Small in-fiber Fabry-Perot low-frequency acoustic pressure sensor with PDMS diaphragm embedded in hollow-core fiber

A small and low cost extrinsic Fabry-Perot (FP) fiber low-frequency acoustic pressure sensor (EFPAS) based on polydimethylsiloxane (PDMS) diaphragm is proposed. The EFPAS is fabricated by fusion splicing a single mode fiber (SMF) with a hollow-core fiber (HCF), which embed a PDMS diaphragm by the capillary effect, to form an air micro-cavity. The structural strength of the whole thin diaphragm based sensing probe is improved by the ingenious design, and a high precision optical fiber cutting system can be used to control the length of air micro-cavity. Simulation proves that the PDMS diaphragm with large radius and small thickness will help to improve the sensitivity, but there is a tradeoff between sensitivity and the mechanical strength. Experimental results show that the sensor has the acoustic pressure sensitivity of 0.427 mV/mPa with a high linear pressure response in the range of 5 mPa-720 mPa. A stable signal-to-noise ratio (SNR) about 80 dB with a noise floor at 0 dB can be realized at 30 Hz and a 0.5 Hz resolution can be observed ranging from 10 to 50 Hz of low-frequency acoustic wave (The relative error of measured frequency is lower than 1%). Its high precision of low-frequency acoustic measurement and simple fabrication process make it an attractive tool for acoustic sensing and photo-acoustic spectroscopy.