JP2000088676A - Temperature sensor - Google Patents

Abstract

(57) [Problem] To provide a temperature sensor capable of performing a stable operation for a long time by suppressing a change over time of an operation characteristic. A temperature sensor according to the present invention has a predetermined operating temperature range, an operation guarantee time, and a temperature resolution, and includes an optical waveguide grating as a sensing unit. Optical waveguide gratings are
Accelerated aging is performed under predetermined conditions. The aging condition is determined such that when the temperature sensor is used at the highest operating temperature in the operating temperature range for the operation assurance time, the temperature deviation of the temperature sensor due to the deterioration over time of the optical waveguide type grating is equal to or less than the temperature resolution. ing. Since the optical waveguide type grating subjected to accelerated aging is used as the sensing unit, the temperature sensor according to the present invention can sufficiently suppress the fluctuation of the operating characteristics due to the deterioration with time of the optical waveguide type grating.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a temperature sensor used for temperature measurement, and more particularly to a temperature sensor having an optical waveguide type grating as a sensing unit.

[0002]

2. Description of the Related Art An optical waveguide grating typified by an optical fiber grating periodically refracts a certain region (often a core region) in an optical waveguide such as an optical fiber along a longitudinal direction of the optical waveguide. Rate change is given. This region where the refractive index changes is called a grating and can transmit or reflect propagating light according to the wavelength. In particular, a Bragg grating generates reflected light having sharp wavelength selectivity centered on light having a Bragg wavelength.

FIG. 12 shows a typical method of manufacturing such an optical waveguide grating. As shown in FIG. 12, the grating 20 is formed by preparing a silica-based optical fiber 10 in which Ge element oxide (GeO ₂ ) is added to at least the core region, and using light 30 having a predetermined wavelength. By irradiating this optical fiber 10 with interference fringes,
It is formed by causing a change in refractive index according to the light energy intensity distribution of the interference fringes. The optical fiber 10
Normally, the optical fiber 10 is covered with a plastic layer (not shown), so that a predetermined portion of the coating is removed and the exposed portion of the optical fiber 10 is irradiated with light 30.
In FIG. 12, a portion having a large increase in the refractive index due to light irradiation is indicated by reference numeral 22, and a portion having a small increase in the refractive index is indicated by reference numeral 2.
4. The grating 20 has this part 2
2 and 24 can be considered as regions arranged alternately and periodically along the longitudinal direction of the optical fiber 10. As described above, the manufacture of the optical fiber grating is completed. In the following, reference numeral 10 may be used to represent the optical fiber grating manufactured as described above.

[0004] One of the uses of such an optical waveguide grating is a temperature sensor. This temperature sensor includes an optical waveguide type Bragg grating as a sensing unit, and measures temperature by utilizing the fact that the Bragg wavelength has temperature dependency. More specifically, in this temperature sensor, the Bragg wavelength is measured at the time of temperature measurement, and the temperature is obtained by comparing the measured value with the previously measured temperature dependence of the Bragg wavelength.

[0005]

As is known in the art, the characteristics of an optical waveguide grating change over time. This is known as deterioration with time of the optical waveguide type grating. In the case of the Bragg grating, the Bragg wavelength for an arbitrary temperature changes with the passage of time (usually, it becomes shorter uniformly). This means that the operating characteristics of the temperature sensor having the optical waveguide type Bragg grating as the sensing unit change over time. If such a change with time of the Bragg wavelength is rapid, for example, one month and three months after the start of use of the temperature sensor, different Bragg wavelengths are measured for the same temperature, and as a result, different temperatures are measured. Will be done.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a temperature sensor capable of suppressing a change over time in operating characteristics and performing stable operation for a long time.

