JP2016102777A - Magnetic field measuring method and magnetic field measuring device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To propose a new approach to identify an original magnetic field situated within a measurement region.SOLUTION: A light source unit 2 radiates linear polarization light of which the travel direction is a z-axis direction and the vibration direction of the magnetic field is a y-axis direction to a gas cell 3 arranged in a measurement region. A polarimeter 4 detects optical characteristics of light that has passed through the gas cell 3. A magnetic field generator 7 applies an artificial magnetic field in which a z-axis component, a y-axis component, and an x-axis component orthogonal to the z-axis direction and y-axis direction can be varied to the measurement region. An arithmetic control unit 5 has the magnetic field generator 7 generate an artificial magnetic field including a plurality of combinations obtained by changing the x-axis, y-axis and z-axis components, and the z-axis component being periodically changed. The arithmetic control unit calculates a magnetization value on the basis of a detection result by the polarimeter 4, and calculates an original magnetic existing field within the measurement region using the artificial magnetic field when the ratio between the time change of the magnetization value and the time change of the z-axis component meets a prescribed extreme value condition.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a magnetic field measurement method for measuring a magnetic field.

  As a device for measuring weak magnetic fields such as the magnetic field generated by the living body (biomagnetic field) such as the magnetic field from the heart (magnetomagnetic field) and the magnetic field from the brain (magnetomagnetic field), linear polarization is applied to the gas cell containing alkali metal atoms. And using an optical pumping type magnetic sensor that measures the magnetic field by rotating the polarization plane is known (see, for example, Patent Document 1).

JP 2013-108833 A

  In the measurement of a weak magnetic field using an optical pumping type magnetic sensor, it is necessary to cancel a magnetic field (referred to as an original magnetic field) due to an environment such as geomagnetism or urban noise existing in a measurement region where a gas cell is arranged. This is because the presence of the original magnetic field reduces the sensitivity to the magnetic field generated by the measurement object due to the influence, and causes a decrease in measurement accuracy.

  The present invention has been made in view of such circumstances, and an object thereof is to propose a new method for specifying an original magnetic field existing in a measurement region.

  SUMMARY An advantage of some aspects of the invention is to solve at least a part of the problems described above, and the invention can be implemented as the following forms or application examples.

  [Application Example] In the magnetic field measurement method according to this application example, the first direction, the second direction, and the third direction are orthogonal to each other, the light source unit that emits linearly polarized light, and the vibration direction of the electric field disposed in the measurement region Is a medium that changes the optical characteristics of the linearly polarized light according to a magnetic field, an optical detector that detects the optical characteristics, and the measurement. A magnetic field measurement device comprising a magnetic field generator for applying an artificial magnetic field to a region is a magnetic field measurement method for measuring a magnetic field in the measurement region, wherein the artificial magnetic field components in the first direction to the third direction are changed. A plurality of combinations of artificial magnetic fields, wherein the magnetic field generator is generated by periodically changing the artificial magnetic field component in the third direction, and the first direction component of the magnetization vector of the medium is used. Detection of a certain magnetization value by the optical detector The measurement using the artificial magnetic field when the ratio between the time change of the magnetization value and the time change of the artificial magnetic field component in the third direction satisfies a predetermined extreme value condition. Calculating an original magnetic field existing in the region.

  According to this application example, it is possible to obtain the original magnetic field derived from the environment in the measurement region where the magnetic field generated from the measurement target exists.

In the magnetic field measurement method according to the application example, the calculation of the original magnetic field is based on the fact that the magnetic field in the measurement region when the predetermined extreme value condition is satisfied is a zero magnetic field. It is preferable to include calculating.
According to this method, the original magnetic field can be accurately obtained.

In the magnetic field measurement method according to the application example, it is preferable that generating the artificial magnetic field includes changing the artificial magnetic field component in the third direction at a period equal to or less than a cutoff angular frequency.
According to this method, the original magnetic field can be obtained simply and accurately.

In the magnetic field measurement method according to the application example, the magnetic field generator generates a magnetic field that is a difference of the original magnetic field with respect to a predetermined target magnetic field, and the measurement target that generates the magnetic field is brought close to the measurement region. And measuring the magnetic field generated by the measurement object using the detection result of the optical detector during the generation of the differential magnetic field.
According to this method, the influence of the original magnetic field in the measurement region can be offset and the magnetic field generated by the measurement object can be accurately measured.

  [Application Example] In the magnetic field measurement apparatus of this application example, the first direction, the second direction, and the third direction are orthogonal to each other, the light source unit that emits linearly polarized light, and the vibration direction of the electric field are the second direction. A medium disposed in a measurement region that is irradiated with the linearly polarized light from the third direction, transmits the irradiated linearly polarized light, and changes in optical characteristics according to a magnetic field, and an optical detector that detects the optical characteristics A magnetic field generator that applies an artificial magnetic field that makes each component in the first direction, the second direction, and the third direction variable to the measurement region, and an artificial magnetic field component in the first direction to the third direction. A plurality of combinations of artificial magnetic fields, wherein the magnetic field generator is configured to generate an artificial magnetic field in which the artificial magnetic field component in the third direction is periodically changed, and the magnetization vector of the medium in the first direction The detection result of the optical detector for the magnetization value as a component Using the artificial magnetic field when the ratio between the time change of the magnetization value and the time change of the artificial magnetic field component in the third direction satisfies a predetermined extreme value condition, And an arithmetic control unit that executes calculation of the existing original magnetic field.

