JP2003194635A - Optical fiber type temperature sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical type temperature sensor sensitive to temperature variation and having a large measurable temperature width. <P>SOLUTION: A long-cycle fiber grating is formed by cyclically forming high- refraction rate parts 3 in the longitudinal direction of a polarized wave-keeping optical fiber provided with a stress imparting part for applying a non- axisymmetrical stress to a core 1. The variation of a loss characteristic of the long-cycle fiber grating with respect to temperature is detected, so that the temperature variation is detected. The variation of the loss characteristic to temperature is adjusted by adjusting the position or size of the stress imparting part, so that the temperature detection sensitivity can be enhanced. By detecting the temperature variation by entering only f-polarized waves into the long-cycle fiber grating, high-sensitivity temperature detection can be carried out. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber type temperature sensor for measuring a temperature change, and more particularly to an optical fiber type temperature sensor utilizing a wavelength change of optical characteristics of an optical fiber grating.

[0002]

2. Description of the Related Art As an optical fiber type sensor, there is known an optical fiber type sensor using a fiber grating in which a core of an optical fiber has a periodic refractive index change. This fiber grating reflects or blocks light in the vicinity of a specific wavelength of light propagating in an optical fiber. Fiber Bragg grating (Fiber Bragg Grating:
Hereinafter referred to as "FBG") and a long-period fiber grating (Long-Period Fiber Gr
ating: hereinafter abbreviated as "LPFG"). Since all of these devices have temperature dependence and tension dependence in optical characteristics, a temperature sensor, a tension sensor, and a strain sensor using them are proposed.

Among them, in the long-period fiber grating, light propagating in the core (hereinafter abbreviated as "guided mode light") is propagated in the clad in the same direction (hereinafter abbreviated as "forward clad mode light"). ) To generate a transmission loss for guided mode light in the vicinity of a specific wavelength. This long-period fiber grating has temperature dependence at the wavelength at which the loss value of transmitted light is maximum (hereinafter abbreviated as "loss center wavelength"), and it can be used as a temperature sensor by detecting this temperature dependence. it can. This temperature dependence of the loss center wavelength is 0.05 nm when using a standard single-mode optical fiber or dispersion-shifted optical fiber.
It is about / ° C, and the temperature dependence of the loss center wavelength is larger than that of FBG. In Japanese Patent Laid-Open No. 2000-164955,
A technique for compensating the temperature dependence of the optical amplifier by utilizing the temperature dependence of the loss center wavelength of LPFG is disclosed. In addition, Opt. Lett. , 21 , 692-694, 1996 (Bhatia
V. et al. ) (Hereinafter referred to as “reference [1]”) proposes a temperature sensor and a tension sensor that utilize the temperature dependence and the tension dependence of the loss center wavelength of LPFG. It is disclosed that the wavelength characteristic of a fiber grating using a fiber has a large temperature dependence of 0.154 nm / ° C.

On the other hand, the fiber Bragg grating is for causing Bragg reflection of the guided mode light in the direction opposite to the traveling direction and coupling it with the backward guided mode light. Hereinafter, abbreviated as "center wavelength of reflected light"), or wavelength at which the loss value of transmitted light is maximum (hereinafter abbreviated as "center wavelength of transmitted light") has temperature dependence, and reflection characteristic or loss characteristic It can be used as a temperature sensor by detecting the change of the temperature with respect to the temperature. The fluctuation of this center wavelength is 0.01 to 0.015n
Although it is about m / ° C, which is a smaller value than that of LPFG, it is possible to have a very steep reflection (loss) characteristic, so it is necessary to monitor the change in reflection intensity or loss value with respect to wavelength. Thus, a sensor sensitive to temperature changes can be manufactured. For example, a loss variation of 20 dB can be given in the width of the wavelength of 0.1 nm, and a large change amount of the loss value with respect to temperature can be given such as 2 to 3 dB / ° C. An example in which a temperature sensor is manufactured by utilizing the temperature dependence of the loss wavelength characteristic of the FBG is described in, for example, Japanese Patent Laid-Open No.
It is disclosed in Japanese Patent Laid-Open No. 00-162444 and Japanese Patent Laid-Open No. 11-218450. Of these, in Japanese Patent Laid-Open No. 2000-162444, F
By bending a portion different from the portion where the BG is formed into a U shape, a technique of measuring the temperature without distorting the FBG portion is disclosed, and JP-A-11-218450 discloses a double ring shape. By using an optical fiber having a core structure of No. 3, there is disclosed a technique of reliably separating tension and temperature and simultaneously measuring them. In addition, IEICE 94 Spring C-372 (M. Shige
hara et al. ) (Hereinafter referred to as “reference [2]”), the FBG is fixed to an aluminum plate, and the fact that the central wavelength fluctuates due to the tension fluctuation due to the thermal expansion of the aluminum plate is utilized.
A temperature sensor having a temperature dependence of 0.026 nm / ° C has been proposed.

