A coherent population trapped cold atomic clock device and control method thereof

Abstract

The present invention discloses a coherent population trapped cold atomic clock device and a control method thereof. The device comprises: a Raman light generation and control module, which is used to generate a Raman laser with adjustable frequency difference and controllable switch; a spatial beam separation and control module, which is used to split the Raman laser into a detection laser and a preparation laser in the vertical direction, and combine the Raman laser beam into a Raman laser beam group to input into a differential detection module; a differential detection module, which is used to split the Raman laser beam group into a signal laser and a reference laser, and perform differential detection and processing on the reference laser and the signal laser to output a Ramsey signal. The present invention obtains the preparation laser and the detection laser by separating the laser beams of the preparation pulse and the detection pulse, and detects the obtained signal laser and reference laser, which can be free to evolve for a longer time, and at the same time, the laser spot size can be made smaller, reducing the background noise caused by the laser, thereby improving the accuracy of the atomic clock.

Description

Technical Field

[0001] The invention relates to the technical field of atomic clocks, and in particular to a coherent population trapped cold atomic clock device and a control method thereof. Background Art

[0002] Coherent Population Trapping (CPT) cold atomic clock is a quantum device that can provide high-precision frequency standards and is widely used in the fields of timing, navigation and positioning, time synchronization, etc. CPT cold atomic clock consists of a laser system, cold atoms and a control system. Its principle is to use Raman laser to trap the atomic population in a quantum superposition state called coherent dark state, and use Ramsey scheme for frequency detection and locking.

[0003] For conventional single-beam CPT cold atomic clocks, Ramsey interferometry is implemented during the free fall of cold atomic clusters captured in a magneto-optical trap. Specifically, the first CPT pulse prepares the cold atomic clusters to a coherent dark state. After a period of free evolution, the second CPT pulse reads out the Ramsey interference fringes to obtain an accurate atomic transition reference. For Ramsey interferometry, the longer the free evolution time, the sharper the interference fringes obtained, and thus a higher frequency accuracy is obtained. However, since the atomic clusters fall due to gravity, they usually move a distance of centimeters within a free evolution time of tens of milliseconds. This requires a single beam of CPT light to have a larger diameter to cover the atomic clusters within the time of the previous and next pulses. This not only requires a large laser cross-sectional diameter, but also introduces greater laser noise while increasing the sharpness of the fringes, which limits the further improvement of the accuracy of CPT cold atomic clocks.

[0004] Therefore, the prior art still needs to be improved and developed. Summary of the invention

[0005] In view of the above-mentioned deficiencies in the prior art, an object of the present invention is to provide a coherent population trapped cold atomic clock device and a control method thereof, so as to solve the problem that the existing single-beam CPT cold atomic clock requires a large laser cross-sectional diameter and introduces greater laser noise.

[0006] The technical solution of the present invention is as follows:

[0007] In a first aspect, the present invention provides a coherent population trapped cold atomic clock device, comprising:

[0008] A Raman light generation and control module is used to generate Raman laser with adjustable frequency difference and controllable switch;

[0009] A spatial beam separation and control module is connected to the Raman light generation and control module, and is used to split the Raman laser into a detection laser and a preparation laser in a vertical direction, and combine the detection laser and the preparation laser into a Raman laser beam group output; wherein the separation distance between the preparation laser and the detection laser is the falling height of the atomic group in the free evolution time of the Ramsey sequence;

[0010] Magneto-optical trap module, used to capture and cool atomic groups;

[0011] A differential detection module is located at the light output side of the spatial beam separation and control module, and is used to split the Raman laser beam component into a signal laser and a reference laser, and perform differential detection and processing on the reference laser and the signal laser to output a Ramsey signal; wherein the signal laser is the laser after interacting with the atomic group in the magneto-optical trap module.

[0012] According to a further configuration of the present invention, the Raman light generation and control module comprises: a phase modulator, a beam splitting module, an acousto-optic modulator, a radio frequency switch, a first convex lens, a quarter wave plate and a first reflector; wherein,

[0013] The phase modulator is used to receive the input laser and perform phase modulation on the input laser to generate Raman laser with adjustable frequency difference;

[0014] The beam splitting module is connected to the phase modulator via an optical fiber, and is connected to the control module via the optical fiber spatial beam separation. The beam splitting module is used to split the Raman laser into two beams of laser light with adjustable ratio, and input one of the Raman laser beams into the acousto-optic modulator;

[0015] The acousto-optic modulator is connected to the radio frequency switch, and the acousto-optic modulator outputs a diffracted light beam according to the radio frequency signal output by the radio frequency switch. The diffracted light beam reaches the first reflector after passing through the first convex lens and the quarter-wave plate in sequence, and is reflected by the first reflector. The beam splitting module outputs a Raman laser with an adjustable frequency difference and controllable switch.

[0016] The radio frequency switch is connected to a Ramsey sequence signal to control the on or off of the radio frequency signal.

[0017] According to a further configuration of the present invention, the beam splitting module comprises: a half-wave plate and a polarization beam splitter;

[0018] The half-wave plate is connected to the phase modulator via an optical fiber and is used to adjust the polarization direction of the Raman laser;

[0019] The polarization beam splitter is located on the light output side of the half-wave plate and on the light input side of the acousto-optic modulator, and is used to split the Raman laser into two laser beams, and input one of the Raman laser beams into the acousto-optic modulator.

[0020] According to a further configuration of the present invention, the spatial beam separation and control module comprises: an aperture, a first beam splitter, a first reflector group, a second reflector group, a beam combiner and a third reflector group; wherein,

[0021] The aperture is connected to the beam splitting module via an optical fiber, and the aperture is used to adjust the spot size of the Raman laser;

[0022] The beam splitter is located at the light exit side of the aperture, and is used to split the Raman laser into a detection laser and a preparation laser, wherein the preparation laser reaches the beam combiner via the first reflector group, and the detection laser reaches the beam combiner via the second reflector group;

[0023] The beam combiner is located at the light exit side of the first reflector group and the second reflector group, and at the light entrance side of the third reflector group, and the beam combiner combines the detection laser and the preparation laser into a Raman laser beam group and inputs the Raman laser beam group to the differential detection module through the third reflector group;

[0024] The separation distance between the preparation laser and the detection laser is adjusted by adjusting the height between the first reflector group and the second reflector group.

