SERF atom magnetometer optical frequency shift virtual magnetic field cross section distribution measurement method

Abstract

According to the SERF atom magnetometer optical frequency shift virtual magnetic field cross section distribution measurement method, a pumping light beam and a detection light beam parallelly pass through an atom gas chamber, the sizes of optical frequency shift virtual magnetic fields at different positions of the pumping light can be detected by using fine detection light, and the distribution of measurement points is preset; the distribution condition of the light frequency shift along the section of the pumping light spot is measured to the maximum extent, when the detection light measures the light frequency shift, the longitudinal magnetic field sizes when the pumping light is left-hand circularly polarized light and right-hand circularly polarized light are measured respectively, then the difference obtained by subtracting the left-hand circularly polarized light and the right-hand circularly polarized light is divided by 2 to obtain the virtual magnetic field value of the light frequency shift, and after all positions are measured, the magnetic field value of the light frequency shift is obtained. Therefore, the distribution condition of the optical frequency shift virtual magnetic field along the section of the pumping light spot can be described, the distribution characteristic of the optical frequency shift can be evaluated, and the optical frequency shift can be better inhibited.

Description

Technical Field

[0001] This invention relates to the field of SERF atomic magnetometer technology, specifically to a method for measuring the cross-sectional distribution of the optical frequency-shifted virtual magnetic field in a SERF atomic magnetometer. Background Technology

[0002] Atomic magnetometers based on the interaction between light and atoms require lasers to polarize atomic spins. Atomic magnetometers show promising applications in areas such as biomagnetic field measurement and the exploration of dark matter candidate particles. SERF atomic magnetometers are currently a mainstream approach. SERF atomic magnetometers require circularly polarized pump light to polarize atomic spins; however, detuned pump light causes a frequency shift, which can be equivalent to the atom experiencing a virtual magnetic field along the pump light direction. Since the laser in a SERF atomic magnetometer system is typically in an open-loop state, it experiences frequency drift, and this frequency shift introduced by the pump light is usually present in the magnetometer. The presence of this virtual magnetic field increases the linewidth of the atomic magnetometer, reducing its performance; when the virtual magnetic field fluctuates, virtual magnetic field noise also affects the performance of the atomic magnetometer. Since pump light is usually not flat-top light, the optical power density distribution is non-uniform, which leads to different optical frequency shifts at different locations on the pump light cross section. In order to evaluate the distribution characteristics of optical frequency shift and better suppress optical frequency shift, it is very important to measure the distribution of the optical frequency shift virtual magnetic field on the pump light cross section. Existing optical frequency shift measurement methods lack the ability to measure the distribution of optical frequency shift along the pump light cross section. This invention addresses this problem.

[0003] Existing methods for measuring optical frequency shift in SERF atomic magnetometers either measure the overall magnitude of the pump light frequency shift or the frequency shift along the direction of the detection light, but there is no method to focus on the measurement and evaluation of the frequency shift distribution along the cross-section of the pump light spot.

[0004] After searching, the relevant existing technologies are listed below:

[0005] CN114485638A discloses a method for decoupling and suppressing transverse optical frequency shift in an atomic spin inertial measurement device: This method involves decoupling and measuring, and then suppressing, the optical frequency shift caused by the circular polarization of the detection light in the atomic spin inertial measurement device. While the previous invention measured the optical frequency shift in the direction of the detection light, this invention measures the optical frequency shift in the direction of the pump light.

[0006] CN119881754A Single-beam optically pumped atomic magnetometer optical frequency shift virtual magnetic field measurement device and method: The device measures the overall optical frequency shift of the single-beam optically pumped atomic magnetometer, but does not measure the distribution of the optical frequency shift.

[0007] CN116500518A describes a device and method for measuring the optical frequency shift gradient in a SERF atomic magnetic field measuring apparatus, the objective of which is to measure the gradient of the optical frequency shift along the propagation direction of the pump light in an atomic magnetometer. This invention, however, aims to measure the distribution of the optical frequency shift on the cross-section of the pump light spot. The two inventions have different objectives and fundamental structures. In the aforementioned invention, the pump light and the detection light are perpendicular to each other, while in this invention, the pump light and the detection light propagate in the same direction.

