Vector vortex beam generation method applied to SERF magnetic field measurement device

Abstract

According to the vector vortex beam generation method applied to the SERF magnetic field measurement device, a vector vortex beam is generated according to a light field distribution formula of the vortex beam in combination with a computer-generated holography method and a polarization mode decomposition method, then iterative optimization is carried out on parameters such as the phase, the amplitude and the polarization direction in the beam in a global NSGA-II algorithm and local SPGD algorithm combination mode, and finally the vector vortex beam is generated. And then judging whether the electron steady-state polarizability uniformity meets the expectation or not, if so, ending the optimization, taking the optimized light beam as a pumping light beam to pump alkali metal atoms, and carrying out subsequent magnetic field measurement, and if not, changing parameters and continuing optimization until the electron steady-state polarizability uniformity meets the expectation. Compared with a traditional Gaussian beam, the atomic polarizability in the air chamber is stabilized at a required value, the atomic spin gradient field of the device is effectively inhibited, the differential performance of the device is enhanced, and the sensitivity of the SERF magnetic field measurement device is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of SERF atomic magnetic field measurement, and in particular to a vector vortex beam generation method applied to a SERF magnetic field measurement device. Background Art

[0002] Ultra-highly sensitive magnetic field measurement devices based on the spin exchange relaxation-free regime (SERF) effect of atomic spins are widely used in magnetic field detection and quantum information. Scalar light fields (such as Gaussian beams) are typically used as pump sources in SERF magnetic field measurement devices. The disadvantages of Gaussian beams include a non-uniform transverse polarization distribution. This Gaussian beam has a strong center-to-center intensity distribution and a weak edge-to-edge intensity distribution, resulting in a Gaussian polarization distribution across the cross-section of the alkali metal vapor cell, forming a transverse polarization gradient. Gaussian beams also cause a decay in the longitudinal polarization. This is because the Gaussian beam is absorbed as it passes through the alkali metal vapor cell at a high optical depth, gradually attenuating the light intensity along the propagation direction. This results in a gradual decrease in the longitudinal polarization from the input to the output end, forming a longitudinal polarization gradient. The optical frequency shift gradient is difficult to suppress, as the non-uniformity of the Gaussian beam amplifies the virtual magnetic field gradient. This gradient varies dynamically with the pump power and frequency, making it impossible to completely eliminate it through static magnetic compensation. Therefore, the Gaussian beam limits the optimization of polarization uniformity, and its inherent distribution characteristics make it difficult to significantly improve the polarization gradient even if the pump beam parameters are optimized.

[0003] For magnetic field measurement devices, polarization gradients can significantly amplify the gradient noise in the differential measurement system, reducing the common-mode rejection ratio and thus weakening the ability to suppress common-mode magnetic noise. Polarization inhomogeneities are converted into optical frequency-shifted virtual magnetic field gradients through the interaction between light and atoms. This dynamic gradient cannot be fully compensated by static gradient coils and will introduce additional spin relaxation, reducing the transverse relaxation time of the atomic ensemble, and ultimately affecting the theoretical sensitivity of magnetic field measurements. The virtual magnetic field gradient caused by the polarization gradient will intensify atomic spin relaxation through the diffusion effect, leading to a broadening of the magnetic resonance linewidth and further reducing the signal-to-noise ratio of the magnetic field measurement. The polarization gradient appears as non-common-mode noise in differential measurements, and its amplitude is inversely proportional to the gradient baseline length, directly limiting the improvement of differential sensitivity.

[0004] As a pump beam, vector vortex beams show the following significant advantages over traditional Gaussian beams in atomic spin polarization: vector vortex beams have a more uniform transverse intensity distribution. By controlling the polarization and phase, some vector vortex beams (such as flat-top vortex beams) can achieve a more uniform transverse intensity distribution and reduce the polarization gradient; the polarization efficiency can be improved through spatial polarization control. The polarization state of some vector vortex beams shows regular changes in space (such as radial or angular polarization), which can be more efficiently coupled with the atomic spin state, and the symmetric polarization distribution can offset the spatial inhomogeneity of the optical frequency shift; the phase vortex will suppress the relaxation effect, and the vortex phase carrying topological charge can induce the orbital angular momentum exchange of atomic spins, suppressing the relaxation caused by diffusion.

[0005] Currently, methods for generating vortex beams include direct generation, mode conversion, spiral phase plate, fiber generation, and computational holography. The advantage of the direct generation method is that it can be generated directly within the laser resonator. However, its disadvantages are that it is difficult to obtain a stable vortex beam and to generate high-order vortex beams. The advantage of the mode conversion method is high conversion efficiency. However, its disadvantages are that the optical structure is relatively complex, device fabrication is difficult, and it is difficult to control the type and parameters of the vortex beam. The advantage of the spiral phase plate method is high conversion efficiency. However, its disadvantages are that it is difficult to control the type and parameters of the vortex beam, and it is difficult to prepare high-quality phase plates. The advantage of the fiber generation method is that it is easy to promote in optical communication systems and the generated vortex beam is relatively stable. However, its disadvantage is that currently only low-order vortex beams can be achieved experimentally, and high-order vortex beams are currently difficult to achieve. The advantage of the computational holography method is that the position, size, and parameters of the vortex beam can be controlled. However, its disadvantage is that it requires the beam to be incident on the center of the hologram, which places strict requirements on the optical path. For application in SERF magnetic field measurement devices, computational holography is more suitable. Combining computational holography with a spatial light modulator allows for flexible beam control outside the resonator by adjusting the optical path structure.

