JPH09189752A - Fiber-optic current and magnetic field sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the measurement accuracy of a fiber-optic current and magnetic field sensor against temperature changes by deciding the length of the optical path of a magneto- optical element from a specific formula by using the Verdet constant of the element and the temperature coefficient of the constant. SOLUTION: The light emitted from a light source 11 and having a wavelength of 850nm is polarized into linearly polarized light by means of a polarizer 12 after being propagated through a first fiber 20a and the polarized light is made incident to a magneto-optical element 13a by which the plane of polarization is rotated by a Farady effect. The linearly polarized light coming out from the element 13a is made incident to a Bi12 GeO20 element 13b by which the plane of polarization is further rotated by a Farady effect and further made incident to an analyzer 14 which gives intensity modification to the light. The linearly polarized light coming out from the analyzer 14 is made incident to an optical receiver 15 after being propagated through a second optical fiber 20b and the receiver 15 detects the light as the electric signal corresponding to the intensity of the light. As a result, the intensity of a magnetic field is measured. The length L of the optical path of the element 13a is decided from formula I when the optical axes of the polarizer 12 and analyzer 14 are parallel to each other or from formula II when the optical axes intersect each other at right angles. The VO and α in the formulae respectively represent the Verdet constant of the element 13a at the center operating temperature and the temperature efficiency of the constant.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber current / magnetic field sensor for measuring an electric current and a magnetic field by utilizing the Faraday effect, and in particular, a magneto-optical element having the Faraday effect and a Bi having the Faraday effect and the natural optical rotatory power. ₁₂ G
In combination with the eO ₂₀ element, the present invention relates to specifying the length of the optical path of the magneto-optical element to increase the measurement accuracy with respect to changes in temperature.

[0002]

2. Description of the Related Art FIG. 2 is a block diagram of a conventional optical fiber current / magnetic field sensor disclosed in Japanese Patent Publication No. 4-24665. The light emitted from the light source 1 is the optical fiber 10a.
Of the light, is incident on the polarizer 2, and is changed to linearly polarized light. This linearly polarized light is incident on the magneto-optical element 3. The magneto-optical element 3 has both the Faraday effect and the natural optical rotatory power, and the optical path length is such that the rotation angle of the linearly polarized light due to the natural optical rotatory power is 45 degrees. Since the sensitivity is maximized, the polarization plane rotates by 45 degrees due to the natural optical rotatory power, and further rotates by a fixed angle in proportion to the strength of the magnetic field acting in parallel to the optical path due to the Faraday effect. The rotation angle Δψ due to the natural optical rotatory power of the plane of polarization and the Faraday effect, the magnetic field strength H, the Verdet constant V of the magneto-optical element 3, the natural optical rotatory power θ, and the optical path length L.
The quantitative relationship existing between is as shown in Equation 1.

[0003]

[Equation 1]

The linearly polarized light emitted from the magneto-optical element 3 enters the analyzer 4, and the rotation angle Δψ of the plane of polarization of the magneto-optical element 3 is changed.
Corresponding to the intensity modulation of light. The linearly polarized light emitted from the analyzer 4 propagates through the optical fiber 10b, enters the optical receiver 5 and is photoelectrically converted, and is detected as an electric signal corresponding to the intensity of the linearly polarized light, that is, the strength H of the magnetic field. This makes it possible to measure the strength of the magnetic field, and if the magnetic field is due to current, the magnitude of the current can be measured.

[0005]

The conventional optical fiber current / magnetic field sensor is constructed as described above, and the length of the optical path of the magneto-optical element 3 is determined by the rotation angle of the plane of polarization of linearly polarized light due to its natural optical rotatory power. The optical axes of the polarizer 2 and the analyzer 4 are geometrically perpendicular to or parallel to each other so as to maximize the sensitivity while the value becomes 45 degrees, but when the ambient temperature changes, the magneto-optical element The Verde constant of 3 and the change in natural optical rotatory power cannot be ignored compared to the change in the length of the optical path,
There is a problem in that the rotation angle of the polarization plane changes, the output error increases, and the measurement accuracy decreases.

