Multi-core fiber grating array with double-helix structure and torsion measurement method

Abstract

The invention discloses a multi-core fiber grating array with a double-helix structure and a torsion measurement method. The array comprises 1 + N + M independent optical fibers which are engraved with gratings and wrapped by a coating layer to form a double-layer spiral structure. The inner layer is composed of a central straight optical fiber and N inner spiral optical fibers wound along a first steering wheel. The outer layer is composed of M outer spiral optical fibers wound in opposite directions. The invention provides two configurations of grating staggering and aligning. According to the invention, bending interference is accurately decoupled by using the same-layer phase difference, the limitation bottleneck of single-spiral measurement is broken through the complementary characteristic of double-layer reverse spiral, omni-directional large-range torsion measurement is realized, and meanwhile, the spatial sampling rate or the micro torsion measurement precision can be improved according to configuration.

Description

Technical Field

[0001] This invention belongs to the field of fiber optic grating array technology, and specifically relates to a double-helix structure multi-core fiber optic grating array and a torsion measurement method. Background Technology

[0002] As high-end equipment and intelligent systems continue to develop towards flexibility, precision, and large scale, unprecedented demands are being placed on the real-time, online, and distributed sensing of complex three-dimensional spatial shapes and multi-dimensional mechanical parameters. Against this backdrop, fiber optic shape sensing technology, with its inherent advantages such as electromagnetic interference resistance, harsh environment tolerance, small size, light weight, and the ability to achieve absolute measurement, has become a key enabling technology in cutting-edge fields such as attitude feedback for robotic dexterity hands and continuous robotic arms, wing deformation monitoring for aerospace vehicles, load analysis of large wind turbine blades, and navigation of cardiovascular and neurointerventional surgical instruments. Among these technologies, fiber Bragg gratings (FBGs), as devices that linearly modulate environmental strain, temperature, and other physical quantities into changes in reflected wavelength, are the core sensing units for achieving discrete or quasi-distributed measurements. Multi-core optical fibers, by integrating multiple spatially separated independent cores within a single cladding, provide a physical carrier for the parallel acquisition of information from multiple spatial points. By inscribing fiber grating arrays into each core of a multi-core fiber, the resulting multi-core fiber grating array effectively integrates the advantages of multi-point measurement and spatial diversity multiplexing, significantly improving the information dimension and reliability of shape reconstruction. It has become one of the most dynamic research directions in the field of fiber optic sensing.

[0003] Current mainstream multi-core fiber grating array (FBG) technologies still face fundamental challenges in sensing complex deformations, especially those accompanied by torsion. First, the most common parallel-core array structure, where cores are symmetrically distributed across the fiber cross-section, is sensitive to bending strain, but for torsional deformation around the fiber axis, the responses of each core are almost identical, making it difficult to effectively decouple the pure torsional component. This results in a sensing "blind zone" or severe calculation errors when monitoring scenarios involving significant torsional deformation, such as robotic arm joint rotation, cable twisting, and helicopter rotor torsional vibration, failing to fully and accurately reconstruct the three-dimensional spatial curve. Second, to improve torsional sensitivity, recent research has proposed introducing helical arrangements within the fiber core. These single-layer helical multi-core fibers, by deviating the core from the neutral axis and extending it in a helical shape, successfully convert torsional strain into axial strain within the core, which can then be sensed by the FBG. However, this approach has significant limitations: First, all fiber cores are helical structures in a single direction, making them more sensitive to the measurement of torsion in the forward direction of the helix, while the measurement range of torsion in the reverse direction of the helix is ​​limited; Second, in order to obtain sufficient spatial resolution to accurately characterize shape (especially curvature) changes, it is usually necessary to inscribe a dense array of gratings (the grating spacing is often on the order of centimeters) on each helical fiber core, which greatly increases the complexity of fabrication and system cost.

