Communication coding and decoding method based on radial carpet light beam optimization spectrum distribution

Abstract

The invention discloses a communication coding and decoding method based on radial carpet light beam optimization spectrum distribution, which comprises the following steps of: loading a radial grating to a spatial light modulator at a transmitting end to generate RC light beams with corresponding mode coefficients, and mapping digital signal information to the RC light beams with different modes; transmitting the RC light beam in an ocean channel to obtain an RC light beam considering the influence of ocean turbulence; and shooting the RC light beam at a receiving end, obtaining optimized spiral spectrum distribution, interpreting a mode coefficient of the RC light beam according to the magnitude of spectrum distribution energy, and recovering digital signal information of a transmitting end. According to the method, the spectral distribution calculation method of the RC light beam after passing through the ocean transmission environment is obtained by constructing the spectral distribution transmission characteristic model of the RC light beam in the ocean turbulence, the transmission reliability of the light beam in a complex medium is improved, and the recognition accuracy of an emission light beam mode is greatly improved through a spiral spectral distribution optimization scheme.

Description

Technical Field

[0001] The invention belongs to the technical field of optical communication, and in particular relates to a communication encoding and decoding method based on radial carpet light beam optimized spectral distribution. Background Art

[0002] With the large-scale commercial use of 5G, the global industry is conducting research on 6G in full swing. In order to achieve the leap from the Internet of Everything to the Internet of Everything, promote the transformation of virtual reality VR to immersive extended reality XR, and implement new services such as holographic communication, intelligent interaction, and digital twins, the 6G network needs to reach at least the transmission level of Tbps. Therefore, low-latency, high-speed and large-capacity communication technology has become the focus of researchers. As one of the important communication means, optical communication has been greatly developed and applied in recent years due to its rich spectrum resources, high transmission rate, no need for frequency authorization, high confidentiality and no electromagnetic radiation. However, optical signals are easily affected by the environment. Especially under harsh conditions such as rain, snow, fog, haze and dust, the signal strength will decay rapidly and the wavefront of the light wave will be damaged. In addition, the spot size of the light beam will gradually widen during the transmission process, and the longitudinal section of the spot will present an inverted cone divergent structure, which makes the beam face challenges in long-distance transmission and beam alignment. Therefore, how to overcome the influence of environmental interference on optical transmission and solve the problem of spot size widening has important theoretical and engineering value for optical communication.

[0003] In recent years, in order to improve the performance of light transmission in resisting environmental interference and solve the problem of spot size widening, the industry has proposed the concept of non-diffraction beams. Non-diffraction beams have the characteristics of time-invariant spot shape and self-healing when encountering obstacles during transmission, which makes this type of beam maintain a more superior transmission performance than the laser beam directly emitted by the traditional laser. In 2018, Rasouli proposed a new type of non-diffraction beam - RC (radial carpet) beam (Saifollah Rasouli, Physical Review A, Vol. 97, 033844). In addition to the non-diffraction, self-healing, time-invariant and self-accelerating characteristics of non-diffraction beams such as Bessel, Airy, Mathieu, Weber and Pearcey, this beam also has multiple additional adjustable parameters such as wavelength, phase factor and mode coefficient, which greatly enhances the controllable freedom of the beam, making it very promising in the fields of photolithography, optical nickel and optical communication. However, current research on RC beams is mostly focused on the generation of light field control and non-diffraction characteristics, and there are few reports on the transmission characteristics of RC beams in complex media and their optical communication applications. Summary of the invention

[0004] The purpose of the present invention is to provide a communication encoding and decoding method based on radial carpet light beam optimized spectral distribution, which solves the problems of low reliability and accuracy faced by existing optical communication beams in long-distance transmission and beam alignment.

[0005] The technical solution adopted by the present invention is: a communication encoding and decoding method based on radial carpet beam optimized spectral distribution, comprising the following steps:

[0006] Step 1: At the transmitting end, a radial grating is loaded into the spatial light modulator to generate an RC beam with a corresponding mode coefficient, and then the digital signal information is mapped to the RC beams of different modes to complete the information encoding;

[0007] Step 2, transmitting the RC beam obtained in step 1 in an ocean channel to obtain an RC beam that takes into account the influence of ocean turbulence;

[0008] Step 3: At the receiving end, the RC beam obtained in step 2 is photographed and the optimized spiral spectrum distribution is obtained. The mode coefficient of the RC beam is determined according to the energy of the spectrum distribution, and the digital signal information of the transmitting end is restored to complete the information decoding.