  According to this application example, it is possible to provide a magnetic field measurement apparatus capable of obtaining an original magnetic field derived from an environment in a measurement region where a magnetic field generated from a measurement target exists.

  A first invention for solving the above-described problems is a light source unit that emits linearly polarized light that is a second direction in which the vibration direction of the electric field is orthogonal to the third direction in the third direction, and transmits the linearly polarized light. And a medium arranged in a measurement region whose optical characteristics change according to a magnetic field, an optical detector for detecting the optical characteristics, the third direction, the second direction, the third direction, and the second direction. A magnetic field measurement method for measuring a magnetic field in the measurement region, comprising: a magnetic field generator that applies an artificial magnetic field that varies each component in a first direction orthogonal to the measurement region; A plurality of combinations of artificial magnetic fields in which artificial magnetic field components in the first direction to the third direction are changed, and generating an artificial magnetic field in which the artificial magnetic field components in the third direction are periodically changed in the magnetic field generator; , Composition of the magnetization vector of the medium in the first direction. And the ratio between the time change of the magnetization value and the time change of the artificial magnetic field component in the third direction satisfies a predetermined extreme value condition. A magnetic field measurement method including calculating an original magnetic field existing in the measurement region using the artificial magnetic field.

  As another invention, in the third direction, a light source unit that emits linearly polarized light that is a second direction in which the vibration direction of the electric field is orthogonal to the third direction, and transmits the linearly polarized light and is optical in accordance with a magnetic field A medium arranged in a measurement region where the characteristics change, an optical detector for detecting the optical characteristics, and a first direction orthogonal to the third direction, the second direction, the third direction, and the second direction. A magnetic field generator that applies an artificial magnetic field that makes each component variable to the measurement region, and a plurality of combinations of artificial magnetic fields in which the artificial magnetic field components in the first direction to the third direction are changed, and an artificial magnetic field in the third direction Generate an artificial magnetic field in which the magnetic field component is periodically changed in the magnetic field generator, and calculate a magnetization value that is a component in the first direction of the magnetization vector of the medium based on a detection result of the optical detector. And temporal change of the magnetization value Calculation control for calculating the original magnetic field existing in the measurement region using the artificial magnetic field when the ratio of the artificial magnetic field component in the third direction to the time change satisfies a predetermined extreme value condition It is good also as comprising a magnetic field measuring device provided with a part.

  According to the first invention and the like, a plurality of combinations of artificial magnetic fields in which the artificial magnetic field components in the first direction to the third direction are changed in the measurement region in which the medium is arranged, the artificial magnetic field components in the third direction Generates an artificial magnetic field that is periodically changed. Then, the magnetization value of the medium is calculated based on the detection results obtained for each of the plurality of artificial magnetic fields, and the ratio between the time change of the magnetization value and the time change of the artificial magnetic field component in the third direction satisfies a predetermined extreme value condition. The original magnetic field existing in the measurement region can be calculated using the artificial magnetic field.

  Further, in a second aspect based on the first aspect, the calculation of the original magnetic field is based on the fact that the magnetic field in the measurement region when the predetermined extreme value condition is satisfied is a zero magnetic field. A magnetic field measurement method including calculating an original magnetic field.

  According to the second aspect of the invention, the original magnetic field existing in the measurement region can be calculated based on the fact that the magnetic field in the measurement region when the extreme value condition is satisfied is a zero magnetic field.

  Further, in a third invention, in the first or second invention, generating the artificial magnetic field includes changing the artificial magnetic field component in the third direction at a period equal to or less than a cutoff angular frequency. This is a magnetic field measurement method.

  According to the third aspect of the invention, it is possible to calculate the original magnetic field by generating the artificial magnetic field by changing the artificial magnetic field component in the third direction at a period equal to or less than the cutoff angular frequency.

  According to a fourth invention, in any one of the first to third inventions, the magnetic field generator generates a magnetic field that is a difference of the original magnetic field with respect to a predetermined target magnetic field, and a magnetic field is generated in the measurement region. Further including approaching a generated measurement object and measuring a magnetic field generated by the measurement object using a detection result of the optical detector while generating the differential magnetic field. This is a magnetic field measurement method.

  According to the fourth aspect of the present invention, the original magnetic field existing in the measurement region is canceled to form a predetermined target magnetic field in the measurement region, and then the magnetic field generated by the measurement target is brought close to the measurement region. It can be measured.

