Photoexcited magnetic sensor and magnetoencephalographic meter including the same

Abstract

To easily adjust an optical system of a photosensor.SOLUTION: A photoexcited magnetic sensor 1 includes: a cell 2 into which an alkali metal vapor is sealed; a pump laser 4 for emitting pump light for exciting alkali metal atoms composing the alkali metal vapor; a probe laser 5 for emitting probe light for detecting a change in an electron spin in an excited state of the alkali metal atom to a sensitivity region crossing pump light in the cell 2; photosensor pairs 43a to 43d for detecting a polarization surface angle of the probe light passing through the sensitivity region; and a read circuit 10 for generating a measurement signal for measuring the strength of a magnetic field in the sensitivity region on the basis of the output of the photosensor pairs 43a to 43d. The read circuit 10 amplifies the output of the photosensor pairs 43a to 43d with predetermined gain under a first condition and transmits it with gain less than predetermined gain under a second condition.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 in a cell are excited by pump light, and 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 conventional optically excited magnetic sensor described above, before measuring the external magnetic field, the optical system of the optical sensor is adjusted to set the incidence state of the probe light to the optical sensor. On the other hand, in order to increase the measurement accuracy of the external magnetic field, it is preferable to have a function to amplify the output of the optical sensor. Therefore, it tends to be difficult to adjust the optical system of the optical sensor.

[0005] Therefore, one aspect of the embodiment has been made in consideration of such problems, and has an object to provide an optically excited magnetic sensor that can facilitate adjustment of the optical system of an optical sensor, and a magnetoencephalograph including the same. [Means for solving the problem]

[0006] An optically excited magnetic sensor according to a first aspect of an embodiment includes a cell in which alkali metal vapor is sealed, a pump laser that emits pump light for exciting alkali metal atoms that constitute the alkali metal vapor, a probe laser that emits probe light for detecting changes in electron spin of the alkali metal atoms in an excited state into a sensitivity region within the cell where the probe light intersects with the pump light, an optical sensor that detects the angle of the polarization plane of the probe light that has passed through the sensitivity region, and a readout circuit that generates a measurement signal for measuring the strength of a magnetic field in the sensitivity region based on an output of the optical sensor, and the readout circuit amplifies the output of the optical sensor with a predetermined gain under a first condition and transmits it with a gain less than the predetermined gain under a second condition.

[0007] Alternatively, a magnetoencephalograph according to a second aspect of the embodiment includes the optically excited magnetic sensor arranged so as to be positioned around a head of a subject, and measures the intensity of a magnetic field emitted from the subject.

[0008] According to the first or second aspect, pump light is emitted to a cell in which an alkali metal is sealed, thereby generating (exciting) electron spins of alkali metal atoms. Furthermore, probe light is emitted to a sensitivity region in the cell where the probe light intersects with the pump light, and an optical sensor detects the polarization plane angle of the probe light that has passed through the sensitivity region. In addition, a measurement signal for measuring the strength of the magnetic field in the sensitivity region is generated by a readout circuit based on the output of the optical sensor. Here, the readout circuit amplifies the output of the optical sensor with a predetermined gain under the first condition, and transmits the output of the optical sensor with a gain less than the predetermined gain under the second condition, thereby improving the measurement accuracy of the magnetic field under the first condition, and easily adjusting the optical system of the optical sensor based on the output of the optical sensor under the second condition.

[0009] In the first aspect, it is preferable to further include a bias magnetic field coil that applies a magnetic field in the same direction as the pump light and determines the resonance frequency of the electron spin, and a tilting means that tilts the direction of the electron spin to a direction perpendicular to the pump light. In this case, by measuring the strength of the magnetic field based on the frequency of the change in the polarization plane angle of the probe light detected using the measurement signal, it is possible to maintain the measurement sensitivity of the magnetic field without being affected by the environmental magnetic field.

[0010] In the first aspect, it is also preferable that the tilting means irradiates an RF signal having the same frequency as the resonance frequency, thereby making it possible to measure the magnetic field based on the frequency of the change in the angle of the polarization plane of the probe light by a simple means.

[0011] In the first aspect, it is also preferable that the tilting means irradiates pulsed light. In this case, it is possible to realize measurement of the magnetic field based on the frequency of the change in the angle of the plane of polarization of the probe light by simple means.

[0012] In the first aspect, it is preferable that there are two or more sensitivity regions where the pump light and the probe light intersect, and the readout circuit generates the measurement signal based on a difference between outputs of the optical sensors corresponding to two adjacent sensitivity regions. In this case, by using the difference between outputs of the optical sensors corresponding to two adjacent sensitivity regions, common mode noise common to the two sensitivity regions is removed, and measurement of a weak magnetic field can be realized.

[0013] Furthermore, in the first aspect, it is also preferable that the readout circuit has an instrumentation amplifier having a predetermined gain, an adjustment amplifier having a gain less than the predetermined gain, and a switch for switching the connection between the output of the optical sensor and the input of the instrumentation amplifier and the input of the adjustment amplifier. In this case, by switching between the first condition and the second condition by switching the switch between measuring the magnetic field and adjusting the optical system of the optical sensor, it is possible to improve the measurement accuracy of the magnetic field and to easily adjust the optical system of the optical sensor based on the output of the optical sensor.

[0014] Furthermore, in the first aspect, it is also preferable that the readout circuit amplifies with a predetermined gain in a frequency band when the strength of the magnetic field is measured, and transmits with a gain less than the predetermined gain in a frequency band lower than the frequency band when the optical system of the optical sensor is adjusted. In this way, in the readout circuit, the output of the optical sensor is amplified with a predetermined gain under a first condition when the magnetic field is measured, and the output of the optical sensor is transmitted with a gain less than the predetermined gain under a second condition when the optical system of the optical sensor is adjusted. As a result, it is possible to improve the measurement accuracy of the magnetic field, and to easily adjust the optical system of the optical sensor based on the output of the optical sensor.

