JPH07209397A - Optical magnetic field sensor - Google Patents

Abstract

PURPOSE:To obtain an optical magnetic field sensor where the linearity of a sensor output for magnetic field is excellent, the phase shift between the magnetic field and the sensor output is small and the modulation factor is large. CONSTITUTION:In a structure where a light emitted from a lens 3 just behind an optical guide fiber 2 on the input side passes through a polarizer 4, magnetic garnet 6, a condenser lens 14 and a polarizer 7 on the output side sequentially and then is led to an optical guide fiber 9 by a lens 8 just before the optical guide fiber 9, a relative angle phi (deg) between the two polarizers 4 and 7 and a Faraday rotation angle theta (deg) of the magnetic garnet 6 are so set that they are in a specific relation.

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 a magnetic field intensity by using the Faraday effect of a magneto-optical element, and particularly to a power transmission line, a distribution line and a power receiving and transforming facility (cubicle), GIS (Gas Insulated Sw)
The present invention relates to an improvement of an optical magnetic field sensor that measures the magnitude of a current by measuring the strength of a magnetic field generated around electric wires such as an itch gear) and a general magnetic field sensor that measures an alternating magnetic field.

[0002]

2. Description of the Related Art A current sensor used in a cubicle or GIS to detect an abnormality by measuring the amount of current flowing through a power transmission line or distribution line, which is a power transportation route from a power plant to a customer. As such, an optical magnetic field sensor utilizing the Faraday effect and having a small size, a light weight, and a non-inductive property is attracting attention. The principle of the optical magnetic field sensor is to measure a magnetic field generated around a conductor, for example, a power transmission line by a current, by using the Faraday effect of a magneto-optical element, and obtain a current value. The features of the optical magnetic field sensor are high withstand voltage, high insulation,
It is non-contact, compact and lightweight, and does not require a power supply or electric circuit on the high voltage side.

FIG. 6 is a schematic diagram showing the basic configuration of an optical magnetic field sensor that has been developed up to now and is used for current measurement. In the figure, 1 is a light source, 2 is an input side optical guide fiber, 3
Is an input side lens, 4 is a polarization beam splitter (hereinafter referred to as PBS) as an input side polarizer, 5 is a half-wave plate, 6
Is a magneto-optical element, 7 is a PBS as an output side polarizer, 8
Is an output side lens, 9 is an output side optical guide fiber, 10 is a photodetector, and 11 is a signal processing circuit. And the light source 1
The light emitted from the input side passes through the input side optical guide fiber 2 and the input side lens 3 and is linearly polarized by the input side PBS 4, and further passes through the half-wave plate 5 so that the plane of polarization of the linearly polarized light is 45 (d).
eg) Rotate and enter the magneto-optical element 6. Polarization plane is 4
When the linearly polarized light rotated by 5 (deg) passes through the magneto-optical element 6, the linearly polarized light is subjected to magnetic rotation according to the strength of the magnetic field to be measured (hereinafter, simply referred to as a magnetic field in principle) and the passing distance, and then is polarized. The face is rotated. When the light whose polarization plane has been rotated passes through the next output-side PBS 7, it has an intensity corresponding to the strength of the magnetic field, which is condensed by the output-side lens 8 on the output-side optical guide fiber 9 to detect light. It is guided to the device 10 and photoelectrically converted. Finally, the signal processing circuit 11 detects a desired magnetic field.

In the above structure, a light emitting diode is generally used as the light source 1. It is also possible to employ a laser diode that emits light with excellent directivity and has high emission intensity. However, the diode laser emitted from the laser diode is almost linearly polarized light, and when the linearly polarized light passes through the input side optical guide fiber 2, if a stress exists in the input side optical guide fiber 2, the polarization plane is not stable, There is a problem that the intensity of the diode laser passing through the input side PBS 4 becomes unstable. Therefore, a light emitting diode that emits unpolarized light is suitable for the light source 1. Further, the half-wave plate 5 includes an input side PBS 4 and an output side PBS.
It is used to set the relative angle of 7 to 45 (deg). In the optical magnetic field sensor with this configuration, the path of linearly polarized light passing through the magneto-optical element 6 and the magnetic field are parallel. Even if the arrangement order of the half-wave plate 5 and the magneto-optical element 6 is reversed, there is no great difference in characteristics.

