Optically pumped magnetometer having lasers and optical systems used to derive an intensity of a magnetic field

An optically pumped magnetometer includes cells configured to form a first cell region and a second cell region on a measurement target, a pump laser, a probe laser, a first optical system configured to cause pump light to be incident on the first cell region, a second optical system configured to cause the pump light having passed through the first cell region to be incident on the second cell region, a third optical system configured to cause first probe light to be incident on the first cell region, a fourth optical system configured to cause second probe light to be incident on the second cell region, detection portions configured to detect the first probe light having passed through the first cell region and the second probe light having passed through the second cell region, and a deriving portion configured to derive an intensity of a magnetic field.

TECHNICAL FIELD

The present disclosure relates to an optically pumped magnetometer.

BACKGROUND

Magnetoencephalography using optically pumped magnetometers is known (for example, refer to Japanese Patent No. 5823195). An optically pumped magnetometer measures a microscopic magnetic field by exciting alkali metal atoms through optical pumping and using spin polarization of the atoms.

SUMMARY

In magnetoencephalography using optically pumped magnetometers, a number of optically pumped magnetometers are disposed with narrow intervals therebetween. For this reason, it is desired to achieve miniaturization of optically pumped magnetometers.

An aspect of the present disclosure provides an optically pumped magnetometer which can be miniaturized.

According to an aspect of the present disclosure, there is provided an optically pumped magnetometer including at least one cell configured to be filled with an alkali metal vapor, be disposed in a first direction along a measurement target, and form a first cell region and a second cell region on the measurement target; a pump laser configured to emit pump light for exciting alkali metal atoms; a probe laser configured to emit probe light including first probe light and second probe light for detecting change in a polarization angle caused by a magnetic field in an excited state of the alkali metal atoms; a first optical system configured to cause the pump light to be incident on the first cell region in the first direction; a second optical system configured to cause the pump light having passed through the first cell region to be incident on the second cell region in the first direction; a third optical system configured to cause the first probe light to be incident on the first cell region in a second direction orthogonal to the first direction; a fourth optical system configured to cause the second probe light to be incident on the second cell region in the second direction; a detection portion configured to detect first probe light having passed through the first cell region and second probe light having passed through the second cell region; and a deriving portion configured to derive an intensity of a magnetic field related to a region having the pump light and the first probe light orthogonal to each other from change in the polarization angle of the first probe light within the first cell region and derive an intensity of a magnetic field related to a region having the pump light having passed through the first cell region and the second probe light orthogonal to each other from change in the polarization angle of the second probe light within the second cell region based on detection results of the detection portion.

In the optically pumped magnetometer according to the aspect of the present disclosure, the pump light is incident on the first cell region in the first direction, and the first probe light is incident on the first cell region in the second direction. Traveling directions of the pump light and the first probe light are orthogonal to each other within the first cell region. Further, the pump light having passed through the first cell region is incident on the second cell region in the first direction, and the second probe light is incident on the second cell region in the second direction. Traveling directions of the pump light having passed through the first cell region and the second probe light are orthogonal to each other within the second cell region. That is, the same pump light is consecutively incident throughout multiple cell regions. Further, the intensity of a magnetic field related to a region having them intersecting each other is derived using the pump light and the first probe light, and the intensity of a magnetic field related to a region having them intersecting each other is derived using the pump light and the second probe light. Accordingly, there is no need to cause the pump light to branch or prepare multiple pump lasers in a manner of corresponding to the number of cell regions. Therefore, the constitution related to the pump light can be simplified. Accordingly, it is possible to provide an optically pumped magnetometer which can be miniaturized.

The first cell region and the second cell region may be formed by multiple cells. Accordingly, each cell can be miniaturized.

The multiple cells may be disposed away from each other in a direction away from the measurement target. The deriving portion may perform noise removal processing by calculating a difference between an intensity of a magnetic field related to an area within the first cell region and an intensity of a magnetic field related to an area within the second cell region. According to such a constitution, since an influence of common-mode noise is manifested in each of the intensity of a magnetic field related to an area within the first cell region and the intensity of a magnetic field related to an area within the second cell region, common-mode noise is removed by calculating the difference therebetween. Accordingly, measurement accuracy of the optically pumped magnetometer can be improved.

The multiple cells may be disposed away from each other in a direction orthogonal to the first direction and extending along the measurement target. According to such a constitution, the cells are adjacent to each other in a horizontal direction along the measurement target. Since the same pump light is incident on cells adjacent to each other, there is no need to prepare a constitution related to the pump light for each cell. In this case, since an interval between cells adjacent to each other can be narrowed, the optically pumped magnetometer can be miniaturized.

The first cell region and the second cell region may be away from each other in a direction away from the measurement target and be formed by one cell. The deriving portion may perform noise removal processing by calculating a difference between an intensity of a magnetic field related to an area within the first cell region and an intensity of a magnetic field related to an area within the second cell region. Accordingly, the cell can have a simple constitution. In addition, since common-mode noise is removed, measurement accuracy of the optically pumped magnetometer can be improved.

At least the one cell may have a pair of end surfaces orthogonal to the first direction with an antireflection film attached to the end surfaces. According to such a constitution, when the pump light is incident on and emitted from the first cell region or the second cell region in the first direction, attenuation due to reflection of the pump light is curbed. Accordingly, electric power of the pump laser can be reduced.

The optically pumped magnetometer may further include an attenuation detection portion configured to detect attenuation of the pump light having passed through the second cell region. The deriving portion may revise at least one of an intensity of a magnetic field related to an area within the first cell region and an intensity of a magnetic field related to an area within the second cell region based on detection results of the attenuation detection portion. According to such a constitution, attenuation of the pump light is taken into consideration, and the intensity of a magnetic field related to an area within each cell region is revised. Accordingly, measurement accuracy of the optically pumped magnetometer can be improved.

The alkali metal may be potassium and rubidium. A density of the rubidium may be lower than a density of the potassium. The pump laser may emit the pump light for exciting atoms of the rubidium and transferring spin polarization of atoms of the rubidium to atoms of the potassium. The probe laser may emit the probe light for detecting change in a polarization angle caused by a magnetic field in an excited state of atoms of the potassium. According to such a constitution, if the pump light excites atoms of the rubidium, spin polarization of atoms of the rubidium is transferred to atoms of the potassium, and thus atoms of the potassium are excited. This phenomenon is caused due to spin exchange interaction between the potassium and the rubidium. Since the pump light excites the rubidium having a lower density, attenuation of the pump light is curbed. As a result, electric power of the pump laser can be reduced.

