An Elliptically Polarized Light SERF Atomic Magnetometer Device and Method Based on Magnetic Field Closed Loop

Abstract

The present invention proposes an ellipsometric SERF atomic magnetometer device and method based on magnetic field closed-loop, providing a feasible closed-loop scheme for the ellipsometric SERF atomic magnetometer, thereby locking the magnetic field sensed by alkali metal atoms in the ellipsometric SERF atomic magnetometer at zero field, significantly increasing the dynamic range and bandwidth of magnetic field measurement, and improving the stability of magnetic field measurement at the same time. The present invention uses a dimensionless operation signal obtained by dividing the first harmonic component of the zero-order resonance signal by the second harmonic component as the input signal of the closed-loop feedback system, weakening the interference of factors such as the ellipticity of the pumping laser, the optical power density, the optical frequency fluctuation, and the temperature fluctuation of the atomic gas chamber on the input signal of the closed-loop feedback system, and improving the accuracy and robustness of the closed loop.

Description

Technical Field

[0001] The present invention relates to an ellipsometric light SERF (Spin-Exchange-Relaxation-Free) atomic magnetometer device and method based on a magnetic field closed-loop, belonging to the technical field of atomic magnetometers. Background Art

[0002] The SERF atomic magnetometer is an ultra-high sensitive quantum precision measurement instrument for measuring extremely weak magnetic fields and has been widely used in fields such as frontier physics research and biomagnetic measurement in recent years. The ellipsometric light SERF atomic magnetometer uses a beam of elliptically polarized light to pump alkali metal atoms and simultaneously uses polarization differential detection technology to detect the optical rotation angle, having the advantages of small volume and high sensitivity. However, the current ellipsometric light SERF atomic magnetometer operates in an open-loop state and directly measures the magnetic field to be measured in the z direction through the first harmonic component of the zero-order resonance signal output therefrom, which not only limits the dynamic range and bandwidth of the atomic magnetometer, but also is vulnerable to interference caused by fluctuations in factors such as the temperature of the atomic gas cell and laser parameters, resulting in drift of the magnetic field measurement signal and limiting the measurement effect of the ellipsometric light SERF atomic magnetometer. Summary of the Invention

[0003] The present invention proposes an ellipsometric light SERF atomic magnetometer device and method based on a magnetic field closed-loop, providing a feasible closed-loop scheme for the ellipsometric light SERF atomic magnetometer, and locking the magnetic field sensed by the sensitive medium - the alkali metal atom spin ensemble in the ellipsometric light SERF atomic magnetometer at zero field in real time, thereby significantly improving its dynamic range and bandwidth. After operating the first harmonic component and the second harmonic component signals of the zero-order resonance signal output by the atomic magnetometer and inputting them into the closed-loop feedback system, the interference of fluctuations in factors such as the ellipticity, optical power density, optical frequency, and temperature of the atomic gas cell of the pumping laser on the input signal of the closed-loop feedback system is weakened, the closed-loop stability is improved, and further the stability and accuracy of the magnetic field measurement are improved.

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

[0005] An ellipsometric SERF atomic magnetometer device based on a magnetic field closed-loop, characterized in that it includes a three-axis magnetic field coil and a polarization differential detection module located inside the housing of the atomic magnetometer head. The laser input end of the polarization differential detection module is connected to the laser output end of the atomic gas cell inside the three-axis magnetic field coil. The laser input end of the atomic gas cell is sequentially connected to a narrow-linewidth semiconductor laser through an elliptical polarizer and a polarization-maintaining optical fiber. The output end of the polarization differential detection module is sequentially connected to a lock-in amplifier, a division circuit, a subtraction circuit, a PID module, and a z-axis function generator, and then connected to the three-axis magnetic field coil. The three-axis magnetic field coil is connected to an xy-axis function generator. The z-axis function generator outputs a DC voltage to drive the three-axis magnetic field coil to generate a magnetic field in the z direction, and the xy-axis function generator outputs a DC voltage to drive the three-axis magnetic field coil to generate magnetic fields in the x direction and the y direction.

[0006] The polarization differential detection module includes a lateral displacement polarization beam splitter prism. The input end of the lateral displacement polarization beam splitter prism is connected to the laser output end of the atomic gas cell through a half-wave plate. The output end of the lateral displacement polarization beam splitter prism is sequentially connected to a photodetector and a differential amplifier circuit, and then connected to the lock-in amplifier.

