Mr. Stefan Kefer Profile

Stefan Kefer

at Aschaffenburg University of Applied Sciences

SPIE Involvement:

Author

Publications (5)

Proceedings Article | 27 November 2023 Poster + Paper

Hydrogen sensor based on PPBG coated with porous Pt/WO3

Jingjing Wang, Wenbin Hu, Minghong Yang, Stefan Kefer, Ralf Hellmann

Proceedings Volume 12771, 1277116 (2023) https://doi.org/10.1117/12.2683838

KEYWORDS: Hydrogen, Sensors, Porosity, Polymers, Bragg gratings, Fiber optics sensors

Read Abstract +

Proceedings Article | 17 March 2023 Poster + Paper

High-precision micromachining of sapphire towards optical waveguiding structures using femtosecond lasers

Stefan Kefer, Julian Zettl, Bernhard Schmauss, Cemal Esen, Ralf Hellmann

Proceedings Volume 12409, 1240919 (2023) https://doi.org/10.1117/12.2648758

KEYWORDS: Sapphire, Waveguides, Refractive index, Fabrication, Micromachining, Photonics, Near field, Laser radiation

Read 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 rod and the practical realization of windmill fibers. In addition, 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.

Proceedings Article | 14 March 2023 Presentation + Paper

Towards pressure sensors based on polymer planar Bragg gratings

Stefan Kefer, Natalie Pape, Nikola Gries, Gian-Luca Roth, Bernhard Schmauss, Ralf Hellmann

Proceedings Volume 12418, 124180A (2023) https://doi.org/10.1117/12.2648723

KEYWORDS: Sensors, Bragg gratings, Polymers, Photonics, Bragg wavelengths, Waveguides, Polymeric sensors, Reflection, Integrated photonics, Deformation

Read Abstract +

Proceedings Article | 19 December 2022 Poster + Paper

Hydrogen sensor based on planar polymer Bragg grating coated with Pd/Ni composite film

Yong Zhou, Wenbin Hu, Yongxin Ye, Jingjing Wang, Jixiang Dai, Minghong Yang, Ralf Hellmann, Stefan Kefer

Proceedings Volume 12321, 1232116 (2022) https://doi.org/10.1117/12.2643877

KEYWORDS: Hydrogen, Sensors

Read Abstract +

Proceedings Article | 12 November 2019 Paper

Enhanced seismic sensing based on epoxy waveguide Bragg gratings

Steffen Hessler, Patrick Bott, Stefan Kefer, Maiko Girschikofsky, Bernhard Schmauß, Ralf Hellmann

Proceedings Volume 11197, 111970Q (2019) https://doi.org/10.1117/12.2542646

KEYWORDS: Sensors, Bragg gratings, Waveguides, Polymers, Epoxies, Seismic sensors, Polymeric sensors, Fiber Bragg gratings, Mechanical sensors, Ultraviolet radiation

Read Abstract +

View contact details

UPDATE YOUR PROFILE

Is this your profile? Update it now.

Sign into your SPIE.org account

Don’t have a profile and want one?

Create an account on SPIE.org

Keywords/Phrases

Search In:

Publication Years