Photoexcitation magnetic sensor and magnetoencephalograph

Abstract

To enhance the sensitivity of magnetic field intensity.SOLUTION: A photoexcitation magnetic sensor 1 comprises: a cell 2 filled with alkali metal vapor; a pump laser 4 and a first optical system 6 for making pump light incident on sensitivity regions ARa-ARd in the cell 2 along a y-axis direction; a probe laser 5 and a second optical system 7 for making probe light incident on the sensitivity regions ARa-ARd along an x-axis direction; a bias magnetic field coil 11 for applying a bias magnetic field By along the y-axis direction to the cell 2, and determining the resonant frequency of an electron spin; bias magnetic field gradient correction coils 12 and 13 for correcting the gradient of the bias magnetic field By; a tilting coil 14 for tilting the rotation axis direction of the electron spin; optical sensor pairs 43a-43d for detecting the probe light passed through the sensitivity regions ARa-ARd; and a reading circuit 10 for measuring magnetic field intensity on the basis of the output of the optical sensor pairs 43a-43d.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] One aspect of the embodiment relates to an optically excited magnetic sensor and a magnetoencephalograph including the same. [Background technology]

[0002] Optically excited magnetic sensors capable of measuring weak external magnetic fields are known (see, for example, Patent Documents 1 and 2). In optically excited magnetic sensors, alkali metal atoms inside a cell are excited by pump light, a probe light irradiated toward the cell so as to intersect with the pump light is measured by an optical sensor, and the strength of the external magnetic field is detected based on the output of the optical sensor. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] U.S. Patent Publication No. 2022 / 0091200 [Patent Document 2] U.S. Patent No. 10,782,368 Summary of the Invention [Problem to be solved by the invention]

[0004] In the optically excited magnetic sensor as described above, for example, as the sensor has become increasingly popular in recent years, there is a demand for improving the sensitivity of the magnetic field intensity to be measured. Therefore, an object of one aspect of the embodiment is to provide an optically excited magnetic sensor and a magnetoencephalograph capable of improving the sensitivity of the magnetic field intensity. [Means for solving the problem]

[0005] An optically excited magnetic sensor according to a first aspect of an embodiment is [1] "an optically excited magnetic sensor comprising: a cell in which alkali metal vapor is sealed; a pump light incident unit that incidents pump light for exciting alkali metal atoms constituting the alkali metal vapor into a sensitivity region inside the cell along a first direction; a probe light incident unit that incidents probe light for detecting changes in electron spin in an excited state of the alkali metal atoms into the sensitivity region along a direction intersecting the first direction; a bias magnetic field coil that applies a bias magnetic field along the first direction to the inside of the cell and determines a resonance frequency of the electron spin; a gradient correction coil that corrects the gradient of the bias magnetic field applied by the bias magnetic field coil; an electron spin tilting unit that tilts the direction of the rotation axis of the electron spin in a direction perpendicular to the first direction; an optical sensor that detects the probe light that has passed through the sensitivity region; and a magnetic field measurement unit that measures a magnetic field intensity related to the sensitivity region based on an output of the optical sensor."

[0006] In an optically excited magnetic sensor, the gradient of the bias magnetic field can be corrected by a gradient correction coil to improve the uniformity of the bias magnetic field and suppress the variation in the resonance frequency of the electron spins, thereby making it possible to increase the sensitivity to the magnetic field strength.

[0007] The optically excited magnetic sensor of the embodiment may be [2] "the optically excited magnetic sensor according to [1] above, wherein the gradient correction coil includes a pair of first coils arranged on one side and the other side of the cell in the first direction, and corrects the amount of change in the bias magnetic field with respect to the amount of change in position in the first direction." In this case, it is possible to improve the uniformity of the bias magnetic field with respect to the position in the first direction.

[0008] The optically excited magnetic sensor of the embodiment may be [3] "the optically excited magnetic sensor according to the above [1] or [2], wherein the gradient correction coil includes a pair of second coils arranged on one side and the other side of the cell in a second direction perpendicular to the first direction, and corrects the amount of change in the bias magnetic field with respect to the amount of change in position in the second direction." In this case, it is possible to improve the uniformity of the bias magnetic field with respect to the position in the second direction.

[0009] The optically excited magnetic sensor of the embodiment may be [4] "the optically excited magnetic sensor according to any one of the above [1] to [3], wherein the gradient correction coil includes a pair of third coils arranged on one side and the other side of the cell in the first direction, and corrects a change in the bias magnetic field with respect to a change in position in a third direction perpendicular to the first direction." In this case, it is possible to improve the uniformity of the bias magnetic field with respect to the position in the third direction.

[0010] The optically excited magnetic sensor of the embodiment may be [5] "the optically excited magnetic sensor according to any one of the above [1] to [4], wherein the gradient correction coil corrects the gradient of the bias magnetic field so that a relaxation time of free induction decay obtained from the output of the optical sensor is longer than a first time when no correction by the gradient correction coil is performed." In this case, it is possible to effectively correct the gradient of the bias magnetic field.

[0011] The optically excited magnetic sensor of the embodiment may be [6] "the optically excited magnetic sensor according to the above [5], wherein the gradient correction coil corrects the gradient of the bias magnetic field so that the relaxation time of the free induction decay obtained from the output of the optical sensor is the longest." In this case, it is possible to more effectively correct the gradient of the bias magnetic field.

[0012] The optically excited magnetic sensor of the embodiment may be [7] "the optically excited magnetic sensor according to any one of the above [1] to [4], wherein the gradient correction coil corrects the gradient of the bias magnetic field so that a peak of a spectrum after fast Fourier transform for the relaxation time of free induction decay obtained from the output of the optical sensor becomes steeper than when correction by the gradient correction coil is not performed." In this case, it is possible to effectively correct the gradient of the bias magnetic field.

[0013] The optically excited magnetic sensor of the embodiment may be [8] "the optically excited magnetic sensor according to the above [7], wherein the gradient correction coil corrects the gradient of the bias magnetic field so that the spectrum peak after fast Fourier transform for the relaxation time of free induction decay obtained from the output of the optical sensor becomes the steepest." In this case, it is possible to correct the gradient of the bias magnetic field more effectively.

[0014] The optically excited magnetic sensor of the embodiment may be [9] "the optically excited magnetic sensor according to any one of the above [1] to [8], comprising one or more mirrors that reflect the probe light traveling within the cell toward the outside of the cell, or traveling from within the cell to the outside of the cell, back into the cell." In this case, the optical path length of the probe light within the cell can be increased, making it possible to reduce, for example, the noise level caused by the probe light.

