JP2007298368A - Fiber optic pressure sensor - Google Patents

Abstract

An optical fiber pressure sensor capable of detecting pressure while suppressing the influence of temperature change is provided.
An optical fiber, a first reflecting member that is provided opposite to one end surface of the optical fiber, reflects a part of light emitted from the optical fiber, and enters the reflected light into the optical fiber; The second reflecting member that reflects part or all of the light transmitted through the first reflecting member toward the first reflecting member, and the light that is interposed between the first reflecting member and the second reflecting member and is incident And a second light guide that has a refractive index temperature coefficient that is negative and a second light guide that has a refractive index temperature coefficient that is negative. Including.
[Selection] Figure 1

Description

  The present invention relates to an optical fiber pressure sensor.

  The optical fiber sensor detects a temperature or an external force applied to the sensing unit as an optical characteristic change and transmits the detected signal through the optical fiber.

  As described above, since the sensing method of the optical fiber sensor uses a change in optical characteristics, no power supply is required for the sensing unit, and the optical fiber used for the path has a small transmission loss, which is advantageous for remote measurement. It has the characteristic of being. Furthermore, from the properties of the optical fiber itself, it also has the advantage of being excellent in small diameter, light weight, explosion proof, and electromagnetic noise resistance.

  Patent Document 1 discloses an optical fiber temperature sensor using a narrow band wavelength filter called an etalon. An etalon is an optical interference type wavelength filter that takes out light of a specific wavelength by multiple reflection of light between two reflecting members. Patent Document 1 discloses the thickness of a member provided between two reflecting members. It is disclosed that the temperature is measured by detecting the change in the temperature.

Patent Document 2 discloses that an optical interference fiber sensor having two opposing reflection films is used for pressure detection.
Japanese Patent Laid-Open No. 05-56813 Japanese Patent Laid-Open No. 10-319241

  However, when the pressure is detected by extracting light of a specific wavelength by multiple reflection of light between the two reflecting members, the interval between the reflecting members changes depending on both pressure and temperature. It was difficult to detect only the pressure.

  Accordingly, an object of the present invention is to provide an optical fiber pressure sensor that can detect the pressure while suppressing the influence of a temperature change.

  In order to achieve the above object, an optical fiber pressure sensor according to the present invention is provided to face an optical fiber and one end face of the optical fiber, and reflects a part of the light emitted from the optical fiber. A first reflecting member that makes the reflected light incident on the optical fiber, a second reflecting member that reflects a part or all of the light transmitted through the first reflecting member toward the first reflecting member, and the first reflecting member. A light guide that is interposed between one reflective member and the second reflective member and that causes multiple interference of incident light and is elastically deformed by an external force, and the light guide has a refractive index temperature coefficient. It comprises a first light guide that is positive and a second light guide that has a negative refractive index temperature coefficient.

  The optical fiber pressure sensor according to the present invention configured as described above includes a first light guide having a positive refractive index temperature coefficient and a second light guide having a negative refractive index temperature coefficient. Therefore, the pressure can be detected while suppressing the influence of the temperature change.

Hereinafter, an optical fiber pressure sensor according to an embodiment of the present invention will be described with reference to the drawings.
Embodiment 1. FIG.
FIG. 1 is a schematic diagram showing a configuration of an optical fiber pressure sensor according to Embodiment 1 of the present invention. The optical fiber pressure sensor includes an optical fiber 11 and a sensing unit 10 optically coupled to one end side of the optical fiber 11 via a lens 14. The sensing unit 10 is configured to reflect a part of incident light, and is provided between the two reflecting mirrors 12a and 12b facing each other and the two reflecting mirrors 12a and 12b. 13 For example, the sensing unit 10 selectively causes light having a wavelength corresponding to the interval between the two reflection mirrors 12a and 12b by causing multiple interference of light incident through the reflection mirror 12a in the light guide 13. The narrow-band wavelength filter that emits light through the reflection mirror 12a or the reflection mirror 12b.
The sensing unit 10 constituted by the reflecting mirrors 12a and 12b and the light guide 13 is generally called an etalon, and such an interferometer is generally called a Fabry-Perot interferometer.

