JP2004219137A - Reflection type optical magnetic field sensor head - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a simple measuring instrument that can accurately measure rotating speeds and has a light weight more inexpensively and in a large amount. <P>SOLUTION: This reflection type optical magnetic field sensor head is constituted so that its optical path may intersect the normal to the main surface of a Faraday rotator at an angle of 5 to 20° by successively arranging a two-core ferrule or glass capillary to which two optical fibers are stuck, a double refraction plate, a lens, the Faraday rotator composed of a bismuth-substituted rare-earth-iron-garnet single-crystal film, and a mirror in this order. Consequently, the size of the sensor head can be reduced and the head hardly causes PDL. In addition, the head has a large SN ratio. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
[Industrial applications]
The present invention relates to an optical magnetic field sensor head utilizing the Faraday effect of a bismuth-substituted rare earth iron garnet film. More specifically, the present invention relates to an improvement in a reflection type optical magnetic field sensor head which has a small number of parts, is small and lightweight, has high reliability, and is easily mass-produced.
[0002]
[Prior art]
At present, many industrial devices and consumer appliances generally used have a rotating device such as a motor or a rotating portion such as a gear. In these devices and devices, advances in science and technology and social demands for global environmental protection and energy saving are increasing. In order to respond to these demands, control of industrial devices such as aircraft and ships, and consumer devices such as passenger cars has been performed with higher precision and higher accuracy. In order to control a rotating device or a rotating device with higher precision and higher accuracy, the rotation speed (number) must be continuously and accurately measured. For that purpose, it is necessary to develop a simple and lightweight measuring device capable of more accurately measuring the rotation speed, and to provide it inexpensively and in large quantities.
[0003]
As a method of measuring the rotation speed (number), a method using electromagnetic induction (sensor technology, December 1986, p. 68) (Non-Patent Document 1) or light utilizing the Faraday effect of a magneto-optical material has already been used. A method using a magnetic field sensor (Applied Optics, Vol. 28, No. 11, p. 1992 (1989)) has been proposed (Non-Patent Document 2).
[0004]
The method using electromagnetic induction has already been used for measuring the rotational speed (number) of an aircraft engine, an automobile engine, and the like. However, the tachometer using electromagnetic induction has a serious drawback that a transmission line (cable) connecting a measurement terminal and a device body is susceptible to electromagnetic noise. In addition, since the magnetic field measurement unit is composed of an electric circuit, it is important to note that explosion-proof measures must be taken for measuring equipment at hazardous material factories and hazardous material handling sites that handle flammable substances such as organic solvents. There is a problem.
[0005]
On the other hand, for example, in the measurement of the rotation speed by the optical magnetic field method using the Faraday effect shown by the magneto-optical material, there is almost no effect of electromagnetic noise or explosion-proof even in places handling flammable substances such as organic solvents. Features such as no need for countermeasures.
[0006]
The basic principle of an optical magnetic field sensor using magneto-optical characteristics of a bismuth-substituted rare earth iron garnet film or the like utilizes a phenomenon that a magnetic domain structure of a magneto-optical material changes due to an external magnetic field. That is, the light transmitted through the magneto-optical material having the magnetic domain structure causes a diffraction phenomenon due to the magnetic domain structure, but the 0th-order diffracted light that is a part of the transmitted light is affected by the difference in the magnetic domain structure changed by the external magnetic field, The polarization plane rotates according to the amount, and the light intensity itself changes. Therefore, the change in the polarization plane is converted into a change in the light intensity, or the change in the light intensity itself is detected and counted to measure the rotation speed (number).
[0007]
Optical magnetic field sensors include a transmission type and a reflection type. Since the transmission type must be arranged so that the directions of incidence and transmission of signal light are on a straight line due to the properties of the components thereof, the installation location is limited. Therefore, it cannot be adopted depending on the purpose of use and the installation location.
[0008]
As a configuration for improving the drawbacks of the transmission type optical magnetic field sensor, the present inventors have developed a small, high-performance reflection type optical magnetic field using a bismuth-substituted rare earth iron garnet single crystal film having a very large Faraday effect as a Faraday rotator. A sensor has been proposed (Patent Document 1).
