JPH04143679A - Optical magnetic field sensor - Google Patents

Abstract

PURPOSE:To enable fluctuation of sensitivity for temperature change to be restricted and a stable and highly sensitive measurement to be made by obtaining strength of a magnetic field to be measured from output difference of two types of measurement lights with different wavelengths which are transmitted through a magnetic optical element. CONSTITUTION:Measurement lights with different wavelengths lambda1 and lambda2 which are emitted from light sources 5 and 5' are synthesized 6 for enabling them to be transmitted through a magnetic optical element and to be emitted from a polarizer 1', they are divided into measurement lights with the wavelengths lambda1 and lambda2 spectrally by a dichroic mirror 7 for detecting with light detectors 8 and 9, difference of detection outputs I1 and I2 is obtained as a sensor output I by a subtraction circuit 10, and then it is converted to a strength of a magnetic field to be measured. In this case, when an output of the light sources 5 and 5' is adjusted to an output ratio (approximately 0.25) of the detectors 8 and 9 where fluctuation of sensor sensitivity for temperature change becomes the minimum, film thickness of a magnetic garnet film of an element 2 is set to approximately 200mum, and the strength of the magnetic field to be measured is changed by + or -40 Oe within a temperature range of -20 - +60 deg.C, sensitivity of the sensor output I becomes nearly constant, the amount of fluctuation is kept to be within + or -2%, and an absolute value itself of sensitivity becomes approximately 0.015, which is highly sensitive.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to an optical magnetic field sensor that uses a magneto-optical element to measure a magnetic field generated from an electric current or a magnet, and is particularly applicable to a weak magnetic field and a non-uniform magnetic field. The present invention relates to a highly sensitive optical magnetic field sensor that can detect

[Conventional technology]

In general, various measurements using light are performed by taking advantage of the characteristics of light, such as electrical insulation, non-induction, and complete explosion-proofness. Among these, there is an optical magnetic field sensor that uses magneto-optical elements, and Fig. 3 shows an example of the configuration of such a conventional optical magnetic field sensor. The planes of polarization of are tilted 45° to each other. 2 is a magneto-optical element made of a magnetic garnet film having a large magneto-optical effect (Faraday effect), and 3 is provided to remove the influence of light diffraction due to the magnetic domains of the magnetic garnet film of the magneto-optical element 2. A condenser lens 4 is a photodetector, and by measuring the amount of measurement light transmitted through the magneto-optical element 2, the intensity of the magnetic field to be measured can be determined. In this case, the measured light amount (hereinafter simply referred to as output) of the photodetector 4 corresponding to the magnetic field to be measured H is given by the following equation.

I=Io(1+H-sin (2θf-d)/Hs)
...(1) However, Io is the output when the magnetic field to be measured is zero, θf is the Faraday rotation coefficient, d is the thickness of the magnetic garnet film, and H
s is the magnetic field strength required to saturate the magnetic garnet film. Therefore, if the change in the above output when the magnetic field to be measured changes by ΔH is Δ■, then the sensitivity of the optical magnetic field sensor Δ■
/ΔH is expressed by the following equation (2).

ΔI/ΔH= I o sin (2θf −d)
/H5... (2) Such optical magnetic field sensors are being put into practical use, for example, in the electric power field, for measuring the current of power transmission lines and determining fault zones. The magneto-optical properties are required to be stable against changes.

That is, the sensitivity expressed by equation (2) is, for example, -20 to +8
It must be constant within the temperature range of 0°C. the above(
In equation 2), the value that changes depending on the temperature is θ[
and Hs, and the temperature change in sensitivity depends only on the magnetic garnet film. In addition, the fluctuation rate of the sensitivity sin (2θf-d)/Hs of the magnetic garnet film used as a current sensor in the above temperature range -20 to +80°C is ±2%.
Since it is only a small amount, there is no problem in practical use.

On the other hand, in the current sensor of the above example, the strength of the magnetic field to be measured is about several hundred e, and the Hs of the magnetic garnet film used therein is about 10,000 e, so its sensitivity sin
(2θ[・d)/H5 is sin (2θf-d)≦1
Therefore, it is about 0.001.

However, with the diversification of devices in recent years, for example, 100
There is a growing demand for a highly sensitive optical magnetic field sensor capable of detecting weak magnetic fields of less than e, that is, capable of detecting weak magnetic fields and minute non-uniformities in magnetic fields. An example of this is the detection of minute inhomogeneities in a magnetic field caused by minute defects in permanent magnets, but the higher the sensitivity of the magnetic garnet film used in such a highly sensitive optical magnetic field sensor, the better (for example, , 0.01 or more). In order to increase the sensitivity of a magnetic garnet film, it is necessary to reduce its Hs. Examples of such a highly sensitive magnetic garnet film include (GdBi)3(FeAIGa)sO+2 or (GaBi)s (FeAI) io+2 etc. are known (13th Japanese Society of Applied Magnetics Academic Conference Abstracts, P2
S5).

