25 April 2008 Optimisation and characterisation of parabolic membrane mirrors
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Abstract
MOEMS-based thin silicon membrane mirrors with a useable diameter of 5mm and fast (up to 1kHz) tunable focal length (80 mm to 1m) have been realized. A ring shaped counter electrode is used to achieve a parabolic membrane deformation by electrostatic forces. A circular kerf at the outer perimeter of the membrane provides a soft suspension to the rim and thus reduces the needed driving voltage. FEM has been used for optimisation of the design, especially of the soft suspension, which is realized by a controlled thinning of the outer rim of the Si-membrane. A critical issue for demanding applications is the membrane distortion induced by material stress and the fabrication process. Membrane residual stress reduction has been obtained by using SOI-technology (c-silicon) and by optimisation of the Al deposition process (Al-coated Si-membrane). For dynamic tests of the optical mirror properties a stroboscopic interferometer has been realized. A pulsed laser diode with a pulse duration of 10μs is used as a light source which is synchronized with the modulated electrical field driving the membrane mirror. The interference pattern is recorded with a CCD and evaluated with conventional phaseshift techniques. The geometry is similar to a Mach-Zehnder interferometer. The reference path length can be varied with a piezoceramic to induce the phase shift.
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T. Hellmuth, T. Hellmuth, K. Khrennikov, K. Khrennikov, W. Kronast, W. Kronast, R. Huster, R. Huster, U. Mescheder, U. Mescheder, } "Optimisation and characterisation of parabolic membrane mirrors", Proc. SPIE 6993, MEMS, MOEMS, and Micromachining III, 699305 (25 April 2008); doi: 10.1117/12.780758; https://doi.org/10.1117/12.780758
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