A fiber-optic silicon-based microstructure underwater acoustic sensor

Abstract

The present invention discloses a fiber-optic silicon-based microstructure underwater acoustic sensor, which is a non-intrinsic F-P air microcavity probe in which the optical fiber end face and the silicon-based thin film form optical signal interference. The silicon-based thin film is sensitive to sound pressure signals to pick up the sound pressure changes in the liquid sound field environment and convert them into changes in the light signal interference phase. The sound pressure signal in the liquid sound field is restored through a detection and demodulation algorithm. The microcavity probe is encapsulated with an organic material thin shell structure to improve the hydrostatic pressure resistance of the underwater acoustic sensor to adapt to applications in deep water environments. The fiber-optic silicon-based microstructure underwater acoustic sensor includes an acoustic sensing silicon microstructure, a microstructure sensing probe, and a sensor pressure-resistant structure. The acoustic characteristics of the acoustic sensing silicon microstructure are designed using materials such as silicon oxide / silicon nitride as sensing materials, and the external stress recovery of the diaphragm is achieved through residual internal stress. The present invention has easy design and process, flexible application, and strong environmental adaptability, and can be used for noise monitoring and target detection in various complex scenarios.

Description

Technical Field

[0001] The present invention belongs to the technical field of optical fiber acoustic sensing, and in particular relates to an optical fiber silicon-based microstructure underwater acoustic sensor. Background Art

[0002] Optical acoustic sensors are sensing devices that efficiently acquire acoustic information from media such as air, water, oil, and gas by converting acoustic signals into changes in optical parameters via optical structures. With the continued implementation of my country's maritime strategy, the exploration and development of oil and gas resources continues to deepen, the construction of important marine and land facilities, and the continuous expansion of urban scale, the requirements for marine national defense, seabed mineral exploration, security monitoring of important facilities, and intelligent urban governance are constantly increasing. Optical acoustic sensing technology, as a passive technology that is easy to deploy on a large scale and has a wide range of applications, is widely applicable to multiple related fields such as temperature, sound, and pressure. Its technical application research and equipment deployment are rapidly advancing.

[0003] Optical underwater acoustic sensors, due to their non-contact, passive, high-sensitivity, and ease of scalable deployment, offer cost-effective advantages over active sensors, including fewer components, enhanced safety, low noise, and superior sensing performance. Currently, optical underwater acoustic sensors are widely used in environmental noise monitoring, road noise surveillance, underwater acoustic listening, radiated noise measurement, and early warning detection. They can be deployed over wide areas, acquire signals over long distances, and rapidly establish target monitoring capabilities in complex environments.

[0004] Conventional optical acoustic sensors primarily include noise measurement probes and fiber optic hydrophones. These typically utilize optical fibers wound or bonded onto a sensitizing structural material to form an acoustically sensitive probe structure, employing the principle of optical interference for signal acquisition and conversion. This type of probe structure typically incorporates multiple optical device structures or grating structures to form optical phase interference. The use of optical devices results in higher sensor costs. Furthermore, the size of the optical devices and the fiber coil winding diameter directly limit the sensor's minimum size, resulting in corresponding volume requirements for its application environment and equipment layout. Furthermore, to fully utilize the wide bandwidth advantages of single-mode optical fiber, a combination of optical splitters and wavelength splitters is employed to load the fiber optic acoustic sensor onto a single fiber, introducing a large number of transmission nodes. This, combined with the aforementioned factors, makes it difficult to promote the widespread, cost-effective, and simple application of this type of optical acoustic sensor.

[0005] Using MEMS process technology, a silicon-based composite microstructure film is processed on a silicon wafer and assembled with an optical fiber into a microcavity structure for acoustic-optical transduction, realizing an acoustic sensor that picks up acoustic signals through a silicon-based film. The acoustic sensor has the advantages of small size, easy mass production, and low batch cost, so it is less restricted by installation conditions and can be easily applied in a variety of complex environments. At the same time, through the regulation of cavity parameters, sensor probes with different sensitivities, light interference beats and other characteristics can be formed. In multiplexing, the frequency division characteristics of the cavity length are fully utilized, and the number of devices can be effectively reduced under the same multiplexing scale. Generally speaking, acoustic sensors using silicon-based microstructures are more flexible and free to use, and are more cost-effective when applied individually or on a large scale.

[0006] At present, silicon microstructure sensors using MEMS process technology are mostly used for airborne sound sensing. In addition, they are also widely used in hydrostatic pressure testing, temperature testing and other fields. Unlike hydrostatic pressure signals, water sound pressure signals are weak variables. Therefore, sensors for measuring hydrostatic pressure are not sensitive enough when used to test water sound pressure signals. The membrane structure sensitive to water sound pressure signals has the characteristics of large diameter (millimeter level) and thin thickness (submicron level). The hydrostatic pressure resistance will be seriously insufficient. Therefore, airborne sound sensors cannot be directly applied to water sound sensing. While ensuring the high sensitivity of the diaphragm, the organic packaging outer cylinder structure is added to achieve resistance to hydrostatic pressure while being sensitive to weak water sound signals. Summary of the Invention

[0007] In response to the above-mentioned technical deficiencies, the purpose of the present invention is to provide a photoacoustic sensor with small size, high sensitivity, large bandwidth, and easy mass production. It forms a micro FP cavity through an optical silicon-based film and the end face of the optical fiber, thereby realizing optical signal interference, and straining the silicon-based sensitive film through signal fields such as sound and vibration, thereby causing changes in the phase or interference spectrum of the interference light signal, thereby completing the sensing signal pickup; a micro-cavity underwater acoustic sensing structure is formed by using a silicon-based diaphragm microstructure and optical fiber precision packaging, and at the same time, a pressure-resistant and sound-transparent outer package is used and filled with sound-transparent liquid to achieve acoustic impedance matching, thereby realizing sensitive acquisition of underwater acoustic signals in the water environment, and having good resistance to hydrostatic pressure. The fiber-optic underwater acoustic sensor has easy design process, flexible application, and strong environmental adaptability. It can be used for noise monitoring, target detection, etc. in various complex scenarios. It comprehensively considers the actual needs of sensing, resistance to hydrostatic pressure and batch consistency, and has a unique design to achieve engineering application capabilities.

