Magnetic field sensor head and magnetic field sensor device

Abstract

A magnetic field sensor head 50 comprises: a Faraday rotator 54 having a plurality of magnetic thin films 56a-56e arranged in a predetermined arrangement direction and a plurality of spacers 57a-57d disposed between the plurality of magnetic thin films 56a-56e; and a mirror element 55 that outputs return light to the Faraday rotator 54 in response to incident light from the Faraday rotator 54. The thickness of each of the plurality of magnetic thin films 56a-56e is determined such that the magnetic resonance frequency of each of the plurality of magnetic thin films 56a-56e is a desired frequency.

Description

Magnetic field sensor head and magnetic field sensor device

[0001] The present invention relates to a magnetic field sensor head and a magnetic field sensor device.

[0002] A magnetic field sensor head is known that uses a Faraday rotor made of n magneto-optical elements made of rare-earth iron garnet, bismuth-substituted rare-earth iron garnet, or orthoferrite stacked together (see, for example, Patent Document 1). The magnetic field sensor head described in Patent Document 1 can detect transmitted light with less diffraction loss by using a magneto-optical element stacked together.

[0003] Patent No. 3044084

[0004] However, the magnetic field sensor head described in Patent Document 1 does not have a controlled magnetic resonance frequency, which may limit the frequency band in which the Faraday rotator, which changes according to the magnetic resonance frequency, can be detected.

[0005] The present invention aims to solve these problems and to provide a magnetic field sensor head having a magnetic field sensor element with a controllable magnetic resonance frequency.

[0006] The magnetic field sensor head according to the present invention includes a Faraday rotor having a plurality of magnetic thin films arranged in a predetermined alignment direction and a plurality of spacers disposed between the plurality of magnetic thin films, and a mirror element that emits reflected light to the Faraday rotor in response to incident light from the Faraday rotor, wherein the thickness of each of the plurality of magnetic thin films is determined such that the magnetic resonance frequency of each of the plurality of magnetic thin films is a desired frequency.

[0007] Furthermore, in the magnetic field sensor head according to the present invention, it is preferable that each of the multiple magnetic thin films is a rare-earth iron garnet thin film.

[0008] Furthermore, in the magnetic field sensor head according to the present invention, it is preferable that each of the multiple spacers has a thickness greater than or equal to the thickness of each of the multiple magnetic thin films arranged adjacent to it.

[0009] Furthermore, in the magnetic field sensor head according to the present invention, it is preferable that at least one of the plurality of magnetic thin films has a different thickness from the other magnetic thin films included in the plurality of magnetic thin films.

[0010] It is preferable that the multiple magnetic thin films be arranged such that their thickness increases along the direction in which the multiple magnetic thin films are arranged.

[0011] Furthermore, in the magnetic field sensor head according to the present invention, it is preferable that the multiple magnetic thin films are arranged such that their thickness increases as they move away from the light guide member. Furthermore, in the magnetic field sensor head according to the present invention, it is preferable that each of the multiple magnetic thin films is formed from a Bi-substituted iron garnet crystal, and that the thickness of each of the multiple magnetic thin films is determined such that the magnetic resonance frequency of each of the multiple magnetic thin films is 320 MHz or more and 450 MHz or less.

[0012] The magnetic field sensor device according to the present invention includes a light-emitting unit that emits incident light, a magnetic field sensor head that emits reflected light corresponding to the magnetic field generated by the flow of current when incident light is incident, a detection signal generation unit that receives the reflected light and outputs a detection signal corresponding to the magnetic field, and an optical path unit that receives incident light from the light-emitting unit and emits the incident light to the magnetic field sensor head, and receives reflected light from the magnetic field sensor head and emits the reflected light to the detection signal generation unit, wherein the magnetic field sensor head includes a Faraday rotor having a plurality of magnetic thin films arranged in a predetermined arrangement direction and a plurality of spacers arranged between the plurality of magnetic thin films, and a mirror element that emits reflected light to the Faraday rotor in response to incident light being incident from the Faraday rotor, wherein the thickness of each of the plurality of magnetic thin films is determined so that the magnetic resonance frequency of each of the plurality of magnetic thin films is a desired frequency.

