A SERF atomic magnetometer based on laser heating and a magnetic field measurement method

Abstract

The application relates to the field of quantum sensing, and provides a SERF atomic magnetometer based on laser heating and a magnetic intensity measuring method. The purpose is to overcome the problem that the magnetic noise introduced by the conventional heating method of the SERF atomic magnetometer interferes with the signal of the magnetometer. The main scheme comprises the following steps: laser emitted by a detection laser and a pumping laser is introduced into a magnetometer probe through an optical fiber; the magnetometer probe comprises a gas chamber, graphene outside the gas chamber, and a three-dimensional magnetic compensation coil outside the gas chamber, and a heat shield composed of aerogel is arranged on the outer layer; a detection laser light path comprises a coupling mirror, a 1 / 2 glass sheet, a reflecting prism, and a PBS (polarizing beam splitter) prism; the detection laser finally passes through the PBS prism and is divided into two beams of light which are respectively irradiated on two photodiodes; a heating laser light path comprises a coupling mirror; the heating laser irradiates the graphene on the lower surface of the glass gas chamber through the coupling mirror; and a pumping laser light path comprises a coupling mirror, a 1 / 4 glass sheet, and a reflecting prism; the pumping laser is irradiated into the glass gas chamber through the reflecting prism.

Description

Technical Field

[0001] This invention relates to the field of quantum sensing and provides a SERF atomic magnetometer based on laser heating and a method for measuring magnetic intensity. Background Technology

[0002] Laser-heated SERF (Spin-Exchange Relaxation Free) atomic magnetometers are a cutting-edge technology based on fundamental principles. They offer advantages such as high precision and small size, and can be used to fabricate single-probe metrological magnetometers as well as miniaturized array magnetometers. Currently, Beijing University of Aeronautics and Astronautics (BUAA) is at the forefront of this technology's research and development, leading the world and opening up a new track for high-precision technology. This technology can be applied to fields such as ultra-high resolution magnetic resonance imaging of the heart and brain. Distinguished from electrocardiography (ECG), it provides a new detection method to achieve functional imaging of heart and brain tissues, providing crucial support and guidance for the study of the mechanisms of major and complex cardiovascular and cerebrovascular diseases and the exploration of new treatment options.

[0003] After alkali metal atoms enter the SERF state within the gas chamber, they undergo Larmor precession under the influence of an external magnetic moment. The Larmor precession frequency is proportional to the strength of the external magnetic field, and the magnetic field strength sensed by the gas chamber can be obtained by detecting the atomic precession angle. The realization of the SERF state requires three key elements: efficient laser pumping, a low magnetic field environment, and a high atomic number density. The SERF state requires the atomic spin exchange rate to be much greater than its Larmor precession frequency. Generally, increasing the atomic spin exchange rate requires increasing the density of alkali metal atoms; decreasing the Larmor precession frequency requires reducing the ambient magnetic field. To increase the alkali metal atom density, the atomic gas chamber needs to be heated to a high temperature, e.g., 150°C. Temperature fluctuations in the gas chamber directly affect the change in alkali metal atom density, which in turn causes a change in the scaling factor of the SERF atomic spin gyroscope. Therefore, to ensure that the scaling factor of the SERF atomic spin gyroscope remains constant, the atomic gas chamber must have high temperature stability, requiring a high-precision heating and temperature control system and a high-performance thermal insulation structure. Summary of the Invention

[0004] The purpose of this invention is to overcome the problem that the magnetic noise introduced by the conventional heating method of the current SERF atomic magnetometer interferes with the magnetometer signal.

[0005] To achieve the above objectives, the present invention employs the following technical means:

[0006] A SERF atomic magnetometer based on laser heating includes a heating laser, a detection laser, a pump laser, an electronic control system, and a magnetometer probe. The lasers emitted by the heating laser, the detection laser, and the pump laser are all guided into the magnetometer probe through optical fibers.

[0007] The magnetometer probe includes a glass chamber, graphene placed outside the chamber, a three-dimensional magnetic compensation coil wrapped outside the chamber, and a heat shield made of aerogel on the outer layer.

