Biomagnetic measurement atomic magnetometer system and method for automatically compensating for external interference magnetic field

Abstract

The present invention discloses a biomagnetic measurement atomic magnetometer system and method for automatically compensating for external interference magnetic fields. The system generates a magnetic field by hyperpolarizing nuclear spins and applies an external constant magnetic field at the same time. The magnitude of the constant magnetic field makes the nuclear spins feel a magnetic field equal to the magnitude of the magnetic field generated by themselves. At this time, the nuclear spins can automatically track and compensate for the external low-frequency interference magnetic field, protect the electron spins from the intrusion of the external low-frequency interference magnetic field, make the electron spins only sensitive to the extremely weak biomagnetic field that changes rapidly, and make the electron spins in the magnetometer only sensitive to high-frequency brain magnetic signals, greatly improving the availability of the atomic magnetometer for brain magnetic measurement, effectively solving the problem of poor brain magnetic signal measurement caused by too large background low-frequency interference magnetic field in current brain magnetic measurement, and hopefully breaking through the limitations of existing brain magnetic measurement under static head state, so that brain magnetism can be effectively measured under the state of head movement.

Description

Technical Field

[0001] The present invention belongs to the technical field of atomic magnetometers and brain magnetism measurement, and specifically relates to a biomagnetic measurement atomic magnetometer system and method that automatically compensates for external interference magnetic fields. The present invention aims to automatically track and compensate for the external low-frequency interference magnetic field of the atomic magnetometer probe system, thereby applying the system to the measurement of brain magnetism and overcoming the defect that the current brain magnetism detection system cannot work under dynamic conditions. Background Art

[0002] With the development of quantum science and technology, an ultra-high sensitive atomic magnetometer (atomic magnetometer) based on the atomic spin exchange collision-free relaxation state (Spin ExchangeRelaxationFreeRegime:SERF) has been proposed and experimentally verified. Its measurement sensitivity exceeds that of the superconducting quantum interference magnetometer, making it the world's most sensitive magnetic field measurement sensor. Due to the ultra-high sensitivity of the atomic magnetometer, it can be used to measure brain magnetism, and it has many advantages over the superconducting quantum interference magnetometer. Since the magnetic field generated by the brain is much smaller than the geomagnetic field, in order to measure brain magnetism, the geomagnetic field needs to be shielded. In addition, the low-frequency variation components in the geomagnetic field also affect the measurement of brain magnetism. For this reason, a magnetic shielding room is generally used for geomagnetic shielding. In order to further reduce the magnetic field of the shielding room, the residual magnetic field is further compensated by placing a compensation coil inside the shielding room. Even so, the signal strength of brain magnetism is still 4 orders of magnitude lower than the residual magnetic field. If the brain magnetism signal of human movement is to be measured, the background magnetic field changes felt by the probe in different shielded rooms are also much greater than the amplitude of the brain magnetism signal. Therefore, it is necessary to develop a method that can compensate for the low-frequency interference magnetic field so that the brain magnetism signal can be detected with noise resistance even in motion. The existing atomic magnetometer brain magnetism detection system only uses compensation coils to dynamically compensate for the interference magnetic field of the brain magnetism probe, which has the problem of poor compensation ability. Summary of the invention

[0003] The present invention provides a biomagnetic measurement atomic magnetometer system and method that automatically compensates for external interference magnetic fields. A magnetic field is generated by hyperpolarized nuclear spins. Under specific conditions, the nuclear spins can automatically track and compensate for external low-frequency interference magnetic fields, so that the electron spins in the magnetometer are only sensitive to high-frequency brain magnetic signals, greatly improving the availability of the atomic magnetometer for brain magnetic measurement.

[0004] To achieve the above-mentioned purpose, the present invention provides a biomagnetic measurement atomic magnetometer system that automatically compensates for external interference magnetic fields, comprising a geomagnetic shielding room, a plurality of atomic magnetometer probes, and a data reading and processing system;

[0005] The atomic magnetometer probe is arranged in the geomagnetic shielding room and is used for scanning and detecting the brain magnetism of the whole brain. The data reading and processing system is located outside the geomagnetic shielding room. The atomic magnetometer probe is connected to the data reading and processing system. The data reading and processing system processes the signals read by the multiple atomic magnetometer probes and images the biomagnetic signals of the whole brain area.

[0006] The atomic magnetometer probe comprises a MEMS alkali metal gas chamber, wherein the MEMS alkali metal gas chamber is filled with alkali metal atoms A, noble gas isotope atoms B and nitrogen C, a first total reflection prism and a second total reflection prism are respectively arranged on both sides of the MEMS alkali metal gas chamber, a main magnetic field coil and a magnetic field modulation coil are arranged on the outer side of the MEMS alkali metal gas chamber, and a second photodetector is arranged below the second total reflection prism; a λ / 4 wave plate, a beam splitter, a polarizer and a first optical lens are arranged in sequence from top to bottom directly below the first total reflection prism, and a first photodetector is arranged on one side of the beam splitter; the first photodetector and the second photodetector are both connected to the input end of a weak photocurrent amplifier;

[0007] The data reading and processing system includes a weak photocurrent amplifier, a signal processing module and a pumping laser. The output end of the weak photocurrent amplifier is connected to the input end of the signal processing module, and the pumping laser is connected to the first optical lens; the signal processing module is used to convert the output signal of the weak photocurrent amplifier into a weak brain magnetic field signal of the brain to be tested.

