A directional underwater acoustic sensor based on asymmetric structure and a directional calculation method thereof

Abstract

This invention provides a directional underwater acoustic sensor based on an asymmetric structure and its directional calculation method, relating to the fields of biomimetic underwater acoustic sensing and microelectromechanical systems (MEMS). The sensor comprises: a biomimetic nerve fiber strut with its top end fixed to the sound pressure-sensitive region of a waterproof asymmetric elliptical tympanic membrane, and its bottom end connected to the outer reflective film of a fully sealed fiber optic Fabry-Perot interferometer cavity; based on the mechanical vibration generated by the waterproof asymmetric elliptical tympanic membrane under the excitation of water sound pressure, the vibration is transmitted axially through the biomimetic nerve fiber strut, resulting in the synchronous displacement of the outer reflective film, thereby changing the physical cavity length of the fully sealed fiber optic Fabry-Perot interferometer cavity. This invention solves the technical bottleneck of traditional multi-node arrays in miniaturized scenarios by integrating the monoauricular directional mechanism of the giant wax moth with a miniature fiber optic hydrophone architecture.

Description

Technical Field

[0001] This invention relates to the field of biomimetic underwater acoustic sensing and microelectromechanical systems (MEMS) technology, and in particular to a directional underwater acoustic sensor based on an asymmetric structure. Background Technology

[0002] In complex underwater environments, sound waves are the core information carrier for achieving effective long-distance transmission. Traditional underwater sound source localization and target detection rely on multi-node hydrophone arrays, which perform beamforming and direction inversion by extracting the binaural time difference or phase difference when sound waves arrive at different spatial node hydrophone units.

[0003] As detection systems evolve towards miniaturization, the time difference generated by hydrophone arrays with micrometer or millimeter-level spacing becomes extremely small. Given the sound wave propagation speed of approximately 1500 m / s in water, this tiny time difference directly leads to a sharp decrease in the directional sensitivity of traditional algorithms and imposes stringent requirements on the resolution of the backend sampling hardware.

[0004] Large arrays of traditional hydrophones can disrupt the hydrodynamic shape of miniature underwater unmanned vehicles, thereby increasing drag and limiting their maneuverability.

[0005] The giant wax moth in nature exhibits monoauricular directional ability, accurately locating spatial sound waves using only one tympanic membrane. Currently, there is a lack of engineering solutions to address the array-based bottleneck in the field of miniature underwater acoustic sensing, which would otherwise be hindered by the giant wax moth's asymmetric auditory mechanism. Summary of the Invention

[0006] This invention provides a directional underwater acoustic sensor based on an asymmetric structure. By integrating the single-ear directional mechanism of the giant wax moth with the architecture of a miniature fiber optic hydrophone, it solves the technical bottleneck of traditional multi-node arrays in miniaturized scenarios.

[0007] To solve the above-mentioned technical problems, the technical solution of the present invention is as follows: In a first aspect, a directional underwater acoustic sensor based on an asymmetric structure includes: Waterproof asymmetric elliptical diaphragm, in contact with water; An eccentric traction mass point is set in a local area of ​​the waterproof asymmetric elliptical tympanic membrane; A biomimetic nerve fiber support extends vertically beneath a waterproof, asymmetrical, elliptical tympanic membrane; A fully sealed fiber optic Fabry-Perot interferometer cavity is positioned below a biomimetic nerve fiber support. The top end of the biomimetic nerve fiber support is fixed to the sound pressure sensitive area of ​​the waterproof asymmetric elliptical tympanic membrane, and the bottom end is connected to the outer reflective film of the fully sealed fiber optic Fabry-Perot interferometer cavity. Based on the mechanical vibration generated by the waterproof asymmetric elliptical tympanic membrane under the excitation of water sound pressure, the vibration is transmitted axially through the biomimetic nerve fiber support, thereby obtaining the synchronous displacement of the outer reflective film and changing the physical cavity length of the fully sealed fiber optic Fabry-Perot interferometer cavity.

[0008] Furthermore, the major axis span of the waterproof asymmetric elliptical tympanic membrane is 1250 μm, and the minor axis span is 930 μm; the surface of the waterproof asymmetric elliptical tympanic membrane is divided into a thick region and a thin region, the thickness of the thin region is 800 nm, and the thickness of the thick region is 1.5 μm. Based on the difference in stiffness distribution between the thick and thin regions, the offset distance of the boundary line between the thick and thin regions relative to the elliptical geometric center of the waterproof asymmetric elliptical tympanic membrane is 28 μm outward.

[0009] Furthermore, the physical distance between the eccentric traction mass point and the geometric center of the waterproof asymmetric elliptical tympanic membrane is 270 μm; Based on the mass distribution adjustment effect of the eccentric traction mass point, the stiffness asymmetry and mass eccentricity characteristics of the waterproof asymmetric elliptical tympanic membrane in the higher-order resonance mode are obtained.

[0010] Furthermore, the fully sealed fiber Fabry-Perot interferometer cavity is composed of a tiny air gap between the end face of a single-mode fiber and the outer reflective film. The external structure is completely sealed by photoresist material; Based on the fully sealed encapsulation structure of the photoresist material, a sealed state is obtained to prevent water medium from entering the microcavity, thus forming an underwater optical interference demodulation unit.

