TY - JOUR A1 - Rausch, Peter A1 - Verpoort, Sven A1 - Wittrock, Ulrich T1 - Unimorph deformable mirror for space telescopes: environmental testing JF - Opt. Expr. N2 - 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´s performance, and discuss the compliance with the demanded requirements KW - active or adaptive optics KW - telescopes thermal effects vibration analysis KW - radiation KW - space optics KW - space instrumentation Y1 - 2016 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:hbz:836-opus-8530 VL - 24 SP - 1528 EP - 1542 ER - TY - JOUR A1 - Rausch, Peter A1 - Verpoort, Sven A1 - Wittrock, Ulrich T1 - Unimorph deformable mirror for space telescopes: design and manufacturing JF - Opt. Expr. N2 - 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. KW - Active or adaptive optics KW - Telescopes KW - Space optics KW - Space instrumentation Y1 - 2015 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:hbz:836-opus-8227 VL - 23 SP - 19469 EP - 19477 ER - TY - JOUR A1 - Wittrock, Ulrich A1 - Vorholt, Christian T1 - Spatial hole burning in Yb:YAG thin-disk lasers JF - Appl. Phys. B N2 - 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. Y1 - 2015 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:hbz:836-opus-8234 VL - 120 SP - 711 EP - 721 ER - TY - JOUR A1 - Vorholt, Christian A1 - Wittrock, Ulrich T1 - Single-frequency oscillation of thin-disk lasers due to phase-matched pumping JF - Opt. Expr. N2 - 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. KW - diode-pumped laser KW - solid-state laser KW - ytterbium laser Y1 - 2017 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:hbz:836-opus-10258 VL - 25 SP - 21388 EP - 21399 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 - Perchermeier, Julian A1 - Wittrock, Ulrich T1 - Precise measurements of the thermo-optical aberrations of an Yb:YAG thin-disk laser JF - Opt. Lett. N2 - 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. Y1 - 2013 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:hbz:836-opus-6854 VL - 38 SP - 2422 EP - 2424 PB - OSA 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 - 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 - JOUR A1 - Wittrock, Ulrich T1 - Laryngeally echolocating bats (Brief Communication Arising) JF - Nature Y1 - 2010 U6 - http://dx.doi.org/10.1038/nature09156 VL - 466 SP - E6 ER -