Multi-core oam mode demultiplexer device and method based on inverse design of spiral transformation

Abstract

The application provides a multi-core OAM mode demultiplexer and method based on spiral transformation reverse design, the application sets an initial spiral coordinate transformation phase plate for reverse design optimization, which is an optical transformation phase plate, a phase correction phase plate and an elliptical lens phase plate; input vortex modes are sequentially propagated through the three phase plates and evolved into output spatially separated Gaussian-like modes, the phase distribution of the three phase plates is optimized by a wavefront matching method, the average overlap integral of the output Gaussian-like mode and the ideal Gaussian mode is taken as an objective function, and iteration is performed until convergence, so that the corresponding three phase plates optimized by reverse design are obtained, the number of modes supported by the photon orbital angular momentum mode demultiplexer after reverse design optimization is improved by nearly one time, and the output beam quality is improved; the application is beneficial to compatibility of 19-core OAM fiber communication, and has important significance for development of the field of super-large capacity dense OAM space division multiplexing.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of optical communication technology, and in particular to a multi-core OAM mode demultiplexer and method based on spiral transformation reverse design. BACKGROUND

[0002] With the rapid development of information technology centered on the Internet, including mobile Internet, Internet of Things, cloud computing, big data, etc., the amount of information is rapidly expanding, and higher requirements are put forward for the transmission capacity of optical communication systems.

[0003] At present, the space for improving the communication capacity of traditional channel multiplexing technologies such as wavelength division multiplexing, time division multiplexing and polarization multiplexing has reached the limit. In order to further greatly improve the communication capacity, space division multiplexing technology has become an important research direction in the current communication field. Due to the characteristics of phase singularity and theoretically infinite orthogonal OAM mode space of vortex beams, it has shown broad application prospects in many fields such as optical communication, optical micro-manipulation, optical imaging and quantum information. In particular, OAM space division multiplexing technology has become a promising technology to cope with the growing communication capacity, and an optical vortex mode sorter, i.e. an OAM mode demultiplexer, is needed to support multiple mode numbers, multiple core numbers and large bandwidths to realize accurate optical coupling between multi-core OAM fibers and single-mode fiber arrays. Several schemes for OAM mode demultiplexing have been proposed, such as Dammann gratings, photonic lanterns, 3D direct writing waveguides, optical coordinate transformation, etc.

[0004] Compared with other OAM demultiplexing technologies, the vortex beam coordinate transformation technology based on the ray model is an efficient and high-resolution solution for dense OAM space division multiplexing. However, the vortex mode demultiplexing performance is limited by the paraxial approximation and the plane wave approximation, and the high-order vortex mode has a large divergence angle, gradually deviating from the approximation condition, causing phase distortion. Thus, the number of modes that can be supported is limited, and the quality of the transformed beam is reduced. Although this problem can be alleviated by increasing the size of the phase plate and by non-paraxial phase correction to compensate for the phase distortion, increasing the size of the phase plate is not suitable for integration and array, and non-paraxial phase correction cannot be used to flexibly adjust the configuration of the input and output beam sizes, which is not suitable for the mutual matching between multi-core multi-mode fibers and single-mode fiber arrays. SUMMARY

[0005] In view of the deficiencies of the prior art, the present application provides a multi-core OAM mode demultiplexer and method based on spiral transformation reverse design, which can be extended to a high-resolution spatial separation of more vortex mode numbers.

[0006] The technical scheme of the present application is: a multi-core OAM mode demultiplexing method based on spiral transformation reverse design, comprising the following steps:

[0007] S1), the helical transformation phase plate obtained by solving based on the ray model is taken as the initial input of the wavefront matching method;

[0008] S2), the perfect vortex mode with a constant central bright ring radius is taken as the incident vortex beam, and the mode field distribution PV of the incident vortex beam and the mode field distribution of the output beam are constructed;

[0009] S3), the mode field distribution of the incident perfect vortex beam with different topological charges propagating forward to the mth phase plate and the mode field distribution of the output ideal Gaussian mode propagating backward to the position of the mth phase plate are calculated by the wavefront matching method;

[0010] S4), when the phases on both sides of the mth phase plate do not match, the phase error of each spatial coordinate point on the mth phase plate is calculated, and the average value of the phase error of each group of input and output modes is taken as the phase distribution of the mth phase plate to be updated;