[0015] The optically excited magnetic sensor according to the embodiment may be

[10] "the optically excited magnetic sensor according to any one of [1] to [9] above, wherein the electron spin tilting part irradiates an RF signal having the same frequency as the resonance frequency." In this case, the direction of the rotation axis of the electron spin can be tilted by utilizing the RF signal.

[0016] The optically excited magnetic sensor according to the embodiment may be

[11] "the optically excited magnetic sensor according to any one of the above [1] to [9], wherein the electron spin tilting part irradiates pulsed light." In this case, the direction of the rotation axis of the electron spin can be tilted by using pulsed light.

[0017] The optically excited magnetic sensor of the embodiment may be

[12] "the optically excited magnetic sensor according to any one of the above [1] to

[11] , wherein there are two or more sensitive regions, and the magnetic field measuring unit measures the magnetic field strength based on a difference between outputs of the optical sensors corresponding to two adjacent sensitive regions." In this case, common mode noise common to the two sensitive regions is removed, and measurement of a weak magnetic field strength can be realized.

[0018] A magnetoencephalograph according to a second aspect of the embodiment is

[13] "a magnetoencephalograph comprising an optically excited magnetic sensor according to any one of [1] to

[12] above, which is positionable around a subject's head and measures the intensity of a magnetic field emitted by the subject."

[0019] This magnetoencephalograph also has the optically excited magnetic sensor, and hence has the above-mentioned advantageous effect of being able to increase the sensitivity to magnetic field strength. Effect of the Invention

[0020] According to one aspect of the present invention, it is possible to provide an optically excited magnetic sensor and a magnetoencephalograph capable of increasing the sensitivity to magnetic field strength. [Brief description of the drawings]

[0021] [Figure 1] 1 is a perspective view showing a configuration of an optically excited magnetic sensor according to an embodiment. [Diagram 2] FIG. 2 is a plan view of the first optical system in FIG. 1 as viewed in the -x-axis direction. [Diagram 3] FIG. 3 is a plan view of the second optical system, the optical sensor group, and the third optical system in FIG. 1, as viewed in the -y-axis direction. [Figure 4] FIG. 4 is a timing chart showing the generation timing of pump light, an RF signal, and a probe light during the measurement process of an external magnetic field by the readout circuit of FIG. [Diagram 5] FIG. 5 is a graph showing the change over time of the FID acquired by the readout circuit of FIG. [Figure 6]FIG. 6 is a perspective view showing the bias magnetic field gradient correction coil of FIG. [Figure 7] Fig. 7(a) is a plan view showing the second coil, and Fig. 7(b) is a plan view showing the first coil and the third coil. [Figure 8] Fig. 8(a) is a graph showing the time change of FID to explain the correction procedure by the bias magnetic field gradient correction coil. Fig. 8(b) is another graph showing the time change of FID to explain the correction procedure by the bias magnetic field gradient correction coil. [Figure 9] FIG. 9 is a perspective view showing a configuration of an optically excited magnetic sensor according to a first modified example. [Figure 10] FIG. 10 is a plan view showing the configuration of the magnetoencephalograph according to the embodiment. [Figure 11] Fig. 11(a) is a cross-sectional view showing a schematic diagram of a cell according to a modified example in which a mirror is provided, and Fig. 11(b) is a cross-sectional view showing a schematic diagram of a cell according to another modified example in which a mirror is provided. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] Hereinafter, an embodiment of the present invention will be described in detail with reference to the accompanying drawings. In the description, the same elements or elements having the same functions will be denoted by the same reference numerals, and duplicated description will be omitted.

[0023] FIG. 1 is a perspective view showing a configuration of an optically excited magnetic sensor 1 according to an embodiment. The optically excited magnetic sensor 1 is a device for measuring an external magnetic field by utilizing optical pumping. In this embodiment, the optically excited magnetic sensor 1 is described as being used for brain magnetism measurement, but the application is not limited thereto. In one example, the optically excited magnetic sensor 1 measures a brain magnetic field. For convenience of explanation, in FIG. 1, the direction along the magnetic field direction of the measurement target is the y axis, and two axes perpendicular to the y axis that constitute a three-dimensional coordinate system together with the y axis are the x axis and the z axis. As will be described later, the direction along the x axis corresponds to the incident direction of the probe light in the sensitivity region, and the direction along the y axis corresponds to the incident direction of the pump light in the sensitivity region. In the following description, the positive and negative directions along the x axis are respectively written as the +x axis direction and the -x axis direction, the positive and negative directions along the y axis are respectively written as the +y axis direction and the -y axis direction, and the positive and negative directions along the z axis are respectively written as the +z axis direction and the -z axis direction. The y-axis direction corresponds to the first direction, the z-axis direction corresponds to the second direction, and the x-axis direction corresponds to the third direction.

[0024] 1, the optically excited magnetic sensor 1 includes a cell 2, a heater 3, a pump laser 4, a probe laser 5, a first optical system 6, a second optical system 7, an optical sensor group 8, a third optical system 9, a readout circuit 10, a bias magnetic field coil 11, bias magnetic field gradient correction coils 12 and 13, and a tilting coil (electron spin tilting unit) 14. The optically excited magnetic sensor 1 is an orthogonal type sensor. The components of the optically excited magnetic sensor 1 will be described in detail below.

[0025] The cell 2 is a container that seals alkali metal vapor. The cell 2 is arranged along the z-axis direction. The cell 2 has a substantially rectangular parallelepiped and bottomed cylindrical shape with a surface substantially parallel to the xz plane. The cross section of the cell 2 in a direction perpendicular to the longitudinal direction of the cell 2 (direction along the xy plane) is, for example, a square. The cell 2 may be made of, for example, quartz, sapphire, silicon, Kovar glass, borosilicate glass, or other material. The cell 2 is optically transparent to pump light and probe light, which will be described later. A heater 3 and the like are attached to the side surface of the cell 2 in the -y-axis direction. A magnetic field Bm of the measurement object generated from the measurement object is incident on the side surface of the cell 2 in the -y-axis direction along the +y-axis direction.

