A system and method for measuring magnetic susceptibility based on atomic magnetometer

Abstract

The application discloses a kind of high-precision magnetic susceptibility measurement systems based on atomic magnetometer, comprising: first coherent light source, second coherent light source, first polarization beam splitter, second polarization beam splitter, third polarization beam splitter, 1 / 4 wave plate, half wave plate, balanced detector, rubidium atomic ensemble, helmholtz coil, magnetic shield, electromagnet, guide tube, guide coil, vacuum pump, electromagnetic valve, air pipe.The application is based on the magneto-optical rotation effect generated in rubidium atomic ensemble, realizes sample polarized magnetic field measurement, and the ratio of polarized magnetic field and the magnetic field generated by electromagnet is sample polarizability.The measurement system disclosed in the application has very important application prospect in the field of precision measurement, geological exploration and biomagnetic field measurement.Compared with traditional magnetic susceptibility meter, the application uses emerging ultra-sensitive atomic magnetometer as magnetization probe, and has great improvement in magnetic field measurement sensitivity.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of nonlinear optics, quantum optics and precision measurement system, and relates to a magnetic susceptibility measurement system and method based on an atomic magnetometer. BACKGROUND

[0002] Magnetic susceptibility is a measure of the magnetization performance of a substance. The measurement of the magnetic susceptibility of a substance has important applications in the detection of non-magnetic materials and environmental magnetism research. With the continuous development of electronic technology, electronic devices are developing towards miniaturization and high precision. Many industries now require the use of non-magnetic materials, such as copper materials, aluminum materials, titanium alloys, ceramics, etc. The magnetic susceptibility and other characteristics of these substances can have a very obvious impact on the precision of the equipment. Therefore, it is necessary to accurately measure the magnetic permeability of the substance. In the natural environment, the magnetic susceptibility performance of the sample can indicate the mineral types contained in the soil, rock, dust and sediment, and trace the formation or transport process of the substance. Magnetic susceptibility measurement has diagnostic significance for specific processes such as burning or soil immersion, and plays an important role in research fields such as archaeology or soil science.

[0003] Traditional magnetic susceptibility measurement is completed by a magnetic balance device developed based on the Guoy principle. In a non-uniform magnetic field, the force acting on a weak magnetic substance is proportional to its magnetic susceptibility, and the magnetic parameters can be obtained by measuring the force acting on the magnetic substance in a non-uniform magnetic field. Therefore, the measurement accuracy of the balance limits the measurement sensitivity and range of the magnetic susceptibility. The most sensitive existing magnetic susceptibility measurement system is realized based on a superconducting quantum magnetometer. The superconducting quantum interference device is a sensor for detecting magnetic flux or any physical quantity that can be converted into magnetic flux. It is connected by connecting two Josephson tunnel junction points with a superconducting ring. Its advantage is high sensitivity in magnetic field measurement and wide range in magnetic susceptibility measurement. However, the superconducting magnetic susceptibility meter has obvious disadvantages. The requirement for a low-temperature environment makes the instrument very expensive and bulky. Therefore, it is necessary to study a small-sized magnetic susceptibility measurement system and method with a wider measurement range and higher measurement accuracy. SUMMARY

[0004] In order to solve the problems in the prior art, the purpose of the present application is to provide a novel magnetic susceptibility measurement system and method based on an atomic magnetometer, which aims to solve the problem of low signal-to-noise ratio of the system and limited magnetic susceptibility measurement range caused by insufficient magnetic field measurement sensitivity in the existing magnetic susceptibility measurement technology. Compared with the traditional magnetic susceptibility meter, the present application applies a new type of atomic magnetometer as a magnetic field probe, which greatly increases the sensitivity of magnetic field measurement. The present application has a wide range of applications in the fields of material detection, biomagnetism, environmental magnetism and other precision measurement fields due to its large magnetic susceptibility measurement range and high precision.

[0005] In order to achieve the above object, the application provides a high-precision magnetic susceptibility measurement system based on an atomic magnetometer, comprising a sample polarization device, a sample transportation device and a polarization magnetic field measurement device; and the specific implementation is as follows:

[0006] A first coherent light source is used to generate a pump light field, and the light field is required to be resonant with atomic transition energy levels;

[0007] A second coherent light source is used to generate a probe light field, and the light field is required to be far detuned from atomic transition energy levels;

[0008] The far detuning refers to that the laser frequency is different from the resonant frequency by tens of GHz.