In the signal processing circuit 11, the AC component and the DC component obtained by photoelectric conversion are separated, and the AC component voltage / DC component voltage is obtained by the divider. And
The effective value of the AC component voltage / DC component voltage represents the magnetic field, that is, the magnetic field to be measured, and this is called the modulation degree. In this way, the reason for detecting the AC component voltage / DC component voltage is to eliminate fluctuations in the intensity of light emitted from the light source 1 and fluctuations in the light amount due to fluctuations of both the input-side and output-side optical guide fibers 2, 9. This is because a desired magnetic field can be detected.

[0006]

As the magneto-optical element 6 in the above-mentioned optical magnetic field sensor, there are lead glass containing lead oxide (PbO), paramagnetic materials such as ZnSe, BGO and BSO, or diamagnetic materials. However, recently, a plan to use the optical magnetic field sensor for the current measurement in power transmission lines and distribution lines, GIS, and current transformers for measuring instruments in cubicles is being actively promoted. Such a sensor is required to have high sensitivity, small size, and low price. Therefore, a magnetic garnet having high mass productivity and high magnetic sensitivity and a magnetic field sensor using Bi-substituted magnetic garnet have been developed.

As described above, a paramagnetic material or a diamagnetic material has been conventionally used as the magneto-optical element 6. In this case, the relative angle φ between the two PBSs (polarizers) 4 and 7 is
At 45 (deg), the modulation degree becomes maximum. In recent years, a highly sensitive RIG has attracted attention as the magneto-optical element 6, and is approaching a practical stage. However, even when the magneto-optical element 6 is replaced with a RIG from a paramagnetic material or a diamagnetic material, the relative angle φ between the two polarizers can be set to a normal value by simply replacing the magneto-optical element 6 in FIG. 6 with the RIG. It is used at 45 (deg). In the optical magnetic field sensor using RIG,
This has the advantage that the sensitivity (modulation degree) of the sensor output with respect to changes in the external magnetic field is high. The higher the modulation degree, the better the signal-noise ratio (S / N ratio) of the sensor output, and the higher the accuracy. It is possible to manufacture a sensor. Therefore, it is desired to increase the degree of modulation also in the optical magnetic field sensor using the RIG.

At present, an optical magnetic field sensor using an RIG as a magneto-optical element has a good sensor output linearity with respect to a magnetic field, and can be manufactured with a small phase shift between the magnetic field and the sensor output (hereinafter referred to as a phase angle). It is possible. However, it is desired to further increase the degree of modulation in terms of cost reduction of the signal processing circuit 11 and measurement of a small magnetic field.

In view of the above-mentioned circumstances, the present invention has an object to provide an optical magnetic field sensor having a good sensor output linearity with respect to a magnetic field, a small phase angle, and a large modulation degree.

[0010]

According to the optical magnetic field sensor of the present invention, the light emitted from the input side lens immediately after the input side optical guide fiber passes through the input side polarizer, the magnetic garnet, the condenser lens and the output side polarizer. In an optical magnetic field sensor that is guided to the output side optical guide fiber by the output side lens just before the output side optical guide fiber after passing, in the Faraday of the magnetic garnet and the relative angle φ (deg) between these two polarizers. The rotation angle θ (deg) is 0 <θ <90, 0 <φ <
When 90, θ + φ = 90, 0 <θ <90, 90 <φ
When <180 and when 90 <θ <180, 0 <φ <90, | φ−θ | = 90 and 90 <θ <180,90 <
The feature is that when φ <180, θ + φ = 270.

[0011]

When the RIG is used for the magneto-optical element 6 and all the diffracted light is taken into the output side optical guide fiber 9, the optical magnetic field sensor shown in FIG. The relationship between the light amount I _r of light transmitted through the output-side PBS 7 as a child and the magnetic field is expressed by the following equation (1). I _r = [1/2] I _in [1 + cos2θ cos2φ + (H / H _s ) sin2θsin2φ] (1) where θ is the Faraday rotation angle of the magneto-optical element 6.
(deg) and φ are relative angles (deg) between the two polarizers 4 and 7,
H _s and H are the saturation magnetic field (Oe), measured magnetic field (Oe), and I, respectively.
_in is the amount of light incident on the RIG.

When measuring an alternating magnetic field, H ₀ is the amplitude of the alternating magnetic field, ω is the angular frequency of the alternating magnetic field (angular frequency of the current), t
And H = H ₀ sin ωt, the output voltage V from the photodetector 10 is expressed by the following equation (2). V = [1/2] aI _in [1 + cos2θ cos2φ + (H ₀ sin ωt / H _s ) sin2θ sin2φ] (2) where a is the incident light amount and output voltage V of the photodetector 10.
Is a conversion coefficient of.

