A method for measuring and suppressing three-axis magnetic field gradient based on SERF common magnetometer

Abstract

A three-axis magnetic field gradient measurement and suppression method based on SERF common magnetometer, in which SERF (Spin-Exchange-Relaxation-Free) is spin-exchange relaxation-free. Firstly, an in-situ measurement and compensation method of longitudinal magnetic field gradient based on atomic spin relaxation mechanism is proposed; then a method for K-Rb- 21 A transverse magnetic field gradient compensation method for a Ne SERF co-magnetometer uses closed-loop control of the self-compensation point to represent the nuclear spin polarization, enabling in-situ measurement of triaxial transverse magnetic field gradients. This method facilitates analysis of the impact of magnetic field gradients on system relaxation mechanisms, improves atomic decoherence time, and enhances signal-to-noise ratio, thereby improving inertial measurement accuracy. Furthermore, the compensation method provided by this technology has the potential to be applied to other atomic sensors, such as SERF magnetometers and nuclear magnetic resonance co-magnetometers.

Description

Technical Field

[0001] The present invention relates to the technical field of magnetic field gradient measurement and suppression in inertial measurement, and in particular to a method for measuring and suppressing three-axis magnetic field gradients based on a SERF common magnetometer. SERF (Spin-Exchange-Relaxation-Free) is a method without spin exchange relaxation. Based on the effect of magnetic field gradients on the relaxation mechanism of the SERF common magnetometer measurement device, a self-compensation point closed-loop control method is used to determine changes in nuclear polarization, thereby achieving in-situ measurement of three-axis transverse magnetic field gradients. Experimental results demonstrate that magnetic field gradient compensation under steady-state polarization conditions is achieved using magnetic field gradient coils. Furthermore, the proposed three-axis magnetic field gradient in-situ measurement and compensation method can also be applied to instruments such as SERF magnetometers and nuclear magnetic resonance gyroscopes. Background Art

[0002] The SERF common magnetometer has long attracted research attention due to its ultra-high theoretical sensitivity limit and long-term stability. It holds significant research value in fundamental physics research, including Lorentz experiments, studies of charge-parity-time (CPT) symmetry, and high-precision atomic spin-inertial measurements. All experiments and applications require the system to operate under steady-state conditions. However, in practical applications, various perturbations can affect its stability, particularly the impact of magnetic field inhomogeneities on the atomic decoherence time. In SERF common magnetometer applications, magnetic field gradients generated by the spatially inhomogeneous remanent magnetic field of the magnetic shielding system, the non-orthogonality of the magnetic compensation system, the coupling between the active magnetic compensation system and the magnetic shielding system, and the magnetism generated by energizing the heated membrane all have a certain impact on the atomic decoherence time, all of which can affect the long-term stability of the SERF common magnetometer. Furthermore, the high density of alkali metal atoms used for optical pumping results in a large optical depth, which leads to an inhomogeneous distribution of the electron spin magnetic field due to optical absorption. SERF common magnetometers based on inert atom hyperpolarization typically require a uniform spatial distribution of spin polarization. Therefore, it is necessary to analyze the principles of inertial measurement and the system dynamics process based on the polarization and relaxation mechanism of inertial measurement, and to study the system's magnetic field gradient measurement method to provide theoretical support and experimental guidance for the subsequent research on in-situ compensation methods.

[0003] Typically, in an environment with low remanent magnetic field and high atomic number density, the spin exchange collision rate of the SERF atomic co-magnetometer is much greater than the Larmor precession frequency of the atoms, and the transverse relaxation time T2 is close to the longitudinal relaxation time T1 of the atomic spins. The above state is called the SERF state. However, there is a big difference between T1 and T2 measured in the experiment. Studies have found that the gradient relaxation caused by the magnetic field gradient is the main source of the difference. The polarization gradient of the electron spin can be suppressed by using the counter-pumping method. For the remanent magnetic field gradient, it can be suppressed by actively applying the magnetic field gradient by designing a three-axis magnetic field gradient coil. Therefore, measuring the influence of the total magnetic field gradient, the electron spin polarization magnetic field gradient and the remanent magnetic field gradient on the relaxation mechanism helps to suppress the influence of magnetic field inhomogeneity on the system, thereby extending the atomic decoherence time and improving the stability of the SERF co-magnetometer. In summary, it is urgently needed to invent a three-axis magnetic field gradient measurement and suppression method based on the SERF co-magnetometer. Summary of the Invention

[0004] The present invention proposes a three-axis magnetic field gradient measurement and suppression method based on a SERF common magnetometer. Through closed-loop control of self-compensation points and gradient optimization methods, etc., it helps to achieve higher longitudinal and transverse relaxation times. The present invention defines the z-axis of the SERF common magnetometer as the pump light direction, the x-axis as the detection light direction, and the y-axis as the sensitive axis direction, thereby increasing the coupling between nuclear spins and electron spins, thereby improving the polarization level and system stability. It can solve the existing technical defects.

