JP2705543B2 - Method for measuring magnetic field strength and optical magnetic field sensor using the same - 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 magnetic field sensor for measuring the intensity of a static magnetic field or an alternating magnetic field using the Faraday effect of a magneto-optical element. In addition, the optical magnetic field sensor of the present invention is particularly applicable to transmission lines, distribution lines and substation facilities (hereinafter, cubicles) for supplying electric power, GIS (GAS INSULAT).
ED SWITCH GEAR) is most suitable for measuring the strength of the magnetic field generated around the electric wire and detecting the magnitude of the current flowing through the electric wire.

[0002]

2. Description of the Related Art It is used in current sensors, cubicles, and GISs, which measure the magnitude of current flowing in power transmission lines and distribution lines, which are power transmission routes from power plants to consumers, and detect abnormalities. Some current sensors have been of the transformer type. This transformer-type current sensor is made by winding an electric wire around an iron core, and is therefore large and heavy.
There were various problems such as poor insulation. In order to solve this problem, a plan is underway to replace the transformer-type current sensor with an optical magnetic field sensor that does not include these problems.

The principle of the optical magnetic field sensor will be described with reference to the drawings. FIG. 5 shows an example of the basic configuration of a current measuring optical magnetic field sensor that has been developed. This optical magnetic field sensor includes a light source 1, a first optical fiber 2, a first lens 3, a first polarizing beam splitter (hereinafter referred to as PBS) 4, a half-wave plate 5, a magneto-optical material 6, Second PBS
7, second lens 8, second optical fiber 9, photodetector 1
0 and the arithmetic unit 11 are provided in this order. In use, at a minimum, the magneto-optical material 6 is placed in a magnetic field to be measured (hereinafter, referred to as a "magnetic field").

[0004] Light emitted from the light source 1 first optical fiber 2 as the first lens 3, passes through the first PBS 4, the
1 is changed to linearly polarized light by the PBS 4 and then the half-wave plate 5
Pass through and enter the magneto-optical material 6. When this light passes through the magneto-optical material 6, it is rotated according to the strength of the magnetic field. As a result, the intensity of the light passing through the second PBS 7 takes a value corresponding to the intensity of the magnetic field. Second PBS
The light passing through 7 is condensed by a second lens 8 and reaches a photodetector 10 via a second optical fiber 9. The light is photoelectrically converted by the photodetector 10.

When the magnetic field is an AC magnetic field, the signal output from the photodetector 10 is the sum of an AC voltage component and a DC voltage component. In the arithmetic unit 11, the signal is first divided into an AC voltage component and a DC voltage component, and then a value obtained by dividing the AC voltage component by the DC voltage component is calculated and output by the divider. The strength of the magnetic field to be measured is represented by the effective value of this output. Here, the reason why the AC voltage component / DC voltage component is detected is to eliminate the fluctuation of the intensity of the emitted light from the light source and the fluctuation of the light amount due to the fluctuation of the fiber and to detect the magnetic field intensity more accurately.

On the other hand, when the magnetic field is a static magnetic field, the output of the detector 10 has only a DC component.

[0007] As can be understood from the above description, the optical magnetic field sensor has large characteristics, such as higher withstand voltage and insulation, and can be reduced in size and weight, as compared with the transformer type current sensor. In order to further utilize this feature and further reduce the cost with higher sensitivity, it is a recent trend to use magnetic garnet, which has high mass productivity and high magnetic sensitivity, or a Bi-substituted magnetic garnet as a magneto-optical material.

One of such magneto-optical materials having high magnetic sensitivity is RIG (Ri: Rare Earth Iron Garnet). Since the RIG has a maze-like magnetic domain structure, when light passes through the RIG, the magnetic domain functions as a phase grating, and the transmitted light causes a diffraction phenomenon. According to the contents described in JP-A-5-126924, in order to improve the linearity between the magnetic field intensity and the output intensity of the sensor, it is necessary to take all these diffracted lights into the photodetector. However, taking all the diffracted light into the photodetector can be realized only by using a specially selected lens and adjusting the positional relationship between the lens and the optical component very precisely. Therefore, the mass productivity of the sensor is low, and the manufacturing cost is high. On the other hand, in order to increase the productivity of the sensor and reduce the manufacturing cost, for example, if only the zero-order light is taken into the detector, the linearity of the relationship between the magnetic field intensity and the output of the sensor can be understood from the following relationship. Is lost.

The relationship between the output obtained by using only the zeroth-order light and using a conventional arithmetic unit and the magnetic field strength is as follows: the input to the magneto-optical element is I _in and the output from the photodetector 10 is I ₀ . Then, I ₀ is approximately obtained by Expression 1. However,
I _in indicates the absolute value of the current value obtained from the photodetector when the amount of light input to the magneto-optical material is directly incident on the photodetector.

