Ultrahigh-sensitivity triaxial closed-loop spin exchange relaxation-free atom magnetometer

Abstract

The invention discloses an ultrahigh-sensitivity three-axis closed-loop spin exchange relaxation-free atom magnetometer, which expands the ultrahigh-sensitivity measurement capability of the atom magnetometer to three axes based on parameter modulation optimization and double detection light, fully inhibits transverse spin relaxation introduced by a modulation magnetic field, provides more high-quality information for weak magnetic measurement, and improves the measurement accuracy. Based on decoupled magnetic field information, a three-axis magnetic field sensitive to an atomic ensemble is locked to zero in real time, and closed-loop feedback quantity is used as magnetic field measurement information, so that the problems of nonlinear offset of a magnetometer calibration coefficient and cross-axis coupling crosstalk caused by magnetic field drift are fundamentally inhibited; the high anti-interference capability of the atom magnetometer in a complex magnetic field environment is enhanced, long-time stable measurement and magnetic field measurement precision of the atom magnetometer are guaranteed, sub-fT-level sensitivity and pT-level precision can be achieved on transverse double axes, and the atom magnetometer is suitable for the fields of extremely weak magnetic measurement testing, biomedical weak magnetic imaging and the like.

Description

Technical Field

[0001] The present invention relates to an ultra-high sensitivity three-axis closed-loop spin-exchange relaxation-free atomic magnetometer, belonging to the technical field of ultra-weak magnetic field precision measurement, particularly relating to the closed-loop detection of three-axis ultra-weak magnetic fields, having the characteristics of ultra-high sensitivity and long-term stability, and being applicable to fields such as ultra-weak magnetic metrology testing and biomedical weak magnetic imaging. Background Art

[0002] An atomic magnetometer is a high-sensitivity device for measuring magnetic fields based on the Zeeman effect of atomic energy levels in a magnetic field, and has important application prospects in fields such as biomedical imaging and space magnetic field detection. The atomic magnetometer can achieve ultra-high measurement sensitivities at the sub-fT / Hz 1 / 2 to fT / Hz 1 / 2 level in a weak magnetic environment. However, residual magnetic field drifts in devices such as magnetic shielding rooms and magnetic shielding cabins can cause errors such as non-linear offset of the scale factor and cross-axis coupling crosstalk in the atomic magnetometer, thereby resulting in a decrease in measurement accuracy. Magnetic field closed-loop can feedback and suppress the drift amount while retaining all magnetic field information, and is a direct and effective error suppression method. However, this method relies on the ultra-high sensitive synchronous measurement and decoupling of the three-axis magnetic field by the atomic magnetometer. Existing schemes apply multi-axis high-frequency rotating magnetic field modulation at the level of hundreds of nT, which will cause a significant increase in spin-exchange relaxation and a decrease in measurement sensitivity. Summary of the Invention

[0003] In view of the deficiencies in the prior art, the present invention proposes an ultra-high sensitivity three-axis closed-loop spin-exchange relaxation-free atomic magnetometer. Different from existing multi-axis high-frequency magnetic field modulation, the scheme is based on transverse weak parameter modulation and a double-detection light configuration. While achieving three-axis real-time measurement, it fully suppresses the spin-exchange relaxation introduced by the modulation magnetic field, providing more high-quality information for weak magnetic measurement. Based on the magnetic field information decoupled by the spin-exchange relaxation-free atomic magnetometer, the three-axis magnetic fields sensitive to the atomic ensemble are locked to zero in real time, and the closed-loop feedback amount is used as the magnetic field measurement information to solve the problems of scale factor offset and cross-axis coupling crosstalk of the atomic magnetometer, ensuring long-term stable measurement and magnetic field measurement accuracy of the ultra-high sensitive atomic magnetometer, and enabling sensitivities at the sub-fT level and accuracies at the pT level to be achieved on the transverse two axes.

[0004] The technical solution of the present invention is as follows:

[0005] An ultra-high sensitivity three-axis closed-loop spin-exchange relaxation-free atomic magnetometer, characterized in that it includes a pump laser acting on the atomic cell along the z-axis direction, a first detection laser passing through the atomic cell along the x-axis direction, and a second detection laser passing through the atomic cell along the y-axis direction. The pump laser is a circularly polarized laser generated by a pump laser and formed after passing through a combined prism, which is close to the resonance frequency of the alkali metal atom D1 line. The first detection laser is a parallel polarized P light formed by splitting a linearly polarized laser from a detection laser that is far detuned from the resonance frequency of the atomic D1 line. The second detection laser is a vertically polarized S light formed by splitting a linearly polarized laser from a detection laser that is far detuned from the resonance frequency of the atomic D1 line. The P light emerging from the atomic cell obtains the optical rotation angle of the detection light along the x-axis that maps the magnetic field in the y-axis direction. The S light emerging from the atomic cell obtains the DC component of the optical rotation angle of the detection light along the y-axis that maps the magnetic field in the x-axis direction, and obtains the first harmonic component of the optical rotation angle of the detection light along the y-axis that maps the magnetic field in the z-axis direction. By respectively decoupling the three-axis magnetic field information and feeding it back into the three-axis magnetic field coils in the atomic magnetometer probe, a compensation magnetic field is generated in a manner that is equal in magnitude and opposite in direction to each axis magnetic field, thereby realizing real-time closed-loop suppression of magnetic field drift.

