Magnetic field vector measurement method based on CPT magnetometer

Abstract

The magnetic field vector measurement method based on the CPT magnetometer adopts a dual-beam solution, uses a servo motor to accurately control the polarization angle, and measures the magnetic field direction by observing the dispersion signal. As the probe part of the CPT vector magnetometer, the multi-color light generated by the modulated vertical cavity surface emitting laser is input into the gas chamber, and the optical signal carrying the magnetic field information is modulated and demodulated by the phase-locked amplifier to obtain multiple dispersion signals. The state of the CPT signal can be analyzed through each dispersion signal, wherein the zero crossing point of the dispersion signal is at the same frequency as the CPT peak, and the larger the amplitude of the dispersion signal, the larger the CPT peak amplitude. The present invention is based on the basic principle that the angle between the magnetic field B and the wave vector k and the polarization e affects the CPT peak amplitude, and performs vector measurement of the magnetic field. The advantage of the present invention is that it can measure the direction of the magnetic field and can effectively avoid the problem of decreased sensitivity due to a specific angle. The present invention has strong extension applicability and high practical value.

Description

Technical Field

[0001] The invention relates to the technical field of interaction between light and alkali metal atoms and automatic control, in particular to a method for measuring magnetic field vector based on a CPT magnetometer. Background Art

[0002] The CPT effect is a quantum coherence effect (CPT, Coherent Population Trapping). The CPT effect and the Zeeman effect can be used to realize a CPT magnetometer that can absolutely measure the external magnetic field. Generally speaking, the magnetic field size of a CPT magnetometer is calculated by measuring the frequency detuning value Δν of the CPT manifold and using the relationship between the frequency detuning value Δν and the external magnetic field size B0 to obtain the size of the external magnetic field:

[0003] Δν=γ|B|

[0004] where γ is the gyromagnetic ratio of the alkali metal atom.

[0005] CPT atomic magnetometer can be widely used in geophysics, medicine and basic physics. CPT magnetometer has high sensitivity potential. It can achieve the advantages of miniaturization, vector measurement and geomagnetic environment measurement. In many application scenarios, understanding the vector component of the magnetic field is crucial to perfect the specific information of the field. However, the current vector measurement scheme for the CPT atomic magnetometer is relatively backward. The above principle can only perform scalar measurement of the magnetic field at the measured point, and cannot obtain the angle information of the magnetic field, so the practical application value is limited. And when the unknown magnetic field B (that is, the external magnetic field, or the magnetic field to be measured) and the wave vector k and the polarization direction e are not at the optimal relative angle, the signal-to-noise ratio does not reach the highest, which will seriously affect the sensitivity of the magnetometer. In extreme cases, it may be reduced by one to two orders of magnitude. In actual application conditions, the magnetic field B is the measured value, the wave vector k is determined by the experimental optical path, and it is crucial to set the adjustable polarization direction e to obtain the direction information of the magnetic field and improve the sensitivity of the magnetometer to a certain extent. Summary of the invention

[0006] The problem solved by the present invention is: a magnetic field vector measurement method based on a CPT magnetometer is proposed. The radio frequency signal generated by the radio frequency signal generator is injected into the VCSEL laser through Bias-T, and the output laser is half-wave modulated to obtain a multi-color laser; the signals generated by the triangle wave signal generator and the sine wave signal generator are superimposed by an adder, and the radio frequency signal is modulated to realize the scanning and modulation of the multi-color light field, the stability of the linear polarized light is ensured by a polarizer, the half-wave plate is accurately controlled by a servo motor to adjust the polarization angle, and the polarization angle information is recorded, and a plurality of CPT dispersion signals are obtained by demodulating the optical signal through an alkali metal gas chamber using a phase-locked amplifier; a suitable magnetically sensitive CPT dispersion signal is selected as an observation target to realize the magnetic field vector measurement.

[0007] The technical solution of the present invention is as follows:

[0008] The magnetic field vector measurement method based on the CPT magnetometer is characterized by comprising the following steps:

[0009] Step 1, starting the CPT magnetometer system to generate a multi-color laser, the multi-color laser is formed into a first beam of light and a second beam of light through a polarization beam splitter prism, the first beam of light passes through a first polarizer and a first half-wave plate in sequence, and then passes through an alkali metal gas chamber in a positive direction along the x-axis to form a first beam of light signal carrying magnetic field information and enters a first input end of a phase-locked amplifier, the second beam of light passes through a first reflector, a second reflector, a second polarizer, and a second half-wave plate in sequence, and then passes through an alkali metal gas chamber in a negative direction along the z-axis to form a second beam of light signal carrying magnetic field information and enters a second input end of the phase-locked amplifier;