[0010]

I ₀ = I _in · (cos θ cos φ + (H ₀ sin ωt / Hs) sin θ sin φ) ² where θ is the Faraday rotation angle of the magneto-optical material, φ is the angle difference between the two polarizers, and Hs is the saturation magnetic field. The intensity, H ₀ is the intensity of the externally applied magnetic field, ω is the frequency of the AC magnetic field (ie, the frequency of the current), and t is the time.

[0011] In the calculator is separated into AC component I _0AC the DC component I _{zero DC} of I _0. Each component is represented by Formulas 2 and 3.

[0012]

## _EQU2 ## I _0DC = I _in · (cos ² θcos ² φ + H ₀ ² sin ² θsin ² φ / 2Hs ² )

Equation 3] I ₁ based on the _{I 0AC = I in · (H} 0 sin2θsin2φsinωt-H 0 2 sin 2 θsin 2 φcos2ωt / 2Hs 2) Number 4 divider circuit in the arithmetic unit is determined. This effective value V ₀ is obtained by Expression 5.

[0013]

## _EQU4 ## I ₁ = I _0AC / I _0DC

(Equation 5) As is apparent from Equations 4 and 5, the effective value V ₀ does not have a linear relationship with the externally applied magnetic field, and the phase at which the measured value (H ₀ sinωt) becomes zero and the phase at which I ₁ = 0 deviate from each other. (Hereinafter, this phase shift is referred to as a phase angle).

[0014]

As described above, in order to obtain an inexpensive optical magnetic field sensor suitable for mass production using RIG as a high-sensitivity magneto-optical material, the magnetic field intensity and the output from the arithmetic unit are required. And the phase angle is increased.

In view of such a situation, the present invention has high sensitivity and high precision (good linearity of magnetic field magnitude and output,
It is an object of the present invention to provide an optical magnetic field sensor which enables measurement of a small phase angle) and which is excellent in mass productivity, and a method of measuring a magnetic field intensity which enables this.

[0016]

According to a method of the present invention, a square root value of an output of a photodetector is obtained, an AC component and a DC component of the obtained value are divided, and the AC component is converted to a DC component. The magnetic field strength is measured from the value obtained by dividing the component. The optical magnetic field sensor of the present invention that solves the above-mentioned problems has a main component of a light source, a first optical fiber, a first lens, a first polarizer, a magnetic garnet, and a second polarized light. Child,
Second lens, the second optical fiber, optical detector, the arithmetic unit or <br/> that consists of light field sensor, from the second optical fiber
In an optical magnetic field sensor that photoelectrically converts an optical signal with a photodetector and converts it into an electric signal, and arithmetically processes the electric signal to obtain an output, a circuit in which the arithmetic unit mainly obtains the square root value of the output of the photodetector is obtained. It comprises a circuit for dividing the AC component and the DC component of the value, and a circuit for dividing the obtained AC component by the DC component. These circuits may be separated from each other and connected to each other to form an optical magnetic field sensor, or a square root operation circuit may be provided on the photodetector side.

The optical magnetic field sensor according to the present invention is particularly effective when not all of the diffracted light from the magnetic garnet film is taken into the photodetector.

[0018]

In the optical magnetic field sensor of the present invention, a square root operation circuit,
Alternatively, the output of the photodetector is squared by a square root calculator, then divided into an AC component and a DC component, the AC component is divided by the DC component, and the obtained value is output. Therefore, even if all of the diffracted light from the magneto-optical material is not taken into the photodetector, for example, only the 0th-order light of the diffracted light is taken into the photodetector, the magnitude of the magnetic field and the output of the optical magnetic field sensor are reduced. Is improved.

In order to intentionally take only the zero-order light into the photodetector, the distance between the magneto-optical element 6 and the second lens 8 in FIG. 1 is increased or the distance between the second lens 8 and the second fiber 9 is increased.
The distance can be easily adjusted by adjusting the distance.

[0020]

FIG. 1 shows an example of an optical magnetic field sensor according to the present invention. Optical magnetic field sensor is a light source 1 of FIG. 1, the first optical fiber 2, the first lens 3, a first PBS 4, the half-wave plate 5, magneto-optical material 6, the second PBS 7, the second lens 8, the Two optical fibers 9, a photodetector 10, and an arithmetic unit 11 having a square root arithmetic circuit are provided in this order.
In the apparatus shown in FIG. 1, a light emitting diode that emits light having a wavelength of 0.85 μm to a light source 1 is connected to a first optical fiber 2 and a second optical fiber 2 .
The optical fiber 9 has a multi-mode fiber, the first lens 3 and the second lens 8 have a self-focusing lens,
RIG with composition of (YbYbBi) ₃ Fe ₅ O ₁₂ for magneto-optical material 6
The photodetector 10 used a Si photodiode.

