Anti-turbulence free space optical communication system based on frozen wave depth scanning

Abstract

The invention belongs to the technical field of optical wireless communication, and particularly relates to an anti-turbulence free space optical communication system based on frozen wave depth scanning. The system comprises a transmitter, a transmission channel and a receiver, at a transmitter end, the GaN visible light laser is integrated with a super-structure device which generates different freezing waves at different diffraction orders; the transmission channel is an underwater turbulence environment with disturbance, namely a turbulence channel; a 1 * 2 PD array is used at a receiver end, and the signal quality is optimized based on an MIMO maximum ratio merging algorithm. According to the invention, a spatial light modulator is used to modulate a plurality of different freezing waves, and each freezing wave detects turbulence information at different positions; and extracting the part with the maximum signal-to-noise ratio from the multiple beams of frozen waves by using a maximum ratio merging algorithm, and merging the part, the signal-to-noise ratio of the final merged signal being higher than that of any signal, thereby improving the transmission capacity of the channel. Experiments show that the problems of mode crosstalk, centroid drifting, light intensity fluctuation and the like in a designated area can be effectively relieved.

Description

Technical Field

[0001] The present invention belongs to the technical field of optical wireless communication, and in particular relates to a free space optical communication system. Background Art

[0002] Free-space optical communication is a key technology with significant technological advancements that is expected to shape the future era of communication networks. Unlike traditional fiber-optic communications, free-space links transcend terrain limitations and therefore provide unparalleled flexibility and operational efficiency in deployment. At present, research on free-space optical communications is mainly focused on transmitters and receivers, and there are relatively mature solutions for the modulation of transmitted signals and the demodulation and equalization of received signals. However, research on wavefront shaping of transmission beams has just begun. Free-space optical communications will inevitably be affected by atmospheric turbulence during long-distance transmission, which mainly comes from atmospheric wind fields, atmospheric temperature gradients, etc. Affected by the turbulent effect, the transmission quality of the light beam will be greatly reduced, resulting in undesirable results such as beam centroid drift, light intensity flicker, and mode crosstalk.

[0003] The present invention proposes a method for shaping the wavefront of a light field, namely, a frozen wave, which uses the characteristic that the focus of the frozen wave can be arbitrarily adjusted to perform a deep scan of atmospheric turbulence. The signals of different segments are combined using a maximum ratio algorithm to extract the part with the largest signal-to-noise ratio, and finally obtain a higher-speed communication signal. The frozen wave, also called a superimposed Bessel beam, is composed of multiple Bessel beams with different wave vectors superimposed according to different weights. The superimposed Bessel beams interact with each other and can eventually be focused anywhere within the working range according to design requirements to obtain a small and bright light spot. The special properties of the frozen wave enable it to be used for deep scanning of turbulence. Since the turbulence intensity, that is, the influence of turbulence on the quality of the light beam, depends on the size of the light spot, the larger the light spot, the more serious the influence of turbulence, and vice versa. Therefore, the arbitrarily focused frozen wave can perform a deep scan of different positions of the turbulent field, and the light beams carrying information of different positions of turbulence can be separated at the receiving end through a post-processing algorithm. In order to obtain better signal quality, the present invention uses the maximum ratio combining algorithm in multiple-input multiple-output (MIMO) to extract and combine the information of the maximum signal-to-noise ratio parts of different frozen waves. The combined signal has a better signal-to-noise ratio than all previous signals, ultimately improving the signal quality in turbulence.

[0004] Some researchers have used frozen waves to measure turbulence intensity [1]. They proposed a turbulence measurement algorithm based on the fundamental mode, but only measured turbulence intensity and did not apply it further. The present invention loads signals on frozen waves to achieve anti-turbulence communication signal transmission. In addition, some researchers have used self-coherence methods to perform anti-turbulence communication, passing the reference beam through the turbulent channel to increase the power of the demodulated signal [2]. However, the author only used ordinary laser beams and did not perform light field shaping on the transmitted beam. Some researchers have simulated and experimented with frozen waves. They verified that the generation of frozen waves can be experimentally achieved through computational holography. They also did not make further applications of frozen waves [3-5]. Summary of the invention

[0005] The purpose of the present invention is to provide an ultra-high-speed, large-bandwidth, high-security and radiation-free underwater complex turbulence resistant free-space optical communication system to alleviate problems such as mode crosstalk, centroid drift and light intensity fluctuation in a specified area.

