Atomic magnetometer for ultra-low-intensity magnetic field measurement based on hyperpolarization 3He

Abstract

The invention discloses an atomic magnetometer for ultra-weak magnetic field measurement based on hyperpolarization 3He. The device comprises a laser device used for emitting laser with the wavelength close to 1083 nm; the automatic shutter is used for controlling the laser to pass through; the optical lens group comprises a beam expanding device, a wave plate and a reflecting mirror; the metastable state activation circuit is used for providing excitation energy, so that atoms in the 3He gas chamber are transited from a ground state to a metastable state and are maintained to work stably; the multi-axis Helmholtz coil can apply an accurate and controllable multi-directional magnetic field; the at least one magnetometer is used for measuring Larmor precession signals of 3He atoms; the 3He gas chamber is filled with 3He gas; the magnetic shielding device has a multi-layer shielding structure. Compared with a traditional atomic magnetometer which is mostly used for measuring the relative change value of a large magnetic field or a low magnetic field, the device can be used for measuring the size of an extremely weak magnetic field, and even can measure the size of an absolute magnetic field close to a zero field.

Description

Technical Field

[0001] The present invention relates to an atomic magnetometer for measuring extremely weak magnetic fields, and particularly to an atomic magnetometer for measuring extremely weak magnetic fields based on hyperpolarized 3 He. Background Art

[0002] An atomic magnetometer is a precision instrument for measuring magnetic fields based on the splitting effect of atomic energy levels in a magnetic field. It has the advantages of high sensitivity, high precision, good stability, etc., and has a wide range of applications in fields such as geophysical exploration, biomedical imaging, and basic physical research.

[0003] In many applications, especially in the measurement of extremely weak magnetic field environments, the accuracy of measurement is crucial. For this reason, many atomic magnetometers for measuring extremely weak magnetic field environments have been proposed. Among them, in an Optical Pumping Magnetometer (OPM), circularly polarized light (optical pumping) is used to excite atoms to specific energy levels, making the atomic spins polarized. In an external magnetic field, the atomic energy levels will undergo Zeeman splitting, and the magnetic field strength is measured by detecting the frequency of atomic spin precession, but it is difficult to measure the exact value in an extremely weak magnetic field.

[0004] In addition, in a Spin-Exchange Relaxation Free (SERF) magnetometer, the spin-exchange collision effect of alkali metal atoms (such as potassium, rubidium, cesium) at extremely low magnetic fields is utilized. Atomic spins are polarized through optical pumping, and the spin precession frequency is detected to measure the magnetic field. This type of magnetometer has extremely high resolution, but has high requirements for the environment, requires precise temperature control and low vibration. It can accurately measure the relative change in the magnetic field, but it is difficult to measure the exact value of the absolute magnetic field.

[0005] There is also a Cold Atom Magnetometer (CAM) that uses laser cooling technology to cool atoms to extremely low temperatures and detects the magnetic field by measuring the energy level splitting of cold atoms in an external magnetic field. Its sensitivity is extremely high (up to the fT / Hz 1 / 2 order of magnitude), but its equipment is complex and requires a laser cooling system.

[0006] There is also a Superconducting Quantum Interference Device (SQUID) that is based on the superconducting quantum interference effect and detects the magnetic field by measuring the change in magnetic flux in a superconducting ring. It can also achieve extremely high sensitivity, but its cost is high and it requires liquid helium or liquid nitrogen cooling. Summary of the Invention

[0007] The purpose of this application is to improve the accuracy and stability of the measurement results of atomic magnetometers for extremely weak magnetic fields.

[0008] The present invention includes:

[0009] A laser for emitting laser light with a wavelength near 1083 nm, whose power is adjustable, and the optical head thereof is combined with a beam expander to make the emitted laser light spot reach a certain size;

[0010] An automated shutter for controlling the passage of laser light, which can be opened and closed by being triggered by a signal preset by a signal generator;

[0011] An optical mirror group includes a beam expander, a wave plate and a mirror. The beam expander is used to expand the laser light spot to a larger size, the wave plate is used to change the linearly polarized pump light into circularly polarized light, and the circularly polarized light is reflected by the mirror after passing through 3 the He gas cell and then irradiated onto 3 the He gas cell again;

[0012] A metastable activation circuit for providing excitation energy to make 3 the atoms in the He gas cell transition from the ground state to the metastable state and maintain their stable operation;

[0013] A multi-axis Helmholtz coil capable of applying precisely controllable multi-directional magnetic fields to provide stable controllable magnetic field conditions for experiments;

[0014] At least one magnetometer for measuring 3 the Larmor precession signal of He atoms;

[0015] 3 A He gas cell filled with 3 He gas, whose shape and size can be adjusted according to needs and is placed in the center of the multi-axis Helmholtz coil;

[0016] A magnetic shielding device with a multi-layer shielding structure, and the entire experimental device is encapsulated therein.

