Method for improving low frequency accuracy of atomic magnetometer

Abstract

The application discloses a method for improving low-frequency precision of an atomic magnetometer, and relates to the technical field of weak magnetic detection. In the method, the closed-loop atomic magnetometer is in a zero-locking state, a direct-current bias system is automatically and accurately adjusted, the atomic magnetometer is caused to work in a critical interval between a zero-locking region and a linear region, the zero-locking region has the suppression on low-frequency magnetic noise, and the linear region has a higher response to a measured magnetic field, so that the low-frequency measurement precision of the atomic magnetometer is improved. The application is suitable for an optical pumping atomic magnetometer system, and is favorable for improving the precision and stability of the atomic magnetometer, and can significantly improve the magnetic field sensitivity in the low-frequency part.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of weak magnetic detection, in particular to a performance optimization technology of an atomic magnetometer based on closed-loop feedback control. BACKGROUND

[0002] Magnetic field measurement is an important means to study physical processes related to magnetic phenomena, and weak magnetism is an important branch of magnetic field measurement. Weak magnetic detection technology has been widely used in medicine, industry, military and many other fields. The spin-exchange relaxation-free (SERF) atomic magnetometer is a new type of weak magnetic field detection sensor system, and is one of the most sensitive magnetic field measurement devices. Its theoretical sensitivity can reach aT level. Compared with other magnetic field measurement devices, the atomic magnetometer has the characteristics of ultra-high sensitivity, small size, etc. It is the most sensitive magnetic field measurement device at present, and is also the current focus of weak magnetic detection research.

[0003] Magnetometer detection has been widely used in biomedical, ocean exploration, earthquake warning and other aspects, and has shown great potential. Many countries have invested huge manpower and financial resources to ensure the research of magnetometer technology. In particular, the atomic magnetometer has become a research hotspot due to its high sensitivity, miniaturization and easy arraying characteristics, and has a promising development prospect and broad application demand.

[0004] Low-frequency noise is mainly composed of current source noise and coupling between magnetic field regulation coil and ferromagnetic material. With the continuous development of atomic magnetometer technology, its sensitivity has been significantly improved, but its sensitivity in the low-frequency part is limited by hardware conditions and is difficult to improve.

[0005] Now, with the development of airborne magnetic gradient systems towards digitization, miniaturization and intelligentization, the upgrade of the more precise optical pumping atomic magnetometer is also imperative. In view of the problem of low sensitivity of the existing atomic magnetometer in the low-frequency part, an optical pumping atomic magnetometer system with larger dynamic range, better stability and higher low-frequency sensitivity is needed. SUMMARY

[0006] The purpose of the present application is to provide a method for improving the low-frequency precision of an atomic magnetometer. In the closed-loop atomic magnetometer in the zero-locking state, the direct current bias system is automatically and accurately adjusted to make it work in the critical interval between the zero-locking region and the linear region. The zero-locking region suppresses the low-frequency magnetic noise, and the linear region has a higher response to the measured magnetic field, thereby improving the low-frequency measurement precision of the atomic magnetometer.

[0007] In order to achieve the above-mentioned target, the present application adopts the following technical solutions:

[0008] The direct current bias system comprises a signal generation module 1, a control module 2, a magnetic field bias coil 3, and a closed-loop atomic magnetometer 4. Further, the specific implementation steps are as follows:

[0009] a. The signal generation module 1 outputs a voltage sweep field signal connected to the magnetic field bias coil 3, where T is the voltage sweep period, and U0 is the maximum amplitude of the voltage sweep. Under the action, the closed-loop atomic magnetometer 4 outputs an amplitude response g(t).

[0010] b. Further, the control module 2 respectively collects the output signals of the closed-loop atomic magnetometer 4 and the signal generation module 1 and , so as to obtain arrays A1[] and B1[]. Further, a continuous period of data A2[] and B2[] is selected from the two arrays respectively.

[0011] c. Further, the control module 2 automatically analyzes the data in the array A2[], and when the following conditions are met and A2[i] is at the falling edge (where i is the position of the data in the array), the control array B2[] is output to the signal generation module 1. Further, the signal generation module 1 channel 2 generates a voltage as a direct current bias signal and outputs it to the magnetic field bias coil 3 of the atomic magnetometer, automatically completing the establishment of the direct current bias state, so that the atomic magnetometer works in the critical region between the lock-in region and the linear region.

[0012] Specifically, the magnetic field bias coil 3 in step a of the technical solution can generate a certain range of uniform magnetic field, the center of the uniform region coincides with the geometric center of the atomic chamber in the closed-loop atomic magnetometer 4, and both are located inside the magnetic shielding cylinder of the closed-loop atomic magnetometer 4.

