JP2009236599A - Optical pumping magnetometer - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical pumping magnetometer capable of detecting a magnetic field with higher sensitivity by enhancing detection signal intensity. <P>SOLUTION: This optical pumping magnetometer 1 equipped with a cell 110 in which an atomic group capable of taking a gas state is enclosed, a pumping light source 130 for generating pumping light for spin-polarizing the atomic group, a probe light source 140 for irradiating probe light to the cell 110, and a detector 160 for detecting rotation of a plane of polarization of the probe light, is also equipped with at least one reflection mirror 120 for reflecting the probe light. The magnetometer 1 is constituted so that the reflection mirror 120 is provided on a position where the probe light crossing the inside of the cell 110 is reflected, and that the probe light is terminated at the detector 160 after crossing the inside of the cell 110 at least twice or more. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an optical pumping magnetometer and a magnetic sensing method.

A highly sensitive optical pumping magnetometer using spin of alkali metal gas or rare gas has been proposed. Patent Document 1 proposes a highly sensitive optical pumping magnetometer including a pump light source and a probe light source.
US Pat. No. 7,038,450

  The above-described optically pumped magnetometer can detect a very small magnetic field in principle, but it has been demanded for higher sensitivity in terms of application.

  Therefore, an object of the present invention is to provide an optical pumping magnetometer and a magnetic sensing method that can detect a magnetic field with higher sensitivity by increasing the detection signal intensity.

  In order to achieve the above object, an optical pumping magnetometer according to the present invention has the following features.

  Specifically, a cell in which atomic groups are sealed in a gas state, a pump light source that generates pump light for spin-polarizing the atomic groups, a probe light source that irradiates the cell with probe light, and polarization of the probe light An optical pumping magnetometer comprising a detector for detecting surface rotation. Furthermore, at least one reflecting mirror for reflecting the probe light is provided, and the reflecting mirror is provided at a position for reflecting the probe light after traversing the inside of the cell, and the probe light is at least twice in the cell. It is configured to terminate at the detector after traversing.

  The magnetic sensing method according to the present invention has the following features.

Specifically, by irradiating pump light to the atomic group contained in the cavity of the cell, the spin directions of the atoms constituting the atomic group are aligned, and the atomic spin directions are aligned. A magnetic sensing method of irradiating the atomic group with linearly polarized light as probe light and measuring a polarization rotation angle of the probe light,
After irradiating the group of atoms in the cell with linearly polarized light as probe light, the probe light that has passed through the cell is folded and passed through the cell again, so that the cell is at least twice or more. A deflection rotation angle of the probe light that has traversed is measured.

  As described above, according to the present invention, it is possible to provide an optical pumping magnetometer capable of detecting a magnetic field with higher sensitivity by increasing the detection signal intensity.

  First, the operation principle of the magnetic detection method in the optical pumping magnetometer according to the present invention will be described.

  An optical pumping magnetometer of the present invention includes a cell in which an atomic group capable of taking a gas state is enclosed, a pump light source that generates pump light for aligning the spin directions of a plurality of atoms constituting the atomic group, and a probe A probe light source for irradiating the cell with light. Here, aligning the spin directions of a plurality of atoms constituting an atomic group means spin-polarization.

  Here, the pump light for the atomic group in the cell is a circularly polarized beam tuned to the absorption line of the atomic group, and the probe light is a linearly polarized beam detuned from the absorption line of the atomic group. The pump light and the probe light are configured to intersect within the cell, and the intersecting position is an area where the magnetic field is measured. The magnetic field is measured by detecting information related to the change in the polarization state of the probe light with a detector. The atomic group introduced into the cell includes substances that are solid at room temperature as well as substances that are gaseous at room temperature. In the case of a substance that is solid at room temperature as in the embodiments described later, when the magnetometer is operated, it is vaporized by heating and used.

  By irradiating with pump light, spin polarization is formed in the atomic group (atomic group). The spin-polarized atomic group precesses by receiving a torque corresponding to the magnetic field in the measurement region. This state is known to be described by the Bloch equation described later. This precession causes a component in the probe light direction in the spin direction of the atomic group. The spin magnitude in the direction of the probe light rotates the polarization plane of the probe light due to the paramagnetic Faraday effect, and information on the measured magnetic field strength can be obtained by measuring this rotation angle.