The figure which shows the example of whole structure of a magnetic field measuring apparatus. The figure which shows the outline of the arrangement | positioning relationship of a light source part, a gas cell, and a polarimeter. The figure explaining rotation of a polarization plane. The figure which shows the relationship between alignment azimuth and the detection result of probe light. It shows a three-dimensional distribution of the differential values ∂M _x / ∂B _z. It shows a two-dimensional distribution of the differential values ∂M _x / ∂B _z. The figure which shows the relationship between the coordinate system of the magnetic field of a measurement area | region, and the coordinate system of an artificial magnetic field. The flowchart which shows the process sequence of a magnetic field formation process.

  Hereinafter, an embodiment for carrying out the magnetic field measurement method and the magnetic field measurement apparatus of the present invention will be described. It should be noted that the present invention is not limited to the embodiments described below, and modes to which the present invention can be applied are not limited to the following embodiments. In the description of the drawings, the same parts are denoted by the same reference numerals.

[overall structure]
FIG. 1 is a diagram illustrating an overall configuration example of a magnetic field measurement apparatus 1 according to the present embodiment. FIG. 2 is a diagram showing an outline of the arrangement relationship of the light source unit 2, the gas cell 3, and the polarimeter 4 constituting the magnetic field measuring apparatus 1. The magnetic field measuring apparatus 1 of this embodiment is used for a magnetocardiograph that measures a magnetocardiograph or a magnetoencephalograph that measures a magnetoencephalogram. The magnetic field measuring apparatus 1 incorporates a so-called one-beam type magnetic sensor that combines pump light irradiation with probe light irradiation as an optical pumping type magnetic sensor, and performs non-linear magneto-optical rotation (NMOR). Use this to measure magnetic fields. Note that the configuration is not limited to the one-beam type, and a so-called two-beam type configuration in which a light source unit for irradiating pump light and a light source unit for irradiating probe light may be separated.

  As shown in FIG. 1, the magnetic field measurement apparatus 1 includes a light source unit 2, a gas cell 3, a polarimeter 4 as an optical detector, an arithmetic control unit 5, and a magnetic field generator 7. Here, as shown in FIG. 2, the third direction, which is the traveling direction of linearly polarized light (irradiated light) irradiated by the light source unit 2 as pump light and probe light, is the z-axis direction, and the vibration direction of the electric field of this linearly polarized light The second direction is defined as the y-axis direction, the first direction orthogonal to the z-axis direction and the y-axis direction is defined as the x-axis direction, and the space in which the light source unit 2, the gas cell 3 and the polarimeter 4 are disposed is defined as three orthogonal axes. Xyz coordinate space.

  The light source unit 2 includes a light source 21 and a polarizing plate 23, and emits linearly polarized light that propagates in the z-axis direction and vibrates along the y-axis direction as irradiation light. The light source 21 is a laser generator that generates a laser beam having a frequency corresponding to the transition of the ultrafine structure level of gas atoms enclosed in the gas cell 3. Specifically, the wavelength of the laser beam depends on the state transition between the ultrafine structure quantum number F and F ′ (= F−1) of the D1 line of a gas atom (for example, cesium, potassium, rubidium, etc.) in the gas cell 3. Corresponding wavelength. The polarizing plate 23 is an element that polarizes the laser beam from the light source 21 in a predetermined direction to form linearly polarized light. Irradiation light emitted from the light source unit 2 is guided by, for example, an optical fiber or the like and is applied to the gas cell 3.

The gas cell 3 is a glass element in which alkali metal atoms such as potassium (K), rubidium (Rb), and cesium (Cs) are sealed in a gaseous state. The alkali metal atoms are excited (optically pumped) by the irradiation light (pump light) from the light source unit 2 and have a property as a medium that rotates the polarization plane of the light transmitted through the gas cell 3 according to the strength of the magnetic field. The gas cell 3 is disposed inside a magnetic shield 8 indicated by a two-dot chain line in FIG. The magnetic shield 8 is for shielding a certain amount of magnetism and forming a space in which the magnetism is reduced compared to the outside of the magnetic shield 8. For measurement, the gas cell 3 is disposed inside the magnetic shield 8. The region of the subject as the measurement object such as the heart or the brain is positioned in the measured region (the peripheral region of the gas cell 3). As described later, the measurement region can be set to a predetermined target magnetic field (for example, zero magnetic field) by the magnetic field generator 7 installed in the magnetic shield 8. The magnetic field measurement apparatus 1 measures the magnetic field generated by the measurement region by placing the measurement region of the subject in the measurement region after setting the magnetic field of the measurement region to the state of the target magnetic field. Therefore, it can be said that the magnetic field measuring device 1 includes a magnetic field forming device. In addition, the gas atom in the gas cell 3 should just be a gaseous state at the time of the measurement of a magnetic field, and does not need to be always a gaseous state. Further, the material of the gas cell 3 is not limited to glass but may be a material that transmits the irradiation light, and may be a resin or the like. In the present specification, a magnetic field existing in a measurement region such as the gas cell 3 is referred to as a magnetic field (B _x , B _y , B _z ) in the measurement region.

