A full polarization resonant fluorescence lidar ultra-narrow band filter

Abstract

The application discloses a full-polarization resonant fluorescence laser radar ultra-narrow band filter and relates to the technical field of laser radar detection. The full-polarization resonant fluorescence laser radar ultra-narrow band filter comprises a first polarization beam splitter, a single-atom vapor cell, a working parameter control device, first and second polarized light optical paths, a plurality of relay lenses and a beam combiner. The optical paths guide the first and second polarized light to be incident from two sides of the single-atom vapor cell in opposite directions, the optical path length of the second polarized light optical path is longer than that of the first polarized light optical path, and the plurality of relay lenses are configured to compensate for the optical path difference, so that the two polarized lights have consistent beam parameters when entering the vapor cell. The single-cell opposite incidence and optical path compensation are adopted, the problems of inconsistent parameters in the double-cell scheme and large energy loss in the single-cell scheme in the prior art are solved, and efficient filtering of full-polarization signals under consistent physical conditions is ensured.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of laser radar detection, in particular to a full polarization resonance fluorescence laser radar ultra-narrow band filter. BACKGROUND

[0002] High-altitude detection laser radar, such as resonance fluorescence laser radar, is an important tool for detecting middle and high-altitude atmospheric parameters, for example, for detecting the atmospheric metal layer at a height of 80 to 105 kilometers. However, when detection is performed during the day, strong solar background light will cause serious interference, which greatly limits the continuous observation capability of the laser radar. In order to realize all-day observation, an atomic filter, in particular an atomic filter based on the Faraday anomalous dispersion effect, is generally used in the prior art to perform ultra-narrow band filtering on the return signal, so as to effectively suppress the background light.

[0003] However, the working mechanism of such an atomic filter strictly depends on a specific polarization state. When the incident radar return signal is non-polarized light, the traditional single-channel filter will directly lose nearly half of the light energy in the polarization process, which is a major loss for extremely weak radar return signals. In order to solve this problem, some technicians have proposed a double-channel scheme, for example, using two independent atomic filters, or adopting a compact design of a double-pass optical aperture magnet and two side-by-side atomic cell, the purpose of which is to process two orthogonal polarization components respectively. However, these double-channel schemes essentially depend on two independent atomic cells. In an actual physical system, it is extremely difficult to ensure that the working temperature, the axial magnetic field strength and the physical parameters of the atomic cell itself, such as the atomic density distribution, of the two atomic cells are completely consistent. Such inconsistency in parameters will cause unpredictable differences in the filtering characteristics, such as the center wavelength and the transmission bandwidth, of the two polarization channels, which will directly affect the accuracy of the detection data and the stability of the entire system.

[0004] Therefore, how to design a filter that can not only avoid energy loss when non-polarized light is incident, but also fundamentally ensure that the two polarization components are filtered under completely consistent physical conditions, has become a technical problem to be solved. SUMMARY

[0005] The main purpose of the present application is to provide a full polarization resonance fluorescence laser radar ultra-narrow band filter, which aims to design a filter that can not only avoid energy loss when non-polarized light is incident, but also fundamentally ensure that the two polarization components are filtered under completely consistent physical conditions.

[0006] In order to achieve the above purpose, the present application provides a full polarization resonance fluorescence laser radar ultra-narrow band filter, comprising:

[0007] a first polarization beam splitter for splitting an incident light beam into a first polarized light and a second polarized light;

[0008] a single-atom cell filled with atomic medium for providing a place for optical activity to occur;

[0009] a working parameter control device for controlling the temperature of the single-atom cell and generating an axial magnetic field, so that the first and second polarized light realize light filtering based on Zeeman effect and Faraday anomalous dispersion effect in the single-atom cell;

[0010] a first polarized light path and a second polarized light path, the light paths being configured to guide the first and second polarized light to be incident from two sides of the single-atom cell, and the optical path length of the second polarized light path being longer than that of the first polarized light path;

[0011] a plurality of relay lenses arranged in the first and second polarized light paths, the plurality of relay lenses being configured to compensate for the difference in beam parameters of the first and second polarized light caused by the difference in optical path lengths of the first and second polarized light paths, so that the first and second polarized light have consistent beam parameters when entering the single-atom cell;

[0012] a combiner for combining and outputting the filtered first and second polarized light.

