阅读说明：本技术 一种单光束serf原子磁强计碱金属原子密度测量方法 (Method for measuring alkali metal atom density of single-beam SERF atomic magnetometer ) 是由 赵立波 马银涛 陈瑶 乔智霞 罗国希 李伟 于明智 王延斌 林启敬 杨萍 王久洪 于 2021-09-01 设计创作，主要内容包括：本发明公开了一种单光束SERF原子磁强计及碱金属原子密度测量方法,所述方法包括以下步骤：首先,将单光束SERF原子磁强计中心位置三个方向的磁场强度补偿到零,加热碱金属气室,使碱金属原子达到SERF态；然后沿一个敏感轴方向施加恒定的直流磁场,沿另一个敏感轴方向施加带调制的偏置磁场,连续变化且过零点的偏置磁场,记录磁强计的输出信号,会得到具有色散线型的磁场共振曲线,进而得到沿X轴方向上不同直流磁场下的磁场共振线宽大小；通过二次函数拟合得到磁场共振线宽与直流磁场大小的关系；最后利用二次函数的二次项系数计算得到碱金属原子密度,从而实现碱金属原子密度的原位测量。(The invention discloses a single-beam SERF atomic magnetometer and an alkali metal atomic density measuring method, which comprises the following steps: firstly, compensating the magnetic field intensity of a single-beam SERF atomic magnetometer in three directions to zero, and heating an alkali metal gas chamber to enable alkali metal atoms to reach an SERF state; then, a constant direct-current magnetic field is applied along one sensitive axis direction, a modulated bias magnetic field is applied along the other sensitive axis direction, the bias magnetic field which changes continuously and crosses zero point is recorded, the output signal of the magnetometer is recorded, a magnetic field resonance curve with a dispersion line type can be obtained, and the magnetic field resonance line width under different direct-current magnetic fields along the X-axis direction is further obtained; obtaining the relation between the magnetic field resonance line width and the size of the direct-current magnetic field through quadratic function fitting; and finally, calculating to obtain the density of the alkali metal atoms by utilizing the quadratic term coefficient of the quadratic function, thereby realizing the in-situ measurement of the density of the alkali metal atoms.)

1. A single-beam SERF atomic magnetometer is characterized by comprising a DFB pump laser (1), a fiber coupler (3), a plano-convex lens (4), a lambda/4 wave plate (5), a photoelectric detector (7), an alkali metal gas chamber (10), a signal processing module and a high-power laser (14);

laser emitted by the DFB pump laser (1) passes through the optical fiber coupler (3), the plano-convex lens (4), the lambda/4 wave plate (5) and the alkali metal air chamber (10) in sequence and is detected by the photoelectric detector (7); the signal processing module is used for collecting and processing the light intensity signal output by the photoelectric detector (7); the high-power laser (14) is used for heating the alkali metal gas chamber (10);

a first coil (61), a second coil (62) and a third coil (63) for applying a magnetic field to the alkali metal gas chamber (10) are arranged outside the alkali metal gas chamber (10); the central axes of the first coil (61), the second coil (62) and the third coil (63) are perpendicular to each other.

2. A single beam SERF atomic magnetometer according to claim 1 characterized in that a single mode polarization maintaining fiber (2) is connected between said DFB pump laser (1) and said fiber coupler (3).

3. Single beam SERF atomic magnetometer according to claim 1, characterized in that said signal processing module comprises a transimpedance amplifier (11), a lock-in amplifier (12) and a data acquisition card (13) connected in sequence.

4. The method for measuring the density of alkali metal atoms based on the atomic magnetometer of claim 1, characterized by comprising the steps of:

s1, heating the alkali metal gas chamber to increase the density of alkali metal atoms and enable the alkali metal atoms to reach a SERF state;

s2, applying a constant direct current magnetic field along the first sensitive axis direction of the atomic magnetometer, applying a bias magnetic field with modulation along the second sensitive axis direction, continuously changing the magnitude of the bias magnetic field with a zero crossing point, recording the light intensity signal of the pumping light of the alkali metal gas chamber 10 passing through the atomic magnetometer, obtaining a magnetic field resonance curve with dispersion line type, and obtaining the resonance line width under the constant direct current magnetic field;

s3, changing the applied direct current magnetic field, and repeating the S2 process, so as to obtain the magnetic field resonance line width under different direct current magnetic fields along the direction of the first sensitive axis;

s4, obtaining a quadratic function of the magnetic field resonance line width and the size of the direct-current magnetic field through quadratic function fitting to obtain a quadratic term coefficient of the quadratic function;

and S5, calculating the density of the alkali metal atoms by utilizing the coefficient of the quadratic term of the quadratic function.