[0004] At the fabrication level, the existing manufacturing processes for high-performance multi-core fiber grating arrays constitute another major bottleneck for their large-scale application. The current mainstream approach involves first fabricating or procuring multi-core fibers with a specific core arrangement (parallel or spiral), and then writing the grating point-by-point into the cores through a complex post-processing procedure. This process typically involves precise removal of the coating layer, point-by-point scanning and writing based on a phase mask and ultraviolet laser, and recoating for protection. This approach has inherent drawbacks: numerous process steps and low production efficiency; the removal and recoating processes may damage the mechanical strength of the fiber; more importantly, it is difficult to ensure a high degree of consistency in the grating quality (such as center wavelength, reflectivity, and bandwidth) across multiple cores, and long-distance writing presents significant challenges in terms of positioning accuracy and repeatability, leading to discrete sensor performance and cumbersome calibration work. Furthermore, this "form first, then etch" approach lacks flexibility in innovating fiber core spatial configurations, making it difficult to economically and reliably achieve complex and high-precision fiber arrangement structures such as the double-layer and anisotropic spiral described in this invention. This limits further improvements in sensor sensing dimensions and performance from the source of the process.

[0005] Therefore, the core contradiction facing existing technologies lies in the following: On the one hand, the application demand for sensing complex three-dimensional deformation is increasingly urgent, especially the clear requirements for high-precision and high signal-to-noise ratio measurement of torsional components; on the other hand, both traditional arrays based on parallel structures and improved schemes based on single-layer helices have significant shortcomings in sensing performance (torsional decoupling capability, anti-interference) or manufacturing cost and feasibility (reliance on dense gratings, complex post-processing). Specific challenges can be summarized as follows: First, the torsional measurement range and accuracy are limited. Existing single-helix fiber gratings can only guarantee a large range of measurements of torsions in the same direction as their helix, making it difficult to achieve a large range of measurements of torsions in the opposite direction of their helix while ensuring high bending measurement accuracy. Furthermore, when facing small torsions under complex force fields, there is a lack of effective structural design to further improve the precision of decoupling and measurement. Second, the system cost is high. To achieve high-performance measurement, dense grating arrays must be used, leading to a surge in writing costs, demodulation equipment complexity, and data processing overhead. Third, the fabrication process is limited. Traditional "post-processing" methods cannot simultaneously achieve high consistency, high efficiency, and flexible fabrication of complex spatial structures. Therefore, there is an urgent need in this field for an innovative design and fabrication method for multi-core fiber Bragg grating arrays. This design should be able to optimize the response characteristics to multi-dimensional strains such as torsion from the perspective of sensing mechanism, and reduce the dependence on high-density gratings through structural innovation, thereby improving performance while controlling the overall system cost, and providing a feasible technical path for high-efficiency and large-scale manufacturing. Summary of the Invention

[0006] To address a series of problems with existing multi-core fiber Bragg grating arrays, such as insufficient torsion sensing capability, coupling between torsion and strain, high cost due to reliance on dense gratings for spatial sampling, and the difficulty in achieving complex high-performance structures using traditional fabrication processes, this invention proposes a double-helix multi-core fiber Bragg grating array and a torsion measurement method. The core design concept of this scheme lies in innovating the sensor's response characteristics to multidimensional strain, especially torsional deformation under complex force fields, from a physical mechanism perspective by constructing a double-layer, anti-helix sensing structure and configuration method. This significantly improves the measurement range while substantially reducing the required density of grating units or improving the accuracy of small-scale torsion measurements.

[0007] The objective of this invention is achieved as follows:

[0008] A double-helix structure multi-core fiber grating array, comprising 1+N+M independent optical fibers, all wrapped together by an external coating layer. The 1+N+M optical fibers are arranged in a double-layer helical structure in space; The double-layer helical structure is specifically composed of: the inner layer structure consists of 1+N optical fibers, including a central straight optical fiber located at the geometric center of the optical fiber cross section and extending in a direction parallel to the optical fiber axis, and N inner helical optical fibers coiled around the central straight optical fiber in a first helical direction; the outer layer structure consists of M outer helical optical fibers, which surround the inner layer structure and coil in a second helical direction opposite to the first helical direction. Where N≥3, M≥3.