[0006] The anti-turbulence free-space optical communication system provided by the present invention is based on the frozen wave deep scanning technology. In the spatial light field, a spatial light modulator is used to modulate a plurality of different frozen waves, and each frozen wave detects turbulence information at different positions; in signal processing, the maximum ratio combining algorithm in the MIMO technology is used to extract the part with the largest signal-to-noise ratio from the multiple frozen waves and combine them. The signal-to-noise ratio of the final combined signal will be higher than that of any signal, thereby improving the transmission capacity of the channel.

[0007] The present invention provides an anti-turbulence free-space optical communication system based on freezing wave depth scanning, comprising a transmitter, a transmission channel, and a receiver; at the transmitter end, a GaN visible light laser is integrated with a brand-new meta-device that can generate different freezing waves at different diffraction orders; the transmission channel is an underwater turbulent environment with disturbance (i.e., a turbulent channel); at the receiver end, a 1x2 PD array is used, and the signal quality is optimized based on a MIMO (multiple input multiple output) maximum ratio combining algorithm; wherein:

[0008] The transmitter end includes an arbitrary signal generator, a laser, a metamaterial, a lens 1, a pinhole aperture, and a lens 2 connected in sequence;

[0009] In order to generate two frozen waves with different focusing depths and realize an ultra-compact receiver, the present invention integrates a metamaterial, specifically a lens, at the front end of a GaN visible light laser; the lens is made of nanomaterials and meets a specific phase distribution.

[0010] The specific instructions are as follows:

[0011] Construct a radial intensity distribution that satisfies |F(z)| 2The non-diffracting beam satisfies the exact solution of Maxwell's equations, and its wave function is:

[0012]

[0013] Among them, k ρn and k zn are the transverse and longitudinal wave vectors respectively, N is the total number of Bessel beams, ρ and φ are the radius and azimuth, A n is the weight of each Bessel beam, J υ is the vth-order Bessel function, z is the radial position, t is the time, and ω represents the angular frequency.

[0014] Considering the designed radial intensity distribution function F(z), the weight A of each Bessel function can be calculated: n for:

[0015]

[0016] Wherein, L is the distance of light beam propagation, 0≤z≤L. The present invention hopes to concentrate the intensity distribution on the propagation axis (ρ=0), therefore, the 0th order Bessel function (v=0) is selected as the superposition of modes.

[0017] In order to generate frozen waves, the present invention uses a spatial light modulator (SLM) to replace the metamaterial to control the wavefront of the light beam. According to the exact solution of the wave equation of the desired light beam, the amplitude generated hologram (CGHs) can be calculated. Once a frozen wave of a specific spatial shape is selected, it can be described by the above formulas (1) and (2). The amplitude calculated generated hologram can reconstruct the complex amplitude field ψ(ρ, φ, z, t) of the frozen wave on the original propagation axis, and its transmittance function can be expressed by the following formula:

[0018]

[0019] Where α(x,y) and φ(x,y) are the amplitude and phase of the frozen wave complex amplitude field ψ(ρ,φ,z,t), β(x,y) is a normalization constant, and ξ and η are the diffraction angles in the x and y directions, respectively.

[0020] In order to reduce the noise of the holographic spectrum signal, the traditional bias function β(x,y)=[1+α 2 (x,y)] / 2 is the soft envelope of amplitude normalization. The off-axis reference plane wave exp(i2π(ξx+ηy)) is used to obtain the complex amplitude field ψ(ρ ,Different diffraction orders are separated from the beams (φ, z, t). In the Fourier plane, the center of the signal information is moved to the spatial frequency (ξ, η) and selected according to the diffraction efficiency and bandwidth of the SLM. The Bessel beam is not infinitely superimposed, it has a maximum superposition value of 2N max +1, given by the formula,

[0021] N max =[L(kQ) / 2π], (4)

[0022] Wherein, [·] is a rounding function, L is the working distance, k is the wave vector, Q is the quality factor of focusing, where Q can be 5e-6, so the maximum superposition value Nmax is equal to 9, and N=8 is taken in the present invention.

