Silicon-based film optical underwater acoustic sensor with pressure-resistant cavity structure

Abstract

The invention discloses a silicon-based grain film optical underwater acoustic sensor with a pressure-resistant cavity structure, which relates to the field of underwater acoustic sensing and comprises a silicon-based sensing grain film, a pressure-resistant spiral cavity, an optical fiber collimation layer, a ceramic ferrule, a transmission optical fiber and the like. An MEMS etching process is adopted, an annular line structure is designed on a silicon-based diaphragm to release film etching stress, and it is guaranteed that a millimeter-level large-size and submicron-level-thickness circular film is achieved; a spiral cavity structure is designed to be packaged between the optical fiber collimation layer and the sensing diaphragm layer, and is provided with an internal cavity with compressed air volume, so that pressure balance of two sides of the sensing diaphragm is achieved, and hydrostatic pressure resistance is improved under the condition that sensitivity is kept; the optical fiber collimation layer is designed, the perpendicularity of the optical fiber insertion core and the sensing surface is ensured, and good parallelism between the optical fiber end surface and the sensing film surface can be realized in combination with polishing of the optical fiber end surface, so that high-quality interference light performance is ensured. The optical underwater acoustic sensor has the performance effects of small size and high sensitivity.

Description

Technical Field

[0001] The present invention relates to the field of underwater acoustic sensing, and particularly to a silicon-based corrugated film optical underwater acoustic sensor with a pressure-resistant cavity structure. Background Art

[0002] Optical underwater acoustic sensors, due to their high sensitivity and passive characteristics, are widely used in scenarios such as underwater acoustic detection, mineral exploration, environmental investigation, and safety monitoring of liquid pipe networks. Currently, optical sensors in the field of underwater acoustic sensing adopt several types such as all-fiber type and optical micro-structure F-P type. The former, typically like fiber optic hydrophones, commonly uses an air-backed structure. The optical fiber is wound around a sensitizing structure. The sensitizing structure receives the underwater acoustic signal, causing expansion and contraction, thereby loading the signal onto the fiber coil, causing changes in the phase or intensity of the optical signal, and realizing the conversion of acoustic-optic information. It has high sensitivity. However, it usually comes with a relatively large volume. The common structural dimensions reach dozens of millimeters in diameter and dozens of millimeters in length. According to literature and patent reports, the minimum winding diameter reaches a few millimeters, but its axial length is much longer than the radial dimension. Due to the sensitivity requirements of the underwater acoustic sensor structure with fiber optic loop sensitization, a specific length L is required. The relationship between it and the coiling diameter R and the number of coil turns N is L = 2πR·N, where N is proportional to the axial length of the sensitizing structure. Therefore, it can be seen that the product of the length and diameter of the fiber optic coil type underwater acoustic sensor structure is a constant (without considering ineffective lamination), thus resulting in the inability to compress the overall size. The optical micro-structure F-P interferometer underwater acoustic sensor, because it adopts MEMS processing technology, can realize acoustic signal sensing in a thin film structure with a sub-millimeter scale and realize acoustic-optic signal conversion through the interference light phase transformation, so it has the advantage of small size. The reported sensing thin film structures include flat films, corrugated films, photonic crystal thin films, etc. The radial dimensions of the usually packaged F-P sensing probes are generally in the order of several millimeters, and the length dimensions are generally within more than ten millimeters. However, at the same time, due to the limitations of the preparation area size and film thickness of its sensing film, it is very difficult to improve the sensitivity. Usually, it is more than 1 order lower than that of the fiber optic interferometer type underwater acoustic sensor. And because the F-P interference cavity structure adopted is either air cavity sealed or an open cavity for the thin film. For the former, due to the compressibility of the air cavity, the sensing film is easily pressed and broken in the water environment. For the latter, because it belongs to a water-permeable cavity, the media and static pressure inside and outside the film are exactly the same, making it difficult to improve its sensitivity. Therefore, there is an inverse correlation between the hydrostatic pressure resistance ability and the sensitivity improvement of the MEMS structure underwater acoustic sensor.

[0003] Conventional fiber optic MEMS type underwater acoustic sensors mainly face the following technical problems:

[0004] (1) Low sensitivity of underwater acoustic sensing

[0005] To achieve the miniaturization and light weight of sensors, an F-P interferometer sensor structure with a silicon-based sensing film was designed. According to the stress-strain theoretical model, the sensitivity of the sensing film is positively correlated with the film diameter R and negatively correlated with the film thickness T. Since a smooth planar sensing film structure is adopted, to obtain high-sensitivity characteristics, such as a sound pressure sensitivity above -150 dB, the film thickness is required to reach around 1 micron, and the film diameter needs to reach more than 1 mm. Due to the surface tension of the liquid in wet etching, the flat film shrinks and cannot form an optical mirror reflection to output an interference signal. On the other hand, the silicon-based film has intrinsic internal stress, which is closely related to the film sensitivity. When the sensing film is in an immersion water environment, due to the hydrostatic pressure, the stress of the film increases. On the sensitivity-versus-hydrostatic-pressure curve, it shows that as the pressure increases, the stress-strain decreases and approaches saturation, so its sensitivity decreases with the increase of the external pressure. Based on the above deficiencies, by designing a circular texture structure on the smooth film, the release of the internal stress and external stress of the film can be achieved, ensuring the stability of the sensing film structure and high sensitivity.

