Embedded mirror symmetry flexible sound transmission sensitive optical fiber sensing cable and manufacturing method thereof

Abstract

The invention relates to an embedded mirror symmetry flexible sound transmission sensitive optical fiber sensing cable and a manufacturing method thereof, the optical fiber sensing cable comprises a core cable, at least one sensing optical fiber, a sound transmission sensitive material and an outer protection layer, and the core cable sequentially comprises a supporting core, a buffer layer and a spiral core column made of the sound transmission sensitive material from inside to outside; an inverted trapezoidal spiral groove spirally extending in the axial direction is formed in the outer surface of the spiral core column, and the bottom of the groove is in a circular arc shape; a flexible special-shaped coating layer in mirror symmetry is coated outside the sensing optical fiber and is embedded into the center of the groove bottom in a torsion-free state, and a symmetric plane of the flexible special-shaped coating layer is in surface contact with the groove bottom; the acoustic transmission sensitive material is filled in the groove and cured, so that semi-embedded fixation of the optical fiber is realized; the outer protection layer wraps the whole structure, the problems of low sensitivity, poor consistency and the like caused by uncontrollable torsion, unstable point contact and structural relaxation of a traditional DAS optical cable are effectively solved, and the acoustic response performance and the long-term reliability are remarkably improved.

Description

Technical Field

[0001] This invention relates to the field of fiber optic sensing technology, specifically to an embedded mirror-symmetric flexible acoustically transparent sensitive fiber optic sensing cable and its manufacturing method. Background Technology

[0002] When sound waves act on optical fibers, they cause changes in the fiber's refractive index, which in turn alters the phase, intensity, or propagation direction of the optical signal propagating within the fiber. Distributed acoustic sensing (DAS) technology utilizes Rayleigh scattering and phase-sensitive optical time-domain reflectometry (TDR) to analyze the phase changes of locally scattered light caused by sound waves through a high-speed acquisition system, thereby enabling the detection of sound wave frequency and location. This type of technology has been widely applied in fields such as earthquake monitoring, geophysical exploration, pipeline monitoring, and underwater acoustic detection.

[0003] To achieve sufficient detection sensitivity, optical fibers in current sensing cables are typically laid out in a helical winding pattern to increase the fiber length per unit length of cable and improve sensitivity to acoustic waves in all directions. However, existing helical sensing cables still have inherent defects in manufacturing processes and structural stability, making the consistency, reliability, and stability of their sensing performance the core bottleneck restricting the further promotion and development of this technology. These bottlenecks mainly focus on the following points:

[0004] 1) Uncontrollable fiber torsion leads to inconsistent performance along the cable: In traditional manufacturing processes, when optical fibers are wound onto the core, they undergo random torsion along their own axis. This torsion is uncontrollable, resulting in uneven cumulative transmission loss and polarization state changes. Consequently, the sensing sensitivity and frequency response characteristics of the final sensor cable cannot be consistent along its length, severely affecting sensing quality, data reliability, and positioning accuracy. 2) Point contact structure leads to poor stability: In traditional designs, optical fibers are typically wound around a circular core, which is then wrapped with a circular coating or sheath. From a cross-sectional perspective, this "circle-to-circle" contact is physically a "point contact," with a small contact area and poor stability. Under vibration or temperature changes, the optical fiber is prone to relative slippage, leading to uneven local tension and inconsistent sensitivity at different locations on the same cable, resulting in substandard cable reliability. 3) Structural looseness leads to reduced sensitivity and poor reliability: To reduce bending loss, some solutions add a loose sheath to the bend-insensitive fiber. While this reduces static loss, it also causes the optical fiber to be in a relaxed state within the sheath, resulting in poorer coupling with the external sound field and a significant decrease in sensing sensitivity. Furthermore, the relaxed fiber is more prone to breakage due to uneven stress when subjected to severe vibration or bending, making it difficult to meet the stringent reliability requirements of field operations.

[0005] In summary, existing spiral-shaped DAS sensing optical cables face insurmountable technical bottlenecks in terms of torsion control, contact stability, and structural compactness. There is an urgent need for a new structural design and supporting manufacturing process that can achieve precise control of fiber attitude, contact fixation, and efficient coupling between acoustic materials and optical fibers while ensuring low transmission loss, thereby comprehensively improving the consistency, sensitivity, and engineering reliability of distributed acoustic sensing optical cables. Summary of the Invention

[0006] To address the common problems in the manufacturing and application of existing helical fiber optic sensing cables for distributed acoustic sensing (DAS), such as uncontrollable fiber torsion, poor stability due to point contact between the fiber and the core cable, and low acoustic coupling efficiency and insufficient reliability caused by structural laxity, this invention provides an embedded mirror-symmetric flexible acoustically transparent sensitive fiber optic sensing cable. This cable achieves surface contact and semi-embedded fixation between the fiber and the slot, effectively eliminating performance inconsistencies along the cable and significantly improving the cable's response sensitivity to sound waves, mechanical stability, and environmental adaptability. This invention also relates to a method for manufacturing an embedded mirror-symmetric flexible acoustically transparent sensitive fiber optic sensing cable.

[0007] The technical solution of the present invention is as follows:

[0008] An embedded mirror-symmetric flexible acoustically transparent sensitive fiber optic sensing cable is characterized by comprising a core cable, at least one sensing fiber, an acoustically transparent sensitive material, and an outer protective layer.

[0009] The core cable comprises, from the inside out, a support core, a buffer layer, and a spiral core column. The support core is composed of multiple reinforcing members with tensile strength. The buffer layer is made of a support material with stress-absorbing function and covers the outer periphery of the support core. The spiral core column is made of a sound-permeable material with acoustic conductivity and covers the outer periphery of the buffer layer.

[0010] The outer surface of the spiral core column is provided with at least one inverted trapezoidal spiral groove extending spirally along its axis. The groove opening width is greater than the groove bottom width, and the groove wall gradually slopes outward from the groove bottom towards the groove opening. The groove bottom is arc-shaped and has a predetermined bending radius, and the groove bottom width is greater than the maximum outer diameter of the optical fiber it accommodates.

[0011] The outer surface of the sensing fiber is covered with a flexible, irregularly shaped coating layer with a mirror-symmetrical cross-section. The sensing fiber is embedded in the bottom center of the inverted trapezoidal spiral groove in a non-twisted state, so that the flexible contact surface of the irregularly shaped coating layer that contacts the bottom of the groove conformally fits the arc surface of the bottom of the inverted trapezoidal spiral groove, thereby forming a surface contact.

[0012] The sound-sensitive material is filled in the inverted trapezoidal spiral groove and wraps the portion of the sensing optical fiber that is coated with an irregular shape without forming a surface contact with the bottom of the groove. After curing, the sensing optical fiber is semi-embedded and fixed to the spiral core column, and its top does not extend beyond the outer peripheral surface of the spiral core column.

[0013] The outer protective layer covers the outer peripheral surface of the spiral core and covers the inverted trapezoidal spiral groove into which the sensing fiber has been embedded and the acoustically sensitive material filling it.

