JP2011220957A - Magnetic field measuring device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve accuracy of magnetic field measurement.SOLUTION: A liquid crystal layer 620 of a liquid crystal panel 62 has a liquid crystal 622 of which orientation varies with a applied voltage. The liquid crystal 622 rotates a plane of polarization of a probe light passing through a cell in a period of time T1 in which no voltage is applied to the liquid crystal 622, and does not rotate the plane of polarization of the probe light in a period of time T2 in which a voltage is applied to the liquid crystal 622. A polarizing plate 625 of the liquid crystal panel 62 passes a polarization component in a predetermined direction of the probe light passing through the liquid crystal layer 620. A detector detects an s-polarization component of the probe light passing through the polarizing plate 625 in the period of time T1, and a p-polarization component of the probe light passing through the polarizing plate 625 in the period of time T2.

Description

  The present invention relates to a technique for measuring a magnetic field.

  Magnetic sensors using optical pumping are used in so-called MRI (magnetic resonance imaging) devices and the like. In this type of magnetic sensor, the pump light having a circularly polarized light component and the probe light having a linearly polarized light component are irradiated to the cell so that they intersect (preferably orthogonally), and further, irradiation of these lights is performed. A magnetic field in a direction orthogonal to the direction is applied (see, for example, Patent Document 1).

JP 2009-14708 A

By the way, in a magnetic sensor using optical pumping, the probe light transmitted through the cell is separated into a p-polarized component and an s-polarized component using, for example, a polarizing beam splitter, and a p-polarized component and s are used using two optical sensors. A polarization component is detected. At this time, unless the inclination of the polarization beam splitter is accurately adjusted, the light amounts of the two polarization components become unbalanced, resulting in a decrease in measurement accuracy. In addition, when two optical sensors are used, measurement accuracy is lowered due to variations in the optical sensors themselves.
An object of this invention is to improve the measurement precision of a magnetic field.

  The present invention includes a cell having therein an atom that rotates a polarization plane of linearly polarized light according to a magnetic field applied from the outside when excited, and a cell having a linearly polarized component with respect to the cell in which the atom is excited. A first irradiation section that irradiates one light; and a liquid crystal whose orientation is changed by application of a voltage, and the first light transmitted through the cell in a first period in which no voltage is applied to the liquid crystal. In the second period in which the polarization plane is rotated by the liquid crystal and a voltage is applied to the liquid crystal, the liquid crystal layer that does not rotate the polarization plane of the first light and the first light transmitted through the liquid crystal layer Among these, a polarization unit that transmits a polarization component in a predetermined direction, and a first polarization component of the first light that has passed through the polarization unit in the first period are detected, and the second In the period, the first that has passed through the polarizing section. Providing a magnetic field measuring device, characterized in that it comprises a detection unit for detecting a second polarized component of light. According to this configuration, the magnetic field measurement accuracy can be improved.

  In the magnetic field measurement apparatus, the detection unit may calculate a difference between the first polarization component and the second polarization component. According to this configuration, the calculation of the difference between the first polarization component and the second polarization component can also be performed in the magnetic field measurement apparatus.

  In the magnetic field measurement apparatus, the liquid crystal layer may include a nematic liquid crystal. According to this configuration, the liquid crystal layer can be configured using nematic liquid crystal.

  In the magnetic field measurement apparatus, the cell includes an atom excited by circularly polarized light, and irradiates the cell with second light having a circularly polarized component so as to intersect the first light. Two irradiating units, and the first irradiating unit may irradiate the cell irradiated with the second light with the first light. According to this configuration, the magnetic field can be measured by a method using circularly polarized light.

Diagram showing the configuration of the magnetic field measurement device The perspective view which shows the structure of a cell Diagram explaining the operation principle of nematic liquid crystal Diagram explaining the operation principle of nematic liquid crystal Diagram explaining the operation principle of nematic liquid crystal Diagram showing the configuration of the liquid crystal panel Diagram showing control of voltage application to liquid crystal

  FIG. 1 is a diagram illustrating a configuration of the magnetic field measurement apparatus 1. The magnetic field measuring apparatus 1 is used for measuring a weak magnetic field generated from a living body such as a magnetocardiogram (magnetism from the heart) and a cerebral magnetism (magnetism from the brain). The magnetic field measurement apparatus 1 includes a cell 10, irradiation units 20 and 30, a magnetic field generation unit 40, and detection units 50 and 60. The magnetic field measuring apparatus 1 is arranged so that the cell 10 is positioned in the magnetic field to be measured.

