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  - CHAP
A1  - Kazasidis, Orestis
A1  - Verpoort, Sven
A1  - Wittrock, Ulrich
T1  - Algorithm design for image-based wavefront control without wavefront sensing
T2  - SPIE Optical Instrument Science, Technology, and Applications, Proc. SPIE
N2  - Active optics is an enabling technology for future large space telescopes. Image-based wavefront control uses an image-sharpness metric to evaluate the optical performance. A control algorithm iteratively adapts a corrective element to maximize this metric, without reconstructing the wavefront. We numerically study a sharpness metric in the space of Zernike modes, and reveal that for large aberrations the Zernike modes are not orthogonal with respect to this metric. The findings are experimentally verified by using a unimorph deformable mirror as
corrective element. We discuss the implications for the correction process and the design of control algorithms.
KW  - active optics, adaptive optics, sharpness metrics, aberration compensation, algorithm design
Y1  - 2018
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:hbz:836-opus-14760
UR  - https://www.spiedigitallibrary.org/conference-proceedings-of-spie/10695/1069502/Algorithm-design-for-image-based-wavefront-control-without-wavefront-sensing/10.1117/12.2312523.full
VL  - 10695
SP  - 1069502
ER  - 
TY  - BOOK
ED  - Wittrock, Ulrich
T1  - Adaptive optics for industry and medicine. Proceedings of the 4th International Workshop on Adaptive Optics for Industry and Medicine
Y1  - 2005
UR  - https://www.springer.com/de/book/9783540239789
SN  - 3-540-23978-2
PB  - Springer
CY  - Berlin
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  - CHAP
A1  - Verpoort, Sven
A1  - Wittrock, Ulrich
T1  - Miniaturized adaptive mirror for solid state laser resonators
T2  - Proceedings of the 7th International Workshop on Adaptive Optics for Industry and Medicine, Shatura, Russia
Y1  - 2009
ER  - 
TY  - JOUR
ED  - Wittrock, Ulrich
ED  - Kynast, Ulrich
ED  - Jüstel, Thomas
ED  - Bredol, Michael
T1  - 6th Laser Ceramics Symposium
T2  - Opt. Mat.
Y1  - 2012
UR  - https://www.sciencedirect.com/journal/optical-materials/vol/34/issue/6
VL  - 34
SP  - 935
EP  - 1002
PB  - Elsevier B.V.
ER  - 
TY  - JOUR
A1  - Wittrock, Ulrich
T1  - Grenzen? Welche Grenzen? Prof. Ulrich Wittrock, Leiter des Labors für Photonik an der Fachhochschule Münster, fragt nach den ultimativen Grenzen der Lasertechnik
JF  - Laser Community - das Lasermagazin von Trumpf
Y1  - 2014
VL  - 02:14
ER  - 
TY  - JOUR
A1  - Wittrock, Ulrich
T1  - Limitations? What Limitations? Prof. Ulrich Wittrock, head of the Photonics Laboratory at the Münster University of Applied Sciences, seeks out the ultimate boundaries of laser technology
JF  - Laser Community - the laser magazine from Trumpf
Y1  - 2014
VL  - 02:14
ER  - 
TY  - CHAP
A1  - Verpoort, Sven
A1  - Wittrock, Ulrich
T1  - Deformable mirrors for high power lasers
T2  - Proceedings of the 8th International Workshop on Adaptive Optics for Industry and Medicine (AOIM)
N2  - It has been shown that the beam quality and the efficiency of high-power solid-state lasers could be enhanced by the use of deformable mirrors in order to compensate for optical aberrations. An intracavity compensation requires a deformable mirror which is capable of handling very high laser intensities. The active diameter of the deformable mirror should be a few millimeters in order to match typical fundamental mode laser beam diameters. There is a wide variety of commercially available deformable mirrors, but neither meets all requirements.
