Free core-based multi-core fiber grating array preparation and multi-parameter sensing method

Abstract

The invention provides a multi-core fiber grating array preparation and multi-parameter sensing method based on a free core aiming at the cross sensitive effect of temperature and strain of a multi-core fiber in the detection process of multiple parameters such as temperature, strain and shape. The multi-core fiber grating array based on the free core is formed by compounding a plurality of grating array optical fibers which are integrally prepared on line by a wire drawing tower, and a loose tube is sleeved outside the grating array optical fiber at the central fiber core position on a neutral surface to form a free core state. The free core fiber bragg grating array is not affected by stress in actual monitoring, single parameter measurement of temperature can be achieved, temperature compensation of other fiber cores can be achieved through the free core fiber bragg grating array, and the cross sensitive effect of temperature and strain is effectively restrained. Through the multi-core fiber grating array based on the free core, simultaneous monitoring of multiple parameters such as temperature, strain and shape can be realized, the monitoring precision is high, and the shape reconstruction effect is good.

Description

Technical Field

[0001] The present invention relates to the field of optical fiber sensing, and in particular to a method for preparing a multi-core optical fiber grating array based on a free core and a multi-parameter sensing method. Background Art

[0002] Fiber Bragg grating is an optical device that forms a periodic refractive index modulation structure in the core of an optical fiber through ultraviolet laser. It achieves precise control of optical signals by reflecting light of a specific wavelength and transmitting other wavelengths. Fiber Bragg grating is widely used in the sensing and monitoring of physical quantities such as temperature, strain, vibration, etc. in various fields such as aerospace and civil engineering. Compared with photoelectric sensors and micro-electromechanical sensors, it has the advantages of no need for power supply, high sensitivity, anti-electromagnetic interference, corrosion resistance and easy reuse.

[0003] With the development of the optical field, the role of fiber Bragg grating in shape sensing technology has been continuously demonstrated.

[0004] In the field of interventional medicine, fiber Bragg gratings are small in size and easy to package. They can achieve shape sensing in a narrow surgical space and accurately deliver surgical instruments or drugs to the required parts. In the field of aerospace, intelligent aircraft based on fiber Bragg grating shape sensing can monitor wing changes in real time and improve aircraft performance and safety. In the field of intelligent robots, fiber Bragg gratings are placed in robot joints and mechanical arms, which can be tracked and feedback controlled in real time through fiber Bragg grating shape sensing.

[0005] Multi-core optical fiber is a special optical fiber that contains multiple independent cores. Each core can be written with a fiber grating separately. By monitoring the wavelength offset of the fiber grating in different cores, it can monitor temperature, strain and vibration. In the field of shape sensing, multi-core fiber grating measures the difference in wavelength changes caused by bending or deformation of each core, and finally realizes the three-dimensional shape reconstruction of the optical fiber by combining algorithms.

[0006] However, in the above detection process, the wavelength of the multi-core fiber Bragg grating is affected by multiple parameters such as temperature, strain, and shape change, resulting in a cross-sensitivity effect between temperature and strain. This effect will significantly reduce the accuracy of each parameter sensing, especially in a complex temperature change environment, where it is difficult to distinguish the effects of temperature and strain on the wavelength of the fiber Bragg grating. Although the existing technology attempts to compensate for temperature interference through dual parameter matrix decoupling or the introduction of a reference grating, there are still problems such as high solution complexity and limited spatial resolution.

[0007] In view of the cross-sensitivity effect of temperature and strain in the multi-parameter detection process of temperature, strain, shape, etc. of the above-mentioned multi-core optical fiber, how to achieve dynamic and flexible temperature compensation, effectively suppress cross-sensitivity, and thus improve the sensing accuracy of multi-core optical fiber grating shape has become a key problem that needs to be solved urgently. The present invention proposes a multi-core optical fiber grating array preparation and multi-parameter sensing method based on free core, wherein the free core multi-core optical fiber includes a plurality of grating array optical fibers prepared online by an integrated drawing tower, and each group of gratings of the plurality of grating array optical fibers is fixed at the same horizontal position and then composited into a multi-core optical fiber through a multi-core binding system, wherein the grating array optical fiber located at the center core position on the neutral plane is covered with a loose tube on the outside to form a free core state. Due to the sensitivity of fiber Bragg grating parameters such as temperature and strain, the free-core multi-core fiber Bragg grating array can realize the monitoring of multiple parameters such as temperature, strain, and shape. The free-core fiber Bragg grating array located at the neutral plane of the central core is protected by a loose tube and will not be affected by stress in actual monitoring. It only needs to monitor the temperature change, and then the temperature compensation of several other cores can be realized through the free-core fiber Bragg grating array, effectively suppressing the cross-sensitivity effect of temperature and strain. Summary of the invention

