Electric current measuring device and electric current measuring method

Abstract

An electric current measuring device according to an embodiment of the present invention comprises a substantially annular magnetic core that has an air gap, an NV sensor element that includes a diamond substrate having an NV center and that is disposed in the air gap such that a main surface of the diamond substrate is perpendicular to a gap thickness direction, a measured winding that is wound around a magnetic core, a reference winding that is wound around the magnetic core, a microwave application unit that applies microwaves to the diamond substrate, an exciting light emitting unit that emits exciting light toward the diamond substrate, a fluorescence detecting unit that detects the intensity of fluorescence of the diamond substrate, and a computing unit that calculates the electric current value of a current being measured from the intensity of the fluorescence detected by the fluorescence detecting unit, the frequency of the microwaves, and the electric current value of a detected current, wherein the main surface of the diamond substrate coincides with the (111) plane of a diamond crystal, and the axis of the NV center coincides with the normal direction of the diamond substrate.

Description

Current measuring device and current measuring method

[0001] The present invention relates to a current measuring device and a current measuring method.

[0002] When a carbon atom in a diamond crystal is replaced with a nitrogen atom, a vacancy that occurs adjacent to this nitrogen atom is called an NV center, and diamond elements having such NV centers are known. NV centers emit red fluorescence when irradiated with green excitation light. When microwaves are irradiated from the outside, the fluorescence intensity of the NV center changes with the frequency of the microwave, and the fluorescence intensity becomes minimum at a specific frequency. When a magnetic field is applied from the outside, the point where this fluorescence intensity becomes minimum splits into two with respect to the microwave frequency, and the width of this split is proportional to the magnetic flux density of the external magnetic field, so diamond elements having NV centers can be used to construct magnetic sensors.

[0003] Also proposed is a current measuring device that uses diamond element with NV center.Specifically, proposed is a current measuring device that is wound around a C-shaped magnetic core with air gap, and is provided with a coil that is applied with the current to be measured and a coil that is applied with a reference current that can set current value, and that places diamond element in the air gap.In this current measuring device, based on the fluorescence measurement of diamond element, search for the condition that the magnetic flux that is formed by the current to be measured and the magnetic flux that is formed by the reference current cancel each other out, and estimate the current value of the current to be measured from the current value of the reference current under this condition (for example, see Patent Document 1).

[0004] Chinese Patent Publication No. 110045310

[0005] Although the detection of magnetic fields and therefore currents by NV centers can be achieved with relatively high accuracy, the measurement accuracy decreases due to the nonlinearity of the Zeeman splitting width near zero magnetic field. Therefore, an object of the present invention is to provide a current measurement device and a current measurement method that can measure currents with high accuracy.

[0006] (1) a reference winding wound around the magnetic core and a reference current applied thereto; a microwave application unit that applies microwaves of varying frequency to the diamond substrate; an excitation light irradiation unit that irradiates the diamond substrate with excitation light; a fluorescence detection unit that detects the intensity of fluorescence from the diamond substrate; and a calculation unit that determines whether or not the magnetic flux formed by the current to be measured and the magnetic flux formed by the reference current are canceled out based on a characteristic frequency difference that is the difference between two frequencies of the microwaves at which the intensity of the fluorescence detected by the fluorescence detection unit is minimized, and calculates the current value of the current to be measured from the current value of the reference current at the time of magnetic flux cancellation;

[0007] (2) The current measuring device of (1) may further include a magnetic flux offset unit that applies an offset magnetic field to the air gap in the gap thickness direction, and the calculation unit may store in advance the characteristic frequency difference when only the offset magnetic field is applied, and may determine that the magnetic flux formed by the current to be measured and the magnetic flux formed by the reference current are canceled out when the characteristic frequency difference during measurement matches the stored characteristic frequency difference.

[0008] (3) In the current measuring device of (2), the magnetic flux offset unit may have a magnetomotive force source that generates a magnetic flux that passes through at least a portion of the magnetic core.

[0009] (4) In the current measuring device of (3), the magnetomotive force source may have an annular permanent magnet fitted into the magnetic core.

[0010] (5) In the current measuring device of (3), the magnetomotive force source may be an offset winding wound around the magnetic core and to which a current that generates the offset magnetic field is applied.

