JP2000206218A - Multilayer film magnetic field sensitive element and optical type magnetic field detector using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a multilayer film magnetic field sensitive element constituted of a multilayer film Faraday rotation element where transmission amount or reflection amount of light is changed by intensity of a magnetic field, and a small-sized optical type magnetic field detector of low cost which has the multilayer film magnetic field sensitive element. SOLUTION: A multilayer film is composed of magnetooptics material and dielectric material and has a periodically repeated structure. A multilayer film magnetic field sensitive element is constituted of a multilayer film Faraday rotation element having a structure where the repeating period is reversed by making the center of the multilayer film as a center of symmetry. The magnetic field sensitive element has performance that transmittance or reflectivity is changed by a magnetic field applied from the outside. An optical type magnetic field detector is constituted by using the multilayer film magnetic field sensitive element. The conventional magnetic field detecting element does not have a function that transmittance is changed in accordance with a magnetic field. In the case that a magnetic field detector is constituted by using the magnetic field sensitive element of this invention, a polarizer and a photo detector become unnecessary, and a small-sized magnetic field detector of low cost can be realized.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a multilayer magnetic field sensitive element comprising a multilayer Faraday rotator having a structure in which a magneto-optical material and a dielectric material are periodically and repeatedly laminated. The present invention relates to a multilayer magnetic field sensitive element in which the transmittance or reflectance of incident light changes according to the strength of a magnetic field, and an optical magnetic field detector including the same.

[0002]

2. Description of the Related Art A magnetic field is generated around a cable through which a large current flows, such as a power cable, according to the magnitude of the current. Such a magnetic field has become an important problem in view of noise in a recent electromagnetic environment and its effect on living bodies. On the other hand, industrially, a current measuring device that measures a current by detecting a magnetic field generated around a cable has been developed. Conventionally, for the detection of such a magnetic field,
An optical magnetic field detection method using the Faraday effect has been developed (for example, Ito et al., Journal of the Japan Society of Applied Magnetics vo
1.19, pp. 209-212, 1995). In this configuration, a Faraday rotator is used as a magnetic field detecting element, and a polarizer and an analyzer are arranged on both sides of the Faraday rotator at an inclination of 45 degrees. When light is incident on this, the Faraday rotation angle changes according to the magnetic field intensity, and the transmission amount of light passing between the polarizer, the rotator and the analyzer changes according to the magnitude. This change in the amount of light is detected and converted into a magnetic field intensity. The main types of optical detectors that have been developed so far include a bulk type using a magneto-optical crystal for the Faraday rotator and an optical detector using an optical fiber made of a glass material having a magneto-optical effect. There is a fiber type (see, for example, Katsuaki Sato "Light and Magnetism" Asakura Shoten (1988)). As described above, the configuration of a conventional magnetic field detector includes a Faraday rotator, a polarizer, and an analyzer as essential elements. In the case of a bulk type using a rare earth iron garnet crystal plate as the Faraday rotator, for example, a magneto-optical crystal such as bismuth-substituted yttrium iron garnet as described later, 3
A thickness of about mm is required. In the fiber type, a lead oxide-based glass material is used. However, such a material requires a length of several tens of cm due to a small Verdet constant. Therefore, the bulk type requires a thickness of several mm from the viewpoint of detection sensitivity, and the fiber type requires a length of several tens of cm. There is a limit to miniaturization.
Furthermore, when constructing a detector, a polarizer,
The analyzer cannot be omitted. Further, since the polarizer and the analyzer must be arranged shifted by 45 degrees, it takes time to adjust the optical axis. As described above, in the conventional optical magnetic field detector, there is a problem that it takes time to assemble and adjust to align the optical axes, and there is a limit to downsizing. Under these circumstances, it has recently been found that an extremely large Faraday rotation angle can be obtained in a multilayer film structure composed of a magneto-optical material and a dielectric material (Inoue, Fujii, Journal of the Japan Society of Applied Magnetics, 21 (1997) ), 187-192).
Such a multilayer Faraday rotator has a periodic repeating structure made of a magneto-optical material and a dielectric material. Due to this periodicity, a large Faraday rotation effect can be obtained. In response to this effect, the inventors
As a result of investigating the relationship between the laminated structure and the transmission characteristics in detail, it was found that, as described later, the light transmittance or the reflectance greatly changed according to the intensity of the externally existing magnetic field. By utilizing this effect, it has been found that such a multilayer film acts as a sensitive element to a magnetic field. Furthermore, the use of this multilayer film eliminates the necessity of a polarizer and analyzer, which are indispensable in conventional detectors, and makes it possible to construct a magnetic field detector having a simple configuration and a low production cost. . Further, since the multilayer film can be manufactured with a thickness of 10 μm or less, which is about 1/100 of that of the conventional device, an extremely small detector can be realized, and the manufacturing cost is greatly reduced by 1/3 or less. It has the advantage of being able to reduce it and is extremely useful.

