JP2003014491A - Optical fiber sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical fiber sensor capable of adjusting a temperature coefficient. SOLUTION: Both sides of a grating part 4 of an FBG 1 of this optical fiber sensor 25 for measuring the temperature are held by a base constituted from a first substrate 20 comprising low thermal expansion material and a second substrate 21 comprising high thermal expansion material, which are arranged in parallel along the expansion-contraction direction. Therefore, the temperature coefficient of the whole base can be changed by changing the length of the second substrate 21. The temperature coefficient of a connection body comprising the low thermal expansion material 36 and the high thermal expansion material 37, 38 for covering a temperature compensating FBG 16 can be adjusted so as to become equal to the temperature coefficient of a compensation object 15 by covering a grating part 18 of the temperature compensating FBG 16 of the optical fiber sensor for measuring the pressure or the strain with the low thermal expansion material 36, by covering optical fibers on both sides of the grating part 18 with the high thermal expansion material 37, 38, and by changing the length of the high thermal expansion material 37, 38.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to temperature change, pressure,
The present invention relates to an optical fiber sensor that measures a physical quantity such as strain.

[0002]

2. Description of the Related Art FIG. 6 is an explanatory view of a fiber Bragg grating, FIG. 7 is a conceptual view showing a conventional example of an optical fiber sensor using the fiber Bragg grating shown in FIG. 6, and FIG. 8 is shown in FIG. It is a conceptual diagram which shows the other prior art example of the optical fiber sensor using the fiber Bragg grating shown.

As shown in FIG. 6, a fiber Bragg grating (hereinafter referred to as “FBG”) 1 is obtained by changing the refractive index of a core 3 of an optical fiber 2 at a constant period P and is incident on the optical fiber 2. The incident light is reflected by the portion (grating portion) 4 in which the refractive index has changed, and is emitted as reflected light. The wavelength of the reflected light is
Since it is determined by the period P and the refractive index of the core 3, the FBG1
Causes the wavelength of the reflected light to change due to a change in the period P due to distortion and a change in the refractive index due to a change in temperature. Therefore, FBG1 is sensitive to strain and temperature, and various sensors for measuring pressure, strain, and temperature have been studied. 5
Is a clad that covers the core 3 and has a lower refractive index than the core 3.

As a method for measuring the temperature using such an FBG 1, a method using the change in the refractive index of the FBG 1 with the temperature and the change in the cycle due to the thermal expansion of the optical fiber, and FIG.
As shown in Fig. 6, FB is used for high thermal expansion materials such as stainless steel, aluminum, polymer materials, etc.
There is one in which G is fixed, and the change in the refractive index of the temperature of the FBG alone, the change in the wavelength of reflected light due to thermal expansion, and the strain due to the thermal expansion of the high thermal expansion material are used.

An optical fiber sensor 6 shown in FIG. 7 has a base 7 made of a high thermal expansion material, a pair of fixing members 8 and 9 provided on both sides of the base 7, and fixing members 8 and 9 each having a grating portion 4 of a grating portion 4. It is composed of an FBG 1 in which optical fibers on both sides are fixed.

The optical fiber 2 of the optical fiber sensor 6 is a measuring device body (not shown) (for example, OTDR: Optical Time).
It is connected to a Domain Reflectometer (optical pulse tester) and detects a change in the refractive index and expansion / contraction (arrow 11) of the grating portion 4 due to thermal expansion (arrow 10) of the base 7 to measure the temperature. There is.

The purpose of using this type of FBG is not only as a temperature sensor but also as a sensor for measuring physical quantities such as pressure and strain. Further, it is used as a temperature compensating FBG of the pressure strain detecting FBG. That is, it is also used for compensating by removing the temperature change from the reflection wavelength change due to pressure, strain, and temperature.

The optical fiber sensor 12 shown in FIG.
This is for measuring the pressure and strain of the compensation target (measurement target) 15 by the pressure strain sensing unit 14 in which G1 is covered with the metal material 13. Since the FBG1 is affected by the temperature change, the FBG1 is compensated for the error due to the temperature change.
An FBG 16 for temperature compensation is connected to G1 in series with an optical fiber 17.

The temperature compensating FBG 16, particularly the grating portion 18, is used by being arranged around the pressure strain sensing portion 14 or fixed to the compensation object 15.