[0007] The elliptical polarizer includes a linear polarizer and a quarter-wave plate, and the included angle between their optical axis directions is α. The polarization direction of the linearly polarized light emitted by the narrow-linewidth semiconductor laser coincides with the optical axis direction of the linear polarizer. The linearly polarized light source is converted into elliptically polarized light with an ellipticity of α through the elliptical polarizer. This elliptically polarized light is the pumping laser of the ellipsometric SERF atomic magnetometer.

[0008] An oven is provided between the atomic gas cell and the three-axis magnetic field coil. The lock-in amplifier inputs the first harmonic component and the second harmonic component to the division circuit respectively. The subtraction circuit subtracts the dimensionless operation signal from the division circuit from the set value, and the obtained difference is used as an error signal and input to the PID (Proportional Integral Derivative) module. The dimensionless operation signal is the quotient obtained by dividing the first harmonic component by the second harmonic component. Both the first harmonic component and the second harmonic component are harmonic components in the zero-order resonance signal of the ellipsometric SERF atomic magnetometer.

[0009] The z-axis function generator outputs a feedback control signal. The feedback control signal is operated with the coil constant in the z direction of the three-axis magnetic field coil to obtain the output signal of the atomic magnetometer, that is, the magnetic field to be measured in the positive z direction.

[0010]

[0011] Where θ w is the first harmonic component, θ 2wis the second - harmonic component, K is the amplification coefficient, I0 is the pump - light power density, OD(v) is the optical depth, n is the atomic number density, c is the speed of light, r is the classical electron radius, f is the resonance intensity of the D1 line of alkali - metal atoms, l is the length of the atomic cell, v0 is the resonance frequency of the D1 line of alkali - metal atoms, v is the frequency of the laser, Г is the pressure broadening of the atomic cell, R op is the pumping rate, R rel is the transverse atomic spin - relaxation rate, s is the spin angular momentum of the pump light, γ e is the gyromagnetic ratio of the electron, J0, J1, and J2 are the Bessel functions of order 0, 1, and 2 respectively, u is the magnetic - field modulation coefficient, and B0 is the magnetic field to be measured in the positive z - axis direction.

[0012]

[0013] where S input is the dimensionless operation signal, S ctrl is the feedback control signal, R op is the pumping rate, R rel is the transverse relaxation rate, J1 and J2 are the Bessel functions of order 1 and 2 respectively, u is the magnetic - field modulation parameter, B0 is the magnetic field to be measured in the positive z - axis direction, Kz is the coil constant of the z - direction of the three - axis magnetic - field coil, and R is the resistance of the three - axis magnetic - field coil in the z - direction.

[0014] A method for using an ellipsometric SERF atomic magnetometer based on a magnetic - field closed - loop, characterized by comprising using the above - mentioned ellipsometric SERF atomic magnetometer device based on a magnetic - field closed - loop, and the following steps:

[0015] Step 1: Adjust the frequency of the laser emitted by the narrow - line - width semiconductor laser to deviate from the resonance frequency of the D1 line of alkali - metal atoms, and the polarization state is linear polarization. Coupling this laser as a light source into a polarization - maintaining optical fiber. The linearly polarized light emitted from the polarization - maintaining optical fiber passes through an ellipsometer and is converted into elliptically polarized light with an ellipticity of α. Using this light as the pump laser to irradiate the atomic cell;

[0016] Step 2: Control the xy - axis function generator and the z - axis function generator. Using three - dimensional magnetic compensation technology to zero the magnetic field sensed by the atomic cell. At the same time, control the z - axis function generator to apply a high - frequency modulation magnetic field in the z - direction, and its frequency is w m ;

[0017] Step 3: Introduce the differential amplification signal output by the polarization - difference detection module into a lock - in amplifier. Using lock - in amplification technology in the lock - in amplifier to demodulate the differential amplification signal at frequencies of the fundamental frequency w m and the second - harmonic frequency 2w m respectively, to obtain the fundamental - harmonic component θ w , and the second - harmonic component θ 2w ;

[0018] Step 4, θ w and θ 2w pass through a division circuit to obtain a dimensionless operation signal S input , and connect S input to a subtraction circuit, such that S input is subtracted from the set value 0, and the obtained difference is used as an error signal and input to the PID module. The output signal of the PID module controls the z-axis function generator to generate a feedback control signal S ctrl ;

[0019] Step 5, S ctrl acts on the z direction of the three-axis magnetic field coil, such that the magnetic field generated by the coil is equal in magnitude and opposite in direction to the magnetic field B0 to be measured in the positive direction of the z axis, thereby locking the magnetic field sensed by the ellipsometric SERF atomic magnetometer in a zero field in real time.

