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@MASTERSTHESIS{Kilinc:607009,
      author       = {Kilinc, Muharrem and Kärtner, Franz},
      othercontributors = {Pergament, Mikhail},
      title        = {{T}hermal and optical characterization of cryogenically
                      cooled {Y}b:{YLF} laser},
      school       = {University of Hamburg},
      type         = {Masterarbeit},
      reportid     = {PUBDB-2024-01714},
      pages        = {93},
      year         = {2024},
      note         = {Masterarbeit, University of Hamburg, 2024},
      abstract     = {This thesis focuses on two fundamental limitations that
                      affect the power scaling of cryogenically cooled high-power
                      solid-state lasers: (i) Fractional thermal load (FTL) and
                      (ii) Thermal and population lensing. For FTL, a novel method
                      is introduced for the direct measurement of fractional
                      thermal load in cryogenically cooled laser crystals. The
                      experimental methodology involves characterizing the liquid
                      nitrogen evaporation rate in a dewar containing the laser
                      crystal, which allows the accurate determination of FTL. The
                      FTL is measured to be 1.7 × quantum defect (QD) for Yb:YLF
                      and 1.5 × QD for Yb:YAG under continuous wave lasing
                      conditions. The measured FTL values are then used to
                      calculate the temperature distribution inside the crystals
                      as a function of pump power, and the simulation results are
                      found to be in a very good agreement with the in-situ
                      temperature measurements using contactless optical
                      luminescence thermometry. For thermal and population
                      lensing, this thesis conducts the first detailed study of
                      thermal and population lensing in cryogenically cooled
                      Yb:YLF. Using a rod-shaped a-cut Yb:YLF crystal, the thermal
                      lens strength is measured as a function of absorbed pump
                      power for both E//a and E//c polarizations for pump power
                      levels up to 600 W. Our experiments clarified that the
                      thermal lensing behavior in Yb:YLF is quite asymmetric. For
                      the E//a polarization, the thermal lens is rather strong and
                      always converging for both horizontal and vertical axes. For
                      the E//c configuration, thermal lensing is rather weak, and
                      its sign could be different in different axes, and
                      furthermore, the sign of thermal lensing is observed to be
                      dependent on the absorbed pump power level. Simulations and
                      detailed experimental measurements show that the observed
                      peculiar behavior is a result of competition between the
                      positive photo-elastic effect and negative thermo-optic
                      effect: as their relative strength varies at different pump
                      power levels. We have also measured the contribution of
                      surface bulging into thermal lensing and found it to be
                      relatively small in our crystals with un-doped end-caps.
                      Lastly, the population lensing is measured in Yb:YLF for the
                      first time, and it is found to be quite weak as well, and
                      the polarizability difference parameter of Yb:YLF is
                      determined as (3 ± 1.5) × 10−34 m3.},
      cin          = {FS-CFEL-2},
      cid          = {I:(DE-H253)FS-CFEL-2-20120731},
      pnm          = {631 - Matter – Dynamics, Mechanisms and Control
                      (POF4-631)},
      pid          = {G:(DE-HGF)POF4-631},
      experiment   = {EXP:(DE-H253)AXSIS-20200101},
      typ          = {PUB:(DE-HGF)19},
      doi          = {10.13140/RG.2.2.20360.43521},
      url          = {https://bib-pubdb1.desy.de/record/607009},
}