Active graded magnetic compensation system and method for unshielded serf atomic magnetometer

Abstract

The present disclosure relates to an active graded magnetic compensation system and method for an unshielded SERF magnetometer. The system comprises: a detection laser, a half-wave plate, a magnetic field compensation coil, a magnetic field modulation coil, a non-magnetic heating assembly, an alkali-metal atomic vapor cell, a polarizing beam splitter prism, a balanced photodetector, a graded feedback control module, a pump laser, a quarter-wave plate, a focusing lens and a photodetector. By means of the present disclosure, a graded compensation system is used to realize high-precision compensation for a magnetic field within a large dynamic range in a geomagnetic environment, thereby ensuring that an instrument carries out high-sensitivity magnetic field measurement under geomagnetic conditions.

Description

Active hierarchical magnetic compensation system and method for unshielded SERF atomic magnetometer

[0001] Related Applications

[0002] This application claims priority to Chinese Patent Application No. 202410536201.7, filed on April 30, 2024, and incorporates by reference the entire disclosure of the aforementioned patent application as part of the present application. TECHNICAL FIELD

[0003] The present disclosure belongs to the technical field of magnetic field measurement sensors, and relates to an active hierarchical magnetic compensation system and method, which is an active hierarchical magnetic compensation system and method for an unshielded SERF atomic magnetometer. BACKGROUND

[0004] A SERF magnetometer realizes the polarization of an atomic ensemble in an alkali metal cell through laser pumping. The polarized atomic ensemble precesses under an external magnetic field, and is coupled with the external magnetic field through light-atom interaction, thereby detecting the external magnetic field information. When the number density of alkali metal atoms is high and in an extremely weak magnetic field environment, the spin exchange rate is much greater than the Larmor precession frequency of electrons. At this time, the spin exchange relaxation effect can be ignored, i.e., the alkali metal atoms are in a SERF state, and the relaxation time is prolonged. The sensitivity measurement limit determined by spin projection noise is inversely proportional to the square root of the relaxation time, so higher sensitivity magnetic field measurement can be achieved.

[0005] In 1973, W. Happer et al. of Columbia University first discovered that the magnetic resonance linewidth is greatly reduced under high atomic density and low magnetic field, which is contrary to intuition and is called "spin exchange relaxation free (SERF) phenomenon". In 2003, Kominis I.K. et al. of Princeton University in the United States first realized a magnetic field measurement sensitivity of 0.54 fT / Hz 1 / 2 in the 28-45 Hz frequency band based on the SERF effect. In 2010, Dang H.B. et al. of the team increased the volume of the sensitive element to realize a sensitivity of 0.16 fT / Hz 1 / 2 . In 2019, the team of Academician Fang Jiancheng of Beijing University of Aeronautics and Astronautics used an atomic spin SERF effect-based ultra-high sensitivity magnetic field measurement platform to realize a magnetic field measurement sensitivity of 0.089 fT / Hz 1 / 2 in the 30-39 Hz frequency band, which was the highest index reported internationally at that time.

[0006] The basic condition of spin-exchange relaxation free is that the Larmor precession frequency of atoms is much smaller than the spin-exchange collision frequency of atoms. This requires that the atomic cell must work in a weak magnetic field, and the intensity of the geomagnetic field is generally about 50 μT. In order to realize the SERF state, the shielding cylinder and the magnetic coil are often used. Therefore, the application of the SERF magnetometer in the field of non-shielded environment, such as resource exploration, earthquake monitoring and geomagnetic field mapping in geology, is limited.

[0007] The Seltzer research group of Princeton University has proposed a technical scheme of a non-shielded SERF magnetometer. The geomagnetic information is obtained by using an additional fluxgate magnetometer, and the compensation of the geomagnetic field is performed by the outermost large Helmholtz coil. After the residual magnetism is weak, the SERF magnetometer principle is used to realize the measurement of the magnetic field. Since this method is only a preliminary technical attempt, the sensitivity index obtained finally cannot be compared with that in the shielded state.

[0008] The domestic research units include: Central South University and Beijing University of Aeronautics and Astronautics. The research of Professor Shang Jintang of Central South University focuses on the miniaturization of elements such as the cell and the magnetic compensation coil, but the sensitivity index of the existing instrument is not ideal due to the limitation of compensation accuracy.

