A method and verification device for using accompanying vortex light to offset orbital angular momentum channel crosstalk

Abstract

The present invention discloses a method and a verification device for using accompanying vortex light to offset orbital angular momentum channel crosstalk. The method comprises the following steps: using laser one and laser two to generate a signal Gaussian light GDB carrier and an accompanying Gaussian light GPB carrier respectively, and loading data onto the carrier to form a signal Gaussian light GDB and an accompanying Gaussian light GPB respectively; the generated GDB and GPB are multiplexed into a Gaussian light HGB mixed with GDB and GPB after passing through an optical separator and synthesizer; the mixed Gaussian light HGB is irradiated into a spiral phase plate VPP, and then multiplexed into a multiplexed light carrying photon orbital angular momentum OAM through a space division multiplexer SDM; after the multiplexed light carrying photon orbital angular momentum OAM passes through an atmospheric turbulence disturbance AT analog channel, a photodetector PD is used to convert the received optical signal with crosstalk phase into an electrical signal, and a useful signal is obtained from the obtained electrical signal. The present invention realizes a method for reducing vortex channel crosstalk and improves the performance of a communication system.

Description

Technical Field

[0001] The present invention belongs to the technical field of signal multiplexing, and in particular relates to a method and a verification device for using accompanying vortex light to offset orbital angular momentum channel crosstalk. Background Art

[0002] Multiplexing technology has always been considered an important way to improve channel capacity and spectrum efficiency, among which physical fields such as wavelength, polarization, time and frequency have been widely used for single or multiple physical field multiplexing. However, the channel capacity based on the above physical field multiplexing will be consumed in the near future and will no longer meet the demand for higher channel capacity of new technologies (for example, autonomous driving, big data, and computing applications). In 1992, Allen's team discovered that a vortex light with a donut-shaped energy profile can carry photon orbital angular momentum (OAM), and there is a singularity caused by a spiral phase in the center of the beam. Such a spiral phase can be expressed as a rotatable phase factor where l and Represent the topological charge and azimuthal phase respectively. Due to the mutual orthogonality between different OAM modes, the OAM channel carried by each vortex light can be regarded as an independent communication channel and can be used for spatial multiplexing (SDM) to improve channel capacity and spectral efficiency. In addition, due to the mutual independence of OAM and other physical fields, these physical fields can be used for multiplexing at the same time, which greatly promotes the improvement of channel capacity and spectral efficiency. Technology based on OAM multiplexing has been widely used in fiber optic communication and free space optical communication.

[0003] In 2012, Wang Jian's research group realized a free-space wireless optical communication system with a rate of 42.8×4Gbit / s, in which 4 OAM modes were multiplexed and an advanced signal modulation method of 16-QAM was adopted. In 2019, Chen Si's research group proposed a scheme to realize the multiplexing of 50 basic radial OAM mode lights, in which a ring-core optical fiber was used as a transmission light guide and a simplified multiple-input multiple-output (MIMO) technology was used to balance various OAM channels. In addition, compared with traditional radio communications, vortex optical communication based on OAM multiplexing has higher channel capacity and higher level of security, and it has received more and more attention. However, the spiral phase carried by vortex light is very sensitive to free-space atmospheric turbulence (AT), and atmospheric turbulence can cause distortion of the spiral phase. The distorted spiral phase can cause crosstalk between OAM modes, causing the transmitted OAM mode light energy to leak into other OAM mode light fields, resulting in crosstalk between different OAM modes, thereby reducing the performance of the communication system.

[0004] In order to reduce the impact of crosstalk on communication performance, scientists and engineers have studied various methods. These methods can be roughly divided into the following types, namely adaptive optical compensation (AO), MIMO channel equalization and deep learning (DL) correction.

[0005] In 2021, Wang et al. proposed a phase compensation method for OAM-encoded quantum key distribution system, which established a mapping relationship between turbulent disturbance and damaged phase by constructing a neural convolutional network (CNN) structure, predicted and corrected the damaged phase, and realized phase compensation. In 2019, Li Si's research group proposed a method to reduce turbulent disturbance based on 2×2 MIMO equalization, using two OAM modes (l=+1 and l=-3) multiplexing to establish an OAM channel link between base stations and unmanned aerial vehicles with a rate of 40Gbit / s. In 2020, Zhao et al. proposed a free space channel using AO compensation method, which used a wavefront phase sensor (WFS), a deformable mirror, a wavefront corrector (WFC) and a control loop, which were used to detect phase, change phase, correct phase and control optical transmission path, respectively, to realize the detection and compensation of turbulent disturbances and reduce the OAM channel crosstalk caused by AT. In 2019, Chang et al. proposed an adaptive phase compensation method based on the phase regression (PRA) algorithm. The original phase is calculated by detecting the damaged energy profile, thereby reducing the OAM channel crosstalk caused by AT. However, the CCN method used above requires the establishment of a large number of training and test cases in advance, which inevitably increases the system complexity and training time. The method of reducing crosstalk by using MIMO channel equalization requires an additional digital signal processor (DSP), and its algorithm complexity is proportional to the number of multiplexed OAM modes, which means that the system complexity continues to increase with the increase in the number of multiplexed OAM channels.