[0003]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a multilayer magnetic field sensitive element in which the amount of transmission or reflection of light is changed by a magnetic field using a multilayer Faraday rotator, and an inexpensive and compact optical device having the same. An object of the present invention is to provide a magnetic field detector.

[0004]

Means for Solving the Problems In order to achieve the above object, the present invention is configured as described in the claims. That is, the present invention provides a multilayer film having a structure in which a magneto-optical material and a dielectric material are periodically repeated and stacked, wherein the repetition period is symmetric with respect to the center of the multilayer film. A multilayer magnetic field sensitive element comprising a Faraday rotator having an inverted structure,
The multilayer magnetic field sensitive element is a multilayer magnetic field sensitive element having a multilayer film whose light transmittance or reflectance changes according to the intensity of a magnetic field applied from the outside. According to a second aspect of the present invention, there is provided a multilayer magnetic field sensitive element according to the first aspect, wherein the multilayer magnetic field sensitive element is a gradient index rod having an input / output port made of an optical fiber. An optical magnetic field detector which is mounted on a lens and has at least means for measuring the intensity of an external magnetic field from the reflectance of light by the multilayer magnetic field sensitive element. According to a third aspect of the present invention, there is provided the multilayer magnetic field sensitive element according to the first aspect, wherein the multilayer magnetic field sensitive element is inserted into an abutting surface of an optical fiber. An optical magnetic field detector comprising at least means for measuring the intensity of an external magnetic field from the light transmittance of the element. The present invention is a multilayer film comprising a magneto-optical material and a dielectric material and having a periodically repeating structure, wherein the repetition period is reversed with the center of the multilayer film being symmetrical. A multilayer Faraday rotator having a structure, which is a multilayer magnetic field sensitive element having a function of changing transmittance or reflectivity by a magnetic field applied from the outside. The Faraday rotator changes the rotation angle according to the magnitude of the magnetic field but does not change the transmittance or reflectance of light. The rate of change has a completely unprecedented effect. Further, as described in the second and third aspects, the main feature is that an optical magnetic field detector is configured using the multilayer magnetic field sensitive element. The conventional magnetic field detecting element does not have a function of changing the transmittance or the reflectance according to the magnetic field, and by using a multilayer magnetic field sensitive element of the present invention to constitute a reflective or transmissive optical magnetic field detector, A detector that does not require a polarizer, analyzer, etc. can be configured,
There is an effect that a small and inexpensive magnetic field detector can be manufactured.