[0008] CN106226713A describes a method for suppressing optical frequency shift in a SERF atomic magnetometer: The method involves measuring the overall optical frequency shift and identifying the zero point of the shift to suppress it. However, it only measures the overall optical frequency shift and does not measure the distribution of the shift. Summary of the Invention

[0009] This invention addresses the deficiencies or shortcomings of existing technologies by providing a method for measuring the virtual magnetic field cross-section distribution of optical frequency shift in a SERF atomic magnetometer. This method involves combining a thin beam of detection light with a pump beam using a depolarizing beam splitter. The directions of the pump and detection beams are adjusted so that they pass parallel to each other through the atomic gas cell. The thin detection light is then used to detect the magnitude of the optical frequency shift at different positions of the pump beam. First, the distribution of measurement points is preset based on the size of the gas cell to maximize the measurement of the optical frequency shift distribution along the cross-section of the pump beam spot. When measuring the optical frequency shift using the detection light, the longitudinal magnetic field magnitudes are measured for both left-handed and right-handed circularly polarized pump light. The longitudinal magnetic field value is obtained by subtracting the longitudinal magnetic field value measured for the left-handed case from the longitudinal magnetic field value measured for the right-handed case, and then dividing by 2. The longitudinal magnetic field is measured using a magnetic compensation method; the negative of the optimal magnetic compensation value during the magnetic compensation process is the longitudinal magnetic field value. A precision displacement stage is used to sequentially move the detection beam spot to the preset measurement positions to measure the virtual magnetic field of the optical frequency shift. Once all locations have been measured, the distribution of the optical frequency-shifted virtual magnetic field along the cross-section of the pump light spot can be depicted.

[0010] The technical solution of the present invention is as follows:

[0011] The method for measuring the cross-sectional distribution of the optical frequency-shifted virtual magnetic field of a SERF atomic magnetometer is characterized by comprising the following steps:

[0012] Step 1: In the SERF atomic magnetometer, the pump laser and the detection laser are configured as a double beam that passes through the atomic gas cell parallel to the z-axis.

[0013] Step 2: Heat the atomic gas chamber using a heating film;

[0014] Step 3: Pre-set several measurement points within the boundary of the pump spot on the xy section of the atomic gas cell. The size of each measurement point matches the size of the detection spot, and the position distribution of the measurement points matches the cross-sectional distribution of the optical frequency shift virtual magnetic field.

[0015] Step 4: Adjust the position of the detection light so that the detection spot is aligned with each preset measurement point in sequence;

[0016] Step 5: Set the pump light to left-hand circularly polarized light and measure the magnitude of the first longitudinal magnetic field using magnetic compensation. Then set the pump light to right-hand circularly polarized light and measure the magnitude of the second longitudinal magnetic field using magnetic compensation.

[0017] Step 6: Divide the difference between the first longitudinal magnetic field and the second longitudinal magnetic field by 2 to obtain the magnitude of the optical frequency shift virtual magnetic field at a measurement point location and record it. Then determine whether the measurement point is the last measurement point traversed. If not, return to step 4; if yes, proceed to step 7.

[0018] Step 7: Based on the measurement results of the light traversing all measurement points, depict the cross-sectional distribution of the optical frequency shift virtual magnetic field.

[0019] Step 3 includes the following expression:

[0020]

[0021] Among them B LS (x,y) represents the distribution of the optical frequency-shifted virtual magnetic field along the cross-section of the pump light spot, I pump (x,y) represents the distribution of pump light power density across the beam cross-section, r e Let f be the classical electron radius, c be the speed of light, and f be the velocity of light. D1 γ is the oscillator strength, tr is the transmittance of the air chamber glass, and γ is the oscillator strength. e Let be the electron gyromagnetic ratio, h be Planck's constant, and Δv be the electron gyromagnetic ratio. pump The frequency difference between the pump laser frequency and the D1 line frequency of the alkali metal atom, v D1 The D1 line frequency of alkali metal atoms, Γ D1 The pressure broadening of alkali metal atoms is represented by s, where s is the photon spin component. When there is a frequency difference between the pump light frequency and the atomic resonance frequency, a virtual magnetic field with optical frequency shift will appear. The magnitude of the virtual magnetic field is directly related to the optical power density. When the optical power density of the pump light is not uniformly distributed, the virtual magnetic field with optical frequency shift will have an uneven distribution on the cross-section of the pump light spot.

[0022] Step 6 includes the following expression:

[0023]

[0024] Among them B zσ+ The longitudinal magnetic field during pumping of left-handed circularly polarized light is measured using a magnetic compensation method; the negative of the optimal magnetic compensation value is the longitudinal magnetic field value. z σ- The longitudinal magnetic field during the pumping of right-handed circularly polarized light is measured using the same magnetic compensation method; B z res The remaining geomagnetic field inside the magnetically shielded container is independent of the pump light state; B LS σ+ B is the virtual magnetic field for frequency shifting during the pumping of left-handed circularly polarized light; LS σ- B is the virtual magnetic field for frequency shifting during the pumping of right-hand circularly polarized light; LS σ+ and B LS σ- They are opposites.