[0009]
FIG. 7 shows a configuration of a reflection type optical magnetic field sensor head described in Japanese Patent Application Laid-Open No. 11-44744. In this reflection type optical magnetic field sensor head, a semiconductor laser or a light emitting diode is suitable as a light source. However, the reflection-type optical rotation sensor head disclosed in the publication uses the polarization element 26 that transmits only light in a specific polarization direction. For this reason, a light emitting diode was used instead of a semiconductor laser because of the restriction that incident light must be close to perfect random polarization or circular polarization. In addition, a light emitting diode in a short wavelength band is preferable because it can be obtained at low cost.
[0010]
[Non-patent document 1]
Sensor Technology, December 1986, p. 68
[Non-patent document 2]
Applied Optics, Vol. 28, No. 11, p. 1992 (1989))
[Patent Document 1]
JP-A-11-44744
[0011]
[Problems to be solved by the invention]
However, since the light emitting diode has a wide wavelength distribution of the oscillation wavelength and the Faraday rotator has a wavelength dependence of the Faraday rotation angle, the reflection type optical magnetic field disclosed in JP-A-11-44744 is disclosed. The limit of the SN ratio of the sensor head was about 13 to 15 dB. In recent years, optical communication systems have become widespread, and semiconductor lasers have become available at low cost. By using a semiconductor laser having almost no oscillation wavelength distribution, the SN ratio is improved to 30 dB or more. However, since the semiconductor laser is a light having a certain polarization direction, in the reflection type optical magnetic field sensor head disclosed in Japanese Patent Application Laid-Open No. 11-44744, a difference occurs in the loss depending on the polarization direction of the incident light. That is, it has a large polarization dependent loss (PLD), which is a problem in configuring an optical magnetic field sensor system.
[0012]
Further, inside the optical magnetic field sensor head, the beam diameter of the light emitted from the lens depends on the numerical aperture of the used optical fiber when the same lens is used. In order to reduce the size, it is effective to use a single-mode optical fiber having a small numerical aperture for the optical magnetic field sensor head. Joining is technically difficult.
[0013]
[Means for Solving the Problems]
FIG. 1 shows an example of the entire configuration of a reflection type optical magnetic field sensor head (hereinafter, referred to as OMS-I) according to the present invention. Further, as described above, the inventors of the present application have conducted intensive studies to solve the problem of the reflection type optical magnetic field sensor using the bismuth-substituted rare earth iron garnet single crystal film. As a result, two optical fibers were fixed. A core ferrule or glass capillary 11, a polarizing element 21, a lens 31, a Faraday rotator 41 made of bismuth-substituted rare earth iron garnet single crystal, and a mirror 51 are arranged in this order, and the optical path and Faraday rotation of light incident on the Faraday rotator. A reflection type optical magnetic field sensor has been devised, wherein the angle formed by the normal to the main surface of the child is in the range of 5 degrees or more and 20 degrees or less.
[0014]
The principle of the reflection type optical magnetic field sensor (OMS-I) of the present invention will be described below. 2 (a) to 2 (d) are diagrams illustrating the operation principle of OMS-I. The bismuth-substituted rare earth iron garnet single crystal film is magnetically saturated or unsaturated depending on the presence or absence of an external magnetic field, and the difference between the saturated state and the unsaturated state appears as a change in the magnetic domain structure. That is, the saturated state is a single magnetic domain structure (also called a single domain) shown in FIGS. 2A and 2B, and the unsaturated state is shown in FIGS. 2C and 2D. It has a multi-domain structure (also called multi-domain). When light passes through the bismuth-substituted rare earth iron garnet single crystal film and is reflected back by a mirror or the like and returns, the polarization plane of light changes depending on whether the garnet single crystal film is single-domain or multi-domain. . As a result, when the returned light passes through the polarizer, the light intensity changes. In OMS-I, a change in the state of the magnetic domain structure is detected by replacing it with a change in light intensity.
