@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{Wittrock2014,
  author    = {Wittrock, Ulrich},
  title     = {Limitations? What Limitations? Prof. Ulrich Wittrock, head of the Photonics Laboratory at the M{\"u}nster University of Applied Sciences, seeks out the ultimate boundaries of laser technology},
  series = {Laser Community - the laser magazine from Trumpf},
  volume    = {02:14},
  journal   = {Laser Community - the laser magazine from Trumpf},
  year      = {2014},
  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}
}
@inproceedings{VorholtWittrock2015,
  author    = {Vorholt, Christian and Wittrock, Ulrich},
  title     = {Wavelength control by angle-tuning of the laser radiation in an intra-cavity pumped Yb:YAG thin-disk laser},
  series = {Advanced Solid State Lasers (ASSL), paper AM5A.39},
  booktitle = {Advanced Solid State Lasers (ASSL), paper AM5A.39},
  doi       = {10.25974/fhms-829},
  url       = {http://nbn-resolving.de/urn:nbn:de:hbz:836-opus-8298},
  year      = {2015},
  abstract  = {In an intra-cavity pumped thin-disk laser the pump radiation forms a standing wave pattern inside the intra-cavity pumped disk.We demonstrate experimentally that the grating period of the standing wave pattern of the pump radiation can control the laser wavelength.},
  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}
}
@incollection{VerpoortWittrock2009,
  author    = {Verpoort, Sven and Wittrock, Ulrich},
  title     = {Miniaturized adaptive mirror for solid state laser resonators},
  series = {Proceedings of the 7th International Workshop on Adaptive Optics for Industry and Medicine, Shatura, Russia},
  booktitle = {Proceedings of the 7th International Workshop on Adaptive Optics for Industry and Medicine, Shatura, Russia},
  year      = {2009},
  language  = {en}
}
@inproceedings{VerpoortWittrock2011,
  author    = {Verpoort, Sven and Wittrock, Ulrich},
  title     = {Deformable mirrors for high power lasers},
  series = {Proceedings of the 8th International Workshop on Adaptive Optics for Industry and Medicine (AOIM)},
  booktitle = {Proceedings of the 8th International Workshop on Adaptive Optics for Industry and Medicine (AOIM)},
  url       = {http://nbn-resolving.de/urn:nbn:de:hbz:836-opus-106953},
  year      = {2011},
  abstract  = {It has been shown that the beam quality and the efficiency of high-power solid-state lasers could be enhanced by the use of deformable mirrors in order to compensate for optical aberrations. An intracavity compensation requires a deformable mirror which is capable of handling very high laser intensities. The active diameter of the deformable mirror should be a few millimeters in order to match typical fundamental mode laser beam diameters. There is a wide variety of commercially available deformable mirrors, but neither meets all requirements.},
  language  = {en}
}
@inproceedings{VerpoortWittrock2011,
  author    = {Verpoort, Sven and Wittrock, Ulrich},
  title     = {Novel unimorph deformable mirror with monolithic tip-tilt functionality for solid state lasers},
  series = {MEMS Adaptive Optics V, Proc. SPIE},
  volume    = {7931},
  booktitle = {MEMS Adaptive Optics V, Proc. SPIE},
  publisher = {SPIE},
  doi       = {10.25974/fhms-647},
  url       = {http://nbn-resolving.de/urn:nbn:de:hbz:836-opus-6471},
  pages     = {793107},
  year      = {2011},
  abstract  = {We present a new type of unimorph deformable mirror with monolithic tip-tilt functionality. The tip-tilt actuation is based on a spiral arm design. The mirror will be used in high-power laser resonators for real-time intracavity phase control. The additional tip-tilt correction with a stroke up to 6 μm simplifies the resonator alignment significantly. The mirror is optimized for a laser beam footprint of about 10 mm. We have modeled and optimized this mirror by finite element calculations and we will present design criteria and tradeoffs for this mirrors. The mirror is manufactured from a super-polished glass substrate with very low surface scattering and excellent dielectric coating.},
  language  = {en}
}
@inproceedings{VerpoortWittrock2010,
  author    = {Verpoort, Sven and Wittrock, Ulrich},
  title     = {Unimorph deformable mirror for telescopes and laser applications in space},
  series = {International Conference on Space Optics (ICSO), Rhodes Island, Greece},
  booktitle = {International Conference on Space Optics (ICSO), Rhodes Island, Greece},
  publisher = {SPIE},
  doi       = {10.25974/fhms-648},
  url       = {http://nbn-resolving.de/urn:nbn:de:hbz:836-opus-6484},
  year      = {2010},
  abstract  = {Over the past 5 years we have developed a new type of unimorph deformable mirror. The main advantages of this mirror technology are · very low surface scattering due to the use of superpolished glass · excellent coatings, even suitable for high power lasers, can be applied · active diameter of the mirrors can be between 10 mm and 100 mm · large strokes can be achieved even for small mirror diameters · integrated monolithic tip/tilt functionality based on a spiral arm design We have modeled these mirrors by analytical models as well as by the finite element method. This allows us to quickly design new mirrors tailored to specific applications. One example is a mirror for laser applications that has a diameter of 10 mm and can achieve a stroke in defocus mode of 5 μm. The stroke for these mirrors scales as the square of the mirror diameter, meaning that we can achieve, for example, a stroke of 125 μm for a mirror of 50 mm diameter. We will present design criteria and tradeoffs for these mirrors. We characterize our mirrors by the maximum stroke they can deliver for various Zernike modes, under the boundary condition that the Zernike mode has to be created with a certain fidelity, usually defined by the Mar{\´e}chal criterion.},
  language  = {en}
}
@inproceedings{VerpoortWelpWittrock2009,
  author    = {Verpoort, Sven and Welp, Petra and Wittrock, Ulrich},
  title     = {Novel unimorph deformable mirror for solid state laser resonators},
  series = {MEMS Adaptive Optics III, Proc. SPIE},
  volume    = {72090N},
  booktitle = {MEMS Adaptive Optics III, Proc. SPIE},
  publisher = {SPIE},
  doi       = {10.25974/fhms-650},
  url       = {http://nbn-resolving.de/urn:nbn:de:hbz:836-opus-6504},
  year      = {2009},
  abstract  = {We present a novel unimorph deformable mirror with a diameter of only 10 mm that will be used in adaptive resonators of high power solid state lasers. The relationship between applied voltage and deformation of a unimorph mirror depends on the piezoelectric material properties, layer thicknesses, boundary conditions, and the electrode pattern. An analytical equation for the deflection of the piezoelectric unimorph structure is derived, based on the electro-elastic and thin plate theory. The validity of the proposed analytical model has been proven by numerical finite-element modelling and experimental results. Our mirror design has been optimized to obtain the highest possible stroke and a high resonance frequency.},
  language  = {en}
}