[0006]

An optical fiber current / magnetic field sensor according to the present invention comprises a light source which emits light having a wavelength of 850 nm, and a polarizer which converts the light from the light source into linearly polarized light.
Magneto-optical elements that rotate the plane of linearly polarized light from this polarizer by the Faraday effect, and oxide crystals that rotate the plane of polarized linearly polarized light from the polarizer by 45 degrees by natural optical rotation and further rotate by the Faraday effect. Bi ₁₂ GeO
₂₀ elements, an analyzer that performs intensity modulation of light in accordance with the rotation angle of the polarization plane of the linearly polarized light in the magneto-optical element and the Bi ₁₂ GeO ₂₀ element, and the linearly polarized light from this analyzer is photoelectrically converted to An optical receiver that detects an electrical signal corresponding to the intensity,
A first optical fiber for transmitting light between the light source and the polarizer,
A second optical fiber for transmitting linearly polarized light is provided between the analyzer and the optical receiver to measure the strength of the magnetic field acting in parallel on each optical path of the magneto-optical element and the Bi ₁₂ GeO ₂₀ element. The natural optical rotatory power of the Bi ₁₂ GeO ₂₀ element at the use center temperature and its temperature coefficient, as well as the Verdet constant and its temperature coefficient are known, and the wavelength of the light from the light source is 850 n.
Therefore, the length of the optical path of the Bi ₁₂ GeO ₂₀ element is uniquely determined by setting the rotation angle of the polarization plane of the linearly polarized light due to the natural optical rotation to 45 degrees, and the Verde constant at the use center temperature of the magneto-optical element. Is Vo and the temperature coefficient of this Verdet constant is α, the length L of the optical path of the magneto-optical element is L = −0.019 / Vo (α−0 when the optical axes of the polarizer and the analyzer are parallel to each other. 0.039) (mm), and when they are perpendicular to each other, L = -0.046 / Vo (α + 0.039) (mm) is set to cancel each constant of the magneto-optical element and the Bi ₁₂ GeO ₂₀ element. There is almost no influence of temperature change on the sensitivity when the intensity of linearly polarized light is modulated by the analyzer.

When the magneto-optical element of the optical fiber current / magnetic field sensor is made of FR-5 glass and the optical path length is 1.8 mm, the intensity of linearly polarized light is modulated by the analyzer. The influence of temperature change on the sensitivity of is almost eliminated.

Further, the magneto-optical element is (Cd _1-x Mn _x ) Te (x = 0.
1) and by setting the length of its optical path to 0.1 mm, the influence of temperature change on the sensitivity when linearly polarized light intensity modulation is performed by the analyzer is almost eliminated.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiment 1 FIG. FIG. 1 is a configuration diagram showing Embodiment 1 of the present invention. In the figure, 11 is a light source that emits light with a wavelength of 850 nm, and 12 is a polarizer that changes the light from the light source 11 into linearly polarized light, which is a polarization beam splitter and uses transmitted light. Reference numeral 13a denotes a magneto-optical element having a Faraday effect, which does not have a natural optical rotatory power, and rotates the polarization plane of linearly polarized light from the polarizer 12 in proportion to the length of the optical path and the strength of the magnetic field acting parallel to the optical path due to the Faraday effect. To do. 13b is an oxide crystal Bi having Faraday effect and natural optical rotatory power.
_{The 12} GeO ₂₀ element rotates the plane of polarization of the linearly polarized light from the magneto-optical element 13a by 45 degrees due to the natural optical rotatory power, and further rotates by the Faraday effect in proportion to the length of the optical path and the strength of the magnetic field acting parallel to the optical path. Let 14 is Bi ₁₂ GeO ₂₀
An analyzer that modulates the intensity of the linearly polarized light in the element 13b according to the rotation angle of the plane of polarization, and an optical receiver 15 that photoelectrically converts the linearly polarized light from the analyzer 14 and detects an electrical signal corresponding to the intensity. , 20a is a first optical fiber for transmitting light between the light source 11 and the polarizer 12, and 20b is an analyzer 1
4 is a second optical fiber for transmitting linearly polarized light between the optical receiver 4 and the optical receiver 15.