[0011] S5), all phase plates are traversed as an iteration of the wavefront matching method, and the overlap integral of each input and output mode is calculated again to obtain the average mode conversion efficiency of this generation, and the difference σ0 between the average mode conversion efficiencies of adjacent two generations is taken as the condition for determining whether the iteration converges;

[0012] S6), three phase plates are obtained by reverse design optimization;

[0013] S7), the incident perfect vortex beams with different topological charges pass through the phase modulation and propagation in turn, and the light field profile gradually evolves into a Gaussian-like mode, and the centers of the vortex beams of each core are aligned with the center position of the transformation phase plate; the input vortex beams with different topological charges are converted into Gaussian beams at different positions on the output plane, realizing the spatial separation of different vortex modes.

[0014] As preferred, in step S1), the initial input phase plates are optical transformation phase plates, phase correction phase plates, and elliptical lens phase plates.

[0015] As preferred, in step S1), the conversion relationship of the spiral coordinate transformation from the (r, θ) coordinate plane to the (u, v) coordinate plane is:

[0016]

[0017] wherein a, r0, β are all spiral line parameters, (r, θ) represents the polar coordinates of the vortex beam, and (u, v) is the position coordinates of the phase correction phase plate.

[0018] Preferably, in step S1), the phase distribution of the optical transformation phase plate and the phase correction phase plate corresponding to the spiral transformation is obtained by solving the following set of partial differential equations based on the ray model:

[0019]

[0020]

[0021] In the formula, P1 and P2 represent the phases of the optical transformation phase plate and the phase correction phase plate, respectively; k = 2π / λ is the wave number; (x, y) are the position coordinates of the optical transformation phase plate; (u, v) are the position coordinates of the phase correction phase plate; and d represents the distance between the optical transformation phase plate and the phase correction phase plate.

[0022] Preferably, in step S1), the elliptical lens phase plate corresponding to the spiral transformation is denoted as P3, and its phase distribution is expressed as follows:

[0023]

[0024] In the formula, f is the focal length of the elliptical lens; (x,y) are the position coordinates of the phase plate of the elliptical lens.

[0025] Preferably, in step S2), the expression for the mode field distribution PV(r,θ) of the incident vortex beam is:

[0026]

[0027] In the formula, (r,θ) represents the polar coordinates of the vortex beam; r max w0 represents the maximum radius of the central bright ring; l represents the topological charge number; and i represents the imaginary unit.

[0028] Preferably, in step S2), the output beam is a Gaussian beam, and the mode field distribution G(r,θ) of the output beam is:

[0029]

[0030] In the formula, w is the waist radius of the Gaussian beam; (r,θ) represents the polar coordinates of the vortex beam.

[0031] Preferably, in step S3), the vortex beam with incident topological charge number l propagates in the forward direction to the mode field distribution of the m-th phase plate. Represented as:

[0032]

[0033] The mode field distribution of the output ideal Gaussian beam, corresponding one-to-one with the input mode, propagating backward to the m-th phase plate. Represented as:

[0034]

[0035] wherein, is the Fourier transform, and respectively represent the forward phase modulation of the mth phase plate and the reverse phase modulation of the m+1th phase plate, H is the beam propagation function; (x, y, z m ) represents the three-dimensional rectangular coordinate system in the physical space; k x , k y , z respectively represent the wave number component of the light wave in the transverse x, y direction and the coordinate of the light wave along the propagation direction.

[0036] As preferred, in step S4), specifically comprises the following steps:

[0037] S41), set the input and output beam power normalization, the input vortex mode forward propagation to the complex amplitude distribution of the mth phase plane is:

[0038]

[0039] In the formula, A f represents the amplitude distribution of the input light field forward propagation to the mth phase plane; i represents the imaginary unit, φ f represents the phase distribution of the input light field forward propagation to the mth phase plane;

[0040] S42), the output ideal Gaussian mode reverse propagation to the complex amplitude distribution of the mth phase plane is:

[0041]

[0042] In the formula, A b represents the amplitude distribution of the output ideal light field reverse propagation to the mth phase plane; φ b represents the phase distribution of the output ideal light field reverse propagation to the mth phase plane;

[0043] S43), calculate the phase distribution that makes the phase mismatch of the two perfect compensation is φ = -φ f + φ b ;

[0044] S44), by calculating the average value of the phase error of each group of input and output modes, as the phase distribution required to update the mth phase plate.