[0010] Step 2: The lock-in amplifier demodulates the first light signal to obtain a dispersion signal, and obtains a magnetically sensitive dispersion signal according to the magnetic field measurement principle that the zero-crossing point of the dispersion signal is consistent with the peak point frequency of the CPT signal;

[0011] Step 3, adjust the polarization direction e of the first beam of light by rotating the first half-wave plate, obtain the change in the size of the peak amplitude of the CPT signal by observing the change in the oscillation amplitude of the dispersion signal, keep the wave vector k fixed, and track the maximum value of the peak amplitude of the CPT signal; define n=k×B, B is the external magnetic field or the magnetic field to be measured, n is the normal vector, since k⊥n, the polarization direction e is adjusted by rotating the half-wave plate, when the signal amplitude reaches a maximum value, then e∥n, thereby obtaining the first plane where the magnetic field B is located through the first beam of light, that is, the plane with n as the normal vector and passing through the wave vector k;

[0012] Step 4, determine the magnitude of the magnetic field scalar using the following relationship:

[0013] Δν=γ|B|

[0014] Where Δv is the frequency difference between the zero crossing points of the dispersion signal, i.e., the optical frequency difference between the two CPT peaks, B is the magnitude of the magnetic field, and γ is the gyromagnetic ratio;

[0015] Step 5, return to step 2, process the second light beam signal through a phase-locked amplifier in a similar manner to the first light beam signal, obtain the second plane where the magnetic field B is located through the second light beam, and the intersection of the first plane and the second plane is the direction of the magnetic field B.

[0016] The first plane where the magnetic field B is located in step 3 has the following expression:

[0017] x+tan(2α)y=0

[0018] where α is the half-wave plate rotation angle.

[0019] Step 3 includes the following expression for the polarization direction e:

[0020]

[0021] Where α is the rotation angle of the half-wave plate, and the polarization direction e starts scanning from the positive direction of the X-axis and scans in the positive direction of the Y-axis.

[0022] The step 5 includes: defining is the angle between e and n, where counterclockwise is Clockwise Defining dependency functions exist That is, when e⊥B, To reach the maximum value, the wave vector k=k1 is selected, where k1 is the set value, and the polarization vector e is obtained by rotating the wave vector k1. The maximum value corresponds to the direction of vector n=[k1×B], that is, the equation of the plane (k1,B) formed by the vectors k1 and B is obtained; repeat the same process in the other direction of the wave vector k=k2, k2 is the set quantity, and the equation of the plane (k2,B) is obtained; the intersection line of the two planes (k1,B) and (k2,B) is the three-dimensional direction of the magnetic field B.

[0023] The first half-wave plate in step 1 is connected to the first servo motor, and the second half-wave plate is connected to the second servo motor.

[0024] The alkali metal gas chamber in step 1 is located in an oven, the oven is located in a Helmholtz coil, and the Helmholtz coil is located in a magnetic shielding device.

[0025] The CPT magnetometer system in step 1 includes a VCSEL laser, the output end of the VCSEL laser is connected to a fiber coupler, the fiber coupler is connected to the input end of a polarization beam splitter prism through a polarization-maintaining fiber and a laser collimator in sequence, the transmission side of the polarization beam splitter prism is connected to a phase-locked amplifier through a first polarizer, a first half-wave plate, an alkali metal gas chamber and a first photodetection amplifier in sequence, the reflection side of the polarization beam splitter prism is connected to a phase-locked amplifier through a first reflector, a second reflector, a second polarizer, a second half-wave plate, an alkali metal gas chamber and a second photodetection amplifier in sequence, the output end of the phase-locked amplifier is connected to an input voltage data acquisition system, the third input end of the phase-locked amplifier is connected to the output end of a sine wave signal generator, the output end of the sine wave signal generator is connected to a first input end of an adder, the second input end of the adder is connected to a triangular wave signal generator, the output end of the adder is connected to a first input end of a T-type bias device through a radio frequency signal generator, the second input end of the T-type bias device is connected to a VCSEL controller, and the output end of the T-type bias device is connected to a VCSEL laser.

[0026] The VCSEL laser locks the laser on the alkali metal atom D1 line, uses a radio frequency signal generator to generate a 3.417GHz signal, and modulates the radio frequency signal with a T-type bias device and adds it to the DC control electrical signal of the VCSEL laser to generate multi-color laser.

