JP2020037492A - Optical fiber member for fiber bragg grating and manufacturing method thereof - Google Patents

Abstract

To provide an optical fiber member which suppresses a deviation of a temperature dependence of a reflection wavelength of an FBG caused by thermal expansion of a silica glass core and is suitable as a material for temperature measurement of a structure whose temperature has been difficult to measure so far.SOLUTION: There is provided an optical fiber member for fiber Bragg grating comprising a silica glass core that has a fluorine content of 1 wt.% or more and 5 wt.% or less and has a change of the thermal expansion coefficient between 0°C to 100°C of Δ0.02×10/K or less.SELECTED DRAWING: None

Description

  The present invention relates to an optical fiber type temperature sensor used for measuring the temperature of a structure that is difficult to measure temperature with an electric sensor.

  Conventionally, an electric sensor is generally used to measure the temperature of an apparatus, a structure, or the like, and a thermocouple or a resistance temperature detector has been used. However, these electric sensors have insufficient accuracy, are difficult to use in a situation affected by electric noise, or have a limited usable environment.

  On the other hand, a fiber Bragg grating (FBG) has attracted attention as an optical sensor. The FBG is an element in which a diffraction grating is formed so that a periodic change in the refractive index occurs in the core of the optical fiber. Of incident light, a wavelength that matches the period of the refractive index modulation is reflected, and other light is reflected. Is transmitted. If a temperature change occurs in the temperature measurement environment by taking advantage of this feature, the reflection wavelength changes due to the periodic change in the refractive index formed in the core, and the temperature applied to the grating is measured by measuring this. Changes can be measured.

  The optical fiber typically has a two-layer structure including a core and a concentric clad covering the core. The core and clad are made of different transparent glass or plastic, respectively, and the core is designed to have a higher refractive index than the clad.By adjusting the relative refractive index of the core and the clad, light is referred to as total internal reflection. Due to the phenomenon, the light propagates while being confined in the core. As an example using such an optical fiber, Patent Document 1 discloses an optical fiber distributed sensor and a method for measuring parameters and a method for calibrating a measured value using the optical fiber distributed sensor.

  U.S. Pat. No. 6,064,097 discloses a chlorine-doped silica core region, an annular stress-adjusting region surrounding the core region and doped with a fluorine dopant at a concentration required to exhibit a similar viscosity as the Cl-doped core region. And a heavily doped F cladding layer provided surrounding the optical fiber, wherein the dopant concentration in the cladding layer is sufficient to provide optical mode confinement in the core region. . Patent Document 2 discloses that an annular stress adjusting region in the optical fiber reduces stress generated during drawing by improving viscosity compatibility between the core and the cladding, and minimizes the amount of optical power present in the cladding. Is disclosed.

  However, the conventional FBG has a problem that, when a temperature change occurs in the temperature measurement environment, not only does the effective refractive index change, but also the core material thermally expands as a whole.

（ただし、式（１）中、λ_Bはブラッグ波長、λは入射波長、αは熱膨張係数、ζは実効屈折率変化、Ｔは温度を表す。） In the case of FBG, it is known that the temperature dependence of the wavelength is represented by the following equation (1).

(However, in the equation (1), λ _B is a Bragg wavelength, λ is an incident wavelength, α is a coefficient of thermal expansion, ζ is a change in effective refractive index, and T is a temperature.)

When forming the FBG using a single mode fiber using silica glass as the ^{core, α = 0.443 × 10 -6 /K,ζ=8.75×10} -6 / K, when lambda = 1550 nm, temperature dependence Can be assumed to be Δλ _B = 14.6 pm / K (Reports Res. Lab. Asahi Glass Co., Ltd., 56 (2006)). However, the thermal expansion coefficient α of the silica glass differs depending on the temperature and does not become a constant value (Reports Res. Lab. Asahi Glass Co., Ltd., 56 (2006)). In other words, since the coefficient of thermal expansion α differs depending on the temperature to be measured, it can be ignored when measuring a very small temperature difference, but when used for measurement in the range of 0 ° C to 100 ° C, for example, the coefficient of thermal expansion is It changes from 0.4 × 10 ⁻⁶ / K to 0.6 × 10 ⁻⁶ / K. In this case, the change in the coefficient of thermal expansion is Δ0.2 × 10 ⁻⁶ / K, the temperature dependency of the FBG on the reflection wavelength changes from 14.2 pm / K to 14.5 pm / K, and the deviation of about 2% Occurs. Since this leads to a measurement error as a highly sensitive and highly reliable optical sensor, improvement is required.

JP-A-2004-69685 JP-A-2006-81067

  The present invention suppresses temperature-dependent deviation of the reflection wavelength of FBG caused by thermal expansion of a silica glass core, and is an optical fiber member suitable as a material for temperature measurement of a structure that has been difficult to measure the temperature. The purpose is to provide.

