Optical excitation magnetic sensor and brain magnetic instrument

Abstract

The invention provides an optical excitation magnetic sensor and a brain magnetic instrument. The optical excitation magnetic sensor includes: a unit; a pump light incidence section that causes pump light to enter a sensitivity region inside the cell in a first direction; a probe light incidence unit for causing probe light for detecting a change in electron spin to enter the sensitivity region in 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 to determine the resonance frequency of the electron spin; a gradient correction coil that corrects the gradient of the bias magnetic field; an electron spin tilting unit that tilts the rotation axis direction of electron spin; a light sensor that detects the probe light that has passed through the sensitivity region; and a magnetic field measurement unit that measures the magnetic field strength of the sensitivity region on the basis of the output of the optical sensor.

Description

Technical Field

[0001] The invention relates to a light-excited magnetoelectric sensor and a magnetoencephalometer comprising the light-excited magnetoelectric sensor. Background Art

[0002] There are known optically excited magnetic sensors that can measure weak external magnetic fields (for example, see U.S. Patent Publication No. 2022 / 0091200 and U.S. Patent No. 10,782,368). In the optically excited magnetic sensor, pump light is used to excite alkali metal atoms inside a cell, and a light sensor is used to measure probe light irradiated to the cell in a manner intersecting with the pump light, and the intensity of the external magnetic field is detected based on the output of the light sensor. Summary of the invention

[0003] As the optically excited magnetic sensors described above have become more and more popular in recent years, for example, there is a demand for improving the sensitivity of the magnetic field strength they measure. Therefore, the technical problem of the present invention is to provide an optically excited magnetic sensor and a magnetoencephalometer that can improve the sensitivity of the magnetic field strength.

[0004] According to one aspect of the present invention, a photoexcitation magnetization sensor is, [1] "a photoexcitation magnetization sensor comprising: a cell in which alkali metal vapor is sealed; a pump light incident portion, which causes pump light to be incident on a sensitive region inside the cell along a first direction, the pump light being used to excite alkali metal atoms constituting the alkali metal vapor; a probe light incident portion, which causes probe light to be incident on the sensitive region along a direction intersecting the first direction, the probe light being used to detect changes in electron spins in an excited state of the alkali metal atoms; a bias magnetic field coil, which applies a bias magnetic field along the first direction to the inside of the cell to determine a resonance frequency of the electron spin; a gradient correction coil, which is used to correct the gradient of the bias magnetic field applied by the bias magnetic field coil; an electron spin tilting portion, which tilts the rotation axis direction of the electron spin in a direction perpendicular to the first direction; a light sensor, which detects the probe light after passing through the sensitive region; and a magnetic field measuring portion, which measures the magnetic field intensity of the sensitive region based on the output of the light sensor."

[0005] In the optically excited magnetic sensor, by correcting the gradient of the bias magnetic field using the gradient correction coil, the uniformity of the bias magnetic field can be improved, and the variation of the resonance frequency of the electron spin can be suppressed, thereby improving the sensitivity of the magnetic field intensity.

[0006] The optically excited magnetic sensor of one aspect of the present invention may also be [2] "the optically excited magnetic sensor as described in [1] above, wherein the gradient correction coil includes a pair of first coils, the pair of first coils being arranged on one side and the other side of the unit in the first direction, and correcting the change in the bias magnetic field with respect to the change in position in the first direction." In this case, the uniformity of the bias magnetic field with respect to the position in the first direction can be improved.

[0007] The optically excited magnetic sensor according to one aspect of the present invention may also be, [3] "the optically excited magnetic sensor as described in [1] or [2] above, wherein the gradient correction coil includes a pair of second coils, the pair of second coils being arranged on one side and the other side of the unit in a second direction perpendicular to the first direction, and correcting the change in the bias magnetic field with respect to the change in position in the second direction." In this case, the uniformity of the bias magnetic field with respect to the position in the second direction can be improved.

[0008] The optically excited magnetic sensor according to one aspect of the present invention may be, [4] "a optically excited magnetic sensor as described in any one of [1] to [3] above, wherein the gradient correction coil includes a pair of third coils arranged on one side and the other side of the unit in the first direction, and corrects the change in the bias magnetic field with respect to the change in position in a third direction, wherein the third direction is perpendicular to the first direction." In this case, the uniformity of the bias magnetic field with respect to the position in the third direction can be improved.

[0009] The optically excited magnetic sensor according to one aspect of the present invention may be, [5] "the optically excited magnetic sensor according to any one of [1] to [4] above, wherein the gradient correction coil corrects the gradient of the bias magnetic field so that the relaxation time of the free induction decay obtained based on the output of the optical sensor is longer than the first time when the correction is not performed using the gradient correction coil." In this case, the gradient of the bias magnetic field can be effectively corrected.

[0010] The optically excited magnetic sensor of one aspect of the present invention may also be, [6] "the optically excited magnetic sensor as described in [5] above, wherein the gradient correction coil corrects the gradient of the bias magnetic field so that the relaxation time of the free induction decay obtained based on the output of the optical sensor becomes the longest." In this case, the gradient of the bias magnetic field can be corrected more effectively.

[0011] The optically excited magnetic sensor of one aspect of the present invention may be, [7] "a optically excited magnetic sensor as described in any one of [1] to [4] above, wherein the gradient correction coil corrects the gradient of the bias magnetic field so that the peak of the spectrum after fast Fourier transformation of the relaxation time of the free induction decay obtained based on the output of the optical sensor is steeper than when the gradient correction coil is not used for correction." In this case, the gradient of the bias magnetic field can be effectively corrected.

[0012] The optically excited magnetic sensor of one aspect of the present invention may also be, [8] "the optically excited magnetic sensor as described in [7] above, wherein the gradient correction coil corrects the gradient of the bias magnetic field so that the peak of the spectrum after fast Fourier transform of the relaxation time of the free induction decay obtained based on the output of the optical sensor is the steepest." In this case, the gradient of the bias magnetic field can be corrected more effectively.