[0008] The present invention provides a method for preparing a free-core multi-core fiber grating array and a multi-parameter sensing method, which can rely on the free-core multi-core fiber grating array to achieve multi-parameter sensing of temperature, strain, and shape, solve the temperature compensation problem in strain monitoring, suppress cross sensitivity, and provide the preparation of the free-core multi-core fiber grating array and a multi-parameter sensing demodulation solution.

[0009] The method for realizing multi-parameter sensing of temperature, strain and shape of the present invention is realized based on a free-core multi-core fiber grating array.

[0010] The free core multi-core fiber grating array is characterized in that it is composed of a plurality of grating array optical fibers prepared online by an integrated drawing tower, wherein the grating array optical fiber located at the center of the neutral plane is covered with a loose sleeve on the outside, i.e., the free core. The remaining plurality of grating array optical fibers are evenly arranged outside the free core in a symmetrical structure. The free core fiber grating array and the fiber gratings of other fiber grating array cores outside are at the same horizontal position. There is cured glue between the free core fiber grating array and other fiber grating array cores outside.

[0011] The temperature, strain, and shape multi-parameter sensing implementation method is characterized in that the free-core fiber grating array located at the center position on the neutral plane is not affected by strain but only by temperature, thereby realizing single-parameter measurement of temperature; the fiber gratings on the cores of other outer fiber grating arrays can be directly temperature compensated according to the temperature measurement result; the wavelength change of other outer fiber grating array cores can obtain the strain amount after temperature compensation, thereby realizing strain monitoring; and the bending radius and bending direction of the grating position on the outer fiber grating array can be determined through the relationship between strain and curvature and the geometric relationship between each core, and the grating position coordinates are obtained accordingly to realize shape reconstruction.

[0012] The specific steps include:

[0013] Step 1: The free-core multi-core fiber grating array based on the above method realizes temperature sensing, characterized in that the free-core fiber grating array at the center core position on the neutral plane is not affected by strain under the protection of the external loose tube, and its wavelength change is only affected by temperature. The relationship between wavelength and temperature is: (1) in is the initial wavelength of the mth grating in the free core fiber grating array, is the wavelength offset of the mth grating in the free core fiber grating array, is the thermal expansion coefficient of the optical fiber, is the optical fiber thermo-optic coefficient, is the temperature change of the mth grating position. Monitor the wavelength change of the mth grating in the free core fiber grating array , based on formula (1), the temperature change at the mth grating position is obtained: Since the cores of the free-core multi-core fiber Bragg grating array are closely arranged, the temperature variation at the mth grating position of the other outer fiber Bragg grating array cores is The same; record the wavelength changes of each fiber grating on the core of other outer fiber grating arrays to further calculate the strain value at that location.

[0014] Step 2: The free-core multi-core fiber grating array based on the above method realizes strain sensing, characterized in that the free-core fiber grating array at the center core position on the neutral plane is only affected by temperature but not strain under the protection of the external loose tube, and the fiber gratings of other fiber grating array cores on the outside are affected by both temperature and strain in actual monitoring, and the relationship between temperature and strain and grating wavelength is: (2) in is the initial wavelength of the mth grating in the nth fiber core, is the wavelength offset of the mth grating in the nth fiber core, is the elastic-optical coefficient of the optical fiber, is the strain value of the mth grating position of the nth fiber core, is the optical fiber thermo-optic coefficient, is the equivalent thermal expansion coefficient of other outer fiber grating array cores. The coefficient is the same for all other outer fiber grating array cores and can be obtained through calibration. is the temperature change of the mth grating position obtained in step 1. The wavelength change of the fiber grating on the core of other fiber grating arrays outside is used to calculate the strain value at the corresponding position. .

