JPH05264603A - Apparatus and method for measuring photomagnetic field - Google Patents

Abstract

PURPOSE:To simplify the structure of a photomagnetic field measuring apparatus and make its size compact by computing the light intensity ratio of a ray component which vibrates with twice as high frequency of a magnetic field frequency and a ray component corresponding to the direct current component among 0-order diffracted rays which pass a magnetooptical element device. CONSTITUTION:Light is led to a magnetooptical element device 1 from a light- emitting element device through an optical fiber, etc., and among the diffracted rays which pass the element device 1, only 0-order diffracted rays are drawn out to an optical fiber through a collimator lens 3' and converted into electric signals. Then, the intensity I (2omega) of a ray component with twice as high frequency of the magnetic field frequency omega of a magnetic field measuring object (electric cable 8) and the intensity I0 corresponding to the direct current component among the electric signals are sent to frequency component detectors 5 and 6, respectively and observed as electric signals. Next, based on these signals, the ratio; k=I(2omega)/I0 is computed by a dividing apparatus 7 and the magnetic field intensity Homega of the measuring object is computed based on the prescribed equation wherein Hs stands for saturated magnetic field of the magnetooptical element device and theta1 for saturated Faraday rotation angle.

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 (current) measuring device and an optical magnetic field (current) measuring method suitable for measuring currents in power transmission lines and distribution lines. The present invention relates to a novel optical magnetic field measuring device and an optical magnetic field measuring method that have an extremely simple structure and do not depend on operating temperature.

[0002]

2. Description of the Related Art Since an optical magnetic field sensor uses light as a medium, it has good insulating properties and is not affected by electromagnetic induction. It is used for etc. FIG. 6 shows an example of a magnetic field measuring device used conventionally. The optical system of this device is composed of a magneto-optical element 1, optical fibers 2, 2'collimator lenses 3 and 3 ', a polarizer 17 and an analyzer 18. The light emitted from the light source 4 becomes a parallel beam by the collimator lens 3, passes through the polarizer 17, and becomes a linearly polarized light. When a magnetic field is applied to the magneto-optical element 1, the plane of polarization of this linearly polarized light rotates in proportion to the strength of the magnetic field due to the Faraday effect. When this light passes through the analyzer 18, the amount of light changes depending on the angle of the plane of polarization, is condensed by the collimator lens 3 ′ onto the optical fiber 2 ′, and is detected by the light receiving element 19. Here, the ratio of the light intensity I (ω) of the light component vibrating at the magnetic field frequency ω of the light passing through the analyzer and the light intensity I ₀ of the light component corresponding to the direct current component is I (ω) / I _O = 2V _r LHω (where Hω is the AC magnetic field strength of the measured object vibrating at the frequency ω, V _r is the Verdet constant of the Faraday element, L is the thickness of the Faraday element), and the magnetic field strength Hω is I
It can be easily determined by observing (ω) / I _O.

However, such a conventional magnetic field measuring device has a drawback in that it is necessary to dispose a polarizer and an analyzer on both sides of the magneto-optical element, which complicates the structure of the sensor itself. In addition, the Verdet constant V _r of the magneto-optical element
Since is a function of temperature, there was also the problem that the measurement environment had to be constant.

As another conventional magnetic field measuring apparatus, for example,
Photocurrent as described in JP-A-1-223359
Magnetic field measuring devices are known. In this device, a constant bias magnetic field different from the AC magnetic field to be measured is separately added to the Faraday element of the sensor head, and the same angular frequency component (Eω) and the AC magnetic field as the AC magnetic field of the light passing through the Faraday rotator are added. The magnetic field is measured by extracting the angular frequency component (E2ω) that is twice as large as that of the above, and determining the relative value of the double angular frequency component (E2ω) with respect to the same angular frequency component (Eω). In this technique, unlike the technique, the V _r is a function of temperature there is an advantage that regardless can know the magnetic field intensity.

However, in the magnetic field measuring device disclosed in Japanese Patent Laid-Open No. 1-223359, it is necessary to dispose the polarizer and the analyzer on both sides of the magneto-optical element as in the above-mentioned conventional technique.
Further, since the bias magnetic field applying means is also required, the structure of the sensor is further complicated.

Therefore, an object of the present invention is to provide a novel optical magnetic field measuring apparatus and method having a simple structure which does not require a polarizer and an analyzer. Further, the present invention is
An object of the present invention is to provide an apparatus and method for measuring a magnetic field of a power transmission line or the like without requiring a polarizer, an analyzer and a bias applying unit.

