Magnetic field sensor and magnetic field measuring device using the same

Abstract

To provide a magnetic field sensor that is highly safe, has high measurement reliability, and exhibits minimal degradation, and a magnetic field measuring device using the same. [Solution] The magnetic field sensor comprises a transmission section consisting of an optical fiber having a core and a cladding covering the outer surface of the core; an optical leakage section connected to the optical fiber and leaking a portion of the light transmitted from the optical fiber; a film formed on the outer surface of the optical leakage section and containing any of the following: a metal film in which surface plasmon resonance occurs upon excitation by evanescent waves, a film containing metal nanoparticles in which localized surface plasmon resonance occurs upon excitation by evanescent waves, or a semiconductor film that causes loss mode resonance; and a magnetic fluid disposed to cover the film.

Description

Technical Field

[0001] The present invention relates to a sensor for detecting a magnetic field, and particularly to a sensor for detecting a magnetic field using an optical fiber and a detection device using the sensor.

Background Art

[0002] Sensors for detecting magnetic fields, so-called magnetic field sensors, are used in various industrial fields. Conventionally, as magnetic field sensors, for example, Hall sensors as described in Patent Document 1 and magnetoresistive effect elements as described in Patent Document 2, so-called MR sensors, which are electrical magnetic field sensors, are used.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0004] When using an electrical magnetic field sensor as described above in a magnetic field measuring device, since it is necessary to supply power to the sensor element itself, a spark can occur in the sensor element or its vicinity. For example, when using an electrical magnetic field sensor in an environment where there is a flammable gas or the like, there is a risk of explosion due to the above-mentioned spark. Although the risk can be reduced by explosion-proof treatment such as sealing the sensor part, the explosion-proof treatment requires additional costs.

[0005] In addition, electrical magnetic field sensors have problems such as the measured value being affected by electromagnetic noise due to changes in the magnetic field, etc., and temperature degradation.

[0006] Therefore, there is a need to provide a magnetic field sensor that is highly safe, has high measurement reliability, and exhibits minimal degradation, as well as a magnetic field measuring device using the same. [Means for solving the problem]

[0007] According to the present invention, a magnetic field sensor is provided, comprising: a transmission section comprising an optical fiber having a core and a cladding covering the outer surface of the core; an optical leakage section connected to the optical fiber and leaking a portion of the light transmitted from the optical fiber; a film formed on the outer surface of the optical leakage section and comprising any of the following: a metal film in which surface plasmon resonance occurs upon excitation by evanescent waves, a film containing metal nanoparticles in which localized surface plasmon resonance occurs upon excitation by evanescent waves, or a semiconductor film that causes loss mode resonance; and a magnetic fluid disposed to cover the film. [Effects of the Invention]

[0008] The magnetic field sensor of the present invention is a sensor consisting of an optical fiber and a magnetic fluid disposed around the optical fiber, and has a configuration that allows changes in surface plasmon resonance, localized surface plasmon resonance, or loss mode resonance to be utilized in response to changes in the refractive index of the magnetic fluid. With such a configuration, it is possible to provide an explosion-proof sensor with excellent safety features, as the sensor part does not have electrical contacts. [Brief explanation of the drawing]

[0009] [Figure 1] This is a schematic cross-sectional view of a magnetic field sensor according to the first embodiment of the present invention. [Figure 2] This is an explanatory diagram showing an example of the configuration of a magnetic field measuring device. [Figure 3] This is a spectral graph showing the spectrum of propagating light at various magnetic field strengths. [Figure 4] This graph shows the change in optical loss for light with a wavelength of 850 nm at various magnetic field strengths. [Figure 5] This is a schematic cross-sectional view of a magnetic field sensor according to the first embodiment of the present invention. [Figure 6]This is an explanatory diagram showing an example of the configuration of a magnetic field measuring device. [Modes for carrying out the invention]

[0010] First, we will describe the overview and operating principle of the magnetic field sensor of the present invention.

[0011] The magnetic field sensor of the present invention is a so-called optical fiber magnetic field sensor that uses an optical fiber consisting of a core and a cladding surrounding the core. In its basic structure, a portion of the optical fiber is surrounded by a film and magnetic fluid, which are arranged on the outer circumference of a part of the optical fiber in the longitudinal direction, and this structure allows light to leak out to the outer circumference of the optical fiber. In other words, it is a structure that allows light to leak from the cladding of the optical fiber.

[0012] The magnetic field measuring device using the magnetic field sensor of the present invention has a magnetic fluid placed near the surface of the film body of the magnetic field sensor, and detects changes in the magnetic field and measures the magnetic field using the change in the refractive index of the magnetic fluid.

