Method and system for improving receiving bandwidth of very-low-frequency atom magnetometer

Abstract

The invention provides a method and a system for improving the receiving bandwidth of a very-low-frequency atom magnetometer. The method comprises the following steps of: polarizing alkali metal atoms in an atom gas chamber along the direction of driving laser by using the driving laser; an external magnetic field at the air chamber is compensated through a three-axis magnetic coil and a three-axis magnetic sensor, a zero field environment is provided, and a main magnetic field B is applied by the coil in the direction of driving light; detecting the precession of the spin projection signal of the polarized atom in the x direction by using the detection laser in the x direction; an external very-low-frequency signal interacts with atoms polarized in the z direction to enable the atoms to generate precession signals on the xOy plane, and therefore the external very-low-frequency signal is detected by detection light. After detection optical signals are collected, the control system feeds back the signals to the atom ensemble through the coil in a damping magnetic field mode, the bandwidth of the system is expanded by adjusting feedback parameters, and meanwhile the sensitivity is kept unchanged. According to the technical scheme, the technical problem that high sensitivity and high bandwidth cannot be achieved at the same time in a very-low-frequency atom magnetometer in the prior art is solved.

Description

Technical Field

[0001] The present invention relates to the field of quantum sensing technology, and in particular to a method and system for improving the receiving bandwidth of a very low frequency atomic magnetometer. Background Art

[0002] In recent years, optically pumped alkali metal atomic magnetometers based on the Larmor precession effect of atomic spins in a magnetic field have been shown to be more sensitive to radio frequency magnetic fields than standard coil detection, especially in low-frequency radio such as the very low frequency band. The very low frequency atomic magnetometer technology based on the atomic magnetometer principle can have a sensitivity three orders of magnitude higher under the same effective sensing volume, providing a new idea for effectively solving the bottleneck problem in the current very low frequency communication field.

[0003] The sensitivity of a very low frequency atomic magnetometer is related to the alkali metal spin relaxation time T2. To achieve a measurement sensitivity as low as ~fT level, a longer alkali metal spin relaxation time T2 (about ms) is usually required. However, a longer relaxation time T2 also means that it takes a longer time for the alkali metal atoms to recover to the standby state after one measurement and start the next measurement, which means that the system response is slower, or the response bandwidth is narrower, and the response bandwidth is characterized by the parameter Γ=1 / T2. Therefore, the conventional very low frequency atomic magnetometer has the problem that high sensitivity and high bandwidth cannot be achieved at the same time. For short-time signals or applications that require good sensitivity at a high bandwidth, it is necessary to solve the problem of how to achieve high-bandwidth measurement capability without excessively losing the sensitivity of the very low frequency atomic magnetometer. Summary of the invention

[0004] The present invention provides a method and system for improving the receiving bandwidth of a very low frequency atomic magnetometer, which can solve the technical problem in the prior art that a very low frequency atomic magnetometer cannot have both high sensitivity and high bandwidth.

[0005] According to one aspect of the present invention, a method for improving the receiving bandwidth of a very low frequency atomic magnetometer is provided. The method comprises: using a driving laser to polarize alkali metal atoms in an atomic gas chamber along the driving laser direction (z direction); the external magnetic field at the atomic gas chamber is compensated by a three-axis magnetic coil and a three-axis magnetic sensor to provide a zero-field environment; then the three-axis magnetic coil applies a main magnetic field B in the driving light direction; using a detection laser in the x direction to detect the precession of the spin projection signal of the polarized atoms in the x direction, the precession amplitude is proportional to the external very low frequency signal amplitude, and the frequency is the same as the external very low frequency signal; the driving laser and the detection laser are used to manipulate the atoms, and the driving laser is σ + Circularly polarized light resonates with the D1 line of alkali metal atoms. Due to the transition selection rule ΔF = 1, under the action of circularly polarized driving laser, the atoms will be pumped to m F=F is in the quantum state with the largest magnetic quantum number, thereby realizing polarization, the system quantization axis points along the driving light, and the polarized atomic spin points in the positive z direction; the main magnetic field is in the same direction as the driving laser, along the positive z direction, with a magnitude of B, and the very low frequency electromagnetic signal B0cos(ωt) propagating along the z direction and vibrating in the y direction from the outside will be absorbed by the polarized atoms, causing the polarization vector to deviate from the z direction and begin to precess around the z axis with a frequency of γB, where γ is the gyromagnetic ratio of the alkali metal atoms; the detection laser is linearly polarized, and the frequency is greatly detuned from the resonance frequency of the alkali metal atoms, so that the absorption of the detection laser by the atomic ensemble can be ignored, the dichroic effect plays a dominant role, and the detection light signal will change with the precession of the atomic spin; by increasing the feedback magnetic field B in the x direction fb =aP x Introducing the damping term aP x , where a is the feedback parameter, and a new kinetic equation of the system is constructed. Based on the new kinetic equation of the system, the bandwidth parameter is changed to Γ eff =Γ+aγP z ; By adjusting the detection laser pumping rate R pr , so that the detection laser pumping rate R prr >>Γ, Γ is the initial bandwidth parameter, which is used to increase the bandwidth without excessively losing sensitivity.