[Problem to be solved by the invention]

However, in conventional optical magnetic field sensors, although the saturation magnetization must be reduced in order to reduce Hs of the magnetic garnet film, when the saturation magnetization is reduced, the Faraday rotation coefficient θf decreases as the Curie temperature decreases.
There is a problem in that the Hs of the magnetic garnet film changes rapidly with respect to temperature. Therefore, simply reducing the Hs of the magnetic garnet film will reduce the fluctuation of the highly sensitive magnetic garnet film with respect to temperature changes in the above-mentioned temperature range -10 to +60°C. Rate is ±30~±
This increases to 40%. Therefore, the allowable range of use of conventional optical magnetic field sensors has been limited to constant temperatures. Furthermore, when using the device in an environment with temperature changes, it is necessary to calibrate the output of the optical magnetic field sensor by attaching a separate temperature sensor.

In view of these circumstances, it is an object of the present invention to provide a highly sensitive optical magnetic field sensor that can particularly reduce fluctuations in sensitivity due to temperature.

[Means to solve the problem]

The optical magnetic field sensor according to the present invention is configured to allow two types of measurement light having different wavelengths to pass through a magneto-optical element, and to determine the intensity of the magnetic field to be measured from the output difference between these measurement lights.

[Effect]

According to the present invention, since the strength of the magnetic field to be measured is determined by the output difference between measurement lights of different wavelengths, it is possible to suppress fluctuations in sensitivity to temperature changes, thereby stably and accurately measuring the magnetic field strength. can be measured.

〔Example〕

Hereinafter, an embodiment of the optical magnetic field sensor according to the present invention will be described based on FIGS. 1 and 2, using the same reference numerals for the same members as in the conventional example. First, in this embodiment, (GaBi)s(
FeAIGa)sO+t is used. And in the figure, l
, 1' is a polarizer, 2 is a magneto-optical element, and 3 is a condensing lens, and these structures are basically the same as in the conventional example, but furthermore, 5 and 5' are wavelengths λ1 and λ, respectively. ! A light source 6 is a gicroic mirror for combining the measurement lights emitted from these light sources 5 and 5'.

The two types of measurement lights with different wavelengths are transmitted to the magneto-optical element 2.
These measurement lights are transmitted by the dichroic mirror 7 and are then split again into measurement lights of wavelength λ1 and wavelength λ2.

In the figure, 8 is a photodetector for detecting the measurement light of wavelength λ1 divided by dichroic mirror 7, 9 is a photodetector for detecting the measurement light of wavelength λ2 divided by dichroic mirror 7, and lO is a rain detector. The outputs 11 and I2 of the detectors 8 and 9 are input, and the difference between the two outputs is I=1.
-1. This is a subtraction circuit configured to calculate . Then, this output difference is determined as the sensor output I, and the intensity of the magnetic field to be measured is converted from the sensor output ■.

In the above case, the light sources 5 and 5' have respective wavelengths λ+
= 1.30 μm and λ2 = 0.83 μm, and photodetectors 8 and 9 are used.
A germanium photodiode and a silicon photodiode were used, respectively.

The optical magnetic field sensor according to the present invention is constructed as described above, and its operation will be explained next. First of all, θLゎ=θ
, ,+anT, the sensor sensitivity S is given by (2) above.
From the equation, it is expressed by the following equation (3).

... (3) However, 1. = and I2- are the wavelength λ when the magnetic field strength is zero
The outputs of the photodetectors 8 and 9 for the measurement light of I and wavelength λ2, 0.1' and θ, 2' are the wavelength λ1 and wavelength λ2, respectively.
The Faraday rotation coefficient at 0°C of the magnetic garnet film for the measurement light is θz −= l 471d, respectively.
eg, and θT2=5135deg. Further, a and b are the temperature coefficients of the magnetic garnet film for the measurement light of wavelength λ1 and wavelength λ2 in the temperature range -20 to +60°C, a = -3,81 deg, / 'C and b
= -13.08 deg, /°C. Hs- and C
are the magnetic field strength and its temperature coefficient required for saturation of the magnetic garnet film at 0°C, and are Hs −=163.20 e and c = −2,180 e/′C. And T
is the temperature (°C).

By the way, aT and bT in the above formula (3) are each θ.

Since this value is extremely small compared to θ and 2′, this (
Equation 3) can be approximated by the following equation (4) by expanding it at the temperature T and summarizing it using first-order terms.

S = C+ [2[12-b -cos2(θt2-
d) −I, −a −cos2(θ++−a)ld[
1! -5in2(θI! -d)! + -5in2(θ,, -d)l c/Hs -)
T...(4) Since C is a constant in this equation (4), the above (
4) The term temperature T in the equation may be set to 0. That is, when the output ratio of the photodetectors 8 and 9 is 1g-/I+-=α, the following equation (5) may be satisfied.

