Space division multiplexing communication coding and decoding method based on petal-shaped perfect vortex array

Abstract

The invention discloses a space division multiplexing communication coding and decoding method based on a petal-shaped perfect vortex array, and the method comprises the steps: determining a topological charge value for generating a perfect vortex beam and a corresponding conjugate perfect vortex beam, obtaining petal-shaped perfect vortex light spots through coaxial superposition, and carrying out the one-to-one mapping of the petal-shaped perfect vortex light spots and a binary sequence to form a coding comparison table; dividing a space domain into a plurality of sub-regions for arranging array space distribution; acquiring gray values of RGB channels of each pixel point of the color image, performing binary conversion and splicing on the gray values, segmenting the gray values into a plurality of 3-bit binary sequences, sequentially arranging the 3-bit binary sequences in a plurality of sub-regions in combination with a coding comparison table, and obtaining a hologram through a holographic phase transfer function; after the light beam passes through the hologram, Fourier transform is carried out to obtain a to-be-decoded petal-shaped perfect vortex light spot array, the to-be-decoded petal-shaped perfect vortex light spot array is collected, and decoding information is obtained in combination with the coding comparison table; the problem that the light beam mode cannot be identified due to the fact that the perfect vortex light spot size is not changed is solved.

Description

Technical Field

[0001] The invention belongs to the field of optoelectronic technology and optical communication technology, and specifically relates to a space division multiplexing communication encoding and decoding method based on a petal-shaped perfect vortex array. Background Art

[0002] In recent years, with the rapid development of emerging bandwidth-consuming businesses such as big data, cloud computing, digital twins, and autonomous driving, the demand for data traffic is growing exponentially, which poses a huge challenge to existing communication systems. In order to cope with the ensuing communication capacity crisis, giving full play to the role of optical communications has become the focus of the communications industry. Thanks to the rich spectrum resources, high-confidential communication performance, and fast and convenient system installation of optical communications, it is expected to provide a new solution for the sustainable growth of communication capacity.

[0003] In order to achieve sustainable expansion of optical transmission systems, the amplitude, phase, polarization, frequency / wavelength, and time dimension resources of light waves have been fully developed and utilized. However, the spatial distribution of light waves has not been fully developed and utilized. Therefore, the spatial dimension resources of light waves are widely favored by researchers. Unlike traditional laser beams, vortex beams carry orbital angular momentum, and their wavefronts present a unique spiral spatial form. Therefore, as a spatial structured light, vortex beams have broad application prospects in the field of optical communications. However, it should be pointed out that the spot size of the vortex beam will continue to widen as the value of the beam topological charge increases, resulting in the need for optical receivers to use large-sized optical antennas when capturing vortex beams corresponding to larger topological charges, which seriously restricts the practical application of vortex beams.

[0004] In order to overcome the influence of the spot size changing with the topological charge value, the perfect vortex beam came into being. As a special spatial structured light, the spot size of the perfect vortex beam is independent of the topological charge value. The perfect vortex beam has the advantage of a fixed spot size, but it should be noted that this also brings another problem, that is, the spot size of beams with different topological charge values ​​is the same, which means that they cannot be distinguished from the spot shape alone, which makes it difficult to identify beams of different modes. At present, the interference method is mostly used to distinguish perfect vortex beams of different modes, but because the interference method has strict requirements on the coherence of the light source and the collimation of the optical path, and the actual environment inevitably has interference factors such as shaking and turbulence, which greatly affects the interference effect. Summary of the invention

[0005] The purpose of the present invention is to provide a space-division multiplexing communication encoding and decoding method based on a petal-shaped perfect vortex array, make full use of the constant size characteristic of the perfect vortex light beam, design a vortex array that does not interfere with each other in space, and propose a petal-shaped perfect vortex, which solves the problem of the inability to identify the light beam mode due to the constant size of the perfect vortex spot, and provides an efficient space-division multiplexing and incoherent encoding and decoding method.

