@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}
}
@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{KazasidisVerpoortWittrock2020,
  author    = {Kazasidis, Orestis and Verpoort, Sven and Wittrock, Ulrich},
  title     = {Sensor for dynamic focus control of a deformable mirror},
  series = {Appl. Opt.},
  journal   = {Appl. Opt.},
  number    = {59},
  doi       = {10.1364/AO.392970},
  pages     = {5625 -- 5630},
  year      = {2020},
  abstract  = {We recently presented a novel unimorph deformable mirror which allows for dynamic focus shift with an actuation rate of 2 kHz. Such mirrors suffer from hysteresis and creep. Therefore, they have to be operated in closed-loop. For this purpose, we developed a defocus sensor based on an astigmatic detection system. In this paper, we present the sensor design and discuss its performance.},
  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{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}
}
@inproceedings{LeitzGerhardsVerpoortetal.2021,
  author    = {Leitz, Sinje and Gerhards, Maximilian and Verpoort, Sven and Wittrock, Ulrich and Freudling, Maximilian and Grzesik, Andreas and Erhard, Markus and Hallibert, Pascal},
  title     = {Vibration and shock testing of a 50 mm aperture unimorph deformable mirror},
  series = {Proceedings of the International Conference on Space Optics (ICSO)},
  volume    = {11852},
  booktitle = {Proceedings of the International Conference on Space Optics (ICSO)},
  organization    = {ESA/ESTEC, Noordwijk},
  doi       = {10.25974/fhms-13740},
  url       = {http://nbn-resolving.de/urn:nbn:de:hbz:836-opus-137401},
  pages     = {118524N},
  year      = {2021},
  abstract  = {We present our latest results on a refined unimorph deformable mirror which was developed in the frame of the ESA GSTP activity "Enabling Technologies for Piezo-Based Deformable Mirrors in Active Optics Correction Chains". The identified baseline concept with the soft piezoceramic material PIC151 successfully sustained all vibration requirements (17.8 gRMS random and 20 g sine) and shock testing (300 g SRS). We cover the mirror design development which reduces the stress in the brittle piezo-ceramic by 90 \% compared to the design from a former GSTP activity. We briefly address the optical characterization of the deformable mirror, namely the achieved Zernike amplitudes as well as the unpowered surface deformation (1.7 µm) and active flattening (12.3 nmRMS). The mirror produces low-order Zernike modes with a stroke of several tens of micrometer over a correction aperture of 50 mm, which makes the mirror a versatile tool for space telescopes.},
  language  = {en}
}
@inproceedings{FreudlingGrzesikErhardetal.2021,
  author    = {Freudling, Maximilian and Grzesik, Andreas and Erhard, Markus and Gerhards, Maximilian and Leitz, Sinje and Verpoort, Sven and Wittrock, Ulrich and Hallibert, Pascal},
  title     = {Space-qualified piezo based deformable mirror for future instruments with active optics},
  series = {Proceedings of the International Conference on Space Optics (ICSO)},
  volume    = {11852},
  booktitle = {Proceedings of the International Conference on Space Optics (ICSO)},
  organization    = {ESA/ESTEC, Noordwijk},
  doi       = {10.25974/fhms-13741},
  url       = {http://nbn-resolving.de/urn:nbn:de:hbz:836-opus-137410},
  pages     = {1185231-11},
  year      = {2021},
  abstract  = {This paper presents the results of the technology development project "Enabling Technologies for Piezo-Based Deformable Mirrors in Active Optics Correction Chains" conducted by OHB System AG together with its partner M{\"u}nster University of Applied Sciences (MUAS). The project was funded by ESA within their General Support Technology Programme (GSTP). We address in this paper mainly the definition, flow-down and verification of the requirements for the Deformable Mirror (DM). The requirements were derived from a set of real space mission applications. The deformation of the mirror is performed by piezo-ceramic actuators in an unimorph configuration. The finally developed DM is able produce Zernike modes with a stroke of several tens of µm over a clear optical aperture of 50 mm in diameter. It underwent successfully a full environmental qualification campaign including thermal cycling, shock- and vibration testing, as well as exposure to proton and γ-ray radiation. Thermal and performance tests were performed in the temperature range from 100 K to 300 K. Furthermore, the DM sustained all vibration (random 17.8 g RMS and sinus) and shock (300 g) testing. Thereby all criticalities which were identified a previous study have been overcome successfully. A Technology Readiness Level (TRL) of 5 is reached, as the component has been validated in relevant environment. Based on the high level of maturity, this deformable mirror is now ready for the incorporation in future flight instruments. The achieved TRL of 5 is sufficient for the status of a PDR at payload level and gives thus a very good basis for all kinds of potential B2, C/D payload developments.},
  language  = {en}
}
@inproceedings{WittrockWelp2006,
  author    = {Wittrock, Ulrich and Welp, Petra},
  title     = {Adaptive laser resonator control with deformable MOEMS mirrors},
  series = {MEMS/MOEMS Components and Their Applications III, Proc. SPIE},
  volume    = {6113},
  booktitle = {MEMS/MOEMS Components and Their Applications III, Proc. SPIE},
  publisher = {SPIE},
  doi       = {10.25974/fhms-653},
  url       = {http://nbn-resolving.de/urn:nbn:de:hbz:836-opus-6537},
  pages     = {61130C},
  year      = {2006},
  abstract  = {Adaptive laser resonators with deformable MOEMS mirrors under closed-loop control are discussed and experimental results are presented. The requirements for deformable mirrors and for closed-loop control systems of these mirrors are analyzed. Several deformable mirrors have been characterized and the results are presented. Currently available membrane mirrors deform under laser load and need further development before they can be used for aberration correction of solid state lasers above some tens of Watts. Nevertheless, the results are encouraging and the requirements are within reach of currently available technology. Finally, we demonstrate an Nd.YVO4-laser with a closed-loop adaptive resonator and more than 6 W of output power. The closed-loop system was able to compensate artificially introduced aberrations from a phase plate.},
  language  = {de}
}
@article{AlbersWittrock2011,
  author    = {Albers, Klaus and Wittrock, Ulrich},
  title     = {Optical pump concepts for highly efficient quasi-three-level lasers},
  series = {Appl. Phys. B},
  volume    = {105},
  journal   = {Appl. Phys. B},
  publisher = {Springer},
  doi       = {10.25974/fhms-646},
  url       = {http://nbn-resolving.de/urn:nbn:de:hbz:836-opus-6467},
  pages     = {245 -- 254},
  year      = {2011},
  abstract  = {Efficient quasi-three-level laser operation requires the generation of the highest possible pump rate from a given pump source. We derive the fundamental scaling laws for the pump rate and we extract optimization criteria for pump concepts from these laws. The analysis is then applied to the thin-disk laser. Based on the results, a novel pump concept for thin-disk lasers is proposed, which allows for several tens of pump beam passes and reduces the optical complexity of conventional pump concepts. Furthermore, the beam quality of the pump source is preserved almost completely, facilitating the highest possible pump rate.},
  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}
}