[0005] The technical solutions of the present invention are as follows:

[0006] A method for measuring and suppressing a three-axis magnetic field gradient based on a SERF common magnetometer, characterized by comprising the following steps:

[0007] Step 1: Apply a magnetic field gradient to the SERF common magnetometer through a first signal generator connected to a gradient coil, wherein the magnetic field gradient includes a longitudinal magnetic field gradient and transverse magnetic field gradient After the atomic spin polarization in the atomic gas cell is stabilized, the three-axis static magnetic field is compensated by a second signal generator connected to the active magnetic compensation coil to keep the system in a stable working state, thereby making the atomic ensemble in the atomic gas cell able to resist external interference;

[0008] Step 2: Apply a magnetic field on the z-axis that is consistent with the direction of the pump light to decouple the precession of electrons and nuclei. After the polarization state of the atomic spin ensemble stabilizes, apply an excitation signal on the y-axis and record the relationship between the laser free precession decay signal and time along the x-axis.

[0009] Step 3: Based on the relationship between the free precession decay signal and time, the longitudinal relaxation time T2 under the set magnetic field gradient is fitted. The reciprocal of T2 is the longitudinal relaxation rate. It is determined whether the value of T2 is the maximum or minimum inflection point with the change of the magnetic field gradient. If so, proceed to step 4; if not, return to step 1.

[0010] Step 4: Obtain the longitudinal magnetic field gradient by quadratic parabola fitting and the nuclear transverse relaxation rate The center of the quadratic parabola is the longitudinal magnetic field gradient of the measured SERF common magnetometer:

[0011]

[0012] in represents the relaxation of the electric quadrupole moment, represents spin-destruction relaxation, Indicates that alkali metal electrons are polarized by spin exchange collisions 21 The nuclear spin exchange rate of Ne, represents the transverse magnetic field gradient relaxation:

[0013]

[0014] Where V is the volume of the atomic gas chamber, D Ne-Ne is the diffusion coefficient of the atomic gas cell, R is the radius of the atomic gas cell, γ n is the nuclear gyromagnetic ratio, and the longitudinal magnetic field gradient of the system is obtained by fitting the relationship between the longitudinal relaxation time T2 and the magnetic field gradient;

[0015] Step 5, longitudinal magnetic field gradient After returning to zero, the change of nuclear polarizability is indirectly reflected by the closed-loop control self-compensation point method, and the transverse magnetic field gradient is Zeroing:

[0016] The minimum value of the transverse relaxation rate is used to express the total magnetic field gradient ΔB in the gas chamber. total , SERF common magnetometer transverse magnetic field gradient and the time derivative of the nuclear longitudinal polarizability The relationship is as follows:

[0017]

[0018] in is the nuclear longitudinal polarizability, t is the time, is the electron longitudinal polarizability, is the initial value of the nuclear longitudinal polarizability, B z represents the z-axis magnetic field;

[0019] Step 6: Use the gradient coil to actively apply a magnetic field gradient to suppress the impact of magnetic field inhomogeneity on the system, extend the atomic decoherence time, and optimize the transverse magnetic field gradient: the frequency of the modulation signal applied by the first signal generator to the transverse magnetic field coil and the phase-locked amplifier is equal to the frequency corresponding to the electron resonance peak. The output signal of the SERF common magnetometer is demodulated by the phase-locked amplifier and input into the PID controller. The phase-locked amplifier performs closed-loop control on the self-compensation point. At this time, the magnetic field control voltage applied to the modulation coil reflects the change in the nuclear polarization rate; based on the quadratic parabola equation, the relationship between the magnetic field gradient and the slope corresponding to the magnetic field control voltage changing with time is fitted, and the transverse magnetic field gradient is extracted from the center of the quadratic parabola.

[0020] The step 1 includes heating the atomic gas chamber to above 170° C. to increase the atomic density. The inert gas inside the atomic gas chamber is used to sense inertia and resist interference from the external magnetic field. The alkali metal electrons inside the atomic gas chamber are used for spin exchange collision polarization of the inert gas nuclear spin. The active magnetic compensation coil and the gradient coil are both flexible thin film coils. The transverse shim coil in the active magnetic compensation coil adopts a saddle-shaped nested form, and the axial shim coil adopts a Maxwell coil.

[0021] The pump light in step 2, ie, the pumping laser, comes from a pumping laser, which is a distributed Bragg reflector laser (DBR). The detection laser in step 2 comes from a detection laser, which is a distributed feedback laser (DFB).

[0022] In step 2, a magnetic field of 1500 nT is applied on the z-axis in the same direction as the pump light.

[0023] The step 4 includes obtaining the relationship between the longitudinal magnetic field gradient and the nuclear transverse relaxation rate by quadratic parabola fitting under three temperature conditions of 170° C., 180° C., and 190° C.

[0024] The transverse magnetic field gradient in step 1 includes four transverse magnetic field gradients: dBx / dz, dBy / dz, dBx / dx and dBy / dy, where Bx and By are the x-axis and y-axis magnetic fields respectively.

[0025] The SERF common magnetometer includes a detection laser, a first spot beam expansion component, a first 1 / 2 wave plate, a first beam splitter, a first liquid crystal phase retarder, a first Grantiera prism, a polarization beam splitter, an atomic gas chamber, a Wollaston prism and an array differential photodetector connected in sequence. The array differential photodetector is respectively connected to a phase-locked amplifier and a DAQ data acquisition system, the DAQ data acquisition system is connected to a host computer, the phase-locked amplifier is respectively connected to a PID controller and a second signal generator, the PID controller is connected to a modulation coil in the coil, the second signal generator is connected to an active magnetic compensation coil in the coil, the gradient coil in the coil is connected to the first signal generator, and the reflective side of the first Grantiera prism is connected to the first liquid crystal phase retarder via a second photodetector and an electronic control unit ECU in sequence.