[0013] The optically excited magnetic sensor of one aspect of the present invention may also be, [9] "a optically excited magnetic sensor as described in any one of [1] to [8] above, comprising one or more reflectors that reflect the detection light that travels from the inside of the cell to the outside of the cell or from the inside of the cell to the outside of the cell so as to return the detection light to the inside of the cell." In this case, the optical path length of the detection light in the cell can be increased, for example, the noise level caused by the detection light can be reduced.

[0014] The optically excited magnetic sensor according to one aspect of the present invention may be,

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

[0015] The optically excited magnetic sensor according to one aspect of the present invention may be

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

[0016] The optically excited magnetic sensor of one aspect of the present invention may also be,

[12] "a optically excited magnetic sensor as described in any one of [1] to

[11] above, wherein the sensitivity regions are two or more, and the magnetic field measuring unit measures the magnetic field intensity based on the difference between the outputs of the optical sensors corresponding to the two adjacent sensitivity regions." In this case, the common mode noise common to the two sensitivity regions can be removed, thereby enabling the measurement of weak magnetic field intensity.

[0017] A magnetoencephalometer according to one aspect of the present invention is,

[13] "a magnetoencephalometer comprising a light-excited magnetoencephalometer as described in any one of [1] to

[12] above, wherein the light-excited magnetoencephalometer is configured to be arranged around the head of a subject to measure the intensity of a magnetic field emitted by the subject."

[0018] Since the magnetoencephalometer includes the above-mentioned light-excited magnetoencephalometer sensor, the above-mentioned effect of improving the sensitivity of the magnetic field intensity can be obtained. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is a perspective view showing the structure of a light-excited magnetic sensor according to an embodiment.

[0020] Figure 2 Looking in the -x direction Figure 1 A top view of the first optical system.

[0021] Figure 3 Looking in the -y direction Figure 1 A top view of the second optical system, the optical sensor group and the third optical system.

[0022] Figure 4A It means to use Figure 1 A timing diagram of the timing of generating pump light when the readout circuit performs external magnetic field measurement processing. Figure 4B It means to use Figure 1 A timing diagram of an RF signal when the read circuit performs a process of measuring an external magnetic field. Figure 4C It means to use Figure 1 A timing diagram of the timing of the generation of the detection light when the reading circuit performs the external magnetic field measurement process.

[0023] Figure 5 It means by Figure 1 A graph showing the time variation of the FID obtained by the reading circuit.

[0024] Figure 6 Yes means Figure 1 A stereogram of the bias magnetic field gradient correction coil.

[0025] Fig. 7A It is a top view showing the second coil. Figure 7B It is a top view showing the first coil and the third coil.

[0026] Fig. 8A : is a graph showing the time change of FID for explaining the correction procedure of the bias magnetic field gradient correction coil. Figure 8B FIG. 4 is another graph showing a time change of FID for explaining the correction procedure of the bias magnetic field gradient correction coil.

[0027] Fig. 9 It is a perspective view showing the structure of the light-excited magnetic sensor according to the first modified example.

[0028] Fig.10 It is a top view showing the structure of the magnetoencephalometer according to the embodiment.

[0029] Fig.11A It is a cross-sectional view schematically showing a unit according to a modified example in which a reflecting mirror is provided. Fig. 11B It is a cross-sectional view schematically showing a unit according to another modified example in which a reflecting mirror is provided. DETAILED DESCRIPTION

[0030] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the description, the same reference numerals are used for the same elements or elements having the same functions, and repeated descriptions are omitted.

[0031] Figure 1 1 is a perspective view showing the structure of the optically excited magnetic sensor 1 of the embodiment. The optically excited magnetic sensor 1 is a device that uses optical pumping to measure the external magnetic field. In this embodiment, although the optically excited magnetic sensor 1 is described as being used for brain magnetometry, its use is not limited to this. In one example, the optically excited magnetic sensor 1 measures the brain magnetic field. For the sake of convenience, Figure 1 In the y-axis, the direction along the magnetic field direction of the measurement object is the y-axis, and the two axes perpendicular to the y-axis and mutually perpendicular to each other that form a three-dimensional coordinate system together with the y-axis are the x-axis and the z-axis. As 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 direction and the negative direction along the x-axis are respectively recorded as the +x-axis direction and the -x-axis direction, the positive direction and the negative direction along the y-axis are respectively recorded as the +y-axis direction and the -y-axis direction, and the positive direction and the negative direction along the z-axis are respectively recorded 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.

[0032] like Figure 1 As shown, the optical excitation magnetization sensor 1 includes a unit 2, a heater 3, a pump laser 4, a detection laser 5, a first optical system 6, a second optical system 7, an optical sensor group 8, a third optical system 9, a reading circuit 10, a bias magnetic field coil 11, bias magnetic field gradient correction coils 12, 13 and a tilting coil (electron spin tilting unit) 14. The optical excitation magnetization sensor 1 is an orthogonal sensor. The components of the optical excitation magnetization sensor 1 are described in detail below.

[0033] Unit 2 is a container for sealing alkali metal vapor. Unit 2 is arranged along the z-axis direction. Unit 2 is in the shape of a roughly rectangular parallelepiped with a surface roughly parallel to the xz plane and a bottomed cylindrical shape. The cross section of unit 2 in a direction perpendicular to the long side direction of unit 2 (in the direction along the xy plane) is, for example, a square. Unit 2 can be made of materials such as quartz, sapphire, silicon, Kovar glass, borosilicate glass, etc. Unit 2 is translucent to the pump light and probe light described later. A heater 3 and the like are installed on the side of unit 2 in the -y-axis direction. On the side of unit 2 in the -y-axis direction, the magnetic field Bm of the measurement object generated from the measurement object is incident in the +y-axis direction.

[0034] The alkali metal constituting the alkali metal vapor sealed in the unit 2 may be, for example, 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 only potassium. The spin-destruction collision relaxation rate of potassium is relatively small among the alkali metals used by the photoexcitation magnetic sensor. The spin-destruction collision relaxation rate of potassium is, for example, smaller than that of cesium and rubidium. Therefore, in the case of using a single alkali metal, the photoexcitation magnetic sensor using only potassium is more sensitive than the photoexcitation magnetic sensor using only cesium or only rubidium.

