JPH04304417A - Magnetooptic element for magnetic field sensor - Google Patents

Abstract

PURPOSE:To reduce the diffraction loss of the magnetooptic element for the magnetic field sensor which uses perpendicularly magnetizing elements. CONSTITUTION:The magnetooptic element for the magnetic sensor has (n) magnetooptic elements put one over another (n;>=2, n: integer); and the total of the Faraday rotation angles of the (n) magnetooptic elements in a saturated magnetic field is >=15 deg., the easy magnetization axis of the magnetooptic element is parallel to the traveling direction of light, and the magnetic layers of respective magnetooptic elements do not contact one another. Consequently, the magnetic field sensor which has low loss and high sensitivity is obtained.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a low-loss, high-sensitivity magneto-optical element for a magnetic field sensor in which a plurality of magneto-optical elements are arranged in isolation from each other.

0002

2. Description of the Related Art A magneto-optical element for a magnetic field sensor is an optical element for measuring magnetic field strength using the Faraday effect. FIG. 1 shows the relationship between the Faraday rotation angle θF and the magnetic field H. In the figure, HS is the saturation magnetic field, θFS
represents the saturated Faraday rotation angle, and the sensitivity of the magnetic field sensor is θ
It is expressed as FS/HS. Therefore, to increase the sensitivity of such an element, either HS can be decreased or θfs can be increased. Although it is possible to reduce HS by changing the material composition, this is not preferable as a magnetic field sensor because the measurable magnetic field range is limited (the measurement range is -HS < H < HS). On the other hand, θfs is the Faraday effect relational expression θf = V×L×
Since it is given by H (in the formula, V is the Verdet constant, L is the optical path length, and H is the magnetic field), θfs can be increased by increasing the thickness L of the element. Faraday rotation ability (θfs/thickness), which represents the Faraday rotation angle per unit thickness
Bismuth-substituted rare earth iron garnet is known to have a large effect.

However, when bismuth-substituted rare earth iron garnet is manufactured by the LPE method, which is suitable for mass production, it usually becomes a perpendicularly magnetized film as shown in FIG. 2, and the magnetic domain structure becomes a multi-domain structure. Below the saturation magnetic field, such a multi-domain structure acts as a diffraction grating, resulting in a diffraction loss of a portion of the incident light as the light passes through it. Therefore, there is a problem in that the intensity of the signal transmitted through the element is reduced.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a highly sensitive magneto-optical element for a magnetic field sensor in which the axis of easy magnetization is parallel to the direction of propagation of light, and in which the diffraction caused by the element is small. .

[0005]

[Means for Solving the Problems] As a result of intensive studies in order to solve the above problems, the inventors of the present invention have found that by arranging a plurality of magneto-optical elements in isolation from each other, the We have found that it is possible to detect transmitted light with less diffraction loss than a single element with a thickness of

That is, the present invention provides a magneto-optical element for a magnetic field sensor in which n magneto-optical elements are stacked (n≧2).
, n: an integer), the total Faraday rotation angle of the n magneto-optical elements in a saturated magnetic field is 15 degrees or more, the axis of easy magnetization of the magneto-optical element is parallel to the traveling direction of light, and each magneto-optic element The present invention provides a magneto-optical element for a magnetic field sensor, characterized in that the magnetic layers of the magnetic layers are not in contact with each other.

Furthermore, the present invention also provides a magnetic field sensor equipped with the above magneto-optical element.

The magneto-optical element used in the present invention uses an element material in which the axis of easy magnetization of the magneto-optical element is parallel to the traveling direction of incident light as shown in FIG. 2 when no magnetic field is applied. . Examples of such element materials include:
Examples include rare earth iron garnet, bismuth-substituted rare earth iron garnet, orthoferrite, and the like. Such materials are
Below the saturation magnetic field, transmission loss usually occurs due to diffraction due to the multi-domain structure as described above, and the present invention can effectively reduce such diffraction loss.

[0009] In the present invention, two or more such material elements are used and are used in isolation from each other. When used in close contact with each other as shown in Figure 3, the multi-domain structure is the same in the two elements as shown in the figure, so as will be described later, the optical This is not preferable because the transmission loss increases. Therefore, in the present invention, as shown in FIG.
For example, the above-mentioned magneto-optical element must be used in isolation using a non-magnetic substrate.

Furthermore, in the present invention, when n pieces of the above elements are arranged in isolation and a magnetic field is applied to the elements to make them saturated magnetize, the Faraday rotation angle of the n pieces must be 15 degrees or more in total. be. This will be explained below. The following equation is a calculation equation for calculating the transmittance T when light is incident on a plurality of perpendicularly magnetized elements as shown in FIG. However, no magnetic field is applied to the element.

[0011]

[Formula 1] T = [COS2 (θfs/n)]n × (tR
) nn: Number of elements tR: Transmittance considering reflection and absorption loss of one element COS2 (θfs/n): Diffraction loss term

0012
] In the above formula, tR = 0.98, n = 1 to 3
FIG. 5 shows the calculated transmittance and the transmission loss for various saturated Faraday rotation angles θfs. It can be seen from the figure that the transmission loss increases as the saturated Faraday rotation angle increases. For example, in a single element, θf
The loss when s=26 degrees is 1 dB. However, θfs
= 13 degrees, the loss is approximately 0.
6 dB, which can significantly reduce loss. Moreover, when the number of elements with θfs=8.7 degrees is reduced to three, the effect becomes even greater. However, as is clear from the figure, the effect of reducing transmission loss using a plurality of elements appears when θfs≧15 degrees. Therefore, in the present invention, the total Faraday rotation angle of the n magneto-optical elements is required to be 15 degrees or more. Although the above calculation is the result when no magnetic field is applied, it also applies to the magneto-optical element for a magnetic field sensor of the present invention used to detect a magnetic field below the saturation magnetic field. That is, in a magnetic field less than the saturation magnetic field, diffraction loss occurs due to the multi-domain structure as described above.