Further, a first PBS 4 and a second PBS
The half-wave plate 5 was placed immediately before the magneto-optical element 6 in order to place 7 on the same plane and to arrange it at 45 degrees. Arithmetic unit 10
Is composed of a square root operation circuit, a circuit for separating a DC component and an AC component by a filter, and an operation circuit for dividing the AC component by the DC component. In the evaluation, the effective value of the output voltage of the arithmetic unit was obtained.

In this optical sensor, only the zero-order light of the diffracted light of the magneto-optical material is incident on the photodetector.

Next, the intensity of the 50 Hz AC magnetic field was varied in the range of 0 to 700 Oe by the magnetic field sensor to obtain an output voltage from the arithmetic unit, and the relationship between the effective value and the magnetic field intensity was obtained.
The result is shown in FIG. In the figure, a (solid line) is the result of the optical magnetic field sensor according to the present invention. It can be seen that the output of the optical magnetic field sensor changes linearly with the change in the magnitude of the magnetic field. The output on the vertical axis is shown as a relative value for the convenience of the detector.

Next, the ratio error R (%) was obtained from Equation 6, and the obtained result is shown by a solid line in FIG.

[0025]

R = ((Kn−K) / K) × 100 where K is a value obtained by dividing the sensor output when a magnetic field of a desired strength is applied by the applied magnetic field strength, and Kn is the saturation. This is a value obtained by dividing the sensor output when the magnetic field is applied by the saturation magnetic field intensity.

Next, the change in the phase angle due to the change in the magnitude of the magnetic field was examined. The result is shown by a solid line in FIG. From FIG.
It can be seen that the phase angle in the range of the magnetic field strength of 700 Oe is almost zero.

In this embodiment, light emitted from the light source passes through an optical fiber, is converted into a parallel light by a lens, and is incident on the first PBS 4. The light linearly polarized by the first PBS 4 passes through the magneto-optical element, is rotated according to the strength of the magnetic field, and passes through the second PBS 7. Then, the light is focused on the cross section of the optical fiber by the lens, and reaches the photodetector 10.

In this apparatus, since only the zero-order light of the diffracted light of the magneto-optical element is focused on the optical fiber, when the input to the magneto-optical material is I _in and the output from the photodetector 10 is I _0. , I ₀ are approximately determined by Equation 7. Where I
_in indicates the absolute value of the current value obtained from the photodetector when the amount of light input to the magneto-optical material is directly incident on the photodetector.

[0029]

I ₀ = I _in · (cos θ cos φ + (H ₀ sinωt / Hs) sin θ sin φ) ² where θ is the Faraday rotation angle of the magneto-optical material, φ is the angle difference between the two polarizers, and Hs is the saturation magnetic field. The intensity, H ₀ is the intensity of the externally applied magnetic field, ω is the frequency of the AC magnetic field (ie, the frequency of the current), and t is the time.

By passing the output of the photodetector through the square root operation circuit as in the present invention, the output I ₂ of the square root operation circuit is given by the following equation (8).

[0031]

Equation 8] and the ^{_{I 2 = I in 1/2 · (}} cosθcosφ + (H 0 sinωt / Hs) sinθsinφ) then the DC component I _2DC and an AC component I _2AC of I ₂ is determined.

[0032]

[Equation 9] I _2DC = cosθcosφ

Equation 10] _{I 2AC = (H 0 sinω /} Hs) tsinθsinφ Finally, as the number 11 by the divider circuit I ₃ = I _2AC / I
_2DC is required.

[0033]

I ₃ = ((H ₀ sin ω / Hs) tsin θ sin φ) / (cos θ cos φ) Then, the effective value V ₁ of I ₃ is expressed by Expression 12.

[0034]

V ₁ = (H ₀ / (2Hs) ^1/2 ) tan θtan φ As is clear from Equations 11 and 12, V ₁ is proportional to the externally applied magnetic field H ₀ , and I ₃ is the phase of the external magnetic field H ₀ sinωt It can be seen that it matches. This can be said to theoretically support the above measurement results.

When the magnetic field is a static magnetic field, the output I ₃ from the photodetector is expressed by Expression 13.

[0036]

I ₃ = I _in · (cos θ cos φ + H / Hs sin θ sin φ) ² where H is a static magnetic field.