[0025] In a further configuration of the present invention, the first reflector group includes: a second reflector, a third reflector and a first optical switch;

[0026] The second reflector is located at the first light output side of the first beam splitter, and the prepared laser reaches the beam combiner via the second reflector, the first optical switch, and the third reflector in sequence;

[0027] The second reflector group includes: a fourth reflector, a fifth reflector and a second optical switch;

[0028] The fourth reflector is located at the second light-emitting side of the first beam splitter, and the detection laser reaches the beam combiner via the fourth reflector, the second optical switch and the fifth reflector in sequence;

[0029] The third reflector group includes: a sixth reflector and a seventh reflector;

[0030] The sixth reflector is located at the light-emitting side of the beam combiner, and the Raman laser beam group is incident on the differential detection module via the sixth reflector and the seventh reflector in sequence.

[0031] According to a further configuration of the present invention, the differential detection module comprises: a second beam splitter, an eighth reflector, a ninth reflector, a tenth reflector, a second convex lens, a third convex lens, a first probe, a second probe and a signal processing unit; wherein,

[0032] The second beam splitter is located at the light-exiting side of the spatial beam separation and control module, and is located at the same horizontal plane as the eighth reflector, and the magneto-optical trap is located between the second beam splitter and the eighth reflector; the ninth reflector is located at the first light-exiting side of the second beam splitter, the magneto-optical trap module is located at the second light-exiting side of the second beam splitter, and the third convex lens is located at the third light-exiting side of the second beam splitter; the second convex lens is located on the reflected light path of the tenth reflector; the first probe is located at the light-exiting side of the second convex lens, and the second probe is located at the light-exiting side of the third convex lens;

[0033] The second beam splitter splits the Raman laser beam into a signal laser and a reference laser; the signal laser reacts with the atomic group in the magneto-optical trap module and is reflected back to the second beam splitter via the eighth reflector, and is focused to the second probe via the third convex lens; the reference laser is sequentially input to the second convex lens via the ninth reflector and the tenth reflector, and is input to the first probe after being focused by the second convex lens;

[0034] The signal processing unit is connected to the first probe and the second probe respectively, and is used to perform differential detection and processing on a first acquisition signal detected by the first probe and a second acquisition signal detected by the second probe, and output a Ramsey signal.

[0035] According to a further configuration of the present invention, the magneto-optical trap module includes a magneto-optical trap and a vacuum cavity; the magneto-optical trap has a through hole, and the through hole is located at the light output side of the second beam splitter; and the vacuum cavity is located between the second beam splitter and the eighth reflector.

[0036] In a second aspect, the present invention further provides a control method for the coherent population trapped cold atomic clock device described above, which comprises the steps of:

[0037] Controlling the input laser, radio frequency signal and Ramsey sequence to be input into the Raman light generation and control module, so as to control the on and off of the radio frequency signal through the Ramsey sequence, so that the Raman light generation and control module can output Raman laser with adjustable frequency difference and controllable switch;

[0038] Controlling the spatial beam separation and control module to split the Raman laser into a detection laser and a preparation laser in a vertical direction, and combining the detection laser and the preparation laser into a Raman laser beam group to input into the differential detection module;

[0039] The Raman laser beam group is divided into a signal laser and a reference laser by the differential detection module, and the reference laser and the signal laser are differentially detected and processed to output a Ramsey signal;

[0040] The microwave frequency of the Raman light generation and control module is adjusted to perform a frequency sweep cycle, so as to perform differential detection and processing on the reference laser and the signal laser to obtain frequency domain Ramsey fringes.

[0041] A further configuration of the present invention is that in the step of controlling the input laser, radio frequency signal and Ramsey sequence to be input into the Raman light generation and control module to control the turning on and off of the radio frequency signal through the Ramsey sequence, so that the Raman light generation and control module outputs a Raman laser with an adjustable frequency difference and controllable switch, the output laser frequency of the Raman laser is the frequency of the input laser minus or plus twice the frequency of the radio frequency signal.

[0042] A further arrangement of the present invention is that in the step of controlling the spatial beam separation and control module to split the Raman laser into a detection laser and a preparation laser in the vertical direction, and combining the detection laser and the preparation laser into a Raman laser beam group and inputting them into the differential detection module, a first optical switch sequence is used to control the turning on and off of the preparation laser, and a second optical switch sequence is used to control the turning on and off of the detection laser, so as to adjust the timing interval between the preparation laser and the detection laser.

[0043] The present invention provides a coherent population trapped cold atomic clock device and a control method thereof, the device comprising: a Raman light generation and control module, used for generating a Raman laser with adjustable frequency difference and controllable switch; a spatial beam separation and control module, connected to the Raman light generation and control module, the spatial beam separation and control module is used for splitting the Raman laser into a detection laser and a preparation laser in a vertical direction, and combining the detection laser and the preparation laser into a Raman laser beam group for output; wherein the separation distance between the preparation laser and the detection laser is the falling height of the atomic group in the free evolution time of the Ramsey sequence; a magneto-optical trap module, used for capturing and cooling the atomic group; a differential detection module, located at the light output side of the spatial beam separation and control module, the differential detection module is used for splitting the Raman laser beam group into a signal laser and a reference laser, and differentially detecting and processing the reference laser and the signal laser to output a Ramsey signal; wherein the signal laser is the laser after interacting with the atomic group in the magneto-optical trap module. The present invention obtains preparation laser and detection laser by separating the laser beams of preparation pulse and detection pulse, and detects the obtained signal laser and reference laser, thereby enabling free evolution for a longer time, and at the same time making the laser spot size smaller, reducing the background noise caused by the laser, thereby improving the accuracy of the atomic clock. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary personnel in this field, other drawings can be obtained based on the structures shown in these drawings without paying any creative work.

[0045] Figure 1 It is a schematic diagram of the principle structure of the coherent population trapped cold atomic clock device in the present invention.

[0046] Figure 2 It is a schematic diagram of the coherent population trapped cold atomic clock device in the present invention.

[0047] Figure 3 It is a timing control principle diagram of the coherent population trapped cold atomic clock device in the present invention.

[0048] Figure 4 This is a diagram of the experimental results of the coherent population trapped cold atomic clock device of the present invention.

