Patent ID: 12203847

DESCRIPTION OF EMBODIMENTS

With reference to the attached drawings, embodiments of the present invention will be described. The embodiments described below are working examples of the present invention, and the present invention is not limited to the following embodiments. In the specification and the drawings, components having the same reference numerals indicate mutually identical components.

Embodiment 1

FIG.1is a drawing describing a shape measurement system of the embodiment. The shape measurement system includes a multi-core optical fiber10having a center core11arranged in a center of a cross section and three or more outer peripheral cores12arranged at equal intervals on an outside of the center core11and in a concentric manner, a measuring device20that measures a backward Brillouin scattering light distribution in a propagation direction of each core of the multi-core optical fiber10, and an analysis device30that computes positional coordinates in a three-dimensional space of a linear structural object having an unknown three-dimensional shape from the backward Brillouin scattering light distribution of the multi-core optical fiber10arranged along the linear structural object having an unknown three-dimensional shape and the multi-core optical fiber10arranged along a linear structural object having an already-known three-dimensional shape to identify its time change.

InFIG.1, an example in which one specific core of the multi-core optical fiber10is connected to the measuring device20that measures the backward Brillouin scattering light distribution is illustrated. Another configuration may be in a form in which the multi-core optical fiber10is connected to the measuring device20via a fan-out mechanism that separates each core of a multi-core optical fiber into single core optical fibers and an optical switch.

The shape measurement system includes the multi-core optical fiber10that is a sensing medium, the measuring device (BOTDR)20that detects a backward Brillouin scattering light in the propagation direction of each core of the multi-core optical fiber10, and the analysis device30that analyzes measurement data obtained by the BOTDR12.

The multi-core optical fiber10is installed along a longitudinal direction of a linear structural object that is a target to be measured. The linear structural object is, for example, a pipeline, a riser (pipe through which a fluid flows from a sea bottom to a facility above sea level in offshore drilling and marine production), a submarine cable, and the like.FIG.2is a drawing describing the cross section of the multi-core optical fiber10. The multi-core optical fiber10has a total of four cores which are a center core located in the center of the cross section and outer peripheral cores arranged mostly at equal intervals mostly on an identical circumference from the center of the cross section. In the embodiment, while cores having a mostly equal refractive index distribution and optical property are used as the four cores, cores may have a structure in which they are arranged such that the refractive index distribution and the optical property of the respective cores are purposely different.

Restrictions of a count of cores are as follows. When the count of cores is three or less (two or less outer peripheral cores), shape identification cannot be performed. On the other hand, when the count of cores is five or more (four or more outer peripheral cores), the aspect of measurement accuracy improves, but a measurement time period increases by the count of cores.

A circle having a certain radius can be surrounded by six circles having an identical radius that are in contact with the circle. In this case, a total count of the circles is seven. That is, a multi-core optical fiber having a core position when seven single core optical fibers having an identical diameter are arranged in a close packed manner can be formed, allowing easily making a FIFO.

On the other hand, when a count of outer peripheral circles becomes seven or more (the total count of circles is eight or more), gaps are generated between the center circle and the outer peripheral circles by arranging seven or more circles around one circle. That is, eight or more single core optical fibers having an identical diameter cannot be arranged in a close packed manner, a cladding diameter of the multi-core optical fiber increases, and it becomes difficult to make a fan-in/fan-out (FIFO).

Therefore, the count of outer peripheral cores in the multi-core optical fiber is preferably three or more and six or less.

The multi-core optical fiber10has a cladding outer diameter (>125 μm) larger than that of a general optical fiber for communication. A center-to-center distance between the center core11arranged in the center of the optical fiber and the outer peripheral cores12arranged at an outer periphery of the center core11is set to larger than 30 μm. The reason is that in a typical multi-core optical fiber having a cladding outer diameter of 125 μm, the center-to-center distance between the center core and the outer peripheral cores that can be arranged to reduce an influence, such as leakage loss, is approximately 30 μm.