[0007]

SUMMARY OF THE INVENTION A temperature sensor according to the present invention has a predetermined operating temperature range, an operation guarantee time, and a temperature resolution, and includes an optical waveguide type grating as a sensing unit. The optical waveguide grating is obtained by accelerated aging under predetermined conditions. The aging condition is determined such that when the temperature sensor is used at the highest operating temperature in the operating temperature range for the operation assurance time, the temperature deviation of the temperature sensor due to the deterioration over time of the optical waveguide type grating is equal to or less than the temperature resolution. ing. Since the optical waveguide grating subjected to accelerated aging under such aging conditions is used as the sensing unit, the temperature sensor according to the present invention can sufficiently suppress the fluctuation of the operating characteristics due to the aging deterioration of the optical waveguide grating. become.

[0008] The above aging condition is determined by using the time t and the parameters A and α depending on the temperature as A · t.
^- it can be determined by using a time degradation curve of the optical waveguide grating represented by alpha. The parameter α is

(Equation 3) Can be expressed as Since these expressions can express the deterioration over time of the optical waveguide grating with sufficient accuracy, determining the aging conditions using these expressions will ensure that the temperature deviation of the temperature sensor can be suppressed below the temperature resolution. it can.

Further, the parameter A is

(Equation 4) Can be expressed as This equation can be suitably used when expressing the deterioration with time of the optical waveguide grating in a high temperature range. Therefore, if the aging condition is determined using this equation, the temperature deviation of the temperature sensor can be reliably suppressed to below the temperature resolution even when the temperature sensor according to the present invention is used at a relatively high temperature.

[0010] The optical waveguide grating provided in the temperature sensor according to the present invention may have a surface etched after accelerated aging. Since the surface of the optical waveguide grating can be removed by etching, the risk of damage to the optical waveguide due to thermal deformation, such as when the temperature sensor is installed in an environment with temperature fluctuations, can be reduced.

Further, the optical waveguide type grating may be provided with a heat-resistant coating material on its surface. The temperature sensor provided with such an optical waveguide grating can be suitably used even at a relatively high temperature.

[0012]

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings. In the following, FIG.
And the optical fiber 10 shown in FIG. 12 as an optical waveguide including a Bragg grating portion, as an optical fiber Bragg grating or an optical fiber grating 10.
Call.

FIG. 1 is an overall configuration diagram of the temperature sensor of the present embodiment. The temperature sensor shown in FIG. 1 includes an optical fiber Bragg grating 10 as a sensing unit,
An optical spectrum measuring system 5 is connected to the optical fiber grating 10 so that the transmitted light spectrum of the Bragg grating unit 20 included in the optical fiber grating can be measured.

The optical spectrum measuring system 5 includes a light source 1
00, spectrum analyzer 300, computer 4
00, and a computer display 500. The light source 100 generates and outputs light over a predetermined wavelength band. One end 21 of the optical fiber grating 10 is optically connected to the light source 100,
0 is incident on the optical fiber grating 10. The optical fiber grating 10
A single-mode optical fiber having a circular cross-section core and a fixed portion 2 of the optical fiber grating 10.
At 0, a Bragg grating is formed. The grating section 20 reflects narrow-band light centered on the Bragg wavelength among the light propagated through the optical fiber grating 10. The light source 100 is configured to output light in a wavelength band including the Bragg wavelength. A spectrum analyzer 300 is optically connected to the end 23 of the optical fiber grating 10,
Out of the light transmitted through the Bragg grating unit 20 is incident on the spectrum analyzer 300. A computer 400 is electrically connected to the spectrum analyzer 300, and the output of the spectrum analyzer 300 is sent to the computer 400. A computer display 500 is electrically connected to the computer 400, and a calculation result of the computer 400 is displayed on the display 500.

In this temperature sensor, light is output from the light source 100 at the time of temperature measurement, and the spectrum of the light transmitted through the grating unit 20 is converted to a spectrum analyzer 300.
Is detected by This detection data is stored in computer 4
00, the computer 400 calculates the Bragg wavelength of the grating section 20 based on this data. The computer 400 has a built-in hard disk drive in which a function representing the temperature dependence of the Bragg wavelength measured in advance is stored. After calculating the Bragg wavelength, the value is substituted into the temperature dependence function to calculate the temperature. Is calculated. This temperature value is displayed on the computer display 500, so that the user of the temperature sensor can know the measured temperature value.