Here, in particular, the optical fiber pressure sensor of the present embodiment includes the first light guide 13a having a positive refractive index temperature coefficient and the second light guide 13b having a negative refractive index temperature coefficient. This makes it possible to detect only changes due to external force by setting the thicknesses La and Lb of the first light guide 13a and the second light guide 13b to appropriate values. become.
Hereinafter, the optical fiber pressure sensor of the present embodiment will be described in detail.

The reflection mirrors 12a and 12b can be formed of a dielectric multilayer film, a metal film, or the like, and are members that reflect some light and transmit the remaining light. The reflection mirror 12a may be directly formed on the optical fiber 11 or the light guide 13, for example, or the optical fiber 11 and the light guide 13 formed by forming a dielectric multilayer film or a metal film on a glass plate. You may make it arrange | position between. For example, the reflection mirror 12a has a reflectance of 0.9 and a transmittance of 0.1, while the reflection mirror 12b has a reflectance of 0.99 and a transmittance of 0.01, for example. The Thus, if the reflectance of the reflection mirror is set higher than that of the reflection mirror 12a and the reflection mirror 12b is set high, the alignment process for re-entering the reflected light of the reflection mirror 12b into the optical fiber 11 is facilitated. More specifically, as the reflection mirror 12a and the reflection mirror 12b, for example, a metal film such as Al (aluminum) or Au (gold), a film such as ZnS which is a relatively high refractive index material, and the high refractive material are used. In contrast, a multilayer dielectric film in which Na ₃ A1F _{6 and the} like having a relatively low refractive index and a low refractive index are alternately formed is used. When such a multilayer dielectric film is used, the thickness is related to the reflectance and the laminated structure, but is set to about 5 μm, for example. The reflection mirror 12a and the reflection mirror 12b are formed by, for example, vapor deposition, ion plating, sputtering, or spin coating.
The reflecting mirrors 12a and 12b may be formed by directly forming a film on the light guide 13. Alternatively, the light guide 13 may be formed by forming a dielectric multilayer film or a metal film on a transparent glass plate. You may form so that it may adhere.
The reflection mirror 12b may reflect all of the incident light.
The lens 14 has a function of condensing the light emitted from the optical fiber 11 toward the reflection mirror 12b and efficiently re-entering the light reflected by the reflection mirror 12b into the optical fiber 11. Without such a lens 14, the light emitted from the optical fiber 11 spreads at an angle corresponding to the numerical aperture of the optical fiber 11, and is lost when the light reflected by the reflecting mirror 12 b is reincident on the optical fiber 11. Is likely to grow. For example, the lens 14 can be appropriately selected from a convex lens, a ball lens, and an aspheric lens, and examples thereof include BK7, TaF3, synthetic quartz, and silicon resin.

Hereinafter, the principle and operation of the optical fiber pressure sensor of this embodiment will be described.
First, in the optical fiber pressure sensor of the embodiment, if light is incident from the reflection mirror 12a, the light is multiple-reflected inside the light guide 13 having a thickness L and is transmitted or reflected by the sensing unit 10. At this time, the peak wavelength λ of light transmitted through the sensing unit 10 (attenuation peak in reflection) is given by the following relational expression.
mλ = 2nL (1)
Here, m is the interference order (integer), n is the refractive index of the glass plate, and L is the thickness of the light guide 13. The incident angle is 90 degrees.

