Integrated cold atom two-dimensional magneto-optical trap optical system

Abstract

An integrated cold atom two-dimensional magneto-optical trap optical system includes a heavy pump cooling module, a push module and a reflection module. The heavy pump cooling module generates collimated and expanded heavy pump cooling light; the push module generates convergent push light; and the reflection module changes the polarization state of the heavy pump cooling light output from the two-dimensional magneto-optical trap cavity and returns it along the original path. At the same time, the V‑GROOVE fiber head is designed to realize the simultaneous input of 4-way light into one optical machine port. The entire system has good optical performance, and the structural design meets the characteristics of high stability, simplification and miniaturization. The present invention is applied to a space ultracold atomic physics experimental device, which can provide the required light field configuration for the two-dimensional magneto-optical trap of cold atoms and generate a high-throughput cold atom beam.

Description

Technical Field

[0001] The invention relates to the technical field of laser cooling, in particular to an integrated cold atom two-dimensional magneto-optical trap optical system. Background Art

[0002] Since the 1980s, the emergence and development of technologies such as laser cooling and evaporative cooling have enabled people to prepare ultracold atomic gases with temperatures as low as nanokelvin in the laboratory. They have been widely used in the research of strong interaction superfluidity, quantum simulation, atom interferometers, cold collisions, quantum information and other fields.

[0003] At present, the main technical route for obtaining ultracold atomic gas is to use a two-dimensional magneto-optical trap (2D-MOT) to collect cold atoms from background vapor and generate a cold atomic beam, inject it into an ultra-high vacuum three-dimensional magneto-optical trap (3D-MOT) for pre-cooling, and then further reduce the temperature by polarization gradient cooling and other methods, and then transfer the atoms to a magnetic trap or optical trap for evaporative cooling to obtain ultracold atomic gas. The laser cooling, state preparation, detection of atoms in the ultracold atomic physics experimental platform, as well as the light source for evaporative cooling and deep cooling experiments based on optical traps are all provided by the optical system, so the optical system is one of the key systems of the space ultracold atomic physics experimental platform. The optical system is mainly composed of three parts: laser, optical platform, and optomechanical system. Among them, the optomechanical system is a key component installed on the physical subsystem to perform operations such as shaping, collimation, imaging, and polarization control on the fed laser.

[0004] The two-dimensional magneto-optical trap of the ultracold atomic physics experimental platform is used for the preparation and pre-cooling of cold atoms. The main purpose is to generate a high-flux cold atom beam through 2D-MOT lateral pre-cooling, thereby increasing the atomic loading rate of 3D-MOT. The light field required for the two-dimensional vacuum chamber includes cooling light, re-pumping light, and pushing light. The cooling light and re-pumping light are used to form a 2D-MOT in the vacuum chamber, thereby 87 Rb and 40 K atoms produce transverse cooling, generating an atomic beam. The push light coincides with the atomic beam and points from the two-dimensional magneto-optical trap cavity to the scientific cavity, with the aim of increasing the axial velocity of the atoms and strengthening the beam intensity.

[0005] Two-dimensional laser cooling has very strict requirements on the intensity distribution uniformity and power stability of the beam. Considering the limited mass and space constraints and the high requirements for optical components, optical path adjustment process and structural materials, all optomechanical structure designs must meet the requirements of integrated stability in order to achieve the purpose of engineering. Summary of the invention

[0006] The present invention provides a high-stability, simple and small integrated cold atom two-dimensional magneto-optical trap optical system. The optical path includes a first heavy pump cooling light, a second heavy pump cooling light, a push light, a first reflected light and a second reflected light. The first heavy pump cooling light and the second heavy pump cooling light have the same optical path design. The heavy pump light and the cooling light are output after being combined by the first optical fiber, and are collimated and expanded through two lenses with focal lengths of -442.53mm and 53.57mm, respectively, to finally output a heavy pump cooling light with a diameter of 13mm; the push light is a convergent light generated by the light output by the second optical fiber through a focusing lens with a focal length of 25.85mm, and its focal spot is located at the right 1 / 4 position of the long axis of the two-dimensional magneto-optical trap cavity, and the spot size is 2.02mm at the position of the differential tube aperture, which can fully cover the aperture (aperture diameter 1.96mm); the V-GROOVE optical fiber head design for the first optical fiber can meet the simultaneous input of 4-way light into one optical machine port. The structural design features of each component corresponding to each light path are that the external dimensions are as small as possible, and that no adjustable elastic mechanical structure is used, thereby improving the stability of the light path and the durability of use.