[0008] To solve the above technical problems, the present invention provides the following technical solutions: a fiber-optic silicon-based microstructure underwater acoustic sensor, which is an extrinsic FP air microcavity probe in which the optical signal interference is formed between the optical fiber end face and the silicon-based thin film. The silicon-based thin film is sensitive to sound pressure signals, and the sound pressure changes in the liquid sound field environment are picked up and converted into changes in the interference phase of the optical signal. The sound pressure signal in the liquid sound field is restored through a specific detection and demodulation algorithm. The microcavity probe is encapsulated in a thin shell structure of organic material to improve the hydrostatic pressure resistance of the underwater acoustic sensor, making it suitable for application in deep water environments.

[0009] The fiber-optic silicon-based microstructure underwater acoustic sensor includes an acoustic sensing silicon microstructure, a microstructure sensing probe, and a sensor pressure-resistant structure.

[0010] The acoustic characteristics design of the acoustic sensing silicon microstructure uses materials such as silicon oxide / silicon nitride as sensing materials, and realizes the external stress recovery of the diaphragm through the internal stress residual;

[0011] According to the theory of silicon-based microstructured films, the relationships between film thickness, diameter, sensitivity, and film thickness, diameter, and resonant frequency are obtained, which are described by the following formulas:

[0012] ; (2-1)

[0013] ; (2-2)

[0014] Where S is the absolute sensitivity, is the fundamental frequency of resonance, a is the radius of the diaphragm, h is the thickness, is the density, is the residual internal stress of the diaphragm, r is the distance from the center of the sensing film;

[0015] The relative sensitivity of the film can be expressed as follows:

[0016] ; (2-3)

[0017] The optical characteristic design of the acoustic sensing silicon microstructure uses optical fiber and silicon-based film to form an FP microcavity, realizing the sound and vibration signal → diaphragm strain signal → optical interference change signal, which is transmitted back through the optical fiber line and the optical detection demodulation scheme is used to realize signal restoration.

[0018] Preferably, the FP microcavity composed of optical fiber and silicon-based film adopts a low-fineness optical FP interferometer structure to obtain the characteristics of stable sinusoidal spectrum and cavity length change. The optical signal coupling coefficient is an important optical parameter of the microcavity, which is related to the quality of optical interference and thus determines the noise and fidelity of the restored signal. The optical coupling efficiency of the FP cavity is limited by three factors: "cavity length, reflection surface inclination, and lateral misalignment". The cavity length is mainly affected by the numerical aperture N of the fiber. AThe diffusion of the optical signal output mode field caused by limitation with the transmission distance, the relationship between the divergence angle after reflection of the diaphragm and the ratio of the mode field size of the optical fiber receiving end face (T1); the inclination angle of the reflection surface is mainly determined by the parallelism between the optical fiber end face and the sensing film, and the coupling efficiency related to the coaxial deviation between the film reflected light and the end face reception (T2); the lateral misalignment mainly increases the eccentricity r, which mainly affects the acoustic characteristics.

[0019] Preferably, the theoretical models corresponding to T1 and T2 are as follows:

[0020] ; (2-4)

[0021] ; (2-5)

[0022] in is the waist radius of the Gaussian beam, and the single-mode step-index fiber used here is The expression is as follows:

[0023] ; (2-6)

[0024] in is the core radius. Taking SMF-28 single-mode fiber as an example, its corresponding value is 4.07 μm; is the normalized frequency:

[0025] ; (2-7)

[0026] is the core refractive index, 1.45205; is the cladding refractive index, 1.44681;

[0027] is the mode field radius of the diffraction spot after the light propagates a distance z, where z should be expressed as 2 times the cavity length , the expression is as follows:

[0028] ; (2-8)

[0029] in is the Rayleigh distance of the Gaussian beam, ; is the incident tilt angle;

[0030] Based on the FP cavity light propagation coupling model, the corresponding optical coupling coefficient is related to the cavity length, tilt angle, etc. In terms of optical design, the cavity length spacing is selected to be within 100μm, so that the coupling coefficient can reach more than 20%, while the spacing within 35μm can reach a coupling coefficient of more than 70%. The tilt angle is designed to be within 2° to ensure a coupling efficiency of more than 80%. The former is achieved by axial distance control, and the latter is achieved by polishing the fiber end face and the sensing film surface and controlling the tilt angle.

[0031] The interference signal contrast of the low-fineness FP interferometer is closely related to the reflection characteristics of the two end faces of the cavity. It can be simplified to the reflectivity relationship between the fiber end face and the diaphragm end face as shown in the following formula. The designed end face reflectivity is about 3.5%, which can achieve a good match between the high-fineness spectral characteristics of multiple interference and the low-fineness spectral characteristics of single interference. Combined with the optimization of the cavity length design, the contrast can be close to 1, providing a good light detection signal.

[0032] ; (2-9)

[0033] in 、 is the reflectivity of the optical fiber end face and the sensing film surface, and T is the coupling coefficient.

[0034] Preferably, the overall shape of the diaphragm of the acoustic sensing silicon microstructure adopts a cylindrical design, which can reduce the stress on the corners of the sensing crystal diaphragm and reduce the circular cross-sectional size of the package; the structural design includes a sensing film structure and a two-stage annular boss. The thickness and diameter structural parameters of the sensing membrane are obtained according to theoretical calculations. It is the key to achieving underwater acoustic sensing sensitivity and frequency bandwidth. Its flatness has a great influence on the tilt coupling coefficient T2, usually reaching within 2.5°; two circular ring platforms are designed to achieve stable and high-precision installation of the fiber FP interferometer. The thickness of the cavity length limiting ring platform is designed according to the desired cavity length, which determines the lower limit of the FP interferometer cavity length. The consistency of micromachining determines the consistency of the interferometer spectral characteristics and determines the cavity length-related coupling efficiency T1; the alignment limiting ring platform mainly realizes the consolidation of the optical fiber ceramic core and the sensing structure, reduces the lateral offset error between the optical fiber axis and the diaphragm center, and has a direct impact on the sensing sensitivity; at the same time, the verticality of the ring platform side wall ensures the parallelism between the optical fiber reflection end face and the diaphragm, thereby affecting its coupling efficiency T2.

