PhD defence Orestis Kazasidis

22 February 2021 10:07 till 01 March 2021 23:00 - Location: online - By: DCSC

"Methods for controlling deformable mirrors with hysteresis"

Fast adaptive optics and comparatively slower active optics are cornerstones of modern-day astronomy. Such systems are installed on most current large ground-based observatories in the visible or infrared and are included in the design of all future observatories. Their role is twofold; first, to compensate for astronomical seeing, and second, to correct for design and manufacturing errors, as well as thermal and mechanical distortions. What's more, the science goals of future large space observatories in the visible or infrared rely on active and adaptive optics systems for reaching the required wavefront accuracy and stability, with imminent examples the folded segmented primary mirror of the James Webb Space Telescope (JWST) and the deformable mirrors of the Roman Space Telescope, previously called the Wide Field Infrared Survey Telescope (WFIRST). Besides astronomy, adaptive optics find laser applications, for aberration correction and for beam shaping.

This thesis was set to explore methods for controlling deformable mirrors with hysteresis, specifically for controlling unimorph deformable mirrors developed and manufactured at the Photonics Laboratory of the FH Münster University of Applied Sciences in Germany. The technology for manufacturing unimorph deformable mirrors has been developed in the past at the Photonics Laboratory and has been expanded in a series of industrial and research projects, both for astronomical and for laser applications. Unimorph deformable mirrors are a promising technology for adaptive and active optics systems, thanks to their paramount mechanical properties and their versatility. However, their piezoelectric actuators exhibit higher hysteresis than most other actuators. The focus of this thesis lies in accurate and precise wavefront control with unimorph deformable mirrors despite their intrinsic hysteresis.  Hysteresis can be compensated with two different approaches. In the feedforward scheme, a mathematical model of the hysteresis is constructed and its inverse model is used in open-loop to drive the deformable mirror. In the feedback scheme, the wavefront deviation — including the hysteresis influence — is measured by a wavefront sensor and the deformable mirror is controlled in closed-loop. These two approaches can be combined for optimal performance. The open-loop compensation using the Prandtl-Ishlinskii formalism has previously been implemented at the Photonics Laboratory and was found to reduce the hysteresis from 15% to about 2%. Nevertheless, the residual uncompensated hysteresis still limits the performance of optical systems that have to be almost diffraction-limited. This thesis consists of two parts that manifest the two activities carried out during this PhD project. The first is image-based aberration correction using extended scenes. This aspires to complement existing technologies for the wavefront control in future space telescopes using active optics. The second is fast defocus sensing for the implementation of a closed-loop focus-shifter, with potential application in laser micromachining.

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Promotor: M. Verhaegen