Next, the intensity of the light that returns from the sensing unit 10 and reenters the optical fiber 11 will be considered.
First, in the optical fiber pressure sensor of FIG. 1, a part of the light incident on the sensing unit 10 from the optical fiber 11 is reflected by the reflection mirror 12a and reenters the optical fiber 11. Here, the reflectance in the reflection mirror 12a is R1. On the other hand, the transmitted light that has passed through the reflection mirror 12a passes through the light guide 13 having a refractive index n, is reflected by the reflection mirror 12b, passes through the light guide 13 again, and reaches the reflection mirror 12a. Here, the reflectance at the reflection mirror 12b is R2. Further, part of the light reaching the reflection mirror 12a is reflected and the remaining part is transmitted. The reflected light passes through the light guide 13 again, is reflected by the reflection mirror 12b, passes through the light guide 13 again, and reaches the reflection mirror 12a. Again, part of the light reaching the reflecting mirror 12a is reflected and partly transmitted. In this way, light is reflected and transmitted repeatedly infinitely. During this time, there is an infinite amount of light that passes through the reflection mirror 12a and reenters the optical fiber 11. All these lights interfere with each other, and the ratio Ir / Ii of the reflected light intensity Ir and the incident light intensity Ii as a whole is expressed by the following equation (2).

Here, φ = 4 nLπ / λ, and L is the thickness of the light guide 13.
As shown in the equation (2), Ir / Ii is expressed as a function of the wavelength λ, the refractive index n of the light guide 13 and the thickness L. The expression (2) is an expression derived by assuming that the reflectance R1 of the reflecting mirror 12a and the reflectance R2 of the reflecting mirror 12b are smaller than 1 (R1 <1, R2 <1).

  For example, when the thickness L of the light guide 13 is 20 μm and Ir / Ii in the vicinity of a wavelength of 1580 nm (1550 nm to 1610 nm) is calculated using the formula (2), it is as shown in FIG. In FIG. 2, the solid line represents the reflectance of the reflecting mirrors 12a and 12b as 0.9 (R1 = R2 = 0.9), and the broken line represents the reflectance of the reflecting mirrors 12a and 12b as 0.5. (R1 = R2 = 0.5), the alternate long and short dash lines are calculated values when the reflectance of the reflecting mirrors 12a and 12b is 0.2 (R1 = R2 = 0.2). .

  As shown in FIG. 2, the ratio Ir / Ii of the reflected light intensity Ir and the incident light intensity Ii has a plurality of attenuation peaks, and the attenuation peak wavelength is φ = π / 2 + mλ (m is 1 or more). Value). On the other hand, since φ = 4 nLπ / λ, the wavelength of the attenuation peak is also expressed as a function of the refractive index n and the thickness L of the light guide 13.

  Therefore, when pressure is applied to the sensing unit 10, the refractive index n and the thickness L of the light guide 13 change due to the photoelastic effect, and the attenuation peak wavelength changes. Therefore, by detecting the change in the peak wavelength, The magnitude of the applied pressure can be detected based on the amount of change.

  However, the refractive index n and length L of the light guide 13 also change depending on the temperature, and when it is desired to sense only the change due to pressure (stress), it is necessary to exclude the influence of temperature. Therefore, in the present invention, in order to cancel the influence of temperature, the light guide 13 having the characteristics with opposite refractive index temperature coefficients is configured using the first light guide 13a and the second light guide 13b. Yes.

  Next, the operation of the sensing unit 10 in the optical fiber pressure sensor of the present embodiment in which the light guide 13 is configured using the first light guide 13a and the second light guide 13b will be described.

First, also in the optical fiber pressure sensor of the present embodiment, if there is no reflection at the boundary between the first light guide 13a and the second light guide 13b, the reflected light intensity Ir and the incident light intensity Ii as a whole are reduced. The ratio Ir / Ii is expressed by the above formula (2). At this time, the optical path length S (= nL) of the entire light guide 13 is equal to the optical path length S _a (= n _a L _a ) of the first light guide 13a and the optical path length S _b (=) of the second light guide 13b. n _b L _b ). That is, S = n _a L _a + n _b L _b .
Here, _{n a} is the refractive index of the first light guide body 13a, _{n b} is the refractive index of the second light guide body 13b, _{L a} is the thickness of the first light guide body 13a, _{L b} is the second It is the thickness of the light guide 13b.