[0013] Specifically, the alignment of magnetic particles in the magnetic fluid changes due to the magnetic field, which in turn changes the refractive index of the magnetic fluid. This change in refractive index changes the intensity of light passing from one end of an optical fiber to the other, or in other words, light transmitted from one end of an optical fiber to the other (hereinafter also referred to as transmitted light or propagated light). In the magnetic field measuring device using the magnetic field sensor of the present invention, the magnetic field is measured by measuring the intensity of this transmitted light.

[0014] The changes in the intensity of the transmitted light described above are greatly influenced by changes in the refractive index near the surface of the film, for example, changes in the resonance conditions of surface plasmon resonance (SPR), localized surface plasmon resonance (LSPR), and loss mode resonance (LMR) in response to changes in the refractive index of materials present near the surface.

[0015] In the magnetic field sensor of the present invention, in particular, the optical fiber has a hetero-core structure in which the core diameter of the portion where the film body is formed is smaller than the core diameter of the optical fiber in other portions. In other words, the magnetic field sensor of the present invention has a configuration in which the core diameter becomes smaller in the middle and then becomes larger again.

[0016] By adopting the above-described hetero-core structure for the optical fiber used in the magnetic field sensor, light can be actively leaked from the core to the outside at the portion of the optical fiber where the core diameter is smaller than that of other portions. As a result, the change in the light loss that changes according to the refractive index of the magnetic fluid, that is, the change in the intensity of the transmitted light, appears more prominently than in a normal optical fiber with a constant core diameter.

[0017] As described above, as the magnetic field sensor, it is desirable to have a hetero-core structure. Basically, in the portion where the thin film is formed, it is sufficient that the propagated light that has propagated from the optical fiber core to the outer periphery can be leaked.

[0018] The magnetic field measuring device using the above magnetic field sensor may measure the magnetic field from the change in the intensity of the emitted light emitted from the other end by irradiating light of a single wavelength from one end of the optical fiber. Further, the magnetic field measuring device using the above magnetic field sensor may measure the magnetic field from the change in the intensity of light of a specific wavelength in the emitted light emitted from the other end or the change in the spectrum of the emitted light by irradiating light having a broad spectrum such as obtained from a white light source using a halogen light source from one end of the optical fiber.

[0019] The material used for the thin film is a material in which surface plasmon resonance (hereinafter, also referred to as SPR or SPR phenomenon) or localized surface plasmon resonance (hereinafter, also simply referred to as LSPR or LSPR phenomenon) excited by evanescent light generated on the cladding surface of the optical fiber occurs. Further, the material used for the thin film is a material that brings about loss mode resonance (hereinafter, also referred to as LMR or LMR phenomenon).

[0020] For thin films exhibiting SPR and LSPR phenomena, metals, specifically precious metals such as gold and silver, and copper can be used as materials. Thin films exhibiting SPR phenomena are thin films consisting solely of elemental metals or alloys, or those primarily composed of these materials. Thin films exhibiting LSPR phenomena are thin films containing particles of elemental metals or alloys. Furthermore, for thin films exhibiting LMR phenomena, semiconductor films, specifically metal oxide semiconductor films such as ITO, IZO, IGZO, ZnO, SnO2, CuO, and TiO2, can be used as materials. [Examples]

[0021] A magnetic field sensor 1 according to Embodiment 1 of the present invention will be described with reference to the drawings. Note that the drawings are stylized to clarify the magnetic field sensor 1 and its components, and do not represent the actual proportions.

[0022] Figure 1 shows a cross-sectional view of the magnetic field sensor 1 cut along the length of the optical fiber. As shown in Figure 1, the magnetic field sensor 1 consists of an optical fiber 4 having a heterocore portion 3 of a predetermined length sandwiched between optical transmission sections 2, 2. The optical transmission section 2 is a multimode optical fiber consisting of a core 5a as a first core or a third core, and a cladding 6a as a first cladding or a third cladding that covers the outer surface of the core 5a, i.e., the outer surface of the core 5a.

[0023] The heterocore section 3 is a single-mode optical fiber consisting of a second core 5b, which is a core with a smaller diameter than core 5a, and a second cladding 6b, which is a cladding that covers the outer surface of core 5b. The heterocore section 3 is an optical leakage section that leaks a portion of the light transmitted from the transmission section 2.

