A beam expander and collimator for a cold atom interferometer and its usage method

Abstract

The present invention provides a beam expander and collimator for a cold atom interferometer and a method for using the same, comprising: a polarization-maintaining optical fiber, a half-wave liquid crystal variable retarder, a lens group, a first 1 / 2 wave plate, a beam splitting mirror, and an optical power monitoring module; the polarization-maintaining optical fiber outputs a divergent linearly polarized laser beam, which undergoes real-time polarization state switching through the half-wave liquid crystal variable retarder, is expanded and collimated into a parallel beam through the lens group, has its initial polarization axis angle adjusted through the first 1 / 2 wave plate, and is split into a reflected light and a transmitted light through the beam splitting mirror; wherein, the reflected light enters the vacuum chamber, and the transmitted light enters the optical power monitoring module. The present invention can switch the polarization state of the laser beam in real time and monitor the changes in the polarization state and optical power of the laser beam in real time. By using one beam expander and collimator, laser beams with different polarization states required in the atomic interference measurement process can be realized, greatly reducing the complexity of cold atom trapping and interference optical arrangement, and at the same time effectively reducing the volume and application cost of the cold atom interferometer.

Description

Technical Field

[0001] The present invention relates to the technical field of cold atom technology, and particularly to a beam expander collimator for a cold atom interferometer and a method for using the same. Background Art

[0002] Cold atom technology is a technology that realizes the research of atomic physical properties and precision measurement by manipulating the change of atomic quantum states, and is applied in many fields such as quantum communication, atomic clocks, atomic gravimeters, and quantum simulations. The cold atom precision measurement process requires circularly polarized cooling light to meet the preparation needs of cold atom clouds, and linearly polarized Raman light to meet the pulsed laser requirements of cold atom interference. The measurement results are presented in the form of atomic quantum state distributions, and their accuracy is directly related to the collimation characteristics, polarization characteristics, and power stability of the laser beam.

[0003] Generally, the laser generated by the laser system commonly used for cold atom preparation is first split into six beams by an optical fiber beam splitter, and then input into six beam expander collimators through six polarization-maintaining optical fibers to form parallel beams and then cool the atoms. Therefore, the performance of the beam expander collimator is directly related to the quality of the prepared cold atom cloud.

[0004] Currently, the widely used beam expander collimators on the market have the following deficiencies:

[0005] 1. A polarizer is not installed in the optical path to optimize the polarization state of the input laser beam, and only the polarization state of the output beam can be adjusted through a wave plate. Therefore, the polarization of the input laser beam cannot be accurately switched and maintained, so that the polarization state of the output laser beam will change significantly with the environment, and it is difficult to generate a super-low temperature cold atom cloud with a stable number of atoms; 2. The traditional beam expander collimator cannot monitor the polarization state and power change of the output laser beam in real time; 3. The traditional beam expander collimator cannot realize the real-time switching of cooling light and Raman light in the same beam expander, resulting in a series of problems such as complex cold atom trapping and interference optical arrangement, large volume, and high application cost.

[0006] In view of this, overcoming the defects of the above-mentioned existing technologies is an urgent problem to be solved in this technical field. Summary of the Invention

[0007] The present invention preferably provides a solution to the technical problem that the traditional beam expander collimator for a cold atom interferometer cannot accurately switch and maintain the polarization state of the output laser beam.

[0008] The present invention further provides a solution to the technical problem that the traditional beam expander collimator for a cold atom interferometer cannot monitor the polarization state and power change of the output laser beam.

[0009] The present invention further provides a solution to the technical problem that the traditional beam expander and collimator for a cold atom interferometer cannot realize real-time switching of cooling light and Raman light in the same beam expander.

