Real-time dual-wavelength digital holographic microscopy for MEMS characterization

We present a method to achieve real-time dual-wavelength digital holographic microscopy (DHM) measurements of micro-electro-mechanical systems (MEMS) with a single camera acquisition. Indeed, while DHM is a technique of choice for MEMS investigation, thanks to its high-speed full-field complex wavefront reconstruction compatible with stroboscopic or pulsed operation modes, the nanometer-resolved phase information available suffers from a so-called phase ambiguity when the optical path length (OPL) induced by the sample is larger than the laser wavelength (typically 400-700nm in the visible range). This measurement range limitation is due to the periodic nature of the phase and, although unwrapping algorithms may be used in some cases, it represents an obstacle to widen DHM applications range. Here we introduce a technique for two-wavelengths DHM, extending the technology field of applications to the micro-meter range, and this with a single hologram acquisition to stay compatible with the exclusive DHM high-speed capabilities described above. Examples of investigation on a 1Hz moving micro-mirror at video frequency are shown to demonstrate the interest of the method for MEMS monitoring.