[0015] Step 3: The shape sensing is realized by a free-core multi-core fiber grating array, which is characterized by using the strain value of other outer fiber grating array cores to solve the bending radius and bending direction of the grating position, and the relationship formula is: (3) in is the strain value of the mth grating position of the nth fiber core, D is the horizontal distance between the two outer symmetrical fiber cores, is the bending radius of the mth grating position of the nth fiber core, is the angle between the bending direction and the first fiber core, and N is the number of other fiber grating array cores on the outside. Subtract the strain value of the mth grating position of the nth fiber core obtained in step 2 from Substituting into formula 3, we get (N-1) equations. Solving these (N-1) equations together, we get the bending radius at the mth grating position of the nth fiber core: The value of, and the angle between the bending direction and the first fiber core The value of the first fiber core can be freely selected, and the other fiber grating cores on the outside are arranged counterclockwise following the first fiber core.

[0016] Step 4: Reconstruct the fiber shape using the Frenet-Serret framework, which is characterized in that the free-core multi-core fiber grating array is regarded as a space curve located in Cartesian coordinates, with point O as the origin, and the parametric equation of the curve is . Where i, j, k are position vectors, s is the arc length, (0≤s≤L, L is the total length of the free core multi-core fiber grating array). In the Frenet-Serret framework, T, N, and B are three unit vectors, which are the tangent axis, normal axis, and binormal axis vectors of the space curve, respectively, and The bending and twisting at each position of the free-core multi-core fiber grating array can be represented by T, N, and B. The rotation angle of T is k(s), and the rotation rate of B around the arc length is the torsion The relationship among T, N and B is: (4) (5) in, , is the unit vector of x and y axis, , K(s) is the curvature vector at the grating point position, and is the component of the curvature vector on the x and y axes, and d is the distance from the other outer core fiber Bragg gratings to the middle free core fiber Bragg grating. Set boundary conditions , , , , , Substitute the above formula and iterate to get the unit T, N, B and the coordinates of each fiber grating point. Inversely integrate T(s) (6) The final three-dimensional curve r(s) is the shape reconstruction curve of the free-core multi-core fiber grating array.

[0017] The method for preparing a free-core multi-core fiber grating array according to the present invention comprises the following steps: Step 1: Select a grating array optical fiber with a loose tube (1), a plurality of grating array optical fibers (2), a fiber feeding device (3), a multi-core binding device (4), a coating cup (5), a curing furnace (6), and a fiber collecting device (7); Step 2: placing a plurality of grating array optical fibers and a grating array optical fiber with a loose tube in a fiber delivery device, the fiber delivery device delivers a plurality of grating array optical fibers and a grating array optical fiber with a loose tube at a uniform speed at the same time, and after each group of grating positions of the plurality of grating array optical fibers is determined at the same horizontal position, the grating array optical fiber with a loose tube is delivered through a multi-core binding device, the multi-core binding device has a plurality of small holes, the grating array optical fiber with a loose tube is delivered through the central hole to ensure that the free core is at the center of the neutral plane during compounding, and each of the remaining grating array optical fibers corresponds to a small hole around the central hole, and the grating array optical fiber with a loose tube and the remaining grating array optical fibers are simultaneously delivered from the small holes and penetrate into the coating cup; Step 3: Glue is poured into the coating cup, and several grating array optical fibers and one grating array optical fiber with a loose tube are coated with glue by the coating cup and then passed through a curing furnace to finally form a multi-core fiber grating array based on a free core; Step 4: The multi-core fiber grating array based on the free core is collected into a disk through a fiber collection device for subsequent use.

[0018] The demodulation scheme of the multi-parameter sensing method of a free-core multi-core fiber grating array described in the present invention has the following characteristics: The invention has a multi-channel data acquisition system (1), a long-distance weak grating demodulator (2), a host computer and demodulation software (3), and connects multiple cores of the multi-core fiber grating array based on the free core through the multi-channel data acquisition system, and connects the collected data to the long-distance weak grating demodulator, so as to realize the demodulation of the wavelength information of the grating points on each core of the long-distance free core multi-core fiber grating array. The wavelength information of each grating point is presented, and the wavelength information is transmitted to the host computer and the demodulation software, so that the wavelength information can be converted into temperature data, strain data, grating position data, and shape reconstruction.

