JP5097646B2 - Temperature compensated optical fiber Bragg grating - Google Patents

Description

  The present invention relates to an optical fiber black grating (hereinafter abbreviated as FBG), and more particularly to a temperature compensated FBG having a structure that stabilizes the temperature characteristics of the FBG and used as an optical communication and strain sensor.

  Since the center wavelength of the FBG changes in response to a temperature change (0.01 nm / ° C.), it is necessary to reduce the temperature sensitivity when it is desired to measure the center wavelength accurately and stably. As a temperature compensation method for the center wavelength of the FBG, a method is known in which an FBG having a positive expansion coefficient is fixed to a substrate having a negative thermal expansion coefficient (see, for example, Patent Documents 1 and 2).

  Patent Document 1 discloses an optical filter element and an auxiliary material having a dimensional temperature dependency opposite to the wavelength temperature dependency of the optical filter element, and a temperature at which the filter wavelength of the optical filter element is a target value. An optical filter characterized by being fixed in is disclosed.

  In Patent Document 2, an optical fiber in which a grating whose refractive index changes periodically is formed is sandwiched between resin-permeable films containing fibers having a negative thermal expansion coefficient. A dependent optical fiber grating is disclosed.

  The structure of a general temperature compensation type FBG is shown in FIG. In the figure, reference numeral 1 denotes a temperature compensation type FBG, 2 denotes an optical fiber, 3 denotes a grating portion, 4 denotes a negative expansion base, and 5 denotes an adhesive. This temperature compensation type FBG 1 has a structure in which a grating portion 3 is formed in a part of an optical fiber 2 and the optical fiber 2 including the grating portion 3 is fixed to a negative expansion base 4. Normally, this type of temperature-compensated FBG1 is fixed to a substrate with a tension applied to adjust the center wavelength (0.013 nm / gf).

Here, the principle of temperature compensation will be described.
As the temperature rises, the center wavelength tends to change to a long wave at a rate of 0.01 nm / ° C. with a change in the refractive index of the FBG. At this time, since the negative expansion base material 4 shrinks as the temperature rises, it is utilized that the tension is relaxed at a rate of 0.013 nm / gf.
JP 2003-344671 A JP 2004-109551 A

However, when an attempt was made in accordance with the technical information disclosed in Patent Documents 1 and 2 described above, the following problems were encountered.
Patent Document 1 discloses, as an example, a structure in which FBG is fixed to a negative expansion base material in which a mixture of glass fiber and Dyneema (registered trademark of Toyobo Co., Ltd.), which is an ultrahigh molecular weight polyethylene fiber, is mixed with resin. However, in this structure, since there is hysteresis, temperature compensation cannot be performed accurately even at the same temperature (refer to <Preliminary Experiment 1> in an example described later). Further, as the heat cycle was performed, a phenomenon in which the center wavelength was shifted to the short wavelength side (see <Preliminary Experiment 2> in Examples described later) was observed.

Patent Document 2 exemplifies a structure in which an FBG is sandwiched between resin penetration films containing Dyneema (registered trademark of Toyobo Co., Ltd.), which is an ultrahigh molecular weight polyethylene fiber, and temperature compensation is performed as an example. However, as a result of demonstrating this structure, the degree of negative expansion of the fiber is too strong and the degree of temperature compensation is poor. That is, the linear expansion coefficient of Dyneema is about −10 × 10 ⁻⁶ , while the linear expansion coefficient of optical fiber is about 0.5 × 10 ^−6, so there is hysteresis, so even at the same temperature, the accuracy is high. The temperature cannot be compensated.

  The present invention has been made in view of the above circumstances, and an object thereof is to provide a temperature-compensated FBG that can perform temperature compensation with high accuracy.

In order to achieve the above object, the present invention provides an FBG in which a grating portion is formed in a part of an optical fiber, and the optical fiber including the grating portion is fixed to a base material.
Provided is a temperature compensated FBG characterized in that an optical fiber including a grating portion is fixed to a base material including a combination of two or more types of negative expansion fibers having different negative linear expansion coefficients .

In the temperature-compensated FBG of the present invention, there are two types of negative expansion fibers, and the difference in linear expansion coefficient between these negative expansion fibers is preferably 5 × 10 ⁻⁶ or less.

In the temperature compensated FBG of the present invention, it is preferable that the linear expansion coefficients of the two types of negative expansion fibers are −10 × 10 ⁻⁶ and −6 × 10 ⁻⁶ , respectively.