[0011] (6) In any of the current measuring devices (1) to (5), the fluorescence detection unit may have a light intensity sensor and a parabolic lens that guides the fluorescence emitted from the diamond substrate to the light intensity sensor, and the NV sensor element may be supported at the tip of the parabolic lens.

[0012] (7) In the current measuring device of (6), the fluorescence detecting unit may further include an auxiliary prism provided on a surface perpendicular to the surface of the NV sensor element supported by the parabolic lens, and which bends the fluorescence emitted from the diamond substrate toward the parabolic lens.

[0013] (8) In any of the current measuring devices of (1) to (7), the excitation light irradiating unit may be arranged on one main surface of the diamond substrate and have an excitation light prism that bends the excitation light incident from a direction parallel to the diamond substrate toward the diamond substrate, and the fluorescence detecting unit may be arranged on the other main surface of the diamond substrate and have a fluorescence prism that bends the fluorescence emitted from the diamond substrate in a direction parallel to the diamond substrate.

[0014] (9) The current measuring device according to any one of (1) to (8) may further include a tilt compensation mechanism that compensates for tilt of the diamond substrate relative to the gap thickness direction.

[0015] (10) In any one of the current measuring devices (1) to (9), the microwave application unit may have a strip-shaped conductor that faces the diamond substrate and is folded back to cover both main surfaces of the diamond substrate.

[0016] (11) In any of the current measuring devices (1) to (10), the microwave application unit may have a support disposed opposite the end face of the diamond substrate, and a transmission coil wound around the support.

[0017] (12) In the current measuring device according to any one of (1) to (11), a region of the magnetic core in the vicinity of the air gap may be formed so that the cross-sectional area decreases toward the air gap.

[0018] (13) In any of the current measuring devices (1) to (12), the excitation light irradiating unit may linearly polarize the excitation light and be arranged so that the polarization plane of the excitation light is perpendicular to the axis of the NV center when it is incident on the diamond substrate.

[0019] (14) A current measurement method according to one aspect of the present invention includes an NV sensor element having a substantially annular magnetic core having an air gap and a diamond substrate having an NV center, the NV sensor element having the diamond substrate arranged in the air gap so as to be perpendicular to the gap thickness direction, a winding to be measured wound around the magnetic core and to which a current to be measured is applied, a reference winding wound around the magnetic core and to which a reference current is applied, a microwave application unit that applies microwaves of varying frequency to the diamond substrate, an excitation light irradiation unit that irradiates excitation light onto the diamond substrate, and and a fluorescence detection unit that detects the intensity of fluorescence from the diamond substrate, wherein the main surface of the diamond substrate coincides with the (111) plane of the diamond crystal and the axis of the NV center coincides with the normal direction of the diamond substrate. Using a current measuring device, the offset between the magnetic flux formed by the current to be measured and the magnetic flux formed by the reference current is determined based on a characteristic frequency difference, which is the difference between the two frequencies of the microwaves at which the intensity of the fluorescence detected by the fluorescence detection unit is minimized, and the current value of the current to be measured is calculated from the current value of the reference current at the time of magnetic flux offset.

[0020] According to the current measuring device and current measuring method of the present invention, current can be measured with high accuracy.

[0021] FIG. 1 is a schematic diagram showing the configuration of a current measuring device according to a first embodiment of the present invention; FIG. 2 is a graph showing the relationship between the intensity of fluorescence from the NV center and the frequency of microwaves; FIG. 3 is a graph showing the relationship between the amount of change in fluorescence intensity and the angle between the axis of the NV center and the polarization direction of excitation light; FIG. 4 is a schematic diagram showing the configuration of a current measuring device according to a second embodiment of the present invention; and FIG. 5 is a schematic diagram showing the configuration of a current measuring device according to a third embodiment of the present invention.

[0022] Hereinafter, embodiments of the present invention will be described with reference to the drawings. Note that the dimensions of various components in the drawings have been adjusted for clarity and convenience. Furthermore, in the embodiments described later, components similar to those in the previously described embodiments will be designated by the same reference numerals, and redundant explanations may be omitted.