[0025] In step 1, the SERF atomic magnetometer includes a depolarizing beam splitter prism disposed on the incident side of the atomic gas cell. The first input side of the depolarizing beam splitter prism is connected to the pump laser via a quarter-wave plate, a mirror, a first collimator, and a first polarization-maintaining fiber. The second input side of the depolarizing beam splitter prism is connected to the detection laser via a second collimator and a second polarization-maintaining fiber. The second collimator is mounted on a first displacement stage. The output side of the depolarizing beam splitter prism, as the pump light transmission side, is connected to an aperture located on a second displacement stage via the atomic gas cell. The aperture intercepts the non-overlapping portion of the pump light and the detection light. The output side of the depolarizing beam splitter prism, as the detection light reflection side, is connected to the data acquisition system via the atomic gas cell, aperture, half-wave plate, polarizing beam splitter prism, differential photodetector, transimpedance amplifier, and lock-in amplifier.

[0026] In step 1, the atomic gas chamber is located inside the heating film, the heating film is located inside the triaxial coil, and the triaxial coil is located inside the magnetic shielding barrel.

[0027] Both the first and second displacement stages are two-degree-of-freedom precision displacement stages.

[0028] The technical effects of this invention are as follows: The method for measuring the cross-sectional distribution of the optical frequency shift virtual magnetic field of the SERF atomic magnetometer of this invention, by making the pump beam and the detection beam pass through the atomic gas cell in parallel, can use a small detection beam to detect the magnitude of the optical frequency shift virtual magnetic field at different positions of the pump beam, and preset the distribution of measurement points to maximize the measurement of the optical frequency shift distribution along the cross-section of the pump beam spot. When the detection beam measures the optical frequency shift, the longitudinal magnetic field magnitudes are measured when the pump beam is left-handed circularly polarized and right-handed circularly polarized, respectively. The difference between the two is then divided by 2 to obtain the value of the optical frequency shift virtual magnetic field. After all positions have been measured, the distribution of the optical frequency shift virtual magnetic field along the cross-section of the pump beam spot can be depicted, which is beneficial for evaluating the distribution characteristics of the optical frequency shift and for better suppressing the optical frequency shift.

[0029] The features and advantages of this invention are as follows:

[0030] 1. This invention is capable of measuring the distribution of optical frequency shift of a SERF atomic magnetometer on the pump spot cross section.

[0031] 2. The results obtained from the measurements of this invention are helpful in assessing the distribution of optical frequency shift.

[0032] 3. The measurement method in this invention is an in-situ measurement under the working condition of the SERF atomic magnetometer. Attached Figure Description

[0033] Figure 1 This is a flowchart of a method for measuring the optical frequency shift virtual magnetic field cross-section distribution of a SERF atomic magnetometer according to the present invention. Figure 1 The process includes: Step 1, setting up the optical path and circuit configuration required for the SERF atomic magnetometer; Step 2, turning on the pump laser and the detection laser; Step 3, aligning the pump laser and the detection laser in parallel; Step 4, heating the atomic gas cell using a heating film; Step 5, designing the distribution of measurement points; Step 6, adjusting the position of the detection light so that the spot of the detection light is aligned with the preset measurement point; Step 7, setting the pump light to left-hand circularly polarized light, measuring the magnitude of the longitudinal magnetic field using magnetic compensation, and recording the result; Step 8, setting the pump light to right-hand circularly polarized light, measuring the magnitude of the longitudinal magnetic field using magnetic compensation, and recording the result; Step 9, subtracting the longitudinal magnetic field of the right-hand pump light from the longitudinal magnetic field of the left-hand pump light and dividing by 2 to obtain the magnitude of the optical frequency shift virtual magnetic field at that location; Step 10, determining whether all measurement points have been traversed; if not, returning to Step 6; if yes, proceeding to Step 11; Step 11, based on the measurement results, depicting the cross-sectional distribution of the optical frequency shift virtual magnetic field.

[0034] Figure 2 This is a schematic diagram of the SERF atomic magnetometer structure involved in implementing the SERF atomic magnetometer optical frequency shift virtual magnetic field cross-section distribution measurement method of the present invention.

[0035] Figure 3 This is a schematic diagram showing the relative positions of the detection light spot and the pumping light spot in this invention. Figure 3 The solid circle represents the range of the pump light spot, and the several dashed circles inside the solid circle represent several preset detection light positions (e.g., 21 detection light positions or 21 measurement points).

[0036] The reference numerals in the attached figures are explained as follows: 1-Pump laser; 2-First polarization-maintaining fiber; 3-First collimator; 4-Mirror; 5-Quarter-wave plate; 6-Detection laser; 7-Second polarization-maintaining fiber; 8-Second collimator; 9-First displacement stage; 10-Depolarization beam splitter prism; 11-Atomic gas cell; 12-Heating film; 13-Triaxial coil; 14-Magnetic shielding barrel; 15-Second displacement stage; 16-Aperture; 17-Half-wave plate; 18-Polarization beam splitter prism; 19-Differential photodetector; 20-Transimpedance amplifier; 21-Lock-in amplifier; 22-Data acquisition system; xyz-Cartesian coordinate system three axes (i.e., x-axis, y-axis, and z-axis). Detailed Implementation

[0037] The following is in conjunction with the attached diagram ( Figures 1-3 The invention will be described in the following sections and examples.