Therefore, in the above formula (2), φ becomes φ = 4π (n _a L _a + n _b L _b ) / λ, and when pressure (external force) is applied to the sensing unit 10, the first light guide 13a Since the refractive index na and the thickness La and the refractive index nb and the thickness Lb of the second light guide 13b are changed and the attenuation peak wavelength is changed, by detecting the change of the peak wavelength, the application is performed based on the change amount. The magnitude of the applied pressure can be detected.

Next, change in peak wavelength due to temperature change will be considered.
First, the optical path length S in a single light guide is defined as S = nL as described above.
Therefore, the optical path length when the temperature changes by ΔT ° C is
S + (dS / dT) LΔT
= (N + (dn / dT) ΔT) (L + αLΔT) −1 · αLΔT
= NL + (dn / dT) ΔTL + nαLΔT + (dn / dT) ΔTαLΔT) −1 · αLΔT
It becomes.
In this equation, dn / dT is a refractive index temperature coefficient, and is δn in the following description. Further, α is a linear expansion coefficient. Further, the fourth term “(dn / dT) ΔTαLΔT” in this equation is an equation including the product of the refractive index temperature coefficient and the linear expansion coefficient, and its value is very small and can be ignored. The fifth term “1 · αLΔT” is a correction term representing the optical path length of the external space reduced when the medium expands.

Solving this for dS / dT,
(DS / dT) LΔT = (δn) ΔTL + nαLΔT−1 · αLΔT
It becomes.
Therefore,
dS / dT = (n−1) α + δn
It becomes.

The light guide 13, when configured by joining a first light guide body 13a and the second light guide 13b, the light guide 13 the total optical path length S is the optical path length _{S a} of the first light guide body 13a since the sum of the optical path length S _b of the second light guide body 13b, the change with temperature of the optical path length S of the light guide 13, the change due to the optical path length S _a temperature of the first light guide body 13a and the second light guide the sum of the variation with temperature of the optical path length S _b of the body 13b.

Therefore, when the light guide 13 is configured by joining the first light guide 13a and the second light guide 13b, the change in the optical path length S due to the temperature change is as follows:
L _a {(n _a −1) α _a + (δn _a )} + L _b {(n _b −1) α _b + (δn _b )}
It becomes.
Here, .DELTA.n _a has a refractive index temperature coefficient of the first light guide body 13a, .DELTA.n _b has a refractive index temperature coefficient of the second light guide body 13b, _{L a} is the thickness of the first light guide body 13a, _{L b} Is the thickness of the second light guide 13b, α _a is the linear expansion coefficient of the first light guide 13a, and α _b is the linear expansion coefficient of the second light guide 13b.

In other words, in the present embodiment, selection of materials constituting the first light guide 13a and the second light guide 13b and the first light guide 13a and the second guide are satisfied so as to satisfy the following expression (3). By setting the thicknesses L _a and L _b of the light body 13b, it is possible to sense only the change due to the external force.
L _a {(n _a −1) α _a + (δn _a )} + L _b {(n _b −1) α _b + (δn _b )} = 0 = 0 (3)

As clearly shown in the equation (3), the degree of influence of the linear expansion coefficient is smaller than the refractive index temperature coefficient.
Therefore, as specific steps for satisfying the expression (3), for example, as a light guide material constituting the first light guide 13a and the second light guide 13b, a refractive index temperature coefficient is first set. select positive material and a negative material, (3) so as to satisfy the equation, the thickness L _a of the first light guide body 13a and the second light guide body _13b, it is preferable to set the L _b.
That is, the refractive index temperature coefficient .DELTA.n a selected light guide _material, the coefficient of L _a in the above formula based .DELTA.n _b and the linear expansion coefficient alpha _a, the _{α b ({(n a -1} ) α a + ( δn _a )}) and the coefficient of L _b ({(n _b −1) α _b + (δn _b )}), and the first light guide 13a and the second light guide so as to satisfy the expression (3). The thicknesses L _a and L _b of the body 13b are set.