[0015] Furthermore, in the first aspect, it is also preferable that the readout circuit has a high-pass filter that amplifies signals above a frequency band used for measuring the strength of the magnetic field with a predetermined gain. In this way, with a simple circuit configuration, it is possible to amplify the output of the optical sensor with a predetermined gain when measuring the magnetic field. The optically excited magnetic sensor according to the embodiment includes: [1] "a cell in which alkali metal vapor is sealed; a pump laser that emits pump light for exciting alkali metal atoms that constitute the alkali metal vapor; a probe laser that emits probe light for detecting a change in electron spin in an excited state of the alkali metal atom to a sensitivity region in the cell where the probe light intersects with the pump light; an optical sensor for detecting an angle of a plane of polarization of the probe light passing through the sensitive region; a readout circuit that generates a measurement signal for measuring the strength of a magnetic field in the sensitivity region based on an output of the optical sensor; The read circuit includes: The output of the optical sensor is amplified by a predetermined gain under a first condition, and is transmitted by a gain less than the predetermined gain under a second condition. The name of the device is "optically excited magnetic sensor."

[0016] The optically excited magnetic sensor according to the embodiment includes: [2] "a bias magnetic field coil that applies a magnetic field in the same direction as the pump light and determines the resonance frequency of the electron spins; and and tilting means for tilting the direction of the electron spins in a direction perpendicular to the pump light. The optically excited magnetic sensor may be the optically excited magnetic sensor described in [1] above.

[0017] In the optically excited magnetic sensor according to the embodiment, [3] "the tilting means irradiates an RF signal having the same frequency as the resonance frequency, The optically excited magnetic sensor may be the optically excited magnetic sensor described in [2] above.

[0018] The optically excited magnetic sensor according to the embodiment has the following features: [4] "The tilting means irradiates pulsed light, The optically excited magnetic sensor may be the optically excited magnetic sensor described in [2] above.

[0019] The optically excited magnetic sensor according to the embodiment includes: [5] "two or more sensitive regions where the pump light and the probe light intersect; the readout circuit generates the measurement signal based on a difference between outputs of the optical sensors corresponding to two adjacent sensitivity regions. The optically excited magnetic sensor may be the one according to any one of the above items [1] to [4].

[0020] In the optically excited magnetic sensor of the embodiment, [6] "the readout circuit has an instrumentation amplifier having the predetermined gain, an adjustment amplifier having a gain less than the predetermined gain, and a switch for switching a connection between an output of the optical sensor and an input of the instrumentation amplifier and an input of the adjustment amplifier. The optically excited magnetic sensor may be the one according to any one of the above items [1] to [5].

[0021] In the optically excited magnetic sensor of the embodiment, [7] "the readout circuit amplifies with a predetermined gain in a frequency band when the strength of the magnetic field is measured, and transmits with a gain less than the predetermined gain in a frequency band lower than the frequency band when the optical system of the optical sensor is adjusted. The optically excited magnetic sensor may be the one according to any one of the above items [1] to [5].

[0022] In the optically excited magnetic sensor according to the embodiment, [8] "the readout circuit has a high-pass filter that amplifies a signal above a frequency band for measuring the strength of the magnetic field by a predetermined gain, The optically excited magnetic sensor may be the one described in [7] above.

[0023] The magnetoencephalograph of the embodiment is [9] "a magnetoencephalograph comprising an optically excited magnetic sensor according to any one of [1] to [8] above, which is positionable around a subject's head and measures the intensity of a magnetic field emitted by the subject." Effect of the Invention

[0024] According to one aspect of the present invention, it is possible to easily adjust the optical system of the optical sensor. [Brief description of the drawings]

[0025] [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 6 in FIG. 1 as viewed in the -x-axis direction. [Diagram 3] FIG. 3 is a plan view of the second optical system 7, the optical sensor group 8, and the third optical system 9 in FIG. 1, as viewed in the -y-axis direction. [Figure 4] FIG. 4 is a timing chart showing the timing of the pump light, the RF signal, and the probe light during the measurement process of the external magnetic field by the readout circuit 10 of FIG. [Diagram 5] FIG. 5 is a graph showing the change over time of the FID acquired by the readout circuit 10 of FIG. [Figure 6] FIG. 6 is a circuit diagram showing a configuration of the reading section 10a of the read circuit 10 of FIG. [Figure 7] FIG. 7 is a circuit diagram showing a configuration of a reading section 10a of a readout circuit 10 according to a modified example. [Figure 8]FIG. 8 is a perspective view showing a configuration of an optically excited magnetic sensor according to a first modified example. [Figure 9] FIG. 9 is a perspective view showing a configuration of an optically excited magnetic sensor according to a second modified example. [Figure 10] FIG. 10 is a plan view showing the configuration of the magnetoencephalograph according to the embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0026] 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.

[0027] Hereinafter, an embodiment of the present invention will be described in detail with reference to the accompanying drawings. In the description of the drawings, the same elements are given the same reference numerals, and duplicated explanations will be omitted.

[0028] FIG. 1 is a perspective view showing a configuration of an optically excited magnetic sensor 1 according to an embodiment. The optically excited magnetic sensor 1 is a device for measuring an external magnetic field by utilizing optical pumping. In this embodiment, the optically excited magnetic sensor 1 is described as being used for brain magnetism measurement, but the application is not limited thereto. In one example, the optically excited magnetic sensor 1 measures a brain magnetic field. For convenience of explanation, in FIG. 1, the direction along the magnetic field direction of the measurement target is the y axis, and two axes perpendicular to the y axis that constitute a three-dimensional coordinate system together with the y axis are the x axis and the z axis. As will be described later, the direction along the x axis corresponds to the incident direction of the probe light in the measurement region, and the direction along the y axis corresponds to the incident direction of the pump light in the measurement region. In the following description, the positive and negative directions along the x axis are respectively written as the +x axis direction and the -x axis direction, the positive and negative directions along the y axis are respectively written as the +y axis direction and the -y axis direction, and the positive and negative directions along the z axis are respectively written as the +z axis direction and the -z axis direction.