The second optical system may cause the pump light having passed through the first cell region to be turned back and be incident on the second cell region. In this case, since a short optical path can be set for the pump light, it is possible to provide an optically pumped magnetometer1which can be miniaturized.

According to the aspect of the present disclosure, it is possible to provide an optically pumped magnetometer which can be miniaturized.

DETAILED DESCRIPTION

Hereinafter, forms for performing the present invention will be described in detail with reference to the accompanying drawings. In description of the drawings, the same reference signs are applied to the same elements, and duplicate description thereof will be omitted.

First Embodiment

FIG.1AandFIG.1Bare views illustrating a constitution of an optically pumped magnetometer1.FIG.1Ais a view illustrating a constitution of the optically pumped magnetometer1when viewed from a side.FIG.1Bis a view illustrating a constitution of the optically pumped magnetometer1when viewed from the front. The optically pumped magnetometer1is a device for measuring a magnetic field utilizing optical pumping. In the present embodiment, the optically pumped magnetometer1will be described as being used for magnetoencephalographic measurement, but the usage is not limited thereto. As an example, a measurement target of the optically pumped magnetometer1is a cerebral magnetic field. InFIG.1AandFIG.1B, an x axis and a y axis are directions along a measurement target, and a z axis is a direction intersecting a measurement target (a direction away from a measurement target). The x axis, the y axis, and the z axis are orthogonal to each other. Hereinafter, a positive direction and a negative direction of the x axis will be generally referred to as “a first direction”. In addition, a positive direction and a negative direction of the y axis will be generally referred to as “a second direction”. Moreover, a positive direction and a negative direction of the z axis will be generally referred to as “a third direction”.

As illustrated inFIG.1A, the optically pumped magnetometer1includes a cell2, a heater3, a thermocouple5, a case6, a pump laser7, a probe laser8, mirrors10and11, dividing portions12, an attenuation detection portion20, a pump connector70, and a probe connector80.

The cell2is a container to be filled with an alkali metal vapor. The cell2is disposed in the first direction along a measurement target. Here, with reference toFIG.2, details of the cell2will be described. The cell2substantially has a rectangular parallelepiped bottomed tubular shape. A cross section of the cell2in a direction perpendicular to a longitudinal direction of the cell2has a square shape, for example. The cell2may be constituted using a material such as quartz, sapphire, silicon, Kovar glass, or borosilicate glass, for example. The cell2allows light transmission with respect to pump light and probe light, which will be described below. The cell2has pump surfaces2aand2b, probe surfaces2cand2d, an upper surface2e, a lower surface2f, and a sealing portion2g.

The pump surfaces2aand2bare a pair of end surfaces orthogonal to the first direction. An antireflection film200is attached to each of the pump surfaces2aand2b. Pump light is incident on the pump surface2ain the first direction. The pump surface2bemits pump light in the first direction. Pump light may be incident on and emitted from the pump surfaces2aand2bin directions opposite to those described above.

The probe surfaces2cand2dare a pair of flat surfaces orthogonal to the second direction. Probe light is incident on the probe surface2cin the second direction. The probe surface2demits probe light in the second direction. Probe light may be incident on and emitted from the probe surfaces2cand2din directions opposite to those described above.

The upper surface2eand the lower surface2fare a pair of flat surfaces orthogonal to the third direction. The heater3(which will be described below) and the like are attached to the upper surface2eof the cell2. A magnetic field generated from a measurement target is incident on the lower surface2fof the cell2in a direction intersecting the measurement target.

The sealing portion2gis an end portion provided when it is filled with an alkali metal vapor. For example, the sealing portion2gis provided on the upper surface2ein a region close to the pump surface2b. The sealing portion2ghaving a base end on the upper surface2ehas a shape protruding away from the upper surface2ein the third direction and gradually reducing in diameter. In a cell substantially having a rectangular parallelepiped shape in the related art, an end portion for filling it with an alkali metal vapor is provided in a manner of protruding in the longitudinal direction of the cell. For this reason, in a cell substantially having a rectangular parallelepiped shape in the related art, a pair of end surfaces cannot be provided in the longitudinal direction of the cell. On the other hand, the sealing portion2gaccording to the present embodiment is provided on the upper surface2e. Accordingly, the pump surfaces2aand2bare provided as a pair of end surfaces in the longitudinal direction of the cell2.

The cell2accommodates an alkali metal vapor. For example, alkali metal may be at least one or more kinds of lithium (Li), sodium (Na), potassium (K), rubidium (Rb), and cesium (Cs). For example, alkali metal may be potassium and rubidium or may be only potassium. Potassium has a comparatively low spin-destruction collision relaxation rate among the kinds of alkali metal used in an optically pumped magnetometer. For example, the spin-destruction collision relaxation rate of potassium is lower than those of cesium, rubidium, and the like. Therefore, when single alkali metal is employed, an optically pumped magnetometer using only potassium has a higher sensitivity than an optically pumped magnetometer using only cesium or only rubidium.

In addition, the cell2accommodates filler gas. The filler gas curbs relaxation of spin polarization of an alkali metal vapor. In addition, the filler gas protects an alkali metal vapor and curbs noise light emission. For example, the filler gas may be an inert gas such as helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), or nitrogen (N2). For example, the filler gas may be helium and nitrogen.

Returning toFIG.1A, the heater3and the thermocouple5are attached to the cell2. The heater3generates heat in accordance with a current supplied from a heater power source (not illustrated). The heater3controls a density of an alkali metal vapor by controlling an internal temperature of the cell2. For example, when potassium and rubidium are accommodated as alkali metal in the cell2, the heater3performs heating such that the internal temperature of the cell2becomes 180° C. For example, the heater3is attached to the upper surface2eof the cell2. The thermocouple5measures the internal temperature of the cell2. For example, the thermocouple5is attached to the probe surface2cor the probe surface2dof the cell2at a position not blocking an optical path of probe light.

The optically pumped magnetometer1has a cell2rhaving a constitution similar to that of the cell2. The cell2rand the cell2are disposed away from each other in a direction away from a measurement target (third direction). The cell2rand the cell2form a first cell region and a second cell region on a measurement target. The first cell region and the second cell region correspond to regions in which the optically pumped magnetometer1measures a magnetic field. In the first embodiment, the first cell region is formed by the cell2r. In addition, the second cell region is formed by the cell2. That is, in the first embodiment, the first cell region and the second cell region are formed by multiple cells2rand2.