[0009] The present invention is also characterized in that:

[0010] Step 1 is specifically as follows: at the transmitting end, the spatial light modulator is irradiated with the fundamental mode Gaussian beam emitted by the helium-neon laser, and a radial grating hologram is generated according to the phase transfer function of the radial grating t(θ)=exp[iγcos(lθ)], where γ represents the grating adjustment coefficient, and a radial grating with M mode coefficients of l=1, 2…, M is selected, and a one-to-one mapping relationship is established between the radial grating and the digital signal. Then, encoding the digital signal at a certain moment is equivalent to loading the corresponding radial grating hologram into the spatial light modulator.

[0011] The light field u(r,θ,z) of the RC beam obtained after transmission in the ocean channel in step 2 is expressed as:

[0012] u(r,θ,z)=u0(r,θ,z)exp[iφ(x,y)]

[0013] Where (r, θ) is the polar coordinate in the cylindrical coordinate system, θ=tan -1 (y / x), i represents an imaginary unit; assuming that the light field of the RC beam transmitted to z without being affected by ocean turbulence is u0(r,θ,z), it can be expressed as:

[0014]

[0015] Where k is the wave number of the light wave, J (ql+1) / 2 (·) represents the (ql+1) / 2 order Bessel function of the first kind, λ is the wavelength of light, l represents the value of the beam mode coefficient;

[0016] φ(x,y) is the turbulence phase added to the RC beam affected by ocean turbulence, expressed as:

[0017]

[0018] In the formula, H(κ x ,κ y ) is an N×N dimensional complex random matrix with mean 0 and variance 1, represents Fourier transform operation; σ 2 (κ x ,κ y ) is the phase screen spectrum variance, expressed as:

[0019]

[0020] Where Δx represents the grid spacing of the phase screen, N is the number of grids along the x or y direction of the phase screen plane, and κ x , κ y Represent the x-axis and y-axis spatial frequencies respectively; Φ(κ x ,κ y ) is the phase spectrum of ocean turbulence, expressed as:

[0021] Φ(κ x ,κ y )=2πk 2 ΔzΦ n (κ x ,κ y )

[0022] In the formula, Φ n (κ x ,κ y ) is the power spectrum of ocean turbulence refractive index fluctuation, expressed as:

[0023] Φ n (κ)=0.388×10 -8 ε -1 / 3 κ -11 / 3 [1+2.35(κη) 2 / 3 ]f(κ,ω,χ T )

[0024]

[0025] δ=8.284(κη) 4 / 3 +12.978(κη) 2

[0026] In the formula, A T, A S and A TS is a constant, κ represents the spatial frequency, ε represents the energy dissipation rate of ocean turbulence, η represents the microscale of ocean turbulence, and χ T represents the temperature variance dissipation rate of turbulence, and ω is the ratio of ocean turbulence caused by seawater temperature and salinity.

[0027] When obtaining the spiral spectrum distribution of the RC beam in step 3, the RC beam is decomposed into a linear superposition of a series of spiral harmonics exp(imθ):

[0028]

[0029] In the formula, The corresponding energy is Therefore, the spectral components corresponding to the RC beam with the mode coefficient value of l are:

[0030]

[0031] Therefore, the spiral harmonic expansion of the RC beam with a mode coefficient of l is performed within a certain range, and the spectral distribution of the RC beam after transmission through ocean turbulence is obtained.

[0032] Step 3 also includes optimizing the obtained spiral spectrum distribution, specifically: first, the obtained RC light beam spectrum distribution after ocean turbulence transmission is divided into three parts according to the mode coefficient values ​​of negative, equal to 0, and positive; then, the energy of the part of the spectrum distribution with the mode coefficient equal to 0 remains unchanged, and the energy corresponding to the part with negative mode values ​​is added one by one to the spectrum energy of the mutually conjugate mode coefficients, and at the same time, the energy corresponding to the part with negative mode values ​​is all set to zero; then, find the mode coefficient l′ corresponding to the maximum value of the spectrum energy outside the mode coefficient of 0; finally, merge the spectral energy with the mode coefficient equal to 0 into l′, and set the spectral energy with the mode coefficient equal to zero to zero.

