Magneto-optical pumping cesium beam atomic clock

Abstract

The invention discloses a magneto-optical pumping cesium beam atomic clock, which comprises a laser, a half-wave plate, a polarization splitting prism, a reflector, a cesium furnace, a state selection magnet, a microwave cavity, a fluorescence collector, a fluorescence detector and a vacuum cesium beam tube, and is characterized in that the cesium furnace, the state selection magnet, the microwave cavity and the fluorescence collector are mounted in the vacuum cesium beam tube; the laser is used for emitting a laser beam, the laser beam is divided into two beams after passing through the half-wave plate and the polarization splitting prism, one beam of laser transmitted by the polarization splitting prism is used as pumping light, and the other beam of laser reflected by the polarization splitting prism is used as detection light; the detection light enters the cesium-beam tube and is received by a fluorescence collector in the cesium-beam tube; the pumping light is reflected by the reflector, then enters the cesium beam tube and acts with cesium atoms emitted by the cesium furnace.

Description

Technical Field

[0001] The present invention relates to the field of atomic frequency standards, and particularly to a magneto-optical pumped cesium beam atomic clock. Background Art

[0002] An atomic clock is a timing device that uses atomic transitions as frequency standards. Atomic clocks have important applications in various fields, from basic scientific measurements such as the determination of physical constants and the verification of theoretical physics, to timekeeping, time dissemination, navigation and positioning, and telecommunication. Among various atomic clocks, the cesium beam atomic clock has a simple structure, high accuracy, and good long-term stability, and is the core of a high-precision and high-stability timekeeping system.

[0003] The working principle of a cesium beam atomic clock is to lock the frequency of a voltage-controlled crystal oscillator to the microwave spectral line of an atomic beam through frequency synthesis. The core component that can form an atomic beam and realize the interaction between the atomic beam and the electromagnetic field is the cesium beam tube. According to different working principles, the current cesium beam tubes can be divided into three categories: magnetic state selection - electron multiplier type, optical pumping - optical detection type, and magnetic state selection - optical detection type.

[0004] The magnetic state selection - electron multiplier type scheme uses two state selection magnets. The first state selection magnet selects atoms with |F = 3, m F > and |F = 4, m F = -4>. In the microwave interaction region, the atoms and microwaves interact. Atoms with |F = 3, m F = 0> transition to |F = 4, m F = 0>. The second state selection magnet selects atoms with |F = 4, m F ≠ -4>. The detection of the atomic state is realized by an electron multiplier. This scheme has good environmental adaptability and a relatively narrow Ramsey spectral linewidth, but the atomic utilization rate is low, resulting in a poor signal-to-noise ratio. In addition, the lifespan problem of the electron multiplier is also a major challenge in the process.

[0005] The optical pumping - optical detection type scheme uses a pumping laser beam resonant with |F = 4> - |F' = 4> in the D2 line of cesium atoms to complete the preparation of the atomic state. In the microwave interaction region, the atoms and microwaves interact. Atoms with |F = 3, m F = 0> transition to |F = 4, m F = 0>. Then, a detection laser beam resonant with |F = 4> - |F' = 5> in the atomic D2 line is used to complete the detection of the atomic state. This scheme has a high atomic utilization rate, so the signal-to-noise ratio is high. However, compared with the magnetic state selection scheme, the Ramsey spectral linewidth is worse. In addition, since the frequencies of the pumping light and the detection light are different, an acousto-optic modulator (AOM) needs to be introduced, resulting in a complex optical system.

[0006] The magnetic state selection - optical detection type scheme uses a state selection magnet to select |F = 3, m F>and |F = 4, m F = -4> atoms, in the microwave interaction region, the atoms and the microwave interact. |F = 3, m F = 0> atoms transition to |F = 4, m F = 0>, and then a laser beam resonant with the |F = 4> - |F' = 5> in the atomic D2 line is used to complete the detection of the atomic state. This scheme has a relatively narrow Ramsey spectral linewidth. Optical detection avoids the lifetime process problems of electron multipliers, but the utilization efficiency of the atomic beam is relatively low, and |F = 4, m F = -4> atoms will introduce additional noise, resulting in a poor signal-to-noise ratio. Summary of the Invention