[0035] Preferably, the microstructure sensing probe optically adopts an FP interferometer structure, which is a low-fineness interference cavity composed of two low-reflectivity smooth planes, a polished optical fiber end face and a microstructure sensing film; the microstructure sensing probe is structurally formed by a ceramic ferrule with a pigtail and a sensing film with a limit ring structure. The cavity length is positioned by adjusting the distance between the two end faces, and then the bonding angle of the ceramic ferrule is adjusted to achieve the parallelism of the two surfaces, ensuring the optical coupling efficiency and the contrast of the interference light signal; at the same time, the ceramic ferrule and the sensing membrane structure are consolidated to form a sealed air cavity. When in an aqueous environment, the outside of the diaphragm is liquid and the inside is air. The internal and external impedance mismatch forms an enhanced sensitivity response of the diaphragm to the alternating underwater acoustic signal.

[0036] Preferably, the reflection spectrum corresponding to the FP cavity interference light path of the microstructure sensor probe is It is expressed as follows:

[0037] ; (2-10)

[0038] in 、 are the light reflectances of the two reflection end faces respectively. Due to the extrinsic FP interference of the fiber-film combination, Contains The transmission coefficient through the incident surface is, is the phase difference corresponding to the interference beam of a specific wavelength λ on the two reflecting surfaces, and the length L of the cavity formed by the reflecting surface cav , the refractive index n of the filling medium in the cavity and the incident light tilt angle θ, the phase difference can be expressed as follows:

[0039] ; (2-11)

[0040] From the above two expressions, it can be seen that the reflection spectrum of the fiber low-fineness FP interferometer is a quasi-sine / cosine function of the light wavelength and the cavity length.

[0041] Preferably, when the sensing film vibrates upon receiving the underwater acoustic signal S, the corresponding FP cavity length changes, and the corresponding interference phase difference between the two reflected light beams changes, thereby causing a change in the reflection spectrum. The process is as follows:

[0042] S→ →Δ → ; (2-12)

[0043] The probe signal is restored to the corresponding inverse process, and the change of the reflection spectrum is inferred to form The difference in the wavelength of the light of the phase difference is deduced based on the adjacent wavelengths and the wavelength difference, according to the physical relationship expressed by formula (2-13), and the change in cavity length is deduced. Combined with the film sensitivity value, the underwater acoustic signal intensity is restored;

[0044] ; (2-13)

[0045] The corresponding process of signal restoration is as follows:

[0046] & → → →S (2-14).

[0047] Preferably, the microstructure sensing probe includes a closed air cavity structure. When in a water environment, the two sides of the sensing film are water medium and air medium respectively. As the depth of the water increases, a pressure difference will be formed on both sides of the film. Due to the good compressibility of air, the pressure formed will be borne by the sensing film; because the sensitivity of the film is inversely correlated with the pressure resistance, the probe film is easily damaged and the pressure resistance is insufficient.

[0048] Preferably, the pressure-resistant structure of the sensor adopts a pressure-resistant and sound-transparent organic columnar shell to encapsulate the microstructure sensor probe. The thickness of the packaging shell is comprehensively designed according to the hydrostatic pressure depth and the attenuation parameters of the hydroacoustic signal, and the sensor is pressure-resistant packaged. The pressure-resistant columnar shell and the plug form a closed cavity that wraps the sensor probe, isolating it from the external hydrostatic pressure, and the gap is filled with matching liquid to achieve acoustic impedance matching, thereby ensuring that the hydroacoustic signal is transmitted to the surface of the probe sensor membrane with low attenuation. The pressure-resistant packaged hydrophone correspondingly realizes resistance to large hydrostatic pressure, and achieves a significant improvement in the hydrostatic pressure resistance under the condition of limited attenuation of the probe sensitivity.

[0049] Compared with the prior art, the present invention has the following beneficial effects:

[0050] 1) Wideband, highly sensitive sensing of acoustic signals. By designing sensing films with micron and submicron thicknesses and millimeter and submillimeter diameters, and based on the composite film stress-strain model, the film diameter is inversely correlated with the resonant frequency, the film diameter is positively correlated with the sensitivity, and the film thickness is inversely correlated with the sensitivity. Through the comprehensive design of film thickness, diameter, and material ratio, a sensing film structure with a large bandwidth (resonance point exceeding 30kHz) and high sensitivity (greater than -160dB) can be obtained, which can support the implementation of MEMS underwater acoustic sensors with a wide operating frequency band, high sensitivity, and low noise. At the same time, the sensing film is light, thin, and small in size, ensuring the implementation of micro-sized probes that can be adapted to applications in complex and restricted scenarios.

[0051] 2) The sensor probe has high performance with consistent batch size. The MEMS fine micromachining technology is used to realize the batch synchronous molding of hundreds of sensor diaphragm structures on a wafer, obtaining a small-sized and thin-thick sensor membrane with consistent structural parameters and good surface flatness (the granularity is at the nm level, far below the diffraction limit of the infrared wavelength), thereby ensuring good consistency of the acoustic sensing sensitivity; the cavity length limiting structure designed and processed on the diaphragm can control the cavity length of the FP interferometer composed of the optical fiber reflection end face and the diaphragm as the end face, with good consistency (the cavity length difference is usually within a few microns) and excellent precision (the precision control can reach within 0.5 microns), thereby ensuring the FP cavity The consistent spectral peak and valley characteristics and periodic characteristics ensure good batch consistency of the probe's operating points, providing engineering support for the time-frequency multiplexing of large-scale probes. A vertical limit installation structure (collimation limit ring) is designed on the diaphragm, which provides a fulcrum for the installation and bonding of the optical fiber ceramic ferrule and the diaphragm, making the probe structure stable and reliable. It also ensures a small inclination and high concentricity between the optical fiber end face and the sensing membrane surface that make up the FP cavity. Combined with the cavity length process control, the two interfering optical signals can have a high coupling coefficient, so that the contrast of the interference light signal reaches above 0.9, which provides a guarantee for high-fidelity and low-noise optical detection and demodulation restoration of the sensing signal.