[0026] The SERF common magnetometer includes a pump laser, a second spot beam expander, a second 1 / 2 wave plate, a second beam splitter, a second liquid crystal phase retarder, a second Granitella prism, a second 1 / 2 wave plate, an optical isolator and an atomic gas chamber connected in sequence. The optical isolator is connected to the second liquid crystal phase retarder through a first photodetector and an electronic control unit ECU in turn.

[0027] A heating film is provided outside the atomic gas chamber, the heating film is located inside the coil, the coil is located inside the ferrite, and the ferrite is located inside the permalloy.

[0028] The technical effects of the present invention are as follows: the present invention provides a three-axis magnetic field gradient measurement and suppression method based on the SERF common magnetometer measurement device, and performs an in-situ measurement and compensation method of the longitudinal magnetic field gradient based on the atomic spin relaxation mechanism; a method for K-Rb- 21 A transverse magnetic field gradient compensation method for a Ne SERF common magnetometer determines changes in nuclear polarizability after actively applying a magnetic field gradient, using closed-loop control at the compensation point. This method enables in-situ measurement of triaxial transverse magnetic field gradients. This method facilitates analysis of the effect of magnetic field gradients on system relaxation mechanisms, thereby improving atomic decoherence time, increasing the signal-to-noise ratio, and ultimately enhancing inertial measurement accuracy. This research could ultimately be used for ultra-high-precision inertial and sensitivity measurements. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 The present invention is a schematic structural diagram of a SERF common magnetometer involved in a three-axis magnetic field gradient measurement and suppression method based on a SERF common magnetometer.

[0030] Figure 2 This is the experimental diagram of the total magnetic field gradient measurement results. Figure 2The method includes obtaining the relationship between the transverse relaxation rate of nuclei by changing the longitudinal magnetic field gradient under three temperature conditions of 170℃, 180℃ and 190℃, thereby measuring the total magnetic field gradient in situ. Figure 2 It can be seen from the graph that the total magnetic field gradient is -14.86 nT / cm at 170°C, -25.62 nT / cm at 180°C, and -43.02 nT / cm at 190°C. The transverse relaxation rate is the reciprocal of the transverse relaxation time. Figure 2 The vertical axis is the nuclear transverse relaxation rate (1 / s), and the vertical axis scale values ​​are 0, 0.01, 0.02, ···, 0.05; the horizontal axis is the longitudinal magnetic field gradient dBz / dz (nT / cm), Bz represents the longitudinal magnetic field, that is, the magnetic field in the z-axis direction, and the horizontal axis scale values ​​are -150, -100, -50, 0, 50, 100. Figure 2 The transverse relaxation rate has a quadratic parabolic relationship with the longitudinal magnetic field gradient.

[0031] Figure 3 This is a diagram showing the measurement results of the transverse magnetic field gradient proposed by the present invention. Figure 3 The horizontal axis represents the SERF common magnetometer transverse magnetic field gradient Figure 3 The figure includes four actively applied transverse magnetic field gradients: dBx / dz (nT / cm), dBy / dz (nT / cm), dBx / dx (nT / cm) and dBy / dy (nT / cm). It can be seen that the corresponding transverse magnetic field gradients obtained by in situ measurement are -1.01nT / cm, 0.37nT / cm, -0.62nT / cm and -0.61nT / cm, respectively, where the positive and negative signs indicate the direction of the magnetic field gradient. Figure 3 The vertical axis represents the nuclear spin closed-loop control voltage V Bn The rate of change over time t (dV Bn / dt), which reflects the time derivative of the nuclear spin polarization. Ultimately, the relationship between the rate of change of the nuclear spin polarization and the transverse magnetic field gradient can be obtained. The relationship between the magnetic field gradient and the slope of the magnetic field control voltage over time is fitted based on the quadratic parabola equation, and the transverse magnetic field gradient corresponding to that direction is extracted from the center of the quadratic parabola.