[0027] The technical effects of the present invention are as follows: The present invention is based on the magnetic field vector measurement method of the CPT magnetometer, adopts a dual-beam solution, uses a servo motor to accurately control the polarization angle, and measures the magnetic field direction by observing the dispersion signal. As the probe part of the coherent population trapping (CPT) vector magnetometer, the polychromatic light generated by the modulated vertical cavity surface emitting laser (VCSEL) is input into the gas chamber, and the optical signal carrying the magnetic field information is converted into a voltage signal by a photodiode and modulated and demodulated by a phase-locked amplifier to obtain multiple dispersion signals. The state of the CPT signal can be analyzed by each dispersion signal, wherein the zero crossing point of the dispersion signal is at the same frequency as the CPT peak, and the larger the amplitude of the dispersion signal, the larger the CPT peak amplitude. The present invention is based on the basic principle that the angle between the magnetic field B and the wave vector k and the polarization e affects the CPT peak amplitude, and the magnetic field is vector-measured. The advantage of the present invention is that it can measure the direction of the magnetic field and can effectively avoid the problem of decreased sensitivity due to a specific angle. The invention has strong extended applicability and high practical value. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 It is a flow chart of the method for measuring magnetic field vector based on CPT magnetometer according to the present invention. Figure 1 The method comprises the steps of: 1, modulating the multicolor laser to generate; 2, the laser passes through the air chamber (forming an optical signal carrying magnetic field information), and demodulating with a phase-locked amplifier to obtain a dispersion signal (the state of the CPT signal is analyzed by each dispersion signal, CPT is the abbreviation of Coherent Population Trapping, i.e., coherent population trapping, wherein the zero crossing point of the dispersion signal and the CPT peak are at the same frequency, and the larger the amplitude of the dispersion signal, the larger the peak amplitude of the CPT signal); 3, rotating the polarization angle, and tracking the maximum value of the signal peak amplitude; 4, processing the dispersion signal, and scalar measuring the magnetic field size (Δν=γ|B|, Δν is the frequency difference between the zero crossing points of the dispersion signal, i.e., the optical frequency difference between the two CPT peaks, B is the magnetic field size, and γ is the gyromagnetic ratio, for example, the gyromagnetic ratio γ of Rb87=7Hz / nT); 5. The dual-beam scheme determines the direction of the magnetic field (through the first light beam, the wave vector k is defined along the Z axis, and n is defined as n=k×B. Since k⊥n, the polarization direction e is adjusted by rotating the half-wave plate. When the signal amplitude reaches a maximum value, e∥n. Thus, the first plane where the magnetic field B is located is obtained through the first light beam, that is, the plane with n as the normal vector and passing through the wave vector k; through the second light beam, the second light beam has a certain angle with the first light beam, and the above process is repeated for measurement to obtain the second plane where the magnetic field B is located. The intersection of the two planes is the direction of the magnetic field B).

[0029] Figure 2 Schematic diagram of polarization scanning. Figure 2 It includes defining the wave vector k along the Z axis (optical path), defining n = k × B, B is the external magnetic field, its actual direction is unknown, adjusting the polarization direction e for scanning, and adjusting the polarization direction e by rotating the half-wave plate. The polarization direction e is perpendicular to the wave vector k (Z axis). Figure 2 The polarization direction e starts scanning from the positive direction of the X axis and scans in the positive direction of the Y axis. According to n=k×B, the plane where the magnetic field B is located is: x+tan(2α)y=0, where α is the rotation angle of the half-wave plate.

[0030] Figure 3 It is a schematic diagram of the structure of a CPT magnetometer system involved in the magnetic field vector measurement method based on a CPT magnetometer of the present invention.

[0031] Figure 4 This is a schematic diagram of the relative relationship between the wave vector k, polarization e and the magnetic field B to be measured. Figure 4 Includes definitions is the angle between e and n, where counterclockwise is positive and clockwise is negative. Define the dependency function exist (i.e. when e⊥B) reaches its maximum value. For the selected wave vector k=k1, by rotating the polarization vector e around the wave vector k1, we obtain The maximum value of corresponds to the direction of the vector n = [k1×B], which gives the equation of the plane (k1, B) formed by the vectors k1 and B. Repeating the same process for the other direction of the wave vector k = k2 gives the equation of the plane (k2, B). The intersection of the two planes (k1, B) and (k2, B) is the three-dimensional direction of the magnetic field B.

[0032] The reference numerals are as follows: 1-VCSEL laser (VCSEL, Vertical-Cavity Surface-Emitting Laser); 2-fiber coupler; 3-polarization-maintaining fiber; 4-laser collimator; 5-polarization beam splitter prism; 6-first polarizer; 7-first half-wave plate; 8-magnetic shielding device; 9-Helmholtz coil; 10-oven; 11-atomic gas chamber (such as Rb gas chamber) or alkali metal gas chamber; 12-first photodetection amplifier; 13-phase-locked amplifier; 14-input voltage data acquisition system; 15-first reflector; 16-second reflector; 17-second polarizer; 18-second half-wave plate; 19-second servo motor; 20-VCSEL controller; 21-T-type bias device (Bias-T, used to mix DC signal and RF signal into the same path, DC is direct current, RF is radio frequency, and T-type bias device outputs RF+DC signal); 22-RF signal generator; 23-adder; 24-triangular wave signal generator; 25-first servo motor; 26-sine wave signal generator; 27-second photodetection amplifier; xyz-three axes of rectangular coordinate system (i.e., x-axis, y-axis and z-axis); k-wave vector; e-polarization or polarization direction or polarization angle or polarization scanning; B-magnetic field to be measured; n-normal vector, n=k×B; -The angle between vectors e and n (counterclockwise is positive, denoted by; clockwise is negative, denoted by ). DETAILED DESCRIPTION

[0033] Below is the attached figure ( Figure 1-Figure 4 ) and Examples illustrate the present invention.