  FIG. 2 is a perspective view showing the configuration of the cell 10. The cell 10 has a hollow cubic shape and is formed of a material that transmits light, such as glass. The shape of the cell 10 may be another three-dimensional shape. Inside the cell 10, for example, alkali metal atoms are sealed in a gaseous state (that is, a gas state). Examples of the alkali metal include lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr). The atoms in the cell 10 are excited by circularly polarized light, and the spins of the outer electrons of the atoms are polarized. The cell 10 typically contains a single type of alkali metal atom, but may contain a plurality of types of alkali metal atoms. Further, the atoms in the cell 10 do not always need to be in a gas state, and may be in a gas state when the magnetic field is measured. Further, the inside of the cell 10 may contain helium (He), nitrogen (N) or the like as a buffer gas in order to moderate the relaxation of alkali metal atoms due to collision with the wall of the cell 10 or the like. Good.

  The irradiation unit 20 (an example of the second irradiation unit) illustrated in FIG. 1 irradiates the cell 10 with pump light (an example of the second light) having a circularly polarized component. The irradiation unit 20 includes a light source 21, a half-wave plate 22, a polarization beam splitter 23, and a quarter-wave plate 24. The light source 21 irradiates non-polarized laser light in the direction of arrow X shown in FIG. The half-wave plate 22 rotates the polarization plane of the light emitted from the light source 21. The deflecting beam splitter 23 transmits the p-polarized component (component parallel to the incident surface) of the light transmitted through the half-wave plate 22 and reflects the s-polarized component (component perpendicular to the incident surface). This s-polarized component may be used for monitoring the output of laser light, for example, or may be absorbed by a member that absorbs light. Note that the reflection direction of the s-polarized component is not limited to the illustrated direction. The quarter-wave plate 24 changes the light transmitted through the deflecting beam splitter 23 into circularly polarized light. Thereby, the light transmitted through the quarter-wave plate 24 becomes pump light having a circularly polarized component.

  The irradiation unit 30 (an example of the first irradiation unit) irradiates the cell 10 with probe light (an example of the first light) having a linearly polarized component. The irradiation unit 30 includes a light source 31, a half-wave plate 32, a deflection beam splitter 33, and a polarizing plate 34. The light source 31 irradiates non-polarized laser light in the arrow Y direction shown in FIG. The half-wave plate 32 rotates the polarization plane of the light emitted from the light source 31. The deflecting beam splitter 33 transmits the p-polarized component of the light transmitted through the half-wave plate 32 and reflects the s-polarized component. The polarizing plate 34 transmits only light polarized in a specific direction out of the light transmitted through the deflecting beam splitter 33. Thereby, the light transmitted through the polarizing plate 34 becomes probe light having a linearly polarized light component. Note that the pump light and the probe light are preferably orthogonal to each other, but may not be completely orthogonal as long as they intersect.

  The magnetic field generator 40 includes a coil 41 and a magnetic shield 42. The coil 41 applies a magnetic field for signal and a magnetic field for correcting a magnetic field from the outside. The magnetic shield 42 is a member for shielding a magnetic field other than the measurement target, and is provided so as to cover the cell 10 and the coil 41. The magnetic shield 42 has a path through which the pump light and the probe light pass.

  The detection unit 50 detects the pump light transmitted through the cell 10. The detection unit 50 includes an optical sensor 51. The optical sensor 51 is, for example, a photodiode, and converts the pump light transmitted through the cell 10 into an electric signal. The detection result of the optical sensor 51 is used for monitoring pump light, for example.

