Method for detecting OAM mode of vortex beam by using polarization characteristic

Abstract

The application discloses a method for detecting OAM modes of vortex beams by utilizing polarization characteristics, and comprises the following steps: by utilizing the unified theory of coherence-polarization, combining the wave propagation theory in random medium and the transmission advantage of LGSMV beams in atmospheric turbulence medium, the change of the polarization direction angle and the ellipticity of LGSMV beams with different radial indexes and different topological charges after the transmission in atmospheric turbulence is researched, and the result shows that the topological charge and the positive and negative of the LGSMV beams can be detected by utilizing the polarization direction angle and the ellipticity of the LGSMV beams. Compared with the prior art, the method directly utilizes the polarization direction angle and the ellipticity distribution diagram to detect the size and direction of the OAM mode, is not limited by experimental conditions, is simple to operate and easy to realize.

Description

Technical Field

[0001] This invention belongs to the field of wireless laser communication technology, and in particular relates to a method for detecting the OAM mode of a vortex beam using polarization characteristics. Background Technology

[0002] With the emergence of new fields such as big data, artificial intelligence, and the Internet of Things, high speed, low energy consumption, intelligence, and enhanced security are inevitable trends in the future development of communication technologies. Orbital Angular Momentum (OAM) multiplexing communication systems, to some extent, reflect the arrival of another era of "multiplexing" in communication technology. This is because vortex beams possess a helical phase structure, their topological charge has infinite values, and vortex beams with different topological charges are orthogonal to each other. Applying them to communication systems can achieve higher data transmission rates than traditional ordinary beams. Furthermore, the uncertain relationship between their topological charge and azimuth angle gives the communication system higher security.

[0003] In OAM multiplexing communication systems, atmospheric turbulence disrupts the spatial structure of multiple vortex beams after OAM multiplexing transmission, causing the OAM modes (equivalent to topological charge numbers) of the multiple vortex beams to need to be detected at the receiving end after demultiplexing to ensure correct demodulation of the information carried by each OAM mode. Therefore, whether the corresponding OAM mode can be detected efficiently after the transmission of vortex beams carrying OAM is one of the key technologies of OAM multiplexing communication. Currently, there are four main methods for detecting OAM modes: (1) using a forked diffraction aperture to convert the vortex beam into a Gaussian beam in the diffraction direction; (2) using the interference pattern of the vortex beam and the Gaussian beam to distinguish the OAM modes; (3) using the diffraction pattern generated by the vortex beam after passing through a slit or various apertures to distinguish the OAM modes; and (4) using optical elements to reconstruct the wavefront of the vortex beam carrying the OAM mode to make it easy to distinguish. Therefore, based on the existing OAM mode detection, it is necessary to seek a detection method for the vortex beam OAM mode that is more adaptable to the transmission of atmospheric turbulence channels, so as to improve the detection probability of the OAM multiplexing communication system. Summary of the Invention

[0004] The purpose of this invention is to provide a method for detecting the OAM mode of a vortex beam using polarization characteristics, so as to solve the problems existing in the prior art.

[0005] To achieve the above objectives, the present invention provides a method for detecting the OAM mode of a vortex beam using polarization characteristics, comprising:

[0006] Calculate the observed cross spectral density matrix of the vortex beam at the receiving plane after transmission through atmospheric turbulence, and calculate the Stokes parameter at the receiving plane based on the observed cross spectral density matrix; calculate the polarization direction angle and ellipticity of the vortex beam after transmission through atmospheric turbulence based on the Stokes parameter.

[0007] Optionally, the process of obtaining the observed cross spectral density matrix includes: obtaining the propagation information of the vortex beam, the position coordinates of the receiving plane, and the position coordinates of the source plane, and constructing the cross spectral density matrix expression of the receiving plane based on the obtained information; obtaining the cross spectral density matrix expression of the source plane; and calculating the observed cross spectral density matrix by combining the atmospheric turbulence power spectrum model and wave structure function. The propagation information includes wavelength, transmission distance, wave number, and amplitude.

[0008] Optionally, the amplitude and waist radius of the vortex beam in the x and y directions are obtained, and the coherence length of the vortex beam at the plane z=0 is obtained; the source plane cross spectral density matrix expression is calculated based on the amplitude and waist radius, Laguerre polynomial, topological charge number, and coherence length.

