Self-interference optical fiber sound sensing unit, sensing device and sensing detection system

Abstract

The invention discloses a self-interference optical fiber acoustic sensor unit, a self-interference optical fiber acoustic sensor device and a self-interference optical fiber acoustic sensor detection system. The sensing optical fiber is spirally wound on the base body, the axial length of the sensing optical fiber wound on the base body is 90-110 cm, and the winding diameter is 4-6 m. According to the self-interference optical fiber acoustic sensing unit provided by the invention, the acoustic signal can be detected only by winding the optical fiber meeting the specific required length and winding radius on the base body, and the self-interference optical fiber acoustic sensing unit is simple in structure, low in cost and easy to process. Through spiral winding, the contact area between the optical fiber and a strain signal is increased, so that the strain transmission efficiency is improved. In addition, the sensitivity can be further improved by increasing the number of winding turns. The spiral structure formed by winding the optical fiber has a remarkable effect of improving the sensing sensitivity of the self-interference optical fiber.

Description

Technical Field

[0001] This invention relates to a self-interfering fiber optic acoustic sensing device based on a layered stretchable structure, belonging to the field of fiber optic sensing technology. The layered stretchable structure enables flexible application in various monitoring environments. The core innovation lies in enhancing the sensitivity of acoustic signal detection through a combination of a layered stretchable skeleton and a self-interfering fiber optic ring. Simultaneously, the integration of a stretchable multi-layered structure on the same sensor achieves integrated multi-channel monitoring and precise damage location of a single sensor. This device can be applied to complex sensing scenarios such as underwater acoustic detection, structural health monitoring, industrial noise detection, and underground rock formations. Background Technology

[0002] Currently, traditional acoustic sensor technology often faces problems such as strong electromagnetic interference and signal susceptibility to environmental disturbances in fields such as oil exploration, underwater listening, and structural health monitoring. While existing fiber optic acoustic sensors have solved some of these problems, they still suffer from drawbacks such as complex manufacturing processes, high costs, and complex demodulation systems. Phase interferometric acoustic sensors with MZI, MI, and SI structures all require sensing fiber arms and reference fiber arms, which are then coupled together by a coupler for interference, resulting in high system costs. FPI type sensors require a diaphragm and fiber end face to form a cavity, making them unsuitable for complex and harsh environments and requiring complex manufacturing processes. Furthermore, traditional point-type fiber optic acoustic sensors, with their single-mode, multi-mode, and single-mode structures, and intensity-type point sensors that typically use diaphragm structures, rely on the diaphragm to receive external acoustic signals, causing vibrations that drive the fiber optic cable for monitoring. This approach struggles to detect acoustic emission signals over a wide frequency range, and cannot achieve telescopic adjustment or precise spatial damage location on a single sensor. Different sensors and multiple deployment points are required for different detection environments, leading to poor adaptability and making it difficult to meet the needs of locating and monitoring damage such as cracks in industrial metals and rocks. Summary of the Invention

[0003] The technical problem to be solved by the present invention is to provide a self-interference fiber optic acoustic sensor unit, device and detection system for acoustic signal detection that is simple in structure, low in cost and easy to manufacture.

[0004] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows: This invention first provides A self-interference fiber optic acoustic sensing unit includes a sensing fiber, the sensing fiber including a helical segment in the elongation direction, the axial length of the helical segment being 80-160cm, and the cross-sectional diameter of the helical segment being 2-6m.

[0005] The axial length of the helical segment is 90-110cm, and the cross-sectional diameter of the helical segment is 4-6m.

[0006] It also includes a cylindrical substrate, on which the helical segment of the sensing optical fiber is wound.

[0007] The present invention also provides a self-interference fiber optic acoustic sensing device, comprising at least two self-interference fiber optic acoustic sensing units; the at least two self-interference fiber optic acoustic sensing units are nested together in a layered telescopic structure.

[0008] The present invention also provides an optical fiber acoustic sensing detection system, comprising: DFB high-power pump source, used to output high-power optical signals; The optical splitter, connected to the DFB high-power pump source, splits the high-power optical signal into multiple paths; Self-interference fiber optic acoustic sensing unit or self-interference fiber optic acoustic sensing device; connected to the output end of the beam splitter, converting external acoustic signals into interference optical signals carrying acoustic signal information; A photodetector, connected to the self-interference fiber optic acoustic sensing unit or the self-interference fiber optic acoustic sensing device, converts the interference optical signal into an electrical signal. The data acquisition module is connected to the photodetector to acquire the electrical signals converted by the photodetector; as well as The signal analysis and processing module is connected to the data acquisition module. It analyzes and processes the data acquired by the data acquisition module and demodulates the characteristic information of the acoustic signal under test and the spatial location information of the damage based on the spatiotemporal differences of the electrical signals of each channel.

