JP4403674B2 - Optical fiber sensor - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical fiber sensor that measures physical quantities such as temperature change, pressure, and strain.
[0002]
[Prior art]
6 is an explanatory diagram of a fiber Bragg grating, FIG. 7 is a conceptual diagram showing a conventional example of an optical fiber sensor using the fiber Bragg grating shown in FIG. 6, and FIG. 8 is a fiber Bragg grating shown in FIG. It is a conceptual diagram which shows the other prior art example of the optical fiber sensor using this.
[0003]
As shown in FIG. 6, a fiber Bragg grating (hereinafter referred to as “FBG”) 1 is obtained by changing the refractive index of the core 3 of the optical fiber 2 with a constant period P, and incident light incident on the optical fiber 2. However, it is reflected by the portion (grating portion) 4 where the refractive index has changed and is emitted as reflected light. Since the wavelength of the reflected light is determined by the period P and the refractive index of the core 3, the FBG 1 changes the wavelength of the reflected light due to a change in the period P due to distortion or a change in refractive index due to a temperature change. Therefore, FBG1 is sensitive to strain and temperature, and various sensors that measure pressure, strain, and temperature have been studied. Reference numeral 5 denotes a clad that covers the core 3 and has a lower refractive index than the core 3.
[0004]
As a method of measuring the temperature using such FBG 1, a method using a change in refractive index due to the temperature of FBG 1 and a periodic change due to thermal expansion of the optical fiber or a coefficient of thermal expansion larger than that of the optical fiber as shown in FIG. Some FBGs are fixed to a high thermal expansion material such as stainless steel, aluminum, polymer material, etc., and the refractive index change of the temperature of the FBG alone, the wavelength change of reflected light due to thermal expansion, and the distortion due to the thermal expansion of the high thermal expansion material are available. .
[0005]
An optical fiber sensor 6 shown in FIG. 7 includes 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 light on both sides of the grating portion 4 on the fixing members 8 and 9. It is comprised with FBG1 to which the fiber was fixed.
[0006]
The optical fiber 2 of the optical fiber sensor 6 is connected to a not-shown measuring instrument body (for example, OTDR: Optical Time Domain Reflectometer, optical pulse tester), and the refractive index of the grating section 4 due to the thermal expansion of the base 7 (arrow 10). Changes and expansion / contraction (arrow 11) are detected, and the temperature is measured.
[0007]
Here, the purpose of using this type of FBG is not only a temperature sensor but also 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 to compensate for the temperature change from the change in reflection wavelength due to pressure, strain and temperature.
[0008]
The optical fiber sensor 12 shown in FIG. 8 is for measuring the pressure and strain of the compensation object (measurement object) 15 by the pressure strain sensing unit 14 in which the FBG 1 is covered with the metal material 13. Since the FBG 1 is affected by a temperature change, a temperature compensating FBG 16 is connected in series to the FBG 1 by an optical fiber 17 in order to compensate for an error corresponding to the temperature change of the FBG 1.
[0009]
The temperature compensating FBG 16, particularly the grating portion 18, is used around the pressure strain sensing portion 14 or fixed to the compensation object 15.
[0010]
[Problems to be solved by the invention]
By the way, generally, the amount of wavelength change (hereinafter referred to as “temperature coefficient”) due to temperature change in the FBG alone is about 10 pm / ° C. (picometer / ° C.). When fixed to a high thermal expansion material, the temperature coefficient is about 30 pm / ° C. for stainless steel, although it varies depending on the material.
[0011]
When simply measuring the temperature around the FBG, if the temperature coefficient is large, the temperature accuracy is improved, but the amount of change in wavelength is correspondingly increased, and the reflected wavelength is measured using a reflected wavelength measuring device in which the used wavelength band is determined. It becomes difficult.
