Optical fiber shape sensor with equivalent spiral structure and manufacturing method thereof

Abstract

The invention provides an equivalent helical structure optical fiber shape sensor. The equivalent helical structure optical fiber shape sensor comprises a multi-core optical fiber and a flexible capillary tube, and the multi-core optical fiber is inserted, pre-twisted and packaged in the capillary tube to form an equivalent spiral structure with a controllable screw pitch. According to the invention, a controllable spiral structure (the screw pitch adjusting range is 20-200mm) is formed in the optical fiber by applying torsion to the parallel optical fiber, compared with a parallel multi-core optical fiber scheme, the preset torsion rate can be accurately matched with the deformation mode of a target detection object, synchronous demodulation of bending curvature (the detection sensitivity is greater than or equal to 3.5 pm / mu epsilon) and torsional deformation (the angular resolution is less than or equal to 0.1 degree / m) is realized, and the detection accuracy is improved. The coupling error problem of a traditional sensor under the combined load is effectively solved.

Description

Technical Field

[0001] The present invention relates to the technical field of the preparation of sensing optical fibers, and in particular, to an equivalent spiral structure optical fiber shape sensor and a manufacturing method thereof. Background Art

[0002] Optical fiber shape sensing technology realizes non-contact measurement of the three-dimensional pose of an object by detecting the bending deformation of an optical fiber. With advantages such as a miniaturized structure, electromagnetic interference resistance, and high resolution, it shows important application value in fields such as minimally invasive surgical navigation and deformation monitoring of aerospace vehicles. The existing technologies mainly adopt the following three implementation methods:

[0003] Multi-fiber integration scheme: A sensing array is constructed by bonding multiple independent optical fibers, and shape reconstruction is achieved by using spatially distributed strain detection. However, this scheme has problems such as the relative positions of the optical fibers being prone to offset and the bonding interface being significantly affected by environmental aging, resulting in poor measurement repeatability and insufficient long-term stability.

[0004] Multi-core optical fiber scheme: A specially drawn multi-core optical fiber is used to replace a discrete optical fiber bundle, and the strain differences of each fiber core are demodulated by optical frequency domain reflectometry. Although this scheme improves the spatial resolution (typical root mean square error ≤ 7.2 mm), the torsional effect generated when the optical fiber bends will introduce coupling errors, seriously affecting the measurement accuracy in complex deformation scenarios.

[0005] Spiral multi-core optical fiber scheme: A spiral structure optical fiber is prepared by applying high-speed rotation during the preform drawing process. In theory, the combined demodulation of bending and torsion can be achieved. However, this process has inherent defects such as a narrow window for regulating drawing parameters (typical pitch control error > ±5%) and high manufacturing costs (the equipment modification cost increases by about 300%), and the geometric parameters of the optical fiber cannot be reconstructed, severely restricting engineering applications.

[0006] The current technical bottlenecks are mainly reflected in two aspects: First, traditional manufacturing processes are difficult to achieve low-cost mass production of high-precision spiral structure optical fibers; second, the existing demodulation algorithms have insufficient compensation capabilities for the bending-torsion coupling effect, resulting in a non-linear increase in the reconstruction error under complex deformations. Therefore, it is urgent to develop new optical fiber processing methods and intelligent decoupling algorithms to improve the measurement accuracy while reducing the manufacturing cost, thereby promoting the industrialization process of optical fiber shape sensing technology. Summary of the Invention

[0007] Aiming at the defects in the prior art, the purpose of the present invention is to provide an equivalent spiral structure optical fiber shape sensor and a manufacturing method thereof.

[0008] An equivalent spiral structure optical fiber shape sensor provided by the present invention includes: a multi-core optical fiber and a flexible capillary; the multi-core optical fiber is inserted and pre-twisted and encapsulated in the capillary to form an equivalent spiral structure with a controllable pitch.

[0009] Preferably, the inner diameter of the flexible capillary is larger than the coating diameter of the multi-core optical fiber. The cladding diameter of the multi-core optical fiber is 125 - 300 μm, the coating diameter is 250 - 600 μm, and the core pitch is 30 - 50 μm; the inner diameter of the capillary is 0.3 - 0.8 mm, and the outer diameter is 0.6 - 1.6 mm; the multi-core optical fiber includes n cores symmetrically distributed around the center of the circle, and n ≥ 3.

[0010] Preferably, the adjustable range of the controllable pitch is 20 - 200 mm;

[0011] The flexible capillary is made of a flexible material, including polytetrafluoroethylene, polyvinyl chloride, polypropylene, silicone rubber, polyurethane, thermoplastic elastomer, and nickel-titanium metal alloy.

[0012] The present invention also provides a manufacturing method of an equivalent spiral structure optical fiber shape sensor. The method is applied to the equivalent spiral structure optical fiber shape sensor described above, and the method includes the following steps:

[0013] Step S1: Insert the multi-core optical fiber into the flexible material capillary and fix one end;

[0014] Step S2: Apply a pre-twist angle to the other end of the multi-core optical fiber and then fix it;

[0015] Step S3: Inject ultraviolet curable glue for encapsulation and curing.

[0016] Preferably, the pre-twist angle in step S2 is 1800° - 18000°, and the deviation rate of the actual pitch from the theoretical pitch ≤ 15%.