  The detection unit 60 (an example of a detection unit) detects the probe light that has passed through the cell 10. The detection unit 60 includes a half-wave plate 61, a liquid crystal panel 62, an optical sensor 63, an A / D converter 64, a memory 65, and a calculation unit 66. The half-wave plate 61 rotates the polarization plane of the probe light transmitted through the cell 10. The liquid crystal panel 62 transmits the p-polarized component or the s-polarized component of the probe light that has passed through the half-wave plate 61 using a liquid crystal called a nematic liquid crystal. The optical sensor 63 is a photodiode, for example, and converts the probe light transmitted through the liquid crystal panel 62 into an electrical signal and outputs the electrical signal. The A / D converter 64 converts the electrical signal output from the optical sensor 63 into digital data and outputs the digital data. The memory 65 stores the data output from the A / D converter 64. The calculation unit 66 uses the data stored in the memory 65 to calculate the difference between the p-polarized component and the s-polarized component of the probe light detected by the optical sensor 63.

  3 to 5 are diagrams for explaining the operation principle of the nematic liquid crystal. The liquid crystal molecules C of the nematic liquid crystal are arranged with gradual regularity along the major axis direction of the molecules when there is no applied voltage (see FIG. 3A). At this time, when the liquid crystal molecules C are brought into contact with the substrate B (alignment film) having fine grooves in a certain direction (see FIG. 3B), the liquid crystal molecules C are aligned along the grooves of the substrate B (FIG. 3 ( c)).

  The liquid crystal molecules C are held between two substrates B as shown in FIG. The substrate B is arranged so that the direction in which the groove of the upper substrate B extends (direction of arrow a) and the direction in which the groove of the lower substrate B extends (direction of arrow b) are orthogonal. At this time, the upper liquid crystal molecules C are aligned in the direction of the arrow a along the groove of the upper substrate B, and the lower liquid crystal molecules C are aligned in the direction of the arrow b along the groove of the lower substrate B. The liquid crystal molecules C in the middle of them are arranged while gradually changing the direction from the direction of the arrow a to the direction of the arrow b. As a result, the alignment direction of the liquid crystal molecules C is twisted by 90 degrees.

  When the light L is passed through the nematic liquid crystal in the absence of an applied voltage, the polarization plane of the light L is rotated by 90 degrees due to the twist in the alignment direction of the liquid crystal molecules C as shown in FIG. This phenomenon is called optical rotation. On the other hand, when a voltage is applied to the nematic liquid crystal, the liquid crystal molecules C are aligned perpendicular to the substrate B as shown in FIG. When the light L is passed through the nematic liquid crystal in this state, the polarization plane of the light L passes through without being rotated.

  FIG. 6 is a diagram showing the configuration of the liquid crystal panel 62. The liquid crystal panel 62 includes a liquid crystal layer 620 and a polarizing plate 625 (an example of a polarizing unit). The liquid crystal layer 620 includes two substrates 621a and 621b, a liquid crystal 622, and a driving unit 623. An alignment film having fine grooves in a certain direction is provided on the surfaces of the substrate 621a and the substrate 621b. As shown in FIG. 4, the substrate 621a and the substrate 621b are arranged so that the extending directions of the grooves of the light distribution film are orthogonal to each other. The liquid crystal 622 is a nematic liquid crystal and is held between the substrate 621a and the substrate 621b. The driver 623 applies a voltage to the substrate 621a and the substrate 621b to drive the liquid crystal 622. The polarizing plate 625 allows only a polarization component in a specific direction to pass through among the probe light transmitted through the liquid crystal layer 620.

  The alignment of the liquid crystal 622 is changed by application of a voltage. When there is no applied voltage, the liquid crystal 622 is arranged so as to be twisted 90 degrees between the substrate 621a and the substrate 621b as shown in FIG. On the other hand, when a voltage is applied, the liquid crystals 622 are aligned in a direction perpendicular to the surfaces of the substrates 621a and 621b as shown in FIG. 6B.