[0052] 3) The underwater acoustic sensor is resistant to high hydrostatic pressure. The sensor's cylindrical housing is made of an organic, acoustically transparent material. The housing's dimensions, thickness, and length are designed based on the hydrostatic pressure environment and acoustic attenuation requirements. The underwater acoustic sensor probe is sealed within the cylindrical housing, and a sealing plug isolates the inner and outer cavities. This allows the hydrostatic pressure to be borne solely by the pressure-resistant cylindrical housing, significantly improving the sensor's pressure resistance. Under increasing hydrostatic pressure, the bare fiber microstructure probe exhibits a dramatic change in cavity length (manifested by a significant shift in the interference spectrum) and ruptures when the hydrostatic pressure reaches 3 MPa. However, when encapsulated with a 0.5 mm thick pressure-resistant impedance seal, the sensor probe's FP cavity length remains largely stable under high hydrostatic pressure, with the diaphragm functioning intact even at 5 MPa. Furthermore, the presence of a matching liquid within the main seal ensures excellent acoustic impedance matching between the inner and outer shells, allowing near-lossless transmission of acoustic signals, thus maintaining the sensor's high sensitivity. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] Figure 1 This is a relationship diagram between the sensitivity, resonant frequency, membrane diameter, and membrane thickness of the sensing diaphragm in the present invention. (a) The membrane thickness determines the relationship between sensitivity, resonant frequency, and membrane diameter; (b) The membrane diameter determines the relationship between sensitivity, resonant frequency, and membrane thickness.

[0054] Figure 2 These are the resonant frequency and vibration mode of the φ900μm-thick 2.5μm film in the present invention.

[0055] Figure 3 The response characteristics of the optical fiber silicon-based thin film (H2.5μm-φ900μm SiO2 membrane) in this invention. (a) Sensitivity response curve; (b) Phase response curve.

[0056] Figure 4 Theoretical analysis of the coupling coefficient of the fiber-silicon-based FP interferometer in this invention: (a) Cavity length-dependent coupling coefficient; (b) Tilt angle-dependent coupling coefficient.

[0057] Figure 5 The relationship between cavity length and reflection spectrum of the fiber low-fineness FP interferometer in the present invention. (a) Cavity length-reflection spectrum; (b) Fine reflection spectrum.

[0058] Figure 6 The sensor membrane structure design of the present invention. (a) Schematic diagram of the cross-section of the probe sensitivity enhancement structure; (b) 3D design diagram of the probe sensitivity enhancement structure.

[0059] Figure 6 Marking instructions: (1) Sensing film; (2) Cavity length limiting ring; (3) Alignment limiting ring; (4) Sensitivity-enhancing FP air cavity; (5) Ceramic ferrule fixing cavity.

[0060] Figure 7 Design of the fiber-optic silicon-based microprobe in this invention. (b) Interference optical path of the fiber-optic FP interferometer; (a) Structure of the fiber-optic FP probe.

[0061] Figure 7 Labeling instructions: (6) Collimating ceramic ferrule; (7) Transmission optical fiber; (8) Optical fiber reflection end face; (9) Sensing membrane reflection surface; (10) Input spectrum; (11) Output interference spectrum; (12) Input detection light; (13) Transmitted light; (14) Sensing membrane reflection light; (15) Sensing membrane transmission light; (16) FP interferometer reflection light.

[0062] Figure 8 This is the spectrum-water pressure test result of the sensor probe in the present invention.

[0063] Figure 9 The design and water pressure resistance test of the packaged sensor in this invention. (a) Sensor pressure-resistant package design; (b) Spectrum-water pressure test results of the pressure-resistant packaged sensor.

[0064] Figure 9 Marking instructions: (17) FP sensor probe; (18) pressure-resistant packaging column shell; (19) sealing packaging plug; (20) matching fluid, (21) center pipe. DETAILED DESCRIPTION

[0065] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention. Example 1

[0066] The present invention integrates the actual requirements of sensing, hydrostatic pressure resistance and batch consistency, and makes a unique design to achieve the capability of engineering application.

[0067] (1) Design of acoustic sensing silicon microstructure

[0068] 1) Acoustic characteristic design

[0069] This research uses materials such as silicon oxide and silicon nitride as sensing materials, and achieves external stress recovery of the diaphragm through residual internal stress. Based on the theory of silicon-based microstructured thin films, the relationships between film thickness, diameter, sensitivity, and film thickness, diameter, and resonant frequency are obtained, which are described by the following formula:

[0070] ; (2-1)

[0071] ; (2-2)

[0072] Where S is the absolute sensitivity, is the fundamental frequency of resonance, a is the radius of the diaphragm, h is the thickness, is the density, is the residual internal stress of the diaphragm, and r is the distance from the center of the sensing film.

[0073] The relative sensitivity of the film can be expressed as follows:

[0074] ; (2-3)

[0075] Theoretical models show that sensitivity is closely related to the membrane diameter, thickness, internal stress, and the distance between the light sensing position and the membrane center; while the resonant frequency of the sensing membrane is related to the material diameter, density, and internal stress, and has nothing to do with thickness.

[0076] Focusing on the working frequency and sensitivity characteristics of the fiber optic underwater acoustic sensor, the basic parameters such as the diameter and thickness of the sensing film are designed and optimized to obtain the required sensing film sensitivity and resonant frequency. Figure 1 (a) To determine the film thickness, the relationship between sensitivity, resonant frequency and film diameter; Figure 1(b) To determine the relationship between film diameter, sensitivity, resonant frequency and film thickness. Based on this, the sensor film is designed with a thickness of 2.5μm and a film diameter of 900μm. The obtained sensitivity M is greater than -160dB (ref rad / μPa) and the upper limit frequency f is m Greater than 40kHz.

[0077] The acoustic characteristics of the theoretically designed sensor film are simulated and analyzed using the finite element method to obtain its resonant mode, harmonic response curve distribution, and phase response characteristic distribution within the frequency band, thereby obtaining the acoustic response performance of the sensor film by numerical simulation. Figure 2 and Figure 3 , showing the characteristics of wide bandwidth, high sensitivity and consistent phase response.

[0078] 2) Optical property design

[0079] In order to output and restore the strain information obtained by the sensing film, a scheme using optical signals as the sensing medium to pick up signals is proposed. An FP microcavity is composed of optical fiber and silicon-based film to realize the conversion of sound, vibration and other signals → diaphragm strain signals → optical interference change signals. The signals are transmitted back via optical fiber lines, and the signal restoration is achieved using an optical detection and demodulation scheme.

