Two-dimensional vector bending sensor and preparation method, two-dimensional vector bending measurement method

Abstract

The present invention discloses a two-dimensional vector bending sensor, comprising an optical fiber having a grating thereon, the grating being tilted along a fixed tilt angle, and the optical fiber being encapsulated in a flexible cylinder at a position deviating from the neutral axis to form an asymmetric structure. The two-dimensional vector bending sensor disclosed in the present invention greatly simplifies the manufacturing difficulty of the sensor, and realizes two-dimensional vector bending sensing through a single optical fiber grating without the need for a complex structure or damage to the optical fiber. At the same time, both the transverse electric (TE) and transverse magnetic (TM) modes acquire one-dimensional bending sensing capabilities. By utilizing the orthogonal relationship between the bending sensitivities of the two modes, it is equivalent to orthogonally connecting two one-dimensional sensors in series, so that the measurement results are unique due to being restricted by the two modes, thereby realizing the function of two-dimensional vector bending sensing and having high detection sensitivity. The sensor has broad application prospects in fields such as structural health monitoring and intelligent machinery in complex environments.

Description

Technical Field

[0001] The present invention relates to the field of two-dimensional vector bending parameter measurement, and in particular to a two-dimensional vector bending sensor and a preparation method, and a two-dimensional vector bending measurement method. Background Art

[0002] Vector bending detection plays an extremely important role in various fields such as structural health monitoring, intelligent machinery, and human health. In recent years, fiber optic sensors have been widely used in vector bending measurement due to their high sensitivity, compact size, resistance to electromagnetic interference, remote sensing, and suitability for harsh environments. According to their directional discrimination capabilities, they can be divided into one-dimensional and two-dimensional sensors. One-dimensional vector bending sensors usually achieve the ability to identify one-dimensional positive and negative directions by destroying the cylindrical symmetry of the optical fiber, such as eccentric fiber Bragg gratings, tilted fiber Bragg gratings, D-type fibers, and laterally offset fibers. Two-dimensional vector bending sensors can be realized by orthogonally connecting two one-dimensional sensors in series, nesting multiple one-dimensional sensors in parallel through multi-core optical fibers, or combining multiple sensing mechanisms.

[0003] Although these sensors have been shown to be able to measure bending curvature and distinguish direction, there are challenges in manufacturing orthogonal series fiber Bragg gratings or nesting multiple one-dimensional sensors in parallel, making it difficult to widely apply them in the field of two-dimensional vector bending detection.

[0004] In addition, the stability of sensors based on fiber interferometers and the sensitivity of sensors based on fiber Bragg gratings are relatively low, which makes the sensitivity of two-dimensional vector bending detection also low. Summary of the Invention

[0005] The present invention aims to provide a two-dimensional vector bending sensor and a preparation method, as well as a two-dimensional vector bending measurement method, to solve the technical problems in the prior art that two-dimensional vector bending sensors are difficult to manufacture and the sensitivity of two-dimensional vector bending detection is low.

[0006] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0007] In a first aspect, the present invention discloses a two-dimensional vector bending sensor, comprising an optical fiber, a grating on the optical fiber, the grating being tilted along a fixed tilt angle, and the optical fiber being encapsulated in a flexible cylinder at a position deviating from the neutral axis to form an asymmetric structure. In the present invention, because the optical fiber with the grating at a fixed tilt angle is embedded in a non-neutral axis position of the cylinder, the sensor composed of the optical fiber and the cylinder is an asymmetric structure, which meets the conditions for one-dimensional vector bending sensing. The two-dimensional vector bending sensor disclosed in the present invention is the first to realize two-dimensional vector bending sensing through a single fiber grating without the need for a complex structure and the destruction of the optical fiber, so that both the transverse electric (TE) and transverse magnetic (TM) modes obtain one-dimensional bending sensing capabilities. By utilizing the orthogonal relationship between the bending sensitivities of the two modes, it is equivalent to orthogonally connecting two one-dimensional sensors in series, so that the measurement results are unique due to being restricted by the two modes, thereby realizing the function of two-dimensional vector bending sensing and having high detection sensitivity.

[0008] Preferably, the optical fiber utilizes an extremely large-angle fiber Bragg grating (ExTFG), wherein the grating tilt angle of the ExTFG is greater than 66.9°. The two-dimensional vector bend sensor employed in the present invention utilizes only a single bare extremely large-angle fiber Bragg grating (ExTFG), resulting in a simple and stable structure, ease of manufacture, and the ability to select different off-axis distances based on actual needs to achieve optimal bend sensitivity.

[0009] Preferably, the grating has a tilt angle of 81° and an axial grating period of 28 μm. The grating tilt angle of the extremely large tilt-angle fiber Bragg grating (ExTFG) is much larger than the typical angle of a standard tilted fiber Bragg grating (TFBG) (typically less than 45°). Due to its very large tilt angle, the axial period (28 μm) of the ExTFG is much larger than that of a standard TFBG, and it exhibits some properties similar to those of a long-period fiber grating (LPFG), such as mode coupling and high sensitivity to specific physical quantities.

[0010] Preferably, the flexible cylindrical material is made of a cured first ultraviolet adhesive.

[0011] Preferably, the surface of the extremely large tilt fiber grating is coated with a second layer of UV glue, which is a low-refractive-index UV glue with a refractive index of 1.327. The cured first UV glue is coated on the periphery of the second UV glue. Because ExTFG is highly sensitive to changes in the refractive index (SRI) of the surrounding medium, changes in the refractive index of the surrounding medium may affect the performance of ExTFG, such as changing its resonant wavelength or affecting its sensing characteristics. The refractive index of the second UV glue used is only 1.327, which is lower than the refractive index of the optical fiber cladding. This can reduce mode coupling or light scattering caused by refractive index discontinuities, help maintain the long-term stability of ExTFG, and ensure that the sensor can maintain its sensing performance under different environmental conditions. The coated second UV glue not only plays a role in refractive index matching, but also provides physical protection for ExTFG to prevent damage caused by environmental factors (such as mechanical scratches, humidity, temperature changes, etc.). The curing process of the UV glue is fast and controllable, which is conducive to the rapid manufacture of sensors.

[0012] Preferably, the cross-sectional diameter of the two-dimensional vector bending sensor ranges from 1 to 5 mm, wherein the ratio of the diameter of the flexible cylinder to the core diameter of the maximum-tilt fiber Bragg grating is in the range of 100:1 to 625:1, and the diameter-to-effective-length ratio of the two-dimensional vector bending sensor is in the range of 1:14 to 1:35. This allows the two-dimensional vector bending sensor to maintain good flexibility and ductility while also maintaining a certain degree of mechanical stability. The flexibility and ductility of the flexible cylinder enable it to return to its original shape after multiple measurements, thereby accommodating multiple measurements and extending its service life.

