JP2000009495A - Physical quantity measuring method and sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make a specific physical quantity measurable stably with high accuracy utilizing the difference of measurements of a plurality of specified wavelengths. SOLUTION: A physical quantity is determined using more than one wavelength having an extremal value of optical intensity in the spectrum of at least one of transmitted light and reflected light of Bragg grating. A spectral analyzer 30 is coupled optically to the end part 23 of an optical fiber 20 for measurement and the light from a light source 10 transmitted through the Bragg grating in the optical fiber 20 for measurement enters the spectral analyzer 30. The spectral analyzer 30 is connected electrically with a computer 40 and the output from the spectral analyzer 30 is delivered to the computer 40. The computer 40 is connected electrically with a computer display 50 and operation results of the computer 40 are presented on the screen of the computer display 50.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a method and a sensor for measuring a predetermined physical quantity using an optical waveguide having an optical waveguide type Bragg grating.

[0002]

2. Description of the Related Art An optical waveguide type Bragg grating is:
A region in the optical waveguide where the refractive index is spatially fluctuated (perturbed) at a predetermined period along a predetermined direction, and generates reflected light having sharp wavelength selectivity centered on light having a Bragg wavelength. A typical example of such an optical waveguide Bragg grating is a fiber Bragg grating formed in an optical fiber.

[0003] With the development of a technique for forming a fiber Bragg grating by irradiating a high intensity interference fringe light to the core portion of an optical fiber, an application using an optical fiber having a fiber Bragg grating has been proposed. I have. One such applied product is an optical fiber sensor such as an optical fiber temperature sensor. An example of an optical fiber temperature sensor is described in "PRForman et.a1., Rev. Sci. Instrum.61 (1
0), 0ctorber1990, pp. 2970-2972 ".

The optical fiber temperature sensor proposed by PRForman et al. Is made of silica-based glass doped with germanium and has an optical fiber having a core in which a fiber Bragg grating is formed, and generates light over a predetermined wavelength range. A light source to be incident on the optical fiber of
A physical quantity measuring instrument such as a spectrum analyzer for measuring the wavelength-intensity distribution of light.

[0005] In the optical fiber used in this optical fiber temperature sensor, the optical fringe interval L (T) of the grating is used.
Is the temperature T: L (T) = n (T) Λ (T) (1) where n (T): average effective refractive index of the grating in the core Λ (T): distance between stripes ( That is, the grating period). Then, of the light traveling in the core, the wavelength λ is λ = 2N · L (T) (2) Here, the light satisfying the condition of N: natural number is efficiently reflected by the grating. A wavelength satisfying the expression (2) is called a Bragg wavelength.

By the way, the temperature dependency of the optical fringe interval L of the grating is as follows: dL (T) / dT = (dn (T) / dT) Λ (T) + n (T) ・ (dΛ (T) / dT) = ((dn (T) / dT) tens n (T) · β) · Λ (T) (3) where β is a linear expansion coefficient of the optical fiber. In the quartz glass to which germanium is added,
(Dn (T) / dT) has small temperature dependency, and
Since (dn (T) / dT) / n (T) is sufficiently smaller than 1, and the linear expansion coefficient β has a small temperature dependency, L (T ₀ + ΔT) = L (T ₀ ) · ( 1+ (dL (T) / dT ) T = T0 · △ T) = L (T 0) · (1 + C L · △ T) ... (4) where, C _L: constant, and the efficiently reflected light Wavelength (Bragg wavelength)
λ _B (T = T ₀ + ΔT) is: λ _B (T ₀ + ΔT) = λ _B (T ₀ ) (1 + Cλ · ΔT) (5) where Cλ: constant.

Therefore, a Bragg wavelength λ _B (T ₀ ) at a specific temperature T ₀ and a constant Cλ are determined in advance, and a known wavelength-intensity generated by a light source is applied to a grating forming unit disposed at a temperature measurement position. Light having a distribution is incident, and a wavelength-intensity distribution (light spectrum) of reflected light or transmitted light is detected by a spectrum analyzer. Then, the optical spectrum is observed to determine the Bragg wavelength, and the Bragg wavelength is substituted for λ _B (T ₀ + ΔT) in equation (5) to determine ΔT, and the temperature T (= T ₀ + ΔT) ) Is calculated.