[0045] As preferred, in step S5), the input mode propagates to the output plane through a series of phase plane modulations, and the corresponding output mode complex amplitude distribution is u1(x, y), and the ideal output mode complex amplitude distribution is u2(x, y), and the overlap integral I of the two is:

[0046]

[0047] In the formula, represents the conjugate output mode complex amplitude distribution;

[0048] The overlap integral of each output mode and the ideal output mode is calculated, and the average value is denoted as the average mode conversion efficiency of this generation.

[0049] As preferred, in step S5), when the average mode conversion efficiency difference σ0 between the adjacent two generations is less than a set threshold, it is determined whether the current iteration optimization converges.

[0050] As preferred, in step S6), three phase plates are obtained by reverse design optimization, and the specific steps are as follows:

[0051] The optical transformation phase plate, the phase correction phase plate and the elliptical lens phase plate obtained by analytical solution of the partial differential equation are used as the initial input phase distribution, and the wavefront matching method is iterated until the set iteration number is reached or the convergence condition is reached, and then the iteration is stopped, to obtain the optical transformation phase plate, the phase correction phase plate and the elliptical lens phase plate after reverse optimization reconstruction.

[0052] As preferred, in step S7), the spatial separation of different vortex modes is realized, and the specific steps include that the doughnut-shaped intensity distribution vortex mode propagates after being modulated by the reverse reconstructed optical transformation phase plate, and is gradually expanded into a long strip light spot, and then sequentially passes through the reverse reconstructed phase correction phase plate and the elliptical lens phase plate, and propagates to the output plane, and the long strip light spot is gradually propagated and evolved into spatially separated different position Gaussian light spots.

[0053] As preferred, the application further provides a multi-core OAM mode demultiplexing device based on spiral transformation reverse design, which comprises a double-sided diffractive optical element, a single-sided diffractive optical element, and an optical transformation phase plate, a phase correction phase plate and an elliptical lens phase plate.

[0054] The optical transformation phase plate and the phase correction phase plate are respectively located on the front and back surfaces of the double-sided diffractive optical element; and the elliptical lens phase plate is integrated on one side surface of the single-sided diffractive optical element.

[0055] As preferred, the optical transformation phase plate, the phase correction phase plate and the elliptical lens phase plate are respectively arranged in a hexagonal array with 19 cores, and the incident 19-core array vortex beam sequentially passes through the arrayed optical transformation phase plate, the phase correction phase plate and the elliptical lens phase plate to be modulated and propagated, and is converted into a spatially separated Gaussian-like beam at the output plane, so that the vortex mode is sorted.

[0056] As preferred, the double-sided diffraction optical element and the single-sided diffraction optical element are quartz plates with different thicknesses, wherein the thickness of the double-sided diffraction optical element is 5 mm, and the thickness of the single-sided diffraction optical element is 1 mm.

[0057] The present application has the following advantages:

[0058] 1. The present application uses reverse design wavefront matching iterative optimization spiral transformation phase distribution, which breaks through the limitation of previous optical coordinate transformation calculation phase distribution based on ray model for paraxial approximation and plane wave approximation;

[0059] 2. Compared with the previous spiral transformation, the optimized optical transformation can be extended to high-resolution spatial separation of more vortex mode numbers, i.e. OAM mode demultiplexing.

[0060] 3. Compared with the previous spiral transformation input-output beam configuration which is not controllable, the 19-core photon orbital angular momentum mode demultiplexing device based on reverse design spiral transformation provided by the present application can be beneficial to compatible 19-core OAM fiber communication, and has important significance for the development of super-capacity dense OAM space division multiplexing field. BRIEF DESCRIPTION OF DRAWINGS

[0061] Figure 1 It is a comparison diagram of three phase plates in the embodiment of the present application, wherein (a) is a schematic diagram of the spiral transformation phase plate; (b) is a schematic diagram of the reverse design optimized phase plate; the phase plates 1-3 are respectively an optical transformation phase plate, a phase correction phase plate and an elliptical lens phase plate.

[0062] Figure 2 It is a comparison diagram of demultiplexing light spots and light intensity profiles in the embodiment of the present application, wherein (a) is a demultiplexing light spot and light intensity profile diagram of the spiral transformation; (b) is an optical transformation demultiplexing light spot and light intensity profile diagram of the reverse design optimization.

[0063] Figure 3 It is a structure schematic diagram of the 19-core photon orbital angular momentum demultiplexing device based on reverse design optimization of spiral transformation in the embodiment of the present application.