[0023] The turbulence channel comprises a 1m free space optical link, a turbulence phase plate 1 and a turbulence phase plate 2;

[0024] The method of generating turbulent channels in the laboratory is to use two freely movable turbulent phase plates. The turbulent phase plates can be moved in a 1m free-space optical link to control the location where turbulence is generated. The atmospheric coherence length of the turbulent phase plates is 1mm. -1 , the smaller the atmospheric coherence length, the stronger the turbulence intensity. Two turbulence phase plates can ensure that turbulence is generated at any position, and can also ensure that turbulence is generated at two different positions at the same time.

[0025] The receiver end includes a lens 3, an avalanche photodiode, and an oscilloscope;

[0026] Since the transmitter emits two different freezing waves, the receiver uses a 1x2 PD (diode) array to receive the corresponding two beams of light. The two beams of light are two channels, which can detect two signals affected by different degrees of turbulence. Therefore, an optimization algorithm is required to receive the signal.

[0027] Maximum ratio combining (MRC) is an optimization technique for combining signals that transmit the same information. Maximum ratio combining is often applied to MIMO or SIMO wireless communication systems. MRC is similar to classical beamforming, where the goal of a classical beamforming solution is to maximize the signal level in a specified direction by adjusting the phase of the signal. MRC includes phase alignment, but the phase alignment may not correspond to a specific direction, and the ultimate goal is to maximize the signal-to-noise ratio of the received signal.

[0028] The symbol s[n] sent by the transmitter can be transmitted in all directions, and each symbol has a different path to reach the receiver. The MRC receiver applies different weights to each received symbol x[n]. In order to determine the best combination, it is necessary to estimate the characteristics of the channel blindly or through pilot signals. Once the channel characteristic information is estimated, more weight is given to receivers with higher signal-to-noise ratios. This weighted receiver combination and phase alignment can mitigate the effects of channel fading and reduce the bit error rate of the communication channel.

[0029] Maximum ratio combining is achieved by multiplying the N diversity signals by a different coefficient ω i ,i=1,2,...,N, and the coefficient is determined by the fading coefficient h of the N-way branch i , i=1,2,...,N. Usually there is the following relationship:

[0030]

[0031] The relationship is derived as follows: Consider an additive white Gaussian noise channel, assuming that the power of the transmitted signal is Es, the noise power spectrum density is N0, and the fading coefficient of the N branches is h b , the combined weight coefficient is ω i , then the signal-to-noise ratio at the receiving end is:

[0032]

[0033] Since Es and N0 are fixed values, to maximize SNR, the combined weighting coefficient ω of the control parameters needs to be i . Calculate the first-order partial derivative of SNR and set it equal to 0.

[0034]

[0035] Simplifying the equation, we finally get the following result:

[0036]

[0037] Substituting in m and n and eliminating the same terms gives:

[0038]

[0039] Therefore, the weighting ratio of the N-path signals is proportional to the fading coefficients of the N-path branches.

[0040] The system of the present invention is one in which each array element, called a light particle, produces two superimposed Bessel beams or "frozen waves". These beams can offset turbulence in different areas during signal transmission. At the receiver, a maximum ratio combining algorithm is used to reconstruct high-quality signals. Theoretical analysis shows that a multiple-input multiple-output (MIMO) system using frozen waves can significantly outperform traditional plane waves and approach the theoretical capacity limit set by Shannon's theorem. Experiments have verified the effectiveness of light particles in reducing turbulence by segmentation, alleviating problems such as mode crosstalk, center of mass drift, and light intensity fluctuations in a specified area. The present invention paves the way for integrating structured light into optical communication systems.

[0041] The technical features and performance advantages of the present invention mainly lie in:

[0042] (1) The frozen wave and maximum ratio combining algorithm can improve the communication quality (Q factor) by 1dB.