The first embodiment is configured as described above, and the light having a wavelength of 850 nm emitted from the light source 11 is emitted from the first fiber 20.
It propagates through a and enters the polarizer 12 to become linearly polarized light. This linearly polarized light is incident on the magneto-optical element 13a, and its plane of polarization is rotated by the Faraday effect in proportion to the strength of the magnetic field acting in parallel to the optical path. The linearly polarized light emitted from the magneto-optical element 13a is incident on the Bi ₁₂ GeO ₂₀ element 13b, the plane of polarization is rotated by 45 degrees due to the natural optical rotatory property, and is further proportional to the strength of the magnetic field acting in parallel to the optical path due to the Faraday effect. And then rotate. The linearly polarized light whose polarization plane has been rotated by the magneto-optical element 13a and the Bi ₁₂ GeO ₂₀ element 13b enters the analyzer 14 and undergoes intensity modulation according to the rotation angle of the polarization plane.
The optical axes of the polarizer 12 and the analyzer 14 are perpendicular or parallel to each other. The linearly polarized light emitted from the analyzer 14 propagates through the second optical fiber 20b, is incident on the optical receiver 15 and is photoelectrically converted, and thus the intensity of the linearly polarized light, and therefore, the optical paths of the magneto-optical element 13a and the Bi ₁₂ GeO ₂₀ element 13b. Is detected as an electric signal corresponding to the strength of the magnetic field acting in parallel with. This allows the strength of the magnetic field to be measured.

Next, the influence of a change in temperature on the sensitivity when the intensity of linearly polarized light is modulated by the analyzer 14 will be described. The electric field vector components Ex and Ey of the intensity of the linearly polarized light intensity-modulated by the analyzer 14 are set to 1 as the intensity of the linearly polarized light incident on the analyzer 14 from the Bi ₁₂ GeO ₂₀ element 13b via the magneto-optical element 13a. And Bi ₁₂ G
The strength of the magnetic field acting in parallel with each optical path of the eO ₂₀ element 13b is H, the Verdet constant of the magneto-optical element 13a is V, the length of the optical path is L, the Verdet constant of the Bi ₁₂ GeO ₂₀ element 13b is Vr, and The natural optical rotation is θ, and the optical path length is L
r and the angle formed by the optical axes of the polarizer 12 and the analyzer 14 is ψ, and is expressed by Equation 2 using a Jones vector matrix.

[0012]

[Equation 2]

The intensity I of the linearly polarized light intensity-modulated by the analyzer 14 is obtained by the mathematical formula 3.

[0014]

(Equation 3)

In Equation 3, the first and second terms in the parentheses on the right side are components that are not affected by the magnetic field, and the third term is a component that is affected by the magnetic field. The former is represented by Idc and the latter is represented by Iac, and the magneto-optical element 13a and Bi ₁₂ GeO ₂₀ element 13b are represented.
Let H be the strength (effective value) of a magnetic field acting in parallel with each of the optical paths of ## EQU1 ## and η be the sensitivity when linearly polarized light intensity is modulated by the analyzer 14, and η = Iac / Idc Equation 4 is obtained.

[0016]

(Equation 4)

The influence of temperature change on the sensitivity η will be considered. What is affected by the temperature change is the Verdet constant of the magneto-optical element 13a, its optical path length, B
i ₁₂ GeO ₂₀ element 13b's Verdet constant, natural optical rotatory power, and the length of its optical path are considered.
Since the temperature characteristic of the optical path length of any of the i ₁₂ GeO ₂₀ elements 13b can be ignored as compared with that of the Verdet constant and the natural optical rotatory power, it is not considered here. The Verde constants of the magneto-optical element 13a at the predetermined temperature and the use center temperature are V and Vo, the temperature coefficient of this Verde constant is α, and the Verde constants of the Bi ₁₂ GeO ₂₀ element 13b at the predetermined temperature and the use center temperature are Vr and Vo, respectively. Vro, the temperature coefficient of this Verdet constant is β, the natural optical rotatory power is also θ and θo at the predetermined temperature and the use center temperature, the temperature coefficient of the natural optical rotatory power is γ, and the temperature difference between the predetermined temperature and the use center temperature is ΔT. Then V,
Vr and θ are represented by Equation 5, Equation 6, and Equation 7, respectively.

