JP2015062020A - Magnetic measuring device and biological state measuring device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance sensitivity in a magnetic measuring device with an optical pumping method.SOLUTION: A magnetic measuring device includes: a cell accommodating a medium that changes an orientation of a polarization plane of light according to magnetic field intensity therein; a light source for emitting light that interacts with the medium to the cell; a mirror for reflecting light having been transmitted through the cell toward the cell; a polarization separator for separating light into a plurality of polarization components including a first polarization component and a second polarization component; a first photodetector for detecting the first polarization component of light having been reflected by the mirror and having been transmitted through the cell; and a second photodetector for detecting the second polarization component of the light having been reflected by the mirror and having been transmitted through the cell.

Description

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

  Non-Patent Documents 1 to 3 disclose a magnetic field measurement method using non-linear magneto-optical rotation (NMOR). For example, FIG. 1 of Non-Patent Document 3 discloses a magnetic measurement system using a gas cell containing rubidium (Rb) and a single laser light source. Patent Document 1 discloses that in a magnetic measurement apparatus using two light sources of pump light and probe light, pumping efficiency is improved by reflecting the pump light with a mirror and irradiating the gas cell twice. (See FIG. 2).

JP 2009-236598 A

D. Budkar, et al., “Resonant nonlinear magneto-optical rotation effect of atoms”, Review of Modern Physics, USA, American Physical Society, October 2002, Vol. 74, No. 4, p. 1153-1201 (D. Budker et al., “Resonant nonlinear magneto-optial effects in atoms”, Rev. Mod. Phys., 74, 1153-1201 (2002)) D. Budkar, et al., “Highly sensitive magnetic field measurement by non-linear magneto-optical rotation”, Physical Review A, USA, American Physical Society, October 2000, Vol. 62, No. 4, p. 043403 (D. Budker et al., “Sensitive magnetometry based on nonlinear magneto-optical rotation”, Phys. Rev. A, 62, 043403 (2000)) D. Budkar, et al., “Non-linear magneto-optical rotation using frequency-modulated light”, Physical Review A, USA, American Physical Society, May 2002, Vol. 65, No. 5, p. 055403 (D. Budker et al., “Nonlinear magneto-optical rotation with frequency-modulated light”, Phys. Rev. A, 65, 055403 (2002))

In the methods described in Non-Patent Documents 1 to 3, the amount of rotation of the polarization plane is small, and it is difficult to increase the sensitivity. Further, Patent Document 1 increases the efficiency of pumping.
The present invention provides a technique for improving sensitivity in an optically-pumped magnetic measuring apparatus.

The present invention includes a cell containing therein a medium that changes the polarization plane direction of light according to the magnetic field strength, a light source that emits light that interacts with the medium to the cell, and the light that has passed through the cell. A mirror that reflects the light toward the cell, a polarization separator that separates the light into a plurality of polarization components including a first polarization component and a second polarization component, and the light reflected by the mirror and transmitted through the cell. A magnetic measurement apparatus comprising: a first photodetector that detects the first polarization component of light; and a second photodetector that detects the second polarization component of the light reflected by the mirror and transmitted through the cell. provide.
According to this magnetometer, the sensitivity to a magnetic field can be improved compared to a configuration in which light is not reflected by a mirror.

In a preferred aspect, the polarization separator may be disposed between the cell and the mirror on the optical path of the light.
According to this magnetometer, the sensitivity to a magnetic field can be improved compared to a configuration in which light is not reflected by a mirror.

In another preferred embodiment, the light may have a linearly polarized component when it first enters the cell, and the polarization planes of the first and second polarized components may be orthogonal to each other.
According to this magnetometer, magnetic field information can be obtained from the difference between the output signal of the first photodetector and the output signal of the second photodetector.

In still another preferred embodiment, the polarization separator may be disposed between the light source and the cell on the optical path of the light.
According to this magnetometer, the sensitivity to a magnetic field can be improved compared to a configuration in which light is not reflected by a mirror.

In still another preferred embodiment, the light has a linearly polarized component when incident on the polarization separator, and the planes of polarization of the first polarized component and the second polarized component are at an angle of 45 °. Good.
According to this magnetometer, magnetic field information can be obtained from the difference between the output signal of the first photodetector and the output signal of the second photodetector.

