A single optical magnetic signal detection system and method based on polarization volume grating

Abstract

This invention provides a single-optical magnetic signal detection system and method based on a polarizing body grating, comprising: a laser, a pre-optical path unit, an atomic gas cell unit, a polarization detection unit, and a signal processing unit. The light emitted by the laser is modulated into elliptically polarized light by the pre-optical path unit and acts on the atomic gas cell unit, causing the alkali metal atoms to spin polarize. The polarization detection unit and signal processing unit then measure the magnitude of the external magnetic field using optical rotation detection. This method combines the advantages of existing single-optical and dual-optical atomic magnetometers, featuring a compact structure and simplified optical path. Differential detection suppresses common-mode noise, improving measurement accuracy and sensitivity. The optically pumped atomic magnetic resonance polarization detection is achieved using a liquid crystal polarizing body grating thin-film device with efficient polarization chiral splitting and large-angle single-order diffraction performance, realizing miniaturized design of the magnetic measurement system and laying the foundation for the system to move towards chip-level design.

Description

Technical Field

[0001] This invention belongs to the field of magnetic measurement technology, and in particular to a single-optical magnetic signal detection system and method based on a polarizing grating. Background Technology

[0002] As a branch of precision measurement, magnetic field measurement has been widely used in recent years in fields such as earth science, space exploration, magnetic anomaly detection, and biomedicine. Compared with other types of magnetometers, optically pumped atomic magnetometers have advantages such as high sensitivity, no zero-point drift, no need for strict orientation, and continuous measurement.

[0003] Based on the number of light sources, optically pumped atomic magnetometers can be divided into two categories: single-beam detection type and dual-beam detection type; among which:

[0004] The single-beam method uses optical absorption detection, where the pump light and the probe light are the same circularly polarized light. Magnetic field information is extracted by detecting the absorption of light by the atomic gas cell. However, this method is easily affected by optical power noise, and the output signal has a large DC quantity, increasing the workload in subsequent signal processing.

[0005] The dual-beam method employs optical rotation detection, with a circularly polarized beam and a linearly polarized beam perpendicular to each other as the light source. The circularly polarized pump beam is tuned to the D1 resonance center to polarize the atomic spins, while the linearly polarized beam is tuned to a non-resonant state to measure the rotational change of the polarization plane before and after passing through the atomic gas cell. Compared with the optical absorption method, the optical rotation mode detects the difference in light intensity, which can suppress common-mode noise and has a higher signal-to-noise ratio, but its system structure is more complex.

[0006] The key to optical rotation detection methods lies in the detection of the rotation angle. Commonly used detection methods include photoelastic modulation, Faraday modulation, and differential detection.

[0007] The first two methods modulate the detection light by adding an external modulator and then demodulate the optical rotation angle signal by using a lock-in amplifier. They generally have disadvantages such as large optical modulation crystals and easy introduction of additional errors. In addition, adding a modulation module will make the optical path complex and the power consumption high.

[0008] The differential detection method utilizes the polarization beam splitting function of a polarization beam splitter (PBS) to differentially detect the detection light. The optical rotation angle is approximately calculated using a differential formula. This method can be implemented without external modulation. However, due to the limitations of the beam splitting principle, the size of the PBS beam splitting device cannot be made very small.

[0009] Liquid crystal planar optical devices utilize high-resolution patterned liquid crystal alignment technology to precisely control the distribution of liquid crystal directores, enabling efficient manipulation of light fields within a thickness of several micrometers. This allows for complex phase and polarization control of wavefronts, earning them the reputation of fourth-generation optical devices. Patent application number 202210204877.7 discloses a small-angle differential detection module and method based on a polarization grating. The patent uses a polarization grating to replace the traditional PBS (propagation beam splitter) to achieve miniaturization of a single device. However, due to Raman-Nath diffraction limitations, the diffraction angle of the emitted light from such polarization gratings is very small (generally less than 15°). This is not conducive to shortening the detection optical path; while liquid crystal polarizer gratings, as a typical representative of liquid crystal polymer films, possess both the large response bandwidth and polarization separation characteristics of Pancharatnam-Berry phase gratings, and the efficient single-order large-angle diffraction capability of Bragg volume gratings (diffraction angle easily reaches 45°); it can be used as a "thin film prism" to achieve efficient polarization chiral separation and unique beam splitting function, with unique advantages of being thin, light, highly transparent, and easy to integrate, and has broad application prospects in scenarios with strict requirements for system size, such as industrial miniaturization equipment and military aerospace.