[0005]

However, in the temperature sensor using the LPFG, since the loss characteristic of the LPFG is broad, there is a problem that the method of observing the loss change at one wavelength has poor sensitivity to the temperature change. there were. For example, LPF with temperature dependence of 0.05 nm / ℃, loss accuracy of ± 0.5 dB, wavelength accuracy of ± 0.02 nm, maximum loss value of 8 dB, and one-sided bandwidth of 12 nm.
In the case of G, the error of the detected temperature in the method of detecting the loss change due to temperature at one wavelength is as large as ± 7 ° C, and the error of the detected temperature in the method of detecting the loss change due to temperature at the loss center wavelength is ± 0.4 ° C. Therefore, in order to improve the detection accuracy, the method is limited to the method of detecting the temperature change by the change of the loss center wavelength.

On the other hand, in the case of the temperature sensor using the FBG, there is a limit to the measurable fluctuation amount, that is, the temperature width.
There is a trade-off between measurement sensitivity and measurable temperature range, for example, temperature dependence of 0.01 nm / ° C, loss accuracy of ± 0.5d.
In the case of an FBG that causes a loss variation of 20 dB in the wavelength range of B and 0.1 nm, according to the method of detecting the loss change in one wavelength, if the temperature error is suppressed to ± 0.25 ° C, the measurable temperature range is ± 5
℃. This measurable temperature range is ± 10 ℃, ± 20 ℃,
When it is widened to ± 50 ° C, the temperature error increases to ± 0.5 ° C, ± 1.0 ° C, and ± 2.5 ° C, respectively, which impairs the measurement accuracy. As described above, in the conventional optical fiber type, when the LPFG is used, the temperature sensitivity is poor and the FBG is
However, the problem that the measurable temperature range is narrow is a problem. The present invention has been made in view of the above problems, and an object thereof is to provide an optical fiber type sensor that is sensitive to temperature changes and has a wide measurable temperature range.

[0007]

In order to solve the above-mentioned problems, the invention according to claim 1 has a length of a polarization-maintaining optical fiber having a stress-applying portion for applying non-axisymmetric stress to a core. A light characterized by forming a long-period fiber grating by providing a high-refractive-index portion that is periodic with respect to the direction, detecting the change in the loss characteristics of this long-period fiber grating with respect to temperature, and detecting the temperature change. It is a fiber type temperature sensor. Accordingly, since it is possible to detect the temperature change by using the polarized wave that is sensitive to temperature and whose loss characteristic changes in the wide temperature range, it is possible to detect the temperature change with high accuracy and in the wide temperature range.

According to a second aspect of the present invention, in the optical fiber temperature sensor according to the first aspect, the size of the stress applying portion is adjusted, or the distance between the stress applying portion and the core is adjusted. The temperature dependence of the loss characteristic of the long-period fiber grating is adjusted. Thereby, the temperature detection sensitivity of the optical fiber type temperature sensor can be adjusted. According to a third aspect of the present invention, in the optical fiber temperature sensor according to the first aspect, the composition of the stress-applying portion is changed to adjust the thermal expansion coefficient difference between the stress and the glass constituting the clad, thereby providing the long-period fiber. It is characterized by adjusting the temperature dependence of the loss characteristic of the grating. Thereby, the temperature detection sensitivity of the optical fiber type temperature sensor can be adjusted.