[0008] The heat shield is equipped with a coupling mirror, a half glass slide, a reflecting prism, and a PBS beam splitter to form the detection laser optical path. The detection laser is finally split into two beams by the PBS beam splitter and illuminates two photodiodes respectively.

[0009] A coupling mirror is installed outside the heat shield to form a heating laser optical path. The heating laser shines on the graphene on the lower surface of the glass gas chamber through the coupling mirror.

[0010] The heat shield is equipped with a coupling mirror, a quarter glass slide, and a reflecting prism to form the pump laser path. The pump laser enters the glass gas chamber after passing through the reflecting prism.

[0011] In the above technical solution, the graphene is graphene that has been completely oxidized at a high temperature of 2000℃.

[0012] In the above technical solution, the detection light needs to pass through the gas chamber, and no graphene is added to these two sides; the pump light needs to enter the gas chamber, and no graphene is added to this side; the heating laser, pump laser, and detection laser are orthogonal to each other. The three sides of the graphene being heated are limited.

[0013] This invention also provides a method for measuring the magnetic intensity of a SERF atomic magnetometer based on laser heating, comprising the following steps:

[0014] Step 1: Magnetometer probe calibration. The heating laser, pump laser and detection laser are all introduced into the magnetometer probe through optical fiber. Before the magnetometer probe is packaged, the detection laser is turned on. The linear polarization direction of the detection light changes after passing through the 1 / 2 wave plate. Rotating the 1 / 2 wave plate makes the readings of the photodiode PD equal. At this time, the polarization direction of the detection light is 45° with the two axes of the PBS beam splitter.

[0015] Step 2: Heating and temperature control of the gas chamber. Turn on the heating laser, and the graphene absorbs the heating laser energy to heat the gas chamber evenly from multiple sides. The aerogel insulation layer reduces heat loss during heating. The Pt1000 platinum resistance thermometer detects the current temperature and controls the power of the heating laser through closed-loop feedback to control the temperature of the gas chamber to about 150°C.

[0016] Step 3: Atomic pump polarization. Turn on the pump laser. The pump laser becomes circularly polarized after passing through a quarter-wave plate. It optically pumps the alkali metal atom ensemble in the gas chamber. After a period of time, the polarizability of the alkali metal atoms becomes saturated.

[0017] Step 4: The precession angle of atoms is detected by differential detection. After the gas cell senses the magnetic field, the atoms inside precess under the influence of the magnetic moment. When the precession of atoms generates an optical rotation angle θ, the intensity of the detection light passing through the gas cell is recorded as I0. The light is split into two, and the intensity of the transmitted light and the intensity of the reflected light passing through the PBS beam splitter are I2, respectively:

[0018]

[0019] get:

[0020] I1-I2=(I1+I2)sin(2θ)

[0021] Since the detected optical rotation angle is extremely small, it can be approximated as:

[0022] I1-I2≈2(I1+I2)θ

[0023]

[0024] Step 5: The output signal expression of the SERF atomic magnetometer is:

[0025]

[0026] Where Vo is the voltage signal output by the magnetometer, G is the conversion coefficient between the difference in input light intensity and the output voltage of the differential photodetector, Ipr is the intensity of the incident detection light, is the attenuation coefficient of the detection light intensity by the air chamber glass, and B y It is the magnetic field strength in the Y-axis direction, ΔB n It is environmental magnetic field noise, Δ r This is optical rotation noise, where OD(vpr) is the optical depth of the detected light, and e is the natural constant. Its expression is:

[0027]

[0028] Among them, v pr It detects the frequency of light, where n is the refractive index, l is the spatial distance the light travels through the air cell, c is the speed of light, and f is the refractive index. D2 It is the resonance intensity of the D2 line of rubidium atoms, v D2 It is the frequency corresponding to line D2, Γ D2 This is the pressure broadening value of line D2, r e It is the electron radius;

[0029] Simplified formula:

[0030]

[0031] in:

[0032]

[0033]

[0034] K is called the calibration coefficient between the magnetometer's input magnetic field and its output voltage signal. This parameter can be obtained through preliminary testing. Ignoring the other two noise interference items, the input magnetic field is proportional to the output voltage signal. Based on the above expression for the output signal of the SERF atomic magnetometer, the magnetic field strength in the Y-axis direction at the probe can be solved inversely.

