Microwave power servo method for an atomic-beam-based microwave atomic clock system

Abstract

The application discloses a microwave power servo method of an atomic-beam-based microwave atomic clock system, and the steps of the method comprise the following steps: 1) dividing a microwave power range to be scanned in a microwave power servo period into 2N+1 scanning points in succession and at equal intervals, and the power interval between adjacent scanning points is a unit; in the clock frequency servo process of the microwave atomic clock system, servo feedback of the clock frequency is paused every certain period of time, a microwave power servo period is started, and the detuning of the microwave frequency of the microwave area in the microwave atomic clock system relative to the clock transition frequency is set to 0; in the clock frequency servo process, servo feedback of the clock frequency is paused every certain period of time, and a microwave power servo period is started; in a microwave power servo period, microwave power is scanned twice, once from low to high and once from high to low; and then the extreme position obtained by third-order fitting in the two directions is averaged to serve as the final microwave power servo locking reference.

Description

Technical Field

[0001] This invention relates to the field of microwave band frequency standards based on atomic beams, specifically to a microwave power locking method for a microwave atomic clock system based on atomic beams. Background Technology

[0002] The essence of an atomic clock is to lock the clock frequency to a highly stable transition energy level inside an atom through electronic means, thereby achieving a frequency output with extremely high stability. In cesium beam microwave clocks, the stability of microwave power significantly affects the long-term stability of the atomic clock frequency, typically requiring power servo locking.

[0003] The traditional mainstream approach is to use a second-order fitting method to lock the microwave power to the extreme value of the curve of Ramsey transition probability versus microwave power (hereinafter referred to as the microwave power curve). This position is called the optimal microwave power. Figure 1 (a) shows b opt The horizontal axis represents the Rabi frequency *b*, which is proportional to the microwave power, and the vertical axis represents the Ramsey transition probability under zero detuning. It can be seen that this curve is not axially symmetric; therefore, the extreme values ​​found through second-order fitting will always deviate slightly from the actual extreme values, resulting in locking offset, such as... Figure 1 (a) shows the comparison results of second-order and third-order curve fitting.

[0004] on the other hand, Figure 1 (a) The transition probability curve, obtained experimentally through the interaction between the laser and atoms, is therefore also affected by the laser power. Laser power drift directly causes a shift in the curve's background, such as... Figure 1 As shown in (b), this further leads to a deviation in the final servo locking result relative to the extreme point of the target, ultimately affecting the frequency stability performance of the cesium clock. Summary of the Invention

[0005] To overcome the inherent locking bias in second-order curve fitting caused by curve asymmetry, this invention proposes a microwave power servoing method for a microwave atomic clock system based on atomic beams. This invention employs a third-order curve fitting method, which can more accurately fit the extreme points of the curve. To overcome the locking bias caused by curve background drift, this invention further proposes a bidirectional third-order fitting method to eliminate the influence of background drift.

[0006] In summary, the technical solution adopted by this invention to solve its technical problem is as follows:

[0007] 1. Within a microwave power range of approximately ±2dB near the extreme value of the curve, 127 points are continuously scanned at equal intervals, and the extreme value of the curve is found by fitting a third-order polynomial.

[0008] 2. Repeat the above process twice. The first time, scan the microwave power from low to high, and the second time, scan the microwave power from high to low. Then, take the average of the extreme value positions obtained by the third-order fitting in the two directions, which is the final microwave power servo locking reference.

[0009] The technical solution of this invention is as follows:

[0010] A microwave power servoing method for a microwave atomic clock system based on atomic beams, comprising the following steps:

[0011] 1) Divide the microwave power range to be scanned within one microwave power servo cycle into 2N+1 consecutive equally spaced scanning points, with the power interval between adjacent scanning points being one unit; During the clock frequency servoing process of the atomic beam-based microwave atomic clock system, pause the clock frequency servo feedback and start one microwave power servo cycle at regular intervals, and set the detuning of the microwave frequency of the microwave region in the atomic beam-based microwave atomic clock system relative to the clock transition frequency to 0; Complete steps 2) to 6) in one microwave power servo cycle;

[0012] 2) Based on the current microwave power x0, reduce it by N units as the starting point of the scan, and then gradually increase it by 1 unit until it increases by 2N units and then stop scanning. At each scan point, collect a fluorescence signal from the detection area to obtain the fluorescence signal values ​​corresponding to 2N+1 microwave powers.

[0013] 3) Based on the 2N+1 fluorescence signal values ​​obtained in step 2), perform third-order curve fitting, and denote the extreme points of the third-order fitted curve as x1;

[0014] 4) Increase the microwave power x0 by N units as the starting point of the scan, and then gradually decrease it by 1 unit until it decreases by 2N units and then stop scanning. At each scan point, a fluorescence signal is collected from the detection area to obtain the fluorescence signal values ​​corresponding to 2N+1 microwave powers.