Next, the output voltage V from the photodetector 10 is
The signal processing circuit 11 separates the DC component V _D and the AC component V _A. The DC component V _D and the AC component V _A are expressed by the following equations (3) and (4) from the equation (2). V _D = (1/2) aI _in (1 + cos2θ cos2φ) ··· (3) V _A = (1/2) (H ₀ / H _s ) aI _in sin2 θsin2φ sin ωt ··· (4) Also, the signal The following formula (5) is used for the divider in the processing circuit 11.
Is calculated. V ₁ = V _A / V _D (5) Then, find the effective value V _r of V ₁ . As mentioned earlier, the effective value V _r of V ₁ was a degree of modulation, a mean square calculation and expression for sin ωt (3) ~ (5 ), the modulation factor V _r
Is expressed by the following equation (6). V _r = H ₀ | sin2θsin2φ | / [√2H _s (1 + cos2θcos2φ)] (6) Equation (6) shows the linear relationship between the modulation degree V _r and the magnetic field.

From equation (6), θ which maximizes the modulation degree V _r
Consider the relationship between and. Therefore, the formula (6) is changed to φ
By differentiating with, the relationship between θ and φ that maximizes the modulation degree V _r can be obtained. Differentiating the expression (6) with respect to φ gives the following expression (7). ∂ V _r / ∂φ = 2H ₀ sin2θ (cos2φ + cos2θ) / [√2H _s (1 + cos2θcos2φ) ² ] ... (7) And the relation between θ and φ is such that ∂ V _r / ∂φ = 0. cos2
φ + cos2θ = 0, therefore, when sin2θsin2φ> 0, θ + φ = 90 (2n −1) and sin2θsin2φ <0 are represented by | φ−θ | = 90 (2n−1). Here n = 1, 2, ...

In FIG. 5, the Faraday rotation angle θ (deg) of the magneto-optical element 6 is plotted on the horizontal axis and the modulation degree V _r is plotted on the vertical axis, and the relative angle φ (deg) between the two polarizers 4 and 7 is shown. As a parameter, the calculation result by the equation (6) is shown. φ =
In the order of 20, 40, 60, and 80, the calculation result is shown by a solid line, a chain line with a large pitch, a chain line with an intermediate pitch, and a chain line with a small pitch. In addition, ● in the figure
The mark is an experimental value described later. From FIG. 5, when 0 <θ <90, 0 <φ <90, θ + φ = 90 When 0 <θ <90, 90 <φ <180, φ−θ = 90 90 <θ <180, 0 <φ <90 , Φ−θ = 90 90 <θ <180, and 90 <φ <180, θ + φ =
It can be seen that the modulation degree V _r becomes maximum at 270. And RI
In the optical magnetic field sensor for AC magnetic field measurement using the G film, by comparing with the generally adopted relative angle φ = 45 (deg), by selecting the value of the relative angle φ that satisfies the above condition, It can be seen that the modulation degree V _r can be further increased.

[0016]

Embodiments of the optical magnetic field sensor according to the present invention will be described below in detail with reference to FIGS. Figure 1
2 is a schematic diagram showing the basic configuration of the optical magnetic field sensor of the first embodiment, and FIG. 2 shows the configuration of the optical magnetic field sensor of the second embodiment that was produced based on the basic configuration shown in FIG. It is a schematic diagram. The same members as those of the above-described conventional technique are designated by the same reference numerals and the description thereof will be omitted.

First Embodiment Unlike the above-described prior art optical magnetic field sensor, the first embodiment
The optical magnetic field sensor in the example has φ and θ such that a large modulation degree V _r can be obtained, and as is clear from the comparison between FIG. 1 and FIG. A condenser lens 14 is arranged. The magneto-optical element 6 is specified as a magnetic garnet.

[0018] In the optical magnetic field sensor of the second embodiment was subjected to the second embodiment experiment, the input-side polarizer 15, the polarizing glass plate on the output side polarizer 16, the magneto-optical element 6 (YbTbBi) ₃ Fe _A magnetic garnet represented by ₅ O ₁₂ was used. A multimode fiber was used for the input side optical guide fiber 2 and the output side optical guide fiber 9, and a gradient index lens was used for the input side lens 3 and the output side lens 8. In addition, the light source 1 has a wavelength of 0.85 (μ
m) light emitting diode, the photodetector 10 uses a Si photodiode, and the light traveling direction is 90 (deg).
In order to change, an optical magnetic field sensor was constructed using the input side total reflection prism 12 and the output side total reflection prism 13.