[0011] Furthermore, the waterproof asymmetric elliptical tympanic membrane, the eccentric traction mass point, the biomimetic nerve fiber support, and the fully sealed fiber Fabry-Perot interference cavity are all made of photoresist material. Based on the two-photon absorption effect of femtosecond laser two-photon 3D printing technology, integrated direct laser writing is performed on the polished end face of a single-mode optical fiber to obtain a seamless micro-nano structure at the microscale.

[0012] Secondly, a method for directional calculation of a directional underwater acoustic sensor includes the following steps: Step 1: Obtain the underwater dynamic underwater acoustic pressure wave propagating to the surface of the waterproof asymmetric elliptical tympanic membrane; based on the acoustic pressure excitation of the underwater dynamic underwater acoustic pressure wave, obtain the local asymmetric mechanical vibration of the waterproof asymmetric elliptical tympanic membrane in the fifth resonant mode. Step 2: Obtain local asymmetric mechanical vibration; based on the axial transmission of local asymmetric mechanical vibration through the biomimetic nerve fiber support, obtain the synchronous displacement of the outer reflective film of the fully sealed fiber Fabry-Perot interferometer cavity, and then obtain the nanoscale minute change of the physical cavity length of the fully sealed fiber Fabry-Perot interferometer cavity. Step 3: Obtain the continuous wave laser from the single-mode fiber input to the fully sealed fiber Fabry-Perot interferometer cavity; based on the multi-beam interference caused by the nanometer-scale change in the physical cavity length, obtain the high-frequency modulated optical signal whose intensity of the reflected light varies with the cavity length. Step 4: Acquire the high-frequency modulated optical signal; Based on the extraction and processing of the high-frequency modulated optical signal by the photodetector and coherent demodulation system, the underwater acoustic response signal at the characteristic frequency of 5.6kHz is obtained. The optical demodulation amplitude of the underwater acoustic response signal has a mapping relationship with the incident angle of the underwater sound wave. Step 5: Obtain the optical demodulation amplitude of the underwater acoustic response signal at the characteristic frequency of 5.6 kHz; calculate the incident azimuth angle of the sound source in the underwater space by comparing the optical demodulation amplitude with the preset underwater acoustic direction response feature library.

[0013] Furthermore, in step 1, the simulated characteristic frequency corresponding to the fifth resonant mode is 5.6 kHz; Based on the 28μm offset of the thickness boundary line inside the waterproof asymmetric elliptical tympanic membrane and the 270μm eccentric traction mass point, the extremely strong stiffness asymmetry and mass eccentricity characteristics of the waterproof asymmetric elliptical tympanic membrane in the 5.6kHz frequency band are obtained, and then the local resonance or warping mode induced by sound pressure waves from different angles in the underwater three-dimensional space is obtained.

[0014] Furthermore, in step 2, the local asymmetric mechanical vibration is manifested as an anisotropic change in the amplitude of the maximum displacement point of the tympanic membrane. Based on the change in the incident angle of the underwater acoustic wave, the anisotropic change in the amplitude of the maximum displacement point of the tympanic membrane is obtained, which drives the outer reflective film to generate synchronous displacement.

[0015] Furthermore, in step 4, the optical demodulation amplitude of the underwater acoustic response signal exhibits a strong mapping relationship with the incident angle of the underwater acoustic wave; by continuously sampling the amplitude difference at the characteristic frequency in the received signal, amplitude difference sequence data characterizing the change of the incident angle of the underwater acoustic wave is obtained.

[0016] Furthermore, in step 5, the preset underwater acoustic direction response feature library stores amplitude difference reference values ​​corresponding to different incident angles. Based on the inverse calculation of the amplitude difference sequence data and the amplitude difference reference value, the incident azimuth angle of the sound source in the underwater space, which is directly inverted from the single-node optical signal, is obtained, realizing the acoustic orientation of the miniature hydrophone without array configuration.

[0017] The above-described solution of the present invention has at least the following beneficial effects: This application employs a single-node micro / nano structure integrating a waterproof, asymmetric elliptical diaphragm with a fully sealed fiber optic Fabry-Perot interferometer cavity. Based on the processing logic of directly inversely calculating the incident azimuth angle of the sound source using the single-node optical signal, a high-precision, fast inversion result of the sound source direction without an array is obtained. This structure significantly reduces the physical volume and hardware power consumption of the underwater acoustic directional system and eliminates the phase mismatch problem faced by multi-node arrays.

[0018] This application divides the surface of a waterproof, asymmetric elliptical diaphragm into thick and thin regions, offsetting the boundary between these regions by 28 μm. Simultaneously, an eccentric traction mass point, 270 μm from the geometric center, is placed in a localized area. Based on this asymmetric design of mass and stiffness distribution, the sensor's fifth-order resonant mode is excited at 5.6 kHz, resulting in extremely strong stiffness asymmetry and mass eccentricity. This characteristic enables the sensor to possess extremely high amplitude sensitivity to subtle changes in underwater acoustic direction, achieving precise acoustic directionality in complex underwater fluid environments.

