@inproceedings{SauvageAmiauxAustinetal.2016,
  author    = {Sauvage, Marc and Amiaux, J{\´e}rome and Austin, James and Bello, Mara and Bianucci, Giovanni and Chesn{\´e}, Simon and Citterio, Oberto and Collette, Christophe and Correia, S{\´e}bastien and Durand, Gilles A. and Molinari, Sergio and Pareschi, Giovanni and Penfornis, Yann and Sironi, Giorgia and Valsecchi, Guiseppe and Verpoort, Sven and Wittrock, Ulrich},
  title     = {A development roadmap for critical technologies needed for TALC: a deployable 20m annular space telescope},
  series = {Space Telescopes and Instrumentation 2016: Optical, Infrared, and Millimeter Wave, Proc. SPIE},
  volume    = {9904},
  booktitle = {Space Telescopes and Instrumentation 2016: Optical, Infrared, and Millimeter Wave, Proc. SPIE},
  doi       = {10.25974/fhms-924},
  url       = {http://nbn-resolving.de/urn:nbn:de:hbz:836-opus-9248},
  pages     = {99041L},
  year      = {2016},
  abstract  = {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: \&\#8226; 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; \&\#8226; 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; \&\#8226; 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.},
  language  = {en}
}
@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}
}
@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}
}
@inproceedings{KazasidisVerpoortWittrock2018,
  author    = {Kazasidis, Orestis and Verpoort, Sven and Wittrock, Ulrich},
  title     = {Algorithm design for image-based wavefront control without wavefront sensing},
  series = {SPIE Optical Instrument Science, Technology, and Applications, Proc. SPIE},
  volume    = {10695},
  booktitle = {SPIE Optical Instrument Science, Technology, and Applications, Proc. SPIE},
  doi       = {10.1117/12.2312523},
  url       = {http://nbn-resolving.de/urn:nbn:de:hbz:836-opus-14760},
  pages     = {1069502},
  year      = {2018},
  abstract  = {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.},
  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}
}
@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}
}
@inproceedings{KazasidisVerpoortWittrock2019,
  author    = {Kazasidis, Orestis and Verpoort, Sven and Wittrock, Ulrich},
  title     = {Image-based wavefront correction for space telescopes},
  series = {International Conference on Space Optics - ICSO 2018},
  booktitle = {International Conference on Space Optics - ICSO 2018},
  publisher = {Proc. SPIE},
  doi       = {10.1117/12.2536206},
  url       = {http://nbn-resolving.de/urn:nbn:de:hbz:836-opus-109036},
  pages     = {111807Z},
  year      = {2019},
  abstract  = {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.},
  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{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}
}
@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}
}