FIG.3is a drawing describing a relationship between the center-to-center distance and strain amounts of the outer peripheral cores12relative to curvatures of the center of the multi-core optical fiber10when (α−β) is assumed to be zero in a relational expression (1). FromFIG.3, it can be seen that when the center-to-center distance between the center core11and the outer peripheral cores12is 120 μm or more, a strain amount of five times or more the strain amount obtained in the typical multi-core optical fiber having a cladding outer diameter of 125 μm can be obtained even in a curvature κ=0.5[1/m]. This means that by lengthening the center-to-center distance between the center core11and the outer peripheral cores12, a sensitivity can improve to a shape change with a small curvature generated in the multi-core optical fiber10.

As described above, since an FIFO device can be easily achieved by bundling existing single mode fibers (cladding diameter of 125 μm), and the center-to-center distance between the center core11and the outer peripheral cores12of the multi-core optical fiber10becomes 125 μm by using the FIFO made of the existing single mode fibers and a sufficient sensitivity can be obtained to a small shape change, in the embodiment, the cladding outer diameter of the multi-core optical fiber10is set to 375 μm (=125 μm×3) to conduct a study.

Next, usingFIG.4, a shape measurement method of measuring a shape change of an object to be measured using the multi-core optical fiber10will be described. The shape measurement method performs arranging the multi-core optical fiber10along a linear structural object (Step S01), measuring a backward Brillouin scattering light distribution in a propagation direction of each core of the multi-core optical fiber10(Step S02), and computing positional coordinates in a three-dimensional space of a linear structural object having an unknown three-dimensional shape from the backward Brillouin scattering light distribution of the multi-core optical fiber10arranged along the linear structural object having the unknown three-dimensional shape and the multi-core optical fiber10arranged along a linear structural object having an already-known three-dimensional shape to identify the time change (Step S03). The measuring device20performs Step S02, and the analysis device30performs Step S03. In Step S02, temperature dependence of a Brillouin frequency shift is preferably corrected.

Step S03will be described in more detail, and a position in the longitudinal direction from the measuring device20to the multi-core optical fiber10is defined as z.

In Step S03, calculating a difference in strain at the position z that is a difference between a strain amount at the position z of each core of the multi-core optical fiber obtained from the backward Brillouin scattering light distribution when the multi-core optical fiber is arranged along the linear structural object having the unknown three-dimensional shape and a strain amount at the position z of each core of the multi-core optical fiber obtained from the backward Brillouin scattering light distribution when the multi-core optical fiber is arranged along the linear structural object having the already-known three-dimensional shape (Step S31), calculating a bending strain ε of each of the outer peripheral cores by subtracting the difference in strain of the center core from the difference in strain of each of the outer peripheral cores (Step S32), calculating a curvature κ and a bending angle β at the position z of the multi-core optical fiber from the bending strain ε of each of the outer peripheral cores using the relational expression (1) (Step S33) and calculating a torsion by differentiating the bending angle β by an arc length, and computing the positional coordinates in the three-dimensional space of the linear structural object having the unknown three-dimensional shape from the curvature κ and the torsion at the position z using Frenet-Serret formulas to identify the time change (Step S34), are performed.

FIG.5is a drawing describing a specific procedure when a linear structural object having an unknown three-dimensional shape is measured by the shape measurement method.

[Step A]

First, the multi-core optical fiber10is laid along a linear structural object having an already-known three-dimensional shape (for example, a water service pipe or the like that is already laid), and a distribution property of a backward Brillouin scattering light of each core in a steady state (reference state) is acquired. This is defined as reference data.

[Step B]

Next, a distribution property of the backward Brillouin scattering light of each core is acquired again in a state where the three-dimensional shape of the linear structural object is changed (for example, a water service pipe or the like that is assumed to be deformed due to an earthquake or the like). This is defined as comparison data.

[Step C]

Next, a difference between the comparison data and the reference data is calculated for each core and for each position z to derive a difference in strain.

[Step D]

Next, the difference in strain of the center core is subtracted from the difference in strain of each of the outer peripheral cores to derive the bending strain ε at the position z for each of the outer peripheral cores.