[0026] The alkali metal constituting the alkali metal vapor sealed in the cell 2 may be at least one of lithium (Li), sodium (Na), potassium (K), rubidium (Rb), and cesium (Cs). For example, the alkali metal may be potassium and rubidium, or may be potassium alone. Potassium has a relatively small spin-destruction collision relaxation rate among alkali metals used in optically excited magnetic sensors. The spin-destruction collision relaxation rate of potassium is smaller than, for example, cesium and rubidium. Therefore, when a single alkali metal is adopted, an optically excited magnetic sensor using only potassium has a higher sensitivity than an optically excited magnetic sensor using only cesium or only rubidium.

[0027] The cell 2 also contains a fill gas. The fill gas suppresses relaxation of the spin polarization of the alkali metal vapor. The fill gas also protects the alkali metal vapor and suppresses noise emission. The fill gas may be, for example, an inert gas such as helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), nitrogen (N2), etc. The fill gas may be, for example, helium and nitrogen.

[0028] As described above, the heater 3 is attached to the cell 2. The heater 3 generates heat in response to a current supplied from a heater power supply (not shown). The heater 3 controls the internal temperature of the cell 2 to control the vapor density of the alkali metal. For example, when potassium is contained as the alkali metal in the cell 2, the heater 3 heats the cell 2 so that the internal temperature of the cell 2 becomes 120°C.

[0029] The pump laser 4 emits pump light in the -y-axis direction for exciting the alkali metal atoms. That is, the pump laser 4 emits linearly polarized pump light, which is converted to circularly polarized light by the quarter-wave plates 27a to 27d. The alkali metal atoms contained in the cell 2 are excited by the pump light in a circularly polarized state, and spin polarization is generated. The wavelength of the pump light is set according to the type of atoms constituting the alkali metal vapor (more specifically, the wavelength of the absorption line). For example, when the atoms constituting the alkali metal vapor are potassium, the wavelength of the pump light is set to 770.11 nm in accordance with the resonance line of potassium. When the alkali metal atoms contained in the cell 2 are potassium and rubidium, the pump laser 4 may emit pump light for exciting the rubidium atoms and transferring the spin polarization of the rubidium atoms to the potassium atoms. In this case, the rubidium atoms are excited by the pump light. Then, due to the spin exchange interaction between potassium and rubidium, the spin polarization of the rubidium atoms is transferred to the potassium atoms, and the potassium atoms are excited.

[0030] Pump light from the pump laser 4 is incident on the cell 2 via the first optical system 6. Fig. 2 is a plan view of the first optical system 6 as viewed in the -x-axis direction. The first optical system 6 includes a lens 21, a half-wave plate 22, polarizing beam splitters 23a, 23b, and 23c, half-wave plates 24a and 24b, a total reflection mirror 25, polarizers 26a, 26b, 26c, and 26d, and quarter-wave plates 27a, 27b, 27c, and 27d.

[0031] The lens 21 is provided adjacent to the pump laser 4 in the -y-axis direction and collects the pump light emitted from the pump laser 4. The half-wave plate 22 is provided adjacent to the lens 21 in the -y-axis direction and rotates the polarization plane so that the power of the pump laser 4 is distributed at 1:3 by the polarizing beam splitter 23a.

[0032] The polarizing beam splitter 23a is provided adjacent to the half-wave plate 22 in the -y-axis direction, and separates two linearly polarized components perpendicular to each other from the polarized components of the pump light transmitted through the half-wave plate 22, transmits one linearly polarized component in the -y-axis direction, and reflects the other linearly polarized component in the -z-axis direction. The polarizing beam splitters 23b and 23c and the total reflection mirror 25 are provided in sequence in the -z-axis direction relative to the polarizing beam splitter 23a. The polarizing beam splitters 23b and 23c each separate two linearly polarized components perpendicular to each other from the pump light transmitted through the polarizing beam splitters 23a and 23b, transmits one linearly polarized component in the -z-axis direction, and reflects the other linearly polarized component in the -y-axis direction. The total reflection mirror 25 reflects the linearly polarized pump light transmitted through the polarizing beam splitter 23c in the -y-axis direction.

[0033] The half-wave plate 24a is provided between the polarizing beam splitter 23a and the polarizing beam splitter 23b, and rotates the polarization plane of the linearly polarized pump light reflected by the polarizing beam splitter 23a. This allows the polarizing beam splitter 23b to split the pump light into two linearly polarized polarization components. The half-wave plate 24b is provided between the polarizing beam splitter 23b and the polarizing beam splitter 23c, and rotates the polarization plane of the linearly polarized pump light transmitted through the polarizing beam splitter 23b. This allows the polarizing beam splitter 23c to split the pump light into two linearly polarized polarization components.

[0034] The polarizer 26a and the quarter-wave plate 27a are provided adjacent to the polarizing beam splitter 23a in the -y-axis direction. The polarizer 26a transmits a specific linearly polarized component of the pump light transmitted through the polarizing beam splitter 23a, and the quarter-wave plate 27a changes the polarization state of the pump light transmitted through the polarizer 26a to circular polarization and transmits it in the -y-axis direction as pump light PLa.

[0035] The polarizers 26b, 26c, and 26d and the quarter-wave plates 27b, 27c, and 27d are disposed adjacent to the polarizing beam splitters 23b and 23c and the total reflection mirror 25 in the -y-axis direction. The polarizers 26b, 26c, and 26d and the quarter-wave plates 27b, 27c, and 27d have the same functions as the polarizer 26a and the quarter-wave plate 27a, respectively, and transmit the pump light PLb, PLc, and PLd in the -y-axis direction.

[0036] The first optical system 6 configured as described above is configured so that the four systems of pump light PLa-PLd transmitted through the four ¼ wavelength plates 27a, 27b, 27c, and 27d can be incident on the sensitivity regions ARa-ARd inside the cell 2, respectively. The pump laser 4 and the first optical system 6 configure a pump light incident section that makes the pump light PLa-PLd incident on the sensitivity regions ARa-ARd along the -y-axis direction.

[0037] The probe laser 5 emits a probe light for detecting the precession of the spin when the electron spin in the excited state of the alkali metal atom is tilted by 90 degrees. That is, the probe laser 5 emits a linearly polarized probe light. When the probe light passes through the alkali metal vapor, the magnetic rotation angle of the probe light changes due to the influence of the spin polarization state of the alkali metal atom. By detecting the change in the magnetic rotation angle, the state of the spin precession can be derived. The wavelength of the probe light is set according to the type of atoms constituting the alkali metal vapor (more specifically, the wavelength of the absorption line). For example, when only potassium is contained as the alkali metal in the cell 2, the wavelength of the probe light is detuned from the wavelength of the pump light (for example, 770.11 nm) and is set to, for example, about 770.05 nm. By detuning the wavelength of the probe light from the wavelength of the pump light, the absorption of the probe light by potassium is suppressed.