[0032] The reference numerals are listed as follows: 1-detection laser (e.g., distributed feedback laser DFB); 2-first spot beam expansion assembly (including two lenses); 3-first 1 / 2 wave plate; 4-first beam splitter; 5-first liquid crystal phase retarder (connected to the second photodetector through the electronic control unit ECU); 6-first Granitella prism; 7-polarization beam splitter; 8-first signal generator (connected to the gradient coil); 9-pump laser (e.g., distributed Bragg reflection laser DBR); Laser); 10-second spot beam expansion assembly (including two lenses); 11-second 1 / 2 wave plate; 12-second Granitella prism; 13-first photodetector; 14-second 1 / 2 wave plate; 15-optical isolator; 16-second photodetector; 17-ferrite; 18-coil (including active magnetic compensation coil, modulation coil, and gradient coil, all of which are three-axis coils); 19-heating film; 20-atomic gas cell; 21-Wollaston prism; 22-array differential photodetector; 23-PID controller (connecting modulation coil Zm, PID, Proportional Integral Derivative, proportional, integral, differential); 24-phase-locked amplifier (receives the reference signal Ref from the second signal generator); 25-second beam splitter; 26-second liquid crystal phase retarder (connected to the first photodetector through the electronic control unit ECU); 27-second signal generator (connected to the three-axis coil, i.e., the active magnetic compensation coil, Xc, Yc, Zc, the x-axis magnetic field of the input coil Xc is B x0 , the y-axis magnetic field (modulation magnetic field) of the input coil Yc is B y0 +B0cos(ωt), the z-axis magnetic field of the input coil Zc is B z0 +Bc, B0 is the static magnetic field, B x0 、B y0 、B z0 are the x-axis, y-axis, and z-axis static magnetic fields, Bc is the compensation magnetic field, ω is the frequency, and t is the time); 28-DAQ data acquisition system; 29-host computer; 30-Permalloy. DETAILED DESCRIPTION

[0033] Below is the attached figure ( Figure 1-Figure 3 ) and Examples illustrate the present invention.

[0034] Figure 1 The present invention is a schematic structural diagram of a SERF common magnetometer involved in a three-axis magnetic field gradient measurement and suppression method based on a SERF common magnetometer. Figure 2 This is the experimental diagram of the total magnetic field gradient measurement results. Figure 3This is a diagram showing the measurement results of the transverse magnetic field gradient proposed by the present invention. Figures 1 to 3 As shown, a method for measuring and suppressing three-axis magnetic field gradient based on SERF common magnetometer includes the following steps: Step 1, applying a magnetic field gradient to the SERF common magnetometer through a first signal generator connected to a gradient coil, wherein the magnetic field gradient includes a longitudinal magnetic field gradient and transverse magnetic field gradient After the atomic spin polarization in the atomic gas chamber is stabilized, the three-axis static magnetic field is compensated by the second signal generator connected to the active magnetic compensation coil to keep the system in a stable working state, so that the atomic ensemble in the atomic gas chamber has the ability to resist external interference; Step 2, apply a magnetic field consistent with the direction of the pump light on the z-axis to decouple the precession of electrons and nuclei, wait for the polarization state of the atomic spin ensemble to stabilize, then apply an excitation signal on the y-axis, and record the relationship between the laser free precession decay signal and time detected in the x-axis direction of the system; Step 3, according to the relationship between the free precession decay signal and time, fit the longitudinal relaxation time T2 under the set magnetic field gradient, the reciprocal of T2 is the longitudinal relaxation rate, and determine whether the value of T2 is the maximum or minimum inflection point with the change of the magnetic field gradient. If so, enter step 4, if not, return to step 1; Step 4, obtain the longitudinal magnetic field gradient by quadratic parabola fitting and the nuclear transverse relaxation rate The center of the quadratic parabola is the longitudinal magnetic field gradient of the measured SERF common magnetometer:

[0035]

[0036] in represents the relaxation of the electric quadrupole moment, represents spin-destruction relaxation, Indicates that alkali metal electrons are polarized by spin exchange collisions 21 The nuclear spin exchange rate of Ne, represents the transverse magnetic field gradient relaxation:

[0037]

[0038] Where V is the volume of the atomic gas chamber, D Ne-Ne is the diffusion coefficient of the atomic gas cell, R is the radius of the atomic gas cell, γ n is the nuclear gyromagnetic ratio, and the longitudinal magnetic field gradient of the system is obtained by fitting the relationship between the longitudinal relaxation time T2 and the magnetic field gradient; Step 5, the longitudinal magnetic field gradient After returning to zero, the change of nuclear polarizability is indirectly reflected by the closed-loop control self-compensation point method, and the transverse magnetic field gradient is Zeroing:

[0039] The minimum value of the transverse relaxation rate is used to express the total magnetic field gradient ΔB in the gas chamber.total , SERF common magnetometer transverse magnetic field gradient and the time derivative of the nuclear longitudinal polarizability The relationship is as follows:

[0040]

[0041] in is the nuclear longitudinal polarizability, t is the time, is the electron longitudinal polarizability, is the initial value of the nuclear longitudinal polarizability, B z represents the z-axis magnetic field; step 6, using the gradient coil to actively apply the magnetic field gradient to suppress the influence of magnetic field inhomogeneity on the system, prolong the atomic decoherence time, and realize the optimization of the transverse magnetic field gradient: the frequency of the modulation signal applied by the first signal generator to the transverse magnetic field coil and the phase-locked amplifier is equal to the frequency corresponding to the electronic resonance peak, the output signal of the SERF common magnetometer is demodulated by the phase-locked amplifier and input into the PID controller, and the phase-locked amplifier performs closed-loop control on the self-compensation point. At this time, the magnetic field control voltage applied to the modulation coil reflects the change of the nuclear polarization rate; based on the quadratic parabola equation, the relationship between the magnetic field gradient and the slope corresponding to the change of the magnetic field control voltage with time is fitted, and the transverse magnetic field gradient is extracted from the center of the quadratic parabola.