[0014] Preferably, the cross-sectional shape of the irregularly shaped coating layer is elliptical, D-shaped, or semi-circular; when the cross-sectional shape of the irregularly shaped coating layer is elliptical, its contact surface conformally fits the arc surface of the bottom of the groove along the short axis; when the cross-sectional shape of the irregularly shaped coating layer is D-shaped, its flat bottom surface conformally fits the arc surface of the bottom of the groove; when the cross-sectional shape of the irregularly shaped coating layer is semi-circular, its diameter plane conformally fits the arc surface of the bottom of the groove.

[0015] Preferably, the spiral core column has multiple inverted trapezoidal spiral grooves, each inverted trapezoidal spiral groove is evenly distributed along the circumference of the spiral core column, and the central angle between the center lines of any two adjacent inverted trapezoidal spiral grooves is equal.

[0016] Preferably, the sensing optical fiber is a bend-insensitive optical fiber; the sound-sensitive material is polyurethane and is cured by heating or ultraviolet irradiation; the outer protective layer is made of polyurethane or high polypropylene material.

[0017] Preferably, the reinforcing member with tensile strength is steel wire or Kevlar rope; the supporting material with stress buffering function is Kevlar reinforcing material; and the acoustically transparent material with acoustic conductivity is high-polypropylene material.

[0018] A method for manufacturing the above-mentioned embedded mirror-symmetric flexible acoustically transparent sensitive fiber optic sensing cable, characterized by comprising the following steps:

[0019] S1: A core cable comprising the support core, buffer layer and spiral core column is formed through the first extrusion process, and at least one inverted trapezoidal spiral groove extending spirally along its axial direction is simultaneously machined on the outer surface of the spiral core column using a milling device.

[0020] S2: Perform the first stress relief treatment on the sensing fiber, and form a flexible irregular coating layer with a mirror-symmetric cross-section on the outer surface of the stress-relieved sensing fiber through a coating process. Perform the second stress relief treatment on the coated sensing fiber and wind it onto the fiber laying tray.

[0021] S3: Release the sensing fiber on the fiber release tray and guide it to the winding cage. Before the sensing fiber enters the inverted trapezoidal spiral groove through the winding cage, the cross-sectional orientation of the irregular coating layer covering the outer surface of the sensing fiber is identified in real time by the high-speed image monitoring system. The twisting orientation of the fiber is dynamically adjusted by the active un-twist mechanism to compensate for the torsional stress of the fiber, so that the designated contact surface of the irregular coating layer moves toward the fixed orientation of the bottom of the inverted trapezoidal spiral groove.

[0022] S4: Using a tangential fiber feeding method, the dynamically adjusted sensing fiber is synchronously embedded into the bottom center position of the inverted trapezoidal spiral groove with a preset tension and angle.

[0023] S5: While the sensing fiber is being embedded, a second extrusion process is used to fill the inverted trapezoidal spiral groove with sound-sensitive material and wrap the uncontacted bottom part of the sensing fiber. After curing, the sensing fiber is semi-embedded and fixed to the spiral core.

[0024] S6: An outer protective layer is coated on the outer circumferential surface of the spiral core through a third extrusion process. The outer protective layer covers the inverted trapezoidal spiral groove into which the sensing fiber has been embedded and the acoustically sensitive material filling it, thereby forming a complete optical fiber sensing cable.

[0025] Preferably, in step S3, the release speed of the sensing optical fiber, the forward speed of the core cable, and the rotation speed of the winding cage are synchronously controlled by a servo motor, and the ratio of the release speed of the sensing optical fiber to the forward speed of the core cable is 2:1, and is adaptively adjusted according to the helical pitch requirements to ensure that the helical pitch of the sensing optical fiber is constant and without additional stress.

[0026] Preferably, in step S5, different types of sound-transmitting sensitive materials are selected to fill the inverted trapezoidal spiral groove according to the acoustic response characteristics requirements of the target monitoring scene, and cured by heating or ultraviolet irradiation, so as to adjust the sensitivity of the sensing optical fiber to sound waves along the axial or radial direction of the optical fiber sensing cable.

[0027] Preferably, in step S3, the active untorsion mechanism drives the fiber feeding disc or guide wheel to rotate in the opposite direction via a servo motor to compensate for the torsional stress of the sensing optical fiber.

[0028] Preferably, in step S4, the tension ranges from 0 to 15 grams, and the angle is from 20 to 30 degrees along the direction tangential to the outer circle of the spiral core.

[0029] The technical effects of this invention are as follows:

