JP2018132348A - Gas cell, magnetic measuring device, and atomic oscillator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a gas cell, a magnetic measuring device, and an atomic oscillator which can efficiently suppress reflection of light and can make a sensitive and accurate measurement.SOLUTION: A gas cell 10 includes: a cell 12 made of a wall 11; an antireflection film 50a containing magnesium fluoride, the film being located on inner surfaces 112 and 113 of the wall 11; a coating layer 32 over the surface of the antireflection film 50a; and an alkali metal gas 13 sealed in the cell 12.SELECTED DRAWING: Figure 3

Description

  The present invention relates to a gas cell, a magnetic measurement device, and an atomic oscillator.

  There is known an optical pumping type magnetic measuring device that irradiates a gas cell filled with an alkali metal gas with linearly polarized light and measures a magnetic field in accordance with a rotation angle of a polarization plane (see, for example, Patent Document 1). Patent Document 1 discloses a configuration of a gas cell in which the inner surface of a closed vessel (cell) wall is coated with chain saturated hydrocarbon (paraffin or the like) and an alkali metal gas is filled inside. The layer of coating material (hereinafter referred to as the coating layer) has a function of suppressing or reducing a change in behavior (for example, spin) when an alkali metal atom directly collides with the cell wall.

JP 2013-7720 A

  However, there is a problem that when the irradiated linearly polarized light passes through the gas cell, light is reflected on the surface of the wall of the gas cell. When a loss occurs in the light transmitted through the gas cell due to the reflection of light, the signal component in the measurement of the magnetic field decreases. Further, the reflected light becomes stray light and becomes a noise component in the measurement of the magnetic field. As a result, the sensitivity in the magnetic field measurement is lowered and the measurement accuracy is lowered. Therefore, there is a need for a gas cell that can effectively suppress reflection on the surface of the cell wall and perform measurement with high sensitivity and high accuracy.

  SUMMARY An advantage of some aspects of the invention is to solve at least a part of the problems described above, and the invention can be implemented as the following forms or application examples.

  Application Example 1 A gas cell according to this application example covers a closed container formed of walls, an antireflection film containing magnesium fluoride provided on an inner surface of the wall, and a surface of the antireflection film. A chain saturated hydrocarbon and an alkali metal gas sealed in the closed vessel are provided.

  According to the configuration of this application example, since the antireflection film containing magnesium fluoride is provided on the inner surface of the wall constituting the closed container, when light passes through the closed container, Reflection of light on the inner surface and the inner surface of the exit side wall is suppressed. In addition, since the surface of the antireflection film is covered with a coating layer made of chain saturated hydrocarbon, the change in behavior when alkali metal atoms sealed in the closed container collide with the wall of the closed container is suppressed or reduced. it can. Thereby, in the gas cell which has a coating layer in the inner surface of a closed container, the fall of the sensitivity resulting from reflection of light and the fall of a measurement precision can be suppressed.

Application Example 2 In the gas cell according to the application example, when the wavelength of light incident on the closed vessel is λ and the optical film thickness of the chain saturated hydrocarbon is nd _c , The coefficient A is represented by Formula 1, and the optical film thickness nd _f of the magnesium fluoride is preferably represented by Formula 2.
A = −1.1067 nd _c +0.2488 (1)
nd _f = 0.25qλ− (0.25−A) λ (2)
However, in Formula 2, q is an odd number (1, 3, 5,...).

According to the configuration of this application example, in the configuration in which the surface of the antireflection film is covered with the coating layer using Formula 1 and Formula 2, reflection is effectively performed according to the optical film thickness nd _c of the coating layer. The optical film thickness of the magnesium fluoride film suitable for the suppression can be calculated.

  Application Example 3 In the gas cell according to the application example, it is preferable that the antireflection film is also provided on an outer surface of the wall.

  According to the configuration of this application example, since the antireflection film is also provided on the outer surface of the wall constituting the closed container, when light passes through the closed container, the inner surface of the incident-side wall In addition to the inner surface of the exit wall, reflection of light on the outer surface of the entrance wall and the outer surface of the exit wall is also suppressed. Thereby, in a gas cell, the fall of the sensitivity resulting from reflection of light and the fall of measurement accuracy can be suppressed more effectively.

  Application Example 4 In the gas cell according to the application example described above, the antireflection film may be a dielectric single layer film.

  According to the configuration of this application example, since the antireflection film is a dielectric single layer film, the antireflection film can be formed more easily than in the case of a dielectric multilayer film.

  Application Example 5 In the gas cell according to the application example described above, the antireflection film may be a dielectric multilayer film.

  According to the configuration of this application example, since the antireflection film is a dielectric multilayer film, the reflectance can be lowered as compared with the case of the dielectric single layer film.

  Application Example 6 A magnetic measurement apparatus according to this application example includes the gas cell described above.

  According to the configuration of this application example, it is possible to provide a magnetic measurement device capable of measuring magnetism with high sensitivity and high accuracy.

  Application Example 7 An atomic oscillator according to this application example includes the gas cell described above.

  According to the configuration of this application example, a highly accurate atomic oscillator can be provided.

The block diagram which shows the structure of the magnetic measuring device which concerns on 1st Embodiment. The sectional side view along the longitudinal direction of the gas cell concerning a 1st embodiment. FIG. 3 is a plan sectional view taken along line A-A ′ of FIG. 2. Sectional drawing along the longitudinal direction of the ampoule which concerns on 1st Embodiment. FIG. 5 is a cross-sectional view taken along line B-B ′ of FIG. 4. Graph showing the relationship between the coefficient of MgF ₂ that the optical thickness of the coating layer and the reflectance is reduced. Graph showing the relationship between the MgF ₂ having an optical thickness nd _f and reflectance. The figure explaining the manufacturing method of the gas cell which concerns on 1st Embodiment. The figure explaining the manufacturing method of the gas cell which concerns on 1st Embodiment. The figure explaining the manufacturing method of the gas cell which concerns on 1st Embodiment. The figure explaining the manufacturing method of the gas cell which concerns on 1st Embodiment. The figure explaining the manufacturing method of the gas cell which concerns on 1st Embodiment. The figure explaining the manufacturing method of the gas cell which concerns on 1st Embodiment. The figure explaining the manufacturing method of the gas cell which concerns on 1st Embodiment. The figure explaining the manufacturing method of the gas cell which concerns on 1st Embodiment. Schematic which shows the structure of the atomic oscillator which concerns on 2nd Embodiment. The figure explaining operation | movement of the atomic oscillator which concerns on 2nd Embodiment. The figure explaining operation | movement of the atomic oscillator which concerns on 2nd Embodiment. The plane sectional view showing an example of the gas cell which is not provided with the conventional antireflection film.

  DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, embodiments of the invention will be described with reference to the drawings. The drawings to be used are appropriately enlarged, reduced or exaggerated so that the part to be described can be recognized. In addition, illustrations of components other than those necessary for the description may be omitted.

(First embodiment)
<Magnetic measuring device>
The configuration of the magnetic measurement apparatus according to the first embodiment will be described with reference to FIG. FIG. 1 is a block diagram showing the configuration of the magnetic measurement apparatus according to the first embodiment. The magnetic measurement device 100 according to the first embodiment is a magnetic measurement device using non-linear optical rotation (NMOR). The magnetic measurement device 100 is, for example, a biological state measurement device (such as a magnetocardiograph or a magnetoencephalograph) that measures a minute magnetic field generated from a living body such as a magnetic field from the heart (magnetomagnetic field) or a magnetic field from the brain (magnetomagnetic field). Used for. The magnetic measuring device 100 can also be used for a metal detector or the like.

  As shown in FIG. 1, the magnetic measurement device 100 includes a light source 1, an optical fiber 2, a connector 3, a polarizing plate 4, a gas cell 10, a polarization separator 5, and a photodetector (Photo Detector: PD) 6. And a light detector 7, a signal processing circuit 8, and a display device 9. In the gas cell 10, an alkali metal gas (a gas-state alkali metal atom) is sealed. As the alkali metal, for example, cesium (Cs), rubidium (Rb), potassium (K), sodium (Na), or the like can be used. Hereinafter, a case where cesium is used as the alkali metal will be described as an example.

  The light source 1 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 1 is so-called CW (Continuous Wave) light having a constant light amount continuously.

  The polarizing plate 4 is an element that polarizes the laser beam in a specific direction to make it linearly polarized light. The optical fiber 2 is a member that guides the laser beam output from the light source 1 to the gas cell 10 side. For the optical fiber 2, for example, a single mode optical fiber that propagates only the fundamental mode is used. The connector 3 is a member for connecting the optical fiber 2 to the polarizing plate 4. The connector 3 is a screw type and connects the optical fiber 2 to the polarizing plate 4.

  The gas cell 10 is a box (cell) having a gap inside, and an alkali metal vapor (alkali metal gas 13 shown in FIG. 2) is sealed in the gap (main chamber 14 shown in FIG. 2). The configuration of the gas cell 10 will be described later.

  The polarization separator 5 is an element that separates an incident laser beam into beams of two polarization components orthogonal to each other. The polarization separator 5 is, for example, a Wollaston prism or a polarization beam splitter. The photodetector 6 and the photodetector 7 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 8. It is desirable that the photodetector 6 and the photodetector 7 are made of a non-magnetic material because they themselves may affect the measurement when a magnetic field is generated. The photodetector 6 and the photodetector 7 are disposed on the same side (downstream side) as the polarization separator 5 as viewed from the gas cell 10.

  The arrangement of each part in the magnetic measuring device 100 will be described along the laser beam path. The light source 1 is located at the uppermost stream of the laser beam path. Hereinafter, the optical fiber 2, the connector 3, and the polarizing plate 4 from the upstream side. The gas cell 10, the polarization separator 5, and the photodetectors 6 and 7 are arranged in this order.

  The laser beam output from the light source 1 is guided to the optical fiber 2 and reaches the polarizing plate 4. The laser beam that has passed through the polarizing plate 4 becomes linearly polarized light having a higher degree of polarization. The laser beam passing through the gas cell 10 excites (optically pumps) the alkali metal atoms sealed in the gas cell 10. 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 10 is separated into two polarized component beams by the polarization separator 5. The light amounts of the two polarized component beams are measured (probing) by the photodetector 6 and the photodetector 7.

  The signal processing circuit 8 receives from each of the signals indicating the light amounts of the beams measured by the photodetector 6 and the photodetector 7. The signal processing circuit 8 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 8 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 9 displays the strength of the magnetic field measured by the signal processing circuit 8.

  Then, the structure of the ampule used for the gas cell which concerns on 1st Embodiment, and a gas cell is demonstrated with reference to FIGS.

<Gas cell>
FIG. 2 is a side sectional view along the longitudinal direction of the gas cell according to the first embodiment. 3 is a cross-sectional plan view taken along the line AA ′ of FIG. 2 and 3, the height direction of the gas cell 10 is taken as the Z axis, and the upper side is taken as the + Z direction. A direction intersecting the Z axis and the longitudinal direction of the gas cell 10 (a direction in which a main chamber 14 and a reservoir 16 to be described later are arranged) is taken as an X axis, and the right side in FIG. And it is a direction which cross | intersects a Z-axis and a X-axis, Comprising: Let the width direction of the gas cell 10 be a Y-axis, and let the right side in FIG. 3 be a + Y direction.

  As shown in FIG. 2, the gas cell 10 according to the first embodiment includes a cell 12 and a lid 19 as a closed container. The cell 12 is a box (cell) having a void inside the wall 11. The wall 11 is a glass plate member made of, for example, quartz glass. The wall 11 (glass plate member) has a thickness of 1 mm to 5 mm, for example, about 1.5 mm.

  The cell 12 has a main chamber 14 and a reservoir 16 whose longitudinal direction is the X-axis direction as an internal space. The main chamber 14 and the reservoir 16 are arranged along the X-axis direction, and communicate with each other through the communication hole 15. The communication hole 15 is provided on the upper side (+ Z direction side) of the main chamber 14 and the reservoir 16. Although not shown, the shape of the communication hole 15 viewed along the X-axis direction is, for example, a circular shape. The inner diameter of the communication hole 15 is, for example, about 0.4 mm to 1 mm.