[0034] Figure 1 It is a flow chart of the method for measuring magnetic field vector based on CPT magnetometer according to the present invention. Figure 2 Schematic diagram of polarization scanning. Figure 3 It is a schematic diagram of the structure of a CPT magnetometer system involved in the magnetic field vector measurement method based on a CPT magnetometer of the present invention. Figure 4 This is a schematic diagram of the relative relationship between the wave vector k, polarization e and the magnetic field B to be measured. Figures 1 to 4As shown, a magnetic field vector measurement method based on a CPT magnetometer includes the following steps: Step 1, starting the CPT magnetometer system to generate a multi-color laser, the multi-color laser forms a first beam of light and a second beam of light through a polarization beam splitter prism, the first beam of light passes through a first polarizer and a first half-wave plate in sequence, and then passes through an alkali metal gas chamber in a positive direction along the x-axis to form a first beam of light signal carrying magnetic field information and enters a first input end of a phase-locked amplifier, the second beam of light passes through a first reflector, a second reflector, a second polarizer, and a second half-wave plate in sequence, and then passes through an alkali metal gas chamber in a negative direction along the z-axis to form a second beam of light signal carrying magnetic field information and enters a second input end of the phase-locked amplifier; Step 2, the phase-locked amplifier demodulates the first beam of light signal to obtain a dispersion signal, and according to The magnetic field measurement principle that the zero-crossing point of the dispersion signal is consistent with the peak point frequency of the CPT signal is used to obtain the magnetically sensitive dispersion signal; step 3, adjusting the polarization direction e of the first beam of light by rotating the first half-wave plate, obtaining the change in the peak amplitude of the CPT signal by observing the change in the oscillation amplitude of the dispersion signal, keeping the wave vector k fixed, and tracking the maximum peak amplitude of the CPT signal; defining n = k × B, B is the external magnetic field or the magnetic field to be measured, n is the normal vector, since k⊥n, the polarization direction e is adjusted by rotating the half-wave plate, when the signal amplitude reaches the maximum value, e∥n, thereby obtaining the first plane where the magnetic field B is located through the first beam of light, that is, the plane with n as the normal vector and passing through the wave vector k; step 4, determining the size of the magnetic field scalar using the following relationship:

[0035] Δv=γ|B|

[0036] Wherein Δv is the frequency difference between the zero crossing points of the dispersion signal, i.e., the optical frequency difference between the two CPT peaks, B is the magnitude of the magnetic field, and γ is the gyromagnetic ratio; Step 5, return to Step 2, process the second light beam signal through the phase-locked amplifier in a manner similar to that of the first light beam signal, obtain the second plane where the magnetic field B is located through the second light beam, and the intersection of the first plane and the second plane is the direction of the magnetic field B.

[0037] The first plane where the magnetic field B is located in step 3 has the following expression:

[0038] x+tan(2α)y=0

[0039] where α is the half-wave plate rotation angle.

[0040] Step 3 includes the following expression for the polarization direction e:

[0041]

[0042] Where α is the rotation angle of the half-wave plate, and the polarization direction e starts scanning from the positive direction of the X-axis and scans in the positive direction of the Y-axis.

[0043] The step 5 includes: defining is the angle between e and n, where counterclockwise is Clockwise Defining dependency functions exist That is, when e⊥B, To reach the maximum value, the wave vector k=k1 is selected, where k1 is the set value, and the polarization vector e is obtained by rotating the wave vector k1. The maximum value corresponds to the direction of vector n=[k1×B], that is, the equation of the plane (k1,B) formed by the vectors k1 and B is obtained; repeat the same process in the other direction of the wave vector k=k2, k2 is the set quantity, and the equation of the plane (k2,B) is obtained; the intersection line of the two planes (k1,B) and (k2,B) is the three-dimensional direction of the magnetic field B.