[0017] Compared with the prior art, the advantages of this application are:

[0018] 1. An automated shutter with extremely fast response can quickly control the passage of laser light by opening or closing the shutter blades at a set time point. Its switching time is extremely short, less than 10 ms, which can minimize the influence brought by laser switching when measuring data to the greatest extent.

[0019] 2. Compared with traditional atomic magnetometers that are mostly used to measure large magnetic fields or relative change values of low magnetic fields, this device can be applied to measure the magnitude of extremely weak magnetic fields, and can even measure the absolute magnetic field magnitude close to zero field.

[0020] 3. An adjustable metastable activation circuit is designed to enable 3 He atoms to stably and highly transition to the metastable state; and a five-layer shielding cylinder is used to isolate the interference of external magnetic fields and noise

[0021] 4. Compared with traditional atomic magnetometers, this device does not require complex conditions, such as heating or cooling, and reduces the influence from other conditions.

[0022] 5. Based on the hyperpolarized 3 He-designed experimental system, the system structure is simple and reliable, and 3 He has a high gyromagnetic ratio and an extremely long relaxation time, so the measured magnetic field value is relatively accurate and the stability is good. Brief Description of the Drawings

[0023] Figure 1 As shown, it is a schematic diagram of an atomic magnetometer for measuring extremely weak magnetic fields based on hyperpolarized 3 He

[0024] Figure 2 As shown, it is the structure of a high-voltage excitation circuit based on an adjustable duty cycle

[0025] Figure 3 Overall design scheme of the high-frequency excitation circuit for the gas chamber

[0026] Figure 4 Schematic diagram of the Larmor precession of the magnetic moment and angular momentum of helium atoms under an external magnetic field Detailed Implementation Modes

[0027] The technical solutions of this application will be described in detail below with reference to the drawings.

[0028] As Figure 1 shown, an embodiment of this application provides an atomic magnetometer device for measuring extremely weak magnetic fields based on hyperpolarized 3 He, which can measure the magnitude of extremely weak magnetic fields relatively accurately.

[0029] This device includes: a 1083 nm laser 1, an automated shutter 2, a beam expander group 3, a quarter-wave plate 4, a three-axis Helmholtz coil 5, two SERF magnetometers 6, 3 a He gas chamber 7, a retroreflector 8, a metastable activation circuit 9, and a magnetic shielding cylinder 10.

[0030] The 1083 nm laser in this embodiment has a power of 10 W, and the laser wavelength is adjustable near 1083 nm. The optical head of this laser incorporates a beam expander, making the size of the emitted laser spot reach 20 mm.

[0031] The automatic shutter in this embodiment. The optical shutter can be triggered by a signal preset by a signal generator to achieve opening and closing, thereby controlling whether the laser passes through.

[0032] The optical lens group in this embodiment includes: an expanding lens group 3, a quarter-wave plate 4, and a retroreflector 8.

[0033] The expanding lens group contains a convex lens and a concave lens, and is used to expand a 20-mm laser spot to 50 mm.

[0034] The quarter-wave plate. The linearly polarized pump light with a spot size of 50 mm becomes circularly polarized light after passing through the quarter-wave plate set at a certain angle.

[0035] The retroreflector. The 1083-nm circularly polarized light is reflected by the retroreflector after passing through 3 the He gas cell, and then irradiates the 3 He gas cell again, thereby making better use of the circularly polarized light to achieve better 3 He polarization efficiency.

[0036] The metastable activation circuit is used to provide excitation energy to cause 3 the atoms in the He gas cell to transition from the ground state to the metastable state. This circuit consists of two parts: a high-voltage excitation circuit and a high-frequency excitation circuit.

[0037] The high-voltage excitation circuit breaks down 3 the He gas cell by providing a relatively high voltage, causing the 3 He atoms in the gas cell to quickly transition from the ground state to the metastable state.

[0038] In a certain example, the structure of the high-voltage excitation circuit is as Figure 2 shown. The core part of this circuit is composed of an NE555 timer, which is used to generate an adjustable oscillating pulse signal. The oscillating pulse signal is amplified by a power transistor and then loaded onto a boost coil, and finally output to 3 the He gas cell. To ensure that the gas cell can be effectively broken down, the circuit design needs to ensure that the voltage output to the gas cell exceeds 1 kV. The start and stop of the high-voltage excitation circuit are controlled by a touch switch to achieve the transient breakdown of the gas. Among them, in order to optimize the output performance of the circuit, the oscillating pulse generated by the NE555 timer is designed to have an adjustable duty cycle, and its duty cycle Q is:

[0039]

[0040] The high-frequency excitation circuit maintains 3 the metastable energy level of the He gas cell at a relatively low voltage to ensure its stable operation. This design not only reduces energy consumption but also improves energy transfer efficiency, while ensuring the safety and reliability of the system.