[0007]

Means for Solving the Problems As a result of earnest studies and researches for solving the above problems, the present inventor has found that a polarizer and an analyzer are required by using a novel magneto-optical element utilizing a diffraction phenomenon. The inventors have succeeded in developing an optical magnetic field measuring device having a simple structure and a novel magnetic field measuring method using the device.

That is, according to the present invention, a magneto-optical element having a multi-domain structure in a state where a magnetic field is not applied, and a magnetic component parallel to the traveling direction of light in the magnetic domain is different from each other in adjacent magnetic domains, and light is applied to the element. Light emitting means for entering and light intensity I of a light component of the 0th-order diffracted light that has passed through the magneto-optical element and vibrates at a frequency twice the magnetic field frequency ω of the magnetic field measured object.
(2ω) and means for obtaining the light intensity I _O of the light component corresponding to the DC component of the 0th-order diffracted light, and the ratio I (2ω) / I _O
And an optical magnetic field measuring device having means for obtaining

According to another aspect of the present invention, the present invention has a multi-domain structure in the vicinity of a magnetic field-measuring object that generates a magnetic field and a light propagation in the magnetic domain when no magnetic field is applied. Light is made incident on magneto-optical elements different from each other in magnetic domains whose magnetization components parallel to the direction are adjacent to each other, and out of the diffracted light that has passed through the magneto-optical element, only the 0th-order diffracted light is extracted, and the magnetic field of the 0th-order diffracted light is extracted. The light intensity I (2ω) of the light component oscillating at a frequency 2ω that is twice the magnetic field frequency ω of the measurement object and the light intensity I _O of the light component corresponding to the direct current component are measured, and their ratio I (2ω) is measured. It is a method of measuring the magnetic field of the magnetic field measured object by obtaining / I _O.

According to another aspect of the present invention, the present invention has a multi-domain structure in the state where no magnetic field is applied, and the magnetic components parallel to the traveling direction of light in the magnetic domains are different from each other in adjacent magnetic domains. An optical element, a light emitting means for making light incident on the element, a magnetic field applying means for applying a constant bias magnetic field to the magneto-optical means, and a zero-order diffracted light passing through the magneto-optical element of a magnetic field measured object. A means for measuring the light intensity I (ω) of the light component oscillating at the magnetic field frequency ω and the light intensity I (2ω) of the light component oscillating at the frequency 2ω, which is twice that, and the ratio I (2
An optical magnetic field measuring apparatus having means for obtaining ω) / I (ω).

According to another aspect of the present invention, the present invention has a multi-domain structure in the vicinity of a magnetic field measuring object that generates a magnetic field in a state where no magnetic field is applied, and has a direction of travel of light in the magnetic domain. In a magnetic domain in which parallel magnetization components are adjacent to each other, light is incident on different magneto-optical elements while applying a constant bias magnetic field, and out of the diffracted light that has passed through the magneto-optical element, only the 0th-order diffracted light is extracted. Light intensity I of the optical component of the diffracted light of the second order that oscillates at the magnetic field frequency ω of the magnetic field measurement object
(ω) and the light intensity I of the light component that oscillates at twice that frequency
(2ω) is measured, respectively, and their ratio I (2ω) / I (ω)
Is a method of measuring the magnetic field strength of the object to be measured.

The magneto-optical element used in the present invention is a material having a multi-domain structure when no magnetic field is applied. As shown in FIG. 3, the magneto-optical element is a magnetic domain in which the magnetization vector component with respect to the traveling direction of incident light is adjacent. They are arranged differently from each other. For example, a magnetic garnet material in which a large amount of Bi has been replaced is usually perpendicularly magnetized and has a multi-domain structure as shown in FIG. When linearly polarized light parallel to the magnetization direction enters such a multi-domain structure, the polarization plane rotates due to the Faraday effect, and the polarization plane of the light is + θ _f in the magnetic domain in which the magnetization is in the same direction as the traveling direction of light. On the other hand, the magnetic domain having the opposite magnetization rotates by −θ _f . In this way, the rotation angle of the plane of polarization differs depending on the location of the magnetic domain, so that diffraction occurs. Such a multi-domain structure remains when a magnetic field smaller than the saturation magnetic field is applied, and when light enters the multi-domain structure, the multi-domain structure acts as a diffraction grating to diffract a part of the incident light. As a result, the detection light causes diffraction loss. Here, the light intensity I of the n = 0th order light, that is, the straight traveling light, due to the occurrence of the diffraction loss, is represented by the following equation (1), which is an approximate expression that favorably reproduces the actual measurement value:

[Number 1] I = P _O in _{^{(cos 2 θ f + (H}} / H s) 2 sin 2 θ f) ··· (1) ( wherein, the theta _f saturated Faraday rotation angle, H is an external magnetic field,
H _s is the saturation magnetic field, P _O is the light intensity above the saturation magnetic field,
| H | ≦ Hs) It should be noted that I = P _O for | H |> Hs. Therefore, the external magnetic field H is obtained by observing the 0th-order diffracted light intensity I from the magneto-optical element.