[0030] This invention provides an embedded mirror-symmetric flexible acoustically sensitive fiber optic sensing cable, comprising a core cable, at least one sensing fiber, an acoustically sensitive material, and an outer protective layer. Through a multi-layer composite structure design, it integrates mechanical support, buffer energy absorption, acoustic sensitivity, and external protection functions into one unit. While ensuring the flexibility and tensile strength of the optical cable, it achieves efficient coupling and stable response to external sound waves, significantly improving the overall performance of the distributed acoustic sensing (DAS) system. The core cable consists of a support core, a buffer layer, and a spiral core column from the inside out. The support core is composed of multiple reinforcing members with tensile strength (such as reinforcing steel wires and Kevlar ropes), which can provide high tensile strength and ensure that the optical cable is not easily broken during laying and service. The buffer layer is made of a support material with stress buffering function (such as Kevlar reinforcement material) and covers the outer periphery of the support core. It is used to absorb external impact and temperature stress, prevent internal structural deformation under pressure, and protect the optical fiber from mechanical damage. The spiral core column is made of a sound-transmitting material with acoustic conductivity (such as high polypropylene) and covers the outer periphery of the buffer layer. It can efficiently transmit external sound wave vibrations to the embedded optical fiber, improve the sound-to-light conversion efficiency, and enhance sensing sensitivity. Furthermore, the outer surface of the helical core column has at least one inverted trapezoidal helical groove extending helically along its axis. This helical extension along the axis allows the optical fiber to be wound at a constant pitch, extending the effective sensing distance per unit length and enhancing the response to acoustic waves in all directions. The cross-section of the inverted trapezoidal helical groove is trapezoidal, wider at the top and narrower at the bottom, facilitating smooth embedding of the sensing fiber under low tension and preventing the fiber from being extruded during subsequent extrusion filling, thus achieving self-positioning. The groove bottom width is greater than the maximum outer diameter of the fiber it accommodates, ensuring stress-free embedding of the fiber while limiting its lateral movement and improving positional stability. The groove bottom is arc-shaped with a predetermined bending radius to match the outer diameter of the sensing fiber, avoiding micro-bending loss at the bottom and providing continuous curved surface support. Further, the outer surface of the sensing fiber is coated with a flexible, irregularly shaped coating layer with a mirror-symmetrical cross-section. This mirror-symmetrical irregularly shaped coating layer (such as D-shaped, elliptical, or semi-circular) breaks the rotational symmetry of traditional circular optical fibers, providing a geometric reference for attitude recognition and control. The sensing fiber is embedded in the bottom center of the inverted trapezoidal spiral groove in a twist-free state. This twist-free embedding ensures high consistency in the polarization state and transmission characteristics of the fiber along its length, eliminating phase noise and sensitivity fluctuations caused by random twisting. Through a mirror-symmetric irregular-shaped coating layer and an active de-twisting process, random twisting of the fiber during winding is completely eliminated, ensuring high consistency in core indicators such as sensing sensitivity and frequency response characteristics along the axial direction of the optical cable. Furthermore, the irregular-shaped coating layer, used as a flexible contact surface with the bottom of the groove, conforms to the arc surface of the bottom of the inverted trapezoidal spiral groove (meaning the two surfaces closely conform to each other's shapes), thus forming a surface contact. This significantly increases the actual contact area between the fiber and the groove, significantly improving mechanical stability compared to traditional "circle-to-circle point contact," effectively suppressing relative slippage caused by temperature changes or vibrations, and ensuring the consistency and reliability of the sensing signal during long-term service.Then, the sound-sensitive material is filled into the inverted trapezoidal spiral groove and wrapped around the sensing optical fiber (the part of the sensing optical fiber that is wrapped with the irregular coating layer does not form a surface contact with the bottom of the groove, while the part of the optical fiber that forms a surface contact with the bottom of the groove is not wrapped by the sound-sensitive material, ensuring that the sound signal is directly transmitted through the contact part). It has both good acoustic conductivity and elasticity. By filling different sound-sensitive materials (such as polyurethane), the sensitivity of the optical cable to sound waves in different directions can be flexibly changed, which can efficiently transmit sound wave vibrations to the optical fiber, while buffering external impacts. After curing, the sensing fiber is semi-embedded and fixed to the spiral core. The semi-embedded design ensures tight coupling between the fiber and the acoustic sensing layer, guaranteeing high sensitivity. At the same time, the semi-embedded fixation ensures that the fiber is neither in a loose state (avoiding coupling failure) nor rigidly bound (preventing stress concentration), achieving optimal acoustic-fiber coupling while maintaining low transmission loss. Furthermore, the cured filling structure firmly locks the fiber in the center of the groove bottom, completely eliminating structural slack and significantly improving the vibration resistance and reliability of the optical cable in complex environments. Through the inverted trapezoidal spiral groove + semi-embedded structure, the combination between the fiber and the core cable is extremely stable, eliminating relative slippage and greatly improving the structural stability and sensing reliability of the optical cable under complex working conditions. Finally, the outer protective layer covers the outer surface of the spiral core column and the inverted trapezoidal spiral groove with the embedded sensing fiber and the sound-sensitive material inside. The outer protective layer (made of polyurethane or high polypropylene) provides overall sealing and mechanical protection, resisting external damage such as wear, moisture, and chemical corrosion. After covering, it forms a smooth and continuous cylindrical outer surface, which facilitates the laying and deployment of optical cables, while maintaining the integrity of the spiral structure. It also works in conjunction with the internal semi-embedded structure to build an integrated design of "internal stability and external protection", which makes the optical cable have high mechanical strength and is not easy to break. It ensures that the optical cable maintains high sensitivity, low noise and high consistency during long-term operation in the field, perfectly meeting the needs of field operations. In summary, this invention fundamentally solves the three major technical bottlenecks of existing DAS optical cables—uncontrollable torsion, unstable point contact, and structural laxity—through an innovative combination of mirror-symmetric irregular optical fiber, inverted trapezoidal spiral groove, surface contact embedding, and semi-embedded solidification. It achieves high consistency in sensing performance, excellent acoustic sensitivity, and outstanding engineering reliability, making it particularly suitable for demanding applications such as earthquake monitoring, oil and gas pipeline safety, perimeter security, and underwater detection.

[0031] This invention achieves a comprehensive improvement in four dimensions of distributed acoustic sensing (DAS) optical cables: performance consistency, acoustic sensitivity, mechanical stability, and engineering reliability, through a collaborative innovation integrating structural design, material selection, and process control. Specifically: 1) The use of mirror-symmetric irregularly coated optical fibers combined with an inverted trapezoidal spiral groove surface contact embedding structure completely eliminates the slippage risk caused by traditional round-to-round contact and the phase noise caused by random torsion, ensuring a high degree of consistency in sensing response along the entire cable length; 2) The semi-embedded fixed structure combined with acoustically sensitive filling material achieves efficient coupling of acoustic energy while avoiding fiber relaxation, significantly improving the system's sensitivity to weak vibrations; 3) The multi-layer core cable structure and synchronous secondary extrusion process provide high-strength support, buffer energy absorption, and external protection, enabling the optical cable to possess excellent tensile strength, vibration resistance, temperature change resistance, and long-term service capability in complex outdoor environments. Compared to existing technologies, this solution eliminates the need for costly tension control or post-calibration, enabling stable output of high-performance DAS optical cables in large-scale production. This significantly lowers the application threshold and provides a reliable, efficient, and engineerable sensing infrastructure for key areas such as earthquake monitoring, oil and gas pipeline safety early warning, perimeter security, and underwater acoustic detection.

[0032] Furthermore, the cross-sectional shape of the irregular coating layer is elliptical, D-shaped, or semi-circular. The D-shaped and semi-circular shapes have flat bottom surfaces, which can form a large-area surface contact with the arc bottom of the inverted trapezoidal spiral groove, greatly improving contact stability and effectively suppressing relative displacement caused by vibration or thermal expansion and contraction. More importantly, its mirror symmetry provides the possibility for the visual system to recognize and control its rotational posture. The elliptical shape (preferably with the short axis aligned with the bottom of the groove) can achieve stable directional embedding while maintaining good bending insensitivity, making it suitable for laying scenarios with higher flexibility requirements. All three shapes are easy to mass-produce through UV curing or thermosetting coating processes and have good interfacial compatibility with acoustically sensitive filling materials, which is conducive to achieving semi-embedded and firm fixation of optical fibers. In summary, by adopting any of the above mirror-symmetric cross-sectional shapes, the sensing consistency, mechanical stability, and acoustic-optical coupling efficiency of the optical cable along its length can be significantly improved while ensuring low transmission loss. This fundamentally solves the core pain points of spiral optical fiber sensing cables in terms of product quality consistency and stability, clearing the way for the large-scale application of high-performance distributed acoustic sensing technology.

[0033] Furthermore, the spiral core column is provided with multiple inverted trapezoidal spiral grooves, which are uniformly distributed along the circumference of the spiral core column, and the central angle between the center lines of any two adjacent inverted trapezoidal spiral grooves is equal. The uniform distribution of multiple grooves allows multiple sensing optical fibers to be symmetrically embedded along the circumference, significantly increasing the effective fiber length per unit length of optical cable, enhancing signal acquisition density and system signal-to-noise ratio without increasing the outer diameter of the optical cable; the circumferentially symmetrical layout (e.g., double grooves at 180°, triple grooves at 120°, and quadruple grooves at 90°) ensures uniform overall mass distribution of the optical cable, avoiding eccentricity or twisting tendencies caused by unilateral fiber laying, and improving the mechanical balance and structural stability of the optical cable during laying and coiling; the uniform groove arrangement ensures that the acoustic environment of each sensing optical fiber is highly consistent, eliminating differences in directional sensitivity, thereby achieving uniformity and isotropy of omnidirectional acoustic response.