The optical fiber member for a fiber Bragg grating of the present invention has a fluorine content of 1 wt% or more and 5 wt% or less, and a change in thermal expansion coefficient between 0 ° C. and 100 ° C. of Δ0.02 × 10 ⁻⁶ /. It is characterized by having a silica glass core having K or less.
In the method for producing an optical fiber member for a fiber Bragg grating according to the present invention, the content of fluorine is 1 wt% or more and 5 wt% or less, and a change in a coefficient of thermal expansion between 0 ° C. and 100 ° C. is Δ0.02 × 10 A step of cooling the silica glass core having a temperature of ^-6 / K or less from a temperature range of 800 to 1000 ° C. at a temperature decreasing rate of 500 ° C./sec or more.

ADVANTAGE OF THE INVENTION In the optical fiber member of this invention, the shift | offset | difference of the temperature dependence with respect to the reflection wavelength of FBG which arises due to the thermal expansion of a silica glass core can be suppressed effectively. That is, it is possible to adjust the change in the effective refractive index for the propagating light even by the temperature change.
When the optical fiber member of the present invention is used, the low thermal expansion is not limited to the range of 0 ° C. to 100 ° C., but lasts up to about 350 ° C. Therefore, it is necessary to perform more accurate measurement than the conventional optical fiber member in the range of about 350 ° C. Can be.
Therefore, in the optical fiber member of the present invention, since the FBG shifts the reflection wavelength in proportion to the temperature change, the FBG is suitable as a material for measuring the temperature of a complicated structure which has been difficult to measure the temperature. For example, a temperature sensor using the optical fiber member has high sensitivity and excellent reliability.

FIG. 1 is a flowchart for producing the optical fiber member of the present invention.

The optical fiber member for a fiber Bragg grating (FBG) of the present invention (hereinafter simply referred to as “optical fiber member”) has a fluorine content of 1 wt% or more and 5 wt% or less, and a heat content of 0 ° C. to 100 ° C. A silica glass core having a change in expansion coefficient of not more than Δ0.02 × 10 ⁻⁶ / K is provided.

  The optical fiber member of the present invention typically has a two-layer structure including a silica glass core made of a low thermal expansion silica glass to which fluorine is added as described above, and a concentric clad covering the core. .

  Heretofore, silica glass for fibers has been produced by a vapor phase axial method (VAD method), and silica glass added with fluorine has also been reported. In the VAD method, however, fluorine is added by baking porous silica (so-called soot) in a fluorine atmosphere, but it is difficult to add fluorine to the inside of the glass, and the content is at most about 1 wt%. (JP-A-2000-239040). This amount of fluorine addition is sufficient for the purpose of providing a refractive index difference between the core and the clad as an optical fiber for communication, but in order to suppress a change in the coefficient of thermal expansion with respect to temperature, the amount of fluorine addition is 1 wt% or more and 5 wt% or less are required. When the fluorine content in the silica glass is within the above range, the density in the glass is lower than that when no fluorine is contained (NEW GLASS, 27, p. 49 (2012)). When the density of the silica glass is reduced, the change in the coefficient of thermal expansion with respect to the temperature can be reduced. This is because, when vibrational movement of the glass skeleton occurs due to thermal expansion, the expansion accompanying the vibrational movement is suppressed by utilizing the gap between the lattices.

  It is known that crystallization hardly occurs in silica because the Si—O—Si bond is very flexible, and the silica melt maintains a stable supercooled liquid state in a wide temperature range of 600 ° C. or more. The fictive temperature of the silica glass according to the present invention is such that the higher the fictive temperature of the silica glass is, the larger the voids present in the network are, and the strain in the glass is taken into account. preferable. The virtual temperature refers to the freezing temperature of the supercooled liquid when the cooling start temperature of the silica glass is lowered. The adjustment of the virtual temperature can be arbitrarily adjusted by adding quench and anneal of the glass body as appropriate.

  The silica glass core according to the present invention is preferably manufactured using a sol-gel method that can be fired at a low temperature. FIG. 1 is a flow chart for producing an optical fiber member of the present invention, including a step by a sol-gel method. In the sol-gel method, a silica raw material and a fluorine compound are mixed in a solvent such as water or alcohol, and hydrolysis and a polymerization reaction are caused in the resulting solution. dry. When the glass is made as in the present invention, it is further heated after drying.