[0015]
Each magnetic domain of the bismuth-substituted rare earth iron garnet single crystal film having multi-domains has magnetic anisotropy perpendicular to the film surface, spreads two-dimensionally in the film surface, and is sufficiently fine for incident light. Have a width. When light is perpendicularly incident on the film surface having the multi-domain structure, a diffraction phenomenon occurs due to the multi-domain structure, and a part of transmitted light is diffracted, but the behavior is extremely complicated. . The so-called zero-order light (light component that does not diffract) that vertically passes through the Faraday rotator having the multi-domain reflects a change in the magnetic domain structure that is proportional to the magnitude of the external magnetic field, and is substantially proportional to the magnitude of the external magnetic field. Will receive the Faraday rotation. Further, the diffracted light has a polarization plane of 90 degrees with respect to the polarization plane of the incident light. For example, if the Faraday rotator has a Faraday rotation angle of 90 degrees, theoretically, the entire light passing through the film becomes light having a polarization plane of 90 degrees with respect to the polarization plane of the incident light, and diffracted light and It cannot be distinguished from the zero-order light. In order to increase the S / N ratio as much as possible, a rotator capable of obtaining a Faraday rotation angle of 45 degrees when light passes through the Faraday rotator only in one way and a 90-degree reciprocation in the reciprocation is preferable, and the present invention also satisfies this condition. Is preferred.
Japanese Patent Application Laid-Open No. 11-44744 and the present invention are characterized in that the optical path has a certain angle with the normal to the main surface of the Faraday rotator.
[0016]
When light enters obliquely with respect to the normal line of the bismuth-substituted rare earth iron garnet single crystal film, passes through the film, is reflected by a mirror, and transmits through the film in the opposite direction, the single domain state (FIG. 2B) ) And in the multi-domain state (FIG. 2D), the polarization plane of the returned light changes. In the case of a single domain, it undergoes Faraday rotation. In the case of multi-domain, it is not substantially subjected to Faraday rotation. This is because, in the case of the multi-domain, when a light beam passes through each magnetic domain, the adjacent magnetic domains give Faraday rotations in opposite directions to each other, so that they interfere and cancel each other as a whole.
[0017]
FIGS. 3A to 3D show the Faraday rotation when light is vertically incident on the Faraday rotator. In the case where the Faraday rotator has a single domain, when a light beam enters in a direction perpendicular to the main surface of the film, the light beam passes through the film and is reflected by the mirror 51, and the light beam is reflected in the opposite direction as shown in FIG. Through the membrane. For example, if the element rotates the Faraday rotation angle by 45 degrees, the reciprocation is 90 degrees. In the case of multi-domain, as shown in FIG. 3D, when a light beam enters in a direction perpendicular to the main surface of the film, the light beam passes through each magnetic domain, is reflected by the mirror 51, and reverses the same magnetic domain. Pass in the direction. Therefore, for example, if the element rotates the Faraday rotation angle by 45 degrees, the reciprocation is 90 degrees. That is, regardless of which magnetic domain passes, the plane of polarization of the reflected light is perpendicular (90 degrees) to the plane of polarization of the incident light, which is no different from the case of the single domain. Further, as described above, the polarization plane of the diffracted light by the multi-domain is 90 degrees with the polarization plane of the incident light. As described above, when a light beam is incident in a direction perpendicular to the main surface of the film, the bismuth-substituted rare earth iron garnet single crystal film is in a magnetically saturated state and an unsaturated state. There is no distinction in the rotation angle.
[0018]
FIG. 4 shows a reflection type optical magnetic field sensor head in which Faraday rotators made of a bismuth-substituted rare earth iron garnet single crystal film are arranged in series at regular intervals. For example, when the magnet approaches the reflection type optical magnetic field sensor, the individual Faraday rotators are sequentially magnetized, and each time the individual Faraday rotator is magnetized, the light intensity of the reflected light changes. The specific position of the magnet can be detected. Using this principle, for example, the position of the magnet inside the cylinder can be detected.
[0019]
FIG. 5 shows the overall configuration of another reflection type optical magnetic field sensor head (hereinafter, referred to as OMS-II) according to the present invention. This reflection type optical magnetic field sensor head has the same structure as the OMS-I except for the Faraday rotator. That is, a two-core ferrule or glass capillary 11 to which two optical fibers are fixed, a birefringent plate 21, a lens 31, a Faraday rotator 41 made of a bismuth-substituted rare earth iron garnet single crystal film, and a mirror 51 are arranged in this order. I do. In this reflection type optical magnetic field sensor head, the optical path of light incident on the Faraday rotator is perpendicular to the main surface of the Faraday rotator, and the Faraday rotator is a Faraday rotator whose magnetic anisotropy has been reduced by heat treatment. The permanent magnets 45 and 46 are arranged so that a constant external magnetic field is applied in a direction parallel to the plane of the Faraday rotator.