[0009] Compared with the prior art, the beneficial effects of the present invention are: 1. The self-interference fiber optic acoustic sensing unit provided by the present invention can detect acoustic signals simply by winding an optical fiber that meets specific requirements for length and winding radius onto a substrate. It has a simple structure, low cost, and is easy to process.

[0010] 2. The self-interference fiber optic acoustic sensing unit provided by this invention increases the contact area between the optical fiber and the strain signal through helical winding, thereby improving the strain transmission efficiency. Furthermore, increasing the number of winding turns can further improve sensitivity. The helical structure formed by optical fiber winding significantly enhances the sensitivity of the self-interference fiber optic sensing.

[0011] 3. The self-interference fiber optic acoustic sensing unit provided by the present invention uses a high-damping material as its substrate. The high-damping material suppresses mechanical resonance, and its frequency response range enables the sensor to have a good response frequency in both low-frequency and high-frequency ultrasonic bands.

[0012] 4. This invention provides a layered telescopic structure self-interference fiber optic acoustic sensing device, used to realize multi-channel measurement of damage signals in different working scenarios using a single instrument. Through the interlocking arrangement of multiple telescopic units, the telescopic units are made of rigid materials. The acoustic signal is transmitted to the sidewall through the solid structure. Combined with the design of self-interference fiber optics, the optical fiber is used as the acoustic signal sensing and receiving device. At the same time, the telescopic unit directly contacts the signal transmitted from the vibration sound source, and the acoustic signal is received by the attached fiber optic ring. The skeleton sensing unit combined with the fiber optic ring forms a structural sensitivity enhancement design. The optical fiber is wound on each layer of telescopic unit according to the engraved pitch to avoid direct mechanical contact between optical fibers and reduce lateral coupling / crosstalk, resulting in a purer response to sound pressure and better linearity. Attached Figure Description

[0013] Figure 1 This is a schematic diagram of the structure of the self-interference fiber optic acoustic sensing unit of the present invention; Figure 2 This is a schematic diagram of the structure of a sensing optical fiber; Figure 3 yes Figure 2 Mid-side approach; Figure 4 This is a structural diagram of the sensing fiber; Figure 5 The image shows the test results for the fiber optic winding length. Figure 6 The image shows the test results for the fiber winding diameter. Figure 7 This is a three-dimensional schematic diagram of the overall assembly of the self-interfering fiber optic acoustic sensing device of the present invention. Figure 8 This is an exploded view of the device described in this invention, showing the disassembled structure of the four-layer telescopic unit; Figure 9 This is a schematic diagram of the top end cap structure of the device described in this invention; Figure 10 This is a side cross-sectional view of the device described in this invention, clearly showing the four-layer nested structure, fiber optic trench, and internal channel layout; Figure 11 This is a three-dimensional cross-sectional view of the device described in this invention, showing the winding path and penetration method of the optical fiber in each layer of the column; Figure 12 This is a connection diagram of the fiber optic acoustic sensing detection system of the present invention. Detailed Implementation

[0014] The present invention will be described in detail below with reference to the accompanying drawings, but this should not be construed as limiting the scope of protection of the present invention.

[0015] Example 1 This embodiment provides a self-interference fiber optic acoustic sensing unit, such as Figure 1 , Figure 2 and Figure 3 As shown, it includes a substrate 100 and a sensing fiber 200. The sensing fiber 200 is spirally wound around the substrate 100 and includes a wound section 1000, an entry section 1001, and an exit section 1002. The axial length L of the wound section 1000 of the sensing fiber 200 on the substrate 100 is 90-110 cm, and the winding diameter R is 4-6 m. The sensing fiber 200 adopts a core-diameter mismatched self-interference structure and is formed by fusion splicing single-mode fiber 201 and multimode fiber 202. See [reference needed]. Figure 4 .

[0016] The substrate 100 is not necessary and can be omitted. The substrate 100 simply provides a fixing carrier for the sensing fiber 200, making it convenient to fix the sensing fiber 200 at the test target location.

[0017] The optical fiber used in the core-diameter mismatch self-interference structure can be a step-index multimode fiber, a multi-clad fiber, a coreless fiber, a hollow-core fiber, a few-mode fiber, a tapered fiber, a multi-core fiber, or a photonic crystal fiber.

[0018] The 200 sensing fiber uses quartz optical fiber, which has the characteristics of corrosion resistance, electromagnetic interference resistance, and high temperature resistance (up to 800℃), making it suitable for harsh environments such as chemical and nuclear power plants.

