@article{WittrockVorholt2015, author = {Wittrock, Ulrich and Vorholt, Christian}, title = {Spatial hole burning in Yb:YAG thin-disk lasers}, series = {Appl. Phys. B}, volume = {120}, journal = {Appl. Phys. B}, doi = {10.25974/fhms-823}, url = {http://nbn-resolving.de/urn:nbn:de:hbz:836-opus-8234}, pages = {711 -- 721}, year = {2015}, abstract = {The spatially varying intensity in a standing wave resonator leads to spatial hole burning in the gain medium of a laser. The spatial hole burning changes the gain of different longitudinal modes and can thus determine the optical spectrum of the laser. We simulate this longitudinal mode competition in standing wave resonators of thin-disk lasers. The resulting optical spectra of the laser are compared to measured optical spectra. We examine two types of resonators: I-resonators and V-resonators with different angles of incidence. In V-resonators, the non-normal incidence of the laser beam on the disk lifts the degeneracy of the polarization. Experiments show that the slight gain advantage for the p-polarization does not lead to polarized emission. For both types of resonators, the measured spectra are in good agreement with the simulated ones. The simulations allow to study the influence of spectral intra-cavity losses on the optical spectrum of a thin-disk laser.}, language = {en} } @article{VorholtWittrock2015, author = {Vorholt, Christian and Wittrock, Ulrich}, title = {Intra-cavity pumped Yb:YAG thin-disk laser with 1.74\% quantum defect}, series = {Opt. Lett.}, volume = {40}, journal = {Opt. Lett.}, doi = {10.25974/fhms-828}, url = {http://nbn-resolving.de/urn:nbn:de:hbz:836-opus-8284}, pages = {4819 -- 4822}, year = {2015}, abstract = {We present, to the best of our knowledge, the first intracavity pumped Yb:YAG thin-disk laser. It operates at 1050.7 nm with a quantum defect of just 1.74\% due to pumping at 1032.4 nm. Low absorption of the pump light at the pump wavelength of 1032.4 nm is compensated for by placing the disk inside the resonator of another Yb:YAG thin-disk laser which is diode-pumped at 940 nm. The intra-cavity pumped laser has an output power of 10.3 W and a slope efficiency of 8.3\%}, language = {en} } @article{VorholtWittrock2017, author = {Vorholt, Christian and Wittrock, Ulrich}, title = {Single-frequency oscillation of thin-disk lasers due to phase-matched pumping}, series = {Opt. Expr.}, volume = {25}, journal = {Opt. Expr.}, doi = {10.25974/fhms-1025}, url = {http://nbn-resolving.de/urn:nbn:de:hbz:836-opus-10258}, pages = {21388 -- 21399}, year = {2017}, abstract = {We present a novel pump concept that should lead to single-frequency operation of thin-disk lasers without the need for etalons or other spectral filters. The single-frequency operation is due to matching the standing wave pattern of partially coherent pump light to the standing wave pattern of the laser light inside the disk. The output power and the optical efficiency of our novel pump concept are compared with conventional pumping. The feasibility of our pump concept was shown in previous experiments.}, language = {en} } @article{VerpoortBittnerWittrock2020, author = {Verpoort, Sven and Bittner, Matthias and Wittrock, Ulrich}, title = {Fast focus-shifter based on a unimorph deformable mirror}, series = {Applied Optics}, volume = {59}, journal = {Applied Optics}, number = {23}, organization = {Optical Society of America}, issn = {1559-128X}, doi = {10.1364/AO.397495}, pages = {6959 -- 6965}, year = {2020}, abstract = {On-the-fly remote laser processing plays an increasingly important role in modern fabrication techniques. These processes require guiding of the focus of a laser beam along the contours of the workpiece in three dimensions. State-of-the-art galvanometer scanners already provide highly dynamic and precise transverse x-y beam steering. However, longitudinal focus shifting ("z-shifting") relying on conventional optics is restricted to a bandwidth of a few hundred Hz. We have developed and manufactured a fast piezo-based z-shifting mirror with diffraction-limited surface fidelity providing a focus shift of 1z> 60 mm with an actuation rate of 2 kHz.}, language = {en} } @article{TrinschekVorholtWittrock2021, author = {Trinschek, Sarah and Vorholt, Christian and Wittrock, Ulrich}, title = {Nonlinear dynamics in intra-cavity pumped thin-disk lasers}, series = {Optics Express}, volume = {29}, journal = {Optics Express}, number = {4}, doi = {https://doi.org/10.1364/OE.417154}, url = {http://nbn-resolving.de/urn:nbn:de:hbz:836-opus-132355}, pages = {5755 -- 5773}, year = {2021}, abstract = {Cross-saturation of the gain media in intra-cavity pumped lasers leads to complex dynamics of the laser power. We present experimental results and a detailed theoretical analysis of this nonlinear dynamics for an intra-cavity pumped Yb:YAG thin-disk laser in the framework of a rate-equation model. The gain medium of this laser is residing in the resonator of a conventional, diode-pumped Yb:YAG thin-disk laser. Continuous-wave operation, periodic pulse trains, and chaotic fluctuations of the optical power of both lasers were observed. The dynamics is not driven by external perturbations but arises naturally in this laser system. Further examination revealed that these modes of operation can be controlled by the resonator length of the diode-pumped laser but that the system can also show hysteresis and multi-stability.