[0019] The beneficial effects of the present invention are:

[0020] 1. The free core multi-core fiber grating array is composed of a grating array fiber with a loose sleeve on the outside and a plurality of grating array fibers. The grating array fiber with a loose sleeve on the outside is located at the center core position on the neutral plane in a free core state and is not affected by strain. In practical applications, the free core is only affected by temperature, and single variable monitoring of temperature can be realized and temperature compensation can be performed on other cores. The wavelength variation of other cores can be calculated after temperature compensation to realize monitoring of strain. The bending radius and bending direction of the grating position can be determined by the relationship between strain and curvature and the geometric relationship between each core, and the grating position coordinates are obtained accordingly to realize shape reconstruction. Based on this, the free core multi-core fiber grating array can realize multi-parameter sensing of temperature, strain and shape.

[0021] 2. Free-core optical fiber is protected by loose tube and located in the neutral plane of multi-core optical fiber. It is not affected by stress in actual detection and can detect the single variable of temperature. The data value is accurate and direct. At the same time, there is no need to add other complex optical fiber links or place redundant fiber gratings to achieve temperature monitoring while performing temperature compensation on other fiber cores. It is simple, fast and easy to deploy.

[0022] 3. The preparation device of the free-core multi-core fiber grating array is simple and easy to operate, and can complete the customization of multi-core optical fiber. The number of multi-core optical fiber cores can be customized, and the selected grating array optical fiber based on the online integrated preparation of the drawing tower can freely select the grating wavelength, grating spacing and other parameters. The multi-core fiber grating array preparation method that replaces the grating on the existing multi-core optical fiber is replaced by multi-core binding. There is no need to position the grating point by point on the core, avoiding the loss caused by the coupling between the single-mode optical fiber and the multi-core optical fiber, greatly reducing the preparation cost of the multi-core fiber grating array, ensuring the grating consistency and writing efficiency, while achieving the needs of low-cost and high-efficiency industrial production, and meeting the industrial preparation conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 Schematic diagram of a free-core multi-core fiber grating array.

[0024] Figure 2 It is a schematic diagram of the structure of the demodulation device of the present invention.

[0025] Figure 3 This is a schematic diagram of the structure of a device for preparing a multi-core fiber grating array based on a free core.

[0026] Figure 4 This is a temperature strain sensing experimental data diagram of the present invention.

[0027] Figure 5 Schematic diagram of the geometric relationship between the four other fiber cores and between the fiber cores and the bending direction.

[0028] Figure 6 Schematic diagram of the Frenet-Serret framework for space curves.

[0029] Figure 7 It is a schematic diagram of shape reconstruction of the present invention. DETAILED DESCRIPTION

[0030] The present invention is further described below in conjunction with the accompanying drawings.

[0031] See attached Figure 1 The present invention discloses a multi-core fiber grating array based on a free core, characterized in that it is composed of a plurality of fiber grating arrays (2) prepared online by an integrated drawing tower, wherein the grating array optical fiber located at the center of the neutral plane is covered with a loose tube to form a free core (1), and the remaining plurality of fiber grating arrays are evenly arranged outside the free core; the fiber grating positions of the free core fiber grating array and the fiber cores of other fiber grating arrays outside are aligned. At the same horizontal position; there is a curing glue (3) between the free core fiber grating array and the fiber cores of other fiber grating arrays outside.

[0032] See attached Figure 2 The present invention discloses a demodulation device for a multi-parameter sensing method of a multi-core fiber grating array based on a free core, comprising a multi-channel data acquisition system (1), a long-distance weak grating demodulator (2), a host computer and demodulation software (3). The multi-channel data acquisition system (1) is used to connect multiple cores of the multi-core fiber grating array based on a free core, and the collected data is connected to the long-distance weak grating demodulator (2), so that the wavelength information of the grating points on each core of the long-distance multi-core optical fiber can be demodulated. The wavelength information of each fiber grating point is presented, and the wavelength information is transmitted to the host computer and the demodulation software (3), so that the wavelength information can be converted into temperature data, strain data, grating position data, and shape reconstruction.