[0019] This application employs femtosecond laser two-photon 3D printing technology to perform integrated direct laser writing on the polished end face of a single-mode optical fiber. Based on the localized high polymerization of photoresist induced by the two-photon absorption effect, a micro / nano structure is obtained that seamlessly connects the fiber end face, a fully sealed Fabry-Perot microcavity, a nerve fiber-like support, and an asymmetric thick-film sheet. This integrated molding process avoids the risk of water leakage and stress concentration problems caused by interlayer gaps in traditional processing methods, ensuring the submicron-level geometric stability and resonant mode reliability of the device in deep-water high hydrostatic pressure environments. Simultaneously, it endows the sensor with extremely low water flow drag resistance and complete electromagnetic interference immunity. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the three-dimensional overall structure of a single-ear directional fiber optic acoustic sensor based on the asymmetric structure of the tympanic membrane of the giant wax moth. Figure 2 It is a topological reference model diagram of the thickness distribution, boundary line offset, and geometric dimensions of the eccentric traction mass point of the asymmetric elliptical tympanic membrane; Figure 3 This is a contour map showing the local amplitude distribution and stiffness response of the sensor in the fifth resonant mode at a frequency of 5.6kHz in a finite element physical field simulation. Figure 4 This is a graph showing the amplitude displacement caused by different support radii. Detailed Implementation

[0021] Exemplary embodiments of the present disclosure will now be described in more detail with reference to the accompanying drawings. While exemplary embodiments of the present disclosure are shown in the drawings, it should be understood that the present disclosure may be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art.

[0022] This embodiment provides a complete three-dimensional structure of a directional underwater acoustic sensor based on an asymmetric structure, as well as the spatial assembly relationship and mechanical transmission path between its constituent units. For example... Figure 1 As shown, the sensor is a single-node micro-nano sensing structure that is directly fabricated and integrated onto the polished end face of a single-mode optical fiber. It can independently complete the underwater sound source direction finding function without relying on traditional hydrophone arrays.

[0023] The top of this directional underwater acoustic sensor features a waterproof, asymmetric elliptical diaphragm, which is directly exposed to the external water medium, serving as the first sensitive interface for fluid-structure interaction acoustic contact with the aquatic environment. An eccentric traction mass point is positioned in a specific localized area on the lower surface of the waterproof, asymmetric elliptical diaphragm to adjust the diaphragm's mass distribution. Below this combined structure of the waterproof, asymmetric elliptical diaphragm and the eccentric traction mass point, a biomimetic nerve fiber support extends vertically. This support extends upwards away from the fiber optic end face, its top end firmly fixed to the acoustic pressure-sensitive area of ​​the waterproof, asymmetric elliptical diaphragm, and its bottom end vertically and firmly connected downwards to the central region of the upper surface of the outer reflective film of a fully sealed fiber optic Fabry-Perot interferometer cavity. The fully sealed fiber optic Fabry-Perot interferometer cavity is located at the very bottom of the entire sensor structure and is directly constructed onto the polished end face of the single-mode fiber.

[0024] In operation, when a dynamic underwater acoustic pressure wave propagating in a specific direction acts on the upper surface of the waterproof asymmetric elliptical tympanic membrane, due to the specific asymmetric mass and stiffness distribution of the membrane, it will generate localized asymmetric mechanical vibration within a specific resonant frequency band after being excited by acoustic pressure. This mechanical vibration forms a direction-dependent displacement response in the acoustic pressure-sensitive region of the waterproof asymmetric elliptical tympanic membrane. This displacement response is then efficiently transmitted axially through the biomimetic nerve fiber support, driving the outer reflective film of the fully sealed fiber Fabry-Perot interferometer cavity to produce a vertical displacement synchronized with the tympanic membrane vibration. Since a tiny air gap is formed between the outer reflective film and the end face of the single-mode fiber, the synchronous displacement of the outer reflective film directly changes the physical cavity length of this air gap, thereby converting the underwater acoustic pressure signal into a nanometer-scale mechanical change in the physical cavity length of the fully sealed fiber Fabry-Perot interferometer cavity.

[0025] The waterproof asymmetric elliptical tympanic membrane, the eccentric traction mass point, the biomimetic nerve fiber support, and the external sealing structure of the fully sealed fiber Fabry-Perot interference cavity are all made of the same high-performance waterproof photoresist material to ensure that each structural unit has completely consistent physicochemical properties, excellent waterproof performance, and seamless interface characteristics.

[0026] The asymmetric structural design and key geometric parameter configuration of the waterproof asymmetric elliptical tympanic membrane in this embodiment are as follows: The waterproof asymmetric elliptical diaphragm is an elliptical planar thin film structure with its major axis span precisely set to 1250μm and its minor axis span precisely set to 930μm. This major-minor axis ratio has been optimized to match the underwater acoustic impedance of a specific frequency band and to excite the required high-order resonant modes.

