JP2002168608A - Optical fiber sensor and strain-measuring method using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical fiber sensor, giving no influence on the intensity characteristics thereof and the attenuation rate of transmission light and an accurate strain-measuring method using it. SOLUTION: The optical fiber sensor is provided with an optical fiber 11, having a straight line provided substantially linearly and a curved line 11t provided in the form of substantial curvature, a pipe 12 storing the straight line of the optical fiber, and a storing part 13 storing the curved line of the optical fiber. The storing part is formed into size, capable of storing the curved line having a radius of curvature larger than the inside diameter of the pipe.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber sensor and a strain measuring method using the same, and
The present invention relates to an optical fiber sensor for measuring and monitoring deformation of a structure such as steel, and a strain measurement method using the same.

[0002]

2. Description of the Related Art Recently, a BOTDR (Brillouin Opti) has been used as a strain measurement sensor using an optical fiber.
cal Time Domain Reflect
eter) or FBG (Fiber Bragg Gra)
ting) have been used.

As a sensor for measuring the deformation of an earth structure,
As shown in Japanese Patent Application Laid-Open No. 10-176965, an optical fiber and a tube having elasticity around the optical fiber are integrated with each other, and the optical fiber is bent at the tip thereof.

[0004]

The optical fiber is used by folding the optical fiber at the tip. If the bending radius of the optical fiber is small at the folded portion, the optical fiber is easily broken. Further, when the bending radius of the optical fiber is small, there may be a portion where the reflection condition in the optical fiber is different from that of other portions, and the attenuation rate of the transmitted light may increase.

On the other hand, when the bending radius of the optical fiber is increased, the thickness of the tube accommodating the optical fiber is increased,
When the optical fiber sensor is buried in the ground, the driving work becomes difficult, and the driving causes the ground to be deformed.

Furthermore, conventionally, since all or most of the longitudinal direction of the optical fiber is fixed in the tube, it has been difficult to detect local deformation with respect to the longitudinal length of the optical fiber sensor.

An object of the present invention is to provide an optical fiber having strength characteristics,
For example, an object of the present invention is to provide an optical fiber sensor that does not affect the following of bending and deformation. It is another object of the present invention to provide an optical fiber sensor that does not affect the attenuation rate of transmission light of an optical fiber. Still another object of the present invention is to provide an optical fiber sensor that is easy to install (drive). Still another object of the present invention is to provide an optical fiber sensor that does not affect a measurement target by installation. Still another object of the present invention is to provide an optical fiber sensor capable of measuring local deformation. Still another object of the present invention is to provide a distortion measuring method using an optical fiber sensor for evaluating a signal light at a position having the most uniform characteristics. Still another object of the present invention is to provide a distortion measuring method using an optical fiber sensor capable of accurately detecting distortion.

[0008]

Means for solving the problem are described as follows. The technical matters corresponding to the claims in the expression are appended with parentheses (), numbers, symbols, and the like. The numbers, symbols, etc. clarify the correspondence / correspondence between the technical matter corresponding to the claim and the technical matter of at least one of the multiple embodiments. It is not to show that the matter is limited to the technical matter of the embodiment.

An optical fiber sensor (10) according to the present invention has an optical fiber (11) having a linear portion provided substantially linearly and a curved portion (11t) provided substantially curvedly.
A tube (12) for accommodating the linear portion of the optical fiber (11); and a curved portion (1) of the optical fiber (11).
1t), and the accommodating portion (13) can accommodate the curved portion (11t) having a bending radius larger than half the inner diameter of the tube (12). It is formed in size.

[0010] In the optical fiber sensor (10) of the present invention, the accommodating portion (13) has a tapered shape.

[0011] In the optical fiber sensor (10) of the present invention, the accommodating portion (13) is formed substantially in an arrowhead shape. Here, the arrowhead shape indicates a flat shape.

An optical fiber sensor (20) according to the present invention comprises: an optical fiber (21) to which measurement light is incident; and a fixing part (22) to which the optical fiber (21) is fixed. The measured light is transmitted to the optical fiber (21).
Has a set length (GL) corresponding to the condition of the measurement light, and the optical fiber (21) is disposed at a fixed length (GL) with respect to the fixed portion (22). It is fixed at the above intervals.

