Low Acoustic Frequency Sensing Based on Ghost Mode of Small Angle Tilted Fiber Bragg Grating

In this paper, we firstly propose a low acoustic frequency sensing system based on small-angle tilted fiber Bragg grating(TFBG) which is used in the sensor head to achieve a high sensitivity. The sensitivity of the proposed diaphragm based acoustic sensing system is theoretically and experimentally proved closely related to the coupling methods and efficiency of ghost mode. In the experiment, the sensing TFBG is tightly bounded on the designed low-frequency polyethylene terephthalate (PET) transducer membrane with a fixed boundary and its ghost mode will have a large wavelength drift and change of amplitude in response to the periodic elastic deformation of the transducer membrane under acoustic pressure. With using the edge filtering demodulation method, the low-frequency acoustic can be effectively and sensitively measured. The theoretical analysis and experimental results indicate that the wavelength and intensity respond linearly to the acoustic pressure. The proposed system can achieve a maximal sensitivity of about 509 mV/Pa in the frequency range of 40 Hz ∼ 85 Hz, a minimum detectable pressure(MDP) of 93 μ Pa/Hz 1/2 @55 Hz and signal-to-noise ratio (SNR) of 59 dB. The sensor also demonstrates a flat frequency response within 85 Hz ∼ 1500 Hz. The sensor operates stably with a peak fluctuation of about ±1 mV over 1 h, and about 1.8% for the percentage standard deviation. The proposed acoustic sensor has advantages of high sensitivity at low frequency, high stability and high SNR, simple fabrication, and excellent repeatability.