5. The method for measuring the density of alkali metal atoms as claimed in claim 4, wherein in S1, after the alkali metal atoms reach the SERF state, the magnetic field strength of the central position of the single-beam SERF atom magnetometer is accurately compensated to zero.

6. The alkali metal atom density measuring method according to claim 4, wherein when the detected light intensity of the photodetector no longer changes, S2 is further performed.

7. The method for measuring the density of alkali metal atoms of a single-beam SERF atomic magnetometer according to claim 4, wherein in S4, the quadratic function is in the form of:wherein, Δ B is the magnetic field resonance line width, a is the quadratic term fitting coefficient, B is the first order fitting coefficient, and c is the constant term.

8. The method according to claim 4, wherein in S5, the alkali metal atom density is calculated by the following formula:

wherein, γ^eIs the gyromagnetic ratio of bare leakage electrons, I is the nuclear spin quantum number of alkali metal atoms, q (0) is the nuclear slowdown factor under the condition of zero polarizability, a is the quadratic fitting coefficient, sigma is_seIs the cross-sectional area of the spin-exchange collision between alkali metal atoms, and v is the relative thermal motion velocity between alkali metal atoms.

Technical Field

The invention belongs to the technical field of quantum precision measurement, and particularly relates to a single-beam SERF atomic magnetometer and an alkali metal atomic density measurement method.

Background

The Spin Exchange Relaxation Free (SERF) atomic magnetometer has great application value in the fields of advanced basic scientific research such as dark matter detection, mineral resource exploration, cardio-cerebral-magnetic source imaging biomedicine and the like as a magnetic field detector with the highest sensitivity at present. SERF atomic magnetometers achieve vector magnetic field measurements by measuring the larmor precession frequency of polarized alkali metal atoms in a magnetic field. The density of alkali metal atoms directly affects the signal-to-noise ratio and the sensitivity of the atomic magnetometer, so that the accurate and rapid in-situ measurement of the density of the alkali metal atoms has important significance. The commonly used atomic density measurement method is laser spectral absorption, but this method is limited by the pressure-induced broadening and therefore does not allow accurate measurement of the atomic density of alkali metals in the sealed gas chamber. At present, no public report is found about the alkali metal atom density measuring method used for a single-beam SERF atom magnetometer.

Meanwhile, the SERF atomic magnetometer configured by the single beam can realize the polarization and detection of atoms at the same time only by using the single pumping and detection beam, and compared with the double-beam configuration, the SERF atomic magnetometer has the advantages of simple structure, low cost, easiness in integration, miniaturization and the like, and is favored by a plurality of domestic and foreign research institutions. In recent years, research on related work is carried out in units such as Beijing aerospace university and SiAn transportation university in China, but due to late domestic start and weak foundation, efforts on improving the sensitivity of the atomic magnetometer are needed.

The theoretical limit sensitivity δ B of a SERF atomic magnetometer, limited by shot noise, is:

wherein γ is the gyromagnetic ratio of electrons,_nis the density of alkali metal atoms, T₂Is the transverse spin relaxation time of the alkali metal atom, V is the alkali metal gas cell volume, and t is the measurement time. As can be seen from the above formula (1), the alkali metal atom density_nThe method is directly related to the sensitivity of the magnetometer, so that the in-situ, accurate and rapid measurement of the alkali metal atom density becomes a key factor for improving the sensitivity of the SERF atomic magnetometer, and provides an important theoretical reference for the realization of the whole atomic magnetometer system and the optimization of the performance.

Disclosure of Invention

The invention provides an alkali metal atom density measuring method suitable for a single-beam SERF atomic magnetometer, aiming at the problem that the alkali metal atom density in the SERF atomic magnetometer directly influences the sensitivity of a magnetometer system, and providing reference and basis for theoretically improving the signal-to-noise ratio and the sensitivity of the MEMS atomic magnetometer.

In order to achieve the purpose, the single-beam SERF atomic magnetometer comprises a DFB pump laser, a fiber coupler, a plano-convex lens, a lambda/4 wave plate, a photoelectric detector, an alkali metal gas chamber, a signal processing module and a high-power laser; laser emitted by the DFB pump laser sequentially passes through the optical fiber coupler, the plano-convex lens, the lambda/4 wave plate and the alkali metal air chamber and is detected by the photoelectric detector; the signal processing module is used for collecting and processing the light intensity signal output by the photoelectric detector; the high-power laser is used for heating the alkali metal gas chamber; a first coil, a second coil and a third coil which are used for applying a magnetic field to the alkali metal gas chamber are arranged outside the alkali metal gas chamber; the central axes of the first coil, the second coil and the third coil are mutually vertical.