[0018]

(Equation 5)

(Equation 6)

(Equation 7)

From Equation 4, Equation 5, Equation 6, and Equation 7, Equation 8 of the sensitivity η can be obtained approximately.

[0020]

(Equation 8)

The coefficient of ΔT in the brackets on the right side of Equation 8 is the temperature coefficient of sensitivity, and this causes an output error when the temperature changes. The temperature coefficient of this sensitivity is represented by δ,
The optical axes of the polarizer 12 and the analyzer 14 are perpendicular to each other (ψ = π
If it is / 2) or parallel (ψ = 0), then Equation 9 is obtained. Regarding the sign of the second term on the right side of Expression 9, minus corresponds to ψ = π / 2, and plus corresponds to ψ = 0.

[0022]

[Equation 9]

By the way, the length of the optical path where the rotation angle of the plane of polarization of linearly polarized light due to the natural optical rotatory power of the Bi ₁₂ GeO ₂₀ element 13b is 45 degrees is from the wavelength of the light of the light source 11 to 9 nm.
It has been experimentally confirmed that there is a proportional relationship in the range of 00 nm, and if the wavelength of light is 850 nm, the length Lr of the optical path is 4.4 mm. This is also described in the specification attached to the application of Japanese Patent Application No. 7-172130. Further, the Verde constant Vro at the use center temperature of the Bi ₁₂ GeO ₂₀ element 13b is 0.188 min / Oe · cm, which is the value published in page 17 of the magazine “Functional Material”, November 1983, and this Verde constant. The temperature coefficient β of is given in the same specification above
It is 65 ppm / ° C. Furthermore, Bi ₁₂ GeO ₂₀ element 13
The natural optical rotation power θo at the use central temperature of b is 9.6 deg / min in the above-mentioned magazine “Functional Material” November 1983, page 17, and the natural optical rotation power θo.
The temperature coefficient γ of o is -25 according to "High accuracy of optical fiber voltage sensor" on page 41 of the material of the Institute of Electrical Engineers of Japan Sensor Technology Study Group (Document number ST-92-19, December 10, 1992).
It is 0 ppm / ° C. Using each of these numerical values, the unit of the Verde constant Vro is min / Gauss · cm, the unit of the temperature coefficient β thereof is% / ° C, the temperature coefficient γ of the natural optical rotation power θo.
When the unit of is converted into% / ° C. and the length L of the optical path of the magneto-optical element 13a in which the temperature coefficient δ of the sensitivity of Expression 9 is zero is obtained, the optical axes of the polarizer 12 and the analyzer 14 are When they are parallel to each other, Equation 10 is obtained, and when their optical axes are perpendicular to each other, Equation 11 is obtained. Note that θoLr = π / 4.

[0024]

(Equation 10)

[Equation 11]

When the material of the magneto-optical element 13a is specifically determined, the temperature coefficient δ of the sensitivity is calculated by the formula 10 or the formula 11.
The length of the optical path at which is zero is determined. If the magneto-optical element 13a having this optical path length is used, even if the ambient temperature changes, the temperature coefficient of sensitivity when the intensity of linearly polarized light is modulated by the analyzer 14 becomes almost zero, and the output error is almost eliminated.
Highly accurate magnetic field measurement is possible.

Embodiment 2 FIG. The second embodiment has the same configuration as that of the first embodiment, and uses a kind of lead glass, FR-5 glass, for the magneto-optical element 13a. FR-5
The Verde constant Vo and its temperature coefficient α at the use center temperature of glass are 0.11 min from the values published in the "Functional Material" magazine, November 1983, page 17, respectively.
/ Os · cm, −2730 ppm / ° C. The unit of the Verdet constant Vo is min / Gauss · cm, and the unit of the temperature coefficient is converted into% / ° C. The polarizer 12 and the analyzer 14
When the respective optical axes of 1 are perpendicular to each other, the length L of the optical path of the magneto-optical element 13a which makes the temperature coefficient δ of the sensitivity zero according to the formula 11 becomes 1.8 mm. If the magneto-optical element 13a made of FR-5 glass having this optical path length is used, the temperature coefficient of sensitivity when the intensity of linearly polarized light is modulated by the analyzer 14 becomes substantially zero even if the ambient temperature changes. The output error is almost eliminated and the magnetic field can be measured with high accuracy.