[0009] Through retrieval, it is found that in the patent document with the publication number CN103412268A, the team of Professor Dong Haifeng of Beijing University of Aeronautics and Astronautics adopts the compensation algorithm of sequentially scanning the magnetic field to find the light intensity extreme value of three axes for preliminary magnetic field compensation, but there is no further fine compensation process.

[0010] Therefore, the present disclosure proposes an active hierarchical magnetic compensation system and method for a non-shielded SERF atomic magnetometer. SUMMARY

[0011] The purpose of the present disclosure is to overcome the shortcomings of the prior art, and to propose an active hierarchical magnetic compensation system and method for a non-shielded SERF atomic magnetometer, which realizes the magnetic field compensation with a large dynamic range and high accuracy in the geomagnetic environment through the hierarchical compensation system, and ensures the high-sensitivity magnetic field measurement of the instrument in the geomagnetic condition.

[0012] The present disclosure solves its practical problems by adopting the following technical scheme:

[0013] An active hierarchical magnetic compensation system for a non-shielded SERF magnetometer, comprising: a detection laser, a 1 / 2 wave plate, a magnetic field compensation coil, a magnetic field modulation coil, a non-magnetic heating assembly, an alkali metal atomic cell, a polarization beam splitter prism, a balanced photodetector, a hierarchical feedback control module, a pumping laser, a 1 / 4 wave plate, a focusing lens and a photodetector.

[0014] The magnetic field compensation coil and the magnetic field modulation coil are composed of two coaxial groups of coils, and are respectively used for realizing the geomagnetic compensation and the modulation of the atomic spin.

[0015] The hierarchical feedback control module is internally provided with four phase-locked amplifiers, three signal generators and three compensation current sources; the first phase-locked amplifier is used for demodulating the output signal of the balanced photoelectric detector, and the second, third and fourth phase-locked amplifiers are used for demodulating the output signals of photoelectric detectors of different frequencies; the three signal generators are respectively used for generating x, y and z direction modulation magnetic field signals and reference signals of the phase-locked amplifiers; and the three current sources are respectively used for generating x, y and z direction compensation magnetic fields.

[0016] After the pumping laser is converted into circularly polarized light by the 1 / 4 wave plate, the pumping laser is incident on the alkali metal atom chamber to polarize the atoms, and the magnetic heating assembly is used for heating and constant temperature treatment of the alkali metal atom chamber.

[0017] The detection laser first passes through the 1 / 2 wave plate to adjust the polarization state, and after passing through the alkali metal atom gas, the detection laser passes through the polarization beam splitter prism to generate mutually orthogonal light components, and the differential signal obtained by the balanced photoelectric detector is the input signal of the primary compensation system.

[0018] The hierarchical feedback control module applies the compensation magnetic field through the magnetic field compensation coil and applies the modulation magnetic field through the magnetic field modulation coil.

[0019] The pumping laser passing through the atom chamber passes through the focusing lens and converges on the photoelectric detector, and the generated signal is related to the residual magnetic field strength and enters the hierarchical feedback control module as the input signal of the secondary compensation system, the hierarchical feedback control module applies the compensation magnetic field through the magnetic field compensation coil and applies the modulation magnetic field through the magnetic field modulation coil.

[0020] An active hierarchical magnetic compensation method for a non-shielded SERF magnetometer, comprising the following steps:

[0021] S1, starting the primary compensation system: using the hierarchical feedback control module, the magnetic resonance frequency is calculated and obtained as a monitoring signal, and the external magnetic field is offset by actively applying a magnetic field, as the total magnetic field decreases, the magnetic resonance frequency also decreases, and when the magnetic resonance frequency decreases to a preset value ω0, the applied primary compensation magnetic field is locked and unchanged;

[0022] S2, starting the secondary compensation system: the three-dimensional modulation magnetic field coil generates a modulation magnetic field, and the three-dimensional magnetic field strength information is obtained through the hierarchical feedback control module, and a secondary compensation magnetic field is further applied to make the total magnetic field strength close to zero field, and the zero magnetic field environment is maintained through closed-loop feedback;

[0023] S3, the total compensation magnetic field is calculated from the total current applied to the three-dimensional compensation coil.