[0006] Currently, the main methods for generating vector beams are intracavity generation and extracavity generation. The advantage of the intracavity generation method is that it can efficiently generate vector beams with specific polarization states. However, its disadvantage is that the generated beam pattern is limited and has limited flexibility. The advantage of the extracavity generation method is that it can flexibly control the beam pattern, but its disadvantage is lower efficiency. Therefore, for applications in SERF magnetic field measurement devices, the extracavity generation method is more suitable.

[0007] In order to be applied to a SERF magnetic field measurement device, the present invention uses a computational holographic method to generate a vortex beam based on a spatial light modulator, and uses a polarization mode decomposition method to generate a vector beam. The generated vector vortex beam is then optimized using a global NSGA-II (Non-Dominated Sorting Genetic Algorithm) and a local SPGD (Stochastic Parallel Gradient Descent Algorithm) algorithm on parameters such as phase, amplitude, and polarization direction. The phase, amplitude, and polarization direction of the beam are precisely controlled to stabilize the atomic polarizability in the gas chamber at a desired value, reduce the influence of the polarization gradient on the sensitivity of the SERF magnetic field measurement device, and improve the sensitivity and stability of the device. Summary of the Invention

[0008] In response to the deficiencies in the prior art, the present invention provides a method for generating a vector vortex beam applied to a SERF magnetic field measurement device. Based on a spatial light modulator, a computational holographic method is used to generate a vortex beam, and a polarization mode decomposition method is used to generate a vector beam. The generated vector vortex beam is then optimized using a global NSGA-II algorithm and a local SPGD algorithm in terms of parameters such as phase, amplitude, and polarization direction. The phase, amplitude, and polarization direction of the beam are precisely controlled to stabilize the atomic polarizability in the gas chamber at a desired value, thereby reducing the influence of the polarization gradient on the sensitivity of the SERF magnetic field measurement device and improving the sensitivity and stability of the device.

[0009] The technical solutions of the present invention are as follows: A vector vortex beam generation method applied to a SERF magnetic field measurement device is characterized in that it includes arranging a combination of a first beam splitter prism, a spatial light modulator and a vortex wave plate on a pumping light path before the pumping light incident side of an atomic gas chamber in the SERF magnetic field measurement device, wherein the exit plane of the spatial light modulator is conjugate with the incident plane of the vortex wave plate, the first beam splitter prism splits the Gaussian light from the pump laser into a first beam of reflected Gaussian light and a second beam of transmitted Gaussian light, the second beam of transmitted Gaussian light irradiates a spatial light modulator loaded with a computer-generated hologram, the spatial light modulator converts the second beam of transmitted Gaussian light into a vortex beam and emits the vortex beam to the first beam splitter prism through its exit plane, and the first beam splitter prism reflects the vortex beam and the first beam of reflected Gaussian light respectively to the vortex wave plate to generate a vector vortex beam.

[0010] The following steps are involved: Step 1: Determine the theoretical model of the vortex beam. First, determine the complex amplitude expression, and then obtain the vortex beam light field distribution expression through Fourier transform. Step 2: The intensity map and phase map of the vortex beam are simulated based on the light field distribution expression. The interference pattern of the plane wave is superimposed using the computational holography method to generate a fork grating pattern. Then, based on the correspondence between the phase value of each pixel and the grayscale value of the spatial light modulator, a grayscale map is generated from the phase map and loaded into the spatial light modulator to generate the required vortex beam. Step 3: For vortex wave plates of order 30 or higher, a polarization mode decomposition method is used to add an additional phase modulation factor to the vortex beam field distribution expression. The originally complex polarization mode of the beam is decomposed into multiple polarization modes, which are dispersed in space. The parameters of the phase modulation factor are then adjusted to reduce the energy density of other polarization modes. Ultimately, only the target polarization mode is retained, thereby achieving the extraction of any polarization mode in the beam and obtaining a vector vortex beam of any order. Step 4: For the obtained vector vortex beam, iteratively optimize the beam in terms of phase, amplitude, and polarization direction parameters. First, perform global NSGA-II algorithm optimization, initialize control parameters, which include beam phase, polarization direction, and amplitude parameters, define multiple objective functions, and the objective functions involve polarization purity and phase error. Then, calculate the multi-objective fitness value, perform non-dominated sorting on the population, and obtain Pareto front solutions of different levels. Then, perform selection, crossover, and mutation, and repeat iterations until the target convergence is reached, thereby obtaining a set of candidate beam generation parameters for the Pareto optimal solution. Step 5: Perform local SPGD optimization on the candidate beams. Select several candidate solutions close to the ideal target from the frontier solutions generated by NSGA-II as the initial input of SPGD. Initialize the control parameters, then define the performance indicators of SPGD. Introduce small perturbations, measure the performance indicators, calculate the performance differences, and then update the control parameters. Iterative optimization is performed. Finally, from the locally optimized solutions, the parameter combination with the best beam quality is selected as the final parameter combination. Step 6, measuring the uniformity of the electronic steady-state polarizability; Step 7: Determine whether the electronic polarization uniformity meets the requirements. If not, return to step 4, modify the topological charge, and continue iterative optimization. If yes, the optimization ends and proceeds to step 8. Step 8: Use the optimized beam as a pumping beam to pump the alkali metal atoms for subsequent magnetic field measurement.