[0013] Specifically, the atomic magnetometer system in step a of the technical solution uses a phase-sensitive detection technology phase lock amplifier, which directly calculates the amplitude and phase of the measured signal according to the orthogonality of the two output components.

[0014] Specifically, the atomic magnetometer closed-loop control system in step a of the technical solution uses the proportional-integral-derivative (PID) closed-loop control principle to feedback adjust the output response signal of the atomic magnetometer phase lock amplifier. The phase lock amplifier outputs the demodulated amplitude response to the PID feedback module, and then the PID feedback module outputs a signal to the modulation coil of the phase lock amplifier, completing the closed-loop feedback control and ensuring that the output response is locked at zero within a certain range.

[0015] Specifically, the signal generation module 1 in step a of the technical solution can be composed of a signal generator or a DDS signal synthesis device, and the control module 2 can be composed of an FPGA or an MCU module.

[0016] Specifically, in step b of this technical solution, the control system filters out TA consecutive data samples for one period. The filtering method is as follows: select TA consecutive sample values ​​A[TA] from A1[], find their maximum and minimum values, and record their data positions a and b. Then, the sample value A2[TA] for one consecutive period is... , ,..., , Record the data positions c and d when B1[] is located at two consecutive rising edges. Then the sample value B2[TA] for a consecutive period is B2[c], B2[c+1], B2[c+2], ..., B2[d-2], B2[d-1].

[0017] Specifically, in step c of this technical solution, the signal from the closed-loop atomic magnetometer 4 is amplified and demodulated by a lock-in amplifier to calculate the noise power spectral density, thereby completing the high-precision measurement of the low-frequency magnetic field.

[0018] The beneficial effects of this invention are as follows: Unlike common methods for suppressing low-frequency noise, such as introducing acousto-optic modulators, this invention, in the zero-locked state of the closed-loop atomic magnetometer, automatically and precisely adjusts the DC bias system to operate within the critical range between the zero-locked and linear regions. This achieves both suppression of low-frequency magnetic noise in the zero-locked region and a high response to the measured magnetic field in the linear region, thereby improving the low-frequency measurement accuracy of the atomic magnetometer. This invention effectively improves the low-frequency measurement sensitivity of the atomic magnetometer, thus suppressing low-frequency magnetic field noise. Furthermore, based on the closed-loop feedback system of the atomic magnetometer, it enhances the stability of the atomic magnetometer system. The process of this invention is mature, easy to manufacture, and compatible with existing atomic magnetometer systems, which is beneficial for further improving sensitivity in the field of weak magnetic field detection. Attached Figure Description

[0019] Figure 1 Diagram of a closed-loop atomic magnetometer system

[0020] Figure 2 To filter the fitted curve of data A2[] with respect to B2[] for a continuous period.

[0021] Figure 3 This is a schematic diagram of the control process.

[0022] Figure 4 This is a flowchart illustrating the control principle.

[0023] Figure 5 The graph shows the sensitivity curves of the closed-loop atomic magnetometer under open-loop and DC bias conditions. Detailed Implementation

[0024] The following specific examples illustrate the installation method of this invention. Those skilled in the art can easily understand the principles and advantages of this invention from the content of this description. Various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of this invention. The invention is further illustrated below with specific examples.

[0025] Example:

[0026] Step (a):

[0027] A closed-loop atomic magnetometer system was constructed. The system consists of three parts: the first part is the light source system, where a 795nm laser emits light, which is reflected by a mirror and then passes through a polarization-maintaining fiber, a fiber collimator, a polarizer, a quarter-wave plate, and then along the x-axis through the atomic gas cell before finally incident on an S16008-33 silicon photodiode. The second part is the sensing system, which, from the outside in, includes a five-layer permalloy magnetic shielding cylinder, a Lewynn coil system, and the atomic gas cell. The five-layer permalloy magnetic shielding cylinder is placed on the outermost layer to shield the geomagnetic field around the two atomic gas cells, reducing the remanent magnetization of the environment to below 10 nT. The coil system is used to compensate for remanent magnetization, provide a modulation magnetic field, and transmit feedback signals. The atomic gas cell is placed at the center of the coil. The third part is the signal processing system, which includes an S16008-33 silicon photodiode, an HF2TA type transimpedance amplifier, an HF2LI type lock-in amplifier, a closed-loop feedback control module, a data acquisition card, and a computer. The overall system structure is illustrated in the diagram. Figure 1 .

[0028] Step (b):

[0029] Using the output voltage sweep signal of the DG5200 function generator The voltage is applied to the DC bias coil of the atom, where T is the voltage sweep period and U0 is the maximum amplitude of the voltage sweep. T is set to 10 and U0 is set to 0.2V. At this time, the closed-loop atomic magnetometer system outputs the response g(t) under the action of the sweep field.