[0044] The first half-wave plate 7 in step 1 is connected to the first servo motor 25, and the second half-wave plate 18 is connected to the second servo motor 19. The alkali metal gas chamber (i.e., atomic gas chamber 11) in step 1 is located in an oven 10, and the oven 10 is located in a Helmholtz coil 9, and the Helmholtz coil 9 is located in a magnetic shielding device 8. The CPT magnetometer system in step 1 includes a VCSEL laser 1, the output end of the VCSEL laser 1 is connected to a fiber coupler 2, and the fiber coupler 2 is connected to the input end of a polarization beam splitter prism 5 through a polarization-maintaining fiber 3 and a laser collimator 4 in sequence, and the transmission side of the polarization beam splitter prism 5 is connected to a phase-locked amplifier 13 through a first polarizer 6, a first half-wave plate 7, an alkali metal gas chamber 11 and a first photodetection amplifier 12 in sequence, and the reflection side of the polarization beam splitter prism 5 is connected to a phase-locked amplifier 13 through a first reflector 15, a second reflector 16, a second polarizer 17, a second half-wave plate 18, an alkali metal gas chamber 11 and a first photodetection amplifier 12 in sequence. The second photodetection amplifier 27 is connected to the phase-locked amplifier 13, the output end of the phase-locked amplifier 13 is connected to the input voltage data acquisition system 14, the third input end of the phase-locked amplifier 13 is connected to the output end of the sine wave signal generator 26, the output end of the sine wave signal generator 26 is connected to the first input end of the adder 23, the second input end of the adder 23 is connected to the triangle wave signal generator 24, the output end of the adder 23 is connected to the first input end of the T-type bias device 21 through the radio frequency signal generator 22, the second input end of the T-type bias device 21 is connected to the VCSEL controller 20, and the output end of the T-type bias device 20 is connected to the VCSEL laser 1. The VCSEL laser 1 locks the laser on the alkali metal atom D1 line, uses the radio frequency signal generator 22 to generate a 3.417GHz signal, and the radio frequency signal is modulated by the T-type bias device 21 and added to the DC control electrical signal of the VCSEL laser 1 to generate multi-color laser.

[0045] The present invention implements a magnetic field vector measurement method based on a CPT magnetometer through five steps.

[0046] Step 1: Multicolor laser generation

[0047] According to the experimental design, two lasers are needed to meet the frequency difference between the two hyperfine energy levels of the Rb87 atomic D1 line. This experiment uses a VCSEL laser to lock the laser on the D1 line, and uses an RF signal generator to generate a 3.417GHz signal. The RF signal is modulated by a T-type bias device and added to the DC control electrical signal of the VCSEL laser to obtain the required multi-color laser.

[0048] Step 2: Modulate and demodulate the CPT signal to obtain the dispersion signal

[0049] This scheme uses a triangular wave function and a sine wave function to modulate the signal generated by the RF signal generator. The function of the triangular wave function is frequency scanning, and the function of the sine wave function is to modulate the optical signal and input it into the phase-locked amplifier as a reference signal. The dispersion signal can be obtained by demodulating the phase-locked amplifier. The dispersion signal and the CPT signal have a functional relationship, in which the zero-crossing point of the dispersion signal corresponds to the peak point frequency of the CPT signal. Based on this principle, the dispersion signal can be used to measure the magnetic field. The external magnetic field forms multiple CPT resonances. For the magnetic quantum number m of the alkali metal atom F The frequency difference of the Zeeman sublevel with a value of ±1 will change with the change of the magnetic field, and the frequency offset will change. Its dispersion signal is called the magnetically sensitive dispersion signal, and m F = 0, the Zeeman sub-level frequency difference remains unchanged and is always equal to the hyperfine level frequency difference, which is called the magnetically insensitive dispersion signal.

[0050] Step 3: Adjust the polarization direction to observe the CPT peak signal amplitude

[0051] The angle between the magnetic field B and the wave vector k and polarization direction e will have a significant impact on the CPT signal. The amplitudes of the seven CPT peaks will change with the angle and reach maximum values ​​at certain angles.

[0052] The dispersion signal is the result of CPT signal demodulation. This paper can obtain the change in the size of the CPT signal peak amplitude by observing the change in the oscillation amplitude of the dispersion signal. The implementation method is to keep the wave vector k fixed, accurately control the polarization direction e, observe the signal, and find the polarization direction e corresponding to the peak amplitude maximum. At this time, the signal-to-noise ratio is the largest, and the magnetic field measurement with the highest sensitivity based on the existing scheme can be obtained. When the magnetic field is unknown, the analytical expression of the plane where the magnetic field direction is located can be obtained through relative relationships.

[0053] Step 4: Magnetic field scalar measurement

[0054] The modulation signal of the RF signal generator is input as a reference signal into the phase-locked amplifier for demodulation to obtain the dispersion signal. The dispersion signal and the CPT signal have a functional relationship, in which the zero crossing point of the dispersion signal corresponds to the peak point frequency of the CPT signal. Based on this principle, the dispersion signal can be used to measure the magnetic field. Under the action of the external magnetic field, multiple CPT resonances are formed. The magnetic flux density can be measured by measuring the frequency shift of the Zeeman CPT resonance:

[0055] Δv=γ|B|

[0056] Among them, Δv represents the optical frequency difference between the two CPT peaks, which is also the frequency difference between the zero crossing points of the dispersion signal in this scheme; γ = 7Hz / nT is the gyromagnetic ratio of Rb87; B represents the magnitude of the magnetic field. This allows magnetic field measurement.