[0006] (2) Poor hydrostatic pressure resistance

[0007] The F-P type underwater acoustic sensor with a silicon-based film usually forms a sealed air cavity package with two reflecting surfaces and the surrounding support structures, as Figure 1 shown in Figures a and 1b, the sensing film adopts a sensitive film structure (1), the input optical fiber (3) passes through the ceramic ferrule (2) and extends into the F-P sealed cavity (4), and both sides of the sensing film are immersed in water and air respectively. When the water depth increases, the corresponding external pressure stress increases significantly, causing the film to be pressed inward and concave until it is damaged. When the smooth planar film with a thickness of 2.5 microns and a diameter of 0.9 mm is tested for pressure resistance up to above 2 MPa, the film is damaged, as Figure 2 shown in Figures a and 2b. The main reason is the pressure mismatch between the inside and outside of the cavity. When the external pressure is too large, it causes the stress fracture of the thin film structure. This problem needs to be solved by increasing the volume of the internal cavity and achieving the internal and external pressure balance by increasing the pressure under a large compression ratio of air, so as to improve the hydrostatic pressure resistance of the sensor.

[0008] (3) Difficulties in sealing the air compression cavity and optical end face packaging of the F-P cavity

[0009] The improvement of the pressure resistance of the underwater acoustic sensor is achieved by introducing a spiral cavity structure and increasing the large volume ratio of compressible air to achieve the internal and external pressure balance. The spiral cavity structure needs to contact the external water environment through a small interface, and the inner cavity cannot be completely infiltrated by water, resulting in the failure of the sensitivity-enhancing cavity structure. Therefore, it is required that the upper and lower end faces of the spiral cavity structure be sealed with the film layer and the collimation layer through a special process.

[0010] Meanwhile, for the F-P cavity formed by the fiber end face and the sensing film face, the parallelism between the two greatly affects the interference light visibility and the overall coupling strength, etc., and it is easy to form an inclined package. Usually, when the inclination angle is greater than 5°, it will cause a significant decrease in the interference effect. Therefore, it is necessary to achieve parallelism retention through a special installation design.

[0011] The above-mentioned difficult problems directly affect the sensing performance, engineering usability, batch production qualification rate, etc. of the MEMS underwater acoustic sensor structure, and have an important impact on the engineering maturity and popularization of this type of underwater acoustic sensor. Summary of the Invention

[0012] The purpose of the present invention is to overcome the deficiencies existing in the prior art, and provide a silicon-based corrugated film optical underwater acoustic sensor with a pressure-resistant cavity structure, which successively includes a corrugated film sensing structure, a pressure-resistant cavity structure, and an optical fiber collimation structure from bottom to top. Among them, the corrugated film sensing structure undertakes the induction of underwater acoustic signals, and the transmission optical fiber undertakes the input of optical signals and the feedback of interference signals. Each part is specially designed for the specific problems of the silicon-based micro-structure underwater acoustic sensor to achieve specific functional performances to support the engineering usability of the optical underwater acoustic sensor.

[0013] The purpose of the present invention is completed through the following technical solutions: This silicon-based corrugated film optical underwater acoustic sensor with a pressure-resistant cavity structure includes:

[0014] The sensing corrugated film layer adopts a disc-shaped structure, with a depression in the center. A sensing smooth planar film is arranged at the center of the depression, and a ring-shaped corrugated film is arranged on the periphery of the sensing smooth planar film. The surface of the sensing corrugated film layer outside the ring-shaped corrugated film serves as a sealing and consolidation surface;

[0015] The spiral air cavity layer has its lower bottom surface in close contact with and consolidated and sealed to the sealing and consolidation surface of the sensing corrugated film layer, so as to form an F-P sealed cavity at the depression. The upper bottom surface of the spiral air cavity layer is etched to form a spiral channel;

[0016] The ceramic ferrule adopts a columnar structure;

[0017] The collimation installation layer embeds the lower part of the ceramic ferrule into the collimation structure layer, and the ceramic ferrule is consolidated with the collimation installation layer. The bottom of the collimation installation layer is closely attached to and consolidated and sealed to the upper bottom surface of the spiral air cavity layer, so as to form a spiral pressure-resistant cavity in the spiral channel. Water-permeable micropores communicating with the spiral channel are opened on the side wall of the spiral air cavity layer for introducing liquid flow into the spiral pressure-resistant cavity;

[0018] An input optical fiber, one end of which is connected to an optical fiber spectral analysis device, and the other end of which sequentially passes through a ceramic ferrule, a collimation mounting layer, and a spiral air cavity layer, then extends into an F-P sealed cavity and is located above a sensing smooth planar film. The sensing smooth planar film and the input optical fiber cooperate to form an F-P interference cavity optical structure to realize acousto-optic information conversion. The optical fiber spectral device observes in real time the spectral characteristics of the F-P interferometer returned by the input optical fiber.

[0019] As a further technical solution, a limiting boss is provided at the center of the lower bottom surface of the spiral air cavity layer for being placed in the depression of the sensing corrugated film layer, and the limiting boss does not contact the sensing smooth planar film and the annular corrugated film.

[0020] As a further technical solution, the lower bottom surface of the spiral air cavity layer is a smooth bottom surface. The sensing corrugated film layer and the spiral air cavity layer are both made of silicon-based materials. The smooth bottom surface and the sealing consolidation surface are consolidated together through a molecular bonding process to achieve sealing.

[0021] As a further technical solution, a structural center hole is opened at the center of the spiral air cavity layer. The structural center hole penetrates the limiting boss, and the spiral channel is communicated with the F-P sealed cavity through the structural center hole.

[0022] As a further technical solution, the end face of the input optical fiber extending into the F-P sealed cavity is a polished end face, and the sensing smooth planar film and the polished end face form the reflecting surfaces of the two beams of light of the F-P interferometer.

