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@PHDTHESIS{Hofmann:1011781,
      author       = {Hofmann, Jörg},
      othercontributors = {Holly, Carlo and Kaierle, Stefan},
      title        = {{K}ompensation thermischer {L}inseneffekte in optischen
                      {S}ystemen für die {L}asermaterialbearbeitung},
      school       = {Rheinisch-Westfälische Technische Hochschule Aachen},
      type         = {Dissertation},
      address      = {Aachen},
      publisher    = {RWTH Aachen University},
      reportid     = {RWTH-2025-04723},
      pages        = {1 Online-Ressource : Illustrationen},
      year         = {2025},
      note         = {Veröffentlicht auf dem Publikationsserver der RWTH Aachen
                      University; Dissertation, Rheinisch-Westfälische Technische
                      Hochschule Aachen, 2025},
      abstract     = {In laser material processing, optical systems are used e.g.
                      for beam guidance and shaping. Particularly with laser
                      powers in the multi-kilowatt range, the absorbed laser light
                      leads to heating of the optical elements. One consequence is
                      the so-called thermal lens effect. The thermal lens effect
                      leads to a change in the focal length of the optical system
                      and thus to a shift in the focal position relative to the
                      workpiece. This can lead to reduced machining quality or
                      even to an abortion of the machining process. This thesis
                      therefore investigates various concepts for active and
                      passive compensation of thermal effects. A passive approach
                      for the compensating of thermal effects is investigated
                      using the example of plastic optics for a laser power of 15
                      W. Compared to glass optics, plastic optics exhibit thermal
                      effects that are about 100 times greater at the same laser
                      power, which is why their range of application has so far
                      been limited to optical systems with laser powers <<1 W. On
                      the other side, they offer considerable cost-saving
                      potential due to their injection molding production. Passive
                      compensation of thermal effects through a different
                      combination of thermoplastics is not possible due to their
                      similar material parameters. This work therefore pursues a
                      passive approach in which the geometry of an existing
                      plastic lens is adapted for a specific operating condition.
                      This approach extends the power range that has been
                      effectively usable to date. To this end, the laser and
                      material parameters relevant to the thermo-optical design
                      are first identified using a sensitivity analysis and
                      measured using various measurement methods. The lens
                      geometry is then optimized for the operating condition based
                      on the laser and material parameters. The adapted lens
                      geometry is produced by injection moulding and then
                      characterized with regard to its optical properties. The
                      compensated lens geometry has a Gaussian intensity
                      distribution in the focal plane but deviates from the target
                      value by $27\%$ in terms of its focal length. The thermal
                      effects during the laser machining process can also be
                      intensified by process-related contamination of the optics,
                      in particular the protective glass. In practice, the
                      protective glass is therefore replaced at regular intervals.
                      By compensating for the thermal effects over time, it would
                      be possible to extend the necessary service intervals or
                      increase the process accuracy. Due to the temporal change of
                      the thermal effects that occur, passive compensation is not
                      possible here. As an alternative, active compensation of
                      thermal effects is therefore demonstrated using a new type
                      of measurement and control system. The measurement of the
                      focus position is based on the evaluation of the
                      characteristic diffraction pattern of an amplitude mask
                      integrated into the beam path, the so-called Bahtinov mask.
                      The diffraction pattern is evaluated using different
                      evaluation algorithms. Following experimental validation,
                      the measurement concept is being further developed for
                      coaxial integration in almost any laser material processing
                      system. Measurements show a determination of the focus
                      position over a range of ±2.5 mm with an average deviation
                      of 42.52 μm.},
      cin          = {418910},
      ddc          = {620},
      cid          = {$I:(DE-82)418910_20140620$},
      typ          = {PUB:(DE-HGF)11},
      doi          = {10.18154/RWTH-2025-04723},
      url          = {https://publications.rwth-aachen.de/record/1011781},
}