[0007] For this reason, the present invention proposes a magneto-optical pumped cesium beam atomic clock. In this scheme, the cesium furnace is fixed on the cesium furnace displacement table for precisely controlling the incident direction of the cesium atomic beam; a state-selecting magnet is used to shift the atomic energy levels. Therefore, a pumping laser beam resonant with the |F = 3> - |F' = 2> in the cesium atomic D2 line can be used to complete the preparation of the atomic state. A reverse state-selecting magnet is used to select |F = 4, m F ≠ -4> atoms. In the microwave interaction region, the atoms and the microwave interact. |F = 4, m F = 0> atoms transition to |F = 3, m F = 0>, and then a laser beam resonant with the |F = 3> - |F' = 2> in the atomic D2 line is used to complete the detection of the atomic state. Compared with the traditional optical pumping clock, the frequencies of the detection laser and the pumping laser are the same, eliminating the need for a laser frequency shifter (AOM), simplifying the physical system, and improving the system stability; the reverse state-selecting magnet selects |F = 4, m F ≠

[0008] -4> atoms, avoiding the noise introduced by stray atoms; the pumping light improves the utilization efficiency of the atoms. The state-selecting magnet can also select the atomic beam velocity, narrowing the linewidth of the Ramsey spectral line and having a higher potential for frequency stability.

[0009] To achieve the objectives of the present invention, the following technical solutions are adopted:

[0010] A magneto-optical pumped cesium beam atomic clock, comprising a laser, a half-wave plate, a polarization beam splitter prism, a mirror, a cesium furnace, a state selection magnet, a microwave cavity, a fluorescence collector, a fluorescence detector, and a vacuum cesium beam tube, wherein: the cesium furnace, the state selection magnet, the microwave cavity, and the fluorescence collector are installed in the vacuum cesium beam tube; the laser is used to emit a laser beam, which is divided into two beams after passing through the half-wave plate and the polarization beam splitter prism. One beam of laser transmitted by the polarization beam splitter prism is used as the pumping light, and the other beam of laser reflected by the polarization beam splitter prism is used as the detection light; the detection light enters the cesium beam tube and is received by the fluorescence collector in the cesium beam tube; the pumping light enters the cesium beam tube after being reflected by the mirror and interacts with the cesium atoms emitted by the cesium furnace.

[0011] The described magneto-optical pumped cesium beam atomic clock, wherein: the state selection magnet is of a two-wire type structure.

[0012] The described magneto-optical pumped cesium beam atomic clock, wherein: the magnetic field magnitude at the position where the pumping light interacts with the atoms is 300 Gauss.

[0013] The described magneto-optical pumped cesium beam atomic clock, wherein a reverse state selection magnet is used to select atoms with |F = 4, m F ≠ -4>, and the laser frequency is locked to the cesium atom D2 line |F = 3> - |F′ = 2>.

[0014] The described magneto-optical pumped cesium beam atomic clock, wherein the optical detection region is composed of a fluorescence collector and a detection coil.

[0015] A method for controlling a magneto-optical pumped cesium beam atomic clock, wherein the optical pumped cesium beam atomic clock is the magneto-optical pumped cesium beam atomic clock described above, and is characterized by including the following steps:

[0016] (1) The laser is first divided into two beams of laser by a half-wave plate and a polarization beam splitter prism. One beam of laser is used as the pumping light, and the other beam of laser is used as the detection light;

[0017] (2) The microwave frequency is pre-adjusted to be near 9.192631770 GHz;

[0018] (3) Only the detection laser is injected into the cesium beam tube, the beam spectrum of the cesium atomic beam is measured, and the laser is locked to the cesium atom D2 line |F = 3> - |F′ = 2>;

[0019] (4) After the laser locking is completed, the pumping laser is injected into the cesium beam tube, and the position of the pumping light is adjusted to make the output fluorescence signal maximum;

[0020] (5) The microwave signal is frequency-modulated, and the output signal of the fluorescence detection circuit is demodulated with the same modulation signal, so as to obtain a frequency discrimination signal. The servo circuit processes the frequency discrimination signal through proportional-integral-differential (PID) and feeds it back to the frequency synthesis circuit to realize the closed-loop locking of the atomic clock. Description of the Drawings

[0021] Appendix Figure 1 Schematic diagram of a novel magneto-optical pumped cesium beam atomic clock;

[0022] Appendix Figure 2 Energy level diagram of the cesium atom D2 line;