[0007] The technical solution of the present invention is as follows:

[0008] An integrated cold atom two-dimensional magneto-optical trap optical system, comprising a heavy pump cooling module, a push module, a reflection module, a differential tube module and a two-dimensional magneto-optical trap cavity;

[0009] The heavy pump cooling module is used to output the first heavy pump cooling light and the second heavy pump cooling light;

[0010] The push module is used to output push light;

[0011] The reflection module is used to generate a first reflected light and a second reflected light;

[0012] The first heavy pump cooling light and the second heavy pump cooling light are perpendicular to each other, wherein the first heavy pump cooling light is incident along the -Z direction of the two-dimensional magneto-optical trap cavity, and the second heavy pump cooling light is incident along the X direction of the two-dimensional magneto-optical trap cavity, and after acting on the atoms in the two-dimensional magneto-optical trap cavity, they are output to the reflection module;

[0013] The first reflected light and the second reflected light are respectively along the Z direction and -X direction of the two-dimensional magneto-optical trap cavity, and the light returning along the original path acts on the atoms again;

[0014] The push light is incident along the long axis -Y direction of the two-dimensional magneto-optical trap cavity and is focused in the two-dimensional magneto-optical trap cavity, and then output to the differential tube and the spot size at the small hole of the differential tube covers the small hole.

[0015] The heavy pump cooling module outputs a first heavy pump cooling light and a second heavy pump cooling light, and has the same optical path, including a light beam output by a first optical fiber passing through a first λ / 2 wave plate and a first PBS and being collimated and expanded by collimating mirrors one and two, and the collimated and expanded light beam passing through a first 45° reflector and a second 45° reflector to fold the light path, and the folded light beam passes through a beam splitter prism BS and a first λ / 4 wave plate to reach a two-dimensional magneto-optical trap cavity.

[0016] The first optical fiber includes a V-GROOVE optical fiber head pressure plate, a V-GROOVE optical fiber head base and four parallel optical fiber ribbons, including a 767nm cooling optical fiber, a 780nm cooling optical fiber, a 767nm re-pumping optical fiber and a 780nm re-pumping optical fiber.

[0017] The reflection module outputs the first reflected light and the second reflected light and has the same optical path, including a second λ / 4 wave plate and a 0° reflector. The light beam output by the heavy pump cooling module passes through the second λ / 4 wave plate to change the polarization state of the light beam, and then passes through the 0° reflector along the original path back to the two-dimensional magneto-optical trap cavity to act on the atoms again.

[0018] The push module includes a light beam outputted from the second optical fiber, which passes through a focusing lens and a second λ / 2 wave plate to reach a third 45° reflector, and a light beam reflected by the third 45° reflector passes through a second PBS and a third λ / 4 wave plate, and passes through a two-dimensional magneto-optical trap cavity to reach a differential tube, wherein the focal spot is located at the right 1 / 4 position of the long axis of the two-dimensional magneto-optical trap cavity, and the differential tube is horn-shaped. The light beam transmitted through the third 45° reflector is monitored for beam quality by a first PD detector and a second PD detector through a dichroic mirror.

[0019] In order to achieve the above requirements, the optical components required for the cooling light, re-pumping light, and pushing light involved in 2D-MOT include: collimator, focusing lens, λ / 2 wave plate, λ / 4 wave plate, PBS, 45° reflector, 0° emission mirror, beam splitter BS, and dichroic mirror. The material of the wave plate is Quartz, the material of the focusing lens is SCHOTT N-SF66, and the material of other optical components is Corning 7980. The application wavelengths are 780nm and 767nm. The performance of the incident fiber includes: NA = 0.1043 @ 780nm, NA = 0.1052 @ 767nm, mode field diameter 5.5 ± 1μm @ 850nm.

[0020] In the above scheme, the re-pumping light and the cooling light are combined into one beam before the incident light, and are outputted by the first optical fiber, collimated and expanded by the first λ / 2 wave plate and the first PBS, and the collimated and expanded light beam passes through the first 45° reflector and the second 45° reflector to fold the light path, and the folded light beam passes through the beam splitter BS and the first λ / 4 wave plate to reach the two-dimensional magneto-optical trap cavity. The light beam outputted from the two-dimensional magneto-optical trap cavity passes through the second λ / 4 wave plate to change the polarization state of the light beam, and then passes through the 0° reflector and returns to the two-dimensional magneto-optical trap cavity along the original path, and acts on the atoms again.