[0013] The magnetic field sensor head according to the present invention can control the magnetic resonance frequency of the magnetic field sensor element.

[0014] This is a block diagram showing the magnetic field sensor device according to an embodiment. This is a diagram showing the magnetic field sensor head shown in Figure 1. This is a diagram showing the relationship between the thickness of the magnetic thin film and the magnetic resonance frequency when Bi:RIG is used as the magnetic thin film. This is a diagram showing the transmission characteristics of the Faraday rotor shown in Figure 1 and the Faraday rotor according to a comparative example. This is a diagram showing the magnetic field sensor head according to the second embodiment. This is a diagram showing the transmission characteristics of the Faraday rotor shown in Figure 5.

[0015] The magnetic field sensor head and magnetic field sensor device according to the present invention will be described below with reference to the drawings. However, it should be noted that the technical scope of the present invention is not limited to these embodiments, but extends to the invention described in the claims and its equivalents.

[0016] (Configuration and function of the magnetic field sensor device according to the embodiment) Figure 1 is a block diagram of the magnetic field sensor device according to the embodiment.

[0017] The magnetic field sensor device 1 includes a light-emitting unit 10, a circulator 20, a first optical element 30, an optical path unit 40, a magnetic field sensor head 50, and a detection signal generation unit 60. The optical path between the light-emitting unit 10, the circulator 20, the first optical element 30, the optical path unit 40, the magnetic field sensor head 50, and the detection signal generation unit 60 is formed by an optical fiber 15, which is a PANDA (Polarization-maintaining AND Absorption-reducing) fiber. In one example, the outer diameter of the optical fiber 15 is 125 μm. However, the optical path between the first optical element 30, the optical path unit 40, the magnetic field sensor head 50, and the detection signal generation unit 60 may be formed by polarization-maintaining optical fibers other than PANDA fibers, such as bow-tie fibers and elliptical jacket fibers.

[0018] The light-emitting unit 10 includes a light-emitting element 11, an isolator 12, and a polarizer 13. The light-emitting element 11 is, for example, a semiconductor laser or a light-emitting diode. Specifically, a Fabry-Perot laser, a superluminescent diode, etc., can be preferably used as the light-emitting element 11.

[0019] The isolator 12 protects the light-emitting element 11 by allowing light incident from the light-emitting element 11 to pass through to the circulator 20 side, while preventing light incident from the circulator 20 from passing through to the light-emitting element 11 side. The isolator 12 is, for example, a polarization-dependent optical isolator, or it may be a polarization-independent optical isolator.

[0020] The polarizer 13 is an optical element that converts the light emitted by the light-emitting element 11 into linearly polarized light, and its type is not particularly limited. The first linearly polarized light obtained by the polarizer 13 is incident on the first optical element 30 via the circulator 20.

[0021] The circulator 20 is an optical splitter that transmits the first linearly polarized light emitted from the light-emitting unit 10 to the first optical element 30, and splits the second linearly polarized light emitted from the first optical element 30 to the detection signal generation unit 60. The circulator 20 is formed, for example, by a Faraday rotor, a half-wave plate, a polarizing beam splitter, and a reflective mirror.

[0022] The first optical element 30 is, for example, a half-wave plate positioned such that its azimuth angle is 22.5 degrees with respect to the polarization plane of the first linearly polarized light incident from the circulator 20. The first optical element 30 rotates the polarization plane of the first linearly polarized light incident from the circulator 20 by 45 degrees and emits the first linearly polarized light into the optical path section 40. The first linearly polarized light whose polarization plane has been rotated by 45 degrees in the first optical element 30 has a first linearly polarized light CW1 which is P-polarized and a second linearly polarized light CCW1 which is S-polarized and orthogonal to the first linearly polarized light CW1.

[0023] Furthermore, the first optical element 30 rotates the polarization plane of the second linearly polarized light, which is incident linearly polarized light from the optical path section 40, by 45 degrees and emits it to the circulator 20.

[0024] The optical path section 40 includes a first beam splitter 41, a second beam splitter 42, a first optical path 43, a second optical path 44, and a second optical element 45.