The relative angle φ (deg) between the input side polarizer 15 and the output side polarizer 16 is 20, 40, 60 and 80 (deg), and the magneto-optical element 6 is a magnetic garnet.
Faraday rotation angle θ (deg) of 10, 30, 50, 7
Test optical magnetic field sensors having the above-described configurations of 0, 90 and 110 (deg) were produced, and experiments were conducted using these test optical magnetic field sensors. First, the amplitude H ₀ is 500 (Oe) and the frequency ω /
Degree of modulation when an AC magnetic field with (2π) of 50 (Hz) is applied
We experimented with V _r . The measurement result in this experiment is
This is indicated by the ● mark in FIG. 5 described above. The experimental value and the calculated value are in good agreement, and in the range of 0 <θ <90, 0 <φ <90, θ + φ = 90, 90 <θ <180, 0 <φ <90.
In the range of, it was demonstrated that the modulation degree V _r becomes maximum when θ−φ = 90.

Further, the following No. 1 to No. 5 optical magnetic field sensors having φ and θ such that the modulation degree V _r becomes maximum, that is,
(No.1) φ = 20; θ = 70, (No.2) φ = 40; θ =
50, (No.3) φ = 60; θ = 30, (No.4) φ = 8
0; θ = 10, (No.5) φ = 20; θ = 110 As a typical example, (No.3) φ = 60; θ = 30 is used, and the frequency ω / (2π) is used. An AC magnetic field of 50 (Hz) was changed in the range of 0 to 700 (Oe), and the relationship between the strength of the magnetic field and the effective value of the output voltage was examined by experiments. The results of this experiment are shown in FIG. It is shown that the output of the optical magnetic field sensor changes linearly with respect to the change of the magnetic field strength, which is consistent with the expression (6) being a linear expression. In this experiment, the relationship between the magnetic field strength and the phase angle was also examined, and the results shown in FIG. 4 were obtained. That is, 0 to 700 (Oe)
It was found that the phase angle was almost zero in the range of the magnetic field strength, and it was proved that one of the objects of the present invention, that is, an optical magnetic field sensor having a small phase angle was achieved.

[0021]

As described above, the optical magnetic field sensor of the present invention has good linearity of the sensor output from the signal processing circuit with respect to the magnetic field over a wide range of magnetic field strength, and the phase shift between the magnetic field and the sensor output. It also has the excellent performance of being small and having a large degree of modulation, with high sensitivity and high accuracy.
The magnetic field can be measured.

[Brief description of drawings]

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

FIG. 2 is a second panel that was manufactured based on the basic configuration of FIG.
It is a schematic diagram showing the composition of the optical magnetic field sensor of an example.

FIG. 3 is a table of magnetic field strength-effective value of output voltage according to the optical magnetic field sensor of the present invention.

FIG. 4 is a chart of magnetic field strength-phase difference angle according to the optical magnetic field sensor of the present invention.

FIG. 5 is a Faraday rotation angle θ-degree of modulation with a relative angle φ including an experimental value relating to the optical magnetic field sensor of the present invention as a parameter.
It is a chart of V _r .

FIG. 6 is a schematic diagram showing a basic configuration of a conventional optical magnetic field sensor.

[Explanation of symbols]

 1 light source 2 input side optical guide fiber 3 input side lens 4 input side polarizer 5 half-wave plate 6 magnetic garnet or magneto-optical element 7 output side polarizer 8 output side lens 9 output side optical guide fiber 10 photodetector 11 signal processing Circuit 12 Input side total reflection prism 13 Output side total reflection prism 14 Condenser lens 15 Input side polarizer 16 Output side polarizer

Claims (1)

[Claims] 1. Light output from an input-side lens immediately after an input-side optical guide fiber passes through an input-side polarizer, a magnetic garnet, a condenser lens, and an output-side polarizer, and then an output immediately before an output-side optical guide fiber. In the optical magnetic field sensor that is guided to the output side optical fiber by the side lens,
When the relative angle φ (deg) between the two polarizers and the Faraday rotation angle θ (deg) of the magnetic garnet are 0 <θ <90 and 0 <φ <90, θ + φ = 90 and 0 <θ < When 90, 90 <φ <180 and 90 <θ <
When 180,0 <φ <90, | φ−θ | = 90, and when 90 <θ <180, 90 <φ <180, θ + φ =
An optical magnetic field sensor characterized in that the optical magnetic field sensor is 270.