  The Faraday effect is an amount proportional to the optical path length that has passed through the polarized atomic group if the spin polarization has no nonuniform spatial distribution. Therefore, by performing measurements such that the probe light passes through the polarized gas multiple times and terminates at the detector, a large polarization rotation angle can be generated in the probe light, greatly increasing the sensitivity of magnetic field measurement. It is possible to raise. Thus, it is a big feature of the optical pumping magnetometer according to the present invention that the probe light is measured by the detector after traversing the cell twice or more. Of course, if it is two or more times, it may be crossed three times or four times, or it may be an even number or odd number of times.

  In addition, as a reflective mirror applied to this invention, a metal mirror, a semiconductor multilayer mirror, and a dielectric multilayer mirror are used, for example. In particular, a dielectric multilayer mirror not containing metal is preferable.

  Next, components common to the optical pumping magnetometer in each embodiment of the present invention will be described in detail.

(A) Cell The cell is made of a material that is transparent to probe light and pump light, such as glass. In the cell, an alkali metal (K, Rb, etc.) is included in a gas state as an atomic group or an atomic group, and is hermetically sealed. For example, by putting potassium metal in a glass cell and heating to about 180 ° C., the glass cell can be filled with potassium metal vapor at a desired concentration.

In addition, the atomic group included in the cell is not limited to an alkali metal atom, and the material is not particularly limited as long as the above-described detection method can be performed. As an atomic group, in addition to K and Rb, Xe that can perform spin exchange with these atoms can also be mixed. Further, in addition to the atomic group, a gas serving as a buffer such as N ₂ or He can be enclosed in the cell. Since the buffer gas suppresses the diffusion of the polarized alkali metal atoms, it is effective for suppressing the spin relaxation due to the collision with the cell wall and increasing the polarization rate.

(B) Pump light Although it is desirable that the polarization state of the pump light is substantially composed of only a circularly polarized component, in the embodiment of the present invention, if the circularly polarized component is included, other polarized components are included. It does not exclude inclusion. The wavelength of the pump light is tuned to a wavelength that can excite an atomic group in the cell, specifically, an alkali metal absorption line.

  This pump light is used to align the spin directions of the atoms constituting the atomic group. In particular, in the case of an alkali metal, this is an electron spin polarization. Specifically, it is known that the spin direction of atoms can be aligned using a phenomenon called circular polarization pumping.

  The reason why spin directions can be aligned in this way is due to the preservation of angular momentum. Since circularly polarized light has angular momentum, when the atom is excited by absorbing light, the ground level and the excited level increase the angular momentum of the atom by a quantum number 1 in the case of clockwise circularly polarized light. Only the pair is excited. Once excited, the atom returns to the ground state by emitting light in a random polarization state by spontaneous emission or by collision with a buffer gas atom. At this time, atoms that return to a state where the quantum number of angular momentum again decreases by 1 and atoms that return to the ground state while maintaining angular momentum are randomly mixed. By repeating such a process, the proportion of ground state atoms that are not excited by this circularly polarized light gradually increases, so the direction of spin of the atoms that make up the atomic group changes the traveling direction of the circularly polarized light. It is aligned in this direction as an axis.

(C) Probe light The probe light is preferably detuned from the resonance frequency of the atoms in order to avoid unnecessary pumping. In addition, it is desirable that the polarization state of the probe light is composed only of linearly polarized light. However, in the embodiment of the present invention, if the linearly polarized light component is included, it excludes that it contains other polarized light components. It is not a thing.

  The magnetometer is configured such that the probe light and the pump light intersect at some or all of the atomic groups in the cell. The irradiation directions of the probe light and the pump light are not particularly limited as long as they intersect with each other, but in general, both the irradiation directions are orthogonal to each other.

  It is known that when linearly polarized light having a wavelength near the resonance is incident on an atomic group polarized in the optical axis direction, the plane of polarization is rotated by a magneto-optic effect known as paramagnetic Faraday rotation. . Linearly polarized probe light can be described as a superposition of both left and right circularly polarized light. As described in the explanation of the pump light, when the atomic densities of the ground states excited by the left and right circularly polarized light are different, the absorption of the left and right circularly polarized light is slightly different depending on the polarization of the atomic group. There is a difference. The difference in the absorption coefficient, which means the imaginary part of the complex refractive index, is also reflected in the difference in the real part of the refractive index felt by the left and right circularly polarized light due to the Kramers-Kronig relationship. After passing through the atomic group, there is a difference in the optical path lengths of the left and right circularly polarized light, so that the polarization plane of linearly polarized light obtained by superimposing these is rotated. There is known a magnetometer utilizing the fact that the rotation angle depends on the magnitude of the magnetic field at the position of the atomic group.