[0080] A low-fineness optical FP interferometer structure is used to obtain the characteristics of stable sinusoidal spectrum and cavity length change. The optical signal coupling coefficient is an important optical parameter of the microcavity, which is related to the quality of optical interference and thus determines the noise and fidelity of the restored signal. The optical coupling efficiency of the FP cavity is limited by three factors: cavity length, reflection surface inclination, and lateral misalignment. The cavity length mainly affects the numerical aperture of the fiber N. A The diffusion of the optical signal output mode field due to limitation with transmission distance, the relationship between the divergence angle after reflection from the diaphragm and the mode field size ratio of the optical fiber receiving end face (T1); the inclination angle of the reflection surface is mainly determined by the parallelism between the optical fiber end face and the sensing film, and is related to the coupling efficiency related to the coaxial deviation between the film reflected light and the end face receiving (T2). The lateral misalignment mainly increases the eccentricity r, which mainly affects the acoustic characteristics. The theoretical model corresponding to T1 and T2 is as follows:

[0081] ; (2-4)

[0082] ; (2-5)

[0083] in is the waist radius of the Gaussian beam, and the single-mode step-index fiber used here is The expression is as follows:

[0084] ; (2-6)

[0085] in is the core radius. Taking SMF-28 single-mode fiber as an example, its corresponding value is 4.07 μm; is the normalized frequency:

[0086] ; (2-7)

[0087] is the core refractive index, 1.45205; is the cladding refractive index, 1.44681.

[0088] is the mode field radius of the diffraction spot after the light propagates a distance z, where z should be expressed as 2 times the cavity length , the expression is as follows:

[0089] ; (2-8)

[0090] in is the Rayleigh distance of the Gaussian beam, ; is the incident tilt angle.

[0091] Based on the FP cavity light propagation coupling model, the relationship between the corresponding light coupling coefficient and the cavity length, tilt angle, etc. is shown in Figure 4 In terms of optical design, the cavity length spacing is selected to be within 100μm, so that the coupling coefficient can reach more than 20%, while the spacing within 35μm can reach more than 70%; the tilt angle design within 2° can ensure a coupling efficiency of more than 80%. The former is achieved by axial distance control, and the latter is achieved by polishing the fiber end face and the sensing film surface and controlling the tilt angle.

[0092] The interference signal contrast of the low-fineness FP interferometer is closely related to the reflection characteristics of the two end faces of the cavity. It can be simplified to the reflectivity relationship between the fiber end face and the diaphragm end face as shown in the following formula. The design end face reflectivity is about 3.5%, which can achieve a good match between the high-fineness spectral characteristics of multiple interference and the low-fineness spectral characteristics of single interference. Combined with the optimization of the cavity length design, the contrast can be close to 1, providing a good light detection signal. Figure 5 .

[0093] ; (2-9)

[0094] in 、 is the reflectivity of the optical fiber end face and the sensing film surface, and T is the coupling coefficient.

[0095] 3) Mechanical structure design

[0096] In terms of structure, in order to ensure the stability of the sensor structure, the convenience of sensor probe assembly process, and the consistency of optical parameters, the sensor film structure is designed for installation adaptability (see Figure 6 The overall shape of the diaphragm is cylindrical, which can reduce the stress on the corners of the sensing crystal diaphragm and reduce the circular cross-sectional size of the package. The structural design includes a sensing film structure and a two-stage annular boss. The thickness and diameter structural parameters of the sensing membrane are obtained according to theoretical calculations. It is the key to achieving underwater acoustic sensing sensitivity and frequency bandwidth. Its flatness has a significant impact on the tilt coupling coefficient T2, usually reaching within 2.5°. Two circular ring stages are designed to achieve stable and high-precision installation of the fiber FP interferometer. The thickness of the cavity length limit ring stage is designed according to the desired cavity length, which determines the lower limit of the FP interferometer cavity length. The consistency of micromachining determines the consistency of the interferometer spectral characteristics and the cavity length-related coupling efficiency T1. The alignment limit ring stage mainly realizes the consolidation of the fiber ceramic core and the sensing structure, reducing the lateral offset error between the fiber axis and the diaphragm center, which has a direct impact on the sensing sensitivity. At the same time, the verticality of the ring stage side wall ensures the parallelism between the fiber reflection end face and the diaphragm, thereby affecting its coupling efficiency T2.

[0097] (2) Microstructure sensor probe solution

[0098] Optically, the microstructure sensing probe is an FP interferometer structure, which is a low-fineness interference cavity composed of two low-reflectivity smooth planes, namely the polished optical fiber end face and the microstructure sensing film. Structurally, it is formed by consolidating a ceramic ferrule with a pigtail and a sensing film with a limit ring structure. By adjusting the distance between the two end faces to position the cavity length, the bonding angle of the ceramic ferrule is adjusted to achieve the parallelism of the two surfaces, ensuring the optical coupling efficiency and the contrast of the interference light signal. At the same time, a sealed air cavity is formed between the ceramic ferrule and the sensing film structure. In an aqueous environment, the outside of the diaphragm is liquid and the inside is air. The internal and external impedance mismatch forms an enhanced sensitivity response of the diaphragm to the alternating underwater acoustic signal. The overall structure of the probe formed is shown in FIG. Figure 7 (a) shown.

[0099] The microstructure probe formed has a low-fineness FP cavity interference optical path. Figure 7 (b) shows the corresponding reflectance spectrum It is expressed as follows:

[0100] ; (2-10)

[0101] in 、 are the light reflectances of the two reflection end faces respectively. Due to the extrinsic FP interference of the fiber-film combination, Contains The transmission coefficient through the incident surface is, is the phase difference corresponding to the interference beam of a specific wavelength λ on the two reflecting surfaces, and the length L of the cavity formed by the reflecting surface cav , the refractive index n of the filling medium in the cavity and the incident light tilt angle θ, the phase difference can be expressed as follows:

[0102] ; (2-11)

[0103] From the above two expressions, it can be seen that the reflection spectrum of the fiber low-fineness FP interferometer is a quasi-sine / cosine function of the light wavelength and the cavity length.