KW  - Deformable mirror, adaptive mirror, unimorph mirror, high-power laser
Y1  - 2011
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:hbz:836-opus-106953
UR  - http://congresos.um.es/aoim/aoim2011/schedConf/presentations
ER  - 
TY  - CHAP
A1  - Kazasidis, Orestis
A1  - Verpoort, Sven
A1  - Wittrock, Ulrich
T1  - Image-based wavefront correction for space telescopes
T2  - International Conference on Space Optics - ICSO 2018
N2  - With a view to future large space telescopes, we investigate image-based wavefront correction with active optics. We use an image-sharpness metric as merit function to evaluate the image quality, and the Zernike modes as control variables. In severely aberrated systems, the Zernike modes are not orthogonal to each other with respect to this merit function. Using wavefront maps, the PSF, and the MTF, we discuss the physical causes for the non-orthogonality of the Zernike modes with respect to the merit function. We show that for combinations of Zernike modes with the same azimuthal order, a flatter wavefront in the central region of the aperture is more important than the RMS wavefront error across the full aperture for achieving a better merit function. The non-orthogonality of the Zernike modes with respect to the merit function should be taken into account when designing the algorithm for image-based wavefront correction, because it may slow down the process or lead to premature convergence.
KW  - active optics, adaptive optics, sharpness metrics, aberration compensation, algorithm design
Y1  - 2019
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:hbz:836-opus-109036
UR  - https://www.spiedigitallibrary.org/conference-proceedings-of-spie/11180/111807Z/Image-based-wavefront-correction-for-space-telescopes/10.1117/12.2536206.full?SSO=1
SP  - 111807Z
PB  - Proc. SPIE
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  - 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  - 
TY  - CHAP
A1  - Verpoort, Sven
A1  - Wittrock, Ulrich
T1  - Unimorph deformable mirror for telescopes and laser applications in space
T2  - International Conference on Space Optics (ICSO), Rhodes Island, Greece
N2  - 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 Maréchal criterion.
Y1  - 2010
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:hbz:836-opus-6484
UR  - www.icsoproceedings.org
PB  - SPIE
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  - CHAP
A1  - Verpoort, Sven
A1  - Welp, Petra
A1  - Wittrock, Ulrich
T1  - Novel unimorph deformable mirror for solid state laser resonators
T2  - MEMS Adaptive Optics III, Proc. SPIE
N2  - 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.
Y1  - 2009
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:hbz:836-opus-6504
VL  - 72090N
PB  - SPIE
ER  - 
TY  - CHAP
A1  - Verpoort, Sven
A1  - Rausch, Peter
A1  - Wittrock, Ulrich
T1  - Novel unimorph deformable mirror for space applications
T2  - International Conference on Space Optics (ICSO), Proc. SPIE
N2  - 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.
Y1  - 2012
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:hbz:836-opus-6516
UR  - https://www.spiedigitallibrary.org/conference-proceedings-of-spie/10564/1056414/Novel-unimorph-deformable-mirror-for-space-applications/10.1117/12.2309089.full
VL  - 10564
SP  - 1056414-1
PB  - SPIE
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  - CHAP
A1  - Rausch, Peter
A1  - Verpoort, Sven
A1  - Wittrock, Ulrich
T1  - Novel unimorph adaptive mirrors for astronomy applications
T2  - Proc. SPIE 8447, Adaptive Optics Systems III, 844764
N2  - We have developed a new type of unimorph deformable mirror for the correction of low-order Zernike modes. The mirror features a clear aperture of 50 mm combined with large peak-to-valley amplitudes of up to 35 μm. Newly developed fabrication processes allow the use of prefabricated, coated, super-polished glass substrates. The mirror's unique features suggest the use in several astronomical applications like the compensation of atmospheric aberrations seen by laser beacons, low light astronomy, and the use in woofer-tweeter systems. Additionally, the design enables an efficient correction of the inevitable wave-front error imposed by the floppy structure of primary mirrors in future large space 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.
Y1  - 2012
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:hbz:836-opus-6203
PB  - SPIE
ER  - 
TY  - CHAP
A1  - Heuck, Hans-Martin
A1  - Wittrock, Ulrich
A1  - Fils, Jérome
A1  - Borneis, Stefan
A1  - Witte, Klaus
A1  - Eisenbart, Udo
A1  - Javorkova, Dasa
A1  - Bagnoud, Vincent
A1  - Götte, Stefan
A1  - Tauschwitz, Andreas
A1  - Onkels, Eckehard
T1  - Adaptive optics at the PHELIX laser
T2  - Adaptive Optics for Laser Systems and Other Applications, Proc. SPIE
N2  - GSI Darmstadt currently builds a high-energy petawatt Nd:glass laser system, called PHELIX (Petawatt High-Energy Laser for Heavy-Ion Experiments). PHELIX will offer the world-wide unique combination of a high current, high-energy heavy-ion beam with an intense laser beam. Aberrations due to the beam transport and due to the amplification process limit the focusability and the intensity at the target. We have investigated the aberrations of the different amplification stages. The pre-amplifier stage consists of three rod-amplifiers which cause mainly defocus, but also a small part of coma and astigmatism. The main amplifier consists of five disk amplifiers with a clear aperture of 315 mm. These large disk-amplifiers cause pump-shot aberrations which occur instantly. After a shot, the disk amplifiers need a cooling time of several hours to relax to their initial state. This limits the repetition rate and causes long-term aberrations. We will present first measurements of the pump-shot and long-term aberrations caused by the pre- and the main amplifier in a single-pass configuration. In this context, we will present the adaptive optics system which is implemented in the PHELIX beam line and discuss its capability to compensate for the pump-shot and long-term aberrations.