[0038] When potassium and rubidium are contained in the cell 2 as alkali metals, the probe laser 5 may emit a probe light for detecting a change in the magnetic rotation angle caused by the spin polarization in the excited state of potassium atoms. The density of rubidium used for excitation is set to be smaller than the density of potassium used for the probe. When the density of rubidium is smaller than the density of potassium, attenuation of the pump light due to excitation is suppressed. This allows the optically excited magnetic sensor 1 to obtain uniform sensitivity within the cell 2.

[0039] The probe light from the probe laser 5 is incident on the cell 2 via the second optical system 7. Fig. 3 is a plan view of the second optical system 7, the optical sensor group 8, and the third optical system 9 as viewed in the -y-axis direction. The second optical system 7 includes half-wave plates 31a, 31b, and 31c, polarizing beam splitters 32a, 32b, and 32c, a total reflection mirror 33, and polarizers 34a, 34b, 34c, and 34d.

[0040] The half-wave plate 31a is provided adjacent to the probe laser 5 in the -z-axis direction, and rotates the polarization plane so that the power of the probe laser 5 is distributed 1:3 by the polarizing beam splitter 32a. The polarizing beam splitters 32a, 32b, and 32c are provided in sequence in the -z-axis direction relative to the half-wave plate 31a, and separate two linearly polarized components perpendicular to each other from the polarized component of the probe light transmitted through the half-wave plate 31a, transmit one linearly polarized component in the -z-axis direction, and reflect the other linearly polarized component in the -x-axis direction. The half-wave plate 31b is provided between the polarizing beam splitter 32a and the polarizing beam splitter 32b, and rotates the polarization plane of the linearly polarized probe light transmitted through the polarizing beam splitter 32a. This allows the polarizing beam splitter 32b to separate the probe light into two linearly polarized polarized components. The half-wave plate 31c is provided between the polarizing beam splitter 32b and the polarizing beam splitter 32c, and rotates the polarization plane of the linearly polarized probe light transmitted through the polarizing beam splitter 32b. This allows the polarizing beam splitter 32c to split the probe light into two linearly polarized polarization components. The total reflection mirror 33 reflects the linearly polarized probe light transmitted through the polarizing beam splitter 32c in the -x-axis direction.

[0041] The polarizers 34a, 34b, 34c, and 34d transmit specific linearly polarized components of the probe light reflected by the polarizing beam splitters 32a, 32b, and 32c and the total reflection mirror 33, respectively.

[0042] The second optical system 7 configured as above is configured so that the four systems of probe light QLa-QLd transmitted through the four polarizers 34a, 34b, 34c, 34d can be incident on the sensitivity regions ARa-ARd inside the cell 2, respectively. The sensitivity regions ARa-ARd are regions spaced apart from each other in the longitudinal direction (z-axis direction) inside the cell 2. The sensitivity regions ARa-ARd are regions that intersect with the pump light PLa-PLd inside the cell 2. The sensitivity regions ARa-ARd are intersection regions where the probe light QLa-QLd and the pump light PLa-PLd intersect. The probe laser 5 and the second optical system 7 thus configured constitute a probe light incident section that makes the probe light QLa-QLd incident on the sensitivity regions ARa-ARd along the x-axis direction.

[0043] Referring to FIG. 1, the bias magnetic field coil 11 is a coil that applies a bias magnetic field By in the y-axis direction inside the cell 2, and determines the resonance frequency of the electron spin of the alkali metal atom excited inside the cell 2. The bias magnetic field coil 11 applies a bias magnetic field By in the same direction as the incident direction of the pump light to the sensitivity areas ARa to ARd inside the cell 2. This aligns the rotation axis of the electron spin of the alkali metal atom excited inside the cell 2 in the y-axis direction. When the magnetic field strength of the bias magnetic field applied by the bias magnetic field coil 11 is 14 μT and the alkali metal atom is potassium, the resonance frequency of the electron spin is 100 kHz. 14 μT is a magnetic field strength that can be easily formed.

[0044] The bias magnetic field gradient correction coils 12, 13 are a coil group for correcting the gradients of the bias magnetic field By in the x-axis direction, the y-axis direction, and the z-axis direction. The bias magnetic field gradient correction coils 12, 13 may be configured to correct the gradient of the bias magnetic field By in one or two directions, the x-axis direction, the y-axis direction, or the z-axis direction, or may be configured to correct the gradient in three directions. The presence of the bias magnetic field gradient correction coils 12, 13 makes it possible to correct the bias magnetic field By to be uniform even when it is not uniform due to the influence of an environmental magnetic field, etc., and thus makes it possible to increase the detection sensitivity of the optically excited magnetic sensor 1 to an external magnetic field.

[0045] The tilting coil 14 generates and irradiates an RF signal Rf in order to tilt the rotation axis direction of the electron spin of the alkali metal atom excited inside the cell 2 in a direction perpendicular to the incident direction of the pump light. Specifically, the tilting coil 14 generates an RF signal Rf with the same frequency as the resonance frequency of the electron spin (100 kHz when the strength of the bias magnetic field By is 14 μT and the alkali metal atom is potassium) and irradiates the RF signal Rf in the -z axis direction with the intensity and length required to tilt the electron spin by 90°. Note that, instead of the tilting coil 14, a means for irradiating another pulsed pump light from a direction perpendicular to the incident direction of the pump light may be provided as an electron spin tilting unit that tilts the direction of the electron spin.

[0046] Returning to FIG. 3, the configurations of the optical sensor group 8 and the third optical system 9 will be described.

[0047] The third optical system 9 includes half-wave plates 41a, 41b, 41c, and 41d, and polarizing beam splitters 42a, 42b, 42c, and 42d. The half-wave plates 41a, 41b, 41c, and 41d are provided adjacent to the cell 2 in the −x-axis direction on the optical paths of the probe lights QLa, QLb, QLc, and QLd that have passed through the sensitivity regions ARa to ARd of the cell 2, and rotate the polarization plane of the linearly polarized light of the probe lights QLa, QLb, QLc, and QLd that have passed through the cell 2. These half-wave plates 41a, 41b, 41c, and 41d are supported rotatably about an axis along the x-axis so that the rotation angle of the polarization plane of the probe light can be adjusted. The polarizing beam splitters 42a, 42b, 42c, and 42d separate two mutually perpendicular linearly polarized components from the polarization components of the probe light QLa, QLb, QLc, and QLd that have passed through the half-wave plates 41a, 41b, 41c, and 41d, respectively, and transmit one linearly polarized component in the -x-axis direction and reflect the other linearly polarized component in the y-axis direction.