The pump laser7emits pump light for exciting alkali metal atoms. Alkali metal atoms accommodated in the cell2are excited by pump light, and spin directions thereof are aligned (spin polarization). A wavelength of pump light is set in accordance with the kind of atoms constituting an alkali metal vapor (more specifically, a wavelength of an absorption line).

When alkali metal accommodated in the cells2and2ris potassium and rubidium, the pump laser7may emit pump light for transferring spin polarization of atoms of the rubidium to atoms of the potassium by exciting atoms of the rubidium. In this case, atoms of the rubidium are in an excited state by the pump light. Further, spin polarization of atoms of the rubidium is transferred to atoms of the potassium due to spin exchange interaction between the potassium and the rubidium, and thus atoms of the potassium are in an excited state.

The pump connector70is a connector for introducing pump light emitted from the pump laser7into a casing of the optically pumped magnetometer1. For example, the pump connector70is disposed at a position close to the pump surface2bof the cell2rin the positive direction of the x axis.

The probe laser8emits probe light for detecting change in a polarization angle caused by a magnetic field in the excited state of alkali metal atoms. The probe light includes a first probe light and a second probe light. The first probe light and the second probe light may be rays of probe light divided from one ray of probe light through fiber branching or the like or may be rays of probe light emitted from multiple probe lasers8. The probe light is influenced by the state of spin polarization of alkali metal atoms when it passes through an alkali metal vapor, and thus the polarization angle thereof changes. The state of spin polarization can be derived upon detection of this change in a polarization angle. A wavelength of probe light is set in accordance with the kind of atoms constituting an alkali metal vapor (more specifically, a wavelength of an absorption line). For example, when only potassium is accommodated as alkali metal in the cell2, the wavelength of probe light is detuned from the wavelength of pump light (for example, 770.1 nm), that is, approximately 770 nm, for example. Due to detuning of the wavelength of probe light from the wavelength of pump light, Absorption of the probe light into the potassium is curbed.

When potassium and rubidium are accommodated as alkali metal in the cell2, the probe laser8may emit probe light for detecting change in a polarization angle caused by a magnetic field in the excited state of atoms of the potassium. A density of rubidium used for excitation is set to be lower than a density of potassium used in a probe. When the density of rubidium is lower than the density of potassium, attenuation of pump light due to excitation is curbed. Accordingly, even if the cell2has a slender shape, pump light arrives at the pump surface2bfrom the pump surface2aor at the pump surface2afrom the pump surface2b. As a result, pump light can uniformly excite rubidium. Thus, the optically pumped magnetometer1can obtain a uniform sensitivity inside the cell2.

The probe connector80is a connector for introducing probe light emitted from the probe laser8into the casing of the optically pumped magnetometer1.

The mirrors10and11are pump light mirrors for reflecting pump light at an angle of 90 degrees. The mirror10is disposed at a position close to the pump surface2aof the cell2rin the negative direction of the x axis. The mirror11is disposed at a position away from the mirror10in the negative direction of the z axis and close to the pump surface2aof the cell2.

The attenuation detection portion20detects attenuation of pump light having passed through the second cell region. The attenuation detection portion20is constituted of a photodiode. For example, the attenuation detection portion20is disposed at a position close to the pump surface2bof the cell2in the positive direction of the x axis. The attenuation detection portion20generates and outputs a signal corresponding to an intensity of pump light after consecutively passing through the multiple cells2rand2.

The dividing portions12divide probe light into multiple rays. For example, the dividing portions12are disposed between the cell2and the cell2rand are disposed side by side away from each other in the negative direction of the x axis with respect to the probe connector80. Probe light is incident on the dividing portions12toward the negative direction of the x axis. The number of divided rays of probe light corresponds to the number of channels (ch) through which the optically pumped magnetometer1can measure a magnetic field. In addition, the number of constituents on optical paths of probe light may also vary in accordance with the number of channels. As an example, the dividing portions12divide probe light into four rays of probe light. In this case, the dividing portions12are constituted of beam splitters12BSa,12BSb, and12BSc and a mirror12M. The beam splitters12BS a,12BSb, and12BSc allow some of incident light components to be transmitted therethrough and output the remaining light components from a surface different from a transmission surface. The mirror12M reflects incident light at an angle of 90 degrees. The beam splitters12BSa,12BSb, and12BSc and the mirror12M are disposed in the negative direction of the x axis in this order.

Transmittances of the beam splitters12BSa,12BSb, and12BSc are different from each other. For example, the transmittances of the beam splitters12BSa,12BSb, and12BSc may be set to 75%, 66.6%, and 50%, respectively. In this case, the beam splitter12BSa allows 75% of the incident light components to be transmitted therethrough in the negative direction of the x axis and outputs the remaining 25% in the negative direction of the y axis. The beam splitter12BSb allows 66.6% of the light components transmitted through the beam splitter12BSa to be transmitted in the negative direction of the x axis and outputs the remaining 33.3% in the negative direction of the y axis. The beam splitter12BSc allows 50% of the light components transmitted through the beam splitter12B Sb to be transmitted in the negative direction of the x axis and outputs the remaining 50% in the negative direction of the y axis. The mirror12M reflects the light components transmitted through the beam splitter12BSc in the negative direction of the y axis. In this manner, the dividing portions12divide probe light into four rays in the negative direction of the y axis. Each of the four rays of probe light has the light components of 25% of the probe light before being divided (before being guided to the dividing portions12).

As illustrated inFIG.1B, the optically pumped magnetometer1further includes mirrors13,14,15, and17, a polarization beam splitter16, a detection portion18, and a deriving portion19. InFIG.1B, the mirrors10and11are omitted.

The mirrors13and14are probe light mirrors for reflecting respective rays of probe light divided by the dividing portions12at an angle of 90 degrees. The mirror13is disposed close to the dividing portions12in the negative direction of the y axis. The mirror14is disposed away from the mirror13in the negative direction of the z axis and close to the probe surface2cof the cell2.

The mirrors15and17are probe light mirrors for reflecting respective rays of probe light having passed through the cell2at an angle of 90 degrees. The mirror15is disposed away from the mirror14in the positive direction of the y axis with the cell2sandwiched therebetween.