[0013] As a preferred dimension for the 2D vector bending sensor, the cross-sectional diameter of the flexible cylinder is 2.5 mm, and the effective length of the 2D vector bending sensor is 7 mm. Although the flexible cylinder is formed by curing UV adhesive, its small diameter maintains its flexibility and ductility, allowing it to adhere well to the surface of an object. Furthermore, the flexibility and ductility of the flexible cylinder allow it to return to its original shape after multiple measurements, accommodating multiple measurements and extending its service life.

[0014] In a second aspect, the present invention further discloses a method for preparing a two-dimensional vector bending sensor, using the two-dimensional vector bending sensor as described above, comprising the following steps:

[0015] S1. Fabricate a mold for preparing a two-dimensional vector bending sensor, the mold comprising a clamping member having a placement position for a cylindrical hose; a first positioning member and a second positioning member are detachably mounted at each end of the clamping member, each of the first positioning member and the second positioning member having corresponding first and second perforations, respectively, with a center line connecting the first and second perforations being parallel to an extension direction of the placement position and located at a position offset from the central axis of the hose;

[0016] S2, the inner diameters of the first and second perforations are interference-fitted with the outer diameter of the optical fiber; the optical fiber with the fixed tilt angle grating is passed through the first perforation, passed through the hose, and then passed out of the second perforation, ensuring that the grating area is located within the hose, and then the optical fiber is kept in a straightened state;

[0017] S3, injecting the first UV glue evenly into the hose from one end of the hose, and then irradiating the hose with UV light until the first UV glue is completely cured;

[0018] S4. Detach the first positioning member and the second positioning member from both ends of the clamping member, remove the hose, and remove the optical fiber coated with the cured first UV adhesive from the hose to obtain a two-dimensional vector bending sensor.

[0019] In a third aspect, the present invention further discloses a two-dimensional vector bending measurement method, using the two-dimensional vector bending sensor as described above, comprising the following steps:

[0020] A1. Build a two-dimensional vector bending parameter detection model based on the pure bending model to obtain the relationship between the curvature of the two-dimensional vector bending sensor, axial strain, and off-axis distance. Determine the off-axis distance based on the required maximum measured curvature of the two-dimensional vector bending and the known maximum axial strain allowed by the optical fiber.

[0021] A2. preparing a two-dimensional vector bending sensor according to the off-axis distance, and performing vector bending sensitivity calibration on the prepared two-dimensional vector bending sensor to obtain a vector bending sensitivity curve of the two-dimensional vector bending sensor;

[0022] A3, placing the two-dimensional vector bending sensor in TE and TM isoexcitation states simultaneously, and simultaneously monitoring the resonant wavelength changes of the TM and TE modes in the isoexcitation states;

[0023] A4. According to the changes in the resonant wavelengths of the TM and TE modes, combined with the vector bending sensitivity fitting functions of the TM and TE modes, the two-dimensional vector bending parameters are calculated.

[0024] Preferably, in step A4, the vector bending sensitivity fitting function is:

[0025]

[0026] Where: ω is the angle between the line connecting the center of the optical fiber and the center point of the cross section of the two-dimensional vector bending sensor relative to the vertical symmetry axis of the cross section, θ is the bending direction of the two-dimensional vector bending sensor, and d eff is the effective off-axis distance, A, y0, B, θ c are all constants.

[0027] The present invention has the following beneficial effects: the two-dimensional vector bending sensor disclosed in the present invention greatly simplifies the manufacturing difficulty of the sensor, realizes two-dimensional vector bending sensing through a single fiber Bragg grating without the need for complex structures and damage to the optical fiber, and at the same time, enables both the transverse electric (TE) and transverse magnetic (TM) modes to obtain one-dimensional bending sensing capabilities. By utilizing the orthogonal relationship between the bending sensitivities of these two modes, it is equivalent to orthogonally connecting two one-dimensional sensors in series, so that the measurement results are unique due to the limitations of the two modes, thereby realizing the function of two-dimensional vector bending sensing and having higher detection sensitivity; compared with sensors based on fiber Bragg gratings, it has higher sensitivity, and compared with sensors based on fiber interferometers, it has higher working stability and also has low-temperature crosstalk characteristics; the sensor has broad application prospects in fields such as structural health monitoring and intelligent machinery in complex environments. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to make the purpose, technical solutions and advantages of the invention more clear, the present invention will be further described in detail below with reference to the accompanying drawings, in which:

[0029] Figure 1 This is a schematic structural diagram of the two-dimensional vector bending sensor of the present invention.

[0030] Figure 2 Schematic diagram of the polar coordinate system of the cross section of the two-dimensional vector bending sensor of the present invention.

[0031] Figure 3 1 is the transmission spectrum of the two-dimensional vector bending sensor before and after packaging of the present invention.

[0032] Figure 4 This is a simulation diagram of the relationship between the double peak interval and SRI of the two-dimensional vector bending sensor of the present invention.

[0033] Figure 5 This is a schematic diagram of the mold structure for preparing a two-dimensional vector bending sensor according to the present invention.

[0034] Figure 6 Schematic diagram of the pure bending model.

[0035] Figure 7 Schematic diagram of the polar coordinate system of the cross section of the two-dimensional vector bending sensor of the present invention when bending.

[0036] Figure 8 This is a polar coordinate simulation diagram of the bending sensitivity of the two-dimensional vector bending sensor when ω=0° according to an embodiment of the present invention.

[0037] Figure 9 This is a polar coordinate simulation diagram of the bending sensitivity of the two-dimensional vector bending sensor when ω=15° according to an embodiment of the present invention.

[0038] Figure 10 This is a polar coordinate simulation diagram of the bending sensitivity of the two-dimensional vector bending sensor when ω=30° according to an embodiment of the present invention.

[0039] Figure 11 This is a polar coordinate simulation diagram of the bending sensitivity of the two-dimensional vector bending sensor when ω=45° according to an embodiment of the present invention.

[0040] Figure 12 Schematic diagram of a vector bending sensitivity calibration system according to an embodiment of the present invention.

[0041] Figure 13 1 is the transmission spectrum of the two-dimensional vector bending sensor at different bending curvatures when the bending direction is 75° according to an embodiment of the present invention.

[0042] Figure 14 1 is the transmission spectrum of the two-dimensional vector bending sensor at different bending curvatures when the bending direction is 165° according to an embodiment of the present invention.

[0043] Figure 15 1 is the transmission spectrum of the two-dimensional vector bending sensor at different bending curvatures when the bending direction is 255° according to an embodiment of the present invention.

[0044] Figure 16 1 is the transmission spectrum of the two-dimensional vector bending sensor at different bending curvatures when the bending direction is 345° according to an embodiment of the present invention.

[0045] Figure 17 FIG. 4 is a polar coordinate diagram of the bending sensitivity of the two-dimensional vector bending sensor according to an embodiment of the present invention.

[0046] Figure 18 1 and 2 are F(θ) / Δλ curves of the TM and TE modes of the embodiment of the present invention.