In still another preferred embodiment, the first photodetector and the second photodetector may be disposed on the opposite side of the polarization separator as viewed from the cell.
According to this magnetometer, the sensitivity to a magnetic field can be improved compared to a configuration in which light is not reflected by a mirror.

In still another preferred embodiment, the magnetic measurement apparatus has two sets of optical systems including the light source, the mirror, the polarization separator, the first photodetector, and the second photodetector, The pair of optical systems may be arranged so that optical paths in the cell are orthogonal to each other.
According to this magnetometer, it is possible to measure magnetic fields having different axial components using two sets of optical systems.

Moreover, this invention provides the biological condition measuring apparatus which has one of the said magnetic measuring apparatuses.
According to this biological state measuring device, the sensitivity to a magnetic field can be improved as compared with a configuration in which light is not reflected by a mirror.

1 shows a configuration of a magnetic sensor 1 according to a first embodiment. The existence probability distribution of angular momentum in the pumping process is shown. The probability distribution of angular momentum in the precession process is shown. The figure explaining a probe process. The structure of the magnetic sensor 2 which concerns on 2nd Embodiment is shown. The precession process in the outgoing laser beam is shown. The polarization state of the laser beam immediately after passing through the polarization separator 60 is shown. The rotating action of the polarization plane in the return laser beam A is shown. The rotating action of the polarization plane in the backward laser beam B is shown. The structure of the magnetic sensor 3 which concerns on 3rd Embodiment is shown. The existence probability distribution of angular momentum in the pumping process is shown. The probability distribution of angular momentum in the precession process is shown. The figure which shows the rotation effect | action of the polarization surface of the laser beam A. FIG. The figure which shows the rotation effect | action of the polarization plane of the laser beam B. FIG. The structure of the magnetic sensor 4 which concerns on 4th Embodiment is shown.

1. First Embodiment FIG. 1 is a block diagram showing a configuration of a magnetic sensor 1 (magnetic measurement device or magnetic field measurement device) according to a first embodiment. In this example, the magnetic sensor 1 is used in a biological state measuring device (such as a magnetocardiograph or a magnetoencephalograph) that measures a magnetic field generated from a living body, such as a magnetocardiogram or a brain magnetism, as an indicator of the state of the living body. The magnetic sensor 1 includes a light source 10, a polarizing plate 20, a half mirror 30, a gas cell 40, a mirror 50, a polarization separator 60, a PD (Photo Detector), and a PD 80. In the following description, an orthogonal coordinate system having an x-axis in the right direction in the drawing, a y-axis in a direction perpendicular to the paper surface, and a z-axis in the upward direction in the drawing is used.

  The gas cell 40 is a box (cell) having a void inside, and an alkali metal atom (cesium (Cs) in this example) is sealed in the void. The gas cell 40 is formed of an inorganic material such as quartz glass or borosilicate glass. At least a part of the alkali metal atoms sealed in the gas cell 40 is vaporized at the time of measurement. In order to lengthen the alignment relaxation time described later, an inert gas such as a rare gas may be mixed with an alkali metal gas, or the inner wall of the gas cell 40 may be coated with paraffin or the like.

  The light source 10 is a device that outputs a laser beam having a wavelength corresponding to the absorption line of cesium (for example, 894 nm corresponding to the D1 line), for example, a tunable laser. The laser beam output from the light source 10 is so-called CW (Continuous Wave) light having a constant light amount continuously. The output of the light source 10 is adjusted so that the amount of light incident on the gas cell 40 is on the order of several tens of μW.

The polarizing plate 20 is an element that polarizes the laser beam in a specific direction to make it linearly polarized light.
The half mirror 30 is an element that transmits a laser beam directed in the negative z-axis direction and reflects the laser beam directed in the positive z-axis direction toward the polarization separator 60. The half mirror 30 is, for example, a partially polarized beam splitter or a non-polarized beam splitter whose transmittance is constant regardless of the polarization direction.
The mirror 50 is an element that reflects the laser beam transmitted through the gas cell 40 and enters the gas cell 40 again. The mirror 50 has a reflective surface using a metal film or a dielectric multilayer film. When the polarization plane rotates before and after reflection on the reflection surface, a bias (offset) is generated in the magnetic field detection signal. Therefore, it is desirable that the mirror 50 has a small rotation angle of the polarization plane on the reflection surface.