[0008] Furthermore, the MEMS alkali metal gas chamber includes a top plate, a bottom plate and side walls, the top plate and the bottom plate are light-transmissive, and the side walls of the MEMS alkali metal gas chamber are made of silicon material.

[0009] Furthermore, the alkali metal atom A is an Rb atom, and the noble gas isotope atom B is 131 Xe.

[0010] Furthermore, the data reading and processing system also includes a heating laser, which is connected to a second optical lens via an optical fiber, and the second optical lens is arranged directly below the MEMS alkali metal gas chamber.

[0011] Furthermore, the MEMS alkali metal gas chamber is arranged in the thermal insulation cavity.

[0012] Furthermore, a vacuum heat-insulating sheet is provided in the heat-insulating cavity.

[0013] Furthermore, a magnetic field compensation coil is provided in the geomagnetic shielding room.

[0014] The biomagnetic measurement method based on the above-mentioned atomic magnetometer system comprises the following steps:

[0015] S1: Lower the geomagnetic shielding room to less than 10nT, adjust the pump laser wavelength to the D1 line absorption wavelength of the alkali metal atom A, heat the MEMS alkali metal gas chamber, and make the optical depth of the alkali metal atom A absorbing the pump light be 1 to 3;

[0016] S2: Adjust the pumping laser power of the pumping laser so that the electron spin polarization rate of the alkali metal atom A reaches 40%-60, and when the inert gas isotope atom B reaches a stable state, find the working point of the automatic compensation interference magnetic field, and use this working point as the working point of the system;

[0017] S3: Apply an AC voltage to the magnetic field modulation coil, the modulation frequency is ω m , then the modulation coil will generate an AC magnetic field signal. Under the action of this AC magnetic field, the electron spin of the alkali metal atom A is modulated by the magnetic field, generating an absorption modulation signal of the pumping laser. The frequency of the absorption modulation signal is the same as the modulation frequency, and the magnitude is affected by the magnitude of the AC magnetic field; the absorption modulation signal is finally received by the second photodetector, and the current differential signal of the second photodetector and the first photodetector after current-to-voltage amplification is read. The one-fold frequency signal output by the weak photocurrent amplifier is extracted through the phase-locked loop circuit in the signal processing module, and the reference signal selects the AC magnetic field signal applied to the modulation coil;

[0018] S4: The magnitude of the main magnetic field generated by the main magnetic field coil is adjusted to the sum of the electron spin magnetic field and the nuclear spin magnetic field, and the direction is adjusted to the opposite direction of the nuclear spin magnetic field and the electron spin magnetic field. At this time, the atomic magnetometer probe works in a state of automatic tracking and compensation of the external interference magnetic field, and measurement begins.

[0019] Furthermore, in S2, the process of making the electron spin polarization rate of the alkali metal atom A reach 40%-60% is as follows:

[0020] The total relaxation rate of electron spins under different pumping light powers was measured, and a straight line was measured to show the relationship between the total relaxation rate of electron spins and the pumping light power. The straight line was fitted to obtain the intercept of the line under 0 pumping light power. The relaxation rate at this time is the part of the total relaxation rate of electron spins that does not include the pumping light, recorded as R1. The pumping light power point when the total relaxation rate of electron spins R2 is 2R1 was found. The pumping light power at this time is the point where the electron spin polarization rate reaches 50%.

[0021] Furthermore, in S2, the process of finding the working point of automatic compensation of interference magnetic field is: first set the size of a main magnetic field, then apply a step magnetic field in a direction perpendicular to the main magnetic field and parallel to the magnetic field generated by the modulation magnetic field coil, record the difference dV between the steady-state amplitude of the step response output of the magnetometer and the steady-state amplitude before the step magnetic field is applied, change the size of the main magnetic field, and when dV is 0, the applied main magnetic field is the point for automatically tracking and compensating the external interference magnetic field.

[0022] Compared with the prior art, the present invention has at least the following beneficial technical effects:

[0023] The invention proposes a biomagnetic measurement atomic magnetometer system that automatically tracks and compensates for external interference magnetic fields. A magnetic field is generated by hyperpolarizing nuclear spins, and an external constant magnetic field is applied at the same time. The size of the constant magnetic field makes the nuclear spin feel the same size as the magnetic field generated by itself. At this time, the nuclear spin can automatically track and compensate for the external low-frequency interference magnetic field, protect the electron spin from the intrusion of the external low-frequency interference magnetic field, and make the electron spin only sensitive to the extremely weak biomagnetic field that changes rapidly. The existence of the nuclear spin is similar to a high-pass filter, and the low-frequency interference magnetic field is effectively suppressed, so that the electron spin in the magnetometer is only sensitive to high-frequency brain magnetic signals, which greatly improves the availability of atomic magnetometers for brain magnetic measurement, effectively solves the problem of poor brain magnetic signal measurement caused by too large background low-frequency interference magnetic field in current brain magnetic measurement, and is expected to break through the limitations of existing brain magnetic measurement under static head state, so that brain magnetism can be effectively measured under the state of head movement. It is expected that the brain magnetic detection system based on atomic magnetometers will perform excellent performance under the condition of movement, effectively measure the biomagnetic field of motor neurons, and greatly improve the availability of atomic spin magnetometers.

[0024] In order to achieve the condition that the nuclear spin can automatically track and compensate for the external interference magnetic field, the present invention proposes an integrated device of an atomic spin magnetometer, so that the gas chamber, heating, thermal insulation, optical path and detection system are highly integrated and small in size.