[0006] The atomic magnetometer probe is connected to a circuit control system. The circuit control system includes a host computer, a narrow-linewidth semiconductor laser, a signal generator, and a power amplifier. The narrow-linewidth semiconductor laser includes the pump laser and the detection laser. The signal generator is used to provide an AC drive voltage and a modulation voltage. The circuit control system includes three channels. The polarization differential detection module along the x-axis is connected to channel I, and after passing through a first low-pass filter with a cut-off frequency lower than the modulation frequency, it is input to the first PID module. The output following voltage drives the y-axis magnetic field coil to generate a compensation magnetic field that is equal in magnitude and opposite in direction. The polarization differential detection module along the y-axis is connected to channel II and channel III through a tee. Channel II is input to the second PID module after passing through a second low-pass filter with a cut-off frequency lower than the modulation frequency. The output following voltage drives the x-axis magnetic field coil to generate a compensation magnetic field that is equal in magnitude and opposite in direction. Channel III is input to the third PID module after passing through a lock-in amplifier with the modulation magnetic field frequency ω as a reference. The output following voltage drives the z-axis magnetic field coil to generate a compensation magnetic field that is equal in magnitude and opposite in direction. The filtering cut-off frequency of the lock-in amplifier is lower than the modulation frequency.

[0007] The atomic magnetometer probe is placed at the center of a magnetic shielding device made of a high-permeability material, and internally includes the combined prism, the boron nitride oven, and the first half-wave plate through which the detection laser is input into the probe via polarization-maintaining optical fiber. After the first half-wave plate, the light path passes through a polarization beam splitter prism. The transmitted side of the polarization beam splitter prism passes through the atomic gas cell and then passes through the second half-wave plate in the polarization difference detection module. After the second half-wave plate, it sequentially passes through the first lateral displacement polarization beam splitter prism, the first photodetector, and the first differential circuit and is connected to channel I in the circuit control system. The reflected side of the polarization beam splitter prism sequentially passes through the first right-angle prism, the second right-angle prism, the atomic gas cell, the third half-wave plate, the second lateral displacement polarization beam splitter prism, the second photodetector, and the second differential circuit and is respectively connected to channel II and channel III in the circuit control system. The combined prism includes a right-angle prism, a linear polarizer, and a quarter-wave plate. A heating film is attached to the outer wall surface of the boron nitride oven. The atomic gas cell is located inside the boron nitride oven.

[0008] The y-axis coil generates a modulation magnetic field, and the output voltages of the three channels of the circuit control system are:

[0009]

[0010]

[0011] S0 = R op / 2(R op +R rel )

[0012] where V x is the optical rotation angle signal of the detection light along the x-axis mapping the y-axis magnetic field, G is the photoelectric conversion coefficient of the differential photodetector, I pr is the intensity of the incident detection light, η is the attenuation coefficient of the detection light intensity by the gas cell glass, e is the natural constant, OD is the optical depth, l is the optical path of the gas cell, n is the atomic number density, r e is the electron radius, c is the speed of light, f D1 is the D1 line resonance frequency, Δ is the detection light detuning, Γ D1 is the pressure broadening of the alkali metal D1 line, S0 is an intermediate quantity, γ e is the electron gyromagnetic ratio, B y is the y-axis magnetic field, R op is the pumping rate, R rel is the relaxation rate is the DC component of the optical rotation angle of the detection light along the y-axis mapping the x-axis magnetic field, B x is the x-axis magnetic field, J is the Bessel function of the first kind, the subscript is the order of the Bessel function, β is proportional to B modModulation factor of / ω, B mod is the amplitude of the modulation magnetic field, ω is the frequency of the modulation magnetic field, is the first harmonic component of the optical rotation angle of the detection light along the y-axis that maps the magnetic field along the z-axis. R is an intermediate quantity, R = R op / q, where q is the nuclear slowdown factor, Γ is the ratio of the total atomic relaxation to the nuclear slowdown factor, and B z is the magnetic field along the z-axis.

[0013] A method for using an ultra-high sensitivity three-axis closed-loop spin-exchange relaxation-free atomic magnetometer, characterized in that the above-mentioned ultra-high sensitivity three-axis closed-loop spin-exchange relaxation-free atomic magnetometer is adopted, including the following steps:

[0014] Step 1: Adjust the external semiconductor laser, adjust the pump laser and detection laser frequencies to be close to the resonance frequency of the alkali metal atom D1 line and far detuned from the atomic D1 line resonance frequency respectively. Start the measurement device, input an AC drive voltage into the heating film, and perform three-axis remanence compensation after the temperature stabilizes, so that the atomic ensemble initially remains in a spin-exchange relaxation-free state;

[0015] Step 2: Use the signal generator in the circuit control system to output a voltage signal to the y-axis magnetic field coil to generate a modulation magnetic field B mod along the y-axis with a frequency of ω and an amplitude of B mod cos(ωt), where t is time, and the modulation frequency is much lower than the Larmor precession frequency. The optimal modulation amplitude is proportional to the modulation frequency;

[0016] Step 3: Connect the polarization differential detection module to the three channels of the circuit control system, decouple the three-axis magnetic fields B x , B y , B z to be measured respectively, and equivalently input the closed-loop feedback voltage into the corresponding uniform magnetic field coils to generate a compensation magnetic field of equal magnitude and opposite direction, so as to compensate the magnetic field sensitive to the atomic ensemble to zero in real time;

[0017] Step 4: Use the PID closed-loop feedback signal as the magnetic field measurement signal, optimize the parameters of the pump light intensity, detection light frequency, and modulation magnetic field amplitude according to the output response of the atomic magnetometer, ensure that the output response of the atomic magnetometer, that is, the closed-loop feedback signal, is maximized, and determine the system parameters;

[0018] Step 5: Apply a calibrated magnetic field with increasing frequency and the same amplitude through three groups of coils, record the amplitude of the closed-loop feedback signal, calibrate the scale factor and effective measurement bandwidth of the spin-exchange relaxation-free atomic magnetometer, synchronously collect the voltages output from the three channels of the circuit control system, convert them into current signals by using high-precision resistors equivalently, and perform power spectral density processing on the upper computer and convert according to the coil constant to evaluate the magnetic field measurement sensitivity of the system.