[0034] This invention also relates to a manufacturing method for an embedded mirror-symmetric flexible acoustically sensitive fiber optic sensing cable. This method achieves high precision, high consistency, and scalable production from materials to finished product through coordinated control of the entire process—structure prefabrication, fiber orientation, dynamic untwisting, precise embedding, synchronous filling, and overall encapsulation—ensuring the final optical cable possesses excellent acoustic response consistency, mechanical stability, and long-term reliability. First, a core cable comprising a support core, a buffer layer, and a spiral core post is formed through a first extrusion process. Then, a milling device is used to simultaneously machine at least one inverted trapezoidal spiral groove extending along the axial direction on the outer surface of the spiral core post. The integral extrusion molding of the core cable ensures a tight bond between the support core, buffer layer, and spiral core post, avoiding delamination or gaps and improving the overall structural strength and acoustic wave transmission continuity. The servo-controlled milling device synchronously processes the inverted trapezoidal spiral groove, which can precisely control the groove depth, groove width, spiral pitch and bottom arc radius, ensuring that the groove geometry parameters are highly consistent, providing a high-precision positioning reference for subsequent stress-free fiber embedding; the "wide at the top and narrow at the bottom" structure of the inverted trapezoidal groove facilitates fiber insertion and prevents it from being pushed out during extrusion, realizing a self-alignment function. The sensing fiber undergoes a first stress-relief treatment, followed by a coating process to form a flexible, irregularly shaped coating layer with a mirror-symmetrical cross-section on its outer surface. A second stress-relief treatment is then performed on the coated sensing fiber before it is wound onto a fiber-laying reel. These two stress-relief treatments (before and after coating) effectively eliminate residual internal stress generated during fiber drawing and coating, preventing shrinkage or micro-bending during subsequent winding or service, thus ensuring stable transmission performance. The irregularly shaped coating layer gives the fiber a mirror-symmetrical cross-section, breaking the rotational symmetry of traditional circular fibers and providing geometric markers for attitude recognition and directional embedding. Maintaining a consistent cross-sectional direction during winding onto the fiber-laying reel provides an initial attitude reference for the subsequent dynamic untwisting system, significantly reducing the difficulty of real-time adjustments and improving winding efficiency and accuracy. The sensing fiber is then released from the fiber release tray and guided to the winding cage. Before the sensing fiber enters the inverted trapezoidal spiral groove through the winding cage, the cross-sectional orientation of the irregularly shaped coating layer covering the outer surface of the sensing fiber is identified in real time by a high-speed image monitoring system. The fiber twisting is dynamically adjusted by a de-twisting mechanism to ensure that the designated contact surface of the irregularly shaped coating layer moves towards the bottom of the inverted trapezoidal spiral groove in a fixed orientation. The high-speed image monitoring system can capture the cross-sectional profile of the fiber in real time and accurately identify the spatial orientation of the mirror symmetry plane. The active de-twisting mechanism dynamically compensates for the random twisting of the fiber during the fiber release and guidance process, ensuring that it enters the spiral groove in a twist-free and fixed orientation. This closed-loop control mechanism fundamentally solves the problem of sensitivity fluctuation and phase noise along the cable caused by uncontrollable twisting in traditional processes, achieving a high degree of consistency in sensing performance across the entire cable.Furthermore, a tangential fiber feeding method is adopted, in which the dynamically adjusted sensing fiber is synchronously embedded into the bottom center of the inverted trapezoidal spiral groove with a preset tension and angle. The tangential fiber feeding method allows the fiber to smoothly transition into the spiral groove with a minimum bending radius, avoiding the introduction of additional macro-bending or micro-bending losses. The preset low tension (e.g., 0–15 g) ensures that the fiber is not subjected to tensile stress during the embedding process, maintaining its original optical properties. The center of the groove bottom is precisely aligned, and the matching of the mirror symmetry plane and the arc groove bottom achieves the maximum surface contact area, laying the foundation for subsequent curing and fixing. Furthermore, during the embedding of the sensing fiber, a second extrusion process is used to fill the inverted trapezoidal spiral groove with a sound-transmitting sensitive material and encapsulate the sensing fiber. After curing, the sensing fiber is semi-embedded and fixed to the spiral core. Synchronous filling and embedding prevent the fiber from shifting before filling, ensuring positional accuracy. The sound-transmitting sensitive material (such as polyurethane) has both high acoustic impedance matching and elasticity, efficiently transmitting external vibrations to the fiber while buffering impacts. The semi-embedded curing fixation ensures that the fiber is neither loose (avoiding coupling failure) nor rigidly bound (preventing stress concentration), achieving optimal acoustic-fiber coupling while maintaining low loss, significantly improving system sensitivity and reliability. Finally, a third extrusion process is used to coat the outer surface of the spiral core with an outer protective layer. This outer protective layer covers the inverted trapezoidal spiral groove with the embedded sensing fiber and the acoustically sensitive material inside, thus forming a complete optical fiber sensing cable. The outer protective layer forms a continuous, smooth cylindrical outer surface, which is convenient for laying, coiling, and splicing. It also provides multiple protections such as waterproofing, abrasion resistance, chemical corrosion resistance, and UV aging resistance, meeting the long-term service requirements of complex outdoor environments. At the same time, it works in conjunction with the internal semi-embedded structure to build an integrated "inner stability and outer protection" encapsulation system, ensuring that the optical cable maintains high stability and high consistency under conditions such as vibration, temperature cycling, and bending.

[0035] In summary, this manufacturing method, through the integration of key technologies such as precision structure prefabrication, active fiber attitude control, low-stress embedding, and synchronous curing and packaging, has for the first time achieved the industrial-scale manufacturing of helical fiber optic sensing cables for DAS with high consistency, high sensitivity, and high reliability. It effectively overcomes the inherent defects of existing technologies such as uncontrollable torsion, unstable contact, and structural relaxation, and provides solid technical support for the large-scale application of distributed acoustic sensing in high-end fields such as earthquake monitoring, pipeline safety, and underwater detection. Attached Figure Description

[0036] Figure 1 This is a cross-sectional view of the embedded mirror-symmetric flexible acoustically transparent sensitive fiber optic sensing cable of the present invention.

[0037] Figure 2 This is a cross-sectional view of the spiral core of the present invention.

[0038] Figure 3This is a cross-sectional view of the inverted trapezoidal spiral groove of the present invention.

[0039] Figure 4 This is a cross-sectional view of the elliptical irregular coating layer of the present invention and its attachment on an inverted trapezoidal spiral groove.

[0040] Figure 5 This is a cross-sectional view of the D-shaped irregular coating layer of the present invention and its attachment on an inverted trapezoidal spiral groove.

[0041] Figure 6 This is a cross-sectional view of the semi-circular irregular coating layer of the present invention and its attachment on an inverted trapezoidal spiral groove.

[0042] Figure 7 This is a schematic diagram of the sensing optical fiber of the present invention wound on a helical core column.

[0043] The labels in the diagram are listed below:

[0044] 1—Supporting core, 2—Buffer layer, 3—Helical core column, 4—Inverted trapezoidal helical groove, 5—Sensing optical fiber, 6—Outer protective layer. Detailed Implementation

[0045] The present invention will now be described with reference to the accompanying drawings.