[0006] In addition, the above-mentioned AO-based crosstalk reduction methods can be divided into two types. One is a system composed of equipment such as WFC and WFS, which is not only expensive but also very complex to implement; the other is a system composed of PRA algorithm, corrector and feedback control loop, which is affected by the resolution of the camera (CCD), and its suppression of OAM channel crosstalk and compensation of damaged phase are very limited. Summary of the invention

[0007] Purpose of the invention: The spiral phase distortion of vortex light caused by atmospheric turbulence disturbance leads to crosstalk between OAM mode channels, resulting in the transmission efficiency and reliability of OAM communication systems, which seriously limits the ability of OAM multiplexing systems to improve channel capacity and spectrum utilization, and restricts the development of vortex optical communication technology. In order to solve the above problems, the present invention proposes for the first time a method of using companion vortex light to offset orbital angular momentum crosstalk, wherein the companion vortex light can generate a conjugate phase of the crosstalk caused by turbulence disturbance, which can offset the crosstalk phase of the signal vortex light, thereby reducing the impact of crosstalk on system performance, having lower complexity and higher economy, and can effectively promote the development of free space vortex optical communication technology.

[0008] Technical solution: On the one hand, the present invention discloses a method for using accompanying vortex light to offset orbital angular momentum channel crosstalk, the method comprising the following steps:

[0009] The first laser and the second laser are used to generate a signal Gaussian light GDB carrier and an accompanying Gaussian light GPB carrier respectively, and data are loaded onto the carrier to form a signal Gaussian light GDB and an accompanying Gaussian light GPB respectively;

[0010] The generated GDB and GPB are multiplexed into a Gaussian light HGB mixed with GDB and GPB after passing through the optical separation and synthesis device;

[0011] The mixed Gaussian light HGB is irradiated into the spiral phase plate VPP, and the spiral phase plate VPP includes n channels. Therefore, the transmitted GPB and GDB are modulated into n vortex companion lights OPB and n vortex signal lights ODB, respectively, and then multiplexed into multiplexed light carrying the orbital angular momentum OAM of photons through the space division multiplexer SDM;

[0012] After the multiplexed light carrying the orbital angular momentum (OAM) of photons passes through the atmospheric turbulence disturbance AT simulation channel, the OPB and ODB are disturbed by the same turbulence, and the damaged ODB and OPB become a series of OAM mode crosstalk groups with different coefficients.

[0013] The photodetector PD is used to convert the received optical signal with crosstalk phase into an electrical signal, and a useful signal is obtained from the obtained electrical signal.

[0014] Further, including:

[0015] The data is loaded onto the carrier to form the signal Gaussian light GDB and the accompanying Gaussian light GPB respectively, specifically including:

[0016] A pseudo-random sequence data transmitter is used to generate random serial PRBS sequence data simulating user information;

[0017] After serial-to-parallel conversion, the generated serial PRBS sequence data is distributed with a length of 4 bits to form hexadecimal data, which are mapped into digital 16-QAM signals through orthogonal amplitude modulators QAM;

[0018] The digital 16-QAM signal generated by the arbitrary wavelength generator AWG is converted into a 16-QAM analog signal;

[0019] The signal Gaussian light GDB carrier generated by laser 1 and the analog signal generated by AWG drive the in-phase orthogonal modulator together, so that the data is loaded onto the carrier to form the signal Gaussian light GDB;

[0020] The Gaussian light GPB carrier generated by the second laser and the analog signal generated by the AWG drive the in-phase quadrature modulator together, so that the data is loaded onto the carrier to form the accompanying Gaussian light GPB.

[0021] Further, including:

[0022] The Gaussian light signal GDB is expressed as:

[0023]

[0024] The accompanying Gaussian light GPB is expressed as:

[0025]

[0026] Among them, d(t,f d ), p(t,f p ), f d and f p They represent data signal, accompanying light signal, Gaussian light signal, the frequency of signal data and the frequency of accompanying signal respectively; LG represents Laguerre Gaussian light, t represents the time component, and x and y represent the components of Laguerre Gaussian light on the x and y coordinate axes respectively.