[0033] See attached Figure 3 The present invention discloses a free-core-based multi-core optical fiber grating array preparation device, comprising a grating array optical fiber (1) with a loose tube, a plurality of grating array optical fibers (2), a fiber feeding device (3), a multi-core binding device (4), a coating cup (5), a curing furnace (6), and a fiber collecting device (7); the free-core-based multi-core optical fiber grating array preparation method comprises the following steps: Step 1: placing a plurality of grating array optical fibers (2) and a grating array optical fiber (1) with a loose tube in a fiber delivery device (3), and the fiber delivery device (3) delivers the plurality of grating array optical fibers (2) and the plurality of grating array optical fibers (1) with a loose tube at a uniform speed at the same time, and after each group of grating positions of the plurality of grating array optical fibers is determined to be at the same horizontal position, the plurality of grating array optical fibers enter a multi-core binding device (4), and the multi-core binding device (4) has a plurality of small holes, and the grating array optical fiber (1) with a loose tube is delivered through the central hole to ensure that the free core is at the center position of the neutral plane during compounding, and each of the remaining grating array optical fibers (2) corresponds to a small hole around the central hole, and the grating array optical fiber (1) with a loose tube and the remaining grating array optical fibers (2) are simultaneously delivered from the small holes and then pass into the coating cup (5); Step 2: Glue is poured into the coating cup (5), and a plurality of grating array optical fibers (2) and a grating array optical fiber (1) with a loose tube are coated (5) and then passed through a curing furnace (6) to finally form a multi-core fiber grating array (8) based on a free core; Step 3: The free-core multi-core fiber grating array is collected into a disk through a fiber collection device (7) for subsequent use.

[0034] See attached Figure 4 The free core fiber grating array located at the center core of the multi-core optical fiber proposed in the present invention can effectively realize the single parameter measurement of temperature and the temperature compensation of the fiber grating arrays with other outer cores. When the free core multi-core optical fiber grating array is subjected to a tensile force of 1.8N, the grating wavelength of the free core NO.1 will not shift; the grating wavelengths of the other outer cores will shift, proving that the free core fiber grating array will not be affected when the multi-core optical fiber is subjected to strain.

[0035] Refer to the attached Figure 5 , Taking a free core fiber grating array and four outer core fiber grating arrays as an example, the grating array fiber located at the center of the neutral plane is the free core fiber grating array, which is numbered 1. The outer cores above the free core fiber grating array are numbered as the second core, and then each outer core is numbered in sequence in reverse order. D is the horizontal distance between the two outer symmetrical cores, is the angle between the bending direction and the second fiber core.

[0036] Refer to the attached Figure 6 , is a schematic diagram of the Frenet-Serret frame of the space curve. In the Frenet-Serret frame, T, N, and B are three unit vectors, which are the tangent axis, normal axis, and binormal axis vectors of the space curve, respectively. .

[0037] Refer to the attached Figure 7 Curve a is the fiber curve of the free-core multi-core fiber Bragg grating in the real state in the experimental environment, and curve b is the fiber curve obtained by shape reconstruction after temperature compensation by the multi-parameter sensing method based on the free-core multi-core fiber Bragg grating array. Curve c is the fiber curve reconstructed by the ordinary multi-core fiber Bragg grating array. Since no temperature compensation is performed, the error of shape reconstruction is large.

[0038] In a specific embodiment of the present invention, the five grating array optical fibers delivered by the fiber delivery device at a speed of 50 m / min are made of conventional common single-mode optical fibers (G625), and the grating array optical fibers located at the center core position are covered with loose tubes---free cores, forming a Figure 1 The free-core-based multi-core fiber grating array shown in the figure has a core diameter of 8.2μm and a cladding diameter of 125μm for the conventional single-mode optical fiber used for preparation. The grating center wavelength of each grating array is 1553.4nm, the bandwidth is 0.15nm, the reflectivity is 0.1%, and the physical spacing between the fiber gratings is 2m. The coating cup length is 15cm, and the fiber diameter of the multi-core fiber grating array formed after coating and curing by the coating cup is 3mm, with a total length of 1km.