[0035]

[0036] Because the present invention employs the above-mentioned technical means, it has the following beneficial effects:

[0037] 1. Laser heating is employed. This completely avoids the magnetic field introduced by the heating source and enables a high-precision, miniaturized gas chamber heating and temperature control system. A 975nm wavelength heating laser is used, which is far detuned from the pump and detection lasers, avoiding interference with them. The power of the heating laser can be controlled by the input voltage, and automatic temperature control can be achieved through hardware circuitry and software programs.

[0038] Second: Using graphene as a photothermal conversion and heat conduction material. Graphene is a two-dimensional nanomaterial with ultra-high thermal conductivity, enabling simultaneous heating of three sides of the gas chamber, improving heating uniformity and reducing the temperature gradient within the chamber. High-temperature (2000℃) fully oxidized graphene is used, which will not peel off under laser irradiation; simultaneously, an adhesive with a melting point above 200℃ is used to fix the graphene. Specially structured graphene is selected on the heated laser-irradiated surface to achieve higher photothermal conversion efficiency and thermal conductivity, improving uniform heating performance.

[0039] Point 3: The use of an aerogel heat shield reduces heat loss and minimizes environmental temperature disturbances within the air chamber. It also reduces the power of the heating laser needed to maintain a stable temperature after reaching the target temperature, thus reducing fluctuations and instability during the heating process. An aerogel layer is added outside the air chamber, with a small hole on each of the four sides requiring light transmission to allow the laser to pass through. Simultaneously, aerogel is filled outside the optical path structure and inside the magnetometer probe housing to further improve heat insulation, reduce heating laser power, improve temperature control accuracy, and prevent burns to patients from excessively high temperatures inside the cardiac and cerebral magnetic probe, ensuring safety. Attached Figure Description

[0040] Figure 1 The diagram shows the structure of the SERF atomic magnetometer, which can be used as a single-probe high-precision metrological magnetometer or as an array magnetometer for cardiac and cerebral magnetic measurements.

[0041] Figure 2 This is a front view of the probe's interior, showing the optical path structure for heating light from the bottom along the positive Y-axis and the optical path structure for detecting light along the X-axis. Figure 3 This is a right-hand view inside the probe, showing the optical path structure for heating light from the bottom along the positive Y-axis and the optical path structure for pumping light along the Z-axis. Figure 4 This is a top view inside the probe, showing the mutually orthogonal detection beams along the X-axis and pump beams along the Z-axis.

[0042] Graphene (without pores) is adhered to the upper, rear, and lower surfaces of the air chamber; for example... Figure 3 The heating laser shines along the positive Y-axis onto the graphene on the lower surface of the glass gas chamber (without entering the chamber); the pump light passes through a reflecting prism from the bottom of the probe and then enters the glass gas chamber along the positive Z-axis. Figure 2 The detection light passes through the bottom of the probe, through the reflective prism, and then through the glass gas cell along the positive X-axis as shown in the figure. It is then transmitted through the reflective prism to the PBS beam splitter, where it is split into two beams that illuminate two photodiodes (PDs) respectively. The sensitive direction of the SERF atomic magnetometer probe for detecting the magnetic field is the Y-axis.

[0043] Figure 5 This is a schematic diagram of a three-dimensional magnetic compensation coil structure. Two symmetrical coils are arranged in each dimension, and each coil is wound multiple turns on a plastic frame. After the wiring is completed, the coil constant is tested; this constant is the ratio of the generated magnetic field to the applied current. Then, the air chamber is placed and fixed in its central position through the frame's opening.

