Mechanical-interference-free magnetic field vector measurement method for miniaturized CPT magnetometer

Abstract

A mechanical-interference-free magnetic field vector measurement method for a miniaturized CPT magnetometer adopts a new polarization state control structure to replace a servo motor part in an original structure, two polarization gratings with the same phase period are used, one polarization grating is regulated and controlled by using small-size piezoelectric ceramics, and the size of the polarization grating is adjusted and controlled by using small-size piezoelectric ceramics. The linear precise control of the light polarization state can be realized, two servo motor mechanical structures with the largest volume in the original system can be omitted, and the polarization state control can be realized by adjusting the relative position of the two polarization gratings by using piezoelectric ceramics with the size of one coin, and the displacement is regulated to be linear movement of several microns. The design volume of the CPT magnetometer is greatly reduced, the CPT magnetometer avoids mechanical vibration caused by the rotation angle of the servo motor, and the CPT magnetometer is verified to be well adaptive to an existing double-beam measurement method, and has relatively strong continuation applicability and relatively high practical value.

Description

Technical Field

[0001] This invention relates to the field of light-alkali metal atom interaction and automatic control technology, and in particular to a mechanically interference-free magnetic field vector measurement method for a miniaturized CPT magnetometer. Background Technology

[0002] CPT atomic magnetometers have wide applications in geophysics, medicine, and fundamental physics (CPT stands for Coherent Population Trapping). CPT magnetometers possess high sensitivity potential (1...). The CPT atomic magnetometer offers advantages such as miniaturization, vector measurement, and geomagnetic environment measurement. In many applications, understanding the vector components of the magnetic field is crucial for a comprehensive understanding of the field's specific information. However, current vector measurement schemes for CPT atomic magnetometers are relatively outdated. The principles described above only allow for scalar measurements of the magnetic field at the measurement point, failing to obtain angular information, thus limiting its practical application value. Furthermore, existing vector measurements often employ servo motors for half-wave plate adjustment, introducing mechanical vibration, amplifying system noise, and severely impacting the magnetometer's sensitivity. In extreme cases, this sensitivity may decrease by one to two orders of magnitude. Additionally, the rotating mechanical structure of the servo motor prevents a reduction in the size of the CPT atomic magnetometer, posing a challenge to its miniaturization.

[0003] The CPT effect is a quantum coherence effect. A CPT magnetometer, utilizing both the CPT and Zeeman effects, can achieve absolute measurement of an external magnetic field. Generally, the magnitude of the magnetic field in a CPT magnetometer is determined by measuring the frequency detuning of the CPT manifold. Utilizing frequency detuning The magnitude of the external magnetic field can be calculated from its relationship with the magnitude of the external magnetic field B0:

[0004]

[0005] in It is the gyromagnetic ratio of alkali metal atoms. Summary of the Invention

[0006] The problem solved by the present application is to provide a novel mechanical interference-free magnetic field vector measurement method for a miniaturized CPT magnetometer, which can replace the servo motor part in the original structure by setting a double-polarization grating control module and a piezoelectric ceramic displacement table combination on the total incident light path to adjust the polarization direction, aiming at the problems of large volume and mechanical interference in the original CPT magnetometer through the servo motor to adjust the polarization state of light, the double-polarization grating control module uses two pieces of polarization gratings with the same phase period in series, and the linear precise control of the polarization state of light can be realized by moving one of the polarization gratings up and down by a small volume piezoelectric ceramic.

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

[0008] A mechanical interference-free magnetic field vector measurement method for a miniaturized CPT magnetometer, characterized by comprising the following steps:

[0009] Step 1, a double-polarization grating control module with a piezoelectric ceramic displacement table is arranged on the total incident light path of the alkali metal cell of the CPT magnetometer system, the double-polarization grating control module is located between the laser collimator and the polarization beam splitter prism, the double-polarization grating control module comprises a first polarization grating and a second polarization grating connected in series, the first polarization grating is connected to the laser collimator to receive collimated polychromatic laser, and the second polarization grating is connected to the input side of the polarization beam splitter prism;

[0010] Step 2, the second polarization grating is driven by the piezoelectric ceramic displacement table to move up and down for adjustment, so as to realize the linear precise control of the polarization state of light, and the precise polarization state laser is emitted to the input side of the polarization beam splitter prism;

[0011] Step 3, the polarization beam splitter prism decomposes the input precise polarization state laser into two beams, the first beam is emitted from the transmission side, and the second beam is emitted from the reflection side, the first beam passes through the alkali metal cell along the x-axis negative direction to form a first beam signal carrying magnetic field information into the first input end of the lock-in amplifier, and the second beam passes through the alkali metal cell along the z-axis positive direction after passing through the first mirror and the second mirror to form a second beam signal carrying magnetic field information into the second input end of the lock-in amplifier;

[0012] Step 4, the lock-in amplifier demodulates the dispersion signals respectively for the first light signal and the second light signal, and determines the size and direction of the external magnetic field or the magnetic field to be measured according to the dispersion signals.

[0013] The first polarization grating and the second polarization grating in step 1 are polarization gratings with the same phase period.