[0027] To construct a spatially non-uniformly distributed stiffness field on the waterproof asymmetric elliptical tympanic membrane, the upper surface of the tympanic membrane is clearly divided into two regions with different thicknesses: a thick region and a thin region. The thickness of the thin region is precisely controlled to 800 nm, and the thickness of the thick region is precisely controlled to 1.5 μm. The thick and thin regions together form a continuous, asymmetrically variable-thickness elliptical membrane, with a smooth curve forming the boundary between the two regions. Due to the significant thickness difference between the thick and thin regions, the tympanic membrane exhibits distinctly different bending stiffness distributions in these regions, with the thick region possessing higher bending stiffness and the thin region possessing lower bending stiffness.

[0028] Based on the difference in stiffness distribution between the thick and thin regions, the boundary line between the thick and thin regions is not located on the standard elliptical geometric center axis of the waterproof asymmetric elliptical tympanic membrane, but rather offset by 28 μm relative to the axial direction of the elliptical geometric center of the waterproof asymmetric elliptical tympanic membrane. This 28 μm offset causes the stiffness center of the tympanic membrane to deviate from its geometric center, thereby introducing an inherent stiffness asymmetry within the membrane.

[0029] Furthermore, a solidified eccentric traction mass point is provided in a specific local area of ​​the waterproof asymmetric elliptical tympanic membrane. This eccentric traction mass point is made of the same photoresist material as the tympanic membrane and is integrally formed with the tympanic membrane through a two-photon 3D printing process. The physical distance between this eccentric traction mass point and the geometric center of the waterproof asymmetric elliptical tympanic membrane is 270 μm. Based on the mass distribution adjustment effect of this eccentric traction mass point, the waterproof asymmetric elliptical tympanic membrane simultaneously possesses the stiffness asymmetry caused by the offset of the 28 μm thickness-to-thin region boundary line and the mass eccentricity characteristic caused by the 270 μm eccentric traction mass point in higher-order resonant modes. This coupling effect of stiffness asymmetry and mass eccentricity characteristic enables the waterproof asymmetric elliptical tympanic membrane to produce distinctly different local resonant responses or warping modes to sound pressure waves from different angles in underwater three-dimensional space at the fifth-order resonant mode of 5.6 kHz, thereby providing the necessary anisotropic vibration basis for single-node acoustic orientation.

[0030] The specific structure, sealing method, and mechanical-optical coupling mechanism of the fully sealed fiber optic Fabry-Perot interferometer cavity and the biomimetic nerve fiber support in this embodiment are as follows: The fully sealed fiber Fabry-Perot interferometer cavity is formed by a tiny air gap between the polished end face of the single-mode fiber and the outer reflective film located directly above it. The outer reflective film is a thin film with optical reflective properties made of photoresist material, and its lower surface is parallel to the end face of the single-mode fiber, maintaining a precise initial spacing between them to form the Fabry-Perot interferometer microcavity. The central region of the upper surface of the outer reflective film is firmly connected to the bottom end of the biomimetic nerve fiber support.

[0031] The biomimetic nerve fiber support is a vertically extending columnar microstructure. Its top end is fixed to the sound pressure-sensitive area on the lower surface of the waterproof asymmetric elliptical tympanic membrane, and its bottom end is connected to the center of the upper surface of the outer reflective film. The axis of the support is substantially coincident with or parallel to the central axis of the single-mode optical fiber to ensure that the vertical displacement component generated by the waterproof asymmetric elliptical tympanic membrane under sound pressure excitation can be efficiently transmitted to the outer reflective film, while suppressing the adverse effects of lateral shear displacement on the cavity stability.

[0032] The entire exterior of the fully sealed fiber Fabry-Perot interferometer cavity is completely encapsulated by photoresist material. Specifically, the circumferential region surrounding the tiny air gap on the end face of the single-mode fiber, as well as the area covering the edge of the outer reflective film, are formed with continuous and dense photoresist sealing structures using a two-photon 3D printing process. Due to this fully sealed encapsulation structure of the photoresist material, water is completely blocked from entering the microcavity under deep-water high hydrostatic pressure conditions, thus ensuring the stable existence of the tiny air gap and forming an optical interferometry demodulation unit capable of long-term stable operation underwater.

[0033] When the waterproof asymmetric elliptical tympanic membrane vibrates mechanically under the excitation of water acoustic pressure, the biomimetic nerve fiber support axially transmits this mechanical vibration, resulting in a synchronous displacement of the outer reflective film. This synchronous displacement directly alters the physical cavity length of the tiny air gap between the single-mode fiber end face and the outer reflective film. The change in this physical cavity length is proportional to the displacement of the waterproof asymmetric elliptical tympanic membrane in the acoustic pressure-sensitive region, and this proportional relationship is jointly determined by the axial stiffness of the biomimetic nerve fiber support and the elastic restoring force of the outer reflective film.

[0034] This embodiment provides a specific integrated micro-nano fabrication method for a directional underwater acoustic sensor based on an asymmetric structure. The waterproof asymmetric elliptical tympanic membrane, the eccentric traction mass point, the biomimetic nerve fiber support, and the fully sealed fiber Fabry-Perot interferometer cavity all utilize the same high-performance waterproof photoresist material.