According to a fourth aspect of the present invention, in the optical fiber type temperature sensor according to the first, second or third aspect, the change amount of the birefringence of the polarization maintaining optical fiber with respect to temperature is -6.5 × 1.
It is characterized by being 0 ^-7 (1 / ° C) or less. As a result, a more sensitive optical fiber type temperature sensor can be realized. The invention according to claim 5 is the invention according to claims 1, 2, and 3.
Alternatively, in the optical fiber type temperature sensor described in 4, the polarization maintaining optical fiber is a PANDA fiber.

The invention according to claim 6 is the invention according to claims 1, 2 and
In the optical fiber type temperature sensor according to 3, 4 or 5,
Of the two polarized waves, only the f-polarized wave, which is a polarized wave having an electric field in the fast axis direction having a high propagation speed, is incident on the long-period fiber grating to detect a temperature change. As a result, the temperature dependence of the residual stress and the temperature dependence of the refractive index of the optical fiber material itself are added together, and the change in the loss characteristics due to temperature changes becomes sensitive. Can be realized.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below.
The optical fiber type temperature sensor of the present invention is provided with a high refractive index portion which is periodic in the longitudinal direction of the polarization maintaining optical fiber having a stress imparting portion for applying a non-axisymmetric stress to the core and has a long period. A fiber grating is formed, and a change in the loss characteristic of the long-period fiber grating with respect to temperature is detected to detect the temperature change. FIG. 1 shows an example of the structure of a long-period fiber grating used as the optical fiber type temperature sensor of the present invention. In FIG. 1, reference numeral 1 is a core, and a clad 2 is provided around the core 1. Reference numeral 3 is a high-refractive index portion formed on the core 1 by irradiation with ultraviolet light. The high refractive index portion 3 is periodically formed in the longitudinal direction of the optical fiber to form the grating portion 4. In this long-period fiber grating, the guided mode light 5 having a specific wavelength is combined with the forward cladding mode light 6 to obtain transmitted light having a transmission loss in this wavelength band.

In the optical fiber type temperature sensor of this example, the long-period fiber grating is formed by irradiating the stress imparting polarization maintaining optical fiber with ultraviolet light. FIG. 2 shows an example of a cross section of the stress imparting polarization maintaining optical fiber in the longitudinal direction. In FIG. 2, reference numeral 1 is a core, and reference numeral 2 is a clad provided around the core 1. Reference numeral 7 is a stress imparting portion symmetrically provided with the core 1 interposed therebetween. In an axially symmetric single mode optical fiber, there are two independent modes polarized in two directions orthogonal to each other in the optical fiber cross section. In a true axisymmetric optical fiber, there is no difference in effective refractive index between these two modes,
There is literally only a single mode, which is said to be degenerate.

However, in an actual optical fiber, there is a slight difference in the effective refractive index between these two modes, so that there is an effect on optical communication utilizing polarization and interference. The polarization-maintaining optical fiber has been proposed to solve such a problem, and is an optical fiber in which the difference in effective refractive index is intentionally increased to improve polarization stability. The polarization-maintaining optical fiber includes a core 1 and a refractive index distribution type in which the refractive index distribution in the vicinity of the core 1 is non-axisymmetric, and a stress providing portion for applying a non-axisymmetric stress to the core 1 is provided. There is a mold. This stress type utilizes the fact that birefringence (difference in effective refractive index) is induced by the photoelastic effect, has higher performance than the gradient index type, and is now common. Hereinafter, this is referred to as a stress type polarization maintaining optical fiber.