The polarization beam splitter16allows a first light component having a first polarization angle included in incident light to be transmitted therethrough and outputs a second light component having other polarization angles from a surface different from the transmission surface. For example, the first polarization angle is an angle inclined by 45 degrees with respect to the polarization angle of probe light emitted from the probe laser8. The second light component is an angle inclined by 90 degrees with respect to the first polarization angle. Thus, when no magnetic field is applied to the cell2, light quantities of probe light having the first and second polarization angles are equivalent to each other. In addition, when a magnetic field is applied to the cell2, spin polarization of alkali metal atoms changes, and a polarization surface thereof changes when probe light passes through the inside of the cell2. Therefore, the balance between the light quantities changes in accordance with the intensity of a magnetic field. The polarization beam splitter16is disposed away from the mirror15in the positive direction of the z axis.

The mirror17is a probe light mirror for reflecting each ray of probe light output by the polarization beam splitter16at an angle of 90 degrees. The mirror17is disposed close to the polarization beam splitter16in the positive direction of the y axis. The mirror17may be disposed close to the polarization beam splitter16in the positive direction of the z axis.

The detection portion18detects the first probe light having passed through the first cell region and the second probe light having passed through the second cell region. The detection portion18is constituted of a pair of photodiodes corresponding to the number of channels. The pair of photodiodes constituting the detection portion18are disposed respectively close to the polarization beam splitter16and the mirror17in the positive direction of the z axis. When the mirror17is disposed in the positive direction of the z axis with respect to the polarization beam splitter16, the detection portion18may be disposed close to the polarization beam splitter16and the mirror17in the positive direction of the y axis. In the detection portion18, the first light component and the second light component transmitted or output by the polarization beam splitter16are respectively incident on the pair of photodiodes. The detection portion18generates and outputs a signal corresponding to the intensity of the first light component and a signal corresponding to the intensity of the second light component.

The optically pumped magnetometer1further includes mirrors13r,14r,15r, and17r, a polarization beam splitter16r, and a detection portion18r. These constituents respectively have functions similar to those of the mirrors13,14,15, and17, the polarization beam splitter16, and the detection portion18described above.

The mirror13ris disposed close to the dividing portions12in the negative direction of the y axis. The mirror14ris disposed away from the mirror13rin the positive direction of the z axis and close to the probe surface2cof the cell2r. The mirror15ris disposed away from the mirror14rin the positive direction of the y axis with the cell2rsandwiched therebetween. The polarization beam splitter16ris disposed away from the mirror15rin the positive direction of the z axis. The mirror17ris disposed close to the polarization beam splitter16rin the negative direction of the y axis. The mirror17rmay be disposed close to the polarization beam splitter16rin the positive direction of the z axis. The pair of photodiodes constituting the detection portion18rare disposed respectively close to the polarization beam splitter16rand the mirror17rin the positive direction of the z axis. When the mirror17ris disposed in the positive direction of the z axis with respect to the polarization beam splitter16r, the detection portion18rmay be disposed close to the polarization beam splitter16rand the mirror17rin the negative direction of the y axis.

The deriving portion19acquires an output signal from the detection portions18and18rand the attenuation detection portion20. The deriving portion19derives the intensity of a magnetic field related to a region having pump light and the first probe light orthogonal to each other from change in a polarization angle of the first probe light within the first cell region based on detection results of the detection portion18r. The deriving portion19derives the intensity of a magnetic field related to a region having pump light having passed through the first cell region and the second probe light orthogonal to each other from change in a polarization angle of the second probe light within the second cell region based on detection results of the detection portion18.

The deriving portion19performs noise removal processing by calculating a difference between the intensity of a magnetic field related to an area within the first cell region and the intensity of a magnetic field related to an area within the second cell region. Such an optically pumped magnetometer1is constituted as a first derivation axis-type gradiometer.

The deriving portion19revises at least one of the intensity of a magnetic field related to an area within the first cell region and the intensity of a magnetic field related to an area within the second cell region based on detection results of the attenuation detection portion20. For example, the deriving portion19derives attenuation of pump light having passed through the cell2based on detection results of the attenuation detection portion20. As an example, the deriving portion19derives attenuation of pump light, for example, based on a signal intensity of pump light output from the pump laser7and a signal intensity of pump light detected by the attenuation detection portion20. The deriving portion19revises at least one of the intensity of a magnetic field related to an area within the first cell region and the intensity of a magnetic field related to an area within the second cell region using the derived attenuation of the pump light and a predetermined function.

The deriving portion19is physically constituted to include a memory such as a RAM or a ROM, a processor (arithmetic circuit) such as a CPU, a communication interface, and a storage portion such as a hard disk. Examples of the deriving portion19include a personal computer, a cloud server, a smartphone, and a tablet terminal. The deriving portion19functions when a program stored in the memory is executed by the CPU in a computer system.

FIG.3AandFIG.3Bare schematic views illustrating optical paths of pump light and probe light according to the first embodiment.FIG.3AandFIG.3Billustrate a simplified constitution of the optically pumped magnetometer1. First, optical paths of pump light will be described with reference toFIG.3A.FIG.3Ais a view of the optically pumped magnetometer1viewed from a side.FIG.3Aillustrates that a magnetic field has been generated from a measurement target in the positive direction of the z axis.

Pump light LP is introduced into the casing of the optically pumped magnetometer1by the pump connector70and is emitted in the negative direction of the x axis. The pump light LP is incident on the pump surface2bof the cell2rin the negative direction of the x axis. In this manner, the pump connector70functions as a first optical system for causing the pump light LP to be incident on the first cell region in the first direction.

The pump light LP passes through the inside of the cell2rin the negative direction of the x axis. The pump light LP is emitted from the pump surface2aof the cell2r. The mirror10reflects the pump light LP having passed through the cell2rin the negative direction of the z axis. Subsequently, the mirror11reflects the pump light LP reflected by the mirror10in the positive direction of the x axis. The pump light LP is incident on the pump surface2aof the cell2in the positive direction of the x axis. In this manner, the mirrors10and11function as a second optical system for causing pump light having passed through the first cell region to be incident on the second cell region in the first direction. In addition, the mirrors10and11cause pump light having passed through the first cell region to be turned back and be incident on the second cell region.

The pump light LP passes through the inside of the cell2in the positive direction of the x axis. The pump light LP is emitted from the pump surface2bof the cell2. The pump light LP having passed through the cell2is incident on the attenuation detection portion20.

Next, optical paths of probe light will be described. Probe light LB1and probe light LB2are introduced into the casing of the optically pumped magnetometer1by the probe connector80and are emitted in the negative direction of the x axis. Each of the probe light LB1and the probe light LB2is divided into four rays of probe light by the dividing portions12, thereby being emitted in the negative direction of the y axis. Hereinafter, in the description ofFIG.3AandFIG.3B, it is assumed that the probe light LB1is divided into four rays and the probe light LB2is also divided into four rays.