  Also in the optical pumping magnetometer of the present invention, a polarized atom group is formed by the pump light described above, linearly polarized probe light is incident on the atom group in the cell, and before and after passing through the cell, the probe Information on the rotation angle of the polarization plane of light is acquired. For example, the rotation angle of the polarization plane of the probe light is measured.

  Here, with respect to the relationship between the rotation of the polarization plane of the probe light and the magnetic detection, the behavior when the optical path of the probe light is folded at the end of the cell as in the present invention can be expressed as follows. Use and describe.

  For simplicity, the optical response of an alkali metal cell in the coordinate system as shown in FIG. 1 is described.

That is, the propagation direction of the probe light, which is linearly polarized light, is on the x-axis (see arrow R1 in the figure), and propagates through the alkali metal population by a distance L. Further, the propagation direction of the pump light positive direction of z, the magnetic field B _y directions to be measured in the y direction.

Macroscopic magnetization vector shown alkali metal population _{m = (m x, m y} , m z) is described by the following optical Bloch equations.
dm / dt = Ω × m−γ _eff m + P (1)
Here, the first term on the right side represents the rotation of the magnetization vector m by a magnetic field B _y, is expressed as _{Ω = (0, Ω L,} 0) = (0, gμ B (Qh) -1 B y, 0) . The second term on the right side is the relaxation of magnetization and is described by the relaxation rate γ _eff . The third term is expressed as P = (0, 0, R _P ) using the optical pumping rate R _P in the pumping of the magnetization in the z direction by the pump light. The parameters used here are the Planck constant h, the g factor, the Bohr magneton μ _B , and the slow down factor Q in the SERF (Spin Exchange Relaxation Free) region. To the static magnetic field B _y, when determining the stationary solution of (1) from the dm / dt = 0, for the x component, solutions such as the following equation is obtained.
m _x = R _p Ω _L / (Ω _L ² + γ _eff ² )
~ R _p Ω _L γ _eff ^-2
_{= R p gμ B (Qh)} -1 γ eff -2 B y (2)
Here, the approximation that the magnetic field is weak and Ω _L ² << γ _eff ² is used. As a result, it can be seen that the x-direction of the magnetization m _x is proportional to the static magnetic field B _y.

Next, Faraday rotation caused by this magnetization will be described. When linearly polarized probe light of wavelength λ in the vacuum travels in the positive x direction, it can be described as a superposition of clockwise circularly polarized light and counterclockwise circularly polarized light. In the polarized alkali metal, the refractive indices for clockwise circularly polarized light and counterclockwise circularly polarized light are n ⁺ and n ⁻ . The rotation angle φ of the polarization plane is equal to the phase difference between the left and right circularly polarized light after propagating the distance L,
^{φ = πL (n + -n -} ) λ -1 = 2πLm x (n 0 -1) λ -1 (3)
It becomes. Here, take only the lowest order term for optical susceptibility, n ⁺ -n ^- = doing approximation to 2m _x (n ₀ -1). In this case, the rotation angle φ of polarization plane is an amount that is proportional to m _x ie B _y.

Here, first, the relationship between mx and n ⁺ −n ⁻ proceeding in the positive direction of _x is shown. Next, a situation in which linearly polarized probe light having a wavelength λ travels in the negative direction of x in the same polarized alkali metal will be described with a suffix R. The angular momentum of clockwise circularly polarized light traveling in the negative direction of x is -1, and the sign is different from the angular momentum of clockwise circularly polarized light +1 traveling in the positive direction. Therefore, when written as φ _R = πL (n _R ⁺ −n _R ⁻ ) λ ⁻¹ , n _R ⁺ = n ⁻ and n _R ⁻ = n ⁺ . The polarization rotation angle of light traveling in the negative x direction is
_{φ R = -2πLm x (n 0} -1) λ -1 = -φ (4)
It is expressed. The state of polarization rotation expressed by Equation (3) and Equation (4) corresponds to FIGS. 1 (a) and 1 (b), respectively. FIG. 1A shows the probe light propagating in the positive direction of x, with the polarization plane rotated by φ, and FIG. 1B shows the probe light propagating in the negative direction of x, The polarization plane is rotated by φ _R = −φ.

(D) Light source Regarding the light source, separate light sources may be used for the pump light and the probe light, respectively, or by using a polarizing plate or the like in common with the light source, the circularly polarized pump light and the linear light are used. It is also possible to produce polarized probe light. A laser light source can be used as a light source for outputting pump light and probe light. For example, when potassium is used as the atomic group or atomic group in the cell, laser light sources having a wavelength of about 770 nm that are detuned from each other in the range of 0.02 nm to 1 nm can be used.