... (5) Now, once the magnetic garnet film to be generally used is decided, θf
1', θf2-1 al bg HS- and C are specified, so in the above equation (5), there are two variables, α and d. Other than these, θ, 1-2θ12+a and b are usually determined by magneto-optical measurement, and Hs and C are determined by magnetic measurement, and their specific values are as described above. Therefore, by substituting these values into equation (5) and performing numerical calculations, we find α and d that most satisfy equation (5). In this case, the incident light intensity of each measurement light with wavelength λ1 and wavelength λ2 or by adjusting the gain of the photodetectors 8, 9 for each measurement light, it is possible to adjust the output ratio α, thereby making the sensor sensitivity almost unaffected by temperature changes. can.

By substituting the above-described specific values of the magnetic properties and magneto-optical properties of the magnetic garnet film used in this example into equation (5), the output of the photodetectors 8 and 9 minimizes the fluctuation of the sensor sensitivity S with respect to temperature changes. Determining the ratio α and the film thickness d of the magnetic garnet film, α=0.25. d=200 μm. Therefore, the thickness of the magnetic garnet film was set to 200 μm, and the output ratio α
The output of the laser diode of the light source 5.5' was adjusted so that The outputs of photodetectors 8 and 9 (L, Ig
) and their output difference obtained by the subtraction circuit 10, that is, the change in the output ■ of the subtraction circuit IO, the sensitivity ΔI, /ΔH
2Δ1. The results of determining /ΔH and Δ1/ΔH are shown in FIG. In FIG. 2, curves a and 2 show changes in sensor sensitivity when the outputs of photodetectors 8 and 9 are used alone, and as is clear from the figure, the temperature range is -20 to +
There is a variation of ±60% or more with respect to 60°C.

On the other hand, the sensitivity of the output of the subtraction circuit 10 and the sensor output ■ is almost constant in the temperature range -20 to +60°C, as shown by curve C, and the amount of variation is suppressed within ±2%, and the absolute sensitivity The value itself is 0.015, which is extremely high sensitivity.

In addition, in the above example, the wavelength of the measurement light is λ1.30 μm.
, λ2=0.83 μm, but this is not limited to this case. Even when using a measuring light of 78 to 1.55 μm, the temperature range is 20 to 20 μm as in the above example.
The amount of variation in sensitivity for temperatures up to +60°C can be suppressed to within ±2%. However, if the difference in wavelength between the two types of measurement light is too small (for example, in the case of 0.78 μm and 0.83 μm), the absolute value of sensitivity tends to become small. In addition, the wavelength of the other measurement light is set to 1. By setting each band in the range of 3 to 1.5 μm, good results can be obtained even in terms of the absolute value of sensitivity.

Furthermore, for the magnetic garnet film constituting the magneto-optical element, a magnetic material having a single domain structure can be used instead of one having a stripe domain structure.

〔Effect of the invention〕

As described above, according to the present invention, an optical magnetic field sensor that measures the intensity of a magnetic field using a magneto-optical element uses two types of measurement lights with different wavelengths, and calculates the output difference after they pass through the magneto-optical element. By doing so, the strength of the magnetic field to be measured can be measured stably and with high accuracy over a wide temperature range.

In addition, temperature compensation for sensor sensitivity can be performed in the signal processing section of the magnetic field sensor. We were able to create an optical magnetic field sensor with high sensitivity and excellent stability even when using

[Brief explanation of the drawing]

FIG. 1 is a schematic overall configuration diagram of an embodiment of the magneto-optical sensor according to the present invention, and FIG. 2 is a comparison of the relationship between temperature change and sensor sensitivity of the magneto-optical sensor according to the present invention with that of a conventional magneto-optical sensor. The graph shown in FIG. 3 is a schematic overall configuration diagram of a conventional optical magnetic field sensor. 1.1'...Polarizer, 2...Magneto-optical element, 3...
- Condensing lens, 5, 5'... Light source, 6, 7... Dichroic mirror 8.9... Detector, IO... Subtraction circuit. Procedural amendment (spontaneous) 1-1゜ Indication of the case Patent application No. 2-267215 2゜ Name of the invention Optical magnetic field sensor 4゜ Agent 5-19 Shinbashi, Minato-ku, Tokyo 105 Telephone Tokyo (432) 4576 5゜Object of amendment 6, contents of amendment (1) Amend the power of attorney as shown in the attached sheet. (2) Correct the description in the patent applicant column of the application as attached. (3) "Also," on page 3, line 14 of the specification, "1, therefore,"
I am corrected. (4) Insert "However, even in this case, it is desirable that the fluctuation in sensitivity be small in the temperature range of -20 to 60° C." before "and" on page 4, line 12 of the same book. (5) Change “-10 to +60” from page 5, lines 7 to 8 of the same book to “
Corrected to -20 to +60J. (6) “Magnetic field strength” on page 8, line 7 of the same book is corrected to “Magnetic field strength at 0°C: 1.” 7. List of attached documents (1) Power of attorney (2 copies)

Claims (1)

[Claims] In a magneto-optical field sensor that measures the intensity of a magnetic field to be measured by measuring measurement light from a light source transmitted through a magneto-optical element, two types of measurement light having different wavelengths are transmitted through the magneto-optical element, and these two types of measurement light are An optical magnetic field sensor characterized in that the intensity of a magnetic field to be measured can be determined from the difference in the output of the measuring light.