Furthermore, a single-mode polarization-maintaining fiber is connected between the DFB pump laser and the fiber coupler.

Furthermore, the signal processing module comprises a transimpedance amplifier, a phase-locked amplifier and a data acquisition card which are connected in sequence.

The alkali metal atom density measuring method based on the atomic magnetometer comprises the following steps:

s1, heating the alkali metal gas chamber to increase the density of alkali metal atoms and enable the alkali metal atoms to reach a SERF state;

s2, applying a constant direct current magnetic field along the first sensitive axis direction of the atomic magnetometer, applying a bias magnetic field with modulation along the second sensitive axis direction, continuously changing the magnitude of the bias magnetic field with a zero crossing point, recording the light intensity signal of the pumping light of the alkali metal gas chamber 10 passing through the atomic magnetometer, obtaining a magnetic field resonance curve with dispersion line type, and obtaining the resonance line width under the constant direct current magnetic field;

s3, changing the applied direct current magnetic field, and repeating the S2 process, so as to obtain the magnetic field resonance line width under different direct current magnetic fields along the direction of the first sensitive axis;

s4, obtaining a quadratic function of the magnetic field resonance line width and the size of the direct-current magnetic field through quadratic function fitting to obtain a quadratic term coefficient of the quadratic function;

and S5, calculating the density of the alkali metal atoms by utilizing the coefficient of the quadratic term of the quadratic function.

Further, in S1, after the alkali metal atoms reach the SERF state, the magnetic field strength at the center of the single-beam SERF atomic magnetometer is accurately compensated to zero.

Further, when the detected light intensity of the photodetector no longer changes, S2 is executed again.

Further, in S4, the quadratic function is in the form:wherein, Delta B is the magnetic field resonance line width,_ais a quadratic fitting coefficient, b is a first order fitting coefficient,_cis a constant term.

Further, in S5, the alkali metal atom density is calculated by the following formula:

wherein, γ^eIs the gyromagnetic ratio of bare leakage electrons, I is the nuclear spin quantum number of alkali metal atoms, q (0) is the nuclear slowdown factor under the condition of zero polarizability, a is the quadratic fitting coefficient, sigma is_seIs the cross-sectional area of the spin-exchange collision between alkali metal atoms, and v is the relative thermal motion velocity between alkali metal atoms.

Compared with the prior art, the invention has at least the following beneficial technical effects:

the atomic magnetometer comprises a DFB pump laser, an optical fiber coupler, a plano-convex lens, a lambda/4 wave plate, a photoelectric detector, an alkali metal air chamber, a coil, a signal processing module and a high-power laser, wherein the DFB pump laser can be used for emitting alkali metal atoms in a laser polarization air chamber, the coil placed in the alkali metal air chamber in different directions is used for applying a direct current magnetic field or a bias magnetic field to the alkali metal air chamber, and the density of the alkali metal atoms is calculated according to the change of output light intensity signals when different magnetic fields are changed, so that the in-situ measurement of the density of the alkali metal atoms is realized.

Furthermore, a single-mode polarization maintaining fiber is connected between the DFB pump laser and the fiber coupler, the single-beam SERF atomic magnetometer system transmits the light beam to the magnetometer probe by using the fiber coupling structure, and the installation position of the sensor probe is slightly restrained, so that the relative position between the sensor probe and the measured magnetic field source can be freely and flexibly determined.

1) The method only needs to measure the magnetic field resonance line width of alkali metal atom response under different direct-current magnetic fields, the established alkali metal density is only related to the fitted resonance line width and the quadratic coefficient of a direct-current magnetic field function, and other parameters are constants, so that the alkali metal atom density can be accurately and quickly measured, and a theoretical and experimental basis is provided for improving the sensitivity of an SERF (serial-exchange reactor) atomic magnetometer;

2) the method can realize the in-situ measurement of the alkali metal density only by utilizing the single-beam magnetometer system, does not need additional optical elements, and has the advantages of simple structure, high integration, freedom, flexibility, low cost, miniaturization and the like.

3) The method provides reference for improving the performance of the single-beam SERF atomic magnetometer and integrating the system, so that the atomic magnetometer system is easier to integrate and miniaturize, and further provides guidance for the application of the chip magnetometer in the biomedical fields of cardiac and cerebral magnetic imaging and the like.

Drawings

FIG. 1 is a schematic diagram of atomic density measurement of a single beam SERF atomic magnetometer;

fig. 2 shows the relationship between the magnetic resonance line width at 393K and the dc magnetic field in the X direction.