[0029] 1, the optically excited magnetic sensor 1 includes a cell 2, a heater 3, a pump laser 4, a probe laser 5, a first optical system 6, a second optical system 7, an optical sensor group 8, a third optical system 9, a readout circuit 10, a bias magnetic field coil 11, bias magnetic field gradient correction coils 12 and 13, and a tilting coil (tilting means) 14. The components of the optically excited magnetic sensor 1 will be described in detail below.

[0030] The cell 2 is a container that seals alkali metal vapor. The cell 2 is arranged along the direction along the z-axis. 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, for example, quartz, sapphire, silicon, Kovar glass, borosilicate glass, or other material. The cell 2 is optically transparent to pump light and probe light, which will be described later. A heater 3 and the like are attached to the side surface of the cell 2 in the -y-axis direction. A magnetic field Bm of the measurement object generated from the measurement object is incident on the side surface of the cell 2 in the -y-axis direction along the +y-axis direction.

[0031] 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.

[0032] 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.

[0033] 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.

[0034] 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. This pump light is finally converted into circularly polarized light by the quarter-wave plates 27a, 27b, 27c, and 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, causing the potassium atoms to enter an excited state.

[0035] Pump light from the pump laser 4 is incident on the cell 2 via the first optical system 6. Fig. 2 is a plan view of the first optical system 6 as viewed in the -x-axis direction. The first optical system 6 includes a lens 21, a half-wave plate 22, polarizing beam splitters 23a, 23b, and 23c, half-wave plates 24a and 24b, a total reflection mirror 25, polarizers 26a, 26b, 26c, and 26d, and quarter-wave plates 27a, 27b, 27c, and 27d.

[0036] The lens 21 is provided adjacent to the pump laser 4 in the -y-axis direction and collects the pump light emitted from the pump laser 4. The half-wave plate 22 is provided adjacent to the lens 21 in the -y-axis direction and rotates the polarization plane so that the power of the pump light is distributed at 1:3 by the polarizing beam splitter 23a.

[0037] 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.

[0038] 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.

[0039] The polarizer 26a and the quarter-wave plate 27a are provided adjacent to the polarizing beam splitter 23a in the -y-axis direction. The polarizer 26a transmits a specific linearly polarized component of the pump light transmitted through the polarizing beam splitter 23a, and the quarter-wave plate 27a changes the polarization state of the pump light transmitted through the polarizer 26a to circular polarization and transmits it in the -y-axis direction as pump light PLa.

[0040] The polarizers 26b, 26c, and 26d and the quarter-wave plates 27b, 27c, and 27d are disposed adjacent to the polarizing beam splitters 23b and 23c and the total reflection mirror 25 in the -y-axis direction. The polarizers 26b, 26c, and 26d and the quarter-wave plates 27b, 27c, and 27d have the same functions as the polarizer 26a and the quarter-wave plate 27a, respectively, and transmit the pump light PLb, PLc, and PLd in the -y-axis direction.

[0041] The first optical system 6 having the above-mentioned configuration is configured so that four systems of pump light PLa to PLd transmitted through the four 1 / 4 wavelength plates 27a, 27b, 27c, and 27d can be incident on regions separated in the longitudinal direction (direction along the z-axis) within the cell 2.

[0042] The probe laser 5 emits a probe light in the -x-axis direction 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 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 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.

[0043] 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.

[0044] The probe light from the probe laser 5 is incident on the cell 2 via the second optical system 7. Fig. 3 is a plan view of the second optical system 7, the optical sensor group 8, and the third optical system 9 as viewed in the -y-axis direction. The second optical system 7 includes half-wave plates 31a, 31b, and 31c, polarizing beam splitters 32a, 32b, and 32c, a total reflection mirror 33, and polarizers 34a, 34b, 34c, and 34d.

[0045] The half-wave plate 31a is provided adjacent to the probe laser 5 in the -z-axis direction, and rotates the polarization plane so that the power of the probe light is distributed 1:3 by the polarizing beam splitter 32a. The polarizing beam splitters 32a, 32b, and 32c are provided in sequence in the -z-axis direction relative to the half-wave plate 31a, and separate two linearly polarized components perpendicular to each other from the polarized component of the probe light transmitted through the half-wave plate 31a, transmit one linearly polarized component in the -z-axis direction, and reflect the other linearly polarized component in the -x-axis direction. The half-wave plate 31b is provided between the polarizing beam splitter 32a and the polarizing beam splitter 32b, and rotates the polarization plane of the linearly polarized probe light transmitted through the polarizing beam splitter 32a. This allows the polarizing beam splitter 32b to separate the probe light into two linearly polarized polarized components. The half-wave plate 31c is provided between the polarizing beam splitter 32b and the polarizing beam splitter 32c, and rotates the polarization plane of the linearly polarized probe light transmitted through the polarizing beam splitter 32b. This allows the polarizing beam splitter 32c to split the probe light into two linearly polarized polarization components. The total reflection mirror 33 reflects the linearly polarized probe light transmitted through the polarizing beam splitter 32c in the -x-axis direction.

[0046] The polarizers 34a, 34b, 34c, and 34d transmit specific linearly polarized components of the probe light reflected by the polarizing beam splitters 32a, 32b, and 32c and the total reflection mirror 33, respectively.

