Optically pumped magnetic sensor

Abstract

An optically pumped magnetic sensor (1) comprises: a pump light source (60) that emits pump light (61); a detector (30) that outputs the result of detecting the pump light (61) emitted from the pump light source (60); two glass cells (11, 12) that are arranged so as to be aligned on the same path of the pump light from the pump light source (60) to the detector (30) and that each include gaseous atoms that are excited by the pump light; a magnetic field coil (21) that applies a first alternating-current magnetic field to one glass cell (11); a magnetic field coil (22) that applies a second alternating-current magnetic field with a phase inverted from that of the first alternating-current magnetic field to the other glass cell (12); and an output device (40) that outputs a value obtained by differentiating the output of the detector (30).

Description

Optical Pumping Magnetometer

[0001] The present disclosure relates to an optical pumping magnetometer.

[0002] Conventionally, as a sensor capable of measuring a weak magnetic field, an optical pumping magnetometer (Optical Pumped Magnetometer, hereinafter also referred to as "OPM") is known. In recent years, with the development of laser technology, the sensitivity of OPM has been improved, and it has become possible to measure a weak magnetic field on the order of femtotesla, which could conventionally be measured only by a superconducting quantum interference device (SQUID; Superconducting Quantum Interference Device), using OPM.

[0003] An OPM includes a light source that emits pumping light, a detector that detects the pumping light, and a cell disposed on the path of the pumping light from the light source to the detector. The cell is filled with gas atoms (excitation species) excited by the pumping light. An OPM measures a weak magnetic field by utilizing the phenomenon that the energy levels of gas atoms (excitation species) excited by the pumping light respond sensitively to the surrounding magnetic field due to the Zeeman effect. As the gas atoms filled in the cell, there are cases where alkali metal atomic vapor is used (see Non-Patent Documents 1 and 2), and cases where metastable state atoms of helium generated by a discharge plasma are used (see Non-Patent Documents 3 and 4).

[0004] The signal detection method of OPM includes types such as a type that measures the change in polarization by irradiating the cell with probe light in addition to the pumping light (see Non-Patent Document 1), and a type that irradiates the cell with pump light and applies an alternating magnetic field to the cell to generate parametric resonance, thereby utilizing the property that the light absorption rate of gas atoms changes depending on the surrounding magnetic field (see Non-Patent Documents 2 and 3). Hereinafter, the type that applies an alternating magnetic field to the cell to generate parametric resonance is also referred to as the "parametric resonance type".

[0005] The parametric resonance type OPM is advantageous for miniaturization and simplification of the structure of the sensor because it does not require a configuration related to the probe light compared to the type that uses pumping light and probe light.

[0006] Tetsuo Kobayashi, "High-Sensitivity Optical Pumping Atomic Magnetic Sensor," Applied Physics Vol. 80, No. 3 (2011) J. DUPONT-ROC et al, "DETECTION OF VERY WEAK MAGNETIC FIELDS (10-9GAUSS) BY 87Rb ZERO-FIELD LEVEL CROSSING RESONANCES," PHYSICS LETTERS Vol. 28A, No. 9, 10 February 1969 WILLIAM FOURCAULT et al, "Helium-4 magnetometers for room-temperature biomedical imaging: toward collective operation and photon-noise limited sensitivity," Optics Express Vol. 29, No. 10, 10 May 2021 S. Morales et al, "Magnetocardiography measurements with 4He vector optically pumped magnetometers at room temperature," Physics in Medicine & Biology, 2017 Phys. Med. Biol. 62 7267-7279

[0007] When measuring weak magnetic fields with an OPM (Operational Magnetic Field), it is desirable to eliminate the influence of the ambient magnetic field around the sensor (a uniform magnetic field generated by the Earth's magnetic field or a distant magnetic field source, hereinafter also referred to as the "ambient ambient magnetic field"). Therefore, in an OPM that measures weak magnetic fields, it is desirable to output a value that does not respond to the ambient ambient magnetic field, but responds to the magnetic field generated by a magnetic field source near the sensor (hereinafter also referred to as the "detection target magnetic field") (hereinafter also referred to as the "gladometer output").

[0008] Conventionally, obtaining a glaziometer output with an OPM required forming two optical paths as the pumping light path, placing a cell and a detection unit in each of the two pumping optical paths, and calculating the difference between the detection units of the two pumping optical paths. Therefore, a configuration was needed to form the two pumping optical paths (such as two light sources, or a configuration that branches the light from one light source), which resulted in the OPM configuration becoming complex and large.

