Sound source localization (SSL) technology is vital in a wide range of applications such as smart robots, unmanned aerial vehicle (UAV) detection, and unmanned driving. Acoustic sensor arrays are the main solution to SSL. However, with the development of small devices, it is difficult for these arrays to simultaneously satisfy the requirements of miniaturization and high precision. Inspired by small animals' auditory organs, bio-mimetic acoustic vector sensors are an alternative to acoustic sensor arrays. The parasitic fly Ormia ochraceainspires mechanical coupling between two membranes with an interaural phase difference (IPD) gain. Bio-mimetic acoustic vector sensors based on mechanical coupling inherit the IPD gain function. The gain effect of the current bio-mimetic acoustic vector sensors is limited to around the eigenfrequency. Meanwhile, electrical sensors are highly susceptible to extreme environments such as strong electromagnetic and high temperatures, while fiber-optic sensors can endure these conditions. We propose a flywheel-like fiber-optic Fabry-Perot (F-P) acoustic vector sensor for wide-range IPD gain based on the diaphragm coupling gain principle. We hope that the diaphragm-coupling fiber-optic F-P acoustic vector sensor can achieve the IPD gain of several kilohertz frequency ranges, adapting to ambiguous sound source direction in extreme environments.

The flywheel-coupling diaphragm is simplified to a two-degree-of-freedom (2-DOF) mass-spring-dashpot system with two shape modes of rocking mode and bending mode. COMSOL Multiphysics is employed to analyze diaphragm vibration characteristics and the structure parameters of the diaphragm are optimized based on the simulation results. The flywheel-coupling structure on stainless steel sheet is produced by ultraviolet laser etching technology. The adjoint spokes of two flywheel vibration units naturally couple to form a simplified intermembrane bridge coupling structure. The vibration units combined with individual fiber form independent fiber-optic F-P sensing units. The displacement of the vibration units changes the light intensity of the F-P sensing units detected by the intensity demodulation system. The intensity demodulation contains a tunable laser, 1×2 fiber splitter, optical circulators, photoelectric detectors, and data acquisition card (Fig. 8). The operating wavelength is determined in a common linear region of two sensors. The real-time IPD calculation is acquired by a phase-sensitive detection algorithm, and the incident angle of the sound wave is localized based on the IPD. The uncoupling two-sensor array is simultaneously subjected to SSL experiments to contrast with the flywheel-coupling acoustic vector sensor.

The proposed sensor has a wide frequency range of IPD gain. The rocking mode and bending mode eigenfrequency is simulated as 7.2 kHz and 7.6 kHz. The simulation results exhibit a significant gain in the frequency range of 5 kHz to 7.4 kHz, with a maximum gain of 4.5 at 7.2 kHz (Fig. 4). The experimental results are in good agreement with simulations conducted in COMSOL Multiphysics (Fig. 10). The measured eigenfrequency is 7.2 kHz and 7.6 kHz with a slight discrepancy. The sensitivities of the sensing units are S1=0.24V/Pa@7.6kHz and S2=0.21V/Pa@7.6kHz. Two-dimensional planar SSL in -90°-90° based on IPD cues is achieved (Fig. 11). The experiment results from 5 kHz to 7.4 kHz present a wide frequency range IPD gain with a maximum gain of 5.05 at 7.2 kHz (Fig. 12). Cavity length and fiber end face inclination affect the spectrum of each sensing unit (Fig. 7). As a result, sensor consistency is difficult to achieve due to unavoidable processing errors. Since the phase-sensitive-detection algorithm is affected by noise, low signal-noise-ratio (SNR) signals may incur high localization errors. Both experimental and simulation results characterize that the sensor has a wide frequency range of IPD amplification effect.

We propose a flywheel-like fiber-optic F-P acoustic vector sensor for wide-range IPD gain based on the diaphragm coupling gain principle. The proposed flywheel-coupling diaphragm has two vibration modes of rocking and bending. The corresponding eigenfrequencies of 7.2 kHz and 7.6 kHz are calculated by COMSOL Multiphysics. The sensor has an obvious IPD amplification effect from 5 kHz to 7.4 kHz in the frequency ranges. The maximum sensitivity and gain are acquired at 7.2 kHz in the simulation. Cavity length and fiber end face inclination affect the spectrum of each sensing unit, limiting the SSL accuracy based on interaural intensity difference. Our paper applies a phase-sensitive-detection algorithm to obtain the phase difference between the two signals in real time. However, the method does not apply to low SNR signals. Meanwhile, the algorithm accuracy is affected by DC components, harmonics, and other factors. Finally, the scheme based on IPD is chosen and a flywheel-coupling diaphragm fiber-optic F-P acoustic vector sensor is fabricated. The first-order eigenfrequency is measured at around 7.2 kHz. The structure achieves SSL with IPD gain in the frequency range from 5 kHz to 7.4 kHz, compared with an uncoupling fiber-optic F-P acoustic sensor array. The measured maximum gain factor of 5.05 is better than the simulation results. The maximum line size of the proposed sensor is smaller than the wavelength of the test acoustic wave to realize a miniaturized acoustic vector sensor with a simple structure and easy processing. The detection method using optical principles can be applied to satisfy SSL needs in extreme environments.