Apparatus and methods for improving the uniformity of electronic polarization in SERF atomic magnetometers

Abstract

This invention discloses an apparatus and method for improving the uniformity of electronic polarization in a SERF atomic magnetometer. The apparatus uses two circularly polarized beams with frequencies deviating from the resonance absorption peaks of alkali metal atoms as pump beams to polarize the atoms. The two pump beams deviate from the resonance absorption peaks by equal amounts of blue and red shifts. On one hand, the deviation of the pump beams from the resonance peaks significantly reduces the atomic polarization gradient caused by alkali metal atom absorption during beam propagation along the gas cell. On the other hand, when the frequency detuning of the pump beams is half the pressure broadening value of the gas cell optical spectral lines, the equivalent virtual magnetic field caused by the optical frequency shift due to the alkali metal atom resonance peaks is insensitive to laser frequency jitter. Therefore, the apparatus and method disclosed in this invention can significantly reduce the electronic polarization gradient, which is beneficial for reducing optical frequency shift noise and improving magnetic field measurement sensitivity. This invention is not only applicable to SERF atomic magnetometers but also to SERF atomic spin inertia and atomic gyroscopes.

Description

Technical Field

[0001] This invention relates to SERF atomic magnetic field measurement technology, specifically to a device and method for improving the uniformity of electronic polarization of a SERF atomic magnetometer. Background Technology

[0002] Spin Exchange Relaxation Free Regime (SERF) atomic spin magnetometers are high-precision devices for measuring extremely weak magnetic fields and have been widely used in fundamental physics research, geomagnetic exploration, and biomedicine. For SERF atomic magnetometers based on optical pumping, the electronic polarizability of the alkali metal atoms (potassium, rubidium, cesium, etc.) in the sensitive medium is a crucial indicator determining the magnetometer's magnetic field measurement sensitivity and signal strength. The uniformity and stability of the electronic polarizability distribution are important factors affecting the system's detection sensitivity. To achieve high sensitivity, SERF magnetometers typically operate at 100℃-200℃ to maintain a high alkali metal density in the gas cell. However, this high alkali metal density causes absorption of the pump light, resulting in a large electronic polarization gradient along the pump direction. Therefore, a device and method to improve the uniformity of electronic polarizability are needed.

[0003] The existing technology (J. Zhao, G. Liu, J. Lu, et al. "Improvement of spin polarization spatial uniformity in optically pumped atomic magnetometers based on counter-propagating pump beams and atomic diffusion," Meas. Sci. Technol. 32, 35902 (2021)) mentions that when the gas chamber temperature is high, two resonant circularly polarized lasers can be used to suppress the polarizability gradient by incident on opposite sides of the gas chamber. However, this method has limited effect on improving the uniformity of electronic polarizability distribution in alkali metal atom pumping schemes. In addition, when pumping atoms at the resonant frequency, the optical frequency shift is quite sensitive to the frequency fluctuations of the pump source, which places high demands on the frequency stability of the laser used. Summary of the Invention

[0004] The purpose of this invention is to provide a device and method for improving the uniformity of electronic polarization in a SERF atomic magnetometer. Utilizing the characteristic that when the pump light frequency in the alkali metal cell deviates from the blue or red shift of the alkali metal atom's resonance absorption peak, the alkali metal atoms experience an equivalent virtual magnetic field with opposite directions of optical frequency shift. Two laser beams are used, with their optical frequencies set to blue and red shifts respectively, each with a detuning amount half the pressure broadening value of the alkali metal atom's resonance absorption spectral line. The two laser beams are then combined and evenly split into two beams, which are then incident on opposite sides of the alkali metal cell to pump polarized alkali metal atoms. This effectively overcomes the aforementioned shortcomings.

[0005] The technical solution of the present invention is as follows:

[0006] A device for improving the uniformity of electronic polarization of a SERF atomic magnetometer is characterized by comprising a first pump laser for outputting a blue-shifted pump beam, a second pump laser for outputting a red-shifted pump beam, and a polarization-maintaining fiber coupler or depolarization-depolarizing beam splitter for combining the blue-shifted and red-shifted pump beams and then splitting them into two pump beams for output. The two pump beams are incident on opposite sides of an alkali metal gas cell to pump polarized alkali metal atoms, so that the equivalent virtual magnetic field of the optical frequency shift experienced by the alkali metal atoms due to frequency detuning is equal in magnitude and opposite in direction, thereby achieving spatial cancellation everywhere.