[0006] The technical solution adopted by the present invention is a space division multiplexing communication encoding and decoding method based on a petal-shaped perfect vortex array, which is specifically implemented according to the following steps:

[0007] Step 1, determine the topological charge value for generating a perfect vortex beam and the corresponding conjugate perfect vortex beam, coaxially superimpose the perfect vortex beam and the conjugate perfect vortex beam to obtain a petal-shaped perfect vortex spot, and form a coding comparison table by corresponding the topological charge value and the petal-shaped perfect vortex spot one by one;

[0008] Step 2, dividing the space domain into n×n sub-areas according to the rectangular coordinate system quadrant division rule;

[0009] Step 3: Obtain the grayscale value of the three channels of RGB for each pixel of the color image, perform binary conversion and splicing on the grayscale values ​​of the three channels, and divide them into multiple 3-bit binary sequences, and arrange them in n×n sub-regions in sequence according to the coding comparison table, and obtain the hologram through the holographic phase transfer function;

[0010] Step 4: After the light beam passes through the hologram, it is Fourier transformed to obtain the petal-shaped perfect vortex spot array to be decoded, and the petal-shaped perfect vortex spot array to be decoded is collected by a CCD camera, and the decoding information is obtained by combining it with the coding comparison table.

[0011] The present invention is also characterized in that:

[0012] The specific process of step 1 is as follows:

[0013] Step 1.1: In the cylindrical coordinate system, the electric field expression of a perfect vortex beam transmitted in free space to a distance z is:

[0014]

[0015] Where ρ and θ represent the polar diameter and polar angle in polar coordinates, ω0 is the beam waist radius, ω represents the width of the bright ring of the spot, R is the radius of the perfect vortex beam, and i represents a unit imaginary number;

[0016] Step 1.2: The topological charge of the perfect vortex beam is l, and then another perfect vortex beam with a topological charge value that is conjugate with it is coaxially superimposed to obtain the superimposed light field E superposition (ρ,θ) is:

[0017]

[0018] Among them, "*" means taking conjugation, is a constant that is independent of the value of the beam topological charge;

[0019] The superimposed light field E superposition(ρ,θ) presents a petal-shaped perfect vortex spot;

[0020] Step 1.3, convert the topological charge value l of the perfect vortex beam into a 3-bit binary sequence, and correspond the topological charge value l of the perfect vortex beam to the petal-shaped perfect vortex spot one by one to obtain a coding comparison table.

[0021] The specific process of step 3 is as follows:

[0022] Step 3.1, obtaining the grayscale values ​​of the three channels of RGB for each pixel of the color image, converting the grayscale values ​​of the three channels into 8-bit binary sequences, and splicing the 8-bit binary sequences in sequence, and then segmenting the spliced ​​sequences with 3-bit binary sequences as the minimum unit to obtain multiple 3-bit binary sequences;

[0023] Step 3.2, convert multiple 3-bit binary sequences into topological charge values ​​to be encoded, and obtain the petal-shaped perfect vortex spots by combining them with the coding comparison table. Arrange them in n×n sub-areas in sequence to obtain the spatial distribution of the petal-shaped perfect vortex array, design the phase transfer function based on the phase grating and the axicon lens, and use the Fourier transform theorem to perform phase shift processing on the phase transfer function to obtain the holographic phase transfer function. After MATLAB simulation, the computational hologram is obtained to complete the information encoding.

[0024] The holographic phase transfer function in step 3.2 is expressed as:

[0025]

[0026] Where t(x,y) represents the phase transfer function, N represents the number of perfect vortices in the array, f = 2π / T, T is the grating constant, (m s ,n s ) represents the position parameter of the sth diffraction lobe perfect vortex, c s is the normalized energy contribution of each diffraction order.

[0027] The phase transfer function expression is:

[0028]

[0029] Among them, l s represents the topological charge value to be encoded, is the phase transfer function of the axicon lens, and D is the focusing parameter of the axicon lens.