[0041] 3 The He gas chamber is filled with high-purity 3 He gas, in which the optical pumping effect and the interaction between atoms and the magnetic field occur. The spherical gas chamber structure can effectively improve the light energy utilization efficiency and reduce the light reflection loss at the same time. Since there is a certain distance between the high-frequency excitation energy generated by the excitation circuit and the gas chamber electrode, coaxial cables with a characteristic impedance of 50 Ω are required for energy transmission.

[0042] In one example, the overall design scheme of the high-frequency excitation circuit is as Figure 3 shown. Considering that the characteristic impedance of the coaxial cable does not match the output impedance of the excitation circuit and the impedance of the gas chamber, impedance matching networks are designed at the source end and the gas chamber end of the excitation circuit respectively to optimize the energy transmission efficiency. In order to maximize the energy transmission efficiency, the capacitance and inductance components of the circuit can be adjusted to minimize the voltage standing wave ratio of the circuit. When the voltage standing wave ratio is closer to 1, the energy reflection of the circuit is smaller. When all the energy is 3 absorbed by the He gas chamber, the metastable state of the 3 He gas chamber reaches the highest level at this time, the light emitted by the gas chamber is the brightest, and the 3 He gas chamber is also the most stable.

[0043] The three-axis Helmholtz coil system, through precise calculation and calibration, can apply precisely controllable multi-directional magnetic fields in the experiment to generate a magnetic field environment with a definite direction and intensity. This coil system is mainly used to compensate for the residual magnetic field and provide a highly stable controllable magnetic field condition for the experiment.

[0044] The SERF magnetometer is used to measure 3 the Larmor precession signal of He atoms

[0045] As an improvement to the above device, 2 SERF magnetometers are used, which are placed perpendicular to each other along the xOz plane to measure the Larmor precession signals in the z-y direction and the x-y direction respectively

[0046] As an improvement to the above device, a 1 Hz low-pass filter is added during the measurement to filter out the influence of other noises and achieve more accurate measurement.

[0047] 3 The He gas chamber, 3 The He atomic gas is contained in a spherical glass bubble with an inner wall coated with paraffin. The size of the bubble is 70 mm. This atomic gas chamber is placed in the center of the entire three-axis Helmholtz coil, 3 and the He gas chamber is the core of the entire device.

[0048] 3The metastable atoms in the He atomic gas chamber will undergo the Zeeman effect under the action of an external magnetic field, resulting in spectral line splitting. The metastable helium atoms split into 3 Zeeman sub-levels, and after interacting with circularly polarized light, they transition to the excited state and achieve hyperpolarization, showing the phenomenon of Larmor precession.

[0049] The 3 He atoms moving in a magnetic field will generate three kinds of magnetic moments: the electron orbital magnetic moment, the electron spin magnetic moment, and the nuclear magnetic moment. Since the electron mass is much smaller than the nuclear mass, the influence of the nuclear magnetic moment can be ignored. Therefore, only the electron orbital magnetic moment and the electron spin magnetic moment need to be considered. Under the action of an external magnetic field, a torque will be generated on the total magnetic moment of the atom, and its magnitude can be expressed as:

[0050] τ = μ × B

[0051] where μ is the magnetic moment and B is the magnetic field strength.

[0052] This torque will cause the angular momentum of the atom to change, making the total magnetic moment of the atom precess around the direction of the magnetic field. As Figure 4 shown, it shows the motion of the atomic magnetic moment under the action of an external magnetic field.

[0053] Figure 4 In, β represents the angle between the magnetic field and the direction of the angular momentum, α represents the angle between the magnetic field and the direction of the atomic magnetic moment, represents the angle of precession of the angular momentum. This precession phenomenon is called Larmor precession, and ω L is the Larmor precession speed.

[0054] By measuring 3 the Larmor precession frequency f of the L He atoms, the magnetic field value B at the position where the 3 He gas chamber is located can be calculated. He

[0055]

[0056] Among them, γ He is 3 the gyromagnetic ratio of the

[0057] He atoms, which is equal to 32.434 100 033(28) Hz / μT.

[0058] The entire experimental device is encapsulated in a magnetic shielding cylinder with a five-layer shielding structure.

[0059] In summary, the embodiments of the present application propose a method based on hyperpolarization 3An atomic magnetometer for measuring the extremely weak magnetic field of He, which significantly improves the measurement accuracy and stability. This solution combines 3 the characteristics of He atoms and the technology of hyperpolarization. By designing an adjustable metastable activation circuit, 3 He atoms can stably and highly transition to the metastable state; and a five-layer magnetic shielding cylinder is used to isolate the interference of external magnetic fields and noise; an automated shutter with extremely fast response is also used to minimize the impact of the laser switching process on the measurement data during the measurement stage, thereby achieving more reliable and accurate measurement of extremely weak magnetic fields.

