@inproceedings{WittrockWelp2006, author = {Wittrock, Ulrich and Welp, Petra}, title = {Adaptive laser resonator control with deformable MOEMS mirrors}, series = {MEMS/MOEMS Components and Their Applications III, Proc. SPIE}, volume = {6113}, booktitle = {MEMS/MOEMS Components and Their Applications III, Proc. SPIE}, publisher = {SPIE}, doi = {10.25974/fhms-653}, url = {http://nbn-resolving.de/urn:nbn:de:hbz:836-opus-6537}, pages = {61130C}, year = {2006}, abstract = {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.}, language = {de} } @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} } @article{BuskeWittrock2006, author = {Buske, Ivo and Wittrock, Ulrich}, title = {Diffraction analysis of aberrated laser resonators}, series = {Appl. Phys. B}, volume = {83}, journal = {Appl. Phys. B}, doi = {10.25974/fhms-825}, url = {http://nbn-resolving.de/urn:nbn:de:hbz:836-opus-8252}, pages = {229 -- 233}, year = {2006}, abstract = {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.}, language = {en} } @inproceedings{HeuckWittrockFilsetal.2007, author = {Heuck, Hans-Martin and Wittrock, Ulrich and Fils, J{\´e}rome and Borneis, Stefan and Witte, Klaus and Eisenbart, Udo and Javorkova, Dasa and Bagnoud, Vincent and G{\"o}tte, Stefan and Tauschwitz, Andreas and Onkels, Eckehard}, title = {Adaptive optics at the PHELIX laser}, series = {Adaptive Optics for Laser Systems and Other Applications, Proc. SPIE}, volume = {6584}, booktitle = {Adaptive Optics for Laser Systems and Other Applications, Proc. SPIE}, publisher = {SPIE}, doi = {10.25974/fhms-621}, url = {http://nbn-resolving.de/urn:nbn:de:hbz:836-opus-6215}, pages = {658402}, year = {2007}, abstract = {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.}, language = {de} } @incollection{VerpoortWittrock2009, author = {Verpoort, Sven and Wittrock, Ulrich}, title = {Miniaturized adaptive mirror for solid state laser resonators}, series = {Proceedings of the 7th International Workshop on Adaptive Optics for Industry and Medicine, Shatura, Russia}, booktitle = {Proceedings of the 7th International Workshop on Adaptive Optics for Industry and Medicine, Shatura, Russia}, year = {2009}, 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} } @article{Wittrock2010, author = {Wittrock, Ulrich}, title = {Laryngeally echolocating bats (Brief Communication Arising)}, series = {Nature}, volume = {466}, journal = {Nature}, doi = {10.1038/nature09156}, pages = {E6}, year = {2010}, language = {mul} } @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} } @article{VerpoortWittrock2010, author = {Verpoort, Sven and Wittrock, Ulrich}, title = {Actuator patterns for unimorph and bimorph deformable mirrors}, series = {Appl. Opt.}, volume = {49}, journal = {Appl. Opt.}, publisher = {OSA}, doi = {10.25974/fhms-649}, url = {http://nbn-resolving.de/urn:nbn:de:hbz:836-opus-6490}, pages = {G37 -- G46}, year = {2010}, abstract = {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.}, language = {de} } @inproceedings{VerpoortWittrock2011, author = {Verpoort, Sven and Wittrock, Ulrich}, title = {Deformable mirrors for high power lasers}, series = {Proceedings of the 8th International Workshop on Adaptive Optics for Industry and Medicine (AOIM)}, booktitle = {Proceedings of the 8th International Workshop on Adaptive Optics for Industry and Medicine (AOIM)}, url = {http://nbn-resolving.de/urn:nbn:de:hbz:836-opus-106953}, year = {2011}, abstract = {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.