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18 May 2009 Deformation measurements of high-speed MEMS with sub-picosecond pulses using combined digital holographic two-wavelength contouring and single phase reconstruction
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Proceedings Volume 7362, Smart Sensors, Actuators, and MEMS IV; 736219 (2009)
Event: SPIE Europe Microtechnologies for the New Millennium, 2009, Dresden, Germany
Circular and rectangular MEMS optical scanners operating at a resonance frequencies close to 1 kHz are investigated by single-pulse digital holographic two-wavelength contouring. Coverage of the range interesting for shape measurements of micro mechanical applications like optical scanning mirrors between 10 - 100 μm using only one laser source is a challenging task. For this purpose, a dual-wavelength laser source was developed that generates two 12 nm and 34 nm separated sub-picosecond pulses around 790 nm. A Twyman-Green interferometer was extended by a polarization encoding sequence to separate the interferograms for the recording process. The two holograms were captured simultaneously introducing two CMOS-cameras in the interferometer setup. The phase difference information of the object within the synthetic wavelength of 40 μm was unambiguously generated and the 3D-shape calculated. To reliably measure the surface deformation of the oscillating mirrors the evaluation of the single phase images is sufficient. The resulting deformation of the mirrors based on the reconstructed single phase from the holograms was captured at a wavelength of λ = 783 nm. The deduced rms-values of the surface shape of the oscillating mirrors at maximum load are only ~50 nm, corresponding to a surface flatness of better than λ/10. This is an excellent result, keeping in mind that the mirror plates of the optical scanners are only ~50 μm thick. These detail information from one interferogram are combined with the coarse shape information deduced from the phase difference map of the two interferograms captured at different wavelengths. Using two-wavelength contouring only for extracting the linear slope of the mirror and combining this with the detail information of the single phase images yields a combined reconstruction. This provides a much more realistic picture of the virtually vanishing deformations of the MEMS optical scanners operated at its resonance. We are not aware of any other method that could provide equally detailed information on such a MEMS structure while simultaneously capturing such a large amplitude of the dynamics.
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Thomas Hansel, Uwe Griebner, Jens Bonitz, and Christian Kaufmann "Deformation measurements of high-speed MEMS with sub-picosecond pulses using combined digital holographic two-wavelength contouring and single phase reconstruction", Proc. SPIE 7362, Smart Sensors, Actuators, and MEMS IV, 736219 (18 May 2009);

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