Examples of the light guide material having a negative refractive index temperature coefficient include, for example, a crown fluoride glass or a phosphate phosphate glass, a glass added with boron trioxide, or an organic polymer such as benzocyclobutene (BCB). Molecular materials can be used. In addition, as a light guide material having a negative refractive index temperature coefficient constituting the second light guide, it is preferable to use glass containing fluorine. Since such a glass containing fluorine can increase the negative refractive index temperature coefficient as compared with other materials, the negative linear expansion coefficient in the material of the first light guide can be reduced. . As a result, according to such a configuration, it is possible to prevent the reflective film from being cracked or peeled off due to mismatch of the linear expansion coefficients. Examples of the light guide material having a positive refractive index temperature coefficient used in combination with this include crystallized glass and translucent ceramics containing niobium oxide. Niobium oxide (Nb ₂ O ₅ ) has a negative linear expansion coefficient. When this material is added as fine particles of 30 nm or less, the linear expansion coefficient and refraction of the base material are maintained while maintaining high transparency depending on the addition rate. The rate temperature coefficient can also be adjusted. Ceramic becomes a polycrystal which is an aggregate of crystal grains by firing, but it has an optically inhomogeneous microstructure, so its transparency is generally lowered by light scattering at heterogeneous interfaces and grain boundaries. . On the other hand, the translucent ceramic is processed densely and homogeneously in the manufacturing process in order to reduce the voids at the intersection of the grain boundaries. For example, Al ₂ O ₃ , Y ₂ O ₃ —ThO _2. In addition to oxides such as MgAl ₂ O ₄ , nitrides, carbides and sulfides are used.

First, as a light guide material constituting the first light guide 13a and the second light guide 13b, a material having a negative linear expansion coefficient and a material having a positive linear expansion coefficient are selected and selected. refractive index temperature coefficient .DELTA.n _a of material, .DELTA.n _b and the linear expansion coefficient alpha _a, the coefficient of _{L a} in the above formula on the basis of _{α b ({(n a -1} ) α a + (δn a)}) The coefficient L of L _b ({(n _b −1) α _b + (δn _b )}) is obtained, and the thickness L of the first light guide 13a and the second light guide 13b is satisfied so as to satisfy the expression (3). _a and L _b may be set.
In addition, as a material whose linear expansion coefficient is negative (refractive index temperature coefficient is positive), TiO ₂ —SiO ₂ (quartz glass doped with TiO ₂ ), Li ₂ O—Al ₂ O ₃ —SiO ₂ , Al There are various things such as crystallized glass (glass ceramics), such as ₂ O ₃ —TiO ₂ , which is obtained by crystallizing glass by heat treatment. Generally, by changing the material composition ratio and the production method, It is possible to continuously change the linear expansion coefficient.
For example, the linear expansion coefficient of Li ₂ O—Al ₂ O ₃ -2SiO ₂ is −8.6 × 10 ⁻⁶ (1 / ° C.), and the linear expansion coefficient of Al ₂ O ₃ —TiO ₂ is −1. .9 × 10 ⁻⁶ (1 / ° C.), both exhibit a negative coefficient of linear expansion.

As can be seen from the above description, in the optical fiber pressure sensor of the present embodiment, based on the characteristics of the light guide material selected to configure the first light guide 13a and the second light guide 13b, By detecting the thicknesses L _a and L _b of the first light guide 13a and the second light guide 13b so as to satisfy the expression (3), the pressure is detected without being affected by the temperature change. Is possible.

<Reflection mitigation at the joint between the first light guide 13a and the second light guide 13b>
As described above, in the optical fiber pressure sensor of the present invention, it is desirable that the reflection at the joint between the first light guide 13a and the second light guide 13b is small.
Therefore, specific means for reducing reflection at the joint between the first light guide 13a and the second light guide 13b will be described below.

  In the present invention, it is desirable that the bonding surfaces of the first light guide body 13a and the second light guide body 13b are such that atoms on each surface are directly bonded at room temperature and there is no dissimilar material such as an adhesive. As a method for realizing such bonding, for example, there is a method in which the surfaces are smoothed by polishing and then cleaned by sputtering, and the surfaces are activated and bonded.