An optical fiber sensor (30) of the present invention accommodates an optical fiber (31) into which measurement light is incident, a guide pipe (34) for accommodating the optical fiber (31), and an accommodating guide pipe (34). And a tube (32),
The incident measurement light has a set length (GL) corresponding to the condition of the measurement light when propagating through the optical fiber (31), and the optical fiber (31) The guide pipe (34) is fixed to the pipe (34) at an interval equal to or longer than the set length (GL), and the guide pipe (34) deforms according to the deformation of the pipe (32) when the pipe (32) deforms. The optical fiber (31) is guided so that the optical fiber (31) is deformed.

The strain measuring method using an optical fiber sensor according to the present invention comprises: (a) installing an optical fiber sensor on an object to be measured; (b) entering light into the optical fiber sensor; Detecting backscattered light from the optical fiber sensor; (d) determining the spectrum width (D) of the backscattered light; and (e) setting the determined spectrum width (D) to a set value. (F) determining the width of the spectrum (D);
Determining the frequency of the backscattered light when is less than or equal to the set value; and (g) measuring the distortion of the measurement object based on the determined frequency.

A guide pipe is fixed along the inner surface of the pile (pipe), and an optical fiber is inserted into the guide pipe and fixed at a predetermined interval. In the folded part at the tip of the pile,
A container having a shape like an arrowhead is fixed, and the optical fiber is folded inside so that the radius does not become less than 30 mm.

In the signal processing, the half width of the spectrum of the backscattered light measured by setting the measurement interval of the signal light to (fixed interval of optical fiber−GL) or less is obtained, and the spectrum of the sample point at which the half width becomes the narrowest is obtained. Find the center frequency from
Convert to distortion value.

Since the tip has an arrowhead shape, the diameter of the pile portion can be reduced, the driving is easy, and the influence on the surroundings at the time of driving can be reduced. Since it is fixed at a fixed point, it can follow local deformation, and since it is in the guide pipe, it is possible to prevent a decrease in sensitivity due to out-of-plane deformation. Further, since the fixed point can be evaluated at the optimum point, it is possible to prevent a problem such as a variation in measured values.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described.

Referring to FIG. 1, an optical fiber sensor according to a first embodiment will be described. In FIG. 1, reference numeral 10 denotes an optical fiber sensor. Optical fiber sensor 10
Is an FBG sensor here. Optical fiber sensor 1
0 is, for example, embedded in the embankment and measures the deformation (strain) generated in the embankment.

The optical fiber sensor 10 includes an optical fiber 11
, A tube 12 and a tip cap 13. One end 11a of the optical fiber 11 is connected to a device (not shown) that receives light into the optical fiber 11 and receives the reflected light. The other end 11b of the optical fiber 11 is
After the upper part of the inner surface of the tube 12 is inserted from the base end side 12a toward the distal end part 12b, and is folded at the place where the distal end part 12b comes out of the tube 12, the lower part of the inner surface of the tube 12 is connected to the base end side 12a. It is inserted toward. Thereby, it is possible to measure the deformation generated in the embankment at the upper position or the lower position of the pipe 12.

The tube 12 has another optical fiber (not shown) in addition to the optical fiber 11 for measuring the deformation of the embankment at the left or right side position of the tube 12.
Is provided. One end of the optical fiber is connected to the above device, and the other end of the optical fiber is inserted through the inner left side of the tube 12 from the base end 12a toward the distal end 12b, and from the distal end 12b to the outside of the tube 12. After being folded back at the protruding portion, the tube 12 is inserted with its right inner surface facing the base end 12a. With this optical fiber and the optical fiber 11, the deformation generated in the embankment in four directions, up, down, left and right, around the pipe 12 can be measured.

The optical fiber 11 is arranged on the inner surface of the tube 12.
The optical fiber 11 is fixed at fixed points 19 at predetermined intervals in the longitudinal direction. An epoxy resin adhesive is used for the fixing means.