[0025] The first beam splitter 41 emits the first linearly polarized light CW1 into the first optical path 43 and the second linearly polarized light CCW1 into the second optical path 44. The first beam splitter 41 also receives the third linearly polarized light CW2 from the second optical path 44 and the fourth linearly polarized light CCW2 from the first optical path 43. The third linearly polarized light CW2 and the fourth linearly polarized light CCW2 are mutually orthogonal polarization components of the second linearly polarized light emitted to the first optical element 30.

[0026] The second beam splitter 42 receives the first linearly polarized light CW1 from the first optical path 43 and the second linearly polarized light CCW1 from the second optical path 44. The second beam splitter 42 also emits the third linearly polarized light CW2 to the second optical path 44 and the fourth linearly polarized light CCW2 to the first optical path 43.

[0027] The first beam splitter 41 and the second beam splitter 42 separate the incident light into a P-polarized component and an S-polarized component, and then combine the P-polarized component and the S-polarized component and emit them. The first beam splitter 41 and the second beam splitter 42 are, for example, prism-type beam splitters, but may also be planar beam splitters or wedge-type beam splitters.

[0028] The first optical path 43 leads the first linearly polarized light CW1 introduced from the first beam splitter 41 to the second beam splitter 42, and also leads the fourth linearly polarized light CCW2 introduced from the second beam splitter 42 to the first beam splitter 41. The second optical path 44 leads the second linearly polarized light CCW1 introduced from the first beam splitter 41 to the second beam splitter 42, and also leads the third linearly polarized light CW2 introduced from the second beam splitter 42 to the first beam splitter 41.

[0029] The first optical path 43 is a PANDA fiber with one end optically connected to the first beam splitter 41 and the other end optically connected to the second beam splitter 42. The second optical path 44 is a PANDA fiber with one end optically connected to the first beam splitter 41 and the other end optically connected to the second beam splitter 42. The first optical path 43 and the second optical path 44 may be polarization-maintaining fibers such as bowtie fibers and elliptical jacket fibers. A second optical element 45 is arranged in the second optical path 44.

[0030] The second optical element 45 includes a first (1 / 4) wave plate 46, a second (1 / 4) wave plate 47, and a 45-degree Faraday rotator 48.

[0031] The first (1 / 4) wave plate 46 is a 1 / 4 wave plate whose optical axis is tilted at 45 degrees with respect to the slow axis and fast axis of the PANDA fiber that forms the second optical path 44. The first (1 / 4) wave plate 46 converts linearly polarized light into circularly polarized light and also converts circularly polarized light into linearly polarized light.

[0032] The second (1 / 4) wave plate 47 is a 1 / 4 wave plate positioned with its optical axis tilted at -45 degrees with respect to the slow axis and fast axis of the PANDA fiber that forms the second optical path 44. The second (1 / 4) wave plate 47 converts circularly polarized light from the 45-degree Faraday rotator 48 into linearly polarized light, and also converts linearly polarized light into circularly polarized light.

[0033] The 45-degree Faraday rotator 48 is a Faraday rotator that changes the rotation angle of circularly polarized light incident from the first (1 / 4) wave plate 46 and the second (1 / 4) wave plate 47, respectively.

[0034] The 45-degree Faraday rotor 48 changes the rotation angle of the circularly polarized light incident on the first (1 / 4) wave plate 46 such that the rotation angle of the second linearly polarized light CCW1 emitted from the second (1 / 4) wave plate 47 is shifted by 45 degrees from the rotation angle of the second linearly polarized light CCW1, which is the linearly polarized light incident on the first (1 / 4) wave plate 46. The 45-degree Faraday rotor 48 also changes the rotation angle of the circularly polarized light such that the rotation angle of the third linearly polarized light CW2 emitted from the first (1 / 4) wave plate 46 is shifted by -45 degrees from the rotation angle of the third linearly polarized light CW2 incident on the second (1 / 4) wave plate 47.

[0035] The magnetic field sensor head 50 is disposed at the distal end of an optical fiber 15, and is optically connected to a second beam splitter 42 via the optical fiber 15. The magnetic field sensor head 50 is a magnetic field sensor head that detects a magnetic field generated by a current flowing through a flat conductor such as a wiring pattern formed on a circuit board or a bus bar. The linearly polarized light emitted from the light emitting unit 10 enters the magnetic field sensor head 50 as incident light, and when the incident light enters via the optical fiber 15, the magnetic field sensor head 50 emits return light whose polarization plane is rotated in accordance with an applied magnetic field.