[0043] In order to better demonstrate the advantages of frozen wave and maximum ratio merging algorithm, the present invention compares frozen wave with Gaussian light. In the experiment, turbulent phase plates are placed at 0.3m and 0.7m, and the atmospheric coherence length of the phase plate is 1mm. -1 (Unit of turbulence size). The experimental results are as follows Figure 2 As shown, it is shown that the frozen wave plus the maximum ratio combining algorithm can improve the communication quality by 1dB.

[0044] (2) Freezing waves can effectively reduce the flicker index and communication bit error rate.

[0045] In order to better demonstrate the performance of frozen waves in turbulence, the present invention slides the position of the turbulence phase plate from 0.2m to 0.8m. A power meter is used to detect the power at the receiving end and calculate the flicker index of the light intensity (see Figure 3 ). The results show that freezing waves can effectively reduce the flicker index. And freezing waves can effectively reduce the communication bit error rate (see Figure 4 ).

[0046] (3) The divergence angle of the freezing wave is small and can achieve any adjustable focus.

[0047] The present invention tests the relative beam widths of the two freezing waves within a range of 1 m. The beams are focused at 0.3 m and 0.7 m, with the smallest beam widths, and the beams do not have large divergence angles at other positions (see Figure 5 shown).

[0048] The present invention demonstrates the comparison between the frozen wave and Gaussian light with adjustable focus. The results show that the fundamental mode ratio of the frozen wave with adaptive focus is basically maintained at more than 80%, which is much higher than that of the Gaussian beam. In addition, the frozen wave has the advantages of small divergence angle and adjustable focus (see Figure 6 shown).

[0049] The system of the present invention is one in which each array element, called a photon, produces two superimposed Bessel beams or "frozen waves". These beams can cancel out turbulence in different areas during signal transmission. At the receiver, a maximum ratio combining algorithm is used to reconstruct high-quality signals. Theoretical analysis shows that multiple-input multiple-output (MIMO) systems using frozen waves can significantly outperform traditional plane waves, approaching the theoretical capacity limit set by Shannon's theorem. Experiments have verified the effectiveness of photons in reducing turbulence by segmentation, alleviating problems such as mode crosstalk, center of mass drift and light intensity fluctuations in a specified area. Our findings pave the way for integrating structured light into optical communication systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Figure 1 It is a structural diagram of the anti-turbulence free-space optical communication system based on frozen wave depth scanning of the present invention.

[0051] Figure 2 The figure is a comparison of the Q quality factor of the freezing wave of the present invention and the Gaussian beam under different driving voltages.

[0052] Figure 3 The figure is a comparison of the scintillation index of the frozen wave method of the present invention and the Gaussian beam at different turbulent locations.

[0053] Figure 4 The figure is a comparison of the bit error rates of the frozen wave method of the present invention and the Gaussian beam at different positions of turbulence.

[0054] Figure 5 is the relative width of the beam of the freezing wave of the present invention propagating along the z-axis during simulation and experiment.

[0055] Figure 6 The figure is a comparison of the fundamental mode ratios of the frozen wave method of the present invention and the Gaussian beam at different turbulent locations. DETAILED DESCRIPTION

[0056] The present invention is further described below in conjunction with specific simulation and experimental methods and results.

[0057] The present invention is based on the frozen wave depth scanning anti-turbulence free space optical communication system, see Figure 1As shown; the light source part adopts a 532nm green laser (PL520) to generate a Gaussian laser beam for the experiment. The laser adopts an internal modulation scheme, and the signal is generated by offline processing with MATLAB software and loaded onto the laser through an arbitrary signal generator. The emitted laser is incident on the spatial light modulator (SLM) after collimation. The spatial light modulator is loaded with a pre-generated computer-generated hologram, and the light beam is modulated by the SLM to generate the required freezing wave complex amplitude light field. The modulated light is spatially filtered through a 4f system, and the 4f system includes a lens 1, a pinhole aperture, and a lens 2 connected in sequence, wherein the focal lengths of lens 1 and lens 2 are 200mm, and the pinhole aperture filters out ±1-level light. Since the modulation effects of ±1-level light are completely opposite, the working distance of the light beam in the present invention is 1m, the focus of the positive first-level light is designed to be 0.3m, and the focus of the negative first-level light is 0.7m. Therefore, the present invention can generate two freezing waves that meet the design requirements after only one spatial modulation.