  The cell 12 (the main chamber 14 and the reservoir 16) is filled with a gas 13 in which alkali metal has evaporated (hereinafter referred to as alkali metal gas). In addition to the alkali metal gas 13, an inert gas such as a rare gas may be present in the main chamber 14 and the reservoir 16.

  An ampoule 20 is accommodated in the reservoir 16. A through hole (opening) 21 is formed in the glass tube 22 of the ampoule 20. The alkali metal gas 13 is obtained by evaporating (gasifying) the alkali metal solid 24 (see FIG. 4) filled in the ampule 20. The configuration of the ampule 20 will be described later.

A film-like coating layer 32 is disposed inside the wall 11 of the cell 12. The coating layer 32 has a function of suppressing or reducing a change in behavior (for example, spin) when an alkali metal atom directly collides with the wall 11 of the cell 12 (main chamber 14). The coating layer 32 is composed of chain saturated hydrocarbon such as paraffin. As an example of paraffin, for example, octatriacontane represented by the chemical formula (CH ₃ (CH ₂ ) ₃₆ CH ₃ ) can be used.

  An opening 18 is provided on the opposite side (−X direction side) of the main chamber 14 and the communication hole 15 in the longitudinal direction of the reservoir 16. The shape of the opening 18 viewed along the X-axis direction is, for example, a circular shape. The inner diameter of the opening 18 is, for example, about 0.4 mm to 1.5 mm. The opening 18 is sealed with a lid 19 joined to the cell 12 via a sealing member 30. Thereby, the cell 12 (the main chamber 14 and the reservoir 16) is sealed.

  In FIG. 3, the longitudinal direction of the cell 12 is illustrated along the vertical direction in FIG. 3. As shown in FIG. 3, an antireflection film 50 a is provided on the inner surface 112 and the surface 113 of the wall 11 of the side portion (part along the XZ plane) of the main chamber 14. The surface of the antireflection film 50 a is covered with the coating layer 32. Further, an antireflection film 50 b is provided on the outer surface 111 and the surface 114 of the side wall 11 of the main chamber 14.

  In the magnetic measurement apparatus 100 (see FIG. 1), the laser beam Lb is irradiated from the −Y direction side to the + Y direction side along the Y axis direction in FIG. When viewed from the irradiation direction of the laser beam Lb, the laser beam Lb is applied to the substantially central portion of the side wall 11 of the main chamber 14 (see FIG. 2) and passes through the main chamber 14. The laser beam Lb is incident on the outer surface 111 of the wall 11 on the −Y direction side substantially perpendicularly, passes through the inner surface 112 from the surface 111, enters the main chamber 14, and passes through the main chamber 14. , The inner surface 113 of the wall 11 on the + Y direction side passes through the outer surface 114 and is injected out of the main chamber 14.

  At this time, the laser beam Lb passes through the antireflection film 50 b provided on the outer surface 111 and the antireflection film 50 a provided on the inner surface 112 in the −Y direction side wall 11. The laser beam Lb passes through the antireflection film 50a provided on the inner surface 113 and the antireflection film 50b provided on the outer surface 114 in the + Y direction side wall 11.

  Here, the effect of the antireflection film (the antireflection film 50a and the antireflection film 50b) included in the gas cell 10 will be described in comparison with a gas cell that does not include a conventional antireflection film. FIG. 19 is a plan cross-sectional view showing an example of a gas cell not provided with a conventional antireflection film. The gas cell 90 shown in FIG. 19 has the same configuration as the gas cell 10 according to the first embodiment, except that the antireflection film (antireflection film 50a and antireflection film 50b) is not provided.

  When the gas cell 90 is irradiated with the laser beam Lb, reflection occurs on the outer surface 111 of the wall 11 on the −Y direction side. Similar reflection occurs on the inner surface 112 of the wall 11 on the −Y direction side, the inner surface 113 and the outer surface 114 of the wall 11 on the + Y direction side. The light loss of the laser beam Lb due to reflection on each surface of the wall 11 is about 4%. Accordingly, when the laser beam Lb passes through the four surfaces of the main chamber, the optical loss of the laser beam Lb is about 15%. As a result, the light amount of the laser beam Lb emitted from the gas cell 90 is reduced to about 85% of the light amount of the laser beam Lb incident on the gas cell 90.

  When a loss occurs in the laser beam Lb that passes through the gas cell 90 due to such reflection, a signal component in the measurement of the magnetic field in the magnetic measurement device 100 is reduced. The reflected light from which the laser beam Lb is reflected becomes stray light and becomes a noise component in the measurement of the magnetic field. As a result, the conventional gas cell 90 causes a decrease in sensitivity and a decrease in measurement accuracy in the measurement of the magnetic field.

  On the other hand, since the gas cell 10 according to the first embodiment includes the antireflection films (the antireflection film 50a and the antireflection film 50b) on the wall 11 of the main chamber 14, the laser beam Lb causes the gas cell 10 to pass through the gas cell 10. During transmission, reflection of light on the outer surface 111 and inner surface 112 of the incident-side wall 11 and the inner surface 113 and outer surface 114 of the emission-side wall 11 is suppressed. Thereby, the fall of the sensitivity resulting from reflection of the laser beam Lb and the fall of a measurement precision can be suppressed. The configuration and effects of the antireflection films (antireflection film 50a and antireflection film 50b) will be described in detail later.

  In the above-described configuration, the antireflection film 50a and the antireflection film 50b are formed on the entire surface 111, 112, 113, 114 of the wall 11, but the antireflection film 50a and the antireflection film 50b are formed on the wall. 11 may be provided only in the region (see FIG. 2) through which the laser beam Lb passes in the center of the surfaces 111, 112, 113, 114.