The core (not shown) of the optical fiber 11 of the optical fiber sensor 10 of the FBG type is provided with a plurality of optical filters (not shown) having different refractive indexes at predetermined intervals in the longitudinal direction. ing. Each filter reflects only light of a specific wavelength out of the incident light. When the tube 12 is deformed by the deformation of the embankment, and the optical fiber 11 expands and contracts according to the deformation of the tube 12, the interval between the gratings of the filter changes, and the wavelength of the reflected light changes with the change. Thereby, the deformation of the embankment can be measured.

The optical fiber 11 has a configuration in which an optical fiber body is coated. The diameter of the optical fiber main body is 125 μm, and the diameter of the optical fiber 11 including the coating is 200 μm to 2 mm.

The tube 12 is made of a substance (metal, resin, etc.) that follows the deformation of the object, for example, vinyl chloride.
The material of the tube 12 is selected so as to have a hardness not to be crushed by the earth pressure and to be deformable following the deformation of the measuring object (for example, embankment). For measuring the deformation of the measurement object in the elongation direction, the tube 12 is made of a substance (metal, resin, etc.) having a shape (bellows structure, etc.) that follows the deformation of the object, and for example, a metal bellows Could be a tube.

At the distal end of the optical fiber sensor 10, there is provided a distal end cap portion 13 which covers a portion of the optical fiber 11 protruding from the distal end portion 12b of the tube 12. The tip cap section 13 has a substrate section 14, a cover main body section 15 provided on the substrate section 14, and a surrounding wall section 16.

The substrate section 14 is formed in a circular flat plate shape. A hole having a diameter equal to or larger than the inner diameter of the tube 12 is formed at the center of the substrate portion 14, and the optical fiber 11 exits from the distal end portion 12 b of the tube 12 through the hole.

On the substrate portion 14, a surrounding wall portion 16 is provided upright. The surrounding wall means a wall formed in a cylindrical shape whose circumferential direction is closed (continuous). Circulating wall 16
The inner peripheral portion 16a is connected to the outer diameter portion 12c of the tube 12 by an adhesive (not shown) over the entire periphery.

The tip 1 of the tube 12 of the optical fiber 11
The bending radius r of the portion protruding from 2b is 30 mm or more. The cover body 15 is formed in a size that can accommodate the folded portion 11t of the optical fiber 11 having the bending radius r in a state where the folded portion 11t is not in contact with the folded portion 11t.

The cover body 15 is formed in an approximately arrowhead shape, and includes a first portion 15a, a second portion 15b, and a third portion 1a.
5c. The first portion 15a is formed in a conical shape with an open bottom. The third portion 15c is a circulating wall portion which is provided upright on the outermost peripheral portion of the substrate portion 14 and is formed so that its diameter gradually decreases. 2nd part 15b
Is formed continuously with the first portion 15a and the third portion 15c so as to connect the first portion 15a and the third portion 15c, and the portion of the third portion 15c from the portion having the largest diameter of the first portion 15a is formed. The surrounding wall portion is formed so that the diameter gradually increases to the portion having the minimum diameter.

According to the first embodiment, since the cover body 15 having the above configuration is used, the folded portion 11t of the optical fiber 11 has a problem with respect to the characteristics of the transmission light of the optical fiber 11 and the strength characteristics of the optical fiber 11. It is possible to bend at a bend radius r (in this embodiment, set to 30 mm or more) which is assumed to be negligible. Further, the cover body 15 is
Since the optical fiber sensor 10 is generally shaped like an arrowhead, it is easily driven when the optical fiber sensor 10 is driven into the ground or the like like a pile.
On the other hand, except for the folded portion 11t of the optical fiber sensor 10, the tube 12 accommodating the optical fiber 11 can be configured to be sufficiently thin. Can be suppressed.

Next, a second embodiment will be described.

The optical fiber sensor 20 of the second embodiment is
Here, it is a BOTDR type. Optical fiber sensor 20
Is, for example, as shown in FIG.
The deformation (strain) generated along the circumferential direction of the peripheral surface SR and generated in the tunnel is measured.

As shown in FIG. 3, the optical fiber sensor 20 includes an optical fiber 21, a tube 22, and a tip cap 23. One end 21a of optical fiber 21
Is connected to a device (not shown) for receiving light into the optical fiber 21 and receiving the reflected light. Optical fiber 2
The other end 21b of the first tube 22 is connected to the inside of the tube 22 by its proximal end 22a.
Through the distal end portion 22b.
From the tube 22 to the outside of the tube 22
2 is inserted toward the base end 22a. The tube 22 has an optical fiber 2 as in the first embodiment.
Another optical fiber (not shown) is provided in addition to the one.