[0047] The second optical system 7 having the above-mentioned configuration is configured so that four systems of probe light QLa to QLd transmitted through the four polarizers 34a, 34b, 34c, and 34d can respectively enter sensitivity regions ARa to ARd that intersect with the pump light PLa to PLd arranged in the longitudinal direction (direction along the z-axis) within the cell 2.

[0048] 1, the bias magnetic field coil 11 is a coil that applies a bias magnetic field By in a direction along the y-axis inside the cell 2, and determines the resonance frequency of the electron spins of the alkali metal atoms excited inside the cell 2. By applying the bias magnetic field By in the same direction as the incident direction of the pump light by the bias magnetic field coil 11, the axes of the electron spins of the alkali metal atoms excited inside the cell 2 are aligned in the direction along the y-axis. When the strength of the bias magnetic field applied by the bias magnetic field coil 11 is 14 μT and the alkali metal atoms are potassium, the resonance frequency of the electron spins is 100 kHz.

[0049] The bias magnetic field gradient correction coils 12 and 13 are a coil group for correcting the gradient of the bias magnetic field By in the direction along the x-axis, the direction along the y-axis, or the direction along the z-axis. The bias magnetic field gradient correction coils 12 and 13 may be configured to correct the gradient of the bias magnetic field By in one or two directions, that is, the direction along the x-axis, the direction along the y-axis, or the direction along the z-axis, or may be configured to correct the gradient in three directions. The presence of the bias magnetic field gradient correction coils 12 and 13 makes it possible to correct the bias magnetic field By to be uniform even when it is not uniform due to the influence of an environmental magnetic field, etc., and thus makes it possible to increase the detection sensitivity of the optically excited magnetic sensor 1 to an external magnetic field.

[0050] The tilting coil 14 generates and irradiates an RF signal Rf in order to tilt the direction of the electron spin of the alkali metal atoms excited in the cell 2 in a direction perpendicular to the incident direction of the pump light. Specifically, the tilting coil 14 generates an RF signal Rf with the same frequency as the resonance frequency of the electron spin (100 kHz when the strength of the bias magnetic field By is 14 μT and the alkali metal atom is potassium) and irradiates the RF signal Rf in the -z axis direction with the intensity and length required to tilt the electron spin by 90 degrees. 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 a tilting means for tilting the direction of the electron spin.

[0051] Returning to FIG. 3, the configurations of the optical sensor group 8 and the third optical system 9 will be described.

[0052] The third optical system 9 includes half-wave plates 41a, 41b, 41c, and 41d, and polarizing beam splitters 42a, 42b, 42c, and 42d. The half-wave plates 41a, 41b, 41c, and 41d are provided adjacent to the cell 2 in the −x-axis direction on the optical paths of the probe lights QLa, QLb, QLc, and QLd that have passed through the sensitivity regions ARa to ARd of the cell 2, and rotate the polarization plane of the linearly polarized light of the probe lights QLa, QLb, QLc, and QLd that have passed through the cell 2. These half-wave plates 41a, 41b, 41c, and 41d are supported rotatably about an axis along the x-axis so that the rotation angle of the polarization plane of the probe light can be adjusted. The polarizing beam splitters 42a, 42b, 42c, and 42d separate two mutually perpendicular linearly polarized components from the polarization components of the probe light QLa, QLb, QLc, and QLd that have passed through the half-wave plates 41a, 41b, 41c, and 41d, respectively, and transmit one linearly polarized component in the -x-axis direction and reflect the other linearly polarized component in the y-axis direction.

[0053] The optical sensor group 8 is an element group that detects the polarization plane angles of the probe light QLa, QLb, QLc, and QLd that have passed through the sensitivity regions ARa to ARd, and is configured to include four optical sensor pairs 43a, 43b, 43c, and 43d. The optical sensor pair 43a has two photodiodes 44a and 45a, and the photodiode 44a outputs a detection signal that detects the intensity of the other linearly polarized component of the probe light QLa, and the photodiode 45a outputs a detection signal that detects the intensity of the other linearly polarized component of the probe light QLa. Similarly, the optical sensor pairs 43b, 43c, and 43d each have two photodiodes 44b and 45b, two photodiodes 44c and 45c, and two photodiodes 44d and 45d, respectively. The two photodiodes 44b, 45b each output a detection signal detecting the intensities of the two linearly polarized components of the probe light QLb, the two photodiodes 44c, 45c each output a detection signal detecting the intensities of the two linearly polarized components of the probe light QLc, and the two photodiodes 44d, 45d each output a detection signal detecting the two linearly polarized components of the probe light QLd.

[0054] 1 again, the readout circuit 10 is a processing circuit electrically connected to the four optical sensor pairs 43a, 43b, 43c, and 43d constituting the optical sensor group 8, and processes the detection signals output from the four optical sensor pairs 43a, 43b, 43c, and 43d. That is, the readout circuit 10 includes a reading unit that reads the detection signals output from the four optical sensor pairs 43a, 43b, 43c, and 43d, and a measuring unit that performs a measurement process of the external magnetic field in 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 the measurement process based on the detection signals output from the optically excited magnetic sensor 1.

[0055] 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 functions of the readout circuit 10 are realized by executing a program stored in the memory by the CPU of a computer system.

[0056] The function of the measurement processing of the external magnetic field by the measurement unit of the readout circuit 10 will be described with reference to Fig. 4 and Fig. 5. Fig. 4 shows the timing of the pump light, the RF signal, and the probe light during the measurement processing of the external magnetic field by the readout circuit 10, with part (a) showing the generation timing of the pump light, part (b) showing the generation timing of the RF signal, and part (c) showing the readout timing of the probe light. Fig. 5 is a graph showing the time change of FID (Free Induction Decay) acquired by the readout circuit 10.