[0011] In step 1, the light field distribution expression of the vortex beam is: , In the formula is the vortex beam light field distribution function, which describes the electric field intensity distribution of the vortex beam at different positions in the polar coordinates, r is the radial coordinate in the polar coordinates, θ is the angular coordinate in the polar coordinates, is an imaginary unit, is the topological charge or orbital angular momentum quantum number, is the waist radius of the Gaussian beam, is half the ring width, is the mode parameter, is the beam radius, is the modified Bessel function of the first kind.

[0012] The computational holography method in step 2 includes generating an interference pattern between a target light field and a reference light field through a computer program based on the principle of interference and diffraction of light to achieve the generation of a vortex light beam. The computational holography method is implemented by generating a computational hologram and combining it with a spatial light modulator. The computational hologram is implemented using a fork grating. The fork grating is used as a holographic film. After being irradiated with a Gaussian plane wave, the expected light field distribution is obtained. The fork grating pattern is loaded into the spatial light modulator so that the Gaussian plane wave is directly incident on the modulator surface, thereby achieving effective control of the output light field and ultimately forming the desired vortex light beam.

[0013] The step 3 includes decomposing and extracting a specific polarization mode from the light beam by simultaneously satisfying the following two conditions: the first condition is that the entangled polarization modes are separated in space; the second condition is that the separated polarization modes have different characteristics, so as to eliminate other polarization modes and retain only the target polarization mode; the first condition is achieved by destroying the modulation symmetry of the light beam. When the phase modulation factor is added, the symmetry is destroyed, and the polarization mode with a higher order is located on the outside and the polarization mode with a lower order is located on the inside; the separated polarization modes are transmitted independently in space; the second condition is achieved based on the energy density of different polarization modes. The mode with a larger topological charge has a greater degree of divergence in space and a lower energy density. By adjusting the parameters of the phase modulation factor, the energy density of the target polarization mode is much greater than the energy density of other polarization modes, and only the target polarization mode is retained.

[0014] The initialization control parameters in step 5 are to use the control parameters used to control the generation of the vector vortex beam as the control variables of the SPGD, including initializing a parameter vector , , to are the first to Nth control variables, where N is a positive integer.

[0015] Defining the performance indicators of SPGD in step 5 includes defining a performance indicator J for measuring the quality of the vector vortex beam as the optimization target of SPGD, and the performance indicators include: (1) mode purity: the matching degree of the polarization and phase distribution of the beam with the target mode; (2) phase and polarization stability: the stability of the phase and polarization distribution of the beam; (3) intensity distribution: the intensity distribution of the beam cross section, ensuring that the intensity characteristics of the vortex in the center of the beam meet the requirements.

[0016] The introduction of small disturbances in step 5 is to apply small random disturbances to the control parameters in each iteration. , generate two perturbation vectors u+ and u- , and applied to the device, including adding small perturbations to the phase pattern of a spatial light modulator, or applying tiny rotations to the angle of a wave plate.

[0017] The performance index measured in step 5 is to measure the two performance indexes of the output vector vortex beam respectively under the condition of applying two different disturbances. and .

[0018] The performance difference calculated in step 5 is calculated using the results of the two perturbation measurements. , ,Will As a basis for optimizing the step size through the following expression: , in is the gain coefficient, is the step size for adjusting the update control parameters.

[0019] The pump laser is connected to the input side of the first beam splitter prism through a first isolator, a first half-wave plate, a first polarization beam splitter prism, a first concave lens, and a first convex lens in sequence; the reflection side of the first polarization beam splitter prism is connected to a first wavelength meter through a first reflector; the reflection side of the first beam splitter prism is connected to the vortex wave plate through a second convex lens and a third convex lens in sequence; the vortex wave plate is connected to the pump light incident side of the atomic gas cell through a fourth convex lens, a fifth convex lens, a second half-wave plate, a second polarization beam splitter prism, and a first quarter-wave plate in sequence. The incident side of the detection light of the atomic gas chamber is connected to the detection laser through the second 1 / 4 wave plate, the Glan Taylor prism, the third reflector, the sixth convex lens, the second concave lens, the third polarization beam splitter prism, the third 1 / 2 wave plate and the second isolator in sequence. The reflection side of the third polarization beam splitter prism is connected to the second wavelength meter through the second reflector. The detection light emitting side of the atomic gas chamber is connected to the host computer through the detector. The atomic gas chamber is located in the magnetic compensation coil, the magnetic compensation coil is located in the oven, the oven is located in the vacuum chamber, and the vacuum chamber is located in the magnetic shielding barrel.

[0020] The technical effects of the present invention are as follows: The present invention applies a vector vortex beam generation method for a SERF magnetic field measurement device. Based on the light field distribution formula of the vortex beam, a computational holographic method and a polarization mode decomposition method are combined to generate a vector vortex beam. Then, a global NSGA-II algorithm and a local SPGD algorithm are combined for iterative optimization of parameters such as phase, amplitude, and polarization direction in the beam. Then, it is determined whether the uniformity of the electronic steady-state polarizability meets expectations. If it meets expectations, the optimization ends and the optimized beam is used as a pump beam to pump alkali metal atoms for subsequent magnetic field measurement. If it does not meet expectations, the parameters are changed and the optimization is continued until the uniformity of the electronic steady-state polarizability meets expectations. Compared with using a traditional pump beam (Gaussian beam), using this as a pump beam stabilizes the atomic polarizability in the gas chamber at the desired value, reduces the influence of the polarization gradient on the sensitivity of the SERF magnetic field measurement device, effectively suppresses the atomic spin gradient field of the device, and enhances the differential performance of the device, which is conducive to improving the sensitivity of the SERF magnetic field measurement device.