[0036] The detection signal generating unit 60 includes a third beam splitter 61, a first light receiving element 62, a second light receiving element 63, and a signal processing circuit 65, and receives the second linearly polarized light split by the circulator 20. The detection signal generating unit 60 separates the second linearly polarized light into an S-polarized component and a P-polarized component, receives the S-polarized component and the P-polarized component, converts them into electrical signals, and performs differential amplification, thereby outputting a detection signal Ed corresponding to the magnetic field applied to the magnetic field sensor head 50. The third beam splitter 61 is a polarizing beam splitter (PBS) of a prism type, a planar type, a wedge substrate type, an optical waveguide type, or the like, and separates the second linearly polarized light split by the circulator 20 into an S-polarized component 66 and a P-polarized component 67.

[0037] Each of the first light receiving element 62 and the second light receiving element 63 is, for example, a PIN photodiode. The first light receiving element 62 receives the S-polarized component 66, and the second light receiving element 63 receives the P-polarized component 67. Each of the first light receiving element 62 and the second light receiving element 63 photoelectrically converts the received light, and outputs an electrical signal corresponding to the light intensity of the received light. The signal processing circuit 65 differentially amplifies the electrical signal representing the S-polarized component and the electrical signal representing the P-polarized component, thereby outputting a detection signal Ed corresponding to the magnetic field applied to the magnetic field sensor head 50.

[0038] (Configuration and Function of Magnetic Field Sensor Head According to First Embodiment) Figure 2 is a diagram showing the magnetic field sensor head 50.

[0039] The magnetic field sensor head 50 includes a head fiber 51, a GI fiber 52, a quarter-wave plate 53, a Faraday rotor 54, and a mirror element 55. The head fiber 51, also called a light guide member, is a PANDA fiber, similar to the optical fiber 15, and is fusion spliced ​​to the optical fiber 15. The head fiber 51 is a light guide member that guides incident light entering from the optical fiber 15 to the GI fiber 52 and guides the reflected light emitted from the GI fiber 52 back to the optical fiber 15.

[0040] The GI fiber 52 is an optical fiber that transmits light introduced into the core from the head fiber 51 as a sine wave with a period of pitch P by radially distributing the refractive index of the core. One end of the GI fiber 52 is optically connected to the head fiber 51. The cladding diameter of the GI fiber 52 is the same as the cladding diameter of the head fiber 51.

[0041] The quarter-wave plate 53 is positioned with its optical axis tilted at 45 degrees with respect to the phase-lagging and phase-advancing axes of the head fiber 51, which is a PANDA fiber. The quarter-wave plate 53 converts the incident light that enters from the head fiber 51 via the GI fiber 52 as linearly polarized light into circularly polarized light and outputs it to the Faraday rotator 54. The quarter-wave plate 53 also converts the reflected light that enters from the Faraday rotator 54 as circularly polarized light into linearly polarized light and outputs it to the head fiber 51 via the GI fiber 52.

[0042] The Faraday rotor 54 has five magnetic thin films 56a to 56e and four spacers 57a to 57d, which are arranged along a predetermined alignment direction that is the direction in which the incident and reflected light are incident. Each of the magnetic thin films 56a to 56e is formed of, for example, a Bi-substituted iron garnet (Bi:RIG) crystal. The compositional formula of the Bi-substituted iron garnet crystal is Bi ３ Fe ５ O １２ It is represented as follows. The Bi-substituted iron garnet crystal may also contain trace amounts of other additives. Note that each of the magnetic thin films 56a to 56e is made of rare earth iron garnet substituted with Y. Ｘ Y ３－Ｘ Fe ５ O １２It may also be a rare earth iron garnet thin film made of a single crystal having a garnet-type crystal structure represented by the composition formula. Here, R is a rare earth metal, which is an element substitutable for Y. R may be Ce in addition to Bi. When R is Ce, Y in the above composition formula ３－Ｘ A small amount among 0 to 3 of is substituted with Y. For example, each of the magnetic thin films 56a to 56e may be formed of a dilute magnetic semiconductor or a granular material in which a magnetic metal is dispersed in a dielectric. The magnetic thin films 56a to 56e have the same thickness. The thickness of the magnetic thin films 56a to 56e is determined such that the magnetic resonance frequency of each of the magnetic thin films 56a to 56e reaches a desired frequency, and is, for example, 30 μm.