[0009] Optionally, the process of calculating the observed cross-spectral density matrix further includes: constructing a wave structure function based on the internal and external scales of atmospheric turbulence and the atmospheric refractive index structure constant; using the wave structure function as the source plane position vector function; substituting the expression of the source plane cross-spectral density matrix into the expression of the receiver plane cross-spectral density matrix; transforming the expression by combining the wave structure function; integrating the source plane variables in the transformed formula to form a new formula; representing the spiral phase factor and Laguerre polynomial in Cartesian coordinates and substituting them into the new formula; and integrating the receiver plane variables in the new formula to obtain the observed cross-spectral density matrix.

[0010] Optionally, the calculation process of the Stokes parameters at the receiving plane includes: obtaining the formula for calculating the initial value of the Stokes parameters, and obtaining the Stokes parameters of the receiving plane z>0 based on the first cross spectral density matrix and the formula for calculating the initial value of the Stokes parameters.

[0011] Optionally, the process of obtaining the polarization direction angle includes: describing the polarization direction angle of the vortex beam according to the Stokes parameter, obtaining a mathematical model of the polarization direction angle, solving the mathematical model of the polarization direction angle according to the observed cross spectral density matrix and the corresponding parameter settings, and obtaining the distribution of the polarization direction angle.

[0012] Optionally, the ellipticity of the vortex beam is described by the Stokes parameters, and a mathematical model of the ellipticity of the vortex beam in the receiving plane, represented by the amplitudes of the primary and secondary half-axis, is obtained. The mathematical model of the ellipticity of the vortex beam is then solved based on the observed cross spectral density matrix and the corresponding parameter settings to obtain the ellipticity distribution.

[0013] The technical effects of this invention are as follows:

[0014] This invention proposes a method for detecting the OAM mode of an LGSMV beam using polarization direction angle and ellipticity. It primarily utilizes the unified theory of coherence and polarization, combined with wave propagation theory in random media and the propagation advantages of LGSMV beams in atmospheric turbulent media. The changes in polarization direction angle and ellipticity of LGSMV beams with different radial indices and topological charges after propagation through atmospheric turbulence were investigated. The results show that the magnitude and sign of the topological charge of an LGSMV beam can be detected using its polarization direction angle and ellipticity. Compared to existing methods, this method directly detects the magnitude and direction of the OAM mode using the polarization direction angle and ellipticity distribution map, is not limited by experimental conditions, and is simple and easy to implement. Attached Figure Description

[0015] The accompanying drawings, which form part of this application, are used to provide a further understanding of this application. The illustrative embodiments and descriptions of this application are used to explain this application and do not constitute an undue limitation of this application. In the drawings:

[0016] Figure 1 This is a diagram of an experimental setup for the application of the method for detecting the OAM mode of an LGSMV beam in an OAM mode multiplexing system, as described in this invention.

[0017] Figure 2 This is a schematic diagram of the polarization direction angle distribution of the vortex beam at the receiving end when the radial index p = 0 and the topological charge l = ±1, ±2, ±3, ±5 in an embodiment of the present invention.

[0018] Figure 3 This is a schematic diagram of the polarization direction angle distribution of the vortex beam at the receiving end when the radial index p = 5, 10, 30 and the topological charge l = 1, 2, 3, 5 in an embodiment of the present invention.

[0019] Figure 4 This is a schematic diagram of the ellipticity distribution of the vortex beam at the receiving end when the radial index p = 0 and the topological charge l = ±1, ±2, ±3, ±5 in an embodiment of the present invention.

[0020] Figure 5 This is a schematic diagram of the ellipticity distribution of the vortex beam at the receiving end when the radial index p = 5, 10, 30 and the topological charge l = 1, 2, 3, 5 in an embodiment of the present invention. Detailed Implementation

[0021] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. This application will now be described in detail with reference to the accompanying drawings and embodiments.

[0022] It should be noted that the steps shown in the flowchart in the accompanying drawings can be executed in a computer system such as a set of computer-executable instructions, and although a logical order is shown in the flowchart, in some cases the steps shown or described may be executed in a different order than that shown here.

[0023] Example 1

[0024] like Figure 1-5 As shown, this embodiment provides a method for detecting the OAM mode of a vortex beam using polarization characteristics, including:

[0025] Step 1: Based on the CSDM expression of the LGSMV beam at the source plane, the atmospheric turbulence power spectrum model, and the wave structure function, the CSDM of the LGSMV beam at the receiving plane after atmospheric turbulence propagation can be calculated.

