JP6459167B2 - Magnetic field measuring apparatus and magnetic field measuring method - Google Patents

Description

  The present invention relates to a technique for measuring a magnetic field.

  There is known an optical pumping type magnetic sensor in which linearly polarized light is incident on a cell in which an alkali metal gas is sealed and a magnetic field is measured according to a rotation angle of a polarization plane. In Patent Document 1, in an optical pumping type magnetic sensor, the influence of a disturbance magnetic field is suppressed by rotating the polarizing plate so that the direction of the disturbance magnetic field matches the vibration direction of the electric field of light incident on the cell. Have been described.

JP 2013-108833 A

  The technique described in Patent Document 1 has a problem that the sensitivity of magnetic field measurement fluctuates when the amount of laser beam incident on the cell changes. An object of the present invention is to measure the strength of a magnetic field when the sensitivity of the measurement of the magnetic field is stable.

The present invention is provided with a light source, a cell containing gas therein, a light guide that guides light emitted from the light source to the cell, and between the cell and the light guide. Measured by the polarization separation unit that separates the polarization component in the first axis direction and the polarization component in the second axis direction, the light amount measurement unit that measures the light amount of the polarization component in the second axis direction, and the light amount measurement unit Provided is a magnetic field measurement device including a magnetic field measurement unit that measures a magnetic field by the amount of light in the first axis direction transmitted through the cell when the light amount or the amount of change in the light amount is equal to or less than a preset threshold value . According to this magnetic field measurement device, it is possible to prevent the magnetic field from being measured when the measured light amount or the amount of change in the measured light amount exceeds a threshold value.

  In another preferred embodiment, the threshold value is determined according to the accuracy of the magnetic field measured by the magnetic field measurement unit. According to this magnetic field measurement apparatus, the strength of the magnetic field is measured when the sensitivity of the magnetic field measurement is stable at a level corresponding to the accuracy of the magnetic field to be measured.

  In another preferred embodiment, the light guide includes an optical fiber. According to this magnetic field measuring apparatus, even when the polarization component changes in the process of guiding light by the optical fiber, the strength of the magnetic field is measured when the sensitivity of the magnetic field measurement is stable.

  In another preferred embodiment, a plurality of the cells are provided, and the polarization separation unit reflects and transmits the polarization component in the first axis direction at different ratios, and the polarization component in the second axis direction. It has a plurality of polarized light separating elements to be transmitted. According to this magnetic field measuring apparatus, a magnetic field can be measured with a plurality of channels.

The present invention also provides a light source, a cell containing gas therein, a light guide that guides light emitted from the light source to the cell, and polarized light in a first axis direction and polarized light in a second axis direction. A magnetic field measurement method in a magnetic field measurement apparatus having a polarization separation unit that separates a component into components, the step of measuring a light amount of a polarization component in the second axis direction among the light separated by the polarization separation unit, and the measurement And a step of measuring the magnetic field by the amount of light in the first axis direction transmitted through the cell when the amount of light or the amount of change in the amount of light is equal to or less than a preset threshold value . According to this magnetic field measurement method, it is possible to prevent the magnetic field from being measured when the measured light amount or the amount of change in the measured light amount exceeds a threshold value .

The block diagram which shows the structure of the magnetic field measuring apparatus which concerns on 1st Embodiment. The flowchart which shows operation | movement of a magnetic field measuring apparatus. The block diagram which shows the structure of the magnetic field measuring apparatus which concerns on 2nd Embodiment. The figure which shows the reflectance R: transmittance | permeability T of each polarization separation element 22. FIG. The block diagram which shows the structure of the magnetic field measuring apparatus which concerns on 3rd Embodiment. The figure which shows the reflectance R: transmittance | permeability T of each non-polarization separation element 23. FIG. The block diagram which shows the structure of the magnetic field measuring apparatus which concerns on a comparative example.