[0023] Appendix Figure 3 Eigenenergy of the ground state energy level of cesium atoms under a magnetic field;

[0024] Appendix Figure 4 Effective magnetic moment of the ground state energy level of cesium atoms under a magnetic field;

[0025] Appendix Figure 5 Magnetic field distribution diagram of the state selection magnet;

[0026] Appendix Figure 6 Schematic diagram of the relationship between pumping efficiency and laser frequency;

[0027] Appendix Figure 7 Schematic diagram of the relationship between the position where the pumping light interacts with the atoms and the pumping efficiency;

[0028] Appendix Figure 8 Schematic diagram of the splitting of the excited state energy level;

[0029] Appendix Figure 9 Schematic diagram of the splitting of the ground state energy level;

[0030] Appendix Figure 10 Schematic diagram of the structure of the cesium furnace displacement stage. Detailed Description of the Invention

[0031] The following will combine with the appendix Figures 1-10 to describe in detail the specific implementation manners of the present invention.

[0032] The basic components of the existing magneto-state selection - optical detection type cesium beam tube include a collimator, a state selection magnet, a microwave cavity, an optical collector, etc. After being heated, cesium atoms are ejected by a folded tape type collimator to form a thermally equilibrated atomic beam. The atomic energy levels are evenly distributed among the 16 magnetic sub-levels of the ground states |F = 3> and |F = 4>. Figure 3 shows the eigenenergy levels of cesium atoms under a magnetic field. Since the atoms are in a magnetic field, the effective magnetic moment of the atoms in the |F,m F > state is

[0033]

[0034] where μ B is the Bohr magneton, g J , g I are the Landé g-factors of the electron and nuclear spins respectively, and I = 7 / 2 is the cesium nuclear spin quantum number, which is defined by the following formula:

[0035]

[0036] E HFS is the hyperfine energy level spacing.

[0037] After the applicant's research on the existing magnetic state selection - optical detection type cesium beam tube, it is found that the atomic beam will deflect under a non - uniform magnetic field. According to the principles of quantum mechanics, the force on a cesium atom in a magnetic field is:

[0038]

[0039] where μ eff (,F,m F ) represents the effective magnetic moment of an atom in the state |F,m F > under the magnetic field B, and its calculation result is as Figure 4 , represents the magnetic field gradient at the r position. From the above formula, the atoms in the states |F = 3,m F = - 3, - 2, …, + 3> and |F = 4,m F = - 4> will deflect towards the direction of the strong magnetic field, while the atoms with |F = 4,m F ≠ - 4> will deflect towards the direction of the weak magnetic field. That is, after the action of the state - selection magnet, the atomic beam will be divided into two beams.

[0040] Based on this discovery, the applicant has proposed the following technical solution.

[0041] As Figure 1 , the magneto - optical pumped cesium beam atomic clock of the present invention includes an 852 nm laser 1, a half - wave plate 2, a polarization beam - splitting prism 3, a mirror 4, a cesium furnace 5, a state - selection magnet (including a state - selection magnet concave pole head 6 and a state - selection magnet convex pole head 7), a microwave cavity 9, a fluorescence collector 10, a detection coil 11, a frequency synthesizer circuit 12, a fluorescence detector 13, a servo system 14, a laser frequency stabilization system 15, a cesium furnace displacement stage 16, etc.; the cesium furnace 5, the state - selection magnet concave pole head 6, the state - selection magnet convex pole head 7, the microwave cavity 9, the fluorescence collector 10, the detection coil 11, and the cesium furnace displacement stage 16 are installed in the vacuum cesium beam tube for adjusting the position of the cesium furnace 5.

[0042] The 852 nm laser 1 is used to emit a laser beam. After passing through a half-wave plate 2 and a polarization beam splitter prism 3, the laser beam is divided into two beams. One beam transmitted by the polarization beam splitter prism is used as the pumping light, and the other beam reflected by the polarization beam splitter prism is used as the detection light. The intensity ratio of the pumping light and the detection light can be controlled by adjusting the angle between the fast axis of the half-wave plate and the polarization beam splitter prism. The pumping light is reflected by a mirror 4 and then enters the cesium beam tube, where it interacts with the cesium atomic beam 8 emitted by the cesium oven 5, and almost all the atoms in the |F = 3> state in the atomic beam are pumped to |F = 4>. The detection light is reflected by the polarization beam splitter prism 3 and enters the cesium beam tube, interacts with the cesium atoms and emits fluorescence. The fluorescence is received by the fluorescence collector 10 inside the cesium beam tube. The fluorescence collector 10 sends the collected fluorescence signal to the fluorescence detector 13 outside the vacuum cesium beam tube, and the fluorescence detector 13 converts the fluorescence signal into an electrical signal.