[0008]

The conventional optical fiber temperature sensor is configured as described above, and measures the value of the Bragg wavelength of the fiber Bragg grating formed on the core of the optical fiber using a measuring instrument such as a spectrum analyzer. I do. However, there is an error in the measuring instrument, and the error usually differs every measurement, so that it is difficult to uniformly correct the error. Therefore, it is difficult for a conventional sensor that obtains a physical quantity such as temperature using the Bragg wavelength itself obtained using a measuring instrument to stably perform highly accurate measurement.

The present invention has been made in view of the above, and an object of the present invention is to provide a method and a sensor capable of stably and accurately measuring a predetermined physical quantity.

[0010]

A physical quantity measuring method according to the present invention uses an optical waveguide having a core having an optical waveguide type Bragg grating and a cladding covering the core and having a lower refractive index than the core. A method for obtaining a predetermined physical quantity, comprising: (a) measuring at least one spectrum of transmitted light and reflected light of a grating; and (b) having an extreme value of light intensity from the measured spectrum, The method includes a step of obtaining values of two or more specific wavelengths that change in accordance with a change in the physical quantity, and (c) a step of calculating a physical quantity using the two or more wavelength values. The error included in each measurement value of the specific wavelength is
The difference can be reduced or eliminated by taking the difference between the measurements at the two specific wavelengths. Therefore, by using the difference between the measured values of the two specific wavelengths, it is possible to stably and accurately measure the physical quantities that affect these wavelength values.

As the specific wavelength, the coupling loss generated between two or more modes among a plurality of modes propagating through the clad and a plurality of modes propagating through the core due to light emission to the clad generated by the grating is maximized. And the Bragg wavelength of the grating. These wavelengths have different sensitivities to changes in the characteristic parameters of the grating (that is, parameters that change the specific wavelength, such as the grating period, the refractive index of the core or cladding of the grating forming portion, and the outer diameter of the cladding of the grating forming portion). Therefore, for example, there is a certain correspondence between the difference between the measured values of these wavelengths and the physical quantity affecting these wavelengths. Therefore, if this correspondence is determined in advance and two or more of the specific wavelengths are measured and the difference between these measured values is determined, the value of the physical quantity can be determined based on the correspondence.

[0012] In the physical quantity measuring method according to the present invention, the two or more wavelength values used for obtaining the physical quantity include two or more specified values obtained from the spectrum measured by the same wavelength scanning of the optical spectrum analyzer. A wavelength value may be included. Since the spectrum of the same wavelength scanning of the optical spectrum analyzer includes an error of almost uniform size, by taking the difference between the values of the two or more specific wavelengths obtained from this spectrum, these errors can be reduced. Can be erased.

Next, the sensor according to the present invention comprises: (a) an optical waveguide including a core having an optical waveguide type Bragg grating, a cladding covering the core and having a lower refractive index than the core, and (b) An optical spectrum measurement system connected to the optical waveguide so that at least one spectrum of transmitted light and reflected light of the grating can be measured,
(C) From the spectrum measured by the optical spectrum measurement system, values of two or more specific wavelengths having an extreme value of the light intensity and changing according to a change in a predetermined physical quantity are obtained. Calculating means for calculating a physical quantity using the value. The error included in the measured value of each of the specific wavelengths can be reduced or eliminated by taking the difference between the measured values of the two specific wavelengths. Can be stably and accurately measured.

The arithmetic means in the sensor according to the present invention is characterized in that, as the specific wavelength, two or more modes out of a plurality of modes propagating in the clad and a plurality of modes propagating in the core by light radiation to the clad generated by the grating. The wavelength at which the coupling loss occurring between them becomes the maximum or the value of the Bragg wavelength of the grating may be obtained.
These wavelengths are characteristic parameters of the grating (that is, parameters that change the specific wavelength, such as the grating period, the refractive index of the core or cladding of the grating forming portion, and the outer diameter of the cladding of the grating forming portion).
For example, there is a certain correspondence between the difference between the measured values of these wavelengths and the physical quantities affecting these wavelengths because of the different sensitivities to changes in the wavelength. Therefore, if this correspondence is determined in advance and two or more of the specific wavelengths are measured and the difference between these measured values is determined, the value of the physical quantity can be determined based on the correspondence.