  The polarimeter 4 includes a polarization separator 41 and two photodetectors 431 and 433, and separates the irradiation light (probe light) transmitted through the gas cell 3 into two polarization components orthogonal to each other, and the respective light intensities. Is detected. The polarization separator 41 is an element that separates the irradiation light from the gas cell 3 into components in the directions of the orthogonal α axis and β axis (see FIG. 3). One separated polarization component is guided to the photodetector 431, and the other polarization component is guided to the photodetector 433. The polarization separator 41 is composed of, for example, a Wollaston prism or a polarization beam splitter. The photodetectors 431 and 433 receive the polarization component separated by the polarization separator 41, generate a signal corresponding to the amount of received light, and output the signal to the signal processing unit 51 of the arithmetic control unit 5.

  The arithmetic control unit 5 is configured using a microprocessor such as a CPU (Central Processing Unit), an ASIC (Application Specific Integrated Circuit), an IC (Integrated Circuit) memory, and the like, and comprehensively controls the operation of each unit of the apparatus. The arithmetic control unit 5 includes a signal processing unit 51, a magnetic field calculation unit 53, and a correction magnetic field setting unit 55, and forms a predetermined target magnetic field (for example, zero magnetic field) in the measurement region (see FIG. 8). Further, the arithmetic control unit 5 has a storage unit such as a flash memory or a hard disk, and stores the program in the storage unit in a readable manner if the configuration is such that the magnetic field forming process is realized by executing the program. .

  In addition, an input unit 61 for inputting necessary information and a display unit 63 for displaying magnetic field measurement results and the like are appropriately connected to the arithmetic control unit 5. The input unit 61 includes various switches such as button switches, lever switches, and dial switches, and input devices such as a touch panel, a keyboard, and a mouse. The display unit 63 includes a display device such as an LCD (Liquid Crystal Display) or an EL display (Electroluminescence display).

The signal processing unit 51 measures the magnetic field in the measurement region by calculating the rotation angle of the polarization plane rotated by the probe light passing through the gas cell 3. The signal processing unit 51 processes the signals from the photodetectors 431 and 433, and according to the following equations (1) and (2), the square sum W ₊ and the square difference W of the components in the respective axial directions of the α axis and the β axis. _- to calculate the and. E _α represents the light intensity of the component in the α-axis direction, and E _β represents the light intensity of the component in the β-axis direction.

The magnetic field calculation unit 53 calculates an original magnetic field existing in the measurement region using an artificial magnetic field when the amplitude of the square difference W ₋ calculated by the signal processing unit 51 satisfies a predetermined extreme value condition. The magnetic field calculation unit 53 includes an artificial magnetic field control unit 531 and an amplitude detection unit 533. The artificial magnetic field control unit 531 controls the magnetic field generator 7 and is a plurality of combinations of artificial magnetic fields obtained by changing the artificial magnetic field components in the x-axis, y-axis, and z-axis directions. An artificial magnetic field in which the z-axis component is periodically changed by superimposing a predetermined AC component on the magnetic field component (referred to as a z-axis component) is sequentially generated in the measurement region. The amplitude detection unit 533 detects the ratio between the time change of the square difference W _{− and} the time change of the z-axis component by extracting the amplitude and phase from the time change of the square difference W ₋ calculated by the signal processing unit 51. To do. The amplitude detector 533 can be configured using a lock-in amplifier or the like.

  The correction magnetic field setting unit 55 sets a correction magnetic field that cancels the original magnetic field calculated by the magnetic field calculation unit 53, and controls the magnetic field generator 7 to generate a correction magnetic field in the measurement region, thereby forming a zero magnetic field in the measurement region. To do.

The magnetic field generator 7 is composed of a three-axis Helmholtz coil for applying a magnetic field in each of the x-axis, y-axis, and z-axis directions, and 1 in each axis direction with the gas cell 3 sandwiched inside the magnetic shield 8. It includes a pair of coils arranged in pairs, and a current supply unit that supplies current to these coils. The magnetic field generator 7 can generate a magnetic field in an arbitrary three-dimensional direction in the measurement region. In this specification, the magnetic field generated by the magnetic field generator 7 is referred to as an artificial magnetic field (A _x , A _y , A _z ).

  The z-axis direction is the third direction in the present invention, but the irradiation light (pump light) is not necessarily emitted from the light source unit 2 in the z-axis direction as shown in FIG. Irradiation light (pump light) may be incident on the gas cell 3 from the z-axis direction after emission.

[principle]
In the magnetic field measurement apparatus 1 configured as described above, when the pump light from the light source unit 2 is irradiated to the gas cell 3, the gas atoms in the gas cell 3 are spin-polarized. The probability distribution of the magnetic moment generated when the energy transitions from the hyperfine quantum number F to F ′ (= F−1) by this spin polarization is an ellipse extending along the y-axis direction that is the vibration direction of the linearly polarized light. It becomes a body shape. This biased probability distribution is called “alignment”, and the occurrence of alignment is called “optical pumping”. When the measurement region is a zero magnetic field, the generated alignment remains along the y-axis direction that is the vibration direction of the pump light, but a magnetic field such as a magnetocardiogram or a magnetoencephalogram generated by a living body, which is a measurement object, is present in the measurement region. When applied, the alignment precesses. As a result, the polarization plane of the linearly polarized light rotates at an angle corresponding to the magnetic field in the measurement region, with the z-axis direction as the traveling direction being the rotation axis.