[0012]
Further, when the FBG is used for temperature compensation of an FBG that performs distortion measurement, the temperature coefficient may be approximately the same as the distortion detection FBG to be compensated. The strain detection FBG to be compensated is used by fixing it to various substances depending on its measurement purpose, such as pressure and strain. Therefore, the thermal coefficient varies depending on each sensor, and a thermal expansion material that has the same temperature coefficient as the strain measurement FBG is selected. It is very difficult to do.
[0013]
As described above, the conventional optical fiber sensor has a problem that a material having an appropriate temperature coefficient must be selected from a variety of materials, and the temperature coefficient cannot be adjusted.
[0014]
Accordingly, an object of the present invention is to provide an optical fiber sensor capable of solving the above-described problems and adjusting the temperature coefficient.
[0015]
[Means for Solving the Problems]
In order to achieve the above object, the optical fiber sensor of the present invention compensates for a fiber Bragg grating in which a refractive index changes with temperature change and a grating portion that expands and contracts is formed on an optical fiber, and the fiber gragg grating is covered with a metal material. A pressure strain sensing unit that is arranged in the object and measures the pressure and strain of the compensation object, and a light in which a fiber Bragg grating for temperature compensation is connected in series to a fiber Bragg grating to compensate for the temperature change of the fiber Bragg grating. In the fiber sensor, the grating portion of the temperature-compensating fiber Bragg grating 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 having a predetermined length.
[0019]
Optical fiber sensor of the present invention in addition to the above configuration, with stainless steel or aluminum as a high thermal expansion material, to use invar or Pyrex as a low thermal expansion material (registered trademark) is preferable.
[0020]
Optical fiber sensor of the present invention in addition to the above arrangement, be adjusted the length of the high thermal expansion material such that the temperature coefficient of the connecting member made of a low thermal expansion material and the high thermal expansion material is equal to the temperature coefficient of the compensation subject Good.
[0021]
According to the present invention, covering the grating portion of the temperature compensating fiber Bragg grating optical fiber sensor for measuring the pressure or distortion of low thermal expansion material, varying the length of the high thermal expansion material is covered with a high thermal expansion material on both sides Thus, the temperature coefficient of the expansion material covering the temperature compensating fiber Bragg grating can be adjusted to be equal to the temperature coefficient of the compensation object.
[0022]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.
[0023]
FIG. 1 is a conceptual diagram showing an embodiment of an optical fiber sensor of the present invention. In addition, the same code | symbol was used for the member similar to a prior art example.
[0024]
One of the first substrates 20 is placed on the first substrate 20 made of a low thermal expansion material having a low thermal expansion coefficient so that the second substrate 21 made of a high thermal expansion material having a high thermal expansion coefficient is parallel to the first substrate 20. One end (right end in the figure) of the second substrate 21 is fixed on the first substrate 20 via the substrate fixing member 22. The fixing member 23 provided at the other end (left end in the figure) of the second substrate 21 and the fixing member 24 provided on the other side (right side in the figure) of the first substrate 20 are arranged on both sides of the grating portion 4 of the FBG 1. The optical fiber 2 is fixed. The fixing members 22, 23, and 24 are arranged along the same line along the FBG1.
[0025]
The FBG 1, the first substrate 20, the second substrate 21, and the fixing members 22 to 24 constitute an optical fiber sensor 25.
[0026]
The present optical fiber sensor 25 is connected to, for example, a measuring instrument body (OTDR) (not shown). The light pulse from the measuring instrument main body propagates through the optical fiber 2, and the reflected pulse light corresponding to the refractive index due to temperature change is reflected by the grating section 4 to return to the OTDR.
[0027]
In addition to the refractive index change due to the temperature change of the FBG alone and the wavelength change due to thermal expansion / contraction (arrow 26), the FBG 1 is subjected to distortion due to the thermal expansion (arrow 27) of the second substrate 21 by the length of the second substrate 21. . By changing the length of the second substrate 21, the temperature coefficient of the entire base can be changed to various values.