[0007] One of the two pump beams passes sequentially through a second beam expander, a first reflector, a second reflector, a third reflector, a first half-wave plate, a first polarizing beam splitter, a fourth reflector, a first polarizer, and a first quarter-wave plate before entering the alkali metal gas chamber from the top. The other beam passes sequentially through a first beam expander, a fifth reflector, a second half-wave plate, a second polarizing beam splitter, a sixth reflector, a second polarizer, and a second quarter-wave plate before entering the alkali metal gas chamber from the bottom. Around the alkali metal gas chamber, from the inside out, are arranged a non-magnetic electric heating system, a triaxial magnetic compensation coil, and a magnetic shielding barrel. The triaxial magnetic compensation coil is connected to a signal acquisition and processing unit via a function generator. The signal acquisition and processing unit is connected to a lock-in amplifier, a first rotating mirror controller, and a second rotating mirror controller. The first rotating mirror controller controls the first half-wave plate via a first rotating mirror mount, and the second rotating mirror controller controls the second half-wave plate via a second rotating mirror mount.

[0008] The lock-in amplifier is connected to the photoelastic modulation crystal via a photoelastic modulator. The photoelastic modulator supplies a reference frequency to the lock-in amplifier. The detection light input side of the photoelastic modulation crystal is connected to the detection laser via a third polarizer, an eighth mirror, and a seventh mirror. The detection light output side of the photoelastic modulation crystal is connected to the lock-in amplifier via a third quarter-wave plate, the alkali metal gas cell, an analyzer, and a photodetector. The input terminal of the lock-in amplifier receives the signal collected by the photodetector and demodulates the photodetector signal using the modulation frequency of the photoelastic modulator as a reference frequency to obtain the response information of the magnetic field to be measured.

[0009] The first polarizer and the first quarter-wave plate convert the light beam into left-handed or right-handed circularly polarized light, which enters the alkali metal gas cell to polarize the alkali metal atoms. The second polarizer and the second quarter-wave plate convert the light beam into right-handed or left-handed circularly polarized light, which enters the alkali metal gas cell to polarize the alkali metal atoms.

[0010] The redshift of the left detuned pump beam from the alkali metal atom resonance absorption peak is equal to the blueshift of the right detuned pump beam from the alkali metal atom resonance absorption peak. This is achieved by coupling two laser beams with equal blueshift and redshift from the alkali metal atom resonance absorption peak into one beam as the pump beam, thus realizing optical frequency shift cancellation.

[0011] A method for improving the uniformity of electronic polarization in a SERF atomic magnetometer, utilizing the aforementioned apparatus for improving the uniformity of electronic polarization in a SERF atomic magnetometer, is characterized by comprising the following steps:

[0012] Step 1: Set the temperature of the alkali metal gas chamber to 100-200℃, and adjust the optical path and the triaxial magnetic compensation coil to make the device work in SERF mode.

[0013] Step 2: Turn off the second pump laser, turn on the first pump laser, and fix the pump light intensity to a certain value I. pump1 The pump light frequency was changed in one direction near the D1 line of alkali metal atoms. At each pump light frequency v, a three-dimensional in-situ magnetic compensation technique was used. The magnetic field generated by the three-dimensional magnetic field coil controlled by the function generator compensated the magnetic field felt by the atoms in the alkali metal gas cell, so that the magnetic field felt by the atoms was 0. The corresponding magnetic compensation value B of the pump light direction was obtained, and the maximum magnetic compensation values ​​B1 and v1 were recorded.