[0027] Further, including:

[0028] The transmitted GPB and GDB are modulated into n vortex companion lights OPB and n vortex signal lights ODB respectively, including:

[0029] First, the vortex companion light OPB is expressed as:

[0030]

[0031] The vortex signal light ODB is expressed as:

[0032]

[0033] Wherein, p represents the radial mode coefficient, if p=0 it represents the basic radial mode, LG represents Laguerre-Gaussian light, and l represents the topological charge number of the vortex light.

[0034] Secondly, the OAM modes carried by OPB and ODB are consistent with the topological charge value of VPP, thus forming l 1 ~l n OAM channels.

[0035] Further, including:

[0036] The structure of the space division multiplexer SDM is a cascade of n-1 optical separators BS, specifically:

[0037] After l 1 and l 2 The signal light is input to the optical splitter BS 1 In the process, the output is the same as that after l 3 The signal light is input to the optical splitter BS 2 until the optical splitter BS n-2 The output and the n The signal light is simultaneously input to the optical splitter BS n-1 The final output is multiplexed into multiplexed light carrying the photon orbital angular momentum OAM.

[0038] Further, including:

[0039] The atmospheric turbulence disturbance AT simulation channel is expressed as:

[0040]

[0041] in, u x ,u y and u z They represent the coordinate values ​​in the Cartesian coordinate system, ξ x and y Respectively represent the anisotropic coordinate values ​​in two directions, u d and u 0 Respectively expressed as u d =c(β) / d 0 and u 0 =2π / D 0 , d 0 and D 0 Denote the inner and outer diameters of the model respectively, c(β) and B(β) are expressed as c(β) = {B(β)Γ[(5-β) / 2]2π / 3} 1 / (β-5) and B(β)=Γ(β-1)cos(πβ2) / 4π 2 , Γ represents the gamma function, β represents the spectral coefficient of the Kolmogorov model, is the model refractive structure coefficient, which indicates the intensity of turbulent disturbance. The higher the value, the stronger the turbulence.

[0042] Further, including:

[0043] The damaged ODB and OPB become a series of OAM mode crosstalk groups with different coefficients, expressed as:

[0044]

[0045]

[0046] Among them, m l and n l represent the complex coefficients of each OAM mode in the crosstalk group of different damaged ODBs and OPBs, W(x,y) represents the crosstalk mode group formed by ODB and OPB interfered by AT, or In theory, since ODB and OPB suffer from the same AT effect, m l With n l are equal, if we ignore the higher-order radial modes p ≥ 1 and almost all the light is received, then

[0047] Further, including:

[0048] The method of converting the received optical signal with crosstalk phase into an electrical signal by using a photodetector PD comprises:

[0049] At the receiving end, the square-rate hybrid detection method is used to convert the received optical signal into an electrical signal, and its photocurrent is expressed as:

[0050]

[0051] The symbols *, |·| and Re(·) represent conjugate operation, modular operation and real number operation respectively; |d(t,f d )| 2 ,|p(t,f p )| 2 and 2Re[d(t,f d )·p(t,f p ) * ] represent the inter-signal jump signal SSB, the DC signal PPB and the intermediate frequency signal Δf(f d -f p ), Δf(f d -f p ) represents the frequency difference between the detection light and the signal light; Indicates the OAM mode group of crosstalk; W(x,y) * represents the conjugated crosstalk OAM mode, due to W(x,y) * and W(x,y) is a conjugate mode group, so ∫∫|W| 2 dxdy can be simplified to because So The above formula is further simplified to:

[0052] I∝|d(t,f d )| 2 +|p(t,f p )| 2 +2Re[d(t,f d )·p(t,f p ) * ]

[0053] The last intermediate frequency signal in the formula is the useful signal that needs to be obtained, and the other two signals are filtered out separately through filters.

[0054] On the other hand, the present invention also provides a device for verifying a method for using accompanying vortex light to offset orbital angular momentum channel crosstalk, characterized in that the device comprises:

[0055] First, according to the polynomial f(x)=x 31 +x 28 +1, the PRBS-31 module generates a pseudo-random sequence for simulating user data. Through the 16-QAM modulation module, the random sequence is first divided into multiple groups of 4-bit sequences, which are mapped into digital 16-QAM signals respectively; using an arbitrary wavelength generator AWG, the digital 16-QAM signal is converted into an analog 16-QAM signal;

[0056] The Gaussian optical carrier generated by the laser ECL1 drives the orthogonal modulator I / Q together with the simulated 16-QAM signal to load data onto the optical carrier to form a GDB signal. After being amplified and filtered by the Er-doped fiber amplifier EDFA and the bandpass filter BPF, the GDB is evenly divided into upper and lower GDB paths by the fiber coupler OC. After delayed transmission through single-mode optical fibers SMF of different lengths, they form two independent upper and lower GDB branch channels.