The arrangement of the magneto-optical element with respect to the incident light is preferably such that the magnetization directions in the adjacent magnetic domains are parallel to the incident light and parallel to each other as shown in FIG. 3, but the magneto-optical element has perpendicular magnetization. The element itself may be arranged obliquely with respect to the incident light. Further, a plurality of magneto-optical elements may be used in superposition for the purpose of adjusting the amount of light attenuation.

Examples of the material having a multi-domain structure without applying a magnetic field as described above include Bi-substituted rare earth iron garnet materials, rare earth iron garnets, orthoferrites and the like produced by the LPE method or the like. However, the material is not particularly limited to these, and various materials having a multi-domain structure can be used within the range in which the object of the present invention can be achieved. By cutting out these materials so that the easy axis of magnetization is in a direction perpendicular to the plane, a thin film with perpendicular magnetization is generally obtained and can be used as the element of the present invention. Particularly, in the case of the Bi-substituted rare earth iron garnet film produced by the LPE method, the film has perpendicular magnetizability as it is without any special treatment due to the growth-induced magnetic anisotropy, and the object of the present invention is achieved. It is preferable to do so. Moreover, since this Bi-substituted rare earth iron garnet material has a large Faraday rotation ability, it is suitable in that a large diffraction loss can be obtained with a small thickness.

FIG. 1 shows a specific example of the optical magnetic field measuring apparatus according to the first aspect of the present invention. Magneto-optical element 1, light emitting means 4 for making light incident on the element, collimator lens 3,
3 ', a light receiving means 19 for receiving the 0th-order diffracted light and converting it into an electric signal, a means 5 for measuring the light intensity of a light component vibrating at a frequency twice the magnetic field frequency ω of the object to be measured, and 0 having a ratio I (2ω) / I _O seek means 7 and means 6 for determining the light intensity I _O of the optical component wherein the I (2 [omega) which corresponds to the DC component of the diffracted light. As the means 5, 2 of the magnetic field frequency ω
It is convenient to measure the light intensity I (2ω) of the light component vibrating at the double frequency as the electric signal intensity I ′ (2ω) through the light receiving means 19. Similarly, as the means 6, a means for measuring the light intensity I _O of the light component corresponding to the DC component of the 0th-order diffracted light as the electric signal intensity I ′ _O through the light receiving means 19 is convenient. In the figure, 2 and 2 ′ are optical fibers. In order to take out only the 0th-order diffracted light, it may be arranged so that the diffracted light of n ≠ 0th order does not enter the optical fiber 2 ′. For example, a single mode fiber is used as the optical fibers 2 and 2 ′, and a collimator lens is used. This can be realized by forming a parallel beam optical system with 3, 3 '. Examples of the light emitting element include a laser and an LED, but the light emitting element is not particularly limited to these and various light sources can be used. Further, as the means 7 for obtaining the ratio I (2ω) / _IO ,
For example, a division circuit can be used.

Such a device is installed in the vicinity of the electric wire cable 8 which is the magnetic field measured object, and the magnetic field is measured by the following operation. First, light is made incident on the magneto-optical element 1 from the light emitting element 4 through an optical fiber or the like, and out of the diffracted light that has passed through the magneto-optical element, only the 0th-order diffracted light is extracted to the optical fiber 2 ′ through the collimator lens 3 ′, It is converted into an electric signal by the light receiving element. Next, the intensity I (2ω) of the frequency component twice the magnetic field frequency ω of the object to be measured and the intensity I _O corresponding to the DC component of this electric signal are input to the frequency component detectors 5 and 6, respectively. Observe as an electric signal. Then to calculate the Hω represented by their ratio k = I (2ω) / I O and the following formula (2) by divider 7 these signals.

## EQU2 ## Hω = (H _S / tan θ _f ) √ {2k / (1-k)} (2) (where, Hω is the magnetic field strength of the object to be measured, and H _S is the magneto-optical element) , The saturated magnetic field, θ _f is the saturated Faraday rotation angle).