[0047] Explanation of the accompanying symbols: 1. optical fiber; 11. core; 12. grating; 13. cladding; 2. flexible cylinder; 3. clamping member; 31. placement position; 4. hose; 5. first positioning member; 51. first perforation; 6. second positioning member; 61. second perforation; 7. calibration system; 70. two-dimensional vector bending sensor; 71. ASE light source; 72. polarizer; 73. polarization controller; 74. optical fiber rotator; 75. spectrometer; 76. standard curvature model. DETAILED DESCRIPTION

[0048] To make the objectives, technical solutions and advantages of the embodiments of the present invention more clear, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention.

[0049] It should be noted that similar reference numerals and letters denote similar items in the following figures. Therefore, once an item is defined in one figure, it does not require further definition or explanation in subsequent figures. In the description of the present invention, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicating orientations or positional relationships, are based on the orientations or positional relationships shown in the figures, or the orientations or positional relationships in which the inventive product is typically placed when in use. These terms are intended solely to facilitate the description of the present invention and simplify the description. They do not indicate or imply that the devices or components referred to must have a specific orientation, be constructed, or operate in a specific orientation, and are therefore not to be construed as limiting the present invention. Furthermore, the terms "first," "second," and "third," etc., are used solely to distinguish descriptions and are not to be construed as indicating or implying relative importance. Furthermore, terms such as "horizontal" and "vertical" do not imply that a component must be absolutely horizontal or overhanging, but rather may be slightly tilted. For example, "horizontal" simply refers to a direction that is more horizontal than "vertical," and does not imply that the structure must be completely horizontal, but rather may be slightly tilted. In the description of the present invention, it should also be noted that, unless otherwise expressly specified or limited, the terms "disposed," "installed," "connected," and "connected" should be understood in a broad sense. For example, they may refer to fixed connections, detachable connections, or integral connections; they may refer to mechanical connections or electrical connections; they may refer to direct connections or indirect connections through an intermediate medium; and they may refer to internal communication between two components. Those skilled in the art will understand the specific meanings of the above terms in the present invention based on the specific circumstances.

[0050] The present invention can be applied to two-dimensional vector bending detection, and solves the technical problems in the prior art of difficulty in manufacturing two-dimensional vector bending sensors and low sensitivity of two-dimensional vector bending detection.

[0051] First, based on the technical issues solved above, please refer to Figure 1 The present invention discloses a two-dimensional vector bending sensor, comprising an optical fiber 1 having a grating 12 on the optical fiber 1, the grating 12 being tilted along a fixed tilt angle, and the optical fiber 1 being encapsulated in a flexible cylinder 2 at a position deviating from the neutral axis to form an asymmetric structure.

[0052] In the present invention, because the optical fiber with a fixed tilt angle grating is embedded in the non-neutral axis position of the cylinder, the sensor composed of the optical fiber and the cylinder is an asymmetric structure, which meets the conditions for one-dimensional vector bending sensing. The two-dimensional vector bending sensor disclosed in the present invention is the first to realize two-dimensional vector bending sensing through a single fiber Bragg grating without the need for a complex structure and damage to the optical fiber, so that both the transverse electric (TE) and transverse magnetic (TM) modes obtain one-dimensional bending sensing capabilities. By utilizing the orthogonal relationship between the bending sensitivities of these two modes, it is equivalent to orthogonally connecting two one-dimensional sensors in series, so that the measurement results are unique due to the limitations of the two modes, thereby realizing the function of two-dimensional vector bending sensing and having high detection sensitivity.

[0053] The maximum measurable curvature of the two-dimensional vector bending sensor of the present invention is determined solely by the off-axis distance d, which refers to the radial distance between the centerline of the optical fiber and the centerline of the flexible cylinder. As the off-axis distance d increases, the sensitivity of the two-dimensional vector bending sensor also increases, while the maximum measurable curvature of the two-dimensional vector bending sensor decreases. Therefore, the off-axis distance d can be adjusted according to actual needs, thereby enabling adjustment of the measurement threshold of the two-dimensional vector bending sensor. This provides high measurement flexibility and simple threshold adjustment.

[0054] Specifically, in the transverse magnetic (TM) mode, the electric field (E field) of the electromagnetic wave is perpendicular to the propagation direction, while the magnetic field (H field) has a component along the propagation direction. In the context of optical fiber, if light propagates along the fiber's axis, the TM mode is characterized by the light's electric field being completely distributed in the fiber's transverse plane, meaning the electric field does not propagate along the fiber's axis. In fiber Bragg gratings, the TM mode typically corresponds to a situation where the electric field interacts strongly with the grating's refractive index variation. In the transverse electric (TE) mode, the electric field (E field) has a component along the propagation direction, while the magnetic field (H field) is completely perpendicular to the propagation direction. In optical fiber, this means the electric field is not entirely located in the transverse plane, but rather has a component propagating along the fiber's axis. In fiber Bragg gratings, the TE mode typically corresponds to a situation where the magnetic field interacts strongly with the grating's refractive index variation.

[0055] Preferably, the optical fiber adopts an extremely large tilt-angle fiber Bragg grating (ExTFG), and the grating tilt angle of the extremely large tilt-angle fiber Bragg grating is greater than 66.9°.

[0056] The two-dimensional vector bending sensor used in the invention uses only one bare large-angle fiber Bragg grating, has a simple and stable structure, is easy to manufacture, and allows different off-axis distances to be selected according to actual needs to achieve optimal bending sensitivity.

[0057] The vector properties of extremely tilted fiber Bragg gratings (ExTFGs) include sensing of vector torsion, transverse loads, vector magnetic fields, vector refractive index, and vector acceleration. In ExTFGs, the grating fringe is tilted at an angle typically exceeding 66.9° relative to the fiber axis, causing coupling between the core mode and the co-propagating cladding mode. Furthermore, the cladding mode resonance peak of an ExTFG splits into two peaks, corresponding to the transverse magnetic (TM) and transverse electric (TE) modes, respectively. Bending can alter the coupling coefficient between the core and cladding modes, and the axial tensile strain induced by bending also causes a blueshift in the resonant wavelength. The bending sensitivity of ExTFGs exceeds that of fiber Bragg gratings (FBGs) and exhibits low-temperature crosstalk. Furthermore, their bending sensitivity exhibits significant directionality and polarization dependence, with the polar plots of the TE and TM mode sensitivities showing orthogonal figure-eight patterns. This opens the possibility of using ExTFGs for two-dimensional vector bending sensing.

[0058] For details, please refer to Figure 1 The extremely large tilt-angle fiber Bragg grating includes a core 11 and a cladding 13, wherein the cladding 13 is coated on the periphery of the core 11, and the grating 12 is located on the core 11; the diameter of the core 11 of the optical fiber 1 of the extremely large tilt-angle fiber Bragg grating is in the range of 8 to 10 μm, and the diameter of the cladding 13 is usually 125 μm.

[0059] Preferably, the grating has a tilt angle of 81° and an axial grating period of 28 μm. The grating tilt angle of the extremely large tilt-angle fiber Bragg grating (ExTFG) is much larger than the typical angle of a standard tilted fiber Bragg grating (TFBG) (typically less than 45°). Due to its very large tilt angle, the axial period (28 μm) of the ExTFG is much larger than that of a standard TFBG, and it exhibits some properties similar to those of a long-period fiber grating (LPFG), such as mode coupling and high sensitivity to specific physical quantities.