[0009] This disclosure was made to solve these problems, and its purpose is to obtain a gradiometer output in a parametric resonant optical pumping magnetic sensor using a single pumping optical path.

[0010] The optical pumping magnetic sensor according to this disclosure comprises a light source that emits pumping light, a first region and a second region arranged side by side on the same path of the pumping light emitted from the light source and each containing gas atoms excited by the pumping light, a detector that detects the pumping light that has passed through the first region and the second region, a first coil that applies a first alternating magnetic field to the first region, a second coil that applies a second alternating magnetic field to the second region with the phase inverted of the first alternating magnetic field, and a first output device that outputs a value indicating the change in the absorption rate of the pumping light by the first region and the second region based on the detection result of the detector.

[0011] According to this disclosure, a parametric resonant optical pumping magnetic sensor can obtain a gradiometer output with a single pumping optical path.

[0012] This is a schematic diagram (1) showing an example of the overall configuration of an optical pumping magnetic sensor. This diagram shows the relationship between the light absorption rate of gas atoms and the magnetic field around the gas atoms. This diagram shows the relationship between the differential value of the light absorption rate of a glass cell and the magnetic field around the glass cell. This is a schematic diagram (2) showing an example of the overall configuration of an optical pumping magnetic sensor. This is a schematic diagram (3) showing an example of the overall configuration of an optical pumping magnetic sensor. This is a schematic diagram (4) showing an example of the overall configuration of an optical pumping magnetic sensor. This is a schematic diagram (5) showing an example of the overall configuration of an optical pumping magnetic sensor. This is a schematic diagram (6) showing an example of the overall configuration of an optical pumping magnetic sensor. This is a schematic diagram (7) showing an example of the overall configuration of an optical pumping magnetic sensor. This is a schematic diagram (8) showing an example of the overall configuration of an optical pumping magnetic sensor. This is a schematic diagram (9) showing an example of the overall configuration of an optical pumping magnetic sensor.

[0013] This embodiment will be described in detail below with reference to the drawings. In the following description, the same or corresponding parts in the drawings will be denoted by the same reference numerals, and their descriptions will not be repeated.

[0014] [Embodiment 1] Figure 1 is a schematic diagram showing an example of the overall configuration of the optical pumping magnetic sensor (OPM) 1 according to this embodiment. The optical pumping magnetic sensor 1 can be used to measure weak magnetic fields with a signal intensity of 1 nanotesla or less, such as brain magnetic fields and cardiac magnetic fields.

[0015] The optical pumping magnetic sensor 1 comprises glass cells 11 and 12, magnetic field coils 21 and 22, a detector 30, an output device (lock-in amplifier) ​​40, an AC power supply 50, and a pump light source 60.

[0016] The pump light source 60 emits pumping light 61. The detector 30 is positioned opposite the pump light source 60 and detects the pumping light 61 emitted from the pump light source 60. The emission of pumping light 61 from the pump light source 60 forms a single pumping light path between the pump light source 60 and the detector 30.

[0017] The glass cells 11 and 12 are arranged side by side along a single pumping optical path from the pump light source 60 to the detector 30. Specifically, glass cell 11 is positioned closer to the detector 30 than glass cell 12 on the single pumping optical path.

[0018] Each glass cell 11, 12 contains gas atoms excited by the pumping light 61. The specifications of the glass cells 11, 12 (size, type and amount of metal atoms sealed inside, pressure, etc.) are predetermined.

[0019] The gas atoms sealed in the glass cells 11 and 12 may be alkali metal atoms or metastable helium atoms. If the gas atoms are alkali metal atoms, a heater (not shown) for heating the alkali metal atoms is provided separately. If the gas atoms are metastable helium atoms, a discharge coil (not shown) for discharging the helium gas is provided separately.

[0020] The magnetic field coils 21 and 22 are placed in the glass cells 11 and 12, respectively. Although Figure 1 shows that each of the magnetic field coils 21 and 22 is in the shape of a Helmholtz coil (a pair of coils sandwiching each cell), the magnetic field coils 21 and 22 are not limited to being in the shape of a Helmholtz coil.

[0021] The magnetic field coils 21 and 22 are connected in series with the AC power supply 50. The alternating current output by the AC power supply 50 flows through the magnetic field coils 21 and 22, causing them to apply alternating magnetic fields to the glass cells 11 and 12 in a direction along the pumping optical path.