[0025] Furthermore, the data reading and processing system also includes a heating laser, which is connected to a second optical lens through an optical fiber. The second optical lens is arranged directly below the MEMS alkali metal gas chamber. The temperature of the MEMS alkali metal gas chamber is adjusted by controlling the light intensity of the heating laser. The advantage of laser heating is that it does not generate electromagnetic interference, thereby ensuring that no background noise signal is generated to drown out weak brain magnetic signals.

[0026] Furthermore, the MEMS alkali metal gas chamber is arranged in a thermal insulation cavity, and the thermal insulation cavity is used to maintain the temperature of the MEMS alkali metal gas chamber, isolate the dissipation of heat, and reduce the total power consumption of the system.

[0027] Furthermore, a vacuum heat-insulating sheet is provided in the heat-insulating cavity to isolate the heat of the MEMS alkali metal gas chamber from heat conduction, thereby reducing the power consumption of the entire system.

[0028] Furthermore, a magnetic field compensation coil is provided in the geomagnetic shielding room, and the magnetic field compensation coil is used to further compensate for the residual magnetic field inside the geomagnetic shielding room to improve the accuracy of the measurement.

[0029] The method described in the present invention, by applying an external constant magnetic field that makes the nuclear spin feel the same magnitude as the magnetic field generated by itself, achieves the condition that the nuclear spin can automatically track and compensate for the external interference magnetic field, and protects the electron spin from the intrusion of the external low-frequency interference magnetic field. The present invention is expected to enable the brain magnetic detection system based on the atomic magnetometer to perform excellent performance under motion conditions and effectively measure the biomagnetic field of motor neurons. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 This is a schematic diagram of the experimental device of the atomic magnetometer system for biomagnetic measurement that automatically tracks and compensates for external interference magnetic fields;

[0031] Figure 2 This is a simplified diagram of the atomic magnetometer probe structure;

[0032] Figure 3 It is a schematic diagram of the MEMS alkali metal gas chamber structure, the types of atoms filled into it, and the magnetic field generated by atomic spin.

[0033] In the attached figure: 1. geomagnetic shielding room, 2. brain to be tested, 3. atomic magnetometer probe, 4. data reading and processing system, 5. magnetic field compensation coil, 6. vacuum insulation system glass window, 7. first total reflection prism, 8. second total reflection prism, 9. MEMS alkali metal gas chamber, 10. thermal insulation sheet, 11. thermal insulation cavity, 12. main magnetic field coil, 13. second photodetector, 14. heating laser, 15. first optical lens, 16. second optical lens, 17. weak photocurrent amplifier, 18. signal processing module, 19. magnetic field modulation coil, 20. spectrometer, 21. first photodetector, 22. polarizer, 23. λ / 4 wave plate, 24. main magnetic field, 25. electron spin magnetic field, 26. nuclear spin magnetic field, 27. pumping laser, 28. optical fiber. DETAILED DESCRIPTION

[0034] In order to make the purpose and technical solution of the present invention clearer and easier to understand, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. The specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0035] In the description of the present invention, it should be understood that the terms "center", "longitudinal", "lateral", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside" and the like indicate positions or positional relationships based on the positions or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as limiting the present invention. In addition, the terms "first" and "second" are used only for descriptive purposes, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Thus, the features defined as "first" and "second" may explicitly or implicitly include one or more of the features. In the description of the present invention, unless otherwise specified, "multiple" means two or more. In the description of the present invention, it should be noted that, unless otherwise clearly specified and limited, the terms "installed", "connected", and "connected" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0036] The present invention discloses a biomagnetic atomic magnetometer that automatically tracks and compensates for external interference magnetic fields. According to the characteristics of the biomagnetic field, firstly, it is extremely weak, with a magnetic field strength of about 1pT. Secondly, the background magnetic field where the biomagnetic field is located is often 8 orders of magnitude larger than the biomagnetic field itself. Therefore, how to extract the weak biomagnetic field is the core research content of biomagnetic measurement. On the basis of magnetic shielding based on high magnetic permeability materials and compensation coils, how to further shield the low-frequency changing magnetic field is the core of biomagnetic detection.

[0037] Reference Figure 1 A biomagnetic measurement atomic magnetometer system for automatically tracking and compensating for external interference magnetic fields includes a geomagnetic shielding room 1, multiple atomic magnetometer probes 3, a data reading and processing system 4, and a magnetic field compensation coil 5.

[0038] Magnetic field compensation coils 5 are arranged on both sides of the geomagnetic shielding room 1, and the magnetic field compensation coils 5 are used to further compensate for the residual magnetic field inside the geomagnetic shielding room. Atomic magnetometer probes 3 are arranged in the geomagnetic shielding room 1, and the brain area to be tested covers multiple atomic magnetometer probes 3, which are used for scanning and detecting the brain magnetism of the whole brain. The data reading and processing system 4 is located outside the geomagnetic shielding room 1, and the atomic magnetometer probes 3 are connected to the data reading and processing system 4. The data reading and processing system 4 processes the signals read by the multiple atomic magnetometer probes 3 and images the biomagnetic signals of the whole brain area.