It will be shown below that k is calculated and the magnetic field strength Hω of the object to be measured is derived by the equation (2). The magnetic field H in the magneto-optical element generated from the electric current of the power transmission line or the like as the DUT is expressed by the following equation:

## EQU3 ## H = Hωsinωt (3) (where ω is the frequency of the AC electric field of the transmission line and Hω is the amplitude of the magnetic field). Substituting the formula (3) into the formula (1). And obtain the following equation.

Equation 4] _{I = I O -I (2ω)} cos2ωt ····· (6) _{Here, I O = P O (cos} 2 θ f + (Hω 2/2
H _S ² ) sin ² θ _f ) I (2ω) = P _O (Hω ² / 2H _S ² ) sin ² θ _f . When Hω is calculated with k = I (2ω) / _IO , the following (2)
Get the expression.

Hω = (H _S / tan θ _f ) · √ {2k / (1-k)} (2)

Equation (2) shows that when k << 1, Hω≈ (H
It becomes even simpler as _S / tan θ _f ) √ (2k). Here, in order to measure Hω stably with respect to temperature, (H _S
It suffices to reduce the temperature dependence of / tan θ _f ). Let T be the temperature,

[0019]

[Equation 6]

For example, in a magneto-optical element having the composition Bi _1.5 Y _1.5 Fe ₅ O ₁₂ ,

[Equation 7] By appropriately adjusting the material composition and the Faraday rotation angle θ _f (that is, the thickness) in this way, the magneto-optical element can be made independent of the ambient temperature.

The method and apparatus of the present invention do not require a polarizer and an analyzer as compared with the prior art, and are further described in JP-A-1.
Since the temperature dependence of the device can be eliminated without applying a bias magnetic field as in the technique of -223359, the magnetic field can be measured more easily. Further, the device itself can be further simplified.

Next, FIG. 4 shows a specific example of the optical magnetic field measuring apparatus according to the second aspect of the present invention. In the figure, in the optical magnetic field measuring apparatus of the present invention of FIG. 1, magnetic field applying means 9 are installed on both sides of the magneto-optical element 1, and I is replaced by I _O of the apparatus of FIG.
A device similar to that of FIG. 1 except that a device for measuring (ω) and a device for obtaining the ratio I (2ω) / I (ω) are provided instead of the device for calculating the ratio I (2ω) / I _O. Is. Here, for example, a cylindrical permanent magnet, a coil, or the like can be used as the magnetic field applying means, but the magnetic field applying means is not particularly limited thereto, and various means can be used as long as they can apply a constant magnetic field to the magneto-optical element 1. Any thing can be used. As a device for measuring I (ω), for example, a frequency component detector similar to I (2ω) is used, and as a device for calculating the ratio I (2ω) / I (ω), for example, a divider is used. be able to.

A device as shown in FIG. 4 is installed in the vicinity of the electric cable 8 which is a magnetic field measured object to measure the magnetic field. The method of measuring the magnetic field is basically the same as in the case of the device shown in FIG. 1, but in this method, the magnetic field applying means applies a constant magnetic field to the magneto-optical element while Observe origami. As the light components of the diffracted light observed here, the light intensity I (ω) and I of the light component oscillating at the frequency ω of the voltage of the transmission line which is the object to be measured and the light component oscillating at the frequency 2ω that is twice that Measure (2ω) respectively. And the following formula (5):

[Equation 8] Hω = (4I (2ω) / I (ω)) Hb (5) (where, Hb is the bias magnetic field strength) is used to calculate the magnetic field strength Hω of the transmission line. be able to. The principle of calculating the magnetic field Hω using the equation (5) will be described below. In this case, since the bias magnetic field is applied to the magneto-optical element separately from the magnetic field from the magnetic field measured object, the magnetic field received by the magneto-optical element is as follows as a whole.

[Equation 9] H = Hb + Hωsinωt This magnetic field H is substituted into the equation (1) for obtaining the 0th-order diffracted light intensity, and the following equation (4) is obtained.

## EQU10 ## I = _IO + I (ω) sin ωt-I (2ω) cos2ωt (4) Here,

Equation 11] ^{_{I O = P O (cos 2}} θ f + {(Hb 2 + Hω 2/2) / H S 2} sin 2 θ f I (ω) = P O (2HbHω / H S 2) sin 2 θ _f I (2ω) = P _O (Hω ² / 2H _S ² ) sin ² θ _f Then, when the ratio of I (ω) and I (2ω) is calculated, I (ω) / I
(2ω) = 4Hb / Hω, and the above equation (5) is obtained.