  FIG. 3 is a diagram for explaining the rotation of the polarization plane. As described above, the alignment precesses according to the magnetic field in the measurement region (the magnetic field received by the gas cell 3). Then, by adding an optical pumping action by the pump light and a relaxation action that occurs when gas atoms collide with the inner wall of the gas cell 3, the alignment is shown by hatched ellipsoids in FIG. A steady state is obtained with the arrangement rotated by an angle (θ) corresponding to the strength of the magnetic field with respect to the y-axis.

By this alignment, the probe light passing through the gas cell 3 is subjected to the action of linear dichroism. Linear dichroism refers to the property that the transmittance of linearly polarized light differs between the direction along the alignment (the direction of Θ _p ) and the direction perpendicular to the alignment (the direction of Θ _s ). Specifically, since the component in the direction perpendicular to the alignment is absorbed more than the direction along the alignment, the polarization plane of the probe light rotates so as to approach the direction along the alignment.

For example, in the present embodiment, the probe light incident on the gas cell 3 is linearly polarized light of the vector E ₀ whose electric field oscillation direction is the y-axis direction, and the alignment transmits the component in the Θ _p direction of the probe light. The light is transmitted at a rate t _p and the component in the Θ _s direction is transmitted at the transmittance t _s . Since t _p > t _s due to linear dichroism, the polarization plane of the probe light transmitted through the gas cell 3 rotates so as to approach the Θ _p direction, and is along the vector E ₁ . Specifically, the component along the alignment of the vector E ₀ is denoted as E _0P , the component along the direction perpendicular to the alignment of the vector E ₀ and the traveling direction of the linearly polarized light is denoted as E _0s, and the vector E ₁ the components along the alignment is denoted by E _1P, if a component along the direction perpendicular to the traveling direction of alignment and the linearly polarized light vector E ₁ was expressed as _{E 1s, E 1P = t p} E 0P and E _1s = t _s E _0s

If the angle formed by the direction along the alignment and the vibration direction of the probe light (hereinafter referred to as “alignment azimuth angle”) is θ, each component of the vector E _{1 in} the Θ _p direction and the Θ _s direction is 3), each component in the coordinate system (α, β) is calculated by the following equation (4).

FIG. 4 is a diagram illustrating the relationship between the alignment azimuth angle θ and the detection result of the probe light. Focusing on the value, the square difference W _{- -} squared difference W 4 vibrates a cycle of 180 degrees with respect to the alignment azimuth theta. Since the square difference W ₋ changes substantially linearly with respect to the alignment azimuth angle θ when the alignment azimuth angle θ is in the range of −45 degrees to 45 degrees, high sensitivity can be obtained. Moreover, since the center of the linear change is 0 degree and the range of the linear change is wider than others (such as the sum of squares W ₊ ), it is suitable for measuring the magnetic field generated in the measurement region. Since the biomagnetic field such as the magnetocardiogram and the magnetoencephalogram is weak and the alignment azimuth angle θ is small, the rotation angle of the polarization plane can be observed with high sensitivity by using the square difference W ₋ . However, as described above, if an original magnetic field is present in the measurement region, the sensitivity is affected and the measurement accuracy is reduced. In the present embodiment, the magnetic field is measured in an environment where the magnetic shield 8 suppresses the magnetic field from entering the measurement region. However, the magnetic shield 8 cannot completely shield the magnetic field from entering. A magnetic shield that can completely shield magnetism is a large-scale device, is expensive, and is expensive to install and operate. Therefore, in this embodiment, the magnetic shield 8 is used. However, in the first place, when the original magnetic field is low or the original magnetic field is stable, this embodiment can be configured without using even the magnetic shield 8.

Here, the square difference W _- is spin polarization of the alignment produced in the gas cell _{3 (M x, M y,} M z) of the x-axis component M _x (hereinafter referred to as "spin polarization M _x". ) Is approximately proportional to the value of), and can be expressed using the spin polarization M _x . Spin polarization _{(M x, M y, M} z) corresponds to the magnetization vector of the gas atoms in the gas cell 3. Hereinafter, the spin polarization M _x is used as the magnetization value indicating the x-axis component of the magnetization vector, and the x-axis component, the y-axis component, and the z-axis component (absolute magnetic flux densities B _x , B _y , The relationship between B _z ) and the spin polarization M _x was examined.