[0057] During the measurement process of the external magnetic field by the readout circuit 10, a control circuit (not shown) controls the pump light to be repeatedly turned on / off in a pulsed manner (part (a) of FIG. 4), and immediately after the pump light transitions from on to off, the RF signal is irradiated in a pulsed manner (part (b) of FIG. 4), and the probe light is read out during a predetermined relaxation time after the irradiation of the RF signal (part (c) of FIG. 4). In such a control state, the readout circuit 10 acquires detection signals output from the four optical sensor pairs 43a, 43b, 43c, and 43d during the relaxation time. Then, the readout circuit 10 acquires the FID by taking the difference between the detection signals of the two photodiodes 44a and 45a constituting the optical sensor pair 43a (FIG. 5). Similarly, the readout circuit 10 acquires the FID by taking the difference between the detection signals of the two photodiodes for the optical sensor pairs 43b, 43c, and 43d. The FID indicates how the electron spin of an alkali metal atom relaxes, and the frequency of the precession of the electron spin changes in response to fluctuations in the external magnetic field in the sensitivity region ARa, so that the frequency of the oscillation changes with fluctuations in the external magnetic field. For example, if the resonance frequency of the electron spin is 100 kHz and the external magnetic field changes from 0 pT to 1 pT, the frequency of the precession changes by about 0.007 Hz. The readout circuit 10 utilizes this property to calculate the following equation for the FID waveform at the beginning of the relaxation time:

number

[0058] 6 is a circuit diagram showing an example of the configuration of the reading unit 10a of the readout circuit 10. Here, the circuit configuration of the reading unit 10a that reads the difference between the detection signals output from the two photodiodes 44a and 45a that constitute the optical sensor pair 43a is shown, and the readout circuit 10 additionally includes circuits of a similar configuration corresponding to each of the three optical sensor pairs 43b, 43c, and 43c.

[0059] The readout section 10a of the readout circuit 10 includes a switch 51, a measurement high-gain amplifier 52 having a predetermined gain, and an adjustment low-gain amplifier 53 having a gain less than the predetermined gain. A positive bias voltage is applied to the cathode of the photodiode 44a, and a negative bias voltage is applied to the anode of the photodiode 45a, and the anode of the photodiode 44a and the cathode of the photodiode 45a are connected to an output node 54. As a result, a current signal that is the difference between the detection signal detected by the photodiode 44a and the detection signal detected by the photodiode 45a is generated at the output node 54.

[0060] The switch 51 is connected between an output node 54 connected to the anode of the photodiode 44a and the cathode of the photodiode 45a, and an input node 55 of the measurement high gain amplifier 52 and an input node 56 of the adjustment low gain amplifier 53. The switch 51 selectively switches the connection between the output node 54 of the optical sensor pair 43a and the input node 55 of the measurement high gain amplifier 52 and the input node 56 of the adjustment low gain amplifier 53. A first condition is a state in which the switch 51 is switched to the input node 55 side, and a second condition is a state in which the switch 51 is switched to the input node 56 side. This switch may be a mechanical switch that can be switched by a manual operation, or may be an electronic switch (e.g., a semiconductor switch) that can be switched by a control signal from an outside.

[0061] The measurement high-gain amplifier 52 includes a differential amplifier 52a and a resistive element 52b. The differential amplifier 52a is configured such that one differential input is connected to an input node 55 and the other differential input is connected to a ground potential. The resistive element 52b has a relatively large resistance value such as 1 MΩ and is connected between one differential input of the differential amplifier 52a and the output of the differential amplifier 52a. The measurement high-gain amplifier 52 configured as described above has its output connected to a measurement unit of the readout circuit 10, and when the switch 51 is switched to the input node 55 side, it amplifies a current signal representing the difference between the detection signals to a voltage signal with a predetermined gain (for example, 1 V / μA) and transmits the voltage signal to the measurement unit of the readout circuit 10.

[0062] The adjustment low-gain amplifier 53 includes a differential amplifier 53a and a resistive element 53b. The differential amplifier 53a is configured such that one differential input is connected to an input node 56 and the other differential input is connected to a ground potential. The resistive element 53b has a relatively small resistance value such as 100 kΩ and is connected between one differential input of the differential amplifier 53a and the output of the differential amplifier 53a. The adjustment low-gain amplifier 53 configured as above has its output connected to a measurement unit of the readout circuit 10, and when the switch 51 is switched to the input node 56 side, it amplifies a current signal of the difference between the detection signals to a voltage signal with a low gain (e.g., 0.1 V / μA) less than the above-mentioned predetermined gain, and transmits the voltage signal to the measurement unit of the readout circuit 10.

[0063] According to the optically excited magnetic sensor 1 according to the present embodiment described above, the pump light is emitted to the cell 2 in which the alkali metal vapor is sealed, thereby generating (exciting) the electron spin of the alkali metal atom. Furthermore, the probe light is emitted to the sensitivity regions ARa to ARd in the cell 2 where the probe light intersects with the pump light, and the optical sensor pairs 43a to 43d detect the polarization plane angle of the probe light that has passed through the sensitivity regions ARa to ARd. In addition, the readout circuit 10 generates a detection signal (measurement signal) for measuring the intensity of the external magnetic field in the sensitivity regions ARa to ARd based on the output of the optical sensor pairs 43a to 43d. Here, when measuring the external magnetic field, the readout circuit 10 amplifies the detection signal of the optical sensor pairs 43a to 43d with a predetermined gain by switching the switch 51 to the input node 55 side (first condition). Furthermore, when adjusting the third optical system 9, which is the optical system of the optical sensor pairs 43a to 43d, the readout circuit 10 transmits the detection signals of the optical sensor pairs 43a to 43d with a gain less than a predetermined gain by switching the switch 51 to the input node 56 side (second condition). This makes it possible to improve the measurement accuracy of the external magnetic field and to easily adjust the third optical system 9 based on the detection signals of the optical sensor pairs 43a to 43d.