[0046] This invention relates to an embedded, mirror-symmetric, flexible, acoustically transparent, and sensitive fiber optic sensing cable, a novel sensing cable solution that represents a collaborative innovation in both structural design and manufacturing process. Its core concept lies in: Structurally, through a special irregularly shaped structure design (inverted trapezoidal spiral groove + irregularly shaped coated fiber), the traditional unstable "point contact" is transformed into a stable, uniform "surface contact" and "embedded" fixation. In terms of manufacturing, through precisely controlled untwisting and synchronous embedding processes, random twisting of the fiber during winding is fundamentally eliminated, ensuring a high degree of consistency in the physical state of the fiber along its axial direction. Figure 1 The structure shown includes an embedded mirror-symmetric flexible acoustically sensitive fiber optic sensing cable comprising a core cable, at least one sensing fiber 5, an acoustically sensitive material, and an outer protective layer 6. The core cable comprises, from the inside out, a support core 1, a buffer layer 2, and a spiral core column 3.

[0047] I. The overall structure of the fiber optic sensing cable specifically includes the following components:

[0048] Support core 1: Composed of multiple high-strength tensile reinforcing wires (such as steel wire or Kevlar rope), with an outer diameter denoted as D1 (e.g., 3 mm). This support core is used to provide axial tensile strength for the optical cable.

[0049] Buffer layer 2: Made of high-strength cylindrical support material (e.g., Kevlar reinforced material) and wrapped around the outer periphery of support core 1, with an outer diameter of D2 (e.g., 5 mm), used to absorb external mechanical impacts and protect the internal structure.

[0050] Spiral core 3: Made of acoustically permeable material (e.g., high polypropylene) and wrapped around the outer periphery of the buffer layer 2, with an outer diameter denoted as D3 (e.g., 17 mm). The outer surface of the spiral core 3 is provided with at least one inverted trapezoidal spiral groove 4 extending spirally along its axial direction.

[0051] Inverted trapezoidal spiral groove 4: It is an innovative skeletal structure, such as Figure 2 and 3 As shown, where, Figure 3 for Figure 1 The image shows a magnified cross-section of the embedded mirror-symmetric flexible acoustic-sensitive fiber optic sensing cable in the AA direction, which is also a magnified cross-section of the inverted trapezoidal spiral groove 4. The groove's structure, the acoustic-sensitive material filling it, and the fit of the irregularly shaped coating on the sensing fiber are clearly visible. The cross-section of the inverted trapezoidal spiral groove 4 is an inverted trapezoid, wider at the top and narrower at the bottom. Its bottom width is denoted as L1 (e.g., 0.9~2.6 mm; different bottom groove widths are selected depending on the application, and the outer coating of the fiber is appropriately thickened, or different types of adhesive are filled into the inverted trapezoidal spiral groove and cured under UV light or heat), its opening width is denoted as L2 (e.g., 3 mm), its angle is φ2 (e.g., 20 degrees), its bottom is arc-shaped with a predetermined bending radius of R1 (e.g., 7.5 mm), and its depth is D3 / 2-R1. The inverted trapezoidal spiral groove 4 has a groove opening width L2 that is greater than the groove bottom width L1, and the groove wall gradually slopes outward from the groove bottom towards the groove opening. The groove bottom width L1 is greater than the maximum outer diameter of the optical fiber it can accommodate, so as to ensure that the optical fiber can be stably placed at the bottom of the inverted trapezoidal spiral groove, providing a precise reference for subsequent embedding and fixing. At the same time, it ensures that the optical fiber is stably wound and solidified on the spiral core after secondary wrapping, ensuring the sensing sensitivity and stability of the optical fiber.

[0052] Sensing fiber 5: This is an innovative sensing element that uses a bend-insensitive fiber and is coated with a flexible, irregularly shaped coating layer with a mirror-symmetrical cross-section. The cross-sectional shape of the irregularly shaped coating layer is elliptical. Figure 4 ), D-shaped ( Figure 5 ) or semi-circular ( Figure 6 This symmetrical structure is a key prerequisite for achieving twist-free winding. The sensing fiber 5 is embedded in the bottom center of the inverted trapezoidal spiral groove 4 in a twist-free state. Figure 3 At point C in the diagram, the flexible contact surface of the irregularly shaped coating layer, which is used to contact the bottom of the groove, conformally fits with the arc surface of the bottom of the inverted trapezoidal spiral groove (meaning that the two surfaces closely conform to each other's shapes), thereby forming a surface contact. Specifically, when the cross-sectional shape of the irregularly shaped coating layer is elliptical, its contact surface conformally fits with the arc surface of the bottom of the groove along the short axis; when the cross-sectional shape of the irregularly shaped coating layer is D-shaped, its flat bottom surface conformally fits with the arc surface of the bottom of the groove; and when the cross-sectional shape of the irregularly shaped coating layer is semi-circular, its diameter plane conformally fits with the arc surface of the bottom of the groove.

[0053] Sound-sensitive material: This material is filled within the inverted trapezoidal spiral groove 4 and wraps around the sensing optical fiber 5. Specifically, it wraps the portion of the sensing optical fiber that is already coated with a special-shaped layer but does not form a surface contact with the bottom of the groove. Depending on the application scenario and requirements, different adhesive materials (sound-sensitive materials) can be uniformly filled onto the surface of the sensing optical fiber 5 and the inverted trapezoidal spiral groove 4 during the embedding process. These materials are then cured by heating or ultraviolet irradiation to adjust the acoustic sensitivity of the sensing optical fiber 5 along the axial and radial directions of the optical fiber sensing cable. For example, polyurethane material can be used. After curing, this material semi-embeds the sensing optical fiber 5 in the spiral core post 3, ensuring that its top does not extend beyond the outer circumferential surface of the spiral core post, thereby increasing the acoustic sensitivity of the optical fiber along the radial direction of the optical cable.

[0054] Outer protective layer 6: Covers the entire outer circumferential surface of the spiral core 3, and covers the inverted trapezoidal spiral groove 4 into which the sensing fiber 5 has been embedded, as well as the acoustically sensitive material filling it. This outer protective layer 6 is made of polyurethane or high-polypropylene materials, etc., and has an outer diameter denoted as D4 (e.g., 19 mm), used to provide environmental protection and mechanical protection.

[0055] II. Structural Characteristics of Sensing Optical Fibers

[0056] like Figures 4 to 6 As shown, the core of the sensing fiber 5 is a standard fiber cladding with a diameter denoted as D6 (e.g., 80 micrometers). Its outer surface is coated with an irregularly shaped layer to provide mirror symmetry, allowing its orientation to be identified via an imaging system during winding, and its rotation angle to be controlled, thus achieving twist-free winding. The specific form is as follows:

[0057] Elliptical coating layer ( Figure 4 ): The short axis length of the elliptical coating is denoted as E1 (e.g., 100 micrometers), and the long axis length is denoted as E2 (e.g., 250 micrometers). The elliptical coating is used as the contact surface that contacts the bottom of the groove, and it is in close contact with the center of the bottom of the inverted trapezoidal spiral groove 4 along the short axis direction.

[0058] D-shaped coating layer ( Figure 5 The distance from the top of the D-shaped coating to the center of the standard fiber cladding is denoted as E3 (e.g., 85 micrometers), the distance from the bottom to the center of the standard fiber cladding is denoted as E4 (e.g., 85 micrometers), and the width is denoted as E5 (e.g., 250 micrometers). The D-shaped coating is used as the contact surface (flat bottom surface) that contacts the bottom of the groove, and it is in close contact with the center of the bottom of the inverted trapezoidal spiral groove 4.