[0057] Step 5: Double beam method to determine the line where the magnetic field is located

[0058] The plane where the magnetic field is located can be determined by step 3 using a single beam. In the magnetic field vector measurement experiment, the relative relationship between the wave vector k, magnetic field B, n and polarization direction e is as follows: Figure 2 As shown. Define the wave vector k as the Z axis, the vertical direction as the X axis, and the Y axis is defined by the three-axis coordinate system. There is an external magnetic field B in the atomic gas chamber, and its actual direction is unknown. To express the principle, Figure 2 As shown in the figure, n = k × B. Since B is unknown, the actual angle is also unknown. However, according to Figure 2 A schematic diagram is drawn to show the direction of the magnetic field, but it is certain that n must be in the XOY plane (k⊥n). By rotating the half-wave plate, polarization scanning is achieved. When the signal amplitude reaches a maximum value, it is the case of e∥n.

[0059] According to the actual situation, the angle between B and n will not be as Figure 2 As shown, the following is an analysis of the process in the experimental measurement. Consistent with the above process, the wave vector k is along the Z axis, and the polarization direction e can be adjusted by rotating the half-wave plate. When the signal amplitude is observed to be a maximum value, the direction of n is found (e∥n). According to the direction of n and n=k×B, the plane where the magnetic field B is located can be found, that is, the plane with n as the normal vector and passing through the wave vector k.

[0060] The polarization direction e is perpendicular to the wave vector k (Z axis) and exists in the XOY plane. When the scanning point is taken as the positive direction of the X axis, the scanning is performed in the positive direction of the Y axis, and the half-wave plate is rotated by an angle of α. According to the properties of the half-wave plate, the actual polarization direction is rotated by an angle of 2α. At this time, the polarization direction e can be expressed as:

[0061]

[0062] At this time, according to n = k × B, we can get the plane where the magnetic field B is located:

[0063] x+tan(2α)y=0

[0064] To sum up, considering the actual experimental situation, if only one beam of light is injected and the wave vector k remains unchanged, adjusting the polarization direction e alone cannot accurately obtain the precise direction of the magnetic field B, but only the plane where the magnetic field B is located.

[0065] On this basis, we split the light beam to obtain a second beam of light with a certain angle to the first beam of light, and repeat the above process to measure. Through data processing, we will get two planes where the magnetic field B is located, and these two planes are not parallel. The intersection of the two planes is the direction of the magnetic field B.

[0066] Figure 3 The principle of the magnetic field vector measurement system and the coordinate system setting of the CPT magnetometer are demonstrated.

[0067] According to the experimental design, two lasers are needed to meet the frequency difference between the two hyperfine energy levels of the Rb87 atomic D1 line. This experiment uses a VCSEL laser to lock the laser on the D1 line, and uses an RF signal generator to generate a 3.417GHz signal. The RF signal is modulated by a T-type bias device and added to the DC control electrical signal of the VCSEL laser to obtain the required multi-color laser.

[0068] This scheme uses a triangular wave function and a sine wave function to modulate the signal generated by the RF signal generator. The function of the triangular wave function is frequency scanning, and the function of the sine wave function is to modulate the optical signal and input it into the phase-locked amplifier as a reference signal. The dispersion signal can be obtained by demodulating the phase-locked amplifier. The dispersion signal and the CPT signal have a functional relationship, in which the zero-crossing point of the dispersion signal corresponds to the peak point frequency of the CPT signal. Based on this principle, the dispersion signal can be used to measure the magnetic field. The external magnetic field forms multiple CPT resonances. For the magnetic quantum number m of the alkali metal atom F The frequency difference of the Zeeman sublevel with a value of ±1 will change with the change of the magnetic field, and the frequency offset will change. Its dispersion signal is called the magnetically sensitive dispersion signal, and m F = 0, the Zeeman sub-level frequency difference remains unchanged and is always equal to the hyperfine level frequency difference, which is called the magnetically insensitive dispersion signal.

[0069] The angle between the magnetic field B and the wave vector k and the polarization direction e (refer to Figure 4) will have a great impact on the CPT signal. The 7 CPT peak amplitudes will change with the angle, and will be maximum at a certain angle. The dispersion signal is the result of the demodulation of the CPT signal. This article can obtain the change in the size of the CPT signal peak amplitude by observing the change in the oscillation amplitude of the dispersion signal. The implementation method is to keep the wave vector k fixed, accurately control the polarization direction e, observe the signal, and find the polarization direction e corresponding to the peak amplitude maximum. At this time, the signal-to-noise ratio is the largest, and the magnetic field measurement with the highest sensitivity based on the existing scheme can be obtained. When the magnetic field is unknown, the analytical expression of the plane where the magnetic field direction is located can be obtained through relative relationships.