[0060] Preferably, the flexible cylindrical material is made of a cured first ultraviolet adhesive.

[0061] Preferably, the surface of the extremely large tilt fiber grating is coated with a second layer of UV glue, which is a low-refractive-index UV glue with a refractive index of 1.327. The cured first UV glue is coated on the periphery of the second UV glue. Because ExTFG is highly sensitive to changes in the refractive index (SRI) of the surrounding medium, changes in the refractive index of the surrounding medium may affect the performance of ExTFG, such as changing its resonant wavelength or affecting its sensing characteristics. The refractive index of the second UV glue used is only 1.327, which is lower than the refractive index of the optical fiber cladding. This can reduce mode coupling or light scattering caused by refractive index discontinuities, help maintain the long-term stability of ExTFG, and ensure that the sensor can maintain its sensing performance under different environmental conditions. The coated second UV glue not only plays a role in refractive index matching, but also provides physical protection for ExTFG to prevent damage caused by environmental factors (such as mechanical scratches, humidity, temperature changes, etc.). The curing process of the UV glue is fast and controllable, which is conducive to the rapid manufacture of sensors.

[0062] Preferably, the cross-sectional diameter of the two-dimensional vector bending sensor ranges from 1 to 5 mm, wherein the ratio of the diameter of the flexible cylinder to the core diameter of the maximum-tilt fiber Bragg grating is in the range of 100:1 to 625:1, and the diameter-to-effective-length ratio of the two-dimensional vector bending sensor is in the range of 1:14 to 1:35. This allows the two-dimensional vector bending sensor to maintain good flexibility and ductility while also maintaining a certain degree of mechanical stability. The flexibility and ductility of the flexible cylinder enable it to return to its original shape after multiple measurements, thereby accommodating multiple measurements and extending its service life.

[0063] Specifically, the effective length of the two-dimensional vector bending sensor refers to the effective length of the optical fiber including the fixed tilt angle grating. The optical fiber also has a pigtail when in use, and the length of the pigtail is determined according to actual needs.

[0064] Preferably, the cross-sectional diameter of the two-dimensional vector bending sensor is in the range of 1 to 5 mm. Within this diameter range, two-dimensional vector bending sensors have better flexibility and ductility, making them applicable to a wider range of scenarios. The flexibility and ductility of the flexible cylinder enable it to return to its original shape after multiple measurements, thus accommodating multiple measurements and extending its service life.

[0065] As a preferred embodiment of the 2D vector bending sensor's dimensions, the cross-sectional diameter of the flexible cylinder is 2.5 mm, and the effective length of the 2D vector bending sensor is 7 mm. Although the flexible cylinder is formed by curing UV adhesive, its small diameter maintains its flexibility and ductility, allowing it to adhere well to the surface of an object. Furthermore, the flexibility and ductility of the flexible cylinder allow it to return to its original shape after multiple measurements, accommodating multiple measurements and extending its service life.

[0066] The working principle of the two-dimensional vector bending sensor is described below.

[0067] Due to the asymmetric grating cross section caused by the introduction of the extremely large-angle tilted grating stripe structure, each cladding mode that meets the resonance conditions in the ExTFG has polarization-dependent degenerate modes of both TM and TE modes.

[0068] The phase matching condition (PMC) is an important formula that describes the mode coupling characteristics of fiber Bragg gratings. The PMC of ExTFG can be expressed as:

[0069]

[0070] In formula (1): res is the resonant wavelength, At wavelength λ res The effective refractive index of the core at At wavelength λ res The effective refractive index of the mth-order TM / TE cladding mode at G is the normal period of the grating, and θ is the tilt angle of the fiber Bragg grating.

[0071] From the theory of elastic mechanics and elasto-optical effect, we know that the effective refractive index of the fundamental mode and cladding mode of the optical fiber and the grating period are all functions of the axial strain. Differentiating equation (1) yields:

[0072]

[0073] In formula (2): is the resonant wavelength of the mth-order TM / TE cladding mode, ε is the axial strain of the optical fiber, and the other characteristics in formula (2) have the same meaning as formula (1).

[0074] When an optical fiber experiences axial strain, the elasto-optic effect will cause the effective refractive index of the core and cladding to change, and the magnitude of the change is related to their respective elasto-optic coefficients. Considering the mode dispersion and waveguide dispersion of the optical fiber, equation (2) can be expressed as:

[0075]

[0076] In formula (3), is the resonant wavelength of the mth-order TM / TE cladding mode, ε is the axial strain of the optical fiber, γ m Fiber waveguide dispersion factor, is the strain sensitivity factor of the resonant wavelength, defined as:

[0077]

[0078] In formula (4), η co and η cl are the elastic-optical coefficients of the core and cladding, respectively, At wavelength λ res The effective refractive index of the core at At wavelength λ res The effective refractive index of the m-th order TM / TE cladding mode at .

[0079] From formula (3), we can see that the magnitude and sign of the axial strain sensitivity of ExTFG are mainly determined by γ m and Since ExTFG is essentially a LPFG, the mode coupling theory of LPFG can be used to analyze the dispersion characteristics of ExTFG, that is, for an LPFG with a specific core elasto-optic coefficient, the sensitivity of the resonant wavelength is A dispersion inflection point appears at m=12. The modes near the dispersion inflection point have greater strain sensitivity, while the modes farther from the inflection point have smaller strain sensitivity.

[0080] When axial strain is applied to ExTFG, the strain sensitivity of the low-order cladding mode (m<12) is >0, and It increases with the increase of mode order m; while the strain sensitivity of high-order cladding mode (m>12) is <0, and It decreases with the increase of mode order m.

[0081] The present invention also explores the relationship between the bending sensitivity polar coordinate diagram of the two-dimensional vector bending sensor and ω, please refer to Figure 2 A polar coordinate system is constructed with the center of the cross section of the flexible cylinder as the origin. The angle between the line connecting the center of the optical fiber and the center point of the cross section of the two-dimensional vector bending sensor and the vertical symmetry axis of the cross section is ω. Both d and ω are adjustable parameters, and different values ​​will affect the performance of the sensor.

[0082] Figure 3The figure shows the changes in the transmission spectrum of the fiber before and after encapsulation. After encapsulation, a red shift is observed in the spectrum, and the low-order mode resonance doublet disappears, while the high-order mode resonance doublet remains. This is because the SRI sensitivity of ExTFG is also related to the mode order, with lower-order modes having higher SRI sensitivity. The sensitivity of lower-order modes increases, and the increase is more rapid, as the SRI increases. Furthermore, the SRI sensitivity of the TM mode is higher than that of the TE mode.