The polarization separator 60 is an element that separates an incident laser beam into beams of two polarization components orthogonal to each other. The polarization separator 60 is, for example, a Wollaston prism or a polarization beam splitter.
The PD 70 and PD 80 are detectors sensitive to the wavelength of the laser beam, and output a current corresponding to the amount of incident light. Since the measurement may be affected when the PD 70 and the PD 80 generate a magnetic field, it is desirable that the PD 70 and the PD 80 are made of a nonmagnetic material. In this example, the PD 70 and the PD 80 are disposed on the same side as the polarization separator 60 when viewed from the gas cell 40.

  If it demonstrates along the path | route of a laser beam, said element will be arrange | positioned as follows. The light source 10 is located on the uppermost stream of the laser beam path. Hereinafter, the polarizing plate 20, the half mirror 30, the gas cell 40, and the mirror 50 are arranged in this order from the upstream side. The above is the outgoing path of the laser beam. Since the laser beam is reflected by the mirror 50, further description will be given along the path after reflection (return path). Is arranged in. That is, the laser beam proceeds as follows. The laser beam output from the light source 10 becomes linearly polarized light having a higher degree of polarization by the polarizing plate 20. The polarized laser beam passes through the half mirror 30 and enters the gas cell 40. The laser beam passing through the gas cell 40 excites (optically pumps) the alkali metal atoms sealed in the gas cell 40. At this time, the polarization plane of the laser beam is rotated by receiving the polarization plane rotation action corresponding to the strength of the magnetic field. The laser beam transmitted through the gas cell 40 is reflected by the mirror 50 and is incident on the gas cell 40 again. The laser beam passing through the gas cell 40 is again subjected to the polarization plane rotating action. The laser beam transmitted through the gas cell 40 is reflected by the half mirror 30, and the traveling direction is converted by 90 °. The laser beam whose traveling direction has been converted is separated into two polarization component beams by the polarization separator 60. The light amounts of the two polarization component beams are measured (probing) by the PD 70 and the PD 80.

  The interaction between atoms and light (polarization plane rotation action) for magnetic field measurement is basically divided into three stages: a pump process, a precession process, and a probe process. Hereinafter, the function of the element at each stage will be described.

FIG. 2 is a diagram showing an existence probability distribution of angular momentum in the pumping process. More precisely, FIG. 2 shows a cross section (shaded portion in the figure) in the xy plane of a region where the existence probability of the spin angular momentum is greater than a certain value. Hereinafter, for simplicity, such a diagram is simply referred to as “a diagram showing the existence probability distribution of angular momentum”. In this example, the laser beam has a wavelength that excites the hyperfine structure quantum number of cesium from the ground state of F = 3 to the excited state of F ′ = 4, and oscillates on the y-axis (electric field vector E _i A linearly polarized beam. The outermost electrons of cesium are excited (optical pumping) by the laser beam, and the angular momentum of the cesium atom (more precisely, the spin angular momentum) has a distribution that is biased along the electric field of the incident light. Since the electric field of incident light is oscillating along the y-axis now, the angular momentum is distributed mainly in the y-axis positive and negative directions as shown in FIG. That is, the optically pumped cesium atom has two anti-parallel angular momentums, the y-axis positive direction and the negative direction. Here, the anisotropy generated in the distribution of angular momentum is widely referred to as “alignment”, and the generation of the anisotropic distribution in angular momentum is referred to as “forming alignment”. In other words, forming alignment is the same as magnetizing.