Y1  - 2007
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:hbz:836-opus-6215
VL  - 6584
SP  - 658402
PB  - SPIE
ER  - 
TY  - CHAP
A1  - Rausch, Peter
A1  - Verpoort, Sven
A1  - Wittrock, Ulrich
T1  - Characterization of a miniaturized unimorph deformable mirror for high power cw-solid state lasers
T2  - Proc. SPIE 8253, MEMS Adaptive Optics VI, 825309
N2  - We have developed a new type of unimorph deformable mirror for real-time intra-cavity phase control of high power cw-lasers. The approach is innovative in its combination of super-polished and pre-coated highly reflective substrates, the miniaturization of the unimorph principle, and the integration of a monolithic tip/tilt functionality. Despite the small optical aperture of only 9 mm diameter, the mirror is able to produce a stroke of several microns for low order Zernike modes, paired with a residual static root-mean-square aberration of less than 0.04 µm. In this paper, the characteristics of the mirror such as the influence functions, the dynamic behavior, and the power handling capability are reported. The mirror was subjected to a maximum of 490 W of laser-light at a wavelength of 1030 nm. Due to the high reflectivity of over 99.998 percent the mirror is able to withstand intensities up to 1.5 MW/cm2.
Y1  - 2012
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:hbz:836-opus-6226
PB  - SPIE
ER  - 
TY  - CHAP
A1  - Rausch, Peter
A1  - Verpoort, Sven
A1  - Wittrock, Ulrich
T1  - Performance verification and environmental testing of a unimorph deformable mirror for space applications.
T2  - Proceedings of the 10th International Conference on Space Optics - ICSO, Tenerife, Spain
N2  - Concepts for future large space telescopes require an active optics system to mitigate aberrations caused by thermal deformation and gravitational release. Such a system would allow on-site correction of wave-front errors and ease the requirements for thermal and gravitational stability of the optical train. In the course of the ESA project "Development of Adaptive Deformable Mirrors for Space Instruments" we have developed a unimorph deformable mirror designed to correct for low-order aberrations and dedicated to be used in space environment. We briefly report on design and manufacturing of the deformable mirror and present results from performance verifications and environmental testing.
KW  - space optics
KW  - adaptive optics
KW  - deformable mirror
KW  - unimorph
Y1  - 2014
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:hbz:836-opus-7716
UR  - http://www.icsoproceedings.org
PB  - FH Münster
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  - CHAP
A1  - Vorholt, Christian
A1  - Wittrock, Ulrich
T1  - Wavelength control by angle-tuning of the laser radiation in an intra-cavity pumped Yb:YAG thin-disk laser
T2  - Advanced Solid State Lasers (ASSL), paper AM5A.39
N2  - In an intra-cavity pumped thin-disk laser the pump radiation forms a standing wave pattern inside the intra-cavity pumped disk.We demonstrate experimentally that the grating period of the standing wave pattern of the pump radiation can control the laser wavelength.
Y1  - 2015
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:hbz:836-opus-8298
UR  - https://www.osapublishing.org/abstract.cfm?uri=ASSL-2015-AM5A.39
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  - CHAP
A1  - Rausch, Peter
A1  - Verpoort, Sven
A1  - Wittrock, Ulrich
T1  - Unimorph piezoelectric deformable mirrors for space telescopes
T2  - Space Telescopes and Instrumentation 2016: Optical, Infrared, and Millimeter Wave, Proc. SPIE
N2  - We have developed, manufactured and tested a unimorph deformable mirror for space applications based on piezoelectric actuation. The mirror was designed for the correction of low-order Zernike modes with a stroke of several tens of micrometers over a clear aperture of 50 mm. It was successfully tested in thermal vacuum, underwent lifetime tests, and was exposed to random vibrations, sinusoidal vibrations, and to ionizing radiation. We report on design considerations, manufacturing of the mirror, and present the test results. Furthermore, we discuss critical design parameters, and how our mirror could be adapted to serve recently proposed space telescopes such as HDST and TALC.