[0023] 1 is a schematic diagram showing the configuration of a current measuring device 1 according to a first embodiment of the present invention. The current measuring device 1 includes a magnetic core 10, an NV sensor element 20, a winding under test 30, a reference winding 40, a magnetic flux offset unit 50, a microwave application unit 60, an excitation light irradiation unit 70, a fluorescence detection unit 80, a calculation unit 90, and a tilt compensation mechanism 100.

[0024] The magnetic core 10 is formed in a generally annular shape having an air gap 11, i.e., in a generally unclosed C-shape. The generally annular overall shape of the magnetic core 10 is not limited to a circular shape as shown in the figure, and may be a polygonal shape or the like. The magnetic core 10 is formed from a soft magnetic material with high magnetic permeability and low coercive force, and by passing a current through the winding under test 30 and the reference winding 40, a magnetic flux is generated that crosses the air gap 11 in the gap thickness direction (the direction in which the end faces of the magnetic core 10 face each other).

[0025] The NV sensor element 20 includes a diamond substrate 21 having an NV center. The NV sensor element 20 may include a transparent substrate 22 supporting the diamond substrate 21. That is, the NV sensor element 20 may include a diamond substrate 21 formed of a thin layer of diamond crystal on the surface of the transparent substrate 22. The NV sensor element 20 is disposed within the air gap 11 so that the principal surface of the diamond substrate 21 is perpendicular to the gap thickness direction. The principal surface of the diamond substrate 21 coincides with the (111) plane of the diamond crystal. This facilitates arranging the (111) plane of the diamond crystal of the diamond substrate 21 perpendicular to the magnetic flux generated by the measured winding 30 and the reference winding 40. Furthermore, the axis of the NV center of the diamond substrate 21 coincides with the normal direction of the diamond substrate 21. This allows the magnetic flux generated by the measured winding 30 and the reference winding 40 to be applied in the axial direction of the NV center of the diamond substrate 21, thereby improving the magnetic sensitivity of the NV center. Furthermore, in crystal growth by CVD (chemical vapor deposition), which is a typical diamond film formation method, the film surface of the diamond crystal is parallel to the (111) plane, and the NV center is precisely oriented in the growth direction of the diamond crystal, that is, in the direction perpendicular to the (111) plane of the diamond crystal, so that the magnetic sensitivity of the NV center can be further improved while suppressing the individual differences of the diamond substrate 21 and, in turn, the NV sensor element 20. In this specification, "coincidence" means that the angle between them is 3° or less, "parallel" means that the angle between them is 20° or less, preferably 10° or less, more preferably 3° or less, and "perpendicular" means that the angle between them is 70° or more and 110° or less, preferably 80° or more and 100° or less, more preferably 87° or more and 93° or less.

[0026] The winding 30 to be measured is a coil wound around the magnetic core 10 and to which a current to be measured, the current value of which is to be measured, is applied. As a result, a magnetic flux component proportional to the current to be measured is induced in the magnetic core 10.

[0027] The reference winding 40 is a coil wound around the magnetic core 10 and to which a reference current supplied from a reference power supply 41 is applied. This induces a magnetic flux component proportional to the reference current in the magnetic core 10. The reference power supply 41 is configured so that the current value of the reference current can be set arbitrarily.

[0028] The magnetic flux offset portion 50 applies an offset magnetic field in the gap thickness direction to the air gap 11 by forming a magnetic flux that passes through at least a portion of the magnetic core 10. In this embodiment, the magnetic flux offset portion 50 has a pair of annular permanent magnets (magnetomotive force sources) 51 fitted adjacent to the air gap 11 of the magnetic core 10. The magnetic flux density of the offset magnetic field formed by the magnetic flux offset portion 50 is preferably such that the influence of the nonlinearity of the Zeeman splitting width of the diamond substrate 21 is minimized, and specifically, can be set to about several mT in the diamond substrate 21.

[0029] The microwave application unit 60 applies microwaves to the diamond substrate 21. The microwave application unit 60 is configured to change the microwave frequency, typically to sweep the frequency, in order to confirm the relationship between the fluorescence intensity of the diamond substrate 21 and the microwave frequency. The microwaves output by the microwave application unit 60 preferably have a magnetic field change direction perpendicular to the axis of the NV center of the diamond substrate 21 in order to increase the change in the fluorescence intensity of the diamond substrate 21.