1. Configuration FIG. 7 is a block diagram illustrating a configuration of a magnetic field measurement apparatus 2 according to a comparative example. The magnetic field measuring device 2 is a magnetic field measuring device using non-linear optical rotation (NMOR). The magnetic field measurement apparatus 2 includes a light source 10, a polarizing plate 20, an optical fiber 30, a connector 40, a gas cell 50, a polarization separator 60, a PD (Photo Detector) 70, a PD 80, and signal processing. The circuit 90 and the display device 100 are included. The gas cell 50 is filled with gaseous alkali metal atoms. The alkali metal atoms are spin-polarized according to the polarization direction of the irradiated light and become magnetized. This magnetization vector is steadily inclined with respect to an axis orthogonal to the measurement axis ((laser beam traveling direction)). When the laser beam passes through the gas cell 50, its plane of polarization rotates due to the Faraday effect. The rotation angle of the polarization plane is proportional to the strength of the magnetic field in the measurement axis direction. The magnetic field is measured by detecting the rotation angle of the polarization plane of the laser beam with PD (Photo Detector) 70 and PD80.

  In the magnetic field measuring apparatus 2, a single mode optical fiber 30 is used for propagation of a laser beam from the light source 10 to the polarizing plate 20. When random birefringence occurs in the optical fiber 30 due to slight distortion of the core or external stress (environmental temperature change or mechanical vibration), the polarization direction of the laser beam passing through the optical fiber 30 changes. When the polarization direction changes, the amount of laser beam incident on the gas cell 50 changes, so that even if the magnetic field is constant, the value of the observed magnetic field changes (that is, the measurement sensitivity changes). See) Further, even when a polarization maintaining fiber (birefringent fiber) is used as the optical fiber 30, it is between the polarization direction of the laser beam emitted from the light source 10 and the high speed axis or the low speed axis of the polarization maintaining fiber. There is a case where a deviation occurs and the polarization direction changes to cause the same problem. The present embodiment addresses such a problem.

  FIG. 1 is a block diagram showing a configuration of a magnetic field measurement apparatus 1A according to the first embodiment of the present invention. In this example, the magnetic field measuring apparatus 1A is used in a biological state measuring apparatus (such as a magnetocardiograph or a magnetoencephalograph) that measures a magnetic field generated from a living body such as a magnetocardiogram or a magnetoencephalogram as an indicator of the state of the living body. The magnetic field measurement apparatus 1A includes a light source 10, an optical fiber 30, a connector 40, a first polarization separator 21, a gas cell 50, a second polarization separator 61, a PD (Photo Detector, a photodetector) 70, A PD 80, a signal processing circuit 90, a display device 100, a light amount monitor 110, a control unit 120, and an amplifier circuit 130 are included.

  The light source 10 is a device that outputs a laser beam having a wavelength corresponding to the absorption line of cesium (for example, 894 nm corresponding to the D1 line), for example, a tunable laser. The laser beam output from the light source 10 is so-called CW (Continuous Wave) light having a constant light amount continuously. In the example of FIG. 1, the light source 10 outputs a laser beam whose polarization component is a P-wave component. The P wave component is an example of a polarization component in the first axis direction.

  The optical fiber 30 (an example of a light guide) is a member that guides the laser beam output from the light source 10 to the gas cell 50 side. As the optical fiber 30, for example, a single mode optical fiber that propagates only the fundamental mode is used. The connector 40 is a member for supporting the optical fiber 30. The first polarization separator 21 (an example of a polarization separation unit) is provided between the gas cell 50 and the optical fiber 30 and separates a laser beam guided by the optical fiber 30 into two polarization components (for example, Wollaston prism). Or a polarizing beam splitter). As described above, the polarization direction of the laser beam output from the light source 10 changes in the process of passing through the optical fiber 30, and the polarization component of the laser beam incident on the first polarization separator 21 includes a P wave component and an S wave component. Is included. The first polarization separator 21 separates the incident laser beam into a P wave component and an S wave component. The S wave component is an example of a polarization component in the second axis direction.