}, 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{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} } @inproceedings{RauschVerpoortWittrock2012, author = {Rausch, Peter and Verpoort, Sven and Wittrock, Ulrich}, title = {Novel unimorph adaptive mirrors for astronomy applications}, series = {Proc. SPIE 8447, Adaptive Optics Systems III, 844764}, booktitle = {Proc. SPIE 8447, Adaptive Optics Systems III, 844764}, publisher = {SPIE}, doi = {10.25974/fhms-620}, url = {http://nbn-resolving.de/urn:nbn:de:hbz:836-opus-6203}, year = {2012}, abstract = {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.}, language = {en} } @inproceedings{RauschVerpoortWittrock2012, author = {Rausch, Peter and Verpoort, Sven and Wittrock, Ulrich}, title = {Characterization of a miniaturized unimorph deformable mirror for high power cw-solid state lasers}, series = {Proc. SPIE 8253, MEMS Adaptive Optics VI, 825309}, booktitle = {Proc. SPIE 8253, MEMS Adaptive Optics VI, 825309}, publisher = {SPIE}, doi = {10.25974/fhms-622}, url = {http://nbn-resolving.de/urn:nbn:de:hbz:836-opus-6226}, year = {2012}, abstract = {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.}, language = {en} } @article{PerchermeierWittrock2013, author = {Perchermeier, Julian and Wittrock, Ulrich}, title = {Precise measurements of the thermo-optical aberrations of an Yb:YAG thin-disk laser}, series = {Opt. Lett.}, volume = {38}, journal = {Opt. Lett.}, publisher = {OSA}, doi = {10.25974/fhms-685}, url = {http://nbn-resolving.de/urn:nbn:de:hbz:836-opus-6854}, pages = {2422 -- 2424}, year = {2013}, abstract = {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.}, language = {en} } @article{Wittrock2014, author = {Wittrock, Ulrich}, title = {Grenzen? Welche Grenzen? Prof. Ulrich Wittrock, Leiter des Labors f{\"u}r Photonik an der Fachhochschule M{\"u}nster, fragt nach den ultimativen Grenzen der Lasertechnik}, series = {Laser Community - das Lasermagazin von Trumpf}, volume = {02:14}, journal = {Laser Community - das Lasermagazin von Trumpf}, year = {2014}, language = {de} } @article{Wittrock2014, author = {Wittrock, Ulrich}, title = {Limitations? What Limitations? Prof. Ulrich Wittrock, head of the Photonics Laboratory at the M{\"u}nster University of Applied Sciences, seeks out the ultimate boundaries of laser technology}, series = {Laser Community - the laser magazine from Trumpf}, volume = {02:14}, journal = {Laser Community - the laser magazine from Trumpf}, year = {2014}, language = {en} } @inproceedings{RauschVerpoortWittrock2014, author = {Rausch, Peter and Verpoort, Sven and Wittrock, Ulrich}, title = {Performance verification and environmental testing of a unimorph deformable mirror for space applications.}, series = {Proceedings of the 10th International Conference on Space Optics - ICSO, Tenerife, Spain}, booktitle = {Proceedings of the 10th International Conference on Space Optics - ICSO, Tenerife, Spain}, publisher = {FH M{\"u}nster}, doi = {10.25974/fhms-771}, url = {http://nbn-resolving.de/urn:nbn:de:hbz:836-opus-7716}, year = {2014}, abstract = {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.}, language = {en} } @article{KazasidisWittrock2014, author = {Kazasidis, Orestis and Wittrock, Ulrich}, title = {Interferometric measurement of the temperature coefficient of the refractive index dn/dT and the coefficient of thermal expansion of Pr:YLF laser crystals}, series = {Opt. Expr.}, volume = {22}, journal = {Opt. Expr.}, publisher = {Optical Society of America}, doi = {10.25974/fhms-772}, url = {http://nbn-resolving.de/urn:nbn:de:hbz:836-opus-7720}, pages = {30683 -- 30696}, year = {2014}, abstract = {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.}, language = {en} }