As described above, Hω is I (2ω) / I (ω), H
It can be obtained from b, and since Equation (5) does not include θ _f and Hs, Hω can be stably measured without depending on the temperature characteristics of the magneto-optical element. However, it is necessary to sufficiently reduce the temperature dependence of Hb itself. The measurable range of the magnetic field Hω is Hω <Hs−Hb.

Although the optical magnetic field measuring apparatus and method of the present invention have been described with reference to FIGS. 1 and 4, the arrangement of the magneto-optical element, the light emitting means, the light receiving means, etc. is not limited to these figures, and various arrangements are possible. Arrangements can be adopted. For example, a mirror may be placed in close contact with or separated from one end of the magneto-optical element, and incident light may be returned to the incident direction by the mirror. Also,
As shown in FIG. 5, it is also possible to dispose the glass prism 12 having a reflecting mirror in the light detecting portion of the magnetic field measuring apparatus. In FIG. 5, the light emitted from the optical fiber becomes a parallel beam by the collimator lens 3, is reflected by the reflecting film 14, passes through the magneto-optical element 1, and is then reflected by the reflecting mirror 13. The reflected light passes through almost the same optical path and is focused on the optical fiber 2 ′ by the collimator lens 3. Further, the number of magneto-optical elements is not limited to one, and a plurality of magneto-optical elements can be stacked and arranged in the light traveling direction. Examples of the present invention will be shown below, but the present invention is not limited thereto.

[0026]

Example 1 An optical magnetic field measuring apparatus of the present invention as shown in FIG. 2 was produced.
In the apparatus shown in the figure, the thickness of the magneto-optical element 1 is 80 μm.
The composition was prepared by LPE method m Bi _1.5 Y _{1. 5} Fe ₅
An optical element of O ₁₂ was used. A reflective film 14 is formed on one end of this element.
Was formed by a vapor deposition method. Adhesive layer 1 on the other end
A lens (SML) was attached via an ultraviolet curing resin of No. 5. An optical fiber for incidence and an optical fiber supported by a glass tube are attached to the other side of this lens, and the glass tube and the optical fiber portion are attached to the SU as shown in FIG.
Surrounded by S-sleeve. The wavelength of the light source is 1.3
LED light of μm was used (not shown). And using a photodiode as the light receiving element (not shown), and the frequency omega of the magnetic field measured sample, 2 [omega and corresponds to the DC component to the intensity of light I (omega), I (2 [omega) and I _O respectively frequency component detection It was observed using a vessel. Further, as means 9 for applying a magnetic field to the magneto-optical element, coils were arranged on both sides of the element as shown in FIG.

[0027] (1) is applied to the apparatus of the magnetic field measuring Figure 2 when no bias magnetic field is an external magnetic field of 50Hz by a magnetic field applying means 9 measures the intensity of light I (2 [omega) and I _O, k =
I (2ω) / _IO was measured. When Hω is small, it was obtained as follows from the relational expression (2) between the amplitude Hω of the external magnetic field and k.

## EQU12 ## Hω (Oe) = 3150 × √k It has been found that this relationship hardly changes depending on the ambient temperature.

(2) Magnetic field measurement in the presence of a bias magnetic field A magnetic field applying means similar to the above (1) with a constant magnetic field of 500 (Oe) being applied as a bias magnetic field to the above apparatus using a permanent magnet. An external magnetic field of 50 to 50 Hz was applied. I (2ω), I (ω) and the ratio I (2ω) / I (ω) are measured, and the following formula is obtained from the relation of formula (5):

[Equation 13] Hω (Oe) = 2000 × I (2ω) / I (ω) was obtained, and it was confirmed that the external magnetic field Hω was easily obtained.

[0029]

The optical magnetic field measuring apparatus of the present invention does not use a polarizer and an analyzer, and therefore has a simple structure, can be miniaturized, and is easy to manufacture. Further, since neither a polarizer nor an analyzer is used, even if the polarization plane of the light propagating through the optical fiber changes, the light quantity does not change, and stable measurement is possible.
In addition, since the ratio of the light quantity is measured as I (2ω) / I (ω) and I (2ω) / I _O , stable measurement is possible even if the light quantity propagating through the optical fiber and the ambient temperature change due to environmental changes. it can.
Further, in the first optical magnetic field measuring apparatus of the present invention, since the temperature can be stabilized without the need for a bias magnetic field, the structure of the apparatus is simpler and the measuring method by it is easier.