Time evolution of spin polarization of alignment caused by optical pumping _{(M x, M y, M} z) is approximated by the following equation (5) to Bloch equations shown in (7) (Bloch Equations). γ _F represents a gyromagnetic ratio determined by the type of gas atom (alkali metal atom) in the gas cell 3. Also, gamma ₀ is spin polarization _{(M x, M y, M} z) represents the rate of relaxation, gamma _p represents an optical pumping speed. The relaxation rate Γ ₀ of the spin polarization and the optical pumping speed Γ _p are expressed in the same unit system as the angular frequency, and specifically have a unit of radians per second (rad / s). The cut-off angular frequency ω _C is the sum of the spin polarization relaxation rate Γ ₀ and the optical pumping rate Γ _p (ω _C = Γ ₀ + Γ _p ).

Pumping light and probe light, for irradiating the gas cell 3 at constant constant power, stationary solutions of spin polarization _{(M x, M y, M} z) , the above equation (5) to (7) It can be solved by setting each left side to zero. The solution is expressed by the following equations (8) to (11).

Differential value ∂M _x / ∂B _z of the spin polarization M _x and differentiating B _z in the formula (8) is expressed by the following equation (12).

The differential value ∂M _x / ∂B _z represented by the above equation (12) indicates the change in the output (magnetization value M _x ) with respect to the change in the detection amount (z component B _{z of the} magnetic field). It means sensitivity. That is, the measurement sensitivity of the magnetic field measurement apparatus 1 is maximized under the condition that the differential value ∂M _x / ∂B _z is maximized. Also the denominator of the equation (12) B _x and B _y are whatever the value go increasing at the fourth power of these values. On the other hand, since the molecules of the above formula (12) varies with the square of B _x and B _y, with respect to the the B _x and B _y, the differential value when the _{B x = B y = 0 ∂M} x / ∂B _z is the maximum. The differential value ∂M _x / ∂B _{z at} this time is expressed by the following equation (13).

When each value of B _x and B _z is changed with B _y = 0 [nT], the result is as shown in FIG. Further, when B _x = B _y = 0 [nT] and each value of B _z is changed, the result is as shown in FIG. As shown in FIGS. 5 and 6, appears one local maximum value in the distribution of the differential values ∂M _x / ∂B _z, differential value ∂M _x / ∂B _z in the case of B _x = B _y = 0 In this case, the maximum (maximum) is obtained when B _z = 0. Therefore, when the measurement region is a zero magnetic field (each value of B _x , B _y , B _z is zero), the spin polarization degree M _x (that is, the change in the magnetic field along the z-axis direction (progression direction of the probe light)) Has the largest change in the square difference W ₋ ), and the sensitivity is maximized. In other words, in order to minimize the original magnetic field such as the external magnetic field in the measurement region, the artificial magnetic field may be adjusted so that the differential value ∂M _x / ∂B _z is maximized.

Here, the differential value ∂M _x / ∂B _z (ratio between the time change of the spin polarization M _{x and} the time change of B _z ) is the ratio between the time change of the square difference W _{− and} the time change of B _z. Can be replaced. Therefore, in the magnetic field forming process of the present embodiment, an artificial magnetic field in which the x-axis component, the y-axis component, and the z-axis component are changed and the z-axis component is periodically changed is measured. An artificial magnetic field in which the differential value ∂M _x / ∂B _z, which is the ratio of the time change of the spin polarization M _{x to} the time change of B _z , is maximized is sequentially generated in the region. At this time, the angular frequency ω of the z-axis component is desirably set to a value equal to or lower than the cutoff angular frequency ω _C. The cut-off angular frequency ω _C is the sum of the relaxation rate Γ ₀ of the spin polarization degree M _x and the optical pumping speed Γ _p (ω _C = Γ ₀ + Γ _p ), and is approximately 100 Hz in this embodiment. . In other words, the z-axis component of the artificial magnetic field preferably satisfies the relationship ω <ω _C = Γ ₀ + Γ _p , and in this embodiment, the angular frequency ω is 100 Hz or less. If the angular frequency ω of the z-axis component is a value less than or equal to the cutoff angular frequency ωc, dM _x / dt can be regarded as almost zero, and it is legal to approximate the left side of the above-described equation (5) to zero. It becomes. That is, when the relationship of ω <ω _C = Γ ₀ + Γ _p is satisfied, the original magnetic field existing in the measurement region can be accurately measured. However, since the magnetic field measuring apparatus 1 behaves like a first-order low-pass filter, the gain and phase in the vicinity of the cutoff angular frequency ω _C gradually decrease as the angular frequency ω increases. Therefore, in practice, the angular frequency ω of the periodic function to be superimposed may be about 10% larger than the cutoff angular frequency ω _C.

