JPH08327712A - Optical magnetic-field sensor and method for measuring strength of magnetic field using this sensor - Google Patents

Abstract

PURPOSE: To provide the optical magnetic-field sensor, which can perform highly sensitive, highly-accurate measurement of a magnetic field, can perform alignment between optical parts readily, has an excellent mass productivity and can achieve the low cost, and the method for measuring the strength of the magnetic field using this sensor. CONSTITUTION: The light emitted from a light source passes through a semiwavelength plate 5 through an optical fiber 2, a lens 3 and a polarizer 4 and enters into a magneto-optical element 6. When the light passes through the magneto-optical element 6, the light is rotated in accordance with the strength of a magnetic field to be measured. After passing through an analyzer 7, the light is condensed with a lens 8 and cast into an optical fiber 9. The light having a strength corresponding to the strength of the magnetic field undergoes photoelectric conversion by a photodetector 10. A signal processing circuit 11 divides the output value of the photodetector 10 into the basic wave component and the second-harmonic-generation component. The value obtained by dividing the second-harmonic-generation component by the basic wave component is outputted.

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 magnetic field strength by utilizing the Faraday effect of a magneto-optical element and a magnetic field strength measuring method using the same.
In particular, power transmission lines and distribution lines, power receiving and transforming facilities (hereinafter referred to as quicles), GIS (GAS INSULATED)
The present invention relates to an optical magnetic field sensor suitable for detecting the magnitude of a current by measuring the strength of a magnetic field generated around an electric wire such as SWITCH GEAR) and a magnetic field strength measuring method using the same.

[0002]

2. Description of the Related Art Heretofore, a transformer type current sensor has been used as a current sensor for detecting an abnormality by measuring the magnitude of current flowing through a transmission line or a distribution line, or a current sensor used in a cubicle or GIS. Has been used. However, the transformer-type current sensor has various problems such as large size, heavy weight, and poor insulation.
Recently, a plan to use an optical magnetic field sensor in place of such a current sensor is being advanced.

When an optical magnetic field sensor is used, a magnetic field generated around a conductor, for example, a current flowing through a transmission line is measured by using the Faraday effect of a magneto-optical material, and the measured magnetic field is applied to the magnetic field. The value of the flowing current is determined, but the characteristics of this case are high breakdown voltage, high insulation, non-contact, small size and light weight, and no need for a power supply or electric circuit on the high voltage side. You can

An example of the basic configuration of such an optical magnetic field sensor will be described with reference to FIG. The light emitted from the light source 1 passes through the optical fiber 2, the lens 3, and the polarizer 4 to become linearly polarized light, and then passes through the half-wave plate 5 and enters the magneto-optical element 6. The linearly polarized light receives optical rotation according to the strength of the magnetic field to be measured (hereinafter abbreviated as magnetic field) when passing through the magneto-optical element 6, and after passing through the analyzer 7, corresponds to the strength of the magnetic field. The resulting intensity is converged by the lens 8 and the optical fiber 9
Incident on.

The light incident on the optical fiber 9 is detected by the photodetector 1.
It is led to 0 and photoelectrically converted. After that, the signal processing circuit 1
In 1, the filter is divided into an AC component and a DC component, and a value obtained by dividing the AC voltage component by the DC voltage component is output by the divider. The measured magnetic field is represented by the effective value calculated in this way. Here, the reason why the value obtained by dividing the AC voltage component by the DC voltage component is detected is to detect the desired magnetic field by eliminating the fluctuation of the intensity of the emitted light of the light source 1 and the fluctuation of the light amount due to the shaking of the optical fibers 2 and 9. This is because it can be done.

In the above structure, it is possible to use a light emitting diode or a laser diode as the light source 1. However, since the light emitted from the laser diode is substantially linearly polarized light, when passing through the optical fiber 2, the polarization plane changes due to the stress-induced birefringence generated in the optical fiber 2 and passes through the polarizer 4. There is a problem that the intensity of the emitted light becomes unstable. Therefore, a light emitting diode that emits unpolarized light is used as the light source 1.