In the drawings: 1. the optical fiber laser comprises a DFB pump laser device, 2 a single-mode polarization-maintaining optical fiber, 3 a fiber coupler, 4 a plano-convex lens, 5 a lambda/4 wave plate, 61 a first coil, 62 a second coil, 63 a third coil, 7 a photoelectric detector, 8 a light filter, 9 a multimode optical fiber, 10 an alkali metal air chamber, 11 a transimpedance amplifier, 12 a phase-locked amplifier, 13 a data acquisition card, 14 and a high-power laser device.

Detailed Description

In order to make the objects and technical solutions of the present invention clearer and easier to understand. The present invention will be described in further detail with reference to the following drawings and examples, wherein the specific examples are provided for illustrative purposes only and are not intended to limit the present invention.

In the description of the present invention, it is to be understood that the terms "center", "longitudinal", "lateral", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", and the like, indicate orientations or positional relationships based on those shown in the drawings, and are used only for convenience in describing the present invention and for simplicity in description, and do not indicate or imply that the referenced devices or elements must have a particular orientation, be constructed and operated in a particular orientation, and thus, are not to be construed as limiting the present invention. Furthermore, the terms "first", "second" and "first" are used for descriptive purposes only and are not to be construed as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of the present invention, "a plurality" means two or more unless otherwise specified. In the description of the present invention, it should be noted that, unless otherwise explicitly specified or limited, the terms "mounted," "connected," and "connected" are to be construed broadly, e.g., as meaning either a fixed connection, a removable connection, or an integral connection; can be mechanically or electrically connected; they may be connected directly or indirectly through intervening media, or they may be interconnected between two elements. The specific meanings of the above terms in the present invention can be understood in specific cases to those skilled in the art.

The method for realizing in-situ measurement of the density of the alkali metal atoms by using the single-beam Cs atom SERF atom magnetometer is specifically described by taking a single-beam Cs atom SERF atom magnetometer as an example.

Referring to fig. 1, an atomic magnetometer includes a DFB pump laser 1, a single-mode polarization maintaining fiber 2, a fiber coupler 3, a planoconvex lens 4, a λ/4 wave plate 5, a first coil 61, a second coil 62, a third coil 63, a photodetector 7, an optical filter 8, a multimode fiber 9, an alkali metal gas cell 10, a transimpedance amplifier 11, a lock-in amplifier 12, a data acquisition card 13, and a high-power laser 14.

Wherein, DFB pump laser 1 is connected through single mode polarization maintaining fiber 2 and fiber coupler 3, and high power laser 14 passes through multimode fiber 9 and optical filter 8 to be connected, and optical filter 8 is installed outside the lateral wall of alkali metal air chamber 10, and six sides of alkali metal air chamber 10 are provided with a coil respectively, are respectively: two first coils 61 respectively located above and below the alkali metal gas cell 10, two second coils 62 respectively located at the front and rear sides of the alkali metal gas cell 10, and two third coils 63 respectively located at the left and right sides of the alkali metal gas cell 10. The plane of the first coil 61 is perpendicular to the X direction, the plane of the second coil 62 is perpendicular to the Y direction, the plane of the third coil 63 is perpendicular to the Z direction, and the Z direction is the pump light propagation direction.

Laser emitted by the DFB pump laser 1 sequentially passes through a single-mode polarization maintaining fiber 2, a fiber coupler 3, a plano-convex lens 4, a lambda/4 wave plate 5, an alkali metal air chamber 10, a photoelectric detector 7, a transimpedance amplifier 11, a phase-locked amplifier 12 and a data acquisition card 13.

The optical fiber coupler 3 is used for transmitting pump light, the plano-convex lens 4 is used for collimating laser beams, and the lambda/4 wave plate 5 is used for converting linearly polarized light into circularly polarized light.

A method for measuring the density of alkali metal atoms of a single-beam SERF atomic magnetometer comprises the following steps:

step 1, building an experimental platform of atomic magnetometer atomic density according to a light path diagram shown in figure 1; the magnetic field strengths of the single-beam SERF atomic magnetometer X, Y and the Z direction are accurately compensated to zero by using the first coil 61, the second coil 62 and the third coil 63, respectively;

step 2, turning on the high-power laser 14, enabling laser emitted by the high-power laser 14 to reach the optical filter 8 through the multimode fiber 9, absorbing light through the optical filter 8 to heat the alkali metal gas chamber 10, heating the alkali metal gas chamber 10 to 393K, and waiting for a period of time until the light intensity detected by the photoelectric detector does not change any more, namely the state of the atomic magnetometer system is kept stable, so that the environment required by the SERF state of the alkali metal atoms is realized;