[0019] The axial length L and winding diameter R of the sensing fiber 200 wound on the substrate 100 are obtained based on the following theories and experiments: The sensing fiber 200 is wound around the substrate 100. The core purpose is to significantly increase the effective length of the interaction between the sensing fiber 200 and the sound field within a limited space, thereby improving the sensor's sensitivity. Since phase changes occur throughout the multimode fiber due to intermodal mismatch caused by the transmission of single-mode light, a fiber ring is used to increase the area of ​​the sound field acting on the multimode fiber. When the sound wave passes through the substrate 100 and acts on the sensing fiber 200, it changes the refractive index of the sensing fiber 200 through the photoelastic effect, and also physically perturbs the length of the sensing fiber 200. Both of these effects change the optical phase of the light passing through this segment of the sensing fiber 200. The specific synergistic sensing principle is as follows: The optical phase φ inside the sensing fiber is represented by: (1) In the formula, β represents the propagation constant of the sensing fiber, and L represents the length of the sensing fiber along the sidewall. The phase change of the fiber signal within the acoustic sensor is expressed as: (2) in, φ1 represents the phase change caused by axial stretching of the optical fiber. φ2 represents the phase change caused by the change in fiber diameter. φ1 and φ2 is defined by equations (3) and (4) respectively: (3) (4) In the formula, L1 represents the change in fiber length directly affected by sound waves, which is related to the sound wave intensity P. L2 represents the change in fiber length due to matrix deformation; N represents the refractive index of the fiber. D represents the change in fiber diameter; P is the pressure of the acoustic emission signal. The change in fiber diameter is typically caused by external acoustic interference. D is very small, approximately zero. Accordingly, φ2 can be ignored. L1 is represented as: (5) In the formula, ε is the strain, E is the Young's modulus of the optical fiber, and ν represents the Poisson's ratio of the optical fiber.

[0020] The coordinate system is established with the center of the bottom surface of the flexible rubber pad structure of the expansion unit as the origin. The X and Y directions lie in the plane of the bottom surface, while the Z direction is perpendicular to this plane. According to elasticity theory, ΔL2 can be expressed as: (6) The optical fiber is firmly fixed to the substrate. Since the diameter of the optical fiber is significantly smaller than the diameter of the substrate, s can be considered as the length variation of an infinitesimally small gap in the optical fiber, where s represents the length variation of an infinitesimally small segment at the edge of the substrate. x Indicates the change in length along X, s y Indicates the change in length along the Y-axis, s z This indicates the change in length along the Z direction.

[0021] (7) In the formula, ν m E is the Poisson's ratio of the matrix. m Let r be the Young's modulus of the matrix, and r be the distance between the initial position of the acoustic emission wave and the infinitesimal segment. Substituting equation (7) into equation (6) yields equation (8): (8) Substituting equations (3), (4), (5), and (8) into equation (2) yields the overall phase change: (9) From equation (9), we can see that The value of φ is related to P, E, and Em It is related to L and g. φ is related to the acoustic wave intensity, the substrate material structure, and the length of the sensing fiber loop. Therefore, the working principle of the skeleton-type ultrasonic sensor formed by winding optical fiber around the substrate in this invention differs from that of a skeletonless ultrasonic sensor. The substrate is not only a simple sound component but also crucial in receiving acoustic emission signals. Without it, the fiber optic ultrasonic sensor cannot function on solid surfaces.

[0022] As can be seen from the above principle section, the phase change of the sensing fiber is related to the length of the fiber. Therefore, it is necessary to determine the winding radius and winding length of the fiber.

[0023] The applicant first conducted an experiment to investigate the length of the bare fiber ring. Using the same laser source as a fixed light source with a power of 20mW, multimode fiber was fabricated into layered fiber rings with a uniform diameter of 4cm. Wideband frequency sweep tests were then performed. By comparing signal strength, and using the same acoustic transmitter with a step parameter of 10kHz, the optimal length of the wound multimode fiber was determined. The applicant conducted multiple sets of experiments, and obtained the optimal solution by arithmetic averaging the test data from multiple sets to eliminate errors caused by single sets of data. The effect of signal strength variation with fiber length is as follows: Figure 4 As shown, all fiber optic loops exhibited significant resonance peaks in the 50-300 kHz frequency sweep test. The 90 cm, 100 cm, and 110 cm fiber optic loops showed significantly better peak intensities at each frequency than other lengths, indicating excellent sensing sensitivity. With increasing fiber length, the main peak intensity gradually decreased, and the peak position shifted towards higher frequencies, while the resonance peak became flatter. This suggests that increasing fiber length weakens the system's selective response to specific frequencies. This is mainly due to the increased sensor length and number of winding turns. Although theoretically increasing the monitoring area coverage should improve sensitivity, the increased winding radius due to the longer fiber optic loop leads to increased bending loss and a relative decrease in transmitted optical power, thus affecting sensor sensitivity. The rising peaks are due to negligible fluctuations in acoustic signal intensity caused by reaching the resonant frequency of the diaphragm inside the acoustic transmitter during the frequency sweep. Based on this, a fiber length of 90-110 cm was determined, with 100 cm showing the best performance.