[0033] The beneficial effects of the present invention are as follows: the communication coding method based on radial carpet light beam optimized spectral distribution of the present invention, by constructing a spectral distribution transmission characteristic model of an RC light beam in ocean turbulence, obtains a method for calculating the spectral distribution of an RC light beam after passing through an ocean transmission environment, thereby improving the reliability of light beam transmission in complex media, and greatly improving the accuracy of emission light beam pattern recognition through a spiral spectral distribution optimization scheme, providing a theoretical basis for studying the propagation and communication problems of non-diffraction light beams in the ocean, and is expected to provide a potential communication coding application solution for existing optical communication technologies. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 It is the original spectrum distribution diagram of the light beam under different transmission conditions;

[0035] Figure 2 is a spectrum distribution optimization flow chart of the present invention;

[0036] Figure 3 is a spectrum distribution diagram of the present invention under different emission mode coefficients and different ocean turbulence intensities;

[0037] Figure 4 It is an implementation scheme architecture diagram of the communication encoding and decoding method based on radial carpet light beam optimized spectral distribution of the present invention;

[0038] Figure 5 The present invention is a schematic flow chart of information encoding and decoding of color images by a communication encoding and decoding method based on radial carpet light beam optimized spectral distribution. DETAILED DESCRIPTION

[0039] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0040] Example 1

[0041] The present invention provides a communication encoding and decoding method based on radial carpet light beam optimized spectral distribution, comprising the following steps:

[0042] 1. Establishing the RC beam transmission model in the ocean turbulence environment

[0043] The commonly used expression for the power spectrum of ocean turbulence refractive index fluctuation is:

[0044] Φ n (κ)=0.388×10 -8 ε -1 / 3 κ -11 / 3 [1+2.35(κη) 2 / 3 ]f(κ,ω,χ T )

[0045]

[0046] δ=8.284(κη) 4 / 3 +12.978(κη) 2

[0047] In the formula, A T , A S and A TS is a constant, and its values ​​are A T =1.863×10 -2 , A s =1.9×10 -4 , A TS =9.41×10 -3 κ represents the spatial frequency, ε represents the energy dissipation rate of ocean turbulence, and the value range is 10-10 m 2 / s 3 ~10 -1 m 2 / s 3 , η represents the microscale of ocean turbulence, χ T It represents the temperature variance dissipation rate of turbulence, and its value range is 10 -10 K 2 / s~10 -4 K 2 / s, ω is the ratio of ocean turbulence caused by seawater temperature and salinity, and its value range is -5 to 0.

[0048] The ocean turbulence phase screen distribution function φ(x,y) is obtained by the power spectrum inversion method, and the power spectrum Φ n (κ x ,κ y ) to obtain the phase spectrum of ocean turbulence Φ(κ x ,κ y ):

[0049] Φ(κ x ,κ y )=2πk 2 ΔzΦ n (κ x ,κ y )

[0050] From this we can get the phase screen spectrum variance:

[0051]

[0052] Where Δx represents the grid spacing of the phase screen, N is the number of grids along the x or y direction of the phase screen plane, and κ x , κ y Represent the x-axis and y-axis spatial frequencies respectively. Using the power spectrum inversion method for the phase screen spectrum variance, the random phase screen expression can be obtained as:

[0053]

[0054] In the formula, H(κ x ,κ y ) is an N×N dimensional complex random matrix with mean 0 and variance 1, Represents the Fourier transform operation.

[0055] The effect of ocean turbulence on RC beams can be simulated by a multi-phase screen. Assuming that the RC light field transmitted to z that is not affected by ocean turbulence is recorded as u0(r,θ,z), the beam affected by ocean turbulence will be added with the turbulence phase φ(x,y), and the light field u(r,θ,z) after transmission is obtained:

[0056] u(r,θ,z)=u0(r,θ,z)exp[iφ(x,y)]

[0057] Where (r, θ) is the polar coordinate in the cylindrical coordinate system, θ=tan -1 (y / x), i represents the imaginary unit.

[0058] In the cylindrical coordinate system, the expression u0(r,θ,z) for the transmission of an RC beam in free space is:

[0059]

[0060] Where k is the wave number of the light wave, J (ql+1) / 2 (·) represents the (ql+1) / 2 order Bessel function of the first kind, λ is the wavelength of light, l represents the value of the beam mode coefficient.