[0006] Furthermore, the new dynamic equation of the system is Among them, P x is the x-direction polarization projection intensity of the alkali metal atom, P y is the y-direction polarization projection intensity of the alkali metal atom, γ is the gyromagnetic ratio of the alkali metal atom, B0 is the amplitude of the very low frequency electromagnetic signal, P z is the polarization projection intensity of the alkali metal atom in z direction, Γ is the initial bandwidth parameter, and a is the feedback parameter.

[0007] Furthermore, the pumping laser generated by the pumping light laser passes through the first Glan Taylor prism and the quarter wave plate in sequence and then enters the atomic gas chamber, and polarizes the alkali metal atoms by interacting with the alkali metal atoms in the atomic gas chamber; the detection laser generated by the detection light laser passes through the second half wave plate, the second Glan Taylor prism and the second reflector in sequence and then enters the atomic gas chamber, the detection laser emitted from the atomic gas chamber is reflected by the first reflector and then passes through the first half wave plate and the Wollaston prism and then enters the balanced zero-beat detector, and the signal collected by the balanced zero-beat detector is output to the data acquisition and control system; a three-axis magnetic field coil and a three-axis flux gate are arranged outside the atomic gas chamber.

[0008] Furthermore, in the case of the very low frequency signal B xWhen cos(ωt) is detected, the data acquisition and control system first controls the ambient magnetic field of the air chamber according to the real-time measurement information of the three-axis flux gate, so that the magnetic fields in the x, y and z directions are 0; on this basis, the three-dimensional magnetic field coil applies a main magnetic field of magnitude B in the z direction; since the Faraday rotation angle θ of the light is proportional to the polarization projection size P of the spin ensemble in the x direction x , that is: θ∝P x At this time, the signal output by the balanced zero-beat detector will be proportional to: P x =Mcos(ωt), where the amplitude M is the amplitude of the atomic precession signal under the action of the very low frequency signal; when the above signal is detected, the data acquisition and control system passes the signal through the three-axis magnetic field coil with a magnitude of B fb =aP x Feedback to the x direction to expand the bandwidth of the VLF atomic magnetometer from Γ to Γ+aγP z .

[0009] Furthermore, the Faraday rotation angle θ can be calculated according to Calculate and obtain, where I1 is the light intensity of the first laser output by the Wollaston prism, I2 is the light intensity of the second laser output by the Wollaston prism, I0=I1+I2, and I0 is the total light intensity of the two laser beams.

[0010] According to another aspect of the present invention, a very low frequency atomic magnetometer receiving bandwidth improvement system is provided. The very low frequency atomic magnetometer receiving bandwidth improvement system based on polarization selective reception uses the very low frequency atomic magnetometer receiving bandwidth improvement method as described above to improve the very low frequency atomic magnetometer receiving bandwidth.

[0011] Furthermore, the receiving bandwidth improvement system of the very low frequency atomic magnetometer includes a pumping light laser, a first Glan-Taylor prism, a quarter wave plate, an atomic gas chamber, a first reflector, a first half wave plate, a Wollaston prism, a balanced zero-beat detector, a data acquisition and control system, a detection light laser, a second half wave plate, a second Glan-Taylor prism, a second reflector, a three-axis magnetic field coil and a three-axis flux gate. The pumping laser generated by the pumping light laser passes through the first Glan-Taylor prism and the quarter wave plate in sequence and then enters the atomic gas chamber, and polarizes the alkali metal atoms by interacting with the alkali metal atoms in the atomic gas chamber; the detection laser generated by the detection light laser passes through the first Glan-Taylor prism and the quarter wave plate in sequence. The second half-wave plate, the second Glan Taylor prism and the second reflector are reflected and enter the atomic gas chamber. The detection laser emitted by the atomic gas chamber is reflected by the first reflector, and then enters the balanced zero-beat detector after passing through the first half-wave plate and the Wollaston prism. The signal collected by the balanced zero-beat detector is output to the data acquisition and control system; the three-axis magnetic field coil and the three-axis flux gate are arranged outside the atomic gas chamber. The three-axis magnetic field coil is used to generate a magnetic field in three-axis directions, which is controlled by the data acquisition and control system to offset the interference of the environmental magnetic field and provide a feedback magnetic field. The three-axis flux gate is used to detect the size of the environmental magnetic field at the atomic gas chamber in real time, and the result is input into the data acquisition and control system as a feedback variable.