  Next, the operation of the magnetic field measuring apparatus 1 will be described. The irradiation unit 20 irradiates the cell 10 with pump light. As a result, the atoms in the cell 10 are excited. At this time, the atoms in the cell 10 precess by receiving torque corresponding to the magnetic field to be measured applied to the cell 10. Subsequently, the irradiation unit 30 irradiates the cell 10 with the probe light. When this probe light passes through the inside of the cell 10, the plane of polarization is rotated by the precession of atoms in the cell 10. The rotation angle of the plane of polarization has a magnitude corresponding to the magnetic field to be measured applied to the cell 10. The polarization plane of the probe light transmitted through the cell 10 is rotated by the half-wave plate 61 and reaches the liquid crystal panel 62.

  At this time, the liquid crystal layer 620 of the liquid crystal panel 62 controls voltage application to the liquid crystal 622 in a time division manner as shown in FIG. Specifically, the driver 623 does not apply a voltage to the substrate 621a and the substrate 621b during the period T1 (an example of the first period). At this time, since no voltage is applied, the liquid crystal 622 is arranged so as to be twisted by 90 degrees between the substrate 621a and the substrate 621b as shown in FIG. When the probe light incident on the liquid crystal layer 620 in the period T1 passes through the liquid crystal layer 620, the plane of polarization is rotated by approximately 90 degrees due to the twist of the alignment direction of the liquid crystal 622. The probe light transmitted through the liquid crystal layer 620 reaches the polarizing plate 625. At this time, the polarizing plate 625 passes only the s-polarized light component of the probe light transmitted through the liquid crystal layer 620.

  Subsequently, the driver 623 applies a voltage to the substrate 621a and the substrate 621b during the period T2 (an example of the second period). At this time, since a voltage is applied, the liquid crystal 622 is aligned in a direction perpendicular to the surfaces of the substrates 621a and 621b as shown in FIG. 6B. The probe light that has entered the liquid crystal layer 620 in the period T2 passes through the liquid crystal layer 620 without rotating the polarization plane. That is, the probe light passes through the liquid crystal layer 620 while maintaining the polarization state before passing through the liquid crystal panel 62. The probe light transmitted through the liquid crystal layer 620 reaches the polarizing plate 625. At this time, the polarizing plate 625 passes only the p-polarized light component of the probe light transmitted through the liquid crystal layer 620.

  The probe light transmitted through the liquid crystal panel 62 reaches the optical sensor 63. The optical sensor 63 detects the s-polarized component (an example of the first polarized component) of the probe light that has passed through the polarizing plate 625 in the period T1, and the p of the probe light that has passed through the polarizing plate 625 in the period T2. A polarization component (an example of a second polarization component) is detected. Then, the optical sensor 63 outputs an electrical signal indicating the detected light amount of the polarization component. The A / D converter 64 converts the electrical signal output from the optical sensor 63 into digital data and outputs the digital data. The memory 65 stores the data output from the A / D converter 64. As a result, the memory 65 stores data indicating the light amount of the s-polarized component of the probe light and data indicating the light amount of the p-polarized component. The calculation unit 66 uses the data stored in the memory 65 to calculate the difference between the p-polarized component and the s-polarized component. Thereby, the rotation angle of the polarization plane of the probe light rotated when passing through the inside of the cell 10 is obtained. As described above, the rotation angle of the plane of polarization has a magnitude corresponding to the magnetic field to be measured applied to the cell 10. Therefore, the magnetic field measuring apparatus 1 can measure the magnetic field to be measured by obtaining the rotation angle of the polarization plane of the probe light.

  According to this embodiment, the probe light transmitted through the cell 10 can be separated into a p-polarized component and an s-polarized component without using a polarizing beam splitter. Therefore, for example, the adjustment of the inclination of the polarization beam splitter is not shifted, so that the measurement accuracy is not lowered. In addition, since the polarizing beam splitter is relatively expensive, the manufacturing cost of the magnetic field measuring apparatus 1 can be reduced by not using the polarizing beam splitter. Further, since the probe light transmitted through the cell 10 is detected by the single optical sensor 63, the variation of the optical sensor does not occur. Therefore, the measurement accuracy of the magnetic field is increased.

[Modification]
The present invention is not limited to the above-described embodiment, and may be carried out by being modified as follows. Further, the following modifications may be combined.