[0048] The optical sensor group 8 is an element group that detects the polarization plane angles of the probe light QLa, QLb, QLc, and QLd that have passed through the sensitivity regions ARa to ARd, and is configured to include four optical sensor pairs (optical sensors) 43a, 43b, 43c, and 43d. The optical sensor pair 43a has two photodiodes 44a and 45a, and the photodiode 44a outputs a detection signal that detects the intensity of the other linearly polarized component of the probe light QLa, and the photodiode 45a outputs a detection signal that detects the intensity of the other linearly polarized component of the probe light QLa. Similarly, the optical sensor pairs 43b, 43c, and 43d each have two photodiodes 44b and 45b, two photodiodes 44c and 45c, and two photodiodes 44d and 45d, respectively. The two photodiodes 44b, 45b each output a detection signal detecting the intensities of the two linearly polarized components of the probe light QLb, the two photodiodes 44c, 45c each output a detection signal detecting the intensities of the two linearly polarized components of the probe light QLc, and the two photodiodes 44d, 45d each output a detection signal detecting the two linearly polarized components of the probe light QLd.

[0049] 1 again, the readout circuit 10 is electrically connected to the four optical sensor pairs 43a, 43b, 43c, and 43d constituting the optical sensor group 8, and is a processing circuit that processes the detection signals output from the four optical sensor pairs 43a, 43b, 43c, and 43d. That is, the readout circuit 10 includes a reading unit that reads the detection signals (outputs of the optical sensor pairs 43a, 43b, 43c, and 43d) output from the four optical sensor pairs 43a, 43b, 43c, and 43d, and a measuring unit that performs measurement processing of the external magnetic field related to the sensitivity regions ARa to ARd based on the detection signals. However, the measuring unit may be provided outside the optically excited magnetic sensor 1 and configured to perform measurement processing based on the detection signals output from the optically excited magnetic sensor 1. The readout circuit 10 constitutes a magnetic field measuring unit.

[0050] The readout circuit 10 is physically configured to include a memory such as a RAM or a ROM, a processor (arithmetic circuit) such as a CPU, a communication interface, and an auxiliary storage device such as a hard disk or a semiconductor memory. The readout circuit 10 can be realized by, for example, a personal computer, a cloud server, a smartphone, a tablet terminal, etc. The function of the readout circuit 10 is realized by executing a program stored in the memory by the CPU of a computer system.

[0051] The function of the measurement processing of the external magnetic field by the measurement unit of the readout circuit 10 will be described with reference to Fig. 4 and Fig. 5. Fig. 4 shows the readout timing of the pump light, the RF signal, and the probe light during the measurement processing of the external magnetic field by the readout circuit 10, with part (a) showing the generation timing of the pump light, part (b) showing the generation timing of the RF signal, and part (c) showing the readout timing of the probe light. Fig. 5 is a graph showing the time change of FID (Free Induction Decay) acquired by the readout circuit 10.

[0052] During the measurement process of the external magnetic field by the readout circuit 10, a control circuit (not shown) controls the pump light to be repeatedly turned on / off in a pulsed manner (part (a) of FIG. 5), the RF signal is irradiated in a pulsed manner immediately after the pump light transitions from on to off (part (b) of FIG. 5), and the probe light is read out during a predetermined relaxation time after the irradiation of the RF signal (part (c) of FIG. 5). In such a control state, the readout circuit 10 acquires detection signals output from the four optical sensor pairs 43a, 43b, 43c, and 43d during the relaxation time. Then, the readout circuit 10 acquires the FID by taking the difference between the detection signals of the two photodiodes 44a and 45a constituting the optical sensor pair 43a (FIG. 5). Similarly, the readout circuit 10 acquires the time change of the FID by taking the difference between the detection signals of the two photodiodes for the optical sensor pairs 43b, 43c, and 43d.

[0053] The FID indicates how the electron spin of an alkali metal atom relaxes, and the frequency of the precession of the electron spin changes in response to fluctuations in the external magnetic field in the sensitivity region ARa, so that the frequency of the oscillation changes with fluctuations in the external magnetic field. For example, if the resonance frequency of the electron spin is 100 kHz and the external magnetic field changes from 0 pT to 1 pT, the frequency of the precession changes by about 0.007 Hz. The readout circuit 10 utilizes this property to calculate the following equation for the FID waveform at the beginning of the relaxation time:

number

[0054] In the optically excited magnetic sensor 1 described above, pump light is emitted to the cell 2 in which alkali metal vapor is sealed, thereby generating (exciting) electron spins of alkali metal atoms. Probe light QLa-QLd is emitted to the sensitivity regions ARa-ARd inside the cell 2, and the optical sensor pairs 43a-43d detect the polarization plane angles of the probe light QLa-QLd that have passed through the sensitivity regions ARa-ARd. The readout circuit 10 generates detection signals (measurement signals) for measuring the strength of the external magnetic field in the sensitivity regions ARa-ARd based on the outputs of the optical sensor pairs 43a-43d, and acquires the FIDs. The readout circuit 10 performs fitting using a function V on the waveform of each FID, and acquires the frequency a1 of each FID as a measurement value of the magnetic field Bm in the -y-axis direction in the sensitivity regions ARa-ARd.

[0055] Next, the bias magnetic field gradient correction coils 12 and 13 will be specifically described.

[0056] For example, in an environment without a magnetic shield, even if the bias magnetic field By is applied uniformly by the bias magnetic field coil 11, it may be difficult to form a uniform magnetic field due to the influence of the environmental magnetic field. If the bias magnetic field By is not uniform, the resonance frequency of the alkali atoms will shift, shortening the relaxation time of the FID. As a result, the fitting accuracy will be insufficient, and high sensitivity may not be achieved. Therefore, the optically excited magnetic sensor 1 is provided with bias magnetic field gradient correction coils (gradient correction coils) 12 and 13 that correct the gradient of the bias magnetic field By by the bias magnetic field coil 11.