  FIG. 3 is a diagram showing an existence probability distribution of angular momentum in the precession process. Here, a case where the static magnetic field B is applied in the z-axis direction in a state where the alignment of the state of FIG. 2 is formed by optical pumping will be described as an example. The magnetic field B is, for example, a magnetic field generated by an object to be measured. Due to the action of the static magnetic field B and alignment, the cesium atom receives a clockwise rotational force with the z axis (direction of the static magnetic field B) as the rotation axis. This rotational force causes the cesium atom to rotate in the xy plane. This is precession. When the cesium atom rotates, the alignment rotates. Here, the rotation angle of the alignment based on the alignment in a state where no magnetic field is applied is represented by α. For a single atom, the angular momentum bias (excited state) caused by pumping decreases with time, that is, the alignment is relaxed. Since the laser beam is CW light, alignment formation and relaxation are repeated in parallel and continuously. As a result, a steady (time-average) alignment is formed as a whole group of atoms. FIG. 3 represents a steady alignment. The rotation angle α of the alignment and the magnitude of the angular momentum depend on the precession frequency (Larmor frequency) and the relaxation rate determined by a plurality of factors.

Due to the steady alignment, the laser beam is subject to linear dichroism. The direction of alignment is the transmission axis, and the polarization component in this direction is mainly transmitted. The direction perpendicular to the alignment direction is the absorption axis, and the polarization component in this direction is mainly absorbed. That is, when the amplitude transmission coefficient of light in the transmission axis and the absorption axis represents the t _‖ and t _⊥, a t _‖> t _⊥. The transmission axis component and the absorption axis component of the electric field Ei of incident light are Eicos α and Eisin α. Transmission axis component and the absorption axis component of the electric field Eo after passing through the gas cell 40 (after interacting with the cesium atom) is a t _‖ E _i cos [alpha] and t _⊥ E _i sin .alpha. t _‖> because it is t _⊥, the electric field vector E _o is rotating the electric field vector E _i as a reference (i.e., the plane of polarization of the laser beam is rotated). This rotation angle is represented by φ. 3 is a rotation angle after passing through the gas cell 40 in the return path of the laser beam.

  More precisely, a phenomenon occurs in which the angular momentum is biased in the propagation direction of the laser beam (Alignment Orientation Conversion, AOC), and as a result, the polarization plane rotation (Faraday effect) due to circular birefringence occurs. Although this happens, this phenomenon is ignored here.

FIG. 4 is a diagram for explaining the probe process. The laser beam whose polarization plane is rotated by the steady alignment is separated into two polarization components by the polarization separator 60. In this example, these two polarization components are separated into components along two axes, a first detection axis and a second detection axis. The first detection axis is inclined + 45 ° with respect to the polarization plane when the polarization plane is not rotated (φ = 0). The second detection axis is inclined by −45 ° with respect to the polarization plane when the polarization plane does not rotate. PD 70 and PD 80 detect the light amounts of the components along the first detection axis and the second detection axis, respectively. The first detection axis component of the electric field vector E _o of the laser beam transmitted through the gas cell 40 is E _o cos (π / 4-φ), and the second detection axis component is E _o sin (π / 4-φ). . According to this configuration, when the rotation of the polarization plane is almost zero (φ≈0), the light amounts of the laser beams incident on the PD 70 and the PD 80 are substantially the same. Conversely, if there is a difference in the amount of laser beams incident on the PD 70 and PD 80, it indicates that the polarization plane is rotating. This means that there is a magnetic field. The difference in the amount of laser beams incident on PD 70 and PD 80 is a function of the rotation angle φ of the polarization plane. By calculating the difference between the output signals of the PD 70 and the PD 80, information on the rotation angle φ can be obtained. The rotation angle φ is a function of the magnetic field B (see, for example, Equation (2) in Non-Patent Document 1. Equation (2) relates to linear optical rotation, but in the case of NMOR, almost the same equation is used. be able to). That is, information on the magnetic field B is obtained from the rotation angle φ.

  According to the magnetic sensor 1, the laser beam passes through the gas cell 40 twice (the forward path and the return path with reference to the reflection at the mirror 50). Compared with a configuration in which the gas cell 40 is passed only once, the optical path length in the gas cell 40 is doubled and the rotation angle φ is further increased. That is, the sensitivity to the magnetic field is improved. Further, according to the magnetic sensor 1, it is possible to place an object to be measured under the mirror 50. Compared with a configuration in which an element such as a PD is disposed under the gas cell, the distance between the object to be measured and the gas cell 40 can be reduced, and the magnetic sensor 1 can detect a larger signal.