[0026] The specific process of step 1 is as follows:

[0027] The expression for calculating the CSDM of the LGSMV beam in the receiving plane is:

[0028]

[0029] Where λ represents the wavelength of the LGSMV beam, z represents the transmission distance, k = 2π / λ represents the wavenumber, ρ1 and ρ2 represent the position coordinates of the LGSMV beam at the receiver, r1 and r2 represent the position coordinates of the LGSMV beam at the transmitter, and Wij(r1,r2,0) represents the CSDM expression of the LGSMV beam at the source plane.<exp[ψ(ρ1,r1)+ψ*(ρ2,r2)]> ≈exp[-Dψ(ρ1-ρ2,r1-r2) / 2];

[0030] The CSDM expression for the LGSMV beam at the source plane is:

[0031]

[0032] Where Ai, Aj and w0i, w0j are the amplitude and beam waist radius in the x and y directions, respectively; Lp(·) is the p-th order Laguerre polynomial; l is the topological charge; δij is the coherence length of the beam at the plane z = 0, and δij = δji; Bij is a constant parameter, and the parameters Bij and δij satisfy the following condition:

[0033]

[0034] D(ρ1-ρ2, r1-r2) is the wave structure function. Since this invention must study the second moment of the light field at the same position vector on the receiving plane, when combined with the modified von Karman spectral model, the wave structure function is explicitly expressed as a function of the source plane position vector, as shown below:

[0035]

[0036] Where κ is the space wavenumber, κm=5.92 / l0, κ0=2π / L0, κm and κ0 are two special space wavenumbers, l0 and L0 are the inner and outer scales of atmospheric turbulence, respectively; C0 represents the atmospheric refractive index structure constant.

[0037] Substituting equation (2) into equation (1), the CSDM of the LGSMV beam propagating from the source plane (z = 0) to the receiver plane (z > 0) in atmospheric turbulence can be given by the following equation:

[0038]

[0039] Substituting equation (4) into equation (5), Wij(ρ,ρ;z) can be transformed into:

[0040]

[0041] In the formula, θ d =θ1-θ2

[0042]

[0043] Using the integral formula

[0044]

[0045] For the variable r in equation (6) c Integrating, we get:

[0046]

[0047] In equation (9), the spiral phase factor exp(-ilqd) can be expressed in Cartesian coordinates as:

[0048] exp(-ilθ d )=[r dx -isgn(l)r dy ] l (10);

[0049]

[0050] Furthermore, the Laguerre polynomial L0 p(r² d / 2d² ij) can be expressed in Cartesian coordinates as:

[0051]

[0052] Substituting equations (10) to (12) into equation (9), and expanding the position vectors ρc, ρd, and rd in the integrand of equation (13) in a rectangular coordinate system, i.e., rc = (rcx, rcy), rd = (rcx, rcy), rd = (rdx, rdy), then equation (9) can be equivalent to:

[0053]

[0054] According to the integral formula:

[0055]

[0056] By integrating the variables rdx and rdy in equation (13) in sequence, and setting ρ1=ρ2=ρc=ρ, ρd=0, we can obtain the CSDM of the LGSMV beam on the observation plane after propagation through atmospheric turbulence as follows:

[0057]

[0058] in,

[0059]

[0060] Step 2: Using the CSDM signal light obtained in Step 1, calculate the four Stokes parameters of the LGSMV beam at the receiving plane: S0, S1, S2, S3.

[0061] Step 2 shall be carried out in accordance with the following specific steps:

[0062] Using the LGSMV beam obtained in step 1, after transmission through atmospheric turbulence, the CSDM of the receiving plane is used to calculate four Stokes parameters according to equation (17): S0, S1, S2, S3.

[0063]

[0064] Based on equation (17) in step 2 and equation (15) in step 1, the Stokes parameters for the receiving plane z>0 are obtained as follows:

[0065]

[0066] Step 3: Based on the four Stokes parameters obtained in Step 2, derive the mathematical model of the polarization direction angle of the LGSMV beam after transmission through atmospheric turbulence.

[0067] Step 3 shall be carried out in accordance with the following specific steps:

[0068] Step 3.1 Using the generalized Stokes parameters to describe the polarization direction angle of the LGSMV beam, the mathematical model of the polarization direction angle of the LGSMV beam receiving plane is expressed as:

[0069]

[0070] The range of θ(ρ,z) is from -π / 2 to π / 2.

