Photoexcited magnetic sensor and magnetoencephalographic meter

Abstract

To enable multi-channelization of measurement of magnetic field strength by a simple configuration.SOLUTION: A photoexcited magnetic sensor 1 includes: a cell 2 into which an alkali metal vapor is sealed; a pump laser 4 and a first optical system 6 for allowing pump light to impinge on multiple sensitivity regions in the cell 2 along the y-axis direction; a probe laser 5 and a second optical system 7 for allowing probe light to impinge on multiple sensitivity regions along the z-axis direction; bias magnetic field coils 11, 15 for determining a resonance frequency of an electron spin by applying a bias magnetic field By along the y-axis direction into the cell 2; a tilting coil 14 for tilting an electron pin in the direction of a rotation axis; photosensor pairs 43a, 43b for detecting probe light passing through multiple sensitivity regions; and a read circuit 10 for measuring magnetic field strength. The bias magnetic field coil 15 applies multiple bias magnetic fields By each having different strength to each of the multiple sensitivity regions.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, it may be desirable to measure the magnetic field strength in a multi-channel manner. In this case, however, the optical system configuration is likely to become complicated. Therefore, an object of one aspect of the embodiment is to provide an optically excited magnetic sensor and a magnetoencephalograph that can measure the magnetic field strength in a multi-channel manner with a simple configuration. [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 plurality of sensitivity regions 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 plurality of sensitivity regions 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; an electron spin tilting unit that tilts the 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 sensitivity region; and a magnetic field measuring unit that measures a magnetic field intensity related to the sensitivity region based on an output of the optical sensor, wherein the bias magnetic field coil applies a plurality of bias magnetic fields having different intensities to each of the plurality of sensitivity regions."

[0006] In this optically excited magnetic sensor, the strength of the bias magnetic field can be changed for each of the multiple sensitivity regions, and the magnetic field strengths for the multiple sensitivity regions can be obtained using one probe light. In other words, multi-channel measurement of magnetic field strength is possible with a simple configuration.

[0007] The optically excited magnetic sensor of the embodiment may be [2] "the optically excited magnetic sensor according to the above [1], in which the multiple sensitivity regions include at least a first sensitivity region and a second sensitivity region, and the bias magnetic field coil includes a first coil corresponding to the first sensitivity region and a second coil corresponding to the second sensitivity region." In this case, it is specifically possible to change the strength of the bias magnetic field for each of the multiple sensitivity regions.

[0008] The optically excited magnetic sensor according to the embodiment may be [3] "the optically excited magnetic sensor according to the above [2], comprising a substrate on which the first coil and the second coil are provided, the substrate having a through hole through which the pump light passes." In this case, it is possible to specifically implement a bias magnetic field coil without preventing the pump light from being incident on the cell.

[0009] The optically excited magnetic sensor of the embodiment may be [4] "the optically excited magnetic sensor according to the above [2] or [3], wherein the first sensitivity region is present at an end of the cell, and the bias magnetic field coil is disposed away from the cell when viewed from the first direction, and includes an extra-cell coil adjacent to the first coil." In this case, the strength of the bias magnetic field applied to the first sensitivity region on the cell end side can be appropriately maintained by the extra-cell coil.

[0010] The optically excited magnetic sensor of the embodiment may be the optically excited magnetic sensor described in [1] above, in which the multiple sensitivity regions include a first sensitivity region, a second sensitivity region adjacent to the first sensitivity region, a third sensitivity region adjacent to the second sensitivity region, and a fourth sensitivity region adjacent to the third sensitivity region, and the bias magnetic field coil includes a pair of first coils arranged to sandwich the first sensitivity region in the first direction and formed with a first number of turns in a first rotation direction, a pair of second coils arranged to sandwich the second sensitivity region in the first direction and formed with a second number of turns in the first rotation direction, a pair of third coils arranged to sandwich the third sensitivity region in the first direction and formed with the second number of turns in a second rotation direction opposite to the first rotation direction, and a pair of fourth coils arranged to sandwich the fourth sensitivity region in the first direction and formed with the first number of turns in the second rotation direction. In this case, it is specifically possible to change the strength of the bias magnetic field for each of the multiple sensitivity regions.

[0011] The optically excited magnetic sensor of the embodiment may be [6] "the optically excited magnetic sensor according to any one of [1] to [5] above, wherein the multiple sensitivity regions include 1st to Nth sensitivity regions (N is an integer of 2 or more), the bias magnetic field coil applies 1st to Nth bias magnetic fields to the 1st to Nth sensitivity regions, respectively, and the resonant frequencies of the 1st to Nth sensitivity regions are set as 1st to Nth resonant frequencies, and the magnetic field measuring unit acquires a mixed waveform of free induction decay including components of the 1st to Nth resonant frequencies based on the output of the optical sensor, filters the mixed waveform by a bandpass filter, acquires 1st to Nth waveforms of free induction decay in each band of the 1st to Nth resonant frequencies, derives each frequency of the 1st to Nth waveforms, and determines the magnetic field strength related to the 1st to Nth sensitivity regions based on the derived frequencies of the 1st to Nth waveforms." In this case, it is possible to specifically measure the magnetic field strength related to the 1st to Nth sensitivity regions.