[0013] Preferably, the working parameter control device comprises a double-channel temperature control module configured to control the temperature of the side wall and the protruding end of the single-atom cell respectively, so as to accurately control the atomic density in the single-atom cell.

[0014] Preferably, at least two polarization purification prisms are further included, which are arranged in the first polarized light path and the second polarized light path respectively, for purifying the polarization state before the light beam enters the single-atom cell and / or after the light beam exits the single-atom cell.

[0015] Preferably, the polarization purification prism is a Glan-Taylor prism, and the background suppression ratio of the filter is .

[0016] Preferably, at least one half-wave plate is further included for rotating the polarization state of the first or second polarized light.

[0017] Preferably, the half-wave plate is configured to be located in both the first polarized light path and the second polarized light path at the same time;

[0018] In the second polarized light path, the half-wave plate is arranged before the single-atom cell incident end, for rotating the polarization state of the second polarized light by 90°, so that the polarization state of the second polarized light when incident on the single-atom cell is consistent with the polarization state when the first polarized light is incident.

[0019] In the first polarized light path, the half-wave plate is arranged after the single-atom cell exit end, for rotating the polarization state of the first polarized light exiting from the single-atom cell by 90°, so that the first polarized light can be transmitted through the subsequent polarization beam splitter.

[0020] Preferably, the first polarized light path and the second polarized light path further comprise a second polarization beam splitter, a third polarization beam splitter and four mirrors; the second polarization beam splitter and the third polarization beam splitter are arranged in the light path for guiding the light beams to enter and exit the single-atom cell; the combiner is a fourth polarization beam splitter; the four mirrors are used for deflecting and guiding the second polarized light path, so that the second polarized light path passes through the second polarization beam splitter, the third polarization beam splitter and finally enters the combiner.

[0021] Preferably, a focusing collimating lens group is further included and arranged before the first polarization beam splitter, for collimating the incident light beam.

[0022] Preferably, a filter is further included and arranged before the first polarization beam splitter, for broadband filtering the incident light beam.

[0023] Preferably, the working parameter control device comprises at least one annular magnet with a single-pass light hole, for generating an axial magnetic field parallel to the single-atom cell; and the single-atom cell is filled with sodium atoms, rubidium atoms or potassium atoms.

[0024] The above technical solution has the following advantages:

[0025] The present application simultaneously processes two orthogonal polarization components by using a single-atom cell, and configures the light path so that the two components are incident from opposite sides of the cell. This design fundamentally ensures that both polarization components are filtered under the same temperature, magnetic field and atomic density conditions, overcoming the technical difficulties of the existing double-cell scheme due to the difficulty in completely consistent parameters, and avoiding the energy loss of nearly half of the non-polarized light in the traditional single-channel filter. In view of the optical path difference between the two light paths caused by the single-cell opposite incidence structure, the present application further provides a plurality of relay lenses to compensate for the optical path difference, ensuring that the two polarized light beams have consistent beam parameters when entering the single-atom cell, thereby ensuring the high consistency and system stability when performing narrowband filtering on the full polarization signal, and significantly improving the transmission efficiency and detection accuracy of the signal. BRIEF DESCRIPTION OF DRAWINGS

[0026] The application will be described in detail below with specific embodiments and drawings, wherein:

[0027] Figure 1 A principle schematic diagram of a full polarization resonant fluorescence lidar ultra-narrow band filter provided by the application.

[0028] Figure 2 A schematic diagram of horizontal polarized light passing through an atomic optical filter in the application.

[0029] Figure 3 A schematic diagram of vertical polarized light passing through an atomic optical filter in the application.

[0030] Figure 4 A simulation verification result of the optical path structure in the application. DETAILED DESCRIPTION

[0031] In order to make the purpose, technical scheme and advantages of the application clearer, the application will be further described in detail below with drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the application, and are not used to limit the scope of the application.