[0071] Step 3.2 Substitute the CSDM expression (Equation (15)) and the corresponding parameter settings (Equation (16)) of the LGSMV beam at the receiving end in Step 1 into Step 3.1 (Equation (19)) to finally obtain the polarization direction angle distribution of the LGSMV beam after transmission through atmospheric turbulence.

[0072] Step 4: Using the four Stokes parameters obtained in Step 2, derive the mathematical model of the ellipticity of the LGSMV beam after transmission through atmospheric turbulence.

[0073] Step 4 shall be carried out in accordance with the following specific steps:

[0074] Using the generalized Stokes parameter to describe the ellipticity of the LGSMV beam, the mathematical model of the ellipticity of the LGSMV beam in the receiving plane is expressed as:

[0075]

[0076] Where Ama and Ami are the amplitudes of the principal and secondary half-axis, respectively, and can be expressed as:

[0077]

[0078] Step 4.2 Substitute the CSDM expression (Equation (15)) and the corresponding parameter settings (Equation (16)) of the LGSMV beam at the receiving end in step 1 into step 3.1 (Equation (21)), and substitute step 3.1 (Equation (21)) into step 3.1 (Equation (20)) to finally obtain the ellipticity distribution of the LGSMV beam after atmospheric turbulence transmission.

[0079] Step 5: Based on the expressions for polarization direction angle and ellipticity obtained in steps 3.2 and 4.2, the polarization direction angle and ellipticity distribution of the LGSMV beam after passing through atmospheric turbulence can be obtained. The changes in polarization direction angle and ellipticity of the LGSMV beam under different topological charge numbers and radial indices are compared and analyzed, realizing the detection of the magnitude and sign of the topological charge number based on the polarization direction angle and ellipticity of the LGSMV beam.

[0080] Step 5 shall be carried out in accordance with the following specific steps:

[0081] Step 5.1: Based on the CSDM and Stokes parameter expressions of the LGSMV beam obtained in Steps 1 and 2, and combined with equation (19) in Step 3.1, numerical simulations and comparative analyses are performed on the polarization direction angle distribution of the LGSMV beam when the radial exponent p = 0, l = ±1, ±2, ±3, ±5, respectively. Figure 2 As shown;

[0082] Step 5.2: Based on the CSDM and Stokes parameter expressions of the LGSMV beam obtained in Steps 1 and 2, and combined with equation (19) in Step 3, numerical simulations and comparative analyses are performed on the polarization direction angle distribution of the LGSMV beam when the radial exponents p = 5, 10, 30, and l = 1, 2, 3, 5, respectively. Figure 3 As shown;

[0083] Step 5.3 Based on the CSDM and Stokes parameter expressions of the LGSMV beam obtained in Steps 1 and 2, and combined with equations (20) and (21) in Step 4, numerical simulations and comparative analyses are performed on the ellipticity distribution of the LGSMV beam when p = 0, l = ±1, ±2, ±3, and ±5, respectively. Figure 4 As shown;

[0084] Step 5.4: Based on the CSDM and Stokes parameter expressions of the LGSMV beam obtained in Steps 1 and 2, and combined with equations (20) and (21) in Step 3, numerical simulations and comparative analyses are performed on the ellipticity distribution of the LGSMV beam when the radial exponents p = 5, 10, 30, and l = 1, 2, 3, 5, respectively. Figure 5 As shown.

[0085] This invention discloses a method for detecting the OAM mode of a Laguerre-Gauss-Schel vortex beam using polarization direction angle and ellipticity, and the experimental setup to which it can be applied is as follows: Figure 1 As shown, the process includes the generation of the Laguerre-Gauss-Sher (LGSMV) beam (using a partially coherent Laguerre-Gauss beam passed through a spatial light modulator (SLM) with a fork grating to generate a partially coherent (vortex beam LGSMV) beam); the LGSMV beam is transmitted through an atmospheric turbulent medium to the receiving end, where a beam quality analyzer is used to detect information such as the beam spot, invert the polarization direction angle and ellipticity of the LGSMV beam, and finally detect the size and direction of its OAM mode.

[0086] Step 1: According to equations (15), (17), and (19), the polarization direction angle of the LGSMV beam when it reaches the receiving plane after being transmitted through atmospheric turbulence can be calculated.

