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    <issue>18</issue>
    <volume>42</volume>
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    <title language="eng">Lattice-Like Waveguides With Integrated Bragg Gratings in Planar Cyclic Olefin Copolymers</title>
    <abstract language="eng">This contribution demonstrates femtosecond laser direct writing of lattice-like waveguides in planar cyclic oleﬁn copolymer substrates. Based on numerical simulation and experimental near-ﬁeld analysis, stable single-mode waveguiding around wavelengths of 1550 nm is demonstrated. The waveguiding mechanism is based on a hexagonal array of laser-induced, positive refractive index modiﬁcation lines. Thus, the lateral extension of the guided mode can be adapted by varying the fabrication parameters and, in consequence, the resulting cross-sectional arrangement of the refractive index perturbations. With an optical attenuationof 2.2 dB·cm-1 around 1550 nm, the fabricated waveguides are well-suited for on-chip integrated photonic devices. Moreover, the waveguides can also be equipped with Bragg gratings to enable the application of the photonic platform as a sensing device. Dependingon their length, the Bragg grating structures exhibit reﬂectivities ofup to 99% and spectral widths down to 0.3 nm. The ﬂexibility of the fabrication process and the sensing capabilities of the lattice-like waveguides with integrated Bragg gratings are underlined by an exemplary application study demonstrating a relative pressure sensor. For that, a photonic platform is micromilled to generate a 300 µm thick diaphragm and a reference pressure chamber. The strain introduced to the diaphragm by external pressure changes can then be quantiﬁed by the integrated photonic structures.This way, absolute pressure sensitivities of up to 38 pm·kPa-1 can be achieved in a relative pressure range from −60 to 100 kPa. The newly-developed lattice-like waveguides with integrated Bragggratings are therefore well-suited for the realization of novel and adaptable photonic devices and sensors.</abstract>
    <parentTitle language="eng">Journal of Lightwave Technology</parentTitle>
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    <author>Stefan Kefer</author>
    <author>Natalie Pape</author>
    <author>Kay Bischoff</author>
    <author>Bernhard Schmauss</author>
    <author>Ralf Hellmann</author>
    <subject>
      <language>eng</language>
      <type>uncontrolled</type>
      <value>Bragg grating</value>
    </subject>
    <subject>
      <language>eng</language>
      <type>uncontrolled</type>
      <value>cyclic olefin copolymers</value>
    </subject>
    <subject>
      <language>eng</language>
      <type>uncontrolled</type>
      <value>femtosecond laser</value>
    </subject>
    <subject>
      <language>eng</language>
      <type>uncontrolled</type>
      <value>integrated photonics</value>
    </subject>
    <subject>
      <language>eng</language>
      <type>uncontrolled</type>
      <value>lattice-like waveguide</value>
    </subject>
    <subject>
      <language>eng</language>
      <type>uncontrolled</type>
      <value>pressure sensor</value>
    </subject>
    <subject>
      <language>deu</language>
      <type>swd</type>
      <value>Wellenleiter</value>
    </subject>
    <subject>
      <language>deu</language>
      <type>swd</type>
      <value>Bragg-Reflektor</value>
    </subject>
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    <collection role="forschungsschwerpunkte" number="">Material Testing &amp; Sensor Technology</collection>
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  </doc>
  <doc>
    <id>2084</id>
    <completedYear>2023</completedYear>
    <publishedYear/>
    <thesisYearAccepted/>
    <language>eng</language>
    <pageFirst/>
    <pageLast/>
    <pageNumber/>
    <edition/>
    <issue/>
    <volume/>
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    <publisherName/>