[0023] As a further technical solution, a circular groove is opened on the collimation mounting layer for placing the ceramic ferrule, and the surface of the circular groove, that is, the inner wall of the collimation structure, is in close contact with and consolidated with the outer wall of the ceramic ferrule to ensure that the axis of the ceramic ferrule is perpendicular to the surface of the sensing smooth planar film.

[0024] As a further technical solution, a central through hole is opened at the center of the ceramic ferrule, and a collimation layer center hole is opened at the center of the circular groove. The input optical fiber sequentially passes through the central through hole and the collimation layer center hole.

[0025] As a further technical solution, the collimation layer bottom surface at the bottom of the collimation mounting layer and the upper bottom surface of the spiral air cavity layer are concentrically fitted and are consolidated and sealed through molecular bonding.

[0026] As a further technical solution, the diameter and thickness of the sensing smooth planar film are determined according to the sensor sensitivity target; the annular corrugated film is formed by wet etching. The cross-sectional structure of the annular corrugation of the annular corrugated film, the corrugation height, the corrugation width, the gap between adjacent corrugations, the radius of the central flat film, and the radius of the corrugated film are determined according to the stress release process requirements during the wet etching process and the stability relationship between the sensor sensitivity and the hydrostatic pressure.

[0027] As a further technical solution, the overall diameter of the spiral pressure-resistant cavity, the width of the spiral channel, and the wall thickness of the side wall of the spiral cavity are determined according to the target hydrostatic pressure of the underwater acoustic sensor, ensuring that the external liquid infiltrates into the spiral channel through the water-permeable micropores and adheres to the side wall of the spiral air cavity layer by surface tension, rather than completely filling the F-P sealed cavity. The water and liquid in the spiral pressure-resistant cavity infiltrate into the spiral channel without injecting into the F-P cavity, which may cause the failure of static pressure balance or the change of the characteristics of the sensing film.

[0028] The beneficial effects of the present invention are as follows:

[0029] 1. Stable and highly sensitive underwater acoustic sensing. The sensing film adopts a central flat film, and multiple concentric rings are etched around it, forming a structure similar to a retractable spring, which can effectively eliminate the influence of the internal stress in the sensing film and the tension of the wet etching developer, realizing a large area and thin thickness of the sensing film without problems such as wrinkling and cracking like a large flat film. According to the theory of the silicon-based sensing film, the area of the film is positively correlated with the sensitivity, and the thickness of the film is inversely correlated with the sensitivity. Therefore, the thin and large sensing film structure can endow the underwater acoustic sensor with high sound pressure sensitivity characteristics. At the same time, due to the annular texture structure, the large-area sensing film releases the internal stress of the silicon-based thin film, making it have the characteristic of low internal stress. Therefore, its sensitivity is independent of the stress of the silicon-based thin film. When in a certain water depth environment, the deformation of the film caused by the hydrostatic pressure is mainly eliminated by the spring-shaped expansion and contraction of the annular texture, and the external stress is also mainly eliminated by the deformation of the ring texture. Therefore, as long as it is still in the elastic deformation process, the sensitivity of the central flat film of the sensing can be kept stable, different from the all-flat film sensing structure, where the hydrostatic pressure causes an increase in film deformation and accumulates a large amount of stress, thus affecting its sensitivity level;

[0030] 2. It has the performance of high hydrostatic pressure resistance. Conventional F-P interferometer type MEMS sensors use a sealed air cavity as a component of the F-P cavity. In the present invention, on the basis of the sealed cavity of the F-P interferometer, an air compression spiral structure is added. There are micropores in it that are connected to the external liquid environment and internally connected to the air cavity of the F-P interferometer. The spiral air cavity is designed as a fine air "runway". Due to the surface tension of the liquid, the air in the cavity is completely sealed into it. At the same time, due to the spiral cavity structure with a width of dozens of microns, there is a sufficiently large volume space. According to the first law of thermodynamics, the product of the air pressure P and the cavity volume V is a constant, that is, P·V = C. Therefore, when the water depth increases, the air in the cavity is compressed, then V↓, and the internal pressure P↑. Since the two sides of the sensing film are respectively the liquid environment and the air environment, and the two are connected through micropores, the internal air pressure and the external water pressure environment pressure are balanced. At this time, there is no unilateral hydrostatic pressure acting on the sensing film, so it can withstand a large hydrostatic pressure. The upper limit of its pressure resistance is that the air cavity is compressed into the F-P cavity and is infiltrated by the external liquid, resulting in the sealing failure, or the sensing film will no longer be able to maintain the sensitivity characteristics. According to the design of the air cavity structure size described above, in a micro-size structure, the static air pressure can be compressed to 3 MPa, and combined with the tensile resistance of the film itself, the sensor can have a hydrostatic pressure resistance ability higher than 3 MPa; through a larger size cavity design, more compressed air space can be provided, enabling the sensor to have a deeper hydrostatic pressure resistance ability.