  The gas cell 50 is a box (cell) having a void inside, and an alkali metal atom (cesium in this example) is sealed in the void. The gas cell 50 is formed of an inorganic material such as quartz glass or borosilicate glass. At least a part of the alkali metal atoms sealed in the gas cell 50 is vaporized at the time of measurement. The alkali metal gas may be mixed with an inert gas such as a rare gas. Further, the inner wall of the gas cell 50 may be coated with paraffin or the like.

  The second polarization separator 61 is an element (for example, a Wollaston prism or a polarization beam splitter) that separates an incident laser beam into laser beams having two polarization components orthogonal to each other. The PD 70 and PD 80 are detectors sensitive to the wavelength of the laser beam, and output a current corresponding to the amount of incident light to the signal processing circuit 90. Since the measurement may be affected when the PD 70 and the PD 80 generate a magnetic field, it is desirable that the PD 70 and the PD 80 are made of a nonmagnetic material. PD 70 and PD 80 are disposed on the same side as the second polarization separator 61 as viewed from the gas cell 50. The amplifier circuit 130 amplifies signals output from the PD 70 and the PD 80.

  If it demonstrates along the path | route of a laser beam, said element will be arrange | positioned as follows. The light source 10 is located in the uppermost stream of the laser beam path. Hereinafter, from the upstream side, the optical fiber 30, the connector 40, the first polarization separator 21, the gas cell 50, the second polarization separator 61, and the PD 70 (PD80). Arranged in order. Explaining the progress of the laser beam, the laser beam output from the light source 10 is guided to the optical fiber 30 and reaches the first polarization separator 21. The laser beam that has reached the first polarization separator 21 is separated into a P wave component and an S wave component. The P-wave component laser beam separated by the first polarization separator 21 enters the gas cell 50. The laser beam passing through the gas cell 50 excites (optically pumps) the alkali metal atoms sealed in the gas cell 50. At this time, the polarization plane of the laser beam is rotated by receiving the polarization plane rotation action corresponding to the strength of the magnetic field. The laser beam transmitted through the gas cell 50 is separated into two polarized light components by the second polarization separator 61. The light amounts of the two polarization component beams are measured (probing) by the PD 70 and the PD 80. Note that the S-wave component of the laser beam separated by the first polarization separator 21 is incident on a light amount monitor 110 described later.

  The signal processing circuit 90 receives signals indicating the light amounts of the beams measured by the PD 70 and PD 80, respectively. The signal processing circuit 90 measures the rotation angle of the polarization plane of the laser beam based on each received signal. The rotation angle of the polarization plane is expressed as a function based on the strength of the magnetic field in the propagation direction of the laser beam (for example, D. Budker, et al., “Resonant nonlinear magneto-optical rotation effect of atoms”, Review of See Equation (2) in Modern Physics, USA, American Physical Society, October 2002, Vol. 74, No. 4, p.1153-11201, although Equation (2) relates to linear optical rotation. In the case of NMOR, almost the same formula can be used). The signal processing circuit 90 measures the strength of the magnetic field in the propagation direction of the laser beam from the rotation angle of the polarization plane. The display device 100 displays the strength of the magnetic field measured by the signal processing circuit 90.

  The light quantity monitor 110 is a light receiving element such as a photodiode that detects the light quantity of the laser beam of the S wave component separated by the first polarization separator 21. When the S wave component is incident on the light amount monitor 110, the light amount monitor 110 outputs a current corresponding to the incident light amount to the control unit 120. The control unit 120 is a computer that controls the operation of each unit of the magnetic field measurement apparatus 1A. The control unit 120 includes a CPU (Central Processing Unit), a ROM (Read Only Memory), and a RAM (Random Access Memory). The control unit 120 controls the measurement of the magnetic field by the signal processing circuit 90 based on the detection result of the light amount by the light amount monitor 110. In FIG. 1, the light amount monitor 110 controlled by the control unit 120 is an example of a light amount measurement unit. The control unit 120 and the signal processing circuit 90 controlled by the control unit 120 are an example of a magnetic field measurement unit.