FIG.3Bis a view of the optically pumped magnetometer1viewed from the front. InFIG.3B, illustration of the pump light LP is omitted. The mirror13rreflects the probe light LB1output from the dividing portions12in the positive direction of the z axis. The mirror14rreflects the probe light LB1reflected by the mirror13rin the positive direction of the y axis. The probe light LB1is incident on the probe surface2cof the cell2rin the positive direction of the y axis. In this manner, the dividing portions12and the mirrors13rand14rfunction as a third optical system for causing the first probe light to be incident on the first cell region in the second direction orthogonal to the first direction.

The probe light LB1passes through the inside of the cell2rin the positive direction of the y axis. The probe light LB1is emitted from the probe surface2dof the cell2r. The mirror15rreflects the probe light LB1having passed through the cell2rin the positive direction of the z axis. The polarization beam splitter16rallows the first light component of the probe light LB1reflected by the mirror15rto be transmitted therethrough in the positive direction of the z axis and outputs the second light component in the negative direction of the y axis. The mirror17rreflects the first light component of the probe light LB1transmitted through the polarization beam splitter16rin the negative direction of the y axis. The first light component and the second light component of the probe light LB1are respectively incident on the pair of photodiodes of the detection portion18r.

The mirror13reflects the probe light LB2output from the dividing portions12in the negative direction of the z axis. The mirror14reflects the probe light LB2reflected by the mirror13in the positive direction of the y axis. The probe light LB2is incident on the probe surface2cof the cell2in the positive direction of the y axis. In this manner, the dividing portions12and the mirrors13and14function as a fourth optical system for causing the second probe light to be incident on the second cell region in the second direction.

The probe light LB2passes through the inside of the cell2in the positive direction of the y axis. The probe light LB2is emitted from the probe surface2dof the cell2. The mirror15reflects the probe light LB2having passed through the cell2in the positive direction of the z axis. The polarization beam splitter16allows the first light component of the probe light LB2reflected by the mirror15to be transmitted therethrough in the positive direction of the z axis and outputs the second light component in the positive direction of the y axis. The mirror17reflects the first light component of the probe light LB2transmitted through the polarization beam splitter16in the positive direction of the y axis. The first light component and the second light component of the probe light LB2are respectively incident on the pair of photodiodes of the detection portion18.

In this manner, the pump light LP proceeds in the negative direction of the x axis (first direction) in the first cell region. The probe light LB1proceeds in the positive direction of the y axis (second direction) in the first cell region. The pump light LP in the first direction and the probe light LB1in the second direction are orthogonal to each other in the first cell region. As a result, a reference region is formed in each of the channels in the first derivation axis-type gradiometer.

In addition, the pump light LP proceeds in the positive direction of the x axis (first direction) in the second cell region. The probe light LB2proceeds in the positive direction of the y axis (second direction) in the second cell region. The pump light LP in the first direction and the probe light LB2in the second direction are orthogonal to each other in the second cell region. As a result, a measurement region is formed in each of the channels in the first derivation axis-type gradiometer.

As illustrated inFIG.3A, a first reference region ch1-ref is formed in the first cell region. In addition, a first measurement region ch1is formed in the second cell region. The first reference region ch1-ref and the first measurement region ch1are away from each other in the third direction. The first measurement region ch1is closer to a measurement target than the first reference region ch1-ref. For this reason, the first measurement region ch1can have a higher sensitivity for a magnetic field generated from a measurement target. Here, both the first reference region ch1-ref and the first measurement region ch1may receive an influence of common-mode noise. In the first derivation axis-type gradiometer, an influence of common-mode noise can be canceled by calculating a difference between the intensity of a magnetic field related to the first reference region ch1-ref and the intensity of a magnetic field related to the first measurement region ch1.

As described above, the optically pumped magnetometer1according to the first embodiment includes at least one cell2configured to be filled with an alkali metal vapor, be disposed in the first direction along a measurement target, and form the first cell region and the second cell region on a measurement target; the pump laser7configured to emit pump light for exciting alkali metal atoms; the probe laser8configured to emit probe light including the first probe light and the second probe light for detecting change in a polarization angle caused by a magnetic field in the excited state of alkali metal atoms; the first optical system configured to cause pump light to be incident on the first cell region in the first direction; the second optical system configured to cause pump light having passed through the first cell region to be incident on the second cell region in the first direction; the third optical system configured to cause the first probe light to be incident on the first cell region in the second direction orthogonal to the first direction; the fourth optical system configured to cause the second probe light to be incident on the second cell region in the second direction; the detection portions18and18rconfigured to detect the first probe light having passed through the first cell region, and the second probe light having passed through the second cell region; and the deriving portion19configured to derive the intensity of a magnetic field related to a region having pump light and the first probe light orthogonal to each other from change in a polarization angle of the first probe light within the first cell region and derive the intensity of a magnetic field related to a region having pump light having passed through the first cell region and the second probe light orthogonal to each other from change in a polarization angle of the second probe light within the second cell region based on detection results of the detection portions18and18r.

In this optically pumped magnetometer1, pump light is incident on the first cell region in the first direction, and the first probe light is incident on the first cell region in the second direction. Traveling directions of pump light and the first probe light are orthogonal to each other within the first cell region. Further, pump light having passed through the first cell region is incident on the second cell region in the first direction, and the second probe light is incident on the second cell region in the second direction. Traveling directions of pump light having passed through the first cell region and the second probe light are orthogonal to each other within the second cell region. That is, the same pump light is consecutively incident throughout multiple cell regions. Further, the intensity of a magnetic field related to a region having them intersecting each other is derived using pump light and the first probe light, and the intensity of a magnetic field related to a region having them intersecting each other is derived using pump light and the second probe light. Accordingly, there is no need to cause pump light to branch or prepare multiple pump lasers7in a manner of corresponding to the number of cell regions. Therefore, the constitution related to pump light can be simplified. Accordingly, it is possible to provide the optically pumped magnetometer1which can be miniaturized.

The first cell region and the second cell region are formed by the multiple cells2rand2. Accordingly, each of the cells2rand2can be miniaturized.

The multiple cells2rand2are disposed away from each other in a direction away from a measurement target. The deriving portion19performs noise removal processing by calculating a difference between the intensity of a magnetic field related to an area within the first cell region and the intensity of a magnetic field related to an area within the second cell region. According to such a constitution, since an influence of common-mode noise is manifested in each of the intensity of a magnetic field related to an area within the first cell region and the intensity of a magnetic field related to an area within the second cell region, common-mode noise is removed by calculating the difference therebetween. Accordingly, measurement accuracy of the optically pumped magnetometer1can be improved.