  As a polishing process at this time, for example, a method called CMP (Chemical Mechanical Polishing) in which a polishing abrasive of ceramics or diamond is mixed in a liquid having a chemical corrosive action is used, and the flatness is 10 μm or less. It is desirable to make it an ultra-smooth surface with a surface roughness of 10 nm or less, and in order to realize better bonding, polishing is performed so as to remove the work-affected layer on the surface. This CMP polishing can be performed in units of several nm and hardly damages the lower part of the polishing layer.

The CMP polishing includes a polishing process and a cleaning process. In the polishing step, the first and second light guides are polished in an ultrapure water using a polishing pad, and then further polished in colloidal silica in which SiO ₂ particles having an average particle size of 30 nm are suspended in an NaOH solution. . The cleaning step includes (a) ultrasonic bath cleaning with alcohol, (b) ultrasonic bath cleaning with ultrapure water, and (c) drying with a spin dryer. Each process is rinsed with ultrapure water.

  After the polishing process, the first light guide body 13a and the second light guide body 13b are opposed to each other in the vacuum chamber, and the ion beam or neutral atoms are irradiated to the respective joint surfaces for cleaning and surface activation. I do. This is because a layer of gas molecules, contaminants and oxide film adhering to the bonding surface in the atmosphere are removed to provide a clean surface. The surface activation is usually performed by irradiating an inert gas (Fast Atom Beam) such as an argon fast atom beam and performing sputtering to etch the material surface. As a result, the inactive surface layer can be removed, and this treatment brings the bonding surface into an active state having a strong bonding force with other atoms.

In this step, the degree of vacuum in the vacuum chamber affects the degree of cleaning of the joint surface, so it is desirable that it be as high as possible. For example, the degree of vacuum is about 10 ⁻² Pa or higher, preferably about 10 ⁻⁴ Pa. To do. Further, the irradiation intensity and irradiation time of the ion beam or neutral atom are appropriately set so that about 10 nm of the surface layer is removed.

  After the above steps, when the activated surface of the first light guide 13a and the activated surface of the second light guide 13b are brought into close contact with each other in vacuum at room temperature, the van der Waals force may Adsorb to. That is, since the cleaned surface of the first light guide 13a and the surface of the second light guide 13b have a lot of orbits in a non-bonded state, that is, dangling bonds, they are naturally brought into close contact with each other. Adsorb. However, since these dangling bonds are very active, if they are exposed to a vacuum atmosphere for a long time before adhesion, they remain in the vacuum chamber even if the vacuum chamber is maintained at a high vacuum. Impurities such as nitrogen, carbon and hydrocarbons chemisorb on the surface and inactivate the surface. Therefore, after the optical element cleaning step, it is necessary to complete the process in a very short time until they are brought into close contact with each other.

  In this joining, it is desirable to apply pressure when the surfaces are brought into close contact with each other in order to strengthen the joining between the first light guide 13a and the second light guide 13b. Here, the pressure is greatly related to the surface roughness of the joint surface. For example, when the surface roughness Ra of the joint surface is about 10 nm, good adhesion between the surfaces can be achieved even with a pressurization as small as 1 kgf.

For example, the first light guide 13a and the second light guide 13b may be joined via an antireflection film made of a multilayer film of an inorganic material. Thus, the reflection of the joint surface can be effectively prevented by applying the antireflection film to the boundary portion.
Specifically, for example, 3 to 5 layers of SiO ₂ and TiO ₂ or SiO ₂ and Ta ₂ O ₅ are alternately formed on the bonding surface of the first light guide 13a and / or the second light guide 13b. The surfaces of the multilayer films to be formed and bonded are activated by the method described above and then bonded.

  In this case, an antireflection film is formed on one joint surface of the first light guide 13a or the second light guide 13b, and a material having a refractive index similar to that of the other light guide is further formed on the surface. If several hundreds of nanometers to several micrometers of film are formed and then removed by bonding several tens of nanometers of the surface layer in the cleaning process, the antireflection film can be bonded without changing the film thickness as much as possible. To work.