[0015] 5) Based on the 2N+1 fluorescence signal values ​​obtained in step 4), perform third-order curve fitting, and denote the extreme points of the third-order fitted curve as x2.

[0016] 6) Update the microwave power setting to (x1+x2) / 2, end the current microwave power servo cycle and restore the clock frequency servo feedback.

[0017] Furthermore, the microwave power range to be scanned is the microwave power range within ±2dB near the extreme value of the microwave power curve; N = 63.

[0018] Furthermore, the microwave power servo cycle is executed via a digital-to-analog converter.

[0019] Furthermore, the microwave atomic clock system based on atomic beams includes an optically pumped cesium beam atomic clock system, an optically pumped rubidium beam atomic clock, and a magnetically separated optical detection atomic clock.

[0020] Furthermore, the time period is 100 seconds, and the microwave power servo cycle is 1 second.

[0021] A microwave atomic clock system based on atomic beams is characterized by using the above-described method for microwave power servoing.

[0022] The beneficial effects of the bidirectional third-order fitting method for servo-locking microwave power in this invention are:

[0023] 1) Compared with the traditional second-order fitting servo locking method, the method of the present invention has almost no locking deviation without increasing the system complexity, thus achieving more accurate locking.

[0024] 2) Within each servo scan cycle, considering the very short time, less than one second, the background drift can be regarded as linear. The locking error caused by the background drift can be eliminated by the bidirectional scan fitting method.

[0025] 3) Through experimental testing, under the same conditions, the long-term stability of the cesium clock is about 2E-14 when using the traditional second-order fitting method to servo lock the microwave power; while the long-term stability of the cesium clock can reach within 1E-14 using the method of the present invention, which is about twice as high. Attached Figure Description

[0026] Figure 1 This is a scan curve and a schematic diagram of curve fitting of Ramsay transition probability relative to microwave power;

[0027] (a) Complete microwave power curve and comparison of second-order and third-order curve fitting.

[0028] (b) Curve extreme value deviation caused by background drift.

[0029] Figure 2 This is a schematic diagram of the structure of an optically pumped cesium clock.

[0030] Figure 3 It is an atomic energy level diagram. Detailed Implementation

[0031] The present invention will be further described below with reference to the accompanying drawings and specific implementation examples in an optically pumped cesium beam atomic clock, but the examples in the drawings do not constitute any limitation on the present invention.

[0032] like Figure 2As shown, in the optically pumped cesium clock system, the cesium beam tube is kept in a vacuum environment, maintained by a titanium pump; the cesium furnace is heated to approximately 100 degrees Celsius, and cesium atoms are ejected from the collimator nozzle at a velocity of 215 m / s (the most probable velocity), forming a cesium atom beam; as... Figure 3 As shown, the cesium atom beam is initially uniformly distributed across the ground state sub-levels F=3 and F=4. After passing through the pump region, all cesium atoms are pumped by the laser to the ground state F=3 sub-level. Then, it enters the Ramsey separation oscillation field, also known as the microwave region. The microwave frequency is obtained by frequency doubling of a 10MHz clock frequency. When cesium atoms interact with the microwave, some atoms transition to the ground state F=4 sub-level. The transition probability is related to the detuning of the microwave frequency; the smaller the microwave frequency detuning, i.e., the closer it is to the transition frequency between the ground state F=3 and F=4, the more atoms return to the ground state F=4 sub-level. The atom beam then enters the detection region. Only atoms at the F=4 sub-level can interact with the detection light, generating fluorescence. By analyzing the intensity of the fluorescence, the deviation information of the microwave frequency, i.e., the deviation information of the clock frequency, can be obtained, further enabling feedback correction.

[0033] In the microwave region, fluctuations in microwave power can affect the frequency locking of atomic clocks through various means, such as the second-order Doppler effect, Rabi traction, and Ramsey traction. Experiments have shown that optically pumped cesium clocks are sensitive to microwave power by approximately 2E-12 / dB, which is fatal for cesium clocks that require long-term stability at the 1E-14 level, necessitating servo control.

[0034] Research has shown that locking the microwave power at the optimal microwave power b opt place (e.g.) Figure 1 The extreme position of the curve shown represents the optimal solution. The specific implementation process is as follows: During the continuous servo feedback of the clock frequency, every certain period (typically 100 seconds), the servo feedback of the clock frequency is paused and replaced with a microwave power servo cycle, approximately 1 second. This 100:1 time-division multiplexing method ensures that the microwave power servoing has almost no impact on the clock frequency servoing, meaning it does not degrade the short-term stability of the clock frequency (within 100 seconds). Furthermore, microwave power drift is itself a long-term process, and its impact on the atomic clock is concentrated on long-term stability (above 10,000 seconds). Therefore, servo locking the microwave power once every 100 seconds is sufficient to achieve the goal.

