Measuring angle-resolved dynamic deformation of micromirrors with digital stroboscopic holography

The efforts to increase the optical resolution of high-speed scanning mirrors require considering two important system parameters: effective mirror diameter, defining the point spread function, and the optical scanning angle, defining the field of view. Another, often not well-investigated parameter affecting the resolution is the intra-cycle mirror deformation. This deformation is becoming more pronounced with bigger diameters and larger scanning angles. This work investigates the angle-resolved dynamic deformation of a state-of-the-art resonating piezoelectric MEMS mirror. Digital stroboscopic holography was employed to systematically analyze the dynamic deformation and measure its angular evolution over the entire 2π phase cycle of a 15.5° total optical scan angle motion. We found the static deformation to be constant and dominant throughout the entire phase cycle. Smaller dynamic deformations for both rocking and bending mode are shown to be dependent on the instantaneous angle. Moreover, a hysteretic dynamic deformation was observed when the mirror was scanned towards the maximum deflection or on its return path towards zero position, indicating a phase shift between the main mode and the intracycle mode. Altogether three devices were studied, and the experimental results show good agreement with the simplified one-dimensional analytical models as well as more comprehensive finite element method (FEM) simulations. Analytically predicted point spread and modulation transfer (PSF and MTF) functions were found to be in good agreement with the experimental ones. The potential benefit in the optical resolution achieved through the optimization of the firing sequence is investigated. First experiments are conducted and discussed.