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    <completedDate>2023-03-21</completedDate>
    <publishedDate>--</publishedDate>
    <thesisDateAccepted>--</thesisDateAccepted>
    <title language="eng">Towards Pressure Sensors Based on Polymer Planar Bragg Gratings</title>
    <abstract language="eng">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.</abstract>
    <parentTitle language="eng">Organic Photonic Materials and Devices XXV</parentTitle>
    <identifier type="isbn">9781510659414</identifier>
    <identifier type="doi">10.1117/12.2648723</identifier>
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    <licence>Keine Lizenz - es gilt das deutsche Urheberrecht</licence>
    <author>Stefan Kefer</author>
    <author>Natalie Pape</author>
    <author>Nikola Gries</author>
    <author>Gian-Luca Roth</author>
    <author>Bernhard Schmauss</author>
    <author>Ralf Hellmann</author>
    <subject>
      <language>eng</language>
      <type>uncontrolled</type>
      <value>Bragg grating</value>
    </subject>
    <subject>
      <language>eng</language>
      <type>uncontrolled</type>
      <value>pressure sensor</value>
    </subject>
    <subject>
      <language>eng</language>
      <type>uncontrolled</type>
      <value>integrated photonics</value>
    </subject>
    <subject>
      <language>eng</language>
      <type>uncontrolled</type>
      <value>cyclic olefin copolymers</value>
    </subject>
    <subject>
      <language>eng</language>
      <type>uncontrolled</type>
      <value>femtosecond laser</value>
    </subject>
    <subject>
      <language>deu</language>
      <type>swd</type>
      <value>Bragg-Reflektor</value>
    </subject>
    <subject>
      <language>deu</language>
      <type>swd</type>
      <value>Drucksensor</value>
    </subject>
    <collection role="institutes" number="">Angewandte Lasertechnik und Photonik (alp)</collection>
    <collection role="forschungsschwerpunkte" number="">Material Testing &amp; Sensor Technology</collection>
  </doc>
  <doc>
    <id>2083</id>
    <completedYear>2023</completedYear>
    <publishedYear/>
    <thesisYearAccepted/>
    <language>eng</language>
    <pageFirst>3</pageFirst>
    <pageLast>11</pageLast>
    <pageNumber>9</pageNumber>
    <edition/>
    <issue/>
    <volume/>
    <type>conferenceobject</type>
    <publisherName/>
    <publisherPlace/>
    <creatingCorporation/>
    <contributingCorporation/>
    <belongsToBibliography>0</belongsToBibliography>
    <completedDate>2023-03-20</completedDate>
    <publishedDate>--</publishedDate>
    <thesisDateAccepted>--</thesisDateAccepted>
    <title language="eng">High-Precision Micromachining of Sapphire Towards Optical Waveguiding Structures using Femtosecond Lasers</title>
    <abstract language="eng">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</abstract>
    <parentTitle language="eng">Laser-based Micro- and Nanoprocessing XVII</parentTitle>
    <identifier type="isbn">9781510659230</identifier>
    <identifier type="doi">10.1117/12.2648758</identifier>
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    <licence>Keine Lizenz - es gilt das deutsche Urheberrecht</licence>
    <author>Stefan Kefer</author>
    <editor>Rainer Kling</editor>
    <author>Julian Zettl</author>
    <editor>Wilhelm Pfleging</editor>
    <author>Bernhard Schmauss</author>
    <editor>Akira Watanabe</editor>
    <author>Cemal Esen</author>
    <author>Ralf Hellmann</author>
    <subject>
      <language>eng</language>
      <type>uncontrolled</type>
      <value>sapphire</value>
    </subject>
    <subject>
      <language>eng</language>
      <type>uncontrolled</type>
      <value>femtosecond laser</value>
    </subject>
    <subject>
      <language>eng</language>
      <type>uncontrolled</type>
      <value>micromachining</value>
    </subject>
    <subject>
      <language>eng</language>
      <type>uncontrolled</type>
      <value>laser lathe</value>
    </subject>
    <subject>