[0039] Based on the free-core-based multi-core fiber Bragg grating shape sensing method, the temperature compensation accuracy of the free-core fiber Bragg grating and the shape reconstruction accuracy of the free-core multi-core fiber Bragg grating are explored. The free-core multi-core fiber Bragg grating array is heated from 25°C to 65°C as a whole and a tensile force of 1.8N is applied to bend it upward. After the experimental environment is stable, the wavelength data of each core is monitored. Since the free core is not affected by strain, its wavelength offset is completely determined by temperature. The wavelength offset of the free-core grating array is 0.4nm. It can be calculated that the temperature at this time is 65°C, which is consistent with the experimental setting temperature. The temperature change obtained in the experiment and the wavelength data of the other outer core fiber Bragg gratings are substituted into the relationship between temperature and strain and grating wavelength (Formula 2) to obtain the grating strain value. By calculation, it can be concluded that the tensile force on the multi-core optical fiber at this time is 1.802N, which is consistent with the actual situation within the error range. Substitute the strain data of each grating point into the shape reconstruction equation group (Formula 3, Formula 4 and Formula 5), ​​and iterate to obtain the unit T, N, B and fiber Bragg grating point coordinates at the location of each fiber Bragg grating point. The three-dimensional curve r(s) is finally obtained by performing reverse integration of T(s) (Formula 6), which is the shape reconstruction curve of the free-core multi-core fiber grating array. Figure 7 The results show that the temperature compensation effect is good, and the shape reconstruction based on the compensated strain value reduces the reconstruction error by 80%.

[0040] The basic principles and main features of the present invention are shown and described above. Without departing from the spirit and scope of the present invention, the present invention may be subject to various changes and improvements, which all fall within the scope of the present invention to be protected.

Claims

1. A multi-parameter sensing method based on a free-core multi-core fiber grating array, characterized in that: The free-core fiber grating array used is composed of a plurality of grating array optical fibers prepared online by an integrated drawing tower, wherein the grating array optical fiber located at the center of the neutral plane is covered with a loose tube on the outside, which is the free core; the remaining plurality of grating array optical fibers are evenly arranged outside the free core in a symmetrical structure; the free-core fiber grating array and the fiber gratings of other fiber grating array cores outside are at the same horizontal position; there is curing glue between the free-core fiber grating array and other fiber grating array cores outside; The multi-core fiber grating array based on free core realizes the multi-parameter sensing method of temperature, strain and shape by realizing single parameter monitoring of temperature through the free core fiber grating array which is not affected by strain; directly temperature compensation is performed on the fiber grating wavelength of other fiber grating array cores outside according to the temperature measurement result; the strain is calculated after the wavelength change of other fiber grating array cores outside is temperature compensated to realize the monitoring of strain; and the bending radius and bending direction of the grating position are determined through the relationship between strain and curvature and the geometric relationship between each fiber core, and the grating position coordinates are obtained accordingly to realize shape reconstruction; the specific steps include the following: Step 1: The free-core multi-core fiber grating array based on the above method realizes temperature sensing, characterized in that the free-core fiber grating array at the center core position on the neutral plane is not affected by strain under the protection of the external loose tube, and its wavelength change is only affected by temperature. The relationship between wavelength and temperature is: （1） in is the initial wavelength of the mth grating in the free core fiber grating array, is the wavelength offset of the mth grating in the free core fiber grating array, is the thermal expansion coefficient of the optical fiber, is the optical fiber thermo-optic coefficient, is the temperature change of the mth grating position; monitors the wavelength change of the mth grating of the free core fiber grating array , based on formula (1), the temperature change at the mth grating position is obtained: Since the cores of the free-core multi-core fiber Bragg grating array are closely arranged, the temperature variation at the mth grating position of the other outer fiber Bragg grating array cores is Same; record the wavelength change of each fiber grating on the core of other fiber grating arrays outside, which is used to further calculate the strain value at that place; Step 2: The free-core multi-core fiber grating array based on the above method realizes strain sensing, characterized in that the free-core fiber grating array at the center core position on the neutral plane is only affected by temperature but not strain under the protection of the external loose tube, and the fiber gratings of other fiber grating array cores on the outside are affected by both temperature and strain in actual monitoring, and the relationship between temperature and strain and grating wavelength is: （2） in is the initial wavelength of the mth grating in the nth fiber core, is the wavelength offset of the mth grating in the nth fiber core, is the elastic-optical coefficient of the optical fiber, is the strain value of the mth grating position of the nth fiber core, is the optical fiber thermo-optic coefficient, is the equivalent thermal expansion coefficient of other outer fiber grating array cores. The coefficient is the same for all other outer fiber grating array cores and can be obtained by calibration. is the temperature change of the mth grating position obtained in step 1; the strain value of the corresponding position is calculated according to the wavelength change of the fiber grating on the fiber core of other fiber grating arrays outside ; Step 3: The shape sensing is realized by a free-core multi-core fiber grating array, which is characterized by using the strain value of other outer fiber grating array cores to solve the bending radius and bending direction of the grating position, and the relationship formula is: （3） in is the strain value of the mth grating position of the nth fiber core, D is the horizontal distance between the two outer symmetrical fiber cores, is the bending radius of the mth grating position of the nth fiber core, is the angle between the bending direction and the first fiber core, N is the number of other fiber grating array cores on the outside; the strain value of the mth grating position of the nth fiber core obtained in step 2 is Substituting into formula 3, we get (N-1) equations. Solving these (N-1) equations together, we get the value of the bending radius at the mth grating position of the nth fiber core, and the angle between the bending direction and the first fiber core. The value of, where the first fiber core can be freely selected, and the other fiber grating fiber cores on the outside are arranged counterclockwise after the first fiber core; Step 4: Reconstruct the fiber shape using the Frenet-Serret framework, characterized in that the free-core multi-core fiber grating array is regarded as a space curve located in Cartesian coordinates, with point O as the origin, and the parametric equation of the curve is; where i, j, k are position vectors, s is the arc length, (0≤s≤L, L is the total length of the free-core multi-core fiber grating array); in the Frenet-Serret framework, T, N, and B are three unit vectors, which are the tangent axis, normal axis, and binormal axis vectors of the space curve, respectively, and The bending and twisting of the free-core multi-core fiber grating array at each position can be represented by T, N, and B. The rotation angle of T is k(s), and the rotation rate of B around the arc length is the torsion. ; The relationship between T, N, and B is: （4） （5） in, is the unit vector of the x and y axes, K(s) is the curvature vector at the position of the grating point, and is the component of the curvature vector on the x and y axes, and d is the distance from the other outer core fiber gratings to the middle free core fiber grating; set the boundary conditions , , , , , Substitute the above formula and iterate to get the unit T, N, B and the coordinates of each fiber grating point. Inversely integrate T(s): （6） The final three-dimensional curve r(s) is the shape reconstruction curve of the free-core multi-core fiber grating array.