[0070] Consider the dependence of the dark resonance amplitude, which is obtained by rotating the polarization vector e around a fixed wave vector k. Figure 4 As shown, the definition is the angle between e and n, where counterclockwise is positive and clockwise is negative. Define the dependency function exist (That is, when e⊥B) it reaches its maximum value.

[0071] The essence of the measurement process can be expressed by the following algorithm. First, for the selected wave vector k = k1, we obtain by rotating the polarization vector e around the wave vector k1: The maximum of this dependence corresponds to the direction of the vector n = [k1 × B], which gives us the equation of the plane (k1, B) formed by the vectors k1 and B. Repeating the same process for the other direction of the wave vector k = k2 gives the equation of the plane (k2, B). The intersection of the two planes (k1, B) and (k2, B) is the three-dimensional direction of the vector B, but the sign cannot be determined.

[0072] This can be seen from the general symmetry of the problem. In fact, suppose there is an arbitrary polychromatic wave propagating in direction k, with all frequency components having the same linear polarization e. Assume that the atomic medium is isotropic in the absence of a light field. Define the signal intensity as a scalar-valued function S(e,B) whose value depends on the mutual orientation of the vectors e and B.

[0073] In the sense of this definition, a general analysis of the Bloch equation yields the following relationship:

[0074] S(e,B)=S(e,-B)=S(-e,B)

[0075] The light field has Polarization. Let's do a mathematical analysis on the plane (k,B). We can realize the polarization vector , but for the pseudo-vector of the magnetic field, it leads to B→(-B). It is known that mathematical reflection does not affect the scalar signal (i.e. another relationship is obtained):

[0076]

[0077]

[0078] That is, the scalar signal is the angle is an even function of . This means (i.e. e⊥B) corresponds to a local extremum, which is obtained by rotating the polarization vector e around the wave vector k. Similar symmetry considerations show that when the vector e lies in the plane (k, B) (i.e., in There are two other extreme values.

[0079] The modulation signal of the RF signal generator is input as a reference signal into the phase-locked amplifier for demodulation to obtain the dispersion signal. The dispersion signal and the CPT signal have a functional relationship, in which the zero crossing point of the dispersion signal corresponds to the peak point frequency of the CPT signal. Based on this principle, the dispersion signal can be used to measure the magnetic field. Under the action of the external magnetic field, multiple CPT resonances are formed. The magnetic flux density can be measured by measuring the frequency shift of the Zeeman CPT resonance:

[0080] Δν=γ|B|

[0081] Among them, Δv represents the optical frequency difference between the two CPT peaks, which is also the frequency difference between the zero crossing points of the dispersion signal in this scheme; γ = 7Hz / nT is the gyromagnetic ratio of Rb87; B represents the magnitude of the magnetic field. This allows magnetic field measurement.

[0082] The laser frequency is modulated, multiple dispersion signals are observed through a phase-locked amplifier, and the center frequency of the modulation signal is adjusted to select the magnetically sensitive dispersion signal. The polarization direction is changed and the dispersion signal is observed, and the polarization direction e corresponding to the maximum peak amplitude of the CPT signal is found, and the magnetic field scalar measurement is performed at this angle. The polarization direction e can be used to obtain the plane where the magnetic field is located. Repeat the above process with another laser that is not parallel to the original laser, and the straight line where the two planes intersect is the direction of the magnetic field.

[0083] The contents not described in detail in the specification of the present invention belong to the prior art known to the professional and technical personnel in the field. It is pointed out here that the above description helps those skilled in the art to understand the invention, but does not limit the protection scope of the invention. Any equivalent replacement, modification and / or simplification of the above description without departing from the essence of the invention falls within the protection scope of the invention.