[0083] Figure 4 Simulation results show how the resonant wavelengths of TE0m and TM0m vary with SRI. The difference in the resonant wavelengths of lower-order modes decreases faster than that of higher-order modes, and therefore they reach zero more quickly. Consequently, the doublets of lower-order modes disappear into singlets more quickly. Although the simulation results don't exactly match the experimental data numerically due to the two-layer waveguide approximation used in the simulation, the trends are remarkably consistent, clearly demonstrating that the gap between the doublets of ExTFG decreases with increasing SRI, with the doublets of lower-order modes disappearing more quickly.

[0084] It is particularly important to note that due to Figure 3 and Figure 4 It is difficult to draw corresponding conclusions from the image using the grayscale image mode. Figure 3 and Figure 4 Displayed with color pictures.

[0085] The drift of the resonant wavelength is affected by the bending curvature and bending direction. When bending occurs, the TM and TE modes have the same bending curvature and bending direction, and only their respective vector bending sensitivities are different. According to the vector bending sensitivity fitting function of the TM and TE modes explained later, combined with the resonant wavelength drift lengths of the two modes, the bending direction θ can be calculated, and then the bending curvature can be obtained.

[0086] In a second aspect, the present invention further discloses a method for preparing a two-dimensional vector bending sensor, using the two-dimensional vector bending sensor as described above, comprising the following steps:

[0087] S1. Make a mold for preparing a two-dimensional vector bending sensor. Figure 5 The mold includes a clamping member 3, which is provided with a placement position 31 for placing a hose 4, and the hose 4 is cylindrical; a first positioning member 5 and a second positioning member 6 are detachably mounted on both ends of the clamping member 3, and the first positioning member 5 and the second positioning member 6 are respectively provided with a first through-hole 51 and a second through-hole 61 corresponding to the position. The center line of the first through-hole 51 and the second through-hole 61 is parallel to the extension direction of the placement position 31 and is located at a position deviated from the central axis of the hose 4;

[0088] S2. The inner diameters of the first through-hole 51 and the second through-hole 61 are interference-fitted with the outer diameter of the optical fiber 1. The optical fiber 1 having the fixed tilt angle grating 12 is passed through the first through-hole 51, passed through the hose 4, and then passed out through the second through-hole 61, ensuring that the grating area is located within the hose 4. The optical fiber 1 is then kept in a straightened state.

[0089] S3, injecting the first UV glue evenly into the hose from one end of the hose, and then irradiating the hose with UV light until the first UV glue is completely cured;

[0090] S4. Detach the first positioning member and the second positioning member from both ends of the clamping member, remove the hose, and remove the optical fiber coated with the cured first UV adhesive from the hose to obtain a two-dimensional vector bending sensor.

[0091] Preferably, in step S1, the optical fiber adopts an extremely large tilt-angle fiber Bragg grating (ExTFG), the tilt angle of the extremely large tilt-angle fiber Bragg grating is 81°, and the grating period in the axial direction is 28 μm.

[0092] Specifically, in step S1, the placement position is a through groove, the through groove extends along the extension direction of the clamping member, the inner wall of the through groove is a curved surface, the interior of the through groove is used to place the hose, and after the hose is placed in the through groove, the center line of the through groove coincides with the center line of the hose.

[0093] Specifically, in step S1, the clamping member, the first positioning member, and the second positioning member are all implemented by 3D printing.

[0094] Specifically, in step S1, the hose is a 2.5 mm silicone hose.

[0095] Preferably, before step S2, the surface of the optical fiber is first coated with a layer of a second UV glue, wherein the second UV glue is a low-refractive-index UV glue, and the refractive index of the second UV glue is 1.327.

[0096] In a third aspect, the present invention further discloses a two-dimensional vector bending measurement method, using the two-dimensional vector bending sensor as described above, comprising the following steps:

[0097] A1. Build a two-dimensional vector bending parameter detection model based on the pure bending model to obtain the relationship between the curvature of the two-dimensional vector bending sensor, axial strain, and off-axis distance. Determine the off-axis distance based on the required maximum measured curvature of the two-dimensional vector bending and the known maximum axial strain allowed by the optical fiber.

[0098] A2. preparing a two-dimensional vector bending sensor according to the off-axis distance, and performing vector bending sensitivity calibration on the prepared two-dimensional vector bending sensor to obtain a vector bending sensitivity curve of the two-dimensional vector bending sensor;

[0099] A3, placing the two-dimensional vector bending sensor in TE and TM isoexcitation states simultaneously, and simultaneously monitoring the resonant wavelength changes of the TM and TE modes in the isoexcitation states;

[0100] A4. According to the changes in the resonant wavelengths of the TM and TE modes, combined with the vector bending sensitivity fitting functions of the TM and TE modes, the two-dimensional vector bending parameters are calculated.

[0101] Specifically, in step A3, monitoring the change of the resonance wavelength of the TM and TE modes refers to monitoring the change of the resonance wavelength drift length of the TM and TE modes.

[0102] The two-dimensional vector bending sensor used in this measurement method has an asymmetric structure, which enables both TM and TE modes to have one-dimensional vector bending sensing capabilities. At the same time, the orthogonal relationship between the two modes is cleverly utilized to obtain two-dimensional vector bending parameters.

[0103] Specifically, in step A4, the two-dimensional vector bending parameters include bending direction and bending curvature.

[0104] Specifically, in step A1, the model is firstly built using the pure bending model in mechanical engineering. The pure bending model is a widely used model for describing bending, such as Figure 6 As shown, a two-dimensional vector bending sensor has a neutral layer. During bending, the length of the neutral layer does not change, meaning there is no axial strain. The two-dimensional vector bending sensor located above the neutral layer experiences stretching, while the two-dimensional vector bending sensor located below the neutral layer experiences compression. In the figure, L is the neutral layer length, d is the distance from the axis, and β is the center angle. The relationship between these parameters can be expressed as Equation (5-8):

[0105] L=R·β (5)

[0106] L±ΔL=(R±d)·β (6)

[0107]

[0108] In formula (5-8), ε is the axial strain of the sensing part, K max is the maximum measurable curvature, ΔL is the change in fiber length, and R is the radius of curvature. The relationship between the curvature of the two-dimensional vector bending sensor and the axial strain and the distance from the axis can be calculated from equation (5-8):

[0109]

[0110] According to formula (9), when the curvature remains constant, the axial strain of the ExTFG located above and below the neutral layer depends only on the off-axis distance d. As the off-axis distance d increases, the sensitivity of the two-dimensional vector bending sensor also increases. If the maximum axial strain that the ExTFG can withstand is known, the maximum measurable curvature K of the two-dimensional vector bending sensor using the off-axis package is max The maximum measurable curvature of a 2D vector bending sensor decreases as d increases. Adjusting the off-axis distance d based on actual needs allows the 2D vector bending sensor's measurement threshold to be adjusted.