[0020]
Generally, a bismuth-substituted rare earth iron garnet single crystal grown by a liquid phase epitaxial method (LPE method) has a strong growth-induced magnetic anisotropy. Due to the growth-induced magnetic anisotropy, a magnetic domain magnetized in a direction perpendicular to the film surface is generated, and has a magnetic domain structure that spreads two-dimensionally in the film surface. The magnetic domain structure is affected by an external magnetic field applied perpendicular to the film surface, and changes according to the magnitude of the external magnetic field. However, it is hardly affected by an external magnetic field in a direction parallel to the film surface. However, it is known that the growth-induced magnetic anisotropy disappears by the heat treatment. The OMS-II uses a bismuth-substituted rare earth iron garnet single crystal from which growth-induced magnetic anisotropy has been removed by heat treatment.
[0021]
The principle of the reflection type optical magnetic field sensor (OMS-II) of the present invention will be described below. FIGS. 6A and 6B are explanatory diagrams of the operation principle of the OMS-II, in which a magnetic field having a component perpendicular to the film surface of the bismuth-substituted rare earth iron garnet single crystal film of FIG. 5 is applied ( FIG. 6A shows a case where a magnetic field at an angle other than perpendicular to the film surface is applied (FIG. 6B).
[0022]
A bismuth-substituted rare-earth iron garnet single crystal film from which the growth-induced magnetic anisotropy has been removed by heat treatment has only the original magnetic anisotropy, and the magnetic field required to magnetize perpendicularly to the film surface, that is, the saturation magnetic field With a magnetic field of about the same degree, it is also magnetized in a direction parallel to the film surface. That is, when an external magnetic field that is equal to or greater than the saturation magnetic field is applied, a single domain is provided, and the magnetization of the single domain is easily inclined in the direction of the external magnetic field.
[0023]
OMS-II uses a bismuth-substituted rare earth iron garnet single crystal film from which magnetic anisotropy has been removed by heat treatment. In the bismuth-substituted rare earth iron garnet single crystal film from which the magnetic anisotropy has been removed, the direction of magnetization of the bismuth-substituted rare earth iron garnet single crystal film is the same as the direction of the external magnetic field. By applying an external magnetic field in a direction parallel to the film surface using a permanent magnet or the like, the direction of magnetization becomes parallel to the film surface. Therefore, even if light is transmitted perpendicular to the film surface, the transmitted light does not receive the Faraday effect (FIG. 6A), but the bismuth-substituted rare earth iron garnet single crystal film has a component perpendicular to the film surface. When a magnetic field is applied, the magnetization of the bismuth-substituted rare earth iron garnet single crystal film is oriented in the direction of the combined magnetic field of the external magnetic field and the magnetic field parallel to the film surface added in advance, so that the permeation is perpendicular to the film surface. The Faraday effect occurs in the light, and the polarization plane of the transmitted light changes (FIG. 6B).
[0024]
In order to magnetize in a direction parallel to the film surface, as described above, an external magnetic field equal to or higher than the saturation magnetic field of the bismuth-substituted rare earth iron garnet single crystal film is required. The magnetization of the bismuth-substituted rare earth iron garnet single crystal film is oriented in the direction of the combined magnetic field of the magnetic field parallel to the film surface and the magnetic field perpendicular to the film surface (the magnetic field to be sensed by the magnetic field sensor). 4 is an operation principle of the optical magnetic field sensor head of the invention. Therefore, the smaller the saturation magnetic field of the bismuth-substituted rare earth iron garnet single crystal used in the present invention, the higher the sensitivity as an optical magnetic field sensor head. Further, a permanent magnet for applying a magnetic field in a direction parallel to the film surface needs to be made of a material that is not affected by the magnetic field to be detected. In addition, the bismuth-substituted rare earth iron garnet single crystal is magnetically It is necessary to determine the size and arrangement so as to give the minimum magnetic field for saturation. In the OMS-II, similarly to the OMS-I, by arranging the Faraday rotators made of a bismuth-substituted rare earth iron garnet single crystal film in series at a constant interval, each time the Faraday rotators are magnetized, Since the light intensity of the reflected light changes, for example, the position of the magnet inside the cylinder can be detected.