[0010] Therefore, based on the above-mentioned technical problems, those skilled in the art urgently need to develop a magnetic signal detection system and method that combines high sensitivity and miniaturized integration advantages. Summary of the Invention

[0011] To address the aforementioned technical problems, this invention provides a single-optical magnetic signal detection system and method based on a polarizing grating. This system is simple in structure, easy to operate, and readily integrated, while also possessing the advantages of both optical absorption and optical rotation detection methods. By using a liquid crystal polarizing grating to construct the differential polarization detection unit, it facilitates system miniaturization and chip-based implementation. The detection method based on this system provides scientifically reliable detection results and is suitable for widespread application.

[0012] A single-optical magnetic signal detection system and method based on a polarizing grating, wherein:

[0013] A single-optical magnetic signal detection system based on a polarizing grating includes: a laser, a pre-optical path unit, an atomic gas cell unit, a polarization detection unit, and a signal processing unit; the light emitted by the laser is modulated into elliptically polarized light by the pre-optical path unit, acts on the atomic gas cell unit, causing the spin polarization of alkali metal atoms, and then the polarization detection unit and the signal processing unit measure the magnitude of the external magnetic field using an optical rotation detection method;

[0014] As an example, the laser is equipped with a controller that regulates the laser to emit a laser beam of a fixed wavelength.

[0015] As an example, the front optical path unit includes: a collimator, a polarizer, and a first quarter-wave plate;

[0016] As an example, the atomic gas chamber unit includes: an atomic gas chamber, a heating assembly, and a Helmholtz coil assembly wound outside the atomic gas chamber.

[0017] As a preferred example, the atomic chamber is filled with alkali metal working atoms and a buffer gas.

[0018] As a preferred example, the heating component is equipped with a temperature control module for precise temperature control.

[0019] As a preferred example, the Helmholtz coil assembly is used to provide a radio frequency magnetic field for the system.

[0020] As an example, the polarization detection unit includes: a second quarter-wave plate, a liquid crystal polarizer grating, a first photodetector, and a second photodetector;

[0021] As an example, the signal processing unit includes: a differential amplifier circuit and a lock-in amplifier;

[0022] As a preferred example, the output terminals of the first photodetector and the second photodetector are connected in parallel and then electrically connected to one end of the differential amplifier circuit, and the other end of the differential amplifier circuit is electrically connected to the input terminal of the lock-in amplifier.

[0023] A method for detecting single-optical magnetic signals based on a polarizing grating includes:

[0024] Step 1: The controller regulates the laser to emit a laser beam of a fixed wavelength, which is coupled into the polarization-maintaining fiber as a light source. After being collimated by the collimator, the beam is incident on the polarizer to achieve polarization purification. Then, by adjusting the angle between the first quarter-wave plate and the polarizer, elliptically polarized light is generated, whose normalized Stokes vector is:

[0025] S Elliptical =[1cos 2 2θ1cos2θ1sin2θ1sin2θ1] T ;

[0026] Wherein: the polarizer's transmission axis is along the X-axis, and θ1 is the angle between the polarizer's transmission axis and the fast axis of the first quarter-wave plate (i.e., the angle between the first quarter-wave plate and the X-axis).

[0027] As an example, when θ1 = π / 8, θ2 = π / 8 or -3π / 8, the elliptically polarized light has a better effect on the atomic gas cell and can ensure that the two diffracted beams emitted from the liquid crystal polarizer grating have equal intensity when the atomic gas cell is not heated.

[0028] Step 2: The elliptically polarized light is irradiated onto the atomic gas cell unit. The circularly polarized component of the elliptically polarized light is used to pump alkali metal atoms, and the linearly polarized component is used to detect the spin polarization of alkali metal atoms, i.e., to generate the optical rotation angle.

[0029] As an example, the alkali metal atoms in the atomic gas cell unit are pumped by circularly polarized light, and their spin polarization is:

[0030]

[0031] Where: R rel R is the spin relaxation rate of alkali metal atoms. pl R is the pump rate of left-handed circularly polarized light. pr is the pump rate of right-handed circularly polarized light.

[0032] As an example, the optical rotation angle φ is proportional to the spin polarizability P. Z The specific form is as follows:

[0033]

[0034] Where n is the density of alkali metal atoms, c is the speed of light, and r e Let f be the electron radius, l be the gas cell length, and f be the electron radius. D1 It represents the oscillation intensity, and D1(δυ) is the normalized absorption coefficient near the D1 line.