[0010] To solve the above technical problems, the present invention adopts the following technical solutions:

[0011] In a first aspect, the present invention provides a beam expander and collimator for a cold atom interferometer, comprising:

[0012] A polarization-maintaining fiber 1, a half-wave liquid crystal variable retarder 2, a lens group 3, a first 1 / 2 wave plate 4, a beam splitting mirror 5, and an optical power monitoring module;

[0013] The polarization-maintaining fiber 1 outputs a divergent linearly polarized laser beam, which undergoes real-time polarization state switching through the half-wave liquid crystal variable retarder 2, is expanded and collimated by the lens group 3 to form a parallel beam, undergoes initial polarization axis angle adjustment through the first 1 / 2 wave plate 4, and is split into a reflected light and a transmitted light by the beam splitting mirror 5; wherein, the reflected light enters the vacuum chamber, and the transmitted light enters the optical power monitoring module.

[0014] Preferably, the optical power monitoring module comprises: a first photodetector 6, a second photodetector 7, a polarization beam splitting prism 8, and a second 1 / 2 wave plate 9;

[0015] The transmitted light is split by the second 1 / 2 wave plate 9 and the polarization beam splitting prism 8 and respectively reaches the first photodetector 6 and the second photodetector 7.

[0016] Preferably, the half-wave liquid crystal variable retarder 2 is electrically controlled, and under the control of the applied alternating voltage, real-time switching of the polarization state of the output laser beam is achieved.

[0017] Preferably, the polarization-maintaining fiber 1 is fixed in the lens barrel through a fiber flange, and the fiber flange can move back and forth in the lens barrel to adjust the optical distance between the polarization-maintaining fiber 1 and the lens group 3.

[0018] Preferably, the polarization-maintaining fiber 1 is a single-mode polarization-maintaining fiber 1, and the lens group 3 is a doublet lens.

[0019] Preferably, the reflectivity to transmittance ratio of the beam splitting mirror 5 is 99:1.

[0020] Preferably, the polarization beam splitting prism 8 comprises a pair of high-precision right-angle prisms, and a polarization beam splitting dielectric film is coated on the hypotenuse of one of the high-precision right-angle prisms.

[0021] In a second aspect, the present invention provides a method for using a beam expander and collimator for a cold atom interferometer, using the beam expander and collimator for a cold atom interferometer described in the first aspect, the method comprising:

[0022] Output the divergent linearly polarized laser beam to the half-wave liquid crystal variable retarder 2 through the polarization-maintaining optical fiber 1;

[0023] After the polarization state of the laser beam is switched in real time by the half-wave liquid crystal variable retarder 2, output it to the lens group 3;

[0024] After the laser beam is expanded and collimated into a parallel beam by the lens group 3, output it to the first 1 / 2 wave plate 4;

[0025] After adjusting the initial polarization axis angle of the linearly polarized laser beam through the first 1 / 2 wave plate 4, output it to the optical beam splitter mirror 5;

[0026] Divide the laser beam into reflected light and transmitted light through the beam splitter mirror 5;

[0027] Output the reflected light to the vacuum chamber for the measurement of atomic interference, and output the transmitted light to the optical power monitoring module for the real-time monitoring of the polarization state and optical power change of the collimated beam.

[0028] Preferably, the optical power monitoring module includes: a first photodetector 6, a second photodetector 7, a polarization beam splitter prism 8, and a second 1 / 2 wave plate 9;

[0029] The transmitted light is split by the second 1 / 2 wave plate 9 and the polarization beam splitter prism 8 and reaches the first photodetector 6 and the second photodetector 7 respectively.

[0030] Preferably, the step of outputting the transmitted light to the optical power monitoring module for the real-time monitoring of the polarization state and optical power change of the collimated beam includes:

[0031] Judge whether the output collimated beam is circularly polarized light or linearly polarized light by the change of the ratio of the optical powers obtained by the first photodetector 6 and the second photodetector 7;

[0032] Judge whether the optical power of the output collimated beam changes by the change of the sum of the optical powers obtained by the first photodetector 6 and the second photodetector 7.