  In the above-described configuration, the inner surface 112 and the surface 113 of the wall 11 of the main chamber 14 are provided with the antireflection film 50a, and the outer surface 111 and the surface 114 are provided with the antireflection film 50b. The surface 111 and the surface 114 may be provided with no antireflection film 50b. In the case of such a configuration, although the antireflection effect is lower than that of the gas cell 10 according to the first embodiment, it is possible to suppress a decrease in sensitivity and a decrease in measurement accuracy as compared with the gas cell 90 not provided with the antireflection film. Can do.

<Ample>
FIG. 4 is a cross-sectional view along the longitudinal direction of the ampoule according to the first embodiment. FIG. 5 is a cross-sectional view taken along the line BB ′ of FIG. As shown in FIG. 4, the ampoule 20 as a solid substance containing an alkali metal according to the present embodiment has a longitudinal direction. FIG. 4 shows an XZ cross section when the ampoule 20 is arranged so that the longitudinal direction thereof is along the X-axis direction. The ampoule 20 is composed of a hollow glass tube 22. The glass tube 22 is made of, for example, borosilicate glass.

  The glass tube 22 extends along one direction (X-axis direction in FIG. 4), and both ends thereof are welded. Thereby, the hollow glass tube 22 is sealed. In addition, the shape of the both ends of the glass tube 22 is not limited to a round shape as shown in FIG. 4, The shape close | similar to a plane, the shape where one part sharpened, etc. may be sufficient. The hollow interior of the glass tube 22 is filled with an alkali metal solid (granular or powdery alkali metal atoms) 24. As the alkali metal solid 24, as described above, rubidium, potassium, and sodium can be used in addition to cesium.

  FIG. 4 shows a state where the ampoule 20 (glass tube 22) is sealed. At the stage where the ampule 20 is manufactured, the glass tube 22 is in a sealed state. However, when the gas cell 10 is completed, a through hole 21 (see FIG. 2) is formed in the glass tube 22 and the sealing is broken. Thereby, the alkali metal solid 24 in the ampoule 20 evaporates and flows into the gas cell 10, and the gap of the cell 12 is filled with the alkali metal gas 13 (see FIG. 2). In addition, a gap of about 1.5 mm is provided in the + Z direction, for example, between the upper surface of the ampoule 20 and the inner surface of the cell 12 so that the alkali metal solid 24 can easily evaporate and flow out from the ampoule 20. (See FIG. 2).

  FIG. 5 shows a YZ cross section in a direction intersecting the longitudinal direction of the ampoule 20. As shown in FIG. 5, the shape of the glass tube 22 in the YZ cross section is, for example, substantially circular, but may be other shapes. The outer diameter φ of the glass tube 22 is 0.3 mm ≦ φ ≦ 1.2 mm. The thickness t of the glass tube 22 is 0.1 mm ≦ t ≦ 0.5 mm, and is preferably about 20% of the outer diameter φ. When the thickness t of the glass tube 22 is less than 0.1 mm, the glass tube 22 is likely to be damaged, and when the thickness t of the glass tube 22 exceeds 0.5 mm, the through hole 21 is formed in the glass tube 22. (Details will be described later).

<Antireflection film>
Next, the configuration of the antireflection film according to the first embodiment will be described. The antireflection film 50a and the antireflection film 50b according to the first embodiment are made of a dielectric thin film (hereinafter referred to as a dielectric film). By forming a dielectric film as an antireflection film 50a and an antireflection film 50b on the surface of the wall 11 constituting the cell 12, reflection of light on the surface of the wall 11 can be suppressed.

In general, the refractive index n of the dielectric film and the square root of the refractive index n _m of the wall 11 (glass plate member), when a quarter of the wavelength of light λ incident optical thickness nd, of the incident light It is known that reflection can be reduced most. That is, the amplitude condition of the dielectric film is expressed by Expression 3, and the phase condition is expressed by Expression 4.

n = √n _m (3)

nd = 0.25qλ (4)
In Equation 4, q is an odd number (1, 3, 5,...), And d is the thickness of the dielectric film.

As a dielectric constituting the dielectric film, for example, magnesium fluoride (MgF ₂ ) can be suitably used. As an antireflection film, for example, a case where a single layer film of MgF ₂ is formed on the surface of the wall 11 made of quartz glass is assumed. The refractive index of quartz glass is 1.45, the refractive index of MgF ₂ is 1.38, the refractive index of air is 1.00, and MgF ₂ is set to nd _f = 0.25λ (ie, q = 1) in Equation 4. When the Fresnel coefficient is calculated to obtain the approximate reflectance of the antireflection film, the reflectance is about 1.8%. Therefore, when configuring the antireflection film is a single layer film of MgF _2, it can be reduced reflectance of 2% or less.

As described above, the effect of reducing the reflectance can be obtained even with a single layer film of MgF ₂ , but in the present embodiment, a configuration capable of further reducing the reflectance is considered as the antireflection film 50a and the antireflection film 50b. . For example, the anti-reflection film is formed of a multilayer film (a laminated structure of dielectric films) in which a dielectric film having a low refractive index and a dielectric film having a high refractive index are overlapped, thereby further improving the reflectance due to the interference effect. It is possible to reduce.

As the antireflection film, for example, a multilayer film composed of three layers of aluminum oxide (Al ₂ O ₃ ), zirconium oxide (ZrO ₂ ), and magnesium fluoride (MgF ₂ ) is formed on the surface of the wall 11 made of quartz glass. Is assumed. When the surface of the wall 11 is formed by sequentially laminating Al ₂ O ₃ with nd _a = 0.25λ, ZrO ₂ with nd _z = 0.5λ, and MgF ₂ with nd _f = 0.25λ from the lower layer side, The rate is about 0.3%. Further, when a multilayered film of three layers is formed by laminating Al ₂ O _{3 in the order} of nd _a = 0.25λ, ZrO ₂ in nd _z = 0.25λ, and MgF ₂ in order of nd _f = 0.25λ from the lower layer side. The reflectance is about 0.5%.

As described above, when the antireflection film is formed of a dielectric multilayer film, the reflectance can be reduced to a value close to zero as compared with the case of the above-described single layer film. Therefore, as the antireflection film 50a and the antireflection film 50b according to this embodiment, a dielectric multilayer film in which the above-described three layers of Al ₂ O ₃ , ZrO ₂ , and MgF ₂ are stacked is employed. The dielectric multilayer film having the above-described configuration can be applied as it is to the antireflection film 50b disposed on the outer surface 111 and the surface 114 of the wall 11.