[0058] Two frozen waves with different focal points are transmitted through a 1m free space link, represented by green beams in the figure. In order to simulate the turbulence phenomenon in the atmosphere, the present invention uses two continuously rotating turbulence phase plates, which are selectively placed at 0.3m and 0.7m in the 1m link to study the influence of atmospheric turbulence on free space optical communication. Finally, the received beam is focused by lens 3, which has a focal length of 25mm, and received by an avalanche photodiode. The final signal is received by an oscilloscope and processed offline by MATLAB software.

[0059] Transmit digital signal processing (DSP): First, use MATLAB to generate a pseudo-random raw binary bit stream. The raw binary bit stream is converted into a baseband signal through quadrature amplitude modulation (QAM), 4x upsampling, and Nyquist filtering. In order to convert the complex signal into a real signal, after IQ separation, the real and imaginary parts of the signal are multiplied by cosine and sine carriers respectively, and the bandpass signal is obtained after carrier modulation.

[0060] Receive digital signal processing (DSP): At the receiving end, the present invention applies offline DSP technology to recover the original data. The received signal is equalized and restored to the waveform signal after DSP, and the restored waveform is demodulated to obtain the constellation points of the signal. Then, after filtering, four-fold downsampling and QAM demapping, the received data is converted into a binary signal, and the bit error rate (BER) is calculated at this stage. As long as the bit error rate is lower than the hard decision threshold, the signal can be considered to be transmitted without error.

[0061] Figure 2What is shown is that we traverse the peak-to-peak ratio (Vpp) of the signal from 0.8V to 1.0V, and test the Q quality factor of the received signal under two turbulent phase plates. As Vpp increases, the Q quality factor continues to increase, because the increase in Vpp brings an improvement in the signal-to-noise ratio. Since Gaussian light has no modulation, it is severely affected by turbulence and has the lowest Q quality factor. Secondly, there are two frozen waves with different focal points. Since turbulent phase plates are placed at 0.3m and 0.7m, the frozen wave can be unaffected at one turbulent location but greatly affected at another location, so the Q quality factor can be slightly better than that of the Gaussian beam. For example, the frozen wave focused at 0.3m is less affected by the turbulent phase plate at 0.3m and more affected by the turbulent phase plate at 0.7m; while the Gaussian light is greatly affected by the turbulent phase plates at both locations. Finally, the signals received by the two frozen waves are combined by maximum ratio. The maximum ratio combining algorithm effectively combines the signals received by the two frozen waves at different locations, extracts the part with the largest signal-to-noise ratio and combines them. Since the signals with the best signal-to-noise ratio at different locations are combined, the final Q quality factor is better than the other three cases. Therefore, the frozen wave plus maximum ratio combining algorithm can improve the communication quality by 1dB.

[0062] Figure 3 The turbulence phase plate was slid from 0.2m to 0.8m, and a power meter was used to detect the power at the receiving end and calculate the light intensity flicker index. The light intensity flicker index is an indicator of the turbulence size. The larger the light intensity flicker index, the greater the light beam is affected by turbulence, which ultimately affects the communication quality. Figure 3 It can be seen that no matter where the turbulent phase plate moves to, the effect on the flicker index of the Gaussian beam is relatively close. For the two frozen waves, when the turbulent phase plate moves to the focus corresponding to the frozen wave, the light intensity flicker index gradually decreases to the lowest point. When the turbulent phase plate is at 0.3m, the flicker index of the frozen wave (f=0.3) is 7E-5. When the turbulent phase plate is at 0.7m, the flicker index of the frozen wave (f=0.7) is 4E-5. Therefore, the frozen wave can effectively reduce the flicker index.

[0063] Figure 4 The bit error rate of the receiver is tested when the turbulence is at different positions. Figure 3 Similarly, when the turbulent phase plate moves to the focus corresponding to the frozen wave, the bit error rate gradually decreases until it drops to the lowest point. When the turbulent phase plate is at 0.3m, the bit error rate of the frozen wave (f=0.3) is 4.8E-3. When the turbulent phase plate is at 0.7m, the bit error rate of the frozen wave (f=0.7) is 3.5E-3. Since Gaussian light is not modulated, it will be greatly affected no matter where the turbulent phase plate is placed, so its bit error rate is the highest, averaging about 9E-3. Therefore, the frozen wave can effectively reduce the communication bit error rate.