[0021] As an optimization of the above solution, the size of the collimator lens 1 is The optical focal length is -442.53@780nm, -442.35@767nm, the convex surface curvature radius is 30.58mm, and the concave surface curvature radius is 25.45mm. The size of the collimator lens 2 is The optical focal length is 53.57@780nm, 53.54@767nm, the convex curvature radius of the front surface is 118.03mm, and the convex curvature radius of the rear surface is 35.39mm. The effective focal length of the collimation system is 61.82@780nm, 61.78@767nm, and the final output beam size is

[0022] As a further optimization of the above solution, the ratios of reflectivity to transmittance of the beam splitter prism BS are 1 / 3, 1 / 2 and total reflection respectively, and the overall shape is a trapezoid with a length of 36 mm and 54 mm, and a width and height of 18 mm.

[0023] As a further optimization of the above solution, the 0° reflector is shaped as two semicircles with a radius of 9.75 mm on both sides, a rectangle with a length of 34.4 mm and a width of 9.75 mm in the middle, and an overall thickness of 54±0.1 mm.

[0024] In the above scheme, a V-GROOVE fiber optic head is designed, which is characterized by satisfying the requirement of simultaneously inputting four-way light into one optical machine port, including a 1ⅹ4 fiber optic ribbon, 4 optical fibers arranged in parallel, and 4 optical fibers arranged vertically in a V-groove.

[0025] As a further optimization of the above scheme, the core arrangement position of the 780nm heavy pump light must be ensured to be at the incident end of the two-dimensional magneto-optical trap cavity, and the slow axis is perpendicular to the V-groove base, so it is placed at the optical fiber (31). The heavy pump light and the cooling light deviate by about 0.5mm, so the 780nm cooling light is placed at the optical fiber (29). The 767nm heavy pump light and the 780nm heavy pump light are spaced 0.25mm apart, so the 767nm heavy pump light is placed at the optical fiber (30), and the 767nm cooling light is placed at the optical fiber (28).

[0026] In the above scheme, a converging push light is designed. The light beam outputted by the second optical fiber passes through the focusing lens and the second λ / 2 wave plate to reach the third 45° reflector. The light beam reflected by the third 45° reflector passes through the second PBS and the third λ / 4 wave plate, and reaches the differential tube through the two-dimensional magneto-optical trap cavity. Its focal spot is located at the right 1 / 4 position of the long axis of the two-dimensional magneto-optical trap cavity, and the differential tube is horn-shaped. The light beam transmitted by the third 45° reflector is monitored by the first PD detector and the second PD detector through the dichroic mirror for beam quality.

[0027] As an optimization of the above solution, the size of the focusing lens is The optical focal length is 25.85@780nm, 25.81@767nm, the front surface is flat, the back surface is convex, and the convex curvature radius is 23.17mm.

[0028] As a further optimization of the above solution, the focal spot size of the light beam is @780nm, @767nm, the spot size at the differential tube aperture is @780nm, @767nm.

[0029] In the above scheme, a mechanical structure of an integrated cold atom two-dimensional magneto-optical trap optical system is designed, and each component meets the following conditions:

[0030] 1) In the design of each subsystem, a fixed length spacer is used to ensure a fixed distance between lenses.

[0031] 2) For the λ / 4 wave plate mechanical structure and the λ / 2 wave plate mechanical structure, a rotatable small hole and a fixed hole are reserved.

[0032] 3) Each small mechanical structure has a glue injection hole to facilitate the glue curing between the lens and the mechanical structure.

[0033] 4) All mechanical structures are made of titanium alloy. The non-metallic materials used in physical optics include quartz glass windows and radiation-resistant glass lenses, BK7 optical glass, AB two-component epoxy structural adhesive, GD414 silicone rubber, etc.