[0057] After the upper branch GDB is regulated by the polarization controller PC2 and the collimator C2, the GDB in the optical fiber is converted into a linearly polarized free-space GDB. The Gaussian optical carrier output by the laser ECL2 is used as the GPB of the upper branch Ch1. After conditioning by PC1 and C1, the upper branch GPB is adjusted to the linearly polarized free-space GPB of Ch1. The reflector M1 is used to adjust the transmission direction of the light beam. In the upper branch, the free-space GDB and GPB are converged into one light beam through the action of the light separator BS1.

[0058] Two prisms L1 and L2 with different focal lengths form a beam expansion system. The focal length of L1 is f, and the focal length of L2 is 2f. When the mixed GDB and GPB pass through this beam expansion system, the beam size is increased. A larger beam size is conducive to improving the efficiency of conversion between Gaussian light and vortex light.

[0059] The polarization controller composed of the polarizer P1 and the half-wave plate HWP1 modulates the GPB and GDB transmitted in the free space into linear polarized light again. Similarly, the lower branch GDBCh2 output by the OC is converted into linearly polarized free-space GDB light after being regulated by the polarization controller PC3 and the collimator C3. The Gaussian optical carrier output by the laser ECL3 is used as the GPB of the lower branch, which is also converted into the lower branch GPB of free-space linear polarization after being regulated by the polarization controller PC4 and the collimator C4. The GDB and GPB of the lower branch are synthesized by the optical separator BS2 and converged into a mixed light. After being adjusted by the lens L3, the lens L4, the polarizer P2 and the half-wave plate HWP, the size of the mixed light of the lower branch is enlarged and the polarization is adjusted to linear polarization respectively.

[0060] Secondly, the mixed light of the upper and lower branches GDB and GPD illuminates the spiral phase plates VPP1 and VPP2 respectively, and then transforms into the vortex light of the upper and lower branches respectively. Among them, the l carried by the upper branches ODB and OPB 1 4; l carried by the lower branch ODB and OPB 2 The upper and lower vortex light branches are multiplexed by BS3 to form a vortex light beam l 1 and l 2 In order to simulate the AT channel, the spatial light modulator SLM loads AT phase diagrams of different intensities to achieve different degrees of AT interference on the incident light. 1 and l 2 When irradiated onto the SLM, their spiral phases are disturbed by turbulence to different degrees, and the spiral phases are distorted in different programs, generating multiple OAM crosstalk modes. Through the above method, the turbulence disturbance of the two OAM channels of space division multiplexing is achieved;

[0061] Finally, the vortex light beam after turbulent disturbance is divided into two groups by the light separator BS4. One group of light is captured by a charge coupled device CCD to observe the energy profile of the vortex light beam after turbulent disturbance and the influence of different intensities AT on the donut-shaped energy profile. The other group of light is focused by lens L5 and incident on a PD to generate relevant electrical signals. The oscilloscope OSC and the linear processing module are used to capture the demodulated electrical signals and the corresponding system performance analysis, respectively.

[0062] Beneficial effects: The present invention proposes a method for using an accompanying vortex light to offset the OAM channel crosstalk, which adopts a method of simultaneously transmitting an accompanying vortex light (OPB) with the same OAM value as the signal vortex light (ODB), wherein the OPB light can generate a conjugate phase of crosstalk at the photoelectric receiver, and these conjugate phases can offset each other with the crosstalk phase of ODB, thereby realizing a method for reducing vortex channel crosstalk and improving the performance of the communication system. The corresponding theoretical analysis proves that since OPB and ODB have the same OAM mode and are subject to the same AT influence, the crosstalk conjugate phase generated by OPB can offset each other with the crosstalk phase of ODB, thereby reducing the impact of the crosstalk caused by AT on the performance of the OAM multiplexing system. In order to verify the feasibility of the above method, the present invention also designs an experimental scheme to analyze the impact of AT on the vortex optical communication transmission system from the aspects of the received vortex light energy profile, bit error rate and constellation diagram, and demonstrates the improvement effect of the present invention on offsetting OAM crosstalk. Theoretical analysis and experimental results prove the theoretical feasibility and practical feasibility of this scheme. BRIEF DESCRIPTION OF THE DRAWINGS

[0063] Figure 1 A conceptual diagram of the method for using accompanying vortex light to offset orbital angular momentum channel crosstalk according to the present invention;

[0064] Figure 2 The process of generating and multiplexing the signal Gaussian light and the accompanying Gaussian light of the present invention;

[0065] Figure 3 This is a structural diagram of n OAM channel multiplexing according to the present invention;

[0066] Figure 4 It is a position distribution diagram of the beating signal SSB, the direct current signal PPB and the intermediate frequency signal in the frequency spectrum of the present invention;