[0010]

By the way, in general, F
The amount of wavelength change (hereinafter referred to as “temperature coefficient”) due to temperature change in BG alone is about 10 pm / ° C. (picometer / ° C.)
Is. When fixed to a high thermal expansion material, the temperature coefficient of stainless is about 30 pm / ° C., although it depends on the material.

When the temperature around the FBG is simply measured, the temperature accuracy is improved if the temperature coefficient is large, but the amount of wavelength change is increased accordingly, and the reflection wavelength is measured by using a reflection wavelength measuring device having a fixed wavelength band to be used. It becomes difficult to measure.

When the FBG is used for temperature compensation of the FBG for strain measurement, the temperature coefficient may be about the same as that of the strain detection FBG to be compensated. Since the strain detection FBG to be compensated is used by being fixed to various substances depending on the measurement purpose such as pressure and strain, the temperature coefficient also differs depending on each sensor and a thermal expansion material with the same temperature coefficient as the strain measurement FBG is selected. Very difficult to do.

As described above, the conventional optical fiber sensor has a problem that a material having an appropriate temperature coefficient has to be selected from among various kinds of materials, and the temperature coefficient cannot be adjusted. It was

Therefore, an object of the present invention is to solve the above problems and provide an optical fiber sensor capable of adjusting the temperature coefficient.

[0015]

In order to achieve the above object, the optical fiber sensor of the present invention has an optical fiber having a grating portion for measuring temperature by changing the refractive index and expanding and contracting due to temperature change. In an optical fiber sensor having a formed fiber Bragg grating and a base in which optical fibers on both sides of the grating portion of the fiber Bragg grating are fixed via a pair of fixing members and expand and contract due to temperature change, the base is a fiber It is composed of a plurality of substrates which are connected in series along the expansion / contraction direction of the Bragg grating and have different temperature coefficients.

The optical fiber sensor of the present invention has a fiber Bragg grating having a grating portion formed in an optical fiber for measuring temperature by changing the refractive index and expanding / contracting due to temperature change, and a grating of the fiber Bragg grating. In an optical fiber sensor having optical fibers on both sides of the section fixed via a pair of fixing members and having a base that expands and contracts due to temperature change, the base is made of a low thermal expansion material and one fixing member is provided. One substrate, and a second substrate between the first substrate and the fiber Bragg grating, which is fixed to the first substrate with a substrate fixing member and provided with the other fixing member so as to be parallel to the first substrate. It is a thing.

The optical fiber sensor of the present invention has a fiber Bragg grating having a grating portion formed in an optical fiber for measuring temperature by changing the refractive index and expanding / contracting due to temperature change, and a grating of the fiber Bragg grating. In an optical fiber sensor having optical fibers on both sides of the section fixed via a pair of fixing members and having a base that expands and contracts due to temperature change, the base includes a lower substrate made of a low thermal expansion material, and a lower substrate. Fixed to the lower substrate with a pair of substrate fixing members so that it is parallel to the lower substrate between the fiber Bragg grating and the optical fiber on both sides of the grating part of the fiber Bragg grating above. A pair of members each provided with a high thermal expansion material And an upper substrate of.

The optical fiber sensor of the present invention includes a fiber Bragg grating in which a grating portion whose refractive index changes and expands and contracts due to temperature change is formed in an optical fiber, and a fiber Bragg grating is covered with a metal material to be compensated. In the optical fiber sensor in which the pressure strain sensing unit arranged to measure the pressure or strain of the compensation target and the temperature compensation fiber Bragg grating are connected in series to the fiber Bragg grating to compensate for the temperature change of the fiber Bragg grating, The grating portion of the fiber Bragg grating for temperature compensation is covered with a low thermal expansion material, and the optical fibers on both sides of the low thermal expansion material are covered with a high thermal expansion material of a predetermined length.

In addition to the above structure, the optical fiber sensor of the present invention preferably uses stainless steel, aluminum or the like as the high thermal expansion material, and Invar or Pyrex as the low thermal expansion material.

In addition to the above structure, in the optical fiber sensor of the present invention, the temperature coefficient of the fiber Bragg grating may be adjusted by adjusting the length of the high thermal expansion material.