[0035] In addition, the cell 2 contains a sealed gas. The sealed gas suppresses the relaxation of the spin polarization of the alkali metal vapor. In addition, the sealed gas protects the alkali metal vapor and suppresses noise emission. The sealed gas can be, for example, an inert gas such as helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), nitrogen (N2), etc. The sealed gas can be, for example, helium and nitrogen.

[0036] As described above, the unit 2 is provided with a heater 3. The heater 3 generates heat in accordance with the current supplied from the heater power supply (not shown). The heater 3 controls the vapor density of the alkali metal by controlling the internal temperature of the unit 2. For example, when potassium is stored in the unit 2 as the alkali metal, the heater 3 heats the unit 2 so that the internal temperature is 120°C.

[0037] The pump laser 4 emits pump light for exciting alkali metal atoms in the -y axis direction. That is, the pump laser 4 emits linearly polarized pump light, which is converted into circularly polarized light by the 1 / 4 wavelength plates 27a to 27d. The alkali metal atoms contained in the unit 2 are excited by the pump light in a polarized state of circular polarization, generating spin polarization. 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 770.11nm, which is consistent with the resonance line of potassium. When the alkali metal atoms contained in the unit 2 are potassium and rubidium, the pump laser 4 can also emit pump light for exciting rubidium atoms and transferring the spin polarization of the rubidium atoms to the potassium atoms. In this case, the rubidium atoms are in an excited state due to the pump light. Furthermore, due to the spin exchange interaction between potassium and rubidium, the spin polarization of the rubidium atom is transferred to the potassium atom, and the potassium atom becomes excited.

[0038] The pump light from the pump laser 4 enters the cell 2 via the first optical system 6 . Figure 2 It is a top view when looking at the first optical system 6 along the -x axis direction. The first optical system 6 includes a lens 21, a 1 / 2 wavelength plate 22, polarization beam splitters 23a, 23b, 23c, 1 / 2 wavelength plates 24a, 24b, a total reflection mirror 25, polarizers 26a, 26b, 26c, 26d and 1 / 4 wavelength plates 27a, 27b, 27c, 27d.

[0039] The lens 21 is disposed adjacent to the pump laser 4 in the -y axis direction to focus the pump light emitted from the pump laser 4. The 1 / 2 wavelength plate 22 is disposed adjacent to the lens 21 in the -y axis direction to rotate the polarization plane by the polarization beam splitter 23a so that the power of the pump laser 4 is distributed in a 1:3 ratio.

[0040] The polarization beam splitter 23a is disposed adjacent to the 1 / 2 wavelength plate 22 in the -y axis direction, and separates two linear polarization light components perpendicular to each other from the polarization component of the pump light that has passed through the 1 / 2 wavelength plate 22, and transmits one linear polarization light component in the -y axis direction, and reflects the other linear polarization light component in the -z axis direction. The polarization beam splitters 23b, 23c and the total reflection mirror 25 are disposed side by side in sequence in the -z axis direction relative to the polarization beam splitter 23a. The polarization beam splitters 23b, 23c separate two linear polarization light components perpendicular to each other from the pump light that has passed through the polarization beam splitters 23a, 23b, respectively, and transmit one linear polarization light component in the -z axis direction, and reflect the other linear polarization light component in the -y axis direction. The total reflection mirror 25 reflects the linear polarization pump light that has passed through the polarization beam splitter 23c in the -y axis direction.

[0041] The 1 / 2 wavelength plate 24a is disposed between the polarization beam splitter 23a and the polarization beam splitter 23b, and rotates the polarization plane of the linearly polarized pump light reflected by the polarization beam splitter 23a. Thus, the polarization beam splitter 23b can be used to separate the pump light into two polarization components of linearly polarized light. The 1 / 2 wavelength plate 24b is disposed between the polarization beam splitter 23b and the polarization beam splitter 23c, and rotates the polarization plane of the linearly polarized pump light that has passed through the polarization beam splitter 23b. Thus, the polarization beam splitter 23c can be used to separate the pump light into two polarization components of linearly polarized light.

[0042] The polarizer 26a and the quarter-wave plate 27a are arranged adjacent to each other in the -y axis direction relative to the polarization beam splitter 23a. The polarizer 26a transmits a specific linearly polarized component of the pump light after passing through the polarization beam splitter 23a, and the quarter-wave plate 27a changes the polarization state of the pump light after passing through the polarizer 26a into circularly polarized light, and transmits it in the -y axis direction as the pump light PLa.

[0043] Polarizers 26b, 26c, 26d and quarter wavelength plates 27b, 27c, 27d are arranged adjacent to each other in the -y axis direction with respect to polarization beam splitters 23b, 23c and total reflection mirror 25. The functions of polarizers 26b, 26c, 26d and quarter wavelength plates 27b, 27c, 27d are the same as those of polarizer 26a and quarter wavelength plate 27a, respectively, and they transmit pump light PLb, PLc, PLd in the -y axis direction.

[0044] The first optical system 6 of the above-mentioned structure adopts a structure in which four systems of pump light PLa to PLd transmitted through four 1 / 4 wavelength plates 27a, 27b, 27c, and 27d can be incident on the sensitivity areas ARa to ARd inside the unit 2, respectively. Such a pump laser 4 and the first optical system 6 constitute a pump light incident part that allows the pump light PLa to PLd to be incident on the sensitivity areas ARa to ARd along the -y axis direction.

[0045] The detection laser 5 emits a detection light for detecting the precession motion of the spin when the electron spin in the excited state of the alkali metal atom is tilted by 90 degrees. That is, the detection laser 5 emits a detection light of linear polarization. When the detection light passes through the alkali metal vapor, it is affected by the state of the spin polarization of the alkali metal atom, and the magneto-optical rotation angle changes. By detecting the change in the magneto-optical rotation angle, the state of the precession motion of the spin can be derived. The wavelength of the detection light is set according to the type of atoms constituting the alkali metal vapor (more specifically, the wavelength of the absorption line). For example, in the case where the unit 2 contains only potassium as an alkali metal, the wavelength of the detection light is detuned from the wavelength of the pump light (for example, 770.11nm), for example, to about 770.05nm. By detuning the wavelength of the detection light from the wavelength of the pump light, the absorption of the detection light by potassium is suppressed.