[0064] Figure 4 It is a demultiplexing light spot diagram of the 19-core photon orbital angular momentum demultiplexing device based on reverse design optimization of spiral transformation in the embodiment of the present application.

[0065] In the figure, 1 - double-sided diffractive optical element; 2 - single-sided diffractive optical element; 3 - optical transform phase plate; 4 - phase correction phase plate; 5 - elliptical lens phase plate. DETAILED DESCRIPTION

[0066] The specific embodiments of the present application will be further described below in conjunction with the accompanying drawings:

[0067] Example 1

[0068] The present embodiment provides a multi-core OAM mode demultiplexing method based on spiral transform inverse design, comprising the following steps:

[0069] S1), the spiral transform phase plate obtained by analytical solution based on the ray model is taken as the initial input of the wavefront matching method; as shown in (a); in the present embodiment, the initial input phase plate is optical transform phase plate 3, phase correction phase plate 4, and elliptical lens phase plate 5, respectively. Figure 1 (a) as shown; in the present embodiment, the initial input phase plate is optical transform phase plate 3, phase correction phase plate 4, and elliptical lens phase plate 5, respectively.

[0070] Wherein, the conversion relationship of spiral coordinate transformation from (r, θ) coordinate plane to (u, v) coordinate plane is:

[0071]

[0072] Wherein, a, r0, β are all spiral line parameters, (r, θ) represents the polar coordinates of vortex beam, and (u, v) is the position coordinates of the phase correction phase plate.

[0073] The phase distribution of the optical transform phase plate and the phase correction phase plate corresponding to the spiral transformation is obtained by solving the following partial differential equation group based on the ray model:

[0074]

[0075]

[0076] Wherein, P1, P2 represent the phase of the optical transform phase plate and the phase correction phase plate, respectively, k = 2π / λ is the wave number; (x, y) is the position coordinates of the optical transform phase plate; (u, v) is the position coordinates of the phase correction phase plate; d represents the distance between the optical transform phase plate and the phase correction phase plate.

[0077] The elliptical lens phase plate corresponding to the spiral transformation is denoted as P3, and its phase distribution is represented as:

[0078]

[0079] Wherein, f is the focal length of the elliptical lens; (x, y) is the position coordinates of the elliptical lens phase plate.

[0080] S2), the perfect vortex mode with a constant central bright ring radius regardless of the change of the topological charge is used as the incident vortex beam; and the mode field distribution PV of the incident vortex beam and the mode field distribution G(r, θ) of the output beam are constructed;

[0081] The mode field distribution PV(r, θ) of the incident vortex beam is expressed as:

[0082]

[0083] In the formula, (r, θ) represents the polar coordinates of the vortex beam; r max represents the maximum radius of the central bright ring; w0 represents the beam ring width; l represents the topological charge; and i represents the imaginary unit.

[0084] The output beam is a Gaussian beam, and the mode field distribution G(r, θ) of the output beam is:

[0085]

[0086] In the formula, w is the waist radius of the Gaussian beam; and (r, θ) represents the polar coordinates of the vortex beam.

[0087] S3), the mode field distribution of the incident perfect vortex beam with different topological charges propagating forward to the m-th phase plate and the mode field distribution of the output ideal Gaussian mode propagating reversely to the position of the m-th phase plate are calculated by using the wavefront matching method.

[0088] In this embodiment, the mode field distribution of the incident vortex beam with a topological charge of l propagating forward to the m-th phase plate is represented as:

[0089]

[0090] The mode field distribution of the output ideal Gaussian beam corresponding to the input mode propagating reversely to the m-th phase plate is represented as:

[0091]

[0092] wherein, is the Fourier transform, and respectively represent the forward phase modulation of the m-th phase plate and the reverse phase modulation of the m+1-th phase plate, H is the beam transfer function; (x, y, z m ) represents the three-dimensional rectangular coordinate system in the physical space; k x , k y , and z respectively represent the wave number components of the light wave in the transverse x and y directions and the coordinate of the light wave along the propagation direction.

[0093] S4), when the phase of the two sides of the mth phase plate does not match, the phase error of each spatial coordinate point on the mth phase plate is calculated; and the average value of the phase error of each input and output mode is taken as the phase distribution of the mth phase plate to be updated; specifically comprising the following steps:

[0094] S41), assuming that the input and output beam powers are normalized, the complex amplitude distribution of the input vortex mode propagating forward to the mth phase plane is

[0095]

[0096] wherein A f represents the amplitude distribution of the input light field propagating forward to the mth phase plane; i represents an imaginary unit, and φ f represents the phase distribution of the input light field propagating forward to the mth phase plane.