[0019] In step 2, the detection optical rotation angle of the spin-exchange relaxation-free atomic magnetometer along the x-axis under weak transverse modulation maps the magnetic field along the y-axis. The DC and primary harmonic components in the detection optical rotation angle along the y-axis map the magnetic fields along the x-axis and z-axis respectively. The scale factor is related to various parameters such as the pumping rate, relaxation rate, gyromagnetic ratio of the atomic spin ensemble, and the modulation factor determined by the modulation amplitude and modulation frequency.

[0020] In step 3, the polarization differential detection module along the x-axis is connected to channel I of the circuit control system. After passing through a low-pass filter with a cut-off frequency lower than the modulation frequency, it is input to the PID module to control the magnetic field along the y-axis. The polarization differential detection module along the y-axis is connected to channel II and channel III of the circuit control system through a tee. Channel II is input to the PID module to control the magnetic field along the x-axis after passing through a low-pass filter with a cut-off frequency lower than the modulation frequency. Channel III is input to the PID module to control the magnetic field along the z-axis after passing through a lock-in amplifier with the modulation magnetic field frequency ω as the reference.

[0021] The technical effects of the present invention are as follows: The present invention relates to an ultra-high-sensitivity three-axis closed-loop spin-exchange relaxation-free atomic magnetometer, belonging to the field of ultra-weak magnetic field precision measurement. Based on parameter modulation optimization and dual detection light, the ultra-high-sensitivity measurement ability of the atomic magnetometer is extended to three axes, and the spin-exchange relaxation introduced by the modulation magnetic field is fully suppressed, providing more high-quality information for weak magnetic measurement. Based on the decoupled magnetic field information, the three-axis magnetic fields sensitive to the atomic ensemble are locked to zero in real time, and the closed-loop feedback quantity is used as the magnetic field measurement information. The present invention fundamentally suppresses the non-linear offset of the scale factor and the cross-axis coupling crosstalk problem of the atomic magnetometer introduced by magnetic field drift, enhances the strong anti-interference ability of the atomic magnetometer in a complex magnetic field environment, ensures the long-term stable measurement and magnetic field measurement accuracy of the ultra-high-sensitivity atomic magnetometer, can achieve sub-fT-level sensitivity and pT-level accuracy in the transverse two axes, and is applicable to fields such as ultra-weak magnetic metrology testing and biomedical weak magnetic imaging. Description of the Drawings

[0022] Figure 1 It is a schematic structural diagram of implementing an ultra-high-sensitivity three-axis closed-loop spin-exchange relaxation-free atomic magnetometer of the present invention.

[0023] Description of the reference numerals: 1 - Circuit control system; 2 - Atomic magnetometer probe; 3 - Magnetic field coil; 4 - Second right-angle prism; 5 - Atomic gas cell; 6 - Heating film; 7 - Polarization differential detection module; 8 - Detection laser; 9 - Pumping laser; 10 - First lateral displacement polarization beam splitter prism; 11 - First half-wave plate; 12 - Polarization beam splitter prism; 13 - Combined prism (including right-angle prism, linear polarizer and quarter-wave plate); 14 - Second half-wave plate; 15 - Boron nitride oven; 16 - Second photodetector; 17 - Second differential circuit; 18 - Narrow-linewidth semiconductor laser (including pumping laser and detection laser); 19 - Polarization-maintaining optical fiber; 20 - Signal generator; 21 - Power amplifier; 22 - First PID module (PID is proportional integral derivative); 23 - Channel I (including first PID module + first low-pass filter); 24 - Channel II (including second PID module + second low-pass filter); 25 - Second low-pass filter; 26 - Channel III (including third PID module + lock-in amplifier); 27 - Lock-in amplifier; 28 - Host computer; xyz - Three axes of a rectangular coordinate system (i.e., x-axis, y-axis, and z-axis). Detailed implementation manners

[0024] The present invention will be described below in conjunction with the accompanying drawings ( Figure 1 ). and embodiments.

[0025] Figure 1 is a schematic structural diagram of an ultra-high sensitivity three-axis closed-loop spin-exchange relaxation-free atomic magnetometer for implementing the present invention. Refer to Figure 1As shown in the figure, a super-high-sensitivity three-axis closed-loop spin-exchange-relaxation-free atomic magnetometer includes a pump laser 9 that acts on an atomic gas cell 5 in the z-axis direction, a first detection laser (i.e., detection laser 8) that passes through the atomic gas cell 5 in the x-axis direction, and a second detection laser that passes through the atomic gas cell 5 in the y-axis direction. The pump laser 9 is a circularly polarized laser generated by a pump laser (i.e., the pump laser in the narrow-linewidth semiconductor laser 18) and formed after passing through a combination prism 13, which is close to the resonance frequency of the D1 line of alkali metal atoms. The first detection laser 8 is a parallel polarized P light formed by splitting a linearly polarized laser from a detection laser (i.e., the detection laser in the narrow-linewidth semiconductor laser 18) that is far off-resonance from the resonance frequency of the atomic D1 line. The second detection laser is a vertically polarized S light formed by splitting a linearly polarized laser from a detection laser (i.e., the detection laser in the narrow-linewidth semiconductor laser 18) that is far off-resonance from the resonance frequency of the atomic D1 line. The P light emitted after passing through the atomic gas cell 5 obtains the optical rotation angle of the detection light along the x-axis that maps the magnetic field in the y-axis. The S light emitted after passing through the atomic gas cell 5 obtains the DC component of the optical rotation angle of the detection light along the y-axis that maps the magnetic field in the x-axis, and obtains the first harmonic component of the optical rotation angle of the detection light along the y-axis that maps the magnetic field in the z-axis. By respectively decoupling the three-axis magnetic field information and feeding it back into the three-axis magnetic field coils 3 in the atomic magnetometer probe 2, a compensation magnetic field is generated in a manner that is equal in magnitude and opposite in direction to each axis magnetic field, thereby realizing real-time closed-loop suppression of magnetic field drift.