  In the temperature compensated FBG of the present invention, the two types of negative expansion fibers are preferably ultrahigh molecular weight polyethylene fibers and polyparaphenylene benzobisoxazole fibers.

  In the temperature compensated FBG of the present invention, the base material is preferably an FRP base material in which two types of negative expansion fibers are fixed with a resin.

  In the temperature-compensated FBG, it is preferable that the resin for fixing the two types of negative expansion fibers is a vinyl ester resin.

The temperature-compensated FBG of the present invention has a structure in which an optical fiber including a grating portion is fixed to a base material including a combination of two or more types of negative expansion fibers, so that the combination of two or more types of negative expansion fibers is optimized. By doing so, it is possible to compensate for the temperature change of the optical fiber with high accuracy, and it is possible to provide an FBG excellent in temperature compensation performance.
The temperature compensated FBG of the present invention has no hysteresis unlike the case of blending a single negative expansion fiber in the prior art, and therefore, the same center wavelength can be obtained at the same temperature and the reliability becomes high.
If the temperature-compensated FBG of the present invention is subjected to a heat cycle of about 10 cycles in advance, the center wavelength will not change even in the subsequent heat cycle, and long-term reliability can be ensured.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 2 is a side view showing a first embodiment of the temperature compensated FBG according to the present invention. In the temperature compensated FBG 10A of the present embodiment, the grating portion 3 is formed in a part of the optical fiber 2, and the optical fiber 2 including the grating portion 3 is mounted on the FRP base material 11 including a combination of two or more types of negative expansion fibers. The FRP base material 11 is fixed at both ends by an adhesive 5 such as an ultraviolet curable adhesive.

In this embodiment, as the FRP base material 11, there are two types of negative expansion fibers, and it is preferable that the difference between the linear expansion coefficients of these negative expansion fibers is 5 × 10 ⁻⁶ or less. More specifically, the linear expansion coefficients of the two types of negative expansion fibers are preferably −10 × 10 ⁻⁶ and −6 × 10 ⁻⁶ , respectively, and the two types of negative expansion fibers are an ultrahigh molecular weight polyethylene fiber and More preferably, it is a polyparaphenylene benzobisoxazole fiber.

  In the present embodiment, the FRP base material 11 is formed by fixing the above-described two types of negative expansion fibers with a resin, and the resin for fixing the two types of negative expansion fibers may be a vinyl ester resin. preferable. The dimensions of the FRP base material 11 can be appropriately set within a range in which the mechanical strength capable of reliably performing temperature compensation of the optical fiber 2 (grating portion 3) is obtained. Further, the shape of the FRP base material 11 is not particularly limited as long as the temperature compensation of the optical fiber 2 can be reliably performed.

Since the temperature compensation type FBG 10A of the present embodiment has a configuration in which the optical fiber 3 including the grating portion 3 is fixed to the FRP base material 11 including a combination of two types of negative expansion fibers, the combination of the two types of negative expansion fibers By optimizing, the temperature change of the optical fiber 2 can be compensated with high accuracy, and an FBG with excellent temperature compensation performance can be realized.
The temperature compensated FBG 10A of the present embodiment has no hysteresis unlike the case of blending a single negative expansion fiber in the prior art, so that the same center wavelength can be obtained at the same temperature, and the reliability is high. .
In the temperature compensated FBG 10A of the present embodiment, if a heat cycle of about 10 cycles is performed in advance, the center wavelength does not change even in the subsequent heat cycle, and long-term reliability can be ensured.

  FIG. 3 is a side view showing a second embodiment of the temperature compensated FBG of the present invention. The temperature compensated FBG 10B of the present embodiment is configured to include the same components as the temperature compensated FBG 10A of the first embodiment shown in FIG. The temperature compensation type FBG 10B of the present embodiment is characterized in that the optical fiber 2 is placed on the surface of the FRP base material 11 and fixed in the entire longitudinal direction. The temperature compensated FBG 10B of the present embodiment can obtain substantially the same effect as the temperature compensated FBG 10A of the first embodiment described above, and further, the fixing portion between the FRP base material 11 and the optical fiber 2 becomes large. The temperature compensation effect by the FRP substrate 11 can be more reliably exhibited.

