@article{KazasidisVerpoortSolovievetal.2018,
  author    = {Kazasidis, Orestis and Verpoort, Sven and Soloviev, Oleg and Vdovin, Gleb and Verhaegen, Michel and Wittrock, Ulrich},
  title     = {Extended-image-based correction of aberrations using a deformable mirror with hysteresis},
  series = {Opt. Expr.},
  volume    = {26},
  journal   = {Opt. Expr.},
  doi       = {10.1364/OE.26.027161},
  url       = {http://nbn-resolving.de/urn:nbn:de:hbz:836-opus-14759},
  pages     = {27161 -- 27178},
  year      = {2018},
  abstract  = {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.},
  language  = {en}
}
@article{KazasidisVerpoortWittrock2019,
  author    = {Kazasidis, Orestis and Verpoort, Sven and Wittrock, Ulrich},
  title     = {Aberration balancing using an image-sharpness metric},
  series = {J. Opt. Soc. Am. A},
  volume    = {36},
  journal   = {J. Opt. Soc. Am. A},
  number    = {8},
  doi       = {10.1364/JOSAA.36.001418},
  url       = {http://nbn-resolving.de/urn:nbn:de:hbz:836-opus-109198},
  pages     = {1418 -- 1422},
  year      = {2019},
  abstract  = {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.},
  language  = {en}
}
@article{KazasidisVerpoortWittrock2020,
  author    = {Kazasidis, Orestis and Verpoort, Sven and Wittrock, Ulrich},
  title     = {Sensor for dynamic focus control of a deformable mirror},
  series = {Appl. Opt.},
  journal   = {Appl. Opt.},
  number    = {59},
  doi       = {10.1364/AO.392970},
  pages     = {5625 -- 5630},
  year      = {2020},
  abstract  = {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.},
  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}
}