2. Operation FIG. 2 is a flowchart showing the operation of the magnetic field measurement apparatus 1. The following process is started, for example, when the user starts up the magnetic field measurement apparatus 1 by operating an input unit (not shown). When the magnetic field measuring apparatus 1 is activated, a laser beam is output from the light source 10. In step S <b> 1, the control unit 120 measures the amount of light incident on the light amount monitor 110. The measurement of the light quantity in step S1 is repeated at predetermined intervals. The control unit 120 stores the measured light amount in the RAM. In step S <b> 2, the control unit 120 calculates a light amount fluctuation amount (hereinafter referred to as “fluctuation amount ΔL”). As the fluctuation amount ΔL, for example, the maximum value of the light amount fluctuation, the average value of the light amount fluctuation, the standard deviation of the light amount fluctuation, or the like in a predetermined period before the process of step S2 is performed. The control unit 120 calculates the fluctuation amount ΔL according to the light amount measured in step S1. The control unit 120 stores the calculated fluctuation amount ΔL in the RAM. In step S3, the control unit 120 determines whether or not the fluctuation amount ΔL is equal to or less than the threshold value Th1. The threshold value Th1 is a value determined according to the accuracy of the magnetic field strength measured by the signal processing circuit 90, and is stored in the ROM in advance. The control unit 120 reads the fluctuation amount ΔL from the RAM and the threshold value Th1 from the ROM, and compares them. When it is determined that the fluctuation amount ΔL is equal to or less than the threshold value Th1 (S3: YES), the control unit 120 shifts the process to step S4. When it is determined that the fluctuation amount ΔL exceeds the threshold Th1 (S3: NO), the control unit 120 shifts the process to step S1. In step S4, the control unit 120 measures the strength of the magnetic field.

  When the processing from step S1 to step S4 is performed, when the change in the light amount of the laser beam incident on the gas cell 50 is small compared to the case where the magnetic field strength is measured when the fluctuation amount ΔL exceeds the threshold Th1 ( That is, when the magnetic field measurement sensitivity is stable), the magnetic field strength is measured. Next, another configuration of the magnetic field measurement apparatus will be described. The same operation as in FIG. 2 is also performed for the magnetic field measurement apparatus described below.

3. Other Configuration FIG. 3 is a block diagram showing a configuration of a magnetic field measurement apparatus 1B according to the second embodiment of the present invention. The magnetic field measurement apparatus 1B is a so-called 4-channel magnetic field measurement apparatus having four gas cells. Therefore, the magnetic field measuring apparatus 1B has four combinations of the gas cell 50, the second polarization separator 61, the PD 70, the PD 80, and the amplifier circuit 130. Hereinafter, the magnetic field measurement apparatus 1B will be described focusing on a configuration different from the magnetic field measurement apparatus 1A. The magnetic field measurement apparatus 1 </ b> B includes four polarization separation elements 22 (22 </ b> A to 22 </ b> D) instead of the first polarization separator 21.

  The polarization separation element 22 reflects and transmits the same polarization component as the polarization component of the laser beam output from the light source 10 among the polarization components included in the incident laser beam. In the example shown in FIG. 3, since the light source 10 outputs a laser beam whose polarization component is a P-wave component, the polarization separation element 22 reflects and transmits the P-wave component included in the incident laser beam. The ratio (reflectance R: transmittance T) when the polarization separation element 22 reflects and transmits a certain polarization component (here, P wave component) is different between the polarization separation elements 22A to 22D.

  The polarization separation element 22 also transmits a polarization component different from the polarization component of the laser beam output from the light source 10 among the polarization components included in the incident laser beam. In the example shown in FIG. 3, since the light source 10 outputs a laser beam whose polarization component is a P-wave component, the polarization separation element 22 transmits an S-wave component included in the incident laser beam.

