Fig. 1. (a) Schematic diagram of the fiber-optic FP acoustic sensor. (b) Electrical-mechanical-acoustic equivalent model of the sensor.

Fig. 2. (a) Effects of connecting hole radius on transfer function when connecting hole length is fixed. (b) Effects of connecting hole length on transfer function when connecting hole radius is fixed. (c) Effects of back cavity volume on transfer function. (d) Effects of back cavity volume on low cut-off frequency.

Fig. 3. (a) Fabrication process of the diaphragm. (b) Fabrication process of the spiral micro-flow hole and assembly of the sensor. (c) Left: picture of the single layer of silicon chip before bonding. Right: scanning electron microscope (SEM) image of the spiral groove structure. (d) Left: picture of the dual layers of large-area silicon wafers after bonding. Right: picture of the fiber-optic FP sensor.

Fig. 4. Infrasound sensor calibration experimental system.

Fig. 5. Frequency spectra of the sensor output signals under the acoustic waves with different frequencies: (a) 0.01 Hz, (b) 0.05 Hz, (c) 0.1 Hz. (d) Sinusoidal fitting of time domain signal of 0.1 Hz. (e) Linearity of the sensor at 5 Hz acoustic wave.

Fig. 6. Acoustic pressure sensitivity of the fiber-optic FP acoustic sensor from 0.01 Hz to 5 kHz.

Fig. 7. Noise test results. (a) Noise spectrum, (b) minimum detectable pressure (MDP) under different frequencies.

Fig. 8. Relationship between diaphragm sensitivity and various parameters. (a) Stress, (b) radius, (c) thickness, (d) density.

Fig. 9. Acoustic test photos of the fiber-optic FP sensor.