[0009] The aforementioned double-helix multi-core fiber Bragg grating array comprises 1+N+M independent optical fibers, all of which contain a series of fiber Bragg gratings. A series of gratings are etched along the fiber axis in all inner helical fibers, forming a first set of periodic measurement points. A series of gratings are etched along the fiber axis in all outer helical fibers, forming a second set of periodic measurement points. A series of gratings are etched along the axis in the central straight fiber. The axial position of each grating in the central straight fiber corresponds one-to-one with the axial position of either the first or second set of periodic measurement points, providing tensile strain and ambient temperature compensation.

[0010] Preferably, in the double-helix multi-core fiber grating array, the centers of a series of gratings etched in all the inner helical fibers are aligned with each other along the fiber axis; the centers of a series of gratings etched in all the outer helical fibers are aligned with each other along the fiber axis; and the first group of periodic measurement points and the second group of periodic measurement points are staggered along the axial direction.

[0011] Preferably, in the double-helix multi-core fiber grating array, the centers of a series of gratings etched in all the inner helical fibers are aligned with each other along the fiber axis; the centers of a series of gratings etched in all the outer helical fibers are aligned with each other along the fiber axis; and the periodic positions of the series of gratings etched in all the inner helical fibers and all the series of gratings etched in all the outer helical fibers are all aligned with each other along the fiber axis.

[0012] A preferred staggered distribution method for a double-helix multi-core fiber grating array is as follows: if the axial distribution period of the grating is set to L, then the gratings in the inner helical fiber are located at positions 0, L, 2L, 3L…, while the gratings in the outer helical fiber are located at positions L / 2, 3L / 2, 5L / 2, 7L / 2…; the gratings in the central straight fiber are located at positions 0, L / 2, L, 3L / 2…; the grating position offset is ±ΔL, and ΔL is not greater than L / 5.

[0013] A preferred alignment distribution method for a double-helix multi-core fiber grating array is as follows: if the axial distribution period of the grating is set to L, then the inner and outer helical fibers and the central straight fiber grating are located at positions 0, L, 2L, 3L…; the grating position offset is ±△L, and △L is not greater than L / 5.

[0014] This invention also provides a method for measuring the torsion of a multi-core fiber grating array based on the above-mentioned double-helix structure, specifically including: Step 1: Bending strain decoupling and initial torsional calculation The grating wavelength offset of multiple inner or outer spiral optical fibers with specific spatial phase differences within the same spiral layer is collected. The spatial distribution characteristics of different fiber cores in the same layer are used for joint calculation to eliminate the additional strain interference caused by bending and to preliminarily calculate the torsional strain of the spiral layer. Step 2: Independent calculation of torsional rate of the two layers Based on the physical configuration of the inner and outer spiral fibers with opposite spiral directions, the sensing signals of the inner and outer spiral layers are processed separately, and the torsion rates of the inner and outer spiral layers are calculated independently. Step 3: Omnidirectional Torsional Synthesis and Range Expansion By utilizing the complementary characteristics of the inner and outer double-layer reverse spiral structures, the torsion rates of the inner spiral layer and the outer spiral layer are weighted, fused, or switched. When unidirectional torsion is detected, causing the single-layer fiber to enter the measurement nonlinear region or blind region, the measurement data of the other reverse spiral layer is used for compensation or replacement, thereby achieving omnidirectional, large-range torsion morphology reconstruction and accurate measurement.

[0015] A method to improve the accuracy of torsion measurement is to use the grating wavelength offset of multiple spiral optical fibers with specific spatial phase differences within the same spiral layer for joint calculation, thereby eliminating the additional strain interference caused by bending and accurately calculating the torsional strain.