[0028]
The second substrate 21 is made of stainless steel, aluminum, a polymer material having a higher temperature coefficient than the optical fiber 2, and the first substrate 20 made of a low thermal expansion material has a temperature coefficient equal to that of the optical fiber 2, or Low invar material or quartz is used.
[0029]
In addition, although the 1st board | substrate 20 has a substantially L-shaped cross-sectional shape in the figure, this invention is not limited to this, Flat plate shape may be sufficient, and the fixing member 24 is sufficient in that case. It is necessary to increase the height. Moreover, although the case where the second substrate 21 is arranged on the upper side of the first substrate 20 has been described, the present invention is not limited to this, and the first substrate 20 and the second substrate 21 are arranged on the same plane. A connected one may be used as a base.
[0030]
Next, specific numerical values will be described with reference to FIG. 2, but the present invention is not limited to this.
[0031]
FIG. 2 is a conceptual diagram showing an embodiment of the optical fiber sensor of the present invention.
[0032]
The optical fiber sensor 25 shown in the figure has the same configuration as the optical fiber sensor 25 shown in FIG.
[0033]
Stainless steel is used as the second substrate 21, and Invar is used as the first substrate 20. Here, the fiber fixing length L1 was 45 mm, the stainless steel length L2 was 5 mm, and 7 mm, and the temperature was changed from 0 ° C. to 40 ° C.
[0034]
FIG. 3 is a diagram showing a change in wavelength when the stainless steel length of the optical fiber sensor shown in FIG. 2 is changed. The horizontal axis is a temperature axis, and the vertical axis is a wavelength change axis.
[0035]
For reference, the case of a single FBG and the case where the FBG is fixed to stainless steel (length: 45 mm, not using invar) as a high thermal expansion material are also illustrated.
[0036]
The temperature coefficient is 9.5 pm / ° C. for FBG alone (characteristic curve La), 27 pm / ° C. for only high thermal expansion material (stainless steel) (characteristic curve Ld), about 10 pm / ° C. for stainless steel length 5 mm (characteristic curve Lb), stainless steel The length was about 12 pm / ° C. at a length of 7 mm (characteristic curve Lc).
[0037]
Thus, the temperature coefficient can be adjusted by configuring the base by combining the high thermal expansion material and the low thermal expansion material.
[0038]
FIG. 4 is a conceptual diagram showing another embodiment of the optical fiber sensor of the present invention.
[0039]
1 is different from the optical fiber sensor shown in FIG. 1 in that a base substrate 30 is made of a low thermal expansion material, and a pair of high thermal expansion materials fixed to the lower substrate 30 by substrate fixing members 31 and 32. This is a point composed of the upper substrates 28 and 29.
[0040]
The optical fiber sensor 35 shown in the figure includes a lower substrate 30 made of a low thermal expansion material, a pair of substrate fixing members 31, 32 provided on the lower substrate 30, and one end of the substrate fixing member 31, A pair of upper substrates 28 and 29 made of a high thermal expansion material, which are respectively fixed to 32 and arranged on the lower substrate 30 in parallel and outward, and fixing members 33 provided outside the upper substrates 28 and 29, 34 and the FBG 1 in which the optical fibers 2 on both sides of the grating portion 4 are fixed to the fixing members 33, 34. Each fixing member 31-34 is arrange | positioned on the same line.
[0041]
In 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 and 29 are arranged so as to be in a different parallel state. However, the present invention is not limited to this and may be configured to be on the same plane. Good.
[0042]
FIG. 5 is a conceptual diagram showing another embodiment of the optical fiber sensor of the present invention.
[0043]
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 covered with a low thermal expansion material 36 and the optical fibers on both sides of the low thermal expansion material 36 are covered with high thermal expansion materials 37 and 38. Is a point.