[0005]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Comparative Example First, the outline of the operation principle of a multilayer Faraday rotator will be described. FIG. 3 shows a structure of a conventional multilayer Faraday rotator. Reference numeral 1 denotes a magneto-optical material, hereinafter referred to as M. Reference numeral 2 denotes a dielectric material, hereinafter referred to as G. The structure of the multilayer film shown in the figure is
Using M (magneto-optical material) and G (dielectric material), [M
G] ⁿ [2M] [GM] ⁿ (n is the number of repetitions). Light is assumed to travel from left to right in the figure, as indicated by the arrows. Further, when a magnetic field is applied in parallel to the traveling direction of light, the M layer becomes optically active, and the plane of polarization of light traveling through this layer is rotated by the Faraday effect. Detailed theoretical treatment is stated in the above mentioned references Inoue et al., Light transmission characteristics of such a multilayer film is represented by a transition matrix F _M and F _G of the M layer and G layer. The multilayer film having the structure shown in FIG. 3 has F = [F _M · F _G ] ⁿ · [F _M ·
F _M] · [it is represented as F _G · F _M] ^n, where F is the transition matrix of the multilayer film (4 × 4 complex matrix). Assuming that the M layer is isotropic and has negligible light absorption,
Using the main dielectric constant e ₁ when no magnetic field is applied and the magnetically induced dielectric constant e ₂ when a magnetic field is applied, the dielectric constants for right and left circularly polarized light are e _p (= e ₁ + e ₂ ) and e, respectively. _n (= e ₁ −e ₂ ). The Faraday rotation is manifested by the effect of the magnetically induced dielectric constant e ₂ , and the transmission characteristics of the multilayer film, that is, the Faraday rotation and the transmittance, compare the state of light at the entrance surface and the exit surface using the transition matrix F. This will make it possible to evaluate. <Embodiment 1> FIG. 1 illustrates [M
G] ⁿ [2M] [GM] For a multilayer rotator having a structure such as ⁿ , the change in transmittance with respect to the relative magnetization of the M layer is shown using the number of repetitions as a parameter. In the multilayer film, the M layer is composed of bismuth-substituted yttrium iron garnet (hereinafter referred to as Bi: YIG), which is a kind of rare earth iron garnet, and the G layer is composed of SiO ₂ . In the present embodiment, Bi ₁ Y ₂ which is the most general-purpose Bi: YIG
Fe ₅ O ₁₂ is used. The wavelength of light is 1.55 μm, which is a wavelength band where this material has no absorption. At this wavelength, e ₁ of the M layer is 5.3, and e ₂ is 0.002 (Faraday The rotation angle is about 0.1 degree / μm). The dielectric constant of the G layer is 2.3. Further, the thickness of each layer is determined by setting the optical path length (refractive index × thickness) to ４ of the wavelength of light.
It is set to be. FIG. 1 shows the relationship between relative magnetization and transmittance for a multilayer film in which the number of repetitions n is 8 to 10. Here, the relative magnetization refers to the ratio between the magnetization generated by an external magnetic field and the saturation magnetization in the M layer, that is, the magnetization / saturation magnetization. The relative magnetization changes almost in proportion to the magnitude of the external magnetic field. That is, assuming that an external magnetic field that causes saturation magnetization in the M layer is a saturation magnetic field, the relative magnetization is proportional to the magnetic field / saturation magnetic field. In Bi: YIG used in the present embodiment, the magnitude of the external magnetic field that causes saturation magnetization is 1.8 kG. As is clear from FIG. 1, the transmittance changes in accordance with the change in the relative magnetization, that is, the magnitude of the external magnetic field. The rate of the change also depends on the number of repetitions n. As n increases, the light transmittance sharply decreases. For example, n = 10
At this time, the transmittance decreases to about 0.5 when the relative magnetization is 0.2. On the other hand, in the multilayer film of n = 8, the transmittance is about 0.5 even when the saturation magnetization is reached. If n = 9, the magnetic field can be measured over a wide range. As described above, the multilayer film having a large n has a large change in transmittance with respect to a change in the external magnetic field, that is, has a high sensitivity, and is suitable for measuring a small magnetic field. On the other hand, when n is small, the sensitivity is low. In this case, a magnetic field of up to 1.8 kG corresponding to the saturation magnetization of the M layer can be measured. As described above, the multilayer films exemplified in the present embodiment have different light transmittances corresponding to the magnitude of the magnetic field. This effect is completely different from the fact that the conventional Faraday rotator changes the rotation angle according to the magnitude of the magnetic field but does not change the transmittance, and the transmittance directly changes according to the magnitude of the magnetic field. It has a function that has never existed before. Although the transmittance changes according to the magnitude of the magnetic field, the light not transmitted is reflected. As described above, the multilayer Faraday rotator constituted by the magneto-optical layer and the dielectric layer has a laminated structure in which the repetitive periodic structure is inverted symmetric at the center of the film, as shown in FIG. . When an external magnetic field is applied to the laminated structure having the inversion symmetric period, the transmittance and the Faraday rotation angle change according to the magnitude of the external magnetic field. That is, this effect is exhibited when the difference between the localization of light due to the periodicity of the multilayer film and the propagation constant of right and left circularly polarized light matches the periodicity. On the other hand, when the external magnetic field is removed, the material having no magnetic hysteresis has a magnetically induced dielectric constant e in the M layer.
₂ becomes zero and no Faraday effect occurs. As a result, this multilayer film becomes the same as a normal dielectric multilayer film, and completely transmits light. Therefore, the transmittance changes continuously with respect to the external magnetic field until the M layer reaches the saturation magnetization, and the magnetic field can be detected by this action. Bi: YIG used in the present embodiment
Is a material with a large Faraday effect that does not absorb in the 1.5 μm band. Moreover, although it is a ferrimagnetic material, the effect of magnetic hysteresis is so small that it can be effectively ignored.
That is, the magnetically induced dielectric constant e ₂ that causes the Faraday rotation is proportional to the relative magnetization, which is the ratio between the magnetization and the saturation magnetization, until the saturation magnetization is reached. That is, e ₂ = α
Can be represented by C. Here, α is a proportionality constant, and C is a relative magnetization (= magnetization / saturation magnetization). Therefore, when the external magnetic field is smaller than the magnetic field that causes the saturation magnetization, e ₂ changes according to the relative magnetization, so that the transmittance of the multilayer film changes and the magnitude of the external magnetic field can be detected. It is.