The optical fiber shown in FIG. 2 is a PANDA (polarization-type) as an example of a stress type polarization maintaining optical fiber.
maintaining and absorption-reducing) fiber. The stress remaining in the stress-type polarization-maintaining optical fiber is generated by the strong contraction of the stress applying part 7 during the cooling process of the optical fiber. The difference in the coefficient of thermal expansion of is used. In the core 1 and the vicinity of the core 1, tension is exerted in the x-axis direction and a compressive force is exerted in the y-axis direction due to this residual stress, and the effective refractive index of each polarized wave propagating due to the photoelastic effect changes. This difference in effective refractive index, that is, the birefringence B can be approximated by the equation (1).

[0015]

[Equation 1]

Here, n _ex and n _ey are polarized waves having an electric field in the x-axis direction (hereinafter referred to as “x-polarized wave”) and polarized waves having an electric field in the y-axis direction (hereinafter referred to as “y-polarized wave”). ) Represents the effective refractive index. Further, C ₁ and C ₂ represent photoelastic coefficients, and σ _x −σ _y represents a stress difference between the x-axis direction and the y-axis direction. In the case of quartz glass, C ₂ −C ₁ = 3.36 × 10 ^-5 . If tension acts in the x-axis direction and compressive force acts in the y-axis direction, x
The effective index of polarization is greater than the effective index of y polarization, so that B has a positive value. In stress-type polarization-maintaining optical fibers, the propagation speed of x-polarization is slower than that of y-polarization, so x
The axis is called the slow axis and the y axis is called the fast axis. A polarized wave having an electric field in the slow axis direction is called an s polarized wave, and a polarized wave having an electric field in the fast axis direction is called an f polarized wave.

Next, the optical characteristics of the LPFG manufactured by using the stress type polarization maintaining optical fiber will be described. When the LPFG is manufactured on a general single mode optical fiber, the transmitted light has a single peak type loss peak as shown in FIG. However, when an LPFG is produced on a polarization maintaining optical fiber and non-polarized light is incident, two effective peaks are given because the effective refractive index differs depending on the polarization of the incident light. The relationship between the central loss wavelength λ _p and the effective refractive index n _e when light enters the LPFG is shown in equation (2).

[0018]

[Equation 2]

In the equation (2), Λ represents the cycle of the formed refractive index change, and n _e1 and n _en represent the effective refractive indexes of the waveguide mode and the cladding mode, respectively. In the stress-type polarization-maintaining optical fiber, since the effective refractive index of the guided mode and the cladding mode are different depending on the polarization, the loss center wavelength is different. Here, _assuming that the difference between the effective refractive indices due to the polarization of the guided mode and the polarization of the cladding mode is B ₁ and B _n , the s polarization and the f
The polarization loss center wavelength Δλ _p can be expressed by equation (3).

[0020]

[Equation 3]

FIG. 4 shows the measurement results of the loss characteristics of LPFG produced in the PANDA fiber for the wavelength band of 1.55 μm. The formed high refractive index portion has a period of 338 μm, which is 110 periods. B ₁ is B
_Since it is larger than _n, the loss peak when the s polarization is incident appears on the longer wavelength side than the loss peak when the f polarization is incident. In this way, when the LPFG is fabricated on the stress-type polarization-maintaining optical fiber, the loss center wavelength when s-polarized light is incident shifts to the longer wavelength side as compared to the case where no stress is exerted, and f-polarized The loss center wavelength of the wave shifts to the short wavelength side. Here, when the temperature rises, the residual stress changes so as to decrease, so that the loss center wavelengths of the respective polarized waves shift toward each other. That is, the loss center wavelength when the s-polarized light is incident is shifted to the short wavelength side, and the loss center wavelength when the f-polarized light is incident is shifted to the long wavelength side. The optical fiber type temperature sensor in this example uses the fact that the loss characteristic for polarized waves changes with temperature, and the monitor light is incident on the long-period fiber grating fabricated on the polarization maintaining optical fiber. , Detects the change in temperature by measuring the change in the transmission loss.