[0017] Preferably, the actual pitch is calibrated by the following formula:

[0018] P 实际 =P 理论 ·(1 + α·μ)

[0019] where α is the friction coefficient between the optical fiber and the capillary, and μ is the torsional slip compensation factor, with a value range of 0.1 - 0.3.

[0020] Preferably, the equivalent helical structure optical fiber shape sensor is used in conjunction with an optical frequency domain reflectometer (OFDR), an optical switch module, and a fan-in / fan-out module; the OFDR measures the strain distribution along the optical fiber by analyzing the Rayleigh scattering optical signal of the measured optical fiber; the optical switch module controls the path of the optical signal by controlling each optical channel to perform sequential measurements on multiple cores; the fan-in / fan-out module combines and separates the signals of multiple cores to perform optical signal fan-in and fan-out operations.

[0021] Preferably, when the optical fiber is bent or twisted, it will cause changes in its internal refractive index and geometric dimensions, thereby causing changes in the Rayleigh scattering spectrum; by detecting the reflected Rayleigh scattering optical signal and interfering it with a reference optical signal, the frequency domain signal is converted into a spatial domain signal using Fourier transform, and cross-correlation operation is performed to calculate the total strain at each position of the optical fiber; through the composition of the total strain, the bending strain and torsional strain are separated, and the curvature and torsion of each position of the optical fiber are further calculated; the three-dimensional shape of the optical fiber shape sensor is reconstructed by the Frenet-Serret frame method.

[0022] Preferably, when the optical fiber is stretched or compressed, all cores are affected by the same degree of stretching or compression; while bending and torsion only affect the strain of the side cores of the multi-core optical fiber, and the strain of the middle core is not affected; a matrix is established based on the geometric structure of the multi-core optical fiber to describe the relationship between the bending, torsion, and stretching of the optical fiber sensor and the strain on each core:

[0023]

[0024] where ε i is the total strain measured at different positions of the optical fiber sensor for the corresponding core i, r i is the radial distance from the middle core to the side core i, θ i is the angle with respect to the normal neutral axis in the direction of the curve bend; K x (z) is a function of the bend in the X-Z plane and the position of the optical fiber sensor, K y (z) is a function of the bend in the Y-Z plane and the position of the optical fiber sensor, Q(z) is a function of the torsional strain of the optical fiber sensor and the position of the optical fiber sensor, and E(z) is a function of the tensile strain applied to the optical fiber sensor and the position of the optical fiber sensor;

[0025] Derive the curvature k(z) and the angle θ b from the core of the side core to the neutral axis according to formula (1):

[0026]

[0027] According to the definition of the torsion τ(z), τ(z) is obtained through θ b(z) is obtained by differentiation; however, the bending direction angle θ b (z) includes the twist angle and cannot be directly differentiated; because at this time each θ b (z) is the θ of the previous position b (z) and the initial twist angle θ 0 (z), so each θ b (z) has to subtract the sum of all previous twist angles and the initial twist angle θ 0 (z), and the formula is as follows:

[0028]

[0029] Among them, the twist angle is obtained by combining the twist strain Q(z) with the twist coefficient β:

[0030]

[0031] Combining with the Frenet - Serret frame:

[0032] T′(z) = k(z)N(z)

[0033] N′(z) = -k(z)T(z) + τ(z)B(z)

[0034] B′(z) = -τ(z)N(z) (5)

[0035] Among them, z represents the unit length of the curve; T(z) represents the unit tangent vector along the movement direction of the curve; N(z) represents the unit normal vector pointing to the bending direction; B(z) represents the unit binormal vector of the space curve; when the expression functions of the curvature k(z) and the torsion τ(z) are known, generally given the boundary conditions r(0) = (0, 0, 0), T(0) = (1, 0, 0), N(0) = (0, 1, 0), B(0) = (0, 0, 1), k(0) = 0, τ(0) = 0 for iterative calculation, to obtain T(z), N(z), B(z) of the entire optical fiber curve, and using T(z) to solve for the space curve r(z), where r 0 represents the initial position of the space curve:

[0036] r(z) = ∫T(z)dz + r 0 (6).

[0037] Preferably, the pre-twisted multi-core optical fiber shape reconstruction step includes: First, a two-dimensional calibration experiment needs to be carried out. The signal processing process of distributed strain sensing based on OFDR generally collects data through a data acquisition card, transforms the local distance domain signals of the two different state measurement signals to the wavelength domain by inverse FFT, performs cross-correlation operation, and obtains the cross-correlation peak shift amount at each position; then, using the proportional relationship K between the frequency shift amount of the backward Rayleigh scattering spectrum and the strain change amount, the strain magnitude is obtained for shape reconstruction; the reference backward Rayleigh scattering spectrum when the optical fiber does not bend and the measured backward Rayleigh scattering after shape change are subjected to cross-correlation operation. Let the frequency shift amount of the backward Rayleigh scattering spectrum be Δv, and K ε is the strain sensitivity coefficient, then there is the following formula:

[0038]

[0039] wherein, R t represents the actual curvature radius of the optical fiber shape; for a standard single-mode optical fiber, when the central wavelength is 1550 nm, the strain sensitivity coefficient is K s =-6.67 με / GHz; when using this value to reconstruct the shape of the pre-twisted multi-core optical fiber, the reconstructed radius R s ; thus, the actual strain sensitivity coefficient K = K s ·(R s / R t ); the initial twist angle θ 0 (z) is calibrated using the reconstructed curves of different diameters;

[0040] The fabricated pre-twisted seven-core optical fiber is wound on a three-dimensional standard tool with a diameter of 10 cm and a pitch of 5 cm. The measurement spectra of the seven cores are obtained using OFDR. Cross-correlation operation is performed on the measurement spectra and the reference spectra to obtain the wavelength shift of the spectra of the seven cores; then, after obtaining the actual total strain values of each core of the optical fiber by combining the wavelength shift with the actual strain sensitivity coefficient, using the bending-torsion-strain theoretical model, the bending strain, torsion strain, and tensile strain are separated from the total strain, and the curvature k and torsion τ are obtained by combining the initial twist angle θ 0 (z); finally, the three-dimensional shape of the optical fiber is reconstructed using the Frenet-Serret frame.