[0024] In this embodiment, a multimode fiber (MMF) (FutureGuide-MM50, manufactured by Fujikura Ltd.), which is a communication optical fiber with a core 5a having a diameter of 50 μm and a cladding 6a having a diameter of 125 μm, was used as the optical transmission unit 2.

[0025] Furthermore, a single-mode fiber (SMF) (manufactured by F-SA Newport, USA) with a core 5b diameter of 3 μm and a cladding 6b diameter of 125 μm was used as the heterocore section 3. The length of the heterocore section 3 was set to 15 mm. The two optical transmission sections 2 and the heterocore section 3 are joined to each other by fusion splicing. The length of the heterocore section 3 can be, for example, several millimeters to several tens of millimeters.

[0026] Furthermore, by joining the optical fiber constituting the optical transmission section 2 and the optical fiber constituting the heterocore section 3, which has a core diameter smaller than that of the optical transmission section 2, by fusion splicing, it is possible to leak more light in the heterocore section 3 than in other parts without reducing the overall diameter of the optical fiber including the cladding. In other words, it is possible to leak more light in the heterocore section 3 than in other parts without reducing the mechanical strength of the sensing section 10.

[0027] The coating film 7, as a film body, is a metal film formed on the outer surface of the cladding 6b of the heterocore portion 3. In this embodiment, the coating film 7 is a 50 nm thin film of gold (Au). The coating film 7 can be formed on the surface of the cladding 6b, for example, by vacuum deposition or sputtering. Naturally, the thickness of the coating film 7 is not limited to 50 nm.

[0028] In the magnetic field sensor 1, when light is incident from the end of the optical transmission section 2, the core diameter of the heterocore section 3 becomes smaller than that of the optical transmission section 2, causing light to leak from the core 5b to the cladding 6b and propagate through the cladding 6b. As a result, evanescent light is generated that seeps out from the outer surface of the cladding 6b. This evanescent light excites surface plasmon resonance in the coating film 7.

[0029] Since the magnetic field sensor of this embodiment measures magnetic fields using plasmon resonance, it is preferable that the material used for the thin film as the coating film 7 is a material that generates surface plasmon resonance (hereinafter also referred to as SPR or SPR phenomenon) or localized surface plasmon resonance (hereinafter simply referred to as LSPR or LSPR phenomenon) due to evanescent light generated on the cladding surface of the optical fiber.

[0030] As the coating film 7 of the magnetic field sensor 1 in this embodiment, a metal can be used, specifically a material that causes SPR or LSPR phenomena when exposed to evanescent light, such as precious metals like gold and silver as described above, or copper. The thin film may be a thin film that causes SPR phenomena using the above material as a base material, or a thin film that causes LSPR phenomena in which the above material is contained as particles in another base material.

[0031] In this embodiment, gold was used as the coating film 7 because gold is a metal with excellent corrosion resistance, which is desirable for the durability of the sensor. Furthermore, although silver, mentioned above as a candidate for the coating film material, is susceptible to corrosion by sulfur, its SPR spectrum is a smooth curve, and effective sensor sensitivity for high refractive indices can be expected, so using silver is also preferable for the magnetic field sensor of the present invention.

[0032] The sealing body 8 has a cylindrical tube 8a that houses the heterocore portion 3 inside, and lids 8b that close the openings at both ends of the tube. In other words, the heterocore portion 3, which is the second optical fiber, is arranged in the hollow portion of the tube 8a, which is a hollow cylindrical body.

[0033] Since the inner surface of the cylindrical body 8a is separated from the outer surface of the cladding 6b and the surface of the coating film 7, a space is formed inside the sealant 8. In this embodiment, a glass tubing was used for the cylindrical body 8a. A resin adhesive was used for the lid 8b. Specifically, in this embodiment, the lid 8b was formed by applying Cemedyne, an epoxy adhesive, to the openings at both ends of the glass tubing and then curing it.

[0034] The magnetic fluid 9 is filled inside the sealant 8 and is arranged to cover the coating film formed on the cladding 6b of the heterocore portion 3. In other words, the magnetic fluid 9 is filled in the hollow portion of the cylindrical body 8a. The magnetic fluid 9 is a fluid in which magnetic nanoparticles are suspended within a fluid carrier such as water or oil.

[0035] In this embodiment, EMG605 (manufactured by Ferrotec Material Technologies Co., Ltd.), in which water is used as the fluid carrier and Fe3O4 is used as the magnetic nanofluid 9, was used. The sealant 8 and the magnetic fluid 9 form a sensitive part 10 that is sensitive to changes in the magnetic field.