[0014] Step 3: Turn off the first pump laser, turn on the second pump laser, and fix the pump light intensity to a certain value I. pumpNear the D1 line of alkali metal atoms, the pump light frequency is changed in the opposite direction to that in step 1. At each pump light frequency v, a three-dimensional in-situ magnetic compensation technique is used. The magnetic field generated by the three-dimensional magnetic field coil controlled by the function generator compensates for the magnetic field experienced by the atoms in the alkali metal gas cell, making the magnetic field experienced by the atoms zero, and obtaining the corresponding magnetic compensation value B in the pump light direction. The maximum magnetic compensation values ​​B2 and v2 are recorded. At frequency v2, the pump light intensity is changed so that the magnetic compensation value B2 = B1, and the pump light intensity I at this time is recorded. pump2 ;

[0015] Step 4: Simultaneously turn on both pump lasers, setting the light intensity and frequency to I. pump1 I pump2 v1 and v2, at this time, the equivalent virtual magnetic field of optical frequency shift caused by frequency detuning is equal in magnitude and opposite in direction, achieving cancellation everywhere in space. Three-dimensional in-situ magnetic compensation technology is used to compensate for the residual magnetism of the system. A calibration magnetic field is added to the sensitive axis Y using a function generator. The half-wave plate on the rotating frame is controlled by the rotation controller to proportionally change the intensity of the two pump laser beams, so that the output magnetic field response signal of the lock-in amplifier is the strongest. At this time, the electronic polarization distribution in the gas chamber is the most uniform.

[0016] The technical effects of this invention are as follows: This invention provides a device and method for improving the uniformity of electronic polarization in a SERF atomic magnetometer. The device uses two circularly polarized beams with frequencies deviating from the resonance absorption peak of alkali metal atoms as pump beams to polarize the atoms. The frequencies of the two pump beams deviate from the resonance absorption peak of the alkali metal atoms by equal amounts of blue shift and red shift. On one hand, the deviation of the pump beam from the resonance peak can significantly reduce the atomic polarization gradient caused by alkali metal atom absorption during beam propagation along the gas cell. On the other hand, when the frequency detuning of the pump beam is half the pressure broadening value of the optical spectral lines in the gas cell, the equivalent virtual magnetic field caused by the optical frequency shift due to the frequency deviation from the resonance peak of the alkali metal atoms is insensitive to laser frequency jitter. Therefore, the device and method disclosed in this invention can significantly reduce the electronic polarization gradient, which is beneficial for reducing optical frequency shift noise caused by laser frequency jitter and improving the sensitivity of magnetic field measurement. This invention is not only applicable to SERF atomic magnetometers but also to SERF atomic spin inertia and atomic gyroscopes.

[0017] The features of this invention are as follows: (1) The use of frequency-detuned pump laser can significantly reduce the attenuation of laser intensity along the pump direction caused by the absorption of alkali metal atoms, thereby improving the uniformity of alkali metal electronic polarization; (2) By combining two laser beams with opposite frequency detuning amounts to polarize alkali metal atoms, the uniformity of polarization can be improved, and optical frequency shift cancellation can be achieved at the same time; (3) The pump light frequency detuning amount is set to half the pressure broadening value of the gas cell spectral line, so the optical frequency shift is not sensitive to laser frequency jitter, which is beneficial to reducing the optical frequency shift noise caused by laser frequency jitter in the system and improving the sensitivity of the device. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the device structure for improving the uniformity of electronic polarization of the SERF atomic magnetometer in accordance with the present invention.

[0019] Figure 2 This is a schematic diagram showing the distribution curves of the electronic polarizability of alkali metal atoms in the gas cell along the pumping direction under different pump light frequencies. Figure 2 The incident pump light intensity is the same, the gas chamber temperature is 200℃, and the point where the pump light intersects the gas chamber surface is defined as the origin of the gas chamber coordinate system. Figure 2 The horizontal axis represents the position of the gas cell (mm, Z-axis value within the gas cell), and the vertical axis represents the electronic polarizability (%). The four curves on the left, from top to bottom, involve pump light frequency detuning of 0 GHz, 20 GHz, 40 GHz, and 60 GHz, respectively. The greater the pump light frequency detuning, the smoother the change in electronic polarizability along the z-axis.

[0020] Figure 3(a) shows the equivalent virtual magnetic field of optical frequency shift generated in the alkali metal gas cell under the same pump light intensity due to pump light frequency detuning, and the variation of its first derivative with the amount of detuning. In Figure 3(a), the horizontal axis represents the amount of frequency detuning (GHz); the left vertical axis represents the equivalent virtual magnetic field B of optical frequency shift. LS (nT), corresponding to the solid curve; the right vertical axis represents the first derivative B of the equivalent virtual magnetic field of the optical frequency shift. LS '(nT / GHz), corresponding to the dashed curve; if the frequency detuning is controlled at -10GHz, the virtual magnetic field of optical frequency shift is +3nT, and the frequency detuning is controlled at +10GHz, the virtual magnetic field of optical frequency shift is -3nT. The positive and negative values ​​cancel each other out, which can eliminate the optical frequency shift.