As another aspect of the present invention, a magnetic field sensor including the above magneto-optical element can be constructed as shown in FIG. 6, for example. The figure shows an example of a magnetic field sensor of the present invention including the above magneto-optical element, including an optical transmitter 1, an optical receiver 2, optical fibers 3a and 3b, gradient index lenses 4a and 4b, and a polarizer. 5, a magneto-optical element 6 for a magnetic field sensor, and an analyzer 7. Light from the optical transmitter 1 is guided to the optical sensor section by an optical fiber 3a. In the optical sensor section, the light from the optical fiber 3a is collimated by a gradient index lens 4a, then converted into linearly polarized light by a polarizer 5, and guided to a magneto-optical element 6 for a magnetic field sensor. Here, if a magnetic field is applied in parallel to the magneto-optical element 6 for a magnetic field sensor, the polarization plane of the linearly polarized light passing through the element 6 rotates in proportion to the intensity of the magnetic field. The degree of rotation is converted into light intensity by a polarizer 5 and an analyzer 7 whose optical axis is tilted by 45 degrees, and the light is narrowed down by a gradient index lens 4b.
It is guided to the optical receiver 2 via the optical fiber 3b. The optical receiver 2 performs an operation of converting an optical signal into an electrical signal and determining the strength of the magnetic field applied to the optical sensor section. When the magnetic field strength is measured, the magnitude of the current is immediately determined. polarizer 5,
As the analyzer 7, a Glan-Thompson prism or a polarizing beam splitter was used.

[0014] The present invention will be explained in detail with reference to Examples below, but the present invention is not limited thereto in any way.

[0015]

EXAMPLES A method of manufacturing a magneto-optical element for a magnetic field sensor according to the present invention will be described below. Example 1 Bi1.4 was deposited on a high lattice constant GGG substrate by LPE method.
A film having a composition of Ho1.6 Fe5 O12 was prepared and polished to a film thickness of 70 μm. Two pieces of this material
They were placed isolated from each other as shown in Figure 4. When a magnetic field was applied to the magneto-optical element constructed in this way and its characteristics were tested, the saturation Faraday rotation angle was 28° and the sensitivity was 0.
The loss was as large as 0.04 degrees/Oe, and the loss when the applied magnetic field was zero was 0.8 dB.

Comparative Example 1 The film thickness of the composition material of Example 1 was 140 μm, and the characteristics were observed using only one film. As a result, the loss was as large as 1.5 dB, which was about twice the loss of Example 1.

Example 2 By the same method as in Example 1, Bi1.5 Y1.5 Fe
5 O12 material was made to have a film thickness of 60 μm,
Three sheets of this material were placed one on top of the other so that they did not touch each other. When a magnetic field was applied to the magneto-optical element constructed in this way and its characteristics were tested, the saturation Faraday rotation angle was 40°.
The sensitivity was 0.06 degrees/Oe and the loss was 1.1 dB.

Comparative Example 2 Using the material of Example 2, the film thickness was set to 180 μm, and the characteristics were observed using only one film. The result is a loss of 2.6dB.
It was big.

[0019]

Effects of the Invention By using the magneto-optical element for a magnetic field sensor of the present invention, a magnetic field sensor with good sensitivity can be obtained because the diffraction loss of the light beam due to the perpendicular magnetization element is reduced.

[Brief explanation of drawings]

FIG. 1 is a graph showing the relationship between a magnetic field and a Faraday rotation angle in a magneto-optical element for a magnetic field sensor.

FIG. 2 represents the multi-domain structure of a bismuth-substituted rare earth iron garnet perpendicular magnetization element.

FIG. 3 represents a magnetic domain structure when two perpendicular magnetization elements of FIG. 2 are arranged in contact with each other.

FIG. 4 shows a magnetic domain structure when the perpendicular magnetization elements of FIG. 2 are arranged so as to be separated from each other by a nonmagnetic substrate.

FIG. 5 is a graph in which the transmittance of a perpendicularly magnetized film is calculated when tR = 0.98 and n = 1 to 3, and the transmission loss is expressed versus the saturation Faraday rotation angle θfs.

FIG. 6 is a specific example of a magnetic field sensor including the magneto-optical element of the present invention.

Claims (2)

[Claims] Claim 1: In a magneto-optical element for a magnetic field sensor in which n magneto-optical elements are superimposed (n≧2, n: an integer), the total Faraday rotation angle of the n magneto-optical elements in a saturated magnetic field is 15 degrees or more. A magneto-optical element for a magnetic field sensor, characterized in that the axis of easy magnetization of the magneto-optic element is parallel to the traveling direction of light, and the magnetic layers of each magneto-optic element are not in contact with each other. 2. A magnetic field sensor comprising the magneto-optical element according to claim 1.