Shape measurement system and shape measurement method

Abstract

The purpose of the present disclosure is to make it possible to identify the shape of an object to be measured over a long distance using a multi-core optical fiber that does not have a core at the center of cladding.　The present disclosure pertains to a shape measurement system comprising: a multi-core optical fiber in which at least four cores are disposed in a portion other than the center of a cross-section thereof; a strain distribution measurement device that measures the strain distribution of each of the cores in the longitudinal direction of the multi-core optical fiber; and an analysis device that calculates the shape of the multi-core optical fiber using the strain distributions of the multi-core optical fiber.

Description

[Technical field]

[0001] The present disclosure relates to a technique for deriving a three-dimensional shape of a measurement object based on measurement and analysis of a strain distribution in the light propagation direction of a multi-core optical fiber installed along the measurement object. [Background technology]

[0002] The reflection spectrum in the frequency domain of each core of a multi-core optical fiber (hereinafter sometimes referred to as MCF (Multi Core Fiber)) is measured by OFDR (Optical Frequency Domain Reflectometry) and the measurement results are analyzed, thereby enabling the 3D shape of the measurement object to be derived (see, for example, Non-Patent Documents 1 and 2). However, although identification of the 3D shape using OFDR can achieve a high resolution on the order of several tens of mm, the measurement distance is limited to approximately several tens of meters.

[0003] A method is known in which a fiber Bragg grating (FBG) is provided along the entire length of a sensing medium to improve the measurement resolution (see, for example, Non-Patent Document 1). However, this method has the problem of reducing the manufacturability and economic efficiency of the sensing medium.

[0004] On the other hand, shape identification can also be performed using quasi-distributed measurements such as the addition of FBG (see, for example, Non-Patent Document 2). However, this technique has a problem in that the number of measurement points is limited, making it difficult to identify shape over long distances.

[0005] Shape identification using MCF has also been proposed (for example, see Non-Patent Document 3). Non-Patent Document 3 uses BOTDR (Brillouin Optical Time Domain Reflectometry), which is capable of measuring long distances of several kilometers to several tens of kilometers. However, Non-Patent Document 3 requires a core at the center of the cladding of the MCF, so it is not possible to use a four-core fiber that does not have a core at the center of the cladding, which is used for communication purposes.

[0006] Furthermore, in Non-Patent Document 3, the distance between the central core and the outer core needs to be increased for shape sensing of small curvatures. However, if the cladding diameter is increased to widen the core spacing, a problem occurs in that the optical fiber becomes more susceptible to breakage due to bending. [Prior art documents] [Non-patent literature]

[0007] [Non-Patent Document 1] https: / / doi.org / 10.1117 / 12.720914(High-accuracy fiber-optic shape sensing) [Non-Patent Document 2] DOI:10.1109 / TMECH.2010.2080360(Real-Time Estimation of 3-D Needle Shape and Deflection for MRI-Guided Interventions) [Non-Patent Document 3] Kobayashi et al., “Basic study of shape sensing of large linear structures using multicore fiber with BOTDR,” OFT2020-19, 2020. Summary of the Invention [Problem to be solved by the invention]

[0008] An object of the present disclosure is to make it possible to identify the shape of a measurement target over a long distance using a multi-core optical fiber that does not have a core in the center of the cladding. [Means for solving the problem]

[0009] Specifically, the shape measurement system of the present disclosure includes: A multi-core optical fiber having four or more cores arranged in a portion other than the center of the cross section; a strain distribution measuring device that measures a strain distribution of each core in a longitudinal direction of the multi-core optical fiber; an analysis device that calculates a shape of the multi-core optical fiber by using a strain distribution of the multi-core optical fiber; Equipped with.