[0039] like Figure 2 As shown, the atomic magnetometer probe 3 comprises: a first optical lens 15, a polarizer 22, a λ / 4 wave plate 23, a beam splitter 20, a first photodetector 21, a vacuum insulation system glass window 6, a first total reflection prism 7, a second total reflection prism 8, a MEMS alkali metal gas chamber 9, a heat insulation sheet 10, a heat insulation cavity 11, a main magnetic field coil 12, a magnetic field modulation coil 19, a second photodetector 13 and a second optical lens 16. Among them, the MEMS alkali metal gas chamber 9 has a nuclear spin magnetic field 26, an electron spin magnetic field 25 and a main magnetic field 24.

[0040] The data reading and processing system 4 includes a weak photocurrent amplifier 17 , a signal processing module 18 , a pumping laser 27 and a heating laser 14 . The output end of the weak photocurrent amplifier 17 is connected to the input end of the signal processing module 18 .

[0041] One side of the thermal insulation cavity 11 is a glass window 6 of a vacuum insulation system, a MEMS alkali metal gas chamber 9 is located in the thermal insulation cavity 11, and a first total reflection prism 7 and a second total reflection prism 8 are arranged opposite to each other on both sides of the MEMS alkali metal gas chamber 9; a main magnetic field coil 12 is attached to the outside of the thermal insulation cavity 11 to provide a DC main magnetic field for the atoms in the MEMS alkali metal gas chamber 9, and the main magnetic field is used to make the atomic magnetometer work in a state of magnetic field self-compensation, so that the inert gas nuclear spin atoms feel a magnetic field that is equal in magnitude to the magnetic field generated by themselves and opposite in direction; a magnetic field modulation coil 19 is arranged on the outside of the MEMS alkali metal gas chamber 9, and the magnetic field modulation coil 19 is used to generate an AC magnetic field perpendicular to the paper surface, and the AC magnetic field is used to modulate the precession direction of the electron spins of the alkali metal atoms, so that the atomic magnetometer is in a correct detection state;

[0042] The inside of the MEMS alkali metal chamber 9 is filled with alkali metal atoms A, inert gas isotope atoms B and nitrogen C; the thermal insulation sheet 10 is attached to the MEMS alkali metal chamber 9 in a vacuum to isolate heat conduction; the λ / 4 wave plate 23, the first optical lens 15, the second optical lens 16, the first photodetector 21, the second photodetector 13, the polarizer 22 and the beam splitter 20 are all located on one side of the vacuum insulation system glass window 6 of the thermal insulation cavity 11; the second optical lens 16 is located directly below the MEMS alkali metal chamber 9 and is connected to the heating laser 14 through the optical fiber 28. The polarizer 22, the beam splitter 20 and the λ / 4 wave plate 23 are arranged from bottom to top between the first optical lens 15 and the vacuum insulation system glass window 6, and the first photodetector 21 is arranged on the outside of the beam splitter 20. The laser emitted by the heating laser 14 enters the second optical lens 16 through the optical fiber 28, and heats the MEMS alkali metal gas chamber 9 after being collimated by the second optical lens 16. The temperature of the MEMS alkali metal gas chamber 9 is adjusted by controlling the light intensity.

[0043] The main structure of the MEMS alkali metal chamber 9 includes a top plate, a bottom plate and a side wall, which are composed of glass and silicon. The atomic packaging is completed by a glass-silicon bonding process. The top plate and the bottom plate at both ends of the MEMS alkali metal chamber 9 are transparent and made of glass. The side wall of the MEMS alkali metal chamber 9 is made of silicon material, and the silicon material cavity structure is cylindrical or cubic, such as Figure 3 As shown, three kinds of atoms are filled into the cavity, namely A (preferably Rb atoms), B (preferably 131 Xe) and C (select N2), the charging amount of B is preferably 5-20Torr, and the charging amount of C is preferably 500-760Torr; the diameter or side length of the light-transmitting glass surface is between 2mm-5mm, and the distance between the two glass surfaces is between 2mm-5mm. The atom A polarized by the pumping laser generates an electron spin magnetic field 25, and the inert gas nuclear spin B hyperpolarized by the polarized electron spin generates a nuclear spin magnetic field 26. The main magnetic field coil 12 generates a main magnetic field 24 of the sum of the electron spin magnetic field 25 and the nuclear spin magnetic field 26, and the direction is opposite to the nuclear spin magnetic field 26 and the electron spin magnetic field 25. The magnitudes of the electron spin magnetic field 25 and the nuclear spin magnetic field 26 are obtained through the coupling dynamic process of the electron spin and the nuclear spin. When the dynamic response time of the coupling system is the shortest, the magnitude B1 of the main magnetic field 24 is the magnitude of the nuclear spin magnetic field 26. The magnitude of the nuclear spin magnetic field 26 can be obtained by reading the magnetic field B1 applied by the main magnetic field coil 24. The magnitude of the main magnetic field coil 24 is adjusted. When the magnetic field self-compensation state is reached, the magnetic field B2 applied by the main magnetic field coil is the sum of the nuclear spin magnetic field 26 and the electron spin magnetic field 25. Therefore, the electron spin magnetic field 25 is B2-B1.

[0044] The heating laser power can be controlled by controlling the current of the heating laser 14. After passing through the second optical lens 16, the heating laser 14 is incident on the MEMS alkali metal gas chamber 9 in parallel. The wavelength of the heating laser 14 is preferably 1550nm. After the silicon material absorbs the light, it is converted into heat energy, so that the temperature of the MEMS alkali metal gas chamber 9 reaches between 100℃ and 200℃.