[0038] Figure 1 This is a flowchart of a method for measuring the optical frequency shift virtual magnetic field cross-section distribution of a SERF atomic magnetometer according to the present invention. Figure 2 This is a schematic diagram of the SERF atomic magnetometer structure involved in implementing the SERF atomic magnetometer optical frequency shift virtual magnetic field cross-section distribution measurement method of the present invention. Figure 3 This is a schematic diagram showing the relative positions of the detection light spot and the pump light spot in this invention. (Reference) Figures 1 to 3As shown, the method for measuring the cross-sectional distribution of the optical frequency-shifted virtual magnetic field in a SERF atomic magnetometer includes the following steps: Step 1, configuring the pump laser and the detection laser in the SERF atomic magnetometer as dual beams that pass parallel to the z-axis through the atomic gas cell; Step 2, heating the atomic gas cell using a heating film; Step 3, pre-setting several measurement points within the boundary of the pump spot on the xy-section of the atomic gas cell, the size of each measurement point matching the size of the detection spot, and the positional distribution of the measurement points matching the cross-sectional distribution of the optical frequency-shifted virtual magnetic field; Step 4, adjusting the position of the detection light so that the detection spot is sequentially aligned with each pre-set measurement point; Step 5 5. Set the pump light to left-hand circularly polarized light and measure the magnitude of the first longitudinal magnetic field using magnetic compensation. Then set the pump light to right-hand circularly polarized light and measure the magnitude of the second longitudinal magnetic field using magnetic compensation. Step 6. Divide the difference between the first and second longitudinal magnetic fields by 2 to obtain the magnitude of the optical frequency shift virtual magnetic field at a measurement point and record it. Then determine whether the measurement point is the last measurement point reached. If not, return to step 4. If yes, proceed to step 7. Step 7. Based on the measurement results of the measurement detection light traversing all measurement points, depict the cross-sectional distribution of the optical frequency shift virtual magnetic field.

[0039] Step 3 includes the following expression:

[0040]

[0041] Among them B LS (x,y) represents the distribution of the optical frequency-shifted virtual magnetic field along the cross-section of the pump light spot, I pump (x,y) represents the distribution of pump light power density across the beam cross-section, r e Let f be the classical electron radius, c be the speed of light, and f be the velocity of light. D1 γ is the oscillator strength, tr is the transmittance of the air chamber glass, and γ is the oscillator strength. e Let be the electron gyromagnetic ratio, h be Planck's constant, and Δv be the electron gyromagnetic ratio. pump The frequency difference between the pump laser frequency and the D1 line frequency of the alkali metal atom, v D1 The D1 line frequency of alkali metal atoms, Γ D1 The pressure broadening of alkali metal atoms is represented by s, where s is the photon spin component. A virtual optical frequency-shifting magnetic field appears when there is a frequency difference between the pump light frequency and the atomic resonance frequency. The magnitude of this virtual magnetic field is directly related to the optical power density; when the pump light power density distribution is non-uniform, the virtual optical frequency-shifting magnetic field exhibits a non-uniform distribution across the pump light spot cross-section.

[0042] Step 6 includes the following expression:

[0043]

[0044] Among them B z σ+The longitudinal magnetic field during pumping of left-handed circularly polarized light is measured using a magnetic compensation method; the negative of the optimal magnetic compensation value is the longitudinal magnetic field value. z σ- The longitudinal magnetic field during the pumping of right-handed circularly polarized light is measured using the same magnetic compensation method; B z res The remaining geomagnetic field inside the magnetically shielded container is independent of the pump light state; B LS σ+ B is the virtual magnetic field for frequency shifting during the pumping of left-handed circularly polarized light; LS σ- B is the virtual magnetic field for frequency shifting during the pumping of right-hand circularly polarized light; LS σ+ and B LS σ- They are opposites.