[0031] 3. The process controllability of the sensor parameters is excellent. The assembly combination of the various structural components of the sensor is achieved through microstructures for fitting. The microstructures are processed by micro-nano etching technology with an accuracy of sub-micron or nano scale. For example, between the sensing film structure and the spiral air cavity structure, the bottom boss of the latter is embedded in the film surface cavity of the former to achieve radial fit and axial limit between the two. Another example is that the limiting and collimating structure processes the ceramic ferrule fixing groove by micro-nano etching method. The perpendicularity deviation of the groove wall can be within 5°, and the groove diameter error is at the micron level. The fiber ceramic ferrule and the limiting and collimating structure can maintain vertical and almost concentric assembly. At the same time, all consolidation structure surfaces adopt the polished surface of the silicon-based wafer to ensure their smoothness and parallelism, such as the consolidation surface between the sensing film structure and the spiral air cavity structure, the consolidation surface between the spiral air cavity structure and the limiting and collimating structure, etc. Finally, the parallelism between the polished fiber end face and the sensing film face is ensured, so that the interference light signal between the two F-P reflecting surfaces has a good signal-to-noise ratio. In addition, the optical fiber is restricted in the central hole of the high-precision ceramic ferrule. The position of the optical fiber in the radial direction is restricted, which can ensure the alignment between the reflecting end face of the optical fiber and the center of the sensing film, obtaining the maximum film sensing sensitivity. The axial position can be monitored and adjusted in real time to achieve the optimal visibility level and then consolidated. The molecular bonding process technology is used to achieve lossless consolidation between the silicon-based film surfaces, realizing the sealing of the upper and lower end faces of the compressed air cavity and ensuring the water pressure resistance performance of the sensor. The optical fiber thermal bonding technology realizes the consolidation between the ceramic ferrule and the silicon-based structure, and between the optical fiber and the ceramic, with almost the same thermal expansion coefficient, thus ensuring the consistency and stability of the optical parameters. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 Figure a is a schematic three-dimensional structure diagram of an existing F-P interferometer type underwater acoustic sensor.

[0033] Figure 1 Figure b is a schematic cross-sectional structure diagram of an existing F-P interferometer type underwater acoustic sensor.

[0034] Figure 2 Figure a is a schematic diagram showing the change of the interference spectrum of the F-P flat film sensor with the increase of hydrostatic pressure.

[0035] Figure 2 Figure b is a schematic diagram of the pressure-strain theoretical curves of the flat film and the corrugated film with hydrostatic pressure.

[0036] Figure 3 Figure a is a schematic three-dimensional structure diagram of the silicon-based corrugated film underwater acoustic sensor with a pressure-resistant cavity structure in the present invention.

[0037] Figure 3 Figure b is a schematic cross-sectional structure diagram of the silicon-based corrugated film underwater acoustic sensor with a pressure-resistant cavity structure in the present invention.

[0038] Figure 4 It is a schematic diagram of the structure of the sensing corrugated film layer in the present invention.

[0039] Figure 5 Schematic diagram of the cross-sectional structure of the sensing film layer in the present invention.

[0040] Figure 6 a is a schematic diagram of the front structure of the spiral air cavity layer in the present invention.

[0041] Figure 6 b is a schematic diagram of the back structure of the spiral air cavity layer in the present invention.

[0042] Figure 7 a is a schematic diagram of the cross-sectional structure of the optical fiber, the ceramic ferrule, and the collimation mounting layer after assembly in the present invention.

[0043] Figure 7 b is a schematic diagram of the structure of the optical fiber and the ceramic ferrule after assembly in the present invention.

[0044] Figure 7 c is a schematic structural diagram of the collimating mounting layer in the present invention.

[0045] Explanation of the accompanying symbols: sensitive diaphragm structure 1, ceramic ferrule 2, input optical fiber 3, FP sealed cavity 4, sensing pattern membrane layer 5, spiral air cavity layer 6, collimation mounting layer 7, FP sealed cavity 8, spiral pressure-resistant cavity 9, sensing smooth flat membrane 10, ring-shaped pattern membrane 11, sealing consolidation surface 12, ring pattern cross-sectional structure 13, ring pattern height 14, ring pattern width 15, adjacent ring pattern gap 16, center flat membrane radius 17, pattern membrane radius 18, spiral cavity side wall 19, spiral channel 20, water-permeable micropore 21, structural center hole 22, upper bottom surface 23, limiting boss 24, smooth bottom surface 25, ceramic ferrule outer wall 26, center through hole 27, polished end face 28, collimation structure inner wall 29, collimation layer center hole 30, collimation layer bottom surface 31. DETAILED DESCRIPTION

[0046] The present invention will be described in detail below with reference to the accompanying drawings:

[0047] Example: As shown in the attached Figure 3As shown in Figs. a - 7c, this high - pressure - resistant cavity - structured silicon - based patterned - film optical hydroacoustic sensor includes a sensitive diaphragm structure 1, a ceramic ferrule 2, an input optical fiber 3, an F - P sealed cavity 4, a sensing patterned - film layer 5, a spiral air cavity layer 6, a collimation installation layer 7, an F - P sealed cavity 8, a spiral - shaped high - pressure - resistant cavity 9, a sensing smooth planar film 10, an annular - shaped patterned film 11, a sealing and consolidation surface 12, an annular - pattern cross - section structure 13, an annular - pattern height 14, an annular - pattern width 15, an adjacent - annular - pattern gap 16, a central planar - film radius 17, a patterned - film radius 18, a spiral - cavity side wall 19, a spiral - shaped channel 20, a water - permeable micropore 21, a structure central hole 22, an upper bottom surface 23, a limiting boss 24, a smooth bottom surface 25, a ceramic - ferrule outer wall 26, a central through - hole 27, a polished end face 28, a collimation - structure inner wall 29, a collimation - layer central hole 30, and a collimation - layer bottom surface 31.

[0048] Reference appendix Figure 4 As shown in Figs., the sensing patterned - film layer 5 adopts a disc - shaped structure, with a depression provided at its center. A sensing smooth planar film 10 is arranged at the center of the depression, and an annular - shaped patterned film 11 is arranged on the periphery of the sensing smooth planar film 10. The surface of the sensing patterned - film layer 5 outside the annular - shaped patterned film 11 serves as the sealing and consolidation surface 12. Preferably, as Figure 5 shown, the diameter and thickness of the sensing smooth planar film 10 are determined according to the sensitivity target of the sensor. The annular - shaped patterned film 11 is formed by wet etching. The annular - pattern cross - section structure 13, annular - pattern height 14, annular - pattern width 15, adjacent - annular - pattern gap 16, central planar - film radius 17, and patterned - film radius 18 of the annular - shaped patterned film 11 are determined according to the stress - release process requirements during the wet - etching process and the stability relationship between the sensor sensitivity and hydrostatic pressure.