[0064] That is, before the measurement process of the external magnetic field, the third optical system 9 needs to be adjusted in order to improve the measurement accuracy of the external magnetic field. This adjustment is performed by, for example, adjusting the rotation angles of each of the half-wave plates 41a, 41b, 41c, and 41d so that the offset of the waveform approaches zero while monitoring the waveform of the FID acquired by the readout circuit 10. As a result of this adjustment, the distribution ratio by the polarizing beam splitters 42a to 42d can be set to approximately 1:1. At this time, since the probe light having a higher intensity than a predetermined value may be incident on the optical sensor pairs 43a to 43d, the adjustment process is facilitated by the adjustment based on the voltage signal acquired at a low gain. On the other hand, during the measurement process of the external magnetic field, the measurement accuracy can be improved by acquiring the measurement value based on the voltage signal acquired at a high gain.

[0065] The adjustment of the third optical system 9 and the measurement of the external magnetic field are performed in the following procedure. The adjustment may be performed manually or automatically by a processing circuit including the readout circuit 10. First, the pump laser 4 is turned off to emit pump light, and the probe laser 5 is turned on to emit probe light. Next, the switch 51 is switched so that the output of the optical sensor pair 43a to 43d is input to the adjustment low gain amplifier 53. After that, the half-wave plates 41a, 41b, 41c, and 41d are rotated while the waveform acquired by the readout circuit 10 based on the output of the adjustment low gain amplifier 53 is monitored using an oscilloscope or the like, and the rotation angles of the half-wave plates 41a, 41b, 41c, and 41d are set so that the offset of the waveform approaches zero. After completing the setting of the rotation angles of all the half-wave plates 41a, 41b, 41c, and 41d, the switch 51 is switched so that the outputs of the optical sensor pairs 43a to 43d are input to the measurement high-gain amplifier 52. Then, the emission of pump light by the pump laser 4 is turned on, and measurement of the external magnetic field is started.

[0066] 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.

[0067] In addition, in this embodiment, a tilting coil 14 that irradiates an RF signal with the same frequency as the resonance frequency is used as the tilting means. This allows measurement of the external magnetic field based on the frequency of the change in the polarization plane angle of the probe light to be realized by simple means. In this embodiment, a light source that irradiates pulsed light can also be used as the tilting means. In this case, too, measurement of the external magnetic field based on the frequency of the change in the polarization plane angle of the probe light can be realized by simple means.

[0068] Moreover, the optically excited magnetic sensor 1 according to this embodiment has four sensitivity regions ARa to ARd divided in the direction along the z-axis. Therefore, common mode noise can be removed by obtaining a difference between the measurement values ​​of the external magnetic field obtained for two adjacent sensitivity regions among the four sensitivity regions ARa to ARd.

[0069] 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.

[0070] For example, the photo-excited magnetic sensor 1 of this embodiment has four divided sensitivity regions ARa to ARd, but may have any number of sensitivity regions as long as the number is two or more.

[0071] In the optically excited magnetic sensor 1 of this embodiment, other configurations may be adopted for the reading section 10a of the readout circuit 10. Fig. 7 is a circuit diagram showing the configuration of a reading section 10a according to a modified example.

[0072] The reading unit 10a according to the modified example includes an adjustment low-gain amplifier 53 and a high-pass filter 57 connected in series in this order to an output node 54 of the optical sensor pair 43a. One differential input of a differential amplifier 53a of the adjustment low-gain amplifier 53 is connected to the output node 54, and an output of the differential amplifier 53a is connected to an input of the high-pass filter 57. The adjustment low-gain amplifier 53 has a role of amplifying the current signal output from the output node 54 into a voltage signal with low gain (for example, 0.1 V / μA) and outputting the voltage signal.

[0073] The high-pass filter 57 includes a differential amplifier 57a, a capacitor 57b, and two resistor elements 57c and 57d. One differential input of the differential amplifier 57a is connected to the ground potential via the capacitor 57b and the resistor element 57c, and is connected to the output terminal of the differential amplifier 57a via the resistor element 57d. The other differential input of the differential amplifier 57a is connected to the output of the adjustment low-gain amplifier 53. For example, the capacitance value of the capacitor 57b is set to 1 μF, and the resistance values ​​of the resistor elements 57c and 57d are set to 470 Ω and 4.3 kΩ, respectively. When a signal in a frequency band equal to or higher than the frequency band of the resonance frequency of the electron spin (100 kHz in this embodiment), that is, a signal in a frequency band equal to or higher than the frequency band when measuring the external magnetic field, is input to the high-pass filter 57, the high-pass filter 57 amplifies and outputs the voltage signal with high gain, and when a signal in a frequency band lower than the frequency band of the resonance frequency (particularly 1 kHz or less) is input, the high-pass filter 57 passes the voltage signal as it is.

[0074] The reader 10a configured as above operates as follows as a whole: When a current signal having a frequency band equal to or higher than the frequency band used for measuring the external magnetic field is input (first condition), the reader 10a amplifies the signal with a predetermined gain and outputs it, and when a current signal having a frequency band lower than the frequency band used for measuring the external magnetic field is input (second condition), the reader 10a amplifies the signal with a low gain less than the predetermined gain and outputs it.

[0075] In this modification, the readout circuit 10 amplifies the detection signals of the optical sensor pairs 43a-43d with a predetermined gain (e.g., 1 V / μA) when measuring the external magnetic field, and transmits the detection signals of the optical sensor pairs 43a-43d with a gain less than the predetermined gain (e.g., 0.1 V / μA) when adjusting the third optical system 9. This makes it possible to improve the measurement accuracy of the external magnetic field and to easily adjust the third optical system 9 based on the detection signals of the optical sensor pairs 43a-43d.