[0046] In the case where potassium and rubidium are contained in the cell 2 as alkali metals, the detection laser 5 can also emit detection light for detecting the change of the magneto-optical rotation angle caused by the spin polarization of the potassium atoms in the excited state. The density of rubidium used in the excitation is set to be smaller than the density of potassium used in the detection. In the case where the density of rubidium is smaller than the density of potassium, the attenuation of the pump light caused by the excitation is suppressed. As a result, the optically excited magnetic sensor 1 can obtain uniform sensitivity in the cell 2.

[0047] The probe light from the probe laser 5 enters the cell 2 via the second optical system 7 . Figure 3 It is a top view when viewing the second optical system 7, the optical sensor group 8 and the third optical system 9 along the -y axis direction. The second optical system 7 includes 1 / 2 wavelength plates 31a, 31b, 31c, polarization beam splitters 32a, 32b, 32c, a total reflection mirror 33, and polarizers 34a, 34b, 34c, 34d.

[0048] The 1 / 2 wavelength plate 31a is arranged adjacent to the probe laser 5 in the -z axis direction, and the polarization plane is rotated by the polarization beam splitter 32a so that the power of the probe laser 5 is distributed at 1:3. The polarization beam splitters 32a, 32b, and 32c are arranged in sequence in the -z axis direction relative to the 1 / 2 wavelength plate 31a, and separate two linear polarized light components perpendicular to each other from the polarization component of the probe light after passing through the 1 / 2 wavelength plate 31a, so that one linear polarized light component is transmitted in the -z axis direction, and the other linear polarized light component is reflected in the -x axis direction. The 1 / 2 wavelength plate 31b is arranged between the polarization beam splitter 32a and the polarization beam splitter 32b, and the polarization plane of the linear polarized probe light after passing through the polarization beam splitter 32a is rotated. In this way, the probe light can be separated into two linear polarized light polarization components by the polarization beam splitter 32b. The 1 / 2 wavelength plate 31c is disposed between the polarization beam splitter 32b and the polarization beam splitter 32c, and rotates the polarization plane of the linearly polarized probe light after passing through the polarization beam splitter 32b. Thus, the polarization beam splitter 32c can be used to separate the probe light into two polarization components of linearly polarized light. The total reflection mirror 33 reflects the linearly polarized probe light passing through the polarization beam splitter 32c in the -x axis direction.

[0049] The polarizers 34 a , 34 b , 34 c , and 34 d transmit specific linearly polarized light components of the probe light reflected by the polarization beam splitters 32 a , 32 b , and 32 c and the total reflection mirror 33 , respectively.

[0050] The second optical system 7 of the above structure is configured so that the four systems of probe lights QLa to QLd that pass through the four polarizers 34a, 34b, 34c, and 34d can be incident on the sensitivity areas ARa to ARd inside the unit 2, respectively. The sensitivity areas ARa to ARd are areas that are arranged at intervals in the long side direction (z-axis direction) inside the unit 2. The sensitivity areas ARa to ARd are areas that intersect with the pump lights PLa to PLd inside the unit 2. The sensitivity areas ARa to ARd are intersection areas where the probe lights QLa to QLd intersect with the pump lights PLa to PLd. Such a probe laser 5 and the second optical system 7 constitute a probe light incident portion that allows the probe lights QLa to QLd to be incident on the sensitivity areas ARa to ARd along the x-axis direction.

[0051] Reference Figure 1 The bias magnetic field coil 11 is a coil that applies a bias magnetic field By in the y-axis direction inside the unit 2, and determines the resonance frequency of the electron spins of the alkali metal atoms excited inside the unit 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 relative to the sensitivity areas ARa to ARd inside the unit 2. As a result, the rotation axis of the electron spins of the alkali metal atoms excited inside the unit 2 is along the y-axis direction. When the magnetic field strength of the bias magnetic field applied by the bias magnetic field coil 11 is 14μT and the alkali metal atom is potassium, the resonance frequency of the electron spin is 100kHz. 14μT is a magnetic field strength that can be easily formed.

[0052] The bias magnetic field gradient correction coils 12 and 13 are coil groups 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 and 13 can be configured to correct the gradients in any one or two directions of the x-axis direction, the y-axis direction, and the z-axis direction of the bias magnetic field By, or can be configured to correct the gradients in three directions. Due to the presence of the bias magnetic field gradient correction coils 12 and 13, even when the bias magnetic field By is not uniform due to the influence of the environmental magnetic field, etc., it is possible to correct the bias magnetic field By to make it uniform, thereby improving the detection sensitivity of the optically excited magnetic sensor 1 to the external magnetic field.

[0053] 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 having the same frequency as the resonance frequency of the electron spin (100 kHz when the intensity 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°. In addition, instead of the tilting coil 14, a technical means for irradiating other pulsed pump light from a direction perpendicular to the incident direction of the pump light may be provided as an electron spin tilting unit for tilting the direction of the electron spin.

[0054] return Figure 3 , the structures of the optical sensor group 8 and the third optical system 9 are described.

[0055] The third optical system 9 includes 1 / 2 wavelength plates 41a, 41b, 41c, 41d and polarization beam splitters 42a, 42b, 42c, 42d. The 1 / 2 wavelength plates 41a, 41b, 41c, 41d are respectively provided adjacent to the unit 2 in the -x axis direction on the optical paths of the probe lights QLa, QLb, QLc, QLd that have passed through the sensitive areas ARa to ARd of the unit 2, and rotate the polarization planes of the linearly polarized lights of the probe lights QLa, QLb, QLc, QLd that have passed through the unit 2. These 1 / 2 wavelength plates 41a, 41b, 41c, 41d are supported so as to be rotatable around an axis along the x axis, and can adjust the rotation angle of the polarization plane of the probe light. Polarization beam splitters 42a, 42b, 42c, and 42d separate the components of two linearly polarized light beams perpendicular to each other from the polarization components of the detection lights QLa, QLb, QLc, and QLd transmitted through the half-wavelength plates 41a, 41b, 41c, and 41d, respectively, so that one component of the linearly polarized light beam is transmitted in the -x-axis direction and the other component of the linearly polarized light beam is reflected in the y-axis direction.