  In addition, although the optical pumping magnetometer was demonstrated above, this invention also includes the magnetic sensing method shown below.

  Specifically, first, the atom groups included in the cavity of the cell are irradiated with pump light to align the spin directions of the atoms constituting the atom groups. In the magnetic sensing method, the atomic group in which the directions of spins of atoms are aligned is irradiated with linearly polarized light as probe light, and the polarization rotation angle of the probe light is measured. Here, after irradiating the group of atoms in the cell with linearly polarized light as probe light, the probe light that has passed through the cell is folded back and passed through the cell again. In this way, the deflection rotation angle of the probe light that has traversed the cell at least twice is measured.

  Also in the magnetic sensing method, the technical matters (A) to (D) described above are applied.

  As the optical pumping magnetometer for performing the magnetic sensing method, a probe light source for outputting the probe light and a pump light source for outputting the pump light are used. Furthermore, a reflection mirror for turning back the probe light and a detector for measuring the deflection rotation angle are used.

  Hereinafter, the optical pumping magnetometer according to the present invention will be described more specifically.

[First Embodiment]
Below, the detail of embodiment of the optical pumping magnetometer of this invention is demonstrated, referring drawings.

  FIG. 2 is a cross-sectional view schematically showing the optical pumping magnetometer in the first embodiment of the present invention.

The alkali metal cell 110 is a glass cell containing buffer gas (He) and quencher (N ₂ ) in addition to potassium (K), and is hermetically sealed. A reflection mirror 120 for reflecting the probe light is provided on the opposite side of the alkali metal cell 110 when viewed from the probe light source 140. As the reflection mirror 120, a dielectric multilayer mirror that does not include a metal layer and can reduce magnetic noise that is said to be caused by Johnson noise or the like is particularly desirable. By flowing heated air from the outside into the oven 118, the alkali metal cell 110 disposed inside the oven 118 is heated. A heat insulating layer 119 surrounds the oven so that the temperature outside the heat insulating layer is maintained at about room temperature. The heat insulation layer 119 may have an exhaust heat mechanism such as a cooling water circulation tube as necessary. Both the heat insulating layer 119 and the oven 118 are provided with windows in portions corresponding to the optical paths of the pump light and the probe light.

  In order to reduce external magnetic noise such as geomagnetism, the above components are installed inside a magnetic shield 113 made of three layers of mu metal. Furthermore, Helmholtz coils 114-117 are provided in order to cancel the residual magnetic field inside the magnetic shield 113. Helmholtz coils indicated by 114 and 115 are used for compensating the residual magnetic field in the x direction, and Helmholtz coils indicated by 116 and 117 are used for compensating the residual magnetic field in the z direction. Similarly, a Helmholtz coil that compensates for the residual magnetic field in the y direction is also provided, but is not shown here.

  The pump light generated by the pump light source 130 is converted into parallel light by the collimator lens 132 and converted into circularly polarized light by the quarter wavelength plate 133. Then, the alkali metal cell 110 is irradiated through the windows and openings of the magnetic shield 113, the heat insulating layer 119, and the oven 118.

  The probe light generated by the probe light source 140 is converted into parallel light by the collimator lens 142. The polarization direction is determined so as to pass through the polarization beam splitter 150. Similar to the pump light, the alkali metal cell 110 is irradiated through the windows and openings of the magnetic shield 113, the heat insulating layer 119, and the oven 118. The probe light is incident on the reflecting surface of the reflecting mirror 120 perpendicularly. After traversing the alkali metal cell 110, the optical path of the probe light is folded back by 180 degrees. Then, it passes through the alkali metal cell 110 again and returns to the polarization beam splitter 150.

  The probe light that has returned to the polarization beam splitter 150 reflects the polarization component orthogonal to the forward path, bends the optical path by 90 degrees, and terminates in the photodetector 160 as a detector. Since the light intensity of the probe light received by the photodetector 160 is proportional to the rotation angle of the polarization plane as described above, a signal corresponding to the magnetic field intensity in the region where the pump light and the probe light intersect can be obtained. It becomes. In this way, for example, a weak magnetic field generated from the specimen 101 arranged in the magnetic shield 113 can be obtained with high sensitivity.

  Here, with reference to FIG. 3 and FIG. 4, the polarization angle at each point on the optical path of the probe light in the optical pumping magnetometer of the present embodiment will be described in order.