2. Second Embodiment FIG. 5 is a block diagram showing a configuration of a magnetic sensor 2 according to a second embodiment. The magnetic sensor 2 includes a light source 10, a polarizing plate 20, a gas cell 40, a mirror 50, a polarization separator 60, a PD 70, and a PD 80. Compared with the configuration of the magnetic sensor 1, the magnetic sensor 2 is different in that it does not have the half mirror 30 and the positional relationship between the elements is different. Specifically, in the magnetic sensor 2, the polarization separator 60 is disposed between the gas cell 40 and the mirror 50 on the optical path of the laser beam. Further, the PD 70 and the PD 80 are disposed on the opposite side to the polarization separator 60 as viewed from the gas cell 40.

  If it demonstrates along the path | route of a laser beam, said element will be arrange | positioned as follows. The light source 10 is located on the uppermost stream of the laser beam path. Hereinafter, the polarizing plate 20, the gas cell 40, the polarization separator 60, and the mirror 50 are arranged in this order from the upstream side. The above is the outgoing path of the laser beam. Since the laser beam is reflected by the mirror 50, it will be further described along the path (return path) after reflection. From the upstream side, the mirror 50, (polarization separator 60), gas cell 40, and PD70 and PD80 are arranged in this order. Has been. That is, the laser beam proceeds as follows. The laser beam output from the light source 10 becomes linearly polarized light having a higher degree of polarization by the polarizing plate 20. The polarized laser beam is incident on the gas cell 40. That is, the laser beam has a linearly polarized component when it first enters the gas cell 40 (outward). The laser beam passing through the gas cell 40 excites (optically pumps) the alkali metal atoms sealed in the gas cell 40. At this time, the polarization plane of the laser beam is rotated by receiving the polarization plane rotation action corresponding to the strength of the magnetic field. The laser beam transmitted through the gas cell 40 is separated into two polarization components (first polarization component and second polarization component) by the polarization separator 60. The planes of polarization of these two polarization components are orthogonal to each other. The two beams are reflected by the mirror 50 and enter the gas cell 40 again. The laser beam passing through the gas cell 40 is again subjected to the polarization plane rotating action. The light quantities of the two laser beams that have passed through the gas cell 40 are measured (probing) by the PD 70 and PD 80.

  FIG. 6 is a diagram showing a precession process in the forward laser beam. The alkali metal atoms are excited by the laser beam, and the alignment shown in FIG. 2 is formed. The forward laser beam that has passed through the gas cell 40 undergoes a rotating action of the polarization plane due to the alignment. The electric field vector Eo of the forward laser beam transmitted through the gas cell 40 is rotated by an angle φ with respect to the electric field vector Ei of the incident light.

FIG. 7 is a diagram showing the polarization state of the laser beam immediately after passing through the polarization separator 60. Incident light is separated into two beams of a laser beam A and a laser beam B by the polarization separator 60. The polarization of the laser beam A is the same as that of the incident light (φ _ao = φ). The polarization of the laser beam B is perpendicular to the polarization of the incident light (φ _bo = π / 2−φ).

FIG. 8 is a diagram showing the rotating action of the polarization plane in the laser beam A on the return path. The plane of polarization of the laser beam A on the return path also rotates due to the linear dichroism effect of alignment formed by optical pumping. The electric field E _ah of the return laser beam A transmitted through the gas cell 40 is rotated by an angle φ _ah with reference to the electric field E _ao of incident light (return laser beam A before passing through the gas cell 40).

FIG. 9 is a diagram showing the rotating action of the polarization plane in the laser beam B on the return path. The plane of polarization of the laser beam B in the return path also rotates due to the linear dichroism effect of alignment formed by optical pumping. The electric field E _bh of the return laser beam B transmitted through the gas cell 40 rotates by an angle φ _bh with reference to the electric field E _bo of the incident light (return laser beam B before passing through the gas cell 40).