Embodiment 3 The third embodiment also has the same configuration as that of the first embodiment, and the magneto-optical element 13a has (Cd
_1-x Mn _x ) Te (x = 0.1) is used.
The Verdet constant Vo at the use center temperature of (Cd _1-x Mn _x ) Te (x = 0.1) and its temperature coefficient α are Tech.
natural Digest of The 9th S
ensor Symposium, 1990. pp55
~ 58, "Fiber-Optical Magnetic-
Field Sensor Using (Cd _1-x Mn
_x ) 3.3 min / Gauss from the values published in Te Single Crystal ”p.56.
-Cm, -1630 ppm / degreeC. Verde constant V
When the unit of o is min / Gauss · cm and the unit of the temperature coefficient is converted to% / ° C and the optical axes of the polarizer 12 and the analyzer 14 are made perpendicular to each other, the temperature coefficient δ of the sensitivity is calculated by Equation 11.
Is zero, the optical path length L of the magneto-optical element 13a is 0.
1 mm. (Cd _1-x Mn _x ) T of this optical path length
If the magneto-optical element 13a made of e (x = 0.1) is used, the temperature coefficient of the sensitivity when the intensity modulation of the linearly polarized light is performed by the analyzer 14 becomes substantially zero even if the ambient temperature changes, and the output error is reduced. Is almost eliminated, and the magnetic field can be measured with high accuracy.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing a first embodiment of the present invention.

FIG. 2 is a configuration diagram of a conventional optical fiber current / magnetic field sensor.

[Explanation of symbols]

11 Light source 12 Polarizer 13a Magneto-optical element 13b Bi ₁₂ G
eO ₂₀ element 14 analyzer 15 optical receiver 20a first optical fiber 20b second optical fiber

Claims (3)

[Claims] 1. A light source that emits light having a wavelength of 850 nm, a polarizer that converts the light from the light source into linearly polarized light, a magneto-optical element that rotates the plane of polarization of the linearly polarized light from the polarizer by the Faraday effect, and the polarizer. Bi ₁₂ GeO ₂₀ element of an oxide crystal that rotates the plane of polarization of the linearly polarized light from 4 by 45 degrees by natural optical rotation and further rotates by the Faraday effect,
An analyzer that modulates the intensity of light in accordance with the rotation angle of the plane of polarization of the linearly polarized light in the magneto-optical element and the Bi ₁₂ GeO ₂₀ element, and linearly polarized light from the analyzer is photoelectrically converted to correspond to the intensity. An optical receiver for detecting an electric signal, a first optical fiber for transmitting light between the light source and the polarizer,
A second optical fiber for transmitting linearly polarized light is provided between the analyzer and the optical receiver, and the magneto-optical element and the Bi ₁₂
In an optical fiber current / magnetic field sensor for measuring the strength of a magnetic field acting in parallel on each optical path of a GeO ₂₀ element, a Verde constant at a use center temperature of the magneto-optical element is Vo,
When the temperature coefficient of this Verdet constant is α, the length L of the optical path of the magneto-optical element is L = −0.019 / Vo (α−0) when the optical axes of the polarizer and the analyzer are parallel to each other. 0.039) (mm), and when the optical axes of the polarizer and the analyzer are perpendicular to each other, L = −0.046 / Vo (α + 0.039) (mm). 2. The optical fiber current / magnetic field sensor according to claim 1, wherein the magneto-optical element is made of FR-5 glass, and its optical path length is 1.8 mm. 3. The magneto-optical element is (Cd _1-x Mn _x ) Te.
(X = 0.1), and the optical path length is 0.1 mm
The optical fiber current / magnetic field sensor according to claim 1.