[0012] By applying the technical solution of the present invention, a method for improving the receiving bandwidth of a very low frequency atomic magnetometer is provided. The method can effectively improve the bandwidth of the very low frequency atomic magnetometer while keeping the sensitivity of the very low frequency atomic magnetometer unchanged. Based on the very low frequency atomic magnetometer with high bandwidth, fast short-time signal capture can be achieved. By changing the feedback parameters, the bandwidth can be further narrowed to achieve a longer coherence time. The system is simple and easy to realize miniaturization and integration. Based on the optical Bloch equation of atomic spin precession, the system bandwidth is increased from Γ to the effective bandwidth Γ by regulating the damping effect. eff At the same time, by adjusting the detection optical power parameters, the detection bandwidth can be broadened without significantly losing the sensitivity of the very low frequency atomic magnetometer. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The included drawings are used to provide a further understanding of the embodiments of the present invention, which constitute a part of the specification, are used to illustrate the embodiments of the present invention, and together with the text description, explain the principles of the present invention. Obviously, the drawings in the following description are only some embodiments of the present invention, and for ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0014] Figure 1 A schematic diagram showing the structure of a very low frequency atomic magnetometer receiving bandwidth enhancement system provided according to a specific embodiment of the present invention

[0015] The above drawings include the following reference numerals:

[0016] 10. Pumping light laser; 20. First Glan-Taylor prism; 30. Quarter wave plate; 40. Atomic gas chamber; 50. First reflector; 60. First half wave plate; 70. Wollaston prism; 80. Balanced zero-beat detector; 90. Data acquisition and control system; 100. Detection light laser; 110. Second half wave plate; 120. Second Glan-Taylor prism; 130. Second reflector; 140. Three-axis magnetic field coil; 150. Three-axis flux gate. DETAILED DESCRIPTION

[0017] It should be noted that, in the absence of conflict, the embodiments in this application and the features in the embodiments can be combined with each other. The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. The following description of at least one exemplary embodiment is actually only illustrative and is by no means intended to limit the present invention and its application or use. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0018] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present application. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprise" and / or "include" are used in this specification, it indicates the presence of features, steps, operations, devices, components and / or combinations thereof.

[0019] Unless otherwise specifically stated, the relative arrangement of the parts and steps described in these embodiments, numerical expressions and numerical values ​​do not limit the scope of the present invention. At the same time, it should be understood that, for ease of description, the sizes of the various parts shown in the accompanying drawings are not drawn according to the actual proportional relationship. The technology, method and equipment known to ordinary technicians in the relevant field may not be discussed in detail, but in appropriate cases, the technology, method and equipment should be regarded as a part of the authorization specification. In all examples shown and discussed here, any specific value should be interpreted as being merely exemplary, rather than as a limitation. Therefore, other examples of exemplary embodiments may have different values. It should be noted that similar reference numerals and letters represent similar items in the following drawings, so once a certain item is defined in an accompanying drawing, it does not need to be further discussed in subsequent drawings.

[0020] like Figure 1As shown, according to a specific embodiment of the present invention, a method for improving the receiving bandwidth of a very low frequency atomic magnetometer is provided. The method for improving the receiving bandwidth of a very low frequency atomic magnetometer comprises: using a driving laser to polarize alkali metal atoms in an atomic gas chamber along the driving laser direction (z direction); the external magnetic field at the atomic gas chamber is compensated by a three-axis magnetic coil and a three-axis magnetic sensor to provide a zero-field environment; then the three-axis magnetic coil applies a main magnetic field B in the driving light direction; using a detection laser in the x direction to detect the precession of the spin projection signal of the polarized atoms in the x direction, the precession amplitude is proportional to the external very low frequency signal amplitude, and the frequency is the same as the external very low frequency signal; the driving laser and the detection laser are used to manipulate the atoms, and the driving laser is σ + Circularly polarized light resonates with the D1 line of alkali metal atoms. Due to the transition selection rule ΔF = 1, under the action of circularly polarized driving laser, the atoms will be pumped to m F =F is in the quantum state with the largest magnetic quantum number, thereby realizing polarization, the system quantization axis points along the driving light, and the polarized atomic spin points in the positive z direction; the main magnetic field is in the same direction as the driving laser, along the positive z direction, with a magnitude of B, and the very low frequency electromagnetic signal B0cos(ωt) propagating along the z direction and vibrating in the y direction from the outside will be absorbed by the polarized atoms, causing the polarization vector to deviate from the z direction and begin to precess around the z axis with a frequency of γB, where γ is the gyromagnetic ratio of the alkali metal atoms; the detection laser is linearly polarized, and the frequency is greatly detuned from the resonance frequency of the alkali metal atoms, so that the absorption of the detection laser by the atomic ensemble can be ignored, the dichroic effect plays a dominant role, and the detection light signal will change with the precession of the atomic spin; by increasing the feedback magnetic field B in the x direction fb =aP x Introducing the damping term aP x , where a is the feedback parameter, and a new kinetic equation of the system is constructed. Based on the new kinetic equation of the system, the bandwidth parameter is changed to Γ eff =Γ+aγP z ; By adjusting the detection laser pumping rate R pr , so that the detection laser pumping rate R prr >>Γ, Γ is the initial bandwidth parameter, which is used to increase the bandwidth without excessively losing sensitivity.