[0022] The winding directions of the magnetic field coils 21 and 22 are opposite to each other. For example, when viewed from the direction of propagation of the pumping light 61, if the winding direction of the magnetic field coil 21 is clockwise, then the winding direction of the magnetic field coil 22 is counterclockwise. By arranging the two magnetic field coils 21 and 22, which are connected in series with the AC power supply 50, in the pumping light path with their winding directions opposite to each other, it is possible to reverse the phase of the AC magnetic field (first AC magnetic field) applied by the magnetic field coil 21 to the glass cell 11 and the phase of the AC magnetic field (second AC magnetic field) applied by the magnetic field coil 22 to the glass cell 12.

[0023] The pumping light 61 emitted from the pump light source 60 passes through the glass cell 12, then through the glass cell 11, and reaches the detector 30. A portion of the pumping light 61 is absorbed by the gas atoms in the glass cells 11 and 12 as it passes through them. Therefore, the detector 30 detects the pumping light 61 after it has been partially absorbed by the gas atoms in the glass cells 11 and 12. Consequently, the output of the detector 30 is a value corresponding to the light absorption rate of the gas atoms in the glass cells 11 and 12.

[0024] By applying an alternating magnetic field to the glass cells 11 and 12 to induce parametric resonance, the light absorption rate of the gas atoms within the glass cells 11 and 12 has the characteristic of changing with respect to the magnetic field.

[0025] Figure 2 shows the relationship between the light absorption rate of gas atoms and the magnetic field around the gas atoms when parametric resonance occurs. As shown in Figure 2, when parametric resonance occurs, the light absorption rate of gas atoms changes significantly depending on the magnetic field around the gas atoms. Specifically, the light absorption rate increases as the magnetic field approaches zero, and reaches a maximum when the magnetic field is zero. The output of the detector 30 will be a value corresponding to the light absorption rate shown in Figure 2.

[0026] Returning to Figure 1, the output of the detector 30 is input to the output device 40. Based on the output of the detector 30 (detection result), the output device 40 outputs a value indicating the change in the absorption rate of the pumping light by the glass cells 11 and 12 (see Figure 2). Specifically, the output device 40 is a lock-in amplifier that outputs the differential value of the output of the detector 30 using the frequency of the AC output from the AC power supply 50 as a reference signal. Since the output of the detector 30 is superimposed on the effect of the change in light absorption rate due to the AC magnetic field applied by the magnetic field coils 21 and 22, the optical pumping magnetic sensor 1 according to this embodiment is configured to obtain a differential value of the light absorption rate that responds to the magnetic field (detection target magnetic field) generated by a magnetic field source near the sensor, rather than responding to the AC magnetic field applied by the magnetic field coils 21 and 22, by arranging an output device (lock-in amplifier) ​​40 that performs lock-in detection using the frequency of the AC that generates the AC magnetic field.

[0027] Furthermore, the optical pumping magnetic sensor 1 according to this embodiment is configured to obtain a gradiometer output using only one pumping optical path. Specifically, in the optical pumping magnetic sensor 1 according to this embodiment, two glass cells 11 and 12 are arranged side by side on a single pumping optical path, and alternating magnetic fields with opposite phases are applied to the two glass cells 11 and 12 from magnetic field coils 21 and 22. As a result, alternating magnetic fields are always applied to the two glass cells 11 and 12 in opposite directions, and the differential value of the light absorption rate of glass cell 11 and the differential value of the light absorption rate of glass cell 12 are inverted values ​​(values ​​with the same absolute value but opposite signs).

[0028] Figure 3 shows the relationship between the differential values ​​of the light absorption rates of glass cells 11 and 12 and the magnetic field around the glass cells 11 and 12 when an AC magnetic field with opposite phases is applied to the glass cells 11 and 12. In Figure 3, waveform L1 shows the differential value of the light absorption rate of glass cell 11, and waveform L2 shows the differential value of the light absorption rate of glass cell 12. As shown in Figure 3, the waveform L1 of the differential value of the light absorption rate of glass cell 11 is the inverted waveform of the waveform L2 of the differential value of the light absorption rate of glass cell 12.