[0020] The three-dimensional magnetic compensation technology in the above-mentioned Step 2 is a technology that controls the three-axis magnetic field coil around the atomic gas cell to generate a magnetic field with the same magnitude and opposite direction as the ambient magnetic field, thereby realizing the zeroing of the ambient magnetic field sensed by the atomic gas cell.

[0021] The technical effects of the present invention are as follows: The ellipsometric SERF atomic magnetometer device and method based on a magnetic field closed-loop of the present invention provide a feasible closed-loop scheme for the ellipsometric SERF atomic magnetometer, lock the magnetic field sensed by the alkali metal atoms in the ellipsometric SERF atomic magnetometer in a zero field, significantly increase its dynamic range and bandwidth, and improve the stability of magnetic field measurement at the same time. The present invention uses the dimensionless operation signal obtained by dividing the first harmonic component of the zero-order resonance signal by the second harmonic component as the input signal of the closed-loop feedback system, weakens the interference of factors such as the ellipticity, optical power density, optical frequency of the pumping laser, and temperature fluctuation of the atomic gas cell on the closed-loop feedback system, and improves the accuracy and robustness of the closed loop.

[0022] The advantages of the present invention compared with the prior art are as follows:

[0023] (1) The conventional ellipsometric SERF atomic magnetometer operates in an open-loop state, resulting in a small measurement bandwidth and dynamic range of the atomic magnetometer, large fluctuations in the measurement signal, and being easily interfered; the present invention provides a closed-loop method, such that the ellipsometric SERF atomic magnetometer can operate in a closed-loop state. Since the ambient magnetic field sensed by the atomic gas cell is compensated to zero field in real time by the three-axis magnetic field coil, the dynamic range and bandwidth of the ellipsometric SERF atomic magnetometer are increased. At the same time, due to the use of the closed-loop control method, the stability of signal measurement is improved;

[0024] (2) Conventional ellipsometric SERF atomic magnetometers use the first harmonic component of the zero-order resonance signal as the output signal of the atomic magnetometer. Directly using the first harmonic component as the input signal for closed-loop feedback will cause the feedback control signal to be interfered by factors such as the parameter fluctuations of the pumping laser and the temperature fluctuations of the atomic cell, resulting in the environmental magnetic field sensed by the atomic cell fluctuating near zero field and reducing the closed-loop effect. This method uses the dimensionless operation signal obtained by dividing the first harmonic component by the second harmonic component as the input signal of the closed-loop feedback system, so that the terms related to the pumping laser and the temperature of the atomic cell in the two components are reduced, thereby weakening the influence of the laser power density, frequency, ellipticity fluctuations of the pumping laser and the temperature fluctuations of the atomic cell on the feedback control signal, and improving the accuracy and robustness of the closed-loop method.

[0025] (3) The present invention does not change the original structure of the ellipsometric SERF atomic magnetometer probe. By extracting the second harmonic component, the acquisition of the feedback control signal is realized, and the advantages of the original compact structure and high sensitivity are retained. Description of the Drawings

[0026] Figure 1 is a schematic structural diagram of an ellipsometric SERF atomic magnetometer device based on magnetic field closed-loop implementing the present invention. SERF (Spin-Exchange Relaxation Free) is spin-exchange relaxation free.

[0027] The descriptions of the reference numerals are as follows: 1 - narrow linewidth semiconductor laser; 2 - xy-axis function generator; 3 - polarization-maintaining fiber; 4 - ellipsometer; 5 - atomic cell; 6 - half-wave plate; 7 - lateral displacement polarization beam splitter prism; 8 - photodetector; 9 - differential amplifier circuit; 10 - oven; 11 - differential amplified signal; 12 - three-axis magnetic field coil; 13 - atomic magnetometer head housing; 14 - lock-in amplifier; 15 - first harmonic component; 16 - second harmonic component; 17 - division circuit; 18 - subtraction circuit; 19 - PID module (PID, Proportion Integration Differentiation, proportional-integral-derivative); 20 - z-axis function generator; 21 - feedback control signal; 22 - closed-loop feedback system; 23 - polarization differential detection module. Detailed Embodiments

[0028] The following describes the present invention in conjunction with the drawings ( Figure 1 ) and embodiments.