[0009] The first polarization beam splitter is used to convert the probe light field into linearly polarized light;

[0010] The second polarization beam splitter is used to separate the polarization direction of the probe light field into horizontal and vertical directions;

[0011] The third polarization beam splitter is used to convert the pump light field into linearly polarized light;

[0012] A 1 / 4 wave plate is used to change the polarization ellipticity of the pump light generated by the first coherent light source after passing through the third polarization beam splitter;

[0013] A half wave plate is used to change the polarization direction of the probe light generated by the second coherent light source after passing through the first polarization beam splitter;

[0014] A balanced detector is used to detect the difference between the light intensities of two probes entering the detector;

[0015] A rubidium atomic ensemble is used to detect the polarization magnetic field generated by the sample;

[0016] A Helmholtz coil is used to generate a uniform bias magnetic field and balance the residual magnetism of a magnetic shield;

[0017] A magnetic shield is used to shield external magnetic field noise and the magnetic field generated by an electromagnet;

[0018] An electromagnet is used to apply a polarization magnetic field;

[0019] A guide tube is used to guide the movement of the sample, and the bottom end of the guide tube is connected with the atomic magnetometer, and the top end is connected with the electromagnet;

[0020] A guide coil is used to guide the magnetization direction of the sample to be the sensitive direction of the rubidium atomic ensemble, i.e., the direction perpendicular to the first coherent light source and the second coherent light source;

[0021] A vacuum pump is used to transport the test sample to and from the electromagnet and the magnetic shield;

[0022] An electromagnetic valve is used to control the on-off of the vacuum pump, and is powered by 220V mains;

[0023] The air tube is used to connect the vacuum pump and the guide tube.

[0024] Lock-in amplifiers are used to demodulate signals;

[0025] The probe is used to convert the detected optical signals into electrical signals.

[0026] The sample tube is used to hold the sample, and its T-shaped structure allows it to be drawn up by an air pump in the guide tube.

[0027] The first coherent light source and the second coherent light source are coherent light sources emitted by the laser and operate in continuous mode; the first coherent light source resonates with the atomic transition energy level; the second coherent light source is far detuned to the atomic transition energy level.

[0028] The first coherent light source and the second coherent light source are perpendicular to each other.

[0029] The bias magnetic field generated by the Helmholtz coil and the guiding magnetic field generated by the guiding coil are parallel to each other and perpendicular to the propagation direction of the first coherent light source and the second coherent light source.

[0030] The solenoid valve is used to control the vacuum pump switch, modulate the spatial position of the sample, and demodulate the detected signal at the same frequency.

[0031] In this invention, a rubidium atom ensemble is used to detect the polarization magnetic field of a sample.

[0032] This invention also proposes a method for measuring magnetic susceptibility based on an atomic magnetometer. A circularly polarized pump beam, nearly resonant with the atomic energy levels, polarizes the atom and transfers angular momentum to the atomic ensemble, generating a long-lasting spin state in the atom's ground state. Then, a linearly polarized probe beam, far detuned to the atomic transitions, is used to measure the atomic state. Simultaneously, because the light field is far detuned to the atomic transition energy levels, it does not affect the atomic state. By measuring the frequency modulated by the polarization of the light field, the precession frequency of the atomic spin can be obtained, thus determining the magnetic field strength of the sample. Specifically, the method includes the following steps:

[0033] Step 1: The laser light from the first coherent source is first filtered by a third polarization beam splitter to ensure that the light field is linearly polarized. Then it passes through a quarter-wave plate to convert the light field into circularly polarized light. Finally, the light field is injected into the rubidium atomic ensemble, transferring the angular momentum of the light field to the atomic ensemble and preparing all the atoms into individual atomic ground-state magnetic sublevels.

[0034] Step Two: The laser light from the second coherent source is filtered by the first polarization beamsplitter to ensure linear polarization before being injected into the rubidium atomic ensemble. As the laser passes through the rubidium atomic ensemble, the polarization, intensity, and phase of the light field are modulated by the atomic medium under the combined influence of the bias magnetic field generated by the Helmholtz coil and the polarization magnetic field generated by the sample, thus altering accordingly. After exiting the rubidium atomic ensemble, the laser light passes through a half-wave plate and a second polarization beamsplitter, splitting into two beams with mutually perpendicular polarization directions. Finally, these beams are incident on a balanced detector, and the polarization of the light field is measured. By measuring the frequency at which the polarization is modulated, the precession frequency of the atomic spins can be obtained, thereby determining the magnetic field strength of the sample.