As a specific measurement method, the x-axis component A _x and the y-axis component A _y of the artificial magnetic field are fixed magnetic fields, the z-axis component is applied with a magnetic field represented by a periodic function, and the time of spin polarization M _x is obtained. The ratio between the change and the time change of B _z is measured. This is measured at various levels to identify an artificial magnetic field that maximizes the differential value ∂M _x / ∂B _z . For example, as a first measurement, A _x = 0, A _y = 0, A _z is an oscillating magnetic field around 0 (for example, A _z = sin ωt), and the first differential value ∂M _x / ∂B _z is measure. Next, as a second measurement, A _x = 0, A _y = 0, and A _z as an oscillating magnetic field around 1 (A _z = 1 + sinωt as an example), and a second differential value ∂M _x / ∂B _z Measure. Next, as a third measurement, A _x = 0, A _y = 0, and A _z as an oscillating magnetic field around −1 (for example, A _z = −1 + sin ωt), and a third differential value ∂M _x / ∂ to measure the B _z. Next, as a fourth measurement, A _x = 0, A _y = 1, and A _z as an oscillating magnetic field around 0 (for example, A _z = sin ωt), the fourth differential value ∂M _x / ∂B _z Measure. Next, as a fifth measurement, A _x = 0, A _y = 1, and A _z as an oscillating magnetic field around 1 (A _z = 1 + sinωt as an example), the fifth differential value ∂M _x / ∂B _z Measure. Next, as a sixth measurement, A _x = 0, A _y = 1, and A _z as an oscillating magnetic field around −1 (for example, A _z = −1 + sin ωt), the sixth differential value ∂M _x / ∂ to measure the B _z. Next, as a seventh measurement, A _x = 0, A _y = −1, A _z is set as an oscillating magnetic field around 0 (for example, A _z = sin ωt), and a seventh differential value ∂M _x / ∂B Measure _z . Next, as an eighth measurement, A _x = 0, A _y = −1, and A _z is an oscillating magnetic field around 1 (for example, A _z = 1 + sinωt), and an eighth differential value ∂M _x / ∂B Measure _z . Next, as a ninth measurement, A _x = 0, A _y = −1, and A _z as an oscillating magnetic field around −1 (for example, A _z = −1 + sin ωt), the ninth differential value ∂M _x / to measure the ∂B _z. In this way, many measurements are repeated centering on A _y = 0, and an artificial magnetic field that maximizes the differential value ∂M _x / ∂B _z is obtained from the many differential values ∂M _x / ∂B _z thus obtained. Identify. In other words, the differential value ∂M _x / ∂ is obtained by extracting the amplitude from the period change of the square difference W ₋ calculated by the signal processing unit 51 based on the detection result of the polarimeter 4 for each of the plurality of artificial magnetic fields of each combination. B _z is detected. Then, the original magnetic field is calculated using the artificial magnetic field when the extreme value condition in which the differential value ∂M _x / ∂B _z becomes the maximum value is satisfied. The process of obtaining the maximum value of the differential value ∂M _x / ∂B _z while changing the artificial magnetic field can be realized by using a known optimization process. As described above, in order to search for the artificial magnetic field having the maximum differential value ∂M _x / ∂B _z , it is preferable from the viewpoint of efficiency to search for the y-axis component A _y of the artificial magnetic field from the vicinity of zero.

FIG. 7 shows a coordinate system of the magnetic field B = (B _x , B _y , B _z ) in the measurement region and a coordinate system of the artificial magnetic field A = (A _x , A _y , A _z ) generated by the magnetic field generator 7. It is a figure which shows the relationship. As shown in FIG. 7, each component of the magnetic field B = (B _x , B _y , B _z ) in the measurement region has an artificial magnetic field A = (A _x ) in the original magnetic field C = (C _x , C _y , C _z ). , A _y , A _z ) can be calculated by vector addition. The calculation formula is shown in the following formula (14).

As described above, when the differential value ∂M _x / ∂B _z becomes a maximum value and satisfies the extreme value condition, that is, when the extracted amplitude becomes maximum, the magnetic field in the measurement region is zero magnetic field (B = 0). Therefore, the original magnetic field C (C _x , C _y , C _z ) is the artificial magnetic field A _h (A _hx , A _hy , A _z ) when the extreme value condition is satisfied from the relationship of the above equation (14). _hz ) is expressed by the following equations (15) to (17).

[Process flow]
FIG. 8 is a flowchart showing the processing procedure of the magnetic field formation processing. The magnetic field measuring apparatus 1 performs the magnetic field forming process shown in FIG. 8 before bringing the subject into the magnetic shield 8 and measuring the biomagnetic field.

As shown in FIG. 8, in the magnetic field formation process, first, the magnetic field calculation unit 53 calculates the original magnetic field C existing in the measurement region (step S1). As a specific processing procedure, first, the artificial magnetic field control unit 531 sequentially generates a plurality of combinations of artificial magnetic fields in the measurement region by the magnetic field generator 7, and the amplitude detection unit 533 is obtained as a measurement result each time. Extract the amplitude of the square difference W ₋ . The magnetic field calculator 53, the square difference W _- calculating the original field C by the above formula (15) to (17) using an artificial magnetic field A _h when the amplitude is maximized.