In this embodiment, the temperature sensor is configured to determine the Bragg wavelength by detecting the transmitted light spectrum of the grating unit 20, but the Bragg wavelength is determined by detecting the reflected light spectrum of the grating unit 20. The required configuration may be adopted. Specifically, an optical coupler is provided between the light source 100 and the grating unit 20, and the spectrum analyzer 300, the computer 400, and the computer display 500 are sequentially connected to the optical coupler. By doing so, the Bragg reflected return light of the grating section 20 can be guided to the spectrum analyzer 300 via the optical coupler,
The Bragg wavelength can be obtained by detecting the spectrum of the reflected return light.

The optical fiber grating 10 used in this embodiment has been subjected to accelerated aging. Accelerated aging is a process in which a heat treatment is usually performed after the production of an optical waveguide grating to accelerate and progress the aging degradation of the optical waveguide grating. Normally, the deterioration with time is sharper at a point near the completion of the manufacture of the optical waveguide grating, and becomes monotonically gradual as time elapses from the manufacture. Therefore, by performing accelerated aging and forcing deterioration with time to proceed, it is possible to avoid rapid deterioration with time and to stabilize the operating characteristics of the optical waveguide grating.

In order to execute accelerated aging of the optical fiber grating 10, it is necessary to determine the aging conditions, specifically, the temperature and time of the heat treatment in advance. Hereinafter, a method for determining such an aging condition will be described.

In the present embodiment, an equation representing a temporal change of the normalized refractive index difference η of the optical fiber grating (this parameter will be described later) is as follows:

(Equation 5) Is assumed. Here, t represents time, and A and α are each a function of temperature. The normalized refractive index difference η is
Predetermined time after forming grating part (reference time)
This is a value obtained by normalizing the difference in the refractive index of the grating portion after a lapse of time t from the difference in the refractive index of the grating portion at this reference time. That is, η =
(Refractive index difference after elapse of time t from reference time) / (refractive index difference at reference time). Here, the difference in the refractive index indicates the difference between the maximum value and the minimum value of the refractive index of the grating portion.
In many cases, if the refractive index difference is measured at a sufficiently short time interval after the formation of the grating portion, the change over time of the refractive index difference is sufficiently small. It can be considered equal to the refractive index difference at completion. Therefore, the normalized refractive index difference η using the refractive index difference thus measured as a standard for standardization is (the refractive index difference after a lapse of time t from the completion of the formation of the grating portion).
/ (Refractive index difference at the completion of the formation of the grating portion).

FIGS. 2 and 3 are graphs showing measured values of the normalized refractive index difference η of the grating portion at various temperatures and showing curves obtained by fitting the measured values by the equation (1). As shown in FIG. 12, the grating portion used for the measurement was written in the optical fiber by irradiating the silica-based optical fiber with GeO ₂ added to the core with ultraviolet light.

More specifically, FIG.
FIG. 3 shows the time-dependent deterioration of the normalized refractive index difference η of the grating portion at 0 ° C., 220 ° C., and 280 ° C., respectively.
The time-dependent deterioration of the normalized refractive index difference η of the grating portion at each 0 ° C. is shown over a longer period. In these figures, each point in the figures shows the measured value of the normalized refractive index difference η, and the solid line is a curve obtained by fitting the measured value with the equation (1). The normalized refractive index difference η is
The refractive index difference Δn of the grating can be determined by normalizing the refractive index difference Δn0 measured after writing the grating and before heating the optical fiber grating to the above temperature. That is,

(Equation 6) It is.

As shown in these figures, the measured values and the fitting curve are in very good agreement. In particular,
At a temperature of 100 ° C. or higher, the correlation coefficient between the actually measured value and the curve of the equation (2) is 0.94 or higher, and good results are obtained. As described above, it is understood that the temperature dependence and the time dependence of the normalized refractive index difference η are suitably represented by the expression (1).