[0043] Figure 3 is a diagram showing the relationship between the thickness of a magnetic thin film and the magnetic resonance frequency when a Bi-substituted iron garnet crystal (Bi:RIG) is used as the magnetic thin film. In FIG. 3, the horizontal axis represents the thickness of the magnetic thin film, and the vertical axis represents the magnetic resonance frequency. In FIG. 3, a curve L101 is an approximate curve showing the dependence of the magnetic resonance frequency of the magnetic thin film on the thickness of the magnetic thin film.

[0044] For the magnetic thin film, when the film thickness is 100 μm, the magnetic resonance frequency is 200 MHz; when the film thickness is 150 μm, the magnetic resonance frequency is 170 MHz; when the film thickness is 200 μm, the magnetic resonance frequency is 140 MHz. For the magnetic thin film, when the film thickness is greater than 100 μm, the magnetic domain structure becomes indistinct. In the case of a magnetic sensor using magnetic domain reversal (domain wall motion), there is a risk that linearity deteriorates. Therefore, it is preferable that the thickness of the magnetic thin film is 100 μm or less. In addition, when the thickness of the magnetic thin film is 50 μm, the magnetic resonance frequency is 250 MHz, which is 1.25 times the magnetic resonance frequency obtained when the film thickness is 100 μm, at which the magnetic resonance frequency is nearly saturated. Therefore, it is more preferable that the thickness of the magnetic thin film is 50 μm or less. Furthermore, when the thickness of the magnetic thin film is 30 μm, the magnetic resonance frequency is 320 MHz. When the thickness of the magnetic thin film is 30 μm, the magnetic resonance frequency is 1.60 times that when the thickness is 100 μm, at which the magnetic sensor operates well. Therefore, it is even more preferable that the thickness of the magnetic thin film is 30 μm or less.

[0045] When the thickness of the magnetic thin film is 10 μm, the magnetic resonance frequency is 550 MHz. While the magnetic resonance frequency is very high at 550 MHz when the thickness is 10 μm, making it too thin may reduce the magnetization and decrease the saturation Faraday rotation angle. Therefore, it is preferable that the thickness of the magnetic thin film be 10 μm or more. Furthermore, when the thickness is 20 μm, the magnetic resonance frequency is 450 MHz. When the thickness of the magnetic thin film is 20 μm, the magnetic resonance frequency becomes very high at 450 MHz, and the risk of a decrease in the saturation Faraday rotation angle is reduced. Therefore, it is even more preferable that the thickness of the magnetic thin film be 20 μm or more.

[0046] Each of the four spacers 57a to 57d has the same thickness and is made of a light-transmitting material such as glass that transmits incident and reflected light. Each of the magnetic thin films 56a to 56e is positioned to sandwich each of the spacers 57a to 57d. Spacer 57a is positioned to be sandwiched between magnetic thin films 56a and 56b, and spacer 57b is positioned to be sandwiched between magnetic thin films 56b and 56c. Spacer 57c is positioned to be sandwiched between magnetic thin films 56c and 56d, and spacer 57d is positioned to be sandwiched between magnetic thin films 56d and 56e. Each of the spacers 57a to 57d has a thickness of 1 / 5 times or more the thickness of the magnetic thin films 56a to 56e and a thickness of 1 time or less the thickness of the magnetic thin films 56a to 56e. When the thickness of spacers 57a to 57d is less than 1 / 5 the thickness of magnetic thin films 56a to 56e, there is a risk that each of the magnetic thin films 56a to 56e will magnetically bond and function as a single magnetic material. When the thickness of spacers 57a to 57d is greater than 1 time the thickness of magnetic thin films 56a to 56e, the optical paths of the incident and reflected light passing through the Faraday rotator 54 become longer, and there is a risk that optical loss will increase.

[0047] The mirror element 55 is formed on the surface of the Faraday rotor 54 opposite to the surface facing the quarter-wave plate 53, and reflects the light that has passed through the Faraday rotor 54 back towards the Faraday rotor 54. As the mirror element 55, for example, a silver (Ag) film, a gold (Au) film, an aluminum (Al) film, or a dielectric multilayer mirror can be used.