[0035] Step 3: Turn on the vacuum pump and position the sample tube at the top of a guide tube, near the electromagnet. The magnetic field generated by the electromagnet can be used to polarize the sample.

[0036] Step 4: Turn off the vacuum pump and simultaneously turn on the guide coil. During the sample's descent, the guiding magnetic field generated by the guide coil will redirect the magnetization direction to the sensitive direction of the rubidium atomic ensemble, allowing it to fall to the bottom of the guide tube. At this point, the sample enters the magnetic shield, with a gap of approximately 1 mm between it and the rubidium atomic ensemble. Next, a linearly polarized probe beam, detuned from the atomic transitions, is used to measure the atomic states. Simultaneously, because the light field is detuned from the atomic transition energy levels, it does not affect the atomic states. By measuring the frequency modulated by the polarization of the light field, the precession frequency of the atomic spin can be obtained, thus determining the magnetic field strength of the sample.

[0037] Step 5: The vacuum pump can be switched on and off using a solenoid valve, and the spatial position of the sample can be modulated. The spatial position of the sample is modulated with a characteristic frequency, and the magnetic field signal measured by the atomic magnetometer will also be modulated with a corresponding characteristic frequency. Finally, a lock-in amplifier is used to demodulate the measurement signal with the same characteristic frequency to obtain the magnitude of the polarized magnetic field generated by the sample.

[0038] Step 6: The ratio of the polarization magnetic field generated by the sample to the magnetic field generated by the electromagnet is the magnetic susceptibility of the sample.

[0039] The rubidium atom ensemble (9) operates at 160°C.

[0040] The beneficial effects of this invention include: Based on the magneto-optical rotation effect generated in the rubidium atomic ensemble, this invention achieves sample polarization magnetic field measurement, where the ratio of the polarization magnetic field to the magnetic field generated by the electromagnet is the sample polarizability. The measurement system described in this invention has significant application prospects in precision measurement, geological exploration, and biomagnetic field measurement. Compared with traditional magnetic susceptibility meters, this invention, on the one hand, utilizes an emerging ultra-sensitive atomic magnetometer as a magnetization probe, greatly improving the sensitivity of magnetic field measurement. On the other hand, it uses a vacuum pump to modulate the spatial position of the sample, and employs modulation and demodulation measurement to improve the signal-to-noise ratio, ultimately achieving high-precision magnetic susceptibility measurement. Attached Figure Description

[0041] Figure 1 This is a schematic diagram of the magnetic susceptibility measurement system based on an atomic magnetometer in this invention.

[0042] Figure 2 This is a flowchart of the magnetic susceptibility measurement method based on an atomic magnetometer in this invention. Detailed Implementation

[0043] The invention will be further described in detail below with reference to the specific embodiments and accompanying drawings. Except for the contents specifically mentioned below, the processes, conditions, and experimental methods for implementing the invention are all common knowledge and general knowledge in the art, and the invention does not have any particular limitations.

[0044] The magnetic susceptibility measurement system described in this invention is as follows: Figure 1 As shown, it includes: a first coherent light source 1, a second coherent light source 2, a first polarization beam splitter 3, a second polarization beam splitter 4, a third polarization beam splitter 5, a quarter-wave plate 6, a half-wave plate 7, a balanced detector 8, a rubidium atom ensemble 9, a Helmholtz coil 10, a magnetic shield 11, an electromagnet 12, a guide tube 13, a guide coil 14, a vacuum pump 15, a solenoid valve 16, and a gas pipe 17.