[0029] The output of the output device 40 corresponds to the sum of the differential value of the light absorption rate of the glass cell 11 and the differential value of the light absorption rate of the glass cell 12. Therefore, when the optical pumping magnetic sensor 1 is placed in a uniform magnetic field, the magnetic field around the glass cell 11 and the magnetic field around the glass cell 12 are the same value, and as can be seen from the waveform shown in Figure 3, the differential value of the light absorption rate of the glass cell 11 is the inverse of the differential value of the light absorption rate of the glass cell 12, so their sum is always zero. In other words, when the optical pumping magnetic sensor 1 is placed in a space where only the ambient magnetic field is present, the output of the output device 40 is zero.

[0030] On the other hand, when the magnetic field around glass cell 11 and the magnetic field around glass cell 12 are different, as can be seen from the waveform shown in Figure 3, the differential value of the light absorption rate of glass cell 11 does not match the inverted differential value of the light absorption rate of glass cell 12, and their sum is a value corresponding to the difference between the magnetic field around glass cell 11 and the magnetic field around glass cell 12. In other words, when the optical pumping magnetic sensor 1 is placed in a space where both the ambient magnetic field and the target magnetic field are present, the output of the output device 40 will not respond to the ambient magnetic field, but will respond to the target magnetic field. By this principle, the optical pumping magnetic sensor 1 can obtain a gradiometer output.

[0031] As described above, the optical pumping magnetic sensor 1 according to this embodiment arranges two glass cells 11 and 12 side by side on a single pumping optical path, applies an alternating magnetic field with opposite phases to the two glass cells 11 and 12, and outputs a value obtained by differentiating the detected value of the pumping light 61 that has passed through the two glass cells 11 and 12 (a value indicating the change in the absorption rate of the pumping light by the glass cells 11 and 12). With this configuration, a gradiometer output can be obtained with just one pumping optical path.

[0032] Furthermore, in the optical pumping magnetic sensor 1 according to this embodiment, only one pump light source 60 is required, there is no need to branch the pumping light 61 from the pump light source 60, and only one detector 30 and one output device 40 are needed. As a result, the configuration of the optical pumping magnetic sensor 1 is simplified, and the optical pumping magnetic sensor 1 can be made smaller and less expensive. In addition, since there is only one pumping optical path, the effort required to adjust the light intensity or optical path length of the pumping light 61 can be reduced.

[0033] <Modification 1 of Embodiment 1> In Embodiment 1 described above, magnetic field coils 21 and 22 are connected in series with the AC power supply 50, and the winding directions of the magnetic field coils 21 and 22 are reversed to apply an AC magnetic field with opposite phase to the glass cells 11 and 12. However, the configuration for applying an AC magnetic field with opposite phase to the glass cells 11 and 12 is not limited to the above configuration.

[0034] Figure 4 is a schematic diagram showing the overall configuration of an optical pumping magnetic sensor 1A according to one example of the modified example 1. In the optical pumping magnetic sensor 1A shown in Figure 4, a phase inversion circuit 51 is provided that outputs AC with the phase inverted of the AC generated by the AC power supply 50. The magnetic field coil 21 is supplied with AC output from the AC power supply 50, and the magnetic field coil 22 is supplied with AC output from the phase inversion circuit 51. The winding directions of the magnetic field coils 21 and 22 are the same. For example, if the winding direction of the magnetic field coil 21 is clockwise when viewed from the direction of propagation of the pumping light 61, the winding direction of the magnetic field coil 22 is also clockwise.

[0035] Figure 5 is a schematic diagram showing the overall configuration of the optical pumping magnetic sensor 1B according to another example of the modified example 1. In the optical pumping magnetic sensor 1B, in addition to the AC power supply 50, an AC power supply 52 is provided. The AC power supply 52 receives a synchronization signal from the AC power supply 50 and outputs an AC with the phase inverted of the AC output of the AC power supply 50. The output of the AC power supply 50 is supplied to the magnetic field coil 21, and the output of the AC power supply 52 is supplied to the magnetic field coil 22. The winding directions of the magnetic field coils 21 and 22 are the same, as described in Figure 4 above.

[0036] In the optical pumping magnetic sensor 1A shown in Figure 4 or the optical pumping magnetic sensor 1B shown in Figure 5, an AC magnetic field with opposite phases can be applied to the glass cells 11 and 12.

[0037] Furthermore, in the optical pumping magnetic sensor 1A shown in Figure 4 or the optical pumping magnetic sensor 1B shown in Figure 5, the application of the AC magnetic field from the magnetic field coil 22 to the glass cell 12 can be stopped by turning off the output of the phase inversion circuit 51 or the AC power supply 52. ​​This allows the output of the output device 40 to be switched from a gradiometer output using magnetic field coils 21 and 22 to a magnetometer output using only the magnetic field coil 21.