[0061] 2. Calculation of the spectral distribution of the RC beam after passing through the ocean turbulence environment

[0062] The RC beam affected by turbulence can be decomposed into a linear superposition of a series of spiral harmonics exp(imθ):

[0063]

[0064] in, The corresponding energy is Therefore, the spectral components corresponding to the RC beam with the mode coefficient value of l are:

[0065]

[0066] Therefore, by performing spiral harmonic expansion on the RC beam with a mode coefficient of l within a certain range, the spectral distribution of the RC beam after transmission through ocean turbulence can be obtained.

[0067] 3. Optimizing the original spectrum distribution of RC beam

[0068] According to the content of the previous step, the spectral distribution of the RC beam after transmission in the ocean turbulence environment can be obtained. From the spiral spectrum formula, it can be seen that the RC beam with the original mode coefficient value l affected by turbulence will diffuse to other modes, which is not conducive to the stable transmission of the RC beam and its application in optical communication. Therefore, in order to further concentrate the energy on the original RC beam mode, the obtained spectral distribution is optimized.

[0069] First, the spectrum distribution of the RC beam after the ocean turbulence transmission is divided into three parts according to the mode coefficients of negative, 0, and positive values; then, the energy of the part of the spectrum distribution with the mode coefficient equal to 0 remains unchanged, and the energy corresponding to the part with negative mode values ​​is added one by one to the spectrum energy of the mutually conjugate mode coefficients, and the energy corresponding to the part with negative mode values ​​is set to zero; then, find the mode coefficient corresponding to the maximum value of the spectrum energy outside the mode coefficient of 0, recorded as l'; finally, merge the spectrum energy with the mode coefficient equal to 0 into l', and set the spectrum energy with the mode of zero to zero. After the above steps, the spectrum distribution of the RC beam after optimization is obtained.

[0070] 4. Information encoding and decoding based on RC beam optimized spectral distribution

[0071] The information encoding and decoding transmission based on the optimized spectrum distribution of RC beams is divided into three modules: the transmitter, the ocean transmission channel and the receiver. The encoding process of information is realized at the transmitter, specifically, the radial grating is loaded into the spatial light modulator to generate the RC beam with the corresponding mode coefficient, and the information is mapped to the beams of different modes. The phase transfer function of the radial grating is:

[0072] t(θ)=exp[iγcos(lθ)]

[0073] Where γ represents the grating adjustment coefficient.

[0074] A radial grating hologram is generated according to the phase transfer function of the radial grating above. Assuming that a radial grating with M mode coefficients of l=1,2…,M is selected as an alternative, the fundamental mode Gaussian beam emitted by the helium-neon laser is used to illuminate the spatial light modulator at the transmitting end, and a one-to-one mapping relationship is established between the radial grating and the digital signal. Then, encoding the digital signal at a certain moment is equivalent to loading the corresponding radial grating hologram into the spatial light modulator. The RC light beam generated after light field regulation is transmitted in the ocean channel, and the transmission process is simulated by the multi-random phase screen described in step one. At the receiving end, the RC light spot after transmission through the ocean turbulent environment is first photographed with a camera, and then the optimized spiral spectrum distribution of the light beam affected by turbulence is obtained based on steps two and three. The mode coefficient of the RC light beam is judged according to the size of the spectrum distribution energy, and finally the transmitted digital signal information is restored.

[0075] Example 2

[0076] The present invention provides a communication encoding and decoding method based on radial carpet light beam optimized spectral distribution, comprising the following steps:

[0077] 1. Constructing a model of light beam propagation in ocean turbulence

[0078] Optical turbulence in the ocean is affected by temperature and salinity and has a complex structure. The multi-phase screen method is used to simulate the ocean transmission environment. The power spectrum of the ocean turbulence refractive index fluctuation is expressed as:

[0079] Φ n (κ)=0.388×10 -8 ε -1 / 3 κ -11 / 3 [1+2.35(κη) 2 / 3 ]f(κ,ω,χ T )

[0080]

[0081] δ=8.284(κη) 4 / 3 +12.978(κη) 2

[0082] In the formula, A T , A S and A TS is a constant, and its values ​​are A T =1.863×10 -2 , A s =1.9×10 -4 , A TS =9.41×10 -3 κ represents the spatial frequency, ε represents the energy dissipation rate of ocean turbulence, and the value range is 10 -10 m 2 / s 3 ~10 -1 m 2 / s 3 , η represents the microscale of ocean turbulence, χ T It represents the temperature variance dissipation rate of turbulence, and its value range is 10 -10 K 2 / s~10 -4 K 2 / s, ω is the ratio of ocean turbulence caused by seawater temperature and salinity, and its value range is -5 to 0.