[0024] Given a fixed fiber length, fiber loops of different diameters were fabricated and tested.

[0025] With a fixed fiber optic ring length, fiber optic rings were fabricated and tested using 100cm fiber with diameters of 2cm, 3cm, 4cm, 5cm, and 6cm. The results are as follows. Figure 6As shown in the figure, the frequency response characteristics of sensing optical fibers with diameters of 2 cm, 3 cm, 4 cm, 5 cm, and 6 cm in the 50–300 kHz frequency band are presented. Overall, it can be seen that the diameter of the fiber loop has a significant impact on the output performance. When the diameter is too small, it is difficult to detect ultrasonic signals, resulting in low sensitivity. Increasing the diameter to 4-5 cm improves the overall signal strength across the entire frequency band, with a significant increase in peak amplitude. Further increasing the diameter leads to a further decrease in the sensor's monitored signal strength. Therefore, the parameters of the sensing optical fiber can be set to a multimode fiber loop with a diameter of approximately 4-5 cm and a length of 100 cm. Similarly, it can be determined that in a layered telescopic structure, a skeleton radius design with an increasing diameter of 4-6 cm achieves a relatively good sensing response.

[0026] In one embodiment, the screenshot of the base 1 is circular.

[0027] In one embodiment, a spiral groove is provided on the surface of a circular substrate 1, and the sensing optical fiber 2 is wound inside the spiral groove.

[0028] Example 2 This embodiment provides a self-interference fiber optic acoustic sensing device, including at least two self-interference fiber optic acoustic sensing units provided in Embodiment 1; the at least two self-interference fiber optic acoustic sensing units are nested together in a layered telescopic structure.

[0029] In one embodiment, the self-interfering fiber optic acoustic sensing device has four layers of telescopic units, such as... Figure 7 As shown, the layered telescopic structure multi-sensor fiber optic integrated sensor is a four-layer coaxial nested cylindrical structure. From bottom to top, it consists of a soft rubber base 1, an outermost telescopic unit 2, a second telescopic unit 3, a third telescopic unit 4, and an innermost telescopic unit 5. The top is fixed and sealed by a top end cap 6. The device has an overall stepped cylindrical shape, with the diameter of each telescopic unit gradually decreasing from the outside to the inside. The outer wall of each telescopic unit is wrapped with a protective shell layer 9, which provides mechanical protection and environmental isolation for the internal structure and optical fiber. The soft rubber base 1 at the bottom is in close contact with the monitoring medium, efficiently receiving external acoustic emission signals through a solid conduction mechanism and stably transmitting them to each telescopic unit.

[0030] like Figure 8As shown, the outermost telescopic unit 2 is a cylindrical structure with layered helical threads 10 only on its inner wall, without any layered helical threads 10 on its outer side wall; the second and third telescopic units 3 and 4 are cylindrical structures with layered helical threads 10 on both their inner and outer sides; the innermost telescopic unit 5 is a solid cylindrical structure with layered helical threads 10 only on its outer side wall. Adjacent telescopic units engage and transmit power through the layered helical threads 10. Specifically, the inner walls of the outermost telescopic unit 2, the inner and outer walls of the second telescopic unit 3, and the inner and outer walls of the third telescopic unit 4 are all machined with continuous, equal-pitch layered helical threads 10. The innermost telescopic unit only has layered helical threads 10 on its side wall. The axial expansion and contraction of each layer can be achieved by rotating any telescopic unit through the structural fixing holes 7, thereby adjusting the spatial monitoring position of each layer of fiber optic rings.

[0031] Each telescopic unit is separated by a protective shell layer 9, which is smooth and threadless. This layer provides structural support between layers while preventing the optical fiber from experiencing additional bending and torsional stress during telescopic movement, effectively protecting the optical fiber rings within the fiber trench from potential mechanical damage during thread twisting. Fiber trenches 11 are formed in a spiral groove on the sidewalls of each telescopic unit, providing a stable winding and fixing path for the optical fiber. When an acoustic signal is transmitted into the solid structure, it causes optical disturbance in the optical fiber within the fiber trench 11, resulting in changes in the light intensity of the multimode fiber and providing a signal basis for sensing and detection.