Then, the output I ₄ after the square root operation circuit is expressed by Expression 14.

[0038]

I ₄ = I _in ^1/2 · (cos θ cos φ + H / Hs sin θ sin φ) As a result, the output from the arithmetic unit passes through the square root arithmetic unit and is proportional to the static magnetic field H. At this time, I ₄
It can also be seen that the sensitivity to H is maximized when θ = φ = 90 degrees. For this reason, the RIG is used as the magneto-optical element and the square root operation circuit is provided on the input side in the arithmetic unit as shown in FIG. 1 so that the magnitude of the static magnetic field and the linearity of the output are excellent, and mass production is excellent. It can be said that an optical magnetic field sensor can be obtained.

In this embodiment, the square root arithmetic circuit is provided in a part of the arithmetic unit. However, a square root arithmetic unit may be provided between the photodetector and the arithmetic unit of the conventional device.

(Conventional example) For comparison with the optical magnetic field sensor of the first embodiment, only the square root arithmetic circuit in the arithmetic unit is removed from the optical magnetic field sensor of FIG. 1 (the same as the conventional product of FIG. 5). Was prepared, and the characteristics were evaluated in the same manner as in Example 1. The results are also shown in FIGS. B (broken line) in each figure is the result. The linearity between the magnetic field and the output is poor (FIG. 2), the ratio error is large (FIG. 3), and the phase angle increases as the magnitude of the magnetic field increases (FIG. 4). This result is as expected and undesirable.

As shown in the above embodiments and the conventional example, the optical magnetic field sensor according to the present invention has good linearity of the magnitude and output of the magnetic field, a small phase angle, and can measure the magnetic field with high sensitivity. Further, since the alignment between the optical components is easy, the mass productivity is excellent.

[0042]

As described above, the optical magnetic field sensor according to the present invention has a high sensitivity and a high accuracy because the magnitude of the magnetic field and the linearity of the output from the arithmetic unit are good and the phase angle is small in a wide magnetic field range. Measurement of a magnetic field. Furthermore, it is excellent in mass productivity and can be reduced in price. Further, the optical magnetic field sensor according to the present invention can measure the magnitude of the static magnetic field with good linearity.

[Brief description of the drawings]

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

FIG. 2 shows the relationship between the magnetic field strength obtained in the embodiment and the sensor output.

FIG. 3 shows the relationship between the magnetic field strength obtained in the example and the ratio error.

FIG. 4 shows the relationship between the magnetic field strength and the phase angle obtained in the example.

FIG. 5 is a configuration diagram of a conventional optical magnetic field sensor.

[Explanation of symbols]

1 ---- light source, 2-- first optical fiber, 3--
1 lens, 4--first polarizer, 5--half-wave plate, 6- magneto-optical material, 7--second polarizer, 8
--- Second lens, 9 ---- Optical fiber, 10 ----
Photo detector, 11

Claims (3)

(57) [Claims] 1. A method for measuring a magnetic field intensity using an optical magnetic field sensor using a magnetic garnet, wherein an optical signal from the magnetic garnet is photoelectrically converted by a photodetector into an electric signal, and a square root value of the electric signal is obtained. A magnetic field intensity measuring method, wherein the AC component and the DC component of the obtained value are divided, and the magnetic field intensity is obtained from a value obtained by dividing the AC component by the DC component. 2. The main components are sequentially arranged on the optical axis.
The, a light source, a first optical fiber, a first lens,
A first polarizer, a magnetic garnet, a second polarizer,
A second lens , a second optical fiber , a photodetector ,
Are calculator Toka et configured to be connected to the optical detector and the second
It such an electric signal by photoelectric conversion by the photodetector an optical signal from the second optical fiber, an electrical signal to said computing unit
In the optical magnetic field sensor to obtain the arithmetic processing and outputs Ri, the calculator divides mainly a circuit for obtaining a square root value of the output of the photodetector, the AC component of the determined value and the DC component An optical magnetic field sensor, comprising: a circuit; and a circuit for dividing the obtained AC component by a DC component. 3. The main components are sequentially arranged on the optical axis.
The, a light source, a first optical fiber, a first lens,
A first polarizer, a magnetic garnet, a second polarizer,
A second lens , a second optical fiber , a photodetector ,
Are calculator Toka et configured to be connected to the optical detector and the second
It such an electric signal by photoelectric conversion by the photodetector an optical signal from the second optical fiber, an electrical signal to said computing unit
In the optical magnetic field sensor to obtain a processing to output Ri, characterized in that the square root values obtained by photoelectric conversion and so as to form an output of the photodetector by the photodetector Optical magnetic field sensor.