[0083] The ocean turbulence phase screen distribution function φ(x,y) is obtained by the power spectrum inversion method, and the power spectrum Φ n (κ x ,κ y ) to obtain the phase spectrum of ocean turbulence Φ(κ x ,κ y ):

[0084] Φ(κ x ,κ y )=2πk 2 ΔzΦn (κ x ,κ y )

[0085] From this we can get the phase screen spectrum variance:

[0086]

[0087] Where Δx represents the grid spacing of the phase screen, N is the number of grids along the x or y direction of the phase screen plane, and κ x , κ y Represent the x-axis and y-axis spatial frequencies respectively. Using the power spectrum inversion method for the phase screen spectrum variance, the random phase screen expression can be obtained as:

[0088]

[0089] In the formula, H(κ x ,κ y ) is an N×N dimensional complex random matrix with mean 0 and variance 1, Represents the Fourier transform operation.

[0090] The effect of ocean turbulence on RC beams can be simulated by a multi-phase screen. Assuming that the RC light field transmitted to z that is not affected by ocean turbulence is recorded as u0(r,θ,z), the beam affected by ocean turbulence will be added with the turbulence phase φ(x,y), and the light field u(r,θ,z) after transmission is obtained:

[0091] u(r,θ,z)=u0(r,θ,z)exp[iφ(x,y)]

[0092] Where (r, θ) is the polar coordinate in the cylindrical coordinate system, θ=tan -1 (y / x), i represents the imaginary unit.

[0093] In the cylindrical coordinate system, the expression u0(r,θ,z) for the transmission of an RC beam in free space is:

[0094]

[0095] Where k is the wave number of the light wave, J (ql+1) / 2 (·) represents the (ql+1) / 2 order Bessel function of the first kind, λ is the wavelength of light, l represents the value of the beam mode coefficient. Combining the above two formulas, we can get the light intensity of the RC beam after passing through the ocean turbulence.

[0096] Without loss of generality, here we set the number of grids along the x or y direction N = 1024, the default mode coefficient of the transmitted RC beam l = 6, and the ocean turbulence parameter χ T=10 -8 K 2 / s, ε=10 -6 m 2 / s 3 ,ω=-3,η=10 -3 m, and use 20 independent random phase screens to simulate the ocean environment transmission process. If there is no subsequent change, the parameter settings given here will be used. Figure 1 The embedded two-dimensional intensity distributions in (b) and (c) show the intensity of the RC beam when it is transmitted to z = 10, 60, and 100 m in free space and in ocean turbulence, respectively.

[0097] 2. Calculate and obtain the RC beam spectrum distribution

[0098] The RC beam affected by turbulence is expanded into a linear superposition of a series of spiral harmonics exp(imθ):

[0099]

[0100] in, The corresponding energy is The spectral components corresponding to the RC beam with the mode coefficient value of l can be obtained as:

[0101]

[0102] Therefore, by performing a spiral harmonic expansion on the RC beam with a mode coefficient of l within a certain range, the spectral distribution of the RC beam after passing through the ocean turbulence can be obtained. Figure 1 As shown in the figure, the spectrum distribution of the RC beam with the mode coefficient l=6 after being transmitted at different distances in the source field and ocean turbulence environment. Figure 1 (a) is the RC beam spectrum distribution at the source field position z = 0. It can be seen from the figure that when not affected by turbulence factors, the spectrum distribution energy is concentrated at l = 6 and its conjugate mode position and is evenly distributed; Figure 1 As shown in (b), after the light beam is transmitted for a certain distance in free space, except that the spectral energy at the position of l=±6 still accounts for the main component, the spectral distribution energy at the position of mode coefficient m=0 is no longer zero, and the spectral energy of this part increases with the increase of the transmission distance; Figure 1 (c) shows the spectral distribution of the light beam after it is transmitted at different distances in the ocean turbulence environment. Different from the situation at the source field, the light beam will undergo mode dispersion due to the turbulence effect.

[0103] From the spectral distribution results obtained above, it can be seen that the spectral distribution of the RC beam with a mode coefficient of l presents a symmetric distribution characteristic about the m=0 mode, and the spectral energy is mainly concentrated at the mode coefficient m=0, ±l position.