[0064] Figure 5 The relative beam widths of the two freezing waves were tested within a range of 1m. The dashed line is the simulation result, and the solid line is the experimental result. Figure 5 It can be seen that the beam is focused at 0.3m and 0.7m, the beam width is the smallest, and the beam does not have a large divergence angle at other positions. The experimental curve is basically consistent with the simulation curve.

[0065] Figure 6 The comparison between frozen waves and Gaussian light with adjustable focus is shown. Since Gaussian light and frozen waves are both generated in the fundamental mode, in the absence of turbulence, the theoretical proportion of the received beam in the fundamental mode is 100%. Figure 6 It can be seen that as the turbulence position continues to move backward, the Gaussian fundamental mode ratio slowly decreases. This is because the Gaussian beam has a certain divergence angle. As the beam diameter increases, the influence of the turbulence phase plate will gradually increase. The freezing wave adopts an adaptive adjustable focus method, and the focus of the freezing wave and the position of the turbulence phase plate are consistent. As can be seen from the figure, the fundamental mode ratio of the freezing wave with adaptive focus is basically maintained at more than 80%, which is much higher than that of the Gaussian beam. In addition, the present invention also compares the results of adding three lenses with different focal lengths to the Gaussian beam. The Gaussian beam can also maintain a high fundamental mode ratio at its focus, but away from the focus, the Gaussian beam diverges rapidly, so that it is severely affected by turbulence and the fundamental mode ratio decreases rapidly. In addition, the Gaussian beam can only be focused by a lens with a fixed focal length, and the focus cannot be flexibly adjusted. Therefore, the freezing wave has the advantages of a small divergence angle and arbitrarily adjustable focus.

[0066] References:

[0067] [1]H.Zhou,

[0068] Nature Communications 14,4701(2023).

[0069] [2]R.Zhang,N.Hu,H.Zhou,K.Zou,X.Su,Y.Zhou,H.Song,K.Pang,H.Song,andA.Minoofar,"Turbulence-resilient pilot-assisted self-coherentfree-spaceoptical communications using automatic optoelectronic mixingof many modes,"Nature Photonics 15,743-750(2021).

[0070] [3]Vieira,Tárcio A.,M.Zamboni-Rached,and M.R.R.Gesualdi."Modelingthespatial shape of nondiffracting beams:Experimental generation ofFrozenWaves via holographic method."Optics Communications 315(2014):374-380.[4]Vieira,T.A.,M.Gesualdi,and M.Zamboni-Rached."Frozen waves:

[0071] experimental generation."Web Science Optical Soc Amer,2012.

[0072] [5]Zamboni-Rached,and Michel."Stationary optical wave fields witharbitrarylongitudinal shape by superposing equal frequency Bessel beams:FrozenWaves."oe / 12 / 17 / oe-12-17-4001.pdf 12.17(2004):4001-0。

Claims

1. A turbulence-resistant free-space optical communication system based on frozen wave depth scanning, characterized in that: A spatial light modulator is used to modulate a variety of different frozen waves, each of which detects turbulence information at different locations; the maximum ratio combining algorithm in MIMO technology is used to extract the part with the largest signal-to-noise ratio from multiple frozen waves and combine them. The final combined signal has a higher signal-to-noise ratio than any other signal, thereby improving the transmission capacity of the channel; specifically, it includes a transmitter, a transmission channel, and a receiver; at the transmitter end, a GaN visible light laser is integrated with a meta-device that generates different frozen waves at different diffraction orders; the transmission channel is an underwater turbulent environment with disturbances, that is, a turbulent channel; at the receiver end, a 1x2 PD array is used, and the signal quality is optimized based on the MIMO maximum ratio combining algorithm.