step 3, under the condition of meeting the low polarizability condition of the minimum output signal of the detectable magnetometer, the DFB pump laser 1 emits laser, the pump light intensity is 10uW at the moment, firstly, direct current is conducted to the first coil 61, so that a constant direct current magnetic field is applied to the alkali metal gas chamber 10, and the direction of the direct current magnetic field is parallel to the X axis; then applying a bias magnetic field with modulation to the direction through a second coil 62, and continuously changing the magnitude of the bias magnetic field in the Y direction to ensure that the bias magnetic field has the change of a zero crossing point, the change step length is 1 nT-10 nT, and the times are more than 10; at this time, laser emitted from the DFB pump laser 1 sequentially passes through the single-mode polarization maintaining fiber 2, the fiber coupler 3, the plano-convex lens 4, the lambda/4 wave plate 5 and the alkali metal gas chamber 10 and then is detected by the photoelectric detector 7, the photoelectric detector 7 outputs a current signal capable of reflecting light intensity, the current signal is converted into a voltage signal through the resistance amplifier 11 and amplified, then the voltage signal is output to the phase-locked amplifier 12, a first harmonic signal in the voltage signal is extracted by the phase-locked amplifier 12 and sent to the data acquisition card (13), and the first harmonic signal is acquired and processed by the data acquisition card 13.

Recording the first harmonic signal acquired by the data acquisition card 13 in the process of changing the magnitude of the bias magnetic field in the Y direction to obtain a magnetic field resonance curve, wherein the relationship between the magnetic field resonance curve and the magnitude of the bias magnetic field in the Y direction has a dispersion linear form, and the magnetic field resonance line width is obtained according to the magnetic field resonance curve, so that a group of relationships between the direct current magnetic field with constant X direction and the magnetic field resonance line width can be obtained; the magnitude of the direct current supplied to the first coil 61 is changed, the magnitude of the constant direct current magnetic field applied to the alkali metal gas chamber 10 is also changed, generally less than 100nT, and the above process is repeated to obtain a plurality of groups of relationships between the constant direct current magnetic field in the X direction and the magnetic field resonance line width.

Under the condition of low polarizability, the pumping light intensity is very low, generally not more than 10uW, and the power broadening linewidth caused by the pumping light can be ignored.

In this step, a constant direct current magnetic field parallel to the Y direction may be applied to the alkali atom gas cell 10, and a bias magnetic field parallel to the X direction may be applied to the alkali atom gas cell 10.

And 4, continuously changing the size of the direct-current magnetic field in the X direction, wherein the size of the direct-current magnetic field is generally less than 100nT, repeating the process to obtain 8 groups of relations between the direct-current magnetic field in the X direction and the magnetic field resonance line width, and fitting the relations between the magnetic field resonance line width and the size of the direct-current magnetic field in the X direction by utilizing a quadratic function, wherein the relations are shown in table 1.

TABLE 1

Given the relationship between the measured resonance linewidth and the fitting function according to table 1, as shown in fig. 2, a fitting curve having the form of a quadratic function whose quadratic coefficient is proportional to the alkali metal atomic density can be obtained, the fitting formula being as follows:

wherein, Δ B is the magnetic field resonance line width, a is the quadratic term fitting coefficient, B is the first order fitting coefficient, and c is the constant term. The coefficient a of the quadratic term is 0.0024 from the fitting function;

the relationship of the resonance linewidth to the magnetic field in the X direction has the form of a quadratic function, where the quadratic coefficient is proportional to the atomic density, the first order coefficient is related to the modulation parameter, and the constant term is the broadening caused by other relaxation mechanisms, including: spin-destruction collision relaxation, air chamber wall collision relaxation, and relaxation by magnetic field gradients, among others.

And step 5, adopting a formula (2), directly determining the corresponding alkali metal atom density at a specific temperature:

wherein, γ^eIs the gyromagnetic ratio of bare leakage electrons, I is the nuclear spin quantum number of alkali metal atoms, q (0) is the nuclear slowing factor under the condition of zero polarizability, sigma_seIs the spin-exchange collision cross-sectional area between alkali metal atoms, and v is the relative thermal motion velocity between alkali metal atoms. a is 0.0024, σ_seIs 2.1 × 10^-14cm²V is 353m/s, gamma^e2 π X28 Hz/nT, I7/2, q (0) 22. The corresponding Cs atom vapor density of 4.28X 10 at 393K was obtained¹³/cm³。

The above-mentioned contents are only for illustrating the technical idea of the present invention, and the protection scope of the present invention is not limited thereby, and any modification made on the basis of the technical idea of the present invention falls within the protection scope of the claims of the present invention.