[0021] The advantages of this technology compared with the existing technology are: (1) the phase, amplitude and polarization direction of the light beam can be precisely controlled, which can effectively suppress the atomic spin gradient field of the device and improve the uniformity of the electron polarization rate in the gas chamber; (2) the introduction of a multi-objective optimization mechanism (NSGA-Ⅱ+SPGD) that combines global and local optimization. Compared with traditional single optimization strategies (such as genetic algorithms, particle swarm algorithms, etc.), this invention can further fine-tune local performance on the basis of ensuring global convergence, significantly improving the accuracy and efficiency of light beam control; (3) through iterative optimization of the feedback loop between the beam structure parameters and the atomic response characteristics, a beam generation method guided by "system performance" is realized, which can be closer to the actual performance index requirements, rather than relying solely on the ideal model parameters of the optical field itself, and has stronger engineering practicality. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 A schematic structural diagram of a SERF magnetic field measuring device according to a method for generating a vector vortex beam applied to a SERF magnetic field measuring device according to the present invention.

[0023] Figure 2 It is a schematic flow chart of a method for generating a vector vortex beam applied to a SERF magnetic field measurement device according to the present invention. Figure 2 The method includes step 1, determining the light field distribution expression of the vortex beam; step 2, generating the vortex beam by computational holography; step 3, generating the vector beam by polarization mode decomposition; step 4, optimizing the beam by global NSGA-Ⅱ algorithm (NSGA, Non-Dominated Sorting Genetic Algorithm); step 5, optimizing the beam by local SPGD algorithm (SPGD, stochastic parallel gradient descent algorithm); step 6, measuring the uniformity of the electronic steady-state polarizability; step 7, judging whether the electronic polarization uniformity meets the requirements; if not, returning to step 4, modifying parameters such as the topological charge, and continuing iterative optimization; if yes, the optimization ends and entering step 8; step 8, using the optimized beam as the pump beam to pump the alkali metal atoms for subsequent magnetic field measurement.

[0024] The reference numerals are as follows: 1-pumping laser; 2-first isolator; 3-first 1 / 2 wave plate; 4-first polarization beam splitter; 5-first reflector; 6-first wavelength meter; 7-first concave lens; 8-first convex lens; 9-first beam splitter; 10-spatial light modulator; 11-second convex lens; 12-third convex lens; 13-vortex wave plate; 14-fourth convex lens; 15-fifth convex lens; 16-second 1 / 2 wave plate; 17-second polarization beam splitter; 18-first 1 / 4 wave plate; 19-second convex lens; 20-second convex lens; 21-second convex lens; 22-second convex lens; 23-second convex lens; 24-second convex lens; 25-second convex lens; 26-second 1 / 2 wave plate; 27-second polarization beam splitter; 28-first 1 / 4 wave plate; 29-second convex lens; 30-second convex lens; 31-second convex lens; 32-second convex lens; 33-second convex lens; 34-second convex lens; 35-second convex lens; 36-second 1 / 2 wave plate; 37-second convex lens; 38-second 1 / 4 wave plate; 39-second convex lens -Detection laser; 20-Second isolator; 21-Third 1 / 2 wave plate; 22-Third polarization splitter prism; 23-Second reflector; 24-Second wavelength meter; 25-Second concave lens; 26-Sixth convex lens; 27-Third reflector; 28-Glan-Taylor prism; 29-Second 1 / 4 wave plate; 30-Magnetic shielding barrel; 31-Vacuum chamber; 32-Oven; 33-Magnetic compensation coil; 34-Atomic gas chamber; 35-Detector; 36-Host computer; XYZ-Cartesian coordinate system (X axis, Y axis, Z axis). DETAILED DESCRIPTION

[0025] Below is the attached figure ( Figure 1-Figure 2 ) and Examples illustrate the present invention.

[0026] Figure 1 A schematic structural diagram of a SERF magnetic field measuring device according to a method for generating a vector vortex beam applied to a SERF magnetic field measuring device according to the present invention. Figure 2 This is a flow chart of a method for generating a vector vortex beam in a SERF magnetic field measurement device according to the present invention. Figures 1 to 2 As shown, a vector vortex beam generation method applied to a SERF magnetic field measuring device includes arranging a combination of a first beam splitter 9, a spatial light modulator 10 and a vortex wave plate 13 on a pumping light path before the pumping light incident side of an atomic gas chamber 34 in the SERF magnetic field measuring device, wherein the exit plane of the spatial light modulator 10 is conjugate with the incident plane of the vortex wave plate 13, the first beam splitter 9 splits the Gaussian light from the pumping laser 1 into a first beam of reflected Gaussian light and a second beam of transmitted Gaussian light, the second beam of transmitted Gaussian light irradiates the spatial light modulator 10 loaded with a computer-generated hologram, the spatial light modulator 10 converts the second beam of transmitted Gaussian light into a vortex beam and emits the vortex beam to the first beam splitter prism 9 through its exit plane, the first beam splitter prism 9 reflects the vortex beam and the first beam of reflected Gaussian light respectively to the vortex wave plate 13 to generate a vector vortex beam.