[0045] The thermal insulation cavity 11 is used to maintain the temperature of the MEMS alkali metal gas chamber 9, isolate the dissipation of heat, and reduce the total power consumption of the system. One side of the thermal insulation cavity is a vacuum insulation system glass window 6. During packaging, the vacuum degree inside the thermal insulation cavity 11 is ensured to be less than 100Pa, which effectively reduces the heat loss caused by heat conduction. The thermal insulation sheet 10 in the vacuum is attached to one side of the MEMS alkali metal gas chamber 9 to isolate the heat of the MEMS alkali metal gas chamber 9 from heat conduction to 11.

[0046] Adjust the temperature of the MEMS alkali metal gas chamber 9 inside the thermal insulation cavity 11. When the optical depth of the A atom absorbing the pumping light is 1 to 3, adjust it to the required working temperature. Tune the wavelength of the pumping laser 27 to the A atom D1 line resonance absorption peak. The laser output by the pumping laser 27 is introduced into the atomic magnetometer probe through the optical fiber 28. After being shaped by the first optical lens 15, the parallel light output is converted into linear polarized light after entering the polarizer 22. After that, part of the light is reflected by the beam splitter 20 and enters the first photodetector 21. Part of the light is transmitted into the λ / 4 wave plate 23 and converted into parallel circularly polarized light. The circularly polarized light passes through the vacuum insulation system glass window 6 and is incident on the first total reflection prism 7. After being folded back 90 degrees, it enters the MEMS The alkali metal gas chamber 9 is reflected again by the second total reflection prism 8 and then enters the second photodetector 13 to be converted into a current signal, that is, the second photodetector 13 receives the light intensity of the transmitted pumping laser, and the first photodetector 21 receives the incident laser sampling light intensity. The two light intensities are converted into photocurrents and simultaneously input into the weak photocurrent amplifier 17 for photocurrent differentiation, so that the output DC photocurrent signal is 0. After that, it is converted into a voltage output signal after weak current amplification, and is converted into the final weak brain magnetic field signal of the brain 2 to be tested through the signal processing circuit 18.

[0047] The main magnetic field coil 12 and the magnetic field modulation coil 19 generate two sets of orthogonal magnetic fields. The magnetic field modulation coil 19 provides an alternating magnetic field for the MEMS alkali metal gas chamber 9, and its direction is perpendicular to the main magnetic field direction and perpendicular to the paper surface, thereby modulating the direction of the electron spin A, and finally achieving the effect of detecting the biomagnetic field such as brain magnetism in the direction perpendicular to the paper surface. The main magnetic field coil 12 provides a DC bias magnetic field, and the magnetic field direction is along the pumping light direction. The alternating magnetic field is used to modulate the precession direction of the electron spin of the alkali metal atoms (mainly to enable the atomic magnetometer composed of the pumping light and the electron spin A generated by the pumping laser 27 to measure the magnetic field correctly and accurately). The specific principle and implementation method are shown in step S3. The amplitude of the alternating magnetic field is 100-2000nT. The electron spin of the A atom is modulated by the magnetic field, and the light intensity of the transmitted pumping light is also modulated. The signal after amplification of the photodetector is read, and the one-fold frequency signal after the weak current amplifier is extracted through the phase-locked loop circuit in the signal processing circuit. The reference signal selects the alternating magnetic field signal applied to the magnetic field modulation coil. Correctly applying the DC main magnetic field can make the system work in a state of automatic tracking and compensating the magnetic field. For the specific implementation method, please see step S2. Then the system works in a normal state and can resist interfering low-frequency magnetic fields and perform normal sensitive measurements on high-frequency brain magnetic signals. The DC magnetic field size is between -2000nT and 2000nT. The size of the applied bias magnetic field is crucial to realizing the function of automatically tracking and compensating for external interfering magnetic fields.

[0048] The principle of the present invention is as follows:

[0049] Assume that the density of atoms A in the MEMS alkali metal gas chamber 9 is n A , the density of atom B is n B , atom A hyperpolarizes atom B through spin exchange collision interaction. In a stable state, it is assumed that the nuclear spin magnetic field 26 generated by the hyperpolarized atom B and felt by atom A is B n , and the equivalent electron spin magnetic field 25 produced by atom A and felt by atom B is B e , the above two magnetic fields are in the direction of the pumping light. At the same time, the main magnetic field 24 is applied by the main magnetic field coil 12 in the direction of the pumping light, which is -(B n +B e ), then atom B feels a magnetic field that is equal in magnitude and opposite in direction to the magnetic field generated by itself. In this state, the nuclear spin can automatically track and compensate for external interfering magnetic fields, such as the changing magnetic field inside the shielding tube caused by changes in the geomagnetic field in the direction perpendicular to the paper.

[0050] Assume that the external input interference magnetic field is B y , assuming that the direction perpendicular to the paper is the y direction, and assuming that the pumping laser direction is the z direction. In this state, the nuclear spin of atom B will stabilize in a new direction and generate a projected magnetic field in the y direction. The magnitude of the projected magnetic field is proportional to B. y equal and opposite in direction, that is, the nuclear spin is affected by the external B y The electron spin will not be able to sense B y Since the dynamic range of nuclear spins can be adjusted, that is, the bandwidth of their compensation magnetic field can be adjusted, the bandwidth of nuclear spins can be adjusted so that nuclear spins can compensate for interfering magnetic fields below a certain frequency, but not for magnetic fields above this frequency. For electron spins, they are sensitive to magnetic fields with high-frequency components. Therefore, this method can be used to compensate for low-frequency interfering magnetic fields, and normal measurements can be performed on higher-frequency components such as brain magnetism. This is the so-called method of automatically tracking and compensating for external interfering magnetic fields.