[0032] like Figure 9 and Figure 10 As shown, each telescopic unit has fiber optic trenches 11 on its sidewalls, the depth and width of which match the diameter of the optical fiber to ensure that the fiber optic cable fits tightly within the trench and does not slip. The two ends of the fiber optic trenches 11 are connected to the top of the telescopic unit through pre-reserved trench channels 12 inside the fiber optic column, forming a continuous path. The pre-reserved trench channels 12 inside the fiber optic column and the fiber optic trenches 11 together constitute the fiber optic cable insertion path, ensuring the continuity of the fiber optic cabling.

[0033] Four large-core multimode optical fibers enter the device through the fiber outlet 8 of the top end cap 6 of each telescopic unit. They spirally wind around the sensing unit along the fiber grooves 11 of each layer to form fiber loops, then pass through the pre-reserved groove channels 12 inside each fiber column, threading between the telescopic units. Finally, they exit from another pre-reserved fiber outlet 8 of each telescopic unit and are fused to single-mode armored patch cords outside the device, thus forming a single-mode-multimode-single-mode (SMS) core diameter mismatch self-interference optical path in each telescopic structure. The number of fiber loops can be flexibly adjusted according to the sensing sensitivity requirements; more loops result in higher strain transfer efficiency and stronger sensing sensitivity. This device can meet the monitoring needs of different scenarios.

[0034] like Figure 8As shown, the top end cap 6 adopts a multi-layer concentric ring structure design, which contains four concentric rings from the inside to the outside, corresponding to the top end faces of the innermost telescopic unit 5, the third telescopic unit 4, the second telescopic unit 3 and the outermost telescopic unit 2, forming a stepped concentric layout to ensure precise fit with the top end face of each telescopic unit.

[0035] The four-layer telescopic unit is made of rigid, high-damping material. It needs to meet the requirement that the damping factor in the material parameters of the matrix is ​​greater than 0.1, which can suppress mechanical resonance and thus achieve a wide-band ultrasonic response from low frequency to high frequency. In addition, each layer of telescopic unit can extend and retract independently, so that each layer of fiber optic ring forms different spatial monitoring points, which can form differentiated responses to the acoustic signals generated by damage, thereby providing a basis for spatial characteristics for damage localization.

[0036] The sensitivity of the fiber optic sensing structure can be adjusted by increasing the number of turns of the fiber in each layer of trenches to form various fiber loops. The self-interfering fiber loops inside the layered structure, wound around the skeleton expansion unit, have a sensitizing effect. The sensing distances of each layer of fiber loops to the damage location are different. There are spatiotemporal differences in the optical disturbances caused by the damage signal from the same sound source. By analyzing these differences, the damage location can be accurately pinpointed.

[0037] The top end cap 6 has evenly distributed structural fixing holes 7 along its circumference, serving a dual function: firstly, for inserting bolts to tightly connect the top end cap to each layer of telescopic units, ensuring the coaxiality of each layer during telescopic movement and preventing additional strain on the optical fiber due to structural misalignment; secondly, facilitating the rotation and twisting of the telescopic structure, allowing operators to easily adjust the axial position of each layer of telescopic units. After the multimode optical fiber is inserted and led out from the optical fiber outlet 8, it is fused with the external single-mode armored optical fiber to form an SMS structure. The armor protection effectively improves the mechanical strength of the optical fiber, preventing bending or breakage at the outlet.

[0038] This device achieves axial expansion and contraction by rotating the various expansion units. Utilizing the pitch characteristics of the layered helical thread 10, the number of rotations has a strictly linear relationship with the amount of axial expansion and contraction, thus enabling precise control of the expansion and contraction. Rotating the outermost expansion unit 2 allows for overall adjustment of the device's outer diameter to accommodate different installation spaces. Rotating the second, third, and innermost expansion units 3 and 4 allows for independent adjustment of the spatial position of each layer, thereby changing the spatial monitoring point positions of each fiber optic ring to meet different monitoring distance requirements.

[0039] Principles of acoustic sensing and damage localization: The telescopic structure design allows for complex spatial analysis of signals, and the telescopic structure can be fitted into different working environments to meet the monitoring needs of different equipment, thus enabling its widespread application in various damage monitoring applications.

[0040] During monitoring, the layered expansion and contraction of the sensors causes the damage signal to vary in spatial location across each layer. A quantitative correlation algorithm is established between the damage acoustic signal, the time difference of light intensity change, and the spatial location of the damage, enabling precise damage localization. The four-layer unit inherently possesses radial differences and axial increments due to axial expansion and contraction, placing each fiber optic ring at a different sensing position. Therefore, the distance, time, and strain response of the damage acoustic signal propagating radially to each layer all exhibit quantifiable differences. The core formula algorithm is as follows: The four-layer unit has an inherent radial radius R after processing. i0 (Inherent initial radial radius of the i-th fiber ring) i=1,2,3,4 (1 = outermost layer, 4 = innermost layer), processing calibration (four layers R) 10 >R 20 >R 30 >R 40 (The natural radial difference is a fixed value) Rotating the helical thread achieves axial expansion and contraction, that is, it generates an axial increment Δ. H i The final actual radial sensing radius is the inherent value plus the radial increase. p The pitch of the interlayer helical thread is a fixed value (equal pitch) for machining design. n i For the first i The number of rotations of the layer telescopic unit.