On the other hand, the antireflection film 50 a disposed on the inner surface 112 and the surface 113 of the wall 11 is covered with the coating layer 32. The refractive index of the material of the coating layer 32 (for example, octatria contane) is almost the same as that of quartz glass, and is higher than the refractive index of MgF ₂ . Therefore, when the coating layer 32 is formed on the surface of the dielectric multilayer film having the above-described configuration, the reflectance is increased and the antireflection effect is lowered. Hereinafter, a configuration of a dielectric multilayer film suitable for the antireflection film 50a covered with the coating layer 32 will be examined.

By disposing the coating layer 32 on the surface of the antireflection film 50a, the optical path length of the transmitted light is changed. Therefore, as the coating layer 32 is disposed, the optical film thickness of any one of the dielectric multilayer films (laminated films of Al ₂ O ₃ , ZrO ₂ , MgF ₂ ) is changed, Reduce reflectivity. Here, the optical film thickness of MgF ₂ is changed. FIG. 6 is a graph showing the relationship between the optical film thickness of the coating layer and the coefficient of MgF ₂ for reducing the reflectance. In FIG. 6, the horizontal axis represents the optical film thickness nd _c of the coating layer 32, and the vertical axis represents the coefficient A of MgF ₂ at which the reflectance decreases at each optical film thickness of the coating layer 32.

From FIG. 6, when the optical film thickness nd _c of the coating layer is used as a parameter, the coefficient A of MgF ₂ that reduces the reflectance is expressed by the approximate expression shown in Equation 5.

A = −1.1067 nd _c +0.2488 (5)

From Equation 5, when the optical film thickness nd _c of the coating layer 32 is 0.081λ, the coefficient A of MgF ₂ is 0.159.

Subsequently, an optical film thickness nd _f of MgF ₂ for reducing the reflectance is obtained. The optical film thickness nd _f of MgF ₂ that reduces the reflectance can be expressed by Equation 6.

nd _f = 0.25qλ− (0.25−A) λ
= 0.25qλ- (0.25-0.159) λ
= 0.25qλ-0.091λ (6)
However, in Formula 6, λ is the wavelength of incident light, and q is an odd number (1, 3, 5,...). The coefficient A of MgF ₂ is set to 0.159 from Equation 5.

FIG. 7 is a graph showing the relationship between the optical film thickness nd _{f of} MgF ₂ and the reflectance. In FIG. 7, the horizontal axis represents the optical film thickness nd _f of MgF ₂ , and the vertical axis represents the reflectance (%). The reflectance is obtained by calculating the Fresnel coefficient from the optical film thickness nd of Al ₂ O ₃ , ZrO ₂ , and MgF ₂ . As shown in FIG. 7, the optical thickness nd _f of MgF ₂ is changed, the reflectance of the MgF ₂ film changes periodically. The minimum reflectance is about 0.2%. Therefore, even when covered with the coating layer 32, the reflectance can be reduced to a value close to zero.

From FIG. 7, the value of q, the optical thickness nd _f of MgF ₂ reflectivity of MgF ₂ is minimized even periodically appear. For example, when the value of q is 1, the optimum value of the optical film thickness nd _f of MgF ₂ , that is, the optical film thickness nd _f of MgF ₂ at which the reflectance is the minimum value is 0.159λ. Further, from FIG. 7, the optical film thickness nd _f of MgF ₂ is preferably in the range of ± 0.054λ with respect to the optimum value (for example, 0.159λ when the value of q is 1), and ± 0 More preferably, it is in the range of 0.036λ. When the optical film thickness nd _{f of} MgF ₂ is in the range of ± 0.054λ with respect to the optimum value, the reflectance is 2% or less, and when it is within the range of ± 0.036λ with respect to the optimum value, the reflectance is 1%. It becomes as follows.

As described above, as the antireflection film 50a covered with the coating layer 32, Al ₂ O ₃ is nd _a = 0.25λ, ZrO ₂ is nd _z = 0.5λ, and MgF ₂ is formed on the surface of the wall 11 from the lower layer side. _{If the} layers are sequentially stacked at nd _f = 0.159λ and the coating layer 32 is formed on the surface thereof with nd _c = 0.081λ, the reflectance can be reduced to a value close to zero (about 0.2%). it can.

In the above example, the value of q has been described as 1. However, the value of q is an odd number and is not limited to 1. Therefore, the optical film thickness nd _f of MgF ₂ at which the reflectance is the minimum value is not limited to 0.159λ.

In the above example, the film thickness of MgF ₂ is changed. However, the optical film thickness of a dielectric film other than MgF ₂ may be changed. For example, when the optical film thickness of Al ₂ O ₃ is changed, as the antireflection film 50a, Al ₂ O ₃ is nd _a = 0.10λ and ZrO ₂ is nd _z = 0 on the surface of the wall 11 from the lower layer side. .5Ramuda, the MgF ₂ is formed by laminating sequentially with nd _f = 0.25λ, when the coating layer 32 on the surface thereof formed with nd _c = 0.081λ, the reflectance close to zero value (about 0.5% ).

<Gas cell manufacturing method>
Next, a gas cell manufacturing method according to the first embodiment will be described with reference to FIGS. 8 to 15 are views for explaining a method of manufacturing a gas cell according to the first embodiment. 8 to 15, FIGS. 9 and 11 are plan sectional views corresponding to FIG. 3, and the other portions are side sectional views corresponding to FIG. 2.

  In the steps shown in FIGS. 8 and 9, the cell 12 is assembled. The cell 12 is configured by combining, for example, a glass plate member made of quartz glass as the wall 11. Although not shown, a glass plate made of quartz glass is cut to prepare glass plate members corresponding to the walls 11 constituting the cell 12. Then, these glass plate members (walls) 11 are assembled, and the glass plate members (walls) 11 are joined to each other by an adhesive or welding, so that the main chamber 14 and the reservoir 16 as shown in FIGS. A cell 12 is obtained.