[0027] The pump laser 1 is connected to the input side of the first beam splitter prism 9 through the first isolator 2, the first 1 / 2 wave plate 3, the first polarization beam splitter prism 4, the first concave lens 7 and the first convex lens 8 in sequence. The reflection side of the first polarization beam splitter prism 4 is connected to the first wavelength meter 6 through the first reflector 5. The reflection side of the first beam splitter prism 9 is connected to the vortex wave plate 13 through the second convex lens 11 and the third convex lens 12 in sequence. The vortex wave plate 13 is connected to the pump light incident side of the atomic gas cell 34 through the fourth convex lens 14, the fifth convex lens 15, the second 1 / 2 wave plate 16, the second polarization beam splitter prism 17 and the first 1 / 4 wave plate 18 in sequence. The atomic gas cell 3 The incident side of the detection light of 4 is connected to the detection laser 19 through the second 1 / 4 wave plate 29, the Glan Taylor prism 28, the third reflector 27, the sixth convex lens 26, the second concave lens 25, the third polarization beam splitter prism 22, the third 1 / 2 wave plate 21 and the second isolator 20 in sequence. The reflection side of the third polarization beam splitter prism 22 is connected to the second wavelength meter 24 through the second reflector 23. The detection light emitting side of the atomic gas chamber 34 is connected to the host computer 36 through the detector 35. The atomic gas chamber 34 is located in the magnetic compensation coil 33, the magnetic compensation coil 33 is located in the oven 32, the oven 32 is located in the vacuum chamber 31, and the vacuum chamber 31 is located in the magnetic shielding barrel 30.

[0028] In order to be applied to a SERF magnetic field measurement device, the present invention uses a computational holographic method to generate a vortex beam based on a spatial light modulator, and a polarization mode decomposition method to generate a vector beam. The generated vector vortex beam is then optimized using a global NSGA-II algorithm and a local SPGD algorithm in terms of parameters such as phase, amplitude, and polarization direction. The phase, amplitude, and polarization direction of the beam are precisely controlled to stabilize the atomic polarizability in the gas chamber at a desired value, reduce the influence of the polarization gradient on the sensitivity of the SERF magnetic field measurement device, and improve the sensitivity and stability of the device.

[0029] A vector vortex beam generation method for a SERF magnetic field measurement device includes the following steps: Step 1: Determine the theoretical model of the vortex beam. First, determine the complex amplitude expression and obtain its light field distribution expression through Fourier transform. Step 2: The intensity map and phase map of the vortex beam are simulated based on the light field distribution expression. The interference pattern of the plane wave is superimposed using the computational holographic method. Finally, the two phase maps are superimposed to generate a fork grating. Then, based on the correspondence between the phase value of each pixel and the grayscale value of the spatial light modulator, a grayscale map is generated from the phase map and loaded into the spatial light modulator to generate the required vortex beam. Step 3: For vortex wave plates of order 30 or higher, a polarization mode decomposition method is used to add an additional phase modulation factor to the light field distribution expression. This decomposes the originally complex polarization mode of the light beam into multiple polarization modes, which are dispersed in space. The parameters of the phase modulation factor are then adjusted to reduce the energy density of other polarization modes. Ultimately, only the target polarization mode is retained, thus enabling the extraction of any polarization mode in the light beam and obtaining a vector beam of any order. Step 4: For the obtained vector vortex beam, iteratively optimize the beam in terms of phase, amplitude, and polarization direction parameters. First, perform global NSGA-II algorithm optimization, initialize the control parameters (including beam phase, polarization direction, and amplitude parameters), define multiple objective functions such as polarization purity and phase error, and then calculate the multi-objective fitness value. Then, perform non-dominated sorting on the population to obtain Pareto frontier solutions of different levels. Then, perform selection, crossover, and mutation, and iterate the above steps until the target convergence is reached. Finally, a set of candidate beam generation parameters for the Pareto optimal solution is obtained. Step 5: Perform local SPGD optimization on the beam. Select several candidate solutions close to the ideal target from the frontier solutions generated by NSGA-II as the initial input of SPGD. Initialize the control parameters, then define the performance indicators of SPGD. Introduce small perturbations, measure the performance indicators, calculate the performance difference, and then update the control parameters. Iterative optimization is performed. Finally, from the locally optimized solutions, the parameter combination with the best beam quality is selected as the final parameter combination. Step 6: Measure the uniformity of the electronic steady-state polarizability.

[0030] Step 7: Determine whether the electronic polarization uniformity meets the requirements. If not, return to step 4, modify parameters such as topological charge, and continue iterative optimization. If yes, the optimization ends and proceeds to step 8.

[0031] Step 8: Use the optimized beam as a pumping beam to pump the alkali metal atoms for subsequent magnetic field measurement.

[0032] In step 1, the light field distribution expression of the vortex beam is: Where, is the beam radius, is the waist radius of the Gaussian beam, It is half the ring width. is a modified Bessel function of the first kind. E(r,θ) is the light field distribution function, which describes the electric field intensity distribution of the vortex beam at different positions in polar coordinates. r is the radial coordinate in polar coordinates, which represents the straight-line distance between a point and the origin (the center of the beam). θ is the angular coordinate in polar coordinates, which represents the rotation angle of a point relative to a reference direction (such as the positive direction of the x-axis). i is the imaginary unit, which is used to describe the light field phase in complex form. l is the topological charge (or orbital angular momentum quantum number). imθ is the imaginary phase factor, which represents the spiral phase structure as a whole. i: imaginary unit, m: mode parameter (radial mode number or angular mode number). The term exp(imθ) is the core feature of the vortex beam, and its spiral phase causes the beam to carry orbital angular momentum.