[0041] Compared with the prior art, the present invention has the following beneficial effects:

[0042] 1. The present invention forms a controllable spiral structure (pitch adjustment range: 20 mm - 200 mm) inside the optical fiber by applying torsion to parallel optical fibers. Compared with the parallel multi-core optical fiber solution, its preset torsion rate can accurately match the deformation mode of the target detection object, realizing the synchronous demodulation of bending curvature (detection sensitivity ≥ 3.5 pm / με) and torsional deformation (angle resolution ≤ 0.1° / m), effectively solving the coupling error problem of traditional sensors under complex loads;

[0043] 2. The present invention combines Rayleigh scattering optical frequency domain reflectometry (OFDR) technology, constructs a three-dimensional strain measurement model using the scattering spectrum eigenvalue of seven-core optical fiber, and the spatial resolution can reach 1 mm at the same measurement distance (10 m) (20 times higher than the FBG solution). Combined with the adaptive curvature-torsion separation algorithm, the root mean square error of three-dimensional shape reconstruction is ≤ 2.1 mm (67% lower than that of the multi-core optical fiber system of the same size), which is especially suitable for high-dynamic deformation monitoring in narrow spaces. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] By reading the detailed description of the non-restrictive embodiments with reference to the following drawings, other features, objects, and advantages of the present invention will become more apparent:

[0045] Figure 1 Schematic diagram of the steps for preparing an equivalent spiral structure multi-core optical fiber in an embodiment of the present invention;

[0046] Figure 2 Schematic diagram of the structure of the equivalent spiral structure multi-core optical fiber shown in an embodiment of the present invention;

[0047] Figure 3 Schematic diagram of the end face image of the structure of the equivalent spiral structure multi-core optical fiber shown in an embodiment of the present invention;

[0048] Figure 4 Schematic diagram of the shape sensing system device of the equivalent spiral structure multi-core optical fiber in an embodiment of the present invention;

[0049] Figure 5 Schematic diagram of the experimental steps for three-dimensional shape sensing of the equivalent spiral structure multi-core optical fiber in an embodiment of the present invention;

[0050] Figure 6 Schematic diagram of the reconstructed shape image of the optical fiber after shape testing of the sensor in an embodiment of the present invention.

[0051] Wherein:

[0052] Multi-core optical fiber 1 Second circumferentially evenly distributed core 6

[0053] Polytetrafluoroethylene capillary 2 Third circumferentially evenly distributed core 7

[0054] Ultraviolet curable glue 3 Fourth circumferentially evenly distributed core 8

[0055] Central core 4, fifth circumferentially equally spaced cores 9

[0056] First circumferentially equally spaced cores 5, sixth circumferentially equally spaced cores 10 Specific embodiments

[0057] The present invention will be described in detail below in conjunction with specific embodiments. The following embodiments will help those skilled in the art to further understand the present invention, but do not limit the present invention in any form. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several changes and improvements can still be made. These all belong to the protection scope of the present invention.

[0058] Example 1:

[0059] Referring to Figure 1 and Figure 2 , an equivalent spiral structure optical fiber shape sensor provided by the present invention includes: a multi-core optical fiber and a flexible capillary; the multi-core optical fiber is inserted and pre-twisted and encapsulated in the capillary to form an equivalent spiral structure with a controllable pitch.

[0060] The inner diameter of the flexible capillary is larger than the coating diameter of the multi-core optical fiber. The cladding diameter of the multi-core optical fiber is 125 - 300 μm, the coating diameter is 250 - 600 μm, and the core pitch is 30 - 50 μm; the inner diameter of the capillary is 0.3 - 0.8 mm, and the outer diameter is 0.6 - 1.6 mm; the multi-core optical fiber includes n cores symmetrically distributed around the center, where n ≥ 3.

[0061] The adjustable range of the controllable pitch is 20 - 200 mm; the flexible capillary is made of flexible materials, including polytetrafluoroethylene, polyvinyl chloride, polypropylene, silicone rubber, polyurethane, thermoplastic elastomer, and nickel-titanium alloy.

[0062] The present invention also provides a method for manufacturing an equivalent spiral structure optical fiber shape sensor. The method is applied to the equivalent spiral structure optical fiber shape sensor described above, and the method includes the following steps:

[0063] Step S1: Insert the multi-core optical fiber into the flexible material capillary and fix one end;

[0064] Step S2: Apply a pre-twist angle to the other end of the multi-core optical fiber and then fix it; the pre-twist angle is 1800° - 18000°, and the deviation rate between the actual pitch and the theoretical pitch ≤ 15%.