[0030] The microwave application unit 60 in this embodiment has a strip-shaped conductor 61 that extends parallel to the diamond substrate 21 so as to face the main surface of the diamond substrate 21, and is folded back to surround the NV sensor element 20 and cover both main surfaces of the diamond substrate 21. The conductor 61 may be, for example, a copper thin plate or a laminate of copper foil and resin. A high-frequency voltage is applied to the conductor 61 from a high-frequency power supply 62, thereby outputting microwaves.

[0031] The excitation light irradiation unit 70 irradiates the diamond substrate 21 with excitation light that causes fluorescence. The excitation light irradiation unit 70 may be configured to include a light source 71 that emits excitation light and an optical fiber 72 that guides the excitation light to the diamond substrate 21. The excitation light emitted by the light source 71 is green light. Examples of the light source 71 include a laser and an LED. The optical fiber 72 is connected to the end face of the diamond substrate 21 and directs the excitation light to the end face of the diamond substrate 21. The optical fiber 72 may also serve as a support for the NV sensor element 20. In this embodiment, the NV sensor element 20 is held within a sealing resin 73 piled on the tip of the optical fiber 72. In order to increase the rate of change in the fluorescence intensity of the NV center with respect to the microwave frequency, the excitation light irradiation unit 70 preferably linearly polarizes the excitation light and is positioned so that the polarization plane of the excitation light is perpendicular to the axis of the NV center when it enters the diamond substrate 21. The polarization plane is a plane that includes the optical axis direction of the excitation light and the vibration direction of the excitation light.

[0032] The fluorescence detection unit 80 detects the intensity of fluorescence from the diamond substrate 21 generated by irradiation with excitation light. The fluorescence detection unit 80 may be disposed independently of the excitation light irradiation unit 70, but the fluorescence detection unit 80 of this embodiment shares the optical fiber 72 with the excitation light irradiation unit 70. Specifically, the fluorescence detection unit 80 of this embodiment has a beam splitter 82 that converts the intensity of the fluorescence into an electrical signal and separates and extracts the fluorescence incident from the diamond substrate 21 into the optical fiber 72 of the excitation light irradiation unit 70 by wavelength. In this way, by sharing a part of the optical system of the excitation light irradiation unit 70 and the optical system of the fluorescence detection unit 80, interference with other components such as the microwave application unit 60 can be easily avoided.

[0033] The calculation unit 90 specifies the current value of the reference current (the set value of the reference power supply 41) to be supplied to the reference winding 40 and the frequency of the microwaves output by the microwave application unit 60 (the frequency of the high-frequency power supply 62), and calculates the current value of the current to be measured based on the relationship between these and the fluorescence intensity detected by the fluorescence detection unit 80.

[0034] It is known that there is a relationship as shown in Figure 2 between the intensity of fluorescence from the NV centers in the diamond substrate 21 and the frequency of the microwave applied to the diamond substrate 21 by the microwave application unit 60. Specifically, the fluorescence intensity of the NV centers decreases at two microwave frequencies. The characteristic frequency difference Δf, which is the difference between the frequencies at which the fluorescence intensity decreases, is proportional to the density of the magnetic flux applied to the diamond substrate 21. Therefore, the density of the magnetic flux applied to the diamond substrate 21 can be estimated from the characteristic frequency difference Δf.

[0035] 3 shows the relationship between the change in fluorescence intensity (the difference between the baseline and the negative peak in the graph of FIG. 2) and the angle between the polarization direction of the excitation light and the axial direction of the NV center when the optical axis of the excitation light is held perpendicular to the axial direction of the NV center and the polarization direction of the excitation light is rotated around the optical axis. The value of the change in fluorescence intensity in FIG. 3 is the output voltage of the light intensity sensor 81 with gain. The change in fluorescence intensity varies sinusoidally with a period of 180° relative to the tilt angle of the polarization direction of the excitation light relative to the axial direction of the NV center, reaching a maximum at an tilt angle of 90°. Therefore, as described above, by making the polarization plane of the excitation light perpendicular to the axis of the NV center, the density of the magnetic flux applied to the diamond substrate 21 can be more accurately estimated.