According to the present invention, the optical fibers on both sides of the grating portion of the fiber Bragg grating of the optical fiber sensor for measuring temperature are composed of the low thermal expansion material and the high thermal expansion material arranged in the expansion / contraction direction. Since it is held by the base, the temperature coefficient of the entire base changes by changing the length of the high thermal expansion material. In addition, by covering the grating part of the fiber Bragg grating for temperature compensation of the optical fiber sensor that measures pressure and strain with low thermal expansion material, covering both sides with high thermal expansion material and changing the length of high thermal expansion material, temperature compensation The temperature coefficient of the expansion material covering the fiber Bragg grating can be adjusted to be equal to the temperature coefficient of the object to be compensated.

[0022]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

FIG. 1 is a conceptual diagram showing an embodiment of the optical fiber sensor of the present invention. The same reference numerals are used for the same members as in the conventional example.

First made of a low thermal expansion material having a low thermal expansion coefficient
On the substrate 20, a second substrate 21 made of a high thermal expansion material having a high thermal expansion coefficient is arranged on one side (left side in the figure) of the first substrate 20 so as to be parallel to the first substrate 20, and One end (right end in the figure) of the substrate 21 is fixed on the first substrate 20 via a substrate fixing member 22. A fixing member 23 provided on the other end (left end in the drawing) of the second substrate 21 and a fixing member 2 provided on the other side (right side in the drawing) of the first substrate 20.
4, the optical fibers 2 on both sides of the grating portion 4 of the FBG 1 are fixed. The respective fixing members 22, 23, 24 are arranged on the same line along the FBG 1.

The optical fiber sensor 25 includes the FBG 1, the first substrate 20, the second substrate 21, and the fixing members 22 to 24.
Is configured.

The optical fiber sensor 25 is connected to, for example, a measuring device body (OTDR) (not shown). The optical pulse from the measuring instrument body propagates in the optical fiber 2, and the grating portion 4 reflects the reflected pulse light corresponding to the refractive index due to the temperature change and returns to the OTDR.

In addition to the refractive index change due to the temperature change of the FBG alone and the wavelength change due to thermal expansion and contraction (arrow 26), the FBG 1 is affected by the thermal expansion (arrow 27) of the second substrate 21 by the length of the second substrate 21. Adds distortion. The temperature coefficient of the entire base can be changed to various values by changing the length of the second substrate 21.

The second substrate 21 is made of stainless steel, aluminum, a polymer material or the like having a higher temperature coefficient than the optical fiber 2, and the first substrate 20 made of a low thermal expansion material has the same temperature coefficient as the optical fiber 2. Alternatively, low Invar material, quartz, or the like is used.

Although the first substrate 20 has a substantially L-shaped cross-sectional shape in the drawing, the present invention is not limited to this and may be a flat plate, in which case It is necessary to increase the height of the fixing member 24. In addition, the first substrate 20
The case where the second substrate 21 is arranged on the upper side of the
The present invention is not limited to this, and the one in which the first substrate 20 and the second substrate 21 are connected on the same plane may be used as a base.

Next, specific numerical values will be described with reference to FIG. 2, but the present invention is not limited thereto.

FIG. 2 is a conceptual diagram showing an embodiment of the optical fiber sensor of the present invention.

The optical fiber sensor 25 shown in FIG.
Since the configuration is the same as that of the optical fiber sensor 25 shown in FIG.

Stainless steel is used as the second substrate 21, and Invar is used as the first substrate 20. Here, the fiber fixed length L1 was set to 45 mm, the stainless steel length L2 was set to 5 mm and 7 mm, and the temperature was changed from 0 ° C to 40 ° C.

FIG. 3 is a diagram showing the wavelength change when the stainless steel length of the optical fiber sensor shown in FIG. 2 is changed, the horizontal axis is the temperature axis, and the vertical axis is the wavelength change axis.

For reference, a case in which the FBG is used alone and a case in which the FBG is fixed to stainless steel (45 mm in length, without using Invar) as a high thermal expansion material are also illustrated.

The temperature coefficient is the FBG alone (characteristic curve La).
9.5 pm / ° C, high thermal expansion material (stainless steel) only (characteristic curve Ld) 27 pm / ° C, stainless steel length 5 mm (characteristic curve Lb) about 10 pm / ° C, stainless steel length 7 mm
The (characteristic curve Lc) was about 12 pm / ° C.

As described above, the temperature coefficient can be adjusted by combining the high thermal expansion material and the low thermal expansion material to form the base.

FIG. 4 is a conceptual diagram showing another embodiment of the optical fiber sensor of the present invention.