[0016] A method to improve the spatial resolution of torsion measurement: Based on the staggered distribution of grating axial positions in the inner and outer spiral fibers, the effective spatial sampling rate of the array as a whole is improved by utilizing the complementary positions of interlayer gratings while keeping the grating writing density of a single fiber constant.

[0017] A method to improve the accuracy of small-range torsion measurement: When the double-helix multi-core fiber grating array adopts a distribution method in which the grating axial positions of the inner and outer helical fibers are aligned, the first torsion rate data calculated by the inner helical fiber layer and the second torsion rate data calculated by the outer helical fiber layer are simultaneously acquired at the same axial position; the first torsion rate data and the second torsion rate data are differentially processed to increase the measurement variation amplitude, reduce random measurement noise, and realize high-precision measurement of small-range micro-torsion.

[0018] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0019] (1) Precise decoupling using phase difference in the same layer to eliminate bending interference: This invention can effectively eliminate the additional strain caused by bending by jointly calculating the strain of multiple spiral optical fibers with specific spatial phase differences (such as three optical fibers with a phase difference of 120°) in the same layer, and accurately extract the pure torsional strain, thus completely breaking through the blind zone of torsional sensing failure of traditional parallel fiber core structure under complex force field.

[0020] (2) Double-layer reverse spiral configuration significantly expands the torsion measurement range: When a single-layer spiral fiber is subjected to a large torsion opposite to its spiral direction, it is very easy for the single-layer spiral fiber to twist into a straight fiber, causing measurement failure, exceeding the lower limit of torsion measurement, and severely limiting the measurement range. This invention introduces a double-layer design with opposite spiral directions, so that under torsional deformation in any direction, one of the inner and outer spiral fibers can always measure the torsional strain in its spiral direction. This complementary mechanism at the structural level completely breaks the limitation of the reverse torsion measurement range of the single spiral structure, and multiplies the effective measurement range of the sensor for the torsion angle.

[0021] (3) Provides flexible grating configuration, balancing system cost and measurement accuracy: This invention designs two grating distribution strategies, staggered and aligned, to adapt to different application requirements. When adopting the "intra-layer alignment, inter-layer staggered" distribution strategy, the effective spatial sampling rate of the overall sensing array is doubled without increasing the grating writing density on a single fiber, significantly saving writing costs and time, and effectively reducing the data processing pressure on the demodulation equipment; when adopting the strategy of "aligned distribution" of inner and outer spiral layers and central straight fiber, a more refined torsion rate can be obtained by performing differential averaging calculation on the torsion rate within the overlapping measurement range of the inner and outer spiral layers, thereby effectively improving the accuracy of small-range torsion measurement. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of a three-dimensional structure of a double-helix multi-core fiber optic grating array according to the present invention; Figure 2 This is a schematic diagram of the end face cross-section of a double-helix structure multi-core fiber grating array according to the present invention; Figure 3 Schematic diagram of a single spiral fiber grating of the present invention; Figure 4 This is a schematic diagram showing the relationship between the inner layer torsion rate and torsional strain of the present invention; Figure 5 This is a schematic diagram showing the axial position of the staggered distribution of fiber gratings on the inner and outer spiral optical fibers in this invention; Figure 6 This is a schematic diagram showing the axial position of the fiber grating alignment distribution on the inner and outer spiral optical fibers in this invention. Detailed Implementation

[0023] The invention will be further described below with reference to the accompanying drawings. The invention discloses a double-helix multi-core fiber grating array, the core of which is an innovative fiber topology and grating configuration scheme, designed to achieve high sensitivity and high signal-to-noise ratio measurement of bending, especially torsional strain.