[0036] The calculation unit 90 determines whether the magnetic flux formed by the current under measurement and the magnetic flux formed by the reference current cancel each other out based on the characteristic frequency difference Δf, which is the difference between the two microwave frequencies at which the intensity of the fluorescence detected by the fluorescence detection unit 80 is minimized, and calculates the current value of the current under measurement from the current value of the reference current at the time of magnetic flux cancellation. That is, the calculation unit 90 changes the set value of the reference current and repeatedly measures the characteristic frequency difference Δf, and calculates the current value of the current under measurement from the set value of the reference current at the time when cancellation between the magnetic flux formed by the current under measurement and the magnetic flux formed by the reference current is determined. Specifically, the current value of the current under measurement is determined by multiplying the set value of the reference current by the ratio (turns ratio) of the number of turns of the reference winding 40 to the number of turns of the winding under measurement 30.

[0037] For this reason, the calculation unit 90 pre-stores the initial feature frequency difference Δf0 when only the offset magnetic field is applied to the diamond substrate 21. The calculation unit 90 determines that the magnetic flux formed by the current to be measured and the magnetic flux formed by the reference current are canceled out when the feature frequency difference Δf confirmed during measurement of the current to be measured matches the stored initial feature frequency difference Δf0.

[0038] The tilt compensation mechanism 100 compensates for the tilt of the normal direction of the diamond substrate 21 relative to the gap thickness direction. The tilt compensation mechanism 100 of this embodiment adjusts the position of a wedge-shaped member 101 that can abut against the NV sensor element 20, thereby elastically deforming the optical fiber 72 that supports the NV sensor element 20 and adjusting the tilt of the NV sensor element 20. Although only a single wedge-shaped member 101 is shown in the figure for simplicity, it is preferable that the tilt compensation mechanism 100 have multiple wedge-shaped members 101 and be configured to be able to tilt the NV sensor element 20 in any direction.

[0039] In the current measuring device 1 having the above configuration, the axis of the NV center of the diamond substrate 21 is perpendicular to the main surface of the diamond substrate 21, so the axis of the NV center can be positioned accurately parallel to the magnetic flux guided by the magnetic core 10. This allows the density of the magnetic flux applied to the diamond substrate 21 to be confirmed with high sensitivity, allowing the current value of the current to be measured that generates the magnetic flux to be measured relatively accurately. Furthermore, in the current measuring device 1, the magnetic flux offset unit 50 confirms the cancellation of the magnetic flux formed by the current to be measured and the magnetic flux formed by the reference current when a constant offset magnetic field is applied to the diamond substrate 21, thereby suppressing errors due to nonlinearity in the low magnetic flux range of the fluorescence measurement of the diamond substrate 21 and allowing the current value of the current to be measured relatively accurately to be measured.

[0040] A current measurement method according to one embodiment of the present invention that can be performed using current measurement device 1 determines whether the magnetic flux formed by the current under measurement and the magnetic flux formed by the reference current are canceled out based on a characteristic frequency difference Δf, which is the difference between two microwave frequencies at which the intensity of the fluorescence detected by fluorescence detection unit 80 is minimized, and calculates the current value of the current under measurement from the current value of the detected current when the magnetic flux is canceled out. This current measurement method allows the current value of the current under measurement to be measured relatively accurately.

[0041] 4 is a schematic diagram showing the configuration of a current measuring device 1A according to a second embodiment of the present invention. The current measuring device 1A includes a magnetic core 10, an NV sensor element 20, a winding under test 30, a reference winding 40, a magnetic flux offset unit 50A, a microwave application unit 60A, an excitation light irradiation unit 70A, a fluorescence detection unit 80A, and a calculation unit 90.

[0042] The magnetic flux offset unit 50A of this embodiment is configured to include an offset winding (magnetomotive force source) 52 that is wound around the magnetic core 10 and generates an offset magnetic field, and an offset power supply 53 that applies an offset current of a constant current value to the offset winding 52. In other words, the magnetic flux offset unit 50A is an electromagnet that generates a magnetic flux that passes through the magnetic core 10 over its entire length by the offset current applied to the offset winding 52, and forms an offset magnetic field that is applied to the diamond substrate 21 placed in the air gap 11.