[0048] (Effects of the magnetic field sensor head according to the first embodiment) In the magnetic field sensor head 50, the Faraday rotor 54 can have a desired magnetic resonance frequency by determining the thickness of the magnetic thin films 56a to 56e so that the magnetic resonance frequency of the magnetic thin films 56a to 56e becomes a desired frequency.

[0049] Figure 4 shows the transmission characteristics of the Faraday rotor 54 and the Faraday rotor according to the comparative example. In Figure 4, the horizontal axis represents the frequency of the incident light incident on the Faraday rotor 54 and the Faraday rotor according to the comparative example, and the vertical axis represents the gain. Also in Figure 4, L201 shows the transmission characteristics of the Faraday rotor 54, and L202 shows the transmission characteristics of the Faraday rotor according to the comparative example.

[0050] The Faraday rotor in the comparative example is formed of Bi:RIG, similar to the Faraday rotor 54, and differs from the Faraday rotor 54 in that it has a single magnetic thin film with a thickness of 150 μmn instead of the magnetic thin films 56a to 56e and spacers 57a to 57d. The configuration and function of the Faraday rotor in the comparative example, other than the magnetic thin film, are the same as those of the Faraday rotor 54, so a detailed explanation is omitted here.

[0051] As shown in L201, the gain of the Faraday rotor 54 tends to increase up to 350 MHz, or 0.35 GHz, which is the magnetic resonance frequency of the magnetic thin films 56a to 56e, and then tends to decrease above 0.35 GHz. The Faraday rotor 54 can have a detection band ranging from 0 GHz to 0.35 GHz, which is the magnetic resonance frequency of the magnetic thin films 56a to 56e.

[0052] On the other hand, as shown in L202, the gain of the Faraday rotator in the comparative example shows an increasing trend up to 180 MHz, or 0.18 GHz, which is the magnetic resonance frequency of a magnetic thin film with a thickness of 150 μm, and then shows a decreasing trend above 0.18 GHz. The Faraday rotator in the comparative example can have a detection band ranging from 0 GHz to 0.18 GHz, which is the magnetic resonance frequency of a magnetic thin film with a thickness of 150 μm.

[0053] In the magnetic field sensor head 50, the Faraday rotator 54 comprises five magnetic thin films 56a to 56e each having a film thickness of 30 µm and a magnetic resonance frequency of 0.35 GHz. By including five magnetic thin films 56a to 56e each having a film thickness of 30 µm, the magnetic field sensor head 50 can have a wider detection band than a Faraday rotator according to a comparative example that includes a single magnetic thin film having a film thickness of 150 µm.

[0054] Furthermore, in the magnetic field sensor head 50, the magnetic thin films 56a to 56e are arranged with spacers 57a to 57d interposed therebetween, each spacer having the same thickness as the corresponding magnetic thin film 56a to 56e. Since the magnetic thin films 56a to 56e are arranged with the spacers 57a to 57d of the same thickness interposed therebetween, the magnetic field sensor head 50 does not function as an integrated magnetic body, and can have a magnetic resonance frequency corresponding to the thickness of each of the magnetic thin films 56a to 56e.

[0055] (Configuration and Function of Magnetic Field Sensor Head According to Second Embodiment) FIG. 5 is a diagram showing the magnetic field sensor head according to the second embodiment.

[0056] The magnetic field sensor head 70 differs from the magnetic field sensor head 50 in that the magnetic field sensor head 70 includes a Faraday rotator 74 instead of the Faraday rotator 54. The configuration and function of the components of the magnetic field sensor head 70 other than the Faraday rotator 74 are the same as those of the components of the magnetic field sensor head 50 denoted by the same reference numerals, and thus detailed description thereof is omitted herein.