      <language>eng</language>
      <type>uncontrolled</type>
      <value>rod-to-fiber</value>
    </subject>
    <subject>
      <language>deu</language>
      <type>swd</type>
      <value>Femtosekundenlaser</value>
    </subject>
    <subject>
      <language>deu</language>
      <type>swd</type>
      <value>Saphir</value>
    </subject>
    <collection role="institutes" number="">Angewandte Lasertechnik und Photonik (alp)</collection>
    <collection role="forschungsschwerpunkte" number="">Innovative Material Processing</collection>
  </doc>
  <doc>
    <id>2082</id>
    <completedYear>2022</completedYear>
    <publishedYear/>
    <thesisYearAccepted/>
    <language>eng</language>
    <pageFirst>6233</pageFirst>
    <pageLast>6233</pageLast>
    <pageNumber/>
    <edition/>
    <issue>18</issue>
    <volume>15</volume>
    <type>article</type>
    <publisherName/>
    <publisherPlace/>
    <creatingCorporation/>
    <contributingCorporation/>
    <belongsToBibliography>0</belongsToBibliography>
    <completedDate>2022-09-08</completedDate>
    <publishedDate>2022-09-08</publishedDate>
    <thesisDateAccepted>--</thesisDateAccepted>
    <title language="eng">Femtosecond Laser-Based Micromachining of Rotational-Symmetric Sapphire Workpieces</title>
    <abstract language="eng">Sapphire is a robust and wear-resistant material. However, efficient and high-quality micromachining is still a challenge. This contribution demonstrates and discusses two novels, previously unreported approaches for femtosecond laser-based micromachining of rotational-symmetric sapphire workpieces, whereas both methods are in principal hybrids of laser scanning and laser turning or laser lathe. The first process, a combination of a sequential linear hatch pattern in parallel to the workpiece’s main axis with a defined incremental workpiece rotation, enables the fabrication of sapphire fibers with diameters of 50 µm over a length of 4.5 mm. Furthermore, sapphire specimens with a diameter of 25 µm over a length of 2 mm can be fabricated whereas an arithmetical mean height, i.e., Sa parameter, of 281 nm is achieved. The second process combines a constant workpiece feed and orthogonal scanning with incremental workpiece rotation. With this approach, workpiece length limitations of the first process are overcome and sapphire fibers with an average diameter of 90 µm over a length of 20 cm are manufactured. Again, the sapphire specimen exhibits a comparable surface roughness with an average Sa value of 249 nm over 20 cm. Based on the obtained results, the proposed manufacturing method paves an innovative and flexible, all laser-based way towards the fabrication or microstructuring of sapphire optical devices, and thus, a promising alternative to chemical processes.</abstract>
    <parentTitle language="eng">Materials</parentTitle>
    <identifier type="url">https://www.mdpi.com/1996-1944/15/18/6233</identifier>
    <identifier type="doi">10.3390/ ma15186233</identifier>
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    <licence>Creative Commons - CC BY - Namensnennung 4.0 International</licence>
    <author>Stefan Kefer</author>
    <author>Julian Zettl</author>
    <author>Cemal Esen</author>
    <author>Ralf Hellmann</author>
    <subject>
      <language>eng</language>
      <type>uncontrolled</type>
      <value>sapphire</value>
    </subject>
    <subject>
      <language>eng</language>
      <type>uncontrolled</type>
      <value>femtosecond laser</value>
    </subject>
    <subject>
      <language>eng</language>
      <type>uncontrolled</type>
      <value>micromachining</value>
    </subject>
    <subject>
      <language>eng</language>
      <type>uncontrolled</type>
      <value>laser turning</value>
    </subject>
    <subject>
      <language>eng</language>
      <type>uncontrolled</type>
      <value>ablation threshold</value>
    </subject>
    <subject>
      <language>deu</language>
      <type>swd</type>