[0026] The atomic magnetometer probe 2 is connected to a circuit control system 1. The circuit control system 1 includes a host computer 28, a narrow-linewidth semiconductor laser 18, a signal generator 20, and a power amplifier 21. The narrow-linewidth semiconductor laser 18 includes the pump laser and the detection laser. The signal generator 20 is used to provide an AC drive voltage and a modulation voltage. The circuit control system 1 includes three channels. The polarization differential detection module 7 along the x-axis is connected to channel I 23, and after passing through a first low-pass filter with a cut-off frequency lower than the modulation frequency, it is input to the first PID module 22. The output follow-up voltage drives the y-axis magnetic field coil to generate a compensation magnetic field that is equal in magnitude and opposite in direction. The polarization differential detection module along the y-axis is respectively connected to channel II 24 and channel III 26 through a tee. Channel II 24 is input to the second PID module after passing through a second low-pass filter 25 with a cut-off frequency lower than the modulation frequency. The output follow-up voltage drives the x-axis magnetic field coil to generate a compensation magnetic field that is equal in magnitude and opposite in direction. Channel III 26 is input to the third PID module after passing through a lock-in amplifier 27 with the modulation magnetic field frequency ω as a reference. The output follow-up voltage drives the z-axis magnetic field coil to generate a compensation magnetic field that is equal in magnitude and opposite in direction. The filtering cut-off frequency of the lock-in amplifier 27 is lower than the modulation frequency.

[0027] The atomic magnetometer probe 2 is placed at the center of a magnetic shielding device made of a high-permeability material, and internally includes the combined prism 13, the boron nitride oven 15, and the first half-wave plate 11 through which the detection laser (i.e., the detection laser in the narrow-linewidth semiconductor laser 18) is input into the probe through the polarization-maintaining optical fiber 19. After the first half-wave plate 11, the light beam passes through the polarization beam splitter prism 12 along the optical path. The transmitted side of the polarization beam splitter prism 12 passes through the atomic gas cell 5 and then passes through the second half-wave plate 14 in the polarization differential detection module 7. After the second half-wave plate 14, it sequentially passes through the first lateral displacement polarization beam splitter prism 10, the first photodetector, and the first differential circuit and is connected to channel I 23 in the circuit control system 1. The reflected side of the polarization beam splitter prism 12 sequentially passes through the first right-angle prism, the second right-angle prism 4, the atomic gas cell 5, the third half-wave plate, the second lateral displacement polarization beam splitter prism, the second photodetector 16, and the second differential circuit 17 and is respectively connected to channel II 24 and channel III 26 in the circuit control system 1. The combined prism 13 includes a right-angle prism, a linear polarizer, and a quarter-wave plate. A heating film 6 is attached to the outer wall surface of the boron nitride oven 15. The atomic gas cell 5 is located inside the boron nitride oven 15.

[0028] The y-axis magnetic field coil generates a modulation magnetic field. The voltages output by the three channels in the circuit control system 1 are respectively:

[0029]

[0030] S0 = R op / 2(R op +R rel ),

[0031] where V x is the optical rotation angle signal of the detection light along the x-axis mapping the y-axis magnetic field, G is the photoelectric conversion coefficient of the differential photodetector, I pr is the intensity of the incident detection light, η is the attenuation coefficient of the detection light intensity by the gas cell glass, e is the natural constant, OD is the optical depth, l is the optical path of the gas cell, n is the atomic number density, r e is the electron radius, c is the speed of light, f D1 is the D1 line resonance frequency, Δ is the detection light detuning, Γ D1 is the pressure broadening of the alkali metal D1 line, S0 is an intermediate quantity, γ e is the electron gyromagnetic ratio, B y is the y-axis magnetic field, R op is the pumping rate, R rel is the relaxation rate, is the DC component of the optical rotation angle of the detection light along the y-axis mapping the x-axis magnetic field, B xis the x-axis magnetic field, J is the first kind of Bessel function, the subscript is the order of the Bessel function, and β is proportional to B mod / ω modulation factor, B mod is the modulation magnetic field amplitude, ω is the modulation magnetic field frequency, is the first harmonic component of the optical rotation angle of the detection light along the y-axis that maps the z-axis magnetic field, R is an intermediate quantity, R = R op / q, q is the nuclear slowdown factor, Γ is the ratio of the total atomic relaxation rate to the nuclear slowdown factor, B z is the z-axis magnetic field.