[0014] Step 1 includes the following expression:

[0015]

[0016] Where Δn is the polarization grating structure coefficient of the dual-polarization grating control module, n p1 is the p-light refractive index of the first polarization grating, n s1 is the s-light refractive index of the first polarization grating, n p2 is the p-light refractive index of the second polarization grating, n s2 is the s-light refractive index of the second polarization grating.

[0017] Step 2 includes the following expression:

[0018]

[0019]

[0020] Where θ is the polarization angle, ΔS is the distance between the central axes of the two polarization gratings, K is the intermediate quantity, and λ is the input laser wavelength.

[0021] Step 3 includes the following expression:

[0022]

[0023] Where e is the polarization direction of the first light or the second light, and the polarization direction e is adjusted to start scanning from the positive direction of the X axis to the positive direction of the Y axis.

[0024] Step 4 includes: observing the change of the oscillation amplitude of the dispersion signal to obtain the change of the CPT signal peak amplitude, tracking the maximum value of the CPT signal peak amplitude, obtaining the first plane where the magnetic field B is located according to the maximum value of the signal amplitude obtained by the first light signal, obtaining the second plane where the magnetic field B is located according to the maximum value of the signal amplitude obtained by the second light signal, and the intersection line of the first plane and the second plane is the direction of the magnetic field B.

[0025] Step 4 includes the following expression:

[0026]

[0027] Where is the frequency difference between the zero-crossing points of the dispersive signal, i.e., the optical frequency difference between the two CPT peaks, is the magnetic field strength, is the gyromagnetic ratio.

[0028] The input side of the laser collimator is connected with the VCSEL laser through a second polarization maintaining optical fiber, an electro-optic phase modulator, a first polarization maintaining optical fiber, an optical coupler and an optical isolator in sequence, the z-axis positive outgoing side of the alkali metal cell is connected with the lock-in amplifier through a second photoelectric detection amplifier, the x-axis negative outgoing side of the alkali metal cell is connected with the lock-in amplifier through a first photoelectric detection amplifier, the lock-in amplifier is connected with a data acquisition system, an arbitrary signal generator and a radio frequency signal generator respectively, and the radio frequency signal generator is connected with the arbitrary signal generator and the electro-optic phase modulator respectively.

[0029] The alkali metal cell is located in an oven, the oven is located in a Helmholtz coil, and the Helmholtz coil is located in a magnetic shielding device.

[0030] The technical effect of the present application is as follows: the mechanical interference-free magnetic field vector measurement method of the miniaturized CPT magnetometer adopts a double-polarization grating linear control light polarization module to replace the traditional servo motor rotating half-wave plate form to control the polarization state of the input laser. A coin-sized piezoelectric ceramic displacement table is used to displace two polarization gratings by millimeters, thereby changing the phase delay of the light passing through it and achieving precise control of the light polarization state. The polarization angle is precisely controlled, and the magnetic field direction is measured by observing the dispersion signal. As the probe part of the Coherent Population Trapping (CPT) vector magnetometer, the modulated Vertical-Cavity Surface-Emitting Laser (VCSEL) generates polychromatic light into the gas chamber, and the light signal carrying the magnetic field information is converted into a voltage signal by a photodiode and demodulated by a lock-in amplifier to obtain multiple dispersion signals. Each dispersion signal can be used to analyze the state of the CPT signal, wherein the zero-crossing point of the dispersion signal is at the same frequency as the CPT peak, and the greater the amplitude of the dispersion signal, the greater the CPT peak amplitude. The present application 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 measures the vector of the magnetic field. The present application can omit the two largest servo motor mechanical structures in the traditional CPT magnetometer system, and instead use a coin-sized piezoelectric ceramic to adjust the relative positions of the two polarization gratings to achieve polarization state control and linear movement with a displacement of several microns. This greatly reduces the design volume of the CPT magnetometer and avoids mechanical vibration caused by the rotation angle of the servo motor. The present application provides a design method for the miniaturization and high precision of the CPT magnetometer.

[0031] The mechanical interference-free magnetic field vector measurement method of the miniaturized CPT magnetometer adopts a new polarization state control structure to replace the servo motor part in the original structure. By using two polarization gratings with the same phase period and a small-volume piezoelectric ceramic to control one of the polarization gratings, linear precise control of the light polarization state is achieved. The two largest servo motor mechanical structures in the original system can be omitted, and instead a coin-sized piezoelectric ceramic is used to adjust the relative positions of the two polarization gratings to achieve polarization state control and linear movement with a displacement of several microns. This greatly reduces the design volume of the CPT magnetometer and avoids mechanical vibration caused by the rotation angle of the servo motor. After verification, it can well adapt to the existing double-beam measurement method, has strong extension applicability and high practical value. BRIEF DESCRIPTION OF DRAWINGS