[0024] Moreover, the specific calculation steps of the primary compensation system include:

[0025] (1) Pumping laser passes through a 1 / 4 wave plate, and then transmits through an atomic cell to polarize atoms; a frequency of ω x is applied to the x-direction modulation magnetic field coil; a positive rotation modulation signal generates a transverse excitation magnetic field to drive the atomic magnetic moment precession; the detection laser transmits through the atomic cell, and the optical polarization signal is detected by a balanced photodetector; the signal is input into a hierarchical feedback control module, and a first phase-locked amplifier extracts the in-phase signal of the signal as the response of the primary compensation system;

[0026] (2) The initial modulation signal frequency ω s is given, and ω x = ω s ; the x-axis magnetic compensation field size is scanned, the compensation magnetic field is generated by controlling the x-direction compensation current source; B x1 is taken as the initial center value of the x-direction magnetic compensation field, ΔB x1 is the scanning step, and the other two-axis compensation magnetic fields of the magnetic compensation coil are kept unchanged, to find B xf that makes the response signal of the primary compensation system closest to zero, and maintain the x-direction initial compensation magnetic field strength as B xf ;

[0027] (3) The same method as step (2) is taken to find the y and z direction initial compensation magnetic field strengths B yf and B zf that make the response signal of the primary compensation system closest to zero;

[0028] (4) Repeat steps (2)-(3) until the response signal of the primary compensation system is zero, at which time the magnetic resonance frequency ω = ω x = ω s ;

[0029] (5) Reduce the modulation magnetic field frequency, i.e. ω x = ω s - Δω s , Δω s is the frequency change;

[0030] (6) Repeat (2)-(5) until the magnetic resonance frequency ω = ω x < ω0. ω0 is a preset value, and the reduction of the magnetic resonance frequency means the reduction of the total magnetic field size.

[0031] Moreover, the compensation algorithm and steps of the secondary compensation system are:

[0032] (1) Maintain the three-dimensional compensation magnetic fields B xf , B yf , B zfThe output is unchanged, and the hierarchical feedback control module begins secondary magnetic compensation. The feedback control module receives signals from the photodetector, and the secondary compensation system is turned on; the x, y, z direction modulation signal generator respectively applies a modulation magnetic field to the x, y, z three axes, and the magnetic field amplitude is B mx , B my , B mz , corresponding to the magnetic field frequency ω mx , ω my , ω mz , ω mx ≠ ω my ≠ ω mz .

[0033] (2) The input optical signal will be used as the input signal of the independent second, third, and fourth lock-in amplifiers, and the reference signals of the second, third, and fourth lock-in amplifiers are input by the x, y, z direction modulation signal generator, consistent with the modulation magnetic field signal; the secondary compensation system signal strength Out x , Out y , Out z is obtained by demodulation in turn.

[0034] (3) The Out x , Out y , Out z signal is used as a control parameter, and by applying a reverse compensation magnetic field, the control parameter is ensured to be zero, and the closed loop maintains the magnetic field size sensed by the atomic cell to be zero. At this time, the total current of the compensation current source is the sum of the primary compensation and secondary compensation currents.

[0035] Moreover, the specific method for maintaining a zero magnetic field environment by closed loop feedback is:

[0036] The Out x , Out y , Out z signal is input to the PID controller, and the output of the x, y, z direction compensation current source is controlled by the PID controller, so that the Out x , Out y , Out z signal remains zero, that is, the magnetic field component in the corresponding direction is zero;

[0037] Run the primary compensation system to control the compensation current source, so that the monitoring magnetic resonance frequency is not greater than the set value, maintain the primary compensation magnetic field strength at this time and enter the secondary compensation system; after entering the secondary compensation system, the geomagnetic field is finely compensated by further adjusting the magnetic compensation current, and it is judged whether the demodulated photodetector signal is zero and the zero magnetic field environment is maintained by closed loop feedback; the total current intensity applied to the three-dimensional compensation coil by the hierarchical compensation system is measured, multiplied by the magnetic field current coefficient of the magnetic field compensation coil in the corresponding direction, and the magnetic field size in that direction is obtained.

[0038] Advantages and beneficial effects of the present disclosure:

[0039] 1. The present disclosure proposes an active hierarchical magnetic compensation system and method for a non-shielded SERF atomic magnetometer. By adopting hierarchical compensation and closed-loop control, the primary compensation system performs a large range low-precision compensation at the initial stage. When the remanence level is low, the principle of single-beam frequency modulation closed-loop SERF magnetometer is adopted for fine compensation and completion of closed-loop SERF state atomic magnetic measurement. The present disclosure can improve the magnetic compensation feedback speed and compensation accuracy of the non-shielded SERF magnetometer, and ensure the use of the non-shielded SERF magnetometer in a complex geomagnetic environment.