The optical fiber 21 is disposed on the inner surface of the tube 22.
In the longitudinal direction of the optical fiber 21, a fixed point 2 is provided at every fixed interval DS set to be equal to or more than GL (gauge length).
9 is fixed. An epoxy resin adhesive is used for the fixing means. In this case, the optical fiber 21
Is in the section between each fixed point 29,
It is fixed in a state where a uniform distortion is generated (for example, a state where a tension of 0.1% is applied).

The optical fiber 21 has a configuration in which an optical fiber main body is covered. The diameter of the optical fiber body is 125 μm, and the diameter of the optical fiber 21 including the coating is 200 μm to 2 mm. The tube 22 is made of vinyl chloride. The material of the tube 22 can be considered in the same manner as the tube 12.

At the distal end of the optical fiber sensor 20, there is provided a distal end cap 23 which covers a portion of the optical fiber 21 protruding from the distal end 22a of the tube 22. The configuration of the distal end cap section 23 is the same as the configuration of the distal end cap section 13 of the first embodiment.

In the present embodiment, the optical fiber 21 is
One of the points is that the interval DS fixed with respect to is not less than the above GL. Hereinafter, this point will be described.

In the above device, an optical pulse is incident on the optical fiber 21 from one end 21a. The incident light pulse propagates in the optical fiber 21 and is scattered in the optical fiber 21. The above device receives the backward scattered light via one end 21a. The above-described device measures the characteristics (presence or absence of deformation) of the optical fiber 21 in the longitudinal direction from the relationship between the light reception timing (time) of the received backscattered light and the light intensity.

That is, as the light receiving time from the incidence to the light reception becomes longer, in other words, the one end 21a
If the intensity of the received light decreases at a constant ratio as the distance in the longitudinal direction of the optical fiber 21 from the optical fiber 21 increases, the longitudinal characteristics of the optical fiber 21 are uniform, and the optical pulse It can be seen that the light propagates while attenuating according to the distance in the longitudinal direction of 21.

The BOTDR device focuses on Brillouin scattered light among the backscattered light. A value considered to be the center of the light intensity of the received Brillouin scattered light is extracted, and the frequency of the center value (center frequency) is detected. If a distortion occurs at the point (measurement section) where the Brillouin scattered light is scattered in the longitudinal direction of the optical fiber 21, it appears as a change in the center frequency. This makes it possible to measure the deformation in the tunnel pit.

As shown in FIG. 4, the BOTDR device 6
The pulse width (duration of the pulse) of the light pulse incident from 1 is 10 nsec. When the optical pulse enters the optical fiber 21 and propagates, the optical pulse has a length of 1 m in the optical fiber 21. In the present embodiment, 1 m is the above-mentioned GL (see FIG. 5). The received Brillouin scattered light includes all the scattered light generated within 1 m. This 1 m is a measurement section at the time of distortion detection.

Here, a case is considered where the optical fiber 21 is fixed to the tube 22 at a distance of less than 1 m, for example, 20 cm. Further, here, it is assumed that distortion occurs in the portion of the optical fiber 21 of 20 cm to which both ends are fixed, and for example, three adjacent distortion levels are different from each other (see FIG. 6). In this case, the Brillouin scattered light corresponding to the 1 m measurement section including the 20 cm portion includes three spectra whose center frequencies are different from each other, as shown in FIG. In this state, it is very difficult to find a single center frequency from the whole including three spectra. It is even more difficult because the backscattered light usually contains noise. From this,
Locally generated for the measurement section (GL section) (1 m
20 cm, ie in a small area relative to the GL)
Deformation was sometimes impossible to measure. It should be noted that for a 1 m measurement section, measurement is impossible unless the distortion is at a uniform level in a range of at least about 50% or more.