[0012] The optically excited magnetic sensor according to the embodiment may be [7] "the optically excited magnetic sensor according to any one of the above [1] to [6], 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.

[0013] The optically excited magnetic sensor according to the embodiment may be [8] "the optically excited magnetic sensor according to any one of the above [1] to [6], 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.

[0014] The optically excited magnetic sensor according to the embodiment may be [9] "the optically excited magnetic sensor according to any one of the above [1] to [8], wherein the magnetic field measuring unit measures the magnetic field strength based on a difference between outputs of the optical sensors corresponding to the two adjacent sensitivity areas." In this case, common mode noise common to the two sensitivity areas is removed, and measurement of a weak magnetic field strength can be realized.

[0015] A magnetoencephalograph according to a second aspect of the embodiment is

[10] "a magnetoencephalograph comprising an optically excited magnetic sensor according to any one of [1] to [9] above that can be positioned around a subject's head and measures the intensity of a magnetic field emitted by the subject."

[0016] This magnetoencephalograph also has the optically excited magnetic sensor, and thus has the above-mentioned advantageous effect of enabling multi-channel measurement of magnetic field strength with a simple configuration. Effect of the Invention

[0017] According to one aspect of the present invention, it is possible to provide an optically excited magnetic sensor and a magnetoencephalograph that are capable of measuring magnetic field strength in a multi-channel manner with a simple configuration. [Brief description of the drawings]

[0018] [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 and the second optical system in FIG. 1 as viewed in the -x-axis direction. [Diagram 3] FIG. 3 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. [Figure 4] FIG. 4 is a graph showing the change over time of the FID acquired by the readout circuit of FIG. [Diagram 5] FIG. 5 is a perspective view showing the bias magnetic field coil of FIG. [Figure 6] Fig. 6(a) is a plan view showing the bias magnetic field coil of Fig. 1. Fig. 6(b) is a side view showing the bias magnetic field coil of Fig. 1. [Figure 7] FIG. 7 is a graph showing the magnetic field strength of the bias magnetic field generated by the bias magnetic field coil. [Figure 8] FIG. 8 is a functional block diagram showing the processing of the read circuit of FIG. [Figure 9]9(a) is a graph showing an output waveform, and FIG. 9(b) is a graph showing an FID waveform in the first resonant frequency band. [Figure 10] FIG. 10 is a perspective view showing a configuration of an optically excited magnetic sensor according to a first modified example. [Figure 11] FIG. 11 is a plan view showing the configuration of the magnetoencephalograph according to the embodiment. [Figure 12] FIG. 12 is a functional block diagram showing the processing of a readout circuit according to a modified example. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] 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.

[0020] 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. In the following explanation, 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.

[0021] 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, bias magnetic field coils 11 and 15, and a tilting coil (electron spin tilting portion) 14. The optically excited magnetic sensor 1 is an axial type sensor. The components of the optically excited magnetic sensor 1 will be described in detail below.

[0022] The cell 2 is a container that seals alkali metal vapor. The cell 2 is arranged along the z-axis direction. Two cells 2 are provided so as to be lined up in the x-axis direction. The cell 2 has a shape of a substantially rectangular parallelepiped and bottomed cylinder having 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 a material such as quartz, sapphire, silicon, Kovar glass, borosilicate glass, etc. The cell 2 is optically transparent to pump light and probe light, which will be described later. A heater 3 or the like is attached to the opposing side surfaces of each of the two cells 2. 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.

[0023] 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.

[0024] 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.

[0025] 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.

[0026] 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.

[0027] Pump light from the pump laser 4 is incident on the cell 2 via a first optical system 6. As shown in Fig. 1 and Fig. 2, 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, and quarter-wave plates 27a, 27b, 27c, and 27d.

[0028] 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 four powers of the pump laser are distributed at a ratio of 1:3 by the polarizing beam splitter 23a.

[0029] 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.

[0030] 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.

[0031] The quarter-wave plate 27a is provided adjacent to the polarizing beam splitter 23a in the -y-axis direction. The quarter-wave plate 27a changes the polarization state of the pump light transmitted through the polarizer 26a to circular polarization and transmits the pump light PLa in the -y-axis direction. The quarter-wave plates 27b, 27c, and 27d are provided adjacent to the polarizing beam splitters 23b and 23c and the total reflection mirror 25 in the -y-axis direction, respectively. The function of the quarter-wave plates 27b, 27c, and 27d is the same as that of the quarter-wave plate 27a, and each transmits the pump light PLb, PLc, and PLd in the -y-axis direction.