[0065] Step S3: Inject ultraviolet curable glue for encapsulation and curing.

[0066] The actual pitch is calibrated by the following formula:

[0067] P 实际 = P 理论 ·(1 + α·μ)

[0068] Where α is the friction coefficient between the optical fiber and the capillary, and μ is the torsional slip compensation factor, with a value range of 0.1 - 0.3.

[0069] The equivalent helical structure optical fiber shape sensor is used in conjunction with an optical frequency domain reflectometer (OFDR), an optical switch module, and a fan-in / fan-out module; the OFDR measures the strain distribution along the optical fiber by analyzing the Rayleigh scattered optical signal of the measurement optical fiber; the optical switch module controls the path of the optical signal by controlling each optical channel to perform sequential measurements on multiple fiber cores; the fan-in / fan-out module combines and separates the signals of multiple fiber cores to perform fan-in and fan-out operations on the optical signal.

[0070] When the optical fiber is bent or twisted, it will cause changes in its internal refractive index and geometric dimensions, thereby causing changes in the Rayleigh scattering spectrum; by detecting the reflected Rayleigh scattered optical signal and interfering it with the reference optical signal, the frequency domain signal is converted into a spatial domain signal using Fourier transform, and cross-correlation operation is performed to calculate the total strain at each position of the optical fiber; through the composition of the total strain, the bending strain and torsional strain are separated, and further the curvature and torsion at each position of the optical fiber are calculated; the three-dimensional shape of the optical fiber shape sensor is reconstructed by the Frenet - Serret frame method.

[0071] When the optical fiber is stretched or compressed, all fiber cores are affected by the same degree of stretching or compression; while bending and torsion only affect the strain of the side cores of the multi-core optical fiber, and the strain of the middle core is not affected; based on the geometric structure of the multi-core optical fiber, the following matrix is established to describe the relationship between the bending, torsion, and stretching of the optical fiber sensor and the strain on each core:

[0072]

[0073] Where ε i is the total strain measured at different positions of the fiber sensor for the corresponding core i, r i is the radial distance from the middle core to the side core i, θ i is the angle with respect to the normal neutral axis of the curve bending direction; K x (z) is a function of the bending in the X - Z plane and the position of the fiber sensor, K y (z) is a function of the bending in the Y - Z plane and the position of the fiber sensor, Q(z) is a function of the torsional strain of the fiber sensor and the position of the fiber sensor, and E(z) is a function of the tensile strain applied to the fiber sensor and the position of the fiber sensor;

[0074] Derive the curvature k(z) and the angle θ between the core of the side core and the neutral axis according to formula (1). b (z):

[0075]

[0076] According to the definition of the torsion τ(z), τ(z) is obtained by differentiating θ b (z); however, the bending direction angle θ b (z) includes the twist angle and cannot be directly differentiated; because at this time each θ b (z) is the sum of θ b (z) at the previous position and the initial twist angle θ 0 (z), so each θ b (z) has to subtract the sum of all the previous twist angles and the initial twist angle θ 0 (z), and the formula is as follows:

[0077]

[0078] Among them, the twist angle is obtained by combining the twist strain Q(z) with the twist coefficient β:

[0079]

[0080] Combined with the Frenet - Serret frame:

[0081] T′(z) = k(z)N(z)

[0082] N′(z) = -k(z)T(z) + τ(z)B(z)

[0083] B′(z) = -τ(z)N(z) (5)

[0084] Among them, z represents the unit length of the curve; T(z) represents the unit tangent vector along the moving direction of the curve; N(z) represents the unit normal vector pointing to the bending direction; B(z) represents the unit binormal vector of the space curve; when the expression functions of the curvature k(z) and the torsion τ(z) are known, generally, boundary conditions r(0) = (0, 0, 0), T(0) = (1, 0, 0), N(0) = (0, 1, 0), B(0) = (0, 0, 1), k(0) = 0, τ(0) = 0 are given for iterative calculation to obtain T(z), N(z), B(z) of the entire optical fiber curve, and the space curve r(z) is solved using T(z), where r 0 represents the initial position of the space curve:

[0085] r(z) = ∫T(z)dz + r 0 (6).

[0086] The steps for shape reconstruction of the pre-twisted multi-core optical fiber include: First, a two-dimensional calibration experiment needs to be carried out. The signal processing process of distributed strain sensing based on OFDR generally involves collecting data through a data acquisition card, converting the local distance-domain signals of the two different state measurement signals to the wavelength domain using the inverse FFT, performing a cross-correlation operation, and obtaining the cross-correlation peak shift at each position; then, using the proportional relationship K between the frequency shift of the backward Rayleigh scattering spectrum and the strain change amount, the strain magnitude is obtained for shape reconstruction; the reference backward Rayleigh scattering spectrum when the optical fiber does not undergo bending changes is cross-correlated with the measured backward Rayleigh scattering after the shape changes. Let the frequency shift of the backward Rayleigh scattering spectrum be Δv, and K ε is the strain sensitivity coefficient, then there is the following formula:

[0087]

[0088] where, R t represents the actual curvature radius of the optical fiber shape; for a standard single-mode optical fiber, when the central wavelength is 1550 nm, the strain sensitivity coefficient is K s =-6.67 με / GHz; when using this value to reconstruct the shape of the pre-twisted multi-core optical fiber, the reconstructed radius R s ; thus, the actual strain sensitivity coefficient K = K s ·(R s / R t ); the initial twist angle θ 0 (z) is calibrated using the reconstructed curves with different diameters;