[0035] The entire servo process is completed by a microcontroller program. The microwave power control is accomplished by a 12-bit digital-to-analog converter (DAC), with a control range of 0 to 4095 units. The specific steps are as follows:

[0036] (1) During the servo process of the clock frequency, pause once every certain period of time (typically 100 seconds) to maintain the voltage of the control terminal of the clock frequency and set the detuning of the microwave frequency relative to the clock transition frequency to 0.

[0037] (2) Divide the range of microwave power to be scanned within a single microwave power servo cycle into 2N+1 consecutive equally spaced scanning points. The power interval between adjacent scanning points is defined as one unit. In practice, N can be 63. Based on the current microwave power (set as x0), first reduce it by 63 units (x0-63), and then gradually increase it by 1 unit until it increases by 126 units and stops (x0+63). After each step setting, the fluorescence signal of the detection area is synchronously collected to obtain the fluorescence signal values ​​corresponding to each of the 127 microwave powers.

[0038] (3) Based on the fluorescence data with a scanning range of ±63 units above, perform third-order curve fitting, calculate the extreme point of the curve, and denot it as x1;

[0039] (4) Repeat the process of (2) and (3) above. The only difference is that the microwave power is first set to x0+63 and then gradually reduced. The extreme point of the curve obtained by fitting is recorded as x2.

[0040] (5) Update the microwave power setting to (x1+x2) / 2, and then restore the servo cycle of the clock frequency.

[0041] After the improvements made in this invention, the long-term stability of the optically pumped cesium beam atomic clock has been improved from 2E-14 to within 1E-14, representing a performance increase of approximately 2 times.

[0042] Finally, it should be noted that the present invention is not limited to optically pumped cesium beam atomic clock systems, but can also be applied to microwave atomic clock systems based on atomic beams, such as optically pumped rubidium beam atomic clocks and magnetically separated optical detection atomic clocks. These systems can all improve the locking effect of microwave power and enhance clock performance through the present invention.

[0043] Although specific embodiments of the invention have been disclosed for illustrative purposes to aid in understanding and implementing the invention, those skilled in the art will understand that various substitutions, variations, and modifications are possible without departing from the spirit and scope of the invention and the appended claims. Therefore, the invention should not be limited to the content disclosed in the preferred embodiments, and the scope of protection claimed by the invention is defined by the claims.

Claims

1. A microwave power servoing method for a microwave atomic clock system based on atomic beams, comprising the following steps: 1) Divide the microwave power range to be scanned within one microwave power servo cycle into 2N+1 consecutive equally spaced scanning points, with a power interval of one unit between adjacent scanning points; during the clock frequency servoing process of the atomic beam-based microwave atomic clock system, pause the clock frequency servo feedback and start one microwave power servo cycle at regular intervals, setting the detuning of the microwave frequency in the microwave region of the atomic beam-based microwave atomic clock system relative to the clock transition frequency to 0; complete steps 2) to 6) within one microwave power servo cycle, wherein the microwave power range to be scanned is the microwave power range within ±2dB near the extreme value of the microwave power curve; N=63; 2) Based on the current microwave power x0, reduce it by N units as the starting point of the scan, and then gradually increase it by 1 unit until it increases by 2N units and then stop scanning. At each scan point, collect a fluorescence signal from the detection area to obtain the fluorescence signal values ​​corresponding to 2N+1 microwave powers. 3) Based on the 2N+1 fluorescence signal values ​​obtained in step 2), perform third-order curve fitting, and denote the extreme points of the third-order fitted curve as x1; 4) Increase the microwave power x0 by N units as the starting point of the scan, and then gradually decrease it by 1 unit until it decreases by 2N units and then stop scanning. At each scan point, a fluorescence signal is collected from the detection area to obtain the fluorescence signal values ​​corresponding to 2N+1 microwave powers. 5) Based on the 2N+1 fluorescence signal values ​​obtained in step 4), perform third-order curve fitting, and denote the extreme points of the third-order fitted curve as x2; 6) Update the microwave power setting to (x1+x2) / 2, end the current microwave power servo cycle and restore the clock frequency servo feedback.

2. The method according to claim 1, characterized in that, The microwave power servo cycle is executed by a digital-to-analog converter.

3. The method according to claim 1 or 2, characterized in that, The microwave atomic clock system based on atomic beams includes an optically pumped cesium beam atomic clock system, an optically pumped rubidium beam atomic clock, and a magnetically separated optical detection atomic clock.

4. The method according to claim 1 or 2, characterized in that, The time period is 100 seconds, and the microwave power servo cycle is 1 second.

5. A microwave atomic clock system based on atomic beams, characterized in that, Microwave power servoing is performed using the method described in claim 1.

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