent flexural experiments reveal a linear response of the integrated sensor's Bragg wavelength to the CFRP specimen's maximum deflection. Additional findings demonstrate that the embedded PPBG can be used to detect plastic deformations of a CFRP workpiece, whereas a linear correlation of plastic deformation to the resulting Bragg signal offset is determined. A plausibility check of the obtained results is delivered by a comparison of three-point flexural experiments on bulk CFRP workpieces, without integrated sensors and additional specimens featuring external optical sensors affixed to their surface. It is found that PPBGs based on cyclic olefin copolymers are able to overcome the temperature-related limitations of traditional polymer-based optical sensors and can thus be directly integrated into commercial-grade composites during production.}, subject = {Bragg-Reflektor}, language = {en} } @article{KeferDaiYangetal.2020, author = {Kefer, Stefan and Dai, Jixiang and Yang, Minghong and Schmauss, Bernhard and Hellmann, Ralf}, title = {Hypersensitive H2 sensor based on polymer planar Bragg gratings coated with Pt-loaded WO3-SiO2}, series = {Optics Letters}, volume = {45}, journal = {Optics Letters}, number = {13}, doi = {https://doi.org/10.1364/OL.395341}, pages = {3601 -- 3604}, year = {2020}, abstract = {This letter demonstrates a novel hydrogen sensor based on a polymer planar Bragg grating coated with Pt-loaded WO3-SiO2. The reflected Bragg signal shows a distinct peak splitting correlated to substrate anisotropies originating from the injection molding process. Especially at low H2 concentrations, both sensing peaks exhibit an outstanding response to the heat generated by the exothermic reaction between hydrogen molecules and coating. Thereby, a hydrogen volume ratio of 50 ppm leads to a Bragg wavelength shift of -37 pm, which yields an outstandingly low detection limit of only 5 ppm H2 in air. Thus, functionalized polymer planar Bragg gratings are eminently suitable for H2 leak detection applications.}, subject = {Gassensor}, language = {en} } @misc{KeferDaiYangetal.2020, author = {Kefer, Stefan and Dai, Jixiang and Yang, Minghong and Schmauss, Bernhard and Hellmann, Ralf}, title = {Hypersensitive H2 sensor based on polymer planar Bragg gratings coated with Pt-loaded WO3-SiO2: erratum}, volume = {45}, number = {16}, doi = {https://doi.org/10.1364/OL.401819}, pages = {4498 -- 4498}, year = {2020}, abstract = {We present an erratum to our Letter [Opt. Lett. 45, 3601 (2020)]. Labeling errors in two figures and an incorrect sentence are revised. The corrections have no influence on the conclusions of the original Letter.}, subject = {Gassensor}, language = {en} }