[0038] <Modification 2 of Embodiment 1> In Embodiment 1 described above, the magnetic field in the direction along the pumping optical path is measured by applying alternating magnetic fields in the direction along the pumping optical path. In contrast, the magnetic field in the direction intersecting the pumping optical path may be measured by applying alternating magnetic fields in the direction intersecting the pumping optical path.

[0039] Figure 6 is a schematic diagram showing the overall configuration of the optical pumping magnetic sensor 1C according to this modified example 2. The optical pumping magnetic sensor 1C is the optical pumping magnetic sensor 1 shown in Figure 1 above, with the magnetic field coils 21 and 22 replaced by magnetic field coils 21a and 22a.

[0040] The magnetic field coils 21a and 22a are arranged to apply an alternating magnetic field to the glass cells 11 and 12 in a direction perpendicular to the pumping optical path. Thereby, a gradiometer output with respect to the magnetic field in the direction perpendicular to the pumping optical path can be obtained.

[0041] Further, by appropriately combining the magnetic field coils 21 and 22 shown in FIG. 1 and the magnetic field coils 21a and 22a shown in FIG. 4, it is also possible to obtain a gradiometer output for a magnetic field of two axes or three or more axes with one OPM.

[0042] <Modification Example 3 of Embodiment 1> In the above-described Embodiment 1, two glass cells 11 and 12 are arranged side by side on one pumping optical path, and alternating magnetic fields having opposite phases are applied to the two glass cells 11 and 12, respectively.

[0043] On the other hand, the same technique as that of the above-described Embodiment 1 can also be applied to a configuration in which one glass cell is arranged on one pumping optical path.

[0044] FIG. 7 is a diagram schematically showing the overall configuration of the optical pumping magnetic sensor 1D according to Modification Example 3 of Embodiment 1. In the optical pumping magnetic sensor 1D shown in FIG. 7, one glass cell 13 is arranged on one pumping optical path, and alternating magnetic fields having opposite phases are applied to two different regions 11a and 12a inside the glass cell 13. Region 11a is a region closer to the detector 30 inside the glass cell 13, and region 12a is a region closer to the pump light source 60 inside the glass cell 13.

[0045] Since the two regions 11a and 12a in the glass cell 13 serve the same role as the two glass cells 11 and 12 in the above-described Embodiment 1, a gradiometer output can be obtained in the same manner as in the above-described Embodiment 1.

[0046] Furthermore, in the optical pumping magnetic sensor 1 shown in FIG. 1 above, it is necessary to make the specifications (size, type, amount, pressure, etc. of metal atoms enclosed inside) of the two glass cells 11 and 12 the same or to adjust the arrangement of the two glass cells 11 and 12. However, in the optical pumping magnetic sensor 1D shown in FIG. 7, only one glass cell 13 is required, so it is advantageous in that the components are reduced and the structure is simplified.

[0047] <Embodiment 1 - Modification 4> FIG. 8 is a diagram schematically showing the overall configuration of an optical pumping magnetic sensor 1E according to Modification 4 of Embodiment 1. The optical pumping magnetic sensor 1E shown in FIG. 8 is obtained by arranging a mirror 31 at the position where the detector 30 was arranged with respect to the optical pumping magnetic sensor 1A shown in FIG. 4 and moving the detector 30 to the position on the side of the pump light source 60.

[0048] In the optical pumping magnetic sensor 1E shown in FIG. 8, the pumping light is configured to reciprocate inside the glass cells 11 and 12 using the mirror 31. Specifically, the pumping light 61 from the pump light source 60 passes through the glass cells 12 and 11, is reflected by the mirror 31, and the pumping light 62 reflected by the mirror 31 passes through the glass cells 11 and 12 again and is detected by the detector 30.

[0049] With such a configuration, a gradiometer output can also be obtained. Furthermore, in the optical pumping magnetic sensor 1E, since the pumping light reciprocates in the glass cells 11 and 12, the light absorption amount of the glass cells 11 and 12 can be amplified. Also, in the optical pumping magnetic sensor 1E, since the size on the mirror 31 side can be reduced by arranging the detector 30 on the side of the pump light source 60, the mirror 31 side can be brought closer to the magnetic field source.

[0050] <Embodiment 2> In the above-described Embodiment 1, a gradiometer output is obtained with one pumping optical path and one output device 40 by applying alternating magnetic fields with opposite phases to the two glass cells 11 and 12.