[0104] 3. Optimizing the original spectrum distribution of RC beam

[0105] According to the fact that the RC beam spectrum distribution is symmetrical about the m=0 mode and the spectrum energy is mainly concentrated at the mode coefficient m=0,±l position, the spectrum distribution is optimized in a targeted manner as follows.

[0106] Figure 2 The spectrum distribution optimization process is given. Figure 2 (a) is a schematic diagram of the intensity distribution of the RC beam after passing through the turbulent phase screen. Figure 2 (b) shows the original spectrum distribution of the RC beam with l = 6 under the influence of ocean turbulence. First, the spectrum distribution in the range of m = -10, -9, ..., 10 is divided into three parts: m = -10, -9, ... -1, m = 0, and m = 1, 2, ..., 10 according to the segmentation standard of m < 0, m = 0, m > 0; then the spectrum energy corresponding to the part with the mode value of negative number m = -10, -9, ..., -1 is added one by one to the spectrum energy of the mutually conjugate mode coefficient m = 10, 9, ..., 1, and at the same time, the energy corresponding to the part of m = -10, -9, ..., -1 is set to zero, and the result is as follows Figure 2 (c) shows the result; then, find the mode corresponding to the maximum spectral energy in the mode coefficient m = 1, 2, ..., 10, and record it as l'; finally, combine the spectral energy corresponding to the mode coefficient m = 0 with the spectral energy of l', and set the spectral energy of m = 0 to zero. After the above steps, the spectral distribution of the RC beam after optimization is obtained, as shown in Figure 2 (d) as shown.

[0107] In order to verify the spectral distribution effect after RC beam optimization, Figure 3 The following is a graph showing the wavelength of a beam of λ = 532 nm in an ocean turbulence parameter set to χ T =10 -8 K 2 / s, ε=10 -6 m 2 / s 3 , ω=-3, the transmission mode coefficients are l=1,2,…,16 respectively, RC beam three-dimensional ( Figure 3 (a)) and two-dimensional spectral distribution ( Figure 3 (b)) The results show that only a small part of the energy of the optimized spectrum distribution is dispersed to other modes, and the transmission mode can be identified without error. In addition, Figure 3 (c) and Figure 3 (d) shows the three-dimensional spectrum distribution of the RC beam with mode coefficient l = 6 under different ocean turbulence intensity conditions and the three-dimensional graph of the spectrum distribution surface. Indicates the intensity of ocean turbulence, set Value range [5×10 -13 ,2×10 -16], it can be seen from the figure that when When , the spectrum distribution energy of l = 6 in the receiving mode coefficient can always maintain the maximum, that is, the transmission mode coefficient can be distinguished in the case of medium-intensity turbulence. In the case of strong turbulence, the transmitted mode coefficient RC beam decays rapidly and spreads to other modes.

[0108] 4. Information encoding and decoding based on RC beam optimized spectral distribution

[0109] like Figure 4 The figure shows the framework diagram of information encoding and decoding based on the optimized spectrum distribution of RC beams. The information encoding and decoding transmission based on the optimized spectrum distribution of RC beams is divided into three modules: the transmitter, the ocean transmission channel and the receiver. The information is encoded at the transmitter, specifically by loading a radial grating to the spatial light modulator to generate an RC beam with the corresponding mode coefficient, and then mapping the information to beams of different modes. The phase transfer function of the radial grating is:

[0110] t(θ)=exp[iγcos(lθ)]

[0111] Where γ represents the grating adjustment coefficient.

[0112] A radial grating hologram is generated according to the phase transfer function of the radial grating above. Assuming that a radial grating with M mode coefficients of l=1,2…,M is selected, the spatial light modulator is illuminated by the fundamental mode Gaussian beam emitted by the helium-neon laser at the transmitting end, and a one-to-one mapping relationship is established between the radial grating and the digital signal. Then, encoding the digital signal at a certain moment is equivalent to loading the corresponding radial grating hologram into the spatial light modulator. The RC light beam generated after light field regulation is transmitted in the ocean channel, and the transmission process is simulated by the multi-random phase screen described in step one. At the receiving end, the RC light spot after transmission through the ocean turbulent environment is first photographed with a camera, and then the optimized spiral spectrum distribution of the light beam affected by turbulence is obtained based on steps two and three. The mode coefficient of the RC light beam is judged according to the size of the spectrum distribution energy, and finally the transmitted digital signal information is restored.