[0042] Step 1 involves heating the atomic chamber to above 170°C to increase the atomic density. The inert gas within the atomic chamber serves to sense inertia and resist interference from external magnetic fields. The alkali metal electrons within the atomic chamber are used for spin exchange collision polarization of the inert gas nuclear spins. The active magnetic compensation coil and gradient coil are both flexible thin-film coils. The transverse shim coils in the active magnetic compensation coils are nested in a saddle shape, and the axial shim coils are Maxwell coils. The pump light in step 2, or the pump laser, comes from a pump laser, which is a distributed Bragg reflector (DBR). The detection laser in step 2 comes from a detection laser, which is a distributed feedback (DFB) laser. In step 2, a magnetic field of 1500 nT is applied along the z-axis, aligned with the direction of the pump light. Step 4 involves obtaining the relationship between the longitudinal magnetic field gradient and the nuclear transverse relaxation rate through quadratic parabola fitting at three temperatures: 170°C, 180°C, and 190°C. The transverse magnetic field gradient in step 1 includes four transverse magnetic field gradients: dBx / dz, dBy / dz, dBx / dx and dBy / dy, where Bx and By are the x-axis and y-axis magnetic fields respectively.

[0043] The SERF common magnetometer includes a detection laser 1, a first spot beam expander 2, a first 1 / 2 wave plate 3, a first beam splitter 4, a first liquid crystal phase retarder 5, a first Grantiera prism 6, a polarization beam splitter 7, an atomic gas chamber 20, a Wollaston prism 21 and an array differential photodetector 22 connected in sequence. The array differential photodetector 22 is respectively connected to a phase-locked amplifier 24 and a DAQ data acquisition system 28, and the DAQ data acquisition system is connected to a host computer 29. The phase-locked amplifier 24 is respectively connected to a PID controller 23 and a second signal generator 27. The PID controller 23 is connected to the modulation coil Zm in the coil 18, the second signal generator is connected to the active magnetic compensation coil in the coil 18, and the gradient coil in the coil 18 is connected to the first signal generator 8. The reflecting side of the first Grantiera prism 6 is connected to the first liquid crystal phase retarder 5 through the second photodetector 16 and the electronic control unit ECU in turn. The SERF common magnetometer includes a sequentially connected pump laser 9, a second beam expander 10, a second half-wave plate 11, a second beam splitter 25, a second liquid crystal phase retarder 26, a second GranTerra prism 12, a second half-wave plate 14, an optical isolator 15, and an atomic gas cell 20. The optical isolator 15 is connected to the second liquid crystal phase retarder 26 via a first photodetector 13 and an electronic control unit (ECU). A heating film 19 is provided on the outside of the atomic gas cell 20. The heating film 19 is located within a coil 18, which is located within a ferrite 17, which is located within a permalloy 30.

[0044] A three-axis magnetic field gradient measurement and suppression method based on SERF common magnetometer, where SERF (Spin-Exchange-Relaxation-Free) is spin-exchange relaxation-free. Firstly, an in-situ measurement and compensation method of longitudinal magnetic field gradient based on atomic spin relaxation mechanism is proposed; then a method for K-Rb- 21 A transverse magnetic field gradient compensation method for a Ne SERF co-magnetometer uses closed-loop control of the self-compensation point to represent the nuclear spin polarization, enabling in-situ measurement of triaxial transverse magnetic field gradients. This method facilitates analysis of the impact of magnetic field gradients on system relaxation mechanisms, improves atomic decoherence time, and enhances signal-to-noise ratio, thereby improving inertial measurement accuracy. Furthermore, the compensation method provided by this technology has the potential to be applied to other atomic sensors, such as SERF magnetometers and nuclear magnetic resonance co-magnetometers.

[0045] A method for measuring and suppressing a three-axis magnetic field gradient based on a SERF common magnetometer comprises the following steps:

[0046] In step 1, magnetic field gradients of varying directions and magnitudes are applied to the SERF common magnetometer. Once the atomic spin polarization stabilizes, active magnetic compensation coils are used to compensate for the three-axis static magnetic field, maintaining a stable operating state. This allows the atomic ensemble in the atomic gas cell to resist external interference.

[0047] In the second step, a large magnetic field consistent with the direction of the pump light is applied on the z-axis to decouple the precession of electrons and nuclei. After the polarization state of the atomic spin ensemble is stabilized, an excitation signal is applied on the y-axis, and the relationship between the signal and time of the laser free precession decay detected in the x-axis direction of the system is recorded.

[0048] Step 3: According to the free precession attenuation method, T2 under the set magnetic field gradient is fitted, and whether the value of T2 is the maximum (or minimum) inflection point of the change with the magnetic field gradient is determined. If so, the longitudinal magnetic field gradient is obtained through the following steps, otherwise repeat step 1.

[0049] Step 4: The relationship between the longitudinal magnetic field gradient and the nuclear transverse relaxation rate is obtained by fitting a quadratic parabola, and the center of the quadratic parabola is the measured longitudinal magnetic field gradient of the SERF common magnetometer.