[Step E]

To the relational expression (1) of the bending strain ε of the outer peripheral cores and the curvature κ and bending angle β of the multi-core optical fiber10, ε derived in Step D is assigned.
ε=r·κ·cos(α−β)  [Relational Expression (1)]

where r is the center-to-center distance between the center core11and the outer peripheral cores12, and α is an angle representing a position of the outer peripheral cores12on the cross section of the multi-core optical fiber10. For example, r is 125 μm, and α is 0°, 120°, and 240° for each of the outer peripheral cores.

In the embodiment, since there are three outer peripheral cores, simultaneous equations with three variables are obtained. From this expression, the curvature κ and the bending angle β at the position z are computed. Here, although the bending strain ε is different for each core, the curvature κ and the bending angle β are equal for any core, and therefore, the curvature κ and the bending angle β at the position z are determined by the least-square method.

[Step F]

Using the Frenet-Serret formulas, from the curvature κ and the bending angle β for each distance z determined in Step E, a position vector (three-dimensional shape) of the multi-core optical fiber10is determined. In order to improve position accuracy, the three-dimensional shape is preferably corrected using an already-known ending point.

Working Example 1

FIG.6is drawings describing a first example in which the three-dimensional shape of the multi-core optical fiber10is measured by the shape measurement method. InFIGS.6(a) to (e), the distance in the longitudinal direction of the multi-core optical fiber10(the position z described above) is expressed in s[m]. InFIG.6(f), the three-dimensional space in which the multi-core optical fiber10is arranged is expressed by an x-axis, a y-axis, and a z-axis. Note that the z-axis in here is different from the position z described above.

In the working example, the reference data of Step A inFIG.5was acquired in a state where the multi-core optical fiber10for sensing was extended in a straight line. Next, a vicinity of the center of the multi-core optical fiber10was rotated clockwise at a constant curvature to acquire the comparison data of Step B inFIG.5.FIGS.6(a), (b), and (c) are results of the reference data of Step A, the comparison data of Step B, and the difference in strain of Step C inFIG.5, respectively. A dotted line, a dashed line, and a one dot chain line in the drawings each represent the difference of the outer peripheral cores.FIGS.6(d) and (e)illustrate evaluation results of the curvature κ and the angle β of the multi-core optical fiber10, respectively.FIG.6(f)illustrates space coordinates of the multi-core optical fiber10in consideration of the position vector, and the curvature given to the vicinity of the center of the multi-core optical fiber10can be accurately detected.

Working Example 2

FIG.7is drawings describing a second example in which the three-dimensional shape of the multi-core optical fiber10is measured by the shape measurement method. InFIGS.7(a) to (e), the distance in the longitudinal direction of the multi-core optical fiber10(the position z described above) is also expressed in s[m]. InFIG.7(f), the three-dimensional space in which the multi-core optical fiber10is arranged is also expressed by the x-axis, the y-axis, and the z-axis. Note that the z-axis in here is different from the position z described above.

In the working example, the reference data of Step A inFIG.5was acquired in a state where the multi-core optical fiber10for sensing was extended in a straight line. Next, the vicinity of the center of the multi-core optical fiber10was rotated counterclockwise at a constant curvature to acquire the comparison data of Step B inFIG.5.FIGS.7(a), (b), and (c)are results of the reference data of Step A, the comparison data of Step B, and the difference in strain of Step C inFIG.5, respectively. A dotted line, a dashed line, and a one dot chain line in the drawings each represent the difference of the outer peripheral cores.FIGS.7(d) and (e)illustrate evaluation results of the curvature κ and the angle β of the multi-core optical fiber10, respectively.FIG.7(f)illustrates the space coordinates of the multi-core optical fiber10in consideration of the position vector, and a shape change in an opposite direction (counterclockwise) ofFIG.6(f)can be accurately detected.

Embodiment 2

When a multi-core optical fiber is placed along a structural object as a sensing medium, an unintended twist may occur, and an error may be caused in a measurement result. Therefore, in the embodiment, a twist is purposely added to the multi-core optical fiber used as the sensing medium. In shape identification of a linear structural object, a strain by an unintended twist, which is its twisting strain or below, can be eliminated by a calculation, and accuracy of the shape identification can be improved.