[0040] 2. The active hierarchical magnetic compensation system and method proposed by the present disclosure can quickly approach the zero magnetic field working state of the non-shielded SERF magnetometer through the primary compensation system, reducing the start-up time of the non-shielded SERF magnetometer in the early stage.

[0041] 3. The active hierarchical magnetic compensation system and method proposed by the present disclosure can significantly improve the compensation accuracy of the hierarchical compensation system through the secondary compensation system. The near-zero field environment of the atomic cell can be better maintained, and the magnetometer can be ensured to work in the SERF state, thereby improving the sensitivity of the magnetometer.

[0042] 4. The active hierarchical magnetic compensation system and method proposed by the present disclosure can improve the instrument bandwidth and response speed, and improve the sensitivity reduction of the instrument caused by geomagnetic field fluctuations. BRIEF DESCRIPTION OF DRAWINGS

[0043] Fig. 1 is a structural diagram of an active hierarchical magnetic compensation system for a non-shielded SERF atomic magnetometer according to the present disclosure;

[0044] Fig. 2 is a closed-loop control timing diagram of the hierarchical feedback control module;

[0045] Fig. 3 is a structural diagram of the hierarchical feedback control module;

[0046] Fig. 4 is an output response curve diagram of the primary compensation system;

[0047] Fig. 5 is an output response curve diagram of the secondary compensation system; DETAILED DESCRIPTION

[0048] The embodiments of the present disclosure are further described in detail below with reference to the accompanying drawings:

[0049] An active hierarchical magnetic compensation system for a non-shielded SERF magnetometer, as shown in Fig. 1, comprises a detection laser 1, a 1 / 2 wave plate 2, a magnetic field compensation coil 3, a magnetic field modulation coil 4, a non-magnetic heating assembly 5, an alkali metal atom cell 6, a polarization beam splitter prism 7, a balanced photodetector 8, a hierarchical feedback control module 9, a pumping laser 10, a 1 / 4 wave plate 11, a focusing lens 12 and a photodetector 13.

[0050] The magnetic field compensation coil 3 and the magnetic field modulation coil 4 are composed of two coaxial coil groups, respectively used for realizing geomagnetic compensation and modulation of atomic spins.

[0051] The hierarchical feedback control module is built-in with four phase-locked amplifiers, three signal generators and three compensation current sources; the specific connection is shown in Fig. 3, wherein the first phase-locked amplifier is used for demodulating the output signal of the balanced photodetector 8, the second, third (i.e. the third) and fourth (i.e. the fourth) phase-locked amplifiers are used for demodulating the output signals of the photodetector 13 with different frequencies. The three signal generators are respectively used for generating x, y and z direction modulation magnetic field signals and reference signals of the phase-locked amplifiers. The three current sources are respectively used for generating x, y and z direction compensation magnetic fields.

[0052] After the pumping laser 10 passes through the 1 / 4 wave plate 11 to be converted into circularly polarized light, it is incident on the alkali metal atom cell 6 to polarize the atoms, and the heating action of the non-magnetic heating assembly 5 increases the atomic number density, and the signal strength is also enhanced.

[0053] The detection laser 1 first passes through the 1 / 2 wave plate 2 to adjust its polarization state, and after passing through the alkali metal atom cell 6, it generates mutually orthogonal light components through the polarization beam splitter prism 7, and the differential signal obtained by the balanced photodetector 8 is the input signal of the primary compensation system;

[0054] The hierarchical feedback control module 9 applies a compensation magnetic field through the magnetic field compensation coil 3 and applies a modulation magnetic field through the magnetic field modulation coil 4.

[0055] The specific control process is shown in the "primary compensation system algorithm and steps" section.

[0056] After the pumping laser 10 passing through the alkali metal atom cell 6 passes through the focusing lens 12, it converges on the photodetector 13; the generated signal is related to the residual magnetic field strength and enters the hierarchical feedback control module 9 as the input signal of the secondary compensation system, and the hierarchical feedback control module 9 applies a compensation magnetic field through the magnetic field compensation coil 3 and applies a modulation magnetic field through the magnetic field modulation coil 4.

[0057] The specific control process is shown in the "secondary compensation system algorithm and steps" section.