[0021] By using this configuration, a method for improving the receiving bandwidth of a very low frequency atomic magnetometer is provided. The method uses a driving laser to polarize the alkali metal atoms in the atomic gas chamber along the driving laser direction (z direction); the external magnetic field in the gas chamber is compensated by a three-axis magnetic coil and a three-axis magnetic sensor to provide a zero-field environment. Then the coil applies a main magnetic field B in the driving light direction; the detection laser is used in the x direction to detect the precession of the spin projection signal of the polarized atoms in the x direction. The precession amplitude is proportional to the amplitude of the external very low frequency signal, and the frequency is the same as the external very low frequency signal, thereby realizing the detection of the very low frequency signal; (4) the external very low frequency signal interacts with the atoms polarized in the z direction to generate a precession signal in the xOy plane, which is detected by the detection light. After the detection light signal is collected, it is fed back to the atomic ensemble by the control system through the coil in the form of a damping magnetic field. By adjusting the feedback parameters, the bandwidth of the system is expanded while the sensitivity remains unchanged. Compared with the prior art, the method for improving the receiving bandwidth of a very low frequency atomic magnetometer provided by the present invention can effectively improve its bandwidth while keeping the sensitivity of the very low frequency atomic magnetometer unchanged; based on the very low frequency atomic magnetometer with high bandwidth, fast short-time signal capture can be achieved; by changing the feedback parameters, the bandwidth can be further narrowed to achieve a longer coherence time; the system is simple and easy to realize miniaturization and integration; based on the optical Bloch equation of atomic spin precession, the system bandwidth is increased from Γ to the effective bandwidth Γ by regulating the damping effect eff At the same time, by adjusting the detection optical power parameters, the detection bandwidth can be broadened without significantly losing the sensitivity of the very low frequency atomic magnetometer.

[0022] Furthermore, in the present invention, the new kinetic equation of the system is Among them, P x is the x-direction polarization projection intensity of the alkali metal atom, P y is the y-direction polarization projection intensity of the alkali metal atom, γ is the gyromagnetic ratio of the alkali metal atom, B0 is the amplitude of the very low frequency electromagnetic signal, P z is the polarization projection intensity of the alkali metal atom in z direction, Γ is the initial bandwidth parameter, and a is the feedback parameter.

[0023] As a specific embodiment of the present invention, Figure 1As shown, the pumping laser generated by the pumping light laser 10 passes through the first Glan Taylor prism 20 and the quarter wave plate 30 in sequence and then enters the atomic gas chamber 40, and polarizes the alkali metal atoms by interacting with the alkali metal atoms in the atomic gas chamber 40; the detection laser generated by the detection light laser 100 passes through the second half wave plate 110, the second Glan Taylor prism 120 and the second reflector 130 in sequence and then enters the atomic gas chamber 40, and the detection laser emitted from the atomic gas chamber 40 is reflected by the first reflector 50 and then passes through the first half wave plate 60 and the Wollaston prism 70 and then enters the balanced zero-beat detector 80, and the signal collected by the balanced zero-beat detector 80 is output to the data acquisition and control system 90; a three-axis magnetic field coil 140 and a three-axis flux gate 150 are arranged outside the atomic gas chamber 40.

[0024] Furthermore, in the present invention, when the very low frequency signal B x When cos(ωt) is detected, the data acquisition and control system first controls the ambient magnetic field of the air chamber according to the real-time measurement information of the three-axis flux gate, so that the magnetic fields in the x, y and z directions are 0; on this basis, the three-dimensional magnetic field coil applies a main magnetic field of magnitude B in the z direction; since the Faraday rotation angle θ of the light is proportional to the polarization projection size P of the spin ensemble in the x direction x , that is: θ∝P x At this time, the signal output by the balanced zero-beat detector will be proportional to: P x =Mcos(ωt), where the amplitude M is the amplitude of the atomic precession signal under the action of the very low frequency signal; when the above signal is detected, the data acquisition and control system passes the signal through the three-axis magnetic field coil with a magnitude of B fb =aP x Feedback to the x direction to expand the bandwidth of the VLF atomic magnetometer from Γ to Γ+aγP z .

[0025] Among them, the Faraday rotation angle θ can be calculated according to Calculate and obtain, where I1 is the light intensity of the first laser output by the Wollaston prism 70, I2 is the light intensity of the second laser output by the Wollaston prism 70, I0=I1+I2, and I0 is the total light intensity of the two laser beams.

[0026] According to another aspect of the present invention, a very low frequency atomic magnetometer receiving bandwidth improvement system is provided. The very low frequency atomic magnetometer receiving bandwidth improvement system based on polarization selective reception uses the very low frequency atomic magnetometer receiving bandwidth improvement method as described above to improve the very low frequency atomic magnetometer receiving bandwidth.

[0027] By applying this configuration, a very low frequency atomic magnetometer receiving bandwidth improvement system is provided, which can effectively improve the bandwidth of the very low frequency atomic magnetometer while maintaining the sensitivity of the very low frequency atomic magnetometer unchanged; based on the high-bandwidth very low frequency atomic magnetometer, fast short-time signal capture can be achieved; by changing the feedback parameters, the bandwidth can be further narrowed to achieve a longer coherence time; the system is simple and easy to realize miniaturization and integration.