[0025]
As described above, both of the reflection type optical magnetic field sensor heads “OMS-I” and “OMS-II” have a polarization plane of light transmitted through the bismuth-substituted rare earth iron garnet single crystal film in the absence of an external magnetic field. Although there is no change, when an external magnetic field is applied, the polarization plane of the transmitted light changes. In both the reflection type optical magnetic field sensor heads "OMS-I" and "OMS-II", the light emitted from the optical fiber on the light incident side is separated by the birefringent plate into light beams whose polarization directions are orthogonal to each other. Each ray is transmitted through a Faraday rotator made of a bismuth-substituted rare earth iron garnet single crystal film, reflected by a mirror, transmitted again through the Faraday rotator, condensed by a lens, and joined by a birefringent plate. Enters the optical fiber on the exit side. At this time, if the deflecting surface of the light transmitted through the Faraday rotator does not change, that is, if no external magnetic field is applied, if the mirror is slightly tilted, it follows almost the same optical path and couples to the optical fiber on the exit side. .
However, when the polarization plane of light transmitted through the Faraday rotator changes, that is, when an external magnetic field is applied to the Faraday rotator, the birefringent plate further separates the optical path by an amount corresponding to the Faraday rotation angle. The light entering the optical fiber is reduced. By detecting this change in the amount of light, it operates as an optical magnetic field sensor.
[0026]
From the above principle, it is sufficient that the separation distance of the birefringent plate is not less than the cladding diameter of the optical fiber at the maximum. In principle, the diameter may be larger than the core diameter, but light entering the clad also propagates as a clad mode if the distance is short. Since the cladding mode causes noise, it is necessary to eliminate the propagation in the cladding mode. Therefore, the separation distance of the birefringent plate is preferably equal to or larger than the cladding diameter.
[0027]
The present invention is characterized in that a birefringent plate is arranged between an optical fiber and a lens. In the present invention, a distributed index lens, a spherical lens, an aspherical lens, or the like can be used. For example, as in the reflection type optical magnetic field sensor head disclosed in Japanese Patent Application No. 10-31057, when a 0.25 pitch distributed refraction plate lens is used and a refraction plate is used after the lens, the light emitted from the lens is Although it differs depending on the type of the lens and the optical fiber, the light becomes parallel light of 0.4 mm to 2 mm or more, and this parallel light is separated by the birefringent plate into light beams whose polarization directions are orthogonal to each other. If the separation is insufficient, when the light reflected by the mirror and returned is merged at the birefringent plate, the light to be separated overlaps with the optical path of the light to be merged and enters the optical fiber on the exit side. And the SN ratio decreases.
However, when a birefringent plate is used between the optical fiber and the lens, the light emitted from the lens overlaps, but is reflected and then separated when condensed, so that the SN ratio does not decrease.
[0028]
As the optical input / output path, a commercially available optical fiber or an optical waveguide patterned in a glass or polymer film base material can be selected. Generally, it is preferable to select an optical fiber from the viewpoint of mass productivity and miniaturization. Further, in order to increase the optical coupling efficiency, the numerical aperture of the optical fiber as the optical input path is preferably small, and a small single mode fiber having a core diameter of 10 to 50 μm or a graded index fiber is preferably used. In addition, as for the light output path, it is preferable that the core diameter of the optical fiber on the light output side is the same as or larger than that on the light input side because of easy assembly. Also, since the shorter the distance between the optical input and output paths, the smaller the diameter of the optical fiber is, the optical fiber having a core diameter of 10 to 100 μm is preferable.
[0029]
The (111) bismuth-substituted rare earth iron garnet single crystal film used for the Faraday rotator is preferably appropriately selected from rare earth iron garnet single crystals having a composition represented by the following general formula.