The reason why the half-wave plate 5 is used is that the polarization plane is rotated by 45 degrees and the relative principal axis angle between the polarizer 4 and the analyzer 7 is set to 45 degrees to maximize the sensitivity of the optical magnetic field sensor. Is. Further, the magnetic field and the path of the light passing through the magneto-optical material 6 are parallel. In this case, even if the positions of the half-wave plate 5 and the magneto-optical material 6 are exchanged, there is no serious problem in terms of characteristics.

As a material for the magneto-optical element 6 used in the optical magnetic field sensor having such a structure, lead glass and ZnS are used.
There are paramagnetic materials such as e, BGO, BSO, etc., or diamagnetic materials, but recently, as mentioned above, current measurement of power lines and distribution lines, GIS, current transformers for measuring instruments in cubicles are also equipped with optical magnetic field sensors. The use of magnetic garnet is highly promoted, and the optical magnetic field sensor is required to have high sensitivity, small size, and low price. The development of the optical magnetic field sensor using is started.

By the way, as the above-mentioned magneto-optical element, R
When using a ferromagnetic material such as IG (Bi: Rare Earth Iron Garnet) that exhibits a multi-domain structure with perpendicular magnetization, RIG is used.
The light transmitted through causes a diffraction phenomenon, and the 0th order light, the 1st order light, ~
It spreads according to the nth order light and the order (however, when the magnetic field is 0, only the odd order light). This is because the magnetic domain serves as a phase grating.

When all of these diffracted lights are taken in, the linearity of the output from the signal processing circuit with respect to the strength of the magnetic field, that is, the output from the optical magnetic field sensor becomes extremely good (see, for example, Japanese Patent Laid-Open No. Hei. -126924).
However, in order to capture all the diffracted light, it is necessary to select an appropriate lens, and moreover, it can be realized only by a precise arrangement of the lens and other optical components (hereinafter referred to as alignment). Therefore, such an optical magnetic field sensor is extremely low in mass productivity.

On the other hand, if not all the diffracted light is taken in, the alignment at the time of manufacturing the optical magnetic field sensor becomes easy, and it becomes possible to realize an inexpensive optical magnetic field sensor. The output from the circuit will not be linear. The reason for this will be described by using mathematical expressions when only the 0th-order light is captured.

When the input to the magneto-optical element is I _in ,
The output I _o from the photodetector 10 is represented by I _o = I _in [cos θ cos φ + (Ho / Hs) sin θ sin φ sin (ωt)] ² (1). Where θ is the Faraday rotation angle of the magneto-optical material, φ is the relative principal axis angle between the two polarizers, Ho is the externally applied magnetic field, Hs is the saturation magnetic field, ω is the frequency of the alternating magnetic field (current frequency), and t is It's time. The output I _o is the direct current component I
Separated into _DC and AC components I _AC , I _DC = I _in [cos ² θcos ² φ + (Ho ² / 2Hs ² ) sin ² θsin ² φ] (2) I _AC = I _in [(Ho / 2Hs) sin2 θsin2 φsin (ωt) − (Ho ² / 2Hs ² ) sin ² θsin ² φcos (2ωt)] (3). Further, I ₁ = I _AC / I _DC (4) is obtained by the divider in the signal processing circuit. Then, the effective value C of I ₁ is expressed by the following equation (5). _{C = V AC / V DC (} 5) where, V _AC is the effective value of I _AC, V _DC represents the effective value of I _DC, represented by each following equation (6) and (7). ^{_{V DC = I in [cos 2}} θcos 2 φ + (Ho 2 / 2Hs 2) sin 2 θsin 2 φ] (6) V AC = I in [(Ho 2 / 8Hs 2) sin 2 2θ sin 2 2φ + (Ho 4 ^{/ 8Hs 4) sin 2 θsin 2} φ] 1/2 (7) equation (5), (6), as is clear from (7), in this case,
C does not have a linear relationship with the strength of the externally applied magnetic field.