[0089] The fabricated pre-twisted seven-core optical fiber is wound around a three-dimensional gauge with a diameter of 10 cm and a pitch of 5 cm. The measured spectra of the seven cores are obtained using OFDR, and the wavelength shifts of the spectra of the seven cores are obtained by performing a cross-correlation operation on the measured spectra and the reference spectra; then, after obtaining the actual total strain values of each core of the optical fiber from the wavelength shifts in combination with the actual strain sensitivity coefficient, using the bending-torsion-strain theoretical model, the bending strain, torsion strain, and tensile strain are separated from the total strain, and the curvature k and torsion τ are obtained in combination with the initial twist angle θ 0 (z); finally, the three-dimensional shape of the optical fiber is reconstructed using the Frenet-Serret frame.

[0090] Example 2:

[0091] The present invention provides an equivalent spiral structure optical fiber shape sensor that can control the pitch size and has a simple manufacturing method. By applying pre-twist to the multi-core optical fiber in advance, the structure of the optical fiber has helicity. When the optical fiber is bent or twisted, the optical signal propagating along the optical fiber will cause changes in the intensity and spectrum of Rayleigh scattering due to changes in the effective refractive index. By detecting the changes in the Rayleigh scattering spectrum, not only can the optical fiber bending measurement and torsion measurement be realized, but also the shape reconstruction can be carried out.

[0092] An equivalent spiral structure optical fiber shape sensor includes a multi-core optical fiber and a polytetrafluoroethylene capillary;

[0093] The multi-core optical fiber comes from FiberHome Corporation, with a cladding diameter of 123μm - 125μm, a coating diameter of 245μm - 255μm, a core pitch of 39μm - 42μm, and a mode field diameter of 8.5μm - 9.5μm, capable of realizing long-distance low-crosstalk space-division multiplexing optical signal transmission; the polytetrafluoroethylene capillary has an inner diameter of 0.3mm and an outer diameter of 0.6mm, which has the functions of temperature resistance, anti-aging, and corrosion resistance. Its manufacturing method includes the following steps:

[0094] Step 1, insert the multi-core optical fiber into the polytetrafluoroethylene capillary and fix one end;

[0095] Step 2, twist the other end of the multi-core optical fiber in the same direction by a corresponding amount and then fix it;

[0096] Step 3, inject ultraviolet curable glue into both ends of the polytetrafluoroethylene capillary and perform encapsulation and curing.

[0097] Furthermore, in Step 2, by applying different pre-twist angles, optical fibers with equivalent spiral structures having different pitches can be obtained, and thus different pitches can be achieved. The size of the pitch can be estimated by a formula. For example, for a 1m long multi-core optical fiber, when a twist angle of 3600° is applied, the theoretical pitch is about 10cm. However, due to the frictional effect between the multi-core optical fiber and the polytetrafluoroethylene capillary, the actually obtained pitch will be slightly larger. Specifically, this friction will cause certain slip and non-uniform strain distribution of the optical fiber when the twist is applied, thus affecting the final pitch value. Therefore, in actual operation, these factors need to be comprehensively considered to ensure the precise manufacturing and performance optimization of the spiral optical fiber.

[0098] Furthermore, the pre-twisted fiber shape sensor needs to be used in conjunction with an optical frequency domain reflectometer (OFDR), an optical switch module, and a fan-in / fan-out module. The OFDR measures the strain distribution along the fiber by analyzing the Rayleigh scattering optical signal of the fiber; the optical switch module controls the path of the optical signal by controlling each optical channel to achieve sequential measurement of multiple cores; the fan-in / fan-out module combines and separates the signals of multiple cores to perform optical signal input (fan-in) and output (fan-out) operations, ensuring signal processing efficiency and the accuracy of shape reconstruction. The principle of the three-dimensional fiber shape sensing is as follows: when the fiber is bent or twisted, it will cause changes in its internal refractive index and geometric dimensions, thereby causing changes in the Rayleigh scattering spectrum. By detecting the reflected Rayleigh scattering optical signal, interfering it with the reference optical signal, converting the frequency domain signal into a spatial domain signal using Fourier transform, and performing cross-correlation operations, the total strain at each position of the fiber can be calculated. Then, through the composition of the total strain, the bending strain and the torsional strain can be separated, and the curvature and torsion of each position of the fiber can be further calculated. Finally, the three-dimensional shape of the fiber shape sensor is reconstructed by the Frenet-Serret frame method.

[0099] The present invention uses a seven-core fiber to prepare a shape sensor. Compared with general three-core fibers or fiber sensors using manual adhesion, it has a stable symmetric structure, significantly improving the performance and reliability of the sensor. Compared with a parallel multi-core fiber shape sensor, this sensor uses the spiral structure brought by pre-twisting, can simultaneously detect the strain distribution under bending and torsional loads, thereby realizing multi-dimensional strain measurement; moreover, the size of the pre-twisting angle can be changed to fabricate fiber shape sensors with different spiral structures, with low cost and simple fabrication methods. The present invention adopts the OFDR fiber sensing technology based on Rayleigh scattering, which has high spatial resolution and high 3D shape sensing accuracy compared with the wavelength division multiplexing technology based on FBG.