[0097] S42), the complex amplitude distribution of the output ideal Gaussian mode propagating backward to the mth phase plane is

[0098]

[0099] wherein A b represents the amplitude distribution of the output ideal light field propagating backward to the mth phase plane; and φ b represents the phase distribution of the output ideal light field propagating backward to the mth phase plane.

[0100] S43), the phase distribution that perfectly compensates for the phase mismatch between the two is calculated as φ = -φ f + φ b .

[0101] S44), by calculating the average value of the phase error of each input and output mode, the phase distribution required to be updated for the mth phase plate is obtained.

[0102] S5), all the phase plates are traversed as an iteration of the once wavefront matching method, and the overlap integral of each input and output mode is calculated again to obtain the average mode conversion efficiency of this generation, and the difference σ0 between the average mode conversion efficiencies of adjacent two generations is taken as the condition for judging whether the iteration converges or not; specifically:

[0103] The input mode propagates through a series of phase planes to the output plane to obtain the corresponding output mode complex amplitude distribution u1(x, y), and the ideal output mode complex amplitude distribution is u2(x, y), and the overlap integral I of the two is:

[0104]

[0105] wherein​​ represents the conjugate output mode complex amplitude distribution.

[0106] The overlap integral of each output mode with the ideal output mode is calculated, averaged, and denoted as the average mode conversion efficiency of this generation.

[0107] When the difference of the average mode conversion efficiency between two adjacent generations σ0 is less than a set threshold, it is determined whether the current iteration optimization converges.

[0108] S6), three phase plates are obtained by inverse design optimization, as shown in Figure 1 (b), the optical transformation phase plate, the phase correction phase plate, and the elliptical lens phase plate obtained by analytically solving the partial differential equation are used as initial input phase distributions, and the wavefront matching method is iterated until a set number of iterations is reached or a convergence condition is reached, and then the optical transformation phase plate, the phase correction phase plate, and the elliptical lens phase plate after inverse optimization reconstruction are obtained.

[0109] S7), the perfect vortex beams with different topological charges are sequentially modulated by the phase and propagate, and the light field profile gradually evolves into a Gaussian-like mode, and the vortex beam center of each core is aligned with the center position of the transformation phase plate; the input vortex beams with different topological charges are converted into Gaussian beams at different positions on the output plane, realizing spatial separation of different vortex modes.

[0110] In this embodiment, the vortex mode with a donut-shaped intensity distribution is modulated by the inverse reconstruction optical transformation phase plate and propagates, gradually expanding into a long strip of light spots, and then sequentially modulates and propagates through the inverse reconstruction phase correction phase plate and the elliptical lens phase plate to the output plane, and the long strip of light spots gradually evolves into spatially separated Gaussian-like light spots at different positions. As shown in Figure 2 (a) and Figure 2 (b), the OAM mode demultiplexing effect achieved by the original spiral transformation is compared, and the inverse design optimization can support more vortex mode numbers (-10 to +10 OAM modes, a total of 21) for high-resolution sorting in space. Compared with the spiral transformation, the supported mode number is nearly doubled.

[0111] The embodiment uses the transformation between the input perfect vortex beam and the output ideal Gaussian vortex mode as the initial input, uses the wavefront matching algorithm of reverse design, iteratively optimizes the conversion between the support-10~+10 order perfect vortex beam and the array of spatially separated Gaussian modes, and provides an arrayed 19-core photonic orbital angular momentum demultiplexing device prepared into a diffractive optical element.

[0112] Embodiment 2

[0113] As shown in Figure 3 , the embodiment provides a multi-core OAM mode demultiplexing device based on spiral transformation reverse design, which comprises a double-sided diffractive optical element 1, a single-sided diffractive optical element 2, and an optical transformation phase plate 3, a phase correction phase plate 4, and an elliptical lens phase plate 5.

[0114] The optical transformation phase plate 3 and the phase correction phase plate 4 are respectively located on the front and back surfaces of the double-sided diffractive optical element 1; and the elliptical lens phase plate 5 is integrated on one side surface of the single-sided diffractive optical element 2.