2. A free-core multi-core fiber grating array according to claim 1, wherein the preparation method comprises the following steps: Step 1: Select a grating array optical fiber with a loose tube, several grating array optical fibers, a fiber feeding device, a multi-core binding device, a coating cup, a curing furnace, and a fiber collection device; Step 2: placing a plurality of grating array optical fibers and a grating array optical fiber with a loose tube in a fiber delivery device, the fiber delivery device delivers a plurality of grating array optical fibers and a grating array optical fiber with a loose tube at a uniform speed at the same time, and after each group of grating positions of the plurality of grating array optical fibers is determined at the same horizontal position, the grating array optical fiber with a loose tube is delivered through a multi-core binding device, the multi-core binding device has a plurality of small holes, the grating array optical fiber with a loose tube is delivered through the central hole to ensure that the free core is at the center of the neutral plane during compounding, and each of the remaining grating array optical fibers corresponds to a small hole around the central hole, and the grating array optical fiber with a loose tube and the remaining grating array optical fibers are simultaneously delivered from the small holes and penetrate into the coating cup; Step 3: Glue is poured into the coating cup, and several grating array optical fibers and one grating array optical fiber with a loose tube are coated with glue by the coating cup and then passed through a curing furnace to finally form a multi-core fiber grating array based on a free core; Step 4: The multi-core fiber grating array based on the free core is collected into a disk through a fiber collection device for subsequent use.

3. A multi-parameter sensing method of a free-core multi-core fiber grating array based on claim 1, wherein the demodulation scheme is characterized in that: it has a multi-channel data acquisition system, a long-distance weak grating demodulator, a host computer and demodulation software, and the multiple cores of the free-core multi-core fiber grating array are connected through the multi-channel data acquisition system, and the collected data are connected to the long-distance weak grating demodulator, so that the wavelength information of the grating points on each core of the long-distance free-core multi-core fiber grating array can be demodulated; the wavelength information of each grating point is presented, and the wavelength information is transmitted to the host computer and the demodulation software, so that the wavelength information can be converted into temperature data, strain data, grating position data and shape reconstruction.

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