[0057] The Faraday rotator 74 comprises five magnetic thin films 76a to 76e arranged such that the thickness thereof decreases as the distance from the mirror element 55 increases in the incident direction of incident light and return light, and four spacers 77a to 77d. Each of the magnetic thin films 76a to 76e is formed of, for example, Bi:RIG, similarly to the magnetic thin films 56a to 56e. Note that, similarly to the magnetic thin films 56a to 56e, the magnetic thin films 76a to 76e are R obtained by substituting Y for rare earth iron garnet Ｘ Y ３－Ｘ Fe ５ O １２The magnetic thin films may also be rare-earth iron garnet thin films made of single crystals having a garnet-type crystal structure represented by the composition formula. The thicknesses of the magnetic thin films 76a to 76e are determined so that the magnetic resonance frequencies of each magnetic thin film 76a to 76e are at a desired frequency. The thickness of magnetic thin film 76a is 20 μm, the thickness of magnetic thin film 76b is 40 μm, the thickness of magnetic thin film 76c is 60 μm, the thickness of magnetic thin film 76d is 80 μm, and the thickness of magnetic thin film 76e is 100 μm.

[0058] Each of the four spacers 77a to 77d is formed from a light-transmitting material such as glass that transmits incident and reflected light, similar to spacers 57a to 57d. Each of the magnetic thin films 76a to 76e is positioned to sandwich each of the spacers 77a to 77d. Spacer 77a is positioned to be sandwiched between magnetic thin films 76a and 76b, and spacer 77b is positioned to be sandwiched between magnetic thin films 76b and 76c. Spacer 77c is positioned to be sandwiched between magnetic thin films 76c and 76d, and spacer 77d is positioned to be sandwiched between magnetic thin films 76d and 76e. The thickness of spacer 77a is 40 μm, the thickness of spacer 77b is 60 μm, the thickness of spacer 77c is 80 μm, and the thickness of spacer 77d is 100 μm. Furthermore, the thickness of each spacer 77a to 77d is sufficient to be greater than or equal to the thickness of the thicker magnetic thin film among the adjacent magnetic thin films 96a to 96e, and less than twice the thickness of the thicker magnetic thin film among the adjacent magnetic thin films 76a to 76e.

[0059] (Effects of the magnetic field sensor head according to the second embodiment) The magnetic field sensor head 70 can emit reflected light having a uniform intensity over a desired band by determining the thickness of the magnetic thin films 56a to 56e such that the magnetic resonance frequencies of the magnetic thin films 76a to 76e are evenly spaced over a desired band.

[0060] Figure 6 shows the transfer characteristics of the Faraday rotor 74. In Figure 8, the horizontal axis represents the frequency of the incident light incident on the Faraday rotor 74, and the vertical axis represents the gain. Also, in Figure 4, L301 shows the transfer characteristics of the magnetic thin film 76a, L302 shows the transfer characteristics of the magnetic thin film 76b, L303 shows the transfer characteristics of the magnetic thin film 76c, L304 shows the transfer characteristics of the magnetic thin film 76d, L305 shows the transfer characteristics of the magnetic thin film 76e, and L306 shows the transfer characteristics of the Faraday rotor 74.

[0061] As shown in L306, the gain of the Faraday rotor 74 has peaks at the magnetic resonance frequencies of the magnetic thin films 76a to 76e, which are 0.20 GHz, 0.22 GHz, 0.25 GHz, 0.28 GHz, and 0.35 GHz. Since the gain of the Faraday rotor 74 is substantially uniform from 0 GHz to 0.35 GHz, it can emit reflected light with substantially uniform intensity over a wide bandwidth from 0 GHz to 0.35 GHz.

[0062] Furthermore, the magnetic field sensor head 70 can suppress the optical loss of incident light passing through the magnetic thin films 76a to 76e by arranging the magnetic thin films 76a to 76e in order of increasing thickness.

[0063] (Modified versions of the magnetic field sensor head according to the embodiment) In the magnetic field sensor heads 50 and 70, the magnetic film is a rare earth iron garnet thin film, but in the magnetic field sensor head according to the embodiment, the magnetic film may be a thin film other than a rare earth iron garnet thin film, such as a dilute magnetic semiconductor or a granular material in which a magnetic metal is dispersed in a dielectric.

[0064] Furthermore, in the magnetic field sensor heads 50 and 70, the Faraday rotors 54 and 74 have five magnetic films, but in the magnetic field sensor head according to this embodiment, the Faraday rotor may have multiple magnetic films. Also, in the magnetic field sensor heads 50 and 70, the Faraday rotors 54 and 74 have four spacers, but in the magnetic field sensor head according to this embodiment, the Faraday rotor may have one or more spacers, which is one less than the number of magnetic films.