[0050] After the longitudinal magnetic field gradient is returned to zero, the change in nuclear polarization is indirectly reflected by the closed-loop control of the self-compensation point, so that the transverse magnetic field gradients in other directions can be returned to zero. The three-axis transverse magnetic field gradient optimization process is as follows:

[0051] In step 5, the signal generator applies a modulation signal to the transverse magnetic field coil and lock-in amplifier at a frequency equal to the frequency corresponding to the electron resonance peak. The SERF common magnetometer's output signal is demodulated by the lock-in amplifier and input into a PID controller, which then performs closed-loop control of the self-compensation point. The magnetic field control voltage applied to the modulation coil reflects the change in nuclear polarizability.

[0052] Step six, fitting the relationship between the magnetic field gradient and the slope corresponding to the change of the magnetic field control voltage over time based on the quadratic parabola equation, and extracting the transverse magnetic field gradient corresponding to the direction from the center of the quadratic parabola.

[0053] The step three includes the following relationship: in represents the nuclear transverse relaxation rate, represents the relaxation of the electric quadrupole moment, represents spin-destruction relaxation, Indicates that alkali metal electrons are polarized by spin exchange collisions 21 The nuclear spin exchange rate of Ne, represents the transverse magnetic field gradient relaxation. By applying a longitudinal gradient magnetic field using a gradient coil and measuring the transverse relaxation time under different longitudinal gradient magnetic field conditions, the quadratic parabolic relationship between the longitudinal magnetic field gradient and the transverse relaxation time is obtained according to the following formula:

[0054]

[0055] Where V is the volume of the atomic gas cell, is the longitudinal magnetic field gradient, D Ne-Ne is the diffusion coefficient of the atomic gas cell, which is inversely proportional to the gas pressure; R is the radius of the atomic gas cell, γ n is the nuclear gyromagnetic ratio, and the minimum value of the transverse relaxation rate is the total magnetic field gradient ΔB in the gas cell. total SERF common magnetometer transverse magnetic field gradient and the time derivative of the nuclear longitudinal polarizability The relationship is as follows.

[0056]

[0057] in express 21 The nuclear spin polarization of Ne is determined by the alkali metal electron spin exchange collision exchange rate, B z represents the main magnetic field, is the transverse magnetic field gradient, and represent the longitudinal polarizabilities of nuclei and electrons, respectively.

[0058] The step 1 includes heating the atomic gas chamber to increase the atomic density, the inert gas inside the atomic gas chamber is used to sense inertia and resist external interference magnetic fields, and the alkali metal electrons inside the atomic gas chamber are used for spin exchange collision polarization of the inert gas nuclear spin.

[0059] The pumping laser in step 2 comes from a pumping laser, which is a distributed Bragg reflector laser (DBR). The detection laser in step 2 comes from a detection laser, which is a distributed feedback laser (DFB). Four transverse magnetic field gradients, dBx / dz (nT / cm), dBy / dz (nT / cm), dBx / dx (nT / cm), and dBy / dy (nT / cm), were actively applied. It can be seen that the corresponding transverse magnetic field gradients obtained by in-situ measurement are -1.01nT / cm, 0.37nT / cm, -0.62nT / cm, and -0.61nT / cm, respectively, where the positive and negative signs indicate the direction of the magnetic field gradient. Figure 3The vertical axis represents the rate of change of the nuclear spin closed-loop control voltage over time, reflecting the time derivative of the nuclear spin polarization. Ultimately, the relationship between the rate of change of the nuclear spin polarization and the transverse magnetic field gradient can be obtained. The relationship between the magnetic field gradient and the slope of the magnetic field control voltage over time is fitted based on the quadratic parabola equation, and the transverse magnetic field gradient corresponding to that direction is extracted from the center of the quadratic parabola.

[0060] The steps 1 and 6 include applying longitudinal and transverse magnetic fields using gradient coils.

[0061] A method for measuring and suppressing a three-axis magnetic field gradient based on a SERF common magnetometer comprises the following steps:

[0062] Step 1, such as Figure 1 and Figure 2 As shown in the figure, when magnetic field gradients of different directions and magnitudes are applied to the SERF common magnetometer, the system maintains a stable working state by compensating the three-axis static magnetic field using the active magnetic compensation coils developed by our research team after the atomic spin polarization stabilizes, thus making the atomic ensemble in the atomic gas cell resistant to external interference.

[0063] In the second step, a large magnetic field consistent with the direction of the pump light is applied on the z-axis through the active magnetic compensation coil to decouple the precession of electrons and nuclei. After the polarization state of the atomic spin ensemble is stabilized, an excitation signal is applied on the y-axis, and the relationship between the signal and time of the laser free precession decay detected in the x-axis direction of the system is recorded.

[0064] Step three, as shown in the figure, obtain T2 under the set magnetic field gradient by fitting according to the free precession attenuation method, and determine whether the value of T2 is at the inflection point of the maximum value (or minimum value) changing with the magnetic field gradient. If so, obtain the longitudinal magnetic field gradient through the following steps, otherwise repeat step one.

[0065] Step 4: Figure 2 As shown in the figure, under the three temperature conditions of 170℃, 180℃ and 190℃, the relationship between the longitudinal magnetic field gradient and the nuclear transverse relaxation rate was obtained by fitting the quadratic parabola, and the center of the quadratic parabola is the measured longitudinal magnetic field gradient of the SERF common magnetometer.