[0033] Aiming at the deficiencies in the prior art, the beneficial effects that the present invention can achieve are:

[0034] Through the provided half-wave liquid crystal variable retarder, the present invention can switch the polarization state of the laser beam incident through the polarization-maintaining optical fiber in real time. The switching process uses an electric control method to realize the circularly polarized light required for cold atom trapping and the linearly polarized light required for cold atom interference, which can greatly improve the switching efficiency.

[0035] Furthermore, through the optical signal intensity obtained by the provided photodetectors, the present invention can monitor the polarization state and optical power change of the collimated beam in real time to ensure the long-term stability of the optical power of the laser beam.

[0036] Furthermore, while the present invention realizes real-time switching of the polarization state of the laser beam and real-time monitoring of the change in optical power, it also realizes the output of the cooling light and the Raman light sharing the same beam expander and collimator.

[0037] Generally speaking, the present invention can switch the polarization state of the laser beam in real time and monitor the changes in the polarization state and optical power of the laser beam in real time. Compared with the configuration of multiple beam expanders and collimators required by traditional cold atom interferometers, the present invention only needs to apply one beam expander and collimator to realize different polarization state laser beams required in the atomic interference measurement process, greatly reducing the complexity of cold atom trapping and interference optical arrangement, and at the same time effectively reducing the volume and application cost of the cold atom interferometer. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required to be used in the embodiments of the present invention will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0039] Figure 1 is an overall schematic diagram of a beam expander and collimator for a cold atom interferometer provided in Embodiment 1;

[0040] Figure 2 is a partial schematic diagram of a beam expander and collimator for a cold atom interferometer provided in Embodiment 1;

[0041] Figure 3 is a diagram showing the usage method of a beam expander and collimator for a cold atom interferometer provided in Embodiment 2;

[0042] Figure 4 is a diagram showing the usage method of a beam expander and collimator for a cold atom interferometer provided in Embodiment 2.

[0043] In the drawings, the same reference numerals are used to represent the same components or structures, where:

[0044] 1 - polarization-maintaining fiber, 2 - half-wave liquid crystal variable retarder, 3 - lens group, 4 - first 1 / 2 wave plate, 5 - beam splitting mirror, 6 - first photodetector, 7 - second photodetector, 8 - polarization beam splitter prism, 9 - second 1 / 2 wave plate; A1 - reflected light, A2 - transmitted light. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0045] In order to make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0046] In the description of the present invention, the orientation or positional relationship indicated by the terms "inner", "outer", "longitudinal", "transverse", "upper", "lower", "top", "bottom", etc. is based on the orientation or positional relationship shown in the drawings. It is only for the convenience of describing the present invention and does not require the present invention to be constructed and operated in a specific orientation. Therefore, it should not be construed as a limitation to the present invention.

[0047] In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0048] Embodiment 1:

[0049] In order to solve the technical problem that the traditional beam expander and collimator for a cold atom interferometer cannot accurately switch and maintain the polarization state of the output laser beam, Embodiment 1 of the present invention provides a beam expander and collimator for a cold atom interferometer, as Figure 1 shown, including: a polarization-maintaining fiber 1, a half-wave liquid crystal variable retarder 2, a lens group 3, a first 1 / 2 wave plate 4, a beam-splitting mirror 5, and an optical power monitoring module; in the optical path transmission, the polarization-maintaining fiber 1 outputs a divergent linearly polarized laser beam, which is subjected to real-time polarization state switching by the half-wave liquid crystal variable retarder 2, expanded and collimated by the lens group 3 to form a parallel beam, and the initial polarization axis angle is adjusted by the first 1 / 2 wave plate 4, and then is split by the beam-splitting mirror 5 into a reflected light (corresponding to Figure 1 A1 therein) and a transmitted light (corresponding to Figure 1 A2 therein); wherein, the reflected light enters the vacuum cavity, and the transmitted light enters the optical power monitoring module; in specific applications, the output linearly polarized laser beam is a 780 nm collimated laser beam, corresponding to the transition energy level of rubidium (Rb) atoms.

