JP3148614B2 - Optical fiber current / magnetic field sensor - Google Patents

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 a current and a magnetic field using the Faraday effect, and in particular, a magneto-optical element having the Faraday effect and a Bi having both the Faraday effect and natural optical rotation. ₁₂ G
The present invention relates to specifying the length of an optical path of a magneto-optical element in combination with an eO ₂₀ element to increase measurement accuracy with respect to a change 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. 24665/1992. The light emitted from the light source 1 is an optical fiber 10a.
And is incident on the polarizer 2 to be changed to linearly polarized light. The 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 rotation. The optical path length is such that the rotation angle of the linearly polarized light due to the natural optical rotation is 45 degrees. Since the sensitivity has the maximum value, the polarization plane rotates by 45 degrees due to the natural optical rotation, and furthermore, rotates by a certain 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 rotation 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 rotation θ, and the optical path length L
The quantitative relationship existing between is as shown in Expression 1.

[0003]

(Equation 1)

[0004] 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.
Receives the intensity modulation of the light corresponding to. The linearly polarized light emitted from the analyzer 4 propagates through the optical fiber 10b, enters the optical receiver 5, 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. Thus, the strength of the magnetic field can be measured, and if the magnetic field is caused by a current, the magnitude of the current can be measured.

[0005]

The conventional optical fiber current / magnetic field sensor is constructed as described above. 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 rotation. The value is set to 45 degrees and the optical axes of the polarizer 2 and the analyzer 4 are set to be perpendicular or parallel to each other geometrically so that the sensitivity is maximized. The change in the Verde constant and the natural optical rotation of 3 cannot be ignored compared to the change in the optical path length,
There has been a problem that the output error increases due to a change in the rotation angle of the polarization plane, and the measurement accuracy decreases.

[0006]

An optical fiber current / magnetic field sensor according to the present invention includes a light source that emits light having a wavelength of 850 nm, a polarizer that converts light from the light source into linearly polarized light,
A magneto-optical element that rotates the plane of polarization of linearly polarized light from the polarizer by the Faraday effect, and an oxide crystal that rotates the plane of polarization of linearly polarized light from the polarizer by 45 degrees by natural optical rotation and further rotates by the Faraday effect Bi ₁₂ GeO
₂₀ element, 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 the linearly polarized light from the analyzer is photoelectrically converted and converted. An optical receiver that detects an electric 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 between the analyzer and the optical receiver is provided to measure the strength of a magnetic field acting in parallel on each optical path of the magneto-optical element and the Bi ₁₂ GeO ₂₀ element. The natural optical rotation power and its temperature coefficient, and the Verdet constant and its temperature coefficient at the operating center temperature of the Bi ₁₂ GeO ₂₀ element are known, and the light wavelength of the light source is 850 n.
m, the length of the optical path of the Bi ₁₂ GeO ₂₀ element is uniquely determined by setting the rotation angle of the plane of polarization of linearly polarized light by natural rotation to 45 degrees, and the Verdet constant at the center temperature at which the magneto-optical element is used. Is Vo, and the temperature coefficient of this Verdet constant is α, and 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. .039) (mm), and when perpendicular to each other, L = −0.046 / Vo (α + 0.039) (mm), whereby the constants of the magneto-optical element and the Bi ₁₂ GeO ₂₀ element cancel each other. The effect of a change in temperature on the sensitivity when the intensity of linearly polarized light is modulated by the analyzer is almost eliminated.

In the optical fiber current / magnetic field sensor, the magneto-optical element is made of FR-5 glass, and the length of the optical path is set to 1.8 mm. The effect of the change in temperature on the sensitivity of is almost eliminated.

Furthermore, the magneto-optical element in the optical fiber current and the magnetic field sensor _{(Cd 1-x Mn x)} Te (x = 0.
By setting the length of the optical path to 0.1 mm, there is almost no effect of temperature change on the sensitivity when the intensity of linearly polarized light is modulated by the analyzer.