On the other hand, in the present embodiment, as described above,
The optical fiber 21 is fixed to the tube 22 at every fixed interval DS set to GL or more. Here, the fixed interval DS is 1 m. As in the case described above, it is assumed that a distortion of 2 mm in length is generated at three locations in a certain portion of the optical fiber 21 and the three locations have different levels of distortion. When only both ends of a section of 1 m (fixed interval DS) are fixed, in a section of 1 m between both ends,
The optical fiber 21 can freely expand and contract (without restriction). Therefore, 1m with both ends fixed
In the portion of the optical fiber 21, the distortion generated within 1 m (here, three different levels of distortion).
Are averaged over the 1 m section. As a result, 1
A section of m will have a constant level of single distortion over substantially all or most of the section. As a result, Brillouin scattered light having a single center frequency can be obtained from the 1 m measurement section, and stable measurement can be performed even if the measurement section (GL section) is locally deformed. Can be.

Next, a third embodiment will be described.

The optical fiber sensor 30 according to the third embodiment comprises:
BOTDR type. As shown in FIG. 8, for example, when the underground K of the tunnel is viewed from the front, as shown in FIG. 8, the optical fiber sensor 30 is radially embedded from the peripheral surface SR of the underground toward the ground, and the deformation ( Measurement).

As shown in FIG. 9, the optical fiber sensor 30 includes an optical fiber 31, a tube 32, a tip cap 33, and a guide pipe.

The guide pipe 34 has substantially the same length as the length of the pipe 32 in the longitudinal direction. The outer diameter of the guide pipe 34 is smaller than the inner diameter of the tube 32, and the inner diameter of the guide pipe 34 is larger than the outer diameter of the optical fiber 31. Two guide pipes 34 are fixed to the upper and lower portions of the inner surface of the pipe 32 when viewed from the side. Each guide pipe 34
Is approximately the entire area in the longitudinal direction of the guide pipe 34,
The inner surface of the tube 32 is fixed with an adhesive 71 (see FIGS. 12 and 13).

The optical fiber 31 has a configuration in which the optical fiber body is covered. The diameter of the optical fiber body is 125 μm, and the diameter of the optical fiber 31 including the coating is 200 μm to 2 mm. The tube 32 is made of vinyl chloride. The material of the tube 32 can be considered in the same manner as the tube 12.

The guide pipe 34 is a pipe made of metal or resin. The guide pipe 34 is made of a material that is deformable following the deformation of the pipe 32. Further, when the guide pipe 34 is deformed, the deformed guide pipe 34
It is necessary that the inner diameter of the guide pipe 34 is not too large than the outer diameter of the optical fiber 31 so that the optical fiber 31 in the guide pipe 34 is deformed along the (as deformed). Further, the inner surface of the guide pipe 34 needs to have a small coefficient of friction (easy to slip) so as not to hinder the expansion and contraction of the optical fiber 31.

The one end 31a of the optical fiber 31 is connected to a device (not shown) for receiving light into the optical fiber 31 and receiving the scattered light. The other end 31b of the optical fiber 31 is inserted through the inside of the first guide pipe 34 from the base end 34a toward the tip 34b, and from the tip 34b to the pipe 32 (or the first guide pipe 34).
After being turned back at a position outside the second guide pipe, the inside of the second guide pipe 34 is inserted toward the base end 34a thereof. The tube 32 is provided with another optical fiber (not shown) in addition to the optical fiber 31 as in the first embodiment.

The optical fiber 31 is placed on the inner surface of each guide pipe 34 in the longitudinal direction of the optical fiber 31.
It is fixed at a fixed point 39 at fixed intervals DS set to L or more. An epoxy resin adhesive 72 (see FIG. 13) is used for the fixing means.

FIG. 12 is a sectional view taken along line X1-X1 of FIG. FIG. 13 is a sectional view taken along line X2-X2 of FIG. FIG.
As shown in the figure, the guide pipe 34 is fixed to the inner surface of the pipe 32 with an adhesive 71 at a position other than the fixing point 39, and the optical fiber 31 is not fixed to the guide pipe 34. . As shown in FIG. 13, at the fixing point 39, the guide pipe 34 is fixed to the inner surface of the pipe 32 with an adhesive 71, and the optical fiber 31 is fixed to the guide pipe 34 with an adhesive 72. Have been.

