An optical measurement system and method for out-of-plane motion testing of micro-structures is presented. Based on a Mirau type microscopic interferometer, the system adopts time-averaged interferometry to realize the measurement of the out-of-plane dynamic properties of movable micro-structures. For mapping the zero-order Bessel function modulating the contrast of two-beam interference fringes, a four-step phase-shift technique is applied. An experiment on a micro-resonator demonstrates that out-of-plane motion can be measured at any frequency with a lateral resolution in the micrometer range and a detection limit around 5nm.

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