  As shown in FIG. 9, an antireflection film 50 a is formed in advance on the inner surface 112 and the surface 113 of the glass plate member that becomes the side wall 11 of the main chamber 14. In addition, an antireflection film 50 b is formed in advance on the outer surface 111 and the surface 114 of the glass plate member that becomes the side wall 11 of the main chamber 14. The antireflection film 50a and the antireflection film 50b can be formed using, for example, a vacuum deposition method or a sputtering method. 8 and 9, the coating layer 32 is not formed. At this stage, the opening 18 of the cell 12 is opened.

  Subsequently, in the process shown in FIGS. 10 and 11, the coating layer 32 is formed on the inner side of the wall 11 of the cell 12. Although illustration is omitted, the assembled cell 12 is sufficiently degassed, and the material of the coating layer 32 is arranged from the opening 18 of the cell 12 into the reservoir 16 using a needle or the like in a reduced pressure environment. . And the opening part 18 is temporarily sealed with a temporary stopper etc., and the whole cell 12 is heated to about 200 degreeC, for example.

  Thereby, as shown in FIG. 10 and FIG. 11, the coating layer 32 which covers the inner side of the wall 11 of the cell 12 (the main chamber 14 and the reservoir | reserver 16) is formed. As a result, as shown in FIG. 11, the antireflection film 50 a is covered with the coating layer 32 on the inner surface 112 and the surface 113 of the side wall 11 of the main chamber 14. In addition, the inside of the cell 12 may be in a state close to a vacuum, and the material of the coating layer 32 may be heated and introduced into the cell 12 through the opening 18 in a gas phase or liquid phase state.

  Subsequently, in the step shown in FIG. 12, the ampoule 20 is stored in the reservoir 16 of the cell 12 having the coating layer 32 formed on the inner surface. The ampule 20 is inserted from an opening 18 provided on the reservoir 16 side of the cell 12 and is disposed in the reservoir 16. At this stage, as shown in FIG. 4, the ampoule 20 is in a state where the hollow glass tube 22 is filled with an alkali metal solid 24 and sealed.

  The ampoule 20 is filled with an alkali metal solid 24 in the hollow portion of the tubular glass tube 22 in a low-pressure environment close to vacuum (ideally in a vacuum), and both ends of the glass tube 22 are welded and sealed. To form. Alkali metals such as cesium used as the alkali metal solid 24 are highly reactive and cannot be handled in the atmosphere, and thus are stored in the cell 12 in a sealed state in the ampoule 20 under a low pressure environment.

  Subsequently, in the process shown in FIG. 13, the cell 12 is sufficiently evacuated, and the cell 12 is sealed in a state where impurities are extremely small in the main chamber 14 and the reservoir 16. For example, the opening 18 of the cell 12 is sealed with the lid 19 using the sealing member 30 (low melting point glass frit) in a low-pressure environment close to vacuum (ideally in vacuum) or in an inert gas. . Thereby, the cell 12 is sealed in a state where the ampoule 20 is housed inside.

  Subsequently, in the step shown in FIG. 14, the through hole 21 is formed in the ampoule 20 (glass tube 22). In this step, the pulsed laser light 40 is condensed by the condenser lens 42 and irradiated onto the glass tube 22 of the ampule 20 from above (+ Z direction) via the cell 12. Since the laser beam is excellent in directivity and convergence, the glass tube 22 can be easily processed. By irradiating the pulse laser beam 40, the processing proceeds from the upper surface (surface) of the glass tube 22 in the depth direction (−Z direction). And when the through-hole 21 is formed in the glass tube 22, the hollow part of the ampoule 20 will connect with the reservoir | reserver 16, and the sealing of the ampoule 20 will be broken.

  As shown in FIG. 15, by forming the through hole 21 in the glass tube 22, the alkali metal solid 24 (see FIG. 4) in the ampoule 20 evaporates and flows into the reservoir 16 as the alkali metal gas 13. . The alkali metal gas 13 flowing out into the reservoir 16 flows into the main chamber 14 of the cell 12 through the communication hole 15 and diffuses. As a result, the main chamber 14 of the cell 12 is filled with the alkali metal gas 13. The gas cell 10 can be manufactured by the above process.

  In the step shown in FIG. 15, it is necessary to form the through hole 21 in the glass tube 22 of the ampoule 20 without damaging the wall 11 of the cell 12. Therefore, when the wall 11 is made of quartz glass and the glass tube 22 is made of borosilicate glass, for example, pulse laser light 40 having a wavelength in the ultraviolet region is used. Light having a wavelength in the ultraviolet region passes through the quartz glass, but is slightly absorbed by the borosilicate glass. Accordingly, the through hole 21 can be formed by selectively processing the glass tube 22 of the ampoule 20 without damaging the wall 11 of the cell 12.

  In this step, the alkali metal solid 24 only needs to evaporate and flow out from the ampoule 20, so that the invention is not limited to the formation of the through hole 21. For example, the ampule 20 is divided by causing a crack in the glass tube 22. Alternatively, the glass tube 22 may be broken.

(Second Embodiment)
<Atomic oscillator>
In the first embodiment, the magnetic measurement device 100 has been described as an apparatus to which the gas cell 10 can be applied. In the second embodiment, an atomic clock or the like is used as an apparatus to which the gas cell 10 according to the first embodiment is applied. The atomic oscillator will be described. FIG. 16 is a schematic diagram illustrating a configuration of an atomic oscillator according to the second embodiment. FIGS. 17 and 18 are diagrams for explaining the operation of the atomic oscillator according to the second embodiment.

  An atomic oscillator (quantum interference device) 101 according to the second embodiment illustrated in FIG. 16 is an atomic oscillator using a quantum interference effect. As illustrated in FIG. 16, the atomic oscillator 101 includes a gas cell 10 according to the first embodiment, a light source 71, optical components 72, 73, 74, and 75, a light detection unit 76, a heater 77, and a temperature sensor. 78, a magnetic field generation unit 79, and a control unit 80.