On the other hand, when the LPFG is manufactured on the standard single mode fiber (SMF) or the dispersion shift fiber (DSF), the loss center wavelength changes due to the temperature change.
This is mainly due to the change in the refractive index of the material of the optical fiber itself. Therefore, we made a stress-type polarization-maintaining optical fiber.
In LPFG, there are two temperatures: the temperature dependence of the residual stress (abbreviated as “temperature dependence A” below) and the temperature dependence of the refractive index of the optical fiber material itself (hereinafter abbreviated as “temperature dependence B”). Due to the dependency, the central loss wavelength changes.

As described above, the temperature dependence A acts in the opposite direction with respect to the loss center wavelengths of the two incident polarized waves.
On the other hand, the temperature dependence B acts equally regardless of polarization. Therefore, these two changes act in a direction in which one polarized wave is added and in a direction in which they cancel each other out. Since the temperature dependence B generally has a positive slope with respect to temperature, the two temperature dependences normally work in a direction in which they are added when f-polarized light is incident. In the case of a polarized wave that works in a direction in which two temperature dependences are added like this f-polarized wave, L
The temperature dependence of the central loss wavelength is greater than that in the case of manufacturing a PFG, and it can be used as a more sensitive temperature sensor.
Also, by increasing the residual stress at room temperature,
It is possible to further increase the stress change due to temperature.

In the stress-type polarization-maintaining optical fiber, the residual stress in the core 1 and the vicinity of the core 1 can be changed by changing the position and size of the stress applying portion 7. Therefore, it is possible to increase the stress by enlarging the stress applying portion 7 or bringing the position of the stress applying portion 7 closer to the core 1. The stress can also be increased by changing the composition of the stress applying portion 7 to increase the difference in coefficient of thermal expansion with the clad glass. As described above, the temperature dependence of the loss center wavelength of the long-period fiber grating can be adjusted by adjusting the size of the stress applying section 7 or adjusting the distance between the stress applying section 7 and the core 1. The temperature dependence of the loss center wavelength of the long-period fiber grating can also be adjusted by changing the composition of the stress-applying portion and adjusting the difference in the coefficient of thermal expansion from the glass forming the cladding.

In the above description, the case where a PANDA fiber is used as the stress-type polarization maintaining optical fiber has been described, but the stress-type polarization maintaining optical fiber that can be used is not limited to this, and Bow can be used. -Tie fiber or elliptical jacket fiber may be used. By manufacturing a long-period fiber grating using these polarization-maintaining optical fibers, an optical fiber type temperature sensor with high measurement accuracy and a wide measurable temperature range can be obtained. Specific examples will be shown below.

(Example) PANDA for wavelength 1.55 μm band
FIG. 5 shows the result of measuring the temperature dependence of the transmission loss characteristic of this LPFG, which was manufactured by using the fiber.
The measurement temperature is -20 ° C, 0 ° C, 25 ° C, 40 ° C, 70 ° C.
Is. As can be seen from FIG. 5, when the f-polarized wave is incident, the loss center wavelength shifts to the long wavelength side as the temperature shifts from the low temperature to the high temperature, and when the s-polarized wave is incident, the loss center wavelength slightly changes. It is shifted to the short wavelength side. FIG. 6 shows a measurement system used to measure the temperature dependence of the transmission loss characteristic of this LPFG. In FIG. 6, reference numeral 11 is a light source, and this light source 11 is connected to one end of a single mode optical fiber 12. The other end of the single mode optical fiber 12 is connected to the input end of the polarizer 13, and the polarization maintaining optical fiber 14 is connected to the output end of the polarizer 13. This polarization-maintaining optical fiber 14 is an LPFG 1 arranged in a thermostat 16.
5 is connected to the input terminal. A spectrum analyzer (OSA) 17 is connected to the output end of the LPFG 15 via a polarization maintaining optical fiber 14.

In this measurement system, the light sent from the light source 11 is polarized and separated by the polarizer 13, and only the f-polarized wave propagates through the polarization-maintaining optical fiber 14 and enters the LPFG 15. Here, a PANDA fiber is used between the polarizer 13 and the spectrum analyzer 17.
Further, a normal single mode optical fiber is used between the light source 11 and the polarizer 13, but a PANDA fiber may be used. Further, when measuring the temperature dependence of the s-polarized wave, 90 ° twist fusion was performed. Table 1 shows the amount of change in the loss center wavelength when each polarized wave is incident.