[0030] The specific process of step 4 is as follows:

[0031] Step 4.1, irradiate the CGH with a laser beam, and perform Fourier transform through a Fourier lens to obtain a petal-shaped perfect vortex array spot to be decoded;

[0032] Step 4.2, use a CCD camera to capture the petal-shaped perfect vortex spot array to be decoded, count the number of petal-shaped perfect vortex spot structures in the petal-shaped perfect vortex spot array to be decoded, and obtain multiple 3-bit binary sequences in combination with the coding comparison table. The multiple 3-bit binary sequences are spliced, and then divided into multiple groups of 8-bit binary sequences, and then converted into decimal to obtain pixel grayscale values, which is the decoding information.

[0033] The beneficial effects of the present invention are:

[0034] (1) The present invention performs coaxial conjugate superposition of perfect vortex beams to obtain a petal-shaped perfect vortex. The petal-shaped perfect vortex can not only maintain the characteristic that the spot size of the perfect vortex beam and the value of the topological charge of the light field are independent of each other, but also has a numerical relationship in which the number of petal-shaped structures of the spot is twice its own topological charge, which can effectively eliminate the problem of difficult pattern recognition caused by the spot size.

[0035] (2) The present invention designs a petal-shaped perfect vortex array, which fully utilizes the characteristic of fixed size of the petal-shaped perfect vortex light spot, effectively avoids the spatial superposition and interference of each unit light spot in the array, and helps to provide a useful reference for the design of other new array light fields.

[0036] (3) A method for applying a petal-shaped perfect vortex array to information encoding and decoding is provided. By observing the shape of the light spot, the light field pattern information can be directly obtained, and then the information incoherent encoding and decoding can be realized. The method is simple and efficient. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 Two-dimensional intensity distribution diagram of the cross section of vortex beams with different topological charge values ​​and perfect vortex beams;

[0038] Figure 2 The 4×4 petal-shaped perfect vortex array distribution and two-dimensional light intensity distribution diagram designed for the present invention;

[0039] Figure 3 Design a process for generating CGH for 4×4 petal-shaped perfect vortex arrays;

[0040] Figure (a) is a spiral phase with mutually conjugated topological charge values; Figure (b) is the result of superposition of spiral phases with mutually conjugated topological charge values; Figure (c) is the phase distribution of the axicon lens; Figure (d) is the result of superposition of phases (b) and (c); Figure (e) is a computational hologram obtained by linearly superposing multiple results of (d);

[0041] Figure 4 The information encoding and decoding lookup table designed based on different forms of the petal-shaped perfect vortex of the present invention;

[0042] Figure 5Schematic diagram of the encoding and decoding process for space division multiplexing communication based on the petal-shaped perfect vortex array designed for the present invention. DETAILED DESCRIPTION

[0043] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0044] Example 1

[0045] The space division multiplexing communication coding method based on the petal-shaped perfect vortex array of the present invention is specifically implemented according to the following steps:

[0046] Determine the topological charge value of the perfect vortex beam and the corresponding conjugate perfect vortex beam, coaxially superimpose the perfect vortex beam and the conjugate perfect vortex beam to obtain a petal-shaped perfect vortex spot, and form a coding comparison table by corresponding the topological charge value and the petal-shaped perfect vortex spot one by one; the specific process is:

[0047] In the cylindrical coordinate system, the electric field expression of the vortex beam when it is transmitted in free space to a distance z is:

[0048]

[0049] In the formula, ρ and θ represent the polar diameter and polar angle in polar coordinates respectively, and ω represents the width of the bright ring of the light spot. is the Laguerre polynomial, where p and l represent the radial coefficient and the topological charge of the light beam, respectively, and k = 2π / λ is the wave number of the light wave.