[0010] Specifically, the shape measurement method of the present disclosure includes: a strain distribution measurement device measures a strain distribution of each core in a longitudinal direction of a multi-core optical fiber using a multi-core optical fiber in which four or more cores are arranged in a portion other than the center of a cross section; An analysis device calculates a shape of the multi-core optical fiber using the strain distribution of the multi-core optical fiber. Effect of the Invention

[0011] The present disclosure can make it possible to realize shape identification of a measurement target over a long distance using a multi-core optical fiber that does not have a core in the center of the cladding. [Brief description of the drawings]

[0012] [Figure 1] 1 illustrates an example of a system configuration according to the present disclosure. [Diagram 2] 1 shows an example of a flow executed by the system of the present disclosure. [Diagram 3] FIG. 2 is an explanatory diagram of a curvature κ in the present disclosure. [Figure 4] 4 is an explanatory diagram of a distance r, an angle α, and a bending angle β in the present disclosure. FIG. [Figure 5A] 13 illustrates a first example of extraction of a three-dimensional shape change in an embodiment of the present disclosure. [Figure 5B] 13 illustrates a first example of extraction of a three-dimensional shape change in an embodiment of the present disclosure. [Figure 5C] 13 illustrates a first example of extraction of a three-dimensional shape change in an embodiment of the present disclosure. [Figure 6A] 13 illustrates a second example of extraction of a three-dimensional shape change in an embodiment of the present disclosure. [Figure 6B] 13 illustrates a second example of extraction of a three-dimensional shape change in an embodiment of the present disclosure. [Figure 6C]13 illustrates a second example of extraction of a three-dimensional shape change in an embodiment of the present disclosure. [Figure 7] An example of the relationship between the distance r and the amount of strain generated in the core is shown below. [Figure 8A] 1 shows an example of a cross-sectional configuration of a solid optical fiber. [Figure 8B] 2 shows an example of a cross-sectional configuration of a hollow optical fiber. [Figure 8C] Illustrates an example of the section modulus relative to the diameter of the cladding. [Figure 9] 1 shows an example of an MCF used in an embodiment of the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Hereinafter, the embodiments of the present disclosure will be described in detail with reference to the drawings. Note that the present disclosure is not limited to the embodiments shown below. These implementation examples are merely illustrative, and the present disclosure can be implemented in various forms with various modifications and improvements based on the knowledge of those skilled in the art. Note that components with the same reference numerals in this specification and drawings are assumed to indicate the same components.

[0014] (Example 1) FIG. 1 shows an example of a system configuration according to the present disclosure. A system for realizing the shape identification method of the present disclosure includes a multi-core optical fiber 91 used to extract shape changes of the object 100 to be measured, a strain distribution measurement device 92 that detects the strain distribution in the propagation direction of each core 11 of the multi-core optical fiber 91, and an analysis device 93 that analyzes the measurement data acquired by the strain distribution measurement device 92.

[0015] The multi-core optical fiber 91 is disposed along the longitudinal direction of the measurement target 100. The multi-core optical fiber 91 has a total of four cores 11 arranged in the cladding 12. In the figure, as an example of an embodiment, the four cores 11 are arranged in a square lattice at approximately equal intervals on approximately the same circumference at a distance r from the center of the cross section of the cladding 12 in a region other than the center of the cross section of the cladding 12.

[0016] In the embodiment of the present disclosure, an example will be described in which the four cores 11 have approximately the same refractive index distribution and optical characteristics. However, the cores 11 in the MCF 91 may be arranged so that the refractive index distribution and optical characteristics of each core are intentionally different. In addition, in the embodiment of the present disclosure, an example in which the number of cores 11 is four is shown, but the number of cores 11 may be four or more. In addition, the cores 11 of the MCF 91 may be arranged at intervals other than equal intervals, but it is preferable that the cores 11 are arranged at a relatively equal interval in the cross section of the MCF 91. For example, in the case of four cores, the cores may be arranged in each of the four quadrants in the Cartesian coordinate system.

[0017] In the MCF 91 of the embodiment of the present disclosure, an MCF having a cladding outer diameter of 125 μm and a core spacing set to 35 μm to 45 μm to reduce crosstalk between cores was used, and a change in the elongation state of the MCF 91 was detected. In addition, a BOTDR device that measures the back Brillouin scattering light distribution was used as the strain distribution measuring device 92. Figure 2 shows an example of an implementation flow of the shape identification method of the present disclosure.