[0049] As Figure 3 shown in Figs. a, 3b, 6a, and 6b, the lower bottom surface of the spiral air cavity layer 6 is in close contact with and fixedly sealed to the sealing and consolidation surface 12 of the sensing patterned - film layer 5, thereby forming an F - P sealed cavity 8 at the depression. The upper bottom surface 23 of the spiral air cavity layer 6 is etched to form a spiral - shaped channel 20. Further, the lower bottom surface of the spiral air cavity layer 6 adopts a smooth bottom surface 25, and a limiting boss 24 is provided at the center of the lower bottom surface of the spiral air cavity layer 6. The limiting boss 24 is placed into the depression of the sensing patterned - film layer 5, and the limiting boss 24 does not contact the sensing smooth planar film 10 and the annular - shaped patterned film 11. Both the sensing patterned - film layer 5 and the spiral air cavity layer 6 are made of silicon - based materials. The smooth bottom surface 25 and the sealing and consolidation surface 12 are fixedly bonded together through a molecular - bonding process to achieve sealing. A structure central hole 22 is opened at the center of the spiral air cavity layer 6. The structure central hole 22 penetrates through the limiting boss 24, and the spiral - shaped channel 20 is communicated with the F - P sealed cavity 8 through the structure central hole 22. Preferably, parameters such as the overall diameter of the spiral - shaped high - pressure - resistant cavity 9, the width of the spiral - shaped channel 20, and the wall thickness of the spiral - cavity side wall 19 can be determined according to the target hydrostatic pressure of the hydroacoustic sensor.

[0050] As shown Figure 7 in FIGS. 7a, 7b, and 7c, a circular groove is formed on the collimation mounting layer 7, and a ceramic ferrule 2 (adopting a columnar structure) is inserted into the circular groove. The inner wall 29 of the collimation structure, i.e., the surface of the circular groove, is in close contact with and solidly bonded to the outer wall 26 of the ceramic ferrule to ensure that the axis of the ceramic ferrule 2 is perpendicular to the surface of the sensing smooth planar film 10. A central through hole 27 is formed in the center of the ceramic ferrule 2, and a collimation layer central hole 30 is formed in the center of the circular groove. The input optical fiber 3 sequentially passes through the central through hole 27 and the collimation layer central hole 30.

[0051] Furthermore, the bottom surface 31 of the collimation mounting layer 7 and the upper surface 23 are concentrically fitted and solidly sealed by molecular bonding. The bottom of the collimation mounting layer 7 is closely attached to and solidly sealed with the upper surface 23 of the spiral air cavity layer 6, so that a spiral pressure-resistant cavity 9 is formed in the spiral channel 20. As shown Figure 6 in FIGS. 6a and 6b, water-permeable micropores 21 communicating with the spiral channel 20 are formed on the side wall of the spiral air cavity layer 6, and the liquid flow can be introduced into the spiral pressure-resistant cavity 9.

[0052] As shown Figure 3 in FIGS. 3a, 7a, one end of the input optical fiber 3 is externally connected to an optical fiber spectral analysis device. The other end of the input optical fiber 3 sequentially passes through the ceramic ferrule 2 and the collimation mounting layer 7 and then extends into the F-P sealed cavity 8 of the spiral air cavity layer 6. Preferably, the end surface of the input optical fiber 3 extending into the F-P sealed cavity 8 is a polished end surface 28, and the sensing smooth planar film 10 and the polished end surface 28 form the reflection surfaces of the two paths of light of the F-P interferometer. The polished end surface 28 is located above the sensing smooth planar film 10, and the sensing smooth planar film 10 and the input optical fiber 3 cooperate to form an F-P interference cavity optical structure, which can obtain an optical interference signal containing the diaphragm vibration information, realize the conversion of acoustic-optical information, and the optical fiber spectral device observes the spectral characteristics of the F-P interferometer returned by the input optical fiber 3 in real time.

[0053] Furthermore, the following preferred solutions are adopted in this embodiment:

[0054] 1) High-sensitivity underwater acoustic induction solution

[0055] It is designed to adopt a corrugated film sensing layer structure to achieve high-sensitivity induction of underwater acoustic signals. This layer structure is as shown Figure 4As shown in the figure, the structure includes a smooth planar film structure, a ring pattern structure, and a solidified planar structure. The three parts of the structure respectively include different parameter designs and functional roles. The planar film structure forms the reflecting surfaces of the two paths of light of the F-P interferometer with the end face of the optical fiber. It has high requirements for surface finish and inclination. For example, the size of impurity points should be less than 1 μm, and the inclination is close to 2°. This is related to the reflectivity of the signal light, and thus affects the quality of the overall interference light signal. At the same time, the thickness T1 and the film diameter R1 of the planar film structure are directly related to the sensitivity of the underwater acoustic sensor. Therefore, to achieve the sensitivity requirements close to engineering practice, the designed film diameter needs to be large and the thickness needs to be thin. Usually, the thickness is close to 1 μm and the diameter is greater than 1 mm. In addition, according to different interference detection schemes, a high-reflection film can be coated on the film surface and the end face of the optical fiber.