[0021] Figure 3(b) illustrates the dependence of pump rate on frequency detuning. In Figure 3(b), the horizontal axis represents the frequency detuning amount (GHz); the vertical axis represents the pump rate R. op (au, relative value, arbitrary unit); as the detuning increases, the pumping rate decreases.

[0022] The reference numerals in the attached figures are listed below: 1-First pump laser; 2-Second pump laser; 3-Polarization-maintaining fiber coupler or depolarization-reducing beam splitter; 4-First beam expander system; 5-Second beam expander system; 6-First reflector; 7-Second reflector; 8-Third reflector; 9-First half-wave plate; 10-First rotating mirror mount (connected to the signal acquisition and processing unit via the first rotating mirror controller); 11-First polarization beam splitter; 12-Fourth reflector; 13-First polarizer; 14-First half-wave plate; 15-Magnetic shielding barrel; 16-Triaxial magnetic compensation coil (connected to the signal acquisition and processing unit via a function generator); 17-Non-magnetic... Electric heating system; 18-Alkali metal gas chamber; 19-Fifth reflecting mirror; 20-Second half-wave plate; 21-Second rotating mirror frame (connected to the signal acquisition and processing unit via the second rotating mirror controller); 22-Second polarizing beam splitter; 23-Sixth reflecting mirror; 24-Second polarizer; 25-Second half-wave plate; 26-Detection laser; 27-Seventh reflecting mirror; 28-Eighth reflecting mirror; 29-Third polarizer; 30-Photoelastic modulation crystal (connected to the lock-in amplifier via the photoelastic modulator, the photoelastic modulator supplies the reference frequency to the lock-in amplifier); 31-Third half-wave plate; 32-Analyzer; 33-Photodetector. Detailed Implementation

[0023] The following is in conjunction with the attached diagram ( Figure 1 - Figure 3b The invention will be described in the following sections and examples.

[0024] Figure 1 This is a schematic diagram of the device structure for improving the uniformity of electronic polarization of the SERF atomic magnetometer in accordance with the present invention. Figure 2 Figure 3(a) shows the distribution curves of electronic polarizability of alkali metal atoms along the pump direction in the gas cell under different pump light frequencies. Figure 3(a) shows the equivalent virtual magnetic field of optical frequency shift generated in the alkali metal gas cell by pump light frequency detuning under the same pump light intensity, and the variation of its first derivative with the amount of detuning. Figure 3(b) shows the dependence of pump rate on frequency detuning. (Reference) Figures 1 to 3(b) As shown, a device for improving the uniformity of electronic polarization of a SERF atomic magnetometer includes a first pump laser 1 for outputting a blue-shifted pump beam, a second pump laser 2 for outputting a red-shifted pump beam, and a polarization-maintaining fiber coupler or depolarization-depolarizing beam splitter 3 for combining the blue-shifted and red-shifted pump beams and then splitting them into two pump beams for output. The two pump beams are incident on opposite sides of an alkali metal gas cell 18 to pump polarized alkali metal atoms, so that the equivalent virtual magnetic field of the optical frequency shift experienced by the alkali metal atoms due to frequency detuning is equal in magnitude and opposite in direction, thereby achieving spatial cancellation everywhere.

[0025] One of the two pump beams passes sequentially through the second beam expander 5, the first reflector 6, the second reflector 7, the third reflector 8, the first half-wave plate 9, the first polarizing beam splitter 11, the fourth reflector 12, the first polarizer 13, and the first half-wave plate 14 before entering the alkali metal gas chamber 18 from the top. The other beam passes sequentially through the first beam expander 4, the fifth reflector 19, the second half-wave plate 20, the second polarizing beam splitter 22, the sixth reflector 23, the second polarizer 24, and the second half-wave plate 25 before entering the alkali metal gas chamber from the bottom. 18. Around the alkali metal gas chamber 18, from the inside out, there are a non-magnetic electric heating system 17, a triaxial magnetic compensation coil 16, and a magnetic shielding barrel 15 arranged sequentially. The triaxial magnetic compensation coil 16 is connected to a signal acquisition and processing unit through a function generator. The signal acquisition and processing unit is connected to a lock-in amplifier, a first rotating mirror controller, and a second rotating mirror controller. The first rotating mirror controller controls the first half-wave plate 9 through a first rotating mirror frame 10, and the second rotating mirror controller controls the second half-wave plate 20 through a second rotating mirror frame 21.