An optical spectrum measuring system in a sensor according to the present invention includes a light source optically connected to an optical waveguide and generating light over a predetermined wavelength band, and an optical spectrum analyzer optically connected to the optical waveguide. At this time, the two or more wavelength values used for calculating the physical quantity include two or more specific wavelengths obtained from the optical spectrum measured by the same wavelength scanning of the optical spectrum analyzer. May be included.
Since the spectrum of the same wavelength scan of the optical spectrum analyzer contains an error of almost uniform size,
These errors can be eliminated by calculating the difference between the values of the two or more specific wavelengths obtained from the spectrum.

The optical waveguide Bragg grating in the sensor of the present invention preferably has an equirefractive index surface inclined with respect to the axis of the core. In the Bragg grating having such a structure, since the generation amount of the higher-order core mode and the respective order cladding modes is large, the coupling loss between these modes and the respective order core modes increases accordingly.
This facilitates detection of these coupling losses, and facilitates measurement of physical quantities using the wavelength value at which these coupling losses are maximized.

The optical waveguide Bragg grating in the sensor of the present invention may be provided in a core having a circular cross section. When the Bragg grating is provided on the core having a shape symmetrical with respect to the axis of the optical waveguide, the wavelength separation between the specific wavelengths is improved, and the detection of each wavelength is facilitated. It becomes easy to measure the physical quantity using the value of.

The optical waveguide in the sensor of the present invention may include a light reflecting material for covering the clad. In this case, the cladding mode generated by the optical waveguide Bragg grating can be reflected by the light reflecting material to prevent the cladding mode from radiating to the outside. As a result, the coupling loss between the cladding mode and each of the core modes becomes large, and the detection becomes easy. Therefore, it becomes easy to measure the physical quantity using the wavelength value at which the coupling loss becomes maximum. .

[0019]

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings. In the description of the drawings, the same elements will be denoted by the same reference symbols, without redundant description. In addition, for convenience of illustration, the dimensional ratios in the drawings do not always match those described.

FIG. 1 is a configuration diagram of the sensor of this embodiment. As shown in FIG. 1, this sensor comprises a light source 10,
Measurement optical fiber 20, spectrum analyzer 30,
And a computer 40 as main components. The light source 10 generates and outputs light over a predetermined wavelength band. The light source 10 includes an end 21 of a measurement optical fiber 20.
Are optically connected, so that the output light of the light source 10 is incident on the measuring optical fiber 20. The measurement optical fiber 20 is a single-mode optical fiber having a core having a circular cross section. As will be described later, the fixed portion 22 of the measurement optical fiber 20 includes a fiber Bragg grating, and narrow band light centered on the Bragg wavelength of the propagation light of the measurement optical fiber 20 is reflected at this portion. It is supposed to be. Hereinafter, this portion 22 will be referred to as a grating forming portion. A spectrum analyzer 30 is optically connected to the end 23 of the measuring optical fiber 20, and light transmitted from the light source 10 and transmitted through the Bragg grating in the measuring optical fiber 20 is incident on the spectrum analyzer 30. It has become. The computer 40 is electrically connected to the spectrum analyzer 30, and the output of the spectrum analyzer 30 is sent to the computer 40. A computer display 50 is electrically connected to the computer 40, and a calculation result of the computer 40 is displayed on a screen of the display 50.

FIG. 2 is a schematic sectional view showing the grating forming part 22 of the measuring optical fiber 20. In this figure, reference numerals 24 and 25 denote the core of the measuring optical fiber 20,
Each shows a clad. Reference numeral 70 denotes an axis of the measurement optical fiber 20. As shown in FIG. 2, a fiber Bragg grating 60 is formed on the core 24 included in the portion 22. The Bragg grating 60 is a region where the refractive index spatially fluctuates at a constant period along the fiber axis 70. Such a structure of the grating 60 can be described using a surface having a uniform refractive index distribution which is a cross section of the core 24 (hereinafter, referred to as an “equi-refractive index surface”). Since the refractive index of the grating 60 fluctuates spatially along the axis 70, the grating 60 is composed of equal refractive index surfaces having various refractive indexes. FIG. 2 shows a surface 62 having a specific refractive index among these equal refractive index surfaces as a representative. These equal refractive index surfaces 62
Have the same refractive index. In the present embodiment, the equal refractive index surface in the grating 60 is orthogonal to the fiber axis 70. The grating 60 can be considered to have a structure in which these equal refractive index surfaces are periodically arranged at regular intervals along the fiber axis 70. The interval between the equal refractive index surfaces having the same refractive index is the period of the grating 60.