  FIG. 4 is a side view showing a third embodiment of the temperature compensated FBG of the present invention. The temperature compensated FBG 10C of the present embodiment is configured to include the same components as the temperature compensated FBG 10A of the first embodiment shown in FIG. 2 except for the shape of the FRP base material. The temperature compensation type FBG 10B of the present embodiment uses a cylindrical FRP base material 11, and accommodates the grating part 3 and both side parts of the optical fiber 2 in the cylinder, and welds the adhesive part or base resin to the FRP. The base 11 and the optical fiber 2 are fixed. The temperature-compensated FBG 10B of this embodiment can obtain substantially the same effect as the temperature-compensated FBG 10A of the first embodiment described above, and furthermore, the grating portion 3 of the optical fiber 2 is provided in the cylindrical FRP base material 11. Is housed and fixed, so that the mechanical strength can be increased.

<Preliminary experiment 1>
Dyneema (registered trademark of Toyobo Co., Ltd., linear expansion coefficient: −10 × 10 ⁻⁶ , hereinafter referred to as “Dyneema”) and glass fiber (linear expansion coefficient: 8 × 10 ⁻⁶ ), which are ultrahigh molecular weight polyethylene fibers Three types of FRP base materials (A to C) prepared by changing the formulation of were prepared. The blending conditions of each fiber of the FRP base material were as follows (mass ratio).
A: Dyneema: Glass fiber = 9: 1
B: Dyneema: Glass fiber = 8: 2
C: Dyneema: Glass fiber = 7.5: 2.5

  An optical fiber having a grating part is placed on each of the FRP substrates A to C produced as described above, and fixed with an ultraviolet curable adhesive in a state where a certain tension is applied (see FIG. 1). Three types of FBGs (A to C) were prepared.

  Each of the FBGs A to C is placed in a temperature-controllable measurement chamber, and while measuring the reflection center wavelength of each FBG, the temperature of the measurement chamber is changed 50 cycles in the range of −20 ° C. to 80 ° C. The reflection center wavelength shift of each FBG in the range of 20 ° C. to 80 ° C. was measured. The waiting time at each temperature was 30 minutes. The measurement results are shown in FIG. The results shown in FIG. 5 are the center wavelength fluctuation values at the 25th cycle with reference to the wavelength at −20 ° C. for each FBG.

From FIG. 5, it is possible to compensate for the temperature by changing the blending of the dyneema and the glass fiber, but because of the hysteresis, the temperature cannot be compensated accurately even at the same temperature. This is because the difference between the linear expansion coefficient of Dyneema (−10 × 10 ⁻⁶ ) and the linear expansion coefficient of glass fiber (8 × 10 ⁻⁶ ) is large. This is thought to be due to deformation (distortion).

<Preliminary experiment 2>
About each FBG of A-C produced in the preliminary experiment 1, it puts in the measurement chamber which can be similarly temperature-controlled, and the temperature of a measurement chamber is the range of -20 degreeC-80 degreeC, measuring the reflection center wavelength of each FBG. Then, the dependence of the center wavelength on the number of heat cycles was examined using the center wavelength at 60 ° C. as a reference. The measurement results are shown in FIG.

As shown in FIG. 6, as the heat cycle is performed, the center wavelength often shifts to the short wavelength side. This is because the difference between the linear expansion coefficient of Dyneema (−10 × 10 ⁻⁶ ) and the linear expansion coefficient of glass fiber (8 × 10 ⁻⁶ ) is large. This is thought to be because the minute deformation (distortion) due to pulling is increasing. For this reason, long-term reliability cannot be expected with the combination of Dyneema + glass fiber.

[Example]
As an example according to the present invention, a temperature compensated FBG shown in FIG. 2 was produced.
(1) Dyneema; coefficient of linear expansion −10 × 10 ⁻⁶ ,
(2) Zylon (registered trademark of Toyobo Co., Ltd., linear expansion coefficient: −6 × 10 ⁻⁶ , hereinafter referred to as “Zylon”), which is a polyparaphenylene benzobisoxazole fiber,
These fibers were arranged in a certain direction, and both were integrated with a vinyl ester resin to prepare an FRP base material. Here, the mixing ratio of (1) and (2) was changed, and three types of FRP base materials D to F were produced.
The blending conditions of each fiber of the FRP base material were as follows.
D: Dyneema: Zylon = 3: 7
E: Dyneema: Zylon = 4: 6
F: Dyneema: Zylon = 5: 5

Each optical fiber having a grating portion is placed on each of the D to F FRP substrates produced as described above, and fixed with an ultraviolet curable adhesive in a state where a certain tension is applied (see FIG. 2). Three types of FBGs (D to F) were prepared. At this time, the ultraviolet curable adhesive was used at both ends of the FRP substrate.
If the FRP substrate is too thin, it will be broken by the tension applied to the FBG, so in this example the outer diameter was 3 mm and the length was 50 mm.