At the distal end of the optical fiber sensor 30, there is provided a distal end cap portion 33 which covers a portion of the optical fiber 31 protruding from the distal end portion 32b (34b) of the pipe 32 (or the guide pipe 34). The configuration of the distal end cap 33 is the same as the configuration of the distal end cap 13 of the first embodiment.

In the present embodiment, the optical fiber 31
When the optical fiber 31 is fixed at the fixed point 39 at a fixed interval DS of L or more, the optical fiber 31 is not directly fixed to the pipe 32 but is fixed to the guide pipe 34 as one of the points. Become. Hereinafter, this point will be described.

As shown in FIG.
Is directly fixed to the tube 32, if the distance between the adjacent fixing points 39 is large, the optical fiber 31 may be deformed even when the tube 32 is bent upward and bent.
Does not deform as the upwardly convex deformation of the tube 32,
As shown by the reference symbol M, it may protrude in a direction (inside) away from the inner surface 32s of the tube 32. In this case, the actual amount of elongation deformation of the optical fiber 31 becomes smaller than the amount of elongation deformation of the optical fiber 31 corresponding to the amount of elongation deformation of the tube 32, or the tube 32 elongates and deforms. Conversely, the optical fiber 31 may be shrunk and deformed. As a result, the distortion of the measurement object cannot be measured accurately.

As shown in FIG. 11, in the present embodiment, in order to solve the above-mentioned problem, the optical fiber 31 is deformed by itself according to the deformation of the tube 32 and the optical fiber 31 is deformed according to the deformation of the tube 32. A guide pipe 34 for guiding and regulating the optical fiber 31 so as to be deformed is employed.

The optical fiber 31 is connected to the guide pipe 34.
On the other hand, the reason for fixing at a fixed interval DS equal to or longer than GL is the same as in the second embodiment.

Next, a fourth embodiment will be described.

The fourth embodiment relates to signal processing in a measurement method such as BOTDR in which measured values of an optical fiber are continuously obtained.

In FIG. 14, reference numeral 41 denotes an optical fiber. The optical fiber 41 is fixed to a tube (not shown) at a predetermined fixed interval DS. In fixed point fixing, the distortion (level) may differ between adjacent fixed points. In this case, the backscattered light (Brillouin scattered light) with the measurement light, which is an optical pulse, straddling between the fixed points adjacent to each other.
Is detected, as shown in FIG. 15, two spectra corresponding to two different levels of distortion overlap, and as a result, it becomes difficult to determine the center frequency of the light to be measured. Furthermore, the measured values differ depending on how the measuring light straddles between those adjacent fixed points.

The above problem occurs because the measurement light does not enter during a single fixed interval DS. As shown in FIG. 15, when the two spectra overlap, the half-value width D of the spectra increases. In the present embodiment, focusing on the width D of the spectrum, signal processing is performed by the following method shown in FIG. 18 to solve the above problem.

As shown in step S1, it is determined whether or not it is near the evaluation point. That is, as shown in FIG. 8, for example, when a long optical fiber is connected to one BOTDR device and measurement values (backscattered light) are continuously obtained, a predetermined evaluation point (distortion measurement) is set. In order to measure only the backscattered light of the target point), only the measured value corresponding to the evaluation point is extracted based on the light receiving timing.

As a result of step S1, if it is determined based on the light receiving timing that it is not near the evaluation point (step S1-N), step S1 is repeated again, and only the measurement light near the evaluation point is measured. , Step S2
Perform the following steps.

If it is determined in step S1 that the measured light is near the evaluation point (step S1-Y), the half width D of the spectrum of the measured light is calculated (step S2).
It is determined whether the calculation result is the minimum (step S3). In this step S3, if it is determined that the value is smaller than the preset value, it can be determined that the value is the minimum, or the steps S1 and S2 can be determined.
Is performed on a plurality of measurement lights, and the smallest one of the plurality of measurement lights can be determined to be the smallest.

As a result of step S3, when it is determined that the measured light is not the minimum (step S3-N), the measuring position for extracting the measuring light is shifted (step S4), and the process returns to step S1 again. As a result of step S3, when it is determined that the frequency is the minimum (S3-Y), the shift amount of the center frequency is calculated (step S5).

As a result of step S3, when the half width D of the spectrum of the measurement light is the minimum, FIGS.
As shown in FIG. 7, the measurement light enters during a fixed interval DS with a uniform spectrum.