[0076] Next, modified examples of the optically excited magnetic sensor of the present disclosure will be described.

[0077] 8 is a perspective view showing the configuration of a photo-excited magnetic sensor 1A according to a first modified example. The photo-excited magnetic sensor 1A differs from the photo-excited magnetic sensor 1 in that the incident direction of the pump light to the cell 2 and the application direction of the bias magnetic field By are set along the Z-axis, and the irradiation direction of the RF signal Rf is set along the x-axis. Only the configuration different from the photo-excited magnetic sensor 1 will be described below.

[0078] The first optical system 6 includes a lens 21, a half-wave plate 22, a polarizing beam splitter 23a, and a quarter-wave plate 27a. The polarizing beam splitter 23a is provided adjacent to the half-wave plate 22 in the -y-axis direction, and reflects the linearly polarized component of the pump light transmitted through the half-wave plate in the -z-axis direction. The quarter-wave plate 27a is provided adjacent to the polarizing beam splitter 23a in the -z direction, and changes the linearly polarized pump light reflected by the polarizing beam splitter 23a to circularly polarized light, and transmits the pump light in the -z-axis direction. The first optical system 6 configured as described above is configured so that one system of pump light can be incident in the longitudinal direction (direction along the z-axis) within the cell 2.

[0079] The second optical system 7 includes the half-wave plates 31a, 31b, and 31c, the polarizing beam splitters 32a, 32b, and 32c, the total reflection mirror 33, the polarizers 34a, 34b, 34c, and 34d, as well as a lens 35 and a total reflection mirror 37. The lens 35 is provided adjacent to the probe laser 5 in the -y-axis direction, and collects the probe light emitted from the probe laser 5 in the -y-axis direction. The total reflection mirror 37 is provided adjacent to the lens 35 in the -y-axis direction, and reflects the probe light transmitted through the lens 35 in the -z-axis direction toward the half-wave plate 31a. The second optical system 7 configured as above is configured so that the four probe light beams QLa to QLd transmitted through the four polarizers 34a, 34b, 34c, and 34d can be incident on four sensitivity regions that intersect with the pump light PL arranged in the longitudinal direction (direction along the z-axis) in the cell 2.

[0080] According to the optically excited magnetic sensor 1A according to the first modification, the measurement value of the external magnetic field Bm in the direction along the z-axis, which is the incident direction of the pump light, can be obtained for four sensitivity areas divided along the z-axis. The optically excited magnetic sensor 1A has four sensitivity areas, but may be modified to have two sensitivity areas. Even in this case, the difference between the measurement values ​​for the two sensitivity areas can be obtained to remove common mode noise and improve the detection accuracy. Furthermore, by providing the readout circuit 10 in the first modification, the measurement accuracy of the external magnetic field can be improved, and the third optical system 9 can be easily adjusted based on the detection signals of the optical sensor pairs 43a to 43d.

[0081] 9 is a perspective view showing the configuration of an optically excited magnetic sensor 1B according to a second modified example. The optically excited magnetic sensor 1B is different from the optically excited magnetic sensor 1 in that it includes two cells 2 arranged in parallel, four systems of pump light are incident on each of the two cells 2, and the probe light is branched and incident on the two cells 2 along their longitudinal directions. Only the configuration different from the optically excited magnetic sensor 1 will be described below.

[0082] The optically excited magnetic sensor 1B is equipped with a pump laser 4 and a first optical system 6 for directing four types of pump light into regions separated in the longitudinal direction (direction along the z-axis) within the cell 2, corresponding to each of the two cells 2.

[0083] The optically excited magnetic sensor 1B also includes a second optical system 7 as an optical system for splitting the probe light into two and making the two beams incident on the two cells 2. 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 incident on one of the cells 2 in its longitudinal direction. The total reflection mirrors 37b and 37c continuously reflect the linearly polarized components of the probe light reflected by the polarizing beam splitter 38 in the -y-axis and -z-axis directions, and cause the probe light to be incident in the longitudinal direction of the other cell 2. The second optical system 7 configured in this way is configured so that each of the two probe light systems can be incident on four sensitivity regions that intersect with the pump light aligned in the longitudinal direction (direction along the z-axis) in the cell 2.

[0084] Furthermore, four pairs of bias magnetic field coils 11a to 11d are provided in each of the two cells 2 included in the optically excited magnetic sensor 1B. These bias magnetic field coils 11a to 11d generate bias magnetic fields with magnetic field strengths that differ stepwise in the four sensitivity regions in the cell 2. This allows the resonance frequencies of electron spins in the four sensitivity regions in the cell 2 to be set to different values, such as 98.5 kHz, 99.5 kHz, 100.5 kHz, and 101.5 kHz.

[0085] The optically excited magnetic sensor 1B includes two third optical systems 9 and two optical sensor pairs 43a and 43b corresponding to the two cells 2. 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 enter the optical sensor pair 43a or 43b, and reflects the other linearly polarized component in the +z direction. The total reflection mirror 49 reflects the other linearly polarized light component reflected by the polarizing beam splitter 48 in the +y-axis direction and makes it incident on the optical sensor pair 43a or the optical sensor pair 43b.

[0086] The optically excited magnetic sensor 1B having the above configuration can also perform measurement processing of the external magnetic field for the eight sensitivity regions of 4×2 by providing a read circuit 10 including a reader 10a. That is, the read circuit 10 extracts a specific frequency band corresponding to the resonance frequency of the electron spin in the sensitivity region from a voltage signal generated based on each detection signal of the optical sensor pair 43a, 43b, and obtains a measurement value of the external magnetic field based on the voltage signal of the extracted frequency band. The read circuit 10 can obtain measurement values ​​corresponding to the eight sensitivity regions by repeating the extraction of the frequency band and the acquisition of the measurement value for the eight sensitivity regions.