[0026] The lock-in amplifier is connected to the photoelastic modulation crystal 30 via a photoelastic modulator. The photoelastic modulator supplies a reference frequency to the lock-in amplifier. The detection light input side of the photoelastic modulation crystal 30 is connected to the detection laser 26 via the third polarizer 29, the eighth reflector 28, and the seventh reflector 27 in sequence. The detection light output side of the photoelastic modulation crystal 30 is connected to the lock-in amplifier via the third quarter-wave plate 31, the alkali metal gas cell 18, the analyzer 32, and the photodetector 33 in sequence. The lock-in amplifier acquires the signal collected by the photodetector 33 and demodulates the photodetector signal using the modulation frequency of the photoelastic modulator as the reference frequency to obtain the response information of the magnetic field to be measured.

[0027] The first polarizer 13 and the first quarter-wave plate 14 convert the light beam into left- or right-hand circularly polarized light, which then enters the alkali metal gas cell 18 to polarize alkali metal atoms. The second polarizer 24 and the second quarter-wave plate 25 convert the light beam into right- or left-hand circularly polarized light, which then enters the alkali metal gas cell 18 to polarize alkali metal atoms. The redshift of the left-detuned pump beam from the alkali metal atom resonance absorption peak is equal to the blueshift of the right-detuned pump beam from the alkali metal atom resonance absorption peak. By coupling two laser beams with equal blueshift and redshift from the alkali metal atom resonance absorption peak into one beam as the pump beam, optical frequency shift cancellation is achieved.

[0028] A method for improving the uniformity of electronic polarization in a SERF atomic magnetometer, utilizing the aforementioned device for improving the uniformity of electronic polarization in a SERF atomic magnetometer, includes the following steps: Step 1, setting the temperature of the alkali metal gas cell to 100–200°C, adjusting the optical path and the triaxial magnetic compensation coil to make the device operate in SERF mode; Step 2, turning off the second pump laser, turning on the first pump laser, and fixing the pump light intensity to a certain value I. pump1 In step 3, the pump light frequency is changed in one direction near the D1 line of alkali metal atoms. At each pump light frequency v, a three-dimensional in-situ magnetic compensation technique is used. The magnetic field generated by the three-dimensional magnetic field coil controlled by the function generator compensates for the magnetic field experienced by the atoms in the alkali metal gas cell, so that the magnetic field experienced by the atoms is 0, and the corresponding magnetic compensation value B in the pump light direction is obtained. The maximum magnetic compensation values ​​B1 and v1 are recorded. In step 4, the first pump laser is turned off and the second pump laser is turned on, and the pump light intensity is fixed to a certain value I. pump Near the D1 line of alkali metal atoms, the pump light frequency is changed in the opposite direction to that in step 1. At each pump light frequency v, a three-dimensional in-situ magnetic compensation technique is used. The magnetic field generated by the three-dimensional magnetic field coil controlled by the function generator compensates for the magnetic field experienced by the atoms in the alkali metal gas cell, making the magnetic field experienced by the atoms zero, and obtaining the corresponding magnetic compensation value B in the pump light direction. The maximum magnetic compensation values ​​B2 and v2 are recorded. At frequency v2, the pump light intensity is changed so that the magnetic compensation value B2 = B1, and the pump light intensity I at this time is recorded. pump2 Step 4: Simultaneously turn on both pump lasers, setting the light intensity and frequency to I. pump1 I pump2 v1 and v2, at this time, the equivalent virtual magnetic field of optical frequency shift caused by frequency detuning is equal in magnitude and opposite in direction, achieving cancellation everywhere in space. Three-dimensional in-situ magnetic compensation technology is used to compensate for the residual magnetism of the system. A calibration magnetic field is added to the sensitive axis Y using a function generator. The half-wave plate on the rotating frame is controlled by the rotation controller to proportionally change the intensity of the two pump laser beams, so that the output magnetic field response signal of the lock-in amplifier is the strongest. At this time, the electronic polarization distribution in the gas chamber is the most uniform.