[0045] In this system, the lasers from the first coherent light source 1 and the second coherent light source 2 are first filtered by the first polarization beamsplitter 3 and the third polarization beamsplitter 5, respectively, to ensure that the light field is linearly polarized. The first coherent light source 1 resonates with the atomic transition energy levels, and after passing through the quarter-wave plate 6, the light field is converted into circularly polarized light. Finally, this light field is injected into the rubidium atom ensemble 9, preparing all the atoms onto the ground-state magnetic quantum energy levels of individual atoms. The second coherent light source 2 is detuned to the atomic transitions. When this laser passes through the rubidium atom ensemble 9, under the combined action of the bias magnetic field generated by the Helmholtz coil 10 and the polarization magnetic field generated by the sample, the polarization of the light field is modulated by the atomic medium, thus changing accordingly. After exiting the rubidium atom ensemble, the laser passes through a half-wave plate 7 and the second polarization beamsplitter 4, and finally enters the balanced detector 8 to measure the polarization of the light field. By measuring the frequency of polarization modulation, the precession frequency of the atomic spin can be obtained, thereby determining the magnetic field strength. The electromagnetic valve 16 can control the vacuum pump 15 to change the spatial position of the sample. When the vacuum pump 15 is operating, the sample is positioned at the top of a guide tube 13 and close to the electromagnet 12. The magnetic field generated by the electromagnet polarizes the sample. When the vacuum pump 15 is turned off, the sample falls to the bottom of the guide tube 13. During the sample's descent, the guide coil 14 is activated, causing the sample's magnetization direction to be parallel to the bias magnetic field generated by the Helmholtz coil 10. At this point, the sample enters the magnetic shield 11, close to the rubidium atom ensemble 9, with a 1mm gap between it and the rubidium atom ensemble 9. Then, a beam of probe light, detuned and linearly polarized to the atomic transitions, is used to measure the atomic states. Simultaneously, because the light field is detuned to the atomic transition energy levels, it does not affect the atomic states. By measuring the frequency modulated by the polarization of the light field, the precession frequency of the atomic spin can be obtained, thus determining the magnetic field strength of the sample. Modulating the spatial position of the sample with a characteristic frequency will also modulate the measured magnetic field signal with a corresponding characteristic frequency. Finally, a lock-in amplifier is used to demodulate the measurement signal with the same characteristic frequency to obtain the magnitude of the polarization magnetic field generated by the sample.

[0046] The high-precision magnetic susceptibility measurement method described in this invention is as follows: Figure 2 As shown, an electromagnet, acting as a sample polarization device, generates a polarization magnetic field. This field passes through a sample transport module consisting of a guide tube, guide coil, vacuum pump, solenoid valve, and gas tubing. Finally, a polarization magnetic field measurement module, comprised of a coherent light source, polarization beam splitter, quarter-wave plate, half-wave plate, balanced detector, rubidium atom ensemble, Helmholtz coil, and magnetic shield, measures the polarization magnetic field. This process is repeated cyclically at a characteristic frequency, and the measurement signal is demodulated at the same characteristic frequency, ultimately achieving precise measurement of magnetic susceptibility.

[0047] The scope of protection of this invention is not limited to the above embodiments. Any variations and advantages that can be conceived by those skilled in the art without departing from the spirit and scope of the inventive concept are included in this invention and are protected by the appended claims.

Claims

1. A high-precision magnetic susceptibility measurement system based on atomic magnetometer, characterized in that, The system comprises: a first coherent light source (1), a second coherent light source (2), a first polarization beam splitter (3), a second polarization beam splitter (4), a third polarization beam splitter (5), a 1 / 4 wave plate (6), a half wave plate (7), a balanced detector (8), a rubidium atomic ensemble (9), a Helmholtz coil (10), a magnetic shield (11), an electromagnet (12), a guide tube (13), a guide coil (14), a vacuum pump (15), an electromagnetic valve (16), a gas tube (17), a lock-in amplifier (18), a probe (19) and a sample tube (20); wherein: The first coherent light source (1) is used to generate a pump light field, which is resonant with atomic transition energy levels; The second coherent light source (2) is used to generate a probe light field, which is far detuned from atomic transition energy levels; the first coherent light source (1) and the second coherent light source (2) are perpendicular to each other; The first polarization beam splitter (3) is used to convert the probe light field into linearly polarized light; The second polarization beam splitter (4) is used to separate the polarization direction of the probe light field into horizontal and vertical directions; The third polarization beam splitter (5) is used to convert the pump light field into linearly polarized light; The 1 / 4 wave plate (6) is used to change the polarization ellipticity of the pump light generated by the first coherent light source (1) after passing through the third polarization beam splitter (5); The half wave plate (7) is used to change the polarization direction of the probe light generated by the second coherent light source (2) after passing through the first polarization beam splitter (3); The balanced detector (8) is used to detect the difference in light intensity between the two probes (19) entering the detector; The rubidium atomic ensemble (9) is used to detect the polarized magnetic field generated by the sample; the rubidium atomic ensemble (9) works at 160℃; The Helmholtz coil (10) is used to generate a uniform bias magnetic field to balance the residual magnetism of the magnetic shield (11); the bias magnetic field generated by the Helmholtz coil (10) is parallel to the guide magnetic field generated by the guide coil (14), and is perpendicular to the propagation directions of the first coherent light source (1) and the second coherent light source (2); The magnetic shield (11) is used to shield external magnetic field noise and the magnetic field generated by the electromagnet (12); The electromagnet (12) is used to apply a polarized magnetic field to the sample; The guide tube (13) is used to guide the movement of the sample, with the bottom end connected to the atomic magnetometer and the top end connected to the electromagnet (12); The guide coil (14) guides the magnetization direction of the sample to the sensitive direction of the rubidium atomic ensemble, i.e. the direction perpendicular to the first coherent light source (1) and the second coherent light source (2); The vacuum pump (15) is used to transport the test sample back and forth between the electromagnet (12) and the magnetic shield (11); The electromagnetic valve (16) controls the opening and closing of the vacuum pump (15), which is powered by 220V mains electricity, modulates the spatial position of the sample, and demodulates the detected signal at the same frequency; The gas tube (17) is used to connect the vacuum pump (15) and the guide tube (13); The lock-in amplifier (18) is used to demodulate the signal; The probe (19) is used for converting the detected optical signal into an electrical signal; The sample tube (20) is used for placing a sample, and the T-shaped structure enables the sample tube to be lifted in a guide tube by an air pump.