The difference from the optical fiber sensor shown in FIG. 1 is that the base is made of a low thermal expansion material and the high thermal expansion material fixed to the lower substrate 30 by substrate fixing members 31 and 32. It is a point composed of a pair of upper substrates 28 and 29.

The optical fiber sensor 35 shown in the figure has a lower substrate 30 made of a low thermal expansion material, a pair of substrate fixing members 31 and 32 provided on the lower substrate 30, and one end thereof fixed to the substrate. Lower substrate 3 fixed to members 31 and 32, respectively
0, a pair of upper substrates 28, 29 made of a high thermal expansion material, which are arranged parallel to each other toward the outside and an upper substrate 28,
The fixing members 33, 34 provided on the outer side of 29 and the optical fibers 2 on both sides of the grating portion 4 are fixed members 33, 3
4 and the FBG 1 fixed to 4. The respective fixing members 31 to 34 are arranged on the same line.

With such an optical fiber sensor 35, the same effect as that of the optical fiber sensor 25 shown in FIG. 1 can be obtained. In the figure, the lower substrate 30 and the upper substrates 28, 29
And are arranged so that they are in different parallel states,
The present invention is not limited to this, and they may be configured to be on the same plane.

FIG. 5 is a conceptual diagram showing another embodiment of the optical fiber sensor of the present invention.

The difference from the optical fiber sensor 12 shown in FIG. 8 is that the grating portion 18 of the temperature compensating FBG 16 is used.
Is covered with the low thermal expansion material 36, and the optical fibers on both sides of the low thermal expansion material 36 are covered with the high thermal expansion materials 37 and 38.

In this optical fiber sensor 39, the FBG 1 is covered with a metal material and the pressure strain sensing section 14 which receives the pressure and strain of the compensation target 15 and the compensation target 15 connected to the FBG 1 through the optical fiber 17 are provided. A temperature compensating FBG 16 for temperature compensation, a low thermal expansion material 36 covering the grating portion 18 of the temperature compensating FBG 16, and a low thermal expansion material 36.
A high thermal expansion material 37 of a predetermined length covering the optical fibers on both sides of
And 38 (in the figure, the high thermal expansion material 3
Although the lengths in the longitudinal direction of 7 and 38 are different, the present invention is not limited to this. ).

When the pressure strain sensing unit 14 receives a pressure, FB
The G1 grating portion 4 contracts and the refractive index changes.
An optical pulse from a measuring device body (OTDR) (not shown) connected to the temperature compensating FBG 16 is a temperature compensating FBG 1
6, propagates in the optical fiber 17 and the FBG 1, is reflected by the grating unit 4 in the pressure strain sensing unit 14, and the optical fiber 17 and the grating unit 18 of the temperature compensating FBG 16.
And return to OTDR. At this time, the pressure strain sensing unit 1
The temperature change of the compensation object 15 around 4 expands and contracts the grating section 4 of the pressure strain sensing section 14, and also expands and contracts the grating section 18 of the temperature compensating FBG 16. However, the grating portion 18 of the temperature compensating FBG 16 includes the low thermal expansion material 36 and the high thermal expansion materials 37, 3 and 3.
Since it is covered with the low thermal expansion material 36 and the high thermal expansion material 3 by changing the lengths of the high thermal expansion materials 37 and 38.
It is possible to adjust the temperature coefficient of the connected body composed of 7, 38.

Here, a method of using the optical fiber sensor, an application system, and the like will be described.

When an FBG temperature sensor is manufactured with a desired temperature accuracy, a temperature coefficient matching the accuracy of the FBG reflection wavelength measuring device is required. When the reflection wavelength measuring device of the FBG has an accuracy of ± 20 pm and an FBG temperature sensor of ± 1 ° C is required, the required temperature coefficient of the FBG temperature sensor is 20 pm
/ ° C or higher. According to the present invention, an FBG temperature sensor having a desired temperature coefficient can be manufactured by adjusting the length of the high thermal expansion material.

In the above, by changing the temperature coefficient of the FBG temperature sensor, an optical fiber sensor with free temperature accuracy can be manufactured. Also, when used as a temperature compensating FBG for another sensor, optimum temperature compensation can be performed according to the temperature coefficient of the sensor to be compensated.

[0049]

In summary, according to the present invention, the following excellent effects are exhibited.

It is possible to provide an optical fiber sensor whose temperature coefficient can be adjusted.