Hereinafter, the temperature measurement operation by the sensor according to the present embodiment will be described. First, before measuring the temperature using the present sensor, a predetermined preliminary measurement is performed. Specifically, the measuring optical fiber 20 is placed in a thermostatic chamber whose internal temperature can be adjusted.
The light is emitted from the light source 10 while the periphery of the portion 22 is maintained at a constant temperature, and the light transmitted through the Bragg grating 60 in the portion 22 is detected by the spectrum analyzer 30. The spectrum analyzer 30 detects the intensity of light of each wavelength while scanning wavelengths within a predetermined range. The wavelength-intensity distribution of light measured in this manner is an optical spectrum.
In this pre-measurement, the surroundings of the grating forming section 22 are set to various temperatures, and the grating 6 for each temperature is set.
A transmitted light spectrum of 0 is measured.

FIG. 3 shows an example of an optical spectrum measured by the spectrum analyzer 30. As shown in this figure, some valleys representing the optical loss appear in the spectrum of the light transmitted through the Bragg grating 60. Specifically, the largest valley 101 is
The light loss due to the light having a wavelength satisfying the Bragg diffraction condition being reflected by the Bragg grating 60 is shown. Next, the valley 102 located on the short wavelength side of the valley 101 is
It is a representation of the coupling loss that occurs between the fundamental mode of the core and the secondary mode of the core. Since the measurement optical fiber 20 is a single mode optical fiber, only the fundamental mode originally exists in the core, but it is known that a higher-order (second order or higher) core mode occurs around the Bragg grating 60. Therefore, coupling loss occurs between these higher-order modes and the fundamental mode. Valley 102
Represents coupling loss between the second-order mode and the fundamental mode among these higher-order modes. Next, the valleys 103 to 105 located on the short wavelength side of the valley 102 represent the coupling loss generated between the fundamental mode of the core and each of the following cladding modes. In the Bragg grating 60, a part of the light propagating through the core is radiated to the cladding and enters a cladding mode. Therefore, coupling loss occurs between the cladding mode and the core mode. The valleys 103 to 105 represent the coupling loss between the fundamental mode of the cladding, the second mode, the third mode, and the fundamental mode of the core generated near the Bragg grating 60, respectively. Normally,
As shown in FIG. 3, the optical loss due to Bragg diffraction is the largest, then the coupling loss between the fundamental mode and the second-order mode of the core is large, and the coupling loss between the fundamental mode of the core and each of the cladding modes is Relatively small. However, FIG. 3 is only an example of the optical spectrum to be measured, and such a magnitude relationship is not always shown.

The spectrum analyzer 30 sequentially sends data, which is a set of the wavelength and the intensity detected for the light of the wavelength, to the computer 40 over the wavelength scanning range of the spectrum analyzer. The computer 40 stores the data of the wavelength and the light intensity in a built-in hard disk device. After this, the computer 40
Calculates the value of the wavelength at which the differential coefficient of the light intensity becomes 0 with respect to the optical spectrum when the surroundings of the grating forming section 22 is set to a predetermined reference temperature (for example, 25 ° C. at room temperature). The wavelength corresponding to the lowest point of the valley, that is, the value of the wavelength having the minimum value of the light intensity is obtained. Further, the computer 40 compares the light intensity values corresponding to the respective calculated wavelength values with each other, and selects three values from the light intensity values in ascending order. In the case of the spectrum of FIG. 3, the smallest value corresponds to the optical loss due to Bragg diffraction, the second smallest value corresponds to the coupling loss between the core fundamental mode and the core second-order mode, and the third smallest value. The value corresponds to the coupling loss between the fundamental mode of the core and the fundamental mode of the cladding. The computer 40 calculates an average value of the minimum value and the second smallest value, and sets this as a first threshold value. In addition, the computer 40
An average value of the second smallest value and the third smallest value is calculated, and this is set as a second threshold value. These thresholds are stored in the internal hard disk device. As described later, these thresholds are used to determine light reflection by Bragg diffraction (101 in FIG. 3) and coupling loss between the fundamental mode and the second-order mode of the core (102 in FIG. 3). .