According to this configuration, when the rotation of the polarization plane is almost zero (φ≈0 °), the light amounts of the laser beams incident on the PD 70 and the PD 80 are substantially the same. Conversely, if there is a difference in the amount of laser beams incident on the PD 70 and PD 80, it indicates that the polarization plane is rotating. As apparent from FIGS. 8 and 9, the magnitudes of the electric field E _ah and the electric field E _bh (that is, the light _amounts of the laser beam A and the laser beam B) are determined by the rotation angle α of the alignment and the polarization angle φ of the forward path of the laser beam. Is a function of Information on the rotation angle φ is obtained from the output signals of the PD 70 and PD 80. The rotation angle φ is a function of the magnetic field B, and information on the magnetic field B can be obtained from the rotation angle φ.

  According to the magnetic sensor 2, the laser beam passes through the gas cell 40 twice (the forward path and the return path with reference to the reflection at the mirror 50). Compared with a configuration in which the gas cell 40 is passed only once, the optical path length in the gas cell 40 is doubled, and the rotation angle is further increased. That is, the sensitivity to the magnetic field is improved. Further, according to the magnetic sensor 2, it is possible to place an object to be measured under the mirror 50. Furthermore, compared with the magnetic sensor 1, the magnetic sensor 2 does not have the half mirror 30, but has a simpler configuration.

3. Third Embodiment FIG. 10 is a block diagram showing a configuration of a magnetic sensor 3 according to a third embodiment. The magnetic sensor 3 includes a light source 10, a polarizing plate 20, a gas cell 40, a mirror 50, a polarization separator 60, a PD 70, and a PD 80. Compared with the configuration of the magnetic sensor 1, the magnetic sensor 3 is different in that it does not have the half mirror 30 and the positional relationship of the elements is different. Compared with the configuration of the magnetic sensor 2, the magnetic sensor 3 is different in that the positional relationship of the elements is different. Specifically, in the magnetic sensor 3, the polarization separator 60 is disposed between the light source 10 and the gas cell 40 on the optical path of the laser beam. PD 70 and PD 80 are disposed on the same side as the polarization separator 60 as viewed from the gas cell 40.

  If it demonstrates along the path | route of a laser beam, said element will be arrange | positioned as follows. The light source 10 is located at the uppermost stream of the laser beam path. Hereinafter, the polarizing plate 20, the polarization separator 60, the gas cell 40, and the mirror 50 are arranged in this order from the upstream side. The above is the outgoing path of the laser beam. Since the laser beam is reflected by the mirror 50, the mirror 50, the gas cell 40, and the PD 70 and PD 80 are arranged in this order from the upstream side when further described along the path after the reflection (return path). That is, the laser beam proceeds as follows. The laser beam output from the light source 10 becomes linearly polarized light having a higher degree of polarization by the polarizing plate 20. The polarized laser beam enters the polarization separator 60. That is, the laser beam has a linearly polarized component when it enters the polarization separator 60. The laser beam is separated into two polarized component beams (laser beam A and laser beam B) by the polarization separator 60. The two laser beams are incident on the gas cell 40. The laser beam passing through the gas cell 40 excites (optically pumps) the alkali metal atoms sealed in the gas cell 40. At this time, the polarization plane of the laser beam is rotated by receiving the polarization plane rotation action corresponding to the strength of the magnetic field. The two beams are reflected by the mirror 50 and enter the gas cell 40 again. The laser beam passing through the gas cell 40 is again subjected to the polarization plane rotating action. The light quantities of the two laser beams that have passed through the gas cell 40 are measured (probing) by the PD 70 and PD 80.

FIG. 11 is a diagram illustrating the existence probability distribution of the angular momentum in the pumping process. In this example, the polarization planes (electric field vectors E _ai and E _bi ) of the laser beam A and the laser beam B are inclined by 45 ° (π / 4). In FIG. 11, the polarization plane of the laser beam A is tilted by + π / 8 with respect to the y axis, and the polarization plane of the laser beam B is tilted by −π / 8.