[0035] At the start of the process, a standard single-mode optical fiber is prepared. A precision fiber cleaver and polishing equipment are used to cut and polish the end face of the single-mode fiber, obtaining an optically grade smooth polished end face with a surface roughness below a certain threshold. The polished end face of the single-mode fiber is then vertically fixed onto the sample stage of a two-photon 3D printing device, and a layer of photoresist material is uniformly coated onto the polished end face.

[0036] Using femtosecond laser two-photon 3D printing technology, integrated direct laser writing is performed on the polished end face of a single-mode optical fiber based on a pre-designed three-dimensional digital model. During the process, a femtosecond pulsed laser is focused by a high numerical aperture objective lens, inducing a two-photon absorption effect in the focal region within the photoresist material. Due to the highly nonlinear characteristics of the two-photon absorption process, it only occurs within a very small spatial range at the focal center where the laser energy density is extremely high, thus enabling nanoscale processing resolution that breaks through the optical diffraction limit.

[0037] Under the influence of the two-photon absorption effect, the photoresist molecules at the focal point undergo a localized high-polymerization reaction, transforming from a liquid to a solid state. By precisely controlling the scanning trajectory of the laser focus in three-dimensional space, the outer reflective film and its circumferentially sealed wrapping structure of the fully sealed fiber Fabry-Perot interferometer cavity are first constructed layer by layer on the polished end face of the single-mode fiber. This ensures that a tiny air gap with a precise initial cavity length is formed between the outer reflective film and the fiber end face, and that the fully sealed wrapping structure completely seals the circumferential edge of the air gap. Subsequently, the laser focus continues to scan upwards, solidifying layer by layer to form the biomimetic nerve fiber pillar, seamlessly connecting its bottom end to the upper surface of the outer reflective film. Finally, the laser focus continues to scan the top of the support column, integrally forming the waterproof asymmetric elliptical tympanic membrane and the eccentric traction mass point. The waterproof asymmetric elliptical tympanic membrane has a major axis span of 1250 μm, a minor axis span of 930 μm, a thin region thickness of 800 nm, a thick region thickness of 1.5 μm, and the boundary line between the thick and thin regions is offset 28 μm outward from the axial direction of the geometric center of the ellipse. The eccentric traction mass point is 270 μm away from the geometric center of the tympanic membrane.

[0038] Based on the localized high polymerization mechanism of photoresist induced by the two-photon absorption effect, a seamlessly connected micro / nano structure at the microscale is finally obtained. There are no physical bonding interfaces or interlayer gaps between the polished end face of the single-mode fiber, the fully sealed Fabry-Perot microcavity, the nerve fiber-like support, and the asymmetric thick-film sheet. This completely avoids the risk of water leakage and stress concentration problems caused by interlayer gaps in traditional multi-step processing techniques, ensuring the sub-micron-level geometric stability and resonant mode reliability of the device in deep-water high hydrostatic pressure environments.

[0039] The complete mechanical response process of the directional underwater acoustic sensor in this embodiment, from receiving underwater sound waves to changing the optical cavity length, is as follows: When a dynamic sound source radiates underwater acoustic pressure waves in the underwater space, these waves propagate through the water at a speed of approximately 1500 m / s to the surface of the waterproof asymmetric elliptical tympanic membrane. Because the waterproof asymmetric elliptical tympanic membrane is in contact with the water medium, the underwater dynamic acoustic pressure waves apply alternating pressure loads to the tympanic membrane surface, thereby inducing mechanical vibration of the waterproof asymmetric elliptical tympanic membrane.

[0040] The core sensitive operating frequency band of this sensor is designed to be the fifth resonant mode of the waterproof asymmetric elliptical tympanic membrane, with a simulated characteristic frequency of 5.6 kHz. When the incident water acoustic pressure wave contains a 5.6 kHz spectral component, the waterproof asymmetric elliptical tympanic membrane generates a strong resonant response at this frequency. Due to the 28 μm offset of the thickness boundary line and the eccentric traction mass point 270 μm away from the geometric center within the waterproof asymmetric elliptical tympanic membrane, the tympanic membrane exhibits extremely strong stiffness asymmetry and mass eccentricity characteristics in the 5.6 kHz frequency band. This asymmetric physical property means that when acoustic pressure waves from different angles in underwater three-dimensional space act on the tympanic membrane surface, the waterproof asymmetric elliptical tympanic membrane does not produce simple piston-like vibrations or symmetrical spherical vibrations, but rather induces distinctly different local resonant modes or warping modes.

[0041] Specifically, the localized asymmetric mechanical vibration of the waterproof asymmetric elliptical tympanic membrane in the fifth-order resonant mode manifests as anisotropic changes in the amplitude of the local maximum displacement point at different azimuth angles on the tympanic membrane surface. When the incident angle of the underwater sound wave changes, the spatial distribution of sound pressure on the tympanic membrane surface and the impedance matching state of the tympanic membrane change accordingly, resulting in a regular variation in the anisotropic change in the amplitude of the local maximum displacement point of the tympanic membrane. This direction-dependent anisotropic vibration is concentrated in the sound pressure-sensitive area fixed at the tip of the biomimetic nerve fiber support.