Further, instead of directly bonding the atoms on the surface, the first light guide 13a or the second light guide 13b may be bonded using an adhesive and an antireflection film.
Specifically, an antireflection coating that suppresses reflection at the interface with the adhesive is applied to each joint surface of the first light guide 13a and the second light guide 13b, and then the first using the adhesive. The light guide 13a and the second light guide 13b are joined. If it does in this way, it will become possible to join using an adhesive.

  Here, as the adhesive, an epoxy-based or acrylic-based ultraviolet curable translucent adhesive is suitable.

Embodiment 2. FIG.
The optical fiber pressure sensor according to the second embodiment of the present invention is an optical fiber pressure sensor that embodies the basic form described in the first embodiment.
That is, the optical fiber pressure sensor 20 of the second embodiment includes a fiber holder (ferrule) 17 and a holder 16, the sensing unit 10 is joined to the tip of the ferrule 17, and the optical fiber 11 is inserted into the through hole of the ferrule 17. It is configured by being inserted.

In the optical fiber pressure sensor of the second embodiment, a refractive index distribution type fiber (hereinafter referred to as graded index fiber) 15 having a lens function and a coreless fiber 18 are provided in the through hole of the ferrule 17. Thereby, the light emitted from the optical fiber 11 enters the sensing unit 10 via the graded index fiber 15 and the coreless fiber 18, and the light reflected by the sensing unit 10 is reflected between the coreless fiber 18 and the graded index fiber 15. It is made to enter the optical fiber 11 via
The graded index fiber 15 is an optical fiber having an axially symmetric refractive index distribution that gradually decreases from the central axis toward the outer periphery, and is generally used for multimode transmission. The refractive index distribution of the graded index fiber is such that the refractive index decreases in proportion to the square of the distance from the center axis of the optical fiber, and has a lens function like the GRIN lens. Therefore, if a graded index fiber having an appropriate length is used, it can function as a condensing lens, and an optical system with good coupling efficiency can be configured.
The coreless fiber 18 is a fiber having a substantially uniform refractive index distribution, and the material thereof is made of, for example, quartz glass. The refractive index is about 1.45, and the transmission loss is 0.35 × 10 ⁻⁶ dB / mm or less. It is preferable that the transmission loss is relatively low. The coreless fiber 18 has an outer diameter equal to that of the optical fiber 11 (for example, a single mode fiber) and the graded index fiber 15 and about 125 μm, and is joined to the graded index fiber 15 by, for example, thermal fusion.
If it does in this way, the 1st reflected light of the reflective mirror 12b can re-enter the optical fiber 11 efficiently by the lens function of the graded index fiber 15.

  That is, the light emitted from the optical fiber 11 spreads, passes through the reflection mirror 12a, and passes through the light guide 13. Therefore, when the light reflected by the reflection mirror 12b recombines with the optical fiber 11, a loss occurs. To do. Therefore, in the second embodiment, the graded index fiber 15 is used to enter the light guide 13 via the reflection mirror 12a so as to collect light while suppressing the spread of the light emitted from the optical fiber 11. Thus, the light reflected by the reflection mirror 12b can be re-incident on the optical fiber 11. Thereby, the optical coupling efficiency between the optical fiber 11 and the sensing unit 10 can be increased.

  As described above, when the efficiency of re-incident on the optical fiber 11 is increased by using the graded index fiber 15, the reflectance described with reference to FIG. 2 in the first embodiment is substantially increased. Measurement with high wavelength selectivity is possible. Therefore, using the graded index fiber 15 enables more accurate measurement.

  In the optical fiber pressure sensors of Embodiments 1 and 2 described above, the light guide 13 is configured by the first light guide 13a and the second light guide 13b, but the present invention is not limited to this, The light guide body 13 may be configured by alternately and repeatedly laminating a first light guide body having a positive refractive index temperature coefficient and a second light guide body having a negative refractive index temperature coefficient. According to such a configuration, it is possible to stack the thin film with a precisely controlled thickness in a thin film forming process using, for example, a vapor deposition apparatus or a sputtering apparatus. As a result, according to this configuration, the thickness of the light guide 13 can be accurately set on the order of nanometers. Furthermore, since the first light guide and the second light guide are alternately laminated, the external force acts uniformly on the first light guide 13a and the second light guide 13b. And reliability is improved.