[0033] In the step 2, the computational holography method used is based on the principle of interference and diffraction of light, and generates an interference pattern between the target light field and the reference light field through a computer program to achieve the generation of a vortex light beam. Specifically, it can be implemented by generating a computational hologram and combining it with a spatial light modulator. The computational hologram can be implemented in the form of a fork grating, and the fork grating is used as a holographic negative. After being irradiated with a Gaussian plane wave, the expected light field distribution can be obtained. Furthermore, the fork grating pattern is loaded into the spatial light modulator so that the Gaussian plane wave is directly incident on the modulator surface, thereby achieving effective regulation of the output light field and ultimately forming the desired vortex light beam.

[0034] In step 3, in order to decompose and extract a specific polarization mode from the light beam, the following two conditions must be met simultaneously: first, the entangled polarization modes are separated in space; second, the separated polarization modes have different characteristics, so that other polarization modes are eliminated and only the target polarization mode is retained. The first condition can be achieved by destroying the modulation symmetry of the light beam. When the phase modulation factor is added, the symmetry is destroyed, and the polarization mode with a higher order is located on the outside, and the polarization mode with a lower order is located on the inside. The separated polarization modes will be transmitted independently in space. The second condition can be achieved based on the energy density of different polarization modes. The mode with a larger topological charge has a greater degree of divergence in space and a lower energy density. By adjusting the parameters of the phase modulation factor, the energy density of the target polarization mode is much greater than the energy density of other polarization modes, and only the target polarization mode is retained.

[0035] In step 5, the initialization control parameters of the SPGD algorithm correction are the parameters used to control the generation of the vector vortex beam as the control variables of the SPGD. For example, the phase distribution of the spatial light modulator and the adjustment angle of the wave plate are initialized into a parameter vector . Parameter vector u: contains all the control variables used to optimize the vector vortex beam. By adjusting these parameters, the phase distribution, polarization state and amplitude distribution of the beam can be precisely controlled. u1: represents the first independent variable generated by the vector vortex beam during the optimization process, representing the grayscale value of a phase modulation area of ​​the spatial light modulator. u N : represents the last independent variable in the parameter vector.

[0036] In step 5, the performance indicator set for the SPGD algorithm correction is to define a performance indicator J that measures the quality of the vector vortex beam, which serves as the optimization target of the SPGD. The performance indicators to be used include: (1) mode purity: the matching degree between the polarization and phase distribution of the beam and the target mode; (2) phase and polarization stability: the stability of the phase and polarization distribution of the beam; and (3) intensity distribution: the intensity distribution of the beam cross section, ensuring that the intensity characteristics of the vortex at the center of the beam meet the requirements.

[0037] In step 5, the introduced disturbance of the SPGD algorithm correction is to apply a small random disturbance to the control parameters in each iteration. , generate two perturbation vectors u+ and u- , and applied to the device. For example, small perturbations can be added to the phase pattern of a spatial light modulator, or small rotations can be applied to the angle of a wave plate.

[0038] In step 5, the measurement performance index of the SPGD algorithm correction is to measure the performance index of the output vector vortex beam respectively under the condition of applying two different disturbances. and .

[0039] In step 5, the performance difference calculated by the SPGD algorithm correction is the performance difference calculated using the results of the two perturbation measurements. , as the basis for optimizing the step size.

[0040] Update control parameters: Use the following update rules to adjust the control parameters: , in is the gain coefficient, which adjusts the update step size.

[0041] In step 8, the pumping optical path in the magnetic field measurement optical path is a pumping laser with a wavelength of 795nm, which is polarized by the pumping laser 1 through the first isolator 2; the first 1 / 2 wave plate 3 and the first polarization beam splitter prism 4 are used to adjust the power ratio; one beam of light passes through the first reflector 5 and enters the first wavelength meter 6 to collect the wavelength of the pumping beam, and the other beam of light is expanded by the beam expansion system composed of the first concave lens 7 and the first convex lens 8; then passes through a first beam splitter prism 9, and the beam is split into two beams after passing through the beam splitter, and one beam of light reaches the spatial light modulator 10. The reflected light passes through the first 4f system composed of the second convex lens 11 and the third convex lens 12 to reach the vortex wave plate 13. The exit plane of the spatial light modulator 10 is conjugate with the incident plane of the vortex wave plate 13. Then, the light passes through a second 4f system consisting of a fourth convex lens 14 and a fifth convex lens 15; then it passes through a system consisting of a second 1 / 2 wave plate 16 and a second polarization beam splitter prism 17 to regulate the power; finally, it passes through a first 1 / 4 wave plate 18 to convert the linearly polarized light into circularly polarized light and enters the magnetic shielding barrel to pump the Rb atoms in the gas chamber.

[0042] In step 8, the detection optical path in the magnetic field measurement optical path is composed of a detection laser 19 generating a detection laser, passing through a second isolator 20, and then passing through a system consisting of a third 1 / 2 wave plate 21 and a third polarization beam splitter prism 22 to regulate the power, and then passing through a beam expansion system consisting of a second concave lens 25 and a sixth convex lens 26 to expand the light, and then passing through a third reflector 27, and then through a Glan Taylor prism 28, and finally through a second 1 / 4 wave plate 29 to be incident into the atomic gas chamber, a lens is used at the other end of the device to focus the light beam, and finally a detector 35 is used to detect the magnetic field signal.