[0032] The full polarization resonant fluorescence lidar ultra-narrow band filter provided by the embodiment of the application aims to solve the technical problem of the application of an atomic optical filter to non-polarized light detection in the prior art. In the background art, for example, a high-altitude detection lidar, in order to realize all-weather observation, an atomic optical filter needs to be used to suppress strong solar background light. However, when processing non-polarized light, the polarization mechanism of the conventional atomic optical filter will directly lose about half of the energy. To solve this problem, some schemes use two independent atomic optical filters to process two polarization components, which use two atomic bubbles and double-pass optical aperture magnets placed side by side. Although this double-channel design has improved, it still relies on two independent atomic vapor cells, and it is difficult to ensure that the temperature, magnetic field and parameters of the atomic vapor cells themselves of the two vapor cells are completely consistent, which will cause differences in the filtering characteristics of the two polarization lights, affecting the detection accuracy and system stability, and the volume, weight and power consumption are still large.

[0033] The core idea of ​​this invention is to use only a single-atom vapor cell 1, and through a special optical path design, to separate the incident beam into two polarization components, which are then incident from opposite sides of the single-atom vapor cell 1. In this way, the two polarization components are filtered within the same atomic vapor cell, the same temperature field, and the same magnetic field, fundamentally ensuring the consistency of various operating parameters. However, this opposite-incident optical path design inevitably results in the two polarized light paths having different optical path lengths. For large beams with a certain divergence angle, such as probe light from an optical fiber, the optical path difference will cause the two beams to have different spot sizes and divergence angles when entering the atomic vapor cell, which introduces new inconsistencies.

[0034] To address this technical challenge, this embodiment proposes setting multiple relay lenses in both polarization optical paths to actively manage beam propagation and compensate for the effects of optical path difference. This ensures that the two polarization components have consistent beam parameters when entering the single-atom vapor cell 1, guaranteeing the consistency and stability of the filtering operation. In this application, the incident beam is the radar echo signal.

[0035] The following is in conjunction with the appendix Figures 1 to 4 Detailed explanation.

[0036] Example 1

[0037] like Figure 1 As shown, this embodiment provides an ultra-narrowband filter for a fully polarized resonant fluorescence lidar. The filter includes a focusing collimating lens group 6, a filter 7, four polarization beam splitters (i.e., first polarization beam splitter 8, second polarization beam splitter 9, third polarization beam splitter 10, and fourth polarization beam splitter 11), four mirrors (i.e., first mirror 12, second mirror 13, third mirror 14, and fourth mirror 15), four GranThompson prisms (i.e., first GranThompson prism 17, second GranThompson prism 18, third GranThompson prism 19, and fourth GranThompson prism 20), a half-wave plate 16, and multiple relay lenses, including first relay lens 21, second relay lens 22, third relay lens 23, fourth relay lens 24, fifth relay lens 25, sixth relay lens 26, and seventh relay lens 27.

[0038] The core filtering unit includes a single-atom vapor cell 1 and a parameter control device for controlling its operating parameters. This parameter control device includes a temperature control box 2, a temperature controller 3, a first annular magnet 4, and a second annular magnet 5.

[0039] At the entrance of the system, the probe beam containing the signal light usually comes from an optical fiber. The parameters of the fiber have important influence on the design of the subsequent optical path, for example, its core diameter is not more than 1.5 mm, preferably 1.5 mm; the numerical aperture is not more than 0.37, preferably 0.26. The light beam emitted by the optical fiber enters the focusing collimating lens group 6. The lens group 6 can be composed of a plano-convex lens and a double-convex lens placed in turn, which functions to collimate the light beam from the optical fiber. The size of the collimated light spot is controlled to be not more than 12 mm, and the beam divergence angle is not more than 2.5°.

[0040] The collimated light beam first passes through the first relay lens 21. The relay lens 21 is preferably a double-convex lens, and its size is preferably 25.4 mm, and its focal length is in the range of 150-300 mm, preferably 200 mm. Subsequently, the light beam passes through the filter 7. The filter 7 is placed between the first relay lens 21 and the first polarization beam splitter 8, and is used for broadband filtering of the echo signal light, and pre-removing most of the out-of-band background light.

[0041] The pre-filtered light beam enters the first polarization beam splitter 8. The size of the splitter 8 is preferably 25.4*25.4 mm, which divides the incident light beam into two polarization components, for example, transmits the horizontally polarized light (P light, as the first polarized light) and reflects the vertically polarized light (S light, as the second polarized light). The two polarization components enter the first polarization light path (horizontal polarization light path) and the second polarization light path (vertical polarization light path), respectively.