[0041] No. i Axial expansion / contraction of layer expansion unit Δ H i The calculation formula is as follows: (10) No. i Distance from the fiber optic loop to the sensor reference surface D i for: (11) H i0 For the first i The initial spatial distance (processing calibration value) from the fiber optic loop to the reference plane, D i This represents the actual spatial position after axial expansion and contraction, and is the core spatial parameter for positioning.

[0042] No. i / j Actual spatial spacing difference of fiber optic rings d ij As shown in the following formula: (12) The interlayer space difference, determined by axial expansion and contraction, is the difference in the propagation distance of the acoustic signal to the two layers of sensing fiber optic rings, and is the physical source of the time difference and strain difference.

[0043] No. i The rate of change of optical intensity in the fiber ring is η i It is dimensionless and only reflects the degree of micro-strain in the optical fiber, as expressed by the following formula: (13) in I i0 The first data acquired by the four-channel detector under non-destructive conditions i The light intensity transmitted through the fiber optic ring of the layer, Δ I i The change in light intensity of the i-th layer transmitted through the optical fiber after monitoring the acoustic signal generated by the damage.

[0044] No. i The micro-strain of the fiber optic ring is denoted as ε i It does not cause fiber stretching or contraction; only the solid-borne acoustic signal causes a change in the refractive index of the fiber in the groove on the sidewall, resulting in a change in light intensity. In the following formulas... ε i The data values ​​that can be calculated during actual monitoring can be expressed as: (14) Where N i Representing the i The number of turns of the fiber optic loop spiral winding is a design value; increasing the number of turns can enhance the sensor's sensitivity. l 0 represents the length of a single fiber loop. d f The diameter of the optical fiber. β The more turns the strain transfer efficiency increases, the closer the strain transfer efficiency is to 1. Since the number of turns of the fiber optic ring is relatively large, it is uniformly taken as 1 in the calculation.

[0045] Therefore, the first i / j The formula for calculating the micro-strain difference of the fiber optic ring can be expressed as: (15) Micro-strain difference is determined by the spatial spacing of each layer d ij Together with the distance S (to be determined) from the damage source to the sensor, it forms the core basis for subsequent positioning calculations.

[0046] Next, the physical model is used to quantify the relationship between microstrain and spatial distance of damage source. The damage acoustic signal propagates in the solid medium in the form of spherical wave. The acoustic power decreases with the square of the propagation distance, which in turn causes the microstrain of the fiber ring to decrease with the square of the propagation distance. The four-layer fiber ring receives different acoustic power due to different spatial positions, resulting in differentiated microstrain. This establishes the quantitative relationship between microstrain and spatial distance of damage source (this is only a physical model formula to illustrate the relationship between strain and damage position. It cannot be actually measured. The calculation data in formula (14) is used when performing the calculation).

[0047] The quantitative correlation between single-layer micro-strain and damage source propagation distance can be expressed as: (16) Where A is the acoustic-strain conversion coefficient of the monitoring medium, which is experimentally calibrated and related to the medium density and elastic modulus; P is the initial acoustic emission power of the damage source; and S is the distance from the damage source to the sensor reference surface (to be determined). S - D i ) is the source of damage to the first i The actual acoustic propagation distance of the fiber optic ring; α This is a correction coefficient for acoustic signal propagation attenuation, which avoids distortion of calculated values ​​due to excessively short propagation distances and improves positioning accuracy.

[0048] The formula for different interlayer microstrain ratios is: (17) After deformation, we get: (18) The value of L can be obtained as follows: (19) It can measure the values ​​of multiple channels S through multi-level cascading, such as S 12 , S 13 , S 14 The optimal solution is then obtained by arithmetic averaging to eliminate the error caused by a single set of data.

[0049] Acoustic emission signals generated by external damage such as metal cracks and rock cracks are transmitted to the expansion units 2, 3, 4, and 5 through the soft rubber base 1 via solid conduction. Due to the inherent radial spacing of the four fiber optic rings and the additional spatial position difference generated by axial expansion and contraction, the propagation distance of the acoustic signal to each fiber optic ring results in quantifiable differences in acoustic signal intensity, thus causing strain differences in each fiber optic ring receiving acoustic signals of different intensities.