FIG. 12 is a diagram showing an existence probability distribution of angular momentum in the precession process. The alignment by the incident light E _ai and E _bi is rotated by an angle α. Hereinafter, for description, the atoms sealed in the gas cell 40 are classified into the following three groups.
Atoms pumped and probed by the same beam (atoms pumped by laser beam A and probed by laser beam A and atoms pumped by laser beam B and probed by laser beam B).
Atoms pumped by laser beam B and probed by laser beam A.
Atoms pumped by laser beam A and probed by laser beam B.
Among the above three groups, atoms having the same pumping light and probe light have the same contribution to the signals detected by the PD 70 and PD 80, and are ignored here. These classifications are hypothetical for explanation, and the atoms sealed in the gas cell 40 are not clearly distinguished.

FIG. 13 is a diagram showing the rotating action of the polarization plane of the laser beam A by the atoms optically pumped by the laser beam B. FIG. The plane of polarization of the laser beam A is rotated by the linear dichroism effect of the alignment formed in the atoms optically pumped by the laser beam B. The electric field E _ao of the laser beam A transmitted through the gas cell 40 (in the return path) is rotated by an angle φ _{ao with} respect to the electric field E _{ai of the} incident light.

FIG. 14 is a diagram showing the rotating action of the polarization plane of the laser beam B by the atoms pumped by the laser beam A. FIG. The plane of polarization of the laser beam B is rotated by the linear dichroism effect of the alignment formed in the atoms optically pumped by the laser beam A. The electric field E _bo of the laser beam B transmitted (in the return path) through the gas cell 40 is rotated by an angle φ _{bo with} respect to the electric field E _{bi of the} incident light.

According to this configuration, when the rotation of the polarization plane is almost zero (φ≈0 °), the light amounts of the laser beams incident on the PD 70 and the PD 80 are substantially the same. Conversely, if there is a difference in the amount of laser beams incident on the PD 70 and PD 80, it indicates that the polarization plane is rotating. As is clear from FIGS. 13 and 14, the magnitudes of the electric field E _ao and the electric field E _bo (that is, the light _amounts of the laser beam A and the laser beam B) are the rotation angle α of the alignment and the polarization angle φ of the forward path of the laser beam. Is a function of Information on the rotation angle φ is obtained from the output signals of the PD 70 and PD 80. The rotation angle φ is a function of the magnetic field B, and information on the magnetic field B can be obtained from the rotation angle φ.

  According to the magnetic sensor 3, the laser beam passes through the gas cell 40 twice (the forward path and the return path with reference to the reflection at the mirror 50). Compared with a configuration in which the gas cell 40 is passed only once, the optical path length in the gas cell 40 is doubled, and the rotation angle is further increased. That is, the sensitivity to the magnetic field is improved. Further, according to the magnetic sensor 3, it is possible to place an object to be measured under the mirror 50. Compared with a configuration in which an element such as a PD is disposed under the gas cell, the distance between the object to be measured and the gas cell 40 can be reduced, and the magnetic sensor 3 can detect a larger signal. Furthermore, compared with the magnetic sensor 1, the magnetic sensor 3 does not have the half mirror 30, and has a simpler configuration. Further, in this configuration, the pump light and the probe light are spatially separated. For example, when considering atoms pumped by the laser beam B and probed by the laser beam A, the optical path of the pump light (laser beam B) and the optical path of the probe light (laser beam A) are in different positions. As a result, since the probe is performed after a relatively long time has passed since excitation, the number of aligned atoms can be increased by the effect of Ramsey resonance, and the sensitivity to the magnetic field is improved.

4). Fourth Embodiment FIG. 15 is a block diagram showing a configuration of a magnetic sensor 4 according to a fourth embodiment. The magnetic sensor 4 has two sets of optical systems including a light source, a polarizing plate, a polarization separator, a mirror, and two PDs. The first set of optical systems includes a light source 10, a polarizing plate 20, a polarization separator 60, a mirror 50, a PD 70, and a PD 80. The second set of optical systems includes a light source 11, a polarizing plate 21, a polarization separator 61, a mirror 51, a PD 71, and a PD 81. The first set of optical systems and the second set of optical systems have the same configuration, but are arranged so as to be orthogonal to each other with respect to the single gas cell 40. The arrangement of elements in each set of optical systems is the same as that of the magnetic sensor 3. According to the magnetic sensor 4, it is possible to detect a magnetic field in two directions of the x direction and the z direction. The magnetic sensor 4 may further include a third set of optical systems orthogonal to the first set of optical systems and the second set of optical systems, and may measure magnetic fields in three directions. In another example, the first set of optical systems and the second set of optical systems may not be orthogonal. The first set of optical systems and the second set of optical systems need only be capable of detecting magnetic fields along two different axes.