[0036] The reason the thickness of the gold thin film of the coating film 7 is set to 50 nm is to increase the fluctuation in refractive index due to changes in the magnetic field and thereby improve the sensitivity of the magnetic field sensor 1. Here, the refractive index of the magnetic fluid 9 is RIU 1.39 when no magnetic field is applied, and it increases as the magnetic field increases.

[0037] In contrast, when a 50 nm gold thin film is used as the coating film 7, the change in the amount of light absorbed by the gold thin film in response to the change in the refractive index of the magnetic fluid 8 becomes large when the refractive index of the material in contact with the surface of the coating film 7 exceeds RIU 1.37.

[0038] Therefore, the region of RIU 1.39 or higher, which is the refractive index change range for MG605, overlaps with the region of RIU 1.37 or higher, which is the refractive index range where the 50 nm gold thin film is highly sensitive. This is because, when the peritoneum 7 is a 50 nm gold thin film, if the refractive index of an object in contact with the surface of the coating film 7 is RIU 1.37 or higher, the resonance wavelength of the surface plasmon resonance appears around 650 nm, and the increase in refractive index leads to a redshift of the resonance wavelength.

[0039] As described above, it is preferable that the thickness of the coating film 7 of the magnetic field sensor 1 has a thickness corresponding to the refractive index change range of the magnetic fluid 8 used, such that the change in the amount of light absorbed in the coating film 7 is large in response to the change in the refractive index of the magnetic fluid 8.

[0040] In other words, in this embodiment of the magnetic field sensor 1, the coating film 7 is made of a 50 nm thin gold film with the aim of increasing the sensor sensitivity of the magnetic field sensor 1 in light of the refractive index characteristics of EMG605.

[0041] Figure 2 shows the configuration of the magnetic field measuring device 20 using the magnetic field sensor 1 described above.

[0042] The light source 21 is a light source device that uses LED elements, LD elements, halogen light sources, etc. The light source 21 is coupled to one end of the optical fiber 4 of the magnetic field sensor 1 and is configured to allow light to be incident into the optical fiber 4 from that end.

[0043] The detection device 22, acting as a light-receiving unit, is a device that has the configuration of a so-called optical spectrum analyzer, which includes a spectroscopic element that spectrally analyzes the light emitted from the other end of the optical fiber 4, and a light-receiving element that receives the light spectrally analyzed by the spectroscopic element. The detection device 22 outputs the measurement result of the spectrum of the received light.

[0044] The analysis device 23, acting as the detection unit, is a device that receives the measurement results of the optical spectrum output from the detection device 22 and records and displays them. The analysis device 23 is, for example, a personal computer with data logger software installed. The analysis device 23 may also be configured to calculate the magnetic field strength by, for example, having an application installed that can calculate the magnetic field strength from the above measurement results. Of course, the analysis device 23 may be a general-purpose computer or a computer specialized for a specific application.

[0045] To summarize the above, the magnetic field measuring device 20 includes a light source that is optically coupled to the optical transmission unit 2 and causes light to be incident on the optical transmission unit 2, a detection device 22 that receives light from the transmission unit 2 which is optically coupled to the heterocore unit 3, and an analysis device that detects the magnetic field based on the received signal generated by the light received by the detection device 22.

[0046] As described above, in the magnetic field sensor 1, light propagating through the core 5a of the optical transmission section 2 leaks into the cladding 6b at the core 5b of the heterocore section 3 and reaches the interface between the coating film 7 and the cladding 6b, generating evanescent light (near-field light) that seeps out from this interface towards the coating film 7.

[0047] In the magnetic fluid 9 placed on the coating film 7, the arrangement of magnetic nanoparticles within the magnetic fluid 9 changes depending on the strength of the magnetic field to which the magnetic fluid 9 is exposed, and the refractive index of the magnetic fluid 9 changes according to the strength of the magnetic field. Specifically, the degree of alignment of the magnetic nanoparticles in the magnetic fluid 9 increases as the magnetic field becomes stronger, and the refractive index increases. Along with the change in the refractive index of the magnetic fluid 9, the resonance wavelength at which the evanescent light excites surface plasmon resonance in the coating film 7 changes.

[0048] As a result, the light intensity of the propagating light emitted from the light source 21, propagating through the optical transmission unit 2, and reaching the detection device 22 changes. By detecting this change in light intensity with the detection device 22 and analyzing it with the analysis device 23, it is possible to detect changes in the magnetic field or measure the strength of the magnetic field.

[0049] Next, a method for manufacturing the magnetic field sensor 1 according to an embodiment of the present invention will be described. In the following description, the case in which a thin film of gold is used as the coating film 7 will be described as an example.