[0030] Step (c):

[0031] The control system is 10 9 The output signal of the Sa / s sampling lock-in amplifier and function generator and Data, namely A1[] and B1[]. Filter data from a continuous period, A2[] and B2[], and output the fitted curve of A2[] and B2[] as shown in the diagram. Figure 2 .

[0032] In satisfying and A2[i] is at a falling edge (where i is the position of the data in the array), the output is to the function generator.

[0033] The function generator outputs a DC bias voltage signal to the feedback coil of the atomic magnetometer system, completing the construction of the closed-loop feedback control system in the critical bias state of the linear region and the lock-in region. The control process diagram and schematic diagram are shown as follows Figure 3 , 4 .

[0034] Step (d):

[0035] Measure the noise power spectral density N f , the conversion coefficient S between the system voltage and the magnetic field is calculated by fitting the data of A2[] about B2[], and the system sensitivity is calibrated by . Turn off the PID control system and the control system, measure the noise power spectral density in the open-loop state, and obtain the system sensitivity in the open-loop mode. The sensitivity curves of the atomic magnetometer in the open-loop and DC bias states are shown as follows Figure 5 , the magnetic field sensitivity at 1 Hz is reduced from 193 in the open-loop state to 43 , the magnetic field sensitivity at 10 Hz is reduced from 69 in the open-loop state to 56 , and the low-frequency sensitivity is significantly improved compared with the open-loop state.

[0036] The above specific embodiments are only illustrative and not intended to limit the present application. Any person skilled in the art can modify or change the above embodiments without departing from the scope of the present application and the protection scope of the claims, which are all within the protection scope of the present application.

Claims

1. A method for improving the low frequency measurement accuracy of an atomic magnetometer, characterized by: The closed-loop atomic magnetometer (4) is in a zero-lock state, and a direct current bias system is automatically and accurately adjusted to make the closed-loop atomic magnetometer work in a critical interval between a zero-lock region and a linear region. The closed-loop atomic magnetometer has the suppression of low-frequency magnetic noise in the zero-lock region and a higher response to a measured magnetic field in the linear region, so that the low-frequency measurement precision of the atomic magnetometer is improved. The implementation is as follows: The method for improving the low-frequency measurement precision of the atomic magnetometer comprises a direct current bias system, and the system comprises a signal generation module (1), a control module (2), a magnetic field bias coil (3) and a closed-loop atomic magnetometer (4). The control steps are as follows: a. The voltage sweep field signal output by the signal generating module (1) channel 1 is connected to the magnetic field bias coil (4), wherein T is the voltage sweep field period, U 0 is the maximum amplitude of the voltage sweep field, whereby the closed loop atomic magnetometer (4) outputs an amplitude response g ( t ). b, the control module (2) respectively collects the output signal of the closed loop atomic magnetometer (4) and the signal generation module (1) g ( t ) and u 1 ( t ), thereby obtaining the array A 1[], B 1[]; respectively from two arrays, screening out a continuous period of data A 2[], B 2[]。 Wherein, from A 1[] select continuous x data, get its maximum and minimum data position a 、 b , screening out a continuous period of sample value A 2[ x ] is: ; record array B 1[] at two consecutive rising edge data position c 、 d , screening out a continuous period of sample value B 2[ x ] is: B 1[ c ], B 1[ c +1], B 1[ c +2],..., B 1[ d -2], B 1[ d -1]; c. Control module (2) automatically analyzes the array A 2[] data, when meet A 2[ i -1]=0 and A 2[ i ] is in the falling edge, output control array i 2[] to signal generation module (1); by signal generation module (1) channel 2 generates B 2 ( u t )= B 2[ i ] voltage as a DC bias signal and output to the magnetic field bias coil (3) of the closed loop atomic magnetometer (4), automatically complete the establishment of DC bias state, make the closed loop atomic magnetometer (4) work in the critical region of lock zero area and linear area.​ 2. The method of claim 1, wherein the method comprises: The magnetic field bias coil (3) can generate a uniform magnetic field in a certain range, the center of the uniform region of the magnetic field bias coil is coincident with the geometric center of an atomic cell in the closed-loop atomic magnetometer system (4), and both are located in the magnetic shielding cylinder of the closed-loop atomic magnetometer (4).

3. The method of claim 1, wherein the method comprises: The signal generation module (1) can be composed of a signal generator or a DDS signal synthesis device, and the control module (2) can be composed of an FPGA or an MCU module.

4. The method of claim 1, wherein the method comprises: The signal of the closed-loop atomic magnetometer (4) is amplified and demodulated by a lock-in amplifier to complete the calculation of the noise power spectral density, so that the high-precision measurement of the low-frequency magnetic field is completed.

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