[0100] The present invention provides a method for fabricating an equivalent spiral structure multi-core fiber, which can control the pitch of the multi-core fiber shape sensor by changing the degree of pre-twisting. The fabrication method is simple, the structure is easy to process and implement, the cost is low, and the shape sensing accuracy is good.

[0101] As Figure 1 shown, the present invention provides the steps for fabricating an equivalent spiral structure multi-core fiber (referred to as a pre-twisted multi-core fiber).

[0102] Step 1: Horizontally insert a 1.2 m multi-core fiber 1 into a 1 m polytetrafluoroethylene capillary 2. After ensuring that there is no additional torsion and bending of the multi-core fiber in the capillary, fix one end of the multi-core fiber.

[0103] Step 2: Fix the other end of the multi-core optical fiber on the glass slide. Rotate the glass slide in the same direction to apply torsion to the multi-core optical fiber until the total torsion angle reaches 5400°, and then fix the other end of the multi-core optical fiber.

[0104] Step 3: Inject the ultraviolet curable glue 3 into both ends of the polytetrafluoroethylene capillary, and use an ultraviolet lamp for encapsulation and curing.

[0105] The structure of the pre-twisted multi-core optical fiber is as Figure 2 shown. From the inside to the outside, they are the seven-core optical fiber 1 and the polytetrafluoroethylene capillary 2.

[0106] The working principle of the shape sensor of the present invention is obtained based on the Frenet-Serret frame theory.

[0107] When the optical fiber is stretched or compressed, all the cores are affected by the same degree of stretching or compression. However, bending and torsion only affect the strain of the side cores of the multi-core optical fiber, and the strain of the middle core is not affected. As Figure 3 shown, combined with the geometric structure of the multi-core optical fiber, the following matrix can be established to describe the relationship between the bending, torsion, and stretching of the optical fiber sensor and the strain on each core:

[0108]

[0109] where ε i is the total strain measured at different positions of the optical fiber sensor for the corresponding core i, r i is the radial distance from the middle core to the side core i, θ i is the angle with respect to the normal neutral axis in the direction of the curve bending. K x (z) is a function of the bending in the X-Z plane and the position of the optical fiber sensor, K y (z) is a function of the bending in the Y-Z plane and the position of the optical fiber sensor, Q(z) is a function of the torsional strain of the optical fiber sensor and the position of the optical fiber sensor, and E(z) is a function of the tensile strain applied to the optical fiber sensor and the position of the optical fiber sensor.

[0110] According to formula (1), the curvature k(z) and the angle θ b (z) between the core of the side core and the neutral axis can be derived:

[0111]

[0112] According to the definition of the torsion τ(z), τ(z) can be obtained by differentiating θ b (z). However, the bending direction angle θ b (z) includes the torsion angle and cannot be directly differentiated. Because at this time, each θ b (z) is θ b(z) and the initial twist angle θ 0 The sum of (z), so for each θ b (z), all previous twist angles have to be subtracted from the total sum and the initial twist angle θ 0 (z), and the formula is as follows:

[0113]

[0114] where the twist angle can be obtained from the twist strain Q(z) combined with the twist coefficient β:

[0115]

[0116] Combined with the Frenet - Serret frame:

[0117] T′(z) = k(z)N(z)

[0118] N′(z) = -k(z)T(z) + τ(z)B(z)

[0119] B′(z) = -τ(z)N(z) (5)

[0120] where z represents the unit length of the curve; T(z) represents the unit tangent vector along the moving direction of the curve; N(z) represents the unit normal vector pointing to the bending direction; B(z) represents the unit binormal vector of the space curve. When the expression functions of the curvature k(z) and the torsion τ(z) are known, generally, boundary conditions r(0) = (0, 0, 0), T(0) = (1, 0, 0), N(0) = (0, 1, 0), B(0) = (0, 0, 1), k(0) = 0, τ(0) = 0 are given for iterative calculation to obtain T(z), N(z), B(z) of the whole optical fiber curve, and using T(z), the space curve r(z) can be solved, where r 0 represents the initial position of the space curve:

[0121] r(z) = ∫T(z)dz + r 0 (6)

[0122] In order to conduct actual shape test and analysis on the shape sensor prepared in this embodiment, an Figure 4 experimental device as shown is built, which consists of an optical frequency domain reflectometer OFDR, an optical switch module, a fan - in / fan - out module, and a pre - twisted multi - core optical fiber.

[0123] The steps for shape reconstruction of the pre - twisted multi - core optical fiber are as follows Figure 5As shown, a two-dimensional calibration experiment needs to be carried out first. The signal processing process of distributed strain sensing based on OFDR generally involves collecting data through a data acquisition card, converting the local distance domain signals of the measurement signals in two different states to the wavelength domain using the inverse FFT, performing a cross-correlation operation, and obtaining the cross-correlation peak shift at each position. Then, using the proportional relationship K between the frequency shift of the backward Rayleigh scattering spectrum and the strain change amount, the strain magnitude is obtained for shape reconstruction. However, in each experimental environment, the Rayleigh scattering spectrum signal will be affected by external noises such as temperature, vibration, and perturbation, resulting in different sensing sensitivities in each experimental process, which will inevitably affect the shape sensing accuracy. Perform a cross-correlation operation on the reference backward Rayleigh scattering spectrum when the optical fiber does not undergo bending changes and the measured backward Rayleigh scattering after shape changes. Let the frequency shift of the backward Rayleigh scattering spectrum be Δv, and K ε is the strain sensitivity coefficient, then there is the following formula:

[0124]

[0125] where, R t represents the actual curvature radius of the optical fiber shape. For a standard single-mode optical fiber, when the central wavelength is 1550 nm, the strain sensitivity coefficient is K s =-6.67 με / GHz. When using this value to reconstruct the shape of the pre-twisted multi-core optical fiber, the reconstructed radius R s . Therefore, the actual strain sensitivity coefficient K = K s ·(R s / R t ). At the same time, the initial twist angle θ 0 (z) can be calibrated using the reconstructed curves with different diameters.