[0032] Reference Figure 1 As shown, a super-high-sensitivity three-axis closed-loop spin-exchange-relaxation-free atomic magnetometer of the present invention includes an atomic magnetometer probe 2 and a circuit control system 1. Three groups of orthogonal uniform magnetic field coils 3 are installed in the atomic magnetometer probe 2 to generate a modulation magnetic field and realize the closed-loop control of the three-axis magnetic field. The laser for pumping and detecting the atomic ensemble in the atomic magnetometer probe 2 is introduced by two independent polarization-maintaining optical fibers 19. After passing through the built optical path and the atomic gas cell 5, the detection laser 8 is input into the polarization differential detection module 7. The circuit control system 1 is connected to the polarization differential detection module 7 and is divided into three channels. After decoupling the three-axis magnetic field information respectively, it is feedback and input into the three corresponding groups of uniform magnetic field coils 3 to realize the real-time closed-loop suppression of the magnetic field drift.

[0033] The optical path is used to generate the pumping laser 9 for polarizing atoms and two detection lasers 8 for detecting the change of atomic spin polarization. A linearly polarized laser close to the resonance frequency of the alkali metal atom D1 line forms a circularly polarized pumping laser 9 through the action of a combined prism 13 (including a right-angle prism, a linear polarizer, and a quarter-wave plate) and passes through the atomic gas cell 5 from top to bottom. This direction is defined as the z-axis. Another linearly polarized laser far detuned from the resonance frequency of the atomic D1 line passes through a half-wave plate 11 and a polarization beam splitter prism 12 in sequence and is decomposed into two independent linearly polarized lasers. The angle of the half-wave plate 11 is adjusted so that the light intensity ratio of the two split lights is 50%, 50%. The P light after splitting is directly vertically incident on the atomic gas cell 5 to generate a polarization rotation angle and is input into the polarization differential detection module 7. The direction is defined as the x-axis. The S light after splitting passes through two right-angle prisms 4 to detect the atomic gas cell 5 and is input into another polarization differential detection module. The direction is defined as the y-axis.

[0034] The atomic gas cell 5 is made of high-borosilicate material with high light transmittance, filled with alkali metal atoms and buffer gas. The outside of the atomic gas cell 5 is wrapped with a boron nitride oven 15. Heating films 6 are pasted on both sides of the oven 15. The heating films 6 are driven by a signal generator 20 and a power amplifier 21 in the circuit control system 1 to apply a high-frequency alternating voltage. The atomic magnetometer probe 2 is placed in the center of a magnetic shielding device made of high-permeability material.

[0035] The modulation magnetic field is applied along the y-axis by the uniform magnetic field coil 3 and is a magnetic field B with a frequency of ω and an amplitude of B mod of B mod cos(ωt). The modulation frequency is much lower than the Larmor precession frequency. Under this modulation, the DC components of the differential detection output are linearly correlated with the magnetic field in the direction of the sensitive axis perpendicular to the pump-detection plane respectively:

[0036]

[0037] where G is the optoelectronic conversion coefficient of the differential photodetector, I pr is the intensity of the incident detection light, η is the attenuation coefficient of the detection light intensity by the gas cell glass, l is the optical path of the gas cell, n is the atomic number density, r e is the electron radius, f D1 is the D1 line resonance frequency, Δ is the detection light detuning, Γ D1 is the pressure broadening of the alkali metal D1 line, γ e is the electron gyromagnetic ratio, J is the Bessel function of the first kind, the subscript is the order of the Bessel function, β is the modulation factor proportional to B mod / ω, OD = ln Rb r e cf D1 (Γ D1 / 2) / (Δ 2 +(Γ D1 / 2) 2 ) is the optical depth, R op and R rel are the pumping rate and relaxation rate of the atomic ensemble respectively, S0 = R op / 2(R op +R rel ).

[0038] In addition, the perturbation method is used to iteratively calculate the spin polarization evolution of the atomic ensemble along the modulation axis in the transverse modulation mode one by one, and its first harmonic component is sensitive to the longitudinal magnetic field B z . Due to the approximate linear relationship between the optical rotation angle and the atomic ensemble polarizability in a weak magnetic field, the first harmonic component of the differential detection output is expressed as:

[0039]

[0040] where R is the intermediate quantity R op / q, where q is the nuclear slowdown factor and Γ is the ratio of the total atomic relaxation rate to the nuclear slowdown factor. For a given system, the fundamental harmonic component of the differential detection output is linearly related to the longitudinal magnetic field. By demodulating the first-order frequency component, the measurement decoupling of extremely weak longitudinal magnetic fields can be achieved. In addition, the Bessel function determined by the modulation factor has an optimal modulation amplitude proportional to the modulation frequency, and the atomic ensemble shows an enhanced response to the longitudinal magnetic field under weak magnetic field modulation.

[0041] Combined with the dual-detection optical configuration, the differential detection output of the spin-exchange-relaxation-free atomic magnetometer is sensitive to the three-axis magnetic field to be measured under weak transverse modulation:

[0042] V y (t) ∝ k1B x (t) + k2B z (t)sin(ωt)

[0043] V x (t) ∝ k3B y (t)

[0044] where V y (t) is the time-dependent y-axis differential detection output component, k1 is the quasi-linear coefficient of the output voltage related to B x (t), B x (t) is the time-dependent x-axis magnetic field to be measured, k2 is the quasi-linear coefficient of the output voltage related to B z (t), B z (t) is the time-dependent z-axis magnetic field to be measured, V x (t) is the time-dependent x-axis differential detection output component, k3 is the quasi-linear coefficient of the output voltage related to B y (t), B y (t) is the time-dependent y-axis magnetic field to be measured.