[0058] Embodiments of the present disclosure provide an active hierarchical magnetic compensation method for a non-shielded SERF magnetometer, as shown in FIG. 2, comprising the following steps:

[0059] S1, start the primary compensation system: use the hierarchical feedback control module to obtain the magnetic resonance frequency and use it as a monitoring signal. The external magnetic field is offset by actively applying a magnetic field. As the total magnetic field decreases, the magnetic resonance frequency also decreases. When the magnetic resonance frequency decreases to a preset value ω0, the applied primary compensation magnetic field is locked unchanged.

[0060] S2, start the secondary compensation system: the three-dimensional modulation magnetic field coil generates a modulation magnetic field. The three-dimensional magnetic field strength information is obtained through the hierarchical feedback control module. The secondary compensation magnetic field is further applied to make the total magnetic field strength close to zero field, and the zero magnetic field environment is maintained through closed-loop feedback.

[0061] S3, the total compensation magnetic field can be calculated from the total current applied to the three-dimensional compensation coil.

[0062] Primary compensation system basic principle:

[0063] The primary compensation system is a total magnetic field strength compensation method. The atom will produce magnetic resonance phenomenon under the joint action of the geomagnetic field and the radio frequency magnetic field, and the resonance frequency ω is proportional to the total external magnetic field B0, that is, ω = γB0, γ is the alkali metal atom gyromagnetic ratio. Apply the x-direction radio frequency magnetic field, the frequency is ω x . The primary compensation system output signal response is shown in FIG. 4. By actively applying a three-dimensional primary compensation magnetic field, the resonance frequency ω can be changed, and when ω x = ω, that is, the magnetic resonance frequency detuning is zero, the response of the primary compensation system is zero. Then continuously reduce the frequency ω x , and through three-dimensional magnetic compensation, the response of the primary compensation system is zero, so that the resonance frequency and the total magnetic field size are continuously reduced.

[0064] The primary compensation system algorithm and steps are:

[0065] (1) The pump laser 10 passes through the 1 / 4 wave plate 11, and then passes through the atom cell 6 to polarize the atom; the x-direction modulation magnetic field coil 4 is applied with a frequency ω x positive rotation modulation signal to generate a transverse excitation magnetic field to drive the atomic magnetic moment precession. Make the detection laser 1 pass through the atom cell 6, and detect the light polarization signal through the balanced photodetector 8; input the signal into the hierarchical feedback control module 9, which can extract the in-phase signal of the signal through the built-in first lock-in amplifier, and use the signal as the response of the primary compensation system.

[0066] (2) Given the initial modulation signal frequency ω s , ω x = ω s; The scanning x-axis magnetic compensation field size, the compensation field is generated by controlling the x-direction compensation current source. With B x1 as the initial compensation x-direction magnetic field starting center value, ΔB x1 is the scanning step, and the other two-axis compensation magnetic fields of the magnetic compensation coil are kept unchanged, find the B xf that makes the response signal of the primary compensation system closest to zero, and maintain the x-direction initial compensation magnetic field strength as B xf .

[0067] (3) Take the same approach as step (2) to find the y and z direction initial compensation magnetic field strength B yf and B zf when the response signal of the primary compensation system is closest to zero.

[0068] (4) Repeat steps 2-3 until the response signal of the primary compensation system is zero. At this time, the magnetic resonance frequency ω = ω x = ω s .

[0069] (5) Reduce the modulation magnetic field frequency, that is, ω x = ω s - Δω s , Δω s is the frequency change.

[0070] (6) Repeat 2-5 until the magnetic resonance frequency ω = ω x < ω0. ω0 is a preset value, and the decrease of the magnetic resonance frequency means the decrease of the total magnetic field size. The above processing process is shown in Figure 3.

[0071] Basic principle of secondary compensation system:

[0072] The secondary compensation system is a three-axis magnetic field compensation method. When the total magnetic field is lower than the set value, it enters the secondary compensation range, and more precise magnetic compensation can be realized by using SERF state atoms. The secondary magnetic compensation is based on the single-beam frequency modulation principle, which requires different frequency signals to be applied on three orthogonal modulation coils for modulation, and then the demodulation signals of corresponding frequencies are obtained through the lock-in amplifier. The demodulated secondary compensation system signal of the hierarchical feedback control module 9 can be expressed as:

[0073] Where Out x , Out y , Out z are the responses of the x, y, z axes of the secondary compensation system obtained by demodulation, B mx , B my , B mz are the amplitudes of the three-dimensional modulation magnetic field, B tx , B ty , B tzrespectively, are the total three-dimensional magnetic field intensity after compensation, R tot are the total relaxation rate and pumping rate, and γ is the alkali metal atom gyromagnetic ratio. The response of the secondary compensation system to the size of the residual magnetic field after compensation is shown in Figure 5. When the components of the residual magnetic field after compensation are close to zero, the response of the secondary compensation system is zero, and the closed-loop control of the response to zero can achieve accurate compensation of the magnetic field.