[0087] Step 2: Unless otherwise specified, set the MATLAB simulation parameters as follows: LGSMV beam wavelength λ = 632.8 nm; coherence length δ xx =0.02m, δ yy =0.01m, δ xy =δ yx =0.025m; electric field amplitude A in the x and y directions x =A y =1; beam waist radius w in the x and y directions 0x =0.01m, w 0y =0.02m; B ij =0.5exp(iπ / 4) reflects the degree of correlation between components and should satisfy B. ij max{δ Sxx ,δ Syy}≤δ Sxy ≤min{δ Sxx / |B xy | 0.5 ,δ Syy / |B xy | 0.5} and max{δ Lxx ,δ Lyy}≤δ Lxy ≤min{δ Lxx / |B xy | 0.5 ,δ Lyy / |B xy | 0.5 Atmospheric refractive index structure constant C0 = 1.7 × 10⁻⁶ -14 m -2 / 3 Transmission distance z = 1000m, internal scale of atmospheric turbulence l0 = 0.01m, external scale of atmospheric turbulence L0 = 100m; Figure 1 It is the polarization direction angle distribution of the LGSMV beam after propagation through atmospheric turbulence with radial exponent p = 0 and topological charge l = ±1, ±2, ±3, ±5; Figure 2 This describes the polarization direction angle distribution of the LGSMV beam after propagation through atmospheric turbulence at radial exponents p = 5, 10, 30 and topological charge l = 1, 2, 3, 5; observation. Figure 1 It can be concluded that the polarization direction angular distribution of the LGSMV beam is petal-shaped, and the number of petals is twice the size of the topological charge. Furthermore, the rotation direction of the petals is related to the sign of the topological charge value; when the topological charge value is positive, the petals rotate counterclockwise; when the topological charge value is negative, the petals rotate clockwise. Observation Figure 2It can be concluded that when the radial exponent is non-zero: in the polarization direction angle distribution diagram of an LGSMV beam with a topological charge l = 2, the petals are divided into two parts; when the topological charge l = 3, the petals are divided into upper, middle, and lower parts; and when the topological charge l = 5, the petals are divided into five parts. Therefore, it can be inferred that when the radial exponent is non-zero, the petals in the polarization direction angle distribution diagram of the LGSMV beam will ultimately be divided into a portion equal to the topological charge l. Figure 1 , Figure 2 The results show that the magnitude and sign of the topological charge number can be detected by using the polarization direction angular distribution of the LGSMV beam.

[0088] Example 2

[0089] Step 1: Based on formulas (15), (17), (20), and (21), the ellipticity of the LGSMV beam when it reaches the receiving plane after being transmitted through atmospheric turbulence can be calculated.

[0090] Step 2: Unless otherwise specified, set the MATLAB simulation parameters as follows: LGSMV beam wavelength λ = 632.8 nm; coherence length δ xx =0.02m, δ yy =0.01m, δ xy =δ yx =0.025m; electric field amplitude A in the x and y directions x =A y =1; beam waist radius w in the x and y directions 0x =0.01m, w 0y =0.02m; B ij =0.5exp(iπ / 4) reflects the degree of correlation between components and should satisfy B. ij max{δ Sxx ,δ Syy}≤δ Sxy ≤min{δ Sxx / |B xy | 0.5 ,δ Syy / |B xy | 0.5} and max{δ Lxx ,δ Lyy}≤δ Lxy ≤min{δ Lxx / |B xy | 0.5 ,δ Lyy / |B xy | 0.5 Atmospheric refractive index structure constant C0 = 1.7 × 10⁻⁶ -14 m-2 / 3 Transmission distance z = 1000m, internal scale of atmospheric turbulence l0 = 0.01m, external scale of atmospheric turbulence L0 = 100m; Figure 3 It is the ellipticity distribution of the LGSMV beam after transmission through atmospheric turbulence with radial exponent p = 0 and topological charge l = ±1, ±2, ±3, ±5. Figure 4 This describes the ellipticity distribution of the LGSMV beam after propagation through atmospheric turbulence at radial exponents p = 5, 10, 30 and topological charge numbers l = 1, 2, 3, 5; observation. Figure 3 It can be concluded that, similar to the polarization direction angular distribution of the LGSMV beam, the ellipticity distribution of the LGSMV beam also exhibits a petal-like pattern, with the number of petals being twice the size of the topological charge. Furthermore, the rotation direction of the petals is related to the sign of the topological charge value; when the topological charge value is positive, the petals rotate counterclockwise; when the topological charge value is negative, the petals rotate clockwise. (Observation) Figure 4 It can be concluded that when the radial exponent is non-zero: the ellipticity distribution diagram of the LGSMV beam with a topological charge l = 2 shows two dark regions dividing the petals into upper, middle, and lower parts; when the topological charge l = 3, the ellipticity distribution diagram shows three dark regions dividing the petals into four parts; and when the topological charge l = 5, the ellipticity distribution diagram shows five dark regions dividing the petals into six parts. Therefore, it can be inferred that when the radial exponent is non-zero, the ellipticity distribution diagram of the LGSMV beam will ultimately show l dark regions dividing the petals into l+1 parts. Figure 3 , Figure 4 The results show that the magnitude and sign of the topological charge number can be detected by using the ellipticity distribution of the LGSMV beam.