Claims

1. A magnetic field vector measurement method based on a CPT magnetometer, characterized in that: The following steps are involved: Step 1, starting the CPT magnetometer system to generate a multi-color laser, the multi-color laser is formed into a first beam of light and a second beam of light through a polarization beam splitter prism, the first beam of light passes through a first polarizer and a first half-wave plate in sequence, and then passes through an alkali metal gas chamber in a positive direction along the x-axis to form a first beam of light signal carrying magnetic field information and enters a first input end of a phase-locked amplifier, the second beam of light passes through a first reflector, a second reflector, a second polarizer, and a second half-wave plate in sequence, and then passes through an alkali metal gas chamber in a negative direction along the z-axis to form a second beam of light signal carrying magnetic field information and enters a second input end of the phase-locked amplifier; Step 2: The lock-in amplifier demodulates the first light signal to obtain a dispersion signal, and obtains a magnetically sensitive dispersion signal according to the magnetic field measurement principle that the zero-crossing point of the dispersion signal is consistent with the peak point frequency of the CPT signal; Step 3, adjust the polarization direction e of the first beam of light by rotating the first half-wave plate, obtain the change in the size of the peak amplitude of the CPT signal by observing the change in the oscillation amplitude of the dispersion signal, keep the wave vector k fixed, and track the maximum value of the peak amplitude of the CPT signal; define n=k×B, B is the external magnetic field or the magnetic field to be measured, n is the normal vector, since k⊥n, the polarization direction e is adjusted by rotating the half-wave plate, when the signal amplitude reaches the maximum value, then e||n, thereby obtaining the first plane where the magnetic field B is located through the first beam of light, that is, the plane with n as the normal vector and passing through the wave vector k; Step 4, determine the magnitude of the magnetic field scalar using the following relationship: Δν=γ|B| Where Δv is the frequency difference between the zero crossing points of the dispersion signal, i.e., the optical frequency difference between the two CPT peaks, B is the magnitude of the magnetic field, and γ is the gyromagnetic ratio; Step 5, return to step 2, process the second light beam signal through a phase-locked amplifier in a similar manner to the first light beam signal, obtain the second plane where the magnetic field B is located through the second light beam, and the intersection of the first plane and the second plane is the direction of the magnetic field B.

2. The magnetic field vector measurement method based on CPT magnetometer according to claim 1, characterized in that: The first plane where the magnetic field B is located in step 3 has the following expression: x+tan(2α)y=0 where α is the half-wave plate rotation angle.

3. The magnetic field vector measurement method based on CPT magnetometer according to claim 1, characterized in that: Step 3 includes the following expression for the polarization direction e: Where α is the rotation angle of the half-wave plate, and the polarization direction e starts scanning from the positive direction of the X-axis and scans in the positive direction of the Y-axis.

4. The magnetic field vector measurement method based on CPT magnetometer according to claim 1, characterized in that: The step 5 includes: defining is the angle between e and n, where counterclockwise is Clockwise Defining dependency functions exist That is, when e⊥B, To reach the maximum value, the wave vector k=k1 is selected, where k1 is the set value, and the polarization vector e is obtained by rotating the wave vector k1. The maximum value corresponds to the direction of vector n=[k1×B], that is, the equation of the plane (k1, B) formed by the vectors k1 and B is obtained; the same process is repeated in the other direction of the wave vector k=k2, k2 is the set quantity, and the equation of the plane (k2, B) is obtained; the intersection line of the two planes (k1, B) and (k2, B) is the three-dimensional direction of the magnetic field B.

5. The magnetic field vector measurement method based on CPT magnetometer according to claim 1, characterized in that: The first half-wave plate in step 1 is connected to the first servo motor, and the second half-wave plate is connected to the second servo motor.

6. The magnetic field vector measurement method based on CPT magnetometer according to claim 1, characterized in that: The alkali metal gas chamber in step 1 is located in an oven, the oven is located in a Helmholtz coil, and the Helmholtz coil is located in a magnetic shielding device.

7. The magnetic field vector measurement method based on CPT magnetometer according to claim 1, characterized in that: The CPT magnetometer system in step 1 includes a VCSEL laser, the output end of the VCSEL laser is connected to a fiber coupler, the fiber coupler is connected to the input end of a polarization beam splitter prism through a polarization-maintaining fiber and a laser collimator in sequence, the transmission side of the polarization beam splitter prism is connected to a phase-locked amplifier through a first polarizer, a first half-wave plate, an alkali metal gas chamber and a first photodetection amplifier in sequence, the reflection side of the polarization beam splitter prism is connected to a phase-locked amplifier through a first reflector, a second reflector, a second polarizer, a second half-wave plate, an alkali metal gas chamber and a second photodetection amplifier in sequence, the output end of the phase-locked amplifier is connected to an input voltage data acquisition system, the third input end of the phase-locked amplifier is connected to the output end of a sine wave signal generator, the output end of the sine wave signal generator is connected to a first input end of an adder, the second input end of the adder is connected to a triangular wave signal generator, the output end of the adder is connected to a first input end of a T-type bias device through a radio frequency signal generator, the second input end of the T-type bias device is connected to a VCSEL controller, and the output end of the T-type bias device is connected to a VCSEL laser.

8. The magnetic field vector measurement method based on CPT magnetometer according to claim 7, characterized in that: The VCSEL laser locks the laser on the alkali metal atom D1 line, uses a radio frequency signal generator to generate a 3.417GHz signal, and modulates the radio frequency signal with a T-type bias device and adds it to the DC control electrical signal of the VCSEL laser to generate multi-color laser.

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