[0049] Figure 5 It is a flow chart of the coherent population trapped cold atomic clock control method in the present invention.

[0050] The symbols in the accompanying drawings are: 1. Raman light generation and control module; 11. phase modulator; 12. beam splitting module; 121. half-wave plate; 122. polarization beam splitter; 13. acousto-optic modulator; 14. radio frequency switch; 15. first convex lens; 16. quarter-wave plate; 17. first reflector; 2. spatial beam separation and control module; 21. aperture; 22. first beam splitter; 23. first reflector group; 231. second reflector; 232. third reflector; 233. first optical switch; 24. second reflector group; 24 1. The fourth reflector; 242. The fifth reflector; 243. The second optical switch; 25. The beam combiner; 26. The third reflector group; 261. The sixth reflector; 262. The seventh reflector; 3. The magneto-optical trap module; 31. The magneto-optical trap; 32. The vacuum chamber; 4. The differential detection module; 41. The second beam splitter; 42. The eighth reflector; 43. The ninth reflector; 44. The tenth reflector; 45. The second convex lens; 46. The third convex lens; 47. The first probe; 48. The second probe; 49. The signal processing unit; 5. The optical fiber. DETAILED DESCRIPTION

[0051] The present invention provides a coherent population trapped cold atomic clock device and a control method thereof. In order to make the purpose, technical solution and effect of the present invention clearer and more specific, the present invention is further described in detail with reference to the accompanying drawings and examples. 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.

[0052] In the embodiments and the scope of the patent application, unless the text specifically defines the article, "a", "an", "the" and "the" may also include plural forms. If there are descriptions involving "first", "second", etc. in the embodiments of the present invention, the descriptions of "first", "second", etc. are only used for descriptive purposes and cannot be understood as indicating or implying their relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" or "second" may explicitly or implicitly include at least one of the features.

[0053] It should be further understood that the term "comprising" used in the specification of the present invention refers to the presence of the features, integers, steps, operations, elements and / or components, but does not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components and / or groups thereof. It should be understood that when an element is said to be "connected" or "coupled" to another element, it can be directly connected or coupled to the other element, or there can also be intermediate elements. In addition, the "connection" or "coupling" used herein can include wireless connection or wireless coupling. The term "and / or" used herein includes all or any unit and all combinations of one or more associated listed items.

[0054] It will be understood by those skilled in the art that, unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as generally understood by those skilled in the art in the art to which the present invention belongs. It should also be understood that terms such as those defined in general dictionaries should be understood to have meanings consistent with the meanings in the context of the prior art, and will not be interpreted with idealized or overly formal meanings unless specifically defined as herein.

[0055] In addition, the technical solutions between the various embodiments can be combined with each other, but it must be based on the fact that ordinary technicians in the field can implement it. When the combination of technical solutions is contradictory or cannot be implemented, it should be deemed that such combination of technical solutions does not exist and is not within the scope of protection required by the present invention.

[0056] Please also see Figures 1 to 4 The present invention provides a preferred embodiment of a coherent population trapped cold atomic clock device.

[0057] In some embodiments, Figure 1As shown, the present invention provides a coherent population trapped cold atomic clock device, which includes: a Raman light generation and control module 1, a spatial beam separation and control module 2, a magneto-optical trap module 3 and a differential detection module 4. Wherein, the Raman light generation and control module 1 is used to generate a Raman laser with adjustable frequency difference and controllable switch; the spatial beam separation and control module 2 is connected to the Raman light generation and control module 1 and the differential detection module 4 respectively, and the spatial beam separation and control module 2 is used to split the Raman laser into a detection laser and a preparation laser in the vertical direction, and combine the detection laser and the preparation laser into a Raman laser beam group and input it into the differential detection module 4; wherein, the separation distance between the preparation laser and the detection laser is the falling height of the atomic cluster in the free evolution time of the Ramsey sequence; the magneto-optical trap module 3 is used to capture and cool the atomic cluster; the differential detection module 4 is located on the light output side of the spatial beam separation and control module 2, and the differential detection module 4 is used to split the Raman laser beam group into a signal laser and a reference laser, and perform differential detection and processing on the reference laser and the signal laser to output a Ramsey signal; wherein, the signal laser is the laser after interacting with the atomic cluster in the magneto-optical trap module.

[0058] In this embodiment, the Raman light generation and control module 1 is connected to the 795nm input laser through the optical fiber 5, and the opening and closing of the radio frequency signal is controlled by the Ramsey sequence to generate a Raman laser with adjustable frequency difference and controllable switch. Among them, the laser frequency that can be output by the Raman light generation and control module 1 is the frequency of the input laser minus or plus twice the frequency of the radio frequency signal. Then, by adjusting the frequency of the radio frequency signal, the frequency of the Raman laser output by the Raman light generation and control module 1 can be made to resonate with the atomic CPT transition frequency, that is, the common mode detuning is about 0. In addition, by adjusting the microwave frequency of the Raman light generation and control module 1, the frequency difference of the Raman laser can be changed, and finally a Raman laser (i.e., a Raman-Raman laser pulse) with adjustable frequency difference and controllable switch can be output. It should be noted that the Raman-Raman laser pulse includes a 500μs preparation pulse and a 300μs detection pulse, which are separated by a free evolution time of 35ms, and also include a 56ms dead time that cannot be used for Ramsey interferometry detection, which is used to capture and cool atomic groups. It should be noted that the frequency-stabilized input laser is provided by the laser system, and the operating rhythm and operating parameters required by the atomic device, such as the RF signal, Ramsey sequence, modulation ripple, etc., are controlled by the digital pulse sequence and analog voltage generated by the control system.

[0059] The magneto-optical trap module 3 can capture and cool the atomic group. The spatial beam separation and control module 2 is connected to the Raman light generation and control module 1 through the optical fiber 5, and can separate the Raman laser into the detection laser and the preparation laser in the vertical direction. The detection laser obtains the signal laser after interacting with the atomic group in the magneto-optical trap module 3, and the signal laser is then input to the differential detection module 4, while the reference laser is directly input to the differential detection module 4.