[0045] In step 1, the SERF atomic magnetometer includes a depolarizing beam splitter 10 disposed on the incident side of the atomic gas cell 11. The first input side of the depolarizing beam splitter 10 is connected to the pump laser 1 via a quarter-wave plate 5, a reflector 4, a first collimator 3, and a first polarization-maintaining fiber 2. The second input side of the depolarizing beam splitter 10 is connected to the detection laser 6 via a second collimator 8 and a second polarization-maintaining fiber 7. The second collimator 8 is mounted on a first displacement stage 9. The output side of the depolarizing beam splitter 10, as the pump light transmission side, is connected to an aperture 16 located on a second displacement stage 15 via the atomic gas cell 11. The aperture 16 intercepts the non-overlapping portion of the pump light and the detection light. The output side of the depolarizing beam splitter 10, as the detection light reflection side, is connected to the data acquisition system 22 via the atomic gas cell 11, aperture 16, a quarter-wave plate 17, a polarizing beam splitter 18, a differential photodetector 19, a transimpedance amplifier 20, and a lock-in amplifier 21. In step 1, the atomic gas chamber 11 is located inside the heating film 12, which is located inside the triaxial coil 13, which is located inside the magnetic shielding barrel 14. Both the first displacement stage 9 and the second displacement stage 15 are two-degree-of-freedom precision displacement stages.

[0046] A method for measuring the optical frequency shift virtual magnetic field cross-section distribution of a SERF atomic magnetometer, the implementation process of which includes:

[0047] Step 1: Configure the optical path and circuitry required for the SERF atomic magnetometer;

[0048] Step 2: Turn on the pump laser and the detection laser;

[0049] Step 3: Align the pump laser and the detection laser parallel to each other;

[0050] Step 4: Heat the atomic gas chamber using a heating film;

[0051] Step 5: Design the distribution of measurement points;

[0052] Step 6: Adjust the position of the detection light so that the spot of the detection light is aligned with the preset measurement point;

[0053] Step 7: Set the pump light to left-hand circularly polarized light, measure the magnitude of the longitudinal magnetic field using magnetic compensation, and record it;

[0054] Step 8: Set the pump light to right-hand circularly polarized light, measure the magnitude of the longitudinal magnetic field using magnetic compensation, and record it;

[0055] Step 9: Subtract the longitudinal magnetic field of the right-hand pump light from the longitudinal magnetic field of the left-hand pump light and divide by 2 to obtain the magnitude of the virtual magnetic field of the optical frequency shift at that position; repeat steps 6, 7, 8, and 9 to make the detection light traverse all preset measurement points.

[0056] Step 10: Based on the measurement results, depict the distribution of optical frequency shift.

[0057] The optical path and circuit configuration required for the SERF atomic magnetometer described in step 1 include: a pump laser 1 for generating pump laser light, equipped with an optical fiber coupler, which is connected to a first polarization-maintaining fiber 2. The pump laser light exits after passing through the first polarization-maintaining fiber 2 and a first collimator 3. The pump laser light changes its beam direction after passing through a reflector 4, the direction of which is finely adjustable. After the beam direction is changed, the pump laser light passes through a quarter-wave plate 5, which converts it into circularly polarized light. Then, after passing through a depolarizing beam splitter 10, it enters the atomic gas cell 11 along the Z-direction, pumping and polarizing alkali metal atoms. The detection laser 6 is also equipped with an optical fiber coupler. The detection laser light exits after passing through a second polarization-maintaining fiber 7 and a second collimator 8 fixed on a first displacement stage 9. After exiting, it passes through the depolarizing beam splitter 10, and the reflected light after passing through the depolarizing beam splitter 10 is also in the Z-direction and enters the atomic gas cell 11. The pump laser and the detection laser are combined at the depolarization beam splitter prism 10, and the combined pump laser and detection laser pass parallel through the gas chamber 11. To achieve the SERF state, the atomic gas chamber 11 is heated by the heating film 12, and the gas chamber 11 is placed in the magnetic shielding barrel 14 to shield the external geomagnetic field. The triaxial coil 13 is located inside the magnetic shielding barrel 14 to compensate for the magnetic field and apply the modulation magnetic field. After the pump laser and the detection laser pass through the atomic gas chamber 11, they are intercepted by the aperture 16, allowing only the detection light to pass through. After being intercepted by the aperture 16, the laser passes through the half-wave plate 17 and the polarization beam splitter prism 18 to convert the optical rotation angle signal into an optical power signal. The optical signal is received by the differential photodetector 19 and converted into a photocurrent. The photocurrent is converted into a voltage signal by the transimpedance amplifier 20. The voltage signal is demodulated by the lock-in amplifier 21. Finally, the demodulated signal is acquired by the data acquisition system 22.

[0058] The modulation magnetic field is applied perpendicular to the pump light direction. Its function is to enable the magnetometer's longitudinal electron spin polarization to linearly sense the measured magnetic field in the direction of the modulation magnetic field near zero field, thus allowing the magnetometer to have a linear sensitivity range near zero field. The lock-in amplifier's demodulation frequency is one harmonic of the modulation magnetic field frequency. The triaxial coil's geometric dimensions are much larger than the gas chamber's dimensions, and the gas chamber is supported by structural components and placed at the geometric center of the triaxial coil, ensuring the uniformity of the magnetic field applied by the triaxial coil to the gas chamber.

