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In this work, a low-frequency acoustic sensing scheme is proposed based on the structure of in-fiber Mach-Zehnder interferometer , in which the refractive index difference between fiber core and cladding is used to form a miniature Mach-Zehnder interferometer through fusion splicing of specialty optical fibers in a multi-mode-ultra-high numerical aperture-multi-mode configuration. This design achieves modal recombination between cladding and core modes, thereby effectively enhancing fiber bending sensitivity. The interferometer structure is then combined with a polyethylene terephthalate (PET) transducer diaphragm, enabling the sensing fiber to undergo curvature changes synchronously with the diaphragm under sound pressure, thereby indirectly increasing the area over which the fiber receives the acoustic field. When external acoustic pressure induces bending modulation on both the sensing fiber and transducer diaphragm, the differential strain distribution between the fiber cladding and core generates an optical path difference. This manifests itself in interference spectrum shifts, enabling the effective detection of low-frequency acoustic signals through demodulating the spectrum variations. In the paper, the theoretical framework for the acoustic sensing system is derived and validated experimentally. The results show that at 65 Hz, the system achieves a signal-to-noise ratio (SNR) of approximately 57 dB and a minimum detectable sound pressure of 267.9 ${\text{μ Pa/H}}{{\text{z}}^{{\text{1/2}}}}$at 65 Hz. In a frequency range of 50 Hz－500 Hz, the system exhibits good acoustic response, with an SNR consistently above 40 dB and a relatively flat signal output. This scheme significantly enhances the acoustic response capability of the sensing system, enabling the effective detection of low-frequency acoustic waves. Additionally, it features simple fabrication and low cost, showing great potential for the development of acoustic wave detection applications.
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Keywords:
- optical fiber sensing /
- Mach-Zehnder interferometer /
- acoustic sensor /
- acoustic measurement
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图 1 (a) 传感光纤结构示意图; (b) 传感光纤光谱图
Figure 1. (a) Schematic diagram of optic sensing fiber structure; (b) spectral diagram of sensing fiber.
图 2 声波传感系统示意图
Figure 2. Schematic diagram of acoustic sensing system.
图 3 不同频率的频谱响应和时域波形 (a)—(d) 55 Hz, 65 Hz, 75 Hz, 85 Hz时对应的时域信号;(e)—(h)55 Hz, 65 Hz, 75 Hz, 85 Hz时对应的频域信号
Figure 3. Frequency response and time domain signal at different frequencies: (a)–(d) Time domain signal at 55 Hz, 65 Hz, 75 Hz, 85 Hz; (e)–(h) frequency response at 55 Hz, 65 Hz, 75 Hz, 85 Hz.
图 4 传感系统对声波频率的频响 (a) 50—220 Hz部分频率的响应频谱图; (b) 35—1000 Hz的信噪比
Figure 4. Frequency response of the sensing system for acoustic wave: (a) The response of frequency spectrum during 50–220 Hz; (b) signal to noise ratio corresponding to different frequencies during 35–1000 Hz.
图 5 不同频率时传感结构对不同声压的灵敏度线性拟合及重复性测试 (a) 55 Hz; (b) 65 Hz; (c) 75 Hz; (d) 85 Hz
Figure 5. Linearly fitted sensitivities of the sensing structure to different acoustic pressures and repeatability test at (a) 55 Hz, (b) 65 Hz, (c) 75 Hz, (d) 85 Hz.
表 1 几种声传感方案的性能比较
Table 1. Performance comparison of several acoustic sensing systems.
传感结构 声压响应 信噪比/dB 最小探测声压/(${\text{μ Pa}} \cdot {\text{H}}{{\text{z}}^{{\text{ - 1/2}}}}$) Tapered Fiber[20] 36 mV/kPa 46.84 21.11×106
@2500 HzGold diaphragm-based FPI with a fiber-optic collimator[21] 12.6 mV/Pa 51 470@150 Hz FP etalon[22] 177.6 mV/Pa 12.7 530@1 kHz LPBG[15] 0.064 nm/kPa 40.6 331.9@550 Hz CMOS micromachined capacitive[23] — — 1.35×106
@2.4 MHzTwo-photon 3 D printed spring-based
Fabry–Pérot cavity resonator[24]0.0883 mV/Vpp 56.2 2390@75 kHz 本工作 0.0549 mV/Vpp 57.21 267.9@65 Hz 
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