Next, the computer 40 obtains the minimum value of the light intensity and the wavelength value having this minimum value for each of the light spectrums measured at various temperatures, and determines these minimum values of the light intensity in the first and the above-described first and second values. Compare with a second threshold. As a result of this comparison, the minimum value smaller than the first threshold value is determined to correspond to light reflection by Bragg diffraction, and the wavelength value having this minimum value is stored in the internal hard disk device as a Bragg wavelength value. The minimum value between the first threshold value and the second threshold value is determined to correspond to the coupling loss between the fundamental mode and the second-order mode of the core. It is stored as the wavelength value at which the loss is maximum.

The wavelength values obtained in this way are parameters that change according to the temperature around the grating forming portion 22, such as the grating period, the refractive index of the core and cladding of the grating forming portion, the outer diameter of the cladding of the grating forming portion, and the like. Depends on. Therefore, there is a certain correspondence between the wavelength value and the ambient temperature. FIG.
Represents the relationship between the ambient temperature of the grating forming section 22 and the two wavelengths obtained as described above. Specifically, reference numeral 110 in FIG. 4 indicates a relationship between the ambient temperature of the grating forming unit 22 and the wavelength of light reflection by Bragg diffraction (that is, Bragg wavelength), and reference numeral 111 indicates the ambient temperature of the grating forming unit 22 and the core. The relationship with the wavelength at which the coupling loss between the fundamental mode and the secondary mode is maximized is shown. As shown in FIG. 4, the amount of change in the wavelength value with respect to the same amount of temperature change differs between the Bragg wavelength and the wavelength at which the coupling loss between the fundamental mode and the secondary mode of the core is maximized. That is, these two wavelengths are
Different sensitivity to temperature change. Therefore, when the difference between these two wavelengths is obtained, a one-to-one correspondence is created between the wavelength difference and the temperature.

In this embodiment, this correspondence is obtained by using the result of the preliminary measurement. That is, the computer 40
Is the difference Δλ (= λ ₁ −λ ₂ ) obtained by subtracting the wavelength λ _{2 at} which the coupling loss between the fundamental mode and the secondary mode of the core is maximum from the Bragg wavelength λ ₁ obtained as described above, at various temperatures. And a function of temperature with the difference Δλ between these wavelengths as a variable is obtained. The derivation of the function can be performed by a regression analysis such as a least square method.

FIG. 5 shows the relationship between the wavelength difference Δλ thus determined and the temperature. The computer 40 saves a temperature function using Δλ as a variable in the internal hard disk device.

In the present embodiment, after the above-described preliminary measurement, the main measurement is performed by disposing the grating forming section 22 at a desired temperature measurement position. Specifically, light is sent from the light source 10 to the measurement optical fiber 20 and the spectrum analyzer 3
0 and the computer 40 are used to measure the Bragg wavelength λ ₁ and the wavelength λ _{2 at} which the coupling loss between the fundamental mode and the secondary mode of the core is maximized, and the value of the difference Δλ between them is determined. The measurement of each wavelength is performed in the same manner as the pre-measurement. The computer 40 converts the wavelength difference Δλ obtained by the preliminary measurement into a function of the temperature,
And the temperature corresponding to the value is calculated. This calculation result is the temperature around the grating forming section 22. The temperature thus calculated is displayed on the screen of the computer display 50. Thus, the temperature measurement according to the present embodiment ends.

The value of the wavelength measured using the spectrum analyzer 30 includes an error. Since the error differs every time the wavelength is scanned by the spectrum analyzer 30, the measured value of the wavelength or the like is used using the measured wavelength value itself. In the method of obtaining the physical quantity, accurate measurement cannot be stably performed. However, since the wavelength error is substantially constant during one wavelength scan, in this embodiment, the error is eliminated by obtaining the difference between the two wavelengths measured by the same wavelength scan. For concrete explanation, the true value of the Bragg wavelength is λ ₁ tru, the true value of the wavelength at which the coupling loss between the fundamental mode and the second-order mode of the core is maximal is λ ₂ tru, and the wavelength error of each is lambda ₁ err, expressed as lambda ₂ err, is measured lambda ₁ of the Bragg wavelength λ ₁ tru + λ ₁ err, is measured lambda ₂ wavelength coupling loss becomes maximum between the core fundamental mode and the second mode of lambda ₂ tru + λ ₂ err. Here, if the value of each wavelength is measured by the same wavelength scanning, λ ₁ err = λ ₂ err holds. In the present embodiment, since the temperature is obtained using the difference Δλ between λ ₁ and λ ₂ measured by the same wavelength scanning, not the measurement values λ ₁ and λ ₂ itself including the error, the influence of the error λ err is canceled. Can be. That is, the errors λ ₁ err and λ ₂ err are canceled out as Δλ = λ ₁ −λ ₂ = (λ ₁ tru + λ ₁ err) − (λ ₂ tru + λ ₂ err) = λ ₁ tru−λ ₂ tru ,
Δλ does not include an error. As described above, according to the present embodiment, the temperature is obtained using the wavelength difference that does not include an error, so that the temperature can be measured stably and accurately.