R3-X BiX Fe5-Z AZ O12
Here, R is at least one selected from the group consisting of Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu, and A is Ga, It is at least one selected from the group consisting of Sc, Al, and In. The range of X is 0.3 ≦ X <2.0, and the range of Z is 0 ≦ Z ≦ 1.0. The bismuth-substituted rare earth iron garnet single crystal of the present invention can be produced by a known method. In particular, a liquid phase epitaxy (LPE) method in which the operation of the manufacturing apparatus is easy and which is excellent in mass productivity is preferable. When the liquid phase epitaxy method is performed, any known substrate can be used as the substrate. Generally, a commercially available lattice constant called a SGGG substrate having a lattice constant of 12.490 ° to 12.515 ° is used. What is necessary is just to select suitably from nonmagnetic garnet [(GdCa) 3 (GaMgZr) 5O12]. When implementing the reflection type optical magnetic field sensor head “OMS-I” of the present invention, it is necessary that the bismuth-substituted rare earth iron garnet single crystal film and the optical path have an inclination angle of 5 degrees or more. This inclination angle is possible if it is less than 90 degrees in principle. However, as the angle increases, the SN ratio as a sensor increases, and the distance from the light input / output end to the mirror increases. In other words, it is not preferable because the length of the sensor head increases. Therefore, the angle of the light beam with respect to the normal line of the bismuth-substituted rare earth iron garnet single crystal film is preferably 20 degrees or less. More preferably, it is 15 degrees or less. FIG. 4 is a schematic diagram illustrating an example in which a plurality of Faraday rotators are inserted into “OMS-I”.
[0030]
Further, when implementing the reflection type optical magnetic field sensor head “OMS-II” of the present invention, it is necessary to reduce the magnetic anisotropy by heat-treating the bismuth-substituted rare earth iron garnet single crystal film. As a means for reducing the magnetic anisotropy, annealing at 900 to 1300 ° C. is effective (“Magnetic Bubble”, Sakurai et al., Ohmsha, 1982), but when practicing the present invention, a temperature of about 1100 ° C. It was confirmed that the magnetic anisotropy can be sufficiently reduced by annealing for more than 10 hours.
[0031]
The mirror may be appropriately selected from commercially available thin film metal mirrors obtained by depositing metal on glass or the like, or dielectric multilayer mirrors formed of a multilayer film of a metal oxide such as SiO 2 or TiO 2 as required. good.
[0032]
EXAMPLES Hereinafter, the present invention will be described specifically and in detail with reference to examples, but the following examples are for the purpose of specifically explaining the present invention, and the embodiments and the invention of the present invention will be described. Is not intended to limit the scope of
[0033]
【Example】
Example 1
A reflection type optical magnetic field sensor head having the structure shown in FIG. 1 was manufactured using the following components. The optical fiber 12 on the light incident side is a single mode optical fiber having an optical fiber outer diameter of 125 μm. The optical fiber 13 on the light output side is a multi-mode optical fiber having a core diameter of 50 μm and an optical fiber outer diameter of 125 μm. The birefringent plate 21 is a 1.265 mm rutile single crystal. The Faraday rotator 41 is a 0.82 μm bismuth-substituted rare earth iron garnet single crystal film manufactured by Mitsubishi Gas Chemical Company. The mirror 51 is an aluminum metal thin film deposited on the glass substrate. The lens 31 is a distributed index lens. The Faraday rotator 41 was arranged at an angle of 18 degrees with respect to the optical path.
[0034]
Light from a semiconductor laser light source having a wavelength of 820 nm is made incident on the single-mode optical fiber 12 on the light incident side of the optical magnetic field sensor head, and the light intensity of light emitted from the optical fiber 13 on the light emitting side is measured by a photodetector. While applying a magnetic field gradually. As a result, when a magnetic field having a magnetic field strength of 260 (Oe) was applied to the Faraday rotator 41, the Faraday rotator was magnetically saturated, and the light output became a constant value. The light intensity difference (that is, SN ratio) between the state where no magnetic field was applied to the Faraday rotator 41 and the state where a magnetic field strength of 260 (Oe) was applied to the Faraday rotator 41 was 28 dB.
[0035]
Example 2
In Example 1, an optical magnetic field sensor head was manufactured in the same manner as in Example 1 except that light was incident so as to form an angle of 6 degrees with the main surface of the Faraday rotator. When the light intensity difference (SN ratio) was measured in the same manner as in Example 1, it was 24 dB.
[0036]
Example 3
A reflection type optical magnetic field sensor head having the structure shown in FIG. 5 was manufactured using the following components. The optical fiber 12 on the light incident side is a single mode optical fiber having an optical fiber outer diameter of 125 μm. The light emitting side optical fiber 13 is a multi-mode optical fiber having a core diameter of 50 μm and an outer diameter of 125 μm. The birefringent plate 21 is a 1.265 mm rutile single crystal. The Faraday rotator 41 is an annealed 1.31 μm bismuth-substituted rare earth iron garnet single crystal film manufactured by Mitsubishi Gas Chemical Company. The mirror 51 is an aluminum metal thin film deposited on a glass substrate. The lens 31 is a hemispherical lens.