[0051] A biomagnetic measurement method for automatically tracking and compensating for an external interfering magnetic field, the method comprising the following steps:

[0052] S1: Adjust the current of the magnetic field compensation coil 5 so that the magnetic field in the area and vicinity of the brain 2 to be tested is fully reduced to less than 10nT, and adjust the wavelength of the pump laser 27 to the D1 line absorption wavelength of the alkali metal atom A. At this time, the interaction between the laser and the atom is the strongest, and the output signal is the strongest. Adjust the power of the heating laser 14 so that the optical depth of the A atom inside the MEMS alkali metal gas chamber 9 absorbing the pump light is 1 to 3, that is, adjust to the required operating temperature;

[0053] S2: Adjust the pumping laser power of the pumping laser 27 so that the electron spin polarization rate of the alkali metal A reaches 40%-60% (preferably the point where the electron spin polarization rate of the alkali metal A reaches 50%), measure the total electron spin relaxation rate under different pumping light powers, measure a straight line between the total electron spin relaxation rate and the pumping light power, fit the straight line, and obtain the intercept of the straight line under 0 pumping light power. The relaxation rate at this time is the part of the total electron spin relaxation rate that does not include the pumping light, recorded as R1, and find the pumping light power point when the total electron spin relaxation rate R2 is 2R1. The pumping light power at this time is the point where the electron spin polarization rate reaches 50%.

[0054] When the nuclear spin B is pumped to a stable state, the working point of the automatic compensation interference magnetic field is searched. The idea of ​​searching the self-compensation working point is to apply an interference magnetic field. When the system does not respond to the DC magnetic field, the nuclear spin automatically tracks and compensates for the external interference magnetic field. The self-compensation state is related to the size of the main magnetic field 24. For this purpose, the size of the main magnetic field 24 is first set. At this time, a step magnetic field is applied in a direction perpendicular to the main magnetic field and parallel to the magnetic field generated by the modulation magnetic field coil 19. This step magnetic field can be regarded as an interfering DC magnetic field. The difference dV between the steady-state amplitude of the step response output of the magnetometer and the steady-state amplitude before the step magnetic field is applied is recorded. The size of the main magnetic field 24 is changed. When dV is 0, the applied main magnetic field 24 is the point of automatic tracking and compensation for the external interference magnetic field, which is the working point of the system.

[0055] S3: Apply an AC voltage to the magnetic field modulation coil 19 through the signal source, and the modulation frequency is ω m , then the modulation coil 19 will generate an AC magnetic field signal. Under the action of this AC magnetic field, the electron spin of atom A is modulated by the magnetic field, and the electron spin polarization rate in the direction of the pumping light will be ω m and 2ω m The frequency of the pump light is modulated. Since the amount of pump light absorbed is proportional to the electron spin polarization rate in the direction of the pump light, when the atomic spin is modulated by an AC magnetic field, the polarization rate of the atomic spin will change sinusoidally in the direction of the pump light, including a frequency component with the same frequency as the modulation frequency and a signal component twice the frequency of the modulation signal. Therefore, an absorption modulation signal of the pump laser will be generated. The frequency of this signal also includes ω m and 2ω mThe absorption modulated signal is finally received by the second photodetector 13 for further signal processing. The absorption modulated signal received by the second photodetector 13 is differentiated from the light intensity signal received by the first photodetector 22, where the sampling light intensity signal received by the first photodetector 21 is the laser emitted by the pumping laser 27 sampled by the beam splitter 20 after the optical lens 15, and the sampling light intensity signal is equal to the DC component of the absorption modulated signal received by the second photodetector 13, which is the so-called photocurrent difference. The differential light absorption modulated signal only contains the frequency ω m and 2ω m The photocurrent amplifier 17 reads the current differential signal of the second photodetector 13 and the first photodetector 22 after current-to-voltage amplification, and extracts the amplitude V of the phase X part of the one-fold frequency signal output by the weak photocurrent amplifier 17 through the phase-locked loop circuit in the signal processing module 18. x (One-fold signal frequency and ω m The reference signal entering the phase-locked loop selects the AC magnetic field signal applied to the modulation coil 19, and the frequency is ω m . Adjust the phase of the phase-locked loop so that V x By changing the voltage input to the modulation coil, the amplitude of the modulation magnetic field is adjusted, so that under the same input magnetic field to be measured, the output V x Maximum, that is, the optimization of the modulation magnetic field amplitude is completed, and the magnetometer works at the optimal modulation magnetic field amplitude point;

[0056] S4: The magnitude of the main magnetic field 24 generated by the main magnetic field coil 12 is adjusted to the sum of the electron spin magnetic field 25 and the nuclear spin magnetic field 26, and the direction is adjusted to the opposite direction of the nuclear spin magnetic field 26 and the electron spin magnetic field 25, and the brain magnetic signal of the brain 2 to be measured is measured through the atomic magnetometer probe 3, wherein the vacuum insulation cavity 11 of the atomic magnetometer probe 3 will directly contact the brain, and the atomic magnetometer probe 3 works in a state of automatically tracking and compensating for external interference magnetic fields. It is insensitive to low-frequency changing interference magnetic fields, but sensitive to higher-frequency weak biological magnetic fields such as brain magnetism, that is, an atomic magnetometer system that automatically tracks and compensates for external interference low-frequency magnetic fields is realized. The weak magnetic field signal measurement results of brain magnetism will be given by the signal processing module 18 and input into the computer to restore the brain magnetic image signal.