[0060] After the detection laser passes through the atomic group, it will carry the information of the coherent dark state of the atomic group, which is manifested as the change of the transmitted light intensity. The coherent dark state of the atom accumulates the phase in the free evolution time, and then the detection laser acts on it. The phase information is detected by the transmission intensity of the detection laser. Because the free evolution time is fixed, the accumulated phase changes accordingly, and the Raman detuning is scanned, and finally a Ramsey interference signal of approximately sinusoidal oscillation is presented. Among them, the separation distance between the preparation laser and the detection laser is the falling height of the atomic group in the free evolution time of the Ramsey sequence. The larger the separation distance, the longer the free evolution time. The differential detection module 4 can collect the reference laser and the signal laser, and output a voltage signal, i.e., a Ramsey signal, after differential detection and processing of the collected signal, and store the Ramsey signal. By setting the Raman detuning of the Raman light generation and control module 1, i.e., adjusting the frequency of the modulated microwave to perform a frequency sweep cycle, the reference laser and the signal laser are differentially detected and processed to obtain frequency domain Ramsey fringes, which can be used to lock microwaves and output atomic clock signals.

[0061] In the above technical solution, the present invention obtains the preparation laser and the detection laser by separating the laser beams of the preparation pulse and the detection pulse, and detects the obtained signal laser and reference laser, so as to obtain sharper interference fringes, which can be free to evolve for a longer time, and at the same time, the laser spot size can be made smaller, reducing the background noise caused by the laser, so that the uncertainty of atomic clock locking is effectively reduced, thereby improving the accuracy of the atomic clock. Figure 4 As shown in the locking results of the cold atomic clock, the corresponding stability at an average time of 1 s is increased from that of the single-beam scheme to Upgraded to dual beam solution , where the parameter τ represents the averaging time.

[0062] In some embodiments, Figure 2As shown, the Raman light generation and control module 1 includes: a phase modulator 11, a beam splitting module 12, an acousto-optic modulator 13, a radio frequency switch 14, a first convex lens 15, a quarter-wave plate 16 and a first reflector 17. Among them, the phase modulator 11 is used to access the input laser and perform phase modulation on the input laser to generate a Raman laser with an adjustable frequency difference; the beam splitting module 12 is connected to the phase modulator 11 through the optical fiber 5, and is connected to the spatial beam separation and control module 2 through the optical fiber 5. The beam splitting module 12 is used to split the Raman laser into two laser beams with adjustable ratios, and input one of the Raman laser beams into the acousto-optic modulator 13; the acousto-optic modulator 13 is connected to the radio frequency switch 14, and the acousto-optic modulator 13 outputs a diffracted light beam according to the radio frequency signal output by the radio frequency switch 14. The diffracted light beam passes through the first convex lens 15 and the quarter-wave plate 16 in turn and reaches the first reflector 17, and is reflected by the first reflector 17, and outputs a Raman laser with an adjustable frequency difference and controllable switch through the beam splitting module 12; the radio frequency switch 14 is connected to a Ramsey sequence signal to control the on or off of the radio frequency signal.

[0063] In this embodiment, the phase modulator 11 can phase modulate the input laser by modulating microwaves to generate Raman laser with adjustable frequency difference. The beam splitting module 12 divides the Raman laser into two laser beams with adjustable ratio to adjust the power of one of the laser beams input to the acousto-optic modulator 13. The RF switch 14 is connected to the RF signal to drive the acousto-optic modulator 13 to work. The acousto-optic modulator 13 outputs a diffracted light beam according to the RF signal output by the RF switch 14. The diffracted light beam is controlled by the RF signal output by the RF switch 14 to switch the -1 pole (or +1 pole). The diffracted light beam passes through the first convex lens 15 and the quarter-wave plate 16 in turn and reaches the second reflector 231, and is reflected back by the second reflector 231 through the beam splitting module 12. The RF switch 14 controls the opening and closing of the RF signal by receiving the Ramsey sequence, so that the Raman light generation and control module 1 outputs Raman-Raman laser pulses, that is, Raman laser with adjustable frequency difference and controllable switch.

[0064] Furthermore, if Figure 2 As shown, the beam splitting module 12 includes: a half-wave plate 121 and a polarization beam splitter 122; the half-wave plate 121 is connected to the phase modulator 11 through an optical fiber 5, and is used to adjust the polarization direction of the Raman laser; the polarization beam splitter 122 is located on the light output side of the half-wave plate 121, and on the light input side of the acousto-optic modulator 13, and is used to split the Raman laser into two laser beams, and input one of the Raman laser beams into the acousto-optic modulator 13.

[0065] In this embodiment, the half-wave plate 121 is connected to the Raman laser through the optical fiber 5 and can adjust the polarization direction of the Raman laser to change the splitting ratio of the polarization beam splitter 122 to achieve the power of the Raman laser input to the acousto-optic modulator 13.

[0066] In the specific implementation, the 795nm input laser is input into the phase modulator 11, the frequency of the radio frequency signal is adjusted to f, and the output laser frequency is tuned to the D1 line of the rubidium-87 atom by using the -1 pole diffraction. arrive The transition frequency corresponding to the transition, where S represents the electron orbit, 5 represents the electron shell, and F represents the ground state. represents the excited state, It means that the electron occupies the S orbit, and the corresponding angular momentum is 0. Considering the electron spin and orbital angular momentum, the total angular momentum is 1 / 2. P represents the electron orbit. Indicates that the electron occupies the P orbital, and the corresponding angular momentum is 0. Considering the electron spin and orbital angular momentum at the same time, the total angular momentum is 1 / 2. Among them, the D1 line ground state of the rubidium-87 atom undergoes cross-coherent population trapping under the lin||lin configuration, requiring the frequency difference of the Raman laser to be near 6.834GHz, that is, the cross-dark state resonance frequency. The frequency of the modulated microwave is adjusted by the control system, and the frequency is set to a frequency v near the cross-dark state resonance frequency. Thereafter, the quarter-wave plate 16 is adjusted so that the laser reflected by the first reflector 17 is completely transmitted through the polarization beam splitter 122, and the beam splitting module is adjusted to have sufficient output light intensity.