[0059] The heating film is adhered to a highly thermally conductive material, which conducts heat to the gas chamber. The position of the aperture is always consistent with the position of the detection light, and the aperture size is also consistent with the size of the detection light spot. Although a small portion of the pump light will still pass through the aperture after being intercepted, due to the circular polarization state of the pump light, the transmitted and reflected light signals after passing through the polarizing beam splitter will cancel each other out. The half-wave plate is used to return the differential light power signal to zero in the zero-field condition. When a magnetic field is present, the polarization state of the linearly polarized detection light will be deflected by the magnetic field. This is the magnetometer system converting the magnetic field signal into an optical rotation angle signal. The change in the optical rotation angle will cause a change in the transmitted and reflected light of the polarizing beam splitter, resulting in a change in the differential light signal. This is the magnetometer system converting the optical rotation angle signal into an optical power signal. The differential photodetector then receives the optical signal, converts it into a photocurrent, and then processes it through the electrical system. Finally, the magnetic field signal is successfully detected. The optical power of the pump laser and the detection laser is adjusted by regulating the coupling efficiency of the fiber optic couplers on the pump laser and the detection laser, respectively. The demodulation frequency of the lock-in amplifier is one harmonic of the modulation magnetic field frequency.

[0060] In step 3, to ensure the parallelism of the pump laser and the detection laser, two points relatively far apart on the light propagation path are selected using a beam quality analyzer. The relative positions of the pump and detection laser spot cross-sections are measured and observed using the beam quality analyzer. The directions of the pump and detection laser beams are adjusted until the relative positions of the pump and detection laser spot cross-sections remain unchanged when observing the spot distribution at different locations. The direction of the pump laser is adjusted using an adjustable angle holder that fixes the first collimator and a reflector; the direction of the detection laser is adjusted using an adjustable angle holder that fixes the second collimator and a depolarizing beam splitter. Through the adjustment of these devices, the pump and detection lasers are made parallel.

[0061] In step 6, the position of the second collimator is adjusted by operating the first displacement stage, thereby adjusting the position of the detection light spot so that the detection light spot moves to the preset measurement point, such as... Figure 3 As shown. Figure 3 In the diagram, the dashed line represents the preset detection light position, and the solid line represents the range of the pump light spot. Figure 3 For illustrative purposes only, the specific settings of the measurement points depend on the sizes of the pump light spot and the detection light spot. During the design phase, careful planning is needed regarding the distance between the measurement points and the center of the pump light spot. While adjusting the position of the detection light spot, the second displacement stage must also be adjusted simultaneously, ensuring that the position of the aperture on the second displacement stage follows the position of the detection light spot, maintaining the aperture diameter aligned with the detection light spot. Both the first and second displacement stages are graduated, two-degree-of-freedom precision displacement stages.

[0062] In steps 7, 8, and 9, the measurement process of the optical frequency-shifted virtual magnetic field can be described by the following formula.

[0063]

[0064] Among them, B z σ+ The longitudinal magnetic field is the pumping force of left-handed circularly polarized light. The longitudinal magnetic field value is measured using a magnetic compensation method, and the negative of the optimal magnetic compensation value is the longitudinal magnetic field value. B z σ- The longitudinal magnetic field is the pumping force of right-handed circularly polarized light. The value of the longitudinal magnetic field is also measured using the magnetic compensation method. z res This refers to the remaining geomagnetic field inside the magnetically shielded container, which is unrelated to the pump light state. (B) LS σ+ B is the virtual magnetic field for frequency shifting during the pumping of left-handed circularly polarized light. LS σ- The virtual magnetic field for frequency shift during right-handed circularly polarized light pumping is an opposite of the virtual magnetic field for right-handed circularly polarized light pumping. Finally, the virtual magnetic field for frequency shift during left-handed circularly polarized light pumping is calculated by subtracting the longitudinal magnetic field for right-handed circularly polarized light pumping from the longitudinal magnetic field for left-handed circularly polarized light pumping, and then dividing by 2. The magnetic compensation employs a frequency doubling method for triaxial magnetic field compensation. The polarization state of the pump light is adjusted using a quarter-wave plate.

[0065] The magnetic compensation method employs a frequency doubling method: When compensating for the magnetic field in the X direction, a sinusoidal magnetic field of n Hz (n is a set number) is applied to the X direction, and the magnitude of the compensation magnetic field in the X direction is adjusted until the magnetometer's response signal to the n Hz sinusoidal magnetic field becomes 2n Hz; the same principle applies when compensating for the magnetic field in the Y direction; when compensating for the magnetic field in the Z direction, a bias magnetic field is first applied to the X direction to make the magnetometer more sensitive to the magnetic field response in the Z direction, and then a sinusoidal magnetic field of n Hz is applied to the Z direction, and the magnitude of the compensation magnetic field in the Z direction is adjusted until the magnetometer's response signal to the n Hz sinusoidal magnetic field becomes 2n Hz. Then, the bias magnetic field in the X direction is removed, and the triaxial magnetic field compensation process is repeated several times to iterate for the optimal compensation value. The more repetitions, the more accurate the optimal compensation value.