[0045] The polarization differential detection module 7 includes a half-wave plate 14, a lateral displacement polarization beam splitter prism 10, and two photodetectors 16 connected to a differential circuit 17. The half-wave plate 14 is adjusted to make the output of the differential circuit 17 zero and fixed after the magnetic field compensation reaches zero. The DC or fundamental harmonic component output by the polarization differential detection module 7 is approximately linearly related to the magnetic field component to be measured. The output port of the polarization differential detection module 7 is input to the circuit control system 1.

[0046] The circuit control system 1 includes three channels. The polarization differential detection module 7 along the x-axis is connected to channel Ⅰ 23 of the circuit control system 1. After passing through a low-pass filter 25 with a cut-off frequency lower than the modulation frequency, it is input into the PID module 22. The output follow-up voltage drives the y-axis magnetic field coil to generate an equal and opposite compensation magnetic field. The polarization differential detection module 7 along the y-axis is connected to channel Ⅱ 24 and channel Ⅲ 26 of the circuit control system 1 through a tee. Channel Ⅱ 24 is input into the PID module 22 after passing through a low-pass filter 25 with a cut-off frequency lower than the modulation frequency. The output follow-up voltage drives the x-axis magnetic field coil to generate an equal and opposite compensation magnetic field. Channel Ⅲ 26 is input into the PID module 22 after passing through a lock-in amplifier 27 with the modulation magnetic field frequency ω as a reference. The output follow-up voltage drives the z-axis magnetic field coil to generate an equal and opposite compensation magnetic field. The filtering cut-off frequency of the lock-in amplifier 27 is lower than the modulation frequency. The circuit control system 1 also includes a host computer 28, a semiconductor laser 18, a signal generator 20, and a power amplifier 21. The signal generator 20 is used to provide an AC drive voltage and a modulation voltage. The modulation signal generator 20 is connected to a resistor and then input into the y-axis magnetic field coil.

[0047] Reference Figure 1 , the usage method of a super-high-sensitivity three-axis closed-loop non-spin-exchange relaxation atomic magnetometer according to the present invention is as follows:

[0048] (1) Adjust the external semiconductor laser 18 to adjust the frequencies of the pump laser 9 and the detection laser 8 to be close to the resonance frequency of the alkali metal atom D1 line and far off-resonance from the atomic D1 line resonance frequency respectively. Place the atomic magnetometer probe 2 at the center of the magnetic shielding device. Start the atomic magnetometer. Input an AC drive voltage into the heating film 6. After the temperature stabilizes, perform three-axis remanence compensation to keep the atomic ensemble initially in a non-spin-exchange relaxation state;

[0049] (2) Use the signal generator 20 in the circuit control system 1 to output a voltage signal to the y-axis magnetic field coil to generate a modulation magnetic field B mod along the y-axis with a frequency of ω and an amplitude of B mod cos(ωt). The modulation frequency is much lower than the Larmor precession frequency. The optimal modulation amplitude is proportional to the modulation frequency. The non-spin-exchange relaxation atomic magnetometer maps the y-axis magnetic field along the x-axis of the detection light rotation angle under weak transverse modulation. The DC and primary harmonic components in the detection light rotation angle along the y-axis map the x-axis and z-axis magnetic fields respectively. The scale factor is related to parameters such as the pumping rate, relaxation rate, gyromagnetic ratio of the atomic spin ensemble, and the modulation factor determined by the modulation amplitude and modulation frequency;

[0050] (3) Connect the polarization differential detection module 7 to the three channels of the circuit control system 1 to decouple the three-axis magnetic fields B x , B y , Bz and the closed-loop feedback voltage is equivalently input into the corresponding uniform magnetic field coil 3 to generate a compensating magnetic field with the same magnitude and opposite direction, so as to compensate the magnetic field sensitive to the atomic ensemble to zero in real time. The polarization differential detection module 7 along the x-axis is connected to channel Ⅰ 23 of the circuit control system 1, and after passing through a low-pass filter 25 with a cut-off frequency lower than the modulation frequency, it is input to the PID module 22 to control the magnetic field along the y-axis. The polarization differential detection module 7 along the y-axis is respectively connected to channel Ⅱ 24 and channel Ⅲ 26 of the circuit control system 1 through a tee. Channel Ⅱ 24 is input to the PID module 22 to control the magnetic field along the x-axis after passing through a low-pass filter 25 with a cut-off frequency lower than the modulation frequency. Channel Ⅲ 26 is input to the PID module 22 to control the magnetic field along the z-axis after passing through a lock-in amplifier 27 with the modulation magnetic field frequency ω as a reference;

[0051] (4) Using the PID closed-loop feedback signal as the magnetic field measurement signal, optimize parameters such as the pumping light intensity, detection light frequency, and modulation magnetic field amplitude according to the output response of the atomic magnetometer to ensure that the output response of the atomic magnetometer, that is, the closed-loop feedback signal, is maximized, and determine various system parameters;

[0052] (5) Apply calibrated magnetic fields with increasing frequencies and the same amplitudes through three groups of coils 3, and record the amplitudes of the closed-loop feedback signals. Calibrate the scale factor and effective measurement bandwidth of the spin-exchange-relaxation-free atomic magnetometer. Synchronously collect the voltages output by the three channels of the circuit control system 1, convert them into current signals by using high-precision resistors equivalently, perform power spectral density processing on the upper computer 28, and evaluate the magnetic field measurement sensitivity of the system according to the coil constant.