[0032] Figure 1It is a flowchart of a mechanical interference-free magnetic field vector measurement method for implementing a miniaturized CPT magnetometer of the present application. Figure 1 The method comprises the following steps: 1, modulated polychromatic laser generation; 2, laser passes through a gas chamber to generate an optical signal carrying magnetic field information, and a lock-in amplifier is used for demodulation to obtain a dispersion signal (the state of the CPT signal is analyzed through 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 is at the same frequency as the CPT peak, and the greater the amplitude of the dispersion signal, the greater the amplitude of the CPT signal peak); 3, rotation of the polarization angle to track the maximum value of the signal peak amplitude; 4, processing of the dispersion signal to measure the magnetic field size (B) , is the frequency difference between the zero-crossing points of the dispersion signal, i.e. the optical frequency difference between the two CPT peaks, is the magnetic field size, is the gyromagnetic ratio, for example, the gyromagnetic ratio of Rb87 ); 5, a double-beam scheme is used to determine the magnetic field direction (through the first beam, the wave vector k is defined along the Z axis, n=k×B is defined, and since , the polarization direction e is adjusted by rotating a half-wave plate, and when the signal amplitude reaches a maximum value, the , thus the first plane in which the magnetic field B is located is obtained through the first beam, i.e. a plane with n as the normal vector and passing through the wave vector k; through the second beam, which has a certain angle with the first beam, the above process is repeated to measure the second plane in which the magnetic field B is located, and the intersection of the two planes is the direction of the magnetic field B).

[0033] Figure 2 It is a polarization scanning diagram. Figure 2 The method comprises the following steps: 1, modulated polychromatic laser generation; 2, laser passes through a gas chamber to generate an optical signal carrying magnetic field information, and a lock-in amplifier is used for demodulation to obtain a dispersion signal (the state of the CPT signal is analyzed through 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 is at the same frequency as the CPT peak, and the greater the amplitude of the dispersion signal, the greater the amplitude of the CPT signal peak); 3, rotation of the polarization angle to track the maximum value of the signal peak amplitude; 4, processing of the dispersion signal to measure the magnetic field size (B) Figure 2 In the method, the polarization direction e starts scanning from the positive direction of the X axis and scans towards the positive direction of the Y axis. According to n=k×B, the plane in which the magnetic field B is located is obtained as: , is the matching polarization angle.

[0034] Figure 3 It is a CPT magnetometer system structure diagram related to the mechanical interference-free magnetic field vector measurement method of the miniaturized CPT magnetometer of the present application.

[0035] Figure 4 It is Figure 3 a double-polarization grating control module structure diagram in the method. Figure 4The dual polarization grating control module consists of two polarization gratings with the same phase period. It is designed to replace the traditional servo motor rotating half-wave plate module with piezoelectric ceramics, and is used for high-precision polarization control of incident light in a small volume.

[0036] Figure 5 This is a schematic diagram showing the relative relationship between the wave vector k, polarization e, and the magnetic field B to be measured. Figure 5 Includes definitions It is the angle between e and n, where counterclockwise is positive and clockwise is negative. Define the dependency function. ,exist = 0, (That is, when e⊥B) it reaches its maximum value. For the selected wave vector By surrounding the wave vector To obtain by rotating the polarization vector e . The maximum value corresponds to the normal vector n = [ The direction of ×B] is obtained from the vector The plane formed by B ( The equation for B). For wave vector k = Repeating the same process in the other direction yields a plane. The equations of B). Two planes ( B) and ( The intersection line of ,B) is the three-dimensional direction of the magnetic field B.

[0037] The reference numerals in the attached figures are explained as follows: 1-VCSEL laser (VCSEL, Vertical-Cavity Surface-Emitting Laser); 2-Optical isolator; 3-Optical coupler; 4-First polarization-maintaining fiber; 5-Electro-optic phase modulator; 6-Second polarization-maintaining fiber; 7-Laser collimator; 8-Dual polarization grating control module; 9-Piezoelectric ceramic displacement stage; 10-Polarization beam splitter prism; 11-First reflecting mirror; 12-Second reflecting mirror; 13-Lock-in amplifier; 14-Data acquisition system; 15-Arbitrary signal generator; 16-RF signal generator; 17-First optical... 18 - Second photoelectric detector amplifier; 19 - Alkali metal gas chamber or atomic gas chamber (e.g., Rb gas chamber); 20 - Oven; 21 - Helmholtz coil; 22 - First polarization grating; 23 - Second polarization grating (which is a vertically movable polarization grating); ΔS - Distance between the central axes of the two polarization gratings; xyz - Three axes of the 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 scan; B - Magnetic field to be measured; n - Normal vector, n=k×B; - The angle between vectors e and n (counterclockwise is positive, denoted as ). ; clockwise is negative, denoted as ). DETAILED DESCRIPTION

[0038] The application will be described below with reference to the accompanying drawings Figures 1-5 ) and examples.