[0043] The laser is locked at the |F = 3> - |F' = 2> of the cesium atom D2 line through the fluorescence signal generated by the interaction between the detection light and the cesium atomic beam (as Figure 2 shown).

[0044] As Figure 1 shown, in the present invention, the cesium oven 5 is fixed on the cesium oven displacement stage 16, which is used to control the incident position of the cesium atomic beam. The state selection magnet adopts a two-line structure, including a state selection magnet concave pole head 6 and a state selection magnet convex pole head 7. The state selection magnet concave pole head 6 is on the top, and the state selection magnet convex pole head 7 is on the bottom. The position and emission direction of the cesium oven 5 can be adjusted through the cesium oven displacement stage 16, so that the incident direction of the cesium atomic beam points to the state selection magnet convex pole head 6, and the cesium atom incident direction points to the convex pole head. At this time, the state selection magnet is a reverse state selection magnet. The two-line structure state selection magnet can be equivalent to two current lines under ideal conditions, and the magnetic field magnitude is

[0045]

[0046] where μ0 is the magnetic medium constant, I Mag is the equivalent current corresponding to the two-line structure magnet, r1 and r2 are the positions between the point to be calculated and the two equivalent current lines respectively, and 2 Mag is the distance between the two equivalent current lines. Through finite element simulation analysis, under the condition that the central magnetic field meets the atomic deflection requirements, the spatial magnetic field distribution of the state selection magnet is as Figure 5 shown. The origin is set on the left end face of the state selection magnet. In the region from 5 mm to 27 mm away from the state selection magnet, the magnetic field intensity changes from 800 Gauss to 0 Gauss.

[0047] Considering the Zeeman effect caused by the magnetic field of the state selection magnet, the ground state energy level and the excited state energy level corresponding to the cesium atom D2 transition line are split into multiple sub-energy levels, and the energy of each sub-energy level is shown in the following formula:

[0048]

[0049] where F, m F , I are the total angular momentum quantum number of the atom, the magnetic quantum number of the total angular momentum, and the nuclear spin quantum number respectively; ΔE0 = hν0, which is the energy difference of hyperfine splitting without magnetic field; μ B is the Bohr magneton, g’ I is the nuclear Landé factor, B is the magnetic induction intensity, and x is the dimensionless value characterizing the magnetic field, defined as

[0050]

[0051] where g J is the electron Landé factor.

[0052] Since the magnetic field change range of the state-selecting magnet in the outer space is from 0 to 800 Gauss, the energy level splittings of the excited state and the ground state are respectively as Figure 8 and Figure 9 shown. When the magnetic field satisfies

[0053] the frequency shift between |F = 3> - |F′ = 3> is 151 MHz, the pumping of the atomic beam can be realized by using the laser resonant with

[0054] |F = 3> - |F′ = 2>. At this time, the magnetic field is about 200 Gauss. The magnitude of this magnetic field can be controlled by adjusting the positions of the light beam, the light-atom interaction region, and the state-selecting magnet. The relationship between the pumping efficiency and the laser frequency is as Figure 6 shown, and the relationship between the pumping efficiency and the light beam position is as Figure 7 shown.

