Miniaturized Fabry-Perot fiber-optic microphone based on capillary tube and hydrogel diaphragm

Material selection and structure design of acoustic transducers are crucial in optimizing the sensitivity of optical microphones. In this study, we propose a high-sensitivity miniaturized Fabry-Perot fiber-optic microphone based on a capillary tube and hydrogel diaphragm. The fiber-optic microphone comprises a cleaved single-mode optical fiber, a cleaved capillary tube, and a hydrogel diaphragm. The diameter and thickness of the hydrogel diaphragm are 75 μm and 24 μm. The sizes of the fiber-optic microphone head are only about 125 μm in diameter and 170 μm in length. A detailed study of the acoustic sensing test of the fabricated fiber-optic microphone depending on the hydrogel concentration is carried out. The fabricated fiber-optic microphone works at 1550 nm wavelength, which exhibits great stability in a frequency response range from 100 Hz to 10 kHz with excellent fidelity and high linearity of ∼1. The highest sensitivities for 40% and 80% hydrogel diaphragms are 122 mV/Pa and 410 mV/Pa, respectively. The noise-limited minimum detectable pressure levels are 66.69 μPa/Hz^1/2@400 Hz and 44.37 μPa/Hz^1/2@400 Hz at 40% and 80% hydrogel concentrations. The fiber-optic microphone fabricated in this work is simple structure, low cost, electromagnetic robust, high compactness and stability, which has great potential in photoacoustic imaging, structure health monitoring, and non-destructive detection applications.