A system configuration of the shape measurement system of the embodiment is different from the system configuration of the shape measurement system described inFIG.1in an already-known twist being provided to the multi-core optical fiber10and in an analysis procedure of the analysis device30.

FIG.8is drawings describing the multi-core optical fiber10.FIG.8(A)describes a bending strain measured in the multi-core optical fiber10in a state where a twist does not exist, andFIG.8(B)describes a bending strain measured in the multi-core optical fiber10in a state where a twist exists. A core 0 is the center core, and a core 1 to a core 3 are the outer peripheral cores.

When the bending strains do not exist, the bending strains become zero for all the cores and overlap irrespective of the existence of a twist. For the center core, the bending strain becomes zero irrespective of the existence of a twist. In a case of the multi-core optical fiber10in the state where a twist does not exist, while the bending strain measured when a bending having a constant curvature is given is a different strain amount for each core as illustrated between an axial position z=2 m and z=12 m inFIG.8(A), the constant strain amounts are observed.

FIG.8(B)describes the bending strain of the multi-core optical fiber10to which a twist is given between the axial position z=2 m and z=12 m. InFIG.8(B), similarly toFIG.8(A), while a bending having a constant curvature is given, the outer peripheral cores switch the positions by the twist in a section where the bending is given, and therefore, inversion of positives and negatives of the strain amounts can be confirmed. When the multi-core optical fiber10is arranged to a linear structural object as a sensing medium, the twist is required for all the section arranged to the linear structural object.

When the positional coordinates are computed, based on a twisting strain generated in the multi-core optical fiber10and a strain by the already-known twist, the analysis device30estimates an unintended twist generated when the multi-core optical fiber10is arranged along the linear structural object having an unknown three-dimensional shape, and removes an influence by the unintended twist.

In the shape measurement method, Step S33described in the shape measurement method ofFIG.4is different from the description of Embodiment 1. That is, in Step S03, calculating a difference in strain at the position z that is a difference between a strain amount at the position z of each core of the multi-core optical fiber obtained from the backward Brillouin scattering light distribution when the multi-core optical fiber is arranged along the linear structural object having an unknown three-dimensional shape and a strain amount at the position z of each core of the multi-core optical fiber obtained from the backward Brillouin scattering light distribution when the multi-core optical fiber is arranged along the linear structural object having an already-known three-dimensional shape (Step S31), calculating a bending strain εbending,iof each of the outer peripheral cores by subtracting the difference in strain of the center core from the difference in strain of each of the outer peripheral cores (Step S32), calculating a curvature κ and a bending angle β at the position z of the multi-core optical fiber from the bending strain εbending,iof each of the outer peripheral cores using a relational expression (2) of removing an influence by the unintended twist (Step S33), and computing the positional coordinates in a three-dimensional space of the linear structural object having an unknown three-dimensional shape from the curvature κ and the bending angle β at the position z using the Frenet-Serret formulas (Step S34), are performed.

FIG.9is a drawing describing a specific procedure when the linear structural object having an unknown three-dimensional shape is measured by the shape measurement method. Here, “i” is a core number. Note that i=0 is the center core, and i of 1 or larger is an outer peripheral core.

[Step A]

First, the multi-core optical fiber10is laid along a linear structural object having an already-known three-dimensional shape (for example, a water service pipe or the like that is already laid), and a distribution property of a backward Brillouin scattering light of each core in a steady state (reference state) is acquired. This is defined as reference data.

[Step B]

Next, a distribution property of the backward Brillouin scattering light of each core is acquired again in a state where the three-dimensional shape of the linear structural object is changed (for example, a water service pipe or the like that is assumed to be deformed due to an earthquake or the like). This is defined as comparison data.

[Step C]

Next, a difference between the comparison data and the reference data is calculated for each core and for each position z to derive a difference in strain.

[Step D]

Next, the difference in strain of the center core is subtracted from the difference in strain of each of the outer peripheral cores to derive a strain εmix,iat the position z for each of the outer peripheral cores (here, i is 1 or larger).