  The light source 71 emits two kinds of light (resonant light L1 and resonant light L2 shown in FIG. 17) having different frequencies, which will be described later, as excitation light LL for exciting alkali metal atoms in the gas cell 10. The light source 71 is composed of, for example, a semiconductor laser such as a vertical cavity surface emitting laser (VCSEL). The optical components 72, 73, 74, and 75 are provided on the optical path of the excitation light LL between the light source 71 and the gas cell 10, respectively, and the optical component 72 (lens) and the optical component from the light source 71 side to the gas cell 10 side. 73 (polarizing plate), optical component 74 (darkening filter), and optical component 75 (λ / 4 wavelength plate) are arranged in this order.

  The light detection unit 76 detects the intensity of the excitation light LL (resonance light L1, L2) that has passed through the gas cell 10. The light detection part 76 is comprised, for example with the solar cell, the photodiode, etc., and is connected to the excitation light control part 82 of the control part 80 mentioned later. The heater 77 (heating unit) heats the gas cell 10 in order to maintain the alkali metal in the gas cell 10 in a gaseous state (as the alkali metal gas 13). The heater 77 (heating unit) is composed of, for example, a heating resistor.

  The temperature sensor 78 detects the temperature of the heater 77 or the gas cell 10 in order to control the amount of heat generated by the heater 77. The temperature sensor 78 includes various known temperature sensors such as a thermistor and a thermocouple. The magnetic field generator 79 generates a magnetic field that causes Zeeman splitting of a plurality of energy levels of the alkali metal in the gas cell 10 that are degenerated. Zeeman splitting can increase the resolution by widening the gap between different energy levels of alkali metal degeneration. As a result, the accuracy of the oscillation frequency of the atomic oscillator 101 can be increased. The magnetic field generator 79 is composed of, for example, a Helmholtz coil or a solenoid coil.

  The control unit 80 controls the excitation light control unit 82 that controls the frequency of the excitation light LL (resonant light L 1, L 2) emitted by the light source 71, and the temperature that controls the energization of the heater 77 based on the detection result of the temperature sensor 78. The controller 81 and the magnetic field controller 83 that controls the magnetic field generated from the magnetic field generator 79 to be constant. The control unit 80 is provided, for example, on an IC chip mounted on a substrate.

  The principle of the atomic oscillator 101 will be briefly described. FIG. 17 is a diagram for explaining the energy state of the alkali metal in the gas cell 10 of the atomic oscillator 101. FIG. 18 shows the frequency difference between the two lights from the light source 71 of the atomic oscillator 101 and the detection intensity at the light detector 76. It is a graph which shows the relationship. As shown in FIG. 17, the alkali metal (alkali metal gas 13) enclosed in the gas cell 10 has a three-level energy level, and two ground states (bases) having different energy levels. There can be three states: a state S1, a ground state S2) and an excited state. Here, the ground state S1 is a lower energy state than the ground state S2.

  When such an alkali metal gas 13 is irradiated with two types of resonant lights L1 and L2 having different frequencies, according to the difference (ω1−ω2) between the frequency ω1 of the resonant light L1 and the frequency ω2 of the resonant light L2. The optical absorptance (light transmittance) of the resonance lights L1 and L2 in the alkali metal gas 13 changes. When the difference (ω1−ω2) between the frequency ω1 of the resonant light L1 and the frequency ω2 of the resonant light L2 matches the frequency corresponding to the energy difference between the ground state S1 and the ground state S2, the ground states S1 and S2 Each excitation to the excited state stops.

  At this time, the resonance lights L1 and L2 are transmitted without being absorbed by the alkali metal gas 13. Such a phenomenon is called a CPT phenomenon or an electromagnetically induced transparency (EIT) phenomenon. The EIT signal, which is a steep signal generated in accordance with the EIT phenomenon, is detected by the light detection unit 76, and the EIT signal is used as a reference signal. From the viewpoint of improving short-term frequency stability, the EIT signal preferably has a small line width (half-value width) and high strength.

  The light source 71 emits two types of light (resonant light L1 and resonant light L2) having different frequencies as described above toward the gas cell 10. Here, for example, when the frequency ω1 of the resonant light L1 is fixed and the frequency ω2 of the resonant light L2 is changed, the difference (ω1−ω2) between the frequency ω1 of the resonant light L1 and the frequency ω2 of the resonant light L2 is obtained. When the frequency coincides with the frequency ω0 corresponding to the energy difference between the ground state S1 and the ground state S2, the detection intensity of the light detection unit 76 increases sharply as shown in FIG.

  Such a steep signal is called an EIT signal. The EIT signal has an eigenvalue determined by the type of alkali metal. In this embodiment, a high-quality and large EIT signal is obtained with a small line width. Therefore, by using such an EIT signal as a reference, a highly accurate atomic oscillator 101 can be realized.

  The gas cell 10 used in the atomic oscillator 101 is required to be small and have a long life and high accuracy. However, according to the configuration of the gas cell of the above embodiment, the gas cell 10 is small and has a long life and high accuracy. Therefore, the atomic oscillator 101 with a small size, high accuracy, and long life can be provided.

  The above-described embodiments merely show one aspect of the present invention, and can be arbitrarily modified and applied within the scope of the present invention. As modifications, for example, the following can be considered.

(Modification 1)
In the gas cell 10 according to the first embodiment, the case where the material of the wall 11 (glass plate member) provided with the antireflection films 50a and 50b is quartz glass is described as an example, but the present invention is in such a form. It is not limited. For example, borosilicate glass may be used as the material of the wall 11.

Even when borosilicate glass is used as the material of the wall 11, a dielectric multilayer film having the same configuration as that of the first embodiment can be applied as the antireflection film. For example, a configuration in which the optical film thickness of MgF ₂ is changed is applied to the antireflection film 50a in order to suppress an increase in reflectance due to the coating layer 32. In this case, on the surface of the wall 11 made of borosilicate glass, Al ₂ O ₃ is nd _a = 0.25λ, ZrO ₂ is nd _z = 0.5λ, and MgF ₂ is nd _f = 0.159λ from the lower layer side. When the antireflection film 50a is formed by laminating and the coating layer 32 is formed on the surface thereof with nd _c = 0.081λ, the reflectance can be reduced to almost zero (about 0.04%).