  Each of the FBGs D to F is placed in a temperature-controllable measurement chamber, and while measuring the reflection center wavelength of each FBG, the temperature of the measurement chamber is changed 50 cycles in the range of −20 ° C. to 80 ° C. The reflection center wavelength shift of each FBG in the range of 20 ° C. to 80 ° C. was measured. The waiting time at each temperature was 30 minutes. The measurement results are shown in FIG. The results shown in FIG. 7 are the center wavelength fluctuation values of the 25th cycle with reference to the wavelength at −20 ° C. for each FBG.

  From FIG. 7, it was demonstrated that FBG can be temperature compensated with high accuracy by changing the combination of Dyneema and Zylon. Further, unlike the case where Dyneema and glass fiber are blended, there is no hysteresis and stable characteristics are obtained. The cause is considered to be that the deformation between the two fibers is small because the linear expansion difference between Dyneema and Zyron is small.

  Each of these FBGs F to F is similarly placed in a temperature-controllable measurement chamber, and while measuring the reflection center wavelength of each FBG, the temperature of the measurement chamber is changed by 50 cycles in the range of -20 ° C to 80 ° C. The dependence of the center wavelength on the number of heat cycles was examined using the center wavelength at 60 ° C. as a reference. The measurement results are shown in FIG.

From FIG. 8, it was found that the center wavelength slightly changed to about 10 cycles, but the change was smaller than the result of the preliminary experiment 2 performed with the combination of Dyneema and glass fiber (FIG. 6). The reason why the center wavelength slightly changes in each of the FBGs D to F is that residual stress when processing Dyneema, Zylon, and resin into FRP, and in an ultraviolet curable adhesive that fixes the optical fiber and the FRP substrate. It is considered that the residual stress of is released.
From this experiment, it was found that long-term reliability can be ensured by performing about 10 cycles in advance in the FBG of this example.

It is a side view which shows an example of the conventional temperature compensation type FBG. It is a side view which shows 1st Embodiment of the temperature compensation type FBG of this invention. It is a side view which shows 2nd Embodiment of the temperature compensation type FBG of this invention. It is a side view which shows 3rd Embodiment of the temperature compensation type FBG of this invention. It is a graph which shows the result of the preliminary experiment 1 based on a prior art. It is a graph which shows the result of the preliminary experiment 2 based on a prior art. It is a graph which shows the result of the Example which concerns on this invention, and shows the measurement result of the temperature characteristic at the time of making a temperature of -20 degreeC into a reference | standard. It is a graph which shows the result of the Example which concerns on this invention, and shows the measurement result of the heat cycle frequency | count dependence degree of the center wavelength in 60 degreeC.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... FBG, 2 ... Optical fiber, 3 ... Grating part, 4 ... Negative expansion base material, 5 ... Adhesive, 10A-10C ... Temperature compensation type FBG, 11 ... FRP base material.

Claims (6)

In an optical fiber Bragg grating in which a grating part is formed in a part of an optical fiber, and the optical fiber including the grating part is fixed to a base material,
A temperature-compensated optical fiber Bragg grating, wherein an optical fiber including a grating portion is fixed to a base material including a combination of two or more types of negative expansion fibers having different negative linear expansion coefficients . 2. The temperature-compensated optical fiber Bragg grating according to claim 1, wherein there are two types of negative expansion fibers, and the difference between the linear expansion coefficients of these negative expansion fibers is 5 × 10 ⁻⁶ or less. Two negative linear expansion coefficient of the expansion fiber -10 × 10 -6, ^{respectively,} the temperature-compensated optical fiber Bragg grating according to claim 1 or 2, characterized in that it is ^-6 × 10 ^-6.  The temperature-compensated optical fiber Bragg grating according to any one of claims 1 to 3, wherein the two types of negative expansion fibers are an ultrahigh molecular weight polyethylene fiber and a polyparaphenylene benzobisoxazole fiber.  The temperature-compensated optical fiber Bragg grating according to any one of claims 1 to 4, wherein the base material is an FRP base material in which two types of negative expansion fibers are fixed with a resin.  6. The temperature-compensated optical fiber Bragg grating according to claim 5, wherein the resin for fixing the two types of negative expansion fibers is a vinyl ester resin.

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