      <value>Femtosekundenlaser</value>
    </subject>
    <subject>
      <language>deu</language>
      <type>swd</type>
      <value>Saphir</value>
    </subject>
    <collection role="institutes" number="">Angewandte Lasertechnik und Photonik (alp)</collection>
    <collection role="forschungsschwerpunkte" number="">Innovative Material Processing</collection>
    <file>https://opus4.kobv.de/opus4-h-ab/files/2082/Kefer2022_OPUS.pdf</file>
  </doc>
  <doc>
    <id>2043</id>
    <completedYear>2022</completedYear>
    <publishedYear/>
    <thesisYearAccepted/>
    <language>eng</language>
    <pageFirst>234</pageFirst>
    <pageLast>234</pageLast>
    <pageNumber/>
    <edition/>
    <issue>4</issue>
    <volume>9</volume>
    <type>article</type>
    <publisherName/>
    <publisherPlace/>
    <creatingCorporation/>
    <contributingCorporation/>
    <belongsToBibliography>0</belongsToBibliography>
    <completedDate>2022-04-01</completedDate>
    <publishedDate>2022-04-01</publishedDate>
    <thesisDateAccepted>--</thesisDateAccepted>
    <title language="eng">Sapphire Photonic Crystal Waveguides with Integrated Bragg Grating Structure</title>
    <abstract language="eng">Abstract: This contribution demonstrates photonic crystal waveguides generated within bulk planar sapphire substrates. A femtosecond laser is used to modify the refractive index in a hexagonal pattern around the pristine waveguide core. Near-field measurements reveal single-mode behavior at a wavelength of 1550 nm and the possibility to adapt the mode-field diameter. Based on farfield examinations, the effective refractive index contrast between the pristine waveguide core and depressed cladding is estimated to 3x10-4. Additionally, Bragg gratings are generated within the waveguide core. Due to the inherent birefringence of Al2O3, the gratings exhibit two distinct wavelengths of main reflection. Each reflection peak exhibits a narrow spectral full width at a half maximum of 130 pm and can be selectively addressed by exciting the birefringent waveguide with appropriately polarized light. Furthermore, a waveguide attenuation of 1 dB cm-1 is determined.</abstract>
    <parentTitle language="eng">Photonics</parentTitle>
    <identifier type="url">https://www.mdpi.com/2304-6732/9/4/234</identifier>
    <identifier type="doi">https://doi.org/10.3390/photonics9040234</identifier>
    <enrichment key="copyright">1</enrichment>
    <enrichment key="opus.source">publish</enrichment>
    <licence>Creative Commons - CC BY - Namensnennung 4.0 International</licence>
    <author>Stefan Kefer</author>
    <subject>
      <language>eng</language>
      <type>uncontrolled</type>
      <value>integrated photonics</value>
    </subject>
    <subject>
      <language>eng</language>
      <type>uncontrolled</type>
      <value>sapphire</value>
    </subject>
    <subject>
      <language>eng</language>
      <type>uncontrolled</type>
      <value>Bragg grating</value>
    </subject>
    <subject>
      <language>eng</language>
      <type>uncontrolled</type>
      <value>femtosecond laser</value>
    </subject>
    <subject>
      <language>eng</language>
      <type>uncontrolled</type>
      <value>photonic crystal waveguide</value>
    </subject>
    <subject>
      <language>deu</language>
      <type>swd</type>
      <value>Wellenleiter</value>
    </subject>
    <subject>
      <language>deu</language>
      <type>swd</type>
      <value>Femtosekundenlaser</value>
    </subject>
    <subject>
      <language>deu</language>
      <type>swd</type>
      <value>Bragg-Reflektor</value>
    </subject>
    <collection role="institutes" number="">Angewandte Lasertechnik und Photonik (alp)</collection>
    <collection role="forschungsschwerpunkte" number="">Material Testing &amp; Sensor Technology</collection>
    <file>https://opus4.kobv.de/opus4-h-ab/files/2043/Kefer2022_Sapphire_Photonic_Crystal_Waveguides.pdf</file>
  </doc>
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