[0042] The single-atom vapor cell 1 of the core filtering unit is a key device for realizing narrowband filtering. Its length is in the range of 20-40 mm, preferably 30 mm, and its effective aperture is in the range of 10-23 mm, preferably 18 mm. The cell can be filled with sodium atoms, rubidium atoms or potassium atoms, etc. to match the specific resonant absorption spectral line required by the laser radar. The single-atom vapor cell 1 is placed in the temperature control box 2 and between the first annular magnet 4 and the second annular magnet 5. Both of the two magnets are annular magnets with a single-pass light hole in the middle, and are axially magnetized to generate an axial magnetic field parallel to the axis of the atom vapor cell 1.

[0043] The temperature controller 3 in the working parameter control device is connected with the temperature control box 2. In a preferred embodiment, the temperature controller 3 is a double-channel temperature control module. The module is configured to control the temperature of the side wall and the protruding end of the single-atom vapor cell 1 through heating sheets in the temperature control box 2, such as silicon rubber heating plates, and heat preservation devices, such as Teflon materials. By this double-temperature control method, the atomic vapor density in the cell 1 can be more accurately controlled.

[0044] In the optical path design, in order to ensure a very high background rejection ratio, polarization purification prisms are arranged in both optical paths, preferably first Glan-Thompson prism 17, second Glan-Thompson prism 18, third Glan-Thompson prism 19 and fourth Glan-Thompson prism 20. The material of these prisms can be a-BBO or Calcite, preferably a-BBO. The effective aperture range is 5-20mm, preferably 14.5mm, and the length L and the clear aperture CA satisfy the relationship L / CA=1.6, preferably the length is about 31mm. Such a configuration can make the rejection ratio reach Even higher, and can filter out unwanted noise with high rejection ratio.

[0045] The second relay lens 22, the third relay lens 23, the fourth relay lens 24, the fifth relay lens 25, the sixth relay lens 26 and the seventh relay lens 27 in the two optical paths are preferably all biconvex lenses, and the size is preferably 25.4mm, and the focal length range is 75-150mm, preferably 100mm.

[0046] Figure 2 The first polarized light path is shown. After the light beam transmits through the first polarization beam splitter 8, it is sequentially purified by the first Glan-Thompson prism 17, then transmits through the second polarization beam splitter 9, and then from one side into the single-atom vapor cell 1 after passing through the second relay lens 22. In the atomic vapor cell 1, the signal light of a specific wavelength is rotated by 90 degrees under the Faraday anomalous dispersion effect, and becomes vertically polarized light. The outgoing light beam passes through the third relay lens 23, and then passes through the half-wave plate 16. The wave plate 16 is placed so that its fast axis or slow axis forms a 45° angle with the incident vertically polarized light, so that its polarization direction is rotated by 90° again, and becomes horizontally polarized light. Subsequently, the light beam transmits through the third polarization beam splitter 10, is purified again by the third Glan-Thompson prism 19, and finally transmits through the fourth polarization beam splitter 11 as a beam combiner. In this optical path, the combination of the first relay lens 21, the second relay lens 22 and the third relay lens 23 can control the spot size of the horizontally polarized light to be no more than 18mm within an optical path of about 540mm.

[0047] Figure 3The second polarized light path is shown. This path is significantly longer than the first light path and is also more complex. After being reflected by the first polarizing beam splitter 8, the light beam first passes through the first mirror 12. The first mirror 12, the second mirror 13, the third mirror 14 and the fourth mirror 15 are preferably 25.4 mm in size and are positioned at angles that are carefully designed, for example, the first mirror 12 and the fourth mirror 15 are positioned at an angle of 45° to the optical axis, while the second mirror 13 and the third mirror 14 are positioned at an angle of 135° to the optical axis. After being reflected by the first mirror 12, the light beam passes through the fourth relay lens 24, the fourth Glan-Thompson prism 20, the fifth relay lens 25, the second mirror 13, and is then reflected by the third polarizing beam splitter 10. Next, the light beam passes through the same half-wave plate 16. At this point, the half-wave plate 16 rotates the polarization state of the second polarized light (vertical) by 90°, so that it becomes horizontally polarized light. This is to ensure that the polarization state of the light when it enters the atomic vapor cell 1 is the same as the polarization state of the first polarized light (horizontal).