[0043] The microwave application unit 60A of this embodiment has a support 63 arranged to face the end face of the diamond substrate 21, a transmission coil 64 wound around the support 63 and having an axis perpendicular to the end face of the diamond substrate 21, and a high-frequency power supply 62 that applies a high-frequency voltage to the transmission coil. The support 63 and the transmission coil 64 are preferably arranged in pairs to sandwich the diamond substrate 21 in order to apply microwaves evenly to the entire diamond substrate 21.

[0044] The excitation light irradiation unit 70A of this embodiment has a light source 71 that emits excitation light, an excitation light prism 74 that is arranged on one of the surfaces of the NV sensor element 20 that is parallel to the diamond substrate 21, and a primary optical fiber 75 that guides the excitation light from the light source 71 to the excitation light prism 74. The excitation light prism 74 bends the excitation light that is incident through the primary optical fiber 75 from a direction parallel to the diamond substrate 21 toward the diamond substrate 21. Therefore, the excitation light irradiation unit 70A causes excitation light to be incident on the main surface of the diamond substrate 21.

[0045] The fluorescence detection unit 80A of this embodiment has a fluorescence prism 83 arranged on the other side of the surface of the NV sensor element 20 parallel to the diamond substrate 21 (the surface opposite to the surface on which the excitation light prism 74 is arranged), a light intensity sensor 81, and a secondary optical fiber 84 connecting the fluorescence prism 83 and the light intensity sensor 81. The fluorescence prism 83 bends the fluorescence emitted from the main surface of the diamond substrate 21 in a direction parallel to the diamond substrate 21, and makes the fluorescence incident on the secondary optical fiber 84 extending parallel to the diamond substrate 21. The secondary optical fiber 84 preferably has a larger diameter than the primary optical fiber 75 in order to efficiently extract the fluorescence from the diamond substrate 21.

[0046] In the current measuring device 1A of this embodiment, by using the excitation light prism 74 and the fluorescence prism 83, the main surface of the diamond substrate 21 is arranged perpendicular to the thickness direction of the gap, while the excitation light can be guided to the diamond substrate 21 by the optical fiber and the fluorescence emitted from the diamond substrate 21 can be efficiently extracted. Therefore, the current measuring device 1A can more accurately confirm the density of the magnetic flux applied to the diamond substrate 21, and therefore can more accurately measure the current value of the current to be measured.

[0047] 5 is a schematic diagram showing the configuration of a current measuring device 1B according to a third embodiment of the present invention. The current measuring device 1B includes a magnetic core 10B, an NV sensor element 20, a winding under test 30, a reference winding 40, a magnetic flux offset unit 50A, a microwave application unit 60A, an excitation light irradiation unit 70B, a fluorescence detection unit 80B, and a calculation unit 90.

[0048] The region of magnetic core 10B near air gap 11 is formed so that its cross-sectional area decreases toward air gap 11. This facilitates the arrangement of other components such as fluorescence detection unit 80B, and also increases the density of magnetic flux applied to NV sensor element 20, improving the measurement accuracy of current measurement device 1B. As shown in the figure, magnetic core 10B is eccentric due to the reduced cross-sectional area, and the diameter of magnetic core 10B may be locally reduced. This allows a sensor head with a condenser lens to be inserted into the gap without reducing the magnetic flux density flowing through the gap.

[0049] The excitation light irradiation unit 70B of this embodiment has a light source 71 that emits excitation light, a collimator 76 that irradiates the end face of the diamond substrate 21 with the excitation light, and a primary optical fiber 75 that guides the excitation light from the light source 71 to the collimator 76. The excitation light irradiation unit 70B irradiates the excitation light using the collimator 76, which can be provided at a distance from the NV sensor element 20, thereby enabling the excitation light to be irradiated relatively evenly over a wide range of the diamond substrate 21. The collimator 76 shown in the figure is disposed inside the magnetic core 10B, but it may also be disposed so as to face another surface of the NV sensor element 20.