[0034] Compared with the prior art, the technical effects of the present invention are:

[0035] 1) The heavy pump cooling light of the present invention folds the light path through the reflection module on the basis of fully meeting the spot size and polarization requirements required by physical experiments, greatly improving the integration of the light path;

[0036] 2) The V-GROOVE optical fiber head of the present invention includes a 1×4 optical fiber ribbon, 4 optical fibers are arranged in parallel, and 4 optical fibers are arranged vertically in the V-groove, so as to meet the requirement of inputting four-way light into one optical-mechanical structure at the same time;

[0037] 3) The push light of the present invention adopts a focused beam, and the focal spot is located at the right 1 / 4 position of the long axis of the two-dimensional magneto-optical trap cavity. The spot size can cover the size of the small hole when it is continuously transmitted to the position of the differential tube small hole, which greatly improves the axial velocity of the atoms and strengthens the beam intensity;

[0038] 4) The mechanical structure design of the present invention meets the requirements of high stability, simplification and miniaturization of engineering optical machinery;

[0039] 5) The overall physical optomechanical design also meets the requirements of fewer optical components, shorter total optical path, smaller volume and loss, and lighter total weight. It greatly improves the integration of all optomechanical components and further realizes the engineering of laser cooling technology, laying a solid foundation for the development of ultracold atomic physics from scientific experimental research to engineering applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 This is the total optical path diagram required for 2D-MOT

[0041] Figure 2 This is the design diagram of the light path of the heavy pump cooling light

[0042] Figure 3 It is the design diagram of the push light path

[0043] Figure 4 This is the design diagram of V-GROOVE fiber head (a) mechanical structure diagram; (b) fiber distribution diagram

[0044] Figure 5 It is the overall diagram of the mechanical structure design model required for 2D-MOT

[0045] Figure 6 This is the cross-sectional view of the mechanical structure design required for 2D-MOT (a) Heavy pump cooling light cross-sectional view; (b) Push light cross-sectional view

[0046] In the figure: 1 - heavy pump cooling module; 2 - push module; 3 - reflection module; 4 - differential tube module; 5 - two-dimensional magneto-optical trap cavity; 6 - first optical fiber; 7 - first λ / 2 wave plate; 8 - first PBS; 9 - collimator mirror 1; 10 - collimator mirror 2; 11 - first 45° reflector; 12 - second 45° reflector; 13 - beam splitter BS; 14 - first λ / 4 wave plate; 15 - second λ / 4 wave plate; 16 - 0° reflector; 17 - second optical fiber; 18 - focusing lens; 19 - second λ / 2 wave plate; 20 - third 45° reflector; 21 - double Color mirror; 22 - first PD detector; 23 - second PD detector; 24 - second PBS; 25 - third λ / 4 wave plate; 26 - V-GROOVE fiber head pressure plate; 27 - V-GROOVE fiber head base; 28 - 767nm cooling fiber; 29 - 780nm cooling fiber; 30 - 767nm heavy pumping fiber; 31 - 780nm heavy pumping fiber; 32 - first heavy pumping cooling optomechanical structure; 33 - second heavy pumping cooling optomechanical structure; 34 - push optomechanical structure; 35 - first reflection optomechanical structure; 36 - second reflection optomechanical structure; DETAILED DESCRIPTION

[0047] In order to enable those skilled in the art to better understand the technical solution of the present invention, the present invention is clearly and completely described below in conjunction with implementation examples and drawings, but this should not limit the protection scope of the present invention.

[0048] See also Figure 1 . Figure 1 It is the total optical path diagram required in the two-dimensional magneto-optical trap. It includes a heavy pump cooling module 1, a push module 2, a reflection module 3, a differential tube module 4 and a two-dimensional magneto-optical trap cavity 5; the heavy pump cooling module 1 is used to output the first heavy pump cooling light and the second heavy pump cooling light, the first heavy pump cooling light and the second heavy pump cooling light are perpendicular to each other, wherein the first heavy pump cooling light is incident along the -Z direction of the two-dimensional magneto-optical trap cavity 5, and the second heavy pump cooling light is incident along the X direction of the two-dimensional magneto-optical trap cavity 5, and after acting on the atoms in the two-dimensional magneto-optical trap cavity 5, they are respectively emitted to the reflection module 3; the reflection module 3 is used to generate the first reflected light and the second reflected light, the first reflected light and the second reflected light are respectively along the Z direction and -X direction of the two-dimensional magneto-optical trap cavity 5, and the light returning along the original path through the reflection module 3 acts on the atoms again; the push module 2 is used to output the push light, the push light is incident along the long axis -Y direction of the two-dimensional magneto-optical trap cavity 5 and after focusing in the two-dimensional magneto-optical trap cavity 5, it is output to the differential tube 4 and the spot size at the small hole of the differential tube 4 covers the small hole.

