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

We report the use of digital holographic microscopy to study harmonic evolution of dynamic deformation of two orthogonal modes of resonating piezoelectric MEMS mirror with a gimbal frame suspension. For the bending mode, the results show a linear relationship between dynamic deformation and the optical scan angle. For the torsional mode, a hysteretic behavior is observed, showing a significant difference depending on the scan direction. The difference was measured to be 45 nm, representing 18% of the total dynamic deformation of this mode. To investigate this effect, a point-by-point Fourier expansion method of the deformation cycle was employed and mechanical harmonics were extracted. From studying the first harmonic, we conclude that the origin of the hysteresis can be attributed to the phase difference in the oscillation of the two extrema points at the edge of the mirror, defining the peak-to-value deformation. In addition, higher-order harmonic terms (3rd and 5th) were identified and are affecting the hysteresis shape. Next, a modulation transfer function, corresponding to the measured angle-resolved dynamic deformation was estimated. Results show small contrast loss originating from the torsional mode, with an almost negligible effect of the hysteresis. The loss of contrast is dominated by the dynamic deformation of bending mode and was estimated to be 96% already at 0.18 normalized spatial frequency.