[0042] The localized asymmetric mechanical vibration is then transmitted axially through the biomimetic nerve fiber support. Since the top of the support is fixed to the acoustic pressure-sensitive region of the tympanic membrane, and the bottom is connected to the outer reflective film of the fully sealed fiber Fabry-Perot interferometer cavity, the vertical displacement of the tympanic membrane in the acoustic pressure-sensitive region is transmitted to the outer reflective film almost without loss, resulting in a synchronous displacement of the outer reflective film. This synchronous displacement directly changes the distance of the tiny air gap between the single-mode fiber end face and the outer reflective film, thereby resulting in a nanometer-scale change in the physical cavity length of the fully sealed fiber Fabry-Perot interferometer cavity. The magnitude of this nanometer-scale change is directly related to the acoustic pressure amplitude of the incident water acoustic wave and the incident angle, where the incident angle determines the anisotropic amplitude amplification factor of the tympanic membrane in the fifth resonant mode.

[0043] The complete signal processing procedure of the directional underwater acoustic sensor in this embodiment, from optical signal conversion to final sound source azimuth angle calculation, is as follows: While the physical cavity length of the fully sealed fiber Fabry-Perot interferometer cavity undergoes a nanometer-scale change due to acoustic pressure excitation, a continuous-wave laser is coupled from an external laser source into the single-mode fiber and input from the single-mode fiber into the fully sealed fiber Fabry-Perot interferometer cavity. The continuous-wave laser undergoes a first partial reflection at the end face of the single-mode fiber, and the remaining transmitted light enters the tiny air gap, undergoing a second reflection at the outer reflective film. The two reflected beams undergo multi-beam interference within the tiny air gap. According to the Fabry-Perot interference principle, when the physical cavity length undergoes a nanometer-scale change, the phase condition of the interference light changes accordingly, resulting in high-frequency modulation of the light intensity reflected back into the single-mode fiber. Therefore, based on the multi-beam interference induced by the nanometer-scale change in the physical cavity length, a high-frequency modulated optical signal is obtained, showing that the intensity of the reflected light varies with the cavity length. The frequency of this high-frequency modulated optical signal's intensity change is consistent with the frequency of the incident acoustic wave, while the depth of intensity modulation is related to the amplitude of the cavity length change.

[0044] The reflected high-frequency modulated optical signal is transmitted to the back-end signal processing system via the single-mode optical fiber. In the back-end signal processing system, the high-frequency modulated optical signal is first converted into an electrical signal using a photodetector, and then the electrical signal is amplified, filtered, and coherently demodulated using a coherent demodulation system. During demodulation, the system performs narrowband extraction on the underwater acoustic response signal at the 5.6kHz characteristic frequency to obtain the underwater acoustic response signal at the 5.6kHz characteristic frequency. Because the fifth-order resonant mode of the waterproof asymmetric elliptical tympanic membrane at 5.6kHz has a significant incident angle dependence, the optical demodulation amplitude of the underwater acoustic response signal exhibits a strong mapping relationship with the incident angle of the underwater sound wave. Specifically, different incident angles of the sound wave cause the tympanic membrane to produce different modes of localized asymmetric vibration, which in turn transmits different amplitudes of cavity length modulation through the support, ultimately resulting in different signal amplitudes after optical demodulation.

[0045] By continuously sampling the optical demodulation amplitude difference at a characteristic frequency of 5.6 kHz in the received signal, an amplitude difference sequence data characterizing the change in the incident angle of underwater acoustic waves is obtained. Subsequently, the optical demodulation amplitude of the underwater acoustic response signal at the 5.6 kHz characteristic frequency is acquired and compared with a preset underwater acoustic directional response feature library. The preset underwater acoustic directional response feature library is obtained in advance through calibration experiments or finite element simulations, and it stores reference values ​​of amplitude difference corresponding to different incident angles. These reference values ​​establish a one-to-one mapping relationship between the incident azimuth angle and the optical demodulation amplitude.

[0046] The direction inversion is calculated by inversely using the amplitude difference sequence data and the amplitude difference reference value. Specifically, the incident angle corresponding to the reference value with the highest matching degree to the measured amplitude difference sequence data is found, thus obtaining the incident azimuth angle of the sound source in the underwater space. This process can complete the direction inversion using only the optical signal collected by a single sensor node, without the need to configure multiple hydrophones to form an array, thereby realizing the acoustic direction of miniature hydrophones without array configuration.

[0047] This embodiment combines finite element physical field simulation results with structural parameter optimization data to further illustrate the resonance characteristics of the directional underwater acoustic sensor at a characteristic frequency of 5.6kHz and the basis for structural optimization of the biomimetic nerve fiber support.

[0048] A full-scale three-dimensional model of the directional underwater acoustic sensor was established using multiphysics finite element simulation software. The model fully incorporates the geometric parameters of the waterproof asymmetric elliptical tympanic membrane: its major axis of 1250 μm and minor axis of 930 μm; the thickness division into a 1.5 μm thick region and an 800 nm thin region; the offset distance of the thick-thin region boundary line relative to the geometric center axis of 28 μm; and the spatial position of the eccentric traction mass point 270 μm from the geometric center. In the simulation, 5.6 kHz acoustic pressure loads from different underwater spatial angles were applied to the model, and the dynamic mechanical response of the waterproof asymmetric elliptical tympanic membrane was calculated.