[0049] See also Figure 2 . Figure 2It is the optical path design diagram of the heavy pump cooling light. It includes the first heavy pump cooling light and the second heavy pump cooling light, and the two lights have the same optical path design. The heavy pump light and the cooling light are combined into one light through the first optical fiber 6 before being incident. The light beam output by the first optical fiber 6 passes through the first λ / 2 wave plate 7 and the first PBS 8 and is collimated and expanded by the collimator 1 9 and the collimator 2 10. The final output beam size is The effective focal length of the collimation system is 61.82@780nm and 61.78@767nm. The light beam after collimation and expansion passes through the first 45° reflector 11 and the second 45° reflector 12 to fold the light path. The folded light beam passes through the beam splitter BS13 with a reflectivity to transmittance ratio of 1 / 3, 1 / 2 and full reflection and the first λ / 4 wave plate 14 to reach the two-dimensional magneto-optical trap cavity 5; the light beam output by the two-dimensional magneto-optical trap cavity 5 passes through the second λ / 4 wave plate 15 to change the polarization state of the light beam, and then passes through the 0° reflector 16 to return to the two-dimensional magneto-optical trap cavity 5 along the original path to act on the atoms again.

[0050] See also Figure 3 . Figure 3 The light beam outputted by the second optical fiber 17 passes through the focusing lens 18 and the second λ / 2 wave plate 19 to reach the third 45° reflector 20. The light beam reflected by the third 45° reflector 20 passes through the second PBS 24 and the third λ / 4 wave plate 25 to reach the differential tube 4. Its focal spot is located at the right 1 / 4 position of the long axis of the two-dimensional magneto-optical trap cavity 5. The focal spot size is @780nm, @767nm, the spot size at the 4th hole of the differential tube is @780nm, @767nm. The differential tube 4 is in a trumpet shape. The light beam transmitted by the third 45° reflector 20 is monitored for beam quality by the first PD detector 22 and the second PD detector 23 through the dichroic mirror 21.

[0051] See also Figure 4 . Figure 4 This is the design diagram of the V-GROOVE fiber head. (a) is the mechanical structure diagram; (b) is the fiber distribution diagram. It includes 1×4 fiber ribbons, 4 optical fibers are arranged in parallel, and 4 optical fibers are arranged vertically in the V-groove, passing through the same optical machine port. The core arrangement position of the 780nm re-pumping light must be guaranteed to be at the incident end of the two-dimensional magneto-optical trap cavity, and the slow axis is perpendicular to the V-groove base, so it is placed at the fiber 31 position. The re-pumping light and the cooling light deviate by about 0.5mm, so the 780mm cooling light is placed at the fiber 29 position. The interval between the 767nm re-pumping light and the 780nm re-pumping light is 0.25mm, so the 767nm re-pumping light is placed at the fiber 30 position, and the 767nm cooling light is placed at the fiber 28 position.

[0052] See also Figure 5 . Figure 5 This is the overall diagram of the 2D-MOT optical machine structure. The overall two-dimensional cavity is a rectangular parallelepiped structure, and the volume of the inner wall of the cavity is about 58mm×40mm×40mm. There are four optical windows on the side of the cavity, with a size of 58mm×18mm, including the first heavy pump cooling optical machine 32, the second heavy pump cooling optical machine 33, the first reflection optical machine 35 and the second reflection optical machine 36. Together with the push optical machine 34, there are a total of five optical windows, each of which is coated with an anti-reflection film T>99.8%, with a wavelength of 765nm-781nm.

[0053] See also Figure 6 . Figure 6 It is a cross-sectional view of the structural design, where (a) is a cross-sectional view of the heavy pump cooling light; (b) is a cross-sectional view of the push light. In the entire optomechanical structure design, the design of each small system uses a fixed length spacer to ensure a fixed distance between the lenses. For the λ / 4 wave plate structure, the λ / 2 wave plate structure has a rotatable small hole. Each small mechanical structure has a glue injection hole to facilitate the glue curing between the lens and the mechanical structure. All optomechanical structures are made of titanium alloy. The non-metallic materials used in physical optics include quartz glass windows and radiation-resistant glass lenses, BK7 optical glass, AB two-component epoxy structural adhesive, GD414 silicone rubber, etc.