[0050] The fluorescence detection unit 80B of this embodiment has a light intensity sensor 81, a parabolic lens 85 that supports one surface of the NV sensor element 20 at its tip (the vertex of the paraboloid) and guides the fluorescence emitted from the diamond substrate 21 toward the light intensity sensor 81 to the light intensity sensor 81, and an auxiliary prism 86 that is provided on a surface of the NV sensor element 20 that is perpendicular to the surface supported by the parabolic lens 85 and bends the fluorescence emitted from the diamond substrate 21 toward the parabolic lens 85.

[0051] The parabolic lens 85 reflects the fluorescence emitted from the NV sensor element 20 in a direction inclined relative to the light intensity sensor 81 and causes it to enter the light intensity sensor 81, thereby improving the detection sensitivity of the light intensity sensor 81 and ultimately the measurement accuracy of the current measuring device 1B. The auxiliary prism 86 bends the fluorescence emitted from the NV sensor element 20 in a direction that differs by 90° from the light intensity sensor 81 and causes it to enter the parabolic lens 85, thereby further improving the measurement accuracy of the current measuring device 1B. The auxiliary prism 86 may be selectively disposed on a surface that does not interfere with the excitation light irradiator 70B, etc. and has a sufficient area.

[0052] In the current measuring device 1B of this embodiment, a fluorescence detection unit 80B having a parabolic lens 85 and an auxiliary prism 86 is adopted, so that the fluorescence from the diamond substrate 21 can be efficiently guided to the light quantity sensor 81, thereby enabling more accurate measurement of the current value of the current to be measured.

[0053] Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments and various modifications and variations are possible. For example, the components of each embodiment can be interchangeable. Furthermore, the magnetic flux offset unit may be omitted from the current measuring device according to the present invention. When no offset magnetic field is applied, it is sufficient to determine that the magnetic flux generated by the current to be measured and the magnetic flux generated by the reference current are canceled out when the characteristic frequency difference becomes zero, so the calculation unit does not need to store the initial characteristic frequency difference. Furthermore, the microwave application unit of the current measuring device according to the present invention can be modified. Specifically, the fluorescence detection unit and microwave application unit can be integrated by combining a transparent electrode such as ITO (indium tin oxide) on the surface of a parabolic lens for extracting fluorescence.

[0054] REFERENCE SIGNS LIST 1, 1A, 1B Current measuring device 10, 10B Magnetic core 11 Air gap 20 NV sensor element 21 Diamond substrate 22 Transparent substrate 30 Winding to be measured 40 Reference winding 41 Reference power supply 50, 50A Magnetic flux offset unit 51 Permanent magnet (magnetomotive force source) 52 Offset winding (magnetomotive force source) 53 Offset power supply 60, 60A Microwave application unit 61 Conductor 62 High frequency power supply 63 Support 64 Transmitting coil 70, 70A, 70B Excitation light irradiation unit 71 Light source 72 Optical fiber 73 Sealing resin 74 Excitation light prism 75 Primary optical fiber 76 Collimator 80, 80A, 80B Fluorescence detection unit 81 Light quantity sensor 82 Beam splitter 83 Fluorescence prism 84 Secondary optical fiber 85 Parabolic lens 86 Auxiliary prism 90 Calculation unit 100 Tilt compensation mechanism 101 Wedge-shaped member

Claims

1. An NV sensor element having an approximately annular magnetic core having an air gap, a diamond substrate having an NV center, and disposed in the air gap so that the diamond substrate is perpendicular to the gap thickness direction, a winding to be measured wound around the magnetic core and to which a current to be measured is applied, a reference winding to which a reference current is applied, a microwave application unit to apply microwaves of varying frequency to the diamond substrate, an excitation light irradiation unit to irradiate the diamond substrate with excitation light, a fluorescence detection unit to detect the intensity of fluorescence from the diamond substrate, and a calculation unit to determine the offset between the magnetic flux formed by the current to be measured and the magnetic flux formed by the reference current based on a characteristic frequency difference that is the difference between two frequencies of the microwave at which the intensity of the fluorescence detected by the fluorescence detection unit is minimized, and to calculate the current value of the current to be measured from the current value of the reference current at the time of magnetic flux offset, wherein the main surface of the diamond substrate coincides with the (111) plane of the diamond crystal, and the axis of the NV center coincides with the normal direction of the diamond substrate. Current measuring device.