[0067] Figure 5 It is a schematic diagram of the structure of the verification device of the present invention;

[0068] Figure 6 It is a schematic diagram of the influence of different intensities of AT on the energy intensity profile of single vortex light and multiplexed vortex light according to the present invention;

[0069] Figure 7 It is a schematic diagram of the influence of AT of different strengths and the method adopted by the present invention on OAM crosstalk;

[0070] Figure 8 It is a schematic diagram of the influence of AT of different strengths and the method adopted by the present invention on the BER and CP of the OAM channel;

[0071] Fig. 9 It is a schematic diagram of the stability of the system BER of the present invention;

[0072] Fig.10 A schematic diagram of bit error rate distribution of different OAM modes according to the present invention;

[0073] Fig.11 This is a schematic diagram of the impact of different transmission rates on the performance of a communication system according to the present invention. DETAILED DESCRIPTION

[0074] The technical method of the present invention is described in detail below.

[0075] like Figure 1 As shown, the present invention provides a method for using accompanying vortex light to offset orbital angular momentum channel crosstalk, specifically:

[0076] Step 1: Generate and multiplex companion Gaussian light (GPB) and signal Gaussian light (GDB)

[0077] exist Figure 1 middle, Figure 1 a1 and Figure 1 The b1 in the figure represents the generated signal Gaussian light and accompanying Gaussian light and their multiplexing process. Figure 2 The detailed generation and modulation process of the above OPB and ODB is described, wherein the pseudo-random sequence (PRBS) data transmitter generates random serial data mainly used to generate analog user information, wherein the polynomial f(x)=x 31 +x 28 +1 is used as a random sequence generator, its total length is 2 32 -1. After serial-to-parallel conversion, the generated serial PRBS sequence is distributed with 4-bit length data to form hexadecimal data. These hexadecimal data are mapped into 16-QAM signals through orthogonal amplitude modulators (QAM). Through modulation by an arbitrary wavelength generator, the generated digital QAM signal is converted into a QAM analog signal. Laser 1 and laser 2 are mainly used to generate carriers respectively, with a frequency difference of 0.1 GHz between them. The GDB carrier generated by laser 1 drives the in-phase orthogonal modulator together with the 16-QAM analog signal generated by the AWG, so that the data is loaded onto the carrier to form a GDB. The generated GDB and GPB can be expressed as equations (1) and (2) respectively:

[0078]

[0079]

[0080] Among them, d(t,f d ), p(t,f p ), f d and f p They represent the data signal, the accompanying light signal, the Gaussian light signal, the frequency of the signal data and the frequency of the accompanying signal respectively.

[0081] The generated GDB and GPB are as follows Figure 1 As shown in a1 and b1 in Figure 2 The light shown is splitter and combiner (BS) and then multiplexed into a Gaussian light (HGB) mixed with GDB and GPB.

[0082] Step 2: Generate and multiplex the companion vortex light (OPB) and the signal vortex light (ODP)

[0083] like Figure 1 As shown, Figure 1 a1 and Figure 1 The HGB after b1 irradiation is Figure 1 After a spiral phase plate (VPP) is applied as shown in c in the figure, the transmitted GPB and GDP are modulated into vortex companion light (OPB) and vortex signal light (ODB), which can be expressed as Equations (3) and (4) respectively.

[0084]

[0085]

[0086] Here, p represents the radial mode coefficient (p=0 represents the basic radial mode).

[0087] The OAM mode carried by OPB and ODB is consistent with the topological charge or OAM mode (l) value of VPP. Although the l value of OAM is an integer and has no limit in theory, in practice, due to the limitation of processing progress, the l value of VPP is currently limited to the range of -10 to +10. Using the above method, Figure 1 a1 and a2 in Figure 1 b1 and b2 in Figure 1 The c in the figure forms a vortex light l carrying OPB 1 Channel (Ch1). Using a similar approach, we can construct 1 ~l n (Ch1~Chn) OAM channels. These n OAM channels are Figure 1 The space division multiplexer (SDM) shown in d in FIG. multiplexes the two signals into a beam of OAM multiplexed light. Figure 3 Describes Figure 1 The internal structure of SDM shown in (d) mainly consists of n-1 BSs in cascade.