[Brief description of drawings]

FIG. 1 is a conceptual diagram showing an embodiment of an optical fiber sensor of the present invention.

FIG. 2 is a conceptual diagram showing an embodiment of an optical fiber sensor of the present invention.

FIG. 3 is a diagram showing a wavelength change when the stainless steel length of the optical fiber sensor shown in FIG. 2 is changed.

FIG. 4 is a conceptual diagram showing another embodiment of the optical fiber sensor of the present invention.

FIG. 5 is a conceptual diagram showing another embodiment of the optical fiber sensor of the present invention.

FIG. 6 is an explanatory diagram of a fiber Bragg grating.

7 is a conceptual diagram showing a conventional example of an optical fiber sensor using the fiber Bragg grating shown in FIG.

8 is a conceptual diagram showing another conventional example of an optical fiber sensor using the fiber Bragg grating shown in FIG.

[Explanation of symbols]

1 Fiber Bragg Grating (FBG) 2 optical fiber 4 Grating part 20 First substrate 21 Second substrate 22 PCB fixing member 23, 24 fixing member 25 optical fiber sensor

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) G02B 6/10 G01D 3/04 DF term (reference) 2F056 VF02 VF03 VF20 2F075 AA03 EE02 2F103 BA01 CA07 EA02 EA16 EA18 EA19 EA25 EB02 EB32 EC09 GA15 2H038 AA05 AA07 BA25 CA52 2H050 AC84 AD00 AD06

Claims (6)

[Claims] 1. A fiber Bragg grating in which an optical fiber has a grating portion for measuring temperature by changing the refractive index and expanding and contracting due to temperature change, and a fiber Bragg grating on both sides of the fiber Bragg grating. In an optical fiber sensor having an optical fiber fixed through a pair of fixing members and expanding and contracting due to temperature change, the bases are connected in series along the expansion and contraction direction of the fiber Bragg grating and have a temperature coefficient. An optical fiber sensor comprising a plurality of different substrates. 2. A fiber Bragg grating in which an optical fiber has a grating portion for measuring temperature by changing the refractive index and expanding and contracting due to temperature change, and a fiber Bragg grating on both sides of the fiber Bragg grating. In an optical fiber sensor having an optical fiber fixed via a pair of fixing members and having a base that expands and contracts due to a temperature change, the base is made of a low thermal expansion material and a first substrate provided with one fixing member is provided. And a second substrate provided between the first substrate and the fiber Bragg grating and fixed to the first substrate with a substrate fixing member and provided with the other fixing member so as to be parallel to the first substrate. Characteristic optical fiber sensor. 3. A fiber Bragg grating in which an optical fiber has a grating portion for measuring temperature by changing the refractive index and expanding / contracting due to a temperature change, and on both sides of the grating portion of the fiber Bragg grating. In an optical fiber sensor having an optical fiber fixed via a pair of fixing members and having a base that expands and contracts due to temperature change, the base comprises a lower substrate made of a low thermal expansion material, the lower substrate and the fiber. It is fixed to the lower substrate by a pair of substrate fixing members so as to be parallel to the lower substrate between the Bragg grating and the optical fibers on both sides of the grating portion of the fiber Bragg grating are fixed to the upper surface. A pair of fixing members are provided and are made of a high thermal expansion material. Optical fiber sensor, characterized in that a side board. 4. A fiber Bragg grating in which a grating portion, whose refractive index changes and expands and contracts due to temperature change, is formed in an optical fiber, and a fiber Bragg grating covered with a metal material and arranged in a compensation object to be compensated. In the optical fiber sensor in which a pressure strain sensing unit that measures the pressure and strain of an object and a fiber Bragg grating for temperature compensation are connected in series to the fiber Bragg grating to compensate for the temperature change of the fiber Bragg grating, the temperature compensation is performed. An optical fiber sensor, characterized in that the grating portion of the fiber Bragg grating for use is covered with a low thermal expansion material, and the optical fibers on both sides of the low thermal expansion material are covered with a high thermal expansion material of a predetermined length. 5. The optical fiber sensor according to claim 1, wherein stainless steel, aluminum or the like is used as the high thermal expansion material and Invar or Pyrex (registered trademark) is used as the low thermal expansion material. 6. The optical fiber sensor according to claim 1, wherein the temperature coefficient of the fiber Bragg grating is adjusted by adjusting the length of the high thermal expansion material.