Moreover, the anti-reflection film 50a, it is also possible to apply the configuration for changing the optical thickness of the Al ₂ O _3. In this case, on the surface of the wall 11 made of borosilicate glass, Al ₂ O ₃ is nd _a = 0.10λ, ZrO ₂ is nd _z = 0.5λ, and MgF ₂ is nd _f = 0.25λ from the lower layer side. When the antireflection film 50a is formed by laminating and the coating layer 32 is formed on the surface thereof with nd _c = 0.081λ, the reflectance can be reduced to a value close to zero (about 0.9%). Therefore, even when borosilicate glass is used as the material of the wall 11, the same antireflection effect as in the first embodiment can be obtained.

(Modification 2)
Further, as in the first modification, borosilicate glass is used as the material of the wall 11, and the dielectric film constituting the antireflection films 50a and 50b further includes a silicon oxide (SiO ₂ ) film. Also good.

For example, in the case where the antireflection film 50a further includes a SiO ₂ film, a configuration in which the optical film thickness of MgF ₂ is changed is applied in order to suppress an increase in reflectance due to the coating layer 32. In this case, on the surface of the wall 11 made of borosilicate glass, from the lower layer side, SiO ₂ is nd _s = 0.25λ, Al ₂ O ₃ is nd _a = 0.25λ, ZrO ₂ is nd _z = 0.5λ, MgF ₂ is sequentially laminated at nd _f = 0.159λ to form the antireflection film 50a, and the coating layer 32 is formed on the surface thereof with nd _c = 0.081λ, the reflectance is close to zero (0.5% Degree).

Further, when the antireflection film 50a further includes a SiO ₂ film, a configuration in which the optical film thickness of Al ₂ O ₃ is changed can be applied. In this case, on the surface of the wall 11 made of borosilicate glass, from the lower layer side, SiO ₂ is nd _s = 0.25λ, Al ₂ O ₃ is nd _a = 0.10λ, ZrO ₂ is nd _z = 0.5λ, MgF ₂ are sequentially laminated at nd _f = 0.25λ to form the antireflection film 50a, and the coating layer 32 is formed on the surface thereof with nd _c = 0.081λ, the reflectance is close to zero (0.2% Degree).

(Modification 3)
In the first embodiment, the first modification, and the second modification, in order to suppress an increase in reflectivity due to the antireflection film 50a being covered with the coating layer 32, a dielectric of either MgF ₂ or Al ₂ O ₃ is used. Although the case where the optical film thickness of the film is changed has been described as an example, the present invention is not limited to such a form. It may be to vary the optical thickness of both dielectric film MgF ₂ and Al ₂ O _3. Further, the optical film thickness of dielectric films other than MgF ₂ and Al ₂ O ₃ may be changed.

(Modification 4)
In the gas cell 10 according to the first embodiment, the ampule 20 is used as a means for generating the alkali metal gas 13 inside the cell 12, but the present invention is not limited to such a configuration. For example, as a means for generating the alkali metal gas 13, a pill that includes an alkali metal compound and an adsorbent and has a cylindrical shape, a rectangular parallelepiped shape, a spherical shape, or the like may be used. Although not shown, when such a pill is irradiated with a continuous wave laser beam having a wavelength in the red to infrared region, an alkali metal is generated by activation of the alkali metal compound, and an impurity released at that time is generated. And impure gas is adsorbed by the adsorbent. As the alkali metal compound, when cesium is used as the alkali metal, for example, a cesium compound such as cesium molybdate or cesium chloride can be used. As the adsorbent, for example, zirconium powder, aluminum or the like can be used.

(Modification 5)
In the gas cell 10 according to the above embodiment, when the coating layer 32 is formed inside the cell 12, the material of the coating layer 32 is disposed in the reservoir 16 from the opening 18 of the cell 12, It is not limited to such a configuration. For example, the tubular member is filled with the material of the coating layer 32, this tubular member is placed in the reservoir 16 from the opening 18 of the cell 12, and the ampoule 20 is also placed in the reservoir 16 and the opening 18 is sealed with the lid 19. After stopping, the entire cell 12 may be heated. In this case, after the coating layer 32 is formed inside the cell 12, the tubular member filled with the material of the coating layer 32 remains in the reservoir 16.

  DESCRIPTION OF SYMBOLS 10,90 ... Gas cell, 11 ... Wall, 12 ... Cell (closed container), 13 ... Alkali metal gas, 32 ... Coating layer (chain saturated hydrocarbon), 50a ... Antireflection film, 50b ... Antireflection film, 100 ... Magnetic measuring device, 101 ... atomic oscillator, 111, 114 ... outer surface, 112, 113 ... inner surface.

Claims (7)

A closed container composed of walls,
An antireflection film containing magnesium fluoride provided on the inner surface of the wall;
A chain saturated hydrocarbon covering the surface of the antireflection film;
An alkali metal gas sealed in the closed container;
A gas cell comprising: The gas cell according to claim 1, wherein
When the wavelength of light incident on the closed container is λ and the optical film thickness of the chain saturated hydrocarbon is nd _c , the coefficient A of the magnesium fluoride is expressed by Equation 1, and the optical properties of the magnesium fluoride The gas cell is characterized in that the film thickness nd _f is expressed by Formula 2.
A = −1.1067 nd _c +0.2488 (1)
nd _f = 0.25qλ− (0.25−A) λ (2)
However, in Formula 2, q is an odd number (1, 3, 5,...). The gas cell according to claim 1 or 2, wherein
The gas cell, wherein the antireflection film is also provided on an outer surface of the wall. A gas cell according to any one of claims 1 to 3,
The gas cell, wherein the antireflection film is a dielectric single layer film. A gas cell according to any one of claims 1 to 3,
The gas cell, wherein the antireflection film is a dielectric multilayer film.   A magnetic measuring device comprising the gas cell according to claim 1.   An atomic oscillator comprising the gas cell according to any one of claims 1 to 5.

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