@article{RauschVerpoortWittrock2016,
  author    = {Rausch, Peter and Verpoort, Sven and Wittrock, Ulrich},
  title     = {Unimorph deformable mirror for space telescopes: environmental testing},
  series = {Opt. Expr.},
  volume    = {24},
  journal   = {Opt. Expr.},
  doi       = {10.25974/fhms-853},
  url       = {http://nbn-resolving.de/urn:nbn:de:hbz:836-opus-8530},
  pages     = {1528 -- 1542},
  year      = {2016},
  abstract  = {We have developed and manufactured a unimorph deformable mirror for space telescopes based on piezoelectric actuation. The mirror features 44 actuators, has an aperture of 50 mm, and is designed to reproduce low-order Zernike modes with a stroke of several tens of µm. We assessed the space compliance by operating the mirror in thermal vacuum, and exposing it to random and sinusoidal vibrations, as well as to ionizing irradiation. Additionally, the operational life time and the laser power handling capability were tested. The mirror was successfully operated in thermal vacuum at 100 K. We report on the conducted tests and the methods used to evaluate the mirror\&\#180;s performance, and discuss the compliance with the demanded requirements},
  language  = {de}
}
@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{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{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{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{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}
}
@article{BuskeWittrock2006,
  author    = {Buske, Ivo and Wittrock, Ulrich},
  title     = {Diffraction analysis of aberrated laser resonators},
  series = {Appl. Phys. B},
  volume    = {83},
  journal   = {Appl. Phys. B},
  doi       = {10.25974/fhms-825},
  url       = {http://nbn-resolving.de/urn:nbn:de:hbz:836-opus-8252},
  pages     = {229 -- 233},
  year      = {2006},
  abstract  = {A numerical analysis of laser resonators with aberrations is presented. {T}he analysis shows that aberrations lead to large diffraction losses of laser resonators which are laid out to produce diffraction-limited beam quality. {S}tatic or dynamic compensation of the aberrations is possible and would yield much higher output power.},
  language  = {en}
}
@article{HeuckNeumayerKuehletal.2006,
  author    = {Heuck, Hans-Martin and Neumayer, Paul and K{\"u}hl, Thomas and Wittrock, Ulrich},
  title     = {Chromatic aberration in petawatt-class lasers},
  series = {Appl. Phys. B},
  volume    = {84},
  journal   = {Appl. Phys. B},
  publisher = {Springer},
  doi       = {10.25974/fhms-652},
  url       = {http://nbn-resolving.de/urn:nbn:de:hbz:836-opus-6523},
  pages     = {421 -- 428},
  year      = {2006},
  abstract  = {In order to avoid optical damage and non-linear effects, high-power, high-energy lasers of the petawatt class like PHELIX (petawatt high-energy laser for heavy-ion experiments) use large-aperture optics. Usually, chromatic aberration associated with these optical elements is neglected. By means of numerical simulations, we show how the chromatic aberration affects the focal intensity pattern. In particular, we make quantitative predictions of how chromatic aberration decreases the focused peak intensity. Furthermore, we prove the feasibility of a new interferometer that measures the temporal pulse front distortions which arise from expansion telescopes. We also propose a scheme that pre-compensates these distortions.},
  language  = {en}
}
@article{VerpoortWittrock2010,
  author    = {Verpoort, Sven and Wittrock, Ulrich},
  title     = {Actuator patterns for unimorph and bimorph deformable mirrors},
  series = {Appl. Opt.},
  volume    = {49},
  journal   = {Appl. Opt.},
  publisher = {OSA},
  doi       = {10.25974/fhms-649},
  url       = {http://nbn-resolving.de/urn:nbn:de:hbz:836-opus-6490},
  pages     = {G37 -- G46},
  year      = {2010},
  abstract  = {The actuator pattern of an adaptive mirror determines the amplitudes and the fidelities of the mirror deformations that can be achieved. In this study, we analyze and compare different electrode patterns of piezoelectric unimorph deformable mirrors using a numerical finite element model. The analysis allows us to determine the optimum actuator pattern, and it is also applicable to bimorph mirrors. The model is verified by comparing its predictions with experimental results of our prototype of a novel unimorph deformable mirror.},
  language  = {de}
}