[0035] As an example, the Mueller matrix of the atomic gas chamber is:

[0036]

[0037] The Mueller matrix of the second quarter-wave plate is:

[0038]

[0039] Where: θ2 is the angle between the transmission axis of the polarizer and the fast axis of the second quarter-wave plate; the normalized Stokes vector of the beam incident on the liquid crystal polarizer grating can be derived as:

[0040] S in =M QWP2 M cell S Elliptical =[1S1'S'2S3'] T ;

[0041] Step 3: The laser beam passing through the atomic gas cell (at this time, the laser beam carries optical rotation angle information) undergoes diffraction after passing through the second quarter-wave plate and the liquid crystal polarizer grating. The +1st and 0th order diffracted beams are received by the first photodetector and the second photodetector, respectively. The distances from the first photodetector and the second photodetector to the center of the liquid crystal polarizer grating are the same, thus ensuring that the optical path lengths of the two diffracted beams are the same.

[0042] As an example, the diffraction characteristics of a liquid crystal polarizer grating are related to the rotation direction of the internal liquid crystal molecules' helices.

[0043] As an example, let's take a left-handed circularly polarized device: When left-handed circularly polarized light is incident, that is, when the incident light is in the same direction as the rotation of the liquid crystal device, a single-order diffraction occurs, all the energy is concentrated in the +1 order, and the rotation of the outgoing circularly polarized light becomes right-handed; when right-handed circularly polarized light is incident, that is, when the incident light is in the opposite direction to the rotation of the device, the beam is emitted directly, and the propagation direction and polarization state do not change; natural light can be regarded as the superposition of left-handed and right-handed circularly polarized light, and the beam is split into two beams of equal energy and in the same direction of rotation, one beam is diffracted, and the other beam is directly transmitted.

[0044] As an example, the intensity diffraction efficiency of each order of a left-handed transmission liquid crystal polarizer grating can be described as follows:

[0045]

[0046] Where Q is a factor determined by the structure of the liquid crystal polarizer grating, and S3' is the normalized Stokes parameter corresponding to the ellipticity of the incident light. It is important to emphasize that factor Q is primarily determined by the orientational distribution pattern of the optical axes of the liquid crystal molecules. When the alignment layer satisfies a periodic, continuous cycloidal pattern, the liquid crystal, after being aligned by the surface alignment layer, forms a birefringent grating. In this case, a specific liquid crystal layer thickness makes factor Q equal to 1 at a certain wavelength, known as the half-wave condition.

[0047] Therefore, the diffraction efficiency distribution of the emitted light is as follows:

[0048]

[0049] When the atomic gas cell is not heated, the optical rotation angle φ = 0; in order to make the intensity of the two outgoing beams equal, the condition sin(2θ1-2θ2)cos2θ1 = 0 must be satisfied. The front optical path unit modulates the laser beam into elliptically polarized light, which is further simplified to sin(2θ1-2θ2) = 0; therefore, the fast axes of the two waveplates should be placed parallel or perpendicular.

[0050] Step 4: The output terminals of the first and second photodetectors convert the optical signal into an electrical signal through a differential amplifier circuit and amplify it differentially. The vibration of the differential electrical signal reflects the change in the optical rotation angle. At the same time, the differentially amplified electrical signal is fed to the input terminal of the lock-in amplifier. The lock-in amplifier demodulates the optical rotation signal according to the external oscillating radio frequency magnetic field. Finally, the magnitude of the magnetic field to be measured is obtained through signal processing.

[0051] As an example, the differentially amplified electrical signal D measured by the first and second photodetectors, after adjusting for +1 and 0 levels, is as follows:

[0052] D=CI0sin(2φ+2θ1-2θ2)cos2θ1=±CI0sin2φcos2θ1≈±2CI0φcos2θ1

[0053] In the formula: I0 is the initial light intensity, C is a constant coefficient, and when θ 1＝ When π / 8, θ2 = π / 8 or -3π / 8; θ1 is a known quantity, so the relationship between the differentially amplified electrical signal and the optical rotation angle φ can be obtained; parallel is +, perpendicular is -.

[0054] The beneficial effects of this invention are:

[0055] This invention provides a single-optical magnetic signal detection system based on a polarizing grating, which combines the advantages of existing single-optical and dual-optical atomic magnetometers. It features a compact structure, simplified optical path, and differential detection to suppress common-mode noise, thereby improving measurement accuracy and sensitivity.

[0056] This invention discloses a single-optical magnetic signal detection method based on a polarization body grating. This method utilizes a liquid crystal polarization body grating thin-film device with efficient polarization chirality splitting and large-angle single-order diffraction performance to perform optically pumped atomic magnetic resonance polarization detection. This enables the miniaturization design of the magnetic measurement system and lays the foundation for the system to move closer to the chip level. Attached Figure Description

[0057] Figure 1 This is a schematic diagram of the overall structure of a single-optical magnetic signal detection system based on a polarizing grating according to the present invention.