[0050] In the cylindrical coordinate system, the electric field expression of a perfect vortex beam transmitted in free space to a distance z is:

[0051]

[0052] Where ω0 is the beam waist radius, R is the radius of the perfect vortex beam, and i represents a unit imaginary number;

[0053] Figure 1 The figure shows the two-dimensional light intensity distribution of the vortex beam and the perfect vortex beam when the radial coefficient p = 0 and the topological charge l = 1, 2, ..., 8. It can be clearly observed from the figure that the spot size of the ordinary vortex beam is related to the topological charge value and shows a positive correlation, while the spot size of the perfect vortex beam remains unchanged. Although the perfect vortex beam can overcome the disadvantage that the spot size of the ordinary vortex beam widens as the topological charge value increases, the fixed spot size brings difficulties to the recognition of the light field pattern. The present invention cleverly designs a petal-shaped perfect vortex to make up for this deficiency. The design process of the petal-shaped perfect vortex is as follows:

[0054] Assuming that the topological charge of the perfect vortex beam is l, where l = 1, 2, ..., 8, then coaxially superimpose another perfect vortex beam with a topological charge value that is conjugate with it, and obtain the superimposed light field E superposition (ρ,θ) is:

[0055]

[0056] Among them, "*" means taking conjugation, is a constant that is independent of the value of the beam topological charge;

[0057] After the coaxial superposition of perfect vortex beams with mutually conjugated topological charges, the resulting beam spot morphology presents a "petal-like" light field in the shape of petals. It is observed that the number of petal-like structures is mathematically related to the number of topological charges of the perfect vortex beam by two times. This phenomenon can be used to directly distinguish the topological charge mode value of the beam, and the spot size of the light field after superposition is independent of the topological charge value. Therefore, the conjugate superposition light field not only maintains the advantage of the fixed spot radius of the perfect vortex beam, but also can quickly identify the light field mode through the spot morphology.

[0058] The topological charge values ​​l of the perfect vortex beam are converted into 3-bit binary sequences respectively, and the topological charge values ​​l of the perfect vortex beam are matched one by one with the petal-shaped perfect vortex spots to obtain a coding comparison table.

[0059] Divide the spatial domain into n×n sub-regions according to the rectangular coordinate system quadrant division rule;

[0060] Obtain the grayscale value of the three channels of RGB for each pixel of the color image, perform binary conversion and splicing on the grayscale values ​​of the three channels, and divide them into multiple 3-bit binary sequences, which are arranged in n×n sub-regions in sequence according to the coding comparison table, and a hologram is obtained through the holographic phase transfer function;

[0061] After the light beam passes through the hologram, it is transformed by Fourier transform to obtain the petal-shaped perfect vortex spot array to be decoded. The petal-shaped perfect vortex spot array to be decoded is collected by a CCD camera, and the decoding information is obtained by combining it with the coding comparison table.

[0062] It can be seen from Example 1 that the spot radius of the petal-shaped perfect vortex is independent of the topological charge value. Therefore, by placing single petal-shaped perfect vortices at different spatial positions, even spots of different modes can effectively avoid superposition interference with each other, thereby combining into a vortex array to achieve space division multiplexing.

[0063] Example 2

[0064] On the basis of Example 1, in this embodiment, the spatial domain is divided into n×n sub-areas according to the rectangular coordinate system quadrant division rule. The specific process is as follows:

[0065] Spatial domain partition planning: Without loss of generality, such as Figure 2 As shown in the figure, the design process of the petal-shaped perfect vortex array is explained by taking 4×4 space division multiplexing as an example. Figure 2 As shown in (a), firstly, the plane is arranged according to the rectangular coordinate system o-xy It is divided into four regions, and each region is distinguished according to the quadrant division rules of the rectangular coordinate system to obtain regions I, II, III, and IV. Then each region is divided according to the same rule, and region I is divided into four sub-regions {I_1, I_2, I_3, and I_4} in turn. Then region II, region III, and finally region IV are divided into {IV_1, IV_2, IV_3, and IV_4}. The spatial domain segmentation planning is completed, and 4×4 sub-regions are obtained.