}, language = {en} } @article{RauschVerpoortWittrock2015, author = {Rausch, Peter and Verpoort, Sven and Wittrock, Ulrich}, title = {Unimorph deformable mirror for space telescopes: design and manufacturing}, series = {Opt. Expr.}, volume = {23}, journal = {Opt. Expr.}, doi = {10.25974/fhms-822}, url = {http://nbn-resolving.de/urn:nbn:de:hbz:836-opus-8227}, pages = {19469 -- 19477}, year = {2015}, abstract = {Large space telescopes made of deployable and lightweight structures suffer from aberrations caused by thermal deformations, gravitational release, and alignment errors which occur during the deployment procedure. An active optics system would allow on-site correction of wave-front errors, and ease the requirements on thermal and mechanical stability of the optical train. In the course of a project funded by the European Space Agency we have developed and manufactured a unimorph deformable mirror based on piezoelectric actuation. The mirror is able to work in space environment and is designed to correct for large aberrations of low order with high surface fidelity. This paper discusses design, manufacturing and performance results of the deformable mirror.}, language = {en} } @article{PuesLaubeFischeretal.2021, author = {Pues, Patrick and Laube, Michael and Fischer, Stefan and Schr{\"o}der, Franziska and Schwung, Sebastian and Rytz, Daniel and Fiehler, Torben and Wittrock, Ulrich and J{\"u}stel, Thomas}, title = {Luminescence and up-conversion of single crystalline Lu3Al5O12:Pr3+}, series = {Journal of Luminescence}, volume = {234}, journal = {Journal of Luminescence}, doi = {10.1016/j.jlumin.2021.117987}, pages = {117987 -- 117995}, year = {2021}, abstract = {This work deals with the spectroscopic properties of praseodymium doped single crystalline lutetium aluminum garnet (LuAG:Pr3+). A special focus was set on temperature- and time-dependent spectroscopy. Beyond the well-known down-conversion luminescence of LuAG:Pr3+, also UV-A/B up-conversion luminescence under excitation with a 488 nm laser was thoroughly investigated. Furthermore, the results of the spectroscopic investigations on the single crystalline material were supplemented and compared with measurements on a microscale powder sample. In addition, to the spectroscopic investigations, mechanistic considerations are presented to obtain a closer look at the up-conversion process in LuAG:Pr3+. We promote the thesis of a temperature-dependent energy transfer up-conversion mechanism.}, language = {en} } @article{PerchermeierWittrock2013, author = {Perchermeier, Julian and Wittrock, Ulrich}, title = {Precise measurements of the thermo-optical aberrations of an Yb:YAG thin-disk laser}, series = {Opt. Lett.}, volume = {38}, journal = {Opt. Lett.}, publisher = {OSA}, doi = {10.25974/fhms-685}, url = {http://nbn-resolving.de/urn:nbn:de:hbz:836-opus-6854}, pages = {2422 -- 2424}, year = {2013}, abstract = {We report on interferometric measurements of the thermo-optical aberrations of the laser medium of an Yb:YAG thin-disk laser in pumped and cw lasing conditions at several pump-power levels with a mean repeatability of 5 nm. These measurements build the basis for future intracavity compensation of the aberrations with our deformable mirror in order to improve the fundamental-mode efficiency.}, language = {en} } @article{KazasidisWittrock2014, author = {Kazasidis, Orestis and Wittrock, Ulrich}, title = {Interferometric measurement of the temperature coefficient of the refractive index dn/dT and the coefficient of thermal expansion of Pr:YLF laser crystals}, series = {Opt. Expr.}, volume = {22}, journal = {Opt. Expr.}, publisher = {Optical Society of America}, doi = {10.25974/fhms-772}, url = {http://nbn-resolving.de/urn:nbn:de:hbz:836-opus-7720}, pages = {30683 -- 30696}, year = {2014}, abstract = {We report interferometric measurements of the temperature coefficient of the refractive index (dn=dT) and the coefficient of thermal expansion (a) of a praseodymium-doped yttrium lithium fluoride (Pr:YLF) crystal and of a fused silica reference sample. Our phase-resolved interferometric method yields a large number of data points and thus allows a precise measurement and a good error estimation. Furthermore, both dn=dT and a are obtained simultaneously from a single measurement which reduces errors that can occur in separate measurements. Over the temperature range from 20 °C to 80 °C, the value of dn=dT of Pr:YLF decreases from -5.2 x 10-6 /K to -6.2 x 10-6 /K for the ordinary refractive index and from -7.6 x 10-6 /K to -8.6 x 10-6 /K for the extraordinary refractive index. The coefficient of thermal expansion for the a-axis of Pr:YLF increases from 16.4 x 10-6 /K to 17.8 x 10-6 /K over the same temperature range.}, language = {en} } @article{KazasidisVerpoortWittrock2019, author = {Kazasidis, Orestis and Verpoort, Sven and Wittrock, Ulrich}, title = {Aberration balancing using an image-sharpness metric}, series = {J. Opt. Soc. Am. A}, volume = {36}, journal = {J. Opt. Soc. Am. A}, number = {8}, doi = {10.1364/JOSAA.36.001418}, url = {http://nbn-resolving.de/urn:nbn:de:hbz:836-opus-109198}, pages = {1418 -- 1422}, year = {2019}, abstract = {Image-sharpness metrics can be used to optimize optical systems and to control wavefront sensorless adaptive optics systems. We show that for an aberrated system, the numerical value of an image-sharpness metric can be improved by adding specific aberrations. The optimum amplitudes of the additional aberrations depend on the power spectral density of the spatial frequencies of the object.}, language = {en} }