TY - JOUR A1 - Kazasidis, Orestis A1 - Verpoort, Sven A1 - Soloviev, Oleg A1 - Vdovin, Gleb A1 - Verhaegen, Michel A1 - Wittrock, Ulrich T1 - Extended-image-based correction of aberrations using a deformable mirror with hysteresis JF - Opt. Expr. N2 - With a view to the next generation of large space telescopes, we investigate guide-star-free, image-based aberration correction using a unimorph deformable mirror in a plane conjugate to the primary mirror. We designed and built a high-resolution imaging testbed to evaluate control algorithms. In this paper we use an algorithm based on the heuristic hill climbing technique and compare the correction in three different domains, namely the voltage domain, the domain of the Zernike modes, and the domain of the singular modes of the deformable mirror. Through our systematic experimental study, we found that successive control in two domains effectively counteracts uncompensated hysteresis of the deformable mirror. KW - active optics, metrics, aberration compensation Y1 - 2018 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:hbz:836-opus-14759 UR - https://www.osapublishing.org/oe/fulltext.cfm?uri=oe-26-21-27161&id=398820 VL - 26 SP - 27161 EP - 27178 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 - 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 - 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 - Verpoort, Sven A1 - Wittrock, Ulrich T1 - Actuator patterns for unimorph and bimorph deformable mirrors JF - Appl. Opt. N2 - 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. Y1 - 2010 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:hbz:836-opus-6490 UR - http://www.opticsinfobase.org VL - 49 SP - G37 EP - G46 PB - OSA ER - TY - JOUR A1 - Heuck, Hans-Martin A1 - Neumayer, Paul A1 - Kühl, Thomas A1 - Wittrock, Ulrich T1 - Chromatic aberration in petawatt-class lasers JF - Appl. Phys. B N2 - 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. Y1 - 2006 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:hbz:836-opus-6523 VL - 84 SP - 421 EP - 428 PB - Springer 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 - Kazasidis, Orestis A1 - Wittrock, Ulrich T1 - Interferometric measurement of the temperature coefficient of the refractive index dn/dT and the coefficient of thermal expansion of Pr:YLF laser crystals JF - Opt. Expr. N2 - We report interferometric measurements of the temperature coefficient of the refractive index (dn=dT) and the coefficient of thermal expansion (a) of a praseodymium-doped yttrium lithium fluoride (Pr:YLF) crystal and of a fused silica reference sample. Our phase-resolved interferometric method yields a large number of data points and thus allows a precise measurement and a good error estimation. Furthermore, both dn=dT and a are obtained simultaneously from a single measurement which reduces errors that can occur in separate measurements. Over the temperature range from 20 °C to 80 °C, the value of dn=dT of Pr:YLF decreases from -5.2 x 10-6 /K to -6.2 x 10-6 /K for the ordinary refractive index and from -7.6 x 10-6 /K to -8.6 x 10-6 /K for the extraordinary refractive index. The coefficient of thermal expansion for the a-axis of Pr:YLF increases from 16.4 x 10-6 /K to 17.8 x 10-6 /K over the same temperature range. KW - refractive index change KW - thermal expansion coefficient KW - laser crystal KW - praseodymium KW - yttrium lithium fluoride Y1 - 2014 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:hbz:836-opus-7720 VL - 22 SP - 30683 EP - 30696 PB - Optical Society of America ER - TY - JOUR A1 - Buske, Ivo A1 - Wittrock, Ulrich T1 - Diffraction analysis of aberrated laser resonators JF - Appl. Phys. B N2 - 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. Y1 - 2006 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:hbz:836-opus-8252 VL - 83 SP - 229 EP - 233 ER - TY - JOUR A1 - Vorholt, Christian A1 - Wittrock, Ulrich T1 - Intra-cavity pumped Yb:YAG thin-disk laser with 1.74% quantum defect JF - Opt. Lett. N2 - We present, to the best of our knowledge, the first intracavity pumped Yb:YAG thin-disk laser. It operates at 1050.7 nm with a quantum defect of just 1.74% due to pumping at 1032.4 nm. Low absorption of the pump light at the pump wavelength of 1032.4 nm is compensated for by placing the disk inside the resonator of another Yb:YAG thin-disk laser which is diode-pumped at 940 nm. The intra-cavity pumped laser has an output power of 10.3 W and a slope efficiency of 8.3% Y1 - 2015 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:hbz:836-opus-8284 VL - 40 SP - 4819 EP - 4822 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 - 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 - 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 -