[0126] Wind the fabricated pre-twisted seven-core optical fiber around a three-dimensional gauge with a diameter of 10 cm and a pitch of 5 cm. Use OFDR to obtain the measurement spectra of the seven cores. Perform a cross-correlation operation on the measurement spectra and the reference spectrum to obtain the wavelength shift of the spectra of the seven cores. Then, after obtaining the actual total strain values of each core of the optical fiber by combining the wavelength shift with the actual strain sensitivity coefficient, use the bending-torsion-strain theoretical model to separate the bending strain, torsion strain, and tensile strain from the total strain, and combine the initial twist angle θ 0 (z) to obtain the curvature k and torsion τ. Finally, use the Frenet-Serret frame to reconstruct the three-dimensional shape of the optical fiber. The result is as Figure 6 shown, and this result shows that the sensor has the ability to demodulate torsion and realize three-dimensional shape sensing of the optical fiber.

[0127] Those skilled in the art can understand this embodiment as a more specific illustration of Embodiment 1.

[0128] Those skilled in the art know that in addition to implementing the system and its various devices, modules, and units provided by the present invention in the form of pure computer-readable program code, the method steps can be logically programmed to enable the system and its various devices, modules, and units provided by the present invention to be implemented in the form of logic gates, switches, application-specific integrated circuits, programmable logic controllers, and embedded microcontrollers, etc. to achieve the same functions. Therefore, the system and its various devices, modules, and units provided by the present invention can be considered as a kind of hardware component, and the devices, modules, and units included therein for implementing various functions can also be regarded as the structures within the hardware component; the devices, modules, and units for implementing various functions can also be regarded as either software modules for implementing the method or structures within the hardware component.

[0129] The specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the above specific embodiments, and those skilled in the art can make various changes or modifications within the scope of the claims, which do not affect the essence of the present invention. Without conflict, the embodiments of the present application and the features in the embodiments can be combined with each other arbitrarily.

Claims

1. An equivalent helical structure optical fiber shape sensor, characterized in that: include: A multi-core optical fiber and a flexible capillary; the multi-core optical fiber is inserted and pre-twisted and packaged in the capillary to form an equivalent spiral structure with a controllable pitch.

2. The equivalent helical structure optical fiber shape sensor according to claim 1, characterized in that: The inner diameter of the flexible capillary is larger than the coating diameter of the multi-core optical fiber. The cladding diameter of the multi-core optical fiber is 125-300 μm, the coating diameter is 250-600 μm, and the core distance is 30-50 μm. The inner diameter of the capillary is 0.3-0.8 mm, and the outer diameter is 0.6-1.6 mm. The multi-core optical fiber includes n cores symmetrically distributed around the center of a circle, and n≥3.

3. The equivalent helical structure optical fiber shape sensor according to claim 1, characterized in that: The adjustment range of the controllable pitch is 20-200mm; The flexible capillary is made of flexible materials including polytetrafluoroethylene, polyvinyl chloride, polypropylene, silicone rubber, polyurethane, thermoplastic elastomer and nickel-titanium alloy.

4. A method for manufacturing an equivalent helical structure optical fiber shape sensor, characterized in that: The method is applied to the equivalent helical structure optical fiber shape sensor according to any one of claims 1 to 3, and the method comprises the following steps: Step S1: inserting a multi-core optical fiber into a flexible material capillary and fixing one end thereof; Step S2: applying a pre-twisted angle to the other end of the multi-core optical fiber and then fixing it; Step S3: injecting UV curing glue to perform encapsulation curing.

5. The method for manufacturing an equivalent helical structure optical fiber shape sensor according to claim 4, characterized in that: The pre-twisting angle in step S2 is 1800°-18000°, and the deviation rate between the actual pitch and the theoretical pitch is ≤15%.

6. The method for manufacturing an equivalent helical structure optical fiber shape sensor according to claim 5, characterized in that: The actual pitch is calibrated by the following formula: P 实际 =P 理论 ·(1+a·m) Among them, α is the friction coefficient between the optical fiber and the capillary, and μ is the torsional slip compensation factor, which ranges from 0.1 to 0.

3.

7. The method for manufacturing an equivalent helical structure optical fiber shape sensor according to claim 4, characterized in that: The equivalent helical structure optical fiber shape sensor is used in conjunction with an optical frequency domain reflectometer (OFDR), an optical switch module, and a fan-in and fan-out module; the optical frequency domain reflectometer (OFDR) measures the strain distribution along the optical fiber by analyzing and measuring the Rayleigh scattered light signal of the optical fiber; the optical switch module controls the path of the optical signal by controlling each optical channel and performs sequential measurements on multiple fiber cores; the fan-in and fan-out module combines and separates the signals of multiple fiber cores to perform fan-in and fan-out operations on the optical signal.