[0058] Figure 2 This is a diagram showing the liquid crystal molecule arrangement structure of a liquid crystal polarizer grating in a single-optical magnetic signal detection system based on a polarizer grating according to the present invention.

[0059] Figure 3 This diagram illustrates the beam deflection characteristics of a liquid crystal polarizer grating in a single-optical magnetic signal detection system based on a polarizer grating, as described in this invention. (Liquid crystal polarizer gratings are classified into two types: reflective and transmissive.) Figure 3 (This is a diagram showing the beam deflection characteristics of a transmission polarizer grating)

[0060] Figure label:

[0061] 1. Laser; 2. Pre-optical path unit; 3. Atomic gas cell unit; 4. Polarization detection unit; 5. Signal processing unit; 6. Controller; 7. Polarization-maintaining fiber; 8. Collimator; 9. Polarizer; 10. First quarter-wave plate; 11. Atomic gas cell; 12. Heating assembly; 13. Helmholtz coil assembly; 14. Temperature control module; 15. Second quarter-wave plate; 16. Liquid crystal polarizer grating; 17. First photodetector; 18. Second photodetector; 19. Differential amplifier circuit; 20. Lock-in amplifier. Detailed Implementation

[0062] Below, for reference Figures 1 to 3 As shown, a single-optical magnetic signal detection system and method based on a polarizing grating are disclosed, wherein:

[0063] A single-optical magnetic signal detection system based on a polarizing grating includes: a laser, a pre-optical path unit, an atomic gas cell unit, a polarization detection unit, and a signal processing unit; the light emitted by the laser is modulated into elliptically polarized light by the pre-optical path unit, acts on the atomic gas cell unit, causing the spin polarization of alkali metal atoms, and then the polarization detection unit and the signal processing unit measure the magnitude of the external magnetic field using an optical rotation detection method;

[0064] As an example, the laser is equipped with a controller that regulates the laser to emit a laser beam of a fixed wavelength.

[0065] As an example, the front optical path unit includes: a collimator, a polarizer, and a first quarter-wave plate;

[0066] As an example, the atomic gas chamber unit includes: an atomic gas chamber, a heating assembly, and a Helmholtz coil assembly wound outside the atomic gas chamber.

[0067] As a preferred example, the atomic chamber is filled with alkali metal working atoms and a buffer gas.

[0068] As a preferred example, the heating component is equipped with a temperature control module for precise temperature control.

[0069] As a preferred example, the Helmholtz coil assembly is used to provide a radio frequency magnetic field for the system.

[0070] As an example, the polarization detection unit includes: a second quarter-wave plate, a liquid crystal polarizer grating, a first photodetector, and a second photodetector;

[0071] As an example, the signal processing unit includes: a differential amplifier circuit and a lock-in amplifier;

[0072] As a preferred example, the output terminals of the first photodetector and the second photodetector are connected in parallel and then electrically connected to one end of the differential amplifier circuit, and the other end of the differential amplifier circuit is electrically connected to the input terminal of the lock-in amplifier.

[0073] A method for detecting single-optical magnetic signals based on a polarizing grating includes:

[0074] Step 1: The controller regulates the laser to emit a laser beam of a fixed wavelength, which is coupled into the polarization-maintaining fiber as a light source. After being collimated by the collimator, the beam is incident on the polarizer to achieve polarization purification. Then, by adjusting the angle between the first quarter-wave plate and the polarizer, elliptically polarized light is generated, whose normalized Stokes vector is:

[0075] S Elliptical =[1cos 2 2θ1cos2θ1sin2θ1sin2θ1] T ;

[0076] Wherein: the polarizer's transmission axis is along the X-axis, and θ1 is the angle between the polarizer's transmission axis and the fast axis of the first quarter-wave plate (i.e., the angle between the first quarter-wave plate and the X-axis).

[0077] As an example, when θ1 = π / 8, θ2 = π / 8 or -3π / 8, the elliptically polarized light has a better effect on the atomic gas cell and can ensure that the two diffracted beams emitted from the liquid crystal polarizer grating have equal intensity when the atomic gas cell is not heated.

[0078] Step 2: The elliptically polarized light is irradiated onto the atomic gas cell unit. The circularly polarized component of the elliptically polarized light is used to pump alkali metal atoms, and the linearly polarized component is used to detect the spin polarization of alkali metal atoms, i.e., to generate the optical rotation angle.

[0079] As an example, the alkali metal atoms in the atomic gas cell unit are pumped by circularly polarized light, and their spin polarization is:

[0080]

[0081] Where: R rel R is the spin relaxation rate of alkali metal atoms. pl R is the pump rate of left-handed circularly polarized light. pr is the pump rate of right-handed circularly polarized light.