[0051] However, the configuration of Embodiment 1 described above has the problem of a narrow dynamic range (the magnetic field range in which a linear response can be obtained). Conventionally, it has been known that OPMs inherently have a narrow dynamic range, and as a countermeasure, a wide dynamic range is achieved by feeding the output back as a magnetic field and canceling the magnetic field entering the OPM (a so-called closed-loop output). However, in the configuration of Embodiment 1 described above, it is not a closed-loop output but an open-loop output, so the dynamic range becomes narrow.

[0052] Furthermore, if the target magnetic field is a weak magnetic field of 1 nanotesla or less, measurement is possible even with an open-loop output configuration like that of Embodiment 1 described above. However, in order to perform OPM operation, a separate mechanism to suppress the overall ambient magnetic field is required.

[0053] To solve the above-mentioned problems, in Embodiment 2, two glass cells 11 and 12 are arranged side by side in a single pumping optical path, and different alternating magnetic fields are applied to the two glass cells 11 and 12 to measure the absorption rate of the pumping light. Then, the differential value of the optical absorption rate with respect to the frequency of the alternating magnetic field applied to one of the two glass cells 11 and 12 (lock-in detection value) is converted into a magnetic field and a feedback loop is formed that provides negative feedback to the two glass cells 11 and 12.

[0054] Figure 9 is a schematic diagram showing an example of the overall configuration of the optical pumping magnetic sensor 1F according to this second embodiment. The optical pumping magnetic sensor 1F shown in Figure 9 is the same as the optical pumping magnetic sensor 1B shown in Figure 5, but with the AC power supply 52 replaced by an AC power supply 53, and with the addition of a feedback coil 23 and an output device 41. The other configurations of the optical pumping magnetic sensor 1F shown in Figure 9 are the same as those of the optical pumping magnetic sensor 1B shown in Figure 5.

[0055] In the optical pumping magnetic sensor 1F shown in Figure 9, in addition to the AC power supply 50, an AC power supply 53 is provided. The AC power supply 53 outputs AC at a frequency different from the AC frequency output by the AC power supply 50. The magnetic field coil 21 is supplied with the output of the AC power supply 50, and the magnetic field coil 22 is supplied with the output of the AC power supply 53. Note that the frequency of the AC output by the AC power supply 53 is not necessarily limited to being different from the frequency output by the AC power supply 50, and may be the same as the frequency output by the AC power supply 50.

[0056] The feedback coil 23 is a Helmholtz coil having a pair of coils that sandwich the glass cells 11 and 12. However, the feedback coil 23 is not limited to being a Helmholtz coil shape.

[0057] The output of the detector 30 is input to the output device 41. The output device 41 is a lock-in amplifier that outputs the derivative of the output of the detector 30, using the frequency of the AC output from the AC power supply 53 as a reference signal.

[0058] The output of the output device 41 is converted into a current signal and output to the feedback coil 23. This creates a feedback loop that converts the lock-in detection output of the optical absorption rate with respect to the frequency of the AC magnetic field applied to one of the two glass cells 11, 12 into a magnetic field and negatively feeds it back to the two glass cells 11, 12. By forming this feedback loop, the magnetic field signal detected by glass cell 12 can be treated as the ambient magnetic field, and the ambient magnetic field can be canceled out from the glass cells 11, 12. As a result, the other glass cell 11 is in a state where the ambient magnetic field is canceled out, and the output of the output device 40 responds only to the magnetic field from a magnetic field source near glass cell 11 (the magnetic field to be detected). Consequently, the output of the output device 40 can be made into a gradiometer output with a wide dynamic range.

[0059] <Modifications of Embodiment 2> Embodiment 2 can also be modified in the same way as Embodiment 1. Modifications of Embodiment 2 will be explained below as examples.

[0060] Figure 10 is a schematic diagram showing the overall configuration of the optical pumping magnetic sensor 1G according to the first modification of Embodiment 2. The optical pumping magnetic sensor 1G shown in Figure 10 is obtained by making the same modifications as in Modification 2 of Embodiment 1 (see Figure 6) to the optical pumping magnetic sensor 1F shown in Figure 9. That is, the optical pumping magnetic sensor 1G shown in Figure 10 is obtained by changing the magnetic field coils 21 and 22 of the optical pumping magnetic sensor 1F shown in Figure 9 to magnetic field coils 21a and 22a. With this modification, the same effects as in Modification 2 of Embodiment 1 can be achieved even in the configuration of Embodiment 2.