[0013]

As described above, an optical magnetic field sensor that uses a ferromagnetic material having a multi-domain structure with perpendicular magnetization such as RIG for a magneto-optical element has high sensitivity but strength of magnetic field. If the linearity of the output from the signal processing circuit is improved, the alignment between the optical components becomes difficult and the mass productivity is reduced. On the contrary, if the configuration is easy, the linearity is deteriorated. In addition, there are various problems such as a large phase angle.

The present invention has been made in order to solve such a problem, and an object thereof is to have a high sensitivity and to have a high output linearity with respect to the strength of a magnetic field. An object of the present invention is to provide an optical magnetic field sensor which has high accuracy and is excellent in mass productivity, and a magnetic field strength measuring method using the same.

[0015]

In order to achieve the above object, the present invention comprises a light source, a polarizer, a magneto-optical element, an analyzer, a photodetector and a signal processing circuit. In the optical magnetic field sensor, the signal processing circuit divides the output value of the photodetector into a fundamental wave component and a second harmonic component,
It is composed of a circuit that divides the second harmonic component by the fundamental component.

Further, in the measuring method by the optical magnetic field sensor of the present invention, the optical signal from the magneto-optical element is photoelectrically converted by the photodetector, and the value obtained as the electric signal is converted into the fundamental wave component and the second harmonic by the signal processing circuit. The magnetic field strength is obtained from a value obtained by dividing the second harmonic component by the fundamental component by dividing the second harmonic component.

[0017]

EXAMPLE A concrete structure of an example of the present invention and a test result thereof will be described with reference to FIGS. FIG. 2 shows the construction of an actually manufactured optical magnetic field sensor of the present embodiment, and the reference numerals in the figure correspond to those shown in FIG.
Therefore, first, the configuration of this prototype embodiment will be specifically described.

The light source 1 is a light emitting diode having a wavelength of 0.85 μm, and the photodetector 10 is a Si diode. The optical fibers 2 and 9 are multimode fibers, and the lenses 3 and 8 are self-focal lenses. Both the polarizer 4 and the analyzer 7 are polarization beam splitters, and (YbTbBi) ₃ Fe ₅ O ₁₂ is adopted as the magneto-optical element 6. Further, in order to place the polarizer 4 and the analyzer 7 on the same surface and to set the relative principal axis angle to 45 degrees, the half-wave plate 5
Is arranged immediately before the magneto-optical element 6.

Further, the signal processing circuit 11 has two bandpass filters and a divider, and the S of the photodetector 10 is
The electric signal input from the i diode is separated into a fundamental wave component and a second harmonic component by two band pass filters, and a value obtained by dividing the second harmonic component by the fundamental wave component by a divider is output. Has become. The distance between the lens 8 and the optical fiber 9 is set so that only the 0th order light is coupled to the optical fiber 9.

Next, the reason why the object of the present invention is achieved by the above-mentioned structure will be described.
It is extremely difficult to take in all the diffracted light from the magneto-optical element 6 as described above, but it is easy to take out only the 0th-order light among the diffracted lights. In order to consciously capture only the 0th-order light into the photodetector 10, the distance between the magneto-optical element 6 and the lens 8 should be separated in the figure, or the distance between the lens 8 and the optical fiber 9 should be adjusted as in this embodiment. Can be easily achieved with.

[0021] When only the 0 order light taken into the light detector 10, an output I _o from the photodetector 10, the alternating current component I _AC of the output I _o, respectively formula as described above (1) wherein It is represented by (3). The AC component I _AC is composed of the following fundamental wave component I _AC1 of frequency ω and second harmonic component I _AC2 of frequency 2ω. I _AC1 = I _in [(Ho / 2Hs) sin2θsin2φsin (ωt)] (8) I _AC2 = I _in [(Ho ² / 2Hs ² ) sin ² θsin ² φcos (2ωt)] (9) Therefore, the second harmonic A value I ₂ obtained by dividing the component I _AC2 by the fundamental wave component I _AC1 is represented by I ₂ = I _AC2 / I _AC1 = Ho tan θ tan φ cos (2ωt) / [4Hs sin (ωt)] (10), and I ₂ It can be seen that the effective value of is obviously proportional to the externally applied magnetic field Ho. Therefore, output linearity is obtained with respect to the strength of the magnetic field.