[0044]
This optical fiber sensor 39 is connected to the FBG 1 via the optical fiber 17 and is subjected to temperature compensation of the compensation object 15, which is connected to the FBG 1 via the optical fiber 17. Temperature compensation FBG 16, a low thermal expansion material 36 that covers the grating portion 18 of the temperature compensation FBG 16, and high thermal expansion materials 37 and 38 of a predetermined length that cover optical fibers on both sides of the low thermal expansion material 36. (In the figure, the lengths in the longitudinal direction of the high thermal expansion materials 37 and 38 are different, but the present invention is not limited to this).
[0045]
When the pressure strain sensing part 14 receives pressure, the grating part 4 of the FBG 1 contracts and the refractive index changes. A light pulse from a measuring instrument body (OTDR) (not shown) connected to the temperature compensating FBG 16 propagates through the temperature compensating FBG 16, the optical fiber 17 and the FBG 1, and is reflected by the grating section 4 in the pressure strain sensing section 14. The optical fiber 17 passes through the grating portion 18 of the temperature compensating FBG 16 and returns to the OTDR. At this time, due to the temperature change of the compensation object 15 around the pressure strain sensitive part 14, the grating part 4 of the pressure strain sensitive part 14 expands and contracts, and the grating part 18 of the temperature compensating FBG 16 similarly expands and contracts. However, since the grating portion 18 of the temperature compensating FBG 16 is covered with the low thermal expansion material 36 and the high thermal expansion materials 37 and 38, the lengths of the high thermal expansion materials 37 and 38 are changed to change the low thermal expansion material 36 and the high thermal expansion material 36 and 38. It is possible to adjust the temperature coefficient of the coupling body composed of the expansion materials 37 and 38.
[0046]
Here, the usage method and application system of the present optical fiber sensor will be described.
[0047]
When an FBG temperature sensor is manufactured with a desired temperature accuracy, a temperature coefficient that matches the accuracy of the FBG reflection wavelength measurement device is required. When the FBG reflected wavelength measuring device accuracy is ± 20 pm and an FBG temperature sensor of ± 1 ° C. is required, the temperature coefficient of the required FBG temperature sensor is 20 pm / ° C. or more. By using the present invention, an FBG temperature sensor having a target temperature coefficient can be manufactured by adjusting the length of the high thermal expansion material.
[0048]
In the above, an optical fiber sensor with free temperature accuracy can be manufactured by changing the temperature coefficient of the FBG temperature sensor. Also, when used as a temperature compensation FBG of another sensor, optimum temperature compensation can be performed in accordance with the temperature coefficient of the sensor to be compensated.
[0049]
【The invention's effect】
In short, according to the present invention, the following excellent effects are exhibited.
[0050]
An optical fiber sensor capable of adjusting the temperature coefficient can be provided.
[Brief description of the 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 the optical fiber sensor of the present invention.
FIG. 3 is a diagram showing a change in wavelength 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.
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 1st board | substrate 21 2nd board | substrate 22 Fixing members 23 and 24 for substrates Fixing member 25 Optical fiber sensor

Claims (3)

  A fiber Bragg grating in which an optical fiber has a grating portion that changes in refractive index and expands and contracts due to a change in temperature, and the fiber gragg grating is covered with a metal material and disposed in the compensation object. In the optical strain sensor in which the temperature compensation fiber Bragg grating is connected in series to the fiber Bragg grating in order to compensate for the temperature change of the fiber Bragg grating, An optical fiber sensor, wherein the grating portion is covered with a low thermal expansion material, and optical fibers on both sides of the low thermal expansion material are covered with a high thermal expansion material having a predetermined length. Optical fiber sensor according to claim 1 in which the use of a stainless steel or aluminum as a high thermal expansion material was used invar or Pyrex as the low thermal expansion material. Light according to claim 1 or 2 the low thermal expansion material and a length of equal way the high thermal expansion material to a temperature coefficient of the temperature coefficient of the connecting member made of the high thermal expansion material the compensation object is adjusted Fiber sensor.

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