At least one cell2has the pair of pump surfaces2aand2bin the first direction with the antireflection films200attached to the pump surfaces2aand2b. According to such a constitution, when pump light is incident on and emitted from the first cell region or the second cell region in the first direction, attenuation due to reflection of pump light is curbed. Accordingly, electric power of the pump laser7can be reduced.

The present embodiment further includes the attenuation detection portion20configured to detect attenuation of pump light having passed through the second cell region. The deriving portion19revises at least one of the intensity of a magnetic field related to an area within the first cell region and the intensity of a magnetic field related to an area within the second cell region based on detection results of the attenuation detection portion20. According to such a constitution, attenuation of pump light is taken into consideration, and the intensity of a magnetic field related to an area within each cell region is revised. Accordingly, measurement accuracy of the optically pumped magnetometer1can be improved.

In the present embodiment, alkali metal accommodated in the cell2is potassium and rubidium. The density of rubidium is lower than the density of potassium. The pump laser7emits pump light for exciting atoms of rubidium and transferring spin polarization of atoms of rubidium to atoms of potassium. The probe laser8emits probe light for detecting change in a polarization angle caused by a magnetic field in the excited state of atoms of potassium. According to such a constitution, if pump light excites atoms of rubidium, spin polarization of atoms of rubidium is transferred to atoms of potassium, and thus atoms of potassium are excited. This phenomenon is caused due to spin exchange interaction between potassium and rubidium. Since pump light excites rubidium having a lower density, attenuation of pump light is curbed. As a result, electric power of the pump laser7can be reduced.

In the present embodiment, the second optical system causes pump light having passed through the first cell region to be turned back and be incident on the second cell region. In this case, since a short optical path can be set for pump light, it is possible to provide the optically pumped magnetometer1which can be miniaturized.

Second Embodiment

With reference toFIGS.4,5A and5B, an optically pumped magnetometer1A according to a second embodiment will be described. The optically pumped magnetometer1A differs from the optically pumped magnetometer1according to the first embodiment in that the first cell region and the second cell region are formed by one cell2A.

FIG.4is a perspective view illustrating an example of the cell2A according to the second embodiment. The cell2A has a function similar to that of the cell2according to the first embodiment but has a different shape. The cell2A substantially has a rectangular parallelepiped bottomed tubular shape. A cross section of the cell2in a direction perpendicular to the longitudinal direction of the cell2A has a rectangular shape, for example. The cell2A is formed to further extend in the third direction than the cell2. The cell2A has a first space201A and a second space202A which are virtually stipulated spaces. The first space201A and the second space202A are away from each other in a direction away from a measurement target. The first space201A forms the first cell region. The second space202A forms the second cell region. That is, in the second embodiment, the first cell region and the second cell region are away from each other in a direction away from a measurement target and are formed by one cell2A.

FIG.5AandFIG.5Bare schematic views illustrating optical paths of pump light and probe light according to the second embodiment.FIG.5AandFIG.5Billustrate a simplified constitution of the optically pumped magnetometer1A. First, optical paths of pump light will be described with reference toFIG.5A.FIG.5Ais a view of the optically pumped magnetometer1A viewed from a side.FIG.5Aillustrates that a magnetic field has been generated from a measurement target in the positive direction of the z axis.

The pump light LP is introduced into the casing of the optically pumped magnetometer1A by the pump connector70and is emitted in the negative direction of the x axis. The pump light LP is incident on the pump surface2bof the cell2A in the negative direction of the x axis.

The pump light LP passes through the first space201A of the cell2A in the negative direction of the x axis. The pump light LP is emitted from the pump surface2aof the cell2A. The mirror10reflects the pump light LP having passed through the first space201A of the cell2A in the negative direction of the z axis. Subsequently, the mirror11reflects the pump light LP reflected by the mirror10in the positive direction of the x axis. The pump light LP is incident on the pump surface2aof the cell2A in the positive direction of the x axis.

The pump light LP passes through the second space202A of the cell2in the positive direction of the x axis. The pump light LP is emitted from the pump surface2bof the cell2A. The pump light LP having passed through the cell2A is incident on the attenuation detection portion20.

Next, optical paths of probe light will be described. Probe light LB is introduced into the casing of the optically pumped magnetometer1A by the probe connector80and is emitted in the negative direction of the x axis. The probe light LB is individually divided into four rays of probe light by the dividing portions12, thereby being emitted in the negative direction of the y axis. Hereinafter, in the description ofFIG.5AandFIG.5B, it is assumed that the probe light LB is divided into four rays.

FIG.5Bis a view of the optically pumped magnetometer1A viewed from the front. InFIG.5B, illustration of the pump light LP is omitted. As illustrated inFIG.5B, the optically pumped magnetometer1A includes a beam splitter14BS. The beam splitter14BS is disposed close to the mirror13rin the negative direction of the z axis.

The mirror13rreflects the probe light LB output from the dividing portions12in the negative direction of the z axis. The beam splitter14BS outputs 50% of the probe light LB reflected by the mirror13ras the probe light LB1in the negative direction of the y axis and allows the remaining 50% to be transmitted therethrough as the probe light LB2in the negative direction of the z axis. The probe light LB1is incident on the probe surface2cof the cell2A in the positive direction of the y axis. In this manner, the dividing portions12, the mirror13r, and the beam splitter14B S function as the third optical system for causing the first probe light to be incident on the first cell region in the second direction orthogonal to the first direction.

The mirror14reflects the probe light LB2output by the beam splitter14BS in the positive direction of the y axis. The probe light LB2is incident on the probe surface2cof the cell2in the positive direction of the y axis. In this manner, the dividing portions12, the mirror13r, the beam splitter14BS, and the mirror14function as the fourth optical system for causing the second probe light to be incident on the second cell region in the second direction.

The probe light LB1passes through the first space201A of the cell2A in the positive direction of the y axis. The probe light LB2passes through the second space202A of the cell2A in the positive direction of the y axis. The probe light LB1and the probe light LB2are emitted from the probe surface2dof the cell2A. The optical paths of the probe light LB1and the probe light LB2after having passed through cell2A are similar to the optical paths of the probe light LB1and the probe light LB2in the optically pumped magnetometer1according to the first embodiment.