[0032] 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.

[0033] 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.

[0034] 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.

[0035] The probe light from the probe laser 5 is incident on the cell 2 via the second optical system 7. The second optical system 7 includes a lens 35, a half-wave plate 36, a polarizing beam splitter 38, and total reflection mirrors 37a to 37c. The polarizing beam splitter 38 is provided adjacent to the half-wave plate 36 in the -y-axis direction, and separates two linearly polarized components perpendicular to each other from the polarized component of the probe light transmitted through the half-wave plate 36, transmits one linearly polarized component in the -y-axis direction, and reflects the other linearly polarized component in the +x-axis direction. The total reflection mirror 37a reflects the linearly polarized component of the probe light transmitted through the polarizing beam splitter 38 in the -z-axis direction and makes it enter one of the cells 2 in its longitudinal direction. The total reflection mirrors 37b and 37c continuously reflect the linearly polarized component of the probe light reflected by the polarizing beam splitter 38 in the -y-axis and -z-axis directions and makes it enter the other cell 2 in its longitudinal direction. The second optical system 7 thus configured is configured so that each of the two systems of probe light QL can be incident on four sensitivity regions ARa to ARd aligned in the longitudinal direction (z-axis direction) inside the cell 2 and intersecting with the pump light.

[0036] The sensitive regions ARa to ARd are regions spaced apart from each other in the longitudinal direction inside the cell 2. The sensitive regions ARa to ARd are regions that intersect with the pump light PLa to PLd inside the cell 2. The sensitive regions ARa to ARd are intersection regions where the probe light QL and the pump light PLa to PLd intersect. The probe laser 5 and second optical system 7 constitute a probe light incident section that makes the probe light QL incident on the sensitive regions ARa to ARd along the x-axis direction.

[0037] The bias magnetic field coils 11 and 15 are coils that apply a bias magnetic field By in the y-axis direction inside the cell 2, and determine the resonance frequency of the electron spins of the alkali metal atoms excited inside the cell 2. The bias magnetic field coil 11 applies a reference bias magnetic field in the same direction as the incidence direction of the pump light on the sensitivity areas ARa to ARd, inside the cell 2. This aligns the rotation axes of the electron spins of the alkali metal atoms excited inside the cell 2 in the y-axis direction.

[0038] The bias magnetic field coil 15 cooperates with the bias magnetic field coil 11 to apply a plurality of bias magnetic fields By with different intensities to each of the sensitivity regions ARa to ARd inside each cell 2. The bias magnetic field coil 15 generates bias magnetic fields By with step-like magnetic field intensities in the four sensitivity regions ARa to ARd inside each cell 2. Details of the bias magnetic field coil 15 will be described later. When the magnetic field intensity of the bias magnetic field applied by the bias magnetic field coils 11 and 15 is 14 μT and the alkali metal atom is potassium, the resonant frequency of the electron spin is 100 kHz. 14 μT is a magnetic field intensity that can be easily formed.

[0039] The tilting coil 14 generates and irradiates an RF signal Rf in order to tilt the direction of the rotation axis 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 of the same frequency as the resonance frequency of the electron spin by the bias magnetic field coil 11 (100 kHz when the strength of the reference bias magnetic field by the bias magnetic field coil 11 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.

[0040] The third optical system 9 includes a total reflection mirror 46, a half-wave plate 47, a polarizing beam splitter 48, and a total reflection mirror 49. The total reflection mirror 46 reflects the probe light that has passed through the cell 2 in the -z-axis direction in the +y-axis direction. The half-wave plate 47 rotates the polarization plane of the probe light reflected by the total reflection mirror 46. The half-wave plate 47 is supported rotatably about an axis along the y-axis so that the rotation angle of the polarization plane of the probe light can be adjusted. The polarizing beam splitter 48 splits the probe light that has passed through the half-wave plate 47 into two linearly polarized components that are perpendicular to each other, transmits one of the linearly polarized components in the +y-axis direction to make it incident on the optical sensor group 8, and reflects the other linearly polarized component in the +z direction. The total reflection mirror 49 reflects the other linearly polarized component reflected by the polarizing beam splitter 48 in the +y-axis direction to make it incident on the optical sensor group 8. Two third optical systems 9 are provided corresponding to the two cells 2.

[0041] The optical sensor group 8 is an element group that detects the polarization plane angle of the probe light QL that has passed through the sensitivity regions ARa to ARd, and includes two optical sensor pairs (optical sensors) 43a, 43b corresponding to the two cells 2, respectively. The optical sensor pair 43a has two photodiodes 44a, 45a. The photodiode 44a outputs a detection signal that detects the intensity of the other linearly polarized component of the probe light QL, and the photodiode 45a outputs a detection signal that detects the intensity of one linearly polarized component of the probe light QL. Similarly, the optical sensor pair 43b has two photodiodes.