[0050] In this embodiment, the polarization-maintaining fiber 1 can be a single-mode polarization-maintaining fiber or a multi-mode polarization-maintaining fiber. In order to reduce the dispersion in the laser beam transmission process and achieve long-distance transmission of the laser beam, in actual applications, preferably, the polarization-maintaining fiber 1 is a single-mode polarization-maintaining fiber 1; the polarization-maintaining fiber 1 is fixed in the lens barrel through a fiber flange, and the fiber flange can move back and forth in the lens barrel to adjust the optical interval between the polarization-maintaining fiber 1 and the lens group 3. Specifically, the fiber flange can also adjust the tilt angle within a certain range. After adjustment, finally, the divergence angle of the laser beam output by the polarization-maintaining fiber 1 can be made as small as possible, and thus the collimation characteristic of the laser beam can be made as good as possible; in specific applications, the polarization-maintaining fiber 1 outputs a 780 nm laser beam, and its mode field diameter is 5.0 ± 1.0 um.

[0051] As one of the implementation manners, the half-wave liquid crystal variable retarder 2 is designed to be electrically controlled. Under the control of the applied alternating voltage, it can realize the real-time switching of the polarization state of the output laser beam, that is, the laser beam can be switched into a linearly polarized state or a circularly polarized state in real time by an electrical control method. Its working principle is as follows: The nematic liquid crystal cell filled with the liquid crystal molecular solution is used as a variable wave plate. The two parallel surfaces of the liquid crystal cell wall are coated with transparent conductive films, and a voltage can be applied to the liquid crystal cell. After applying the alternating voltage, the liquid crystal molecules will change their default arrangement direction according to the applied voltage. Since there are no moving parts, a fast response time in the microsecond level can be achieved. Therefore, by changing the applied voltage, the retardation of the liquid crystal variable retarder can be actively controlled, and then the switching of the polarization state of the laser beam can be realized. In specific applications, the clear aperture of the half-wave liquid crystal variable retarder 2 is The wavelength λ ranges from 650 to 1050 nm, and the retardation ranges from 0 nm to λ / 2.

[0052] In this embodiment, in order to avoid the difficulty in correcting the uncertain aberrations of the half-wave liquid crystal variable retarder 2 caused by a complex optical system, preferably, the lens group 3 is a doublet lens, which is obtained by gluing two lenses together, so that the lens group 3 has the advantages of a short focal length and a large magnification. During the correction process, the uncertain aberrations of the half-wave liquid crystal variable retarder 2 can be compensated by adjusting the distance between the fiber optic flange and the lens group 3. In specific applications, the diameter of the lens 3 is 38 mm, the clear aperture is greater than 200 mm, the effective focal length is 160 mm, and the diameter of the laser beam after beam expansion and collimation by the lens group 3 is 34 mm.

[0053] In this embodiment, the first 1 / 2 wave plate 4 and the second 1 / 2 wave plate 9 are made of a quartz crystal substrate, and the crystal thickness is just such that the optical path difference between the o-ray and the e-ray is λ / 2. When the first 1 / 2 wave plate 4 or the second 1 / 2 wave plate 9 rotates by an angle θ, the linearly polarized light is still linearly polarized light after passing through the first 1 / 2 wave plate 4 or the second 1 / 2 wave plate 9, but the vibration direction rotates by 2θ with respect to the original direction. The circularly polarized light is still circularly polarized light after passing through the first 1 / 2 wave plate 4 or the second 1 / 2 wave plate 9, but the vibration direction is opposite to the original direction. Therefore, by rotating the first 1 / 2 wave plate 4 or the second 1 / 2 wave plate 9, the vibration direction of the linearly polarized light can be changed, so that the vibration direction of the laser beam incident on the beam splitting mirror 5 is parallel or perpendicular to the cross section of the beam splitting mirror 5. To a certain extent, the depolarization degree caused by reflection and transmission can be reduced.