[0082] As an example, the optical rotation angle φ is proportional to the spin polarizability P. Z The specific form is as follows:

[0083]

[0084] Where n is the density of alkali metal atoms, c is the speed of light, and r e Let f be the electron radius, l be the gas cell length, and f be the electron radius. D1 It represents the oscillation intensity, and D1(δυ) is the normalized absorption coefficient near the D1 line.

[0085] As an example, the Mueller matrix of the atomic gas chamber is:

[0086]

[0087] The Mueller matrix of the second quarter-wave plate is:

[0088]

[0089] Where: θ2 is the angle between the transmission axis of the polarizer and the fast axis of the second quarter-wave plate; the normalized Stokes vector of the beam incident on the liquid crystal polarizer grating can be derived as:

[0090] S in =M QWP2 M cell S Elliptical =[1S1'S'2S3'] T ;

[0091] Step 3: The laser beam passing through the atomic gas cell (at this time, the laser beam carries optical rotation angle information) undergoes diffraction after passing through the second quarter-wave plate and the liquid crystal polarizer grating. The +1st and 0th order diffracted beams are received by the first photodetector and the second photodetector, respectively. The distances from the first photodetector and the second photodetector to the center of the liquid crystal polarizer grating are the same, thus ensuring that the optical path lengths of the two diffracted beams are the same.

[0092] As an example, the diffraction characteristics of a liquid crystal polarizer grating are related to the rotation direction of the internal liquid crystal molecules' helices.

[0093] As an example, let's take a left-handed circularly polarized device: When left-handed circularly polarized light is incident, that is, when the incident light is in the same direction as the rotation of the liquid crystal device, a single-order diffraction occurs, all the energy is concentrated in the +1 order, and the rotation of the outgoing circularly polarized light becomes right-handed; when right-handed circularly polarized light is incident, that is, when the incident light is in the opposite direction to the rotation of the device, the beam is emitted directly, and the propagation direction and polarization state do not change; natural light can be regarded as the superposition of left-handed and right-handed circularly polarized light, and the beam is split into two beams of equal energy and in the same direction of rotation, one beam is diffracted, and the other beam is directly transmitted.

[0094] As an example, the intensity diffraction efficiency of each order of a left-handed transmission liquid crystal polarizer grating can be described as follows:

[0095]

[0096] Where Q is a factor determined by the structure of the liquid crystal polarizer grating, and S3' is the normalized Stokes parameter corresponding to the ellipticity of the incident light. It is important to emphasize that factor Q is primarily determined by the orientational distribution pattern of the optical axes of the liquid crystal molecules. When the alignment layer satisfies a periodic, continuous cycloidal pattern, the liquid crystal, after being aligned by the surface alignment layer, forms a birefringent grating. In this case, a specific liquid crystal layer thickness makes factor Q equal to 1 at a certain wavelength, known as the half-wave condition.

[0097] Therefore, the diffraction efficiency distribution of the emitted light is as follows:

[0098]

[0099] When the atomic gas cell is not heated, the optical rotation angle φ = 0; in order to make the intensity of the two outgoing beams equal, the condition sin(2θ1-2θ2)cos2θ1 = 0 must be satisfied. The front optical path unit modulates the laser beam into elliptically polarized light, which is further simplified to sin(2θ1-2θ2) = 0; therefore, the fast axes of the two waveplates should be placed parallel or perpendicular.

[0100] Step 4: The output terminals of the first and second photodetectors convert the optical signal into an electrical signal through a differential amplifier circuit and amplify it differentially. The vibration of the differential electrical signal reflects the change in the optical rotation angle. At the same time, the differentially amplified electrical signal is fed to the input terminal of the lock-in amplifier. The lock-in amplifier demodulates the optical rotation signal according to the external oscillating radio frequency magnetic field. Finally, the magnitude of the magnetic field to be measured is obtained through signal processing.

[0101] As an example, the differentially amplified electrical signal D measured by the first and second photodetectors, after adjusting for +1 and 0 levels, is as follows:

[0102] D=CI0sin(2φ+2θ1-2θ2)cos2θ1=±CI0sin2φcos2θ1≈±2CI0φcos2θ1

[0103] In the formula: I0 is the initial light intensity, C is a constant coefficient, and when θ 1＝ When π / 8, θ2 = π / 8 or -3π / 8; θ1 is a known quantity, so the relationship between the differentially amplified electrical signal and the optical rotation angle φ can be obtained; parallel is +, perpendicular is -;

[0104] This single-beam optical path can pump and detect alkali metal atom gas cells. After obtaining the light intensity difference signal through a photodetector, the optical rotation angle can be calculated, thereby obtaining the spin polarization rate to achieve magnetic field measurement.