[0042] The readout circuit 10 is a processing circuit electrically connected to the two optical sensor pairs 43a, 43b constituting the optical sensor group 8, and processes the detection signals output from the two optical sensor pairs 43a, 43b. That is, the readout circuit 10 includes a reading unit that reads the detection signals output from the two optical sensor pairs 43a, 43b (outputs of the optical sensor pairs 43a, 43b), 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.

[0043] 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.

[0044] 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. 3 and Fig. 4. Fig. 3 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. 4 is a graph showing the time change of FID (Free Induction Decay) acquired by the readout circuit 10.

[0045] 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 and 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 two optical sensor pairs 43a and 43b during the relaxation time. Then, the readout circuit 10 acquires the time change of the FID by taking the difference between the detection signals of the two photodiodes 44a and 45a constituting the optical sensor pair 43a (see FIG. 4). 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 pair 43b.

[0046] The FID waveform acquired in this embodiment is a mixed waveform including components of the first to fourth resonance frequencies corresponding to the sensitivity regions ARa to ARd. Here, an FID waveform including a resonance frequency component corresponding to the sensitivity region ARa among the mixed waveforms of the FID will be described as an example. The FID shows how the electron spin of an alkali metal atom relaxes, and the frequency of the precession of the electron spin changes in response to the fluctuation of the external magnetic field in the sensitivity region ARa, so that the frequency of the vibration changes due to the fluctuation of 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 formula for the FID waveform at the beginning of the relaxation time;

number

[0047] As shown in Fig. 5 and Fig. 6, the bias magnetic field coil 15 includes a first coil 15a, a second coil 15b, a third coil 15c, a fourth coil 15d, and extra-cell coils 16a and 16b. The first coil 15a is a coil corresponding to a sensitivity region (first sensitivity region) ARa present at the end of each cell 2 in the +z-axis direction, and is arranged in a pair so as to sandwich the sensitivity region ARa in the y-axis direction. The first coil 15a is formed with a first number of turns in one rotation direction (first rotation direction). The first coil 15a is provided so as to bridge a pair of cells 2 when viewed from the y-axis direction. The first coil 15a is formed so as to surround the sensitivity regions ARa of each cell 2 together. The first number of turns is, for example, 3.

[0048] The second coil 15b is a coil corresponding to the sensitivity region (second sensitivity region) ARb of each cell 2, and is arranged in a pair so as to sandwich the sensitivity region ARb in the y-axis direction. The second coil 15b is formed with a second number of turns in one rotation direction. The second coil 15b is provided so as to bridge the pair of cells 2 when viewed from the y-axis direction. The second coil 15b is formed so as to surround the sensitivity regions ARb of each cell 2 together when viewed from the y-axis direction. The second number of turns is, for example, 1.

[0049] The third coil 15c is a coil corresponding to the sensitivity region (third sensitivity region) ARc of each cell 2, and is arranged in a pair so as to sandwich the sensitivity region ARc in the y-axis direction. The third coil 15c is formed with a second number of turns in another rotation direction (second rotation direction) opposite to the first rotation direction. The third coil 15c is provided so as to bridge the pair of cells 2 when viewed from the y-axis direction. The third coil 15c is formed so as to surround the sensitivity regions ARc of each cell 2 collectively when viewed from the y-axis direction.

[0050] The fourth coil 15d corresponds to the sensitivity area (fourth sensitivity area) ARd at the end of each cell 2 in the -z-axis direction, and is arranged in a pair to sandwich the sensitivity area ARd in the y-axis direction. The fourth coil 15d is formed with the first number of turns in the other rotation direction. The fourth coil 15d is provided so as to bridge the pair of cells 2 when viewed from the y-axis direction. The fourth coil 15d is formed so as to surround the sensitivity areas ARd of each cell 2 together when viewed from the y-axis direction.

[0051] The extracell coil 16a is disposed away from the pair of cells 2 in the +z-axis direction as viewed from the y-axis direction. The extracell coils 16a are provided in a pair spaced apart in the y-axis direction. One of the extracell coils 16a is adjacent to the first coil 15a in the +z-axis direction and is located at the same position in the y-axis direction. The other extracell coil 16a is adjacent to the other first coil 15a in the +z-axis direction and is located at the same position in the y-axis direction. The extracell coil 16b is disposed away from the pair of cells 2 in the -z-axis direction as viewed from the y-axis direction. The extracell coils 16b are provided in a pair spaced apart in the y-axis direction. One of the extracell coils 16b is adjacent to one of the fourth coils 15d in the -z-axis direction and is located at the same position in the y-axis direction. The other extracell coil 16b is adjacent to the other fourth coil 15d in the -z-axis direction and is located at the same position in the y-axis direction.

[0052] The first to fourth coils 15a to 15d generate a bias magnetic field By inside each cell 2, which varies stepwise, as shown in Fig. 7. The bias magnetic field By has constant (flat) intensity ranges F1 to F4 with different intensities at each z-axis position corresponding to the four sensitivity regions ARa to ARd. The bias magnetic field By varies stepwise symmetrically with respect to the reference bias magnetic field α.