5. Other Embodiments In any of the magnetic sensors of the first to fourth embodiments, a cell (gas cell 40) containing alkali metal atoms, which is an example of a medium that changes the polarization plane orientation of light in accordance with the magnetic field strength. A light source (light source 10) that emits light (laser beam) that interacts with alkali metal atoms to the cell, a mirror (mirror 50) that reflects the light transmitted through the cell toward the cell, A polarization separator (polarization separator 60) that separates into a plurality of polarization components including a first polarization component and a second polarization component; a first photodetector (PD70) that detects the amount of light of the first polarization component; It is a magnetic measurement apparatus which has a 2nd photodetector (PD80) which detects the light quantity of a polarization component. As long as these elements are included, the arrangement of the elements is not limited to that described in the first to fourth embodiments. The wavelength of the laser beam and the elements enclosed in the gas cell 40 are merely examples, and are not limited to those described in the embodiment. Any atom (medium) may be used as long as it changes the plane of polarization of light according to the magnetic field strength. Furthermore, the angle formed by the two polarization planes is merely an example. For example, even if the angle formed by the polarization plane is described as 90 ° in the embodiment, the angle formed by the polarization plane is not strictly 90 ° and may include an error.

  Some of the components described in the embodiments may be omitted. For example, the magnetic sensor may not have the polarizing plate 20. In addition to the components described in the embodiments, the magnetic sensor may have other components. For example, the magnetic sensor may have a computing device that includes a processor and a storage device. Recording and analysis of signals output from the PD 70 and PD 80 may be performed by this computer apparatus.

  DESCRIPTION OF SYMBOLS 1 ... Magnetic sensor, 2 ... Magnetic sensor, 3 ... Magnetic sensor, 4 ... Magnetic sensor, 10 ... Light source, 11 ... Light source, 20 ... Polarizing plate, 21 ... Polarizing plate, 30 ... Half mirror, 40 ... Gas cell, 50 ... Mirror , 51 ... mirror, 60 ... polarization separator, 61 ... polarization separator, 70 ... PD, 71 ... PD, 80 ... PD, 81 ... PD.

Claims (8)

A cell containing therein a medium that changes the plane of polarization of light according to the magnetic field strength;
A light source that emits light that interacts with the medium to the cell;
A mirror that reflects the light transmitted through the cell toward the cell;
A polarization separator that separates the light into a plurality of polarization components including a first polarization component and a second polarization component;
A first photodetector for detecting the first polarization component of the light reflected by the mirror and transmitted through the cell;
And a second photodetector for detecting the second polarization component of the light reflected by the mirror and transmitted through the cell. The magnetic measurement apparatus according to claim 1, wherein the polarization separator is disposed between the cell and the mirror on an optical path of the light. The light has a linearly polarized component when it first enters the cell;
The magnetic measurement apparatus according to claim 2, wherein polarization planes of the first polarization component and the second polarization component are orthogonal to each other. The magnetic measurement apparatus according to claim 1, wherein the polarization separator is disposed between the light source and the cell on an optical path of the light. The light has a linearly polarized component when incident on the polarization separator;
The magnetic measurement apparatus according to claim 4, wherein the polarization planes of the first polarization component and the second polarization component form an angle of 45 °. The magnetic measurement according to any one of claims 1 to 5, wherein the first photodetector and the second photodetector are arranged on the opposite side of the polarization separator as viewed from the cell. apparatus. Two sets of optical systems including the light source, the mirror, the polarization separator, the first photodetector, and the second photodetector,
The magnetic measurement apparatus according to any one of claims 1 to 6, wherein the two sets of optical systems are arranged such that optical paths in the cell are orthogonal to each other.   A biological state measurement device comprising the magnetic measurement device according to claim 1.

Images