[0087] The second modification also improves detection accuracy by removing common mode noise by obtaining the difference between the measurement values ​​of two adjacent sensitivity regions. It also improves the measurement accuracy of the external magnetic field and facilitates adjustment of the two third optical systems 9 based on the detection signals of the optical sensor pair 43a, 43b.

[0088] In the above embodiment and the first and second modified examples, the linearly polarized pump light is converted into circularly polarized light, but the pump laser may emit circularly polarized pump light.

[0089] FIG. 10 is a plan view showing the configuration of a magnetoencephalograph 100 according to an embodiment. The magnetoencephalograph 100 includes a plurality of optically excited magnetic sensors 1 and a plurality of optically excited magnetic sensors 1A that are arranged around the head (subject) S of a subject. Two of the plurality of optically excited magnetic sensors 1 are arranged so that their four sensitive regions are arranged in parallel in a predetermined direction (y-axis direction) at positions close to and apart from the head S. Two of the plurality of optically excited magnetic sensors 1A are also arranged so that their four sensitive regions are arranged in parallel in a direction (x-axis direction) approximately perpendicular to the arrangement direction of the sensitive regions in the optically excited magnetic sensor 1 at positions close to and apart from the head S. In addition, a bias magnetic field coil 11, bias magnetic field gradient correction coils 12, 13, and a tilting coil 14 are shared between the plurality of optically excited magnetic sensors 1 and the plurality of optically excited magnetic sensors 1A. A bias magnetic field By is applied in the x-axis direction by the bias magnetic field coil 11.

[0090] The magnetoencephalograph 100 having the above configuration can obtain measurement values ​​of a weak magnetic field from the head S. Specifically, according to the arrangement example of Fig. 10, it is possible to obtain measurement values ​​of the magnetic field in the x-axis direction in 8 x 4 = 32 sensitivity regions. 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 a weak brain magnetism from which common mode noise has been removed.

[0091] In the magnetoencephalograph 100, it is also possible to arrange the optically excited magnetic sensors 1 and 1A only in the vicinity of the head S, and not in parallel. In this case, by obtaining the difference in the measurement values ​​of two adjacent sensitivity areas of one optically excited magnetic sensor 1, 1A, it is possible to measure a weak brain magnetism from which common mode noise has been removed.

[0092] In the magnetoencephalograph 100, the optical system is adjusted before the placement of the optically excited magnetic sensors 1 and 1A. In this case, the third optical system 9 can be easily adjusted based on the detection signals of the optical sensor pairs 43a to 43d by the function of the readout section 10a of the readout circuit 10. [Explanation of symbols]

[0093] 1, 1A, 1B...optically excited magnetic sensor, 2...cell, 4...pump laser, 5...probe laser, 8...optical sensor group, 9...third optical system, 10...readout circuit, 10a...readout section, 11, 11a to 11d...bias magnetic field coil, 14...tilting coil (tilting means), 51...switch, 52...measurement high gain amplifier (measurement amplifier), 53...adjustment low gain amplifier (adjustment amplifier), 57...high pass filter, 100...magnetoencephalograph, ARa to ARd...sensitivity region, Bm...magnetic field, By...bias magnetic field, PL, PLa to PLd...pump light, QLa to QLd...probe light, Rf...RF signal, S...head (subject).

Claims

1. A cell containing alkali metal vapor; a pump laser that emits pump light for exciting alkali metal atoms that constitute the alkali metal vapor; a probe laser that emits probe light for detecting a change in electron spin in an excited state of the alkali metal atom to a sensitivity region in the cell where the probe light intersects with the pump light; an optical sensor for detecting an angle of a plane of polarization of the probe light passing through the sensitive region; a readout circuit that generates a measurement signal for measuring the strength of a magnetic field in the sensitivity region based on an output of the optical sensor; The read circuit includes: amplifying an output of the optical sensor by a predetermined gain under a first condition, and transmitting the output by a gain less than the predetermined gain under a second condition; Optically excited magnetic sensor.

2. a bias magnetic field coil that applies a magnetic field in the same direction as the pump light and determines the resonance frequency of the electron spins; and tilting means for tilting the direction of the electron spins in a direction perpendicular to the pump light.

2. The optically excited magnetic sensor according to claim 1.

3. The tilting means irradiates an RF signal having the same frequency as the resonance frequency.

3. The optically excited magnetic sensor according to claim 2.

4. The tilting means irradiates pulsed light.

3. The optically excited magnetic sensor according to claim 2.

5. There are two or more sensitivity regions where the pump light and the probe light intersect, the readout circuit generates the measurement signal based on a difference between outputs of the optical sensors corresponding to two adjacent sensitivity regions.

5. The optically excited magnetic sensor according to claim 1.

6. the readout circuit includes an instrumentation amplifier having the predetermined gain, an adjustment amplifier having a gain less than the predetermined gain, and a switch for switching a connection between an output of the optical sensor and an input of the instrumentation amplifier and an input of the adjustment amplifier.

5. The optically excited magnetic sensor according to claim 1.

7. the readout circuit amplifies with a predetermined gain in a frequency band when the strength of the magnetic field is measured, and transmits with a gain less than the predetermined gain in a frequency band lower than the frequency band when the optical system of the optical sensor is adjusted.

5. The optically excited magnetic sensor according to claim 1.

8. the readout circuit has a high-pass filter that amplifies signals above a frequency band used for measuring the strength of the magnetic field by a predetermined gain; 8. An optically excited magnetic sensor according to claim 7.

9. a photoexcited magnetic sensor according to any one of claims 1 to 4, which is arranged around a head of a subject and measures the intensity of a magnetic field emitted from the subject; Magnetoencephalography.

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