[0028] Specifically, in the present invention, the receiving bandwidth improvement system of the very low frequency atomic magnetometer includes a pumping light laser 10, a first Glan Taylor prism 20, a quarter wave plate 30, an atomic gas chamber 40, a first reflector 50, a first half wave plate 60, a Wollaston prism 70, a balanced zero-beat detector 80, a data acquisition and control system 90, a detection light laser 100, a second half wave plate 110, a second Glan Taylor prism 120, a second reflector 130, a three-axis magnetic field coil 140 and a three-axis fluxgate 150. The pumping laser generated by the pumping light laser 10 passes through the first Glan Taylor prism 20 and the quarter wave plate 30 in sequence and then enters the atomic gas chamber 40, and polarizes the alkali metal atoms by interacting with the alkali metal atoms in the atomic gas chamber 40. The detection light generated by the detection light laser 100 The laser is reflected by the second half-wave plate 110, the second Glan Taylor prism 120 and the second reflector 130 in sequence and then enters the atomic gas chamber 40. The detection laser emitted by the atomic gas chamber 40 is reflected by the first reflector 50, and then enters the balanced zero-beat detector 80 after passing through the first half-wave plate 60 and the Wollaston prism 70. The signal collected by the balanced zero-beat detector 80 is output to the data acquisition and control system 90; the three-axis magnetic field coil 140 and the three-axis flux gate 150 are arranged outside the atomic gas chamber 40, the three-axis magnetic field coil 140 is used to generate a magnetic field in three-axis directions, which is controlled by the data acquisition and control system 90 to offset the interference of the ambient magnetic field and provide a feedback magnetic field. The three-axis flux gate 150 is used to detect the size of the ambient magnetic field at the atomic gas chamber in real time, and the result is input into the data acquisition and control system 90 as a feedback variable.

[0029] In order to further understand the present invention, the following Figure 1 The method for improving the receiving bandwidth of a very low frequency atomic magnetometer provided by the present invention is described in detail.

[0030] like Figure 1 As shown, the present invention is based on the optical Bloch equation of atomic spin precession, and proposes a method of using feedback damping to increase the system bandwidth from Γ to the effective bandwidth Γ by regulating the damping effect. eff At the same time, by adjusting the detection optical power parameters, the detection bandwidth can be broadened without significantly losing the sensitivity of the very low frequency atomic magnetometer.

[0031] The very low frequency (VLF) atomic magnetometer receives and detects very low frequency (VLF) signals based on the Larmor precession resonance principle of atomic spins. Compared with traditional very low frequency (VLF) magnetic field receiving antennas, it has huge potential advantages in sensitivity and small volume integration. However, as mentioned above, since the VLF atomic magnetometer detects very low frequency signals through the resonance principle, its effective bandwidth is limited by the resonance bandwidth, which brings challenges to the design and implementation of high-bandwidth VLF atomic magnetometers. In response to the above problems, the present invention proposes a high-bandwidth VLF atomic magnetometer implementation method and system, which can effectively improve the receiving bandwidth of the VLF atomic magnetometer while ensuring that the receiving sensitivity of the VLF atomic magnetometer remains unchanged.

[0032] The technical scheme adopted to achieve the above objectives is as follows: (1) The driving laser is used to polarize the alkali metal atoms in the atomic gas chamber along the driving laser direction (z direction); (2) The external magnetic field in the gas chamber is compensated by the three-axis magnetic coil and the three-axis magnetic sensor to provide a zero-field environment. The main magnetic field B is then applied by the coil in the driving light direction; (3) The detection laser is used in the x direction to detect the precession of the spin projection signal of the polarized atoms in the x direction. The precession amplitude is proportional to the amplitude of the external very low frequency signal, and the frequency is the same as the external very low frequency signal, thereby realizing the detection of the very low frequency signal; (4) The external very low frequency signal interacts with the atoms polarized in the z direction to generate a precession signal in the xOy plane, which is then detected by the detection light. After the detection light signal is collected, it is fed back to the atomic ensemble by the control system through the coil in the form of a damping magnetic field. The system bandwidth is expanded by adjusting the feedback parameters while keeping the sensitivity unchanged.

[0033] The beneficial effects of the present invention are: the bandwidth of a very low frequency atomic magnetometer can be effectively improved while maintaining the sensitivity of the very low frequency atomic magnetometer unchanged; and fast short-time signal capture can be achieved based on a high-bandwidth very low frequency atomic magnetometer.

[0034] Additional beneficial effects: By changing the feedback parameters, the bandwidth can be further narrowed to achieve a longer coherence time; the system is simple and easy to achieve miniaturized integration.

