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| Dissertation / PhD Thesis | PUBDB-2018-05902 |
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2019
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Please use a persistent id in citations: doi:10.3204/PUBDB-2018-05902
Abstract: The subject of this thesis was the development of a thin-disk laser with a continuous waveintracavity intensity of more than $10^{10} {W}{cm^{-2}}$ for the adiabatic alignment ofmolecules. In particular, this necessitates an intracavity power of more than 100 kW, aninternal focus with a radius of 20 µm, a well-defined and stable polarization, and benefitsfrom fundamental transverse mode operation. For this purpose, ytterbium-doped host materials werecompared in various resonators, exploring their losses and their suitability for such a lasersystem. The study focused on the possible implementation of such a laser setup, notably for pumppowers below 500 W, which allow resonator losses of approx. $5 \times 10^{-3}$. A characterizationof YAG and LuO in an efficient and short linear multi-transverse-mode resonator revealed resonatorinternal losses in the order of $2 \times 10^{-4}$. No dependency on the gain material could be resolved.Measurements of the losses with more than 20 thin disks pointed towards losses originating at theirsurface. An intracavity power of 130 kW could be demonstrated. This corresponds to anenhancement by a factor of 2500 with respect to the incident pump power of 54 W. Furtherscaling of the pump intensity was restrained by optical damage, mostly in the form of nodular pointdefects, which were observed at optics and disks. The optical damage is believed to be caused bydefects in the highly reflective coatings. The resonator internal losses for linearsingle-transverse-mode resonators increased up to $9 \times 10^{-4}$, which might have been caused byadditional diffraction losses. The application of a Brewster window to ensure a stable polarizationadditionally led to an increase of losses to $1.5 \times 10^{-3}$. Supposedly, it was caused bystress-induced birefringence in the isotropic gain materials. This induced a rotation of thepolarization and eventually contributed to significant reflection losses at the Brewster plate.Additionally, an intrinsic polarization was measured, which was less stable than the beforementioned polarization but avoided additional losses. Similar behaviors were detected for foldedresonators. Those were more suitable for acquiring a tight internal focus. Imaging the Rayleighscattering at atmospheric molecules allowed an in vitro determination of the caustic. Here, anintracavity-focus-radius in the order of 20 µm was measured. However, resonator losses ofapprox. 5 % and the prevalent optical damage obstructed the achievement of the requiredintracavity powers with the available pump powers. Comparisons and calculations of possible lossessuggested the diffraction losses, caused by astigmatic distortions of the disks, being responsible.To accomplish the goals, a combination of a decrease of losses and an increase of the pump powermight be necessary.
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