[0029] Figure 1 is a schematic structural diagram of an ellipsometric SERF atomic magnetometer device based on magnetic field closed-loop implementing the present invention. Refer to Figure 1As shown in the figure, an ellipsometric SERF atomic magnetometer device based on a magnetic field closed loop includes a three-axis magnetic field coil 12 and a polarization differential detection module 23 located inside the housing 13 of the atomic magnetometer head. The laser input end of the polarization differential detection module 23 is connected to the laser output end of the atomic gas cell 5 inside the three-axis magnetic field coil 12. The laser input end of the atomic gas cell 5 is sequentially connected to a narrow-linewidth semiconductor laser 1 through an elliptical polarizer 4 and a polarization-maintaining optical fiber 3. The output end of the polarization differential detection module 23 is sequentially connected to the three-axis magnetic field coil 12 through a lock-in amplifier 14, a division circuit 17, a subtraction circuit 18, a PID module 19, and a z-axis function generator 20. The three-axis magnetic field coil 12 is connected to an xy-axis function generator 2. The z-axis function generator 20 is used to control the three-axis magnetic field coil 12 to compensate the ambient magnetic field in the z direction to zero, and the xy-axis function generator 2 is used to control the three-axis magnetic field coil 12 to compensate the ambient magnetic field in the x direction and the y direction to zero.

[0030] The polarization differential detection module 23 includes a lateral displacement polarization beam splitter prism 7. The input end of the lateral displacement polarization beam splitter prism 7 is connected to the laser output end of the atomic gas cell 5 through a half-wave plate 6. The output end of the lateral displacement polarization beam splitter prism 7 is sequentially connected to the lock-in amplifier 14 through a photodetector 8 and a differential amplifier circuit 9. The linearly polarized light source emitted by the narrow-linewidth semiconductor laser 1 is transmitted to the elliptical polarizer 4 through the polarization-maintaining optical fiber 3. The elliptical polarizer 4 includes a linear polarizer and a quarter-wave plate, and the included angle between the optical axis directions of the two is α. The polarization direction of the linearly polarized light source coincides with the optical axis direction of the linear polarizer. The linearly polarized light source is converted into an elliptically polarized light with an ellipticity of α through the elliptical polarizer 4, and this elliptically polarized light is the pumping laser of the ellipsometric SERF atomic magnetometer.

[0031] An oven 10 is provided between the atomic gas cell 5 and the three-axis magnetic field coil 12. The lock-in amplifier 14 inputs a fundamental harmonic component 15 and a second harmonic component 16 to the division circuit 17 respectively. The subtraction circuit 18 subtracts the dimensionless operation signal from the division circuit 17 from the set value 0, and the obtained difference is used as an error signal and input to the PID module 19. The dimensionless operation signal is the quotient (a / b) obtained by dividing the fundamental harmonic component 15 (a) by the second harmonic component 16 (b). Both the fundamental harmonic component 15 and the second harmonic component 16 are harmonic components in the zero-order resonance signal of the ellipsometric SERF atomic magnetometer. The z-axis function generator 20 outputs a feedback control signal 21, and the feedback control signal 21 is operated with the coil constant in the z direction of the three-axis magnetic field coil 12 to obtain the output signal of the atomic magnetometer, that is, the magnetic field measurement value in the z direction.

[0032]

[0033]

[0034] The present invention provides an ellipsometric SERF atomic magnetometer device and method based on a magnetic field closed-loop. A dimensionless operation signal obtained by dividing the first harmonic component by the second harmonic component is used as the input signal of the closed-loop feedback system to control the three-dimensional magnetic field coil so that the ambient magnetic field sensed by the atomic gas cell is compensated to zero field in real time, thereby operating in a closed-loop state. The present invention will be described in detail below in conjunction with the accompanying drawings and specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and not to limit the scope of the present invention. After reading the present invention, various equivalent modifications made by those skilled in the art fall within the scope defined by the appended claims of this application.

[0035] Refer to Figure 1 , the present invention provides an ellipsometric SERF atomic magnetometer device and method based on a magnetic field closed-loop. As Figure 1 shown, an ellipsometric SERF atomic magnetometer device and method based on a magnetic field closed-loop includes: a narrow linewidth semiconductor laser 1, an xy-axis function generator 2, a polarization-maintaining optical fiber 3, an elliptical polarizer 4, an atomic gas cell 5, a half-wave plate 6, a lateral displacement polarization beam splitter prism 7, a photodetector 8, a differential amplifier circuit 9, an oven 10, a differential amplified signal 11, a three-axis magnetic field coil 12, an atomic magnetometer head housing 13, a lock-in amplifier 14, a first harmonic component 15, a second harmonic component 16, a division circuit 17, a subtraction circuit 18, a PID module 19, a z-axis function generator 20, a feedback control signal 21, a closed-loop feedback system 22, and a polarization differential detection module 23;