  FIG. 4 is a diagram showing the reflectance R: transmittance T of each polarization separation element 22 with respect to the P wave component and the S wave component. Regarding the P-wave component, in the magnetic field measurement apparatus 1B, the reflectance R: transmittance T of each polarization separation element 22A to 22D is determined so that the amount of light incident on each of the plurality of gas cells 50 is equal. Specifically, the reflectance R: transmittance T for the P wave component of each of the polarization separation elements 22A to 22D is 25:75 for the polarization separation element 22A, 33:67 for the polarization separation element 22B, and 50 for the polarization separation element 22C. : 50, and the polarization separation element 22D is 100: 0. Further, the reflectivity R: transmittance T with respect to the S wave component of each polarization separation element 22A to 22D is 0: 100.

  FIG. 5 is a block diagram showing a configuration of a magnetic field measurement apparatus 1C according to the third embodiment of the present invention. The magnetic field measurement apparatus 1C is a so-called 16-channel magnetic field measurement apparatus having 16 gas cells. Therefore, the magnetic field measurement apparatus 1C has 16 combinations of the gas cell 50, the second polarization separator 61, the PD 70, the PD 80, and the amplifier circuit 130. In FIG. 5, illustration of combinations for 16 channels is omitted for convenience of explanation. The magnetic field measurement apparatus 1C includes 16 non-polarization separation elements 23 (23An, 23Bn, 23Cn, 23Dn) in addition to the configuration of the magnetic field measurement apparatus 1B.

  The laser beam reflected by the polarization separation element 22 is incident on the non-polarization separation element 23. In the example shown in FIG. 5, the light source 10 outputs a laser beam whose polarization component is a P-wave component, and the polarization separation element 22 reflects the P-wave component. Therefore, a P-wave component laser beam is incident on the non-polarization separation element 23. The non-polarization separation element 23 does not change the polarization component of the incident laser beam, and the laser beam toward the gas cell 50 (laser beam toward the z-axis positive direction) and the laser beam toward the other adjacent non-polarization separation element 23. (Laser beam directed in the positive x-axis direction). That is, a part of the laser beam separated by the non-polarization separation element 23 is incident on the gas cell 50 and the rest is incident on another non-polarization separation element 23 adjacent to the gas cell 50.

  FIG. 6 is a diagram showing the reflectance R: transmittance T of each non-polarized light separating element 23. In the magnetic field measurement apparatus 1 </ b> C, the reflectance R: transmittance T of each non-polarization separation element 23 is determined so that the light amounts of the P-wave components incident on each of the plurality of gas cells 50 are equal. Specifically, the reflectance R: transmittance T of each non-polarized light separating element 23 is 25:75 for the non-polarized light separating element 23An (23A1 to 23A4), and 33:67 for the non-polarized light separating element 23Bn (23B1 to 23B4). The non-polarization separation element 23Cn (23C1 to 23C4) is 50:50, and the non-polarization separation element 23Dn (23D1 to 23D4) is 100: 0. In addition, reflectance R: transmittance T of each polarization separation element 22A to 22D is the same as that in FIG.

4). Modifications The present invention is not limited to the above-described embodiment, and various modifications can be made. Hereinafter, some modifications will be described. Two or more of the modifications described below may be used in combination.

The configuration of the magnetic field measuring apparatus to which the present invention is applied is not limited to that shown in FIGS. For example, the number of channels of the magnetic field measurement apparatus may be different from the number of channels shown in the embodiment. Further, the reflectance R: transmittance T of each polarization separation element 22 and each non-polarization separation element 23 may be a ratio different from the ratio shown in the embodiment. Furthermore, instead of the non-polarization separation element 23, a polarization separation element that separates the linearly polarized light of the S wave component or the P wave component at a predetermined ratio may be used.
The optical fiber 30 is not limited to a single mode optical fiber. The optical fiber 30 may be a polarization maintaining fiber, for example. Further, the light guide is not limited to the optical fiber 30.
Analysis of the strength of the magnetic field based on the signals output from the PD 70 and the PD 80 may be performed by the control unit 120. In this case, the signal processing circuit 90 is omitted.