[0050] The strain caused by acoustic signals in the optical fiber induces a change in the refractive index of the multimode fiber in the self-interfering fiber structure, resulting in a corresponding change in the intensity of the self-interfering light. The intensity change signals of each layer are synchronously acquired through four channels, and then substituted into the damage localization algorithm formula by the signal analysis module. This yields the radial distance from the damage source to the sensor reference surface.

[0051] The subsequent process can be corrected by combining the time difference correction of the acoustic signal propagation and solving the simultaneous solution. Similarly, the circumferential azimuth of the damage source can be obtained by fitting the circumferential micro-strain distribution. It can also accurately locate the damage source in two dimensions in the radial and circumferential directions, providing accurate data support for fault early warning of structural damage.

[0052] The outermost telescopic unit 2, the second telescopic unit 3, the third telescopic unit 4, and the innermost telescopic unit 5 of the device are all made of rigid, high-damping material, which can effectively suppress mechanical resonance and achieve a wide-band ultrasonic response from low to high frequencies, enabling the monitoring of different types of acoustic emission signals. The sensing fiber uses quartz fiber, which has the characteristics of corrosion resistance, electromagnetic interference resistance, and high temperature resistance (up to 800℃), allowing for stable application in complex monitoring environments such as chemical plants, nuclear power plants, underwater environments, and underground rock formations. The device has an overall closed cylindrical shape, and the shell protective layer 9 further enhances the device's waterproof, dustproof, and impact-resistant capabilities, meeting the complex application needs of industrial sites and possessing wide applicability to various scenarios.

[0053] Example 3 This embodiment provides an optical fiber acoustic sensing detection system, and the system connection diagram is shown below. Figure 12 As shown, it includes: DFB high-power pump source is used to output high-power optical signals; the power of DFB high-power pump source is preferably above 400mW, and high-power light is used to reduce the low sensitivity of monitoring signal caused by the decrease in optical power signal due to fiber self-interference. The optical splitter, connected to the DFB high-power pump source, splits the high-power optical signal into multiple paths; The self-interfering fiber optic acoustic sensing unit provided in Example 1 or the self-interfering fiber optic acoustic sensing device provided in Example 2 are connected to the output end of the beam splitter to convert external acoustic signals into interference optical signals carrying acoustic signal information. A photodetector, connected to the self-interference fiber optic acoustic sensing unit or the self-interference fiber optic acoustic sensing device, converts the interference optical signal into an electrical signal; a multi-channel photoelectric balanced detector with a bandwidth of 5MHz or more can achieve full demodulation across high and low frequency ranges. The data acquisition module is connected to the photodetector to acquire the electrical signals converted by the photodetector; the data acquisition card is a low-speed acquisition card with a bandwidth of 2MHz, requiring four channels for synchronous acquisition; as well as The signal analysis and processing module is connected to the data acquisition module. It analyzes and processes the data acquired by the data acquisition module and demodulates the characteristic information of the acoustic signal under test and the spatial location information of the damage based on the spatiotemporal differences of the electrical signals of each channel.

[0054] Generated by a DFB high-power pump source laser, the beam is split into multiple beams after entering a beam splitter for different sensing channels. The beams enter a layered, telescopic multi-sensor fiber optic integrated sensor. When external acoustic waves act on the sensor, they modulate the intensity of the light waves in the fiber. The modulated signal is received by a four-channel photoelectric balanced detector, which converts the optical signal into a voltage signal and suppresses common-mode noise. The weak electrical signal is amplified by a preamplifier. The amplified electrical signal is then converted into a digital signal by a computer data acquisition card and stored in the computer. Finally, the signal analysis module demodulates and performs algorithmic analysis on the acquired digital signal to reconstruct the acoustic information.

[0055] The method for detecting acoustic signals using a fiber optic acoustic sensing system includes the following steps: S1: The DFB high-power pump source outputs a high-power optical signal, which is split into multiple paths by a beam splitter and then input to multiple layered telescopic self-interference fiber optic acoustic sensing devices.

[0056] S2: Acoustic signals generated by external damage are transmitted to the optical fiber via a solid-state conduction mechanism through a soft rubber substrate, causing changes in light intensity within the fiber that carry information about spatial damage. The acoustic signals are analyzed by demodulating these changes in light intensity.

[0057] S3: The interference light signal is transmitted to the photodetector and converted into a corresponding electrical signal. This electrical signal is amplified by a preamplifier and then synchronously acquired by a data acquisition card across four channels.

[0058] S4: The signal analysis module processes the acquired electrical signals, analyzes the spatiotemporal differences of signals from each channel, demodulates the characteristic information of the acoustic signal under test, and achieves accurate location of the damage, thereby completing acoustic signal detection and damage localization.

[0059] In step S2, the external acoustic signal is the acoustic emission signal generated by damage such as metal cracks and rock cracks.

