@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} } @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} } @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} } @inproceedings{VerpoortRauschWittrock2012, author = {Verpoort, Sven and Rausch, Peter and Wittrock, Ulrich}, title = {Novel unimorph deformable mirror for space applications}, series = {International Conference on Space Optics (ICSO), Proc. SPIE}, volume = {10564}, booktitle = {International Conference on Space Optics (ICSO), Proc. SPIE}, publisher = {SPIE}, doi = {10.25974/fhms-651}, url = {http://nbn-resolving.de/urn:nbn:de:hbz:836-opus-6516}, pages = {1056414-1}, year = {2012}, abstract = {We have developed a new type of unimorph deformable mirror, designed to correct for low-order Zernike modes. The mirror has a clear optical aperture of 50 mm combined with large peak-to-valley Zernike amplitudes of up to 35 μm. Newly developed fabrication processes allow the use of prefabricated super-polished and coated glass substrates. The mirror's unique features suggest the use in several stronomical applications like the precompensation of atmospheric aberrations seen by laser beacons and the use in woofer-tweeter systems. Additionally, the design enables an efficient correction of the inevitable wavefront error imposed by the floppy structure of primary mirrors in future large space-based telescopes. We have modeled the mirror by using analytical as well as finite element models. We will present design, key features and manufacturing steps of the deformable mirror.}, 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} }