In this embodiment, the fiber Bragg grating is provided in a core having a circular cross section. When the optical waveguide type Bragg grating is provided on the core having a shape symmetrical with respect to the axis of the optical waveguide as described above, the separation of the wavelength having the extreme value of the light intensity in the spectrum is improved, and each wavelength is set as described above. It can be suitably used for physical quantity measurement.

The present invention is not limited to the above embodiment, and various modifications are possible. For example, the sensor of the above-described embodiment is intended to measure temperature. However, in addition to temperature, if a physical quantity (such as pressure) that affects a specific wavelength having an extreme value of light intensity, the physical quantity is identified by preliminary measurement. Measurement is possible by determining the relationship with the wavelength difference.

In the above embodiment, the physical quantity is measured using the wavelength at which the coupling loss between the fundamental mode and the secondary mode of the core is maximized and the Bragg wavelength. Any object that changes depending on the physical quantity to be measured can be used. For example, 10 in FIG.
Wavelengths at which the coupling loss between the core mode and the cladding mode is maximized as indicated by 3 to 105 also vary depending on physical quantities such as temperature and pressure, and can be used for these measurements.

Next, in the above embodiment, the fiber Bragg grating in the optical fiber for measurement was constituted by an equal refractive index surface perpendicular to the fiber axis, but the fiber Bragg grating in the optical fiber for measurement was:
It may have a structure in which equi-refractive index surfaces inclined with respect to the axis of the core are arranged parallel to each other. FIG. 6 is a schematic cross-sectional view showing a portion 22 'in the measuring optical fiber including such a Bragg grating 60'. In FIG.
Surface 6 having a specific refractive index among these equal refractive index surfaces
2 'is shown as a representative. Also, reference numeral 72 in FIG.
Indicates the normal line of the isorefractive index surface 62 '. In the Bragg grating composed of the equal refractive index surfaces inclined with respect to the fiber axis 70 in this manner, the amount of generation of higher-order modes and each-order cladding mode of the core increases, and accordingly, these modes and Coupling loss with the next core mode also increases. This makes it possible to easily detect these coupling losses, so that the physical quantity can be easily measured using the wavelength at which these coupling losses are maximized.

Next, the measuring optical fiber used in the present invention may be provided with a light reflecting material (for example, aluminum) that closely surrounds the cladding of the Bragg grating forming portion. The light reflecting material functions to reflect the clad mode reaching the outer surface of the clad, thereby preventing the clad mode from radiating outside. As a result, the coupling loss between the cladding mode and each of the core modes can be efficiently detected, and the measurement of the present invention can be easily performed using the coupling loss.

Next, in the above embodiment, only the spectrum of the light transmitted through the grating was measured, and the wavelength having the minimum value of the light intensity was obtained from the spectrum. However, the present invention is not necessarily limited to such a method. Do not mean. For example, light reflection by a grating can also be detected by measuring the spectrum of light reflected by the grating. In the reflected light spectrum of the grating, light reflection by the grating appears as a peak of light intensity. Therefore, the Bragg wavelength can be obtained by obtaining the maximum value of the light intensity from the reflected light spectrum.

FIG. 7 is a configuration diagram of an example of a sensor for obtaining the Bragg wavelength from the spectrum of the reflected light from the grating. In this sensor, a first terminal of an optical circulator 80 as an optical path setting means is connected to a light source 1 via an optical fiber 82.
0, and the second terminal of the optical circulator 80 is connected to the measuring optical fiber 20. The third terminal of the optical circulator 80 is connected to the optical spectrum analyzer 30 via the optical fiber 84. A computer 40 is connected to the optical spectrum analyzer 30 as in the sensor of FIG.
0 is connected to a computer display 50. The end face 26 of the optical fiber 20 is a reflection end face that reflects light at a constant reflectance.