Second Embodiment Based on the first embodiment, a multilayer film having a structure represented by [MG] ⁿ [2M] [GM] ⁿ with a repetition number n = 8 was manufactured.
The substrate is 1.5 inch diameter quartz glass (1mm thick)
Bi: YIG was used for the M layer, and SiO ₂ was used for the G layer. Each target for the M layer and the G layer was set using a binary sputtering apparatus. While monitoring the thickness with a film thickness meter, the M layer
A total of 34 layers and a thickness of 7.118 μm were deposited to a thickness of 0.1683 μm and a G layer of 0.2555 μm. About the obtained multilayer film, the transmittance | permeability with and without a magnetic field was measured using the optical spectrum analyzer. When light is vertically incident on the multilayer film, the permeability is 97% when there is no magnetic field, and when there is a magnetic field (in this measurement, 1.5 kG).
Has a transmittance of 0.49. These values almost match the estimates shown in FIG.

Third Embodiment FIG. 2 is a schematic diagram showing a cross-sectional structure of an optical magnetic field detector formed by using the n = 8 multilayer film manufactured in the second embodiment. In the figure, 3 is a multilayer film, 4 is a gradient index rod lens (GRIN rod lens), 5a is an input optical fiber, and 5b is an output optical fiber. When the input optical fiber 5a is used as an input port and a magnetic field is applied to the input optical fiber in parallel with the optical axis direction of the GRIN rod lens 4, the light incident from the port 5a becomes as shown by a dashed line in the figure. Take the route. Depending on the magnitude of the magnetic field, the transmittance or reflectance of the multilayer film of 3 changes, so when there is a magnetic field, part of the light incident from the port 5a is reflected by the multilayer film and is output to the output port of 5b. Is output. Also, the transmitted light exits the detector. Using the measurement system described in the second embodiment, the transmittance from 5a to 5b when a magnetic field was applied was examined. Set the external magnetic field to 0.
5, 1.0 and 1.5 kG, the transmittance is 0.14,
0.13 and 0.47. The transmittance from 5a to 5b when the magnetic field was turned off was 0. Since the connection loss is expected to be about 0.3% in each optical path,
This reflects the transmission characteristics of the multilayer film measured in the second embodiment. In the present embodiment, the configuration of the magnetic field detector is a reflection type. This is large in the configuration example of the present invention because a collimating system is required as shown by the GRIN rod lens of 4. As described in the second embodiment, since the multilayer film of the magnetic field sensitive element has a thickness of 10 μm or less, it can be directly sandwiched between the butting surfaces of the optical fibers. That is, the fiber is 10μ
Even if they are separated by about m, the loss due to diffraction is negligibly small if the optical axes coincide. Therefore, if the multilayer film is inserted between the abutting surfaces of the fibers, a small-sized detector can be formed. In this case, the transmission amount of the multilayer film is measured. In the above-described embodiment, Bi: YIG is used as the magneto-optical material, and the wavelength is 1.
It describes about 55 μm. At the same time, by appropriately combining the number of repetitions n and the basic structure, other magneto-optical materials, for example, other rare earth iron garnets such as Ce-substituted yttrium iron garnet, diluted magnetic semiconductors such as HgMnTe, glass such as Faraday rotating glass It has been confirmed that the magnetic field sensitive element of the present invention can be manufactured even when a material is used. Similarly, in the embodiment, SiO ₂ is used as the dielectric material. However, by appropriately combining the number of repetitions n and the basic structure, A1 ₂ O ₃ can be used as another dielectric material with negligible light absorption. Crystal materials, such as
It has been confirmed that a magnetic field sensitive element comprising a multilayer Faraday rotator having the effects of the present invention can be realized even when a semiconductor material such as CdTe or a glass material such as fluoride glass is used. In addition, by appropriately selecting the combination of the number of repetitions n and the basic structure in addition to the types of the magneto-optical material and the dielectric material within the scope of the present invention, the magnetic field sensitivity of the multilayer rotor can be increased even in the wavelength region of 1.55 μm. It has been confirmed that the effect of the present invention, which is an element, can be obtained.