[0029] A method for improving the uniformity of electronic polarization in a SERF atomic magnetometer, utilizing the aforementioned apparatus for improving the uniformity of electronic polarization in a SERF atomic magnetometer, includes the following steps:

[0030] Step 1: Demagnetize the magnetic shielding barrel 10 so that the residual magnetism of each shaft inside the barrel is less than 1nT;

[0031] Step 2: Use fluxgate magnetization to calibrate the coil constant of the triaxial magnetic compensation coil;

[0032] Step 3: Heat the alkali metal atom gas chamber 18 to 100-200°C using a non-magnetic electric heating system 17;

[0033] Step 4: Turn off pump laser 2 and turn on pump laser 1. Fix the pump light intensity to a fixed value I. pump1 The pump light frequency was changed in one direction near the D1 line of alkali metal atoms. At each pump light frequency v, a three-dimensional in-situ magnetic compensation technique was used. The magnetic field generated by the three-dimensional magnetic field coil controlled by the function generator compensated the magnetic field felt by the atoms in the alkali metal gas cell, so that the magnetic field felt by the atoms was 0. The corresponding magnetic compensation value B of the pump light direction was obtained, and the maximum magnetic compensation values ​​B1 and v1 were recorded.

[0034] Step 5: Turn off pump laser 1 and turn on pump laser 2. Fix the pump light intensity to a fixed value I. pump Near the D1 line of alkali metal atoms, the pump light frequency is changed in the opposite direction to that in step 1. At each pump light frequency v, a three-dimensional in-situ magnetic compensation technique is used. The magnetic field generated by the three-dimensional magnetic field coil controlled by the function generator compensates for the magnetic field experienced by the atoms in the alkali metal gas cell, making the magnetic field experienced by the atoms zero, and the corresponding magnetic compensation value B for the pump light direction is obtained. The maximum magnetic compensation values ​​B2 and v2 are recorded. At frequency v2, the pump light intensity is changed so that the magnetic compensation value B2 = B1, and the pump light intensity I at this time is recorded. pump2 ;

[0035] Step 6: Simultaneously turn on both pump lasers, setting the light intensity and frequency to I. pump1 I pump2 v1, v2, at this point the optical frequency shift cancels out. Three-dimensional in-situ magnetic compensation technology is used to compensate for the residual magnetism of the system. A calibration magnetic field is added to the sensitive axis Y using a function generator. The light intensities of the two pump lasers are changed proportionally to make the output magnetic field response signal of the lock-in amplifier the strongest. At this point, the electronic polarization distribution of the corresponding alkali metal is the most uniform and about 50%, and the system sensitivity is the highest.

[0036] The principle of the device and method for improving the uniformity of electronic polarization of the SERF atomic magnetometer is as follows:

[0037] In the SERF atomic magnetometer, circularly polarized pump light is used to polarize alkali metal atoms. When a beam of circularly polarized light passes through the gas cell, the pump rate Rop(Z) follows the following variation:

[0038]

[0039] Wherein, the Lambert W function is f(W) = We W The inverse function of R is also called the Ω function or the product logarithm function; OP (0) represents the initial pumping rate, R rel n is the relaxation rate; A σ(v) is the alkali metal density; σ(v) is the pump light absorption cross section.

[0040] The initial pumping rate R of the pumping light oP The relationship of (0) is as follows:

[0041]

[0042] Among them, I pu,p0 Let c be the initial light intensity of the pump light, and r be the speed of light. e f is the classical electron radius. D1 Let v be the oscillation intensity of the alkali metal D1 line, ν be Planck's constant, and v be the oscillation intensity. D1 v is the center frequency of the alkali metal D1 line. pump For the pump light frequency, Γ D1 This represents the pressure broadening value of the alkali metal D1 line.