Of the light emitted from the light source 10, the light reflected by the grating in the optical fiber 20 passes through the optical circulator 80 and enters the optical spectrum analyzer 30. The light transmitted through the grating is reflected by the end face 26, passes through the grating again, passes through the optical circulator 80, and enters the optical spectrum analyzer 30. Therefore, the optical spectrum analyzer 30
The spectrum of the transmitted light and the reflected light of the grating can be measured simultaneously. From the combined spectrum of the transmitted light and the reflected light of the grating measured in this way, the wavelength or the Bragg wavelength that maximizes the various coupling losses described above can be obtained. Therefore, the optical measurement according to the present invention can be performed by using the sensor shown in FIG. If the reflectivity of the end face 26 of the optical fiber 20 is too large, it is difficult to detect light reflection by the grating, and if the reflectivity is too small, it is difficult to detect various coupling losses. The reflectivity of the end face 26 may be set to an appropriate value in consideration of these matters.

In the above-described embodiment, the wavelength having the extreme value of the light intensity is directly obtained in the spectrum of the transmitted light or the reflected light of the grating, but the center wavelength of the peak or the valley of the spectrum (the wavelength located at the center of the half width) is obtained. ) May be determined, and this may be regarded as a wavelength having an extreme value of light intensity.

[0040]

FIG. 8 is a diagram showing a transmitted light spectrum of an optical fiber Bragg grating actually measured using the sensor having the structure shown in FIGS. The measurement of the spectrum was performed with the ambient temperature of the grating forming part 22 set to 25 ° C. The valley 201 in this spectrum represents light reflection due to Bragg diffraction.
Next, valleys 202 to 209 located on the short wavelength side of the valley 201
Represents the coupling loss occurring between the fundamental mode of the core and each of the following cladding modes. In this measured spectrum, no valley representing the coupling loss occurring between the fundamental mode of the core and the secondary mode of the core did not appear. This is probably because the second-order mode of the core is relatively unlikely to occur because the equi-refractive index surface of the Bragg grating included in the measuring optical fiber 20 is orthogonal to the axis of the optical fiber. When it is desired to use the coupling loss between the fundamental mode of the core and the second-order mode of the core, as shown in FIG. 6, a light including a grating having an equi-refractive index surface inclined with respect to the axis of the optical fiber is used. Fiber may be used.

Using the method described above with reference to FIGS. 1 to 5, the present inventor has determined the values of specific wavelengths having the minimum values of the light intensity included in the valleys 201 and 203 at various temperatures. Was. FIG. 9 is a graph showing these specific wavelength values with respect to temperature. In FIG. 9, the vertical axis on the left is a valley 201
The vertical axis on the right side indicates the wavelength value having the light intensity minimum value of the valley 203. Each point in the figure shows measured values for various temperatures, and the solid line in the figure is a straight line obtained by fitting these measured values by the least squares method. As shown in this figure, the two fitting straight lines have different slopes.
This indicates that the temperature dependence of the two wavelengths is different. Specifically, when the temperature is represented by x and the wavelength is represented by y, the fitting straight line 210 is y = 0.007x + 1552.
2 (correlation coefficient R ² = 0.9572), and the fitting straight line 211 is y = 0.0074x + 154.
It is expressed as 3.6 (correlation coefficient R ² = 0.9594).

FIG. 10 is a graph showing the relationship between the difference between two actually measured wavelength values and the temperature. Each point in the figure shows the value of the wavelength difference for various temperatures, and the solid line in the figure is a straight line obtained by fitting these values by the least square method.
When the horizontal axis in FIG. 10 is represented by x and the vertical axis is represented by y, this fitting straight line is represented by y = −0.0005x + 8.6012. The correlation coefficient R ² is 0.9849, which is close to 1 the correlation coefficient than the fitting straight line in Figure 9. Thus, by taking the difference between the wavelength values, the deviation between the measured value and the fitting straight line is reduced as a whole. Therefore, by using this fitting straight line, the measurement of the physical quantity can be performed with high accuracy.