[0056] A ring pattern is designed around the planar film. The cross-sectional structure is shown in Figure 5 , and the main design parameters of the ring pattern are the cross-sectional shape (rectangle, wavy line), the layer thickness T2, the ring diameter R2, and the spacing dR, etc. The design of the annular ring pattern can effectively reduce the influence of the internal stress of the film and the surface tension of the developer, thus ensuring that the central planar film is flat and smooth. It is an important stress release mechanism. The ring pattern can expand the diameter of the sub-micron-thick sensing film to several millimeters. At the same time, the ring pattern also affects the sound pressure sensing sensitivity characteristics of the film. As the hydrostatic pressure increases, the film almost maintains a stable sensitivity, while the smooth planar film shows a decreasing characteristic. The solidified plane forms a radial relative positioning and an axial sealed consolidation with the spiral hollow pressure-resistant structure to form a sealed F-P cavity. And its flatness also affects the parallelism between the end face of the optical fiber and the sensing film surface, making the internal light transmission performance stable. The sensing structure only needs to support the sensing ribbed film, so the overall thickness is designed to be more than 0.1 mm to support this structural design.

[0057] 2) Underwater acoustic sensor structure scheme

[0058] The underwater acoustic sensor works immersed in a liquid environment and needs to face water depth environments ranging from several meters to hundreds of meters, corresponding to hydrostatic pressures from several kPa to several MPa. If no measures are taken, a film with a diameter of 0.9 mm and a thickness of 2.5 microns will burst at pressures above 3 MPa. Therefore, existing patents use the method of covering a pressure-resistant outer shell for protection, which has high requirements for the outer shell material, thickness, acoustic matching performance, etc., and the packaging is complex. This achievement etches a spiral air cavity on a silicon wafer. The etched surface is a spiral structure with an upper opening, and micropores are designed on the side wall (see Figure 6), the air cavity as a whole is sealed and encapsulated through the upper and lower structural planes, and is connected to the FP interference cavity to form a compressible air cavity. The cavity is connected to the external liquid environment through the micropores on the side wall. When the structure is immersed in water or other liquids, due to the action of the internal air pressure, a closed air cavity structure with a water seal at the orifice is formed, and the pressures of the inner and outer cavities are equal. According to the first law of thermodynamics, the product of the air pressure P0 and the air volume V0 is a constant, then P0·V0=C. When the water depth of the sensor gradually increases and the hydrostatic pressure continues to increase, the air volume is compressed to V1. Due to the interconnection effect of the micropores on the side wall, the air pressure keeps increasing synchronously with the water pressure to P1, and the product of the two remains equal to the original, that is, P0·V0=P1·V1. Therefore, only a sufficient compressed air volume is required to reach the target water depth pressure. At this time, the air pressure inside the sensing diaphragm is equal to the water pressure outside, which ensures its pressure resistance and safety. Taking a diaphragm with a thickness of 0.5mm and an outer diameter of 5mm as an example, the groove wall thickness involves 30 microns and a gap of 50 microns. Combined with the FP cavity, the maximum air volume can be compressed nearly 30 times, and the air pressure can reach 30 atmospheres, or 3MPa. At this point, the sensor can withstand water depths of 300m. Combined with the diaphragm's own internal stress resistance to external pressure, the sensing membrane has sufficient pressure resistance engineering margin. A boss structure is designed on the bottom side of the spiral cavity structure to match the sensing structure cavity, limiting radial movement between the two structures.

[0059] 3) Optical underwater acoustic sensor assembly solution

[0060] The design adopts a collimating structure, such as Figure 6 As shown in a and 6b, the bottom surface is a smooth plane, which is used to consolidate with the upper surface of the spiral cavity structure opening to form a closed cavity structure; the upper bottom surface is a cylindrical groove structure that matches the ceramic ferrule, with the side wall vertically tilted less than 5° and the bottom horizontally tilted less than 5°. The groove and the ceramic ferrule form a small tolerance match to ensure stable low-angle installation. After removing the coating layer, the optical fiber is consolidated in the central through hole of the ceramic ferrule. The through hole has a weak positive tolerance compared to the optical fiber. The optical fiber end face serves as a reflective end face of the FP interferometer. The end face is made into a smooth mirror surface through grinding and polishing to facilitate light signal reflection. According to the specific detection scheme, the end face reflectivity can be improved by coating and other methods. According to the characteristics of the coupled interference light signal, that is, the consistency requirements of the sensor, the relative position between the optical fiber end face and the sensing film surface is adjusted by telescoping so that the interference light signal has good sine and cosine characteristics, see Figure 7 At this time, the optical fiber will pass through the center hole of the collimating structure and the center hole of the spiral cavity structure and be in a suspended state, so that the end face of the optical fiber maintains an appropriate distance from the sensing membrane surface.

[0061] Implementation process of the present invention:

[0062] 1. Design and implementation of various structural parameters of underwater acoustic sensors

[0063] The design of the underwater acoustic sensor includes the design of the overall performance and the design of each component structure. The overall performance design mainly includes the underwater acoustic sensor sensitivity, optical interference visibility and FP interferometer fineness, sensor pressure resistance, etc. According to the sensor sensitivity target, the diameter and thickness of the sensing film layer 5 are determined. The stress release process requirements during the wet etching process of the diaphragm and the stability relationship between sensitivity and hydrostatic pressure (see Figure 2 (b)) Determine the parameters of the ring pattern cross-sectional structure 13, ring pattern height 14, ring pattern width 15, adjacent ring pattern gap 16, center flat film radius 17 and pattern film radius 18 in the sensing film structure, and construct the pattern film sensing structure by wet etching.