[0056] The optical sensor group 8 is an element group that detects the polarization plane angle of the detection light QLa, QLb, QLc, and QLd after passing through the sensitivity areas ARa to ARd, and is composed of 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 for detecting the intensity of the other linear polarized light component of the detection light QLa, and the photodiode 45a outputs a detection signal for detecting the intensity of the one linear polarized light component of the detection light QLa. Similarly, the optical sensor pairs 43b, 43c, and 43d have two photodiodes 44b and 45b, two photodiodes 44c and 45c, and two photodiodes 44d and 45d, respectively. The two photodiodes 44b and 45b respectively output detection signals for detecting the intensity of the components of the two linear polarized light beams of the detection light QLb, the two photodiodes 44c and 45c respectively output detection signals for detecting the intensity of the components of the two linear polarized light beams of the detection light QLc, and the two photodiodes 44d and 45d respectively output detection signals for detecting the intensity of the components of the two linear polarized light beams of the detection light QLd.

[0057] Refer again Figure 1 The reading 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 reading circuit 10 includes: a reading unit that reads the detection signals output from the four optical sensor pairs 43a, 43b, 43c, and 43d (outputs of the optical sensor pairs 43a, 43b, 43c, and 43d); and a measuring unit that performs measurement processing of the external magnetic field of the sensitivity area ARa to ARd based on the detection signals. However, the measuring unit may be provided outside the optical excitation magnetic sensor 1, and performs measurement processing based on the detection signals output from the optical excitation magnetic sensor 1. The reading circuit 10 constitutes a magnetic field measuring unit.

[0058] The reading circuit 10 physically includes a memory such as RAM, ROM, a processor such as CPU (calculation circuit), a communication interface, a hard disk, an auxiliary storage device such as a semiconductor memory, etc. The reading circuit 10 can be implemented by, for example, a personal computer, a cloud server, a smart phone, a tablet terminal, etc. The function of the reading circuit 10 is implemented by the CPU of the computer system executing a program stored in the memory.

[0059] Reference Figure 4A , Figure 4B , Figure 4C and Figure 5 , the function of the external magnetic field measurement processing performed by the measuring unit of the reading circuit 10 is described. Figure 4Arepresents the generation time of the pump light when the reading circuit 10 performs the external magnetic field measurement process, Figure 4B represents the generation time of the RF signal when the reading circuit 10 performs the external magnetic field measurement process, Figure 4C The figure shows the reading time of the probe light when the reading circuit 10 performs the measurement process of the external magnetic field. Figure 5 This is a graph showing the temporal change of FID (Free Induction Decay) obtained by the reading circuit 10 .

[0060] When the reading circuit 10 performs the external magnetic field measurement process, the control circuit (not shown) controls the pump light to be repeatedly turned on and off in a pulsed manner (see Figure 4A ), immediately after the pump light is switched from on to off, the RF signal is pulsed (refer to Figure 4B ), read the probe light during the specified relaxation time after irradiating the RF signal (refer to Figure 4C ). Under such control, the reading circuit 10 obtains the detection signals output from the four light sensor pairs 43a, 43b, 43c, and 43d during the relaxation time. Furthermore, the reading circuit 10 obtains the FID (see Figure 5 ) Similarly, the reading circuit 10 also obtains the time change of FID by obtaining the difference between the detection signals of the two photodiodes for the light sensor pairs 43b, 43c, and 43d.

[0061] The above FID represents the relaxation of the electron spin of the alkali metal atom. The frequency of the precession motion of the electron spin changes according to the change of the external magnetic field in the sensitivity area ARa, and thus the frequency of the vibration changes due to the change of the external magnetic field. For example, when the resonance frequency of the electron spin is 100kHz and the external magnetic field changes from 0pT to 1pT, the frequency of the precession motion changes by 0.007Hz. The reading circuit 10 uses this property to fit the waveform of the initial FID of the relaxation time by using the function V shown in the following formula (in the following formula, t represents the time, and a0, a1, and a2 represent the specified parameters.),

[0062] [Number 1]

[0063]

[0064] The frequency a1 of the vibration of the FID is derived and the frequency a1 is obtained as the measured value of the external magnetic field of the sensitivity area ARa. Similarly, the reading circuit 10 obtains the measured values ​​of the external magnetic field of the sensitivity areas ARb, ARc, and ARd based on the detection signals output by the light sensor pairs 43b, 43c, and 43d. Furthermore, the reading circuit 10 can obtain time series data of the measured values ​​by repeatedly performing this operation at a frequency of 100 to 1000 Hz.

[0065] In the above-described optically excited magnetic sensor 1, by emitting pump light to the unit 2 sealed with alkali metal vapor, the electron spin of the alkali metal atom can be generated (excited). The detection light QLa~QLd is emitted to the sensitive area ARa~ARd inside the unit 2, and the polarization plane angle of the detection light QLa~QLd after passing through the sensitive area ARa~ARd is detected by the optical sensor pair 43a~43d. Through the reading circuit 10, a detection signal (measurement signal) for measuring the intensity of the external magnetic field of the sensitive area ARa~ARd can be generated based on the output of the optical sensor pair 43a~43d, and the FID can be obtained. Through the reading circuit 10, the waveform of each FID can be fitted using the function V, and the frequency a1 of each FID is obtained as the measured value of the magnetic field Bm in the -y axis direction of the sensitive area ARa~ARd.

[0066] Next, the bias magnetic field gradient correction coils 12 and 13 will be described in detail.

[0067] For example, in an environment without magnetic shielding, even if the bias magnetic field By is uniformly applied by the bias magnetic field coil 11, it is 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 is offset, so the relaxation time of the FID becomes shorter. As a result, there is a problem that the fitting accuracy is insufficient and it is difficult to obtain high sensitivity. Therefore, the optically excited magnetic sensor 1 includes: bias magnetic field gradient correction coils (gradient correction coils) 12, 13 for correcting the gradient of the bias magnetic field By of the bias magnetic field coil 11.