  The polarization component of the laser beam output from the light source 10 is not limited to the P wave component. The light source 10 may output a laser beam having an S wave component. In this case, the first polarization separator 21, the polarization separation element 22, and the non-polarization separation element 23 separate the laser beam so that the S wave component is incident on the gas cell 50 and the P wave component is incident on the light amount monitor 110. To do.

  The operation of the magnetic field measuring apparatus 1 is not limited to the operation shown in FIG. For example, the processing from step S1 to step S3 may be performed after the measurement of the magnetic field is started. In this case, when it is determined that the fluctuation amount ΔL is equal to or less than the threshold value Th1, the measurement of the magnetic field may be stopped. Further, the method for calculating the fluctuation amount ΔL is not limited to the method described in the embodiment.

  The method in which the control unit 120 controls the measurement of the magnetic field based on the light amount detection result by the light amount monitor 110 is not limited to the method illustrated in FIG. For example, the control unit 120 may measure the strength of the magnetic field when the light amount detected by the light amount monitor 110 is equal to or less than the threshold Th2. In this case, the fluctuation amount ΔL is not calculated.

DESCRIPTION OF SYMBOLS 1,2 ... Magnetic field measuring apparatus, 10 ... Light source, 20 ... Polarizing plate, 21 ... 1st polarization separator, 22 ... Polarization separation element, 23 ... Non-polarization separation element, 30 ... Optical fiber, 40 ... Connector, 50 ... Gas cell, DESCRIPTION OF SYMBOLS 60 ... Polarization separator, 61 ... 2nd polarization separator, 70,80 ... PD, 90 ... Signal processing circuit, 100 ... Display apparatus, 110 ... Light quantity monitor, 120 ... Control part, 130 ... Amplification circuit

Claims (5)

A light source;
A cell containing gas inside,
A light guide that guides the light emitted by the light source to the cell;
A polarization separation unit that is provided between the cell and the light guide and separates the light into a polarization component in a first axis direction and a polarization component in a second axis direction;
A light amount measuring unit for measuring the light amount of the polarization component in the second axis direction;
A magnetic field measuring unit that measures a magnetic field by the amount of light in the first axis direction transmitted through the cell when the light amount measured by the light amount measuring unit or the amount of change in the light amount is equal to or less than a preset threshold value. And
The amount of change, a magnetic field measuring device that is determined based on the fluctuation of the light intensity at each measurement, which is measured repeatedly at predetermined intervals. The magnetic field measurement apparatus according to claim 1 , wherein the threshold value is determined according to accuracy of a magnetic field measured by the magnetic field measurement unit. The light guide, the magnetic field measuring apparatus according to claim 1 or 2, characterized in that it comprises an optical fiber. A plurality of the cells are provided,
The polarization separation unit includes a plurality of polarization separation elements that reflect and transmit the polarization components in the first axis direction at different ratios and transmit the polarization components in the second axis direction. Item 4. The magnetic field measurement apparatus according to any one of Items 1 to 3 . A light source, a cell containing gas therein, a light guide that guides light emitted from the light source to the cell, and polarized light that separates the light into a polarization component in a first axis direction and a polarization component in a second axis direction A magnetic field measurement method in a magnetic field measurement apparatus having a separation unit,
Measuring the amount of the polarized light component in the second axis direction of the light separated by the polarization separation unit;
Wherein when the amount of change in the measured light intensity or light amount is equal to or less than the predetermined threshold value, possess and measuring the magnetic field by the amount of the first axial transmitted through the cell,
The amount of change, is that the magnetic field measuring method determined based on the change of light intensity at each measurement every is repeatedly measured at predetermined intervals.

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