[0018] First, MCF 91 is laid in the measurement target 100, and the strain distribution characteristic of each core 11 in a steady state is acquired using a strain distribution measuring device 92 (step S1). As a result, an analysis device 93 acquires the strain distribution when the core has a reference shape. Next, in a state where the three-dimensional shape of the measurement target 100 has changed, the strain distribution characteristic of each core 11 is acquired again using the strain distribution measurement device 92 (step S2). In this way, the analysis device 93 acquires the strain distribution when the shape is unknown. Next, using the data acquired in step S1 and the data acquired in step S2, the differential strain in step S2 with respect to step S1 is derived at each point of the MCF 91 where the strain distribution measuring device 92 measures the strain (step S3). i is the bending strain ε of MCF91 b and the longitudinal axial strain ε of MCF91 a It is the sum of.

[0019] When MCF91 is bent as shown in Fig. 3, the bending strain ε b,i The curvature κ and the bending angle β are expressed by the relation (1). ε b,i = r κ cos(α i -β) (1) In addition, r is the central axis C of the cladding 12. C Distance from to Core 11, α i represents the angle of the core 11 in the cross section of the MCF 91.

[0020] FIG. 4 shows specific examples of the distance r, the angle α, and the bending angle β. The angle α1 is the distance between the central axis C of the core C1 and the MCF 91. C The line connecting the center axis C of the core C4 adjacent to the core C1 and the MCF91 C In this embodiment, since the cores C1 to C4 are arranged on concentric circles at approximately equal intervals, the angles α1 to α4 are 90 degrees.

[0021] Here, the axial strain ε a occurs uniformly in the cross section of the MCF 91, and is constant regardless of the core number i. Therefore, the differential strain ε i is expressed by equation (2). ε i =ε a +ε b,i (2)

[0022] In this embodiment, since there are four cores 11, a four-variable simultaneous equation is obtained. a The curvature κ and bending angle β in bending strain are equal for all cores 11. Therefore, the differential strain ε i (Step S3)

[0023] Finally, using the Frenet-Serret integral formula, the differential strain ε iA position vector having a size according to the distance z is then determined (step S4). This makes it possible to determine the direction and extent of deformation of the multi-core optical fiber 91 at each point of the distance z, thereby making it possible to measure the shape of the multi-core optical fiber 91 after deformation from the steady state.

[0024] Incidentally, the position accuracy can be improved by correcting the start point and the end point of the measurement section at the distance z of the multi-core optical fiber 91. In addition, in this embodiment, the BOTDR is used for the strain distribution measurement, but any means capable of measuring the strain of an optical fiber, such as OFDR (Optical Frequency Domain Reflectometry), may be used.

[0025] 5A to 5C show an example of extraction of a three-dimensional shape change in an embodiment of the present disclosure. In the embodiment of the present disclosure, the MCF 91 for sensing was stretched linearly, and strain distribution data was obtained when the MCF 91 had a reference shape in step S1 of FIG. 2. Next, the measurement target 100 arranged near the center of the MCF 91 was wound once counterclockwise on the xy plane with a constant curvature as shown in FIG. 5A, and step S2 was performed. As a result, strain distribution data was obtained when the MCF 91 had an unknown shape in FIG. 5B. The solid lines C1, C2, C3, and C4 in FIG. 5B each represent a difference in the core number i. FIG. 5C shows the result of calculating the shape of the MCF 91 changed from the reference shape based on the strain distribution data in steps S1 and S2, and identifying the shape. It can be seen from this result that the bending curvature near the center of the measurement target 100 can be detected with high accuracy.