As a silica raw material, an alkoxysilane such as tetramethoxysilane or tetraethoxysilane is generally used. An alcohol such as methanol, ethanol, or 2-propanol is used as a solvent as a reaction field. In addition, water and fluorine compounds used for hydrolysis are used. The fluorine compound also functions as a catalyst for promoting hydrolysis. As such a catalyst, a fluoride acid catalyst or a basic catalyst is used, and specific examples thereof include, but are not limited to, hydrofluoric acid (HFaq), hydrofluoric acid (H ₂ SiF ₆ aq). Borofluoric acid (HBF ₄ aq), ammonium fluoride (NH ₄ F), ammonium hexafluorosilicate (F ₆ H ₈ N ₂ Si), and the like. Of these, under acidic conditions, in addition to being able to form a flexible chain-like silica oligomer, a nucleophilic reaction in which fluoride ions (F ⁻ ) attack Si proceeds simultaneously, and polycondensation proceeds three-dimensionally. Hydrofluoric acid is more preferable in that a gel having a large pore diameter composed of a crosslinked polymer can be formed.

  The alkoxysilane and the fluorine compound are mixed with a solvent such as water or alcohol to prepare a fluorinated silica sol (Step S1 in FIG. 1). In the sol-gel reaction, since the reaction rate and spinnability are determined by the molar ratio of the raw materials and the catalyst, it is important to determine an appropriate range of the mixing ratio of alkoxysilane, fluorine compound, water and alcohol.

  Here, the molar ratio of the alkoxysilane to the fluorine compound is 1: 0.05 to 1: 0.3, preferably 1: 0.1 to 1: 0.15. When the proportion of the fluorine compound is less than 0.05, the density of the obtained silica glass core is not sufficiently reduced, and the change in the thermal expansion coefficient with respect to the temperature may not be reduced.

  On the other hand, when the ratio of the fluorine compound is larger than 0.3, the progress of the sol-gel reaction is accelerated, and the uniformity of the distribution of fluorine in the silica glass core is reduced. As a result, a difference in the refractive index between silica glass and fluorine occurs in the silica glass core, resulting in striae, or fluorine is desorbed during firing for vitrification, and the change in thermal expansion coefficient with respect to temperature is small. A silica glass core may not be obtained. In the present invention, preferred specific examples of the mixing ratio of the raw materials and the catalyst are 1 to 4 mol of water, 1 to 4 mol of ethanol, 1 to 3 mol of ethanol, and 0.05 to 0.3 mol of hydrofluoric acid per mol of tetraethoxysilane. Is a mole.

  The prepared sol forms a core fiber having a desired diameter by an electrospinning method in which an electric field is applied to a discharge portion and a target portion, or a spinning method in which the sol is discharged from pores and elongated. During the spinning process, the reaction further proceeds to change the sol into a gel (step S2 in FIG. 1).

  Further, the obtained core fiber is fired (Step S3 in FIG. 1). The firing temperature at this time is 600 to 1200 ° C, preferably 800 to 1000 ° C. After calcination, it is further cooled from a temperature range of 800 to 1000 ° C. at a rate of 100 ° C./sec or more, preferably 500 to 1000 ° C./sec (Step S4 in FIG. 1). By having such a quenching step, the effect of reducing the change in thermal expansion from 0 ° C. to 100 ° C. is exhibited.

The silica glass thus obtained has a fluorine content of 1 wt% or more and 5 wt% or less, preferably 1.5 wt% or more and 3.0 wt% or less. When the fluorine content is within the above range, the density of the silica glass core is sufficiently reduced, and the change in the coefficient of thermal expansion with respect to temperature can be reduced. The change in the coefficient of thermal expansion between 0 ° C. and 100 ° C. is Δ0.02 × 10 ⁻⁶ / K or less, and preferably Δ0.01 × 10 ⁻⁶ / K or less. The fact that the amount of change in the coefficient of thermal expansion is within the above range suggests that the change caused by the thermal expansion can be suppressed.

The optical fiber member of the present invention typically has a two-layer structure including a core made of the silica glass and a concentric clad covering the core. Specifically, the outer peripheral portion of the silica glass core is covered with a clad material by a method such as heat shrinkage (Step S5 in FIG. 1). For the cladding, a resin having a lower refractive index than the core is used. By setting the refractive index of the core and the cladding in this way, light propagates in a state of being totally reflected and confined in the core, and can be used as an optical fiber member. As the material of such a clad, silica glass having a lower refractive index than the above silica glass may be used, or a fluorine-based polymer such as perfluoroalkyl (meth) acrylate and trifluoroethyl methacrylate, silicone, polyimide and polyethylene A thermoplastic resin such as terephthalate or a thermosetting resin such as an epoxy resin and a phenol resin may be used.
The thickness of the cladding coating layer is usually 400 to 600% of the diameter of the silica glass core. The material of the clad may be selected according to the use environment and the refractive index may be controlled.