2. The magnetic susceptibility measurement system of claim 1, wherein, The first coherent light source (1) and the second coherent light source (2) are coherent light sources emitted by lasers, and work in a continuous mode.

3. A high-precision magnetic susceptibility measurement method based on an atomic magnetometer, characterized by, The method comprises the following steps: Step one: the laser of the first coherent light source (1) is first filtered by the third polarization beam splitter (5) to ensure that the light field is linearly polarized light, then passes through the 1 / 4 wave plate (6) to convert the light field into circularly polarized light, and finally is injected into the rubidium atomic ensemble (9) to transfer the angular momentum of the light field to the rubidium atomic ensemble and prepare all the atoms to a single atomic ground state magnetic sublevel; Step two: the laser of the second coherent light source (2) is filtered by the first polarization beam splitter (3) to ensure that the light field is linearly polarized light, and is injected into the rubidium atomic ensemble; when the laser passes through the rubidium atomic ensemble (9), the polarization, intensity and phase of the light field are all modulated by the atomic medium under the joint action of the bias magnetic field generated by the Helmholtz coil (10) and the polarization magnetic field generated by the sample, and then the laser passes through a half wave plate (7) and the second polarization beam splitter (4) to be divided into two beams with perpendicular polarization directions; finally, the laser enters the balanced detector (8) and the polarization of the light field is measured; the precession frequency of the atomic spin is obtained by measuring the frequency of the polarization modulation, so that the magnetic field strength of the sample is known; Step three: the vacuum pump (15) is turned on, the sample tube (20) is located at the top of a guide tube (13) and near the electromagnet (12); the sample is polarized by the magnetic field generated by the electromagnet (12); Step four: when the vacuum pump (15) is turned off, the sample falls to the bottom of the guide tube (13); the guide coil (14) is turned on during the falling process of the sample, so that the magnetization direction of the sample is changed to be parallel to the bias magnetic field generated by the Helmholtz coil (10) by the guide magnetic field generated by the guide coil (14); at this time, the sample enters the magnetic shielding cover (11) and has a 1mm gap with the rubidium atomic ensemble (9); then a probe light which is far detuned from the atomic transition and linearly polarized is used to measure the atomic state, and at the same time, the atomic state is not affected because the light field is far detuned from the atomic transition level; the precession frequency of the atomic spin is obtained by measuring the frequency of the polarization modulation of the light field, so that the magnetic field strength of the sample is known; Step five: the on-off of the vacuum pump (15) is controlled by the electromagnetic valve (16), and the spatial position of the sample is modulated; the spatial position of the sample is modulated at a characteristic frequency, and the magnetic field signal measured by the atomic magnetometer is also modulated at the corresponding characteristic frequency; finally, the lock-in amplifier (18) is used to demodulate the measured signal at the same characteristic frequency, and the size of the polarization magnetic field generated by the sample is obtained; Step six: the ratio of the polarization magnetic field generated by the sample to the magnetic field generated by the electromagnet is the magnetic susceptibility of the sample.

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