[Brief description of drawings]

FIG. 1 shows a specific example of a first optical magnetic field measuring apparatus of the present invention.

FIG. 2 shows a sensor unit of an optical magnetic field measuring apparatus used in an example of the present invention.

FIG. 3 shows a magneto-optical element having a multi-domain structure used in the present invention.

FIG. 4 shows a specific example of a second optical magnetic field measuring apparatus of the present invention which applies a bias magnetic field.

FIG. 5 shows a sensor section of the optical magnetic field measuring apparatus of the present invention using a glass prism provided with a reflecting mirror.

FIG. 6 is a diagram showing an arrangement of a conventional optical magnetic field sensor using a polarizer, an analyzer and a Faraday rotator.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Magneto-optical element 2 Optical fiber 3 Collimator lens 4 Light emitting element (light source) 5,6 Frequency component detector 7 Divider 7 8 Electric wire cable 9 Magnetic field applying means 12 Prism 13 Reflector 14 Reflective film 15 Adhesive layer 17 Polarizer 18 Analyzer 19 Light receiving element 20 Transformer 21 Preamplifier 22 DC amplifier 23 AC amplifier 24 Divider 25 Glass tube 26 SUS sleeve

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[Procedure amendment]

[Submission date] March 10, 1992

[Procedure Amendment 1]

[Document name to be corrected] Drawing

[Correction target item name] All drawings

[Correction method] Change

[Correction content]

[Figure 1]

[Fig. 2]

[Figure 3]

[Figure 4]

[Figure 5]

[Figure 6]

Claims (5)

[Claims] 1. A magneto-optical element having a multi-domain structure in a state where a magnetic field is not applied, and the magnetic components parallel to the traveling direction of light in the magnetic domain are different from each other in adjacent magnetic domains, and light is incident on the element. Of the 0th-order diffracted light passing through the magneto-optical element, and the light intensity II (2ω) of the light component vibrating at a frequency twice the magnetic field frequency ω of the magnetic field measurement target and the 0
Light intensity I of the light component corresponding to the DC component of the next-order diffracted light
An optical magnetic field measuring apparatus having means for obtaining _O and means for obtaining the ratio I (2ω) / I _O. 2. A magnetic field having a multi-domain structure in the vicinity of a magnetic field measurement object that generates a magnetic field when no magnetic field is applied, and magnetic components parallel to the traveling direction of light in the magnetic domain are different from each other in adjacent magnetic domains. Light is incident on the optical element, and out of the diffracted light that has passed through the magneto-optical element, only the 0th-order diffracted light is extracted and oscillates at a frequency 2ω that is twice the magnetic field frequency ω of the magnetic field measured object in the 0th-order diffracted light. Intensity of light component I (2
ω) and the intensity I _{O of the} light component corresponding to the DC component, and the ratio I (2ω) / I _O of them is measured to measure the magnetic field of the magnetic field measurement target. 3. A magneto-optical element having a multi-domain structure in a state where a magnetic field is not applied, and the magnetic components parallel to the traveling direction of light in the magnetic domain are different from each other in adjacent magnetic domains, and light is incident on the element. Light emitting means, a magnetic field applying means for applying a constant bias magnetic field to the magneto-optical means, and a light intensity I of a light component of the 0th-order diffracted light passing through the magneto-optical element that vibrates at the magnetic field frequency ω of the magnetic field measured object. (ω) and a means for measuring the light intensity I (2ω) of a light component oscillating at twice the frequency 2ω, and a means for determining the ratio I (2ω) / I (ω) .. 4. A magnetic field having a multi-domain structure in the vicinity of a magnetic field measuring object that generates a magnetic field and a magnetic component parallel to the traveling direction of light in the magnetic domain is different from each other in the adjacent magnetic domains when no magnetic field is applied. Light is incident on the optical element while applying a constant bias magnetic field, and only 0th-order diffracted light is extracted from the diffracted light that has passed through the magneto-optical element. The light intensity I (ω) of the light component oscillating at and the light intensity I (2ω) of the light component oscillating at twice the frequency are measured, respectively, and their ratio I (2
A method for measuring the magnetic field strength of a magnetic field subject by determining ω) / I (ω). 5. A Bi-substituted rare earth iron garnet is used as the magneto-optical element, and is used with a thickness such that the temperature change of H _S / tan θ _f becomes small when the Faraday rotation angle is θ _f and the saturation magnetic field is H _S. The optical magnetic field measuring device according to claim 1.