Fiber-optic EFPI acoustic emission sensor with bulk silicon micromachined membrane

A fiber-optic extrinsic Fabry-Pérot interferometer (EFPI) sensor was developed by implementing a micromachined membrane, as a second reflecting surface, to the end facet of fiber-optic connector. The membrane was fabricated by depositing 3 layers (Si₃N₄, Ti, and Au, with thicknesses of 500, 10, and 200 nm) on a silicon wafer and using bulk silicon micromachining technique. The material and dimensions were carefully chosen with an FEM simulation to achieve sensitive and robust action in the target frequency range of 20~120 kHz. The prepared membranes could be easily implemented into the right position on sensor frame, which formed Fabry-Pérot cavities with accurate cavity length control due to the novel micromachining design. The constructed EFPI sensors successfully performed preliminary acoustic emission measurements and demonstrated high sensitivity with no noticeable damage to the membranes during the packaging process and the acoustic pressure measurements. The results of preliminary time difference experiments were closely matched with the ideal values, promising the feasibility of accurate time difference of arrival (TDOA) 3D localization.