[0043]

As described above in detail, in the present invention, the physical quantity is obtained by using two or more wavelengths having an extreme value of the light intensity in at least one of the transmitted light and the reflected light of the Bragg grating. , A predetermined physical quantity can be stably and accurately measured.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a sensor according to an embodiment of the present invention.

FIG. 2 is a schematic cross-sectional view showing a grating forming part 22 of the measuring optical fiber 20.

FIG. 3 is a diagram showing an optical spectrum measured by a spectrum analyzer 30.

FIG. 4 is a graph showing a relationship between temperature and wavelength.

FIG. 5 is a graph showing a relationship between a wavelength difference and a temperature.

FIG. 6 shows an equal refractive index surface 6 inclined with respect to the fiber axis.
FIG. 3 is a schematic cross-sectional view showing a portion 22 ′ in a measurement optical fiber on which a Bragg grating having 2 ′ is formed.

FIG. 7 is a configuration diagram of another embodiment of the sensor according to the present invention.

8 is a diagram showing a transmitted light spectrum of an optical fiber Bragg grating actually measured using the sensor having the configuration shown in FIGS. 1 and 2. FIG.

FIG. 9 is a graph showing the relationship between temperature and wavelength obtained from the spectrum of FIG.

FIG. 10 is a graph showing a relationship between a temperature and a wavelength difference obtained from the spectrum of FIG.

[Explanation of symbols]

10 light source, 20 measurement optical fiber, 21 and 23
Ends of optical fiber for measurement, 22 and 22 ': grating forming part, 24: core, 25: clad, 30: spectrum analyzer, 40: computer, 50: computer display, 60 and 60': fiber Bragg grating, 62 and 62 ': equi-refractive index surface, 7
0: Fiber axis, 72: Normal line of iso-refractive index surface.

Claims (9)

[Claims] 1. A physical quantity measuring method for obtaining a predetermined physical quantity using an optical waveguide including a core having an optical waveguide type Bragg grating and a cladding covering the core and having a lower refractive index than the core. Measuring the spectrum of at least one of the transmitted light and the reflected light of the grating; and from the measured spectrum, two or more identifications having an extreme value of light intensity and changing according to the change of the physical quantity A physical quantity measurement method, comprising: a step of obtaining a wavelength value; and a step of obtaining the physical quantity by using two or more wavelength values. 2. The coupling device according to claim 1, wherein the specific wavelength has a coupling loss generated between two or more modes among a plurality of modes propagating through the clad and a plurality of modes propagating through the core due to light radiation to the clad generated by the grating. The physical quantity measuring method according to claim 1, wherein the wavelength is one of a maximum wavelength and a Bragg wavelength of the grating. 3. The two or more wavelength values used for obtaining the physical quantity include values of the two or more specific wavelengths obtained from a spectrum measured by the same wavelength scan of an optical spectrum analyzer. 2. The physical quantity measuring method according to claim 1, wherein the physical quantity is included. 4. An optical waveguide including a core having an optical waveguide Bragg grating, a cladding covering the core, and having a lower refractive index than the core, and at least one of transmitted light and reflected light of the grating. An optical spectrum measurement system connected to the optical waveguide so that a spectrum can be measured; and the spectrum measured by the optical spectrum measurement system has an extreme value of light intensity, and changes according to a change in a predetermined physical quantity. And calculating means for calculating the physical quantity using the two or more wavelength values. 5. The specific wavelength has a coupling loss generated between two or more modes among a plurality of modes propagating through the clad and a plurality of modes propagating through the core due to light radiation to the clad generated by the grating. 5. The sensor according to claim 4, wherein the wavelength is one of a maximum wavelength and a Bragg wavelength of the grating. 6. The optical spectrum measurement system includes a light source optically connected to the optical waveguide and generating light over a predetermined wavelength band, and an optical spectrum analyzer optically connected to the optical waveguide. The two or more wavelength values used for calculating the physical quantity include two or more values of the specific wavelength determined from a spectrum measured by the same wavelength scanning of the optical spectrum analyzer. 5. The method according to claim 4, wherein
The sensor as described. 7. The sensor according to claim 4, wherein the optical waveguide Bragg grating has an equi-refractive index surface inclined with respect to an axis of the core. 8. The sensor according to claim 4, wherein the optical waveguide Bragg grating is provided in a core having a circular cross section. 9. The sensor according to claim 4, wherein the optical waveguide includes a light reflecting material that covers the cladding.