[0036] As Figure 1 shown, the specific implementation steps of the present invention are as follows:

[0037] (1) Adjust the frequency of the linearly polarized laser emitted by the narrow linewidth semiconductor laser 1 to deviate from the resonance frequency of the D1 line of alkali metal atoms by 50 GHz. This laser is used as a light source and coupled into the polarization-maintaining optical fiber 3. The linearly polarized light emitted from the polarization-maintaining optical fiber 3 passes through the elliptical polarizer 4 and is converted into elliptically polarized light with an ellipticity of 22.5°. This light is used as the pumping laser to irradiate the atomic gas cell 5;

[0038] (2) After completing step (1), adjust the output voltages of the xy-axis function generator 2 and the z-axis function generator 20, and use three-dimensional magnetic compensation technology to zero the surrounding magnetic field sensed by the atomic gas cell 5; control the z-axis function generator 20 to apply a high-frequency modulation magnetic field in the z direction, with a frequency of 1 kHz and an amplitude of 80 nT;

[0039] (3) After completing step (2), the differential amplified signal 11 output by the polarization differential detection module 23 is introduced into the lock-in amplifier 14. In the lock-in amplifier, the lock-in amplification technique is used to demodulate the differential amplified signal 11 at frequencies of 1 kHz and 2 kHz respectively to obtain the first harmonic component 15, denoted as θ w , the second harmonic component 16, denoted as θ 2w ;

[0040]

[0041] where K is the amplification coefficient, I0 is the pump light power density, OD(v) is the optical depth, n is the atomic number density, c is the speed of light, r is the classical electron radius, f is the resonance intensity of the D1 line of alkali metal atoms, l is the length of the atomic cell, v0 is the resonance frequency of the D1 line of alkali metal atoms, v is the frequency of the laser, Г is the pressure broadening of the atomic cell, R op is the pumping rate, R rel is the transverse relaxation rate, s is the spin angular momentum of the pump light, γ e is the gyromagnetic ratio of a single electron, J0, J1, and J2 are the Bessel functions of the 0th, 1st, and 2nd orders respectively, u is the magnetic field modulation parameter, and B0 is the magnetic field to be measured in the positive direction of the z-axis;

[0042] (4) The first harmonic component 15θ w and the second harmonic component 16θ 2w obtained in step (3) are passed through the division circuit 17 to obtain the dimensionless operation signal S input = θ w / θ 2w . S input is connected to the subtraction circuit 18 to subtract it from the set value of 0, and the resulting difference is used as the error signal and input to the PID module 19. The output signal of the PID module 19 controls the z-axis function generator 20 to generate the feedback control signal 21, denoted as S ctrl ;

[0043]

[0044] where S input is the dimensionless operation signal, S ctrl is the feedback control signal, R op is the pumping rate, R rel is the transverse relaxation rate, J1 and J2 are the Bessel functions of the 1st and 2nd orders respectively, u is the magnetic field modulation parameter, and B0 is the magnetic field to be measured in the positive direction of the z-axis;

[0045] (5) Using the S ctrlActing on the z - direction of the three - axis magnetic field coil 12, making the magnetic field generated by the coil equal in magnitude and opposite in direction to the magnetic field to be measured in the positive z - axis direction, so as to lock the magnetic field sensed by the ellipsometric SERF atomic magnetometer at zero field. Kz and S ctrl Calculate the magnetic field B0 to be measured in the positive z - axis direction through operation;

[0046] %0 = -KzS ctrl / R

[0047] In the formula, Kz is the coil constant in the z - direction of the three - axis magnetic field coil 12, R is the resistance in the z - direction of the three - axis magnetic field coil, and B0 is the magnetic field to be measured in the positive z - axis direction.

[0048] In summary, the present invention provides an ellipsometric SERF atomic magnetometer device and method based on a magnetic field closed - loop. Using the dimensionless operation signal obtained by dividing the first - harmonic component by the second - harmonic component as the input signal of the closed - loop feedback system weakens the influence of the parameter fluctuations of the pumping laser and the temperature fluctuations of the atomic gas chamber on the feedback control signal, and improves the accuracy and robustness of the closed - loop method. By controlling the three - dimensional magnetic field coil through the closed - loop method, the ambient magnetic field sensed by the atomic gas chamber is compensated to zero field in real time, improving the dynamic range and bandwidth of the measurement.