[0054] In this embodiment, the beam-splitting mirror 5 is made of a fused silica substrate, and its shape is a rectangular planar beam-splitting mirror. Specifically, when applied, a polarization-maintaining beam-splitting film with a wavelength of 780 nm is deposited on the beam-splitting surface of the beam-splitting mirror 5. At the same time, an anti-reflection film with a wavelength of 780 nm is deposited on the second surface; in order to output as much laser as possible into the vacuum chamber for atomic interference measurement, preferably, the ratio of the reflectance to the transmittance of the beam-splitting mirror 5 is 99:1, that is, the reflectance of the beam-splitting mirror 5 is 99%, and the transmittance is 1%. That is to say, 99% of the optical power of the laser beam becomes the reflected light A1, and 1% of the optical power becomes the transmitted light A2, where the reflected light A1 and the transmitted light A2 are at 90 degrees.

[0055] In this embodiment, the polarization beam-splitting prism 8 includes a pair of high-precision right-angled prisms. In order to increase the reflectance of the polarization beam-splitting prism 8, a polarization beam-splitting dielectric film is deposited on the hypotenuse of one of the high-precision right-angled prisms, so as to achieve high reflectance in a specific wavelength range; specifically, when applied, the side length of the high-precision right-angled prism selected for the polarization beam-splitting prism 8 is 1 inch. Similarly, the polarization beam-splitting prism 8 is also glued together.

[0056] In order to solve the technical problem that the traditional beam expander and collimator for a cold atom interferometer cannot monitor the polarization state and power change of the output laser beam, as Figure 2 shown, the optical power monitoring module includes: a first photodetector 6, a second photodetector 7, a polarization beam-splitting prism 8, and a second 1 / 2 wave plate 9; among them, the first photodetector 6 and the second photodetector 7 can convert optical signals into electrical signals. The polarization beam-splitting prism 8 and the second 1 / 2 wave plate 9 form a beam-splitting device. The transmitted light is split by the second 1 / 2 wave plate 9 and the polarization beam-splitting prism 8 and reaches the first photodetector 6 and the second photodetector 7 respectively, and the polarization state and optical power change of the collimated beam are monitored in real time through the first photodetector 6 and the second photodetector 7; specifically, when applied, the first photodetector 6 and the second photodetector 7 are visible light photodetectors doped with silicon (Si) material, meeting the 780 nm fluorescence detection requirements.

[0057] Embodiment 1 provides a beam expander and collimator for a cold atom interferometer, which realizes the real-time switching of the polarization state of the laser beam and the real-time monitoring of the optical power change, and also realizes the integration of the on-line switching and monitoring of the laser beam, improving the operation efficiency. It also realizes the real-time switching of the circularly polarized light required for cold atom trapping and the linearly polarized light required for cold atom interference in the same beam expander and collimator, reducing the complexity of the cold atom trapping and interference optical arrangement, and reducing the volume and application cost of the cold atom interferometer.

[0058] Embodiment 2:

[0059] Based on the same general inventive concept as in Embodiment 1, Embodiment 2 provides a method for using a beam expander and collimator for a cold atom interferometer, using the beam expander and collimator for a cold atom interferometer described in Embodiment 1, as Figure 3 shown, the method comprising:

[0060] S10, outputting a divergent linearly polarized laser beam through a polarization-maintaining fiber 1 to a half-wave liquid crystal variable retarder 2.

[0061] In the actual application process, the polarization-maintaining fiber 1 is preferably a single-mode polarization-maintaining fiber.

[0062] S20, outputting the laser beam to a lens group 3 after the polarization state of the laser beam is switched in real time by the half-wave liquid crystal variable retarder 2.

[0063] In the actual application process, the half-wave liquid crystal variable retarder 2 is preferably electrically controlled.

[0064] S30, expanding and collimating the laser beam through the lens group 3 to form a parallel beam and then outputting it to a first 1 / 2 wave plate 4.

[0065] In the actual application process, the lens group 3 is preferably a doublet lens.

[0066] S40, adjusting the initial polarization axis angle of the linearly polarized laser beam through the first 1 / 2 wave plate 4 and then outputting it to an optical beam splitter mirror 5.