[0028]

[Table 1]

The central loss wavelength changes linearly with temperature change, and when f-polarized light is incident, +
A large change amount of the loss center wavelength of about 0.16 nm / ° C is obtained. The temperature error is ± 0 when the wavelength accuracy is 0.02 nm.
It is within 13 ℃, and the sensitivity is higher than that of the temperature sensor using FBG.

As described above, since the temperature dependence of the loss characteristic of the LPFG can be increased by bringing the stress-applying portion of the polarization-maintaining optical fiber to be used closer to each other, this method can be used to increase the temperature dependence to 1.55. A high birefringence PANDA fiber having a larger birefringence than the PANDA fiber for the μm band was produced, LPFG was formed on this high birefringence PANDA fiber, and the temperature dependence of its loss characteristics was investigated. This PANDA
The birefringence of the fiber is −20 to 70 ° C.
It has a change with temperature of about 6.5 × 10 ^-7 (1 / ° C).
Table 1 shows the temperature dependence of the loss characteristic when the f-polarized wave is incident on the LPFG.

In FIG. 7 and FIG. 8, L produced in this way is shown.
The transmission loss characteristic of PFG and the temperature dependence of the loss center wavelength are shown. The formed high refractive index portion has a period of 354 μm.
The central loss wavelength is about +0.22 nm / ° C with respect to temperature,
It has a larger change amount with respect to temperature change. The temperature error is ± 0.09 ℃ when the wavelength accuracy is 0.02nm, and the temperature error is ± 0.23 ℃ even when the wavelength accuracy is 0.05nm.
Higher sensitivity than the temperature sensor using is obtained. In this way, if the amount of change in birefringence is −6.5 × 10 ⁻⁷ (1 / ° C.) or less, a higher sensitivity than a temperature sensor using an FBG can be obtained with respect to temperature change.

According to the optical fiber type temperature sensor of this example, the high refractive index which is periodic in the longitudinal direction of the polarization maintaining optical fiber having the stress applying portion 7 for applying the stress which is not axisymmetric to the core 1. By providing the portion 3 to form a long-period fiber grating, and detecting a change in the loss characteristic of this long-period fiber grating with respect to temperature, and detecting the temperature change, the loss is sensitive to temperature and loss in a wide temperature range. Since the temperature change can be detected using the polarized wave whose characteristics change, the temperature change can be detected with high accuracy and in a wide temperature range. Further, by adjusting the size of the stress applying portion 7 or adjusting the distance between the stress applying portion 7 and the core 1, the temperature dependency of the loss characteristic of the long-period fiber grating is adjusted to improve the temperature detection sensitivity. Can be adjusted. Such adjustment of the temperature detection sensitivity can also be performed by changing the composition of the stress applying portion 7 and adjusting the difference in the coefficient of thermal expansion from the glass forming the cladding 2.

Further, by setting the amount of change in birefringence of the polarization-maintaining optical fiber with respect to temperature to be −6.5 × 10 ⁻⁷ (1 / ° C.) or less, an optical fiber type temperature sensor with higher sensitivity can be realized. it can. Further, by detecting the temperature change by injecting only the f-polarized wave into the long-period fiber grating, the temperature dependence of the residual stress and the temperature dependence of the refractive index of the optical fiber material itself are added, and the temperature change Since the change in the loss characteristic becomes sensitive, it is possible to realize a more sensitive optical fiber type temperature sensor.