[0104] When the sensing film vibrates when receiving the underwater acoustic signal S, the corresponding FP cavity length changes, and the interference phase difference between the two corresponding reflected light beams changes, which in turn causes the change of the reflection spectrum. The process is as follows:

[0105] S→ →Δ → ; (2-12)

[0106] The probe signal is restored to the corresponding inverse process, and the change of the reflection spectrum is inferred to form The difference in the wavelength of light due to the phase difference is used to deduce the change in cavity length based on the adjacent wavelengths and wavelength difference, according to the physical relationship expressed by formula (2-13). Combined with the film sensitivity value, the underwater acoustic signal intensity is restored.

[0107] ; (2-13)

[0108] The corresponding process of signal restoration is as follows:

[0109] & → → →S. (2-14)

[0110] (3) Sensor pressure resistance solution

[0111] The microstructure sensor probe contains a sealed air cavity structure. When in a water environment, the two sides of the sensing film are water medium and air medium respectively. As the depth of the water increases, a pressure difference will be formed on the two sides of the film. Due to the good compressibility of air, the pressure generated will be borne by the sensing film. Because the sensitivity of the film is inversely related to the pressure resistance, the probe film is easily damaged and the pressure resistance is insufficient. Figure 8 .

[0112] The microstructure sensor probe is encapsulated using a pressure-resistant and sound-transparent organic columnar shell. The thickness of the encapsulation shell is comprehensively designed based on parameters such as hydrostatic pressure depth and underwater acoustic signal attenuation. Figure 9(a) Sensor pressure-resistant packaging design. The pressure-resistant cylindrical shell and the plug form a closed cavity that wraps the sensor probe, isolating it from the external hydrostatic pressure. The gap is filled with matching liquid to achieve acoustic impedance matching, thereby ensuring that the underwater acoustic signal is transmitted to the surface of the probe sensing membrane with low attenuation. The pressure-resistant packaged hydrophone achieves high hydrostatic pressure resistance, see Figure 9 (b) Significant improvement in hydrostatic pressure resistance is achieved with limited degradation of probe sensitivity. Example 2

[0113] A specific implementation scheme of a fiber-optic silicon-based microstructure underwater acoustic sensor is as follows:

[0114] 1. Fiber-optic silicon-based sensing structure:

[0115] A theoretical model of stress-strain response of silicon-based thin films was established. Based on the target technical indicators of underwater acoustic sensitivity, the performance relationship between the thickness, diameter, and material of the sensing diaphragm, such as resonant frequency and strain sensitivity, was analyzed and obtained. Then, the structural parameters of the sensing diaphragm that met the indicators were selected. Figure 1 Based on the sensor structure set by the theoretical model, a finite element simulation model is established to obtain its resonance mode, sensitivity and phase frequency band characteristics, and a numerical experimental evaluation is performed on the acoustic response characteristics of the sensor membrane structure. Figure 2 、 Figure 3 .

[0116] Using the light transmission theory, a transmission waveguide model of FP light transmission, diffusion, reflection, coupling and interference between the fiber end face and the sensing film surface is established, aiming at the high-performance detection and demodulation of the interference spectrum fineness characteristics and interference spectrum contrast. Figure 5 , and then design the related parameters of the FP interferometer coupling coefficient (see Figure 4 ), including the end face reflectivity, FP cavity length, parallelism of the reflecting end face, and concentricity of the optical fiber and the sensing film.

[0117] 2. Fiber-optic silicon-based microstructure sensing probe:

[0118] Through the above theoretical analysis and modeling simulation, the key optical and structural parameters corresponding to the high-performance photoacoustic sensing characteristics of the fiber FP probe were determined. In order to achieve the engineering goals of process convenience, easy batch assembly, and process consistency, the structural design of the FP interferometer process assembly was carried out based on the designed probe parameters. Figure 6 On the basis of the sensing film (1), a cavity length limiting ring (2) is designed to limit the distance between the two reflecting surfaces of the FP cavity (4) to avoid damage to the sensing film during assembly, and an alignment limiting ring (3) is designed to facilitate the stable installation of the optical fiber alignment ceramic ferrule (6) in the fixed cavity (5).

[0119] Using MEMS processing technology, the designed sensing membrane structure is processed in batches (see Figure 6 (b) Through the design and control of etching process parameters, a composite silicon-based sensing film (1) with submicron precision in structural parameters such as diameter, thickness, roundness, and surface smoothness is obtained. The cavity length limiting ring (2) has a height precision of several microns, while the diameter precision of the ceramic insert fixing cavity (5) reaches within 1 mm, and the verticality reaches within 5°.

[0120] The probe is installed using precision assembly technology (see Figure 7 ), fix the transmission optical fiber (7) in the collimating ceramic ferrule (6), grind the optical fiber reflection end face (8) to ensure that its flatness is within 2°, connect the transmission optical fiber (7) pigtail to the spectrometer, and insert the collimating ceramic ferrule (6) into the sensing membrane structure ( Figure 6 ) in the ceramic ferrule fixed cavity (5) of the optical fiber, the spectral characteristics are obtained in real time by the spectrometer, and the installation tilt angle and step length are adjusted by feedback, so as to achieve good consistency in the parameters such as the installation distance, parallelism, and concentricity between the assembled optical fiber reflection end face (8) and the sensing film reflection surface (9).

[0121] During the assembly process, the transmission optical fiber is connected to the broadband light source and the spectrometer. The light source inputs a detection light signal (12) with a wide spectral characteristic (10), forming reflected light and transmitted light (13) on the end face of the optical fiber. The transmitted light (13) forms sensing film reflected light (14) and sensing film transmitted light (15) at the sensing film reflective surface (9). The former is reflected to the optical fiber end face and re-coupled into the optical fiber, forming the reflected light (16) of the FP interferometer with the direct reflected light from the end face. The reflected light is returned to the spectrometer by the transmission optical fiber (7), and the output interference spectrum (11) is obtained. The corresponding peak-to-valley distribution, contrast, spectral intensity and other parameters directly reflect the assembly parameters and accuracy of the optical fiber reflective end face (8) and the sensing film reflective surface (9) in the FP interferometer probe, and determine the acoustic and optical sensing characteristics of the sensor.