[0043] In magnetic field measurement devices, to improve measurement sensitivity, the alkali metal density is typically maximized while reducing relaxation. When these two parameters are fixed, the pump light absorption cross-section can be altered by adjusting the pump light frequency, reducing the attenuation rate of the pump rate in the propagation direction, and thus improving the uniformity of the alkali metal electronic polarization distribution along the pump axis. Figure 2 As can be seen from the figure, the greater the frequency detuning of the pump light, the more uniform the unsaturated polarizability in the central region interacting with the detection light. However, this also causes a decrease in the initial pump rate entering the gas cell surface. That is, under the same power, the initial polarizability decreases with the increase of detuning, as shown in Figure 3(b). The decrease in initial polarizability can be compensated for by increasing the pump light intensity without affecting the attenuation rate of the pump light along the gas cell.

[0044] The alkali metal polarizability P(Z) and the pumping rate R OP The relationship of (Z) is as follows:

[0045]

[0046] Circularly polarized light, while pumping alkali metal atoms, also induces a frequency shift, which acts as a "virtual" magnetic field for the magnetic field measuring device. When the linewidth of the pump laser (in the kHz to MHz range) is much smaller than the pressure broadening value (in the GHz range), the theoretical expression for the equivalent virtual magnetic field caused by the frequency shift of the pump light due to its deviation from the center frequency of the alkali metal D1 line is:

[0047]

[0048] in It is the optical frequency shift equivalent virtual magnetic field, φ pump Photon flux per unit area, which is related to pump intensity I. pump Proportional, γ e Electron gyromagnetic ratio, This represents the degree of circular polarization of light, because the pump light is a circularly polarized beam.

[0049] When the pump light frequency is tuned to the center frequency of the alkali metal D1 line, the absorption of the laser by the gas cell is most intense, and the optical frequency shift is 0. However, at this point, the optical frequency shift is quite sensitive to laser frequency jitter, as shown in Figure 3(a). When the detuning is ±Γ D1 At a frequency of / 2, the absolute value of the equivalent virtual magnetic field of optical frequency shift experienced by alkali metal atoms reaches its maximum. However, at this time, the optical frequency shift is not sensitive to laser frequency jitter, thus eliminating the increase in optical frequency shift noise caused by laser frequency jitter. As shown in Figure 3(a), when the pump light frequency deviates from the resonance absorption peak of alkali metal atoms by equal amounts of blue shift and red shift, the generated equivalent virtual magnetic field of optical frequency shift is equal in magnitude and opposite in direction. Therefore, optical frequency shift cancellation can be achieved by coupling two laser beams with equal amounts of blue shift and red shift from the resonance absorption peak of alkali metal atoms into one beam as the pump light.

[0050] In this invention, the polarization-maintaining fiber coupler can be replaced by a polarization-depolarizing beam splitter, which can also achieve the purpose of combining and splitting two frequencies of laser beams.

[0051] 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 device for improving the uniformity of electronic polarization in a SERF atomic magnetometer, characterized in that, It includes a first pump laser for outputting a left-detuned pump beam, a second pump laser for outputting a right-detuned pump beam, and a polarization-maintaining fiber coupler or depolarization-reducing beam splitter for combining the left-detuned pump beam and the right-detuned pump beam and then splitting them into two pump beams for output. The two pump beams are incident on the alkali metal gas cell in opposite directions to pump polarized alkali metal atoms, so that the equivalent virtual magnetic field sensed by the alkali metal atoms due to frequency detuning is equal in magnitude and opposite in direction, thereby achieving spatial cancellation everywhere to effectively improve the uniformity of electronic polarization. The redshift of the left detuned pump beam from the alkali metal atom resonance absorption peak is equal to the blueshift of the right detuned pump beam from the alkali metal atom resonance absorption peak. This is achieved by coupling two laser beams with equal blueshift and redshift from the alkali metal atom resonance absorption peak into one beam as the pump beam, thus realizing optical frequency shift cancellation.

2. The device for improving the uniformity of electronic polarization of a SERF atomic magnetometer according to claim 1, characterized in that, One of the two pump beams passes sequentially through a second beam expander, a first reflector, a second reflector, a third reflector, a first half-wave plate, a first polarizing beam splitter, a fourth reflector, a first polarizer, and a first quarter-wave plate before entering the alkali metal gas chamber from the top. The other beam passes sequentially through a first beam expander, a fifth reflector, a second half-wave plate, a second polarizing beam splitter, a sixth reflector, a second polarizer, and a second quarter-wave plate before entering the alkali metal gas chamber from the bottom. Around the alkali metal gas chamber, from the inside out, are arranged a non-magnetic electric heating system, a triaxial magnetic compensation coil, and a magnetic shielding barrel. The triaxial magnetic compensation coil is connected to a signal acquisition and processing unit via a function generator. The signal acquisition and processing unit is connected to a lock-in amplifier, a first rotating mirror controller, and a second rotating mirror controller. The first rotating mirror controller controls the first half-wave plate via a first rotating mirror mount, and the second rotating mirror controller controls the second half-wave plate via a second rotating mirror mount.

