JPH0248868B2 - - Google Patents

Description

[Detailed Description of the Invention] (1) Field of Application of the Invention The present invention provides a magnetic field measuring device that utilizes an optical fiber and the rotatability of the polarization plane of light (Faraday rotatability), in which the magnetic field detection section is made of a magnetic garnet thin film. The present invention relates to a magnetic field measurement method that makes it possible to increase magnetic field detection sensitivity.

(2) Prior Art Conventionally, in a magnetic field detection device that combines an optical fiber and a magnetic garnet thin film, the thickness of the magnetic garnet thin film has been increased in order to improve the detection sensitivity. This is because the Faraday rotation amount F (radians) of the magnetic garnet thin film can be expressed as F=V ₀ Hh ……(1) V ₀ : Proportionality constant inherent to the material H : Magnetic field strength h : Thickness of the magnetic garnet thin film This is because the Faraday rotation angle F increases as the film thickness h increases. However, magnetic garnet thin films have poor light transmittance at short wavelengths, for example at wavelengths of 0.6 μm and 0.8 μm.
The thickness of the magnetic garnet thin film was regulated. When the light absorption coefficient is α, the transmittance is exp(−αh). Further, let R be the reflectance from the thin film. Therefore, the total output, excluding the proportionality constant, is generally expressed by the following equation.

P _put ∝F・exp(−αh)(1−R) ² ……(2) ∝V ₀ Hh・exp(−αh)(1−R) ² Considering equation (2) as a function of film thickness h, The output is maximum when h=α ^-1 . This is usually called the figure of merit.

Now, as the strength of the magnetic field applied to the magnetic garnet thin film is increased, the amount of Faraday rotation increases according to equation (1), but above a certain value, the increase stops completely. Let H ₀ be the strength of the magnetic field that causes this stationary state. This saturation magnetic field H ₀ is the magnetic garnet thin film h
Figure 1 shows the relationship between the thickness h of the magnetic garnet, the characteristic length l of the magnetic garnet, and the total magnetization 4πM ₀ . From this, we can understand the following two points.

1. When the thickness of the magnetic garnet is thin, when the thickness h changes, the value of H ₀ also changes significantly.

2 As the thickness of the magnetic garnet increases, the thickness h
The amount of change in H 0 with respect to H ₀ is very small.

With the above results in mind, for a magnetic garnet thin film with thickness h, saturation magnetic field H ₀ , and absorption coefficient α, the output when the thickness is multiplied by n, and the output when n pieces of the above magnetic garnet thin films are stacked. Think about the output.

When n pieces of magnetic garnet thin films are stacked, the output P _{a is} P _a = P _put · n exp (-(n-1) αh) × (1-R) ²ⁿ ... (3) becomes. Here, R is the reflectance of the plane.

Next, when the thickness is increased by n times, H ₀
Assume that the increase becomes mH ₀ . At this time, the output P _b becomes P _b =P _put n/mexp (-(n-1) αh) (1-R) ² (4).

Comparing equations (3) and (4), it becomes P _a /P _b = m(1-R) ^2(n-1) ...(5). This means that it is generally more effective to stack a plurality of magnetic thin films than to simply increase their thickness. However, since there is absorption and loss due to reflection in the thin film, as can be seen from equation (5), if a large number of layers are stacked, the output will become smaller, and increasing the thickness will be more effective.

As described above, an effective effect cannot be obtained simply by increasing the thickness of the thin film, as in the conventional method.

(3) Purpose of the Invention The purpose of the present invention is to provide a method for increasing output efficiency so that a minute magnetic field can be detected.

(4) General explanation of the invention In order to maximize the output as shown in equation (5), when m(1-R) ^2(n-1) >, n pieces of magnetic garnet thin films are stacked and used. Good, m(1-R)
When ^2(n-1) <1, the film thickness of the magnetic garnet is increased to be n times thicker. In either case, it is necessary to use it near the figure of merit explained in equation (2).

(5) Examples Hereinafter, the present invention will be explained in detail with reference to examples.

FIG. 2 is a configuration diagram of a magnetic field detector according to the present invention. Light source 1 has an output of 60mW and a wavelength of 0.8μm.
A light emitting diode was used, and the light was incident on a plastic-clad quartz optical fiber 2 with a diameter of 600 μm, and was guided in parallel to the detection section A via a lens 3. The magnetic field detection section A consists of a polarizing element 4, a magnetic thin film 5, and a Wollaston prism 6, and is arranged so that the vibration direction of the light of the polarizer 4 (direction of linearly polarized light) and the main axis of the Wollaston prism 6 are opposed to each other at an angle of 45 degrees. is set to . The magnetic garnet thin film has (YSmLuCa) ₃
A 4 μm thick magnetic garnet thin film made of (FeGe) ₃ O ₁₂ was grown on both sides of a 0.35 mm thick gadolinium-gallium-garnet Gd ₃ Ga ₅ O ₁₂ using a liquid phase growth method so that the magnetization direction was perpendicular to the above surfaces. The thus-formed material was used. FIG. 2 shows a case where four magnetic thin films are used. The Wollaston prism separates the incident light into two linearly polarized waves, and the lens 3
-1, 3-2 optical fiber 2-1,
It entered 2-2. Each output was detected by a PIN photodiode, and an electrical calculation circuit calculated S=P ₁ −P ₂ /P ₁ +P ₂ to monitor the output.

As shown in Fig. 2, the output when the magnetic field H is applied in the direction of propagation of light is determined by the number of magnetic thin films 5, 5, 3,
Measurements were made by changing the number of sheets and then two sheets. The results are shown in FIG. As can be seen from the figure, the output increases as the number of stacked magnetic thin films increases.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the relationship between the saturation magnetic field H ₀ in a magnetic garnet thin film and the thickness h of the magnetic garnet thin film, where l is the characteristic length and 4πM _s is the total magnetization. Fig. 2 is an explanatory diagram showing the configuration of a magnetic field detection method in an embodiment of the present invention, and Fig. 3 shows the output and magnetic field strength when the number of magnetic garnet thin films is changed in an embodiment of the present invention. It is a graph showing the relationship between. 1...Light source, 2,2-1,2-2...Optical fiber,
3, 3-1, 3-2... Condenser lens, 4... Polarizer,
5...Magnetic thin film, 6...Wollaston prism.

Claims (1)

[Scope of Claims] 1. A light source, a magnetic field detection section having a medium having Faraday rotation ability, a measurement section that measures light from the detection section, and an optical system that connects the light source, the detection section, and the measurement section. In a magnetic field measurement device consisting of ^an optical transmission line coupled to Optical magnetic field measurement device (where m is the thickness of the medium
(R is the rate of change of the saturation magnetic field when it is multiplied, and R is the reflectance of the medium). 2. The optical magnetic field measuring device according to claim 1, wherein the thickness of the medium in the light transmission direction is set so that the figure of merit is maximized. 3. The optical magnetic field measurement device according to claim 1 or 2, which includes a prism for separating light from a medium having Faraday rotation ability into two linearly polarized lights, and a prism for measuring each of the two linearly polarized lights. An optical magnetic field measuring device comprising a measuring section and a calculation means for calculating the ratio of the sum and difference of the outputs of the measuring section. 4. The optical magnetic field measuring device according to any one of claims 1 to 3, wherein the medium is a magnetic garnet.