[0088] Step 3: Construct the atmospheric turbulence disturbance simulation channel

[0089] Figure 1 The e in the figure describes the AT simulation channel based on the Kolmogorov model. The Kolmogorov model is a turbulence disturbance model defined by the foreign name "Kolmogorov". Its expression is shown in formula (5):

[0090]

[0091] in, u x ,u y and u z They represent the coordinate values ​​in the Cartesian coordinate system, ξ x and y Respectively represent the anisotropic coordinate values ​​in two directions, u d and u 0 can be expressed as u d =c(β) / d 0 and u 0 =2π / D 0 , d 0 and D 0 Respectively represent the inner and outer diameters of the model. c(β) and B(β) can be expressed as c(β) = {B(β)Γ[(5-β) / 2]2π / 3} 1 / (β-5) and B(β)=Γ(β-1)cos(πβ / 2) / 4π 2 , where Γ represents the gamma function and β represents the spectral coefficient of the Kolmogorov model. is the model refractive structure coefficient, which indicates the intensity of turbulent disturbance. The value indicates that the intensity of turbulence disturbance is stronger. According to the above description, Figure 1 The e in the equation describes the turbulence intensity. AT phase.

[0092] Step 4: Vortex optical crosstalk caused by AT

[0093] like Figure 1As shown in a3 in the figure, the phases of ODB and OPB affected by AT are distorted. Due to the influence of the same AT, OPB and OPB carrying the same OAM mode are disturbed by the same turbulence. The damaged ODB and OPB are no longer a single OAM mode, but a series of OAM mode crosstalk groups with different coefficients, which can be expressed as

[0094]

[0095]

[0096] Among them, m l and n l denote the complex coefficients of each OAM mode in the crosstalk group of different impaired ODBs and OPBs, or Indicates the crosstalk pattern group formed by AT interference between ODB and OPB. In theory, since ODB and OPB are affected by the same AT, m l With n l If high-order radial modes (p ≥ 1) are not considered and almost all the light is received, then

[0097] Step 5: Cancel the crosstalk phase generated by AT

[0098] At the receiving end, the square-rate hybrid detection method is used, and the photodetector (PD) converts the received optical signal into an electrical signal. Its photocurrent is shown in equation (8):

[0099]

[0100] The symbols *, |·|, and Re(·) represent conjugation operation, modular operation, and real number operation, respectively; |d(t,f d )| 2 ,|p(t,f p )| 2 and 2Re[d(t,f d )·p(t,f p ) * ] represent the inter-signal jump signal (SSB), DC signal (PPB) and intermediate frequency signal Δf (f d -f p ). In formula (8), Indicates the OAM mode group of crosstalk; W(x,y) * represents the conjugated crosstalk OAM mode, whose conjugated phase is Figure 1 As shown in b3 in . Since W(x,y) * and W(x,y) is a conjugate mode group, then the last term in (8) ∫∫|W|2 dxdy can be simplified to According to the discussion in step 4, since So Then formula (8) can be further simplified as:

[0101] I∝|d(t,f d )| 2 +|p(t,f p )| 2 +2Re[d(t,f d )·p(t,f p ) * ] (9)

[0102] Among them, the last intermediate frequency signal in formula (9) is the useful signal we want to obtain, and the other two signals can be filtered out by filters respectively. Their positions in the spectrum are shown in the following figure:

[0103] like Figure 5 As shown, in order to verify the feasibility and correctness of the above method, we established an experimental verification platform. The specific experimental steps are as follows:

[0104] First, according to the polynomial f(x)=x 31 +x 28+1, PRBS-31 module generates pseudo-random sequence for simulating user data. Through 16-QAM modulation module, the random sequence is first divided into multiple groups of sequences with 4 bits in length, and these sequences are mapped into digital 16-QAM signals respectively. Using an AWG, the digital 16-QAM signal is converted into an analog 16-QAM signal. The Gaussian optical carrier generated by ECL1 laser drives I / Q modulator together with the analog 16-QAM signal to load data into the optical carrier and form a GDB signal. After GDB is amplified and filtered by Er-doped fiber amplifier (EDFA) and bandpass filter (BPF), the quality of GDB is significantly improved. Subsequently, GDB is evenly divided into upper and lower GDB paths by fiber coupler (OC), which form two independent upper and lower GDB branch channels after delayed transmission through single-mode fiber (SMF) of different lengths. After the upper branch GDB (Ch1) is regulated by polarization controller (PC2) and collimator (C2), the GDB in the optical fiber is converted into linearly polarized free-space GDB. The Gaussian optical carrier output by ECL2 is used as the GPB of the upper branch (Ch1). After conditioning by PC1 and C1, the upper branch GPB is adjusted to the linearly polarized free-space Ch1 GPB, where the mirror (M1) is used to adjust the transmission direction of the beam. In the upper branch, the free-space GDB and GPB are converged into a beam by the action of BS1. Two prisms L1 (focal length f) and L2 (focal length 2f) with different focal lengths form a "3f" beam expansion system. When the mixed GDB and GPB pass through this "3f" system, their beam size is increased. The larger beam size is conducive to improving the efficiency of conversion between Gaussian light and vortex light. The polarization controller composed of a polarizer (P1) and a half-wave plate (HWP1) modulates the free-space transmitted GPB and GDB into linearly polarized light again. Similarly, the lower branch GDB (Ch2) output by OC is converted into linearly polarized free-space GDB light after being regulated by PC3 and C3. The Gaussian optical carrier output by ECL3 is used as the GPB of the lower branch. After being regulated by PC4 and C4, it is also transformed into the lower branch GPB of free space linear polarization. After being synthesized by BS2, the GDB and GPB of the lower branch are also converged into a mixed light. After being adjusted by L3, L4, P2 and HWP, the size of the mixed light of the lower branch is enlarged and the polarization is adjusted to linear polarization.