[0111] When measuring the actual bending direction θ and the actual bending curvature K, the vector bending sensitivity of the two-dimensional vector bending sensor can be approximately fitted with a sine function. After off-axis packaging, different bending directions will result in different effective off-axis distances (d eff ),like Figure 7 As shown, θ represents the bending direction, and ω is the angle between the center of the fiber and the center of the cross section of the two-dimensional vector bending sensor relative to the vertical symmetry axis of the cross section. When the bending direction is between -ω and 180°-ω, the bending causes axial compressive strain, so a negative sign is required for d eff However, when the bending direction is between 180°-ω and 360°-ω, the bending causes axial tensile strain, so there is no need to make corrections to d eff Make corrections (0<ω<90°). d eff The relationship between and the off-axis distance (d) is expressed by formula (10). Therefore, the vector bending sensitivity of the packaged two-dimensional vector bending sensor can be fitted by formula (11).

[0112]

[0113] In equations (10) and (11), ω is the angle between the line connecting the center of the optical fiber and the center of the cross section of the two-dimensional vector bending sensor relative to the vertical symmetry axis of the cross section, θ is the bending direction of the two-dimensional vector bending sensor, and d eff is the effective off-axis distance, A, y0, B, θ c are all constants.

[0114] It is known that the drift of the resonant wavelength is affected by the bending curvature and bending direction, which can be expressed by formula (12):

[0115] Δλ=F(θ)·K (12)

[0116] In formula (12), F(θ) is the function after vector bending sensitivity fitting. Referring to formulas (10) and (11), when bending occurs, the TM and TE modes have the same bending curvature and bending direction, and only their respective vector bending sensitivities are different. Therefore, it can be seen from formula (12) that:

[0117]

[0118] In formula (13), F TM (θ) and F TE (θ) are the vector bending sensitivity fitting functions of the TM and TE modes, respectively. The vector bending sensitivity fitting functions of the TM and TE modes are known. Therefore, the actual bending direction θ can be calculated by knowing the resonant wavelength drift length of the two modes, and then the actual bending curvature K can be obtained.

[0119] The two-dimensional vector bending sensor, preparation method, and two-dimensional vector bending measurement method disclosed in the present invention have the following technical effects: the two-dimensional vector bending sensor based on ExTFG greatly simplifies the manufacturing difficulty of the sensor. Compared with sensors based on fiber Bragg gratings, it has higher sensitivity, and compared with sensors based on fiber interferometers, it has higher working stability and also has low-temperature crosstalk characteristics. The greater the distance from the neutral axis, the higher the sensitivity and the smaller the maximum measurable curvature. We can choose the most appropriate off-axis distance according to actual needs. This sensor has broad application prospects in fields such as structural health monitoring and intelligent machinery in complex environments.

[0120] Example

[0121] To further illustrate the two-dimensional vector bending sensor and two-dimensional vector bending detection method of the present invention, the present invention discloses the following application embodiment. This embodiment uses the following method to measure two-dimensional vector bending parameters, including the following steps:

[0122] D1. Build a two-dimensional vector bending parameter detection model based on the pure bending model to obtain the relationship between the curvature of the two-dimensional vector bending sensor, axial strain, and off-axis distance. Determine the off-axis distance based on the required maximum measured curvature of the two-dimensional vector bending and the known maximum axial strain allowed by the optical fiber.

[0123] D2. preparing a two-dimensional vector bending sensor according to the off-axis distance, and performing vector bending sensitivity calibration on the prepared two-dimensional vector bending sensor to obtain a vector bending sensitivity curve graph of the two-dimensional vector bending sensor;

[0124] D3, placing the two-dimensional vector bending sensor in TE and TM isoexcitation states simultaneously, and simultaneously monitoring the resonant wavelength changes of the TM and TE modes in the isoexcitation states;

[0125] D4. According to the changes in the resonant wavelengths of the TM and TE modes, combined with the vector bending sensitivity fitting functions of the TM and TE modes, the two-dimensional vector bending parameters are calculated.

[0126] Specifically, in step D1, please refer to Figure 1 and Figure 5 The two-dimensional vector bending sensor of this embodiment is prepared by the following preparation method, including the following steps: first, a mold for preparing the two-dimensional vector bending sensor is made, the mold including a clamping member, the clamping member is provided with a placement position for placing a hose, the hose is cylindrical; the two ends of the clamping member are detachably mounted with a first positioning member and a second positioning member, the first positioning member and the second positioning member are respectively provided with a first through-hole and a second through-hole corresponding to the position, the center line of the first through-hole and the second through-hole is parallel to the extension direction of the placement position, and is located at a position deviated from the central axis of the hose The inner diameters of the first and second perforations are interference fit with the outer diameter of the optical fiber; then the optical fiber with a fixed tilt angle grating is passed through the first perforation, passes through the hose, and then passes through the second perforation, ensuring that the grating area is located inside the hose, and then the optical fiber is kept in a straightened state; then, the first ultraviolet glue is evenly injected into the hose from one end, and then the hose is irradiated with ultraviolet light until the first ultraviolet glue is completely cured; then the first positioning member and the second positioning member are detached from both ends of the clamping member, and the hose is taken out, and the optical fiber coated with the cured first ultraviolet glue is taken out from the hose, thereby obtaining a two-dimensional vector bending sensor.

[0127] The two-dimensional vector bending sensor used in this embodiment is prepared with an extremely large tilt-angle fiber Bragg grating (ExTFG, hereinafter referred to as optical fiber), the tilt angle of the optical fiber is 81°, and the grating period in the axial direction is 28μm; the outer periphery of the optical fiber is coated with a layer of second ultraviolet glue, the second ultraviolet glue is a low-refractive-index ultraviolet glue, the refractive index of the second ultraviolet glue is 1.327, and the outer periphery of the second ultraviolet glue is coated with a flexible cylinder, and the flexible cylinder is made of cured first ultraviolet glue; the fast axis of the optical fiber is parallel to the extension direction of the two-dimensional vector bending sensor, and the optical fiber is located at the flexible An asymmetric structure is formed at the position deviating from the neutral axis in the flexible cylinder, and the deviation distance is the off-axis distance d, and the off-axis distance d refers to the radial distance between the center line of the optical fiber and the center line of the flexible cylinder. In this embodiment, the diameter of the flexible cylinder of the two-dimensional vector bending sensor used in this embodiment is 2.5 mm and the length is 7 cm. It is known that the maximum axial strain that a single-mode optical fiber can withstand is about 0.01333ε. If the off-axis distance d = 0.5 mm is selected, then according to formula (9), the theoretical maximum measurable curvature of the two-dimensional vector bending sensor used in this embodiment is 26.58m -1 .

[0128] In a polar coordinate system constructed with the center of the cross section of the flexible cylinder as the origin, the angle ω between the line connecting the center point of the cross section of the optical fiber and the origin of the polar coordinate system and the vertical direction is 45°. In this embodiment, the cladding mode order m of the optical fiber in the resonance peak of the 1300-1350 nm band is 37, so its strain sensitivity is <0, that is, under the action of axial tensile strain, the spectrum will undergo a blue shift, and conversely, under the action of axial compressive strain, the spectrum will undergo a red shift.