[0024] See Figure 1 , Figure 2 , Figure 3The present invention discloses a double-helix multi-core fiber grating array, comprising an inner helical layer 1, an outer helical layer 2, a central straight fiber 3, a coating layer 4, and a fiber Bragg grating 5. Its main body is a double-helix multi-core fiber wrapped by the coating layer 4. Internally, it integrates a total of (1+N+M) independent fibers, each inscribed with a fiber Bragg grating 5. Here, N and M are both integers not less than 3 to ensure that each helical structure provides sufficient spatial phase information for reliable calculation. These (1+N+M) fibers are arranged in a unique double-helix structure in space. Specifically, the inner helical layer 1 consists of (1+N) fibers, including a central straight fiber 3 located at the geometric center of the fiber cross-section, and N inner helical fibers uniformly coiled around the central straight fiber in a first helical direction (e.g., right-handed), namely inner helical fiber 1-1, inner helical fiber 1-2, inner helical fiber 1-3, ..., inner helical fiber 1-N. The outer spiral layer 2 consists of M optical fibers, namely outer spiral fiber 2-1, outer spiral fiber 2-2, outer spiral fiber 2-3, ..., outer spiral fiber 2-M. They surround the inner spiral layer 1 and are coiled in a second spiral direction opposite to that of the inner spiral fibers 1-1, 1-2, 1-3, etc. (e.g., left-handed). This configuration of reverse spirals between the inner and outer layers is the physical basis for sensing torsional deformation. The inner spiral fibers 1-1, 1-2, ..., 1-N, the outer spiral fibers 2-1, 2-2, ..., 2-M, and the central straight fiber 3 are all engraved with a series of fiber Bragg gratings 5, including fiber Bragg grating 5-1, fiber Bragg grating 5-2, ..., fiber Bragg grating 5-K, where K is an integer not less than 3. The positions of each fiber Bragg grating 5-1, 5-2, ..., 5-K in the central straight fiber 3 are aligned with each fiber Bragg grating 5-1, 5-2, ..., 5-K in the inner spiral layer 1 or the outer spiral layer 2, mainly for strain and temperature compensation.

[0025] The working principle of this invention is as follows: When an optical fiber undergoes natural bending, fibers at different distances from the neutral axis of the bend will experience differentiated strains, resulting in different changes in their grating Bragg wavelengths. The bending shape can be reconstructed by calculating these changes. When the optical fiber twists around its axis, the torsional strain induces strains with opposite signs on the inner and outer spiral fibers due to the opposite directions of the inner and outer spirals. Specifically, for a right-handed inner spiral fiber, forward twisting leads to effective pitch compression, generating compressive strain and causing a "blue shift" in the grating wavelength; for a left-handed outer spiral fiber, the same forward twisting leads to effective pitch stretching, generating tensile strain and causing a "red shift" in the wavelength. By processing the signals from the inner and outer layers, the number of torsional response signals is increased, thereby achieving high signal-to-noise ratio torsional measurement. To address different application requirements, this invention further optimizes measurement performance through two grating layout methods: the staggered distribution method ensures that effective measurement data of at least one layer of helical fiber can be obtained near any axial position, thereby achieving high-resolution capture of deformation gradients with a smaller total number of gratings; while the aligned distribution method ensures that both inner and outer helical layers have measurement nodes at the same axial position, facilitating differential averaging of torsion data within the overlapping range, thus significantly improving the measurement accuracy of small-range micro-torsion. Details are as follows:

[0026] When optical fibers are bent and twisted, each fiber experiences different strains due to its varying distance from and azimuth angle to the neutral axis of the bend. For a helical optical fiber, the strain can be expressed as: (1) In equation (1), r is the helical radius, i.e., the center-to-center distance between the helical fiber and the central straight fiber, and R is the bending radius. It is a spiral phase. The bending direction angle, This refers to the additional strain caused by torsion.

[0027] The strain caused by bending can be eliminated by utilizing the phase difference between the spiral fibers. For example, for three spiral fibers with a phase difference of 120°, the strain when they are subjected to bending and torsion is... Then the torsional strain is: (2) As described in equations (1) and (2), the decoupling of bending and torsional strain in this invention is achieved by summing the strains of multiple helical optical fibers with specific phase differences within the same helical layer. This algorithm can accurately eliminate the strain influence caused by bending, and by combining it with temperature and axial tension compensation of the central straight optical fiber, the pure torsional strain can be calculated.