[0066] Petal-shaped perfect vortex beam arrangement: As the petal-shaped perfect vortex spot size is not affected by the topological charge value, a single petal-shaped perfect vortex beam is placed in the center of each sub-region in the four regions I, II, III, and IV in sequence, and the order of placement is consistent with the order of regional division. Figure 2 As shown in (b), the final result of the distribution of a 4×4 petal-shaped perfect vortex array is shown, in which petal-shaped perfect vortices with topological charge values ​​of {l=1, l=2, l=3, l=4}, {l=5, l=6, l=7, l=8}, {l=1, l=2, l=3, l=4}, {l=5, l=6, l=7, l=8} are placed in 16 sub-regions {Ⅰ_1, Ⅰ_2, Ⅰ_3, Ⅰ_4}, {Ⅱ_1, Ⅱ_2, Ⅱ_3, Ⅱ_4}, {Ⅲ_1, Ⅲ_2, Ⅲ_3, Ⅲ_4}, {Ⅳ_1, Ⅳ_2, Ⅳ_3, Ⅳ_4} respectively divided into the four regions Ⅰ, Ⅱ, Ⅲ, and Ⅳ.

[0067] Example 3

[0068] On the basis of Example 1 and Example 2, in this embodiment, the specific process of generating a petal-shaped perfect vortex array computer-generated hologram is as follows:

[0069] Obtain the grayscale value of the three channels of RGB for each pixel of the color image, convert the grayscale values ​​of the three channels into 8-bit binary, and splice the 8-bit binary in sequence, and then divide the spliced ​​sequence into 3-bit binary sequences as the minimum unit to obtain multiple 3-bit binary sequences;

[0070] Multiple 3-bit binary sequences are converted into topological charge values ​​to be encoded, and the petal-shaped perfect vortex spots are obtained by combining them with the coding comparison table. They are arranged in n×n sub-areas in sequence to obtain the spatial distribution of the petal-shaped perfect vortex array.

[0071] According to the Fourier transform theorem, the phase shift in the spatial domain is equivalent to the translation in the frequency domain. Therefore, if one wants to place petal-shaped perfect vortex beams at different spatial positions in Example 2 and then obtain a 4×4 petal-shaped perfect vortex array, this goal can be achieved through phase shift operation.

[0072] like Figure 3 The flowchart of obtaining the petal-shaped perfect vortex array computational hologram by phase shift operation is shown in FIG. Figure 3 The two phase diagrams in (a) represent the spiral phase distributions with topological charge values ​​of l = 6 and l = -6, respectively. It is known that the spiral phase transfer function with topological charge l is exp(ilθ). The two spiral phases are coaxially superimposed to obtain Figure 3 The superimposed phase shown in (b) is then combined with Figure 3 The phase transfer function of the axicon lens is shown in (c). D is the focusing parameter of the axicon lens, thus we get Figure 3 The superposition result shown in (d) is that the phase transfer function t(x,y) obtained at this time can be expressed as:

[0073]

[0074] Among them, l s represents the topological charge value to be encoded, is the phase transfer function of the axicon lens, and D is the focusing parameter of the axicon lens.

[0075] So far, the CGH of a single petal-shaped perfect vortex has been obtained. In order to obtain the CGH of a petal-shaped perfect vortex array, the Fourier transform theorem is used to Figure 3 (d) The corresponding phase transfer function is phase shifted. Assume that the final holographic phase transfer function is t CGH (x,y), then its expression is:

[0076]

[0077] Where t(x,y) represents the phase transfer function, N represents the number of perfect vortices in the array, f = 2π / T, T is the grating constant, (m s ,n s ) represents the position parameter of the sth diffraction lobe perfect vortex, c s is the normalized energy contribution of each diffraction order.

[0078] Set N = 16, then the topological charge takes the value l s The position coordinates of the sth (s∈{1,2,…,16})th petal-shaped perfect vortex are (m s ,n s), the corresponding relationship between the position coordinates and the spatial region is set as follows: (m1, n1) ~ (m4, n4) correspond to the regions {Ⅰ_1, Ⅰ_2, Ⅰ_3, Ⅰ_4}, (m5, n5) ~ (m8, n8) correspond to the regions {Ⅱ_1, Ⅱ_2, Ⅱ_3, Ⅱ_4}, (m9, n9) ~ (m 12 ,n 12 ) corresponds to the regions {Ⅲ_1, Ⅲ_2, Ⅲ_3, Ⅲ_4}, (m 13 ,n 13 )~(m 16 ,n 16 ) corresponds to the region {Ⅳ_1, Ⅳ_2, Ⅳ_3, Ⅳ_4}. s Based on this formula, we can get the following through MATLAB simulation: Figure 3 (e) The computational hologram of the petal-shaped perfect vortex array shown.