[0035] Implementation principle

[0036] The specific implementation principle of this solution is as follows:

[0037] Very low frequency atomic magnetometer uses the Larmor precession of atomic spin under the resonance of the main magnetic field to detect the magnetic field component of the very low frequency radio signal, so as to receive the very low frequency radio signal. The basic principle is as follows Figure 1 As shown, the atoms are manipulated by pump light and probe light, and the pump light is σ + Circularly polarized light resonates with the D1 line of alkali metal atoms. Due to the transition selection rule ΔF = 1, under the action of circularly polarized pump light, the atoms will be pumped to m F=F is in the quantum state with the largest magnetic quantum number, thereby realizing polarization, the system quantization axis points along the driving light, and the polarized atomic spin points in the positive z direction. The main magnetic field is in the same direction as the driving light, along the positive z direction, and has a magnitude of B. The very low frequency electromagnetic signal B0cos(ωt) propagating in the z direction and vibrating in the y direction will be absorbed by the polarized atoms, causing the polarization vector to deviate from the z direction and begin to precess around the z axis at a frequency of γB, where γ is the gyromagnetic ratio of the alkali metal atom. The detection laser is linearly polarized, and the frequency is greatly detuned from the resonance frequency of the alkali metal atom, so that the absorption of the detection laser by the atomic ensemble can be ignored, the dichroic effect plays a dominant role, and the detection light signal will change with the precession of the atomic spin. Its dynamic evolution is determined by the Bloch equation:

[0038]

[0039] Its bandwidth is the parameter Γ in the equation, and P x is the x-direction polarization projection intensity of the alkali metal atom, P y is the polarization projection intensity of the alkali metal atom in the y direction, P z is the polarization projection intensity of the alkali metal atom in the z direction, γ is the gyromagnetic ratio of the alkali metal atom, and B0 is the amplitude of the very low frequency electromagnetic signal.

[0040] In the above general Bloch equation, the present invention increases the feedback magnetic field B in the x direction. fb =aP x Introducing the damping term aP x , where a is the feedback parameter, and the new kinetic equation of the system is constructed as:

[0041]

[0042] Rewrite the above formula as:

[0043]

[0044] Comparing the above equation with the original equation, we can see that by using the negative feedback mechanism and adding the damping field, the bandwidth parameter in the equation - P x The coefficient before Γ changes from Γ to Γ eff =Γ+aγP z , thereby increasing bandwidth.

[0045] The expression of sensitivity is:

[0046] Where n is the atomic density of the alkali metal, V is the atomic vapor volume, R pr is the detection laser pumping rate (proportional to the detection optical power), OD is the optical density, and η is the detector quantum efficiency. By adjusting the parameter R pr , that is, to detect the laser power, so that R pr>>Γ, so the sensitivity expression can be approximately simplified to: It is independent of Γ, thus achieving an increase in bandwidth without excessive loss of sensitivity.

[0047] Physical Implementation

[0048] A typical physical implementation structure is as follows Figure 1 As shown, the system mainly includes a pumping light laser, a detection light laser, a quarter wave plate, a half wave plate, a Glan Taylor prism, a Wollaston prism, a three-axis magnetic compensation coil, a three-axis flux gate, an atomic gas chamber, a reflector, a balanced zero-beat detector, and a data acquisition and control system.

[0049] Function of each component

[0050] Pumping laser: generates pumping laser; Detection laser: generates detection laser; Quarter wave plate: changes laser polarization, mainly used here to convert linear polarized light into circular polarized light; Half wave plate: changes laser polarization, mainly used here to change the polarization direction of linear polarized light; Glan Taylor prism: performs polarization splitting on the light beam, here used to purify the polarization of the transmitted laser to ensure that it is horizontally polarized (polarized in the x direction); Wollaston prism: performs polarization splitting on the light beam, so that the vertical (y direction) and horizontal (x direction) polarized light are separated in spatial propagation; Three-axis Magnetic field coil: generates a magnetic field in three-axis directions, which is controlled by the acquisition and control system to offset the interference of the environmental magnetic field and provide feedback magnetic field; three-axis flux gate: used to detect the size of the environmental magnetic field in the gas chamber in real time, and the result is input into the acquisition and control system as a feedback variable; atomic gas chamber: core working device, used to detect the atomic ensemble of very low frequency signals; reflector: changes the direction of the light beam; balanced zero-beat detector: converts the precession information detected by the detection light into a differential electrical signal for output; data acquisition and control system: collects and processes data, and performs feedback control on the system.

[0051] How it works

[0052] After the pump laser generated by the pump light laser is polarized and purified by the Glan-Taylor prism, the transmitted light polarized in the x direction is converted into the circularly polarized laser required by the very low frequency atomic magnetometer system through a quarter-wave plate and emitted into the gas cell, where it interacts with the alkali metal atoms in the gas cell to achieve their polarization.

[0053] The laser beam emitted from the detection light laser is polarized and purified by the Glan Taylor mirror, and then enters the gas chamber through the reflector in the polarization state of linear polarization in the x direction. The transmitted light through the gas chamber is adjusted by a half-wave plate, and then divided into two laser beams with orthogonal polarization directions by the Wollaston prism. The light intensity of the two laser beams is related to the Faraday rotation angle after passing through the gas chamber, which can be specifically expressed as:

[0054]

[0055] Where I0=I1+I2 is the sum of the light intensities of the two laser beams, I1 is the light intensity of laser 1, I2 is the light intensity of laser 2, and θ is the Faraday rotation angle after the laser passes through the gas cell, which is related to the polarization intensity of the atoms in the gas cell in the direction of laser propagation. The light intensities of the two laser beams are received by the two photodetectors of the balanced zero-beat detector behind them, and the Faraday rotation angle can be obtained from the signals of the two photodetectors:

[0056]

[0057] Therefore, the differential result (I2-I1) output by the balanced zero-beat detector is directly proportional to the Faraday rotation angle θ. I0 needs to be adjusted to be strong enough to ensure R pr >>Γ condition is established. According to the above principle, under this condition, the sensitivity of the very low frequency atomic magnetometer is independent of the bandwidth.