In this manner, the pump light LP in the first direction and the probe light LB1in the second direction are orthogonal to each other in the first space201A (first cell region) of the cell2A. As a result, the reference region of each of the channels in the first derivation axis-type gradiometer is formed. As an example, as illustrated inFIG.5A, the first reference region ch1-ref is formed in the first cell region.

In addition, the pump light LP in the first direction and the probe light LB2in the second direction are orthogonal to each other in the second space202A (second cell region) of the cell2A. As a result, the measurement region of each of the channels in the first derivation axis-type gradiometer is formed. As an example, the first measurement region ch1is formed in the second cell region.

As described above, the optically pumped magnetometer1A according to the second embodiment includes the cell2A. The first cell region and the second cell region are away from each other in a direction away from a measurement target and are formed by one cell2A. The deriving portion19performs noise removal processing by calculating a difference between the intensity of a magnetic field related to an area within the first cell region and the intensity of a magnetic field related to an area within the second cell region. Accordingly, the cell2A can have a simple constitution. In addition, since common-mode noise is removed, measurement accuracy of the optically pumped magnetometer1A can be improved.

Third Embodiment

with reference toFIG.6, an optically pumped magnetometer1B according to a third embodiment will be described. The optically pumped magnetometer1B differs from the optically pumped magnetometer1according to the first embodiment in that multiple cells forming the first cell region and the second cell region are disposed away from each other in the second direction.

FIG.6is a schematic view illustrating optical paths of pump light and probe light according to the third embodiment.FIG.6is a view of the optically pumped magnetometer1A viewed from above.FIG.6illustrates a simplified constitution of the optically pumped magnetometer1B.

The optically pumped magnetometer1B includes cells21,22,23, and24having constitutions similar to that of the cell2according to the first embodiment. The cells21,22,23, and24are disposed away from each other in the positive direction of the y axis in this order. That is, the multiple cells are disposed away from each other in a direction orthogonal to the first direction and extending along a measurement target (second direction). In the third embodiment, the first cell region and the second cell region are formed by a pair of cells which are disposed in order. For example, the cell21forms the first cell region, and the cell22forms the second cell region. In addition, the cell22forms the first cell region, and the cell23forms the second cell region. In this manner, when there are three or more cells, the first cell region and the second cell region may be formed with two cells constituted as a pair.

The pump light LP is introduced into the casing of the optically pumped magnetometer1B by the pump connector70and is emitted in the negative direction of the x axis. The pump light LP is incident on the pump surface2bof the cell21in the negative direction of the x axis.

The pump light LP passes through the inside of the cell21in the negative direction of the x axis. The pump light LP is emitted from the pump surface2aof the cell21. The mirror10reflects the pump light LP having passed through the cell21in the positive direction of the y axis. Subsequently, the mirror11reflects the pump light LP reflected by the mirror10in the positive direction of the x axis. The pump light LP is incident on the pump surface2aof the cell22in the positive direction of the x axis.

The pump light LP passes through the inside of in the positive direction of the x axis the cell22. The pump light LP is emitted from the pump surface2bof the cell22. The mirror10reflects the pump light LP having passed through the cell22in the positive direction of the y axis. Subsequently, the mirror11reflects the pump light LP reflected by the mirror10in the negative direction of the x axis. The pump light LP is incident on the pump surface2bof the cell23in the negative direction of the x axis.

Similar to the optical paths in the cell21and the cell22described above, the pump light LP passes through the cell23and the cell24. The pump light LP having passed through the cell24is incident on the attenuation detection portion20.

The probe laser8may emit probe light including the probe light LB1, the probe light LB2, probe light LB3, and probe light LB4. The probe laser8may divide emitted probe light into the probe light LB1, the probe light LB2, the probe light LB3, and the probe light LB4through fiber branching or the like. Alternatively, the optically pumped magnetometer1B may include multiple probe lasers8. The probe light LB1, the probe light LB2, the probe light LB3, and the probe light LB4are introduced into the casing of the optically pumped magnetometer1A by the probe connector80and are individually emitted in the negative direction of the x axis. Each of the probe light LB1, the probe light LB2, the probe light LB3, and the probe light LB4is divided into four rays of probe light by the dividing portions12, thereby being emitted in the negative direction of the y axis. Hereinafter, in the description ofFIG.6, it is assumed that each of the probe light LB1, the probe light LB2, the probe light LB3, and the probe light LB4is divided into four rays. The probe light LB1, the probe light LB2, the probe light LB3, and the probe light LB4are respectively guided into the cells21,22,23, and24by the mirrors13and14(refer toFIG.3B).

In this manner, the pump light LP in the first direction and the probe light LB1in the second direction are orthogonal to each other in the cell21. The pump light LP in the first direction and the probe light LB2in the second direction are orthogonal to each other in the cell22. The pump light LP in the first direction and the probe light LB3in the second direction are orthogonal to each other in the cell23. The pump light LP in the first direction and the probe light LB4in the second direction are orthogonal to each other in the cell24. As a result, the measurement region of each of the channels is formed. As an example, a first measurement region ch11is formed inside the cell21disposed first in order. In addition, a measurement region ch14of a fourth channel is formed inside the cell21. Similarly, a first measurement region ch41is formed inside the cell24disposed fourth in order. In addition, a measurement region ch44of a fourth channel is formed inside the cell24.

As described above, the optically pumped magnetometer1B according to the third embodiment includes the cells21,22,23, and24. The multiple cells21,22,23, and24may be disposed away from each other in a direction orthogonal to the first direction and extending along a measurement target. According to such a constitution, the cells21,22,23, and24are adjacent to each other in a horizontal direction along a measurement target. Since the same pump light LP is incident on cells adjacent to each other, there is no need to prepare a constitution related to pump light for each cell. In this case, since an interval between cells adjacent to each other can be narrowed, the optically pumped magnetometer1B can be miniaturized.

Modification Example

The present disclosure is not limited to the embodiments described above, and various modifications can be made as described below within a range not departing from the gist of the present disclosure.

FIG.7AandFIG.7Bare views illustrating a constitution of an optically pumped magnetometer1C according to a first modification example.FIG.7Ais a view illustrating a constitution of the optically pumped magnetometer1C when viewed from a side.FIG.7Bis a view illustrating a constitution of the optically pumped magnetometer1C when viewed from the front. The optically pumped magnetometer1C has four sets of 4 ch optically pumped magnetometers1inside one casing. That is, the optically pumped magnetometer1C derives magnetic fields corresponding to 16 channels in total. InFIG.7AandFIG.7B, illustration of the pump laser7, the probe laser8, and the deriving portion19is omitted.