[0129] This embodiment studies the relationship between the bending sensitivity polar coordinate diagram of the two-dimensional vector bending sensor and ω, and uses the above data to perform fitting analysis. The results are as follows: Figures 8 to 11 As shown in the figure, the polar coordinate simulation diagrams of the bending sensitivity of the two-dimensional vector bending sensor are shown when ω=0°, ω=15°, ω=30°, and ω=45°. It is particularly important to note that due to Figures 8 to 11 It is difficult to draw corresponding conclusions from the image using the grayscale image mode. Figures 8 to 11 Displayed with color pictures.

[0130] The bending sensitivity of the TM and TE modes can be positive or negative, and the polar plots exhibit odd symmetry between the positive and negative regions. When ω = 0°, the sensitivity polar plot exhibits even symmetry. Therefore, when bending occurs in one direction, there exists another bending direction with the same curvature that induces the same change in the transmission spectrum. Therefore, the sensor's bending range is -90° to 90°.

[0131] like Figures 9 to 11 As shown, when ω is not equal to 0°, the sensitivity polar coordinate diagram no longer has even symmetry. The theoretical sensing range is 360° (except when the ExTFG is located in a bent neutral layer, the bending sensitivity is 0). In addition, as ω gradually increases to 45°, taking the positive region as an example, the difference between the two curves to the right of the right intersection gradually increases, while the difference between the two curves to the left of the left intersection gradually decreases until there is no intersection. This means that as ω increases, the bending sensitivity difference between the TM and TE modes becomes larger. When ω = 45°, the difference reaches a maximum value and there is only one intersection. This means that a unique bending direction and curvature can be reconstructed based on the direction and amplitude of the resonant wavelength shift of the TM and TE modes. Although in some bending directions, there may still be two bending directions with different bending curvatures that cause the same transmission spectrum change during the demodulation process, the difference between the two bending directions and curvatures is the largest at this time. In this case, using the change in the resonance peak intensity can help determine the unique bending direction and curvature.

[0132] Specifically, in step D2, the vector bending sensitivity calibration is performed as follows: Figure 12The calibration system 7 shown is carried out, and the calibration system 7 includes a two-dimensional vector bending sensor 70, an ASE light source 71, a polarizer 72, a polarization controller 73, a fiber rotator 74, and a spectrometer 75;

[0133] The light source emitting end of the ASE light source 71 is connected to the input end of the polarizer 72, the output end of the polarizer 72 is connected to the input end of the polarization controller 73, the output end of the polarization controller 73 is connected to the input end of the fiber rotator 74, the output end of the fiber rotator 74 is connected to the input end of the two-dimensional vector bending sensor 70, and the two-dimensional vector bending sensor is attached to the surface of the standard curvature model 76; the output end of the two-dimensional vector bending sensor 70 is connected to the input end of the spectrometer 75.

[0134] The calibration system 7 operates as follows: an ASE light source 71 (1250 nm to 650 nm) emits light, which passes sequentially through a polarizer 72, a polarization controller 73, and a fiber rotator 74. The two-dimensional vector bending sensor is placed in excitation states such as TE and TM by adjusting relevant parameters. The two-dimensional vector bending sensor is then attached to the surface of a 3D-printed standard curvature model. Finally, the spectrometer 75 (OSA) outputs the spectrum of the bend-modulated transmitted light.

[0135] During the vector bend sensitivity calibration experiment, the 2D vector bend sensor was subjected to different bending directions and curvatures by adjusting the fiber rotator and selecting curvature models with varying curvatures. Each time the fiber rotator was rotated, the PC was adjusted to maintain the ExTFG in an isotropic state. This calibrated the vector bend sensitivity of the 2D vector bend sensor.

[0136] The sensor is rotated in steps of 15° to measure the bending sensitivity in the range of 0 to 360°. Figures 13 to 16 Shows the curvature range of 0m -1 ~12m -1 The spectrum of the sensor changes in different directions. It is particularly important to note that due to Figures 13 to 16 It is difficult to draw corresponding conclusions from the image using the grayscale image mode. Figures 13 to 16 Displayed with color pictures.

[0137] When the bending direction is 75° ( Figure 13 ), the wavelength of the resonant double peak red shifts and the intensity decreases; when the bending direction is 165° ( Figure 14 ), the wavelength of the resonant double peak blue shifts and the intensity increases; when the bending direction is 255° ( Figure 15 ), the wavelength of the resonant double peak blue-shifts and the intensity decreases; when the bending direction is 345° ( Figure 16 ), the wavelength of the resonant double peak red-shifts and the intensity increases.

[0138] From the above results, we can see that the resonance doublet redshifts when the bending direction is between -45° and 135°, and blueshifts when the bending direction is between 135° and 315°. The resonance doublet decreases in intensity between 0° and 90°, increases between 90° and 180°, decreases between 180° and 270°, and increases between 270° and 360°. Therefore, we can use the changes in resonance intensity to help determine the unique bending direction and curvature.

[0139] The vector bending sensitivity of the two-dimensional vector bending sensor and its fitting curve are obtained by experimental measurement. Figure 17 As shown. It is particularly important to note that due to Figure 17 It is difficult to draw corresponding conclusions from the image using the grayscale image mode. Figure 17 Displayed with color pictures.

[0140] The fitted curve and Figure 9 The simulated vector bending sensitivity polar coordinate diagrams are basically the same. The slight difference is that the double peak wavelength between 1300-1350nm was selected in this experiment, while the double peak wavelength between 1550-1600nm was selected in the previous simulated vector bending sensitivity measurement. The order of the resonance peak mode selected in the two experiments is different, so the sensitivity of the TM and TE modes will be slightly different. The maximum curvature sensitivity of the two-dimensional vector bending sensor measured in the experiment is -105pm / m. -1 and 105.5pm / m -1 , -70.25pm / m -1 and 67.5pm / m -1 .

[0141] Taking the experimental data as an example, when the bending direction is 45° and the bending size is 2m -1 When Δλ TM is 0.165nm, Δλ TE is 0.135nm, the resonance intensity becomes smaller. Figure 18 As shown, the possible bending directions are 43.1°, 107°, 223.3°, and 287°. Because the wavelength shift is positive, that is, the bending sensitivity is positive, 223.3° and 287° can be ruled out. At the same time, the resonance intensity changes at 43.1° and 107° are different, so 107° can be ruled out. The bending direction can be uniquely determined to be 43.1°, and the curvature at this time is 2.0236m. -1 The reconstructed azimuth error is 4.2% and the curvature error is 1.18%. These values ​​can be averaged over multiple reconstructions to obtain more accurate estimates. Figure 18It is difficult to draw corresponding conclusions from the image using the grayscale image mode. Figure 18 Displayed with color pictures.

[0142] Table 1 summarizes and compares the sensing mechanisms and sensitivities of several 2D vector bending sensors. The 2D vector bending sensor employed in this embodiment utilizes only an ExTFG fabricated from a conventional single-mode optical fiber, significantly simplifying its structure and reducing manufacturing complexity. Furthermore, ExTFG-based sensors exhibit significantly enhanced sensitivity compared to FBG-based sensors. While their sensitivity is inferior to that of orthogonal LPGs or multi-core fiber interferometers, fiber Bragg gratings offer superior stability. Furthermore, the manufacturing complexity of this sensor is far lower than that of orthogonal LPGs.

