TY - JOUR A1 - Kazasidis, Orestis A1 - Verpoort, Sven A1 - Wittrock, Ulrich T1 - Aberration balancing using an image-sharpness metric JF - J. Opt. Soc. Am. A N2 - 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. KW - adaptive optics; image-sharpness Y1 - 2019 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:hbz:836-opus-109198 UR - https://www.osapublishing.org/josaa/abstract.cfm?uri=josaa-36-8-1418 VL - 36 IS - 8 SP - 1418 EP - 1422 ER - TY - JOUR A1 - Verpoort, Sven A1 - Bittner, Matthias A1 - Wittrock, Ulrich T1 - Fast focus-shifter based on a unimorph deformable mirror JF - Applied Optics N2 - 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. KW - beam steering KW - deformable mirrors KW - optical components KW - laser machining KW - laser materials processing Y1 - 2020 UR - https://www.osapublishing.org/ao/abstract.cfm?uri=ao-59-23-6959 U6 - http://dx.doi.org/10.1364/AO.397495 SN - 1559-128X VL - 59 IS - 23 SP - 6959 EP - 6965 ER - TY - JOUR A1 - Kazasidis, Orestis A1 - Verpoort, Sven A1 - Wittrock, Ulrich T1 - Sensor for dynamic focus control of a deformable mirror JF - Appl. Opt. N2 - 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. KW - deformable mirrors KW - diode lasers KW - laser beams KW - laser machining KW - sensor performance Y1 - 2020 UR - https://www.osapublishing.org/ao/abstract.cfm?uri=ao-59-18-5625 U6 - http://dx.doi.org/10.1364/AO.392970 IS - 59 SP - 5625 EP - 5630 ER - TY - JOUR A1 - Trinschek, Sarah A1 - Vorholt, Christian A1 - Wittrock, Ulrich T1 - Nonlinear dynamics in intra-cavity pumped thin-disk lasers JF - Optics Express N2 - 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. Y1 - 2021 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:hbz:836-opus-132355 UR - https://www.osapublishing.org/oe/fulltext.cfm?uri=oe-29-4-5755&id=447544 VL - 29 IS - 4 SP - 5755 EP - 5773 ER - TY - JOUR A1 - Pues, Patrick A1 - Laube, Michael A1 - Fischer, Stefan A1 - Schröder, Franziska A1 - Schwung, Sebastian A1 - Rytz, Daniel A1 - Fiehler, Torben A1 - Wittrock, Ulrich A1 - Jüstel, Thomas T1 - Luminescence and up-conversion of single crystalline Lu3Al5O12:Pr3+ JF - Journal of Luminescence N2 - 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. KW - Pr3+ luminescence KW - Single crystal KW - Photoluminescence KW - Up-conversion KW - Temperature-dependent spectroscopy Y1 - 2021 U6 - http://dx.doi.org/10.1016/j.jlumin.2021.117987 VL - 234 SP - 117987 EP - 117995 ER - TY - CHAP A1 - Leitz, Sinje A1 - Gerhards, Maximilian A1 - Verpoort, Sven A1 - Wittrock, Ulrich A1 - Freudling, Maximilian A1 - Grzesik, Andreas A1 - Erhard, Markus A1 - Hallibert, Pascal T1 - Vibration and shock testing of a 50 mm aperture unimorph deformable mirror T2 - Proceedings of the International Conference on Space Optics (ICSO) N2 - 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. KW - active optics KW - adaptive optics KW - deformable mirror KW - vibration damping KW - space telescopes Y1 - 2021 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:hbz:836-opus-137401 UR - https://icso2020.com/ VL - 11852 SP - 118524N ER - TY - CHAP A1 - Freudling, Maximilian A1 - Grzesik, Andreas A1 - Erhard, Markus A1 - Gerhards, Maximilian A1 - Leitz, Sinje A1 - Verpoort, Sven A1 - Wittrock, Ulrich A1 - Hallibert, Pascal T1 - Space-qualified piezo based deformable mirror for future instruments with active optics T2 - Proceedings of the International Conference on Space Optics (ICSO) N2 - 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 Münster 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. KW - deformable mirror KW - active optics KW - space qualification KW - space telescopes KW - adaptive optics Y1 - 2021 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:hbz:836-opus-137410 UR - https://www.spiedigitallibrary.org/conference-proceedings-of-spie/11852/1185231/Space-qualified-Piezo-Based-Deformable-Mirror-for-future-Instruments-with/10.1117/12.2599467.full VL - 11852 SP - 1185231-11 ER - TY - CHAP A1 - Wittrock, Ulrich A1 - Welp, Petra T1 - Adaptive laser resonator control with deformable MOEMS mirrors T2 - MEMS/MOEMS Components and Their Applications III, Proc. SPIE N2 - 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. Y1 - 2006 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:hbz:836-opus-6537 VL - 6113 SP - 61130C PB - SPIE ER - TY - JOUR A1 - Albers, Klaus A1 - Wittrock, Ulrich T1 - Optical pump concepts for highly efficient quasi-three-level lasers JF - Appl. Phys. B N2 - 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. Y1 - 2011 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:hbz:836-opus-6467 VL - 105 SP - 245 EP - 254 PB - Springer ER - TY - CHAP A1 - Verpoort, Sven A1 - Wittrock, Ulrich T1 - Novel unimorph deformable mirror with monolithic tip-tilt functionality for solid state lasers T2 - MEMS Adaptive Optics V, Proc. SPIE N2 - 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. Y1 - 2011 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:hbz:836-opus-6471 VL - 7931 SP - 793107 PB - SPIE ER -