[0066] After the longitudinal magnetic field gradient is returned to zero, the change in nuclear polarization is expressed by the closed-loop control self-compensation point method, so that the transverse magnetic field gradients in other directions can be returned to zero. The three-axis transverse magnetic field gradient optimization process is as follows:

[0067] Step five, such as Figure 1As shown in the figure, the frequency of the modulation signal applied by the signal generator to the transverse magnetic field coil and the lock-in amplifier is equal to the frequency corresponding to the electron resonance peak. The output signal of the SERF common magnetometer is demodulated by the lock-in amplifier and input into the PID controller, which then performs closed-loop control of the self-compensation point. The magnetic field control voltage applied to the modulation coil reflects the change in nuclear polarizability.

[0068] Step six, such as Figure 3 As shown in the figure, the relationship between the longitudinal polarizability of the nuclei and the transverse magnetic field gradient is obtained by processing and analyzing the data, thereby obtaining the transverse magnetic field gradient of the system in the stable polarization state. Four transverse magnetic field gradients of dBx / dz (nT / cm), dBy / dz (nT / cm), dBx / dx (nT / cm), and dBy / dy (nT / cm) are actively applied. It can be seen that the corresponding transverse magnetic field gradients obtained by in situ measurement are -1.01nT / cm, 0.37nT / cm, -0.62nT / cm, and -0.61nT / cm, respectively, where the positive and negative signs indicate the direction of the magnetic field gradient. Figure 3 The vertical axis represents the rate of change of the nuclear spin closed-loop control voltage over time, reflecting the time derivative of the nuclear spin polarization. Ultimately, the relationship between the rate of change of the nuclear spin polarization and the transverse magnetic field gradient can be obtained. The relationship between the magnetic field gradient and the slope of the magnetic field control voltage over time is fitted based on the quadratic parabola equation, and the transverse magnetic field gradient corresponding to that direction is extracted from the center of the quadratic parabola.

[0069] Step 1 involves heating the atomic chamber to above 170°C to increase the atomic density. The inert gas within the atomic chamber serves as a sensitive inertial sensor and protects against external magnetic fields. The alkali metal electrons within the atomic chamber are used for spin exchange collisions to polarize the inert gas nuclear spins. Both the active magnetic compensation coil and magnetic field gradient coil are self-developed flexible thin-film coils. The transverse shim coils in the active magnetic compensation coils are nested in a saddle shape, while the axial shim coils are an improved design based on Maxwell coils, resulting in a more uniform coil.

[0070] The pumping laser in step 2 comes from a pumping laser, which is a distributed Bragg reflector laser (DBR). The detection laser in step 2 comes from a detection laser, which is a distributed feedback laser (DFB).

[0071] The PID controller used in step 5 is a self-developed controller, and the PID control program written in STM32 performs closed-loop control of the self-compensation point. The active magnetic compensation coil applies a static magnetic field to the SERF common magnetometer, and the modulation coil is used to compensate the magnetic field during PID control.

[0072] Any content not described in detail in this specification is prior art known to those skilled in the art. It should be noted that the above description is intended to help those skilled in the art understand the present invention, but does not limit the scope of protection of the present invention. Any equivalent substitution, modification, improvement, and / or simplification of the above description that does not depart from the essence of the present invention shall fall within the scope of protection of the present invention.

Claims

1. A method for measuring and suppressing three-axis magnetic field gradient based on a SERF common magnetometer, characterized in that: The following steps are involved: Step 1: Apply a magnetic field gradient to the SERF common magnetometer through a first signal generator connected to a gradient coil, wherein the magnetic field gradient includes a longitudinal magnetic field gradient and transverse magnetic field gradient After the atomic spin polarization in the atomic gas cell is stabilized, the three-axis static magnetic field is compensated by a second signal generator connected to the active magnetic compensation coil to keep the system in a stable working state, thereby making the atomic ensemble in the atomic gas cell able to resist external interference; Step 2: Apply a magnetic field on the z-axis that is consistent with the direction of the pump light to decouple the precession of electrons and nuclei. After the polarization state of the atomic spin ensemble stabilizes, apply an excitation signal on the y-axis and record the relationship between the laser free precession decay signal and time along the x-axis. Step 3: Based on the relationship between the free precession decay signal and time, the longitudinal relaxation time T2 under the set magnetic field gradient is fitted. The reciprocal of T2 is the longitudinal relaxation rate. It is determined whether the value of T2 is the maximum or minimum inflection point with the change of the magnetic field gradient. If so, proceed to step 4; if not, return to step 1. Step 4: Obtain the longitudinal magnetic field gradient by quadratic parabola fitting and the nuclear transverse relaxation rate The center of the quadratic parabola is the longitudinal magnetic field gradient of the measured SERF common magnetometer: in represents the relaxation of the electric quadrupole moment, represents spin-destruction relaxation, Indicates that alkali metal electrons are polarized by spin exchange collisions 21 The nuclear spin exchange rate of Ne, represents the transverse magnetic field gradient relaxation: Where V is the volume of the atomic gas chamber, D Ne-Ne is the diffusion coefficient of the atomic gas cell, R is the radius of the atomic gas cell, γ n is the nuclear gyromagnetic ratio, and the longitudinal magnetic field gradient of the system is obtained by fitting the relationship between the longitudinal relaxation time T2 and the magnetic field gradient; Step 5, longitudinal magnetic field gradient After returning to zero, the change of nuclear polarizability is indirectly reflected by the closed-loop control self-compensation point method, and the transverse magnetic field gradient is Zeroing: The minimum value of the transverse relaxation rate is used to express the total magnetic field gradient ΔB in the gas chamber. total , SERF common magnetometer transverse magnetic field gradient and the time derivative of the nuclear longitudinal polarizability The relationship is as follows: in is the nuclear longitudinal polarizability, t is the time, is the electron longitudinal polarizability, is the initial value of the nuclear longitudinal polarizability, B z represents the z-axis magnetic field, yes 21 The nuclear spin polarization of Ne is exchanged via the alkali metal electron spin exchange collision rate; Step 6: Use the gradient coil to actively apply a magnetic field gradient to suppress the impact of magnetic field inhomogeneity on the system, extend the atomic decoherence time, and optimize the transverse magnetic field gradient: the frequency of the modulation signal applied by the first signal generator to the transverse magnetic field coil and the phase-locked amplifier is equal to the frequency corresponding to the electron resonance peak. The output signal of the SERF common magnetometer is demodulated by the phase-locked amplifier and input into the PID controller. The phase-locked amplifier performs closed-loop control on the self-compensation point. At this time, the magnetic field control voltage applied to the modulation coil reflects the change in the nuclear polarization rate; based on the quadratic parabola equation, the relationship between the magnetic field gradient and the slope corresponding to the magnetic field control voltage changing with time is fitted, and the transverse magnetic field gradient is extracted from the center of the quadratic parabola.