[0113] Example 3

[0114] The present invention provides a communication encoding and decoding method based on radial carpet beam optimized spectral distribution, such as Figure 5As shown in the figure, in order to more clearly demonstrate the communication encoding and decoding process based on the optimized spectrum distribution of the RC beam, the transmission of 24-bit color Lena image information is taken as an example to describe the specific encoding and decoding process. First, the color image is divided into three color channels: red (R), green (G), and blue (B). At this time, the 24-bit color image is decomposed into three independent 8-bit grayscale images. Therefore, any point on each color channel image has a unique grayscale value, and the grayscale value range is 0 to 255. Then the decimal grayscale value is converted into a hexadecimal number. Obviously, the transmission of an 8-bit binary data requires a combination of 2 hexadecimal numbers to achieve it. Therefore, two hexadecimal numbers are selected here to represent the pixel information of any point on the three color channel images of red (R), green (G), and blue (B) after the color image is decomposed. Without loss of generality, here the RC beams with mode coefficients l = 1, 2, ..., 16 are selected to correspond to the hexadecimal values ​​0, 1, ..., f one by one. Then, the encoding of any point information on a color channel image can be achieved by the continuous transmission of two RC beams.

[0115] Assume that the information of a certain point on a 24-bit color image needs to be encoded and transmitted. After decomposing the point into three color channels, R, G, and B, the gray value R(210) corresponding to the position of the point on each color channel is read respectively. D 、G(114) D 、B(102) D , and then convert the decimal grayscale data to hexadecimal: R(d2) H 、G(72) H 、B(66) H Furthermore, according to the correspondence between the hexadecimal values ​​0, 1, ..., f and the RC beam mode coefficients specified above, the grayscale information R(d2) on the three color channels can be obtained: H 、G(72) H 、B(66) H , the corresponding RC beam mode coefficient combinations are {l1=14,l2=3}, {l1=8,l2=3}, {l1=7,l2=7}, and radial gratings corresponding to different modes are generated accordingly. The radial gratings are loaded onto the spatial light modulator in turn, and the RC beams with corresponding mode coefficients will be generated after light field control.

[0116] After the grayscale value of each color channel image is encoded and mapped, an RC beam composed of two specific pattern coefficients is obtained. In the same way, the mapping relationship between all pixels of the three color channel images and the RC beam is completed, and the encoding of the color image information is completed. It should be pointed out that the order of information encoding on the three RGB color channels can be determined by yourself. Here, it is set to encode in the order of the three RGB channels, and the grayscale value encoding is performed in the order of the point position from top to bottom and from left to right for each color channel image.

[0117] For the decoding process, after the RC beam generated by the control is transmitted through the ocean turbulence, the spectral distribution corresponding to each transmitted light field is obtained through the aforementioned spiral spectrum decomposition and spectrum distribution optimization algorithm, and the mode coefficient corresponding to the maximum position of the spectrum energy is read out accordingly. Figure 5 The mode coefficients of the three color channels can be read from the spectrum distribution diagram as {l1=14,l2=3}, {l1=8,l2=3}, {l1=7,l2=7}, and the mode coefficients are converted into hexadecimal numbers and combined in pairs to obtain R(d2) H 、G(72) H 、B(66) H , and then convert the two hexadecimal numbers as a whole into decimal data R(210) D 、G(114) D 、B(102) D , the decimal value is the grayscale value of the corresponding image, thus restoring the data information of the point on the three color channel images; finally, the three grayscale values ​​are combined to obtain the information of the point in the color image. Similarly, by operating all the points on the 24-bit color image, the decoding of the entire color image information is completed.

Claims

1. A communication coding method based on radial carpet beam optimized spectral distribution, characterized in that: The following steps are involved: Step 1: At the transmitting end, a radial grating is loaded into the spatial light modulator to generate an RC beam with a corresponding mode coefficient, and then the digital signal information is mapped to the RC beams of different modes to complete the information encoding; Step 2, transmitting the RC beam obtained in step 1 in an ocean channel to obtain an RC beam that takes into account the influence of ocean turbulence; Step 3: At the receiving end, the RC beam obtained in step 2 is photographed and the optimized spiral spectrum distribution is obtained. The mode coefficient of the RC beam is determined according to the energy of the spectrum distribution, and the digital signal information of the transmitting end is restored to complete the information decoding.