[0074] The compensation algorithm and steps of the secondary compensation system are as follows:

[0075] (1) Maintain the three-dimensional compensation magnetic field B xf , B yf , B zf of the primary compensation system unchanged, and the hierarchical feedback control module 9 starts the secondary magnetic compensation. The feedback control module receives signals from the photoelectric detector 13, and the secondary compensation system is started. The x, y, z direction modulation signal generator respectively applies a modulation magnetic field to the x, y, z three-axis, and the magnetic field amplitude is B mx , B my , B mz , and the corresponding magnetic field frequency is ω mx , ω my , ω mz , ω mx ≠ ω my ≠ ω mz .

[0076] (2) The input photoelectric signal will be divided into three paths as the input signals of the independent second, third, and fourth lock-in amplifiers, and the reference signals of the second, third, and fourth lock-in amplifiers are input by the x, y, z direction modulation signal generator, consistent with the modulation magnetic field signal. The secondary compensation system signal intensity Out x , Out y , Out z is obtained by demodulation in turn.

[0077] (3) The Out x , Out y , Out z signal is used as a control parameter, and by applying a reverse compensation magnetic field, the control parameter is ensured to be zero, and the closed-loop maintains the magnetic field size sensed by the atomic cell to be zero. At this time, the total current of the compensation current source is the sum of the primary compensation and secondary compensation currents.

[0078] Moreover, the specific method of maintaining a zero magnetic field environment by closed-loop feedback is as follows: the Out x , Out y , Out z signal is input to the PID controller, and the output of the x, y, z direction compensation current source is controlled by the PID controller, so that Out x , Out y , Outz The signal is kept zero, i.e. the corresponding direction component of the magnetic field is guaranteed to be zero. The above process is shown in detail in Fig. 3.

[0079] The specific method for maintaining a zero magnetic field environment and obtaining the environmental magnetic field strength through closed-loop feedback is as follows:

[0080] The closed-loop control timing of the hierarchical feedback control module is shown in Fig. 2. The primary compensation system is run, the compensation current source is controlled, the monitoring magnetic resonance frequency is not greater than the set value, the primary compensation magnetic field strength at this time is maintained, and the secondary compensation system is entered. After entering the secondary compensation system, the geomagnetic field is finely compensated by further adjusting the magnetic compensation current, and it is judged whether the photodetector signal is zero and a closed loop is formed. The total current intensity applied to the three-dimensional compensation coil by the hierarchical compensation system is measured, multiplied by the magnetic field current coefficient of the magnetic field compensation coil in the corresponding direction, and the magnetic field size in the direction is obtained.

[0081] The disclosure is further described below through specific examples:

[0082] Consider that the Rb atom is filled in the atomic cell. First, the primary compensation is performed, so that the magnetic field is rapidly reduced. Then, the secondary compensation is performed, the magnetic field is further finely compensated, and the zero field environment is maintained through closed-loop control, to ensure the normal work of the SERF magnetometer.

[0083] (1) A positive rotating modulation signal with a frequency of 100 kHz is applied to the x-direction modulation magnetic field coil, and the atomic polarization and detection are completed. The primary compensation system detection signal is input into the first lock-in amplifier through the balanced photodetector 8. The first lock-in amplifier can extract the in-phase signal of the signal as the response signal of the primary compensation system.

[0084] (2) The initial three-dimensional primary compensation magnetic field size is set as B xf = B yf = B zf = 10 μT. The x-axis magnetic compensation magnetic field size is scanned, with 10 μT as the starting value, 0.1 μT as the scanning step, and the scanning range being 10-50 μT. The y and z-axis compensation magnetic fields of the magnetic compensation coil are kept unchanged. When the response signal of the primary compensation system is closest to zero, the x-direction primary compensation magnetic field strength B xf at this time is maintained.

[0085] (3) The same method as in (2) is adopted to find the y and z-direction primary compensation magnetic field strengths B yf and B zf when the response signal of the primary compensation system is closest to zero.