[0105] Secondly, the mixed light of the upper and lower branches GDB and GPD illuminates the spiral phase plates VPP1 and VPP2 respectively, and then transforms into the vortex light of the upper and lower branches respectively. Among them, the l carried by the upper branches ODB and OPB 1 4; l carried by the lower branch ODB and OPB 2 The upper and lower vortex light branches are multiplexed into a vortex light beam (l 1 and l2 ). In order to simulate the AT channel, the spatial light modulator (SLM) loads AT phase diagrams of different intensities to achieve different degrees of AT interference on the incident light. 1 and l 2 ) are irradiated onto the SLM, and their spiral phases are disturbed by turbulence to different degrees, causing different distortions of the spiral phases, generating multiple OAM crosstalk modes. Through the above method, turbulence disturbance of the two OAM channels of space division multiplexing is achieved.

[0106] Finally, the vortex beam after turbulence disturbance is divided into two groups by BS4, one of which is captured by a charge coupled device (CCD) to observe the energy profile of the vortex beam after turbulence disturbance and the effect of different intensities AT on the donut-shaped energy profile. The other group of light is focused by L5 and incident on a PD to generate relevant electrical signals. The oscilloscope (OSC) and linear processing module are used to capture the demodulated electrical signals and the corresponding system performance analysis, respectively.

[0107] In order to verify the correctness of the proposed scheme, we first observed the impact of atmospheric turbulence on the energy profile of the vortex light ring. When a single vortex light and two multiplexed vortex lights pass through the turbulence perturbation phase screen (AT channel) of different intensities simulated by SLM, Figure 6 Describes l 1 =4 and l 2 =6 and l 1 +l 2 Energy profiles of different vortex light combinations transmitted by the channel. W / O AT, weak AT, moderate AT and serious AT represent the AT intensities of no AT interference, weak AT interference, moderate AT interference and strong AT interference, respectively. Their corresponding refractive structure coefficients are divided into and Obviously, as AT increases, the distortion of the energy profile of the vortex light becomes more and more serious. Since the vortex light with a larger topological charge has a larger energy ring, the AT interference intensity it is subject to is greater. Compared with a single channel, the space-division multiplexing channel is more sensitive to AT and is more affected.

[0108] In order to quantitatively analyze the effect of different AT intensities on vortex beam transmission, the measured Figure 7 The influence of different AT strengths on OAM crosstalk and the effect of the compensation method proposed in the present invention are described. Figure 7 (a) shows that in the absence of accompanying light to cancel crosstalk, as the intensity of AT increases, the OAM crosstalk caused by AT becomes more and more serious, and the main transmitted l 1=4OAM channel energy and other OAM modes of crosstalk, resulting in a decrease in the required OAM mode energy ratio and a significant increase in the unwanted OAM mode energy ratio, leading to a deterioration in the performance of the communication system. Compared with the no AT scenario, the l 1 =4 channel energy ratios decreased by 0.35, 0.4 and 0.5 respectively. Figure 7 (c) in the figure indicates that 2 = The distribution of mode spectrum (crosstalk) of 6 modes under different AT conditions. Compared with the no AT scenario, the l 1 =6 channel energy ratios decreased by 0.36, 0.44 and 0.54 respectively. Figure 7 (b) in FIG. 4 shows the improvement of OAM crosstalk after using the method of the present invention. Figure 7 Compared with (b), after using the present invention, l 1 =4 channel energy ratios are increased by 0.23, 0.35 and 0.44 respectively, effectively suppressing the impact of AT on OAM crosstalk. Figure 7 (d) in FIG. 4 shows the improvement of OAM crosstalk after using the method of the present invention. Figure 7 Compared with (b) in FIG. 1 , after using the present invention, l under weak AT interference, medium AT interference and strong AT interference 2 =6 channel energy ratios are increased by 0.26, 0.39 and 0.47 respectively, effectively suppressing the influence of AT on OAM crosstalk. In summary, after using the present invention, the crosstalk caused by turbulence disturbance of the two channels is significantly reduced, which is beneficial to improving the performance of the communication system.