[0067] In some embodiments, Figure 2 As shown, the spatial beam separation and control module 2 includes: an aperture 21 , a first beam splitter 22 , a first reflector group 23 , a second reflector group 24 , a beam combiner 25 and a third reflector group 26 . The aperture 21 is connected to the beam splitting module 12 through an optical fiber 5, and the aperture 21 is used to adjust the spot size of the Raman laser; the first beam splitter 22 is located on the light exit side of the aperture 21, and is used to split the Raman laser into a detection laser and a preparation laser, wherein the preparation laser reaches the beam combiner 25 via the first reflector group 23, and the detection laser reaches the beam combiner 25 via the second reflector group 24; the beam combiner 25 is located on the light exit side of the first reflector group 23 and the second reflector group 24, and on the light incident side of the third reflector group 26, and the beam combiner 25 combines the detection laser and the preparation laser into a Raman laser beam group and inputs it to the differential detection module 4 via the third reflector group 26; wherein the separation distance between the preparation laser and the detection laser is adjusted by adjusting the height between the first reflector group 23 and the second reflector group 24.

[0068] In this embodiment, the Raman laser output by the optical fiber 5 is adjusted to a spot size by the aperture 21, and then is divided into a first Raman laser (i.e., a preparation laser) and a second Raman laser (i.e., a detection laser) by the first beam splitter 22. The preparation laser reaches the beam combiner 25 through the first reflector group 23, and the detection laser reaches the beam combiner 25 through the second reflector group 24. The Raman laser beam group is input to the differential detection module 4 through the third reflector group 26. The vertical spacing between the detection laser and the preparation laser can be adjusted by adjusting the height of the first reflector group 23 and the second reflector group 24, and the preparation laser can be ensured to act on the atomic group before falling by adjusting the third reflector group 26. The Raman laser beam group is required to be strictly parallel to the bias magnetic field to improve the signal-to-noise ratio of the coherent population trapped signal.

[0069] Furthermore, if Figure 2 As shown, the first reflector group 23 includes: a second reflector 231, a third reflector 232 and a first optical switch 233; the second reflector 231 is located at the first light output side of the first beam splitter 22, and the prepared laser reaches the beam combiner 25 via the second reflector 231, the first optical switch 233 and the third reflector 232 in sequence.

[0070] Specifically, the first optical switch 233 is turned on or off by the control of the first optical switch sequence, wherein the first optical switch sequence is turned on when the Ramsey sequence prepares the pulse. Figure 3 When the first optical switch 233 is turned on, the preparation laser is input to the beam combiner 25 via the second reflector 231 , the first optical switch 233 and the third reflector 232 .

[0071] Furthermore, the second reflector group 24 includes: a fourth reflector 241, a fifth reflector 242 and a second optical switch 243; the fourth reflector 241 is located at the second light output side of the first beam splitter 22, and the reference laser reaches the beam combiner 25 via the fourth reflector 241, the second optical switch 243 and the fifth reflector 242 in sequence.

[0072] Specifically, the second optical switch 243 is turned on or off by the control of the second optical switch sequence, wherein the second optical switch sequence is turned on when the Ramsey sequence detects a pulse. Figure 3 When the second optical switch 243 is turned on, the detection laser is input to the beam combiner 25 via the fourth reflector 241 , the second optical switch 243 and the fifth reflector 242 .

[0073] In this embodiment, the vertical spacing between the preparation laser and the detection laser can be achieved by adjusting the height of the second reflector 231 and the third reflector 232 and adjusting the height of the fourth reflector 241 and the fifth reflector 242, so that the first Raman laser and the second Raman laser generate a Raman laser beam group with a vertical spacing of h after passing through the beam combiner 25. The distance h is the distance that the atoms fall due to gravity during the free evolution time. ,in is the Ramsey free evolution time, and g is the gravitational acceleration.

[0074] In some embodiments, Figure 2 As shown, the third reflector group 26 includes: a sixth reflector 261 and a seventh reflector 262; the sixth reflector 261 is located at the light exit side of the beam combiner 25, and the Raman laser beam group is incident on the differential detection module 4 through the sixth reflector 261 and the seventh reflector 262 in sequence. Specifically, the Raman laser beam group is input to the differential detection module 4 through the sixth reflector 261 and the seventh reflector 262, and by adjusting the relative height of the sixth reflector 261 and the seventh reflector 262, it can be ensured that the preparation laser can act on the atomic group before falling.

[0075] In some embodiments, Figure 2As shown, the differential detection module 4 includes: a second beam splitter 41, an eighth reflector 42, a ninth reflector 43, a tenth reflector 44, a second convex lens 45, a third convex lens 46, a first probe 47, a second probe 48 and a signal processing unit 49. The second beam splitter 41 is located at the light-exiting side of the spatial beam separation and control module 2, and is located at the same horizontal plane as the eighth reflector 42. The magneto-optical trap module 3 is located between the second beam splitter 41 and the eighth reflector 42; the ninth reflector 43 is located at the first light-exiting side of the second beam splitter 41, the magneto-optical trap module 3 is located at the second light-exiting side of the second beam splitter 41, the second convex lens 45 is located at the third light-exiting side of the second beam splitter 41; the second convex lens 45 is located on the reflected light path of the tenth reflector 44; the first probe 47 is located at the light-exiting side of the second convex lens 45, the second probe 48 is located at the light-exiting side of the third convex lens 46; the second beam splitter 41 The Raman laser beam component is split into a signal laser and a reference laser; after the signal laser interacts with the atomic group in the magneto-optical trap module 3, it is reflected back to the second beam splitter 41 through the eighth reflector 42, and is focused to the second probe 48 through the third convex lens 46; the reference laser is input to the second convex lens 45 through the ninth reflector 43 and the tenth reflector 44 in turn, and is input to the first probe 47 after being focused by the second convex lens 45; the signal processing unit 49 is respectively connected to the first probe 47 and the second probe 48, and is used to perform differential detection and processing on the first acquisition signal detected by the first probe 47 and the second acquisition signal detected by the second probe 48, and output a Ramsey signal.

[0076] In this embodiment, the second beam splitter 41 divides the input Raman laser beam into a signal laser beam and a reference laser beam. The reference laser beam is incident on the second convex lens 45 via the ninth reflector 43 and the tenth reflector 44, and is focused on the first probe 47 by the second convex lens 45. The signal laser beam enters the magneto-optical trap module 3, reacts with the atomic groups in the magneto-optical trap module 3, and then returns to the second beam splitter 41 via the eighth reflector 42, and is incident on the third convex lens 46 via the second beam splitter 41, and is further focused on the second probe 48 by the third convex lens 46.