[0060] The overall size of the device can be adjusted by rotating the nested telescopic units to adapt to different testing environments and installation spaces.

Claims

1. A self-interference fiber optic acoustic sensing unit, comprising a sensing fiber, characterized in that, The sensing optical fiber includes a helical segment in the elongation direction, the axial length of the helical segment is 80-160cm, and the cross-sectional diameter of the helical segment is 2-6m.

2. The self-interference fiber optic acoustic sensing unit according to claim 1, characterized in that, The axial length of the helical segment is 90-110cm, and the cross-sectional diameter of the helical segment is 4-6m.

3. The self-interference fiber optic acoustic sensing unit according to claim 1, characterized in that, It also includes a cylindrical substrate, on which the helical segment of the sensing optical fiber is wound.

4. The self-interference fiber optic acoustic sensing unit according to claim 3, characterized in that, A spiral groove is provided on the surface of the cylindrical substrate, and the sensing optical fiber is wound inside the spiral groove.

5. The self-interference fiber optic acoustic sensing unit according to claim 1, characterized in that, The sensing fiber adopts a core-diameter mismatch self-interference structure, which is formed by splicing single-mode fiber and multimode fiber.

6. A self-interference fiber optic acoustic sensing device, characterized in that, It includes at least two self-interference fiber optic acoustic sensing units as described in any one of claims 1-5; the at least two self-interference fiber optic acoustic sensing units are nested together in a layered telescopic structure.

7. The self-interference fiber optic acoustic sensing device according to claim 6, characterized in that, The self-interference fiber optic acoustic sensing units are connected by threads, and the extension length can be adjusted by rotating the interference fiber optic acoustic sensing units.

8. The self-interference fiber optic acoustic sensing device according to claim 6, characterized in that, It also includes a soft rubber base and a top end cap; the self-interference fiber optic acoustic sensing unit consists of four units, which form a four-layer coaxial nested telescopic unit. The sensing fibers of the four self-interference fiber optic acoustic sensing units form a four-channel fiber optic sensing network. The four telescopic units, from the outside to the inside, are the outermost telescopic unit, the second telescopic unit, the third telescopic unit, and the innermost telescopic unit. The soft rubber base is located at the bottom of the outermost telescopic unit and can receive external acoustic signals through a solid conduction mechanism and transmit them to the telescopic unit. The diameter of each telescopic unit decreases from the outside to the inside. Adjacent telescopic units are engaged by layered helical threads to achieve axial telescopic movement. By adjusting the telescopic amount of each layer, the spatial monitoring position of the fiber optic sensing network can be changed, thereby achieving spatial localization of the damage source.

9. The self-interference fiber optic acoustic sensing device according to claim 8, characterized in that, The outermost telescopic unit is a cylinder with a cylindrical space, and its outer wall is provided with a shell protective layer. A surrounding fiber optic trench is opened on the side wall. The two ends of the trench are connected to the top of the unit through a U-shaped channel, thus forming the fiber optic inlet and outlet ports. The inner and outer walls of the second and third telescopic units are provided with helical threads. The innermost telescopic unit is a solid cylinder with helical threads only on its side wall. The fiber optic trenches on the side walls of the second, third and innermost telescopic units and the internal axial through-hole structure are consistent with the outermost telescopic unit. The optical fiber sensing network adopts a core-diameter mismatch self-interference structure, which is formed by fusion splicing single-mode and multimode optical fibers. The large-core multimode optical fiber enters from the top port of each layer of the telescopic unit, is spirally wound along the groove of each layer to form an optical fiber loop, and is led out from the top port of each layer through the internal channel, and is fused with the armored patch cord of the single-mode optical fiber, forming a single-mode-multimode-single-mode self-interference optical path in each layer of the telescopic structure.

10. A fiber optic acoustic sensing and detection system, characterized in that, include: DFB high-power pump source, used to output high-power optical signals; The optical splitter, connected to the DFB high-power pump source, splits the high-power optical signal into multiple paths; The self-interference fiber optic acoustic sensing unit according to any one of claims 1-5 or the self-interference fiber optic acoustic sensing device according to any one of claims 6-9; connected to the output end of the beam splitter, converting external acoustic signals into interference optical signals carrying acoustic signal information; A photodetector, connected to the self-interference fiber optic acoustic sensing unit or the self-interference fiber optic acoustic sensing device, converts the interference optical signal into an electrical signal. The data acquisition module is connected to the photodetector to acquire the electrical signals converted by the photodetector; as well as The signal analysis and processing module is connected to the data acquisition module. It analyzes and processes the data acquired by the data acquisition module and demodulates the characteristic information of the acoustic signal under test and the spatial location information of the damage based on the spatiotemporal differences of the electrical signals of each channel.

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