FIG. 3 is a diagram schematically showing the optical path of the probe light in the optical pumping magnetometer of the present embodiment. 4 shows the state of rotation of the polarization plane of the probe light at each point on the optical path 141 of the probe light shown in FIG. 3, and the arrows in the coordinates indicate the direction of the electric field in the yz plane of the probe light. Show. Coordinate axes the same manner as described above, the propagation direction of the z-direction of the pump light (see arrow R2 in FIG. 3), a propagation direction x-direction of the probe light, and the magnetic field B _y direction comprising a y-direction measured.

  The probe light is linearly polarized at the point A after passing through the polarization beam splitter 150, as shown in FIG. 4A, with the electric field directed in the y direction. At point B after passing through the alkali metal cell 110, the polarization angle rotates by an angle φ corresponding to the polarization rotation shown in FIG. 1A (see FIG. 4B). Even at the point C where the propagation direction is reversed by the reflection mirror 120, as shown in FIG. 4C, the direction of the electric field does not change, and is linearly polarized light rotated by an angle φ when viewed in the yz plane. Since the polarization angle returns to the original state with every rotation of 180 degrees, in the case of normal incidence, the phase shift due to reflection does not affect the polarization angle. After traversing the alkali metal cell 110 again in the negative x direction, the polarization angle rotates by an angle −φ corresponding to FIG. 1B. Therefore, the polarization angle at the point D shown in FIG. In the polarization beam splitter 150, only the component of the linearly polarized light whose electric field is directed in the z direction is bent by 90 degrees and reaches the point E, so that only the z component of the polarized light is extracted (FIG. 4 ( e)).

  As a measurement system for measuring the Faraday rotation angle, a configuration such as a balanced polarimeter or a crossed Nicol polarimeter can be used.

  For example, as shown in FIG. 5, the incident light path and the reflected light path of the probe light reflecting mirror 120 may be shifted.

  In the configuration example shown in FIG. 5, two photodetectors 161 a and 161 b are provided for the polarizing beam splitter 151. When the polarization beam splitter 151 is configured as a balanced polarimeter, the direction of the half-wave plate 143 is rotated so that the magnitudes of the output signals of the two photodetectors 161a and 161b coincide when the magnetic field is zero. And the plane of polarization of incident light may be controlled.

  In addition, as shown in FIG. 6, a measurement system using a Faraday modulator may be used.

In this case, a Faraday modulator 144 and a polarizer 145 are arranged on the incident light path of the probe light with respect to the reflection mirror 120. The Faraday modulator 144 is connected to a drive circuit 146 for the Faraday modulator 144. An AC modulation signal having a frequency f ₀ is added to the DC bias current of the Faraday modulator 144, and feedback control is performed so that the signal intensity of the component oscillating at the output frequency f ₀ of the photodetector 162 becomes zero. Thus, a DC bias current value corresponding to the Faraday rotation angle can be obtained.

  As described above, according to the optical pumping magnetometer of the present embodiment, the probe light is reciprocated across the cell by the reflection mirror and then terminated at the detector, so that the magnetic force of a single optical path can be obtained with the same configuration. It is possible to obtain a Faraday rotation angle that is twice that of the total. Thereby, the signal intensity with respect to the weak signal can be increased, and an optical pumping magnetometer capable of highly sensitive magnetic field detection can be realized.

  In the optical pumping magnetometer of this embodiment, the light source, the optical component, and the detection system are arranged on the same side of the cell on the optical axis of the probe light, and the optical path of the probe light is a reflection mirror, that is, the cell It is folded at the end. Therefore, the optical pumping magnetometer of the present embodiment is also advantageous in that the degree of freedom of arrangement around the cell can be increased. For example, it is possible to relax the restriction due to the requirement not to cross the optical path related to the place where the specimen is placed.

[Second Embodiment]
Next, an optical pumping magnetometer according to the second embodiment of the present invention will be described.

  FIG. 7A is a cross-sectional view schematically showing a characteristic configuration of the optical pumping magnetometer of the present embodiment, and FIG. 7B is a schematic view showing the optical path of the probe light in the alkali metal cell. FIG. FIG. 8 is a perspective view schematically showing a part of the optical pumping magnetometer of the present embodiment, and shows only the optical system on the detection side.

  Forms other than those described below, for example, a magnetic shield, a Helmholtz coil, an oven, a heat insulating layer, etc., may be configured as all or part of the same as in the first embodiment, so that it can act as an optical pumping magnetometer. Is possible.