[0059] Semi-circular coating layer ( Figure 6The distance from the top of the semicircular coating to the center of the standard optical fiber cladding is denoted as E6 (e.g., 85 micrometers), the height is denoted as E7 (e.g., 172 micrometers), the radius of the semicircular coating is denoted as R2 (e.g., 170 micrometers), and the width of the semicircular coating is denoted as E8 = 2R2. The semicircular coating plane is used as the contact surface (diameter plane) for contacting the bottom of the groove, and it is in close contact with the center of the bottom of the inverted trapezoidal spiral groove 4.

[0060] III. Arrangement and Geometric Parameters of Spiral Grooves

[0061] like Figure 2 As shown, multiple inverted trapezoidal spiral grooves 4 (e.g., three) can be formed on the outer surface of the spiral core 3. Each inverted trapezoidal spiral groove 4 is evenly distributed along the circumference of the spiral core 3. The central angle φ1 between the center lines of any two adjacent inverted trapezoidal spiral grooves 4 is equal, and the circumferential angle between adjacent grooves is 360° / n (n is the number of grooves). For example, it is 180° for two grooves and 120° for three grooves. Taking three inverted trapezoidal spiral grooves as an example, the central angle φ1 between two adjacent grooves is 60 degrees (i.e., one groove every 120 degrees). Figure 7 The diagram shows three sensing fibers 5 spirally wound around a helical core post 3. The helical pitch of each sensing fiber 5 is denoted as L3 (e.g., 27 mm), and the helical pitch between different sensing fibers (e.g., fiber 1 and fiber 2, fiber 2 and fiber 3) is denoted as L4 (e.g., 9 mm), satisfying L3 = 3L4 (see...). Figure 7 The sensing fiber is spirally wound with a cross-sectional diameter of D5 (e.g., 15.1 mm). This design allows multiple sensing fibers to be spirally wound in parallel, improving space utilization and signal redundancy.

[0062] This invention also relates to a method for manufacturing the above-mentioned embedded mirror-symmetric flexible acoustically transparent sensitive fiber optic sensing cable, comprising the following steps:

[0063] Step S1: Core Cable Forming and Inverted Trapezoidal Helical Groove Machining. This step prepares a core cable substrate with a precise geometric structure, providing a stable carrier for subsequent fiber embedding. Through a first extrusion process, a basic core cable comprising a support core 1, a buffer layer 2, and a helical core post 3 is continuously formed. Simultaneously, a milling device is used to machine at least one inverted trapezoidal helical groove extending helically along the axial direction onto the outer surface of the helical core post.

[0064] Specifically, firstly, multiple high-strength load-bearing materials (such as steel wire or Kevlar rope) are twisted together to form a support core 1, with an outer diameter denoted as D1 (e.g., 3 mm). Around the support core 1, a cylindrical high-strength material (such as Kevlar reinforcement) is extruded to form a buffer layer 2, with an outer diameter denoted as D2 (e.g., 5 mm). This layer absorbs external impacts and prevents internal structural deformation under pressure. Next, around the buffer layer 2, a spiral core column 3 made of sound-sensitive material (such as high-polymer polypropylene) is extruded again, with an outer diameter denoted as D3 (e.g., 17 mm). These three layers (support core + buffer layer + spiral core column) are integrally formed through a single continuous extrusion process to form the base cable.

[0065] Subsequently, the core cable is fed into a high-precision servo-controlled machining system. This system includes a rotating auger and a synchronously moving milling device (milling cutter). The auger drives the core cable to rotate at a constant angular velocity, while the milling cutter advances at a uniform speed along the core cable axis. The two are synchronously controlled by the servo to form a helical motion, milling the surface of the helical core post 3 to produce at least one inverted trapezoidal helical groove 4. The groove cross-section of the inverted trapezoidal helical groove 4 is a trapezoid with a wider top and narrower bottom (i.e., "inverted trapezoid"). Its bottom width is greater than the maximum outer diameter of the optical fiber it accommodates, ensuring that the optical fiber can be stably placed at the bottom of the groove, providing a precise reference for subsequent embedding and fixing. Bottom width L1 = 0.9~2.6 mm (adapted according to the outer diameter of the optical fiber); top width L2 = 3 mm; opening angle φ2 = 20°; the bottom of the groove is arc-shaped with a predetermined bending radius, bending radius R1 = 7.5 mm; groove depth = D3 / 2-R1 ≈ 1 mm. The entire processing is a continuous production process to ensure the consistency of the groove geometry parameters (L1, L2, φ2, R1).

[0066] Step S2: Sensor Fiber Pretreatment and Winding. This step prepares a sensor fiber with a mirror-symmetric cross-section and no internal stress, laying the foundation for twist-free winding. The sensor fiber undergoes a first stress-relief treatment, and a flexible, irregularly shaped coating layer with a mirror-symmetric cross-section is formed on the outer surface of the stress-relieved fiber using a coating process. The coated sensor fiber then undergoes a second stress-relief treatment and is wound onto a fiber feeding reel.

[0067] Specifically, firstly, standard bend-insensitive fiber is selected as the substrate. This fiber undergoes a first stress-relief splitting process: residual stress accumulated during fiber drawing or storage is released through heating or mechanical means to prevent deformation or additional loss during subsequent winding. Then, a shaped coating layer with a mirror-symmetric cross-section is created on the outer surface of the stress-relieved fiber. After coating, the bend-insensitive fiber undergoes a second stress-relief splitting process to ensure no internal stress exists between the coating layer and the fiber body. Finally, the treated bend-insensitive fiber is uniformly and tightly wound onto a dedicated fiber distribution plate according to its cross-sectional direction, maintaining the direction of its mirror symmetry axis during winding (e.g., the minor axis of all elliptical fibers is perpendicular to the plate surface).

[0068] Step S3: Image Monitoring and Active Un-Twist Control. This step involves real-time correction of the optical fiber's rotational attitude before it enters the spiral groove, ensuring "no twisting or entanglement." The sensing fiber is released from the fiber release tray and guided to the winding cage. Before the sensing fiber enters the inverted trapezoidal spiral groove through the winding cage, a high-speed image monitoring system identifies the cross-sectional attitude of the irregularly shaped coating layer covering the outer surface of the sensing fiber in real time. An active un-twist mechanism dynamically adjusts the fiber's twisting attitude to compensate for torsional stress, ensuring that the designated contact surface of the irregularly shaped coating layer advances towards the fixed attitude of the bottom of the inverted trapezoidal spiral groove.

[0069] Specifically, a fiber-laying reel wound with bend-insensitive fiber is mounted on a fiber-laying mechanism. After the bend-insensitive fiber is released from the reel, it passes through a set of guide wheels and enters the winding cage. Before the bend-insensitive fiber enters the inverted trapezoidal spiral groove through the winding cage, a high-speed image monitoring system (such as a CCD camera + image processing unit) is set up to capture cross-sectional images of the irregularly shaped coating layer covering the outer surface of the bend-insensitive fiber in real time. The image processing unit identifies the cross-sectional orientation of the irregularly shaped coating layer covering the current outer surface of the fiber in real time (e.g., the direction of the major axis of the ellipse, the orientation of the plane of the D-shape, etc.). If the system detects that the fiber is rotating around its own axis (i.e., "torsion"), it immediately sends a command to the anti-torsion servo motor. The anti-torsion servo motor drives the fiber-laying reel or intermediate guide wheel to rotate in the opposite direction, dynamically compensating for the torsion angle (compensating for the torsion generated by the fiber during transmission), so that the bend-insensitive fiber always maintains the preset orientation (e.g., the plane of the D-shaped fiber always faces downward). This closed-loop control ensures that when the optical fiber enters the inverted trapezoidal spiral groove 4, the designated contact surface of the irregular coating layer (i.e. the contact surface that contacts the bottom of the inverted trapezoidal spiral groove) moves towards the center of the bottom of the inverted trapezoidal spiral groove 4 in a fixed posture, achieving true "twisting-free winding".