[0057] As shown in FIG. 1, FIG. 6 and FIG. 7(a), the bias magnetic field gradient correction coil 12 is a parallel plate type coil and includes a second coil 12A. The second coil 12A is provided on each of a pair of substrates facing each other in the z-axis direction via the cell 2. The second coil 12A is disposed on one side and the other side of the cell 2 in the z-axis direction. The second coil 12A corrects the amount of change in the bias magnetic field By corresponding to the amount of change in position in the z-axis direction. In the illustrated example, the second coil 12A is a parallel four-wire coil.

[0058] As shown in FIG. 1, FIG. 6 and FIG. 7(b), the bias magnetic field gradient correction coil 13 is a parallel plate type coil and includes a first coil 13A and a third coil 13B. The first coil 13A and the third coil 13B are provided on a pair of substrates facing each other in the y-axis direction via the cell 2. The first coil 13A is disposed on one side and the other side of the cell 2 in the y-axis direction. The first coil 13A corrects the amount of change in the bias magnetic field By corresponding to the amount of change in the position in the x-axis direction. In the illustrated example, the first coil 13A is a parallel four coil. The third coil 13B is disposed on one side and the other side of the cell 2 in the y-axis direction. The third coil 13B corrects the amount of change in the bias magnetic field By corresponding to the amount of change in the position in the y-axis direction. In the illustrated example, the first coil 13A is a Maxell coil.

[0059] The first to third coils 13A, 12A, and 13B are coils that correct the primary gradient magnetic field. The first to third coils 13A, 12A, and 13B may be coils that further correct the secondary gradient magnetic field, in which case the uniformity of the bias magnetic field By can be further improved. The arrows shown in the first to third coils 13A, 12A, and 13B in FIG. 6 indicate the direction of the applied current.

[0060] Such bias magnetic field gradient correction coils 12, 13 correct the gradient of the bias magnetic field By so that the relaxation time TH (see FIG. 8(b)) of the FID is longer than a first time TH1 (see FIG. 8(a)) when no correction is made by the bias magnetic field gradient correction coils 12, 13. In this embodiment, the bias magnetic field gradient correction coils 12, 13 correct the gradient of the bias magnetic field By so that the relaxation time of the FID is the longest.

[0061] An example of a correction procedure for correcting the gradient of the bias magnetic field By using the bias magnetic field gradient correction coils 12, 13 will be specifically described. First, the FID is monitored in the absence of the magnetic field Bm to be measured. The FID can be monitored with an oscilloscope or the like. While observing the spectrum after fast Fourier transform (hereinafter also referred to as the "FFT spectrum") of the FID, the current flowing through the bias magnetic field coil 11 is adjusted so that the peak position is at a desired frequency (for example, 100 kHz).

[0062] Next, current is applied to the first to third coils 13A, 12A, and 13B, and the current value is adjusted so that the relaxation time TH of the FID is the longest. At this time, if the peak position in the FFT spectrum of the FID is shifted from the desired frequency, the current flowing through the bias magnetic field coil 11 is adjusted again. This is repeated as necessary to complete the correction of the gradient of the bias magnetic field By.

[0063] As described above, in this embodiment, the bias magnetic field By gradient correction coils 12 and 13 are used to correct the bias magnetic field By gradient, thereby improving the uniformity of the bias magnetic field By and suppressing the variation in the resonance frequency of the electron spins. This makes it possible to improve the sensitivity of the magnetic field strength.

[0064] In this embodiment, the bias magnetic field gradient correction coil 13 includes a first coil 13A. In this case, it is possible to improve the uniformity of the bias magnetic field By with respect to the position in the x-axis direction. In this embodiment, the bias magnetic field gradient correction coil 12 includes a second coil 12A. In this case, it is possible to improve the uniformity of the bias magnetic field By with respect to the position in the z-axis direction. In this embodiment, the bias magnetic field gradient correction coil 13 includes a third coil 13B. In this case, it is possible to improve the uniformity of the bias magnetic field By with respect to the position in the y-axis direction.

[0065] In this embodiment, the bias magnetic field gradient correction coils 12 and 13 correct the gradient of the bias magnetic field By so that the relaxation time of the FID is longer than the first time TH1. In this case, it is possible to effectively correct the gradient of the bias magnetic field By.

[0066] In this embodiment, the bias magnetic field gradient correction coils 12 and 13 correct the gradient of the bias magnetic field By so as to maximize the relaxation time of the FID. In this case, it is possible to correct the gradient of the bias magnetic field By more effectively.

[0067] In this embodiment, a bias magnetic field coil 11 is provided that applies a bias magnetic field By in the same direction as the pump light and determines the resonance frequency of the electron spin, and a tilting coil 14 is provided that tilts the direction of the electron spin in a direction perpendicular to the pump light. In this case, the strength of the external magnetic field is measured based on the frequency of the change in the angle of the polarization plane of the probe light detected using the detection signal, so that the measurement sensitivity of the external magnetic field can be maintained without being affected by the environmental magnetic field.

[0068] In this embodiment, a tilting coil 14 is used that irradiates an RF signal with the same frequency as the resonance frequency. In this case, the direction of the rotation axis of the electron spin can be tilted by utilizing the RF signal. By simple means, it is possible to realize measurement of an external magnetic field based on the frequency of the change in the polarization plane angle of the probe light. Note that in this embodiment, a light source that irradiates pulsed light can also be used. In this case, the direction of the rotation axis of the electron spin can be tilted by utilizing the pulsed light. By simple means, it is possible to realize measurement of an external magnetic field based on the frequency of the change in the polarization plane angle of the probe light.

[0069] In this embodiment, the sensitivity regions ARa to ARd are divided into four in the z-axis direction. Therefore, common mode noise can be removed by obtaining the difference between the measurement values ​​of the external magnetic field obtained for two adjacent sensitivity regions among the four divided sensitivity regions ARa to ARd. That is, in this embodiment, there are two or more sensitivity regions, and the readout circuit 10 can measure the magnetic field strength based on the difference between the outputs of the optical sensors corresponding to the two adjacent sensitivity regions. In this case, common mode noise common to the two adjacent sensitivity regions is removed, and measurement of a weak magnetic field strength can be realized, and the sensitivity of the magnetic field strength can be further improved. The effect of increasing the sensitivity is particularly noticeable when there is no magnetic shield, and for example, the detection limit of 20 pT / rHz can be reduced by about two orders of magnitude to 300 ft / rHz or less.