The optical paths of pump light and probe light in the optically pumped magnetometer1C are similar to the optical paths of pump light and probe light in the optically pumped magnetometer1described above (refer toFIG.3AandFIG.3B). With this optically pumped magnetometer1C as well, there is no need to cause pump light to branch or prepare multiple pump lasers7in a manner of corresponding to the number of cell regions. Therefore, the constitution related to pump light can be simplified. Accordingly, it is possible to provide the optically pumped magnetometer1C which can be miniaturized.

FIG.8AandFIG.8Bare views illustrating a constitution of an optically pumped magnetometer1D according to a second modification example.FIG.8Ais a view illustrating a constitution of the optically pumped magnetometer1D when viewed from a side.FIG.8Bis a view illustrating a constitution of the optically pumped magnetometer1D when viewed from the front. As illustrated inFIG.8A, the optically pumped magnetometer1D differs from the optically pumped magnetometer1C in a disposed position of the pump connector70and a reflection direction of pump light by the mirror10. InFIG.8AandFIG.8B, illustration of the pump laser7, the probe laser8, and the deriving portion19is omitted.

For example, the pump connector70in the optically pumped magnetometer1D is constituted to include two pump lasers7. The mirror10reflects pump light in the y axis direction such that it is incident on the mirror11of the optically pumped magnetometer1adjacent thereto in the y axis direction. According to such reflection, the optical paths of pump light in the optically pumped magnetometer1D become similar to those of pump light in the optically pumped magnetometer1B described above (refer toFIG.6). As a result, as illustrated inFIG.8B, in the optically pumped magnetometer1D, the same pump light is consecutively incident on the cells2and2radjacent to each other in the y axis direction. That is, regarding the optical paths of pump light in the optically pumped magnetometer1D, the optical paths of pump light in the optically pumped magnetometer1B are applied for both the reference region and the measurement region of each of the channels in the first derivation axis-type gradiometer. In the optically pumped magnetometer1D, the pump connector70is disposed close to the cell2or the cell2ron which pump light is incident first among the four cells2or cells2rin the positive direction of the x axis. In the optically pumped magnetometer1D, the attenuation detection portion20is disposed at a position on which pump light after having passed through the four cells2or cells2ris incident. The optical paths of probe light in the optically pumped magnetometer1D are similar to those of probe light in the optically pumped magnetometer1C described above (refer toFIG.7AandFIG.7B). In regard to the optical paths of pump light and probe light, it can be said that the optically pumped magnetometer1D is a combination of the optically pumped magnetometer1B and the optically pumped magnetometer1C. With this optically pumped magnetometer1D as well, there is no need to cause pump light to branch or prepare multiple pump lasers7in a manner of corresponding to the number of cell regions. Therefore, the constitution related to pump light can be simplified. Accordingly, it is possible to provide the optically pumped magnetometer1D which can be miniaturized.

FIG.9is a schematic view illustrating a bias magnetic field. For example, in the optically pumped magnetometer1D illustrated inFIG.9, four sets of constituents similar to those of the optically pumped magnetometer1are disposed at intervals of 10 mm in the y axis direction in a footprint of 60×60 mm2and derive magnetic fields corresponding to 16 channels. The optically pumped magnetometer1D may include a coil25for forming a bias magnetic field. The coil25for forming a bias magnetic field generates a bias magnetic field B in a region having the cells2disposed in accordance with a current supplied from a coil power source (not illustrated). For example, the coil25for forming a bias magnetic field can serve as a coil system surrounding the optically pumped magnetometer1D. For example, a direction of the bias magnetic field B is the same direction as the optical paths of pump light passing through the inside of the cell2(positive direction of the x axis). A peak frequency of the sensitivity of a magnetic field in the optically pumped magnetometer1D can be adjusted in accordance with the intensity of the bias magnetic field B. The peak frequency may be changed in accordance with a target to be measured by the optically pumped magnetometer1D. For example, when the optically pumped magnetometer1D is used for magnetoencephalographic measurement, the peak frequency of the sensitivity of a magnetic field is within several to several hundred Hz, which is a frequency band of a cerebral magnetic field. As an example, if the coil25for forming a bias magnetic field forms the bias magnetic field B of 7 nT, the peak frequency of the optically pumped magnetometer1D is adjusted to 50 Hz.

FIG.10is a schematic view illustrating a magnetoencephalography100using the optically pumped magnetometer1C. The magnetoencephalograph100includes multiple optically pumped magnetometers1C and a non-magnetic frame26. For example, the multiple optically pumped magnetometers1C are disposed along a measurement target with predetermined intervals therebetween. When optically pumped magnetometers1C are disposed, the magnetoencephalograph100derives magnetic fields corresponding to 192 channels in total. In the optically pumped magnetometer1, temperatures of the cells2and2rmay be adjusted to 180° C., for example. The magnetoencephalograph100may include the optically pumped magnetometers1D in place of the optically pumped magnetometers1C.

The non-magnetic frame26is a helmet-type frame for covering the entire region of the head of a measurement object person in magnetoencephalographic measurement. The non-magnetic frame26is constituted using a non-magnetic material such as graphite. The multiple optically pumped magnetometers1C are fixed to the non-magnetic frame26close to the head of a measurement object person. The non-magnetic frame26curbs heat transfer to the head of a measurement object person by means of a heat insulating material or the like.

The optically pumped magnetometers1C includes a reading circuit27. The reading circuit is a circuit for acquiring detection results of the optically pumped magnetometer1. For example, the temperature of the reading circuit27may be adjusted using a heat insulating material or the like such that it becomes 25° C. The reading circuit27may be the detection portions18and18r. In the magnetoencephalograph100, the deriving portion19may be disposed outside. Such a deriving portion19may acquire detection results from multiple reading circuits27or collectively derive magnetic fields in the multiple optically pumped magnetometers1C.

In the embodiments, magnitudes of pump light and probe light are not stipulated, but they may be individually formed to have an arbitrary magnitude. For example, probe light may be formed such that a height thereof becomes smaller than a width thereof. A height indicates a size of the optical path in the third direction regarding probe light while being guided to the cell2. A width indicates a size of the optical path in the first direction regarding probe light while being guided to the cell2. In this case, the sizes of the mirrors13,13r,14,14r,15, and15rserving as probe light mirrors can be reduced. Accordingly, it is possible to provide the optically pumped magnetometer1which can be miniaturized.