[0077] The signal processing unit 49 can perform differential detection on the two sampling signals output by the first probe 47 and the second probe 48 and process them to output a Ramsey signal. The specific implementation process is as follows: during the specific implementation, the first probe 47 and the second probe 48 continuously perform signal acquisition, and the signal processing unit 49 performs real-time processing; when the second Ramsey pulse (the first pulse is the preparation pulse, and the second pulse is the detection pulse) is turned on, the differential amplification output voltage is read, and this operation lasts for 300μs to obtain a series of sample points; the sampling points within the response time of the first probe 47 and the second probe 48 to the input are eliminated, and then the next sample points of about 13μs are taken and averaged to obtain the atomic Ramsey interference signal value corresponding to the frequency v; in addition, the sample points between 100μs and 300μs are taken and averaged to obtain the atomic CPT signal value corresponding to the frequency v. Thereafter, the atomic Ramsey interference signal value obtained above is divided by the CPT signal value to eliminate the influence of the change in the number of atoms, and the Ramsey signal is output.

[0078] In some embodiments, Figure 2 As shown, the MOT module 3 includes a MOT 31 and a vacuum chamber 32 ; the MOT 31 has a through hole, which is located at the light-emitting side of the second beam splitter 41 ; the vacuum chamber 32 is located between the second beam splitter 41 and the eighth reflector 42 .

[0079] Specifically, the magneto-optical trap 31 has an anti-Helmholtz coil, a counter-cooling laser, a pump laser and a Helmholtz coil, and the vacuum cavity 32 is arranged in the magneto-optical trap 31. The vacuum cavity 32 has a rubidium source, and a high vacuum of about 10-7Pa is maintained by a vacuum system, and the rubidium-87 atomic vapor can be controlled. The anti-Helmholtz coil is used to generate a gradient magnetic field. The counter-cooling laser is a six-beam counter-cooling laser with a wavelength of about 780nm, which acts on the D2 line of the rubidium-87 atom. arrive Transition, where S represents the electron orbital, 5 represents the electron shell, and F represents the ground state. represents the excited state, It means that the electron occupies the S orbit, and the corresponding angular momentum is 0. Considering the electron spin and orbital angular momentum, the total angular momentum is 1 / 2. P represents the electron orbit. It means that the electron occupies the P orbital, and the corresponding angular momentum is 0. Considering the electron spin and orbital angular momentum, the total angular momentum is 3 / 2. The pump laser is two beams of about 795nm wavelength that are vertically directed to the D1 line of the rubidium-87 atom. arrive Helmholtz coils are used to generate the bias magnetic field.

[0080] The magneto-optical trap can continuously obtain cold rubidium-87 atoms in a vacuum cavity. After capturing the atoms, polarization gradient cooling is performed to make the temperature of the atoms reach 10μK, so as to prevent the diffusion and expansion range of the atoms after falling from being too large and exceeding the coverage range of the detection laser beam. The smaller diffusion can appropriately reduce the diameter of the laser beam to minimize laser noise. In one implementation, the magneto-optical trap captures about 10 atoms each time. The diameter of the atomic cluster is about 1 mm, and the temperature is about 10 μK. In this embodiment, the magneto-optical trap module uses the D1 line of the rubidium-87 atom to construct a coherent population trapped dark state of the lin||lin configuration and a Ramsey detection scheme to obtain the output signal. When a single-beam detection scheme is used, the stability of the atomic clock obtained is .

[0081] In some embodiments, Figure 5 As shown, the present invention also provides a control method for the coherent population trapped cold atomic clock device based on the above-mentioned method, which comprises the steps of:

[0082] S100, controlling the input laser, radio frequency signal and Ramsey sequence to be input into the Raman light generation and control module, so as to control the on and off of the radio frequency signal through the Ramsey sequence, so that the Raman light generation and control module outputs a Raman laser with an adjustable frequency difference and controllable switch;

[0083] Specifically, the 795nm frequency-stabilized laser generated by the laser system is input to the phase modulator, and the RF signal input by the RF switch and the Ramsey sequence are generated by the control system. The output laser frequency of the Raman laser is the frequency of the input laser minus or plus twice the frequency of the RF signal. In specific implementation, the 795nm input laser is input to the phase modulator, the frequency of the RF signal is adjusted to f, and the output laser frequency is tuned to the D1 line of the rubidium-87 atom by using the -1 pole diffraction. arrive The transition frequency corresponding to the transition is adjusted to adjust the quarter wave plate so that the laser reflected by the second reflector is fully transmitted through the polarization beam splitter, and the beam splitter module is adjusted to have sufficient output light intensity. The Ramsey sequence is input into the RF switch to control the on and off of the RF signal, thereby making the Raman light generation and control module output Raman-Raman laser pulses.

[0084] S200, controlling the spatial beam separation and control module to split the Raman laser into a detection laser and a preparation laser in a vertical direction, and combining the detection laser and the preparation laser into a Raman laser beam group to input to the differential detection module;

[0085] Specifically, before separating the Raman lasers, the heights of the second reflector and the third reflector can be adjusted, as well as the heights of the fourth reflector and the fifth reflector, so that the first Raman laser and the second Raman laser generate a Raman laser beam group with a vertical spacing of h after passing through the beam combiner. The distance h is the distance that the atoms fall due to gravity during the free evolution time. ,in is the Ramsey free evolution time, and the positions of the sixth reflector and the seventh reflector are further adjusted to ensure that the preparation laser can act on the atomic clusters before falling.

[0086] In each Ramsey cycle, the first optical switch sequence is used to control the first optical switch to control the on and off of the preparation laser, and the second optical switch sequence is used to control the second optical switch to control the on and off of the detection laser. In addition, the first optical switch sequence is turned on only when the preparation pulse is on and is turned off at other times, and the second optical switch sequence is turned on only when the detection pulse is on and is turned off at other times, so that the timing interval between the preparation laser and the detection laser can be adjusted. After the Raman laser is separated into the detection laser and the preparation laser and combined into a Raman laser beam group, it is input into the differential detection module, wherein the Raman laser beam is parallel to the bias magnetic field of the magneto-optical trap, so that the signal-to-noise ratio of the coherent population trapped signal can be improved.