[0049] An ellipsometric SERF atomic magnetometer device and method based on a magnetic field closed loop, characterized in that: a narrow-linewidth semiconductor laser (1), an xy-axis function generator (2), a polarization-maintaining optical fiber (3), an ellipsometer (4), an atomic cell (5), a half-wave plate (6), a lateral displacement polarization beam splitter prism (7), a photodetector (8), a differential amplifier circuit (9), an oven (10), a differential amplified signal (11), a three-axis magnetic field coil (12), an atomic magnetometer head housing (13), a lock-in amplifier (14), a first harmonic component (15), a second harmonic component (16), a division circuit (17), a subtraction circuit (18), a PID module (19), a z-axis function generator (20), a feedback control signal (21), a closed-loop feedback system (22), and a polarization differential detection module (23). Adjust the frequency of the laser emitted by the narrow-linewidth semiconductor laser (1) to deviate from the resonance frequency of the D1 line of alkali metal atoms, couple this laser as a light source into the polarization-maintaining optical fiber (3), and convert it into elliptically polarized light through the ellipsometer (4), and use this light as the pumping laser to irradiate the atomic cell (5); control the xy-axis function generator (2) and the z-axis function generator (20), use three-dimensional magnetic compensation technology to zero the magnetic field sensed by the atomic cell (5), and control the z-axis function generator (20) to apply a high-frequency modulation magnetic field in the z direction; use lock-in amplification technology in the lock-in amplifier to demodulate the differential amplified signal (11) at the first harmonic frequency and the second harmonic frequency respectively to obtain the first harmonic component (15) and the second harmonic component (16); divide the two components to obtain a dimensionless operation signal and subtract it from 0, and input the obtained difference as an error signal into the PID module (19), and use the output signal of the PID module (19) to control the z-axis function generator (20) to generate a feedback control signal (21) and act on the z direction of the three-axis magnetic field coil (12), so that the coil generates a magnetic field with the same magnitude and opposite direction to the magnetic field to be measured in the positive direction of the z axis, thereby locking the magnetic field sensed by the ellipsometric SERF atomic magnetometer at zero field.

[0050] It also includes a narrow-linewidth semiconductor laser (1), a polarization-maintaining optical fiber (3), and an ellipsometer (4); the narrow-linewidth semiconductor laser (1) is used to provide a linearly polarized laser light source; the polarization-maintaining optical fiber (3) is used to transmit the laser light source; the ellipsometer (4) is used to convert the linearly polarized light into elliptically polarized light with a specific ellipticity, and this elliptically polarized light is used as the pumping laser to polarize alkali metal atoms.

[0051] It also includes an xy-axis function generator (2), a z-axis function generator (20), and a three-axis magnetic field coil (12); the xy-axis function generator (2) is used to control the three-axis magnetic field coil (12) to compensate the ambient magnetic field in the x and y directions to zero; the z-axis function generator (20) is used to control the three-axis magnetic field coil (12) to compensate the ambient magnetic field in the z direction to zero.

[0052] It also includes an atomic gas cell (5) and an oven (10); the atomic gas cell (5) serves as a sensing element, which is filled with alkali metal atoms and is in a weak magnetic environment; the oven (10) is wrapped around the atomic gas cell (5) to heat the atomic gas cell (5) until the number density of alkali metal atoms reaches 10 13 ~10 14 per cm 3 .

[0053] It also includes a half-wave plate (6), a lateral displacement polarization beam splitter prism (7), a photodetector (8), a differential amplifier circuit (9), and a polarization differential detection module (23); the half-wave plate (6) adjusts the splitting ratio of the two orthogonal components of the lateral displacement polarization beam splitter prism (7) to 50%, 50% before the alkali metal atoms are heated; the lateral displacement polarization beam splitter prism (7) is used to decompose the pump laser into two linearly polarized lights with orthogonal polarization directions; the photodetector (8) is used to convert the light intensity information of the two linearly polarized lights passing through the lateral displacement polarization beam splitter prism (7) into current signals; the differential amplifier circuit (9) is used to subtract and amplify the two current signals output by the photodetector (8); the polarization differential detection module (23) includes a half-wave plate (6), a lateral displacement polarization beam splitter prism (7), a photodetector (8), and a differential amplifier circuit (9) to implement the polarization differential detection technique.