[0115] In the embodiment, the optical transformation phase plate 3, the phase correction phase plate 4, and the elliptical lens phase plate 5 are respectively arranged in a hexagonal array of 19 cores, and the incident 19-core array vortex beam is sequentially modulated and propagated through the arrayed optical transformation phase plate 3, the phase correction phase plate 4, and the elliptical lens phase plate 5, and is converted into spatially separated Gaussian-like beams at the output plane, thereby realizing the sorting of vortex modes.

[0116] In the embodiment, the double-sided diffractive optical element 1 and the single-sided diffractive optical element 2 are quartz plates with different thicknesses, wherein the thickness of the double-sided diffractive optical element 1 is 5 mm, and the thickness of the single-sided diffractive optical element 2 is 1 mm. The demultiplexed spots output after the 21 vortex beams with different topological charges pass through each core of the device are as shown in Figure 4 .

[0117] The above embodiments and descriptions are only to illustrate the principles and the best embodiments of the present application, and various changes and improvements can be made without departing from the spirit and scope of the present application, and these changes and improvements all fall within the scope of the claimed present application.

Claims

1. A multi-core OAM mode demultiplexing method based on inverse design of spiral transformation, characterized in that, Comprising the following steps: S1), the helical transformation phase plate obtained by analytical solution based on ray model is taken as the initial input of wavefront matching method; the initial input phase plate is optical transformation phase plate (3), phase correction phase plate (4), and elliptical lens phase plate (5) respectively; S2), perfect vortex mode with constant central bright ring radius is taken as incident vortex beam; and the mode field distribution of incident vortex beam and the mode field distribution of output beam are calculated; S3), the mode field distribution of each incident perfect vortex beam with different topological charge propagating forward to the mth phase plate and the mode field distribution of output ideal Gaussian mode propagating backward to the position of the mth phase plate are calculated by wavefront matching method; S4), when the phase on both sides of the mth phase plate is not matched, the phase error of each spatial coordinate point on the mth phase plate is calculated; and the average value of the phase error of each group of input and output mode is taken as the phase distribution that needs to be updated for the mth phase plate; S5), all phase plates are traversed as an iteration of wavefront matching method, and the overlap integral of each input and output mode is calculated again to obtain the average mode conversion efficiency of this generation; the difference σ0 between the average mode conversion efficiencies of adjacent two generations is taken as the condition for judging whether the iteration converges or not; S6), three phase plates are obtained by reverse design optimization; specifically as follows: The optical transformation phase plate (3), the phase correction phase plate (4), and the elliptical lens phase plate (5) obtained by analytical solution of partial differential equation are taken as initial input phase distribution; the wavefront matching method is iterated until the set iteration number is reached or the convergence condition is reached, and then the iteration is stopped; the optical transformation phase plate (3), the phase correction phase plate (4), and the elliptical lens phase plate (5) after reverse optimization reconstruction are obtained; S7), incident perfect vortex beams with different topological charges are modulated and propagated in turn, and the light field profile gradually evolves into a Gaussian-like mode; the vortex beam center of each core is aligned with the center position of the transformation phase plate; the input vortex beams with different topological charges are converted into Gaussian beams at different positions on the output plane, realizing the spatial separation of different vortex modes; specifically including the following steps: the doughnut-shaped intensity distribution vortex mode is modulated and propagated after the optical transformation phase plate after reverse reconstruction, gradually expanding into a long strip of light spot, and then modulated and propagated through the phase correction phase plate and the elliptical lens phase plate after reverse reconstruction in turn to the output plane, and the long strip of light spot gradually evolves into a spatially separated Gaussian-like light spot at different positions.

2. The multi-core OAM mode demultiplexing method based on inverse design of spiral transformation according to claim 1, characterized in that: In step S1), the helical transformation phase plate obtained by analytical solution based on ray model comprises the following steps: S11), the conversion relationship of helical coordinate transformation from (r, θ) coordinate plane to (u, v) coordinate plane is: Wherein, a, r0, β are all helix parameters, (r, θ) represents the polar coordinates of vortex beam, and (u, v) is the position coordinates of phase correction phase plate; S12), the phase distribution of optical transformation phase plate (3) and phase correction phase plate (4) corresponding to helical transformation is obtained by solving the following partial differential equation group based on ray model: In the formula, P1 and P2 represent the phases of the optical transformation phase plate (3) and the phase correction phase plate (4) respectively, k=2π / λ is the wave number, (x, y) is the position coordinate of the optical transformation phase plate (3), (u, v) is the position coordinate of the phase correction phase plate (4), and d represents the distance between the optical transformation phase plate (3) and the phase correction phase plate (4). The phase distribution of the spiral transformation corresponding elliptical lens phase plate (5) is denoted as P3, and is expressed as: In the formula, f is the focal length of the elliptical lens, and (x, y) is the position coordinate of the elliptical lens phase plate (5).