2. A current measuring device as described in claim 1, further comprising a magnetic flux offset unit which applies an offset magnetic field to the air gap in the gap thickness direction, wherein the calculation unit pre-stores the characteristic frequency difference when only the offset magnetic field is applied, and determines that the magnetic flux formed by the measured current and the magnetic flux formed by the reference current are offset when the characteristic frequency difference during measurement matches the stored characteristic frequency difference.

3. The current measuring device according to claim 2, wherein the magnetic flux offset portion has a magnetomotive force source that generates a magnetic flux that passes through at least a portion of the magnetic core.

4. The current measuring device according to claim 3, wherein said magnetomotive force source is an annular permanent magnet fitted into said magnetic core.

5. The current measuring device according to claim 3, wherein said magnetomotive force source is an offset winding wound around said magnetic core and to which a current that generates said offset magnetic field is applied.

6. A current measuring device as described in any one of claims 1 to 5, wherein the fluorescence detection unit has a light quantity sensor and a parabolic lens that guides the fluorescence emitted from the diamond substrate to the light quantity sensor, and the NV sensor element is supported at the tip of the parabolic lens.

7. A current measuring device as described in claim 6, wherein the fluorescence detection unit is provided on a surface perpendicular to the surface supported by the parabolic lens of the NV sensor element, and further includes an auxiliary prism that bends the fluorescence emitted from the diamond substrate toward the parabolic lens.

8. A current measuring device as described in claim 1 or 2, wherein the excitation light irradiation unit is arranged on one main surface of the diamond substrate and has an excitation light prism that bends the excitation light incident from a direction parallel to the diamond substrate toward the diamond substrate, and the fluorescence detection unit is arranged on the other main surface of the diamond substrate and has a fluorescence prism that bends the fluorescence emitted from the diamond substrate in a direction parallel to the diamond substrate.

9. A current measuring device according to any one of claims 1 to 5, further comprising a tilt compensation mechanism for compensating for a tilt of said diamond substrate with respect to said gap thickness direction.

10. A current measuring device as described in any one of claims 1 to 5, wherein the microwave application section faces the diamond substrate and has a band-shaped conductor folded back so as to cover both main surfaces of the diamond substrate.

11. A current measuring device as described in any one of claims 1 to 5, wherein the microwave application unit has a support arranged to face the end face of the diamond substrate, and a transmission coil wound around the support.

12. A current measuring device according to any one of claims 1 to 5, wherein a region of the magnetic core adjacent to the air gap is formed so as to reduce in cross-sectional area toward the air gap.

13. A current measuring device as described in any one of claims 1 to 5, wherein the excitation light irradiation unit linearly polarizes the excitation light and is arranged so that the polarization plane of the excitation light is perpendicular to the axis of the NV center when it is incident on the diamond substrate.

14. An NV sensor element having an approximately annular magnetic core having an air gap, and a diamond substrate having an NV center, the diamond substrate being disposed within the air gap so as to be perpendicular to the gap thickness direction, a winding to be measured wound around the magnetic core and to which a current to be measured is applied, a reference winding to which a reference current is applied, the winding being wound around the magnetic core and to which a reference current is applied, a microwave application unit that applies microwaves of varying frequency to the diamond substrate, an excitation light irradiation unit that irradiates the diamond substrate with excitation light, and a fluorescence detection unit that detects the intensity of fluorescence from the diamond substrate, wherein a main surface of the diamond substrate coincides with the (111) plane of a diamond crystal, and the axis of the NV center coincides with the normal direction of the diamond substrate, A current measurement method comprising: determining cancellation between a magnetic flux formed by the current to be measured and a magnetic flux formed by the reference current based on a characteristic frequency difference, which is the difference between two frequencies of the microwaves at which the intensity of the fluorescence detected by the fluorescence detection unit is minimized; and calculating a current value of the current to be measured from a current value of the reference current at the time of magnetic flux cancellation.

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