[0053] For example, by passing a predetermined current through the first to fourth coils 15a to 15d, the bias magnetic field By is added by 0.14 μT to the reference bias magnetic field α in the first coil 15a, added by 0.07 μT to the reference bias magnetic field α in the second coil 15b, subtracted by 0.07 μT from the reference bias magnetic field α in the third coil 15c, and subtracted by 0.14 μT from the reference bias magnetic field α in the fourth coil 15d. As described above, the bias magnetic field coil 15 can set the resonance frequencies of the electron spins in the four sensitivity regions ARa to ARd to the first to fourth resonance frequencies that are different from each other. For example, when the alkali metal atom is potassium, the first resonance frequency is 98.5 kHz, the second resonance frequency is 99.5 kHz, the third resonance frequency is 100.5 kHz, and the fourth resonance frequency is 101.5 kHz.

[0054] As shown in Figures 5 and 6, a substrate 17 is disposed on one side and the other side of each cell 2 in the y-axis direction. The substrate 17 is a printed circuit board whose thickness direction is in the y-axis direction. The substrate 17 has a rectangular shape that is elongated in the z-axis direction. The substrate 17 is disposed so as to bridge a pair of cells 2 in the x-axis direction.

[0055] The first coil 15a, the second coil 15b, the third coil 15c, the fourth coil 15d, the extra-cell coil 16a, and the extra-cell coil 16b are provided on the substrate 17 on one side. The first coil 15a, the second coil 15b, the third coil 15c, the fourth coil 15d, the extra-cell coil 16a, and the extra-cell coil 16b are provided on the substrate 17 on the other side.

[0056] The substrate 17 is formed with through holes 19 through which the pump light passes. The through holes 19 are shaped like a track with the longitudinal direction in the x-axis direction. Four through holes 19 are formed corresponding to the sensitivity regions ARa to ARd. The four through holes 19 are located inside the first to fourth coils 15a to 15d, respectively. The four through holes 19 are arranged so as to overlap with the sensitivity regions ARa to ARd of each cell 2 when viewed from the y-axis direction, and expose the sensitivity regions ARa to ARd of each cell 2. The through holes 19 may be formed in the substrate 17 arranged on the entrance surface side or the exit surface side of the pump light in the cell 2.

[0057] As described above, the readout circuit 10 acquires, as an output waveform, a mixed waveform of the FID including components of the first to fourth resonance frequencies, which are the resonance frequencies of the electron spins in the sensitivity areas ARa to ARd, based on the outputs of the optical sensor pair 43a, 43b. As shown in Fig. 8, the readout circuit 10 filters the acquired output waveform WI using a bandpass filter BF to acquire FID waveforms (first to fourth waveforms) WO in each band of the first to fourth resonance frequencies.

[0058] As an example, the readout circuit 10 obtains an output waveform WI (see FIG. 9(a)), which is a mixed waveform including a plurality of frequency components reflecting different bias magnetic fields By in each of the sensitivity regions ARa to ARd. The output waveform WI is a sum of waveforms including components of first to fourth resonance frequencies different from each other. The readout circuit 10 applies a bandpass filter BF of a frequency band (fi±Δf) corresponding to the first to fourth resonance frequencies of each of the sensitivity regions ARa to ARd. As a result, the readout circuit 10 extracts a waveform WO (see FIG. 9(b)) of FID in each band of the first to fourth resonance frequencies.

[0059] Then, the readout circuit 10 performs fitting using the above-mentioned function V for the waveform WO of each FID to derive the frequency a1 of the waveform WO of each FID. The derived frequency a1 is acquired as a measurement value of the external magnetic field in the sensitivity regions ARa to ARd. Note that the FID waveform shown in FIG. 9(b) is in the band of the first resonance frequency (e.g., 98.2 kHz to 98.8 kHz), and the fitting result indicates the magnetic field strength corresponding to the sensitivity region ARa. For example, when the measurement magnetic field is 100 pT or less, a band width of 0.6 kHz is sufficient for measurement and there is no interference with adjacent frequencies.

[0060] In the optically excited magnetic sensor 1 described above, pump light is emitted into each cell 2 in which alkali metal vapor is sealed, thereby generating (exciting) electron spins of alkali metal atoms. Probe light QL is emitted into sensitivity regions ARa to ARd inside each cell 2, and the optical sensor pair 43a, 43b detects the polarization plane angle of the probe light QL that has passed through the sensitivity regions ARa to ARd. At this time, the bias magnetic field coil 15 generates a bias magnetic field By that varies stepwise inside each cell 2, and the resonance frequencies of the electron spins in the sensitivity regions ARa to ARd are set to first to fourth resonance frequencies that are different from each other.

