Acoustic sensors are widely applied in industrial production, photoacoustic spectroscopy, nuclear power plant pipeline leakage, seismic monitoring, and many other fields. Traditional piezoelectric and capacitive acoustic sensors utilize the extracted electrical signal to achieve sound detection. However, due to the limitation of the principle based on the piezoelectric effect, traditional electroacoustic sensors are difficult to use in harsh and complex environments such as high temperature and high pressure, strong corrosion, and strong radiation. It is hard to avoid the influence of electromagnetic interference. Compared with traditional electroacoustic sensors, fiber optic sensors possess the advantages of miniaturization, high sensitivity, and higher signal-to-noise ratio. Meanwhile, they can avoid electromagnetic interference and can be applied to complex and harsh environments like flammable and explosive, high temperature, and high pressure. The objective of our research is to enhance the detection sensitivity of fiber optic Fabry-Perot (F-P) acoustic sensors for low-frequency acoustic signals by optimizing the design, processing, and fabrication of the sensing diaphragm of the sensors, conducting acoustic experimental tests and studying its application in the field of photoacoustic spectroscopy. This will provide a certain theoretical and technological accumulation in the field of photoacoustic spectroscopy gas detection and promote research and technological development in relevant fields.

To improve the detection sensitivity of fiber optic F-P sensors for low-frequency acoustic signals, we propose a π-shaped cantilever structure composed of two narrow beams connected to a center sensing diaphragm. First, we employ COMSOL to conduct finite element analysis on the acoustic characteristics of the structure, and the effects of the angle, length, width, and thickness of the L-shaped cantilever on the resonance frequency and frequency response of the diaphragm are explored to complete the optimization of the π-shaped cantilever structure. Then, the dimensions of each part are determined in combination with the experimental effect. The diaphragm is printed on 304 stainless steel by a laser and assembled into a fiber-optic acoustic sensor with the capillary, optical fiber, quartz tube, and other parts. Also, the length of its static F-P interferometric cavity is adjusted so that the sensor operates in orthogonality to ensure that the signals are not distorted. After that, we conduct frequency response experiments on the sensor and compared it with a rectangular cantilever structure of the same size (4 mm×2 mm). Finally, we use the sensor for the detection of photoacoustic spectral signals of acetylene.

The designed π-shaped cantilever is utilized to fabricate an optical fiber acoustic sensor (Fig. 6), and an acoustic test system is constructed to test the performance of the sensor at low-frequency acoustic signals (Fig. 7). The sensor has a homogeneous time-domain response under the acoustic pressure at different frequencies and has a high signal-to-noise ratio without other octave signals (Fig. 8). The sensitivity of the sensor is 178.76 nm/Pa at the first-order resonance frequency of 660 Hz (Fig. 9), and its sensitivity at 500 Hz is 5.21 nm/Pa, which is 1.8 times higher than that of the rectangular cantilever structure (Fig. 10). In the photoacoustic spectroscopy gas detection experiments for different volume fractions of acetylene, the response to the acetylene volume fraction can reach 1.97 pm/10^-6, and the linearity is up to 0.9901 (Fig. 11).

We propose a fiber sound wave sensor based on π-shaped cantilever structure. The sensor film is made of 304 stainless steel and is carved by laser processing technology. The processing process is simple and the cost is low. To optimize the structure of the sensor, through the finite element analysis software, we simulate the inherent frequency and frequency response characteristics of the diaphragm. Considering theoretical analysis and experimental effects, the size of each part of the π-shaped cantilever is determined: the outer diameter of the diaphragm is 10 mm; the diameter of the vibration structure is 6 mm; the width of the L-shaped cantilever is 0.5 mm; the length of the central square diaphragm is 2 mm; the thickness of the diaphragm is 15 μm. In the sound wave testing experiment, compared with the rectangular cantilever structure of the same structure size (4 mm×2 mm), the experimental structure shows that the resonance frequency of the sensor is 660 Hz, and the sensitivity at the resonance frequency is 178.76 nm/Pa, which is twice higher than that of the rectangular cantilever structure, and the sensitivity at 500 Hz is 5.21 nm/Pa, which is 1.8 times higher than the rectangular cantilever structure. Finally, the sensor is used for photoacoustic spectroscopic gas detection experiments, and the result shows that the response of the structure to acetylene is 1.97 pm/10^-6. In summary, the designed sensor has better application advantages in the field of low-frequency acoustic signal detection and photoacoustic spectroscopy. However, its detection capability for high-frequency acoustic signals is relatively weak. Future work will focus on further improving the sensor structure and exploring novel composite materials or micro/nanostructures. Additionally, the experimental scope will be expanded to include acoustic waves of varying frequency ranges and a wide range of gas concentrations, in order to comprehensively assess the sensor’s dynamic response and performance limits.