[0086] (4) Repeat 2-3 until the response signal of the primary compensation system is zero. At this time, the magnetic resonance frequency ω = ω x = 100 kHz.

[0087] (5) Reduce the x-axis modulation magnetic field frequency ω x , let ω x = 99, 98,..., 1 kHz, repeat 2-4, until the magnetic resonance frequency ω = ω x = 1 kHz. Maintain the x, y, z direction initial compensation magnetic field strength B xf , B yf , B zf unchanged, that is, maintain the primary compensation magnetic field unchanged.

[0088] (6) Turn on the secondary compensation, the hierarchical feedback control module 9 will receive the signal from the photodetector 13 as the input signal of the phase-locked amplifier 2-4. Different frequency signals ω mx = 50 kHz, ω my = 60 kHz, ω mz = 70 kHz are input to the magnetic field modulation coil 4 to modulate the atomic spin, and at the same time as the reference signal of the second, third and fourth phase-locked amplifiers to demodulate.

[0089] (7) Let the second, third and fourth phase-locked amplifiers output amplitude signals, and take the signals as closed-loop control signals. The secondary compensation magnetic field can be formed by applying a feedback current on the magnetic field compensation coil 3, so that the closed-loop control signal is zero, that is, the active magnetic compensation of the non-shielded SERF magnetometer is realized. At this time, the measured external magnetic field is the same size and opposite direction as the total compensation magnetic field.

[0090] It should be emphasized that the embodiments of the present disclosure are illustrative rather than limiting, and therefore the present disclosure includes and is not limited to the embodiments in the specific embodiments. Any other embodiments derived by those skilled in the art according to the technical solutions of the present disclosure also belong to the scope of protection of the present disclosure.

Claims

1. An active hierarchical magnetic compensation system for a non-shielded SERF magnetometer, characterized in that: It comprises: a detection laser, a 1 / 2 wave plate, a magnetic field compensation coil, a magnetic field modulation coil, a non-magnetic heating assembly, an alkali metal atom gas chamber, a polarization beam splitter prism, a balanced photodetector, a hierarchical feedback control module, a pumping laser, a 1 / 4 wave plate, a focusing lens and a photodetector; The magnetic field compensation coil and the magnetic field modulation coil are composed of two coaxial coil groups, which are used to realize the geomagnetic compensation and the modulation of atomic spins, respectively. The hierarchical feedback control module is internally provided with four phase-locked amplifiers, three signal generators and three compensation current sources; the first phase-locked amplifier is used to demodulate the output signal of the balanced photodetector, and the second, third and fourth phase-locked amplifiers are used to demodulate the output signals of the photodetectors with different frequencies; the three signal generators are used to generate x, y and z direction modulation magnetic field signals and reference signals of the phase-locked amplifiers, respectively; and the three current sources are used to generate x, y and z direction compensation magnetic fields, respectively. The pumping laser is converted into circularly polarized light after passing through the 1 / 4 wave plate, and then is incident on the alkali metal atom gas chamber to polarize the atoms; and the non-magnetic heating assembly is used to heat and maintain the temperature of the alkali metal atom gas chamber. The detection laser first passes through the 1 / 2 wave plate to adjust its polarization state, and then passes through the polarization beam splitter prism to generate mutually orthogonal light components after passing through the alkali metal atom gas; and the differential signal obtained by the balanced photodetector is the input signal of the primary compensation system. The hierarchical feedback control module applies a compensation magnetic field through the magnetic field compensation coil and applies a modulation magnetic field through the magnetic field modulation coil. The pumping laser passing through the atom gas chamber is converged on the photodetector through the focusing lens, and the generated signal is related to the residual magnetic field strength and is input into the hierarchical feedback control module as the input signal of the secondary compensation system; the hierarchical feedback control module applies a compensation magnetic field through the magnetic field compensation coil and applies a modulation magnetic field through the magnetic field modulation coil.

2. An active hierarchical magnetic compensation method for a non-shielded SERF magnetometer, characterized in that: The method comprises the following steps: S1, starting the primary compensation system: using the hierarchical feedback control module to obtain the magnetic resonance frequency as a monitoring signal, and offsetting the external magnetic field by actively applying a magnetic field; as the total magnetic field decreases, the magnetic resonance frequency also decreases; when the magnetic resonance frequency decreases to a preset value ω0, the applied primary compensation magnetic field is locked and unchanged; S2, starting the secondary compensation system: the three-dimensional modulation magnetic field coil generates a modulation magnetic field, and the hierarchical feedback control module obtains three-dimensional magnetic field strength information to further apply a secondary compensation magnetic field to make the total magnetic field strength close to zero field, and to maintain a zero magnetic field environment through closed-loop feedback; S3, the total compensation magnetic field is calculated from the total current applied to the three-dimensional compensation coil.