[0061] The readout circuit 10 acquires an output waveform WI, which is a mixed waveform of the FID including components of the first to fourth resonant frequencies for each cell 2, based on the outputs of the optical sensor pair 43a, 43b. The readout circuit 10 filters the output waveform WI to acquire an FID waveform WO in each band of the first to fourth resonant frequencies. The readout circuit 10 performs fitting using a function V for the waveform WO 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 areas ARa to ARd.

[0062] As described above, in this embodiment, the strength of the bias magnetic field By is changed for each of the multiple sensitivity regions ARa to ARd, and the magnetic field strength for the multiple sensitivity regions ARa to ARd can be obtained using one probe light QL. That is, multi-channel measurement of magnetic field strength is possible with a simple configuration. In other words, by changing the strength of the bias magnetic field By for each of the multiple sensitivity regions ARa to ARd, multi-channel measurement of the magnetic field Bm is possible with one probe light QL. This embodiment, which can reduce the number of probe lights QL, is important in terms of miniaturization.

[0063] In this embodiment, the bias magnetic field coil 15 includes first to fourth coils 15a to 15d corresponding to the sensitivity regions ARa to ARd, respectively. In this case, it is possible to specifically realize changing the strength of the bias magnetic field for each of the sensitivity regions ARa to ARd.

[0064] This embodiment includes a substrate 17 on which the first to fourth coils 15a to 15d are provided, and a passage hole 19 through which the pump light passes is formed in the substrate 17. In this case, it is possible to specifically implement the bias magnetic field coil 15 without preventing the pump light from being incident on the cell 2.

[0065] In this embodiment, the bias magnetic field coil 15 includes extracellular coils 16a and 16b arranged adjacent to the sensitivity areas ARa and ARd at the ends of the cell 2 and away from the cell 2 when viewed from the y-axis direction. In this case, the strength of the bias magnetic field By applied to the sensitivity areas ARa and ARd at one end side and the other end side of the cell 2 can be appropriately maintained by the presence of the extracellular coils 16a and 16b. Specifically, in the example of the bias magnetic field By shown in FIG. 7, the flat intensity range F1 located at the furthest side in the -z axis direction can be prevented from dropping (decreasing and not becoming flat) on the -z axis side. In addition, the flat intensity range F4 located at the furthest side in the -z axis direction can be prevented from rising (increasing and not becoming flat) on the +z axis side.

[0066] In this embodiment, the first coil 15a is formed with a first number of turns in one rotation direction, the second coil 15b is formed with a second number of turns in one rotation direction, the third coil 15c is formed with a second number of turns in the other rotation direction, and the fourth coil 15d is formed with the first number of turns in the other rotation direction. In this case, it is specifically possible to change the strength of the bias magnetic field for each of the sensitivity regions ARa to ARd. It is possible to realize a bias magnetic field By with a stepwise difference in magnetic field strength in the four sensitivity regions ARa to ARd in the cell 2.

[0067] In this embodiment, the readout circuit 10 acquires an output waveform WI, which is a mixed waveform of the FID including components of the first to fourth resonance frequencies, filters the output waveform WI using a bandpass filter BF, and acquires a waveform WO of each FID in each band of the first to fourth resonance frequencies. Then, the readout circuit 10 determines the magnetic field strength for the sensitivity regions ARa to ARd based on the frequency of the waveform WO of each FID. In this case, it becomes possible to specifically measure the magnetic field strength for the sensitivity regions ARa to ARd.

[0068] In this embodiment, a bias magnetic field coil 11 is provided to apply a bias magnetic field in the same direction as the pump light and determine the resonance frequency of the electron spin, and a tilting coil 14 is provided to tilt 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.

[0069] 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.

[0070] 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.

[0071] 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 beam diameter of the pump light is preferably 4 mm and the beam diameter of the probe light is preferably 3 mm. When providing four sensitivity regions at intervals of 10 mm, due to the size limitations of the polarizing beam splitters 23a, 23b, and 23c that split the pump light, it is desirable to shape the pump light into a substantially rectangular shape (for example, 10 mm×4 mm) and increase the area that intersects with the probe light while restricting the pump light in a direction perpendicular to the area.

[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] Fig. 10 is a perspective view showing the configuration of an optically excited magnetic sensor 1A according to a first modified example. The optically excited magnetic sensor 1A differs from the optically excited magnetic sensor 1 in that the incident direction of the probe light to the cell 2 is the -z-axis direction, the application direction of the bias magnetic field By is the +x-axis direction, and the irradiation direction of the RF signal Rf is the -y-axis direction. Only the configuration different from the optically excited magnetic sensor 1 will be described below. Note that, for convenience, the heater 3, the bias magnetic field coil 11, and the tilting coil 14 are omitted in Fig. 10.