[0070] In this embodiment, it is important to widen the sensitivity regions ARa to ARd in order to improve the sensitivity. In this respect, the inner diameter of the cell 2 is preferably 10×10 mm, and the diameter of the pump light is preferably 10 mm and the diameter of the probe light is preferably 5 mm. In the case where four sensitivity regions ARa to ARd are provided at intervals of 10 mm in the orthogonal type optically excited magnetic sensor 1, it is preferable to shape the pump light into a substantially rectangular shape (for example, 10 mm×5 mm) due to the size limitations of the polarizing beam splitters 23a, 23b, and 23c that split the pump light, and to enlarge the area that intersects with the probe light while restricting the probe light in the direction perpendicular to the area.

[0071] The optically excited magnetic sensor 1 of this embodiment constitutes a scalar magnetic sensor. A scalar magnetic sensor has the great advantage of being able to measure a magnetic field without a magnetic shield, but if the uniformity of the bias magnetic field By is poor, the sensitivity may be easily reduced. According to this embodiment, the uniformity of the bias magnetic field By is improved, the relaxation time of the FID is lengthened, and high sensitivity of, for example, 300 fT / fHz or less can be achieved without a magnetic shield. By using a scalar magnetic sensor, the need for an expensive magnetic shield can be reduced.

[0072] Various embodiments of the present invention have been described above, but the present invention is not limited to the above-described embodiments, and may be modified or applied to other things without departing from the spirit of the invention as described in each claim.

[0073] 9 is a perspective view showing the configuration of a photo-excited magnetic sensor 1A according to a first modified example. The photo-excited magnetic sensor 1A differs from the photo-excited magnetic sensor 1 in that the incident direction of the pump light to the cell 2 and the application direction of the bias magnetic field By are set in the z-axis direction, and the irradiation direction of the RF signal Rf is set in the x-axis direction. Only the configuration different from the photo-excited magnetic sensor 1 will be described below.

[0074] The first optical system 6 includes a lens 21, a half-wave plate 22, a polarizing beam splitter 23a, and a quarter-wave plate 27a. The polarizing beam splitter 23a is provided adjacent to the half-wave plate 22 in the -y-axis direction, and reflects the linearly polarized component of the pump light transmitted through the half-wave plate in the -z-axis direction. The quarter-wave plate 27a is provided adjacent to the polarizing beam splitter 23a in the -z direction, and changes the linearly polarized pump light reflected by the polarizing beam splitter 23a to circularly polarized light, and transmits the pump light in the -z-axis direction. The first optical system 6 configured as described above is configured so that one system of pump light can be incident on the inside of the cell 2 in the longitudinal direction (z-axis direction).

[0075] The second optical system 7 includes the half-wave plates 31a, 31b, and 31c, the polarizing beam splitters 32a, 32b, and 32c, the total reflection mirror 33, the polarizers 34a, 34b, 34c, and 34d, as well as a lens 35, a half-wave plate 36, and a total reflection mirror 37. The lens 35 is provided adjacent to the probe laser 5 in the -y-axis direction, and collects the probe light emitted from the probe laser 5 in the -y-axis direction. The total reflection mirror 37 is provided adjacent to the half-wave plate 36 in the -y-axis direction, and reflects the probe light transmitted through the half-wave plate 36 in the -z-axis direction toward the half-wave plate 31a. The second optical system 7 thus configured is configured such that the four probe light beams QLa to QLd transmitted through the four polarizers 34a, 34b, 34c, and 34d can be incident on four sensitivity regions that are aligned in the longitudinal direction inside the cell 2 and intersect with the pump light PL.

[0076] According to the axial type photoexcitation magnetic sensor 1A of the first modification, it is possible to obtain the measurement value of the magnetic field Bm in the z-axis direction, which is the incident direction of the pump light, for four sensitivity areas divided along the z-axis direction. The photoexcitation magnetic sensor 1A also has the above-mentioned advantageous effect of being able to increase the sensitivity of the magnetic field strength. Although the photoexcitation magnetic sensor 1A has four sensitivity areas, it may be changed to a configuration having two sensitivity areas. Even in this case, it is possible to remove common mode noise and increase the detection accuracy by obtaining the difference between the measurement values ​​for the two sensitivity areas. The number of sensitivity areas may be one, three, five or more.

[0077] FIG. 10 is a plan view showing the configuration of a magnetoencephalograph 100 according to an embodiment. The magnetoencephalograph 100 includes a plurality of optically excited magnetic sensors 1 and a plurality of optically excited magnetic sensors 1A that are arranged around the head (subject) S of a subject. Two of the plurality of optically excited magnetic sensors 1 are arranged so that their four sensitive regions are arranged in parallel in a predetermined direction (y-axis direction) at positions close to and apart from the head S. Two of the plurality of optically excited magnetic sensors 1A are also arranged so that their four sensitive regions are arranged in parallel in a direction (x-axis direction) approximately perpendicular to the arrangement direction of the sensitive regions in the optically excited magnetic sensor 1 at positions close to and apart from the head S. In addition, a bias magnetic field coil 11, bias magnetic field gradient correction coils 12, 13, and a tilting coil 14 are shared between the plurality of optically excited magnetic sensors 1 and the plurality of optically excited magnetic sensors 1A. A bias magnetic field By is applied in the x-axis direction by the bias magnetic field coil 11. The optically excited magnetic sensors 1 and 1A measure the strength of the magnetic field emitted from the head S.

[0078] The magnetoencephalograph 100 having the above configuration can obtain measurement values ​​of weak magnetic fields from the head S. Specifically, according to the arrangement example of Fig. 11, measurement values ​​of the magnetic field in the x-axis direction in 8 x 4 = 32 sensitivity regions can be obtained. Also, by obtaining the difference between the measurement value of a sensitivity region close to the head S and the measurement value of a sensitivity region adjacent to the outside of the head S with respect to the sensitivity region, it is possible to measure weak brain magnetism from which common mode noise has been removed.

[0079] In addition, the magnetoencephalograph 100 may also be configured such that the optically excited magnetic sensors 1 and 1A are arranged only in the vicinity of the head S, and are not arranged in parallel. In this case, a weak brain magnetism from which common mode noise has been removed can be measured by obtaining the difference between the measurement values ​​of two adjacent sensitivity areas of one optically excited magnetic sensor 1, 1A. The magnetoencephalograph 100 also has the above-mentioned effect of being able to increase the sensitivity of the magnetic field strength, since it is provided with the optically excited magnetic sensors 1, 1A.