2. The turbulence-resistant free-space optical communication system according to claim 1, characterized in that: Features: The transmitter end includes an arbitrary signal generator, a laser, a metamaterial, a first lens, a pinhole aperture, and a second lens connected in sequence; In order to generate two frozen waves with different focusing depths and realize an ultra-compact receiver, a metamaterial, specifically a lens, is integrated at the front end of the GaN visible light laser. The lens is made of nanomaterials. Construct a radial intensity distribution that satisfies |F(z)| 2 The non-diffracting beam satisfies the exact solution of Maxwell's equations, and its wave function is: Among them, k ρn and k zn are the transverse and longitudinal wave vectors respectively, N is the total number of Bessel beams, ρ and φ are the radius and azimuth, A n is the weight of each Bessel beam, J υ is the vth-order Bessel function, z is the radial position, t is the time, and ω represents the angular frequency; Considering the designed radial intensity distribution function F(z), the weight A of each Bessel function is calculated. n for: Where L is the distance of beam propagation, 0≤z≤L; it is hoped that this intensity distribution will be concentrated on the propagation axis, i.e., ρ=0, so the 0th-order Bessel function is selected as the superposition of modes; In order to generate frozen waves, a spatial light modulator (SLM) is used to replace the metamaterial to control the wavefront of the light beam. The amplitude generative holograms (CGHs) are calculated based on the exact solution of the wave equation of the desired light beam. Once a frozen wave of a specific spatial shape is selected, it can be described by the above formulas (1) and (2). The amplitude calculated generative hologram can reconstruct the complex amplitude field ψ(ρ, φ, z, t) of the frozen wave on the original propagation axis. Its transmittance function is expressed as follows: where α(x,y) and φ(x,y) are the complex amplitude fields of the freezing wave ψ(ρ , φ, z, t) is the amplitude and phase of the diffraction; β(x, y) is the normalization constant, ξ, η are the diffraction angles in the x and y directions respectively; In order to reduce the noise of the holographic spectrum signal, the traditional bias function β(x,y)=[1+α 2 (x,y)] / 2 is the amplitude-normalized soft envelope; different diffraction orders are separated from the encoded complex amplitude field ψ(ρ,φ,z,t) using an off-axis reference plane wave exp(i2π(ξx+ηy)); in the Fourier plane, the center of the signal information is moved to the spatial frequency (ξ,η) and selected based on the diffraction efficiency and bandwidth of the SLM; the Bessel beam is not infinitely superimposed, it has a maximum superposition value of 2N max +1, given by the formula, N max =[L(k-Q) / 2π], (4) Where [·] is the floor function, L is the working distance, k is the wave vector; Q is the focusing quality factor; The turbulence channel comprises a 1m free space optical link, a turbulence phase plate 1 and a turbulence phase plate 2; Two turbulence phase plates can be moved in a 1m free-space optical link to control the location where turbulence is generated; the atmospheric coherence length of the turbulence phase plate is 1mm -1 ; Two turbulence phase plates are used to ensure that turbulence is generated at any position, or to ensure that turbulence is generated at two different positions at the same time; The receiver end includes a lens 3, an avalanche photodiode, and an oscilloscope; The transmitter sends out two different freezing waves, and the receiver uses a 1x2 PD array to receive the corresponding two beams of light; the two beams of light are two channels, which are used to detect two signals affected by turbulence to different degrees; therefore, an optimization algorithm is needed to receive the signal.

3. The turbulence-resistant free-space optical communication system according to claim 2, characterized in that: At the receiver, an optimization algorithm is used to receive the signal. Specifically, the maximum ratio combining (MRC) technique is used to combine signals that transmit the same information. MRC includes phase alignment, and the ultimate goal is to maximize the signal-to-noise ratio of the received signal. The symbol s[n] sent by the transmitter is transmitted in all directions, and each symbol has a different path to the receiver; MR applies different weights to each received symbol x[n]; in order to determine the best combination, it is necessary to estimate the characteristics of the channel blindly or through pilot signals; once the channel characteristic information is estimated, more weight is given to the receiver with a higher signal-to-noise ratio; This weighted receiver combination and phase alignment mitigates the effects of channel fading and reduces the bit error rate of the communication channel; Maximum ratio combining is achieved by multiplying the N diversity signals by a different coefficient ω i ,i=1,2,...,N, and the coefficient is determined by the fading coefficient h of the N-way branch i , i=1,2,...,N; usually there is the following relationship:

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