Y1  - 2016
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:hbz:836-opus-9230
VL  - 9904
SP  - 990468
ER  - 
TY  - CHAP
A1  - Sauvage, Marc
A1  - Amiaux, Jérome
A1  - Austin, James
A1  - Bello, Mara
A1  - Bianucci, Giovanni
A1  - Chesné, Simon
A1  - Citterio, Oberto
A1  - Collette, Christophe
A1  - Correia, Sébastien
A1  - Durand, Gilles A.
A1  - Molinari, Sergio
A1  - Pareschi, Giovanni
A1  - Penfornis, Yann
A1  - Sironi, Giorgia
A1  - Valsecchi, Guiseppe
A1  - Verpoort, Sven
A1  - Wittrock, Ulrich
T1  - A development roadmap for critical technologies needed for TALC: a deployable 20m annular space telescope
T2  - Space Telescopes and Instrumentation 2016: Optical, Infrared, and Millimeter Wave, Proc. SPIE
N2  - Astronomy is driven by the quest for higher sensitivity and improved angular resolution in order to detect fainter or smaller objects. The far-infrared to submillimeter domain is a unique probe of the cold and obscured Universe, harboring for instance the precious signatures of key elements such as water. Space observations are mandatory given the blocking effect of our atmosphere. However the methods we have relied on so far to develop increasingly larger telescopes are now reaching a hard limit, with the JWST illustrating this in more than one way (e.g. it will be launched by one of the most powerful rocket, it requires the largest existing facility on Earth to be qualified). With the Thinned Aperture Light Collector (TALC) project, a concept of a deployable 20 m annular telescope, we propose to break out of this deadlock by developing novel technologies for space telescopes, which are disruptive in three aspects: • An innovative deployable mirror whose topology, based on stacking rather than folding, leads to an optimum ratio of collecting area over volume, and creates a telescope with an eight times larger collecting area and three times higher angular resolution compared to JWST from the same pre-deployed volume; • An ultra-light weight segmented primary mirror, based on electrodeposited Nickel, Composite and Honeycomb stacks, built with a replica process to control costs and mitigate the industrial risks; • An active optics control layer based on piezo-electric layers incorporated into the mirror rear shell allowing control of the shape by internal stress rather than by reaction on a structure. We present in this paper the roadmap we have built to bring these three disruptive technologies to technology readiness level 3. We will achieve this goal through design and realization of representative elements: segments of mirrors for optical quality verification, active optics implemented on representative mirror stacks to characterize the shape correction capabilities, and mechanical models for validation of the deployment concept. Accompanying these developments, a strong system activity will ensure that the ultimate goal of having an integrated system can be met, especially in terms of (a) scalability toward a larger structure, and (b) verification philosophy.
Y1  - 2016
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:hbz:836-opus-9248
VL  - 9904
SP  - 99041L
ER  - 
TY  - CHAP
A1  - Durand, G.
A1  - Amiaux, J.
A1  - Sauvage, M.
A1  - Austin, J.
A1  - Chesne, S.
A1  - Collette, C.
A1  - Helgouashl, S.
A1  - Pareschi, J.
A1  - Penfornis, Y.
A1  - Valsecchi, G.
A1  - Wittrock, U.
T1  - TALC a far-infrared 20m space telescope and the ELICSIR consortium to reach TRL 3
T2  - Proceedings of the 37th ESA Antenna Workshop, Noordwijk, Netherlands
N2  - Further space exploration in the far-infrared (FIR) requires larger apertures in order to improve the spatial resolution of captured images. To this purpose, the Thinned Aperture Light Collector (TALC) concept of a deployable annular telescope has been recently developed at CEA, which offers novel perspectives for FIR space missions. The consortium ELICSIR consortium of European institutes and companies has been created to improve the technological readiness level (TRL) of its key systems and components.
Y1  - 2016
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:hbz:836-opus-9253
UR  - http://esaconferencebureau.com/2016-events/16c18/introduction
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  -