[0034]

As described above, according to the present invention,
A long-period fiber grating is formed by providing a high-refractive-index portion that is periodic in the longitudinal direction of a polarization-maintaining optical fiber having a stress-applying portion for applying a non-axisymmetric stress to a core. By detecting the change in the loss characteristic of the grating with respect to temperature and detecting the change in temperature, it is possible to detect the change in temperature using polarization that is sensitive to temperature and changes the loss characteristic in a wide temperature range. Therefore, it is possible to detect a temperature change with high accuracy and in a wide temperature range. Also, by adjusting the size of the stress applying portion, or by adjusting the spacing between the stress applying portion and the core,
The temperature detection sensitivity can be adjusted by adjusting the temperature dependence of the loss characteristic of the long-period fiber grating. Such adjustment of the temperature detection sensitivity can also be performed by changing the composition of the stress applying portion and adjusting the difference in the coefficient of thermal expansion from the glass forming the cladding.

Further, by setting the amount of change in birefringence of the polarization-maintaining optical fiber with respect to temperature to be −6.5 × 10 ⁻⁷ (1 / ° C.) or less, it is possible to realize a more sensitive optical fiber type temperature sensor. it can. Further, by detecting the temperature change by injecting only the f-polarized wave into the long-period fiber grating, the temperature dependence of the residual stress and the temperature dependence of the refractive index of the optical fiber material itself are added, and the temperature change Since the change in the loss characteristic becomes sensitive, it is possible to realize a more sensitive optical fiber type temperature sensor.

[Brief description of drawings]

FIG. 1 is a diagram showing a side section of a long-period fiber grating used as an optical fiber type temperature sensor of the present invention.

FIG. 2 is a diagram showing an example of a cross section in the longitudinal direction of a polarization maintaining optical fiber used in a long period fiber grating used as an optical fiber type temperature sensor of the present invention.

FIG. 3 is a diagram showing a loss spectrum when a long-period fiber grating is manufactured using a normal single-mode optical fiber.

FIG. 4 is a diagram showing loss spectra of s-polarized light and f-polarized light when a long-period fiber grating is manufactured using a polarization-maintaining optical fiber.

FIG. 5 is a diagram showing temperature dependence of loss characteristics of the optical fiber type temperature sensor of the present invention.

FIG. 6 is a diagram showing a measurement system for evaluating the characteristics of the optical fiber type temperature sensor of the present invention.

[Fig. 7] Birefringence is -6.5 in the temperature range of -20 to 70 ° C.
High birefringence PAND with a variation of about 10 ^-7 (1 / ° C)
It is a figure which shows the temperature dependence of the loss characteristic at the time of producing a long period fiber grating using A fiber.

FIG. 8 is a diagram showing the temperature dependence of the loss center wavelength of the optical fiber type temperature sensor of the present invention.

[Explanation of symbols]

1 ... Core, 2 ... Clad, 3 ... High refractive index part, 4 ... Grating part, 5 ... Guided mode light, 6 ... Forward clad mode light, 7 ... Stress applying part.

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Claims (6)

[Claims] 1. A long-period optical fiber grating is formed by providing a high-refractive-index portion that is periodic in the longitudinal direction of a polarization-maintaining optical fiber that has a stress-applying portion for applying non-axisymmetric stress to a core. An optical fiber type temperature sensor characterized by detecting a change in the loss characteristic of this long-period fiber grating with respect to temperature and detecting the temperature change. 2. The temperature dependence of the loss characteristics of the long-period fiber grating is adjusted by adjusting the size of the stress applying section or adjusting the distance between the stress applying section and the core. The optical fiber type temperature sensor according to claim 1. 3. The temperature dependence of the loss characteristic of the long-period fiber grating is adjusted by changing the composition of the stress-applying portion and adjusting the difference in the coefficient of thermal expansion with the glass forming the cladding. The optical fiber type temperature sensor according to claim 1. 4. The optical fiber according to claim 1, wherein the change amount of birefringence of the polarization-maintaining optical fiber with respect to temperature is −6.5 × 10 ⁻⁷ (1 / ° C.) or less. Mold temperature sensor. 5. The polarization maintaining optical fiber is a PANDA fiber.
The optical fiber type temperature sensor described. 6. The fast having the highest propagation speed of the two polarized waves
6. The optical fiber type according to claim 1, wherein only the f-polarized wave which is a polarized wave having an electric field in the axial direction is incident on the long-period fiber grating to detect a temperature change. Temperature sensor.