[0122] 3. Hydrostatic pressure resistant fiber optic silicon-based sensor:

[0123] The encapsulated column shell (18) is designed and processed to achieve the target small size and hydrostatic pressure resistance. The inner diameter of the pressure-resistant encapsulated column shell is slightly larger than the outer diameter of the finished acoustic sensor probe (17). The acoustic impedance matching liquid (20) is injected into the column shell (18), and the probe (17) is installed flatly into the column shell (18). The matching liquid (20) wets and wraps the probe (17), and the excess matching is also discharged from the gap between the two. The sealing plug (19) is sealed with glue or other methods to achieve sealing with the column shell (18). The transmission optical fiber (7) and the excess matching liquid (20) are guided to the outside of the water interface through the central pipe (21). By comparing the optical fiber silicon-based probe (7) before water pressure packaging with the sensor (17) after pressure packaging, the sealing plug (19) is sealed with the column shell (18). Figure 9(a) Comparison of hydrostatic pressure resistance was conducted. The former broke the membrane above 3MPa, while the latter maintained the probe structure and optical properties above 5MPa. Figure 8 and Figure 9 (b).

[0124] This invention proposes a fiber-optic extrinsic FP interferometric underwater acoustic sensor based on silicon-based microstructures. A comprehensive design, processing, and evaluation methodology are developed to create a high-performance, engineered, and highly reliable small-sized underwater acoustic sensor. The invention utilizes membrane vibration theory to model the correlation between the sensitivity and resonant frequency of the sensing membrane and structural parameters. Furthermore, through optical transmission coupling theory modeling, the correlation between the interference spectrum distribution characteristics and spectral contrast and optical parameters such as the reflectivity and coupling coefficient of the FP cavity is determined. Based on this, key acoustic, optical, and structural parameters such as the thickness and diameter of the sensing membrane, the membrane surface reflectivity, the interference cavity length, and the inclination angle of the reflecting surface are designed, and finite element numerical simulation is used to evaluate the sensing performance. A limiting additional structure for the sensing membrane is designed to ensure the assembly parallelism accuracy of the FP interferometer cavity length and the reflecting surface pair, process consistency, and mass production convenience. Silicon etching technology is used to fabricate the silicon-based sensing microstructure, combined with optical precision assembly technology to form a micro-opto-electromechanical underwater acoustic probe. To address the problem of insufficient water pressure resistance of silicon-based sensor probes, a water-pressure-resistant cylindrical shell designed with organic sound-transmitting materials is used to seal the probe. Through finite element numerical simulation optimization and iteration, a balance is achieved between the shell's pressure resistance and sound transmission performance. The sealed structure is immersed in a specific matching liquid to achieve sound conduction, ultimately realizing a small-sized, water-pressure-resistant, and highly sensitive fiber-optic silicon-based underwater acoustic sensor. This sensor has the advantages of simple process, easy batch production, and high cost-effectiveness. It has been applied to fiber-optic fine-diameter underwater acoustic sensor arrays, significantly reducing the size and aperture limitations of linear arrays. It can also be applied to underwater acoustic surface arrays and volume arrays, achieving thinness and weight reduction, effectively reducing installation requirements, and improving overall sensing performance. It has good application prospects in underwater acoustic detection, underwater environmental research, mineral exploration and other fields.

[0125] Although specific embodiments of the present invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the present invention, and the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A fiber-optic silicon-based microstructure underwater acoustic sensor, characterized in that: The FP air microcavity probe is a non-intrinsic optical fiber end face that forms optical signal interference with a silicon-based thin film. By utilizing the silicon-based thin film's sensitivity to sound pressure signals, it picks up changes in sound pressure in the liquid acoustic field and converts them into changes in the optical signal interference phase. Through a detection and demodulation algorithm, the sound pressure signal in the liquid acoustic field is restored. The microcavity probe is encapsulated in an organic material thin shell structure, improving the hydrostatic pressure resistance of the underwater acoustic sensor to adapt to deepwater applications. The fiber-optic silicon-based microstructure underwater acoustic sensor includes an acoustic sensing silicon microstructure, a microstructure sensing probe, and a sensor pressure-resistant structure. The acoustic characteristics of the acoustic sensing silicon microstructure are designed using silicon oxide / silicon nitride as the sensing material, and the external stress recovery of the diaphragm is achieved through the internal stress residual; According to the theory of silicon-based microstructured films, the relationships between film thickness, diameter, sensitivity, and film thickness, diameter, and resonant frequency are obtained, which are described by the following formulas: ； ； Where S is the absolute sensitivity, ω0 is the fundamental resonant frequency, a is the diaphragm radius, h is the thickness, ρ is the density, σ is the residual internal stress of the diaphragm, and r is the distance from the center of the sensing film; The relative sensitivity of the film is expressed as follows: ； The optical characteristic design of the acoustic sensing silicon microstructure uses optical fiber and silicon-based thin film to form an FP microcavity, realizing the sound and vibration signal → diaphragm strain signal → optical interference change signal, which is transmitted back through the optical fiber line and restored by the optical detection and demodulation scheme; The FP microcavity composed of optical fiber and silicon-based thin film adopts a low-fineness optical FP interferometer structure to obtain the characteristics of sinusoidal spectrum and stable cavity length change. The optical signal coupling coefficient is an important optical parameter of the microcavity, which is related to the quality of optical interference and thus determines the noise and fidelity of the restored signal. The optical coupling efficiency of the FP cavity is limited by three factors: cavity length, reflection surface inclination, and lateral misalignment. The cavity length affects the numerical aperture N of the fiber. A The diffusion of the optical signal output mode field caused by the limitation with the transmission distance, the relationship between the divergence angle after the diaphragm reflection and the mode field size of the optical fiber receiving end face is T1; the inclination angle of the reflection surface is determined by the parallelism between the optical fiber end face and the sensing film, and the coupling efficiency T2 related to the coaxial deviation between the light reflected by the film and the end face reception. The lateral misalignment increases the eccentricity r, affecting the acoustic characteristics; The theoretical models corresponding to T1 and T2 are as follows: ； ； Where ω0 is the waist radius of the Gaussian beam. For the single-mode step-index fiber used, the expression of ω0 is as follows: ； Where a is the core radius, which corresponds to 4.07 μm; V is the normalized frequency: ； n1 is the core refractive index, 1.45205; n2 is the cladding refractive index, 1.44681; ω is the mode field radius of the diffraction spot after the light propagates a distance z, where z is twice the cavity length L cav , the expression is as follows: ； Where z0 is the Rayleigh distance of the Gaussian beam, θ is the incident tilt angle; Based on the FP cavity light propagation coupling model, the corresponding optical coupling coefficient is related to the cavity length and tilt angle. In terms of optical design, the cavity length spacing is selected to be within 100μm, so that the coupling coefficient reaches more than 20%, while the spacing within 35μm achieves a coupling coefficient of more than 70%. The tilt angle is designed to be within 2° to ensure a coupling efficiency of more than 80%. The former is achieved by axial distance control, and the latter is achieved by polishing the fiber end face and the sensing film surface and controlling the tilt angle. The interference signal contrast of the low-fineness FP interferometer is closely related to the reflection characteristics of the two end faces of the cavity. It can be simplified to the reflectivity relationship between the fiber end face and the diaphragm end face as shown in the following formula. The end face reflectivity is designed to be approximately 3.5%, so that the high-fineness spectral characteristics of multiple interference are consistent with the low-fineness spectral characteristics of single interference. Combined with the optimization of the cavity length design, the contrast is close to 1, providing a light detection signal. ； Where R1 and R2 are the reflectivity of the optical fiber end face and the sensing film surface, and T is the coupling coefficient.