[0028] Based on the decoupling of torsional strain, the inner and outer double-layered reverse spiral structure plays a decisive role in expanding the measurement range. If large-amplitude torsion occurs under complex force fields, a single spiral structure can only measure forward and a small range of reverse torsion. The present invention expands the torsion measurement range based on double-helix optical fiber as follows:

[0029] For torsion calculations, taking a segment of a spiral fiber with a pitch of as an example, the calculation principles for the torsional strain of the inner and outer spiral layers are the same, and are expressed as follows: (3) (4) Among them is The torsional strain experienced by the inner spiral layer, This refers to the torsional strain experienced by the outer spiral layer. It is the original arc length of the helical fiber with the inner helix layer. It is the arc length of the helical optical fiber in the inner helical layer after twisting. It is the original arc length of the helical fiber in the outer spiral layer. It is the arc length of the spiral fiber in the outer spiral layer after twisting.

[0030] and According to the Pythagorean theorem, this can be converted to: (5) (6)

[0031] After being twisted And can be converted to: (7) (8) in, The helical radius of the helical fiber with the inner helical layer is given. The tangential strain of the helical optical fiber with inner helical layer; The helical radius of the helical fiber is the outer helical layer. This represents the tangential strain of the helical optical fiber with the outer helical layer.

[0032] Since the inner and outer spirals are in opposite directions, the effective arc length changes after twisting exhibit an inverse relationship of addition and subtraction, specifically expressed as the addition in formula (7) and the subtraction in formula (8). The torsion rate in the initial state can be expressed as: (9) The torsional rate of the inner spiral layer under torsional conditions can be expressed as: (10)

[0033] The torsional rate of the outer spiral layer can be expressed as: (11)

[0034] By combining equations (3), (5), (7), (9), and (10), we can obtain: (12)

[0035] By combining equations (4), (6), (8), (9), and (11), we can obtain: (13)

[0036] Equations (12) and (13) can be solved using the quadratic formula to calculate the torsion ratios of the inner and outer spiral layers under torsion conditions. Then, by integrating the arc length of the spiral fiber in the inner spiral layer or the arc length in the spiral fiber in the outer spiral layer, the torsion angle of the spiral fiber can be obtained. (14)

[0037] The torsional ratio and torsional strain in equation (12) belong to a hyperbolic relationship defined by a quadratic equation in two variables, such as... Figure 4 As shown. The lower half of the curve is meaningless; in the upper half of the curve, when... When >0, the helical fiber of the inner helical layer is subjected to positive torsion; when < When <0, the helical fiber of the inner helical layer is subjected to reverse torsion; when At this time, the torsional strain experienced by the helical fiber in the inner helical layer is... This makes it a straight fiber. It is evident that the reverse twist measurement range of a single helical fiber has limitations; therefore, using a helical fiber grating with a double helix structure having opposite inner and outer helical directions can complement the twist measurement range. It should be noted that... Figure 4 Although the solution relationship is illustrated using the inner spiral layer as an example, this physical law also applies to the outer spiral layer.

[0038] To more clearly illustrate the details of the principle of the present invention, the following provides a specific implementation method for the distribution of two double-helix multi-core fiber grating arrays.

[0039] like Figure 5The first distribution method shown is an interleaved distribution of inner and outer spiral fiber Bragg gratings. In the grating axial configuration, the grating distribution period of a single spiral fiber is set to L. The gratings of the inner N right-hand spiral fibers are aligned at positions 0, L, 2L, 3L…; the gratings of the outer M left-hand spiral fibers are aligned at positions L / 2, 3L / 2, 5L / 2…; and the grating of the central straight fiber is located at all positions 0, L / 2, L, 3L / 2… This distribution method greatly alleviates the data processing pressure on the demodulation equipment while doubling the overall effective spatial sampling rate of the fiber Bragg grating array.