[0105] To better illustrate the design principles of this invention, specific embodiments are provided below:

[0106] Example:

[0107] See Figure 1 As shown;

[0108] A single-optical magnetic signal detection system based on a polarizing grating includes a measurement system comprising: a laser 1, a pre-optical path unit 2, an atomic gas cell unit 3, a polarization detection unit 4, and a signal processing unit 5. The light emitted by the laser is modulated into elliptically polarized light by the pre-optical path unit and acts on the atomic gas cell unit 3, causing the spin polarization of alkali metal atoms. The light then passes through the polarization detection unit 4 and the signal processing unit 5, and the magnitude of the external magnetic field is measured using an optical rotation detection method.

[0109] Specifically, the front-end optical path unit in this embodiment includes a collimator 8, a polarizer 9, and a first quarter-wave plate 10. The controller 6 adjusts the operating current and operating temperature to control the laser beam of a specific fixed wavelength from the laser 1, which is coupled into the polarization-maintaining fiber 7 as a light source. After being collimated by the collimator 8, the beam is incident on the polarizer to achieve polarization purification. Then, by adjusting the angle between the first quarter-wave plate 10 and the polarizer 9 to 22.5°, elliptically polarized light is generated and irradiated onto the atomic gas cell 11. The circular polarization component of the elliptically polarized light is used to pump alkali metal atoms, and the linear polarization component is used to detect the spin polarization of the atoms, i.e., to generate the optical rotation angle.

[0110] The atomic chamber unit 3 of this embodiment includes an atomic chamber 11 filled with alkali metal working atoms and a buffer gas, a heating component 12 whose temperature is precisely controlled by a temperature control module 14, and a Helmholtz coil assembly 13 wound around the atomic chamber 11 to provide a radio frequency magnetic field for the system. The temperature control module 14 in this embodiment uses DC heating to avoid the introduction of AC noise. The chamber temperature is detected in real time by a thermistor placed on the chamber handle, and the heating rate is controlled by a PID control algorithm. The heating temperature in this embodiment is between 70°C and 120°C, and the environmental conditions are not harsh.

[0111] The polarization detection unit 4 in this embodiment includes a second quarter-wave plate 15, a liquid crystal polarizer grating 16, a first photodetector 17, and a second photodetector 18. Light passing through the atomic gas cell 11 undergoes diffraction after passing through the second quarter-wave plate 15 and the liquid crystal polarizer grating 16. The +1st and 0th order diffracted lights are received by the first and second photodetectors, respectively, and the distances from the two photodetectors to the center of the liquid crystal polarizer grating 16 remain the same, thus ensuring that the optical path lengths of the two diffracted lights are the same.

[0112] In this embodiment, the liquid crystal molecule arrangement structure of the liquid crystal polarizer grating is shown in [reference needed]. Figure 2Each rod-shaped component represents a liquid crystal molecule, whose optical axis (long axis) rotates periodically in the x-direction, continuously changing 180° within one period. When observing the liquid crystal molecules in the xz direction, a cycloidal pattern can be seen. As incident light passes through the optical axis of the liquid crystal molecules at different rotation angles, different polarization states occur. Therefore, this spatial structure can introduce a continuously changing geometric phase into the incident light, namely the Pancharatnam-Berry phase. Under the control of this phase, the liquid crystal polarizer grating can not only deflect the light beam but also be sensitive to the polarization state of the incident light, thereby achieving polarization chiral splitting. In the y-axis direction, a new optical axis rotation is introduced by adding a chiral agent, and the liquid crystal molecules rotate continuously around the y-axis, rotating 180° within the longitudinal period Λy.

[0113] See the beam deflection characteristics of liquid crystal polarizer gratings. Figure 3 The beam deflection angle is determined by the transverse period Λx and the application center wavelength λ, satisfying the planar grating formula:

[0114]

[0115] Where m is the diffraction order of the emitted light. In terms of diffraction angle, the diffraction angle of a liquid crystal polarizing grating is generally less than 15°, while the diffraction angle of a liquid crystal polarizing grating can easily exceed 45°.

[0116] In this embodiment, the output terminal of the photodetector converts the optical signal into an electrical signal and amplifies it differentially through the differential amplifier circuit 19. The vibration of the differential electrical signal reflects the change in the optical rotation angle. At the same time, the differentially amplified electrical signal is fed to the input terminal of the lock-in amplifier 20. The lock-in amplifier demodulates the optical rotation signal according to the external oscillating radio frequency magnetic field, and finally obtains the magnitude of the magnetic field to be measured through signal processing.