2. The optical fiber silicon-based microstructure underwater acoustic sensor according to claim 1, characterized in that: The overall shape of the diaphragm of the acoustic sensing silicon microstructure adopts a cylindrical design to reduce stress on the corners of the sensing crystal diaphragm and the circular cross-sectional size of the package. The structural design includes a sensing film structure and a two-stage annular boss. The thickness and diameter structural parameters of the sensing membrane are obtained according to theoretical calculations. The key to achieving underwater acoustic sensing sensitivity and frequency bandwidth is its flatness, which affects the tilt coupling coefficient T2 and reaches within 2.5°. Two circular ring stages are designed to achieve stable and high-precision installation of the fiber FP interferometer. The thickness of the cavity length limit ring stage is designed according to the desired cavity length and determines the lower limit of the FP interferometer cavity length. The consistency of micromachining determines the consistency of the interferometer spectral characteristics and the cavity length-related coupling efficiency T1. The alignment limit ring stage realizes the consolidation of the fiber ceramic core and the sensing structure, reducing the lateral offset error between the fiber axis and the diaphragm center, which has a direct impact on the sensing sensitivity. At the same time, the verticality of the ring stage sidewall ensures the parallelism between the fiber reflection end face and the diaphragm, thereby affecting its coupling efficiency T2.

3. The optical fiber silicon-based microstructure underwater acoustic sensor according to claim 1, characterized in that: The microstructure sensing probe optically adopts an FP interferometer structure, which is a low-fineness interference cavity composed of two low-reflectivity smooth planes: the polished optical fiber end face and the microstructure sensing film. The microstructure sensing probe is structurally composed of a ceramic ferrule with a pigtail and a sensing film with a limit ring structure. The cavity length is positioned by adjusting the distance between the two end faces, and then the bonding angle of the ceramic ferrule is adjusted to achieve the parallelism of the two surfaces, ensuring the optical coupling efficiency and the contrast of the interference light signal. At the same time, a sealed air cavity is formed between the ceramic ferrule and the sensing membrane structure. When in an aqueous environment, the outside of the diaphragm is liquid and the inside is air. The internal and external impedance mismatch forms an enhanced sensitivity response of the diaphragm to the alternating underwater acoustic signal.

4. The optical fiber silicon-based microstructure underwater acoustic sensor according to claim 1, characterized in that: Reflection spectrum R corresponding to the FP cavity interference optical path of the microstructure sensor probe F-P It is expressed as follows: ； Where R1 and R2 are the light reflectances of the two reflective end faces respectively. Due to the extrinsic FP interference of the optical fiber-film combination, R2 includes the transmission coefficient of 1-R1 through the incident surface. is the phase difference corresponding to the interference beam of wavelength λ of the two reflecting surfaces, and the length L of the cavity formed by it and the reflecting surface cav , the refractive index n of the medium filling the cavity and the incident light tilt angle θ, the phase difference is expressed as follows: ； From the above two expressions, it can be seen that the reflection spectrum of the fiber low-fineness FP interferometer is a quasi-sine / cosine function of the optical wavelength and cavity length; When the sensing film vibrates when receiving the underwater acoustic signal S, the corresponding FP cavity length changes, and the interference phase difference between the two corresponding reflected light beams changes, which in turn causes the change of the reflection spectrum. The process is as follows: ； The probe signal is restored by the corresponding inverse process. The difference in the wavelength of light that forms the 2π phase difference is inferred from the change in the reflection spectrum. According to the adjacent wavelengths and the wavelength difference, the change in the cavity length is deduced according to the physical relationship expressed by formula 2-13. Combined with the film sensitivity value, the underwater acoustic signal intensity is restored. ； The corresponding process of signal restoration is as follows: 。 5. The optical fiber silicon-based microstructure underwater acoustic sensor according to claim 4, characterized in that: The microstructure sensing probe contains a closed air cavity structure. When in a water environment, there are water medium and air medium on both sides of the sensing film respectively. As the depth of the water increases, a pressure difference will be formed on both sides of the film. Due to the good compressibility of air, the pressure formed will be borne by the sensing film. Since the sensitivity of the film is inversely correlated with the pressure resistance, the probe film is easily damaged and the pressure resistance is insufficient. The pressure-resistant structure of the sensor adopts a pressure-resistant and sound-transparent organic columnar shell to encapsulate the microstructure sensing probe. The thickness of the encapsulation shell is comprehensively designed according to the hydrostatic pressure depth and the attenuation parameters of the hydroacoustic signal. The sensor pressure-resistant packaging design; the pressure-resistant columnar shell and the plug form a closed cavity that wraps the sensing probe, isolating it from the external hydrostatic pressure, and the gap is filled with matching liquid to achieve acoustic impedance matching, thereby ensuring that the hydroacoustic signal is transmitted to the surface of the probe sensing membrane with low attenuation. The pressure-resistant encapsulated hydrophone correspondingly realizes high hydrostatic pressure resistance, and significantly improves the hydrostatic pressure resistance under the condition of limited attenuation of the probe sensitivity.

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