[0048] Subsequently, the light beam passes through the third relay lens 23 and enters the single-atom vapor cell 1 from the other side, i.e., in the opposite direction to the first polarized light. After experiencing the same temperature and magnetic field conditions, the polarization plane of the signal light is also rotated by 90°. The outgoing light beam passes through the second relay lens 22 and enters the second polarizing beam splitter 9. Since the signal light is now vertically polarized, it is reflected by the second polarizing beam splitter 9. Subsequently, the light beam passes through the third mirror 14, the sixth relay lens 26, the second Glan-Thompson prism 18, the seventh relay lens 27 and the fourth mirror 15, and finally enters the fourth polarizing beam splitter 11.

[0049] The fourth polarizing beam splitter 11 acts as a beam combiner. The (horizontal) signal light from the first polarized light path is transmitted by it, while the (vertical) signal light from the second polarized light path is reflected by it. The two polarized components of the signal light are combined here and output along the same optical axis to the subsequent detector. In this light path, the fourth relay lens 24, the fifth relay lens 25, the sixth relay lens 26 and the seventh relay lens 27 are used in combination with the shared second relay lens 22 and the third relay lens 23, so that the vertically polarized light can be controlled to have a spot size of no more than 18 mm over a path length of about 1450 mm.

[0050] According to an embodiment of the present disclosure, the horizontal polarized light path in Example One is shown in Table 1 below.

[0051] Table 1

[0052]

[0053] According to the embodiments of this disclosure, the vertically polarized light path in Embodiment 1 is shown in Table 2 below.

[0054] Table 2

[0055]

[0056] In the table, R is the radius of curvature of each lens surface, d is the center-to-center distance between each lens surface, nd is the refractive index of the lens material, and vd is the Abbe number of the lens material.

[0057] The ingenuity of this embodiment lies in its successful compensation for the impact of the optical path difference on the beam parameters by configuring seven relay lenses with specific focal lengths in two optical paths with significantly different optical path lengths. For example... Figure 4 Simulation results show that the spot sizes of horizontally polarized light (blue path) at the incident and exit atomic filters are 15.285 mm and 15.187 mm, respectively, with a maximum beam angle of 2.953°; while the spot sizes of vertically polarized light (green path) at the incident and exit atomic filters are 15.472 mm and 15.286 mm, respectively, with a maximum beam angle of 2.952°. The consistent spot size and angle of both beams in the atomic filters ensure that the two polarized lights pass through the same positions and act on the same atoms, thus guaranteeing the consistency of operating parameters. Simultaneously, simulations show that the spot sizes of the two beams passing through the Gran Thompson prism in both paths are 11.564 mm, 7.705 mm, 11.508 mm, and 12.073 mm, respectively, all smaller than its preferred effective aperture of 14.5 mm, verifying the feasibility of the design.

[0058] Example 2

[0059] This invention is not limited to the specific embodiments described above. Based on the structure of Embodiment 1, those skilled in the art can make various modifications.

[0060] For example, the specific configuration of the focusing collimating lens group 6 can be changed; for instance, the plano-convex lens can be replaced with a biconvex lens.

[0061] The materials for the first GranThompson prism 17, the second GranThompson prism 18, the third GranThompson prism 19, and the fourth GranThompson prism 20 can be either a-BBO or Calcite, depending on the cost and the required extinction ratio.

[0062] The material filled in the single-atom vapor cell 1 can be sodium atoms, rubidium atoms, or potassium atoms, etc., selected according to the specific atmospheric metal layer resonance spectral lines that the lidar needs to detect.

[0063] The focal length, size and type of the relay lens and collimating lens, such as double convex or plano-convex, can be adjusted within a preferred range, for example, R1 focal length 150-300mm, R2-R7 focal length 75-150mm, to adapt to different input fiber parameters or overall system size constraints, as long as the final goal of beam parameter consistency of the two beams at the single-atom cell 1 can be achieved.

[0064] In summary, the embodiment of the present application fundamentally ensures the consistency of the double-channel filtering by adopting a single-atom cell and an opposite incidence light path structure, and innovatively introduces multiple relay lenses to compensate for the optical path difference, solving the inherent technical problem of beam parameter inconsistency in the single-cell opposite incidence scheme. This design not only avoids 50% energy loss, improves transmission efficiency, but also guarantees high background suppression ratio, and has significant advantages in consistency, stability, volume and power consumption compared to the double-cell scheme.