[0054] It also includes a differential amplified signal (11), a lock-in amplifier (14), a first harmonic component (15), a second harmonic component (16), and a division circuit (17); the lock-in amplifier (14) is used to implement the lock-in amplification technique to obtain harmonic components; the first harmonic component (15) and the second harmonic component (16) are both harmonic components in the zero-order resonance signal of the elliptical polarization SERF atomic magnetometer, and the two pass through the division circuit (17) to obtain a dimensionless operation signal.

[0055] It also includes a subtraction circuit (18), a PID module (19), a z-axis function generator (20), a feedback control signal (21), and a closed-loop feedback system (22); the closed-loop feedback system (22) includes a subtraction circuit (18), a PID module (19), a z-axis function generator (20), and a feedback control signal (21), where the subtraction circuit (18) subtracts the dimensionless operation signal from the set value 0, and the obtained difference is used as an error signal and input to the PID module (19). The output signal of the PID module (19) is connected to the z-axis function generator (20), and the z-axis function generator (20) generates a feedback control signal (21) for controlling the z-direction magnetic field of the three-axis magnetic field coil (12).

[0056] The end of the polarization-maintaining optical fiber (3), the elliptical polarizer (4), the atomic vapor cell (5), the oven (10), the three-axis magnetic field coil (12), and the polarization difference detection module (23) are placed in the atomic magnetometer head housing (13), and together with the atomic magnetometer head housing (13), they form the atomic magnetometer head.

[0057] The optical axis direction of the lateral displacement polarization beam splitter prism (7) is aligned with the fast axis direction of the polarization-maintaining optical fiber (3).

[0058] The narrow linewidth semiconductor laser (1) emits a linearly polarized light source whose emission frequency deviates from the resonance frequency of the D1 line of alkali metal atoms. By aligning the polarization direction of this linearly polarized light with the fast axis direction of the polarization-maintaining optical fiber (3), the light source is coupled into the polarization-maintaining optical fiber (3).

[0059] The elliptical polarizer (4) includes a linear polarizer and a quarter-wave plate, and the included angle between their optical axis directions is α. The polarization direction of the linearly polarized light source coincides with the optical axis direction of the linear polarizer. The linearly polarized light source is converted into an elliptically polarized light with an ellipticity of α through the elliptical polarizer (4), and this light is the pumping laser of the elliptical polarization SERF atomic magnetometer.

[0060] The feedback control signal (21) and the coil constant in the z direction of the three-axis magnetic field coil (12) are operated to obtain the output signal of the atomic magnetometer, that is, the magnetic field measurement value in the z direction.

[0061] 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 specified 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 ellipsometric SERF atomic magnetometer device based on a magnetic field closed-loop, characterized in that, It includes a three-axis magnetic field coil and a polarization differential detection module located inside the head shell of an atomic magnetometer. The laser input end of the polarization differential detection module is connected to the laser output end of the atomic gas cell inside the three-axis magnetic field coil. The laser input end of the atomic gas cell is sequentially connected to a narrow linewidth semiconductor laser through an elliptical polarizer and a polarization-maintaining optical fiber. The output end of the polarization differential detection module is sequentially connected to the three-axis magnetic field coil through a lock-in amplifier, a division circuit, a subtraction circuit, a PID module, and a z-axis function generator. The three-axis magnetic field coil is connected to an xy-axis function generator. The z-axis function generator outputs a DC voltage to drive the three-axis magnetic field coil to generate a magnetic field in the z direction, and the xy-axis function generator outputs a DC voltage to drive the three-axis magnetic field coil to generate magnetic fields in the x direction and the y direction; The polarization differential detection module includes a lateral displacement polarization beam splitter prism. The input end of the lateral displacement polarization beam splitter prism is connected to the laser output end of the atomic gas cell through a half-wave plate. The output end of the lateral displacement polarization beam splitter prism is sequentially connected to the lock-in amplifier through a photodetector and a differential amplifier circuit; The elliptical polarizer includes a linear polarizer and a quarter-wave plate, and the included angle between their optical axis directions is α. The polarization direction of the linearly polarized light source coincides with the optical axis direction of the linear polarizer. The linearly polarized light source is converted into elliptically polarized light with an ellipticity of α through the elliptical polarizer, and this elliptically polarized light is the pumping laser of the ellipsometric SERF atomic magnetometer; An oven is provided between the atomic gas cell and the three-axis magnetic field coil. The lock-in amplifier inputs the first harmonic component and the second harmonic component to the division circuit respectively. The subtraction circuit subtracts the dimensionless operation signal from the division circuit from the set value of 0, and the obtained difference is input to the PID module as an error signal. The dimensionless operation signal is the quotient obtained by dividing the first harmonic component by the second harmonic component. Both the first harmonic component and the second harmonic component are harmonic components in the zero-order resonance signal of the ellipsometric SERF atomic magnetometer; The z-axis function generator outputs a feedback control signal, and the feedback control signal is operated with the coil constant in the z direction of the three-axis magnetic field coil to obtain the output signal of the atomic magnetometer, that is, the magnetic field to be measured in the positive z direction.