As an example of the present invention, the pressure sensor shown in FIG. 3 is manufactured and evaluated.
The optical fiber 11 uses a single mode fiber having a mode field diameter of 10.5 μm, and the fiber holder 17 uses a zirconia ferrule. The light guide 13 is formed by depositing an antireflection film on each bonding surface of the first light guide 13a made of crystallized glass and the second light guide 13b made of crown fluoride glass, and then adhered by room temperature bonding. I let you. The partial reflection mirror 12a is a dielectric multilayer film having a reflectance of 90% and a transmittance of 10%, and the partial reflection mirror 12b is a dielectric multilayer film having a reflectance of 99.9%.
An antireflection film is formed at a thickness of 230 nm on the bonding interface between the first light guide body 13a and the second light guide body 13b, and the surface layer is removed until the thickness reaches about 220 nm, followed by bonding at room temperature bonding. .

The crystallized glass that is the material of the first light guide has a linear expansion coefficient of −1.1 × 10 ⁻⁶ (1 / ° C.), a refractive index temperature coefficient of 13 × 10 ⁻⁶ (1 / ° C.), The refractive index is 1.516.
Accordingly, the dS / dT of the first light guide is 12.4 × 10 ⁻⁶ (1 / ° C.).
In addition, the crown fluoride glass, which is the material of the first light guide, has a linear expansion coefficient of 16.5 × 10 ⁻⁶ (1 / ° C.) and a refractive index temperature coefficient of −7.4 × 10 ⁻⁶ ( 1 / ° C.) and a refractive index of 1.446. Therefore, dS / dT of the second light guide is −0.041 × 10 ⁻⁶ (1 / ° C.).

  Based on the optical characteristics of the first light guide 13a and the second light guide 13b, the thickness La of the first light guide 13a is 9090 μm and the second light guide 13b so as to satisfy the expression (3). The thickness Lb is 30 μm.

DS / dT of the light guide 13 manufactured in this way can be ± 0.02 × 10 ⁻⁶ (1 / ° C.) or less, and pressure can be detected without being affected by changes in temperature. .

It is a schematic diagram which shows the structure of the optical fiber pressure sensor of Embodiment 1 which concerns on this invention. It is a figure which shows the reflected light intensity (Ir / Ii) with respect to input light intensity in wavelength 1580nm vicinity of the optical fiber pressure sensor of Embodiment 1. FIG. It is a schematic diagram which shows the structure of the optical fiber pressure sensor of Embodiment 2 which concerns on this invention.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10 Sensing part, 11 Optical fiber, 12a, 12b Reflection mirror, 13 Light guide, 13a 1st light guide, 13b 2nd light guide, 15 Graded index fiber (index distribution type fiber), 16 Holder, 17 Fiber holder, 18 coreless fiber, 20 optical fiber pressure sensor.

Claims (4)

Optical fiber,
A first reflecting member that is provided facing one end surface of the optical fiber, reflects a part of the light emitted from the optical fiber, and enters the reflected light into the optical fiber;
A second reflecting member that reflects part or all of the light transmitted through the first reflecting member toward the first reflecting member;
A light guide that is interposed between the first reflecting member and the second reflecting member and that causes multiple interference of incident light and elastically deforms by an external force;
The optical fiber pressure sensor, wherein the light guide includes a first light guide having a positive refractive index temperature coefficient and a second light guide having a negative refractive index temperature coefficient.   The optical fiber pressure sensor according to claim 1, wherein the first light guide and the second light guide are alternately arranged in the light guide.   The optical fiber pressure sensor according to claim 1, wherein the second light guide is made of glass containing fluorine.   The optical fiber pressure sensor according to any one of claims 1 to 3, further comprising a gradient index fiber between one end face of the optical fiber and the first reflecting member.

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