[0117] In this embodiment, the transmission axis of the polarizer 9 is along the X-axis, the first quarter-wave plate 10 has an angle of θ1 with the X-axis, and the second quarter-wave plate 15 has an angle of θ2 with the X-axis. When θ1 = π / 8, θ2 = π / 8 or -3π / 8, the elliptically polarized light has a better effect on the atomic gas cell, and can ensure that the two diffracted beams emitted from the liquid crystal polarizer grating 16 have equal intensity when the gas cell is not heated.

[0118] A method for detecting single-optical magnetic signals based on a polarizing grating includes:

[0119] By adjusting the operating current and temperature using controller 6, laser 1 emits a laser beam corresponding to the wavelength near the potassium alkali metal D1 transition line. Based on the relationship between the absorption and dispersion components in negative refractive index, the light absorption is strongest at the magneto-optical resonance point, indicating a high atomic polarizability. However, the dispersion is zero at this point, meaning the detection signal is weakest. Considering all factors, a slight detuning of the optical frequency is needed to bring the optical rotation angle close to its maximum. This wavelength range is precisely where laser frequency jitter has the least impact on the optical rotation angle. At this point, the differential photocurrent signals received by the pair of photodetectors show a significant absorption peak.

[0120] In the laboratory coordinate system, the Z-axis is defined along the pump light direction. The temperature is DC heated to 85°C to 120°C via temperature control module 14. Simultaneously, a radio frequency magnetic field is applied along the Y-axis via a Helmholtz coil assembly. The magnetic measurement system is placed under the magnetic field to be measured. By scanning the radio frequency magnetic field frequency, the differential signal generated by the photomagnetic resonance, i.e., the optical rotation angle signal, can be observed on an oscilloscope. The radio frequency ω corresponding to the absorption peak corresponds to the Larmor precession frequency of potassium atoms under the magnetic field to be measured, ω = γB0, where γ = 7.01 Hz / nT is the gyromagnetic ratio of potassium atoms. Therefore, the magnitude of the external magnetic field can be preliminarily calculated based on the radio frequency frequency corresponding to the absorption peak.

[0121] Secondly, to accurately extract the optical rotation angle signal, a lock-in amplifier is used to modulate the radio frequency excitation signal, and the differential light intensity signal is low-pass filtered and phase-sensitively detected to obtain the optical rotation angle signal curve R, as well as its in-phase signal X and orthogonal phase signal Y. Demodulation reveals that the optical rotation angle signal contains an in-phase signal exhibiting absorption lines and an orthogonal phase signal exhibiting dispersion lines. When the radio frequency equals the Larmor precession frequency of the atom under an external magnetic field, the magnetic resonance signal reaches its maximum value. At this point, its orthogonal phase signal crosses zero, and the radio frequency magnetic field frequency value corresponding to the zero-crossing point is the external magnetic field strength. Real-time acquisition of a series of external magnetic field strengths yields the magnetic field time-domain signal.

[0122] As an example, the liquid crystal polarizer grating referred to in this patent specifically refers to a transmissive liquid crystal polarizer grating.

[0123] The above description is only a preferred embodiment of the present invention. It should be understood that the above description of the embodiments is only for the purpose of helping to understand the method and core idea of ​​the present invention, and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, etc. made within the idea and principle of the present invention should be included within the scope of protection of the present invention.