3. The active hierarchical magnetic compensation method for a non-shielded SERF magnetometer according to claim 2, characterized in that: The specific calculation steps of the primary compensation system comprise: (1) Pumping laser passes through 1 / 4 wave plate, and then transmits through atomic cell to polarize atoms; frequency ω x The positive rotation modulation signal generates a transverse excitation magnetic field, which drives the atomic magnetic moment precession; the detection laser transmits through the atomic cell, and the optical polarization signal is detected by a balanced photodetector; the signal is input into a hierarchical feedback control module, and a first lock-in amplifier extracts the in-phase signal of the signal, which is used as the response of the primary compensation system; (2) Given the initial modulation signal frequency ω s , let ω x = ω s ; scan the x-axis magnetic compensation field size, the compensation field is generated by controlling the x-direction compensation current source; take B x1 as the initial compensation x-direction magnetic field starting center value, ΔB x1 as the scanning step, and keep the other two-axis compensation magnetic fields of the magnetic compensation coil unchanged, find B xf that makes the response signal of the primary compensation system closest to zero, and maintain the x-direction initial compensation magnetic field strength as B xf at this time; (3) The same approach as in step (2) is taken to find the y, z direction primary compensation magnetic field strengths B at which the response signal of the primary compensation system is closest to zero yf and B zf ; (4) repeating steps (2)-(3) until the response signal of the primary compensation system is zero, at which time the magnetic resonance frequency ω = ω x = ω s ; (5) reducing the frequency of the modulation magnetic field, i.e. ω x = ω s - Δω s , Δω s is the frequency change (6) repeating (2) - (5) until the magnetic resonance frequency ω = ω x < ω0; ω0 is a pre-set value, and a decrease in the magnetic resonance frequency means a decrease in the total magnetic field size.

4. The active hierarchical magnetic compensation method for a nonshielded SERF magnetometer according to claim 2, characterized in that: The compensation algorithm and steps of the secondary compensation system are: (1) maintain the three-dimensional compensation magnetic field B of the primary compensation system xf , B yf , B zf Output is constant, hierarchical feedback control module begins secondary magnetic compensation; feedback control module receives signals from the photodetector, open secondary compensation system; by x, y, z modulation signal generator respectively to x, y, z three-axis modulation magnetic field, magnetic field amplitude B mx , B my , B mz , corresponding to the magnetic field frequency ω mx , ω my , ω mz , ω mx ≠ ω my ≠ ω mz ; (2) The input photoelectric signal will be the input signal of the independent second, third and fourth lock-in amplifiers, and the reference signals of the second, third and fourth lock-in amplifiers are input from the x, y and z direction modulation signal generators respectively, which are consistent with the modulation magnetic field signals; and the secondary compensation system signal intensity Out x , Out y , Out z is obtained by demodulation in turn. (3) Out x , Out y , Out z signal as a control variable, by applying a reverse compensation magnetic field, to ensure that the control variable is zero, closed loop to maintain the size of the magnetic field sensitive to the atomic gas chamber is zero; at this time, the total current of the compensation current source is the sum of the primary compensation and secondary compensation current.

5. The active hierarchical magnetic compensation method for a non-shielded SERF magnetometer according to claim 2, characterized in that: The specific method for maintaining a zero magnetic field environment through closed-loop feedback is: The primary compensation system is operated to control the compensation current source, so that the monitoring magnetic resonance frequency is not greater than a set value, the primary compensation magnetic field strength at this time is maintained, and the secondary compensation system is entered. After entering the secondary compensation system, the geomagnetic field is finely compensated by further adjusting the magnetic compensation current, it is judged whether the demodulated photoelectric detector signal is zero, and the zero magnetic field environment is maintained through closed-loop feedback. The total current intensity applied to the three-dimensional compensation coil by the hierarchical compensation system is measured, multiplied by the magnetic field current coefficient of the magnetic field compensation coil in the corresponding direction, and the magnetic field size in the direction is obtained.

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