[0039] Figure 1 is a flowchart of a mechanical interference-free magnetic field vector measurement method for a miniaturized CPT magnetometer according to the application. Figure 2 is a schematic diagram of polarization scanning. Figure 3 is a schematic diagram of the system structure of a CPT magnetometer according to the application. Figure 4 is a schematic diagram of the structure of a dual-polarization grating control module in Figure 3 . Figure 5 is a schematic diagram of the relative relationship between the wave vector k, the polarization e, and the magnetic field to be measured B. Referring to Figures 1 to 5 , a mechanical interference-free magnetic field vector measurement method for a miniaturized CPT magnetometer, comprising the following steps: step 1, a dual-polarization grating control module 8 with a piezoelectric ceramic displacement stage 9 is arranged on the total incident light path of the alkali metal gas chamber 19 of the CPT magnetometer system, the dual-polarization grating control module 8 is located between the laser collimator 7 and the polarization beam splitter prism 10, the dual-polarization grating control module 8 comprises a first polarization grating 22 and a second polarization grating 23 connected in series, the first polarization grating 22 is connected to the laser collimator 7 to receive collimated polychromatic laser, and the second polarization grating 23 is connected to the input side of the polarization beam splitter prism 10; step 2, the piezoelectric ceramic displacement stage 9 is used to drive the second polarization grating 23 to adjust the up and down displacement to realize the accurate control of the light polarization state, and the accurate polarization state laser is emitted to the input side of the polarization beam splitter prism 10; step 3, the polarization beam splitter prism 10 decomposes the input accurate polarization state laser into two beams, the first beam is emitted from the transmission side, and the second beam is emitted from the reflection side, the first beam passes through the alkali metal gas chamber 19 along the x-axis negative direction to form a first beam signal carrying magnetic field information into the first input end of the lock-in amplifier 13, and the second beam passes through the alkali metal gas chamber 19 along the z-axis positive direction after passing through the first mirror 11 and the second mirror 12 to form a second beam signal carrying magnetic field information into the second input end of the lock-in amplifier 13; step 4, the lock-in amplifier 13 demodulates the first beam signal and the second beam signal respectively to obtain a dispersion signal, and determines the size and direction of the external magnetic field or the magnetic field to be measured according to the dispersion signal.

[0040] The first polarization grating 22 and the second polarization grating 23 in step 1 are polarization gratings with the same phase period.

[0041] Step 1 includes the following expression:

[0042]

[0043] where Δn is the polarization grating structure coefficient of the dual-polarization grating control module, n p1 is the p-light refractive index of the first polarization grating, n s1 is the s-light refractive index of the first polarization grating, n p2 is the p-light refractive index of the second polarization grating, n s2 is the s-light refractive index of the second polarization grating.

[0044] The following expression is included in step 2:

[0045]

[0046]

[0047] where θ is the polarization angle, ΔS is the distance between the central axes of the two polarization gratings, K is an intermediate quantity, and λ is the input laser wavelength.

[0048] The following expression is included in step 3:

[0049]

[0050] where e is the polarization direction of the first beam or the second beam, and the polarization direction e is adjusted to start scanning from the positive direction of the X-axis as the starting point and scan towards the positive direction of the Y-axis.

[0051] In step 4, the following is included: by observing the changes in the oscillation amplitude of the dispersion signal, the size of the CPT signal peak amplitude is obtained, the maximum value of the CPT signal peak amplitude is tracked, the first plane where the magnetic field B is located is obtained according to the maximum value of the signal amplitude obtained by the first beam signal, the second plane where the magnetic field B is located is obtained according to the maximum value of the signal amplitude obtained by the second beam signal, and the intersection of the first plane and the second plane is the direction of the magnetic field B.

[0052] The following expression is included in step 4:

[0053]

[0054] where is the frequency difference between the zero-crossing points of the dispersion signal, i.e., the optical frequency difference between the two CPT peaks, is the magnetic field size, is the gyromagnetic ratio.

[0055] The input side of the laser collimator 7 is connected with the VCSEL laser 1 in turn through the second polarization maintaining optical fiber 6, the electro-optic phase modulator 5, the first polarization maintaining optical fiber 4, the optical fiber coupler 3 and the optical isolator 2, the z-axis positive direction emission side of the alkali metal cell 19 is connected with the phase-locked amplifier 13 through the second photoelectric detection amplifier 18, the x-axis negative direction emission side of the alkali metal cell 19 is connected with the phase-locked amplifier 13 through the first photoelectric detection amplifier 17, the phase-locked amplifier 13 is connected with the data acquisition system 14, the arbitrary signal generator 15 and the radio frequency signal generator 16 respectively, and the radio frequency signal generator 16 is connected with the arbitrary signal generator 15 and the electro-optic phase modulator 5 respectively. The alkali metal cell 19 is located in the oven 20, the oven 20 is located in the Helmholtz coil 21, and the Helmholtz coil 21 is located in the magnetic shielding device.

[0056] The application discloses a mechanical interference-free magnetic field vector measurement method of a miniaturized CPT magnetometer.

[0057] The application discloses a mechanical interference-free magnetic field vector measurement method of a miniaturized CPT magnetometer.

[0058] Step 1, the VCSEL laser is started to generate polychromatic laser, the polychromatic laser enters the optical fiber coupler after passing through the optical isolator, passes through the first polarization maintaining optical fiber, the electro-optic modulator, the second polarization maintaining optical fiber, the laser collimator and the polarization grating control module, and then enters the polarization beam splitter to form a first light and a second light, the first light passes through the alkali metal cell along the x-axis positive direction to form a first light signal carrying magnetic field information into the first input end of the phase-locked amplifier, and the second light sequentially passes through the first mirror, the second mirror, and then passes through the alkali metal cell along the z-axis negative direction to form a second light signal carrying magnetic field information into the second input end of the phase-locked amplifier.

[0059] Step 2: The lock-in amplifier demodulates the first beam of light to obtain a dispersive signal. Based on the principle of magnetic field measurement that the zero-crossing point of the dispersive signal is consistent with the peak frequency of the CPT signal, a magnetically sensitive dispersive signal is obtained.