  In the present embodiment, dielectric multilayer mirrors 220a and 220b that do not include a metal layer are used as a reflection mirror for reflecting the probe light. Dielectric multilayer mirrors 220 a and 220 b are formed directly on the glass surface outside alkali metal cell 210. The dielectric multilayer mirror 220b, which is the upper side in FIG. 7A, is formed in a stripe shape only at the center of the glass surface.

  Further, as shown in the perspective view of FIG. 8, each component is an array device using an elongated alkali metal cell 210, an elongated polarizing beam splitter 250, a one-dimensional photodetector array 260a, 260b, and the like along the z-axis. Yes. The circularly polarized pump light is incident on the alkali metal cell 210 along the long axis direction (z-axis direction). The pump light generates polarization in the atomic group in the alkali metal cell 210 as in the previous embodiment.

  The probe light from the probe light source 240 is shaped into a desired elongated beam shape by the lens optical system 242 and collimated as parallel light, and the polarization direction is adjusted by the half-wave plate 243, and the beam stop 244 is set. Through the alkali metal cell 210. The probe light incident on the alkali metal cell 210 is reflected by the lower dielectric multilayer mirror 220a as viewed in FIG. 7A, and is reflected again by the upper dielectric multilayer mirror 220b. Further, after being reflected by the lower dielectric multilayer mirror 220a, that is, after traversing the alkali metal cell 210 four times, the probe light is outside the alkali metal cell 210 from the side where the probe light source 240 is installed. To be taken out. The rotation ratio of the polarization plane when transmitted through the alkali metal cell 210 is measured by reading and calculating the light splitting ratio at the polarizing beam splitter 250 with the two photodetector arrays 260a and 260b, and along the z axis. The magnetic field strength at each position can be measured simultaneously.

  As described above, when the optical path of the probe light includes a plurality of turns, the probe light may need to be incident on the reflecting surface of the reflecting mirror in an inclined manner. Even in such a case, it is desirable that the optical path of the pump light realizes a state perpendicular to a plane including the incident optical path and the reflected optical path of the probe light with respect to the reflection mirror. In the present embodiment, the incident angle of the probe light is tilted from the x axis in the xy plane, while the pump light and the probe light are kept orthogonal to each other by making the pump light incident along the z axis. It becomes possible.

In the layout of this embodiment, as shown in FIG. 7B, the probe light undergoes Faraday rotation by the magnetization vectors m1 and m3 in the downward optical paths P1 and P3, and is magnetized in the upward optical paths P2 and P4. Faraday rotation by the vectors m2 and m4 is received. Here, the magnetization vector m1-m4 represents the magnitude of the magnetization of the alkali metal populations in each location, its x component is determined by the magnetic field B _y at each location, the mutually same sign. Further, the y component of the magnetization vector is determined by the magnetic field B _x at each location, m1 and m3 have the same sign, and m2 and m4 contribute with signs opposite to m1. That is, when the rotation angle is added along the optical axis of the probe light, the components due to B _y are added because of the same sign, and the components due to B _x cancel each other. Accordingly, the optical path of the thus probe light be inclined from the x axis for folding can be an optical pumping magnetometer for detecting the substantially magnetic field B _y component. In addition, polarization rotation with an optical path length four times that of a single optical path magnetometer of the same alkali metal cell size can be obtained, so that the signal intensity can be greatly increased.

  Here, the dielectric multilayer mirrors 220 a and 220 b may be formed inside the alkali metal cell 210.

[Third Embodiment]
The optical pumping magnetometer of the present invention does not necessarily have to be terminated in a detection system in which the probe light is arranged on the same side as the light source with respect to the cell as in the above-described embodiment. The polarimeter portion that is the end of the optical path of the probe light may be disposed on the opposite side of the cell as viewed from the probe light source. Below, with reference to FIG. 9, 3rd Embodiment of the optical pumping magnetometer in this invention is described.

  FIG. 9A is a cross-sectional view schematically showing a characteristic configuration of the optical pumping magnetometer according to the third embodiment. Moreover, FIG.9 (b) is the schematic diagram which showed only the pump light source system and cell part of this embodiment from the y direction of Fig.9 (a).