[0053] In summary, the present invention proposes a super-high-sensitivity three-axis closed-loop spin-exchange-relaxation-free atomic magnetometer. Based on weak parameter modulation optimization and three-axis closed-loop feedback, it realizes the real-time high-sensitivity measurement and closed-loop feedback of the three-axis magnetic field by the atomic magnetometer, enhances the strong anti-interference ability of the atomic magnetometer in a complex magnetic field environment, and improves the measurement accuracy and long-term measurement stability of the super-high-sensitivity atomic magnetometer.

[0054] The content not described in detail in the specification of the present invention belongs to the prior art well-known to those skilled in the art. It is hereby indicated that the above description helps those skilled in the art to understand the present invention, but does not limit the protection scope of the present invention. Any implementation that makes equivalent replacements, modifications and improvements, and / or simplifies the above description without departing from the essence of the present invention falls within the protection scope of the present invention.

Claims

1. An ultra-high sensitivity three-axis closed-loop non-spin-exchange relaxation atomic magnetometer, characterized in that, Including a pump laser acting on the atomic gas cell in the z-axis direction, a first detection laser passing through the atomic gas cell in the x-axis direction, and a second detection laser passing through the atomic gas cell in the y-axis direction. The pump laser is a circularly polarized laser generated by a pump laser and formed after passing through a combined prism, with a frequency close to the resonance frequency of the alkali metal atom D1 line. The first detection laser is a parallel polarized P light formed by splitting a linearly polarized laser from a detection laser that is far detuned from the resonance frequency of the atomic D1 line. The second detection laser is a vertically polarized S light formed by splitting a linearly polarized laser from a detection laser that is far detuned from the resonance frequency of the atomic D1 line. The P light emerging from the atomic gas cell obtains the rotation angle of the detection light along the x-axis mapping the y-axis magnetic field. The S light emerging from the atomic gas cell obtains the DC component of the rotation angle of the detection light along the y-axis mapping the x-axis magnetic field, and obtains the first harmonic component of the rotation angle of the detection light along the y-axis mapping the z-axis magnetic field. By respectively decoupling the three-axis magnetic field information and feeding it back into the three-axis magnetic field coils in the atomic magnetometer probe, a compensation magnetic field is generated in a manner equal in magnitude and opposite in direction to each axis magnetic field, thereby realizing real-time closed-loop suppression of magnetic field drift.

2. The ultra-high sensitivity three-axis closed-loop non-spin-exchange relaxation atomic magnetometer according to claim 1, characterized in that, The atomic magnetometer probe is connected to a circuit control system. The circuit control system includes a host computer, a narrow-linewidth semiconductor laser, a signal generator, and a power amplifier. The narrow-linewidth semiconductor laser includes the pump laser and the detection laser. The signal generator is used to provide an AC drive voltage and a modulation voltage.

3. The ultra-high sensitivity three-axis closed-loop spin-exchange relaxation-free atomic magnetometer according to claim 1, wherein The circuit control system includes three channels. The polarization differential detection module along the x-axis is connected to Channel I, and after passing through a first low-pass filter with a cut-off frequency lower than the modulation frequency, it is input to the first PID module. The output follow-up voltage drives the y-axis magnetic field coil to generate a compensation magnetic field equal in magnitude and opposite in direction. The polarization differential detection module along the y-axis is connected to Channel II and Channel III through a tee. Channel II passes through a second low-pass filter with a cut-off frequency lower than the modulation frequency and is then input to the second PID module. The output follow-up voltage drives the x-axis magnetic field coil to generate a compensation magnetic field equal in magnitude and opposite in direction. Channel III passes through a lock-in amplifier with the modulation magnetic field frequency ω as a reference and is then input to the third PID module. The output follow-up voltage drives the z-axis magnetic field coil to generate a compensation magnetic field equal in magnitude and opposite in direction. The filtering cut-off frequency of the lock-in amplifier is lower than the modulation frequency.

4. The ultra-high sensitivity three-axis closed-loop non-spin-exchange relaxation atomic magnetometer according to claim 1, wherein The atomic magnetometer probe is placed at the center of a magnetic shielding device made of high-permeability material, and internally includes the combined prism, boron nitride oven, and a first half-wave plate through which the detection laser input is connected to the probe by polarization-maintaining optical fiber. After the first half-wave plate, the light path passes through a polarization beam splitter prism. The transmission side of the polarization beam splitter prism passes through the atomic gas cell and then passes through a second half-wave plate in the polarization difference detection module. After the second half-wave plate, it sequentially passes through a first lateral displacement polarization beam splitter prism, a first photodetector, and a first differential circuit and is connected to channel I in the circuit control system. The reflection side of the polarization beam splitter prism sequentially passes through a first right-angle prism, a second right-angle prism, the atomic gas cell, a third half-wave plate, a second lateral displacement polarization beam splitter prism, a second photodetector, and a second differential circuit and is respectively connected to channel II and channel III in the circuit control system. The combined prism includes a right-angle prism, a linear polarizer, and a quarter-wave plate. A heating film is attached to the outer wall surface of the boron nitride oven. The atomic gas cell is located inside the boron nitride oven.