Claims

1. A polarization volume grating-based single optical magnetic signal detection method, characterized in that, Comprise: Step one, the controller regulates the laser to emit a fixed wavelength laser beam as a light source coupled into a polarization maintaining optical fiber, collimated by a collimator and incident on a polarizer to realize the purification of polarization; then by adjusting the angle between the first quarter wave plate and the polarizer, an elliptical polarized light is generated, and its normalized Stokes vector is: ； Wherein: the polarizer light transmission axis is along the X axis direction, θ1 is the angle between the polarizer light transmission axis and the fast axis of the first quarter wave plate; Step two, the elliptical polarized light irradiates on the atomic cell unit, and the circularly polarized component of the elliptical polarized light is used to pump alkali atoms, and the linearly polarized component is used to detect the spin polarization of alkali atoms, that is, to generate optical rotation angle; Step three, the laser beam passing through the atomic cell passes through the second quarter wave plate and the liquid crystal polarization volume grating and is diffracted, the +1 order and 0 order diffracted light is received by the first photodetector and the second photodetector respectively, and the distance from the first photodetector and the second photodetector to the center of the liquid crystal polarization volume grating is the same, so as to ensure that the optical path of the two diffracted lights is the same; Step four, the output ends of the first photodetector and the second photodetector are connected to one end of the differential amplification circuit through the differential amplification circuit to convert the optical signal into an electrical signal and differential amplification, the vibration of the differential electrical signal reflects the change of the optical rotation angle, and the differential amplified electrical signal is fed to the input end of the lock-in amplifier, the lock-in amplifier demodulates the optical rotation signal according to the external oscillation radio frequency magnetic field, and finally the size of the measured magnetic field is obtained through signal processing; When θ1=π / 8, θ2=π / 8 or-3π / 8, the elliptical polarized light has good effect on the atomic cell, and the intensity of the two diffracted lights emitted from the liquid crystal polarization volume grating is equal when the atomic cell is not heated; The alkali metal atoms in the atomic cell unit are pumped by circularly polarized light, and the spin polarization rate is: ； where: R rel is the spin-lattice relaxation rate of the alkali metal atom, R pl is the left-handed circularly polarized light pumping rate, R pr is the right-handed circularly polarized light pumping rate; The optical rotation angle is proportional to the spin polarization P Z , in particular: ； where n is the alkali metal atomic density, c is the speed of light, r e is the electron radius, l is the cell length, f D1 is the oscillation strength, D1(δν) is the normalized absorption coefficient near the D1 line; The Mueller matrix of the atomic cell is: the Mueller matrix of the second quarter- wave plate is: where: 02 is the angle between the polarizer pass axis and the fast axis of the second quarter-wave plate; it can be derived that the normalized Stokes vector of the light beam incident to the liquid crystal polarization volume grating is: ; the diffraction characteristics of liquid crystal polarization volume grating are related to the handedness of the internal liquid crystal molecule helix; for a left-handed transmissive device: when left-handed circularly polarized light is incident, i.e. the incident light is consistent with the handedness of the liquid crystal device, single-order diffraction occurs, all the energy is concentrated in the +1 order, and the handedness of the outgoing circularly polarized light becomes right-handed; when right-handed circularly polarized light is incident, i.e. the incident light is opposite to the handedness of the device, the light beam is directly transmitted, and neither the propagation direction nor the polarization state is changed; natural light can be regarded as the superposition of left- and right-handed circularly polarized light, the light beam is divided into two beams of equal energy and consistent handedness, one of which is diffracted and the other is directly transmitted; The light intensity diffraction efficiency of each order of the left-handed transmission liquid crystal polarization volume grating is: ; ; wherein: Q is a factor determined by the structure of the liquid crystal polarization volume grating, is a normalized Stokes parameter corresponding to the ellipticity of the incident light; Therefore, the exit light diffraction efficiency distribution is: ; ; when the atomic cell is not heated, the optical rotation angle ; in order to make the intensity of the two exit lights equal, the condition ; the pre-optical path unit modulates the laser beam into elliptical polarized light, which is further simplified as ; therefore, the two wave plate fast axes should be parallel or perpendicular; the differential amplified electrical signal D of the +1 order and the 0 order measured by the first photodetector and the second photodetector is: where I0is the initial light intensity, C is a constant, and θ is the rotation angle. When θ = 0, the equation becomes I = I0C. When θ = π / 2, the equation becomes I = -I0C. When θ = π, the equation becomes I = -I0. 1= π / 8, θ2= π / 8 or -3π / 8; θ1is a known quantity, so the relationship between the differential amplified electrical signal and the rotation angle θ can be obtained; the plane is +, and the vertical is -. where I0is the initial light intensity, C is a constant, and θ is the rotation angle. When θ = 0, the equation becomes I = I0C. When θ = π / 2, 2. A polarization volume grating based single optical magnetic signal detection system for implementing the detection method of claim 1, characterized in that, Comprise: Laser, pre-light path unit, atomic cell unit, polarization detection unit and signal processing unit; the light emitted by the laser is modulated into elliptical polarized light by the pre-light path unit, acts on the atomic cell unit, makes the alkali metal atoms spin polarization, and then the polarization detection unit and the signal processing unit measure the size of the external magnetic field by using the optical rotation detection method; The laser is provided with a controller, and the controller regulates the laser to emit a fixed wavelength laser beam; The pre-light path unit comprises a collimator, a polarizer and a first quarter wave plate; The atomic cell unit comprises an atomic cell, a heating assembly and a Helmholtz coil assembly wound outside the atomic cell; The atomic cell is filled with alkali metal working atoms and buffer gas inside; The heating assembly is provided with a temperature control module for precise temperature control; The Helmholtz coil assembly is used to provide a radio frequency magnetic field for the system; The polarization detection unit comprises a second quarter wave plate, a liquid crystal polarization volume grating, a first photodetector and a second photodetector; the output ends of the first photodetector and the second photodetector are connected to one end of the differential amplification circuit in parallel, and the other end of the differential amplification circuit is connected to the input end of the lock-in amplifier. The signal processing unit comprises a differential amplification circuit and a phase-locked amplifier.

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