[0080] The optically excited magnetic sensor 1, 1A of the above embodiment may include a mirror 55 that travels from inside the cell 2 to outside the cell 2, as shown in FIG. 11(a). The mirror 55 may be a metal film such as aluminum deposited on the inner wall of the cell 2, or a dielectric multilayer mirror. The mirror 55 may be provided on the outer wall of the cell 2. Alternatively, the optically excited magnetic sensor 1, 1A of the above embodiment may include one or more mirrors 55A that reflect the probe light QL that travels from inside the cell 2 to outside the cell 2 back into the cell 2, as shown in FIG. 11(b). The mirror 55A is installed outside the cell 2. In this way, by reflecting the probe light QL multiple times using the mirrors 55, 55A, the optical path length of the probe light QL in the cell 2 can be increased, the rotation angle (Faraday rotation angle) of the polarization plane of the probe light QL becomes larger, and the response of the readout circuit 10 becomes larger. As a result, it is possible to relatively reduce the noise level caused by the probe light QL and the readout circuit 10, for example.

[0081] In the above embodiment, the bias magnetic field gradient correction coils 12, 13 may correct the gradient of the bias magnetic field By so that the peak of the FFT spectrum for the relaxation time of the FID becomes steeper than when the bias magnetic field gradient correction coils 12, 13 are not corrected. In this case, it is possible to effectively correct the gradient of the bias magnetic field By. In the above embodiment, the bias magnetic field gradient correction coils 12, 13 may correct the gradient of the bias magnetic field By so that the peak of the FFT spectrum for the relaxation time of the FID becomes the steepest. In this case, it is possible to more effectively correct the gradient of the bias magnetic field By. In the above embodiment, linearly polarized light may be converted into circularly polarized light before use, but circularly polarized light may be used from the beginning without the conversion. [Explanation of symbols]

[0082] 1,1A...optically excited magnetic sensor, 2...cell, 4...pump laser (pump light input section), 5...probe laser (probe light input section), 6...first optical system (pump light input section), 7...second optical system (probe light input section), 8...optical sensor group, 9...third optical system, 10...readout circuit (magnetic field measurement section), 11...bias magnetic field coil, 12,13...bias magnetic field gradient correction coil (gradient correction coil), 12A...second coil, 13A...first coil, 13B...third coil, 14...tilting coil (electron spin tilt section), 43a to 43d...optical sensor pair (optical sensor), 55...mirror, 55A...mirror, 100...magnetoencephalograph, ARa to ARd...sensitivity region, Bm...magnetic field, By...bias magnetic field, PL,PLa to PLd...pump light, QL,QLa to QLd...probe light, Rf...RF signal, S...head (subject).

Claims

1. A cell containing alkali metal vapor; a pump light incidence section that causes pump light for exciting alkali metal atoms constituting the alkali metal vapor to be incident along a first direction into a sensitive region inside the cell; a probe light incidence unit that causes a probe light for detecting a change in electron spin in an excited state of the alkali metal atom to be incident on the sensitivity region along a direction intersecting with the first direction; a bias magnetic field coil that applies a bias magnetic field along the first direction to the inside of the cell and determines a resonance frequency of the electron spins; a gradient correction coil for correcting the gradient of the bias magnetic field applied by the bias magnetic field coil; an electron spin tilting unit that tilts a rotation axis direction of the electron spin in a direction perpendicular to the first direction; an optical sensor that detects the probe light that has passed through the sensitive area; and a magnetic field measuring unit that measures a magnetic field strength related to the sensitivity region based on an output of the optical sensor.

2. The gradient correction coil is 2. The optically excited magnetic sensor according to claim 1, further comprising a pair of first coils arranged on one side and the other side of the cell in the first direction, for compensating for a change in the bias magnetic field relative to a change in position in the first direction.

3. The gradient correction coil is 3. The optically excited magnetic sensor according to claim 1, further comprising a pair of second coils arranged on one side and the other side of the cell in a second direction perpendicular to the first direction, and configured to correct a change in the bias magnetic field relative to a change in position in the second direction.

4. The gradient correction coil is 3. The optically excited magnetic sensor according to claim 1, further comprising a pair of third coils arranged on one side and the other side of the cell in the first direction, for compensating for a change in the bias magnetic field relative to a change in position in a third direction perpendicular to the first direction.

5. 3. The optically excited magnetic sensor according to claim 1, wherein the gradient correction coil corrects the gradient of the bias magnetic field so that a relaxation time of free induction decay obtained from an output of the optical sensor is longer than a first time when no correction by the gradient correction coil is performed.

6. 6. The optically excited magnetic sensor according to claim 5, wherein the gradient correction coil corrects the gradient of the bias magnetic field so that a relaxation time of free induction decay obtained from an output of the optical sensor is maximized.

7. 3. The optically excited magnetic sensor according to claim 1, wherein the gradient correction coil corrects a gradient of the bias magnetic field so that a peak of a spectrum after a fast Fourier transform for a relaxation time of free induction decay obtained from an output of the optical sensor becomes steeper than when correction by the gradient correction coil is not performed.

8. 8. The optically excited magnetic sensor according to claim 7, wherein the gradient correction coil corrects the gradient of the bias magnetic field so that a spectrum peak after fast Fourier transform for a relaxation time of free induction decay obtained from the output of the optical sensor becomes the steepest.

9. 3. The optically excited magnetic sensor according to claim 1, further comprising one or more mirrors that reflect the probe light traveling within the cell toward the outside of the cell, or traveling from within the cell to the outside of the cell, back into the cell.

10. 3. The optically excited magnetic sensor according to claim 1, wherein the electron spin tilting portion irradiates an RF signal having the same frequency as the resonance frequency.

11. 3. The optically excited magnetic sensor according to claim 1, wherein the electron spin tilting portion irradiates a pulsed light.

12. The sensitivity regions are two or more, 3 . The optically excited magnetic sensor according to claim 1 , wherein the magnetic field measuring section measures the magnetic field strength based on a difference between outputs of the optical sensors corresponding to two adjacent sensitivity regions.

13. 3. A magnetoencephalograph comprising the optically excited magnetic sensor according to claim 1 or 2, which is arranged so as to be able to be placed around a head of a subject and measures the intensity of a magnetic field emitted from the subject.

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