[0054] The method specifically used in the present invention is:

[0055] This system is an integrated optimization design of a two-dimensional magneto-optical trap optical-mechanical system applied to an ultracold atomic physics experimental platform, which is used to obtain a high-flux cold atom beam to meet the high loading rate of cold atoms in the subsequent three-dimensional magneto-optical trap. It mainly includes: designing an optical path system with good optical performance; designing a structural system that meets the requirements of high stability, simplification, and miniaturization of engineering optical machinery. This invention provides an experimental basis and design reference for the development of ultracold atomic physics from scientific experimental research to engineering applications.

Claims

1. An integrated cold atom two-dimensional magneto-optical trap optical system, comprising a heavy pump cooling module (1), a push module (2), a reflection module (3), a differential tube module (4) and a two-dimensional magneto-optical trap cavity (5); The heavy pump cooling module (1) is used to output a first heavy pump cooling light and a second heavy pump cooling light; The pushing module (2) is used to output pushing light; The reflection module (3) is used to generate a first reflected light and a second reflected light; The first heavy pump cooling light and the second heavy pump cooling light are perpendicular to each other, wherein the first heavy pump cooling light is incident along the -Z direction of the two-dimensional magneto-optical trap cavity (5), and the second heavy pump cooling light is incident along the X direction of the two-dimensional magneto-optical trap cavity (5), and after acting on the atoms in the two-dimensional magneto-optical trap cavity (5), they are output to the reflection module (3); The first reflected light and the second reflected light are respectively along the Z direction and -X direction of the two-dimensional magneto-optical trap cavity (5), and the light returning along the original path acts on the atoms again; The push light is incident along the long axis -Y direction of the two-dimensional magneto-optical trap cavity (5) and is focused in the two-dimensional magneto-optical trap cavity (5), and then output to the differential tube (4), and the light spot size at the small hole of the differential tube (4) covers the small hole.

2. The integrated cold atom two-dimensional magneto-optical trap optical system according to claim 1, characterized in that: The heavy pump cooling module (1) outputs a first heavy pump cooling light and a second heavy pump cooling light and has the same optical path, including a light beam output by a first optical fiber (6) passing through a first λ / 2 wave plate (7) and a first PBS (8) and being collimated and expanded by a collimator 1 (9) and a collimator 2 (10), the collimated and expanded light beam passing through a first 45° reflector (11) and a second 45° reflector (12) for optical path folding, and the folded light beam passing through a beam splitter BS (13) and a first λ / 4 wave plate (14) to reach a two-dimensional magneto-optical trap cavity (5).

3. The integrated cold atom two-dimensional magneto-optical trap optical system according to claim 2, characterized in that: The first optical fiber (6) includes a V-GROOVE optical fiber head pressure plate (26), a V-GROOVE optical fiber head base (27) and four parallel optical fiber ribbons, including a 767nm cooling optical fiber (28), a 780nm cooling optical fiber (29), a 767nm re-pumping optical fiber (30) and a 780nm re-pumping optical fiber (31).

4. The integrated cold atom two-dimensional magneto-optical trap optical system according to claim 1, characterized in that: The reflection module (3) outputs a first reflected light and a second reflected light and has the same optical path, including a second λ / 4 wave plate (15) and a 0° reflector (16). The light beam output by the heavy pump cooling module (1) passes through the second λ / 4 wave plate (15) to change the polarization state of the light beam, and then passes through the 0° reflector (16) along the original path back to the two-dimensional magneto-optical trap cavity (5), and acts on the atoms again.

5. The integrated cold atom two-dimensional magneto-optical trap optical system according to claim 1, characterized in that: The push module (2) comprises a light beam output by the second optical fiber (17) passing through a focusing lens (18) and a second λ / 2 wave plate (19) to reach a third 45° reflector (20); the light beam reflected by the third 45° reflector (20) passes through a second PBS (24) and a third λ / 4 wave plate (25), passes through a two-dimensional magneto-optical trap cavity (5) to reach a differential tube (4); the focal spot is located at the right 1 / 4 position of the long axis of the two-dimensional magneto-optical trap cavity (5); the differential tube (4) is horn-shaped; the light beam transmitted by the third 45° reflector (20) passes through a dichroic mirror (21) and is monitored by a first PD detector (22) and a second PD detector (23) for beam quality.

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