[0064] Based on the basic requirements of photoelectric detection, such as detection visibility, demodulation sensitivity, dynamic range, etc., the reflectivity requirements of the ground end face 28 and the film surface of the sensing smooth plane film 10 are determined, so that the process methods such as grinding and polishing or metallization are selected to achieve the interference spectrum fineness and demodulation sensitivity level; by designing the distance and inclination between the ground end face 28 and the film surface of the sensing smooth plane film 10, the interference signal visibility target is achieved, and then the verticality requirements of the inner wall 29 of the collimation structure and the bottom surface 31 of the collimation layer are designed.

[0065] In order to achieve the goal of hydrostatic pressure resistance of the water-resistant acoustic sensor, the compression volume V0 is designed to be P1·V1 / P0, that is, the minimum allowable volume of the compressed air cavity. The height of V0 is limited by the thickness of the wafer diaphragm, and P0 is the atmospheric pressure, and P1 is the target hydrostatic pressure. Therefore, parameters such as the overall diameter of the spiral pressure-resistant cavity 9, the width of the spiral channel 20, and the wall thickness of the spiral cavity sidewall 19 are determined to ensure sufficient compressible cavity volume. Each silicon-based microstructure is formed by wet etching.

[0066] 2. Assembly and consolidation of various structures of the underwater acoustic sensor

[0067] When the MEMS processing technology completes the processing of each microstructure, the structures of the underwater acoustic sensor are Figure 3The complete structure is assembled from bottom to top. The limiting boss 24 of the spiral air cavity layer 6 is installed into the F-P sealed cavity 8 (recess) of the sensing smooth planar film 10, without contacting the annular pattern and the sensing planar film. At this time, the smooth bottom surface 25 of the spiral air cavity layer 6 is in close contact with the upper bottom surface (sealing consolidation surface 12) of the sensing corrugated film layer 5. By using the molecular bonding technology, the two silicon-based structures are completely consolidated together to achieve the sealing of the lower bottom surface of the spiral cavity, and the F-P sealed cavity 8 of the sensing film structure is communicated with the spiral pressure-resistant cavity 9 through the structural central hole 22. The spiral cavity structure is etched on the upper bottom surface 23 to form a spiral air "runway" (i.e., the spiral channel 20). The upper bottom surface 23 is an open structure, and the bottom surface 31 of the collimating layer of the collimating silicon-based structure is concentrically attached to the upper bottom surface 23 of the spiral cavity structure. Since they are both silicon-based materials, the upper bottom surface 23 of the spiral "runway" is sealed by the molecular bonding method. At this time, only the water-permeable micropores 21 on the side can allow the liquid to flow in. During assembly, the optical fiber is not installed in the central hole 27 of the ceramic ferrule first. The ceramic ferrule 2 is directly inserted into the limiting groove (recess) of the collimating installation layer 7 and consolidated by the thermosetting adhesive with a low coefficient of thermal expansion, keeping the outer wall 26 of the ceramic ferrule of the ceramic ferrule 2 in close contact with the inner wall 29 of the collimating structure, so as to ensure perpendicularity to the horizontal sensing smooth planar film 10. Then, the tail end of the transmission optical fiber 3 is connected to the optical fiber spectral analysis device. After removing the outer coating of the transmission optical fiber, it is inserted into the central through hole 27 of the ceramic ferrule. The end for the F-P end face is polished smoothly to form a polished end face 28. The optical fiber is introduced into the cavity of the sensing film structure (F-P sealed cavity 8) through the central hole 30 of the collimating layer of the silicon-based collimating structure and the structural central hole 22 of the spiral air cavity. The spectral characteristics of the F-P interferometer returned by the optical fiber are observed in real time through the optical fiber spectral device, including the reflectivity, visibility, the number of waveform fluctuations, etc. According to the set target of the interference spectral characteristic parameters and the target of the sensor consistency deviation, the optical fiber and the ceramic ferrule 2 are consolidated by the thermosetting adhesive, and then an F-P interferometer underwater acoustic sensor with a specific cavity length is formed. Its assembled complete structure is as shown in Figure 3 shown.

[0068] The present invention designs a pressure-resistant cavity structure silicon-based grooved film optical hydroacoustic sensor, which includes components such as a silicon-based sensing grooved film, a pressure-resistant spiral cavity, an optical fiber collimation layer, a ceramic ferrule, and a transmission optical fiber. Using the MEMS etching process, a circular pattern structure is designed on the silicon-based diaphragm to release the etching stress of the thin film, which can ensure the realization of a large-sized circular thin film with a millimeter scale and a sub-micron scale thickness, ensuring the high-sensitivity sensing structure of the sensor; a spiral cavity structure is designed and encapsulated between the optical fiber collimation layer and the sensing diaphragm layer, so that there is an internal cavity with space compression, and the pressures at both ends of the sensing diaphragm are balanced, realizing the improvement of hydrostatic pressure resistance while maintaining the sensitivity; an optical fiber collimation layer is designed, which ensures the perpendicularity of the optical fiber ferrule to the sensing surface. Combining with the polishing of the optical fiber end face, the parallelism between the optical fiber end face and the sensing thin film surface can be realized, and it has high-quality interference light performance. The hydroacoustic sensor designed by this intellectual achievement has a small overall size and high-sensitivity performance, and can support applications in thin and light, and slender hydroacoustic devices, greatly reducing the size and weight of the load.

[0069] It can be understood that for those skilled in the art, equivalent replacement or change of the technical solutions and inventive concepts of the present invention should fall within the protection scope of the appended claims of the present invention.