[0060] Step 3: Adjust the relative distance between the two polarization gratings by adjusting the piezoelectric ceramic translation stage, thereby adjusting the polarization direction e of the first and second beams. Observe the changes in the oscillation amplitude of the dispersive signal to obtain the changes in the peak amplitude of the CPT signal. Keep the wave vector k fixed and track the maximum value of the CPT signal peak amplitude. Define n = k × B, where B is the external magnetic field or the magnetic field to be measured, and n is the normal vector. The polarization direction e is electrically adjusted by regulating the piezoelectric ceramic translation stage. When the signal amplitude reaches its maximum value, then... Thus, the first plane containing the magnetic field B is obtained through the first beam of light, that is, the plane with n as the normal vector and passing through the wave vector k.

[0061] Step 4: Determine the magnitude of the magnetic field scalar using the following relationship:

[0062]

[0063] in It is the frequency difference between the zero-crossing points of the dispersive signal, that is, the optical frequency difference between the two CPT peaks. It is the magnitude of the magnetic field. It is the gyromagnetic ratio;

[0064] Step 5, similar to step 2, uses a lock-in amplifier to process the second light signal in a similar way to the first light signal. The second plane containing the magnetic field B is obtained through the second light, and the intersection of the first plane and the second plane is the direction of the magnetic field B.

[0065] The first plane containing magnetic field B in step 3 has the following expression:

[0066]

[0067] in By controlling the distance between the two polarization gratings The corresponding polarization state of the obtained light.

[0068] Step 3 includes the following expression for the polarization direction e:

[0069]

[0070] in It is one of them By controlling the distance between the two polarization gratings We obtain the following expression:

[0071]

[0072] in As a fixed value, K is derived from the following formula:

[0073]

[0074] For the input laser wavelength, The expression for the polarization grating structure coefficient is as follows:

[0075]

[0076] in The grating's refractive index for p-rays and s-rays. The polarization direction e is scanned from the positive X-axis to the positive Y-axis.

[0077] Step 5 includes: defining It is the angle between e and n, where counterclockwise is . clockwise Define dependency functions ,exist = 0, That is, when e⊥B, When the maximum value is reached, select the wave vector. , It is a set quantity, obtained by means of the wave vector. To obtain by rotating the polarization vector e , The maximum value corresponds to the vector n = [ The direction of ×B] is obtained from the vector The plane formed by B ( The equation for B); the wave vector k = Repeat the same process in the other direction. It is a set quantity, to obtain a plane ( The equations of B); two planes ( B) and ( The intersection line of B) is the three-dimensional direction of the magnetic field B.

[0078] The relative distance between the two polarization gratings is adjusted by adjusting the piezoelectric ceramic translation stage, thereby adjusting the polarization direction e of the first and second beams of light.

[0079] like Figure 3As shown, the polarization grating phase modulation module 8 in step 1 is connected to the piezoelectric ceramic displacement stage 9. The alkali metal gas chamber (i.e., the atomic gas chamber 19) is located inside the oven 20, which is located inside the Helmholtz coil 21, which is located inside the magnetic shielding device.

[0080] like Figure 3 As shown, the CPT magnetometer system in step 1 includes a VCSEL laser 1. The output of the VCSEL laser is sequentially connected to an optical isolator 2 and an optical fiber coupler 3. The optical fiber coupler sequentially passes through a first polarization-maintaining fiber 4, an electro-optic phase modulator 5, a second polarization-maintaining fiber 6, and a laser collimator 7. The laser collimator is connected to a polarization grating phase modulation module 8 and then enters the input of a polarization beam splitter prism 10. The first beam of light passes through an alkali metal gas cell 19 along the positive x-axis to form a first light signal carrying magnetic field information, which enters a first photodetector amplifier 17. The second beam of light passes through an alkali metal gas cell along the negative z-axis to form a second light signal carrying magnetic field information, which enters a second photodetector amplifier 18. The two photodetectors are connected to the input of a lock-in amplifier 13. The lock-in amplifier is connected to an input data acquisition system 14. The second input of the lock-in amplifier is connected to the first output of an arbitrary signal generator 15, and the third input of the lock-in amplifier is connected to the output of a radio frequency signal generator 16. The second output terminal of the arbitrary signal generator 15 is connected to the input terminal of the radio frequency signal generator 16, and the output terminal of the radio frequency signal generator 16 is connected to the electro-optic phase modulator 5 to achieve closed-loop control.

[0081] The VCSEL laser locks the laser onto the D1 line of an alkali metal atom. A 3.417 GHz signal is generated using an RF signal generator. The RF signal is modulated by the RF signal generator and added to the DC control signal of the VCSEL laser to generate multicolor laser.

[0082] like Figure 1 As shown, this invention realizes a magnetic field vector measurement method based on a CPT magnetometer through 5 steps, and verifies the feasibility of using a polarization grating module and a small-volume piezoelectric ceramic displacement stage for a miniaturized, mechanically vibration-free CPT magnetometer measurement scheme.

