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@PHDTHESIS{Baer:983709,
      author       = {Baer, Patrick Helmut Bernd},
      othercontributors = {Häfner, Constantin Leon and Graf, Thomas},
      title        = {{E}rhöhung der durch nichtlineare {E}ffekte verursachten
                      {L}eistungsgrenzen von {F}aserlasern durch besondere
                      {F}asergeometrien; 1. {A}uflage},
      school       = {RWTH Aachen University},
      type         = {Dissertation},
      address      = {Aachen},
      publisher    = {Apprimus Verlag},
      reportid     = {RWTH-2024-03643},
      isbn         = {978-3-98555-207-8},
      series       = {Ergebnisse aus der Lasertechnik},
      pages        = {1 Online-Ressource : Illustrationen},
      year         = {2024},
      note         = {Druckausgabe: 2024. - Auch veröffentlicht auf dem
                      Publikationsserver der RWTH Aachen University; Dissertation,
                      RWTH Aachen University, 2023},
      abstract     = {The output power of diffraction limited single-mode fiber
                      lasers is fundamentally limited by several effects, such as
                      nonlinear effects, thermal effects, and transverse mode
                      instabilities. In this work, several main concepts are
                      analyzed for the further power scaling of fiber laser
                      systems: single-mode fibers with rectangular and annular
                      core geometries to suppress transverse mode instabilities,
                      and fiber-Bragg-gratings (FBG) in highly multi-mode fibers
                      to lower the influence of power-limiting effects by a high
                      number of modes.To increase nonlinear thresholds and
                      simultaneously suppress transverse mode instabilities,
                      single-mode fibers with unconventional core geometries are
                      investigated, which can enable modal distributions that
                      differ from those of conventional, cylindrically symmetric
                      fibers. For single-mode fibers with a rectangular core
                      geometry small, but no fundamental advantages can be
                      observed. Since e.g., the modal distribution and the bending
                      characteristics are in high accordance between simulated and
                      experimental results of manufactured fibers, the simulation
                      can be validated. When a pedestal is used as an additional
                      structure within the fiber, the bending sensitivity can be
                      improved, which is simulated and experimentally
                      validated.The validated simulation is used to investigate
                      single-mode fibers with an annular core geometry. Here,
                      fundamental differences are demonstrated in comparison to
                      fibers with conventional, cylindrically symmetric core
                      geometry. For the examined parameters, the nonlinear
                      threshold of passive fibers is improved by a factor of
                      approximately 8, while the nonlinear threshold for actively
                      doped fibers is enhanced by a factor of approximately 25 due
                      to the high core area. Fiber amplifiers are analyzed
                      thermally and optically, which results in a potential for
                      power scaling up to 60 kW or to more than 100 kW with
                      respect to the chosen fiber design. For a few-mode fiber
                      concept the nonlinear threshold can be further improved by
                      30 $\%,$ while transverse mode instabilities are no longer
                      fundamentally suppressed. A tolerance analysis is performed
                      to identify the required accuracy for the manufacturing of
                      such a fiber.FBGs in multi-mode fibers can be used to enable
                      fully fiber-integrated resonators, which have a higher
                      non-linear threshold due to a larger mode field area and a
                      higher number of modes. For the analysis, a simulation based
                      on the coupled-mode-theory is developed. To enable
                      multi-mode FBGs with a high spectral reflectivity a concept
                      based on a chirped lattice constant is developed. With this
                      concept, a high modal reflectivity can be achieved at a
                      chosen wavelength for all guided modes of the fiber. To
                      experimentally verify the concept, a highly reflective FBG
                      is manufactured and used to setup a monolithic multi-mode
                      resonator with more than 60 modes. Additionally,
                      inhomogeneous FBGs are investigated to influence the beam
                      quality of resonators, which is experimentally validated in
                      a few-mode fiber. Based on these concepts, novel multimode
                      resonators are developed.},
      cin          = {418710},
      ddc          = {620},
      cid          = {$I:(DE-82)418710_20140620$},
      typ          = {PUB:(DE-HGF)11 / PUB:(DE-HGF)3},
      doi          = {10.18154/RWTH-2024-03643},
      url          = {https://publications.rwth-aachen.de/record/983709},
}