5. The ultra-high sensitivity three-axis closed-loop non-spin-exchange relaxation atomic magnetometer according to claim 1, characterized in that The y-axis magnetic field coil generates a modulation magnetic field. The voltages output by the three channels in the circuit control system are respectively: S0 = R op / 2(R op +R rel ) where V x is the optical rotation angle signal of the detection light along the x-axis that maps the magnetic field of the y-axis, G is the optoelectronic conversion coefficient of the differential photodetector, I pr is the intensity of the incident detection light, η is the attenuation coefficient of the detection light intensity by the cell glass, e is the natural constant, OD is the optical depth, l is the cell optical path, n is the atomic number density, r e is the electron radius, c is the speed of light, f D1 is the D1 line resonance frequency, Δ is the detection light detuning, Γ D1 is the pressure broadening of the alkali metal D1 line, S0 is an intermediate quantity, γ e is the electron gyromagnetic ratio, B y is the magnetic field of the y-axis, R op is the pumping rate, R rel is the relaxation rate, is the DC component of the optical rotation angle of the detection light along the y-axis that maps the magnetic field of the x-axis, B x is the magnetic field of the x-axis, J is the Bessel function of the first kind, the subscript is the order of the Bessel function, β is the modulation factor proportional to B mod / ω, B mod is the amplitude of the modulation magnetic field, ω is the modulation magnetic field frequency, is the first harmonic component of the optical rotation angle of the detection light along the y-axis that maps the magnetic field of the z-axis, R is an intermediate quantity, R = R op / q, q is the nuclear slowing factor, Γ is the ratio of the total atomic relaxation rate to the nuclear slowing factor, B z is the magnetic field of the z-axis.

6. The ultra-high sensitivity three-axis closed-loop non-spin-exchange relaxation atomic magnetometer according to claim 1, wherein The differential detection output is sensitive to the three-axis magnetic field to be measured under weak transverse modulation: V y (t) is proportional to k1B x (t) + k2B z (t) sin(ωt) V x (t) is proportional to k3B y (t) Where V y (t) is the time-related y-axis differential detection output component, k1 is the quasi-linear coefficient of the output voltage with respect to B x (t), B x (t) is the time-related x-axis magnetic field to be measured, k2 is the quasi-linear coefficient of the output voltage with respect to B z (t), B z (t) is the time-related z-axis magnetic field to be measured, V x (t) is the time-related x-axis differential detection output component, k3 is the quasi-linear coefficient of the output voltage with respect to B y (t), B y (t) is the time-related y-axis magnetic field to be measured.

7. A method for using an ultra-high sensitivity three-axis closed-loop non-spin-exchange relaxation atomic magnetometer, characterized in that, Using the ultra-high-sensitivity three-axis closed-loop spin-exchange-relaxation-free atomic magnetometer described in any one of claims 1-6 above, includes the following steps: Step 1, adjust the external semiconductor laser, adjust the pump laser and detection laser frequencies to be close to the resonance frequency of the alkali metal atom D1 line and far detuned from the resonance frequency of the atomic D1 line respectively. Start the measuring device, input an AC drive voltage into the heating film, and perform three-axis remanence compensation after the temperature stabilizes to keep the atomic ensemble initially in a spin-exchange-relaxation-free state; Step 2: Use the signal generator in the circuit control system to output a voltage signal to the y-axis magnetic field coil, generating a modulated magnetic field B along the y-axis with a frequency of ω and an amplitude of B mod of B mod cos(ωt), where t is time, the modulation frequency is much lower than the Larmor precession frequency, and the optimal modulation amplitude is proportional to the modulation frequency; Step 3: Connect the polarization differential detection module to the three channels of the circuit control system, and decouple the magnetic fields B to be measured on three axes respectively x , B y , B z , and equivalently input the closed-loop feedback voltage into the corresponding uniform magnetic field coil to generate a compensating magnetic field with the same magnitude but opposite direction, so as to compensate the magnetic field sensitive to the atomic ensemble to zero in real time; Step 4, use the PID closed-loop feedback signal as the magnetic field measurement signal, optimize the parameters of the pump light intensity, detection light frequency, and modulation magnetic field amplitude according to the output response of the atomic magnetometer to ensure that the output response of the atomic magnetometer, that is, the closed-loop feedback signal, is maximized, and determine various system parameters; Step 5, apply a calibrated magnetic field with increasing frequency and the same amplitude through three groups of coils, and record the amplitude of the closed-loop feedback signal. Calibrate the scale factor and effective measurement bandwidth of the spin-exchange-relaxation-free atomic magnetometer, synchronously collect the voltages output by the three channels of the circuit control system, convert them into current signals by using a high-precision resistor for equivalent conversion, perform power spectral density processing on the upper computer, and evaluate the magnetic field measurement sensitivity of the system according to the coil constant.

8. The method for using an ultra-high sensitivity three-axis closed-loop non-spin exchange relaxation atomic magnetometer according to claim 7, characterized in that Step 2 includes: Under weak transverse modulation, the rotation angle of the detection light of the spin-exchange-relaxation-free atomic magnetometer along the x-axis maps the y-axis magnetic field, and the DC and primary harmonic components in the rotation angle of the detection light along the y-axis respectively map the x-axis and z-axis magnetic fields. The scale factor is related to various parameters such as the pumping rate, relaxation rate, gyromagnetic ratio of the atomic spin ensemble, and the modulation factor determined by the modulation amplitude and modulation frequency.

9. The method for using an ultra-high sensitivity three-axis closed-loop spin-exchange relaxation-free atomic magnetometer according to claim 7, characterized in that Step 3 includes: The polarization differential detection module along the x-axis is connected to Channel I of the circuit control system, and after passing through a low-pass filter with a cut-off frequency lower than the modulation frequency, it is input to the PID module to control the magnetic field along the y-axis. The polarization differential detection module along the y-axis is respectively connected to Channel II and Channel III of the circuit control system through a tee. Channel II is input to the PID module to control the magnetic field along the x-axis after passing through a low-pass filter with a cut-off frequency lower than the modulation frequency. Channel III is input to the PID module to control the magnetic field along the z-axis after passing through a lock-in amplifier with the modulation magnetic field frequency ω as a reference.

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