[0083] Step 1: Generation of multicolor lasers

[0084] According to the experimental design, two lasers are needed to satisfy the frequency difference between the two hyperfine energy levels of the D1 line of Rb87 atoms. In this experiment, a VCSEL laser is used to lock the laser on the D1 line, and a 3.417 GHz signal is generated by an RF signal generator. The RF signal is modulated by a T-type bias and added to the DC control signal of the VCSEL laser to obtain the required multicolor laser.

[0085] Step 2: Modulate and demodulate the CPT signal to obtain the dispersive signal

[0086] The present scheme uses a triangular wave function and a sine wave function to modulate the signal generated by the radio frequency signal generator. The function of the triangular wave is frequency scanning, and the function of the sine wave is to modulate the optical signal and serve as a reference signal input into the phase-locked amplifier. The dispersion signal can be obtained by demodulation of the phase-locked amplifier. The dispersion signal and the CPT signal have a functional relationship, wherein the zero-crossing point of the dispersion signal corresponds to the peak point frequency of the CPT signal. The magnetic field can be measured by using the dispersion signal according to this principle. Under the action of the external magnetic field, multiple CPT resonances are formed. For alkali metal atoms with magnetic quantum number m F =±1, the frequency difference of the Zeeman sub-level will change with the magnetic field, and the frequency shift will change. The dispersion signal is called a magnetic sensitive dispersion signal, while m F =0, the frequency difference of the Zeeman sub-level remains unchanged and is always equal to the hyperfine level frequency difference, which is called a magnetic insensitive dispersion signal.

[0087] Step three: adjust the polarization direction to observe the CPT peak signal amplitude

[0088] The angle between the magnetic field B and the wave vector k and the polarization direction e has a great influence on the CPT signal. The amplitudes of the seven CPT peaks will change with the angle respectively, and will reach the maximum at a certain angle respectively.

[0089] The dispersion signal is the result of demodulation of the CPT signal. In this paper, the change of the CPT signal peak amplitude can be obtained by observing the change of the oscillation amplitude of the dispersion signal. The implementation method is to keep the wave vector k fixed, to accurately control the polarization direction e, to observe the signal, and to find the polarization direction e corresponding to the maximum peak amplitude. At this time, the signal-to-noise ratio is the largest, and the highest sensitivity of the magnetic field measurement based on the existing scheme can be obtained. In the case where the magnetic field is unknown, the magnetic field direction can be obtained by the relative relationship.

[0090] Step four: magnetic field scalar measurement

[0091] The modulation signal of the radio frequency signal generator is input into the phase-locked amplifier as a reference signal for demodulation, and the dispersion signal can be obtained. The dispersion signal and the CPT signal have a functional relationship, wherein the zero-crossing point of the dispersion signal corresponds to the peak point frequency of the CPT signal. The magnetic field can be measured by using the dispersion signal according to this principle. 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:

[0092]

[0093] wherein, represents the optical frequency difference between two CPT peaks, which is also the frequency difference between the zero-crossing points of the dispersion signal in the present scheme; is the gyromagnetic ratio of Rb87; |B| represents the magnetic field magnitude. Thus, the magnetic field measurement can be performed.

[0094] Step five: Double-beam method determines the straight line where the magnetic field lies

[0095] The plane where the magnetic field lies can be determined by step three from a single beam. The relative relationship between the wave vector k, the magnetic field B, n and the polarization direction e in the vector measurement experiment is shown in Figure 2 . The wave vector k is defined as the Z axis, the X axis is vertically upward, and the Y axis is defined by the three-axis coordinate system. There is an external magnetic field B at the atomic cell, and its actual direction is unknown. For the purpose of description, it has been drawn in Figure 2 . n=k×B Since B is unknown, the actual angle is also unknown, but according to Figure 2 , the magnetic field direction is drawn in the schematic diagram, but it can be determined that n must be in the XOY plane ( ), by changing the voltage of the liquid crystal polarization direction rotator to further change the polarization state of the light, polarization scanning is achieved, and when the signal amplitude reaches the maximum value, it is the case.

[0096] According to the actual situation, the angle between B and n will not be as shown in Figure 2 , the following is the process analysis in the experiment. Consistent with the above process is that the wave vector k is along the Z axis, and the polarization direction e can be adjusted by changing the voltage of the liquid crystal polarization direction rotator. When the maximum value of the signal amplitude is observed, the direction of n is found ( ), according to the direction of n and n=k×B, the plane where the magnetic field B lies can be found, that is, the plane with n as the normal vector and passing through the wave vector k.

[0097] The polarization direction e is perpendicular to the wave vector k (Z axis) and exists in the XOY plane. When the positive direction of the X axis is taken as the starting scanning point and scanning is performed in the positive direction of the Y axis, the polarization direction e can be represented as:

[0098]

[0099] At this time, according to n=k×B, the plane where the magnetic field B lies is:

[0100]

[0101] In summary, considering the actual situation of the experiment, only one beam of light is injected and the wave vector k remains unchanged, and only the polarization direction e is adjusted. The accurate direction of the magnetic field B cannot be accurately obtained, and only the plane where the magnetic field B lies can be obtained.

[0102] On this basis, we get the second beam of light with a certain angle with the first beam of light by splitting the light, and repeat the above process to measure. Through data processing, we will get two planes where the magnetic field B is located, and the two planes will not be parallel. The intersection of the two planes is the direction of the magnetic field B.