[0070] Step S4: Low-tension tangential synchronous embedding. This step precisely and stress-free embeds the stable optical fiber into the bottom of the helical groove. Using a tangential fiber feeding method, the dynamically adjusted sensing optical fiber is synchronously embedded into the center position of the bottom of the inverted trapezoidal helical groove with a preset tension and angle.

[0071] Specifically, using a dedicated winding cage, stable, bend-insensitive optical fibers are wound onto the helical core post 3 using a tangential feeding method, with preset low tension (0–15 g controllable) and angle (typically a 20°–30° angle), and precisely embedded into the bottom center of the inverted trapezoidal helical groove 4. This means the stable, bend-insensitive optical fibers are synchronously embedded into the bottom center of the inverted trapezoidal helical groove 4 at a direction tangential to the outer circle of the helical core post 3 (typically a 20°–30° angle), avoiding lateral scraping or compression. The fiber feeding tension is adjusted by a precision tension controller, ranging from 0–15 g (grams of force), which is extremely low tension, minimizing additional stress on the optical fiber. During this process, the fiber release speed, core cable advance speed, and winding cage rotation speed are precisely synchronized through a servo system to ensure a constant fiber pitch (helical pitch) L3 per turn, and that the fiber smoothly conforms to the bottom of the groove without slack or excessive tightness. The optical fiber is finally precisely guided to the bottom center position (point C) of the inverted trapezoidal helical groove 4, achieving surface contact.

[0072] Step S5: Secondary extrusion, filling and curing, and outer sheath forming. This permanently fixes the optical fiber in the groove and forms a complete optical cable structure. Simultaneously with the embedding of the sensing optical fiber, a second extrusion process fills the inverted trapezoidal spiral groove with a sound-sensitive material, wrapping the uncontacted bottom portion of the sensing optical fiber. After curing, the sensing optical fiber is semi-embedded and fixed to the spiral core post.

[0073] Specifically, while the bend-insensitive optical fiber is embedded in the inverted trapezoidal spiral groove 4, a second extrusion process is performed: high-temperature molten acoustic-sensitive material (such as polyurethane) is injected into the inverted trapezoidal spiral groove 4 and completely encapsulates the sensing optical fiber 5. Depending on design requirements, a gel material (such as polyurethane) can be pre-coated uniformly inside the groove or on the surface of the optical fiber before extrusion to regulate acoustic coupling characteristics. After filling, the acoustic-sensitive material is cured by heating or ultraviolet irradiation, and the sensing optical fiber 5 is semi-embedded and fixed to the spiral core post 3, achieving a semi-embedded thermal fusion fixation between the sensing optical fiber 5 and the spiral core post 3, forming a "semi-embedded" structure—neither loose nor fully encapsulated, but tightly fitted with uniform stress. Finally, the sensing optical fiber embedded in the inverted trapezoidal spiral groove 4 and the core cable form a seamless and tightly integrated whole.

[0074] Step S6: An outer protective layer 6 (made of polyurethane or high-polymer polypropylene) is applied to the outer surface of the spiral core 3 through a third extrusion process. The outer protective layer 6 covers the inverted trapezoidal spiral groove 4, in which the sensing fiber has been embedded, and the acoustically transparent sensitive material filling it. That is, the spiral core 3, the sensing fiber 5 embedded in the inverted trapezoidal spiral groove 4, and the acoustically transparent encapsulating material are all covered by the outer protective layer 6. The final product is a complete embedded mirror-symmetric flexible acoustically transparent sensitive fiber sensing cable with high consistency, high stability, and high sensitivity.

[0075] The core innovation of this invention, a manufacturing method for an embedded mirror-symmetric flexible acoustically transparent sensitive fiber optic sensing cable, lies in overcoming the pain points of "fiber torsion misalignment" and "poor fixation stability" in traditional optical cable manufacturing through key technologies such as "twist-relief stress-reducing precise attitude control" and "synchronous embedding and hot-melt fixation." Before and after winding, the irregularly shaped fiber undergoes stress-relief fiber separation. During the winding process, a real-time untwist process is performed using an image monitoring system to ensure that the symmetry axis of the irregularly shaped fiber remains fixed when entering the inverted trapezoidal spiral groove, achieving twist-free winding of the central axis. Employing a multi-layer synchronous extrusion process, the fiber is precisely wound into the inverted trapezoidal spiral groove (synchronous embedding) while being firmly and uniformly fixed to the core cable through hot-melt extrusion. This achieves a leap from "point contact" to "surface contact" and then to "embedded integration." Thus, this invention successfully manufactures a novel spiral sensing optical cable—an embedded mirror-symmetric flexible acoustically transparent sensitive fiber optic sensing cable—that is highly uniform, stable, and reliable in both physical structure and optical performance.

[0076] Preferably, in step S3, the release speed of the sensing optical fiber, the forward speed of the core cable, and the rotation speed of the winding cage are synchronously controlled by a servo motor, and the ratio of the release speed of the sensing optical fiber to the forward speed of the core cable is 2:1, which can be adaptively adjusted according to the helical pitch requirements to ensure that the helical pitch of the sensing optical fiber is constant and without additional stress.

[0077] Preferably, in step S5, different types of sound-transmitting sensitive materials are selected to fill the inverted trapezoidal spiral groove according to the acoustic response characteristics requirements of the target monitoring scene, and cured by heating or ultraviolet irradiation, so as to adjust the sensitivity of the sensing fiber to sound waves along the axial or radial direction of the optical fiber sensing cable.

[0078] Preferably, in step S3, the active untorsion mechanism drives the fiber feeding disc or guide wheel to rotate in the opposite direction via a servo motor to compensate for the torsional stress of the sensing optical fiber.

[0079] Preferably, in step S4, the tension ranges from 0 to 15 grams, and the angle is 20 to 30 degrees along the tangential direction to the outer circle of the spiral core. By employing low tension (controllable 0-15g) and a tangential fiber feeding method, the lateral and axial torsional stresses on the optical fiber are minimized, thereby reducing the additional loss of the optical fiber to a minimum.

[0080] This invention provides an objective and scientific embedded mirror-symmetric flexible acoustically transparent sensitive fiber optic sensing cable and its manufacturing method. Through collaborative innovation integrating structural design, material selection, and process control, it achieves a comprehensive improvement in four dimensions of distributed acoustic sensing (DAS) optical cables: performance consistency, acoustic sensitivity, mechanical stability, and engineering reliability. Compared to existing technologies, this solution eliminates the need for costly tension control or post-calibration, enabling stable output of high-performance DAS optical cables in large-scale production. This significantly lowers the application threshold and provides a reliable, efficient, and engineerable sensing infrastructure for key areas such as earthquake monitoring, oil and gas pipeline safety early warning, perimeter security, and underwater acoustic detection.