FIG. 4 is a table showing parameters A and α for the fitting curves shown in FIGS. 2 and 3 at respective temperatures. Each column of this chart indicates, in order, a temperature in degrees Celsius, an absolute temperature, a reciprocal of the absolute temperature, a parameter α, and a parameter A. As shown in this table, the parameter A has a constant value (almost 1) regardless of the temperature.

Next, the temperature dependence of the parameter α will be considered. FIG. 5 is a graph showing the relationship between the parameter α obtained by the above measurement and the absolute temperature T. In this figure, the vertical axis represents the parameter α in logarithmic notation, and the horizontal axis represents the reciprocal (1 / T) of the absolute temperature T. The upper scale of this figure also shows the temperature in degrees Celsius for reference. As can be seen from this figure, the parameter α well follows the Arrhenius law for the absolute temperature T. Therefore,
The parameter α is expressed by the following general formula

(Equation 7) Can be approximately expressed by In the above equation, α0
And Eα are temperature independent constants, and T is the absolute temperature. According to the result of the above measurement, α0 is 2.791
4. Eα is 1963.2. Therefore, 16.32 kJ / mol is obtained as the activation energy.

FIGS. 6 and 7 are graphs showing the change over time of the normalized refractive index difference η of the grating measured over a wider temperature range than those shown in FIGS. Specifically, FIG.
7 shows the time-dependent deterioration of the normalized refractive index difference η at 140 ° C., 140 ° C., 170 ° C., 220 ° C. and 280 ° C., respectively.
Shows the time-dependent deterioration of the normalized refractive index difference η at the temperatures of 400 ° C., 500 ° C., 600 ° C., 700 ° C., and 800 ° C. In these figures, each point in the figures is
The measured value of the normalized refractive index difference η is shown, and the solid line is a line obtained by fitting the measured value by the equation (1). Since the vertical and horizontal axes in FIGS. 6 and 7 are expressed in logarithms, the lines obtained by fitting the measured values are straight lines.

FIG. 8 is a graph showing the relationship between the parameters A and α corresponding to the fitting lines in FIGS. 6 and 7 and the temperature. As shown in this graph, about 30
At temperatures below 0 ° C., parameter A is substantially independent of temperature and is approximately unity. Α well follows the Arrhenius law with respect to the absolute temperature T, and can be expressed by the above equation (3). According to the measured values in FIGS. 6 and 7,
α0 is 1.4131 and Eα is 1633. These values are different from the above values obtained from the actually measured values in FIGS. 2 and 3 because the temperature range in which the fitting is performed is different.

As shown in FIGS. 2 to 8, the above (1)
The equation can properly express the aging of the optical fiber grating over a wide temperature range from 75 ° C to 800 ° C. The parameter α included in the equation (1) is
In this temperature range, an Arrhenius-type temperature dependence is exhibited.

In the case where the deterioration over time of the optical fiber grating is represented by the above equation (1), at a temperature of 300 ° C. or less, the parameter A is a constant value having no temperature dependency (here, 1).
Can be considered. However, if it is necessary to consider temperatures above 300 ° C.,
It is not appropriate to think. Therefore, the present inventor proposes that η
Is a non-dimensional parameter, and a parameter τ having a time dimension is introduced to express as A = τα. That is,

(Equation 8) It is.

The parameter τ shows an Arrhenius-type temperature dependence in the same temperature range as the parameter α in the temperature range of 75 ° C. to 800 ° C. where the inventor performed the measurement. That is, τ
Is represented by the following equation.

[0030]

(Equation 9) Here, τ0 and Eτ are constants independent of temperature.

FIG. 9 is a graph showing the relationship between the parameter α and the temperature, and FIG. 10 is a graph showing the relationship between the parameter τ and the temperature. Each point in each figure is α
And τ are shown, and the solid line is a line obtained by fitting the measured values by the equations (3) and (5), respectively. In FIG. 9, α0 = 1.4131, Eα = 163
3.3, and τ0 = 3.358e−6 in FIG.
(Min), Eτ = −6483.4. Note that the measured values of τ shown in FIG.
= Τα.