[0103] Figure 3 The principle of the CPT magnetometer magnetic field vector measurement system and the coordinate system setting are shown.

[0104] According to the experimental scheme design, two lasers are needed to meet the frequency difference of the two hyperfine energy levels of the D1 line of Rb87 atoms. In this experiment, a VCSEL laser is used to lock the laser on the D1 line, a 3.417 GHz signal is generated by a radio frequency signal generator, and the signal is modulated by a T-type biasing device. The direct current control signal of the VCSEL laser is added to the signal, and the required polychromatic laser is obtained.

[0105] This scheme uses triangular wave function and sine wave function to modulate the signal generated by the radio frequency 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 serve as a reference signal input into the lock-in amplifier. The dispersion signal can be obtained by demodulation of the lock-in amplifier. The dispersion signal and the CPT signal have a functional relationship, and the zero-crossing point of the dispersion signal corresponds to the peak point frequency of the CPT signal. According to this principle, the magnetic field can be measured using the dispersion signal. The external magnetic field forms multiple CPT resonances. For alkali metal atoms, the frequency difference of the Zeeman sub-levels with magnetic quantum number m F =±1 will change with the magnetic field, and the frequency offset will change. Its dispersion signal is called magnetic sensitive dispersion signal, while the frequency difference of the Zeeman sub-levels with magnetic quantum number m F =0 remains unchanged and is always equal to the hyperfine energy level frequency difference, which is called magnetic insensitive dispersion signal.

[0106] The angle between the magnetic field B and the wave vector k and the polarization direction e (reference Figure 5 ) will have a great influence on the CPT signal, and the amplitude of the 7 CPT peaks will change with the angle respectively, and will reach the maximum value at a certain angle. The dispersion signal is the result of demodulation of the CPT signal. In this paper, the change of the oscillation amplitude of the dispersion signal can be obtained by observing the change of the oscillation amplitude of the dispersion signal. The method is to keep the wave vector k fixed, control the polarization direction e accurately, observe the signal, and find the polarization direction e corresponding to the maximum peak amplitude. At this time, the signal-to-noise ratio is the largest, and the highest sensitivity of the existing scheme can be obtained for magnetic field measurement. In the case where the magnetic field is unknown, the plane equation of the magnetic field direction can be obtained by relative relationship.

[0107] Considering the dependence of the dark resonance amplitude, which is obtained by rotating the polarization vector e around the fixed wave vector k. As Figure 5 shown, the function is defined as It is the angle between e and n, where counterclockwise is positive and clockwise is negative. Define the dependency function. ,exist = 0, (That is, when e⊥B) it reaches its maximum value.

[0108] The essence of the measurement process can be represented by the following algorithm. First, for the selected wave vector... We do this by surrounding the wave vector To obtain by rotating the polarization vector e The maximum value of this dependency corresponds to the vector n = [ The direction of [×B] gives us the direction of the vector. The plane formed by B ( The equation for B). For wave vector k = Repeating the same process in the other direction yields a plane. The equations of B). Two planes ( B) and ( The intersection line of vectors B is the three-dimensional direction of vector B, but the sign cannot be determined.

[0109] This can be seen from the general symmetry of the problem. In fact, suppose there is an arbitrary polychromatic wave propagating along direction k, all frequency components have the same linear polarization e. Assume the atomic medium is isotropic in the absence of an optical field. Define signal intensity as a scalar-valued function. The scalar value depends on the relative directions of vectors e and B.

[0110] In this sense of definition, a general analysis of the Bloch equation yields the following relationship:

[0111]

[0112] The light field has Polarization. Let's do a thing about a plane. Mathematical analysis. Polarization vectors can be realized. The replacement, but for the pseudo-vector of the magnetic field, it leads to It is known that mathematical reflection does not affect scalar signals (i.e., it yields another relationship):

[0113]

[0114]

[0115] That is, a scalar signal is an angle. It is an even function. This means that... (i.e. e⊥B) Corresponding to local extrema, this is obtained by rotating the polarization vector e around the wave vector k. Similar symmetry considerations show that there are two other extrema when the vector e lies in the plane (k, B) (i.e. at

[0116] The modulation signal of the radio frequency signal generator is inputted into the phase-locked amplifier as a reference signal to demodulate the dispersion signal. The dispersion signal and the CPT signal have a function relationship, wherein the zero-crossing point of the dispersion signal corresponds to the peak point frequency of the CPT signal, and the magnetic field can be measured according to the principle by using the dispersion signal. Under the action of the external magnetic field, a plurality of CPT resonances are formed. The magnetic flux density can be measured by measuring the frequency shift of the Zeeman CPT resonance:

[0117]

[0118] wherein, represents the optical frequency difference between two CPT peaks, and in the scheme, it is also the frequency difference between the zero-crossing points of the dispersion signal; is the gyromagnetic ratio of Rb87; represents the magnetic field size. Thus, the magnetic field measurement can be performed.

[0119] The laser frequency is modulated, a plurality of dispersion signals are observed through the phase-locked amplifier, the center frequency of the modulation signal is adjusted to select the magnetic sensitive dispersion signal. The polarization direction is changed and the dispersion signal is observed, the polarization direction e corresponding to the maximum value of the CPT signal peak amplitude is found, and the magnetic field scalar measurement is realized at the angle, the plane where the magnetic field is located can be obtained by using the polarization direction e, another laser which is not parallel to the original laser is used to repeat the above process, and the intersection line of the two planes is the magnetic field direction.

