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 - 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 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 -