[0068] like Figure 1 , Figure 6 and Fig. 7A As shown, the bias magnetic field gradient correction coil 12 includes a parallel flat plate type second coil 12A. The second coil 12A is provided on each substrate of a pair of substrates that are opposite to each other in the z-axis direction across the unit 2. The second coil 12A is arranged on one side and the other side of the unit 2 in the z-axis direction. The second coil 12A corrects the variation of the bias magnetic field By corresponding to the variation of the position in the z-axis direction. In the example shown in the figure, the second coil 12A is a parallel quadrifilar coil.

[0069] like Figure 1 , Figure 6 and Figure 7B As shown, the bias magnetic field gradient correction coil 13 includes a parallel flat plate type first coil 13A and a third coil 13B. The first coil 13A and the third coil 13B are arranged on each substrate of a pair of substrates that are opposite to each other in the y-axis direction across the unit 2. The first coil 13A is arranged on one side and the other side of the unit 2 in the y-axis direction. The first coil 13A corrects the change in the bias magnetic field By corresponding to the change in position in the x-axis direction. In the example shown in the figure, the first coil 13A is a parallel quadrifilar coil. The third coil 13B is arranged on one side and the other side of the unit 2 in the y-axis direction. The third coil 13B corrects the change in the bias magnetic field By corresponding to the change in position in the y-axis direction. In the example shown in the figure, the third coil 13B is a Maxell coil.

[0070] The first to third coils 13A, 12A, and 13B are coils for correcting the primary gradient magnetic field. However, the first to third coils 13A, 12A, and 13B may also be coils for correcting the secondary gradient magnetic field. In this case, the uniformity of the bias magnetic field By can be further improved. Figure 6 The arrows shown in the first to third coils 13A, 12A, and 13B in FIG. 1 indicate the directions of the applied currents.

[0071] The bias magnetic field gradient correction coils 12 and 13 correct the gradient of the bias magnetic field By so that the relaxation time TH of the FID (see Figure 8B ) is longer than the first time TH1 (refer to Fig. 8A In the present 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 becomes the longest.

[0072] 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 and 13 is specifically described. First, the FID is monitored in a state where there is no magnetic field Bm of the measurement object. The FID can be monitored using an oscilloscope or the like. While observing the spectrum after high-speed Fourier transformation of the FID (hereinafter also referred to as "FFT spectrum"), the current flowing in the bias magnetic field coil 11 is adjusted so that the peak position is the desired frequency (e.g., 100kHz).

[0073] 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 becomes 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 in the bias magnetic field coil 11 is adjusted again. This is repeated as needed to complete the correction of the gradient of the bias magnetic field By.

[0074] As described above, in this embodiment, by correcting the gradient of the bias magnetic field By using the bias magnetic field gradient correction coils 12 and 13, the uniformity of the bias magnetic field By can be improved, and the variation of the resonance frequency of the electron spins can be suppressed. Thus, the sensitivity of the magnetic field intensity can be improved.

[0075] In the present embodiment, the bias magnetic field gradient correction coil 13 includes a first coil 13A. In this case, the uniformity of the bias magnetic field By with respect to the position in the x-axis direction can be improved. In the present embodiment, the bias magnetic field gradient correction coil 12 includes a second coil 12A. In this case, the uniformity of the bias magnetic field By with respect to the position in the z-axis direction can be improved. In the present embodiment, the bias magnetic field gradient correction coil 13 includes a third coil 13B. In this case, the uniformity of the bias magnetic field By with respect to the position in the y-axis direction can be improved.

[0076] 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, the gradient of the bias magnetic field By can be corrected effectively.

[0077] 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 becomes the longest. In this case, the gradient of the bias magnetic field By can be corrected more effectively.

[0078] In this embodiment, a bias magnetic field coil 11 is provided for applying a bias magnetic field By in the same direction as the pump light to determine the resonance frequency of the electron spin, and a tilting coil 14 is provided for tilting the direction of the electron spin in a direction perpendicular to the pump light. In this case, by measuring the intensity of the external magnetic field based on the frequency of the change in the polarization plane angle of the probe light detected using the detection signal, the measurement sensitivity of the external magnetic field can be maintained without being affected by the ambient magnetic field.

[0079] In this embodiment, a tilting coil 14 that irradiates an RF signal with the same frequency as the resonance frequency is used. In this case, the RF signal can be used to tilt the rotation axis direction of the electron spin. Simple technical means can be used to achieve the measurement of the external magnetic field based on the frequency of the change in the polarization plane angle of the detection light. In addition, in this embodiment, a light source that irradiates pulsed light can also be used. In this case, the pulsed light can be used to tilt the rotation axis direction of the electron spin. Simple technical means can be used to achieve the measurement of the external magnetic field based on the frequency of the change in the polarization plane angle of the detection light.

[0080] In the present embodiment, there are sensitivity areas ARa to ARd divided into four parts in the z-axis direction. Therefore, by obtaining the difference in the measured values ​​of the external magnetic field obtained from two adjacent sensitivity areas in the sensitivity areas ARa to ARd divided into four parts, the common mode noise can be removed. That is, in the present embodiment, there are more than two sensitivity areas, and the reading circuit 10 can measure the magnetic field strength based on the difference in the output of the optical sensor corresponding to the two adjacent sensitivity areas. In this case, the common mode noise of the two adjacent sensitivity areas can be removed, and the measurement of the weak magnetic field strength can be achieved, which can further improve the sensitivity of the magnetic field strength. This effect of improving the sensitivity is particularly significant in the absence of magnetic shielding. For example, the detection limit of 20pT / rHz can be reduced to less than 300ft / rHz, which can be reduced by about 2 digits.