[0120] The contents not described in detail in the specification of the present application belong to the prior art known to the person skilled in the art. It is indicated herein that the above description is helpful for the person skilled in the art to understand the present application, but is not limited to the protection scope of the present application. Any implementation of the above description, equivalent replacement, modification, improvement and / or deletion of the above description without departing from the essential content of the present application falls within the protection scope of the present application.​

Claims

1. A method for mechanical disturbance-free magnetic field vector measurement of a miniaturized CPT magnetometer, characterized by The method comprises the following steps: Step 1: A double-polarization grating control module with a piezoelectric ceramic displacement platform is arranged on the total incident light path of the alkali metal gas chamber of the CPT magnetometer system, the double-polarization grating control module is located between the laser collimator and the polarization beam splitter prism, the double-polarization grating control module comprises a first polarization grating and a second polarization grating connected in series, the first polarization grating is connected to the laser collimator to receive collimated polychromatic laser, and the second polarization grating is connected to the input side of the polarization beam splitter prism; Step 2: The second polarization grating is driven by the piezoelectric ceramic displacement platform to perform up-down displacement adjustment, so as to realize linear and accurate control of the polarization state of light, and the accurate polarization state laser is output to the input side of the polarization beam splitter prism; Step 3: The polarization beam splitter prism decomposes the input accurate polarization state laser into two beams, a first beam is output from the transmission side, and a second beam is output from the reflection side, the first beam passes through the alkali metal gas chamber along the x-axis negative direction to form a first beam signal carrying magnetic field information into the first input end of the lock-in amplifier, and the second beam passes through the alkali metal gas chamber along the z-axis positive direction after sequentially passing through the first reflecting mirror and the second reflecting mirror to form a second beam signal carrying magnetic field information into the second input end of the lock-in amplifier; Step 4: The lock-in amplifier demodulates the first beam signal and the second beam signal respectively to obtain a dispersion signal, and the size and direction of the external magnetic field or the magnetic field to be measured are determined according to the dispersion signal.

2. The method of claim 1, wherein the method is a mechanical disturbance-free magnetic field vector measurement method of a miniaturized CPT magnetometer, characterized by, The first polarization grating and the second polarization grating in step 1 are polarization gratings with the same phase period.

3. The method of claim 1, wherein the method is a mechanical disturbance-free magnetic field vector measurement method of a miniaturized CPT magnetometer, characterized by, In step 1, the following expressions are included: where Δn is the polarization grating structure coefficient of the dual-polarization grating control module, n p1 is the p-light refractive index of the first polarization grating, n s1 is the s-light refractive index of the first polarization grating, n p2 is the p-light refractive index of the second polarization grating, n s2 is the s-light refractive index of the second polarization grating.

4. The method of claim 3, wherein the method is a mechanical disturbance-free magnetic field vector measurement method of a miniaturized CPT magnetometer, characterized by, In step 2, the following expressions are included: Wherein θ is a polarization angle, ΔS is the distance between the central axes of the two polarization gratings, K is an intermediate quantity, and λ is the wavelength of the input laser.

5. The method of claim 4, wherein the method is a mechanical disturbance-free magnetic field vector measurement method of a miniaturized CPT magnetometer, characterized by, In step 3, the following expressions are included: Wherein e is the polarization direction of the first beam or the second beam, and the polarization direction e is adjusted to start scanning from the positive direction of the X-axis as the starting scanning point and scan towards the positive direction of the Y-axis.

6. The method of claim 1, wherein the method is a mechanical disturbance-free magnetic field vector measurement method of a miniaturized CPT magnetometer, characterized by, In step 4, the following expressions are included:

7. The method of claim 1, wherein the method is a mechanical disturbance-free magnetic field vector measurement method of a miniaturized CPT magnetometer, characterized by, The input side of the laser collimator is connected to the VCSEL laser through a second polarization maintaining optical fiber, an electro-optic phase modulator, a first polarization maintaining optical fiber, an optical coupler and an optical isolator in sequence, the z-axis positive output side of the alkali metal gas chamber is connected to the lock-in amplifier through a second photoelectric detection amplifier, the x-axis negative output side of the alkali metal gas chamber is connected to the lock-in amplifier through a first photoelectric detection amplifier, the lock-in amplifier is connected to a data acquisition system, an arbitrary signal generator and a radio frequency signal generator, and the radio frequency signal generator is connected to the arbitrary signal generator and the electro-optic phase modulator. wherein is the frequency difference between the zero crossings of the dispersive signal, i.e. the optical frequency difference between the two CPT peaks, is the magnetic field size, is the gyromagnetic ratio.

8. The method of claim 1, wherein the miniaturized CPT magnetometer is free of mechanical disturbances. ​ 9. The method of claim 1, wherein the method is a mechanical disturbance-free magnetic field vector measurement method of a miniaturized CPT magnetometer, characterized by, The alkali metal cell is located within an oven, which is located within a Helmholtz coil, which is located within a magnetic shielding device.

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