[0067] S50, splitting the laser beam into a reflected light and a transmitted light through the beam splitter mirror 5.

[0068] In the actual application process, the ratio of the reflectance to the transmittance of the beam splitter mirror 5 is preferably 99:1.

[0069] S60, outputting the reflected light to a vacuum chamber for atomic interference measurement, and outputting the transmitted light to an optical power monitoring module for real-time monitoring of the polarization state and optical power change of the collimated beam.

[0070] Wherein, the optical power monitoring module includes: a first photodetector 6, a second photodetector 7, a polarization beam splitter prism 8 and a second 1 / 2 wave plate 9; the transmitted light is split by the second 1 / 2 wave plate 9 and the polarization beam splitter prism 8 and respectively reaches the first photodetector 6 and the second photodetector 7.

[0071] In the actual application process, the polarization beam splitter prism 8 is formed by gluing a pair of high-precision right-angle prisms, and a polarization beam splitting dielectric film is coated on the hypotenuse of one of the high-precision right-angle prisms.

[0072] Due to the different polarization states of the laser beam, the optical powers reaching the first photodetector 6 and the second photodetector 7 are different. In the specific implementation process, the transmitted light is output to the optical power monitoring module for real-time monitoring of the polarization state and optical power change of the collimated beam. As Figure 4 shown, it includes:

[0073] S61, judging whether the output collimated beam is circularly polarized light or linearly polarized light by the change of the ratio of the optical powers obtained by the first photodetector 6 and the second photodetector 7.

[0074] Electrically controlled switching by the half-wave liquid crystal variable retarder 2. When the laser beam is linearly polarized light, the laser beam passes through the rotating second 1 / 2 wave plate 9 to make its vibration direction parallel to the cross section of the polarization beam splitter prism 8, then the optical power W of the transmitted light A2 can all pass through the polarization beam splitter prism 8. At this time, the optical power W1 received by the first photodetector 6 is 0, and the optical power W2 received by the second photodetector 7 is W; Electrically controlled switching by the half-wave liquid crystal variable retarder 2. When the laser beam is circularly polarized light, the laser beam is evenly distributed to the first photodetector 6 and the second photodetector 7 after passing through the polarization beam splitter prism 8. At this time, the optical power W1 received by the first photodetector 6 is W / 2, and the optical power W2 received by the second photodetector 7 is W / 2, that is:

[0075] When W1:W2 = 0:1, the output collimated beam is linearly polarized light;

[0076] When W1:W2 = 1:1, the output collimated beam is circularly polarized light.

[0077] S62, judging whether the optical power of the output collimated beam changes by the change of the sum of the optical powers obtained by the first photodetector 6 and the second photodetector 7.

[0078] Referring to the application example in S61, by monitoring the change of the sum of W1+W2, the change of the optical power of the laser beam output by the beam expander and collimator can be monitored.

[0079] In summary, the present invention provides a beam expander and collimator for a cold atom interferometer and its usage method, which can switch the polarization state of the laser beam in real time and perform real-time monitoring on the polarization state and optical power change of the laser beam. Compared with the traditional cold atom interferometer that requires the configuration of multiple beam expanders and collimators, the present invention only needs to apply one beam expander and collimator to realize different polarization state laser beams required in the atomic interference measurement process, greatly reducing the complexity of cold atom trapping and interference optical arrangement, and at the same time effectively reducing the volume and application cost of the cold atom interferometer.

[0080] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included within the protection scope of the present invention.

Claims

1. An expanding and collimating device for a cold atom interferometer, characterized in that, Including: Polarization-maintaining optical fiber (1), half-wave liquid crystal variable retarder (2), lens group (3), first 1 / 2 waveplate (4), beam splitting mirror (5) and optical power monitoring module; The polarization-maintaining optical fiber (1) outputs a divergent linearly polarized laser beam, which undergoes real-time polarization state switching through the half-wave liquid crystal variable retarder (2), is expanded and collimated by the lens group (3) to form a parallel beam, undergoes initial polarization axis angle adjustment through the first 1 / 2 waveplate (4), and is split into reflected light and transmitted light by the beam splitting mirror (5); among them, the reflected light enters the vacuum chamber, and the transmitted light enters the optical power monitoring module.