When it is necessary to use a temperature sensor in a high-temperature environment where it is inappropriate to regard the parameter A of the equation (1) as a constant value, the time-dependent deterioration curve of the optical waveguide type grating is expressed by the equation (4). It is appropriate to determine the aging condition in accordance with

FIG. 11 is a diagram for explaining a method of predicting the deterioration with time after performing the accelerated aging using the equation (4). In this figure, T1 represents the temperature of the heat treatment for accelerated aging, and T2 represents the upper limit of the operating temperature of the temperature sensor, that is, the maximum operating temperature.
In this embodiment, T1> T2. 1 in FIG.
The solid line shows the change over time of the normalized refractive index difference η when accelerated aging is performed on the optical fiber grating at the heating temperature T1 and then the optical fiber grating is placed in the environment of the maximum operating temperature T2. Further, two dashed lines in FIG. 11 indicate the change over time of the normalized refractive index difference η when the optical fiber grating is placed in the environment of the temperatures T1 and T2, respectively, after the formation of the grating portion. The results predicted based on equations (3) and (5) are shown.

First, a value η1 of the normalized refractive index difference when the accelerated aging (heating treatment under the heating temperature T1 and the heating time t1) is completed is predicted. This η1 is calculated by the above (4)
The next expression based on the expression

(Equation 10) Can be obtained by using Where τ _T1 and α _T1
Is, according to equations (5) and (3), respectively:

[Equation 11] It is represented as

Subsequently, when it is assumed that the optical fiber grating is placed in the environment of the maximum operating temperature T2 without aging after the formation of the grating portion, the time required for the normalized refractive index difference η to become the above value η1. t2 (FIG. 11)
See). The value of the time t2 can be obtained from the following equation.

[0036]

(Equation 12) This equation is based on the definition of the time t2 described above and the above (3) to
It can be derived from equation (5).

The normalized refractive index difference η shown by the solid line in FIG.
Before the time t1 coincides with the temporal change at the temperature T1, and after the time t1 (after the aging treatment), after the normalized refractive index difference η becomes η1 among the temporal changes at the temperature T2. , Ie, a change after time t2. For this reason, if the amount of change in η occurring over time from time t2 to time t2 + t3 (t3 is the operation guarantee time of the temperature sensor) of the change with time at temperature T2 is predicted,
This means that the amount of change in η with time when the operation assurance time t3 has elapsed under the maximum operating temperature T2 after aging has been completed is predicted.

Therefore, the amount of change η of η with the passage of the operation guarantee time t3 at the maximum operating temperature T2 is as follows:
Using equation (4),

(Equation 13) It is represented as here,

[Equation 14] It is. Further, t2 is as represented by the above equation (9). By using these equations (10) to (12), δη can be predicted.

On the other hand, from the equation (2),

(Equation 15) Holds. Here, δ (Δn) is the maximum operating temperature T2
Is the amount of change with time of the refractive index difference Δn of the grating part due to the elapse of the operation assurance time t3 under the following conditions. In the present embodiment, the Bragg wavelength λ _B is

(Equation 16) Think of it as: Here, Λ is the period of the grating portion, and n is the effective refractive index of the optical waveguide before the grating portion is formed. In this case, the time-dependent change amount δλ _B of the Bragg wavelength λ _B when the time t 3 has elapsed under the temperature T 2 is calculated by using the Bragg wavelength λ at the time t 2.
_When standardized by _B (t = t2),

[Equation 17] Becomes The above approximation took advantage of Δn being sufficiently small relative to n and that n is substantially invariant. Combining equations (13) and (15) gives:

(Equation 18) Becomes

There is a time-dependent shift in the temperature indicated by the temperature sensor corresponding to δλ _B. The temperature shift amount δT _d corresponding to δλ _B expressed by the equation (16) is obtained by examining the temperature dependence of the Bragg wavelength of the optical fiber grating after aging, and comparing this temperature dependence with δλ _B. You can ask. Specifically, when the temperature dependence of the Bragg wavelength is represented by a function T _d = aλ _B (where a is a constant), the temperature deviation amount δT _d is

[Equation 19] It is represented as

[0041] In this embodiment, as the temperature shift amount? T _d is equal to or less than the temperature resolution T _R of the temperature sensor, i.e.