[0043] refer to Figures 1 to 2A vector vortex beam generation device and method for use in a SERF magnetic field measurement device includes a beam expansion system consisting of a first concave lens 7 and a first convex lens 8, a first beam splitter prism 9 that splits light to a spatial light modulator (SLM), a spatial light modulator 10 for applying a phase pattern, two 4f systems consisting of a second convex lens 11, a third convex lens 12, a fourth convex lens 14, and a fifth convex lens 15, a vortex wave plate 13 for generating a high-order vector beam, as well as a pumping optical system, a detection optical system, and an active and passive magnetic field control system. The beam expansion system consisting of the first concave lens 7 and the first convex lens 8 is used to increase the beam spot radius. The first beam splitter prism 9 is used to split the expanded beam into two beams, one of which is transmitted and irradiated onto the SLM, while the light reflected by the SLM is then reflected by the beam splitter prism. The SLM 10 is used to apply a phase pattern to generate a vortex beam and, in combination with the vortex wave plate 13, generates a vector beam. The two 4f systems are used to filter the beam, and the vortex wave plate 13 is used to generate a vector beam with an arbitrary polarization pattern. The pumping optical system pumps alkali metal atoms, ie, low-density potassium atoms, and the detection optical system detects the optical rotation angle of the dichroism of rubidium atoms to obtain the magnitude of the magnetic field to be measured.

[0044] A vector vortex beam generating method applied to a SERF magnetic field measuring device utilizes the above-mentioned vector vortex beam generating device applied to the SERF magnetic field measuring device.

[0045] Any content not described in detail in this specification is prior art known to those skilled in the art. It should be noted that the above description is intended to help those skilled in the art understand the present invention, but does not limit the scope of protection of the present invention. Any equivalent substitution, modification, improvement, and / or simplification of the above description that does not depart from the essence of the present invention shall fall within the scope of protection of the present invention.

Claims

1. A vector vortex beam generation method for a SERF magnetic field measurement device, characterized in that: The invention comprises a combination of a first beam splitter prism, a spatial light modulator and a vortex wave plate arranged on a pumping light path before the pumping light incident side of an atomic gas chamber in a SERF magnetic field measurement device, wherein the exit plane of the spatial light modulator is conjugate with the incident plane of the vortex wave plate, the first beam splitter prism splits the Gaussian light from the pumping laser into a first beam of reflected Gaussian light and a second beam of transmitted Gaussian light, the second beam of transmitted Gaussian light irradiates a spatial light modulator loaded with a computer-generated hologram, the spatial light modulator converts the second beam of transmitted Gaussian light into a vortex beam and emits the vortex beam to the first beam splitter prism through its exit plane, the first beam splitter prism reflects the vortex beam and the first beam of reflected Gaussian light respectively to the vortex wave plate to generate a vector vortex beam.

2. The vector vortex beam generation method for a SERF magnetic field measurement device according to claim 1, characterized in that: The following steps are involved: Step 1: Determine the theoretical model of the vortex beam. First, determine the complex amplitude expression, and then obtain the vortex beam light field distribution expression through Fourier transform. Step 2: The intensity map and phase map of the vortex beam are simulated based on the light field distribution expression. The interference pattern of the plane wave is superimposed using the computational holography method to generate a fork grating pattern. Then, based on the correspondence between the phase value of each pixel and the grayscale value of the spatial light modulator, a grayscale map is generated from the phase map and loaded into the spatial light modulator to generate the required vortex beam. Step 3: For vortex wave plates of order 30 or higher, a polarization mode decomposition method is used to add an additional phase modulation factor to the vortex beam field distribution expression. The originally complex polarization mode of the beam is decomposed into multiple polarization modes, which are dispersed in space. The parameters of the phase modulation factor are then adjusted to reduce the energy density of other polarization modes. Ultimately, only the target polarization mode is retained, thereby achieving the extraction of any polarization mode in the beam and obtaining a vector vortex beam of any order. Step 4: For the obtained vector vortex beam, iteratively optimize the beam in terms of phase, amplitude, and polarization direction parameters. First, perform global NSGA-II algorithm optimization, initialize control parameters, which include beam phase, polarization direction, and amplitude parameters, define multiple objective functions, and the objective functions involve polarization purity and phase error. Then, calculate the multi-objective fitness value, perform non-dominated sorting on the population, and obtain Pareto front solutions of different levels. Then, perform selection, crossover, and mutation, and repeat iterations until the target convergence is reached, thereby obtaining a set of candidate beam generation parameters for the Pareto optimal solution. Step 5: Perform local SPGD optimization on the candidate beams. Select several candidate solutions close to the ideal target from the frontier solutions generated by NSGA-II as the initial input of SPGD. Initialize the control parameters, then define the performance indicators of SPGD. Introduce small perturbations, measure the performance indicators, calculate the performance differences, and then update the control parameters. Iterative optimization is performed. Finally, from the locally optimized solutions, the parameter combination with the best beam quality is selected as the final parameter combination. Step 6, measuring the uniformity of the electronic steady-state polarizability; Step 7: Determine whether the electronic polarization uniformity meets the requirements. If not, return to step 4, modify the topological charge, and continue iterative optimization. If yes, the optimization ends and proceeds to step 8. Step 8: Use the optimized beam as a pumping beam to pump the alkali metal atoms for subsequent magnetic field measurement.