Next, the test results of this embodiment having the above structure will be described. An AC magnetic field 50H _Z to optical magnetic field sensor of the present embodiment is varied within a range of 0～700O _e is applied, the signal processing circuit 11 to changes in the intensity of the magnetic field
The result of examining the effective value of the output voltage from is shown in FIG. In FIG. 3, the solid line a is the result, and it can be seen that the output from the signal processing circuit 11, that is, the output of the optical magnetic field sensor obviously changes linearly.

FIG. 4 shows the ratio error R obtained by comparing the above actual measurement result with a perfect straight line. In FIG. 4, the solid line a is the ratio error obtained by the optical magnetic field sensor of this embodiment. As can be seen from this figure, the ratio error of this embodiment is 0.
%, And a perfect linear relationship is obtained between the change in magnetic field and the change in output.

In order to make a comparison with the optical magnetic field sensor of this embodiment, the signal processing circuit of the above-mentioned conventional example, that is, the electric signal photoelectrically converted and input is divided into an AC component and a DC component by a filter and divided. The characteristics were evaluated by using a signal processing circuit that outputs a value obtained by dividing the AC component by the DC component by the detector. All other conditions are the same as in this embodiment. The results are as shown by broken lines b in FIGS. 3 and 4, respectively. Linearity clearly worse, the ratio error magnetic field strength was as high as -2.3% at 35o _e.

The arrangement of the optical components of the optical magnetic field sensor of the present invention is not limited to that shown in FIG. 2, but may be the arrangement shown in FIG. In addition, the present invention is not limited to the above-described configuration having all the units. Furthermore, the present invention is not limited to the above-mentioned measurement of a transmission line and the like,
It goes without saying that it can be applied to general AC magnetic field measurement.

[0026]

As described above, according to the present invention, the output can be obtained linearly according to the strength of the magnetic field, and the magnetic field with high sensitivity and high accuracy can be measured. Moreover, since it is possible only by taking in the 0th-order light into the photodetector, alignment between optical components is easy, mass productivity is excellent, and cost reduction is possible.

[Brief description of drawings]

FIG. 1 is a basic configuration diagram of an optical magnetic field sensor necessary for explaining the present invention.

FIG. 2 is a configuration diagram of an optical magnetic field sensor that is an embodiment of the present invention.

FIG. 3 is a diagram showing the relationship between the strength of a magnetic field and the output of an optical magnetic field sensor.

FIG. 4 is a diagram showing a relationship between a magnetic field strength and a ratio error.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 light source 2,9 optical fiber 3,8 lens 4 polarizer 5 half-wave plate 6 magneto-optical element 7 analyzer 10 photodetector 11 signal processing circuit

Claims (2)

[Claims] 1. An optical magnetic field sensor having a light source, a polarizer, a magneto-optical element, an analyzer, a photodetector, and a signal processing circuit, wherein the signal processing circuit outputs the output value of the photodetector. An optical magnetic field sensor comprising a circuit that divides a fundamental wave component and a second-order harmonic component and divides the second-order harmonic component by the fundamental wave component. 2. In a magneto-optical sensor using a magneto-optical element, an optical signal from the magneto-optical element is photoelectrically converted by a photo detector, and a value obtained as an electric signal is converted into a fundamental wave component and a secondary wave by a signal processing circuit. A magnetic field strength measuring method, characterized in that the magnetic field strength is obtained from a value obtained by dividing the second harmonic wave component by the fundamental wave component by dividing the second harmonic wave component.