[0079] The CGH is illuminated with a laser beam and Fourier transformed through a Fourier lens to obtain a petal-shaped perfect vortex array spot to be decoded;

[0080] A CCD camera is used to collect the petal-shaped perfect vortex spot array to be decoded, the number of petal-shaped perfect vortex spot structures in the petal-shaped perfect vortex spot array to be decoded is counted, and multiple 3-bit binary sequences are obtained by combining the coding comparison table. The multiple 3-bit binary sequences are spliced ​​and then divided into multiple groups of 8-bit binary sequences, and then converted into decimal to obtain the pixel grayscale value, which is the decoding information.

[0081] In this embodiment, since the number of petals of the petal-shaped perfect vortex spot is equal to twice the topological charge of the light beam, the petal-shaped perfect vortex is used to transmit information based on this feature, and the information can be directly mapped and encoded through the spot pattern. This incoherent detection method based on the spot morphology can effectively avoid the complexity of using coherent methods such as interference to identify the spot pattern, and improve the detection efficiency. Furthermore, the petal-shaped perfect vortex array can flexibly and efficiently utilize space resources to achieve space division multiplexing.

[0082] like Figure 4 As shown in FIG. 1 , a coding comparison table is given, which gives the spot images of the petal-shaped perfect vortex corresponding to the topological charge l = 1, 2, …, 8. If these 8 modes are used as coding resources, the petal-shaped perfect vortex of each mode can represent a 3-bit binary sequence. It is stipulated that the petal-shaped perfect vortex with l = 1 represents the binary sequence "000", the petal-shaped perfect vortex with l = 2 represents the binary sequence "001", and so on. l = 8 represents the binary sequence "111". At this point, the petal-shaped perfect vortex and the binary sequence have achieved a one-to-one mapping.

[0083] In the 4×4 petal-shaped perfect vortex array set in this embodiment, each sub-area spot can be arbitrarily selected from the modes l=1, 2, ..., 8, so the petal-shaped perfect vortex array can represent 48 bits of information. Figure 2 The order of area division maps the coding bits to the corresponding pattern of light spots. The specific process is to divide the 48-bit binary sequence into 16 groups from front to back. Each group obtains the topological charge value of the petal-shaped perfect vortex according to the coding comparison table, so that the 48-bit data is converted into 16 topological charges, and then the 16 topological charges are assigned to l as the data set. s (s∈{1,2,…,16}), l s Substituting into the transfer function t CGH (x, y) to obtain a 4×4 petal-shaped perfect vortex array of computer-generated holograms to achieve data encoding. In the experiment, a laser beam is used to illuminate the computer-generated hologram, and after passing through a Fourier lens and Fourier transform, a 4×4 petal-shaped perfect vortex array spot is obtained. When decoding, a CCD camera is first used to collect the light spot transmitted to the receiving end, and the number of light spot petal structures in each area of ​​the array is counted to obtain the topological charge value of each light spot, and then each topological charge l is converted into a 3-bit binary sequence, finally realizing the conversion of the light field pattern and the bit data stream to achieve decoding.

[0084] Example 4

[0085] Figure 5 Taking the transmission of color image information as an example, the encoding and decoding process is given. A 24-bit color image can be decomposed into three color channels: red (R), green (G), and blue (B). The data size of each pixel in each color channel image is 8 bits, so the grayscale value of each pixel ranges from 0 to 255. A 4×4 petal-shaped perfect vortex array can represent 48 bits of information, so a vortex array can encode 2 pixels on a 24-bit color image. After two pixels on the existing color image are decomposed into RGB primary color channels, the corresponding grayscale values ​​are [229 222 142] and [142 120 60] respectively. The grayscale values ​​are converted into binary sequences to obtain [11100101 11011110 10001110] and [10001110 01111000 00111100]. The converted binary sequences are concatenated, and then 16 groups of binary sequences are obtained with 3 binary sequences as a group. Each group of binary sequences is mapped with the coding comparison table to obtain a data set composed of 16 topological charges {l} = {8, 2, 4, 6; 8, 3, 2, 7; 5, 4, 5, 8; 5, 1, 8, 5}. The topological charges are assigned to l in turn. s (s∈{1,2,…,16}), and l s Substitute t CGH(x, y), a petal-shaped perfect vortex array hologram is generated. The laser beam illuminates the hologram and undergoes Fourier transform to obtain a 4×4 petal-shaped perfect vortex array, thus completing the information encoding of two pixels of the color image.