[0008]

As described above, a multilayer film made of a magneto-optical material and a dielectric material, which has a centrally symmetric repetitive periodic structure, can be applied to a multilayer Faraday rotator according to the magnitude of an external magnetic field. Light transmittance or reflectance can be changed. By configuring an optical magnetic field detector using this multilayer magnetic field sensitive element, it is possible to configure a magnetic field detector that does not require a polarizer and an analyzer. There is.

[Brief description of the drawings]

FIG. 1 illustrates [MG] exemplified in Embodiments 1 and 2 of the present invention.
FIG. 11 is a diagram showing a change in transmittance of a multilayer film having a structure represented by ⁿ [2M] [GM] ⁿ with respect to relative magnetization of a multilayer film having a repetition number n of 8 to 10;

FIG. 2 shows the number of repetitions n exemplified in Embodiment 3 of the present invention.
FIG. 9 is a schematic diagram showing the structure of a reflection type optical magnetic field detector constituted by using a multilayer film having the structure of FIG.

FIG. 3 is a schematic diagram showing a basic structure of a conventional multilayer Faraday rotator made of a magneto-optical material and a dielectric material.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Magneto-optical material (M) 2 ... Dielectric material (G) 3 ... Multilayer film 4 ... Gradient index type rod lens (GRIN rod lens) 5a ... Input optical fiber 5b ... Output optical fiber 6 ... Light path

Claims (3)

[Claims] 1. A Faraday rotator having a structure in which a magneto-optical material and a dielectric material are periodically repeated and laminated, wherein the Faraday rotator has a structure in which the repetition period is inverted with respect to the center of the multilayer film. A multilayer magnetic field sensitive element comprising: a multilayer magnetic field sensitive element, wherein the multilayer magnetic field sensitive element has a multilayer film whose light transmittance or reflectivity changes according to the intensity of a magnetic field applied from the outside. Film magnetic field sensitive element. 2. The multilayer magnetic field sensitive element according to claim 1, wherein said multilayer magnetic field sensitive element is mounted on a gradient index rod lens having an input / output port made of an optical fiber.
An optical magnetic field detector comprising at least means for measuring the intensity of an external magnetic field from the reflectance of light from the multilayer magnetic field sensitive element. 3. A multilayer magnetic field sensitive element according to claim 1, wherein said multilayer magnetic field sensitive element is inserted into a butt surface of an optical fiber, and an external magnetic field is determined based on the light transmittance of said multilayer magnetic field sensitive element. An optical magnetic field detector comprising at least means for measuring the intensity of a magnetic field.