[0081] It should be noted that the specific embodiments described above enable those skilled in the art to more fully understand the present invention, but do not limit the present invention in any way. Therefore, although the present invention has been described in detail with reference to the accompanying drawings and embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the present invention. In short, all technical solutions and improvements that do not depart from the spirit and scope of the present invention should be covered within the protection scope of the present invention patent.

Claims

1. An embedded mirror-symmetric flexible acoustically transparent sensitive fiber optic sensing cable, characterized in that, It includes a core cable, at least one sensing optical fiber, sound-sensitive material, and an outer protective layer; The core cable comprises, from the inside out, a support core, a buffer layer, and a spiral core column. The support core is composed of multiple reinforcing members with tensile strength. The buffer layer is made of a support material with stress-absorbing function and covers the outer periphery of the support core. The spiral core column is made of a sound-permeable material with acoustic conductivity and covers the outer periphery of the buffer layer. The outer surface of the spiral core column is provided with at least one inverted trapezoidal spiral groove extending spirally along its axis. The groove opening width is greater than the groove bottom width, and the groove wall gradually slopes outward from the groove bottom towards the groove opening. The groove bottom is arc-shaped and has a predetermined bending radius, and the groove bottom width is greater than the maximum outer diameter of the optical fiber it accommodates. The outer surface of the sensing fiber is covered with a flexible, irregularly shaped coating layer with a mirror-symmetrical cross-section. The sensing fiber is embedded in the bottom center of the inverted trapezoidal spiral groove in a non-twisted state, so that the flexible contact surface of the irregularly shaped coating layer that contacts the bottom of the groove conformally fits the arc surface of the bottom of the inverted trapezoidal spiral groove, thereby forming a surface contact. The sound-sensitive material is filled in the inverted trapezoidal spiral groove and wraps the portion of the sensing optical fiber that is coated with an irregular shape without forming a surface contact with the bottom of the groove. After curing, the sensing optical fiber is semi-embedded and fixed to the spiral core column, and its top does not extend beyond the outer peripheral surface of the spiral core column. The outer protective layer covers the outer peripheral surface of the spiral core and covers the inverted trapezoidal spiral groove into which the sensing fiber has been embedded and the acoustically sensitive material filling it.

2. The embedded mirror-symmetric flexible acoustically transparent sensitive fiber optic sensing cable according to claim 1, characterized in that, The cross-sectional shape of the irregularly shaped coating layer is elliptical, D-shaped, or semi-circular; when the cross-sectional shape of the irregularly shaped coating layer is elliptical, its contact surface conformally fits the arc surface of the bottom of the groove along the short axis; when the cross-sectional shape of the irregularly shaped coating layer is D-shaped, its flat bottom surface conformally fits the arc surface of the bottom of the groove; when the cross-sectional shape of the irregularly shaped coating layer is semi-circular, its diameter plane conformally fits the arc surface of the bottom of the groove.

3. The embedded mirror-symmetric flexible acoustically transparent sensitive fiber optic sensing cable according to claim 1, characterized in that, The spiral core column has multiple inverted trapezoidal spiral grooves, which are evenly distributed along the circumference of the spiral core column, and the central angle between the center lines of any two adjacent inverted trapezoidal spiral grooves is equal.

4. The embedded mirror-symmetric flexible acoustically transparent sensitive fiber optic sensing cable according to claim 1, characterized in that, The sensing optical fiber is a bend-insensitive optical fiber; the sound-sensitive material is polyurethane, which is cured by heating or ultraviolet irradiation; the outer protective layer is made of polyurethane or high polypropylene material.

5. The embedded mirror-symmetric flexible acoustically transparent sensitive fiber optic sensing cable according to claim 1, characterized in that, The tensile-strength reinforcing member is steel wire or Kevlar rope; the stress-absorbing support material is Kevlar-reinforcing material; and the acoustically conductive material is high-polypropylene material.

6. A method for manufacturing an embedded mirror-symmetric flexible acoustically transparent sensitive fiber optic sensing cable as described in any one of claims 1 to 5, characterized in that, Includes the following steps: S1: A core cable comprising the support core, buffer layer and spiral core column is formed through the first extrusion process, and at least one inverted trapezoidal spiral groove extending spirally along its axial direction is simultaneously machined on the outer surface of the spiral core column using a milling device. S2: Perform the first stress relief treatment on the sensing fiber, and form a flexible irregular coating layer with a mirror-symmetric cross-section on the outer surface of the stress-relieved sensing fiber through a coating process. Perform the second stress relief treatment on the coated sensing fiber and wind it onto the fiber laying tray. S3: Release the sensing fiber on the fiber release tray and guide it to the winding cage. Before the sensing fiber enters the inverted trapezoidal spiral groove through the winding cage, the cross-sectional orientation of the irregular coating layer covering the outer surface of the sensing fiber is identified in real time by the high-speed image monitoring system. The twisting orientation of the fiber is dynamically adjusted by the active un-twist mechanism to compensate for the torsional stress of the fiber, so that the designated contact surface of the irregular coating layer moves toward the fixed orientation of the bottom of the inverted trapezoidal spiral groove. S4: Using a tangential fiber feeding method, the dynamically adjusted sensing fiber is synchronously embedded into the bottom center position of the inverted trapezoidal spiral groove with a preset tension and angle. S5: While the sensing fiber is being embedded, a second extrusion process is used to fill the inverted trapezoidal spiral groove with sound-sensitive material and wrap the uncontacted bottom part of the sensing fiber. After curing, the sensing fiber is semi-embedded and fixed to the spiral core. S6: An outer protective layer is coated on the outer circumferential surface of the spiral core through a third extrusion process. The outer protective layer covers the inverted trapezoidal spiral groove into which the sensing fiber has been embedded and the acoustically sensitive material filling it, thereby forming a complete optical fiber sensing cable.

7. The manufacturing method according to claim 6, characterized in that, In step S3, the release speed of the sensing optical fiber, the forward speed of the core cable, and the rotation speed of the winding cage are synchronously controlled by a servo motor. The ratio of the release speed of the sensing optical fiber to the forward speed of the core cable is 2:1, and is adaptively adjusted according to the helical pitch requirements to ensure that the helical pitch of the sensing optical fiber is constant and without additional stress.

8. The manufacturing method according to claim 6, characterized in that, In step S5, based on the acoustic response characteristics requirements of the target monitoring scene, different types of sound-transmitting sensitive materials are selected to fill the inverted trapezoidal spiral groove, and then cured by heating or ultraviolet irradiation to adjust the sensitivity of the sensing fiber to sound waves along the axial or radial direction of the fiber sensing cable.

9. The manufacturing method according to claim 6, characterized in that, In step S3, the active untorsion mechanism drives the fiber feeding disc or guide wheel to rotate in the opposite direction via a servo motor to compensate for the torsional stress of the sensing optical fiber.

10. The manufacturing method according to claim 6, characterized in that, In step S4, the tension ranges from 0 to 15 grams, and the angle is from 20 to 30 degrees along the direction tangential to the outer circle of the spiral core.

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