[0044] Explanation of reference numerals in the attached figures:

[0045] 1-Electrical control system, 2-Heating laser, 3-Detection laser, 4-Pump laser, 5-Magnetometer probe, 6-Insulation layer, 7-Three-dimensional magnetic compensation coil, 8-Graphene, 9-PT1000 temperature measuring resistor, 10-Gas chamber, 11-Emitting prism, 12-Heating laser, 13-Detection laser, 14-Pump laser, 15-Coupled mirror, 16-1 / 2 wave plate, 17-1 / 4 glass slide, 18-PBS beam splitter prism, 19-Photodiode, A-Computer, B-Atomic magnetometer, C-Probe. Detailed Implementation

[0046] The embodiments of the present invention will be described in detail below. Although the present invention will be described and illustrated in conjunction with some specific embodiments, it should be noted that the present invention is not limited to these embodiments. On the contrary, any modifications or equivalent substitutions made to the present invention should be covered within the scope of the claims of the present invention.

[0047] Furthermore, to better illustrate the present invention, numerous specific details are set forth in the following detailed embodiments. Those skilled in the art will understand that the present invention can be practiced without these specific details.

[0048] This invention aims to overcome the problem of magnetic noise interference in the magnetometer signal caused by conventional heating methods in current SERF atomic magnetometers. The mainstream gas chamber heating method in current SERF atomic magnetometers is high-frequency electric heating. Although the influence of magnetic noise is reduced through high-frequency AC power supply and symmetrical winding structure, a certain amount of magnetic noise still exists. As SERF atomic magnetometers develop towards higher precision, it is necessary to further optimize the heating method to avoid introducing magnetic field interference. This invention innovatively adopts a laser heating temperature control method to further reduce magnetic field noise interference, and designs a novel, high-precision SERF atomic magnetometer structure, realizing a magnetic field-free, low-noise, and small-volume alkali metal gas chamber heating device, which is expected to further improve the magnetic field measurement sensitivity of SERF atomic magnetometers.

[0049] The pump polarization of alkali metal atoms utilizes circularly polarized laser light with a wavelength near 770 nm at the potassium D1 line. This 770 nm linearly polarized pump laser is guided into the probe via an optical fiber, its spot size reduced by a coupling mirror, and then converted to circular polarization by a quarter-wave plate before entering the gas cell to optically pump the atomic ensemble inside. The detection light uses linearly polarized light with a wavelength near 795 nm at the rubidium D1 line, and the optical rotation angle is detected using differential detection. A 795 nm linearly polarized detection laser is guided into the probe via an optical fiber. Rotating the half-wave plate changes the polarization direction of the detection light to 45° with both axes of the PBS beam splitter, at which point the readings of the two photodiodes 19 are equal. When the atomic ensemble precesses due to the magnetic field, the change in the polarization angle of the detection light passing through the gas cell can be detected by the readings of the two photodiodes 19, allowing the calculation of the magnitude of the magnetic field experienced by the gas cell based on existing theories. A triaxial active magnetic compensation coil is used to compensate for residual magnetism in the gas cell region within the shielding cylinder. Due to the influence of the Earth's magnetic field, current SERF atomic magnetometers typically operate within a magnetic field shielding cylinder. However, the shielding cylinder cannot completely isolate the magnetic field, and the residual magnetism in the gas chamber region is compensated by a three-dimensional coil. Simultaneously, no new magnetic field should be introduced during the heating process. Laser heating is employed, with the heating laser beam guided through an optical fiber to the probe, illuminating graphene adhered to the gas chamber wall. The graphene converts the light energy into heat energy, conducting heat to the gas chamber. Optimizing the internal optical path structure by arranging reflective prisms reduces the probe's size. An aerogel heat shield minimizes heat loss. An aerogel layer is applied outside the gas chamber, with small holes on the four sides requiring light transmission to allow the laser to pass through, reducing fluctuations and instabilities during the heating process. Furthermore, aerogel is filled outside the optical path structure and inside the magnetometer probe shell to further improve heat insulation and prevent burns to patients from excessively high temperatures inside the cardiac and cerebral magnetic field probe, ensuring safety.