[0055] The incident position of cesium atoms determines the atomic motion trajectory. Different motion trajectories correspond to different magnetic field magnitudes, which will then select the atomic velocity, thereby affecting the number of atoms and the Ramsey linewidth, and ultimately reflected in the frequency stability of the atomic clock. In a conventional atomic clock, the position of the cesium furnace is fixed and non-adjustable, resulting in the inability to improve the performance by optimizing the position of the cesium furnace, restricting the flexibility of adjustment and the optimization space. Therefore, this patent adds a cesium furnace displacement stage 16. As Figure 10As shown in the figure, the cesium furnace displacement stage 16 includes a base 101, a lateral movement mechanism installed on the base 101, a longitudinal movement mechanism installed on the lateral movement component, and a pan-tilt head installed on the longitudinal movement mechanism. The cesium furnace is installed on the pan-tilt head. The lateral movement mechanism includes a base 101, a first bracket 102 and a second bracket 102 installed on the left and right sides of the upper surface of the base 101, a lead screw 103 installed between the first bracket 102 and the second bracket 102, a nut 106 threadedly coupled to the lead screw 103, a lateral slide 108 fixedly connected to the nut 106. The bottom of the lateral slide 108 is provided with a chute or a slide rail, which cooperates with the slide rail or the slide on the upper surface of the base 101, so that the lateral slide 108 can move smoothly left and right. The lead screw 106 is driven by a first motor (not shown in the figure). The forward and reverse rotation of the first motor drives the forward and reverse rotation of the lead screw 103, so that the nut 106 moves left or right along the lead screw 106, and the lateral slide 108 moves left or right accordingly.

[0056] The longitudinal movement mechanism includes a support frame 107 and a longitudinal slider 104. The support frame 107 is installed on the lateral slide 108, and the longitudinal slider 104 is installed on the support frame 107. The installation method is a chute and slide rail cooperation method. For example, a chute is machined on the support frame 107, and a slide rail that cooperates with it is machined on the longitudinal slider 104; or a slide rail is machined on the support frame 107, and a chute that cooperates with it is machined on the longitudinal slider 104. The support frame 107 is also provided with a screw rod (not shown in the figure). The screw rod is arranged perpendicular to the lateral slide 108. A threaded hole is machined on the longitudinal slider 104, and the longitudinal slider 104 is sleeved on the screw rod through this threaded hole; the screw rod is driven by a second motor (not shown in the figure). The forward and reverse rotation of the second motor drives the forward and reverse rotation of the screw rod, and the longitudinal slider 104 moves up or down along the screw rod.

[0057] The pan-tilt head 105 is installed on the longitudinal slider 104, and the cesium furnace 5 is installed on the pan-tilt head 105. When the longitudinal slider 104 moves up and down, the pan-tilt head 105 moves up and down accordingly, and the cesium furnace 5 also moves up and down accordingly. By controlling the driving mechanism of the pan-tilt head 105, the rotation angle of the cesium furnace 5 can be adjusted.

[0058] When the cesium furnace displacement stage 16 is working, the first motor, the second motor, and the pan-tilt head 105 are controlled to work respectively through an external control device, and the horizontal and vertical positions and the rotation angle of the cesium furnace can be adjusted to the required optimal positions.

[0059] After the cesium atoms are heated by the cesium atomic furnace, they are ejected through a collimator and interact with the pumping light. At the position where the pumping light interacts with the cesium atoms, the magnetic field generated by the state selection magnet causes the atomic energy level to shift, so as to lock on the cesium atomic D2 line |F = 3> - |F' = 2> to achieve laser pumping, which has a similar pumping efficiency to the traditional cesium atomic D2 line |F = 3> - |F' = 3> laser pumping.

[0060] After pumping, under the action of the concave pole head 6 and the convex pole head 7 of the state-selecting magnet, the atoms in the state of |F = 4, m F ≠ -4> will be deflected by the magnetic force and enter the U-shaped microwave cavity 9 to complete the preparation of the atomic state.

[0061] A static magnetic field (C field) is introduced into the U-shaped microwave cavity 9. Through the interaction with the microwave, the atoms initially in the state of | = 4, m F = 0> will undergo Ramsey resonance and transition to the state of |F = 3, m F = 0>.

[0062] When detecting with a laser corresponding to the frequency of the |F = 3> - |F' = 2> of the cesium atom D2 line, the cesium atom beam spectrum corresponds to the fluorescence signal released by the atoms in the state of |F = 3>. After laser pumping, the cesium atom beam spectrum increases. Adjust the position of the interaction between the laser and the atoms to make the atom beam spectrum maximum, that is, the position corresponding to the maximum laser pumping efficiency. As Figure 7 shown, it describes the relationship between the position of the interaction between the pumping light and the atoms and the pumping efficiency of the laser locked to the |F = 3> - |F' = 2> of the cesium atom D2 line. The relative position refers to the position with the best relative pumping efficiency.