3. The vector vortex beam generation method for a SERF magnetic field measurement device according to claim 2, characterized in that: In step 1, the light field distribution expression of the vortex beam is: ， In the formula is the vortex beam light field distribution function, which describes the electric field intensity distribution of the vortex beam at different positions in the polar coordinates, r is the radial coordinate in the polar coordinates, θ is the angular coordinate in the polar coordinates, is an imaginary unit, is the topological charge or orbital angular momentum quantum number, is the waist radius of the Gaussian beam, is half the ring width, is the mode parameter, is the beam radius, is the modified Bessel function of the first kind.

4. The vector vortex beam generation method for a SERF magnetic field measurement device according to claim 2, characterized in that: The computational holography method in step 2 includes generating an interference pattern between a target light field and a reference light field through a computer program based on the principle of interference and diffraction of light to achieve the generation of a vortex light beam. The computational holography method is implemented by generating a computational hologram and combining it with a spatial light modulator. The computational hologram is implemented using a fork grating. The fork grating is used as a holographic film. After being irradiated with a Gaussian plane wave, the expected light field distribution is obtained. The fork grating pattern is loaded into the spatial light modulator so that the Gaussian plane wave is directly incident on the modulator surface, thereby achieving effective control of the output light field and ultimately forming the desired vortex light beam.

5. The vector vortex beam generation method for a SERF magnetic field measurement device according to claim 2, characterized in that: The step 3 includes decomposing and extracting a specific polarization mode from the light beam by simultaneously satisfying the following two conditions: the first condition is that the entangled polarization modes are separated in space; the second condition is that the separated polarization modes have different characteristics, so as to eliminate other polarization modes and retain only the target polarization mode; the first condition is achieved by destroying the modulation symmetry of the light beam. When the phase modulation factor is added, the symmetry is destroyed, and the polarization mode with a higher order is located on the outside and the polarization mode with a lower order is located on the inside; the separated polarization modes are transmitted independently in space; the second condition is achieved based on the energy density of different polarization modes. The mode with a larger topological charge has a greater degree of divergence in space and a lower energy density. By adjusting the parameters of the phase modulation factor, the energy density of the target polarization mode is much greater than the energy density of other polarization modes, and only the target polarization mode is retained.

6. The method for generating a vector vortex beam for use in a SERF magnetic field measurement device according to claim 2, wherein: The initialization control parameters in step 5 are to use the control parameters used to control the generation of the vector vortex beam as the control variables of the SPGD, including initializing a parameter vector , , to are the first to Nth control variables, where N is a positive integer.

7. The vector vortex beam generation method for a SERF magnetic field measurement device according to claim 2, characterized in that: Defining the performance indicators of SPGD in step 5 includes defining a performance indicator J for measuring the quality of the vector vortex beam as the optimization target of SPGD, and the performance indicators include: (1) mode purity: the matching degree of the polarization and phase distribution of the beam with the target mode; (2) phase and polarization stability: the stability of the phase and polarization distribution of the beam; (3) intensity distribution: the intensity distribution of the beam cross section, ensuring that the intensity characteristics of the vortex in the center of the beam meet the requirements.

8. The method for generating a vector vortex beam for use in a SERF magnetic field measurement device according to claim 2, wherein: The introduction of small disturbances in step 5 is to apply small random disturbances to the control parameters in each iteration. , generate two perturbation vectors u+ and u- , and applied to the device, including adding small perturbations to the phase pattern of a spatial light modulator, or applying tiny rotations to the angle of a wave plate.

9. The method for generating a vector vortex beam for use in a SERF magnetic field measurement device according to claim 2, wherein: The performance index measured in step 5 is to measure the two performance indexes of the output vector vortex beam respectively under the condition of applying two different disturbances. and ; The performance difference calculated in step 5 is calculated using the results of the two perturbation measurements. , ,Will As a basis for optimizing the step size through the following expression: ， in is the gain coefficient, Is to adjust and update the control parameters The step size.

10. The method for generating a vector vortex beam for use in a SERF magnetic field measurement device according to claim 1, wherein: The pump laser is connected to the input side of the first beam splitter prism through a first isolator, a first half-wave plate, a first polarization beam splitter prism, a first concave lens, and a first convex lens in sequence; the reflection side of the first polarization beam splitter prism is connected to a first wavelength meter through a first reflector; the reflection side of the first beam splitter prism is connected to the vortex wave plate through a second convex lens and a third convex lens in sequence; the vortex wave plate is connected to the pump light incident side of the atomic gas cell through a fourth convex lens, a fifth convex lens, a second half-wave plate, a second polarization beam splitter prism, and a first quarter-wave plate in sequence. The incident side of the detection light of the atomic gas chamber is connected to the detection laser through the second 1 / 4 wave plate, the Glan Taylor prism, the third reflector, the sixth convex lens, the second concave lens, the third polarization beam splitter prism, the third 1 / 2 wave plate and the second isolator in sequence. The reflection side of the third polarization beam splitter prism is connected to the second wavelength meter through the second reflector. The detection light emitting side of the atomic gas chamber is connected to the host computer through the detector. The atomic gas chamber is located in the magnetic compensation coil, the magnetic compensation coil is located in the oven, the oven is located in the vacuum chamber, and the vacuum chamber is located in the magnetic shielding barrel.

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