[0081] In this embodiment, it is important to expand the sensitivity region ARa to ARd to improve the sensitivity. In this regard, the inner diameter of the unit 2 is preferably 10×10 mm, the diameter of the pump light is preferably 10 mm, and the diameter of the probe light is preferably 5 mm. In addition, in the orthogonal optical excitation magnetic sensor 1, when four sensitive regions ARa to ARd are arranged at intervals of 10 mm, due to the size limitation of the polarization beam splitters 23a, 23b, and 23c that branch the pump light, it is preferred to form the pump light into a roughly rectangular shape (e.g., 10 mm×5 mm), increase the area intersecting with the probe light, and limit the pump light in the direction orthogonal thereto.

[0082] The light-excited magnetic sensor 1 of this embodiment constitutes a scalar magnetic sensor. The scalar magnetic sensor has the advantage of being able to measure large magnetic fields without magnetic shielding, but when the uniformity of the bias magnetic field By is poor, there is a possibility that the sensitivity is easily reduced. According to this embodiment, the uniformity of the bias magnetic field By can be improved, and the relaxation time of the FID can be extended. For example, a high sensitivity of less than 300fT / fHz can be achieved without magnetic shielding. By using a scalar magnetic sensor, the necessity of using expensive magnetic shielding can be reduced.

[0083] Although various embodiments of the present invention have been described above, the present invention is not limited to the above embodiments, and can be modified or applied to other modes within the scope of the gist of each claim.

[0084] Fig. 91 is a perspective view showing the structure of the optical excitation magnetization sensor 1A of the first variant. The optical excitation magnetization sensor 1A is different from the optical excitation magnetization sensor 1 in that the incident direction of the pump light to the unit 2 and the application direction of the bias magnetic field By are set to the z-axis direction, and the irradiation direction of the RF signal Rf is set to the x-axis direction. In the following, only the structure that is different from the optical excitation magnetization sensor 1 is described.

[0085] The first optical system 6 includes a lens 21, a 1 / 2 wavelength plate 22, a polarization beam splitter 23a, and a 1 / 4 wavelength plate 27a. The polarization beam splitter 23a is disposed adjacent to the 1 / 2 wavelength plate 22 in the -y axis direction, so that the linearly polarized light component of the pump light after passing through the 1 / 2 wavelength plate is reflected in the -z axis direction. The 1 / 4 wavelength plate 27a is disposed adjacent to the polarization beam splitter 23a in the -z direction, and changes the linearly polarized pump light reflected by the polarization beam splitter 23a into circularly polarized light, so that the pump light is transmitted in the -z axis direction. The first optical system 6 of the above structure is configured so that the pump light of the first system can be incident in the long side direction (z axis direction) inside the unit 2.

[0086] The second optical system 7 includes a lens 35, a half-wave plate 36, and a total reflection mirror 37 in addition to the half-wave plates 31a, 31b, and 31c, polarization beam splitters 32a, 32b, and 32c, a total reflection mirror 33, and polarizers 34a, 34b, 34c, and 34d. The lens 35 is disposed adjacent to the probe laser 5 in the -y axis direction, and focuses the probe light emitted from the probe laser 5 in the -y axis direction. The total reflection mirror 37 is disposed adjacent to the half-wave plate 36 in the -y axis direction, and reflects the probe light transmitted through the half-wave plate 36 toward the half-wave plate 31a in the -z axis direction. The second optical system 7 having such a configuration is configured so that the four systems of probe light QLa to QLd transmitted through the four polarizers 34a, 34b, 34c, and 34d can be incident on four sensitive regions arranged in the longitudinal direction inside the cell 2 and intersecting with the pump light PL.

[0087] According to the axial-type photoexcitation magnetoelectric sensor 1A of the first variant, the measurement value of the magnetic field Bm in the z-axis direction, which is the incident direction of the pump light, can be obtained for the four sensitivity regions divided along the z-axis direction. In the photoexcitation magnetoelectric sensor 1A, the above-mentioned effect of improving the sensitivity of the magnetic field intensity can also be obtained. Among them, the photoexcitation magnetoelectric sensor 1A has four sensitivity regions, but it can also be changed to a structure with two sensitivity regions. In this case, by obtaining the difference between the measurement values ​​of the two sensitivity regions, the common mode noise can also be removed to improve the detection accuracy. The sensitivity region can also be set to 1, 3 or 5 or more.

[0088] Fig.101 is a top view showing the structure of a magnetoencephalometer 100 according to an embodiment. The magnetoencephalometer 100 includes: a plurality of optically excited magnetoencephalometer sensors 1 and a plurality of optically excited magnetoencephalometer sensors 1A, which are arranged so as to be arranged around the head (subject) S of a subject. Two of the plurality of optically excited magnetoencephalometer sensors 1 are arranged so that their four sensitivity regions are arranged side by side in a predetermined direction (y-axis direction) at positions close to the head S and at positions separated therefrom. Two of the plurality of optically excited magnetoencephalometer sensors 1A are also arranged so that their four sensitivity regions are arranged side by side in a direction (x-axis direction) substantially perpendicular to the arrangement direction of the sensitivity regions in the optically excited magnetoencephalometer sensors 1 at positions close to the head S and at positions separated therefrom. 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 magnetoencephalometer sensors 1 and the plurality of optically excited magnetoencephalometer sensors 1A. The bias magnetic field coil 11 can be used to apply a bias magnetic field By in the x-axis direction. The optically excited magnetic sensor 1 , 1A measures the intensity of the magnetic field emitted from the head S.

[0089] According to the magnetoencephalometer 100 of the above structure, it is possible to obtain a measurement value obtained by measuring a weak magnetic field from the head S. Specifically, according to Fig.10 In the configuration example, the measured values ​​of the magnetic field in the x-axis direction of 8×4=32 sensitive areas can be obtained. In addition, by obtaining the difference between the measured value of the sensitive area close to the head S and the measured value of the sensitive area adjacent to the sensitive area on the outside of the head S, it is possible to measure weak brain magnetism with common mode noise removed.