[0065] Furthermore, while the magnetic field sensor heads 50 and 70 have a head fiber 51, a GI fiber 52, and a quarter-wave plate 53, the magnetic field sensor head according to this embodiment may have the head fiber 51, GI fiber 52, and quarter-wave plate 53 as separate components.

[0066] Furthermore, in the magnetic field sensor head 70, the magnetic thin films 76a to 76e are arranged such that their thickness decreases as they move away from the mirror element 55 along the arrangement direction. However, in the magnetic field sensor head according to the embodiment, the magnetic thin films 76a to 76e may be arranged such that their thickness increases as they move away from the mirror element 55 along the arrangement direction. Also, in the magnetic field sensor head according to the embodiment, the magnetic thin films 76a to 76e may be arranged such that their thickness changes randomly in the direction in which the incident light and reflected light are incident.

[0067] Furthermore, in the magnetic field sensor head 70, the magnetic thin films 76a to 76e have different thicknesses from each other, but in the magnetic field sensor head according to this embodiment, it is sufficient that at least one of the plurality of thin films has a different thickness from the other magnetic thin films included in the plurality of magnetic thin films.

[0068] 1 Magnetic field sensor device 50, 70 Magnetic field sensor head 51 Head fiber 52 GI fiber 53 Quarter wave plate 54, 74 Faraday rotor 55 Mirror element 56a-56e, 76a-76e Magnetic thin film 57a-57e, 77a-77e Spacer

Claims

1. A magnetic field sensor head comprising: a Faraday rotor having a plurality of magnetic thin films arranged in a predetermined arrangement direction and a plurality of spacers disposed between the plurality of magnetic thin films; and a mirror element that emits reflected light to the Faraday rotor in response to incident light entering from the Faraday rotor, wherein the thickness of each of the plurality of magnetic thin films is determined such that the magnetic resonance frequency of each of the plurality of magnetic thin films is a desired frequency.

2. The magnetic field sensor head according to claim 1, wherein each of the plurality of magnetic thin films is a rare earth iron garnet thin film.

3. The magnetic field sensor head according to claim 2, wherein each of the plurality of spacers has a thickness equal to or greater than the thickness of each of the plurality of magnetic thin films arranged adjacent to it.

4. The magnetic field sensor head according to claim 3, wherein at least one of the plurality of magnetic thin films has a thickness different from the thickness of the other magnetic thin films included in the plurality of magnetic thin films.

5. The magnetic field sensor head according to claim 4, wherein the plurality of magnetic thin films are arranged such that their thickness increases along the direction in which the plurality of magnetic thin films are arranged.

6. The magnetic field sensor head according to claim 5, wherein the plurality of magnetic thin films are arranged such that their thickness decreases as they move away from the mirror element.

7. The magnetic field sensor head according to claim 1, wherein each of the plurality of magnetic thin films is formed of a Bi-substituted iron garnet crystal, and the thickness of each of the plurality of magnetic thin films is determined such that the magnetic resonance frequency of each of the plurality of magnetic thin films is 320 MHz or more and 450 MHz or less.

8. A magnetic field sensor device comprising: a light-emitting unit that emits incident light; a magnetic field sensor head that emits reflected light corresponding to the magnetic field generated by the flow of current when incident light is incident on it; a detection signal generating unit that receives the reflected light and outputs a detection signal corresponding to the magnetic field; and an optical path unit that receives the incident light from the light-emitting unit and emits the incident light to the magnetic field sensor head, and receives the reflected light from the magnetic field sensor head and emits the reflected light to the detection signal generating unit, wherein the magnetic field sensor head comprises a Faraday rotor having a plurality of magnetic thin films arranged in a predetermined arrangement direction and a plurality of spacers disposed between the plurality of magnetic thin films; and a mirror element that emits the reflected light to the Faraday rotor in response to the incident light being incident on it from the Faraday rotor, wherein the thickness of each of the plurality of magnetic thin films is determined so that the magnetic resonance frequency of each of the plurality of magnetic thin films is a desired frequency.

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