[0066] The optical frequency shift virtual magnetic field B mentioned above LS The theoretical expression for the lateral distribution on the cross-section of the pump light spot is:

[0067]

[0068] Among them, B LS (x,y) represents the distribution of the optical frequency-shifted virtual magnetic field along the cross-section of the pump light spot, I pump (x,y) represents the distribution of pump light power density across the beam cross-section, re Let f be the classical electron radius, c be the speed of light, and f be the velocity of light. D1 γ is the oscillator strength, tr is the transmittance of the air chamber glass, and γ is the oscillator strength. e For electron gyromagnetic ratio, Δv pump The frequency difference between the pump laser frequency and the D1 line frequency of the alkali metal atom, v D1 The D1 line frequency of alkali metal atoms, Γ D1 Let s represent the pressure broadening of alkali metal atoms and s be the photon spin component. The formula shows that a frequency-shifted virtual magnetic field appears when there is a frequency difference between the pump light frequency and the atomic resonance frequency. Since lasers in atomic magnetometers typically do not undergo closed-loop frequency stabilization, a frequency difference usually exists between the pump laser and the resonance frequency, resulting in the appearance of a frequency-shifted virtual magnetic field in the atomic magnetometer. The formula also shows that the magnitude of the virtual magnetic field is directly related to the optical power density; when the pump light power density distribution is non-uniform, the frequency-shifted virtual magnetic field also exhibits a non-uniform distribution. Due to the inherent characteristics of lasers, the light produced by lasers is usually not flat-top light, and converting it to flat-top light requires additional devices. This results in non-uniform pump light, and consequently, a non-uniform frequency-shifted virtual magnetic field. Therefore, measuring the distribution of the frequency-shifted virtual magnetic field using the method described in this invention is crucial for evaluating its distribution along the pump light cross-section.

[0069] Contents not described in detail in this specification are prior art known to those skilled in the art. It is hereby indicated that the above description is intended to help those skilled in the art understand this invention, but does not limit the scope of protection of this invention. Any equivalent substitutions, modifications, improvements, and / or simplifications of the above descriptions that do not depart from the essential content of this invention fall within the scope of protection of this invention.

Claims

1. A method for measuring the cross-sectional distribution of the optical frequency-shifted virtual magnetic field in a SERF atomic magnetometer, characterized in that, Includes the following steps: Step 1: In the SERF atomic magnetometer, the pump laser and the detection laser are configured as a double beam that passes through the atomic gas cell parallel to the z-axis. Step 2: Heat the atomic gas chamber using a heating film; Step 3: Pre-set several measurement points within the boundary of the pump spot on the xy section of the atomic gas cell. The size of each measurement point matches the size of the detection spot, and the position distribution of the measurement points matches the cross-sectional distribution of the optical frequency shift virtual magnetic field. Step 4: Adjust the position of the detection light so that the detection spot is aligned with each preset measurement point in sequence; Step 5: Set the pump light to left-hand circularly polarized light and measure the magnitude of the first longitudinal magnetic field using magnetic compensation. Then set the pump light to right-hand circularly polarized light and measure the magnitude of the second longitudinal magnetic field using magnetic compensation. Step 6: Divide the difference between the first longitudinal magnetic field and the second longitudinal magnetic field by 2 to obtain the magnitude of the optical frequency shift virtual magnetic field at a measurement point location and record it. Then determine whether the measurement point is the last measurement point traversed. If not, return to step 4; if yes, proceed to step 7. Step 7: Based on the measurement results of the light traversing all measurement points, depict the cross-sectional distribution of the optical frequency shift virtual magnetic field.

2. The method for measuring the cross-sectional distribution of the optical frequency shift virtual magnetic field of a SERF atomic magnetometer according to claim 1, characterized in that, Its features are, Step 3 includes the following expression: Among them B LS (x,y) represents the distribution of the optical frequency-shifted virtual magnetic field along the cross-section of the pump light spot, I pump (x,y) represents the distribution of pump light power density across the beam cross-section, r e Let f be the classical electron radius, c be the speed of light, and f be the velocity of light. D1 γ is the oscillator strength, tr is the transmittance of the air chamber glass, and γ is the oscillator strength. e Let be the electron gyromagnetic ratio, h be Planck's constant, and Δv be the electron gyromagnetic ratio. pump The frequency difference between the pump laser frequency and the D1 line frequency of the alkali metal atom, v D1 The D1 line frequency of alkali metal atoms, Γ D1 The pressure broadening of alkali metal atoms is represented by s, where s is the photon spin component. When there is a frequency difference between the pump light frequency and the atomic resonance frequency, a virtual magnetic field with optical frequency shift will appear. The magnitude of the virtual magnetic field is directly related to the optical power density. When the optical power density of the pump light is not uniformly distributed, the virtual magnetic field with optical frequency shift will have an uneven distribution on the cross-section of the pump light spot.