[0049] like Figure 3 The finite element physical field simulation results clearly show that the sensor operates precisely at the fifth resonant mode of the waterproof asymmetric elliptical tympanic membrane at a frequency of 5.6 kHz. In this resonant mode, due to the combined effects of the offset of the 28 μm thick-thin region boundary and the 270 μm eccentric traction mass point, the local amplitude distribution of the waterproof asymmetric elliptical tympanic membrane exhibits significant asymmetry and direction sensitivity. The vibration modes excited on the tympanic membrane surface by sound pressure waves from different directions are fundamentally different. Specifically, the position and amplitude of the local maximum displacement point on the tympanic membrane surface change regularly with the incident angle, forming the aforementioned local resonance or warping mode. This simulation result verifies that the asymmetric tympanic membrane structure can generate direction-dependent anisotropic mechanical vibrations at a single node.

[0050] In addition, such as Figure 4As shown, by systematically changing the radius parameters of the biomimetic nerve fiber support, the amplitude displacement of the outer reflective film under different support radius configurations was simulated and calculated. Simulation data shows that the radius of the support directly affects the displacement transmission efficiency between the waterproof asymmetric elliptical tympanic membrane and the outer reflective film. When the support radius is too small, the axial flexibility of the support itself is too large, leading to excessive energy loss during tympanic membrane vibration transmission, and a decrease in the synchronous displacement of the outer reflective film. When the support radius is too large, although the axial stiffness of the support increases, its additional mass effect changes the overall resonance characteristics of the tympanic membrane, and the excessively large contact area may suppress the asymmetric mode shape of the tympanic membrane. Figure 4 The amplitude displacement data resulting from different support radii are shown. The support radius that enables the outer reflective film to obtain the maximum synchronous displacement without significantly degrading the fifth-order resonant mode characteristics of the tympanic membrane is selected as the optimal design parameter.

[0051] Through the above finite element simulation verification and support parameter optimization, it is ensured that the directional underwater acoustic sensor has the best acoustic sensitivity and direction resolution at the characteristic frequency of 5.6kHz. Furthermore, the biomimetic nerve fiber support can efficiently transmit the anisotropic asymmetric vibration of the tympanic membrane to the fully sealed fiber Fabry-Perot interferometer cavity, providing a reliable physical basis for high-precision underwater sound source orientation at a single node. Figure 1 In the diagram, 101 is an asymmetric biomimetic thin film, with the area to the right of the dividing line being a thick region and the area to the left being a thin region. 102 is a neural fiber-like support connecting the thin film. 103 consists of three crossbeams, which connect to a mass block 104. The thin film and cavity in the middle of the mass block 104 form a Fabry-Perot cavity after the optical fiber enters. The base 105 is built on a ceramic ferrule. Figure 2 The ellipse represents the thin film, with the central boundary marking the thickness difference, and the outer sphere representing the fluid medium; the x-axis, y-axis, and z-axis are the coordinate axes in space. Figure 3 The x-axis, y-axis, and z-axis are the spatial coordinate axes under the thin film plane; Figure 4 This represents the vibration displacement of different support radii in the range of 0.8-1.2MHz, reaching its maximum at 4µm.

[0052] The above are preferred embodiments of the present invention. It should be noted that, for those skilled in the art, several improvements and modifications can be made without departing from the principles described in the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A directional underwater acoustic sensor based on an asymmetric structure, characterized in that, include: Waterproof asymmetric elliptical diaphragm, in contact with water; An eccentric traction mass point is set in a local area of ​​the waterproof asymmetric elliptical tympanic membrane; A biomimetic nerve fiber support extends vertically beneath a waterproof, asymmetrical, elliptical tympanic membrane; A fully sealed fiber optic Fabry-Perot interferometer cavity is positioned below a biomimetic nerve fiber support. Among them, the top end of the biomimetic nerve fiber support is fixed to the sound pressure sensitive area of ​​the waterproof asymmetric elliptical tympanic membrane, and the bottom end is connected to the outer reflective film of the fully sealed fiber optic Fabry-Perot interferometer cavity; Based on the mechanical vibration generated by the waterproof asymmetric elliptical tympanic membrane under the excitation of water acoustic pressure, the vibration is transmitted axially through a biomimetic nerve fiber support to obtain the synchronous displacement of the outer reflective film, thereby changing the physical cavity length of the fully sealed fiber Fabry-Perot interferometer cavity.

2. The directional underwater acoustic sensor based on an asymmetric structure according to claim 1, characterized in that, The waterproof asymmetric elliptical tympanic membrane has a major axis span of 1250 μm and a minor axis span of 930 μm; the surface of the waterproof asymmetric elliptical tympanic membrane is divided into a thick region and a thin region, the thickness of the thin region is 800 nm, and the thickness of the thick region is 1.5 μm. Based on the difference in stiffness distribution between the thick and thin regions, the offset distance of the boundary line between the thick and thin regions relative to the elliptical geometric center of the waterproof asymmetric elliptical tympanic membrane is 28 μm outward.