Hereinafter, the present invention will be specifically described based on examples, but the present invention is not limited to the following examples.
[Example 1]
1 mol of tetraethoxysilane, 2 mol of water, 3 mol of ethanol, and 0.1 mol of hydrofluoric acid are mixed and stirred at 80 ° C., and then the mixture is concentrated in an atmosphere of 50 ° C. to obtain 0.65 Pa · s. Viscous. Thereafter, the fiber was discharged from the discharge port of a spinning apparatus provided with a die having a discharge port of φ20 μm under pressure using N ₂ gas, and stretched downstream to obtain a fiber gel of φ18 μm. Thereafter, firing was performed at 1200 ° C., and quenching was performed at a temperature lowering rate of 1000 ° C. to 500 ° C./sec to obtain a silica glass core having a diameter of 10 μm. The fluorine concentration in the silica glass core was 2.0% by weight. An optical fiber was obtained by coating the silica glass core with a fluorinated alkyl methacrylate. The coefficient of thermal expansion of the glass body is 0.30 × 10 ⁻⁶ / K at 0 ° C. and 0.31 × 10 ⁻⁶ / K at 100 ° C., and the change in the coefficient of thermal expansion is Δ0.01 × 10 ⁻⁶ / K. It was K.
The temperature dependence of the fiber Bragg grating (FBG) on the reflection wavelength when measured in the range of 0 ° C. to 100 ° C. at an incident wavelength of 1550 nm changes from 14.03 pm / K to 14.04 pm / K, and is about 0.07. %.
The results are shown in Table 1.

[Example 2]
Silica glass was produced in the same manner as in Example 1 except that the amount of hydrofluoric acid was changed to 0.2 mol in the process of Example 1, and a silica glass core having a diameter of 12 μm was obtained. The fluorine concentration in the silica glass core was 2.3% by weight. An optical fiber was obtained by coating the silica glass core with a fluorinated alkyl methacrylate. The coefficient of thermal expansion of the glass body is 0.27 × 10 ⁻⁶ / K at 0 ° C. and 0.28 × 10 ⁻⁶ / K at 100 ° C., and the change in the coefficient of thermal expansion is Δ0.01 × 10 ⁻⁶ / K. there were. The temperature dependence of the fiber Bragg grating (FBG) with respect to the reflection wavelength when measured in the range of 0 ° C. to 100 ° C. at an incident wavelength of 1550 nm changes from 13.98 pm / K to 13.40 pm / K, and is about 0.14. %.
The results are shown in Table 1.

[Comparative Example 1]
Except that the amount of hydrofluoric acid was changed to 0.01 mol in the process of Example 1, an attempt was made to produce silica glass in the same manner as in Example 1, but discharge was performed under pressure using N ₂ gas. During the elongation, a break occurred during the elongation, and a fiber could not be produced.
The results are shown in Table 1.

[Comparative Example 2]
1 mol of tetraethoxysilane, 2 mol of water, 3 mol of ethanol, and 0.1 mol of hydrofluoric acid are mixed and stirred at 80 ° C., and then the mixture is concentrated in an atmosphere of 50 ° C. to reduce the viscosity of the solution to 0. .65 Pa · s. Thereafter, the fiber was discharged by pressurizing using a N ₂ gas from a discharge port of a spinning device provided with a die having a discharge port of φ20 μm and stretched downstream to obtain a fiber gel of φ18 μm. Thereafter, firing was performed at 1200 ° C. to obtain a silica glass core having a diameter of 10 μm. The fluorine concentration in the silica glass core was 2.0% by weight. An optical fiber was obtained by coating the silica glass core with a fluorinated alkyl methacrylate. The coefficient of thermal expansion of the glass body is 0.47 × 10 ⁻⁶ / K at 0 ° C. and 0.43 × 10 ⁻⁶ / K at 100 ° C., and the change in the coefficient of thermal expansion is Δ0.04 × 10 ⁻⁶ / K. there were.
The temperature dependence of the fiber Bragg grating on the reflection wavelength when measured in the range of 0 ° C. to 100 ° C. at an incident wavelength of 1550 nm changes from 14.29 pm / K to 14.23 pm / K, and is shifted by about 0.28%. Occurred.
The results are shown in Table 1.

  INDUSTRIAL APPLICABILITY The optical fiber member of the present invention is suitably used for a fiber Bragg grating used for measuring the temperature of a structure that is difficult to measure with an electric sensor.

Claims (2)

A silica glass core having a fluorine content of 1 wt% or more and 5 wt% or less and a change in coefficient of thermal expansion between 0 ° C. and 100 ° C. of Δ0.02 × 10 ⁻⁶ / K or less. Characteristic optical fiber member for fiber Bragg grating. A silica glass core having a fluorine content of 1% by weight or more and 5% by weight or less and a change in coefficient of thermal expansion between 0 ° C. and 100 ° C. of Δ0.02 × 10 ⁻⁶ / K or less is 800 ° C. or more. A method for producing an optical fiber member for a fiber Bragg grating, comprising a step of cooling at a temperature lowering rate of 100 ° C./h or more from a temperature range of 1000 ° C. or less.

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