2. The beam expander and collimator for a cold atom interferometer according to claim 1, characterized in that, The optical power monitoring module includes: first photodetector (6), second photodetector (7), polarization beam splitting prism (8) and second 1 / 2 waveplate (9); The transmitted light is split by the second 1 / 2 waveplate (9) and the polarization beam splitting prism (8) and respectively reaches the first photodetector (6) and the second photodetector (7).

3. The beam expander and collimator for a cold atom interferometer according to claim 1, characterized in that, The half-wave liquid crystal variable retarder (2) is electrically controlled and, under the control of the applied alternating voltage, realizes real-time switching of the polarization state of the output laser beam.

4. The beam expander and collimator for a cold atom interferometer according to claim 1, characterized in that, The polarization-maintaining optical fiber (1) is fixed in the lens barrel through an optical fiber flange, and the optical fiber flange can move back and forth in the lens barrel to adjust the optical interval between the polarization-maintaining optical fiber (1) and the lens group (3).

5. The beam expander and collimator for a cold atom interferometer according to claim 4, characterized in that, The polarization-maintaining optical fiber (1) is a single-mode polarization-maintaining optical fiber (1), and the lens group (3) is a doublet lens.

6. The beam expander and collimator for a cold atom interferometer according to claim 1, characterized in that, The reflectance-to-transmittance ratio of the beam splitting mirror (5) is 99:

1.

7. The beam expander and collimator for a cold atom interferometer according to claim 2, characterized in that, The polarization beam splitting prism (8) includes a pair of high-precision right-angle prisms, and a polarization beam splitting dielectric film is coated on the hypotenuse of one of the high-precision right-angle prisms.

8. A method for using a beam expander and collimator for a cold atom interferometer, characterized in that, Using the beam expander and collimator for a cold atom interferometer according to any one of claims 1-7, the method includes: Outputting a divergent linearly polarized laser beam through the polarization-maintaining optical fiber (1) to the half-wave liquid crystal variable retarder (2); Outputting the laser beam to the lens group (3) after real-time switching of the polarization state of the laser beam through the half-wave liquid crystal variable retarder (2); Outputting the laser beam to the first 1 / 2 waveplate (4) after expanding and collimating the laser beam into a parallel beam by the lens group (3); Outputting the linearly polarized laser beam to the optical beam splitting mirror (5) after adjusting the initial polarization axis angle of the linearly polarized laser beam through the first 1 / 2 waveplate (4); Splitting the laser beam into reflected light and transmitted light by the beam splitting mirror (5); Outputting the reflected light to the vacuum chamber for atomic interference measurement, and outputting the transmitted light to the optical power monitoring module for real-time monitoring of the polarization state and optical power change of the collimated beam.

9. The method for using a beam expander and collimator for a cold atom interferometer according to claim 8, characterized in that, The optical power monitoring module includes: first photodetector (6), second photodetector (7), polarization beam splitting prism (8) and second 1 / 2 waveplate (9); The transmitted light is split by the second 1 / 2 waveplate (9) and the polarization beam splitting prism (8) and respectively reaches the first photodetector (6) and the second photodetector (7).

10. The method for using the beam expander and collimator for a cold atom interferometer according to claim 9, characterized in that, The step of outputting the transmitted light to the optical power monitoring module for real-time monitoring of the polarization state and optical power change of the collimated beam includes: Judging whether the output collimated beam is circularly polarized light or linearly polarized light by the change in the ratio of the optical powers obtained by the first photodetector (6) and the second photodetector (7); Judge whether the optical power of the collimated beam output changes by the change of the sum of the optical powers obtained by the first photodetector (6) and the second photodetector (7).

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