  In the present embodiment, dielectric multilayer mirrors 320 a and 320 b that do not include a metal layer are directly formed on the glass surface outside the alkali metal cell 310 as a reflection mirror. This multilayer film is not formed in a portion where incident light and transmitted light from the alkali metal cell 310 are extracted. The configuration and operation of the probe light source 240, the lens optical system 242 for shaping the laser beam, the half-wave plate 243 for controlling the polarization plane of the probe light, the beam stop 244, and the like are the same as in the second embodiment. Other configurations and operations are the same as those in the first embodiment. Further, the polarization beam splitter 250, which is a balanced polarimeter portion constituted by the photodetector arrays 260a and 260b, has the same configuration as that of the second embodiment. As in the second embodiment, the circularly polarized beam of the pump light is incident from the direction perpendicular to the paper surface in FIG. 9A, that is, the z direction.

  Pump light from the pump light source 130 is converted into parallel light by the collimator lens 132 and converted into circularly polarized light by the quarter wavelength plate 133. Here, also in the pump light, as shown in FIG. 9B, a folding mirror 370 is disposed on the optical path opposite to the alkali metal cell 310 when viewed from the pump light source side, and the transmitted light is again transmitted to the alkali metal cell. It is good also as a structure returned to 310. FIG. Since the circularly polarized light reflected in the vertical direction by the folding mirror 370 returns to the alkali metal cell 310 by turning the clockwise circular polarized light into the counterclockwise circular polarized light or turning the counterclockwise circular polarized light into the clockwise circular polarized light. The pump light causes spin polarization in the same direction in both the forward path and the return path. With this configuration, the efficiency of the pump can be increased and the uniformity of polarization can be increased.

  The probe light incident on the alkali metal cell 310 is reflected by the lower dielectric multilayer mirror 320a as viewed in FIG. 9A and is reflected again by the upper dielectric multilayer mirror 320b. Then, after traversing the alkali metal cell 310 three times, the probe light is extracted from the probe light source 240 to the opposite side of the alkali metal cell 310. The rotation angle of the polarization plane caused by the Faraday rotation received during this time is measured by reading and calculating the light splitting ratio at the polarization beam splitter 250 by the two photodetector arrays 260a and 260b. In this way, the strength of the magnetic field at each position along the z axis can be measured simultaneously.

[Fourth Embodiment]
Next, a magnetic field gradient meter operating on the principle of the optical pumping magnetometer of the present invention, which is a fourth embodiment of the present invention, will be described.

  FIG. 10 is a schematic diagram showing only an alkali metal cell portion in the magnetic field gradient meter. The configuration other than the alkali metal cell is the same as that of the first embodiment.

  In this embodiment, the dielectric multilayer mirror 420 that does not include a metal layer for reflecting the probe light is directly formed on the glass surface inside the alkali metal cell 410 in which potassium is enclosed. In addition, two folding mirrors 470a and 470b for folding the pump light are provided on the opposite side of the alkali metal cell 410 when viewed from the pump light source side. Arrows 441 and 431 represent optical paths of pump light and probe light, respectively.

As in the first embodiment, this embodiment constitutes an optical path that turns back the probe light. In the alkali metal cell 410, a region having spin polarization in a certain direction and its direction are And a region having spin polarization in the opposite direction. For this purpose, the pump light is folded back by two of the folding mirrors 470a and 470b and transmitted through the alkali metal cell 410 in the opposite direction. The pump light reflected twice by the folding mirrors 470a and 470b has the same circularly polarized light, but the traveling direction is opposite, so that spin polarization in the opposite direction is formed. As shown in FIG. 10, the case where the same linearly polarized light is used for the probe light using the same coordinate system as in the first embodiment will be described. Direction perpendicular to the plane, that is, when there is a magnetic field B _y in the y-direction, in the region S1 in the figure, the rotation angle of the polarization plane of the probe light becomes a negative value, while the rotation of the polarization plane in the region S2 in Fig. The corner is a positive value. Further, the rotation of the polarization plane is added to the probe light folded back by the reflection mirror 420 in the same manner as in the first embodiment.

  As a whole, this magnetometer causes the rotation of the polarization plane in the opposite direction with respect to the same magnetic field in the two regions S1 and S2 where the optical path 431 of the pump light and the optical path 441 of the probe light intersect with each other. It can be seen that it operates as a gradiometer that detects the difference in magnetic field from the region S2. In this manner, a magnetic sensor having two measurement positions in the alkali metal gas and capable of directly measuring the difference in magnetic field strength at the two measurement positions as a difference in polarization rotation angle is configured.

  Further, a polarizer and a quarter-wave plate may be inserted on the optical path so that the folded pump light maintains more appropriate circularly polarized light.