[0065] The above only describes the preferred embodiments of the present application and is not intended to limit the present application. Any modification, equivalent replacement and improvement made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. An ultra-narrowband filter for a fully polarized resonant fluorescence lidar, characterized in that, include: The first polarization beam splitter is used to split the incident beam into first polarized light and second polarized light. A single-atom vapor cell, filled with an atomic medium, is used to provide a site for optical rotation; The working parameter control device is used to control the temperature of the single-atom vapor cell and generate an axial magnetic field, so that the first polarized light and the second polarized light are filtered in the single-atom vapor cell based on the Zeeman effect and the Faraday anomalous dispersion effect. A first polarized light path and a second polarized light path are configured to guide the first polarized light and the second polarized light to be incident on opposite sides of the single-atom vapor cell, with the optical path length of the second polarized light path being longer than that of the first polarized light path. The first and second polarized light paths also include a second polarization beam splitter, a third polarization beam splitter, and four mirrors. The second and third polarization beam splitters are disposed in the optical paths to guide the light beam into and out of the single-atom vapor cell. The beam combiner is a fourth polarization beam splitter. The four mirrors are used to deflect and guide the second polarized light path, causing it to pass through the second and third polarization beam splitters and finally enter the beam combiner. Multiple relay lenses are disposed in the first polarized light optical path and the second polarized light optical path. The multiple relay lenses are configured to compensate for the difference in beam parameters of the first and second polarized light caused by the optical path difference, so that the first polarized light and the second polarized light have consistent beam parameters when entering the single-atom vapor cell. The consistent beam parameters include that the spot size is controlled to be no more than 18 mm. A beam combiner is used to combine the filtered first polarized light and the second polarized light into a single beam for output.

2. The ultra-narrowband filter for a fully polarized resonant fluorescence lidar as described in claim 1, characterized in that, The operating parameter control device includes a dual-channel temperature control module, which is configured to control the temperature of the sidewall and protruding end of the single-atom vapor pool respectively, so as to precisely control the atomic density in the single-atom vapor pool.

3. The ultra-narrowband filter for a fully polarized resonant fluorescence lidar as described in claim 1, characterized in that, It also includes at least two polarization purification prisms, respectively disposed in the first polarization light path and the second polarization light path, for purifying the polarization state before the light beam enters the monoatomic vapor cell and / or after it leaves the monoatomic vapor cell.

4. The ultra-narrowband filter for a fully polarized resonant fluorescence lidar as described in claim 3, characterized in that, The polarization purification prism is a Gran Thompson prism, and the background suppression ratio of the filter is [missing information]. .

5. The ultra-narrowband filter for a fully polarized resonant fluorescence lidar as described in claim 1, characterized in that, It also includes at least one half-wave plate for rotating the polarization state of the first polarized light or the second polarized light.

6. The ultra-narrowband filter for a fully polarized resonant fluorescence lidar as described in claim 5, characterized in that, The half-wave plate is configured to be located simultaneously in the first polarization optical path and the second polarization optical path; In the second polarized light path, the half-wave plate is placed before the incident end of the monoatomic vapor cell to rotate the polarization state of the second polarized light by 90° so that its polarization state when it enters the monoatomic vapor cell is consistent with the polarization state of the first polarized light when it enters the cell. In the first polarized light path, the half-wave plate is disposed after the exit end of the monoatomic vapor cell to rotate the polarization state of the first polarized light emitted from the monoatomic vapor cell by 90°, so that it can be transmitted through the subsequent polarization beam splitter.

7. The ultra-narrowband filter for a fully polarized resonant fluorescence lidar as described in claim 1, characterized in that, It also includes a focusing and collimating lens group, which is positioned before the first polarizing beam splitter to collimate the incident beam.

8. The ultra-narrowband filter for a fully polarized resonant fluorescence lidar as described in claim 1, characterized in that, It also includes a filter, disposed before the first polarizing beam splitter, for broadband filtering of the incident beam.

9. The ultra-narrowband filter for a fully polarized resonant fluorescence lidar as described in claim 1, characterized in that, The operating parameter control device includes at least one annular magnet with a single light-transmitting hole for generating an axial magnetic field parallel to the single-atom vapor pool; and the single-atom vapor pool is filled with sodium atoms, rubidium atoms, or potassium atoms.

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