[0050] like Figure 1 As shown in Figures 2, 3, 4, and 5, the specific implementation steps of the present invention are as follows:

[0051] Step 1: Device Calibration. The heating laser, pump laser, and detection laser are all guided into the magnetometer probe through optical fibers. Before device encapsulation, the detection laser 3 is turned on. The linear polarization direction of the detection light changes after passing through the 1 / 2 wave plate 16. Rotating the 1 / 2 wave plate causes the readings of the photodiode 19 to be equal. At this time, the polarization direction of the detection light is at 45° to the two axes of the PBS beam splitter 18.

[0052] Step 2: Heating and temperature control of the gas chamber. Turn on the heating laser 2, the graphene 8 absorbs the heating laser energy and heats the gas chamber 10 uniformly from multiple sides; the aerogel insulation layer 6 reduces heat loss during heating; the Pt1000 temperature sensing resistor 9 detects the current temperature and controls the power of the heating laser through closed-loop feedback to control the temperature of the gas chamber to about 150℃.

[0053] Step 3: Atomic pump polarization. Turn on the pump laser 4. The pump laser becomes circularly polarized after passing through the quarter-wave plate 17. It optically pumps the ensemble of alkali metal atoms in the gas chamber. After a period of time, the polarizability of the alkali metal atoms becomes saturated.

[0054] Step 4: Detect the atomic precession angle using the differential detection method. After the gas chamber senses the magnetic field, the atoms inside precess under the influence of the magnetic moment. When the atomic precession generates an optical rotation angle θ, the intensity of the detection light passing through the gas chamber is denoted as I0. This light is then split into two, and the transmitted light intensity and reflected light intensity I2 passing through the PBS beam splitter are respectively:

[0055]

[0056] get:

[0057] I1-I2=(I1+I2)sin(2θ)

[0058] Since the detected optical rotation angle is extremely small, it can be approximated as:

[0059] I1-I2≈2(I1+I2)θ

[0060]

[0061] Step 5: The output signal expression of the SERF atomic magnetometer is:

[0062]

[0063] Where Vo is the voltage signal output by the magnetometer, G is the conversion coefficient between the difference in input light intensity and the output voltage of the differential photodetector, Ipr is the intensity of the incident detection light, is the attenuation coefficient of the detection light intensity by the air chamber glass, and B y It is the magnetic field strength in the Y-axis direction, ΔB n It is environmental magnetic field noise, Δ rThis is optical rotation noise, where OD(vpr) is the optical depth of the detected light, and e is the natural constant. Its expression is:

[0064]

[0065] Among them, v pr It detects the frequency of light, where n is the refractive index, l is the spatial distance the light travels through the air cell, c is the speed of light, and f is the refractive index. D2 It is the resonance intensity of the D2 line of rubidium atoms, v D2 It is the frequency corresponding to line D2, Γ D2 This is the pressure broadening value of line D2, r e It is the electron radius.

[0066] Simplified formula:

[0067]

[0068] in:

[0069]

[0070]

[0071] K is called the calibration coefficient between the magnetometer's input magnetic field and its output voltage signal. This parameter can be obtained through preliminary testing. Ignoring the other two noise interference items, the input magnetic field is directly proportional to the output voltage signal. Based on the above expression for the SERF atomic magnetometer's output signal, the magnetic field strength in the Y-axis direction at the probe can be calculated:

[0072]