As shown in FIG. 16, the measurement interval of the signal light is defined as (the fixed interval DS of the optical fiber 41 -the length G of the measurement light.
L) The half-width D of the spectrum of the measured backscattered light is determined as follows, the center frequency is determined from the spectrum of the sample point at which the half-width D is narrowest, and converted to a distortion value.

As described above, in the above embodiment,
The following four points are some of the points of the above embodiment in a sensor having a finite GL, such as a BOTDR or FBG, in which a sensor can be connected serially on an optical fiber and an axial strain can be measured. It is said.

The first point is a sensor for measuring the deformation of a pile by attaching an optical fiber to a driving pile (pipe). The folded part at the tip of the pile is housed in an arrowhead-shaped part, and the pile is bent. The point is that the diameter is reduced and it is easier to drive. Conventionally, the bending radius is required to be 30 mm or more due to the characteristics of the transmission light and the strength characteristics of the optical fiber, and the shape of the pile itself has been increased. As a result, the driving was difficult, and the driving caused an influence such as an elevation of the ground.

The second point is that the fixed interval inside the pile is GL.
Or more than that. In the past, since the entire surface was fixed, local deformation could not be detected.

The third point is that an optical fiber is inserted into a guide pipe so that an optical fiber fixed at a predetermined interval causes an axial elongation while following the deformation of an object to be measured. It is.

The fourth point is that, in a measurement method such as BOTDR in which a continuous measurement value of an optical fiber is obtained, the measurement interval of the signal light is set to be equal to or less than (fixed interval−GL) of the optical fiber, and the fixed section is fixed. When evaluating the measured values of the above, the characteristic is that the characteristic of the signal light is evaluated at the most uniform position. Conventionally, in the case of a method in which measured values are continuously obtained, such as BOTDR, when fixed points are fixed, distortion may be different between adjacent fixed points, and the measured values may vary. This has a problem in how to determine the distortion value of the measuring device,
It is difficult to obtain the signal light generated by the light when the measurement light falls within one fixed section, and when determining the center frequency of the light to be measured, the signal light generated in the adjacent fixed section is mixed and measured. That's because.

[0074]

According to the optical fiber sensor of the present invention, it is possible to provide a sensor having a small diameter of a pile, a small influence on a driving portion, and a high sensitivity.

[Brief description of the drawings]

FIG. 1 is a side view showing an optical fiber sensor according to a first embodiment of the present invention.

FIG. 2 is a front view showing a state where an optical fiber sensor is used in a second embodiment of the present invention.

FIG. 3 is a side view showing an optical fiber sensor according to a second embodiment of the present invention.

FIG. 4 is a side view for explaining a state in which an optical pulse is incident on the optical fiber sensor according to the second embodiment of the present invention from a BOTDR device.

FIG. 5 is a diagram illustrating a relationship between an incident light pulse and GL in an optical fiber sensor according to a second embodiment of the present invention.

FIG. 6 is a diagram for explaining a case where a plurality of distortions having different levels are generated in a single measurement section in a conventional general optical fiber sensor.

FIG. 7 is a diagram showing three spectra having different frequencies in a conventional general optical fiber sensor.

FIG. 8 is a front view showing a state where an optical fiber sensor according to a third embodiment of the present invention is used.

FIG. 9 is a side view showing an optical fiber sensor according to a third embodiment of the present invention.

FIG. 10 is a front view showing a state in which deformation of an optical fiber does not follow deformation of a tube in a conventional general optical fiber sensor.

FIG. 11 is a front view of the optical fiber sensor according to the third embodiment of the present invention, showing a state where the problem of FIG. 10 has been solved.

FIG. 12 is a sectional view taken along line X1-X1 of FIG. 9;

FIG. 13 is a sectional view taken along line X2-X2 in FIG. 9;

FIG. 14 is a side view of a conventional general optical fiber sensor, showing a state in which measurement light straddles between adjacent fixed points.

FIG. 15 is a diagram showing a state where two spectra overlap in the case of FIG. 14;

FIG. 16 is a side view of the fourth embodiment of the present invention, showing a state where measurement light is between single fixed points.