[0091] The above description is merely a preferred embodiment of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A method for detecting the OAM mode of a vortex beam using polarization characteristics, characterized in that, Includes the following steps: Calculate the observation cross-spectral density matrix of the vortex beam at the receiving plane after transmission through atmospheric turbulence, and calculate the Stokes parameter at the receiving plane based on the observation cross-spectral density matrix; calculate the polarization direction angle and ellipticity of the vortex beam after transmission through atmospheric turbulence based on the Stokes parameter, and perform OAM mode detection based on the polarization direction angle and ellipticity.

2. The method for detecting the OAM mode of a vortex beam using polarization characteristics according to claim 1, characterized in that, The process of obtaining the observed cross spectral density matrix includes: obtaining the propagation information of the vortex beam, the position coordinates of the receiving plane, and the position coordinates of the source plane, and constructing the cross spectral density matrix expression of the receiving plane based on the obtained information; obtaining the cross spectral density matrix expression of the source plane; and calculating the observed cross spectral density matrix by combining the atmospheric turbulence power spectrum model and wave structure function. The propagation information includes wavelength, transmission distance, wave number, and amplitude.

3. The method for detecting the OAM mode of a vortex beam using polarization characteristics according to claim 2, characterized in that, The amplitude and waist radius of the vortex beam in the x and y directions are obtained, and the coherence length of the vortex beam at the plane z=0 is obtained. Based on the amplitude, waist radius, Laguerre polynomial, topological charge, and coherence length, the expression of the source plane cross spectral density matrix is ​​calculated.

4. The method for detecting the OAM mode of a vortex beam using polarization characteristics according to claim 2, characterized in that, The process of calculating the observed cross-spectral density matrix also includes: constructing a wave structure function based on the internal and external scales of atmospheric turbulence and the atmospheric refractive index structure constant; using the wave structure function as the source plane position vector function; substituting the expression of the source plane cross-spectral density matrix into the expression of the receiver plane cross-spectral density matrix; transforming the expression by combining the wave structure function; integrating the source plane variables in the transformed formula to form a new formula; representing the spiral phase factor and Laguerre polynomial in Cartesian coordinates and substituting them into the new formula; and integrating the receiver plane variables in the new formula to obtain the observed cross-spectral density matrix.

5. The method for detecting the OAM mode of a vortex beam using polarization characteristics according to claim 1, characterized in that, The calculation process of the Stokes parameters at the receiving plane includes: obtaining the formula for calculating the initial value of the Stokes parameters, and obtaining the Stokes parameters of the receiving plane z>0 based on the observed cross spectral density matrix and the formula for calculating the initial value of the Stokes parameters.

6. The method for detecting the OAM mode of a vortex beam using polarization characteristics according to claim 1, characterized in that, The process of obtaining the polarization direction angle includes: describing the polarization direction angle of the vortex beam according to the Stokes parameter, obtaining a mathematical model of the polarization direction angle, solving the mathematical model of the polarization direction angle according to the observed cross spectral density matrix and the corresponding parameter settings, and obtaining the distribution of the polarization direction angle.

7. The method for detecting the OAM mode of a vortex beam using polarization characteristics according to claim 1, characterized in that, The ellipticity of the vortex beam is described by the Stokes parameters. A mathematical model of the ellipticity of the vortex beam in the receiving plane, represented by the amplitudes of the main and secondary half-axis, is obtained. The mathematical model of the ellipticity of the vortex beam is solved according to the observed cross spectral density matrix and the corresponding parameter settings to obtain the ellipticity distribution.

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