[0040] like Figure 6 The distribution shown is an aligned fiber grating arrangement of the inner and outer spiral layers and the central straight fiber. In terms of grating axial configuration, the grating distribution period of a single spiral fiber is set to L. The gratings of the inner N right-hand spiral fibers are aligned at positions 0, L, 2L, 3L…; the gratings of the outer M left-hand spiral fibers and the central straight fiber are also aligned at positions 0, L, 2L, 3L… This distribution allows for differential averaging of the torsion rate within the measured range of the overlapping inner and outer spiral layers, resulting in a more refined torsion rate and improving the accuracy of small-range torsion measurements.

[0041] Both distribution methods increase the measurement range of torsion. However, the alignment distribution of the gratings of the inner and outer spiral layers and the central straight fiber only improves the measurement accuracy of the torsion rate in the overlapping part of the torsion measurement range of the inner and outer spiral layers.

[0042] It should be noted that the specific axial coordinates of the inner and outer spiral fiber gratings given in the above embodiments of the present invention are theoretical numerical positions. In actual multi-core fiber drawing and grating writing processes, due to limitations in manufacturing precision and equipment tolerances, the actual axial position of the grating is allowed to have a certain range of process errors or positional offsets (e.g., offsets of ±ΔL). The scope of protection of the present invention is not limited to the ideal situation where the gratings are strictly and absolutely written at mathematically theoretical points; in actual products, as long as the grating sets of the inner spiral fiber and the grating sets of the outer spiral fiber macroscopically satisfy the characteristics of 'periodic distribution within the layer' and 'interlayer staggered or aligned distribution', they should all be considered equivalent implementations and fall within the patent protection scope of the present invention.

[0043] The inner and outer double-layer reverse spiral structure of this invention solves the problem of limited measurement range of this unidirectional spiral torsion, and improves the performance of torsion measurement by different fiber grating distribution methods. The staggered distribution of inner and outer spiral layer fiber gratings effectively reduces the data processing pressure of demodulation equipment while doubling the overall effective spatial sampling rate of the fiber grating array. The aligned distribution of fiber gratings of inner and outer spiral layers and central straight fiber improves the accuracy of small-range torsion measurement.

[0044] This invention uses specific examples to illustrate the principles and implementation methods of the invention. The descriptions of the above embodiments are only for the purpose of helping to understand the method and core ideas of the invention. Furthermore, those skilled in the art will recognize that, based on the ideas of this invention, there will be changes in the specific implementation methods and application scope. Therefore, the content of this specification should not be construed as a limitation of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claimed invention.

Claims

1. A double-helix multi-core fiber grating array, characterized in that: It contains 1+N+M independent optical fibers, all wrapped together by an external coating layer; The 1+N+M optical fibers are arranged in a double-layer helical structure in space; The double-layer helical structure is specifically composed of: the inner layer structure consists of 1+N optical fibers, including a central straight optical fiber located at the geometric center of the optical fiber cross section and extending in a direction parallel to the optical fiber axis, and N inner helical optical fibers coiled around the central straight optical fiber in a first helical direction; the outer layer structure consists of M outer helical optical fibers, which surround the inner layer structure and coil in a second helical direction opposite to the first helical direction. Where N≥3, M≥3.

2. The double-helix multi-core fiber grating array according to claim 1, characterized in that: It contains 1+N+M independent optical fibers, and all optical fibers contain a series of fiber Bragg gratings. A series of gratings are etched along the fiber axis in all internally spiral optical fibers to form the first set of periodic measurement points; All external spiral fibers are inscribed with a series of gratings along the fiber axis to form a second set of periodic measurement points; A series of gratings are etched along the axial direction in the central straight optical fiber. The axial position of each grating in the central straight optical fiber corresponds one-to-one with the axial position of the first group of periodic measurement points or the second group of periodic measurement points, which is used to provide tensile strain and environmental temperature compensation.