[0037]
Light from a semiconductor laser light source having a wavelength of 1310 nm is made incident on the single-mode optical fiber 12 on the light incident side of the optical magnetic field sensor head, and the intensity of the light from the optical fiber 13 on the light emitting side is measured by a photodetector. Was gradually applied.
As a result, when a magnetic field having a magnetic field strength of 230 (Oe) was applied to the Faraday rotator 41, the Faraday rotator 41 was magnetically saturated and the light output became a constant value. The light intensity difference between the state where no magnetic field was applied to the Faraday rotator and the state where a magnetic field of a magnetic field strength of 230 (Oe) was applied to the Faraday rotator 41 was 32 dB.
[0038]
Comparative Example 1
In Example 1, an optical magnetic field sensor head was manufactured in the same manner as in Example 1 except that light was incident perpendicularly to the main surface of the Faraday rotator. Was 4.2 dB.
[0039]
【The invention's effect】
A simple and lightweight measuring device for accurately measuring the rotation speed can be provided at a lower cost and in large quantities. The SN ratio of the detection signal is improved.
[Brief description of the drawings]
FIG. 1 is a schematic diagram showing an example of the entire configuration of a reflection type optical magnetic field sensor head “OMS-I” according to the present invention.
FIGS. 2 (a), (b), (c) and (d) are diagrams showing the operation principle of a reflection type optical magnetic field sensor head “OMS-I” according to the present invention.
FIGS. 3 (a), (b), (c) and (d) are diagrams illustrating Faraday rotation when light is vertically incident on a multi-domain Faraday rotator.
FIG. 4 is a schematic diagram showing an example in which a plurality of Faraday rotators are inserted into a reflection type optical magnetic field sensor head “OMS-I” according to the present invention.
FIG. 5 is a schematic diagram showing an example of the overall configuration of a reflection type optical magnetic field sensor head “OMS-II” according to the present invention.
6A and 6B are diagrams showing the operation principle of the reflection type optical magnetic field sensor head “OMS-II” according to the present invention, wherein FIG. 6A shows a case where a magnetic field having a component perpendicular to the film surface is applied; b) shows a case where a magnetic field at an angle other than perpendicular to the film surface is applied.
FIG. 7 is a schematic diagram showing an example of the entire configuration of a reflection type optical magnetic field sensor head disclosed in Japanese Patent Application Laid-Open No. 11-44744.
[Explanation of symbols]
11 optical fiber holders such as ferrules and glass capillaries, 12, 13 optical fibers, 21 birefringent plates, 26 glass polarizing elements, 31 lenses, 41, 42 Faraday rotators, 45, 46 permanent magnets 51 mirrors.

Claims (5)

In a reflection type optical magnetic field sensor head, a two-core ferrule or glass capillary to which two optical fibers are fixed, a lens, a polarizing element, a Faraday rotator made of a bismuth-substituted rare earth iron garnet single crystal film, and a mirror are arranged in this order. A reflection type optical magnetic field sensor head, wherein a birefringent plate is used instead of a polarizing element, and the birefringent plate is arranged between the ferrule or the glass capillary and the lens. 2. The reflection type optical magnetic field sensor head according to claim 1, wherein a direction of a normal to the main surface of the Faraday rotator forms an angle of 5 degrees or more and 20 degrees or less with respect to an optical path. The reflection type optical magnetic field sensor head according to claim 3, wherein two or more Faraday rotators made of a bismuth-substituted rare earth iron garnet single crystal film are arranged in series. The Faraday rotator is one in which growth-induced magnetic anisotropy is substantially eliminated by heat treatment, and a direction of a normal line of the main surface of the Faraday rotator and a direction of an optical path are substantially the same. 2. The reflection type optical magnetic field sensor head according to claim 1, wherein a permanent magnet is arranged so that an external magnetic field is applied in a direction parallel to a main surface of the Faraday rotator. 5. The reflection type optical magnetic field sensor head according to claim 4, wherein two or more pairs of said Faraday rotator and permanent magnet are arranged in series.

Images