  The optical system for forming spin polarizations in opposite directions in the region S1 and the region S2 may have a configuration other than that described here. For example, after splitting the pump light into two beams and making them a pair of clockwise circularly polarized light and counterclockwise circularly polarized light, it is possible to make two beams incident from one of the alkali metal cells. is there.

  Further, as in the modification of the optical pumping magnetometer in the first embodiment shown in FIG. 5, the same magnetic field gradient meter is configured so that the incident light path to the reflection mirror of the probe light does not overlap with the reflected light path. It is also possible to do. An example of the configuration is shown in FIG.

  Here, the dielectric multilayer mirror 421 is disposed outside the alkali metal cell 411 in which potassium is enclosed, and the folding mirrors 470a and 470b for folding the pump light and the optical path 431 of the pump light are the same as described above. It is the composition. Also, 442 represents the optical path of the probe light incident so that the incident optical path and the reflected optical path do not overlap.

  The optical pumping magnetometer according to the present invention described above can also be expressed as an atomic magnetometer.

It is a perspective view for demonstrating rotation of the polarization plane by paramagnetic Faraday rotation. It is sectional drawing which shows typically the optical pumping magnetometer in the 1st Embodiment of this invention. It is a schematic diagram which shows the optical path of the probe light of the optical pumping magnetometer in the 1st Embodiment of this invention. It is a schematic diagram which shows the mode of rotation of the polarization plane of probe light. It is sectional drawing which shows typically a part of modification of the optical pumping magnetometer in the 1st Embodiment of this invention. It is sectional drawing which shows typically a part of modification of the optical pumping magnetometer in the 1st Embodiment of this invention. It is sectional drawing which shows typically a part of optical pumping magnetometer in the 2nd Embodiment of this invention. It is sectional drawing which shows typically a part of optical pumping magnetometer in the 2nd Embodiment of this invention. It is sectional drawing which shows typically a part of optical pumping magnetometer in the 3rd Embodiment of this invention. It is sectional drawing which shows typically a part of magnetic field gradient meter in the 4th Embodiment of this invention. It is sectional drawing which shows typically a part of modification of the magnetic field gradient meter in the 4th Embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Optical pumping magnetometer 110 Alkali metal cell 120 Reflection mirror 130 Pump light source 140 Probe light source 160 Photo detector

Claims (8)

A cell in which an atomic group capable of taking a gas state is enclosed;
A pump light source that generates pump light for spin-polarizing the atomic group;
A probe light source for irradiating the cell with probe light;
A detector for detecting rotation of a polarization plane of the probe light;
In an optically pumped magnetometer with
Comprising at least one reflecting mirror for reflecting the probe light;
The reflection mirror is provided at a position to reflect the probe light after traversing the cell, and the probe light terminates at the detector after traversing the cell at least twice. An optical pumping magnetometer, characterized in that it is configured as follows.   The optical pumping magnetometer according to claim 1, wherein the probe light is incident on the reflecting surface of the reflecting mirror perpendicularly.   The optical pumping magnetometer according to claim 2, further comprising a polarizing beam splitter, wherein the polarizing beam splitter is disposed between the probe light source and the cell.   The probe light is incident on the reflection surface of the reflection mirror at an angle, and the pump light is on a plane including an incident optical path and a reflection optical path of the probe light with respect to the reflection mirror. The optically pumped magnetometer according to claim 1, wherein the optically pumped magnetometer is configured to be perpendicularly incident.   5. The optical pumping magnetometer according to claim 1, wherein the reflection mirror is a dielectric multilayer mirror that does not include a metal layer. 6.   The optical pumping magnetometer according to claim 1, further comprising a one-dimensional photodetector array capable of measuring a spatial distribution of a magnetic field in the cell. By irradiating pump light to the atomic group contained in the cavity of the cell, the direction of the spin of the atom constituting the atomic group is aligned, and the probe is applied to the atomic group in which the atomic spin direction is aligned. A magnetic sensing method of irradiating linearly polarized light as light and measuring the polarization rotation angle of the probe light,
After irradiating the group of atoms in the cell with linearly polarized light as probe light, the probe light that has passed through the cell is folded and passed through the cell again, so that the cell is at least twice or more. A magnetic sensing method, comprising: measuring a deflection rotation angle of the probe light that has traversed.   In order to perform the magnetic sensing method according to claim 7, a probe light source for outputting the probe light, a pump light source for outputting the pump light, a reflection mirror for folding the probe light, and An optical pumping magnetometer comprising a detector for measuring a deflection rotation angle.

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