3. The double-helix multi-core fiber grating array according to claim 2, characterized in that: The centers of a series of gratings etched in all the internal spiral optical fibers are aligned with each other along the fiber axis. The centers of a series of gratings etched in all external spiral optical fibers are aligned with each other along the fiber axis. The first group of periodic measurement points and the second group of periodic measurement points are staggered along the axial direction.

4. The double-helix multi-core fiber grating array according to claim 3, characterized in that: The axial distribution period of the grating is set to L, and a series of gratings in the inner spiral fiber are located at positions 0, L, 2L, 3L…; A series of gratings in the outer spiral optical fiber are located at positions L / 2, 3L / 2, 5L / 2, 7L / 2…; A series of gratings in the central straight optical fiber are located at positions 0, L / 2, L, 3L / 2…; The position offset of each grating is ±△L, and △L is not greater than L / 5.

5. The double-helix multi-core fiber grating array according to claim 2, characterized in that: The centers of a series of gratings etched in all the internal spiral optical fibers are aligned with each other along the fiber axis. The centers of a series of gratings etched in all external spiral optical fibers are aligned with each other along the fiber axis. All the gratings etched in the inner spiral fibers and all the gratings etched in the outer spiral fibers are periodically aligned with each other along the fiber axis.

6. The double-helix multi-core fiber grating array according to claim 5, characterized in that: The axial distribution period of the grating is set to L, and a series of gratings of the inner spiral fiber, the outer spiral fiber and the central straight fiber are all located at positions 0, L, 2L, 3L...; The position offset of each grating is ±△L, and △L is not greater than L / 5.

7. A method for measuring the torsion of a multi-core fiber optic grating array with a double-helix structure as described in any one of claims 1 to 6, characterized in that, Includes the following steps: Step 1: Bending strain decoupling and initial torsional calculation The grating wavelength offset of multiple inner or outer spiral optical fibers with specific spatial phase differences within the same spiral layer is collected. The spatial distribution characteristics of different fiber cores in the same layer are used for joint calculation to eliminate the additional strain interference caused by bending and to preliminarily calculate the torsional strain of the spiral layer. Step 2: Independent calculation of double-layer torsion ratio Based on the physical configuration of the inner and outer spiral fibers with opposite spiral directions, the sensing signals of the inner and outer spiral layers are processed separately, and the torsion rates of the inner and outer spiral layers are calculated independently. Step 3: Omnidirectional Torsional Synthesis and Range Expansion By utilizing the complementary characteristics of the inner and outer double-layer reverse spiral structures, the torsion rates of the inner spiral layer and the outer spiral layer are weighted, fused, or switched. When unidirectional torsion is detected, causing the single-layer fiber to enter the measurement nonlinear region or blind region, the measurement data of the other reverse spiral layer is used for compensation or replacement, thereby achieving omnidirectional, large-range torsion morphology reconstruction and accurate measurement.

8. The torsion measurement method according to claim 7, characterized in that: By employing the staggered distribution of grating axial positions in the inner and outer spiral optical fibers as described in claims 3 and 4, the effective spatial sampling rate of the entire array is improved by utilizing the complementary positions of the interlayer gratings while maintaining the unchanged grating writing density in a single fiber.

9. The torsion measurement method according to claim 7, characterized in that: When the double-helix multi-core fiber grating array, as described in claims 5 and 6, adopts a distribution pattern in which the grating axial positions of the inner and outer helical fibers are aligned, the first torsion rate data calculated by the inner helical fiber layer and the second torsion rate data calculated by the outer helical fiber layer are synchronously acquired at the same axial position. The first torsion rate data and the second torsion rate data are differentially processed to increase the measurement variation amplitude, reduce random measurement noise, and achieve high-precision measurement of small-range micro-torsion.

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