2. The communication encoding and decoding method based on radial carpet beam optimized spectral distribution as claimed in claim 1, characterized in that: The step 1 is specifically as follows: at the transmitting end, a fundamental mode Gaussian beam emitted by a helium-neon laser is used to illuminate a spatial light modulator, a radial grating hologram is generated according to the phase transfer function t(θ)=exp[iγcos(lθ)] of the radial grating, γ represents the grating adjustment coefficient, a radial grating with M mode coefficients of l=1, 2…, M is selected, a one-to-one mapping relationship is established between the radial grating and the digital signal, and encoding the digital signal at a certain moment is equivalent to loading the corresponding radial grating hologram into the spatial light modulator.

3. The communication encoding and decoding method based on radial carpet beam optimized spectral distribution as claimed in claim 1, characterized in that: The light field u(r, θ, z) of the RC light beam obtained after transmission in the ocean channel in step 2 is expressed as: u(r,θ,z)=u0(r,θ,z)exp[iφ(x,y)] Where (r, θ) is the polar coordinate in the cylindrical coordinate system, θ=tan -1 (y / x), i represents an imaginary unit; assuming that the light field of the RC beam transmitted to z without being affected by ocean turbulence is u0(r,θ,z), it can be expressed as: Where k is the wave number of the light wave, J (ql+1) / 2 (·) represents the (ql+1) / 2 order Bessel function of the first kind, λ is the wavelength of light, l represents the value of the beam mode coefficient; φ(x,y) is the turbulence phase added to the RC beam affected by ocean turbulence, expressed as: φ(x,y)=F[H(κ x ,k y )s 2 (k x ,k y )] In the formula, H(κ x ,κ y ) is an N×N dimensional complex random matrix with mean 0 and variance 1, F(·) represents the Fourier transform operation; σ 2 (κ x ,κ y ) is the phase screen spectrum variance, expressed as: Where Δx represents the grid spacing of the phase screen, N is the number of grids along the x or y direction of the phase screen plane, and κ x , κ y Represents the x-axis and y-axis spatial frequencies respectively; Φ(κ x ,κ y ) is the phase spectrum of ocean turbulence, expressed as: F(k x ,k y )=2πk 2 DzΦ n (k x ,k y ) In the formula, Φ n (κ x ,κ y ) is the power spectrum of ocean turbulence refractive index fluctuation, expressed as: F n (k)=0.388×10 -8 e -1 / 3 k -11 / 3 [1+2.35(k) 2 / 3 ]f(k,w,x T ) δ=8.284(κη) 4 / 3 +12.978(ki) 2 In the formula, A T , A S and A TS is a constant, κ represents the spatial frequency, ε represents the energy dissipation rate of ocean turbulence, η represents the microscale of ocean turbulence, and χ T represents the temperature variance dissipation rate of turbulence, and ω is the ratio of ocean turbulence caused by seawater temperature and salinity.

4. The communication encoding and decoding method based on radial carpet beam optimized spectral distribution as claimed in claim 3, characterized in that: When obtaining the spiral spectrum distribution of the RC beam in step 3, the RC beam is decomposed into a linear superposition of a series of spiral harmonics exp(imθ): In the formula, The corresponding energy is Therefore, the spectral components corresponding to the RC beam with the mode coefficient value of l are: Therefore, the spiral harmonic expansion of the RC beam with a mode coefficient of l is performed within a certain range, and the spectral distribution of the RC beam after transmission through ocean turbulence is obtained.

5. The communication encoding and decoding method based on radial carpet beam optimized spectral distribution as claimed in claim 4, characterized in that: The step 3 also includes optimizing the obtained spiral spectrum distribution, specifically: first, dividing the obtained RC light beam spectrum distribution after ocean turbulence transmission into three parts according to the mode coefficient values ​​of negative, equal to 0, and positive; then, the energy of the part of the spectrum distribution with the mode coefficient equal to 0 remains unchanged, and the energy corresponding to the part with negative mode values ​​is added one by one to the spectrum energy of the mutually conjugate mode coefficients, and at the same time, the energy corresponding to the part with negative mode values ​​is all set to zero; then, find out the mode coefficient l′ corresponding to the maximum value of the spectrum energy outside the mode coefficient of 0; finally, merge the spectral energy with the mode coefficient equal to 0 into l′, and set the spectral energy with the mode coefficient equal to zero to zero.

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