[0086] At the receiving end, a CCD camera is used to capture the transmitted array spot. The spot size received by the CCD camera is a color picture of 256×256×3. The color picture is first grayed out, so the picture is converted into a grayscale image of 256×256×1, and then the topological charge of each area spot in the array is output by combining deep learning image recognition. The convolutional neural network used consists of 6 layers (excluding the input layer), including 2 convolutional layers, 2 pooling layers, 1 fully connected layer, and 1 output layer. The input layer first performs image segmentation, and the image is equally divided into 4×4 modules horizontally and vertically. The size of each module is 64×64×1. Each module first passes through the first convolution layer, which uses 16 5×5 convolution kernels to obtain 16 64×64 feature maps; the second layer uses pooling to compress the image data, and performs 2×2 pooling on the 16 64×64 feature maps to obtain 16 32×32 maps; the third layer uses 32 5×5 convolution kernels to obtain 32 32×32 feature maps; similar to the second layer, the fourth layer performs 2×2 pooling on the 32 32×32 maps to obtain 32 16×16 maps; the fifth layer is a fully connected layer, and the sixth output layer has a total of 8 nodes, representing 1 to 8, that is, representing the topological charge of the light spot in the corresponding area. Identify all topological charges {l}={8,2,4,6; 8,3,2,7; 5,4,5,8; 5,1,8,5} of the light spot in the 4×4 array, convert each light field pattern into a 3-bit binary sequence according to the coding comparison table and combine them together, then segment them in groups of 8 bits to obtain [1110010111011110 10001110 10001110 01111000 00111100], and then convert the 8-bit binary sequence into a decimal value [229 222 142 142 120 60], and then obtain the grayscale values ​​of two pixels of the color image, and then restore the original image information to achieve decoding. The present invention selects 8 kinds of topological charges, and the array size is 4×4. In fact, more topological charge values ​​can be set as needed, and the array scale can be expanded, so that a petal-shaped perfect vortex array image can complete a larger capacity of information encoding and decoding.

[0087] Through the above-mentioned manner, the present invention provides a space-division multiplexing communication encoding and decoding method based on a petal-shaped perfect vortex array, which makes full use of the invariant size characteristic of the perfect vortex beam, designs a vortex array that does not interfere with each other in space, and superimposes perfect vortex beams of mutually conjugated modes to obtain a petal-shaped form, further overcoming the disadvantage that the beam pattern cannot be identified due to the invariant spot size, thereby designing a space-division multiplexing communication method, and finally providing a space-division multiplexing communication encoding and decoding method based on a petal-shaped perfect vortex array.

Claims

1. A space division multiplexing communication coding method based on a petal-shaped perfect vortex array, characterized in that: Follow the steps below to implement it: Step 1, determine the topological charge value for generating a perfect vortex beam and the corresponding conjugate perfect vortex beam, coaxially superimpose the perfect vortex beam and the conjugate perfect vortex beam to obtain a petal-shaped perfect vortex spot, and form a coding comparison table by corresponding the topological charge value and the petal-shaped perfect vortex spot one by one; Step 2, dividing the space domain into n×n sub-areas according to the rectangular coordinate system quadrant division rule; Step 3: Obtain the grayscale value of the three channels of RGB for each pixel of the color image, perform binary conversion and splicing on the grayscale values ​​of the three channels, and divide them into multiple 3-bit binary sequences, and arrange them in n×n sub-regions in sequence according to the coding comparison table, and obtain the hologram through the holographic phase transfer function; Step 4: After the light beam passes through the hologram, it is Fourier transformed to obtain the petal-shaped perfect vortex spot array to be decoded, and the petal-shaped perfect vortex spot array to be decoded is collected by a CCD camera, and the decoding information is obtained by combining it with the coding comparison table.