[0143] Table 1 Comparison of sensing mechanisms and sensitivities of several two-dimensional vector bending sensors

[0144]

[0145] The following conclusions can be drawn from this embodiment: the ExTFG-based two-dimensional vector bending sensor used in this embodiment, in which the ExTFG is packaged in a cylindrical structure with a diameter of 2.5 mm in an off-axis manner. In this embodiment, under the condition of an off-axis distance of d = 0.5 mm, the theoretical maximum measurable curvature of the two-dimensional vector bending sensor used in this embodiment is 26.58 mm. -1 . The two-dimensional vector bending sensor of this embodiment calibrates the bending response of the resonance peaks of the TM and TE modes in all 360° directions with a step size of 15°. The bending sensitivity of the two modes shows a strong direction dependence. By simply monitoring the resonant wavelength and transmission intensity of the resonant double peaks in the transmission spectrum, the bending direction and curvature in any direction can be sensitively and accurately measured. The transmission spectra of the TM and TE modes vary greatly due to the different curvatures and directions of the sensor bending, and the maximum bending sensitivities are -105pm / m and -105pm / m, respectively. -1 and 105.5pm / m -1 、-70.25pm / m -1 and 67.5pm / m -1 . The direction and curvature of the vector bending can be reconstructed by the resonant wavelength offset of the TM and TE modes. The relative error of the reconstructed azimuth is about 4.2%, and the relative error of the reconstructed curvature is about 1.18%. The proposed sensing mechanism greatly simplifies the structure of the vector bending sensor and reduces the manufacturing complexity. Since ExTFG has low-temperature crosstalk characteristics and does not make any changes to the optical fiber itself, the stability of the sensor is greatly improved. The sensor has important application prospects in structural health monitoring and intelligent machinery in complex environments.

[0146] It will be understood that the present invention is described through some embodiments, and it is known to those skilled in the art that various changes or equivalent substitutions may be made to these features and embodiments without departing from the spirit and scope of the present invention. Under the guidance of the present invention, these features and embodiments may be modified to adapt to specific circumstances and materials without departing from the spirit and scope of the present invention. The embodiments described in the present invention are some embodiments of the present invention, not all embodiments. The components of the embodiments of the present invention generally described and shown in the drawings herein may be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the drawings is not intended to limit the scope of the claimed invention, but merely represents selected embodiments of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed herein, and all other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making creative work are within the scope of protection of the present invention.

Claims

1. Two-dimensional vector bending sensor, characterized in that The optical fiber comprises an optical fiber having a grating thereon, the grating being tilted at a fixed tilt angle, and the optical fiber being encapsulated in a flexible cylinder at a position deviating from the neutral axis to form an asymmetric structure; The tilt angle of the grating is 81°, and the grating period in the axial direction is 28μm; the flexible cylinder is made of a solidified first UV glue; the surface of the extremely large-angle fiber grating is coated with a layer of second UV glue, which is a low-refractive-index UV glue with a refractive index of 1.327, and the solidified first UV glue is coated on the outer periphery of the second UV glue; the cross-sectional diameter of the two-dimensional vector bending sensor is in the range of 1 to 5mm, wherein the ratio of the diameter of the flexible cylinder to the core diameter of the extremely large-angle fiber grating is in the range of 100:1 to 625:1, and the ratio of the diameter of the two-dimensional vector bending sensor to the effective length is 1:14 to 1:

35.

2. The two-dimensional vector bending sensor according to claim 1, characterized in that: The cross-sectional diameter of the flexible cylinder is 2.5 mm, and the effective length of the two-dimensional vector bending sensor is 7 mm.

3. A method for preparing a two-dimensional vector bending sensor, comprising preparing the two-dimensional vector bending sensor as claimed in claim 1, characterized in that: The following steps are involved: S1. Fabricate a mold for preparing a two-dimensional vector bending sensor, the mold comprising a clamping member having a placement position for a cylindrical hose; a first positioning member and a second positioning member are detachably mounted at each end of the clamping member, each of the first positioning member and the second positioning member having corresponding first and second perforations, respectively, with a center line connecting the first and second perforations being parallel to an extension direction of the placement position and located at a position offset from the central axis of the hose; S2, the inner diameters of the first and second perforations are interference-fitted with the outer diameter of the optical fiber; the optical fiber with the fixed tilt angle grating is passed through the first perforation, passed through the hose, and then passed out of the second perforation, ensuring that the grating area is located within the hose, and then the optical fiber is kept in a straightened state; S3, injecting the first UV glue evenly into the hose from one end of the hose, and then irradiating the hose with UV light until the first UV glue is completely cured; S4, detaching the first positioning member and the second positioning member from both ends of the clamping member, removing the hose, and removing the optical fiber coated with the cured first UV adhesive from the hose, thereby obtaining a two-dimensional vector bending sensor; The tilt angle of the grating is 81°, and the grating period in the axial direction is 28μm; the flexible cylinder is made of a solidified first UV glue; the surface of the extremely large-angle fiber grating is coated with a layer of second UV glue, the second UV glue is a low-refractive-index UV glue, and the refractive index of the second UV glue is 1.327, and the solidified first UV glue is coated on the outer periphery of the second UV glue; the cross-sectional diameter of the two-dimensional vector bending sensor is in the range of 1 to 5mm, wherein the ratio of the diameter of the flexible cylinder to the core diameter of the extremely large-angle fiber grating is in the range of 100:1 to 625:1, and the ratio of the diameter of the two-dimensional vector bending sensor to the effective length is 1:14 to 1:

35.

4. A two-dimensional vector bending measurement method, using the two-dimensional vector bending sensor according to claim 1, characterized in that: The following steps are involved: A1. Build a two-dimensional vector bending parameter detection model based on the pure bending model to obtain the relationship between the curvature of the two-dimensional vector bending sensor, axial strain, and off-axis distance. Determine the off-axis distance based on the required maximum measured curvature of the two-dimensional vector bending and the known maximum axial strain allowed by the optical fiber. A2. preparing a two-dimensional vector bending sensor according to the off-axis distance, and performing vector bending sensitivity calibration on the prepared two-dimensional vector bending sensor to obtain a vector bending sensitivity curve of the two-dimensional vector bending sensor; A3, placing the two-dimensional vector bending sensor in TE and TM isoexcitation states simultaneously, and simultaneously monitoring the resonant wavelength changes of the TM and TE modes in the isoexcitation states; A4. According to the changes in the resonant wavelengths of the TM and TE modes, combined with the vector bending sensitivity fitting functions of the TM and TE modes, the two-dimensional vector bending parameters are calculated; the vector bending sensitivity fitting function is: Where: ω is the angle between the line connecting the center of the optical fiber and the center point of the cross section of the two-dimensional vector bending sensor relative to the vertical symmetry axis of the cross section, θ is the bending direction of the two-dimensional vector bending sensor, and d eff is the effective off-axis distance, A, y0, B, θ c are all constants.

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