[0063] Finally, perform the closed-loop locking of the microwave. First, pre-adjust the microwave frequency, then input the detection laser, and lock the laser to the cesium atom D2 line

[0064] |F = 3> - |F' = 2> using the cesium atom beam spectrum. Then input the pumping light and adjust the position of the pumping light to make the output fluorescence signal maximum, that is, the corresponding pumping efficiency is maximum.

[0065] Frequency modulate the microwave signal and demodulate the output signal of the fluorescence detection circuit 13 with the same modulation signal to obtain the frequency discrimination signal. The servo system 14 processes the frequency discrimination signal through proportional-integral-differential (PID) and feeds it back to the frequency synthesis circuit 12 to achieve the closed-loop locking of the atomic clock.

[0066] The above is only a preferred embodiment of the present invention and does not impose any form of limitation on the present invention. Although the present invention has been disclosed as above with a preferred embodiment, it is not intended to limit the present invention. Any person skilled in the art can make some changes or modifications to equivalent embodiments by using the disclosed technical content within the scope of the technical solution of the present invention. However, as long as it does not depart from the content of the technical solution of the present invention, any simple modification, equivalent change and modification made to the above embodiments based on the technical essence of the present invention still fall within the scope of the technical solution of the present invention.

Claims

1. A magneto-optical pumped cesium beam atomic clock, comprising a laser, a half-wave plate, a polarization beam splitter prism, a mirror, a cesium furnace, a state selection magnet, a microwave cavity, a fluorescence collector, a detection coil, a fluorescence detector, and a vacuum cesium beam tube, characterized in that: A cesium furnace, a state-selective magnet, a microwave cavity, a fluorescence collector, and a detection coil are installed in a vacuum cesium beam tube; a laser is used to emit a laser beam, which is split into two beams after passing through a half-wave plate and a polarization beam splitter prism. One beam of laser transmitted by the polarization beam splitter prism is used as pumping light, and the other beam of laser reflected by the polarization beam splitter prism is used as detection light; the pumping light enters the cesium beam tube after being reflected by a mirror and interacts with the cesium atoms emitted by the cesium furnace to complete the preparation of the atomic state; the detection light enters the cesium beam tube and interacts with the cesium atomic beam to release fluorescence, which is collected by the fluorescence collector to complete the detection of the atomic state.

2. The magneto-optical pumped cesium beam atomic clock according to claim 1, wherein: The state-selective magnet has a two-line structure and includes a state-selective magnet concave pole head and a state-selective magnet convex pole head.

3. A magneto-optical pumped cesium beam atomic clock according to claim 1, characterized in that: The magnetic field magnitude at the position where the pumping light interacts with the atoms is 300 Gauss.

4. A magneto-optical pumped cesium beam atomic clock according to claim 1, characterized in that The incident direction of cesium atoms points to the salient pole head, that is, the reverse state-selective magnet selects atoms with |F = 4, m F ≠ -4>, and the laser frequency is locked to the D2 line of cesium atoms |F = 3> - |F ′ = 2>.

5. A magneto-optical pumped cesium beam atomic clock according to claim 1, characterized in that The fluorescence collector and the detection coil form an optical detection region.

6. A method for controlling a magneto-optical pumped cesium beam atomic clock, wherein the optical pumped cesium beam atomic clock is the magneto-optical pumped cesium beam atomic clock according to any one of claims 1-5, characterized in that It includes the following steps: (1) The laser is first split into two laser beams by a half-wave plate and a polarization beam splitter prism. One beam of laser is used as pumping light, and the other beam of laser is used as detection light; (2) The microwave frequency is pre-adjusted to be near 9.192631770 GHz; (3) Only inject the detection laser into the cesium beam tube, measure the beam spectrum of the cesium atomic beam, and lock the laser on the cesium atomic D2 line |F = 3> - |F ′ = 2>; (4) After the laser locking is completed, the pumping laser is injected into the cesium beam tube, and the position of the pumping light is adjusted to make the output fluorescence signal maximum; (5) The microwave signal is frequency-modulated, and the output signal of the fluorescence detection circuit is demodulated with the same modulation signal, and then the frequency discrimination signal can be obtained. The servo circuit processes the frequency discrimination signal through proportional-integral-derivative (PID) and feeds it back to the frequency synthesis circuit to realize the closed-loop locking of the atomic clock.

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