(Equation 20) The aging condition is determined so that is satisfied. (18)
Substituting equations (16) and (17) into the equation,

(Equation 21) Becomes Each parameter constituting the right side of the equation (19) is
This is a predetermined value or a value that can be known in advance by measurement. This right side is the threshold value δη _th of δη. As described above, this threshold value δη _th is such that the temperature deviation amount when the temperature sensor of the present embodiment is placed in the environment of the maximum operating temperature for the operation guarantee time is equal to or less than the predetermined temperature resolution of the temperature sensor. It was decided to be.

In this embodiment, in determining the aging condition, the value of the normalized refractive index difference η1 at the time of completion of aging that satisfies the above equation (19) is obtained. here,
It should be noted that the left side of Expression (19) is expressed by using Expressions (10) to (12), and t2 appearing in Expression (10) is expressed by Expression (9). Next, accelerated aging of the optical waveguide grating, that is, heat treatment is performed until the normalized refractive index difference η reaches the value η1 determined in this manner. This can be realized by performing the heat treatment while monitoring the normalized refractive index difference η.
Further, the value of η1 obtained as described above can be obtained by using equation (6) and equations (7) and (8).
The values of 1 and T1 may be determined, and the heat treatment may be performed using these values as the heating time and the heating temperature, respectively. If the temperature sensor according to the present embodiment is configured using the optical waveguide grating that has been accelerated and aged as described above, it is possible to obtain a temperature sensor in which the amount of time-dependent temperature deviation is suppressed to the temperature resolution or less.

As described above, the temperature sensor of this embodiment is
Since the optical fiber grating subjected to such aging processing is provided as a sensing unit, the amount of temperature deviation over the operation guarantee time at the maximum operating temperature is suppressed to the temperature resolution or less, and the sufficient time over the operation guarantee time is obtained. Accuracy can be maintained. The optical fiber grating included in the temperature sensor according to the present embodiment is subjected to the accelerated aging process under the conditions determined based on the time-dependent deterioration curve with sufficient accuracy as expressed by the above equation (4). Therefore, the amount of temperature deviation of the temperature sensor can be surely suppressed to the temperature resolution or less.

The present invention is not limited to the above embodiment, and various modifications are possible. For example, the coating of the region including the grating portion of the optical waveguide type grating is removed (if the coating is removed during the formation of the grating as in the method shown in FIG. 1), the surface of the optical waveguide is removed. It is also possible to use an optical waveguide type grating from which surface flaws are removed by etching. If the optical waveguide grating is used as a temperature sensor after removing the surface damage in this way, the risk of damage to the optical waveguide due to thermal deformation, such as when the temperature sensor is installed in an environment with temperature fluctuations, is reduced. can do. In this case, sufficient reliability can be obtained even when the optical waveguide type grating is used as a sensing unit of a temperature sensor without providing a coating again.

When the temperature sensor is used at a relatively high temperature, it is preferable to cover the region including the grating portion of the optical waveguide type grating with a heat resistant material. As the material of the coating, those generally used in heat-resistant fibers and the like can be arbitrarily used,
For example, a polyimide resin or a metal can be used.