[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, reference numeral 11 denotes a light source that emits light having a wavelength of 850 nm, and reference numeral 12 denotes a polarizer that converts light from the light source 11 into linearly polarized light, and includes a polarization beam splitter and uses transmitted light. Reference numeral 13a denotes a magneto-optical element having a Faraday effect, which has no natural optical rotation, and rotates the plane of polarization of linearly polarized light from the polarizer 12 in proportion to the length of an optical path and the strength of a magnetic field acting in parallel to the optical path due to the Faraday effect. I do. 13b is an oxide crystal Bi having Faraday effect and natural optical rotation
_{The 12} GeO ₂₀ element rotates the plane of polarization of linearly polarized light from the magneto-optical element 13a by 45 degrees by natural rotation, 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 it. 14 is Bi ₁₂ GeO ₂₀
An analyzer 15 that performs intensity modulation corresponding to the rotation angle of the plane of polarization of linearly polarized light in the element 13b, and 15 is an optical receiver that photoelectrically converts 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 that transmits linearly polarized light between the optical receiver 4 and the optical receiver 15.

The first embodiment is configured as described above, and the light of wavelength 850 nm emitted from the light source 11
a, and enters the polarizer 12 to become linearly polarized light. The linearly polarized light enters the magneto-optical element 13a, and the plane of polarization rotates in proportion to the strength of the magnetic field acting in parallel to the optical path due to the Faraday effect. The linearly polarized light emitted from the magneto-optical element 13a is incident on the Bi ₁₂ GeO ₂₀ element 13b, the polarization plane is rotated by 45 degrees due to natural optical rotation, and is proportional to the strength of the magnetic field acting parallel to the optical path due to the Faraday effect. And 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 corresponding 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, enters the optical receiver 15, is photoelectrically converted, and has the intensity of the linearly polarized light. Therefore, each optical path 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 to Thereby, the strength of the magnetic field can be measured.

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

[0012]

(Equation 2)

The intensity I of the linearly polarized light whose intensity has been modulated by the analyzer 14 can be obtained by Expression 3.

[0014]

(Equation 3)

In Equation 3, the first and second terms in 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 by Iac, and the magneto-optical element 13a and the Bi ₁₂ GeO ₂₀ element 13b
The intensity (effective value) of a magnetic field acting in parallel to each optical path of the above as an alternating magnetic field is represented by H, and the sensitivity when linearly polarized light intensity is modulated by the analyzer 14 is represented by η, and is defined by η = Iac / Idc. Equation 4 is obtained.

[0016]

(Equation 4)

The influence of a change in temperature on the sensitivity η will be considered. The influences of the temperature change include the Verdet constant of the magneto-optical element 13a, its optical path length, and B
The Verdet constant, natural optical rotation, and the length of the optical path of the i ₁₂ GeO ₂₀ element 13b can be considered.
In any of the i ₁₂ GeO ₂₀ elements 13b, the temperature characteristics of the optical path length can be neglected as compared with those of the Verdet constant and the natural optical rotation power, and are not considered here. The Verde constants of the magneto-optical element 13a at the predetermined temperature and the center temperature of use are V and Vo, respectively, and the temperature coefficient of the Verde constant is α, and the Verde constants of the Bi ₁₂ GeO ₂₀ element 13b at the predetermined temperature and the center temperature of use are Vr and Vo, respectively. Vro, β is the temperature coefficient of this Verde constant, θ and θo are the natural optical rotation power at a predetermined temperature and the center temperature of use, γ is the temperature coefficient of the natural optical rotation power, and ΔT is the temperature difference between the predetermined temperature and the center temperature of use. Then, V,
Vr and θ are represented by Equations 5, 6, and 7, respectively.

[0018]

(Equation 5)

(Equation 6)

(Equation 7)

Equations (8), (5), (6) and (7) are approximately obtained as the sensitivity η.

[0020]

(Equation 8)

The coefficient of ΔT in brackets on the right side of Equation 8 is a temperature coefficient of sensitivity, and when the temperature changes, an output error occurs. 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
/ 2) or parallel (ψ = 0), Equation 9 is obtained. As for 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 the linearly polarized light due to the natural optical rotation of the Bi ₁₂ GeO ₂₀ element 13b is 45 degrees is from 9 nm to 800 nm.
It has been confirmed by an experiment 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 described in the specification attached to the application of Japanese Patent Application No. 7-172130. Further, the Verde constant Vro at the operating center temperature of the Bi ₁₂ GeO ₂₀ element 13b is 0.188 min / Oe · cm as published in the magazine “Functional Materials”, November 1983, p. Has a temperature coefficient β of 1 in the same specification.
65 ppm / ° C. In addition, Bi ₁₂ GeO ₂₀ element 13
The natural optical rotation θo at the center temperature of use of b is 9.6 deg / min, which is published in the above-mentioned magazine “Functional Materials”, November 1983, page 17, and is 9.6 deg / min.
The temperature coefficient γ of o is -25 according to page 41 of “Improvement of accuracy of optical fiber voltage sensor” on page 41 of the IEICE Technical Committee on Sensor Technology (document number ST-92-19, December 10, 1992).
0 ppm / ° C. Using these numerical values, the unit of the Verde constant Vro is min / Gauss · cm, the unit of the temperature coefficient β is% / ° C., and the temperature coefficient γ of the natural optical rotation θo.
Is converted to% / ° 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 respective optical axes of the polarizer 12 and the analyzer 14 are mutually Equation 10 is obtained when the optical axes are parallel, and equation 11 is obtained when the optical axes are perpendicular to each other. 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 sensitivity
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 when the ambient temperature changes, the temperature coefficient of sensitivity when the intensity modulation of linearly polarized light is performed by the analyzer 14 becomes almost zero, and the output error hardly occurs.
It is possible to measure a magnetic field with high accuracy.