2. The method for measuring and suppressing three-axis magnetic field gradient based on SERF common magnetometer according to claim 1, characterized in that: The step 1 includes heating the atomic gas chamber to above 170° C. to increase the atomic density. The inert gas inside the atomic gas chamber is used to sense inertia and resist interference from the external magnetic field. The alkali metal electrons inside the atomic gas chamber are used for spin exchange collision polarization of the inert gas nuclear spin. The active magnetic compensation coil and the gradient coil are both flexible thin film coils. The transverse shim coil in the active magnetic compensation coil adopts a saddle-shaped nested form, and the axial shim coil adopts a Maxwell coil.

3. The method for measuring and suppressing three-axis magnetic field gradient based on SERF common magnetometer according to claim 1, characterized in that: The pump light in step 2, ie, the pumping laser, comes from a pumping laser, which is a distributed Bragg reflector laser (DBR). The detection laser in step 2 comes from a detection laser, which is a distributed feedback laser (DFB).

4. The method for measuring and suppressing three-axis magnetic field gradient based on SERF common magnetometer according to claim 1, characterized in that: In step 2, a magnetic field of 1500 nT is applied on the z-axis in the same direction as the pump light.

5. The method for measuring and suppressing three-axis magnetic field gradient based on SERF common magnetometer according to claim 1, characterized in that: The step 4 includes obtaining the relationship between the longitudinal magnetic field gradient and the nuclear transverse relaxation rate by quadratic parabola fitting under three temperature conditions of 170° C., 180° C., and 190° C.

6. The method for measuring and suppressing three-axis magnetic field gradient based on SERF common magnetometer according to claim 1, characterized in that: The transverse magnetic field gradient in step 1 includes four transverse magnetic field gradients: dBx / dz, dBy / dz, dBx / dx and dBy / dy, where Bx and By are the x-axis and y-axis magnetic fields respectively.

7. The method for measuring and suppressing three-axis magnetic field gradient based on SERF common magnetometer according to claim 1, characterized in that: The SERF common magnetometer includes a detection laser, a first spot beam expansion component, a first 1 / 2 wave plate, a first beam splitter, a first liquid crystal phase retarder, a first Grantiera prism, a polarization beam splitter, an atomic gas chamber, a Wollaston prism and an array differential photodetector connected in sequence. The array differential photodetector is respectively connected to a phase-locked amplifier and a DAQ data acquisition system, the DAQ data acquisition system is connected to a host computer, the phase-locked amplifier is respectively connected to a PID controller and a second signal generator, the PID controller is connected to a modulation coil in the coil, the second signal generator is connected to an active magnetic compensation coil in the coil, the gradient coil in the coil is connected to the first signal generator, and the reflective side of the first Grantiera prism is connected to the first liquid crystal phase retarder via a second photodetector and an electronic control unit ECU in sequence.

8. The method for measuring and suppressing three-axis magnetic field gradient based on SERF common magnetometer according to claim 1, characterized in that: The SERF common magnetometer includes a pump laser, a second spot beam expander, a second 1 / 2 wave plate, a second beam splitter, a second liquid crystal phase retarder, a second Granitella prism, a second 1 / 2 wave plate, an optical isolator and an atomic gas chamber connected in sequence. The optical isolator is connected to the second liquid crystal phase retarder through a first photodetector and an electronic control unit ECU in turn.

9. The method for measuring and suppressing three-axis magnetic field gradient based on SERF common magnetometer according to claim 1, characterized in that: A heating film is provided outside the atomic gas chamber, the heating film is located inside the coil, the coil is located inside the ferrite, and the ferrite is located inside the permalloy.

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