[0046]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The inventor of the present invention applied accelerated aging to an optical fiber grating having a reflectivity of 99.999% by applying heat at 600 ° C. for 24 hours. As a result, the difference in the refractive index of the grating was reduced, and the reflectance of the optical fiber grating was reduced to 30%. The temperature sensor shown in FIG. 1 is constituted by using the processed optical fiber grating, and this temperature sensor is placed in a temperature environment maintained at 450 ° C. (maximum operating temperature) or lower for one year (operation guarantee time). As a result, the amount of temperature deviation due to the change with time was 0.
It was measured to be 015 nm. The temperature dependence of the Bragg wavelength of an optical fiber grating is usually 0.01 to 0.0
Since it was 25 nm / ° C., a target temperature resolution of 1.5 ° C. could be secured.

[0047]

As described above in detail, the temperature sensor according to the present invention is determined such that the temperature deviation amount is equal to or less than the temperature resolution when the temperature sensor is used over the operation guarantee time at the maximum operating temperature. Since the optical waveguide type grating subjected to accelerated aging under the conditions is provided as a sensing unit, a change over time in operation characteristics is suppressed, and stable operation can be performed for a long period of time.

[Brief description of the drawings]

FIG. 1 is a schematic diagram illustrating a configuration of a temperature sensor according to an embodiment.

FIG. 2: Temperatures of 120 ° C., 170 ° C., 220 ° C. and 28
It is a graph which shows the time-dependent degradation of the normalized refractive index difference η of the grating at each 0 ° C.

FIG. 3. Temperatures of 75 ° C., 85 ° C., 100 ° C. and 120 ° C.
4 is a graph showing the time-dependent deterioration of the normalized refractive index difference η of the grating in each case.

FIG. 4 is a table showing parameters A and n for the fitting curves shown in FIGS. 2 and 3 at respective temperatures.

FIG. 5 is a graph showing a relationship between a parameter α and a temperature.

FIG. 6: Temperatures of 100 ° C., 120 ° C., 140 ° C., 170
It is a graph which shows the time-dependent deterioration of the normalization refractive index difference (eta) of the grating in each of ℃, 220 ℃ and 280 ℃.

FIG. 7: temperature 400 ° C., 500 ° C., 600 ° C., 700 ° C.
6 is a graph showing the time-dependent deterioration of the normalized refractive index difference η of the grating at each of the temperature and 800 ° C.

FIG. 8 is a graph showing the relationship between parameters A and n corresponding to the fitting curves of FIGS. 6 and 7, and temperature.

FIG. 9 is a graph showing a relationship between a parameter α and a temperature.

FIG. 10 is a graph showing a relationship between a parameter τ and a temperature.

FIG. 11 is a diagram for explaining a method for predicting deterioration with time after aging.

FIG. 12 is a diagram for explaining a method of writing a grating on an optical fiber.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 10 ... Optical fiber grating, 20 ... Grating part, 100 ... Light source, 300 ... Optical spectrum analyzer, 400 ... Computer, 500 ... Computer display.

Claims (6)

[Claims] 1. A temperature sensor having a predetermined operating temperature range, an operation assurance time, and a temperature resolution, and including an optical waveguide grating as a sensing unit, wherein the optical waveguide grating is accelerated and aged under predetermined conditions. The aging condition is such that when the temperature sensor is used at the highest operating temperature in the operating temperature range for the operation assurance time, the temperature shift amount of the temperature sensor due to the deterioration with time of the optical waveguide grating is reduced. A temperature sensor which is determined to be equal to or lower than the temperature resolution. 2. The aging condition is defined as A · t using time t and temperature-dependent parameters A and α.
^- Temperature sensor according to claim 1, wherein those determined using a time degradation curve of the optical waveguide grating represented by alpha. 3. The parameter α is given by: 3. The temperature sensor according to claim 2, wherein the temperature sensor is expressed as follows. 4. The parameter A is given by: 3. The temperature sensor according to claim 2, wherein the temperature sensor is expressed as follows. 5. The temperature sensor according to claim 1, wherein the surface of the optical waveguide grating is etched after the accelerated aging. 6. The temperature sensor according to claim 1, wherein the optical waveguide grating has a heat-resistant coating material on a surface thereof.