8. The method for manufacturing an equivalent helical structure optical fiber shape sensor according to claim 7, characterized in that: When the optical fiber is bent or twisted, its internal refractive index and geometric dimensions will change, causing changes in the Rayleigh scattering spectrum. By detecting the reflected Rayleigh scattered light signal and interfering it with the reference light signal, the frequency domain signal is converted into a spatial domain signal using Fourier transform, and a cross-correlation operation is performed to calculate the total strain at each position of the optical fiber. Through the composition of the total strain, the bending strain and torsional strain are separated, and the curvature and torsion of each position of the optical fiber are further calculated. The three-dimensional shape of the optical fiber shape sensor is reconstructed using the Frenet-Serret frame method.

9. The method for manufacturing an equivalent helical structure optical fiber shape sensor according to claim 8, characterized in that: When the optical fiber is stretched or compressed, all the cores are affected by the same degree of stretching or compression; bending and twisting will only affect the side core strain of the multi-core optical fiber, and the strain of the middle core is not affected; combined with the geometric structure of the multi-core optical fiber, the following matrix is ​​established to describe the bending, twisting and stretching of the optical fiber sensor and the strain on each core: Among them, ε i is the total strain measured at different positions of the optical fiber sensor corresponding to core i, r i is the radial distance from the middle core to the side core i, θ i K is the angle of the normal neutral axis relative to the bending direction of the curve; x (z) is a function of the bending in the XZ plane and the position of the fiber sensor, K y (z) is the function of the bending in the YZ plane and the position of the optical fiber sensor, Q(z) is the function of the torsional strain of the optical fiber sensor and the position of the optical fiber sensor, and E(z) is the function of the tensile strain applied to the optical fiber sensor and the position of the optical fiber sensor; According to formula (1), the curvature k(z) and the angle θ between the core of the side core and the neutral axis are derived: b (z): According to the definition of torsion τ(z), τ(z) is expressed by θ b (z) is derived; but the bending direction angle θ b (z) contains the torsion angle and cannot be directly differentiated; because at this time each θ b (z) is the θ of the previous position b (z) and the initial torsion angle θ0(z), so each θ b (z) must subtract all previous torsion angles The sum of and the initial torsion angle θ0(z) is given by the following formula: Among them, the torsion angle The torsional strain Q(z) combined with the torsional coefficient β gives: Combined with the Frenet-Serret framework: T′(z)=k(z)N(z) N′(z)=-k(z)T(z)+τ(z)B(z) B′(z)=-τ(z)N(z) (5) Wherein, z represents the unit length of the curve; T(z) represents the unit tangent vector along the direction of motion of the curve; N(z) represents the unit normal vector pointing to the bending direction; B(z) represents the unit binormal vector of the space curve; when the expression functions of the curvature k(z) and the torsion τ(z) are known, the boundary conditions r(0)=(0,0,0), T(0)=(1,0,0), N(0)=(0,1,0), B(0)=(0,0,1), k(0)=0, τ(0)=0 are generally given for iterative calculation to obtain T(z), N(z), B(z) of the entire optical fiber curve, and T(z) is used to solve the space curve r(z), where r0 represents the initial position of the space curve: r(z)=∫T(z)dz+r0 (6).

10. The method for manufacturing an equivalent helical structure optical fiber shape sensor according to claim 8, characterized in that: The steps of reconstructing the shape of pre-twisted multi-core optical fiber include: first, a two-dimensional calibration experiment is required. The signal processing process of OFDR-based distributed strain sensing is generally to collect data through an acquisition card, convert the local distance domain signals of the two different state measurement signals into the wavelength domain using FFT inverse transform, perform cross-correlation operations, and obtain the cross-correlation peak shift at each position; then use the back Rayleigh scattering spectrum frequency offset to be proportional to the strain change K, and obtain the strain size for shape reconstruction; perform cross-correlation operations on the reference back Rayleigh scattering spectrum when the optical fiber has not been bent and the measured back Rayleigh scattering after the shape change, assuming that the back Rayleigh scattering spectrum frequency shift is Δv, K ε is the strain sensitivity coefficient, then there is the following formula: Among them, R t Indicates the actual curvature radius of the optical fiber shape; for standard single-mode optical fiber, when the central wavelength is 1550nm, the strain sensitivity coefficient is K s = -6.67με / GHZ; when this value is used to reconstruct the shape of the pre-twisted multi-core optical fiber, the reconstruction radius R s ; Therefore, the actual strain sensitivity coefficient K = K s ·(R s / R t ); calibrate the initial torsion angle θ0(z) using the reconstruction curves of different diameters; The fabricated pre-twisted seven-core optical fiber was wound on a three-dimensional etalon with a diameter of 10 cm and a pitch of 5 cm. The measured spectra of the seven cores were obtained using OFDR, and the wavelength shift of the spectra of the seven cores was obtained by cross-correlating the measured spectrum with the reference spectrum. Then, the actual total strain value of each core of the optical fiber was calculated by combining the wavelength shift with the actual strain sensitivity coefficient. The bending-torsion-strain theoretical model was used to separate the bending strain, torsional strain and tensile strain from the total strain, and the curvature k and torsion τ were calculated in combination with the initial torsion angle θ0(z). Finally, the Frenet-Serret frame was used to reconstruct the three-dimensional shape of the optical fiber.

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