2. The ellipsometric SERF atomic magnetometer device based on a magnetic field closed-loop according to claim 1, characterized in that, It includes: ， where θ w is the fundamental harmonic component, θ 2w is the second harmonic component, K is the amplification coefficient, I0 is the pump optical power density, OD(v) is the optical depth, n is the atomic number density, c is the speed of light, r is the classical electron radius, f is the resonance intensity of the D1 line of the alkali metal atom, l is the length of the atomic cell, v0 is the resonance frequency of the D1 line of the alkali metal atom, v is the frequency of the laser, Г is the pressure broadening of the atomic cell, R op is the pumping rate, R rel is the transverse relaxation rate, s is the spin angular momentum of the pump light, γ e is the gyromagnetic ratio of a single electron, J0, J1, and J2 are the Bessel functions of the 0th, 1st, and 2nd orders respectively, u is the magnetic field modulation parameter, and B0 is the magnetic field to be measured in the positive direction of the z-axis.

3. The ellipsometric SERF atomic magnetometer device based on a magnetic field closed-loop according to claim 1, characterized in that, It includes: ， where S input is a dimensionless operation signal, S ctrl is a feedback control signal, R op is the pumping rate, R rel is the transverse relaxation rate, J1 and J2 are the Bessel functions of the first and second orders respectively, u is the magnetic field modulation parameter, B0 is the magnetic field to be measured in the positive direction of the z-axis, Kz is the coil constant in the z direction of the three-axis magnetic field coil, and R is the resistance in the z direction of the three-axis magnetic field coil.

4. A method for operating an ellipsometric SERF atomic magnetometer based on a magnetic field closed-loop, characterized in that, It includes the ellipsometric SERF atomic magnetometer device based on magnetic field closed-loop described in any one of the above claims 1-3, and the following steps: Step 1: Adjust the frequency of the laser emitted by the narrow linewidth semiconductor laser to deviate from the resonance frequency of the alkali metal atom D1 line, and the polarization state is linearly polarized. This laser is used as a light source and coupled into the polarization-maintaining optical fiber. The linearly polarized light emerging from the polarization-maintaining optical fiber passes through the elliptical polarizer and is converted into elliptically polarized light with an ellipticity of α. This light is used as the pumping laser to irradiate the atomic gas cell; Step 2: Control the xy-axis function generator and the z-axis function generator, and use three-dimensional magnetic compensation technology to zero the magnetic field sensed by the atomic gas cell. Control the z-axis function generator to apply a high-frequency modulation magnetic field in the z direction, and its frequency is w m ; Step 3: Introduce the differential amplified signal output from the polarization differential detection module into a lock-in amplifier. In the lock-in amplifier, use lock-in amplification technology to demodulate the differential amplified signal at frequencies of fundamental frequency ω m , second harmonic frequency 2ω m respectively, to obtain the fundamental harmonic component θ w , and the second harmonic component θ 2w ; Step 4, θ w and θ 2w pass through a division circuit to obtain a dimensionless operation signal S input = θ w / θ 2wt , connect S input to a subtraction circuit, subtract S input from the set value 0, and use the obtained difference as an error signal to input to the PID module. The output signal of the PID module controls the z-axis function generator to generate a feedback control signal S ctrl ; Step 5, S ctrl Act on the z-direction of the three-axis magnetic field coil, so that the magnetic field generated by the coil is equal in magnitude and opposite in direction to the magnetic field B0 to be measured in the positive direction of the z-axis, thereby locking the magnetic field sensed by the ellipsometric SERF atomic magnetometer at zero field.

5. The method for operating an ellipsometric SERF atomic magnetometer based on a magnetic field closed-loop according to claim 4, characterized in that, The three-dimensional magnetic compensation technology in Step 2 is a technology that controls the three-axis magnetic field coil around the atomic gas cell to generate a magnetic field with the same magnitude and opposite direction as the ambient magnetic field, so as to achieve the zeroing of the ambient magnetic field sensed by the atomic gas cell.

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