3. The method of claim 1, wherein the method is a multi-core OAM mode demultiplexing method based on inverse design of spiral transformation. In step S2), the mode field distribution PV(r, θ) of the incident vortex beam is expressed as: where (r, θ) represents the polar coordinates of the vortex beam; r max represents the maximum radius of the central bright ring; w0represents the beam ring width; l is the topological charge; i represents the imaginary unit; The output beam is a Gaussian beam, and the mode field distribution G(r, θ) of the output beam is: In the formula, w is the waist radius of the Gaussian beam, and (r, θ) represents the polar coordinates of the vortex beam.

4. The multi-core OAM mode demultiplexing method based on inverse design of spiral transformation according to claim 1, characterized in that: In step S3), the incident vortex beam with topological charge number of l is forward propagated to the mode field distribution of the mth phase plate is represented as: The output mode corresponding to the input mode is an ideal Gaussian beam, and the mode field distribution of the output mode propagating reversely to the mth phase plate is shown in the following table: is shown as: wherein, is the Fourier transform, and respectively represent the forward phase modulation of the mth phase plate and the reverse phase modulation of the m+1th phase plate, H is the beam transfer function; (x, y, z m ) represents a three-dimensional rectangular coordinate system in the physical space; k x , k y , and z respectively represent the wave number components of the light wave in the transverse x, y directions and the coordinate of the light wave along the propagation direction.

5. The method of claim 1, wherein the method is a multi-core OAM mode demultiplexing method based on inverse design of spiral transformation. In step S4), the following steps are specifically included: S41), set input and output beam power normalization, input vortex mode forward propagation to the complex amplitude distribution of the mth block phase plane is: wherein A f represents the amplitude distribution of the input light field forward propagating to the m-th block phase plane; i represents the imaginary unit, φ f represents the phase distribution of the input light field forward propagating to the m-th block phase plane; S42), output the ideal Gaussian mode back propagation to the complex amplitude distribution of the mth block phase plane is: In the formula, A b represents the amplitude distribution of the output ideal light field backpropagating to the mth block phase plane; φ b represents the phase distribution of the output ideal light field backpropagating to the mth block phase plane; S43), the phase distribution φ = -φ is calculated which perfectly compensates for the phase mismatch of both f +φ b ; S44), the phase error average value of each group of input and output modes is calculated as the phase distribution required to update the mth phase plate.

6. The multi-core OAM mode demultiplexing method based on inverse design of spiral transformation according to claim 1, characterized in that: In step S5), the input mode propagates through a series of phase planes to the output plane, and the corresponding output mode complex amplitude distribution is u1(x, y), and the ideal output mode complex amplitude distribution is u2(x, y). The overlap integral I of the two is: wherein represents the conjugated output mode complex amplitude distribution; The average value of the overlap integral of each output mode and the ideal output mode is calculated, and is denoted as the average mode conversion efficiency of this generation. When the average mode conversion efficiency difference σ0 between adjacent two generations is less than a set threshold, it is determined whether the current iteration optimization converges. 7.A multi-core OAM mode demultiplexing device based on inverse design of spiral transformation, characterized in that: The device is used in the method of any one of claims 1-6, and the device comprises a double-sided diffractive optical element (1), a single-sided diffractive optical element (2), and an optical transformation phase plate (3), a phase correction phase plate (4), and an elliptical lens phase plate (5). The optical transformation phase plate (3) and the phase correction phase plate (4) are respectively located on the front and back surfaces of the double-sided diffractive optical element (1). The elliptical lens phase plate (5) is integrated on one side of the single-sided diffractive optical element (2). The optical transformation phase plate (3), the phase correction phase plate (4), and the elliptical lens phase plate (5) are each arranged in a hexagonal array with 19 cores. After the incident 19-core array vortex beam is modulated and propagated through the arrayed optical transformation phase plate (3), the phase correction phase plate (4), and the elliptical lens phase plate (5) in turn, the vortex mode is sorted into spatially separated Gaussian-like beams at the output plane.

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