Subsequently, the correction magnetic field setting unit 55 sets the correction magnetic field TC by subtracting the original magnetic field C calculated in step S1 from the target magnetic field T ( _Tx , _Ty , _Tz ) (step S3). Then, the correction magnetic field setting unit 55 causes the magnetic field generator 7 to generate the set correction magnetic field TC, thereby canceling the original magnetic field C and forming the target magnetic field T in the measurement region (step S5). In the present embodiment, as an example, the correction magnetic field TC is set as the target magnetic field T = (0, 0, 0), and a zero magnetic field is formed in the measurement region.

  As described above, according to the present embodiment, the original magnetic field existing in the measurement region can be calculated. Further, the zero magnetic field can be formed by canceling the original magnetic field in the measurement region, and then the subject can be carried into the magnetic shield 8 and the biomagnetic field can be measured. This makes it possible to measure the biomagnetic field with high sensitivity and high accuracy.

  In the above-described embodiment, the gas cell 3 in which alkali metal atoms are sealed in a gaseous state is used as a medium that causes polarization of the magnetic moment and rotates the polarization plane of transmitted light according to the strength of the magnetic field. However, media other than alkali metal atoms may be used. For example, a structure using a solid element such as diamond provided with a lattice defect by nitrogen may be used. Further, the magnetic field measurement method and the magnetic field measurement apparatus of the present invention can be applied to an atomic oscillator using a small gas cell of a millimeter size in addition to a magnetic sensor.

  Moreover, although embodiment which makes a target magnetic field a zero magnetic field was demonstrated, the target magnetic field can be made into arbitrary magnetic fields other than a zero magnetic field.

  DESCRIPTION OF SYMBOLS 1 ... Magnetic field measuring apparatus, 2 ... Light source part, 3 ... Gas cell, 4 ... Polarimeter as an optical detector, 5 ... Operation control part, 7 ... Magnetic field generator, 8 ... Magnetic shield, 21 ... Light source, 23 ... Polarizing plate , 41 ... polarization separator, 51 ... signal processing unit, 53 ... magnetic field calculation unit, 55 ... correction magnetic field setting unit, 61 ... input unit, 63 ... display unit, 431, 433 ... photodetector, 531 ... artificial magnetic field control unit 533, an amplitude detector.

Claims (5)

The first direction, the second direction, and the third direction are orthogonal to each other, the light source unit that emits linearly polarized light, and the linearly polarized light that is disposed in the measurement region and the vibration direction of the electric field is the second direction is the third direction. A medium that is irradiated along the direction and changes the optical characteristics of the linearly polarized light according to the magnetic field; an optical detector that detects the optical characteristics; and a magnetic field generator that applies an artificial magnetic field to the measurement region. The magnetic field measurement apparatus is a magnetic field measurement method for measuring the magnetic field in the measurement region,
A plurality of combinations of artificial magnetic fields in which the first to third artificial magnetic field components are changed, and an artificial magnetic field in which the artificial magnetic field components in the third direction are periodically changed is generated in the magnetic field generator. When,
Calculating a magnetization value that is a component in the first direction of the magnetization vector of the medium based on a detection result of the optical detector;
The original magnetic field existing in the measurement region is calculated using the artificial magnetic field when the ratio between the time change of the magnetization value and the time change of the artificial magnetic field component in the third direction satisfies a predetermined extreme value condition. And
Magnetic field measurement method including Calculating the original magnetic field includes calculating the original magnetic field based on the fact that the magnetic field in the measurement region when the predetermined extreme value condition is satisfied is a zero magnetic field,
The magnetic field measurement method according to claim 1. Generating the artificial magnetic field includes changing the artificial magnetic field component in the third direction at a period equal to or less than a cutoff angular frequency.
The magnetic field measurement method according to claim 1 or 2. Causing the magnetic field generator to generate a magnetic field that is a difference of the original magnetic field with respect to a predetermined target magnetic field;
Approaching a measurement object that generates a magnetic field to the measurement region;
Using the detection result of the optical detector while generating the difference magnetic field, measuring the magnetic field generated by the measurement object;
The magnetic field measurement method according to any one of claims 1 to 3, further comprising: The first direction, the second direction, and the third direction are orthogonal to each other,
A light source that emits linearly polarized light;
A medium disposed in a measurement region in which the linearly polarized light whose vibration direction is the second direction is irradiated from the third direction, the irradiated linearly polarized light is transmitted, and the optical characteristics change according to the magnetic field. When,
An optical detector for detecting the optical property;
A magnetic field generator configured to apply an artificial magnetic field that makes each component in the first direction, the second direction, and the third direction variable, to the measurement region;
A plurality of combinations of artificial magnetic fields in which the first to third artificial magnetic field components are changed, and an artificial magnetic field in which the artificial magnetic field components in the third direction are periodically changed is generated in the magnetic field generator. Calculating a magnetization value that is a component in the first direction of the magnetization vector of the medium based on a detection result of the optical detector, and a temporal change in the magnetization value and an artificial magnetic field component in the third direction. An arithmetic control unit that executes calculation of an original magnetic field existing in the measurement region using the artificial magnetic field when a ratio with a time change satisfies a predetermined extreme value condition;
Magnetic field measuring device equipped with.