[0026] 6A to 6C show an example of extraction of a three-dimensional shape change in an embodiment of the present disclosure. As in the case of FIGS. 5A to 5C, the MCF 91 for sensing was stretched in a straight line, and strain distribution data was obtained when the MCF 91 had the reference shape in step S1 of FIG. 2. Next, as shown in FIG. 6A, the MCF 91 was deformed from its end along the measurement target 100 that swirls on the xy plane, and step S2 was performed. As a result, strain distribution data was obtained when the MCF 91 had the unknown shape in FIG. 6B. The solid lines C1, C2, C3, and C4 in FIG. 6B each represent a difference in the core number i. FIG. 6C shows the result of shape identification, in which the shape of the MCF 91 that has changed from the reference shape is calculated based on the strain distribution data in steps S1 and S2. From this result, it can be seen that the shape change in which the curvature gradually decreases on the end side of the MCF 91 can be detected with high accuracy.

[0027] (Example 2) FIG. 7 shows an example of the amount of distortion occurring in the core 11 with respect to the distance r. The different line types in FIG. 7 show the different curvatures. C It can be seen that the farther the distance r is from the center axis C of the cladding 12, the greater the amount of distortion, making it easier to extract minute changes. C To increase the distance r between the center of the cladding 12 and the center of the core 11, it is necessary to increase the diameter of the cladding 12. Increasing the cladding diameter increases the rigidity of the optical fiber, making it harder for the optical fiber to bend and more likely to break. The rigidity of the optical fiber depends on the cross-sectional shape of the optical fiber, and in the case of a solid optical fiber in which the entire optical fiber is made of glass as shown in Figure 8A, the section modulus is expressed by equation (3).

number

[0028] On the other hand, when the present disclosure is used, the cladding central axis C C Since the core 11 is not required, the central axis C of the cladding 12 as shown in FIG. CIn this case, the section modulus is expressed by equation (4).

number

[0029] The section modulus versus diameter of the cladding 12 calculated using formulas (3) and (4) is shown in Figure 8C. Non-Patent Document 3 shows that it is possible to perform shape identification without fracturing an MCF with a cladding 12 diameter of 375 µm. It can be seen that if the hollow area is set to 80%, the rigidity can be maintained at the same level as when the cladding 12 diameter is 375 µm, even if the cladding 12 diameter is 430 µm. Since the present disclosure is capable of using a hollow MCF, the cladding central axis C is shorter than that of the solid type shown in Figure 8A. C Therefore, the distance r of the core 11 can be increased. Therefore, the present disclosure makes it possible to detect a minute amount of strain.

[0030] (Example 3) When extracting a three-dimensional shape change using the MCF91, unintended twisting may occur, which may cause an error in the measurement result. Therefore, in this embodiment, as shown in FIG. 9, when preparing an MCF to be used for shape identification, a certain number of twists per unit length is intentionally applied. Here, the figure shows a case where the number of cores is three for easy understanding. As a result, when an unintended twisting is applied that is equal to or less than the number of twists applied in advance, the MCF91 of this embodiment can estimate the unintended twisting applied later. Therefore, it is possible to eliminate distortion due to unintended twisting, and the accuracy of shape identification can be improved.

[0031] The system configuration of the shape measurement system of this embodiment differs from the configuration described in Fig. 1 in that a known torsion is applied to the MCF 91 and in the accompanying analysis procedure of the analysis device 93. Specifically, the shape measurement method of this embodiment differs in the analyses of steps S3 and S4 shown in Fig. 2.

[0032] In step S3 of FIG. 2, a differential strain ε i is the bending strain ε b,i and axial strain ε a In this embodiment, since the MCF 91 is twisted, the differential strain ε i is the bending strain ε b,i and axial strain ε a and torsional strain ε t In addition, the bending strain ε b,i is expressed by equation (5), and each coefficient is expressed by (6) to (9).

number

[0033] k1, k2 are torsional correction coefficients, ω i is the angle representing the position of the core number i, ν is the Poisson's ratio, p is the spin rate of the core 11 on the circumference, and φ is the specific twist angle. Here, the spin rate is the number of twists [times / m] applied to the MCF 91.