[0058] In the case of very low frequency signal B x When cos(ωt) is detected, the data acquisition and control system first controls the ambient magnetic field of the air chamber according to the real-time measurement information of the three-axis flux gate, so that the magnetic fields in the x, y and z directions are 0; on this basis, the main magnetic field of magnitude B is applied in the z direction by the three-dimensional magnetic field lines. Since the Faraday rotation angle θ of light is proportional to the polarization projection size P of the spin ensemble in the x direction, x , that is: θ∝P x At this time, the signal output by the balanced zero-beat detector will be proportional to: P x =Mcos(ωt), where the amplitude M is the amplitude of the atomic precession signal under the action of the very low frequency signal. When the above signal is detected, the data acquisition and control system passes the signal through the three-axis magnetic field coil with a magnitude of B fb =aP x Feedback to the x direction, described by the above principle, thus successfully extending the bandwidth of the VLF atomic magnetometer from Γ to Γ+aγP z .

[0059] Based on the optical Bloch equation of atomic spin precession, the present invention proposes a method for improving the receiving bandwidth of a very low frequency atomic magnetometer using feedback damping. By regulating the damping effect, the system bandwidth is increased from Γ to the effective bandwidth Γ eff At the same time, by adjusting the detection optical power parameters, the detection bandwidth can be broadened without significantly losing the sensitivity of the very low frequency atomic magnetometer.

[0060] For ease of description, spatially relative terms such as "above", "above", "on the upper surface of", "above", etc. may be used here to describe the spatial positional relationship between a device or feature and other devices or features as shown in the figure. It should be understood that spatially relative terms are intended to include different orientations of the device in use or operation in addition to the orientation described in the figure. For example, if the device in the accompanying drawings is inverted, the device described as "above other devices or structures" or "above other devices or structures" will be positioned as "below other devices or structures" or "below other devices or structures". Thus, the exemplary term "above" can include both "above" and "below". The device can also be positioned in other different ways (rotated 90 degrees or in other orientations), and the spatially relative descriptions used here are interpreted accordingly.

[0061] In addition, it should be noted that the use of terms such as "first" and "second" to limit components is only for the convenience of distinguishing the corresponding components. If not otherwise stated, the above terms have no special meaning and therefore cannot be understood as limiting the scope of protection of the present invention.

[0062] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for improving the receiving bandwidth of a very low frequency atomic magnetometer, characterized in that: The method for improving the receiving bandwidth of a very low frequency atomic magnetometer comprises: The alkali metal atoms in the atomic gas chamber are polarized along the driving laser direction (z direction) by using the driving laser; The external magnetic field at the atomic gas chamber is compensated by the three-axis magnetic coil and the three-axis magnetic sensor to provide a zero-field environment; then the three-axis magnetic coil applies the main magnetic field B in the direction of the driving light; In the x direction, the detection laser is used to detect the precession of the spin projection signal of the polarized atom in the x direction. The precession amplitude is proportional to the amplitude of the external very low frequency signal, and the frequency is the same as the external very low frequency signal. The atoms are manipulated by driving laser and detecting laser. The driving laser is σ + Circularly polarized light resonates with the D1 line of alkali metal atoms. Due to the transition selection rule ΔF = 1, under the action of circularly polarized driving laser, the atoms will be pumped to m F =F is in the quantum state with the largest magnetic quantum number, thereby realizing polarization, the system quantization axis points along the driving light, and the polarized atomic spin points to the positive z direction; the main magnetic field is in the same direction as the driving laser, along the positive z direction, with a magnitude of B, and the very low frequency electromagnetic signal B0cos(ωt) propagating along the z direction and vibrating in the y direction from the outside will be absorbed by the polarized atoms, causing the polarization vector to deviate from the z direction and begin to precess around the z axis at a frequency of γB, where γ is the gyromagnetic ratio of the alkali metal atom; the detection laser is linearly polarized, and the frequency is greatly detuned from the resonance frequency of the alkali metal atom, so that the absorption of the detection laser by the atomic ensemble can be ignored, the dichroic effect plays a leading role, and the detection light signal will change with the precession of the atomic spin; By increasing the feedback magnetic field B in the x direction fb =aP x Introducing the damping term aP x , where a is the feedback parameter, and a new kinetic equation of the system is constructed. Based on the new kinetic equation of the system, the bandwidth parameter is changed to Γ eff =Γ+aγP z ; By adjusting the detection laser pumping rate R pr , so that the detection laser pumping rate R prr >>Γ, Γ is the initial bandwidth parameter, which is used to increase the bandwidth without excessively losing sensitivity.