Claims

1. A silicon-based corrugated film optical underwater acoustic sensor with a pressure-resistant cavity structure, characterized in that, Comprising: A sensing striated film layer (5), which adopts a disc-shaped structure, has a depression at its center, a sensing smooth planar film (10) is arranged at the center of the depression, a ring-shaped striated film (11) is arranged on the periphery of the sensing smooth planar film (10), and the surface of the sensing striated film layer (5) outside the ring-shaped striated film (11) serves as a sealing and consolidation surface (12); A spiral air cavity layer (6), whose lower bottom surface is in close contact with and consolidated and sealed to the sealing and consolidation surface (12) of the sensing striated film layer (5), thereby forming an F-P sealed cavity (8) at the depression, and the upper bottom surface (23) of the spiral air cavity layer (6) is etched to form a spiral channel (20); A ceramic ferrule (2), which adopts a columnar structure; A collimation installation layer (7), the lower part of the ceramic ferrule (2) is embedded in the collimation structure layer (7), and the ceramic ferrule (2) is consolidated with the collimation installation layer (7). The bottom of the collimation installation layer (7) is closely attached to and consolidated and sealed with the upper bottom surface (23) of the spiral air cavity layer (6), so that a spiral pressure-resistant cavity (9) is formed in the spiral channel (20). Water-permeable micropores (21) communicating with the spiral channel (20) are opened on the side wall of the spiral air cavity layer (6) for introducing a liquid flow into the spiral pressure-resistant cavity (9); and An input optical fiber (3), one end of which is connected to an optical fiber spectral analysis device, and the other end sequentially passes through the ceramic ferrule (2), the collimation installation layer (7) and the spiral air cavity layer (6) and then extends into the F-P sealed cavity (8) and is located above the sensing smooth planar film (10). The sensing smooth planar film (10) and the input optical fiber (3) cooperate to form an F-P interference cavity optical structure to realize acousto-optic information conversion, and the optical fiber spectral device observes the spectral characteristics of the F-P interferometer returned by the input optical fiber (3) in real time.

2. The pressure-resistant cavity structure silicon-based striped film optical underwater acoustic sensor according to claim 1, characterized in that: A limiting boss (24) is arranged at the center of the lower bottom surface of the spiral air cavity layer (6) for being placed in the depression of the sensing striated film layer (5), and the limiting boss (24) does not contact the sensing smooth planar film (10) and the ring-shaped striated film (11).

3. The pressure-resistant cavity-structured silicon-based corrugated film optical hydroacoustic sensor according to claim 2, wherein: The lower bottom surface of the spiral air cavity layer (6) adopts a smooth bottom surface (25). Both the sensing striated film layer (5) and the spiral air cavity layer (6) are made of silicon-based materials. The smooth bottom surface (25) and the sealing and consolidation surface (12) are consolidated together through a molecular bonding process to achieve sealing.

4. The pressure-resistant cavity structure silicon-based corrugated film optical hydroacoustic sensor according to claim 3, wherein: A structure center hole (22) is opened at the center of the spiral air cavity layer (6). The structure center hole (22) penetrates through the limiting boss (24), and the spiral channel (20) and the F-P sealed cavity (8) are communicated through the structure center hole (22).

5. The pressure-resistant cavity structure silicon-based corrugated film optical hydroacoustic sensor according to claim 1, wherein: The end surface of the input optical fiber (3) extending into the F-P sealed cavity (8) is a polished end surface (28), and the sensing smooth planar film (10) and the polished end surface (28) form the reflection surfaces of the two beams of light of the F-P interferometer.

6. The pressure-resistant cavity structure silicon-based striped film optical underwater acoustic sensor according to claim 1, wherein: A circular groove is opened on the collimation installation layer (7) for placing the ceramic ferrule (2), and the surface of the circular groove, i.e., the inner wall (29) of the collimation structure, is in close contact with and consolidated with the outer wall (26) of the ceramic ferrule to ensure that the axis of the ceramic ferrule (2) is perpendicular to the surface of the sensing smooth planar film (10).

7. The pressure-resistant cavity structure silicon-based corrugated film optical underwater acoustic sensor according to claim 6, characterized in that: A central through hole (27) is provided in the center of the ceramic ferrule (2), and a collimation layer central hole (30) is provided in the center of the circular groove. The input optical fiber (3) sequentially passes through the central through hole (27) and the collimation layer central hole (30).

8. The pressure-resistant cavity structure silicon-based striped film optical hydroacoustic sensor according to claim 1, wherein: The bottom surface (31) of the collimation layer at the bottom of the collimation mounting layer (7) is concentrically attached to the upper surface (23) of the spiral air cavity layer (6), and consolidation sealing is achieved through molecular bonding.

9. The pressure-resistant cavity structure silicon-based corrugated film optical underwater acoustic sensor according to claim 1, wherein: The diameter and thickness of the sensing smooth planar film (10) are determined according to the sensor sensitivity target; the ring-shaped corrugated film (11) is formed by wet etching. The cross-sectional structure (13) of the annular corrugation, the corrugation height (14), the corrugation width (15), the adjacent corrugation gap (16), the radius of the central planar film (17), and the radius of the corrugated film (18) are determined according to the stress release process requirements during wet etching and the stability relationship between the sensor sensitivity and the hydrostatic pressure.

10. The pressure-resistant cavity structure silicon-based corrugated film optical underwater acoustic sensor according to claim 1, characterized in that: The overall diameter of the spiral pressure-resistant cavity (9), the width of the spiral channel (20), and the wall thickness of the spiral cavity side wall (19) are determined according to the target hydrostatic pressure of the underwater acoustic sensor, ensuring that the external liquid infiltrates into the spiral channel (20) through the water-permeable micropores (21) and adheres to the side wall of the spiral air cavity layer (6) by surface tension, rather than completely filling the F-P sealed cavity (8).

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