  The s-polarized component and the p-polarized component of the probe light detected by the optical sensor 63 may be held as an analog electric signal by an analog circuit, for example. In this case, the circuit that holds the electric signal separately holds the electric signal that indicates the light amount of the s-polarized component and the electric signal that indicates the light amount of the p-polarized component.

  In the above-described embodiment, the s-polarized component of the probe light is detected during the period T1 when there is no applied voltage, and the p-polarized component is detected during the period T2 where the applied voltage is present, but this is not restrictive. For example, by changing the direction of the liquid crystal panel 62, the p-polarized component of the probe light may be detected during the period T1 when there is no applied voltage, and the s-polarized component may be detected during the period T2 when the applied voltage is present.

  The pump light and the probe light irradiated on the cell 10 do not need to have a constant traveling direction from when they are emitted until they enter the cell 10. That is, the pump light and the probe light only need to be in a predetermined direction when entering the cell 10, and may be reflected by a mirror or the like in the middle to change the traveling direction. Therefore, for example, it is possible to concentrate the light irradiation position and the detection position in one place.

  The magnetic field measuring apparatus 1 does not have to detect the pump light transmitted through the cell 10. In this case, the above-described optical sensor 51 is not necessary. In this case, the magnetic field measurement apparatus 1 may be provided with a member that absorbs the pump light transmitted through the cell 10. This member is more preferably in the form of a sheet.

  The magnetic field measurement apparatus 1 may include a means for visualizing a signal corresponding to the light detected by the detection unit 60 and displaying the signal. Or the magnetic field measuring apparatus 1 may be provided only with the structure which produces | generates the signal according to the light detected by the detection part 60, and outputs this to an external device (a display apparatus, a computer apparatus, etc.).

  The liquid crystal 622 held in the liquid crystal layer 620 is not limited to nematic liquid crystal. For example, the liquid crystal layer 620 may include cholesteric liquid crystal instead of nematic liquid crystal.

  The method by which the magnetic field measuring apparatus 1 measures the magnetic field is not limited to the method described in the above embodiment. For example, the magnetic field measuring apparatus 1 may measure the magnetic field by a known method that uses the principle of the magneto-optical effect and does not use pump light.

  The apparatus in which the liquid crystal panel 62 is used is not limited to the magnetic field measuring apparatus 1. For example, the liquid crystal panel 62 may be used in place of the polarization beam splitter in an apparatus including a polarization beam splitter that separates light into two polarization components.

DESCRIPTION OF SYMBOLS 1 ... Magnetic field measuring apparatus, 10 ... Cell, 20, 30 ... Irradiation part, 40 ... Magnetic field generation part, 50, 60 ... Detection part, 62 ... Liquid crystal panel, 620 ... Liquid crystal layer, 625 ... Polarizing plate

Claims (4)

A cell having inside an atom that rotates a polarization plane of linearly polarized light according to a magnetic field applied from the outside when excited,
A first irradiating unit that irradiates the cell in which the atoms are excited with a first light having a linearly polarized component;
In a first period in which the orientation of the liquid crystal is changed by applying a voltage and no voltage is applied to the liquid crystal, the liquid crystal rotates the polarization plane of the first light transmitted through the cell. In the second period in which the voltage is applied, a liquid crystal layer that does not rotate the polarization plane of the first light;
Of the first light transmitted through the liquid crystal layer, a polarization unit that transmits a polarization component in a predetermined direction;
In the first period, a first polarization component of the first light that has passed through the polarization unit is detected, and in the second period, the first polarization component of the first light that has passed through the polarization unit is detected. A magnetic field measuring apparatus comprising: a detection unit that detects two polarization components. The magnetic field measurement apparatus according to claim 1, wherein the detection unit calculates a difference between the first polarization component and the second polarization component. The magnetic field measurement apparatus according to claim 1, wherein the liquid crystal layer includes a nematic liquid crystal. The cell has atoms inside that are excited by circularly polarized light,
A second irradiation unit configured to irradiate the cell with second light having a circularly polarized component so as to intersect the first light;
4. The magnetic field measurement apparatus according to claim 1, wherein the first irradiation unit irradiates the cell irradiated with the second light with the first light. 5.

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