[0050] First, the optical fiber 4 is prepared. Next, a coating film made of a thin gold film is formed on the surface of the cladding 6b of the heterocore portion 3 of the optical fiber 4. Then, the optical fiber 4 with the coating film 7 is passed through a glass tube, and magnetic fluid is filled into the glass tube to form magnetic fluid 9. This completes the magnetic field sensor 1.

[0051] [Manufacturing method] As an example, a magnetic field sensor 1 was fabricated. Specifically, similar to the example described above, a multimode fiber (MMF) (FutureGuide-MM50, manufactured by Fujikura Ltd.), which is a communication optical fiber with a core 5a having a diameter of 50 μm and a cladding 6a having a diameter of 125 μm, was used as the optical transmission section 2.

[0052] As the heterocore section 3, a single-mode fiber (SMF) (manufactured by F-SA Newport) was used, with a core 5b having a diameter of 3 μm and a cladding 6b having a diameter of 125 μm. The length of the heterocore section 3 was set to 15 mm. The two optical transmission sections 2 and the heterocore section 3 were joined by fusing the two optical transmission sections 2 to each end of the heterocore section 3.

[0053] Next, a 50 nm thin gold film was deposited on the portion of the optical fiber 4 including the heterocore 3 by sputtering, so that the entire circumference of the cladding was covered.

[0054] Next, the optical fiber 4 is passed through the cylindrical body 8a made of glass tubing, and the cylindrical body 8a is positioned so as to cover the portion including the heterocore 3. Then, the inside of the cylindrical body 8a is filled with magnetic fluid 9, and the openings at both ends of the cylindrical body 8a are sealed and closed to form a sensing part 10 consisting of the sealing body 8 and the magnetic fluid 9, and the magnetic field sensor 1 is completed.

[0055] In forming the sensing part 10, specifically, magnetic fluid is filled into the cylindrical body 8a, and the optical fiber 4 is inserted into the cylindrical body 8a until the heterocore portion 3 is contained within the cylindrical body 8a. After the heterocore portion 3 is contained within the cylindrical body 8a and immersed in the magnetic fluid, the epoxy adhesive (for example, Cemedyne) described above is applied to seal the openings at both ends of the cylindrical body 8a and allowed to harden to form the lid 8b. Note that the material used to form the lid 8b is not limited to epoxy adhesive, but may be other resin materials, such as silicone adhesive.

[0056] [Measurement experiment] (Experimental method) Using the magnetic field sensor 1 according to the embodiment described above, a magnetic field measuring device 20 was constructed and a measurement experiment was conducted. For the light source 20, a white light source device AQ4305 (manufactured by Yokogawa Electric Corporation), consisting of a halogen light source emitting light with a wavelength of 400-1600 nm, was used, and for the detection device 22, a CCS200 / M miniature spectrometer (manufactured by Thorlabs) was used. In the measurement experiment, the magnetic field was generated so that the extension direction of the optical fiber of the sensing part 10 and the direction of magnetic field application were perpendicular, that is, perpendicular to the magnetic field lines of the generated magnetic field.

[0057] In this measurement experiment, the magnetic field magnitude was varied to 0 mT, 4.12 mT, and 8.21 mT. The magnitude of the applied magnetic field was confirmed by measuring the magnetic field near the sensor using a Tesla meter PA-400 (manufactured by Tesla Corporation) as a reference sensor.

[0058] In this measurement experiment, the magnetic field magnitude was gradually increased in the order of 0 mT, 4.12 mT, and 8.21 mT, and this process was repeated three times. The propagation light spectrum measured by the detection device 22 of the magnetic field measuring device 20 was then measured. The spectrum was measured 30 seconds after the magnetic field reached each of the above values.

[0059] (Experimental results) Figure 3 is a spectral graph showing the average propagation light spectrum at each magnetic field strength when the magnetic field strength is changed stepwise from 0mT, 4mT, 8mT, and 12mT, as described above, and this is repeated three times. Note that in the spectral graph of Figure 3, the propagation light spectrum is normalized based on the case where the optical sensor 1, with the sealant 8 and magnetic fluid 9 removed, i.e., with only the coating film 7 formed on the heterocore portion 3, is placed in air.

[0060] As shown in Figure 3, the propagation light spectrum reveals a significant decrease in light intensity around 720 nm. This is thought to be due to the excitation of surface plasmon resonance in the Au thin film, resulting in the absorption of evanescent light corresponding to the refractive index of the magnetic fluid. Furthermore, it was confirmed that the light intensity increased with increasing magnetic field at wavelengths longer than 720 nm, where the decrease in light intensity peaks.