3. The method for measuring the cross-sectional distribution of the optical frequency shift virtual magnetic field of a SERF atomic magnetometer according to claim 1, characterized in that, Step 6 includes the following expression: Among them B z σ+ The longitudinal magnetic field during pumping of left-handed circularly polarized light is measured using a magnetic compensation method; the negative of the optimal magnetic compensation value is the longitudinal magnetic field value. z σ- The longitudinal magnetic field during the pumping of right-handed circularly polarized light is measured using the same magnetic compensation method; B z res The remaining geomagnetic field inside the magnetic shielding barrel is unrelated to the pump light state. B LS σ+ B is the virtual magnetic field for frequency shifting during the pumping of left-handed circularly polarized light; LS σ- B is the virtual magnetic field for frequency shifting during the pumping of right-hand circularly polarized light; LS σ+ and B LS σ- They are opposites.

4. The method for measuring the cross-sectional distribution of the optical frequency shift virtual magnetic field of a SERF atomic magnetometer according to claim 1, characterized in that, In step 1, the SERF atomic magnetometer includes a depolarizing beam splitter prism positioned on the incident side of the atomic gas cell. The first input side of the depolarizing beam splitter prism is connected to the pump laser via a quarter-wave plate, a reflector, a first collimator, and a first polarization-maintaining fiber. The second input side of the depolarizing beam splitter prism is connected to the detection laser via a second collimator and a second polarization-maintaining fiber. The second collimator is mounted on a first displacement stage. The output side of the depolarizing beam splitter prism, serving as the pump light transmission side, is connected to an aperture located on a second displacement stage via the atomic gas cell. The aperture intercepts the non-overlapping portion of the pump light and the detection light. Although some pump light will still pass through the aperture after interception, due to the circular polarization state of the pump light, the transmitted and reflected light signals after passing through the polarizing beam splitter prism will cancel each other out. The output side of the depolarizing beam splitter prism, serving as the detection light reflection side, is connected to the data acquisition system via the atomic gas cell, aperture, quarter-wave plate, polarizing beam splitter prism, differential photodetector, transimpedance amplifier, and lock-in amplifier.

5. The method for measuring the cross-sectional distribution of the optical frequency shift virtual magnetic field of a SERF atomic magnetometer according to claim 1, characterized in that, In step 1, the atomic gas chamber is located inside the heating film, the heating film is located inside the triaxial coil, and the triaxial coil is located inside the magnetic shielding barrel.

6. The method for measuring the cross-sectional distribution of the optical frequency shift virtual magnetic field of a SERF atomic magnetometer according to claim 1, characterized in that, In step 1, to ensure the parallelism of the pump laser and the detection laser, two points relatively far apart on the light propagation path are selected using a beam quality analyzer. The relative positions of the pump and detection laser spot cross-sections are measured and observed using the beam quality analyzer. The directions of the pump and detection laser beams are adjusted until the relative positions of the pump and detection laser spot cross-sections remain unchanged when observing the spot distribution at different locations. The direction of the pump laser is adjusted using an adjustable angle holder that fixes the first collimator and a reflector; the direction of the detection laser is adjusted using an adjustable angle holder that fixes the second collimator and a depolarizing beam splitter. Through the adjustment of these devices, the pump and detection lasers are made parallel.

7. The method for measuring the cross-sectional distribution of the optical frequency shift virtual magnetic field of a SERF atomic magnetometer according to claim 1, characterized in that, In step 4, the position of the second collimator is adjusted by operating the first displacement stage, thereby adjusting the position of the detection light spot so that it moves to the preset measurement point. The specific setting of the measurement point depends on the size of the pump light spot and the detection light spot. When designing, attention should be paid to planning the distance between the measurement point and the center of the pump light spot. While adjusting the position of the detection light spot, the second displacement stage needs to be adjusted simultaneously so that the position of the aperture on the second displacement stage moves with the position of the detection light spot, keeping the aperture diameter coincident with the detection light spot.

8. The method for measuring the cross-sectional distribution of the optical frequency shift virtual magnetic field of a SERF atomic magnetometer according to claim 4, characterized in that, Both the first and second displacement stages are two-degree-of-freedom precision displacement stages.

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