[0090] In addition, in the magnetoencephalometer 100, such a structure can also be applied: the optical excitation magnetometry sensor 1 and the optical excitation magnetometry sensor 1A are only arranged at a position close to the head S, and are not arranged side by side. In this case, by obtaining the difference in the measured values ​​of two adjacent sensitivity areas of one optical excitation magnetometry sensor 1, 1A, it is possible to measure weak brain magnetism with common mode noise removed. In the magnetoencephalometer 100, since the optical excitation magnetometry sensors 1, 1A are included, the above-mentioned effect of being able to improve the sensitivity of the magnetic field strength can be obtained.

[0091] The optically excited magnetic sensor 1, 1A of the above-mentioned embodiment may also be Fig.11A As shown in FIG. 1 , a reflector 55 is provided in the cell 2 to reflect the detection light traveling toward the outside of the cell 2. The reflector 55 may be a metal film such as aluminum vapor-deposited on the inner wall of the cell 2, or may be a dielectric multilayer reflector. The reflector 55 may be provided on the outer wall of the cell 2. Alternatively, the optically excited magnetic sensor 1, 1A of the above-mentioned embodiment may also be provided as Fig. 11BAs shown in FIG. 1 , one or more reflectors 55A are provided to reflect the probe light QL traveling from the inside of the unit 2 to the outside of the unit 2 and return it to the inside of the unit 2. The reflector 55A is provided outside the unit 2. In this way, by using the reflectors 55 and 55A to reflect the probe light QL multiple times, the optical path length of the probe light QL in the unit 2 can be increased, the rotation angle (Faraday rotation angle) of the polarization plane of the probe light QL can be increased, and the response of the readout circuit 10 can be increased. As a result, for example, the noise level generated by the probe light QL and the readout circuit 10 can be relatively reduced.

[0092] In the above-mentioned embodiment, the bias magnetic field gradient correction coils 12 and 13 may also correct the gradient of the bias magnetic field By in such a manner that the peak of the FFT spectrum regarding the relaxation time of the FID becomes steeper than when the bias magnetic field gradient correction coils 12 and 13 are not corrected. In this case, the gradient of the bias magnetic field By can be effectively corrected. In addition, in the above-mentioned embodiment, the bias magnetic field gradient correction coils 12 and 13 may also correct the gradient of the bias magnetic field By in such a manner that the peak of the FFT spectrum regarding the relaxation time of the FID becomes the steepest. In this case, the gradient of the bias magnetic field By can be more effectively corrected. In the above-mentioned embodiment, there is a case where linearly polarized light is converted into circularly polarized light for use, but it is also possible to use circularly polarized light from the beginning without performing this conversion.

[0093] According to the present invention, a light-excited magnetoencephalometer and a magnetoencephalometer capable of improving the sensitivity of magnetic field intensity can be provided.

Claims

1. A light-excited magnetic sensor, comprising: A cell enclosed with alkali metal vapor; a pump light incident portion for causing pump light to be incident along a first direction into a sensitive region inside the cell, wherein the pump light is used to excite alkali metal atoms constituting the alkali metal vapor; a probe light incident portion for causing a probe light to be incident on the sensitive region along a direction intersecting the first direction, the probe light being used to detect a change in the electron spin of the alkali metal atom in an excited state; a bias magnetic field coil for applying a bias magnetic field along the first direction to the interior of the unit to determine the resonance frequency of the electron spin; a gradient correction coil for correcting the gradient of the bias magnetic field applied by the bias magnetic field coil; an electron spin tilting portion for tilting the rotation axis of the electron spin in a direction perpendicular to the first direction; a light sensor that detects the detection light after passing through the sensitivity area; and A magnetic field measuring unit measures the magnetic field intensity of the sensitive area based on the output of the optical sensor.

2. The optically excited magnetic sensor according to claim 1, wherein: The gradient correction coil includes a pair of first coils disposed on one side and the other side of the unit in the first direction and correcting a change in the bias magnetic field with respect to a change in position in the first direction.

3. The optically excited magnetic sensor according to claim 1 or 2, wherein: The gradient correction coil includes a pair of second coils disposed on one side and the other side of the unit in a second direction perpendicular to the first direction, and corrects a change in the bias magnetic field with respect to a change in position in the second direction.

4. The optically excited magnetic sensor according to claim 1 or 2, wherein: The gradient correction coil includes a pair of third coils arranged on one side and the other side of the unit in the first direction, and corrects a change in the bias magnetic field with respect to a change in position in a third direction, wherein the third direction is perpendicular to the first direction.

5. The optically excited magnetic sensor according to claim 1 or 2, wherein: The gradient correction coil corrects the gradient of the bias magnetic field so that a relaxation time of free induction decay obtained based on the output of the optical sensor becomes longer than a first time when the correction is not performed by the gradient correction coil.

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 the output of the photosensor becomes the longest.

7. The optically excited magnetic sensor according to claim 1 or 2, wherein: The gradient correction coil corrects the gradient of the bias magnetic field so that a peak of a spectrum obtained by fast Fourier transforming a relaxation time of free induction decay obtained from an output of the optical sensor becomes steeper than when the gradient correction coil is not used for correction.

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 peak of a spectrum obtained by fast Fourier transforming a relaxation time of free induction decay obtained from an output of the optical sensor becomes steepest.

9. The optically excited magnetic sensor according to claim 1 or 2, wherein: One or more reflecting mirrors are included, which reflect the detection light that travels from the inside of the unit to the outside of the unit or travels from the inside of the unit to the outside of the unit, so as to return the detection light to the inside of the unit.

10. The optically excited magnetic sensor according to claim 1 or 2, wherein: The electron spin tilting unit irradiates an RF signal having the same frequency as the resonance frequency.

11. The optically excited magnetic sensor according to claim 1 or 2, wherein: The electron spin tilting unit irradiates pulsed light.

12. The optically excited magnetic sensor according to claim 1 or 2, wherein: There are more than two sensitivity areas. The magnetic field measurement unit measures the magnetic field intensity based on a difference between outputs of the optical sensors corresponding to two adjacent sensitivity regions.

13. A magnetoencephalometer, comprising the optically excited magnetoencephalometer according to claim 1 or 2, wherein the optically excited magnetoencephalometer is configured to be arranged around the head of a subject to measure the intensity of the magnetic field emitted by the subject.

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