[0057] The following is an explanation of the ability to compensate for the interfering magnetic field. Assume that an interfering magnetic field B is applied in the y direction. y Cos(ωt), then both the electron spin and the nuclear spin will respond to this interfering magnetic field, and there is coupling between the electron spin and the nuclear spin, and the detection system only detects the electron spin, so the response of the nuclear spin to this AC magnetic field also exists in the final output. The magnitude of the magnetic field felt by the nuclear spin is Bn , so define the characteristic frequency ω n ,ω n＝ γ n B n , where γ n is the gyromagnetic ratio of the nuclear spin. In the state of self-compensation of the nuclear spin magnetic field, the magnetic field B of the equivalent input system eff for:

[0058]

[0059] Among them, c is a constant factor, which is mainly related to the relaxation rate of the electron spin and the magnetic field generated by the electron spin. From the above formula, it can be seen that when the frequency of the interfering magnetic field tends to 0, the input interfering magnetic field is attenuated by ω / ω n , that is, under the input of constant interference magnetic field, the frequency tends to 0, and the equivalent magnetic field input is 0, that is, the constant magnetic field is suppressed. For high-frequency magnetic field, the suppression ability will become worse, and the sensitivity of high-frequency magnetic field measurement can be maintained.

[0060] Assume that the modulation magnetic field of the modulation coil in the y direction is B m Cos(ω m t), due to the effect of the modulation magnetic field, in the stable state, the polarization rate of the electron spin in the z direction will also be modulated, and the modulation frequency includes ω m and 2ω m The two components, and then the intensity of the pump light will also be modulated, because the intensity of the pump light absorbed is proportional to the electron spin polarization rate P in the z direction A, through the signal processing module 18, mainly a phase-locked loop circuit. m Cos(ω m t) The modulation signal is the reference signal, and the absorption signal of the pumping laser is demodulated. After demodulation, ω m The amplitude V of the doubled frequency signal and the input modulation magnetic field B y The relationship of Cos(ωt) is:

[0061]

[0062] Among them, P z e is the polarizability of the electron spin of atom A in the z direction, which is generally 0.5, γ e is the gyromagnetic ratio of the electron spin, R tot is the total relaxation rate of the electron spin of atom A, including spin destruction collision relaxation, spin exchange collision relaxation and optical pumping rate. k1 is a coefficient, which indicates the attenuation of the output equivalent electron spin polarization rate due to modulation, which is generally around 0.8. k2 is the coefficient of the electron spin polarization rate converted to light absorption, which is related to the conversion efficiency of the photodetector, the current-voltage amplification factor of the photodetector amplifier, etc. is the phase difference between the output signal and the input interfering low-frequency magnetic field. By demodulating the amplitude V of the one-fold frequency signal, where the reference signal is B m Cos(ω m t), the reading V1 of the in-phase term (X) of the lock-in amplifier can be obtained as:

[0063]

[0064] It can be seen from the above formula that the output of the same phase term of the lock-in amplifier in the signal processing module 18 is the output signal of the system and the input B y Proportional to the magnetic field in the y direction, that is, the weak brain magnetic signal that needs to be measured (magnetic field in the y direction) can be measured.

[0065] Those skilled in the art can understand that the above are only preferred examples of the invention and are not intended to limit the invention. Although the invention is described in detail with reference to the above examples, those skilled in the art can still modify the technical solutions recorded in the above examples or replace some of the technical features therein with equivalents. Any modification, equivalent replacement, etc. made within the spirit and principle of the invention shall be included in the protection scope of the invention.

[0066] The above contents are only for explaining the technical idea of ​​the present invention and cannot be used to limit the protection scope of the present invention. Any changes made on the basis of the technical solution in accordance with the technical idea proposed by the present invention shall fall within the protection scope of the claims of the present invention.

Claims

1. A biomagnetic measurement atomic magnetometer system that automatically compensates for external interference magnetic fields, characterized in that: It comprises a geomagnetic shielding room (1), a plurality of atomic magnetometer probes (3) and a data reading and processing system (4); The atomic magnetometer probe (3) is arranged in the geomagnetic shielding room (1) and is used for scanning and detecting the brain magnetism of the entire brain. The data reading and processing system (4) is located outside the geomagnetic shielding room (1). The atomic magnetometer probe (3) and the data reading and processing system (4) are connected. The data reading and processing system (4) processes the signals read by the multiple atomic magnetometer probes (3) and images the biomagnetic signals of the entire brain area. The atomic magnetometer probe (3) comprises a MEMS alkali metal gas chamber (9), wherein the MEMS alkali metal gas chamber (9) is filled with alkali metal atoms A, noble gas isotope atoms B and nitrogen C; a first total reflection prism (7) and a second total reflection prism (8) are respectively arranged on both sides of the MEMS alkali metal gas chamber (9); a main magnetic field coil (12) and a magnetic field modulation coil (19) are arranged outside the MEMS alkali metal gas chamber (9); a second photodetector (13) is arranged below the second total reflection prism (8); a λ / 4 wave plate (23), a beam splitter (20), a polarizing plate (22) and a first optical lens (15) are arranged in sequence from top to bottom directly below the first total reflection prism (7); a first photodetector (21) is arranged on one side of the beam splitter (20); the first photodetector (21) and the second photodetector (13) are both connected to the input end of a weak photocurrent amplifier (17); The data reading and processing system (4) comprises a weak photocurrent amplifier (17), a signal processing module (18) and a pumping laser (27), wherein the output end of the weak photocurrent amplifier (17) is connected to the input end of the signal processing module (18), and the pumping laser (27) is connected to the first optical lens (15); the signal processing module (18) is used to convert the output signal of the weak photocurrent amplifier (17) into a weak brain magnetic field signal of the brain to be measured (2).