[0061] Here, the inventors attempted to measure the strength or change in the magnetic field from the change in optical loss of 850 nm near-infrared light, which is a common light source for optical fibers.

[0062] Figure 4 is a graph showing the change in optical loss of 850 nm light at magnetic field strengths of 0 mT, 4 mT, 8 mT, and 12 mT, as described above. Each value is the average of the measured values ​​obtained from the three measurements described above. Table 1 shows the optical loss value and standard deviation for each magnetic field strength. The dashed line in the graph is a linear approximation line using least squares approximation.

[0063] [Table 1]

[0064] As shown in Figure 4 and Table 1, the amount of optical loss decreased as the magnetic field increased. Furthermore, from the linear approximation line in Figure 4 described above, it was found that the sensitivity was 0.055 dB / mT. From these findings, it was confirmed that the magnetic field sensor of the present invention is responsive to changes in the magnetic field and has a sensitivity of 0.055 dB / mT at least in magnetic field strengths between 0 and 12 mT.

[0065] As mentioned above, since the magnetic field strength can be measured from the amount of light loss at a specific wavelength (850 nm in the above example), the light source 21 does not have to be a white light source like the one used in the magnetic field measuring device 20, but can also be one that emits light at a specific wavelength. As mentioned above, for example, the light source 21 may be an LED or LD element that emits only near-infrared light such as 850 nm.

[0066] The standard deviations shown in Table 1 are 1.62 × 10⁻¹⁰ for magnetic field strengths of 0 mT, 4 mT, 8 mT, and 12 mT, respectively. -2 , 2.12 × 10 -2 , 1.33 × 10 -2 , 2.26 × 10 -2 The result was a full-scale error of 5.89%. It should be noted that this error is thought to be caused by the fact that it takes time for the arrangement of magnetic nanoparticles in the magnetic fluid to stabilize due to the application of a magnetic field, and the applied magnetic field was changed before it had stabilized, resulting in the error. [Examples]

[0067] A magnetic field sensor 101 according to a second embodiment of the present invention will be described with reference to the drawings. Note that the drawings are simplified to clarify the magnetic field sensor 2 and its components, and do not represent actual proportions. Furthermore, configurations similar to those in the first embodiment will be described using the same notations.

[0068] Figure 5 shows a cross-sectional view of the magnetic field sensor 101 according to the second embodiment, cut along the length of the optical fiber. As shown in Figure 5, the magnetic field sensor 101 includes an optical fiber 4 having a heterocore portion 3 of a predetermined length joined to the end of the optical transmission section 2. The optical transmission section 2 is a multimode optical fiber consisting of a core 5a as a first core and a cladding 6a as a first cladding covering the outer surface of the core 5a.

[0069] The heterocore section 3 is a single-mode optical fiber consisting of a core 5b, which is a second core with a smaller diameter than the core 5a, and a cladding 6b, which is a second cladding covering the outer surface of the core 5b. In Embodiment 2, the heterocore section 3 is fused to one end of the optical transmission section 2, and the optical fiber 4 is terminated at the heterocore section 3.

[0070] In this embodiment, as in Embodiment 1 above, a multimode fiber (MMF) (FutureGuide-MM50, manufactured by Fujikura Ltd.), which is a communication optical fiber with a core 5a having a diameter of 50 μm and a cladding 6a having a diameter of 125 μm, was used as the optical transmission unit 2.

[0071] Furthermore, a single-mode fiber (SMF) (manufactured by F-SA Newport, USA) with a core 5b diameter of 3 μm and a cladding 6b diameter of 125 μm was used as the heterocore section 3. The length of the heterocore section 3 was set to 15 mm. The optical transmission section 2 and the heterocore section 3 are joined by fusion splicing. Note that the length of the heterocore section 3 can be, for example, several millimeters to several tens of millimeters.

[0072] The coating film 7 is a metal film that covers the outer circumferential surface of the cladding 6b of the heterocore portion 3, and the end face 3a opposite to the end face joined to the optical transmission portion 2 of the heterocore portion 3. In this embodiment, the coating film 7 is a 50 nm thin film of gold (Au). The coating film 7 can be formed, for example, on the surface of the cladding 6b by vacuum deposition or sputtering.