[0034] Since the relative positions of the cores 11 do not change, ε b,i The sum of these becomes zero. Therefore, the differential distortion ε i The sum is given by equation (10).

number

[0035] Curvature κ, bending angle β, and axial strain ε that satisfy Equation (5) and Equation (10) a , torsional strain ε t Finally, the Frenet-Serret integral formula is used to determine the differential strain ε of the multi-core optical fiber in the direction corresponding to the curvature κ and bending angle β at each point of the distance z where the strain was measured. iIn this way, it is possible to determine the direction and extent of deformation at each point of the distance z of the multi-core optical fiber 91, and therefore it is possible to measure the shape of the multi-core optical fiber 91 after deformation from the steady state.

[0036] (Effects of the present disclosure) According to the present disclosure, shape identification is possible using a core arrangement structure in which there is no core at the center of a multi-core optical fiber used to detect shape changes. Also, shape identification of several kilometers or tens of kilometers is possible, whereas OFDR was several tens of meters. Furthermore, according to the present disclosure, since a hollow optical fiber can be used for the MCF 91, the outer diameter of the MCF can be made thicker to expand the distance between the cladding center and the core center, thereby increasing the distortion and improving the measurement sensitivity to measure minute shapes. [Industrial Applicability]

[0037] This disclosure can be used to combat the deterioration of infrastructure by deriving the three-dimensional shape of the object being measured. [Explanation of symbols]

[0038] 11: Core 12: Clad 91: Multi-core optical fiber 92: Strain distribution measuring device 93: Analysis device 100: Measurement target

Claims

1. a multi-core optical fiber in which no core is disposed at the center of a cross section, and four or more cores are disposed at approximately equal intervals on approximately the same circumference at a predetermined distance from the center of the cross section; a strain distribution measuring device for measuring a strain distribution of each core in a longitudinal direction of the multi-core optical fiber; an analysis device that uses the strain distribution of the multi-core optical fiber to solve simultaneous equations based on the axial strains in the cores being equal to each other, thereby determining a curvature and a bending angle at each point of the multi-core optical fiber and calculating a shape of the multi-core optical fiber; A shape measurement system comprising:

2. the strain distribution measurement device measures in advance the strain distribution when the multi-core optical fiber has a reference shape; the analysis device calculates a shape of the multi-core optical fiber that has changed from the reference shape by comparing the strain distribution measured by the strain distribution measurement device with the strain distribution when the multi-core optical fiber has the reference shape. The shape measurement system of claim 1 .

3. The analysis device includes: A differential strain is obtained using the strain distribution measured by the strain distribution measuring device and the strain distribution when the reference shape is obtained; Using the differential strain, the curvature and the bending angle are obtained at each point of the multi-core optical fiber where the strain distribution is measured by the strain distribution measurement device; calculating a shape of the multi-core optical fiber in a three-dimensional space using a position vector having a magnitude according to the differential strain in a direction corresponding to the curvature and the bending angle; The shape measurement system of claim 2 .

4. the multi-core optical fiber is provided with a known twist; the analysis device estimates an unintended torsional strain caused by a change in shape of the multi-core optical fiber based on the known torsional strain, and excludes the estimated unintended torsional strain from a strain distribution of the multi-core optical fiber; 2. The shape measurement system according to claim 1 .

5. The multi-core optical fiber has a cladding diameter of 125 μm and has cores arranged on a square lattice at intervals of 35 μm to 45 μm.

2. The shape measurement system according to claim 1 .

6. The multi-core optical fiber is characterized in that the central axis thereof is hollow. The shape measurement system of claim 1 .

7. 2. The shape measurement system according to claim 1, wherein the strain distribution measurement device measures a distribution of back Brillouin scattered light generated in the multi-core optical fiber.

8. a strain distribution measurement device measures a strain distribution of each core in a longitudinal direction of a multi-core optical fiber using a multi-core optical fiber in which no core is arranged at the center of a cross section and four or more cores are arranged at approximately equal intervals on approximately the same circumference at a predetermined distance from the center of the cross section; an analysis device uses the strain distribution of the multi-core optical fiber to solve simultaneous equations based on the axial strains in the cores being equal to each other, thereby determining the curvature and bending angle at each point of the multi-core optical fiber and calculating the shape of the multi-core optical fiber; Shape measurement method.

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