2. According to claim 1, the space division multiplexing communication encoding and decoding method based on the petal-shaped perfect vortex array is characterized in that: The specific process of step 1 is as follows: Step 1.1: In the cylindrical coordinate system, the electric field expression of a perfect vortex beam transmitted in free space to a distance z is: Where ρ and θ represent the polar diameter and polar angle in polar coordinates, ω0 is the beam waist radius, ω represents the width of the bright ring of the spot, R is the radius of the perfect vortex beam, and i represents a unit imaginary number; Step 1.2: The topological charge of the perfect vortex beam is l, where l = 1, 2, ..., 8. Then, another perfect vortex beam with a topological charge value that is conjugate with it is coaxially superimposed to obtain the superimposed light field E. superposition (ρ,θ) is: Among them, "*" means taking conjugation, is a constant that is independent of the value of the beam topological charge; The superimposed light field E superposition (ρ,θ) presents a petal-shaped perfect vortex spot; Step 1.3, convert the topological charge value l of the perfect vortex beam into a 3-bit binary sequence, and correspond the topological charge value l of the perfect vortex beam to the petal-shaped perfect vortex spot one by one to obtain a coding comparison table.

3. According to claim 1, the space division multiplexing communication encoding and decoding method based on the petal-shaped perfect vortex array is characterized in that: The specific process of step 3 is as follows: Step 3.1, obtaining the grayscale values ​​of the three channels of RGB for each pixel of the color image, converting the grayscale values ​​of the three channels into 8-bit binary sequences, and splicing the 8-bit binary sequences in sequence, and then performing 3-bit binary sequence segmentation on the spliced ​​sequences to obtain multiple 3-bit binary sequences; Step 3.2, convert multiple 3-bit binary sequences into topological charge values ​​to be encoded, and obtain the petal-shaped perfect vortex spots by combining them with the coding comparison table. Arrange them in n×n sub-areas in sequence to obtain the spatial distribution of the petal-shaped perfect vortex array, design the phase transfer function based on the phase grating and the axicon lens, and use the Fourier transform theorem to perform phase shift processing on the phase transfer function to obtain the holographic phase transfer function. After MATLAB simulation, the computational hologram is obtained to complete the information encoding.

4. The space division multiplexing communication encoding and decoding method based on the petal-shaped perfect vortex array according to claim 3 is characterized in that: The holographic phase transfer function described in step 3.2 is expressed as: Where t(x,y) represents the phase transfer function, N represents the number of perfect vortices in the array, f = 2π / T, T is the grating constant, (m s ,n s ) indicates the s The position parameter of a diffraction lobe perfect vortex, c s is the normalized energy contribution of each diffraction order.

5. The space division multiplexing communication encoding and decoding method based on the petal-shaped perfect vortex array according to claim 3 is characterized in that: The phase transfer function expression is: Among them, l s represents the topological charge value to be encoded, is the phase transfer function of the axicon lens, and D is the focusing parameter of the axicon lens.

6. According to claim 3, the space division multiplexing communication encoding and decoding method based on the petal-shaped perfect vortex array is characterized in that: The specific process of step 4 is as follows: Step 4.1, irradiate the CGH with a laser beam, and perform Fourier transform through a Fourier lens to obtain a petal-shaped perfect vortex array spot to be decoded; Step 4.2, use a CCD camera to capture the petal-shaped perfect vortex spot array to be decoded, count the number of petal-shaped perfect vortex spot structures in the petal-shaped perfect vortex spot array to be decoded, and obtain multiple 3-bit binary sequences in combination with the coding comparison table. The multiple 3-bit binary sequences are spliced, and then divided into multiple groups of 8-bit binary sequences, and then converted into decimal to obtain pixel grayscale values, which is the decoding information.

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