2. The biomagnetic measurement atomic magnetometer system for automatically compensating for external interference magnetic fields according to claim 1, characterized in that: The MEMS alkali metal chamber (9) comprises a top plate, a bottom plate and a side wall, the top plate and the bottom plate are light-transmissive, and the side wall of the MEMS alkali metal chamber (9) is made of silicon material.

3. The biomagnetic measurement atomic magnetometer system for automatically compensating for external interference magnetic fields according to claim 1 or 2, characterized in that: The alkali metal atom A is Rb atom, and the noble gas isotope atom B is 131 Xe.

4. The biomagnetic measurement atomic magnetometer system for automatically compensating for external interference magnetic fields according to claim 1, characterized in that: The data reading and processing system (4) further comprises a heating laser (14), wherein the heating laser (14) is connected to a second optical lens (16) via an optical fiber, and the second optical lens (16) is arranged directly below the MEMS alkali metal gas chamber (9).

5. The biomagnetic measurement atomic magnetometer system for automatically compensating for external interference magnetic fields according to claim 1, characterized in that: The MEMS alkali metal gas chamber (9) is arranged in a thermal insulation cavity (11).

6. The biomagnetic measurement atomic magnetometer system for automatically compensating for external interference magnetic fields according to claim 5, characterized in that: A vacuum heat-insulating sheet (10) is arranged in the heat-insulating cavity (11).

7. The biomagnetic measurement atomic magnetometer system for automatically compensating for external interference magnetic fields according to claim 1, characterized in that: A magnetic field compensation coil (5) is arranged in the geomagnetic shielding room (1).

8. The biomagnetic measurement method based on the atomic magnetometer system according to claim 1, characterized in that: The following steps are involved: S1: lowering the geomagnetic shielding room (1) to less than 10 nT, adjusting the wavelength of the pumping laser (27) to the D1 line absorption wavelength of the alkali metal atom A, heating the MEMS alkali metal gas chamber (9), and making the optical depth of the alkali metal atom A absorbing the pumping light be 1 to 3; S2: adjusting the pumping laser power of the pumping laser (27) so that the electron spin polarization rate of the alkali metal atom A reaches 40%-60, and finding the automatic compensation interference magnetic field working point when the inert gas isotope atom B reaches a stable state, and using this working point as the working point of the system; S3: Apply an AC voltage to the magnetic field modulation coil (19) with a modulation frequency of ω m , the modulation coil (19) will generate an AC magnetic field signal. Under the action of the AC magnetic field, the electron spin of the alkali metal atom A is modulated by the magnetic field, generating an absorption modulation signal of the pumping laser. The frequency of the absorption modulation signal is the same as the modulation frequency, and the magnitude is affected by the magnitude of the AC magnetic field. The absorption modulation signal is finally received by the second photodetector (13), and the current differential signal of the second photodetector (13) and the first photodetector (22) after current-to-voltage amplification is read. The phase-locked loop circuit in the signal processing module (18) extracts the frequency-doubled signal output by the weak photocurrent amplifier (17), and the reference signal selects the AC magnetic field signal applied to the modulation coil (19); S4: The magnitude of the main magnetic field (24) generated by the main magnetic field coil (12) is adjusted to the sum of the electron spin magnetic field (25) and the nuclear spin magnetic field (26), and the direction is adjusted to the opposite direction of the nuclear spin magnetic field (26) and the electron spin magnetic field (25). At this time, the atomic magnetometer probe works in a state of automatically tracking and compensating for the external interference magnetic field, and measurement begins.

9. The biomagnetic measurement method according to claim 8, characterized in that: In S2, the process of making the electron spin polarization rate of the alkali metal atom A reach 40%-60% is as follows: The total relaxation rate of electron spins under different pumping light powers was measured, and a straight line was measured to show the relationship between the total relaxation rate of electron spins and the pumping light power. The straight line was fitted to obtain the intercept of the line under 0 pumping light power. The relaxation rate at this time is the part of the total relaxation rate of electron spins that does not include the pumping light, recorded as R1. The pumping light power point when the total relaxation rate of electron spins R2 is 2R1 was found. The pumping light power at this time is the point where the electron spin polarization rate reaches 50%.

10. The biomagnetic measurement method according to claim 8, characterized in that: In S2, the process of finding the working point of automatically compensating the interference magnetic field is as follows: first, the size of a main magnetic field (24) is set, and then a step magnetic field is applied in a direction perpendicular to the main magnetic field and parallel to the magnetic field generated by the modulation magnetic field coil (19), and the difference dV between the step response output steady-state amplitude of the magnetometer and the steady-state amplitude before the step magnetic field is applied is recorded, and the size of the main magnetic field (24) is changed. When dV is 0, the applied main magnetic field (24) is the point for automatically tracking and compensating the external interference magnetic field.

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