[0074] The first optical system 6 includes a total reflection mirror 29. The total reflection mirror 29 reflects the pump light transmitted through the half-wave plate 22 in the -z-axis direction and makes it incident on the polarizing beam splitter 23a. The first optical system 6 is configured such that the four systems of pump light transmitted through the four quarter-wave plates 27a, 27b, 27c, and 27d can be incident on the sensitivity regions ARa to ARd inside the cell 2 along the x-axis direction.

[0075] The optically excited magnetic sensor 1A includes a bias magnetic field coil 115 instead of the bias magnetic field coil 15 (see FIG. 1). The bias magnetic field coil 115 has a function similar to that of the bias magnetic field coil 15, and generates a bias magnetic field whose magnetic field strength differs stepwise in four sensitivity areas ARa to ARd in each cell 2. The bias magnetic field coil 115 includes first to fourth coils 115a to 115d provided in each of the two cells 2.

[0076] The first coil 115a is a coil corresponding to the sensitive region ARa, and is arranged in a pair to sandwich the sensitive region ARa in the x-axis direction. The first coil 115a is formed so as to surround the sensitive region ARa when viewed from the x-axis direction. The second coil 115b is a coil corresponding to the sensitive region ARb, and is arranged in a pair to sandwich the sensitive region ARb in the x-axis direction. The second coil 115b is formed so as to surround the sensitive region ARb when viewed from the x-axis direction. The third coil 115c is a coil corresponding to the sensitive region ARc, and is arranged in a pair to sandwich the sensitive region ARc in the x-axis direction. The third coil 115c is formed so as to surround the sensitive region ARc when viewed from the x-axis direction. The fourth coil 115d is a coil corresponding to the sensitive region ARd, and is arranged in a pair to sandwich the sensitive region ARd in the x-axis direction. The fourth coil 115d is formed so as to surround the sensitive region ARd when viewed from the x-axis direction.

[0077] The optically excited magnetic sensor 1A can also obtain a measurement value of the magnetic field Bm (see FIG. 2) in the -y-axis direction for the sensitivity areas ARa to ARd. The optically excited magnetic sensor 1A also has the above-mentioned advantageous effect of enabling multi-channel measurement of magnetic field strength with a simple configuration. In the optically excited magnetic sensor 1A, the pump light is irradiated so as to cross the multiple cells 2, so that the multiple cells 2 can be excited simultaneously. For example, when adopting a multi-channel module configuration such as 4x4 (a configuration in which four sensitivity areas are set for each of the two cells 2), the branching of the pump light can be minimized to facilitate the configuration, and the optically excited magnetic sensor 1A can be made smaller.

[0078] 11 is a plan view showing a configuration of a magnetoencephalograph 100 according to an embodiment. The magnetoencephalograph 100 includes a plurality of optically excited magnetic sensors 1 or 1A arranged so as to be arranged around the head (subject) S of a subject. In the illustrated example, the plurality of optically excited magnetic sensors 1 or 1A included in the magnetoencephalograph 100 are arranged to face each other in the y-axis direction across the head S, and are arranged to face each other in the x-axis direction across the head S. A pair of optically excited magnetic sensors 1 or 1A facing each other in the y-axis direction are arranged such that two cells 2 are longitudinally aligned in the x-axis direction and aligned in the y-axis direction. A pair of optically excited magnetic sensors 1 or 1A facing each other in the x-axis direction are arranged such that two cells 2 are longitudinally aligned in the y-axis direction and aligned in the x-axis direction.

[0079] In addition, the bias magnetic field coil 11, the bias magnetic field gradient correction coils 12, 13, and the tilting coil 14 are shared between the multiple optically excited magnetic sensors 1 and the multiple 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, 1A measure the strength of the magnetic field emitted from the head S.

[0080] 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.

[0081] 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. Since the magnetoencephalograph 100 also includes the optically excited magnetic sensors 1, 1A, it has the above-mentioned advantageous effect of enabling multi-channel measurement of magnetic field strength with a simple configuration.

[0082] In the above embodiment, the readout circuit 10 obtains the waveform WO of the FID in each band of the first to fourth resonant frequencies by filtering the obtained output waveform WI with the bandpass filter BF, but the present invention is not limited to this. For example, the following signal processing procedure using quadrature detection may be adopted. That is, as shown in FIG. 12, the readout circuit 10 obtains the output waveform WI, which is a mixed waveform including a plurality of frequency components reflecting different bias magnetic fields By in each of the sensitivity regions ARa to ARd. The readout circuit 10 generates a reference signal (a sine wave or cosine wave of a predetermined frequency) and multiplies the output waveform WI. The readout circuit 10 applies a low-pass filter LF with a cutoff frequency. As a result, a waveform after quadrature detection WT, which is a waveform of the FID reflecting a frequency difference with the reference signal, is obtained. Fitting is performed on the waveform after quadrature detection WT to obtain a frequency difference with the reference signal in each of the bands of the first to fourth resonant frequencies.