[0087] S300, the Raman laser beam group is divided into a signal laser and a reference laser by the differential detection module, and the reference laser and the signal laser are differentially detected and processed to output a Ramsey signal;

[0088] Specifically, the first probe continuously samples the reference laser, the second probe continuously samples the signal laser, and the signal processing unit processes the sampled signal in real time; when the second Ramsey pulse (the first pulse is the preparation pulse, and the second pulse is the detection pulse) is turned on, the differential amplifier output voltage is read, and this operation lasts for 300μs to obtain a series of sample points; the sampling points within the response time of the first and second probes to the input are eliminated, and then the next 13μs of sample points are taken and averaged to obtain the atomic Ramsey interference signal value corresponding to the frequency v; in addition, the sample points between 100μs and 300μs are taken and averaged to obtain the atomic CPT signal value corresponding to the frequency v. Thereafter, the atomic Ramsey interference signal value obtained above is divided by the CPT signal value to eliminate the influence of the change in the number of atoms, and the Ramsey signal is output.

[0089] S400, adjusting the microwave frequency of the Raman light generation and control module to perform a frequency sweep cycle, so as to perform differential detection and processing on the reference laser and the signal laser to obtain frequency domain Ramsey fringes.

[0090] Specifically, the ground state of the D1 line of the rubidium-87 atom undergoes cross-coherent population trapping in the lin||lin configuration, which requires the frequency difference of the Raman light to be near 6.834 GHz, i.e., the cross-dark state resonance frequency. The frequency of the modulated microwave is adjusted by the control system, and the frequency is set to a frequency v near the cross-dark state resonance frequency. In this way, by changing the frequency of the modulated microwave and repeatedly sampling the reference laser and the signal laser after interacting with the atomic group in the magneto-optical trap, high-contrast Ramsey fringes can be obtained, and the frequency domain Ramsey fringes can be used to lock the microwave and output the atomic clock signal.

[0091] In order to better understand the spatially separated dual beam solution provided by the present invention, the present invention is described using a Ramsey interferometry embodiment:

[0092] First, the cold atomic cluster is captured in the magneto-optical trap. This process takes 50ms. Then, the magneto-optical trap magnetic field is turned off, the red detuning of the cooling light is increased, and the intensity of the cooling light is reduced. Polarization gradient cooling is performed for 5ms to further cool the atomic cluster. Then, the cooling laser is turned off, and the atomic cluster is allowed to relax to a free state for 1ms. At this time, the first pulse of the Ramsey sequence (preparation pulse) is turned on, and the reference laser is turned on at the same time. It lasts for 0.5ms to prepare the atomic cluster to a dark state. Then, the reference laser is turned off, and the atomic cluster is allowed to evolve freely for 35ms. Finally, the second pulse of the Ramsey sequence (detection pulse) is turned on, and the atomic signal is read using the signal laser. The above is a Ramsey cycle. In the magneto-optical trap, the bias magnetic field with the same direction as the laser and a magnitude of about 72mG is always turned on to generate Zeeman splitting of the atomic energy level. When the next Ramsey cycle is performed, the Raman detuning amount of the Ramsey pulse is changed, and the above Ramsey interferometry measurement steps are repeated. After multiple cycles, Ramsey fringes are finally obtained and used to lock the atomic clock.

[0093] In summary, the coherent population trapped cold atomic clock device and control method thereof provided by the present invention have the following beneficial effects:

[0094] A spatial beam separation and control module is used to generate two laser beams separated in the vertical direction, which are used as the coherent population trapped dark state preparation laser (i.e., the first Raman laser) and the detection laser (i.e., the second Raman laser) respectively, acting on the atomic clusters in free fall in a vacuum, and the spot size and switching timing of the two laser beams are controlled to reduce the noise of the detection light, and a differential detection module is used to perform differential amplification processing on the laser beams (signal laser and reference laser) acting on and not acting on the atomic clusters to suppress common mode noise. The present invention realizes the separation of the beams of the preparation pulse and the detection pulse, can split the Raman laser into the preparation laser and the detection laser, detect the obtained signal laser and reference laser, so that a longer Ramsey interferometer measurement can be performed, and the spot size of the laser is reduced, the background noise caused by the laser is reduced, and the accuracy of the atomic clock is improved.

[0095] It should be understood that the application of the present invention is not limited to the above examples. For ordinary technicians in this field, improvements or changes can be made based on the above description. All these improvements and changes should fall within the scope of protection of the claims attached to the present invention.

Claims

1. A coherent population trapped cold atomic clock device, characterized in that: include: A Raman light generation and control module is used to generate Raman laser with adjustable frequency difference and controllable switch; A spatial beam separation and control module is connected to the Raman light generation and control module, and is used to split the Raman laser into a detection laser and a preparation laser in a vertical direction, and combine the detection laser and the preparation laser into a Raman laser beam group output; wherein the separation distance between the preparation laser and the detection laser is the falling height of the atomic group in the free evolution time of the Ramsey sequence; Magneto-optical trap module, used to capture and cool atomic groups; A differential detection module is located at the light output side of the spatial beam separation and control module, and is used to split the Raman laser beam component into a signal laser and a reference laser, and perform differential detection and processing on the reference laser and the signal laser to output a Ramsey signal; wherein the signal laser is the laser after interacting with the atomic group in the magneto-optical trap module.

2. The coherent population trapped cold atomic clock device according to claim 1, characterized in that: The Raman light generation and control module includes: a phase modulator, a beam splitting module, an acousto-optic modulator, a radio frequency switch, a first convex lens, a quarter wave plate and a first reflector; wherein, The phase modulator is used to receive the input laser and perform phase modulation on the input laser to generate Raman laser with adjustable frequency difference; The beam splitting module is connected to the phase modulator through an optical fiber, and is connected to the spatial beam separation and control module through an optical fiber. The beam splitting module is used to split the Raman laser into two laser beams with adjustable ratios, and input one of the Raman laser beams into the acousto-optic modulator; The acousto-optic modulator is connected to the radio frequency switch, and the acousto-optic modulator outputs a diffracted light beam according to the radio frequency signal output by the radio frequency switch. The diffracted light beam reaches the first reflector after passing through the first convex lens and the quarter-wave plate in sequence, and is reflected by the first reflector. The beam splitting module outputs a Raman laser with an adjustable frequency difference and controllable switch. The radio frequency switch is connected to a Ramsey sequence signal to control the on or off of the radio frequency signal.