[Step D-α]

The strain εmix,iincludes the bending strain εbending,iand a twisting strain εtwisting(here, i is 1 or larger). Since a relative positional relationship of the outer peripheral cores does not change, the twisting strain εtwistingat the position z is computed as by the following formula by using the fact that adding the bending strains εbending,iof all the outer peripheral cores i becomes zero. max{i} means a maximum core number.

[Math.7]εtwisting=∑i=1iεmix,imax⁢{i}(7)

The twisting strain εtwistingcomputed by Formula (7) is assigned to Formula (5) to compute a specific angle of twist φ(z) at the position z.FIG.11is a conceptual diagram describing a specific angle of twist φ. The specific angle of twist φ is a twist angle per unit length.

[Math.5]ϕ⁡(z)=εtwistingk2⁢r(5)

Here, r is a distance from the center on the cross section of the multi-core optical fiber10to the centers of the outer peripheral cores. k2 is a correction coefficient of a twist and expressed by the following formula. p is a spin rate (twist amount per unit length) of the outer peripheral cores and a value on design or a value obtained by a measurement method described later.

[Math.6]k2=2⁢π⁢pr(2⁢π⁢pr)2+1(6)
[Step E]

The bending strain εbending,iof the outer peripheral core i is computed by subtracting εtwistingderived in Step D-α from the strain εmix,iof each of the outer peripheral cores.

Then, the specific angle of twist φ(z) computed by Formula (5) is assigned to Formula (4) to compute an angle ωirepresenting a position of the outer peripheral core i at the position z. aiis an initial angle of the outer peripheral core i.

[Math. 4]
ωi=αi+∫0s(2πp+ϕ(z))dz(4)

Furthermore, the bending strain εbending,iof the outer peripheral core i and the angle ωiderived by Formula (4) are assigned to the relational expression (2) of the curvature κ and the bending angle β of the multi-core optical fiber10.
Σbending,i=k1κrcos(ωi−β)  [Relational Expression (2)]

Where k1is a correction coefficient of a twist expressed by Formula (3).

[Math.3]k1=1-v⁡(2⁢π⁢pr)2(2⁢π⁢pr)2+1(3)

where ν is a Poisson's ratio of the multi-core optical fiber.

In the embodiment, since there are three outer peripheral cores, simultaneous equations with three variables are obtained. From the relational expression (2), the curvature κ and the bending angle β at the position z are computed. Here, although the bending strain ε is different for each core, the curvature κ and the bending angle β are equal for any core, and therefore, the curvature κ and the bending angle β at the position z are determined by the least-square method.

FIG.10is conceptual diagrams describing a radius r, a bending direction (curvature κ), and a bending angle β of a multi-core optical fiber having three outer peripheral cores.FIG.10(A)is a drawing viewed from a side surface of the multi-core optical fiber, andFIG.10(B)is a cross-sectional view of the multi-core optical fiber.

[Step F]

Using the Frenet-Serret formulas, from the curvature κ and the bending angle β for each distance z determined in Step E, a position vector (three-dimensional shape) of the multi-core optical fiber10is determined. In order to improve position accuracy, the three-dimensional shape is preferably corrected using an already-known ending point.

(Measurement Method of Spin Rate)

Here, a method of actually measuring the spin rate will be described.

FIG.12is a drawing illustrating a result of measuring in a z direction a strain of each core of a multi-core optical fiber to which a twist is applied. When the multi-core optical fiber is twisted, an error occurs between the number of twists (spin rate) and a design value due to a manufacturing error and the like. In view of this, a strain cycle fluctuates for each core. Therefore, the number of twists (spin rate) is computed from a cyclic strain fluctuation by a short-time Fourier transform (STFT). In the above-described Step D-α, a value of a spin rate p computed from the actually measured strain cycle can be used.

(Effect)

The shape measurement system according to the present invention can achieve a measurement dynamic range of several kilometers to several tens of kilometers by using a backward Brillouin scattering light in shape identification of a linear structural object. Additionally, by setting the center-to-center distance between a center core and an outer peripheral core of a multi-core optical fiber used for detection of a shape change to 120 μm or more, a slight change of a curvature of 0.5[1/m] or less can be detected.

REFERENCE SIGNS LIST

10Multi-core optical fiber11Center core12Outer peripheral core20Measuring device30Analysis device