[0073] In the magnetic field sensor 101 of this embodiment, as with the magnetic field sensor 1 of Embodiment 1, when light is incident from the end of the optical transmission section 2, the core diameter of the heterocore section 3 becomes smaller than that of the optical transmission section 2, causing light to leak from the core 5b to the cladding 6b and propagate through the cladding 6b. As a result, evanescent light seeps out from the outer surface of the cladding 6b. This evanescent light excites surface plasmon resonance in the coating film 7.

[0074] The material of the coating film 7 of the magnetic field sensor 101 in this embodiment is preferably the same material as the magnetic field sensor 1 in Embodiment 1, that is, a material that causes surface plasmon resonance (hereinafter also referred to as SPR or SPR phenomenon) or localized surface plasmon resonance (hereinafter simply referred to as LSPR or LSPR phenomenon) due to evanescent light generated on the cladding surface of the optical fiber.

[0075] The sealant 8 has a cylindrical body 8a that houses the heterocore portion 3 and lids 8b that close the openings at both ends of the cylindrical body. Since the inner surface of the cylindrical body 8a of the sealant 8 is separated from the outer surface of the cladding 6b and the surface of the coating film 7, a space is formed inside the sealant 8. In this embodiment, a glass tubing was used for the cylindrical body 8a. A resin adhesive was used for the lids 8b. In this embodiment, the optical fiber 4 does not pass through the sealant 8 but is terminated inside the sealant.

[0076] The magnetic fluid 9 is filled inside the sealant 8 and is a magnetic fluid in which magnetic nanoparticles are suspended within a fluid carrier such as water or oil. In this embodiment, the same material as in Example 1 is used as the magnetic fluid 9. The sealant 8 and the magnetic fluid 9 form a sensitive part 10 that is sensitive to changes in the magnetic field.

[0077] As described above, in the magnetic field sensor 101 of this embodiment, the fiber 4 is terminated at the heterocore portion 3, and the magnetic fluid 9 is configured to cover the outer circumferential surface and end surface of the heterocore portion 3 at the termination portion of the fiber 4.

[0078] Figure 6 shows a magnetic field measuring device 40 using the magnetic field sensor 101 described above. As shown in the figure, the magnetic field measuring device 40 has a main optical fiber 41 and individual optical fibers 42 branched from the main optical fiber 41. In the magnetic field measuring device 40, the magnetic field sensor 101 is joined to the tip of an individual optical fiber 42.

[0079] The main optical fiber 41 and the individual optical fibers 42 are made of multimode optical fibers similar to the optical transmission section 2 of Example 1. The individual optical fiber 42 may also serve as the transmission section 2. That is, the heterocore section 3 may be directly fused to the end of the individual optical fiber 42.

[0080] The reflected / backscattered light intensity distribution measuring device 43 (hereinafter also simply referred to as the scattering intensity distribution measuring device) is, for example, an optical time-domain reflectometer (OTDR) connected to the main optical fiber 41. It emits optical pulses towards multiple magnetic field sensors 101 via the main optical fiber 41 and individual optical fibers 42, and measures the position and intensity of the backscattered light at each magnetic field sensor 101.

[0081] The analysis device 23 is a device that receives the measurement results of the position and intensity of backscattered light output from the scattering intensity distribution measuring device 43, and records and displays them. The analysis device 23 is, for example, a personal computer with data logger software installed.

[0082] Furthermore, the analysis device 23 may be configured to calculate the magnetic field strength by, for example, having an application installed that can calculate the magnetic field strength from the above measurement results. The magnetic field measuring device 40 measures the magnetic field strength at the location where each magnetic field sensor is placed based on the position and intensity of the backscattered light and outputs the measurement result. Of course, the analysis device 23 may be a general-purpose computer or a computer specialized for a specific application.

[0083] Examples 1 and 2 primarily described the case where a film that induces surface plasmon resonance is used as the coating film 7. However, as mentioned above, a film containing metal nanoparticles that induce localized surface plasmon resonance may be used as the coating film 7. Furthermore, as mentioned above, oxide semiconductor films such as ITO, IZO, IGZO, ZnO, or other semiconductor films that induce loss mode resonance may be used as the coating film 7.

[0084] In these cases as well, it is possible to calculate the magnetic field strength from the change in optical loss, the change in the intensity of propagated light, and the change in the spectrum, which correspond to the change in the refractive index of the magnetic fluid 8 in the sensing part 10 due to the change in the strength of the magnetic field.

[0085] In the above-described embodiment, the case in which a heterocore portion 3 with a smaller core diameter than other portions is formed in the sensitive portion 10 of the fiber 4 was mainly explained. However, in order to function as a magnetic field sensor, it is sufficient that light leaks out of the fiber in the sensitive portion 10, and the sensitive portion 10 of the fiber 4 does not necessarily have to be a heterocore portion 3. In other words, the fiber 4 may have a uniform core diameter including the sensitive portion 10 without having a heterocore portion 3.