[0061] Figure 11 is a schematic diagram showing the overall configuration of the optical pumping magnetic sensor 1H according to a second modification of Embodiment 2. The optical pumping magnetic sensor 1H shown in Figure 11 is modified from the optical pumping magnetic sensor 1F shown in Figure 9, with the same modifications as in Modification 3 of Embodiment 1 (see Figure 7). That is, the optical pumping magnetic sensor 1H shown in Figure 11 replaces the two glass cells 11 and 12 of the optical pumping magnetic sensor 1F shown in Figure 9 with two regions 11a and 12a within a single glass cell 13. This replacement allows the same effects as in Modification 3 of Embodiment 1 to be achieved even in the configuration of Embodiment 2.

[0062] [Embodiments] The embodiments and their modifications described above will be understood by those skilled in the art to be specific examples of the following embodiments.

[0063] (Section 1) The optical pumping magnetic sensor according to this disclosure comprises a light source that emits pumping light, a first region and a second region arranged side by side on the same path of the pumping light emitted from the light source and each containing gas atoms excited by the pumping light, a detector that detects the pumping light that has passed through the first region and the second region, a first coil that applies a first alternating magnetic field to the first region, a second coil that applies a second alternating magnetic field to the second region with the phase inverted of the first alternating magnetic field, and a first output device that outputs a value indicating the change in the absorption rate of the pumping light by the first region and the second region based on the detection result of the detector.

[0064] According to the optical pumping magnetic sensor described in Section 1, a first region and a second region, each containing gas atoms, are arranged side by side along the same path of the pumping light. An alternating magnetic field with opposite phases is applied to the first and second regions, respectively. As a result, the value indicating the change in light absorption rate of the first region and the value indicating the change in light absorption rate of the second region are inverted values ​​(the same absolute value but opposite signs). The first output device then outputs a value indicating the change in the absorption rate of the pumping light due to the first and second regions. Therefore, the output of the first output device does not respond to the ambient magnetic field, but rather to the magnetic field being detected. Consequently, a gradiometer output can be obtained using only one pumping optical path.

[0065] (Section 2) The optical pumping magnetic sensor described in Section 1 further comprises a first power supply that outputs alternating current. The first coil and the second coil are connected in series with respect to the first power supply. The winding direction of the first coil and the winding direction of the second coil are opposite to each other.

[0066] According to the optical pumping magnetic sensor described in paragraph 2, an AC magnetic field with opposite phases can be applied to the first region and the second region, respectively, using the first power supply.

[0067] (Clause 3) The optical pumping magnetic sensor described in Clause 1 further comprises a first power supply that outputs alternating current and an inverting circuit that outputs alternating current with the phase inverted of the alternating current output by the first power supply. The output of the first power supply is supplied to the first coil. The output of the inverting circuit is supplied to the second coil.

[0068] According to the optical pumping magnetic sensor described in paragraph 3, an AC magnetic field with opposite phases can be applied to the first region and the second region, respectively, using a first power supply and an inverting circuit.

[0069] (Clause 4) The optical pumping magnetic sensor described in Clause 1 further comprises a first power supply that outputs alternating current and a second power supply that outputs alternating current with the phase inverted of the alternating current output by the first power supply. The output of the first power supply is supplied to the first coil. The output of the second power supply is supplied to the second coil.

[0070] According to the optical pumping magnetic sensor described in Section 4, an AC magnetic field with opposite phases can be applied to the first region and the second region, respectively, using a first power supply and a second power supply.

[0071] (Clause 5) In the optical pumping magnetic sensor described in any of Clauses 2 to 4, the first output device is a lock-in amplifier that outputs a value obtained by differentiating the detection result of the detector, using the output of the first power supply as a reference signal.

[0072] According to the optical pumping magnetic sensor described in Section 5, lock-in detection (output of differential value) of the detector's detection result can be performed using the AC frequency of the first power supply as a reference signal.

[0073] (Clause 6) The optical pumping magnetic sensor described in Clause 1 further comprises a first power supply that outputs alternating current and a second power supply that outputs alternating current with a frequency different from or the same as that of the alternating current output by the first power supply. The output of the first power supply is supplied to the first coil. The output of the second power supply is supplied to the second coil. Furthermore, the optical pumping magnetic sensor described in Clause 1 further comprises a second output device that outputs a value obtained by differentiating the output of the detector using the output of the second power supply as a reference signal, and a feedback coil that negatively feeds back a magnetic field opposite to the magnetic field of the second region to the first and second regions based on the output of the second output device.