3. The device for improving the uniformity of electronic polarization of a SERF atomic magnetometer according to claim 2, characterized in that, The lock-in amplifier is connected to the photoelastic modulation crystal via a photoelastic modulator. The photoelastic modulator supplies a reference frequency to the lock-in amplifier. The detection light input side of the photoelastic modulation crystal is connected to the detection laser in sequence via a third polarizer, an eighth mirror, and a seventh mirror. The detection light output side of the photoelastic modulation crystal is connected to the lock-in amplifier in sequence via a third quarter-wave plate, the alkali metal gas cell, an analyzer, and a photodetector. The lock-in amplifier acquires the signal collected by the photodetector and demodulates the photodetector signal using the modulation frequency of the photoelastic modulator as a reference frequency to obtain the optical rotation angle signal.

4. The device for improving the uniformity of electronic polarization of a SERF atomic magnetometer according to claim 2, characterized in that, The first polarizer and the first quarter-wave plate convert the light beam into left-handed or right-handed circularly polarized light, which enters the alkali metal gas cell to polarize the alkali metal atoms. The second polarizer and the second quarter-wave plate convert the light beam into right-handed or left-handed circularly polarized light, which enters the alkali metal gas cell to polarize the alkali metal atoms.

5. A method for improving the uniformity of electronic polarization of a SERF atomic magnetometer, comprising the apparatus for improving the uniformity of electronic polarization of a SERF atomic magnetometer as described in any one of claims 1-4, characterized in that, Includes the following steps: Step 1: Set the temperature of the alkali metal gas chamber to 180-200℃, and adjust the optical path and the triaxial magnetic compensation coil to make the device work in SERF mode. Step 2: Turn off the second pump laser and turn on the first pump laser. By adjusting the current of the first pump laser, fix the pump light intensity to a certain value I. pump1 The pump light frequency was changed in one direction near the D1 line of alkali metal atoms. At each pump light frequency v, the optical rotation angle signal was used to generate a magnetic field in three-dimensional in-situ magnetic compensation technology. The magnetic field generated by the three-dimensional magnetic field coil was controlled by the function generator to compensate the magnetic field felt by the atoms in the alkali metal gas cell, so that the magnetic field felt by the atoms was 0. The corresponding magnetic compensation value B of the pump light direction was obtained, and the maximum magnetic compensation value B1 and v1 were recorded. Step 3: Turn off the first pump laser, turn on the second pump laser, and fix the pump light intensity to a certain value I by adjusting the current of the second pump laser. pump Near the D1 line of alkali metal atoms, the pump light frequency is changed in the opposite direction to that in step 1. At each pump light frequency v, using the optical rotation angle signal, a three-dimensional in-situ magnetic compensation technique is employed. The magnetic field generated by the three-dimensional magnetic field coil controlled by the function generator compensates for the magnetic field experienced by the atoms in the alkali metal gas cell, making the magnetic field experienced by the atoms zero. The corresponding magnetic compensation value B for the pump light direction is obtained, and the maximum magnetic compensation values ​​B2 and v2 are recorded. At frequency v2, the pump light intensity is changed so that the magnetic compensation value B2 = B1, and the pump light intensity I at this time is recorded. pump2 ; Step 4: Simultaneously turn on both pump lasers, setting the light intensity and frequency to I. pump1 I pump2 v1 and v2, at this time, the virtual magnetic fields caused by frequency detuning are equal in magnitude and opposite in direction, achieving cancellation everywhere in space. Three-dimensional in-situ magnetic compensation technology is used to compensate for the residual magnetism of the system. A calibration magnetic field is added to the sensitive axis Y using a function generator. The half-wave plate on the rotating frame is controlled by the rotation controller to proportionally change the intensity of the two pump laser beams, so that the output rotation angle signal of the lock-in amplifier is the strongest.

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