[0060] The above is only the specific implementation manner of the present invention and is not limited thereto. Any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed in this patent should be covered within the protection scope of the present invention.

Claims

1. An atomic magnetometer based on the measurement of extremely weak magnetic fields of hyperpolarized 3 He, characterized in that Including: A laser for emitting laser light with a wavelength near 1083 nm, whose power is adjustable, and the optical head thereof is combined with a beam expander to make the emitted laser beam spot reach a certain size; An automated shutter for controlling the passage of the laser, which can be opened and closed by being triggered by a signal preset by a signal generator; Optical mirror group, including a beam expander, a wave plate and a reflector, the beam expander is used to expand the laser spot to a larger size, the wave plate is used to change the linearly polarized pump light into circularly polarized light, and the circularly polarized light passes through 3 the He gas chamber and is reflected by the reflector, and then irradiates the 3 He gas chamber again; A metastable state activation circuit, which is used to provide excitation energy to cause 3 the atoms in the He gas chamber to transition from the ground state to the metastable state and maintain their stable operation; A multi-axis Helmholtz coil capable of applying precisely controllable multi-directional magnetic fields to provide stable controllable magnetic field conditions for the experiment; At least one magnetometer for measuring 3 the Larmor precession signal of He atoms; 3 He gas chamber, filled with 3 He gas, whose shape and size can be adjusted as needed, is placed in the center of a multi-axis Helmholtz coil; A magnetic shielding device with a multi-layer shielding structure, and the entire experimental device is encapsulated therein.

2. The atomic magnetometer based on the measurement of extremely weak magnetic fields of hyperpolarized 3 He, characterized in that The power of the laser is 10 W, and the laser wavelength can be adjusted near 1083 nm. The beam expander can expand the laser beam spot from 20 mm to 50 mm.

3. The atomic magnetometer for measuring extremely weak magnetic fields based on hyperpolarized 3He according to claim 1, characterized in that, The switching time of the automated shutter is less than 10 ms, which can minimize the influence of laser switching on the measured data.

4. The atomic magnetometer for measuring extremely weak magnetic fields based on hyperpolarized 3He according to claim 1, characterized in that, The metastable activation circuit includes: A high-voltage excitation circuit for breaking down the 3He gas chamber by providing a high voltage to rapidly transition 3He atoms from the ground state to the metastable state; A high-frequency excitation circuit for maintaining the metastable energy level of the 3He gas chamber at a lower voltage to ensure its stable operation.

5. The atomic magnetometer for measuring extremely weak magnetic fields based on hyperpolarized 3He according to claim 4, characterized in that, The high-voltage excitation circuit is composed of an NE555 timer for generating an oscillation pulse signal with an adjustable duty cycle. The oscillation pulse signal is amplified by a power tube and then loaded onto a step-up coil, and finally output to the 3He gas chamber, with an output voltage exceeding 1 kV.

6. The atomic magnetometer for measuring extremely weak magnetic fields based on hyperpolarized 3He according to claim 4 or 5, characterized in that, The high-frequency excitation circuit is connected to the 3He gas chamber through a coaxial cable with a characteristic impedance of 50 Ω, and impedance matching networks are designed at the source end and the gas chamber end of the excitation circuit respectively to optimize the energy transmission efficiency.

7. The atomic magnetometer for measuring extremely weak magnetic fields based on hyperpolarized 3He according to claim 1, characterized in that, The 3He gas chamber is a spherical glass bubble with its inner wall coated with paraffin. The size of the bubble is 70 mm, and the purity of the 3He gas is greater than 99.9%.

8. The atomic magnetometer for measuring extremely weak magnetic fields based on hyperpolarized 3He according to claim 1, characterized in that, The three-axis Helmholtz coil has been precisely calculated and calibrated, and can apply precisely controllable multi-directional magnetic fields in the experiment to compensate for the residual magnetic field and provide highly stable controllable magnetic field conditions for the experiment.

9. The atomic magnetometer for measuring extremely weak magnetic fields based on hyperpolarized 3He according to claim 1, characterized in that, The magnetometers are two SERF magnetometers, which are arranged perpendicular to each other along the xOz plane to measure the Larmor precession signals in the z-y direction and the x-y direction respectively.

10. The atomic magnetometer for measuring extremely weak magnetic fields based on hyperpolarized 3He according to claim 1, characterized in that, The magnetic shielding device has a five-layer shielding structure and can effectively isolate the interference of external magnetic fields and noise.

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