[0074] According to the optical pumping magnetic sensor described in Section 6, a feedback loop is formed by providing a second output device and a feedback coil, which negatively feeds back the magnetic field of the second region to the first and second regions. This feedback loop allows the magnetic field of the second region to be treated as the ambient magnetic field, canceling out the ambient magnetic field from the first and second regions. As a result, the first region is in a state where the ambient magnetic field is canceled out, and the output of the first output device responds only to the magnetic field from a magnetic field source near the first region (the magnetic field to be detected). Consequently, the output of the first output device can be made into a gradiometer output with a wide dynamic range.

[0075] (Clause 7) The optical pumping magnetic sensor described in Clause 1 comprises a first cell and a second cell, each containing gas atoms. The first region is the region of the first cell. The second region is the region of the second cell.

[0076] According to the optical pumping magnetic sensor described in Section 7, the first and second regions can be composed of two first and second cells.

[0077] (Clause 8) The optical pumping magnetic sensor described in paragraph 1 comprises a cell containing gas atoms. The first region is a part of the cell. The second region is another region in the cell that is different from the first region.

[0078] According to the optical pumping magnetic sensor described in Section 8, the first and second regions can be composed of two different regions within a single cell.

[0079] The embodiments disclosed herein should be considered in all respects to be illustrative and not restrictive. The scope of the present invention is indicated by the claims rather than by the description of the embodiments above, and all modifications within the meaning and scope of the claims are intended to be included.

[0080] 1, 1A to 1H Optical pumping magnetic sensor, 11, 12, 13 Glass cell, 11a, 12a Region, 21, 21a, 22, 22a Magnetic field coil, 23 Feedback coil, 30 Detector, 31 Mirror, 40, 41 Output device, 50, 52, 53 AC power supply, 51 Phase inversion circuit, 60 Pump light source, 61, 62 Pumping light.

Claims

1. An optical pumping magnetic sensor comprising: a light source that emits pumping light; a first region and a second region arranged side by side on the same path of the pumping light emitted from the light source, each containing gas atoms excited by the pumping light; a detector that detects the pumping light that has passed through the first region and the second region; a first coil that applies a first alternating magnetic field to the first region; a second coil that applies a second alternating magnetic field to the second region, with the phase of the first alternating magnetic field inverted; and a first output device that outputs a value indicating the change in the absorption rate of the pumping light by the first region and the second region based on the detection result of the detector.

2. The optical pumping magnetic sensor according to claim 1, further comprising a first power supply that outputs alternating current, wherein the first coil and the second coil are connected in series with respect to the first power supply, and the winding direction of the first coil and the winding direction of the second coil are opposite to each other.

3. The optical pumping magnetic sensor according to claim 1, further comprising a first power supply that outputs AC, and an inverting circuit that outputs AC with the phase inverted of the AC output by the first power supply, wherein the output of the first power supply is supplied to the first coil, and the output of the inverting circuit is supplied to the second coil.

4. The optical pumping magnetic sensor according to claim 1, further comprising a first power supply that outputs AC, and a second power supply that outputs AC with the phase inverted of the AC output by the first power supply, wherein the output of the first power supply is supplied to the first coil and the output of the second power supply is supplied to the second coil.

5. The optical pumping magnetic sensor according to any one of claims 2 to 4, wherein the first output device is a lock-in amplifier that outputs a value obtained by differentiating the detection result of the detector using the output of the first power supply as a reference signal.

6. The optical pumping magnetic sensor according to claim 1, further comprising: a first power supply that outputs AC; a second power supply that outputs AC at a frequency different from or the same as the frequency of AC output by the first power supply; the output of the first power supply being supplied to the first coil; the output of the second power supply being supplied to the second coil; a second output device that outputs a value obtained by differentiating the output of the detector using the output of the second power supply as a reference signal; and a feedback coil that negatively feeds back a magnetic field opposite to the magnetic field of the second region to the first and second regions based on the output of the second output device.

7. The optical pumping magnetic sensor according to claim 1, comprising a first cell and a second cell, each containing the gas atoms, wherein the first region is the region of the first cell and the second region is the region of the second cell.

8. The optical pumping magnetic sensor according to claim 1, comprising a cell for enclosing the gas atoms, wherein the first region is a part of the cell and the second region is another region in the cell different from the first region.

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