[0086] For example, in a fiber with a uniform core diameter, the portion corresponding to the sensitive part 10 of the present invention may be an unclad fiber without cladding, or a fiber in which the cladding is thinner than that of other parts.

[0087] Even in this case, since a lot of light leaks out of the core to the fiber in the sensing part 10, it is possible to measure the magnetic field or detect changes in the magnetic field by observing the light loss which changes due to the change in the refractive index of the magnetic fluid 8 caused by the change in the magnetic field.

[0088] Furthermore, as mentioned above, if a portion is created by eliminating or thinning the cladding, the diameter of the fiber itself in that portion becomes smaller, resulting in a decrease in mechanical strength. In contrast to such a configuration, the structure of the present invention, which includes the heterocore portion 3, reduces only the core diameter in the sensing portion 10, eliminating the need to reduce the fiber diameter including the cladding. This makes it possible to maintain mechanical strength while more actively leaking light in the sensing portion 10 than in other portions.

[0089] Furthermore, the core diameter may be larger in the sensing portion 10 of the heterocore fiber 3 than in other portions. Even in this case, since a lot of light leaks out of the core to the fiber in the sensing portion 10, it is possible to measure the magnetic field or detect changes in the magnetic field by observing the optical loss that changes due to the change in refractive index of the magnetic fluid 8 caused by the change in the magnetic field.

[0090] Furthermore, although a coating film 7 is formed in the above embodiment, the magnetic fluid 8 can also function as a magnetic field sensor in a configuration where the magnetic fluid 8 is in direct contact with the surface of the fiber in the heterocore portion 3 without forming a coating film 7. In this case as well, the change in the refractive index of the magnetic fluid 8 changes the light leaking outward from the heterocore portion 3, and thus changes the loss of transmitted light.

[0091] The various configurations and dimensions in the above-described embodiments are merely examples and can be modified as appropriate depending on the application. [Explanation of Symbols]

[0092] 1,101…Magnetic field sensor, 2…Optical transmission section, 3…Heterocore section, 4…Optical fiber, 5a,5b…Core, 6a,6b…Cladding, 7…Coating film, 8…Sealing body, 9…Magnetic fluid, 10…Sensing section, Magnetic field measuring device…20,40, 21…Light source, 22…Detection device, 23…Analysis device, 41…Main optical fiber, 42…Individual optical fiber, 43…Intensity distribution measuring device / analysis device

Claims

1. A transmission section comprising an optical fiber having a core and a cladding covering the outer surface of the core, An optical leakage unit connected to the optical fiber, which leaks a portion of the light transmitted from the optical fiber, A film body formed on the outer circumferential surface of the light leakage portion, comprising a metal film in which surface plasmon resonance occurs upon excitation by evanescent waves, a film containing metal nanoparticles in which localized surface plasmon resonance occurs upon excitation by evanescent waves, or a semiconductor film that induces loss mode resonance, A magnetic fluid is arranged to cover the aforementioned membrane, A magnetic field sensor having

2. A magnetic field sensor according to claim 1, It has a hollow cylindrical body, A magnetic field sensor in which the light leakage portion is disposed in the hollow portion of the hollow cylindrical body, and the magnetic fluid is filled in the hollow portion of the cylindrical body.

3. A magnetic field sensor according to claim 1, wherein the metal film forming the film body is a film containing a noble metal, and the semiconductor film forming the film body is a metal oxide semiconductor film.

4. A magnetic field sensor according to claim 1, wherein the optical leakage portion comprises an optical fiber having a core smaller in diameter than the core of the optical fiber of the transmission portion and a cladding covering the outer surface of the core.

5. A magnetic field sensor according to claim 4, A magnetic field sensor in which the optical fiber in the transmission section is a multimode fiber and the optical fiber in the leakage section is a single-mode fiber.

6. A magnetic field sensor according to claim 1, characterized in that the thin film has a thickness corresponding to the refractive index change range of the magnetic fluid.

7. The magnetic field sensor according to claim 1, A light source that is optically coupled to the optical fiber of the transmission unit and causes light to be incident on the optical fiber of the transmission unit, A light receiving unit that receives light transmitted through the aforementioned light leakage unit, A detection unit that detects a magnetic field based on a light-receiving signal generated by the light-receiving unit, A magnetic field measuring device having the following features.

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