Claims

1. A method for measuring magnetic intensity using a laser-heated SERF atomic magnetometer, characterized in that... Includes the following steps: Step 1: Magnetometer probe calibration. The heating laser, pump laser and detection laser are all introduced into the magnetometer probe through optical fiber. Before the magnetometer probe is packaged, the detection laser (3) is turned on. The linear polarization direction of the detection light changes after passing through the 1 / 2 wave plate (16). Rotating the 1 / 2 wave plate makes the readings of the photodiode PD (19) equal. At this time, the polarization direction of the detection light is 45° with the two axes of the PBS beam splitter (18). Step 2: Heating and temperature control of the gas chamber. Turn on the heating laser (2), and the graphene (8) absorbs the heating laser energy and heats the gas chamber (10) uniformly from multiple sides. The aerogel insulation layer (6) reduces heat loss during heating. The Pt1000 platinum resistance (9) detects the current temperature and controls the power of the heating laser through closed-loop feedback to control the temperature of the gas chamber to about 150°C. Step 3: Atomic pump polarization, turn on the pump laser (4), the pump laser becomes circularly polarized light after passing through the 1 / 4 wave plate (17), and optically pumps the alkali metal atom ensemble in the gas chamber. After a period of time, the polarizability of the alkali metal atoms is saturated. Step 4: The precession angle of atoms is detected by differential detection. After the gas cell senses the magnetic field, the atoms inside precess under the influence of the magnetic moment. When the precession of atoms generates an optical rotation angle θ, the intensity of the detection light passing through the gas cell is recorded as I0. The light is split into two, and the intensity of the transmitted light and the intensity of the reflected light passing through the PBS beam splitter are I2, respectively: get: Since the detected optical rotation angle is extremely small, it can be approximated as: Step 5: The output signal expression of the SERF atomic magnetometer is: in, This is the voltage signal output by the magnetometer. This is the conversion coefficient between the difference in input light intensity and the output voltage of the differential photodetector. The intensity of the incident detection light. denoted as the attenuation coefficient of the gas chamber glass for the intensity of the detected light. It is the magnetic field strength in the Y-axis direction. It is environmental magnetic field noise. It's optical rotation noise. To detect the optical depth of light. Let be a natural constant, and its expression is: in, It detects the light frequency. It is the refractive index. It measures the spatial distance the light travels through the air chamber. It's the speed of light. It is the resonance intensity of the D2 line of rubidium atoms. This corresponds to the frequency of line D2. This is the pressure broadening value of line D2. It is the electron radius; Simplified formula: in: This is referred to as the calibration coefficient between the magnetometer's input magnetic field and its output voltage signal. This parameter can be obtained through preliminary testing. Ignoring the other two noise interference items, the input magnetic field is proportional to the output voltage signal. Based on the above expression for the output signal of the SERF atomic magnetometer, the magnetic field strength in the Y-axis direction at the probe can be calculated inversely. 。 2. A laser-heated SERF atomic magnetometer applied to the method described in claim 1, characterized in that, It includes a heating laser (2), a detection laser (3), a pump laser (4), an electrical control system (1), and a magnetometer probe (5). The lasers emitted by the heating laser (2), the detection laser (3), and the pump laser (4) are all guided into the magnetometer probe (5) through optical fibers. The magnetometer probe (5) includes a gas chamber (10), a graphene (8) placed outside the gas chamber (10), a three-dimensional magnetic compensation coil (7) wrapped around the gas chamber (10), and then a heat insulation cover made of aerogel is used on the outer layer. The heat shield is equipped with a coupling mirror (15), a half glass slide (16), a reflecting prism (11), and a PBS beam splitter (18) for forming the detection laser optical path. The detection laser is finally split into two beams by the PBS beam splitter (18) and shines on two photodiodes (19) respectively. A coupling mirror (15) is provided outside the heat shield to form a heating laser optical path. The heating laser is irradiated onto the graphene (8) on the lower surface of the glass gas chamber through the coupling mirror (15). The heat shield is equipped with a coupling mirror (15), a quarter glass slide (17), and a reflecting prism (11) to form the pump laser optical path. The pump laser enters the glass gas chamber after passing through the reflecting prism.

3. The SERF atomic magnetometer based on laser heating according to claim 2, characterized in that, Graphene (8) is graphene that has been fully oxidized at a high temperature of 2000℃.

4. A SERF atomic magnetometer based on laser heating according to claim 2, characterized in that, The heating laser, pumping laser, and detection laser are three mutually orthogonal laser beams.

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