3. The directional underwater acoustic sensor based on an asymmetric structure according to claim 1, characterized in that, The physical distance between the eccentric traction mass point and the geometric center of the waterproof asymmetric elliptical tympanic membrane is 270 μm; Based on the mass distribution adjustment effect of the eccentric traction mass point, the stiffness asymmetry and mass eccentricity characteristics of the waterproof asymmetric elliptical tympanic membrane in the higher-order resonance mode are obtained.

4. The directional underwater acoustic sensor based on an asymmetric structure according to claim 1, characterized in that, The fully sealed fiber Fabry-Perot interferometer cavity is composed of a tiny air gap between the end face of a single-mode fiber and the outer reflective film. The external structure is completely sealed by photoresist material; Based on the fully sealed encapsulation structure of the photoresist material, a sealed state is obtained to prevent water medium from entering the microcavity, thus forming an underwater optical interference demodulation unit.

5. The directional underwater acoustic sensor based on an asymmetric structure according to any one of claims 1 to 4, characterized in that, The waterproof asymmetric elliptical tympanic membrane, the eccentric traction mass point, the biomimetic nerve fiber support, and the fully sealed fiber Fabry-Perot interference cavity are all made of photoresist material. Based on the two-photon absorption effect of femtosecond laser two-photon 3D printing technology, integrated direct laser writing is performed on the polished end face of a single-mode optical fiber to obtain a seamless micro-nano structure at the microscale.

6. A directional solution method based on the directional underwater acoustic sensor according to any one of claims 1 to 5, characterized in that, Includes the following steps: Step 1: Obtain the underwater dynamic underwater acoustic pressure wave propagating to the surface of the waterproof asymmetric elliptical tympanic membrane; based on the acoustic pressure excitation of the underwater dynamic underwater acoustic pressure wave, obtain the local asymmetric mechanical vibration of the waterproof asymmetric elliptical tympanic membrane in the fifth resonant mode. Step 2: Obtain local asymmetric mechanical vibration; based on the axial transmission of local asymmetric mechanical vibration through the biomimetic nerve fiber support, obtain the synchronous displacement of the outer reflective film of the fully sealed fiber Fabry-Perot interferometer cavity, and then obtain the nanoscale minute change of the physical cavity length of the fully sealed fiber Fabry-Perot interferometer cavity. Step 3: Obtain the continuous wave laser from the single-mode fiber input to the fully sealed fiber Fabry-Perot interferometer cavity; based on the multi-beam interference caused by the nanometer-scale change in the physical cavity length, obtain the high-frequency modulated optical signal whose intensity of the reflected light varies with the cavity length. Step 4: Acquire the high-frequency modulated optical signal; Based on the extraction and processing of the high-frequency modulated optical signal by the photodetector and coherent demodulation system, the underwater acoustic response signal at the characteristic frequency of 5.6kHz is obtained. The optical demodulation amplitude of the underwater acoustic response signal has a mapping relationship with the incident angle of the underwater sound wave. Step 5: Obtain the optical demodulation amplitude of the underwater acoustic response signal at the characteristic frequency of 5.6 kHz; calculate the incident azimuth angle of the sound source in the underwater space by comparing the optical demodulation amplitude with the preset underwater acoustic direction response feature library.

7. The directional solution method according to claim 6, characterized in that, In step 1, the simulated characteristic frequency corresponding to the fifth resonant mode is 5.6 kHz; Based on the 28μm offset of the thickness boundary line inside the waterproof asymmetric elliptical tympanic membrane and the 270μm eccentric traction mass point, the extremely strong stiffness asymmetry and mass eccentricity characteristics of the waterproof asymmetric elliptical tympanic membrane in the 5.6kHz frequency band are obtained, and then the local resonance or warping mode induced by sound pressure waves from different angles in the underwater three-dimensional space is obtained.

8. The directional solution method according to claim 6, characterized in that, In step 2, the local asymmetric mechanical vibration is manifested as an anisotropic change in the amplitude of the maximum displacement point of the tympanic membrane. Based on the change in the incident angle of the underwater acoustic wave, the anisotropic change in the amplitude of the maximum displacement point of the tympanic membrane is obtained, which drives the outer reflective film to generate synchronous displacement.

9. The directional solution method according to claim 6, characterized in that, In step 4, the optical demodulation amplitude of the underwater acoustic response signal exhibits a strong mapping relationship with the incident angle of the underwater acoustic wave; by continuously sampling the amplitude difference at the characteristic frequency in the received signal, amplitude difference sequence data characterizing the change of the incident angle of the underwater acoustic wave is obtained.

10. The directional solution method according to claim 6, characterized in that, In step 5, the preset underwater acoustic direction response feature library stores the amplitude difference reference values ​​corresponding to different incident angles; Based on the inverse calculation of the amplitude difference sequence data and the amplitude difference reference value, the incident azimuth angle of the sound source in the underwater space, which is directly inverted from the single-node optical signal, is obtained, realizing the acoustic orientation of the miniature hydrophone without array configuration.

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