[0083] In the above embodiment, the four sensitivity regions ARa to ARd are provided, but the configuration may be changed to include two sensitivity regions. Even in this case, common mode noise can be removed and detection accuracy can be improved by obtaining the difference between the measurement values ​​related to the two sensitivity regions. In the above embodiment, one, three, or five or more sensitivity regions may be provided. In other words, the multiple sensitivity regions may include first to N sensitivity regions (N is an integer of 2 or more). In this case, the bias magnetic field coils 15 and 115 may apply first to N bias magnetic fields to the first to N sensitivity regions, respectively, and the resonance frequencies of the first to N sensitivity regions may be the first to N resonance frequencies. The readout circuit 10 may acquire a mixed waveform of FID including components of the first to N resonance frequencies, filter the acquired mixed waveform of FID, acquire first to N waveforms of FID in each band of the first to N resonance frequencies, derive each frequency of the first to N waveforms, and obtain the magnetic field strength related to the first to N sensitivity regions based on each frequency of the derived first to N waveforms.

[0084] The above embodiment is configured to include two cells 2, but may be configured to include one or three or more cells 2. In the above embodiment, as long as one heater 3 can sufficiently heat the two cells 2, there may be only one heater 3. In this case, the two cells 2 may be disposed close to each other, and the interval between the two cells 2 may be 15 mm, for example.

[0085] In the above embodiment, it is preferable that the gradient of the bias magnetic field By in the x-axis direction and the z-axis direction is small in order to achieve high sensitivity. Therefore, the bias magnetic field gradient correction coil, such as a parallel quadrifilar coil, may be provided to correct the gradient of the bias magnetic field By and improve the uniformity of the bias magnetic field By. In this case, the bias magnetic field gradient correction coil may correct the gradient so that the relaxation time of the FID in each of the first to fourth resonant frequency bands becomes long. 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]

[0086] 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, 14...tilting coil (electron spin tilt section), 15,115...bias magnetic field coil, 15a,115a...first coil, 15b,115b...second coil, 15c,115c...third coil, 15d,115d...fourth coil, 43a,43b...optical sensor pair (optical sensor) 100...magnetoencephalograph, ARa to ARd...sensitivity region, Bm...magnetic field, By...bias magnetic field, PL,PLa to PLd...pump light, QL...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 on a plurality of sensitivity regions 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 plurality of sensitivity regions 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; 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; a magnetic field measuring unit that measures a magnetic field intensity related to the sensitivity region based on an output of the optical sensor; The bias magnetic field coil includes: An optically excited magnetic sensor, comprising: a plurality of bias magnetic fields each having a different strength applied to each of the plurality of sensitive regions.

2. The plurality of sensitive regions include at least a first sensitive region and a second sensitive region, The bias magnetic field coil includes: A first coil corresponding to the first sensitive area; and a second coil corresponding to the second sensitive area.

3. a substrate on which the first coil and the second coil are provided, 3. The optically excited magnetic sensor according to claim 2, wherein the substrate is formed with a through hole through which the pump light passes.

4. the first sensitive area is at an edge of the cell; The bias magnetic field coil includes:

4. The optically excited magnetic sensor of claim 2, further comprising an extracellular coil adjacent to the first coil and spaced from the cell when viewed in the first direction.

5. the plurality of sensitive regions include a first sensitive region, a second sensitive region adjacent to the first sensitive region, a third sensitive region adjacent to the second sensitive region, and a fourth sensitive region adjacent to the third sensitive region; The bias magnetic field coil includes: a pair of first coils arranged to sandwich the first sensitive region in the first direction and formed with a first number of turns in a first rotation direction; a pair of second coils arranged to sandwich the second sensitive region in the first direction and formed with a second number of turns in the first rotation direction; a pair of third coils arranged to sandwich the third sensitive region in the first direction and formed with the second number of turns in a second rotation direction opposite to the first rotation direction; 2. The optically excited magnetic sensor according to claim 1, further comprising: a pair of fourth coils arranged to sandwich the fourth sensitivity region in the first direction and formed with the first number of turns in the second rotation direction.

6. The plurality of sensitivity regions include first to Nth sensitivity regions (N is an integer of 2 or more), the bias magnetic field coil applies first to N bias magnetic fields to the first to N sensitivity regions, respectively, and defines the resonant frequencies of the first to N sensitivity regions as first to N resonant frequencies; The magnetic field measuring unit is A mixed waveform of free induction decay including components of the first to Nth resonant frequencies is obtained based on an output of the optical sensor; Filtering the mixed waveform with a bandpass filter to obtain first to Nth waveforms of free induction decay in each band of the first to Nth resonant frequencies, and deriving the frequencies of the first to Nth waveforms; 3. The optically excited magnetic sensor according to claim 1, wherein the magnetic field strengths related to the first to N sensitivity regions are determined based on the frequencies of the derived first to N waveforms.

7. 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.

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

9. 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.

10. 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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