3. The coherent population trapped cold atomic clock device according to claim 2, characterized in that: The beam splitting module includes: a half-wave plate and a polarization beam splitter; The half-wave plate is connected to the phase modulator via an optical fiber and is used to adjust the polarization direction of the Raman laser; The polarization beam splitter is located on the light output side of the half-wave plate and on the light input side of the acousto-optic modulator, and is used to split the Raman laser into two laser beams, and input one of the Raman laser beams into the acousto-optic modulator.

4. The coherent population trapped cold atomic clock device according to claim 2, characterized in that: The spatial beam separation and control module includes: an aperture, a first beam splitter, a first reflector group, a second reflector group, a beam combiner and a third reflector group; wherein, The aperture is connected to the beam splitting module via an optical fiber, and the aperture is used to adjust the spot size of the Raman laser; The first beam splitter is located at the light exit side of the aperture, and is used to split the Raman laser into a detection laser and a preparation laser, wherein the preparation laser reaches the beam combiner via the first reflector group, and the detection laser reaches the beam combiner via the second reflector group; The beam combiner is located at the light exit side of the first reflector group and the second reflector group, and at the light entrance side of the third reflector group, and the beam combiner combines the detection laser and the preparation laser into a Raman laser beam group and inputs the Raman laser beam group to the differential detection module through the third reflector group; The separation distance between the preparation laser and the detection laser is adjusted by adjusting the height between the first reflector group and the second reflector group.

5. The coherent population trapped cold atomic clock device according to claim 4, characterized in that: The first reflector group includes: a second reflector, a third reflector and a first optical switch; The second reflector is located at the first light output side of the first beam splitter, and the prepared laser reaches the beam combiner via the second reflector, the first optical switch, and the third reflector in sequence; The second reflector group includes: a fourth reflector, a fifth reflector and a second optical switch; The fourth reflector is located at the second light-emitting side of the first beam splitter, and the detection laser reaches the beam combiner via the fourth reflector, the second optical switch and the fifth reflector in sequence; The third reflector group includes: a sixth reflector and a seventh reflector; The sixth reflector is located at the light-emitting side of the beam combiner, and the Raman laser beam group is incident on the differential detection module via the sixth reflector and the seventh reflector in sequence.

6. The coherent population trapped cold atomic clock device according to claim 1, characterized in that: The differential detection module includes: a second beam splitter, an eighth reflector, a ninth reflector, a tenth reflector, a second convex lens, a third convex lens, a first probe, a second probe and a signal processing unit; wherein, The second beam splitter is located at the light-exiting side of the spatial beam separation and control module, and is located at the same horizontal plane as the eighth reflector, and the magneto-optical trap module is located between the second beam splitter and the eighth reflector; the ninth reflector is located at the first light-exiting side of the second beam splitter, the magneto-optical trap module is located at the second light-exiting side of the second beam splitter, and the third convex lens is located at the third light-exiting side of the second beam splitter; the second convex lens is located on the reflected light path of the tenth reflector; the first probe is located at the light-exiting side of the second convex lens, and the second probe is located at the light-exiting side of the third convex lens; The second beam splitter splits the Raman laser beam into a signal laser and a reference laser; the signal laser reacts with the atomic group in the magneto-optical trap module and is reflected back to the second beam splitter via the eighth reflector, and is focused to the second probe via the third convex lens; the reference laser is sequentially input to the second convex lens via the ninth reflector and the tenth reflector, and is input to the first probe after being focused by the second convex lens; The signal processing unit is connected to the first probe and the second probe respectively, and is used to perform differential detection and processing on a first acquisition signal detected by the first probe and a second acquisition signal detected by the second probe, and output a Ramsey signal.

7. The coherent population trapped cold atomic clock device according to claim 6, characterized in that: The magneto-optical trap module comprises a magneto-optical trap and a vacuum cavity; the magneto-optical trap has a through hole, and the through hole is located at the light-emitting side of the second beam splitter; the vacuum cavity is located between the second beam splitter and the eighth reflector.

8. A control method for a coherent population trapped cold atomic clock device according to any one of claims 1 to 7, characterized in that: Includes steps: Controlling the input laser, radio frequency signal and Ramsey sequence to be input into the Raman light generation and control module, so as to control the on and off of the radio frequency signal through the Ramsey sequence, so that the Raman light generation and control module can output Raman laser with adjustable frequency difference and controllable switch; Controlling the spatial beam separation and control module to split the Raman laser into a detection laser and a preparation laser in a vertical direction, and combining the detection laser and the preparation laser into a Raman laser beam group to input into a differential detection module; The Raman laser beam group is divided into a signal laser and a reference laser by the differential detection module, and the reference laser and the signal laser are differentially detected and processed to output a Ramsey signal; The microwave frequency of the Raman light generation and control module is adjusted to perform a frequency sweep cycle, so as to perform differential detection and processing on the reference laser and the signal laser to obtain frequency domain Ramsey fringes.

9. The control method of the coherent population trapped cold atomic clock device according to claim 8, characterized in that: In the step of controlling the input laser, the radio frequency signal and the Ramsey sequence to be input into the Raman light generation and control module, so as to control the turning on and off of the radio frequency signal through the Ramsey sequence, so that the Raman light generation and control module outputs a Raman laser with an adjustable frequency difference and controllable switch, the output laser frequency of the Raman laser is the frequency of the input laser minus or plus twice the frequency of the radio frequency signal.

10. The control method of the coherent population trapped cold atomic clock device according to claim 8, characterized in that: In the step of controlling the spatial beam separation and control module to split the Raman laser into a detection laser and a preparation laser in the vertical direction, and combining the detection laser and the preparation laser into a Raman laser beam group to input into the differential detection module, a first optical switch sequence is used to control the turning on and off of the preparation laser, and a second optical switch sequence is used to control the turning on and off of the detection laser, so as to adjust the timing interval between the preparation laser and the detection laser.

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