FIG. 17 is a diagram showing a state where there is only a single spectrum in the case of FIG. 15;

FIG. 18 is a flowchart showing an operation flow according to the fourth embodiment of the present invention.

[Explanation of symbols]

 Reference Signs List 10 optical fiber sensor 11 optical fiber 11a one end 11b other end 11t folded portion 12 tube 12a base end 12b tip end 12c outer diameter portion 13 tip cap portion 14 substrate portion 15 cover body portion 15a first portion 15b second portion 15c Third portion 16 Circumferential wall portion 16a Inner peripheral portion 19 Fixed point 20 Optical fiber sensor 21 Optical fiber 21a One end 21b Other end 22 Tube 22a Base end 22b Tip 23 Tip cap 29 Fixed point 30 Optical fiber sensor 31 Light Fiber 31a One end 31b Other end 32 Tube 32S Tube inner surface 33 Tip cap 34 Guide pipe 34a Base end 34b Tip 39 Fixed point 41 Optical fiber 61 BOTDR device 71 Adhesive 72 Adhesive D Spectrum half width DS Fixed interval GL Gauge Length K Tonne Underground r radius SR peripheral surface

Continued on front page (72) Inventor Taketoshi Yamaura 5-717-1, Fukahori-cho, Nagasaki-shi, Nagasaki Pref. In Nagasaki Research Laboratory, Mitsubishi Heavy Industries, Ltd. (72) Inventor Hiromasa Ito 12 Nishikicho, Naka-ku, Yokohama-shi, Yokohama (72) Inventor Hiroshi Akiyama 12 Nishikicho, Naka-ku, Yokohama-shi, Kanagawa Prefecture Mitsubishi Heavy Industries, Ltd.Yokohama Works (72) Inventor Kenichi Kojima 2-8 Hikaricho, Kokubunji-shi, Tokyo 38 Foundation Inside the Railway Technical Research Institute (72) Osamu Murata Inventor, 2-8-8 Hikaricho, Kokubunji, Tokyo Metropolitan Institute of Technology (72) Inventor Shiro Tanamura, 2-8-8 Hikarimachi, Kokubunji, Tokyo 38 Foundation F-Term in Railway Technical Research Institute (reference) 2F065 AA65 BB05 CC40 FF00 FF41 FF48 GG08 LL00 LL02 LL21 LL42 PP01 QQ01 QQ31 2F076 BB09 BD06

Claims (6)

[Claims] An optical fiber having a linear portion provided in a substantially straight line and a curved portion provided in a substantially curved shape; a tube accommodating the linear portion of the optical fiber; and the curved portion of the optical fiber And an accommodating portion for accommodating the curved portion having a bending radius larger than half of the inner diameter of the tube. 2. The optical fiber sensor according to claim 1, wherein the housing has a tapered shape. 3. The optical fiber sensor according to claim 1, wherein the housing portion is formed in a generally arrowhead shape. 4. An optical fiber into which measurement light is incident, and a fixing portion to which the optical fiber is fixed, wherein the incident measurement light is transmitted through the optical fiber when the measurement light propagates through the optical fiber. An optical fiber sensor having a set length corresponding to a condition of the measurement light, wherein the optical fiber is fixed to the fixing portion at an interval equal to or longer than the set length. 5. An optical fiber into which measurement light is incident, a guide pipe for accommodating the optical fiber, and a tube for accommodating the guide pipe, wherein the incident measurement light propagates through the optical fiber. Has a set length corresponding to the condition of the measurement light, the optical fiber is fixed to the guide pipe at an interval equal to or longer than the set length, and the guide pipe is An optical fiber sensor that guides the optical fiber so that the optical fiber is deformed in response to the deformation of the tube when deformed. 6. An optical fiber sensor is installed on an object to be measured, (b) light is incident on the optical fiber sensor, and (c) backscattered light is detected from the optical fiber sensor. (D) determining the width of the spectrum of the backscattered light; (e) determining whether the determined width of the spectrum is equal to or less than a set value; and (f) determining the width of the spectrum. Finding the frequency of the backscattered light when the width is equal to or less than the set value,
(G) A strain measuring method using an optical fiber sensor, comprising measuring a strain of the object to be measured based on the determined frequency.