Embodiment 2 FIG. The second embodiment has the same configuration as the first embodiment, and uses a type of lead glass, FR-5 glass, for the magneto-optical element 13a. FR-5
The Verdet constant Vo and its temperature coefficient α at the center temperature of use of glass are 0.11 min, respectively, from the values published in the magazine “Functional Materials”, November 1983, page 17, respectively.
/ Os · cm, -2730 ppm / ° C. The unit of the Verde constant Vo is min / Gauss · cm, and the unit of the temperature coefficient is% / ° C., and the polarizer 12 and the analyzer 14 are converted.
When the respective optical axes are perpendicular to each other, the length L of the optical path of the magneto-optical element 13a in which the temperature coefficient δ of the sensitivity is zero according to Expression 11 is 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 linearly polarized light intensity is modulated by the analyzer 14 becomes almost zero even when the ambient temperature changes. The output error is almost eliminated, and a highly accurate magnetic field measurement can be performed.

Embodiment 3 The third embodiment also has the same configuration as the first embodiment, and (Cd
Those using _{1-x Mn x) Te (} x = 0.1).
The Verdet constant Vo and its temperature coefficient α at the operating center temperature of (Cd _1−x M n _x ) Te (x = 0.1) are determined by Tech.
nical Digest of The 9th S
sensor Symposium, 1990. pp55
~ 58, "Fiber-OpticalMagnetic-
Field Sensor Using (Cd _1-x Mn
_x ) 3.3 min / Gauss respectively from the numerical values published in "Te Single Crystal" p.56.
Cm, -1630 ppm / ° C. 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 respective optical axes of the polarizer 12 and the analyzer 14 are perpendicular to each other, the temperature coefficient δ of the sensitivity is obtained by Expression 11.
Is zero, the length L of the optical path of the magneto-optical element 13a is 0.
1 mm. The optical path length of _{(Cd 1-x Mn x)} T
When the magneto-optical element 13a made of e (x = 0.1) is used, the temperature coefficient of sensitivity when linearly polarized light intensity is modulated by the analyzer 14 becomes almost zero even when the ambient temperature changes, and the output error is reduced. Is almost eliminated, and a highly accurate magnetic field measurement can be performed.

[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]

Reference Signs List 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

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-61-243380 (JP, A) JP-A-63-182573 (JP, A) JP-A-3-44585 (JP, A) JP-A 61-243585 250572 (JP, A) JP-A-55-121422 (JP, A) JP-A-7-248368 (JP, A) JP-A-59-35156 (JP, A) JP-A-2-132389 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) G01R 33/00-33/18 G01R 15/24

Claims (3)

(57) [Claims] 1. A light source that emits light having a wavelength of 850 nm, a polarizer that converts light from the light source into linearly polarized light, a magneto-optical element that rotates a plane of polarization of linearly polarized light from the polarizer by the Faraday effect, and the polarizer. Bi ₁₂ GeO ₂₀ element of an oxide crystal in which the plane of polarization of linearly polarized light from is rotated 45 degrees by natural optical rotation and further rotated 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, corresponding to the intensity by photoelectrically converting the linearly polarized light from the analyzer An optical receiver for detecting the electrical signal, a first optical fiber for transmitting light between the light source and the polarizer,
A second optical fiber for transmitting linearly polarized light between the analyzer and the optical receiver, wherein 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 on each optical path of a GeO ₂₀ element in parallel, the Verdet constant at the operating 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. .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 has an optical path length of 1.8 mm. 3. A magneto-optical element _{(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.