2. The method for improving the receiving bandwidth of a very low frequency atomic magnetometer according to claim 1, characterized in that: The new kinetic equation of the system is: Among them, P x is the x-direction polarization projection intensity of the alkali metal atom, P y is the y-direction polarization projection intensity of the alkali metal atom, γ is the gyromagnetic ratio of the alkali metal atom, B0 is the amplitude of the very low frequency electromagnetic signal, P z is the polarization projection intensity of the alkali metal atom in z direction, Γ is the initial bandwidth parameter, and a is the feedback parameter.

3. The method for improving the receiving bandwidth of a very low frequency atomic magnetometer according to claim 2, characterized in that: The pumping laser light generated by the pumping light laser (10) passes through a first Glan Taylor prism (20) and a quarter wave plate (30) in sequence and then enters an atomic gas chamber (40), and polarizes the alkali metal atoms by interacting with the alkali metal atoms in the atomic gas chamber (40); the detection laser light generated by the detection light laser (100) passes through a second half wave plate (110), a second Glan Taylor prism (120) and a second reflector (130) in sequence and then enters the atomic gas chamber (40); the detection laser light emitted from the atomic gas chamber (40) is reflected by a first reflector (50) and then passes through a first half wave plate (60) and a Wollaston prism (70) and then enters a balanced zero-beat detector (80); the signal collected by the balanced zero-beat detector (80) is output to a data acquisition and control system (90); and a three-axis magnetic field coil (140) and a three-axis flux gate (150) are arranged outside the atomic gas chamber (40).

4. The method for improving the receiving bandwidth of a very low frequency atomic magnetometer according to claim 3, characterized in that: In the case of very low frequency signal B x When cos(ωt) is detected, the data acquisition and control system first controls the ambient magnetic field of the air chamber according to the real-time measurement information of the three-axis flux gate, so that the magnetic fields in the x, y and z directions are 0; on this basis, the three-dimensional magnetic field coil applies a main magnetic field of magnitude B in the z direction; since the Faraday rotation angle θ of the light is proportional to the polarization projection size P of the spin ensemble in the x direction x , that is: θ∝P x At this time, the signal output by the balanced zero-beat detector will be proportional to: P x =Mcos(ωt), where the amplitude M is the amplitude of the atomic precession signal under the action of the very low frequency signal; when the above signal is detected, the data acquisition and control system passes the signal through the three-axis magnetic field coil with a magnitude of B fb =aP x Feedback to the x direction to expand the bandwidth of the VLF atomic magnetometer from Γ to Γ+aγP z .

5. The method for improving the receiving bandwidth of a very low frequency atomic magnetometer according to claim 4, characterized in that: The Faraday rotation angle θ can be calculated according to Calculate and obtain, wherein I1 is the light intensity of the first laser output by the Wollaston prism (70), I2 is the light intensity of the second laser output by the Wollaston prism (70), I0=I1+I2, and I0 is the total light intensity of the two laser beams.

6. A very low frequency atomic magnetometer receiving bandwidth improvement system, characterized in that: The very low frequency atomic magnetometer receiving bandwidth improvement system based on polarization selective reception uses the very low frequency atomic magnetometer receiving bandwidth improvement method as described in claims 1 to 5 to improve the very low frequency atomic magnetometer receiving bandwidth.

7. The very low frequency atomic magnetometer receiving bandwidth improvement system according to claim 6, characterized in that: The receiving bandwidth improvement system of a very low frequency atomic magnetometer comprises a pumping light laser (10), a first Glan-Taylor prism (20), a quarter wave plate (30), an atomic gas chamber (40), a first reflector (50), a first half wave plate (60), a Wollaston prism (70), a balanced zero-beat detector (80), a data acquisition and control system (90), a detection light laser (100), a second half wave plate (110), a second Glan-Taylor prism (120), a second reflector (130), a three-axis magnetic field coil (140) and a three-axis fluxgate (150). The pumping laser light generated by the pumping light laser (10) passes through the first Glan-Taylor prism (20) and the quarter wave plate (30) in sequence and then enters the atomic gas chamber (40), and polarizes the alkali metal atoms by interacting with the alkali metal atoms in the atomic gas chamber (40). The detection laser light generated by the detection light laser (100) is reflected by the second half-wave plate (110), the second Glan Taylor prism (120) and the second reflector (130) in sequence and then enters the atomic gas chamber (40). The detection laser light emitted by the atomic gas chamber (40) is reflected by the first reflector (50), and then enters the balanced zero-beat detector (80) after passing through the first half-wave plate (60) and the Wollaston prism (70). The signal collected by the balanced zero-beat detector (80) The three-axis magnetic field coil (140) and the three-axis fluxgate (150) are arranged outside the atomic gas chamber (40); the three-axis magnetic field coil (140) is used to generate a magnetic field in three directions, which is controlled by the data acquisition and control system (90) to offset the interference of the environmental magnetic field and provide a feedback magnetic field; the three-axis fluxgate (150) is used to detect the size of the environmental magnetic field at the atomic gas chamber in real time, and the result is input into the data acquisition and control system (90) as a feedback variable.

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