[0109] The above crosstalk analysis is mainly aimed at the research in the field of light field, but the present invention is also very important and necessary for the research on the performance of communication system. In the communication system, the experimental bit error rate (BER) and constellation diagram (CP) are usually used to describe the data transmission performance of the system. In the present invention, we also use the above BER and CP to analyze the impact of different ATs on the performance of the communication system and the improvement of the system performance by the present invention. Figure 8 (a) describes the effect of different AT intensities on l 1 =4 The impact of channel BER distribution and the compensation of the present invention on BER distribution. As the AT strength increases, the BER under weak AT interference, medium AT interference and strong AT interference is 3.8×10 -3 The BER performance under weak AT interference, medium AT interference and strong AT interference is improved by 0.94dBm, 1.71dBm and 2.62dBm respectively. Figure 8 (c) shows the effect of different AT strengths on l 1 = The influence of CP distribution of 4 channels and the improvement effect of the present invention on the constellation diagram. As the AT strength increases, the 16QAM constellation diagram becomes more and more blurred and divergent, indicating that the system is seriously interfered. After using the present invention, the 16QAM constellation diagram becomes clearer and more concentrated, effectively reducing the influence of AT. Figure 8 (b) describes the effect of different AT strengths on l 2 =6 The impact of channel BER distribution and the compensation of the present invention on BER distribution. As the AT strength increases, the BER under weak AT interference, medium AT interference and strong AT interference is 3.8×10 -3 The BER performance under weak AT interference, medium AT interference and strong AT interference is improved by 1.01dBm, 2.01dBm and 3.24dBm respectively. Figure 8 (d) shows the effect of different AT strengths on l 2 =The influence of CP distribution of 6 channels and the improvement effect of the present invention on the constellation diagram. As the AT strength increases, the 16QAM constellation diagram becomes more and more scattered, indicating that the system is seriously interfered. After using the present invention, the 16QAM constellation diagram becomes clearer and more concentrated, effectively reducing the influence of AT. The above results show that the present invention can effectively improve the performance of the communication system under AT interference.

[0110] In order to verify the stability of the present invention in improving the communication system, we conducted a half-hour BER stability measurement and analysis. Fig. 9 It shows that under the condition of -11.5dBm transmission power, when a random AT strength test is performed (random AT between weak AT and strong AT), the bit error rate distribution obtained by the present invention within 30 minutes is less than 3.8×10 -3 , which demonstrates the stability of the present invention in AT compensation, and can be stably applied to free-space OAM communication systems.

[0111] In addition, in order to verify the non-dependence of the present invention on OAM channels, the bit error rates of channels with different OAM values ​​were also tested. Fig.10 The bit error rate distribution under the influence of severe AT under different OAM channels (l = 1 to 8) is shown. The measurement results show that the bit error rate does not increase with the increase of l value, but the bit error rate of all measured channels is lower than 3.8×10 -3 , the resulting bit errors are all correctable.

[0112] The transmission rate of a communication system is also an important parameter for measuring the quality of a communication system. The present invention also studies the influence of different transmission rates on the BER performance of the system after using the present invention. Fig.11 It shows that as the transmission rate increases, the BER performance tends to decrease. Compared with the transmission rate of 1Gbps, l 1 =4 The bit error rate performance at 10Gbps and 20Gbps of the channel transmission rate decreased by 0.79 and 1.72dBm respectively. Figure 8 In comparison, although the rate is not increased to 20 Gbps, the bit error rate distribution using the present invention is still superior to the bit error rate distribution without using the present invention and under weak AT conditions.

[0113] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the technical principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for using accompanying vortex light to offset orbital angular momentum channel crosstalk, characterized in that: The method comprises the following steps: The first laser and the second laser are used to generate a signal Gaussian light GDB carrier and an accompanying Gaussian light GPB carrier respectively, and data are loaded onto the carrier to form a signal Gaussian light GDB and an accompanying Gaussian light GPB respectively; The generated GDB and GPB are multiplexed into a Gaussian light HGB mixed with GDB and GPB after passing through the optical separation and synthesis device; The mixed Gaussian light HGB is irradiated into the spiral phase plate VPP, and the spiral phase plate VPP includes n channels. Therefore, the transmitted GPB and GDB are modulated into n vortex companion lights OPB and n vortex signal lights ODB, respectively, and then multiplexed into multiplexed light carrying the orbital angular momentum OAM of photons through the space division multiplexer SDM; After the multiplexed light carrying the orbital angular momentum (OAM) of photons passes through the atmospheric turbulence disturbance AT simulation channel, the OPB and ODB are disturbed by the same turbulence, and the damaged ODB and OPB become a series of OAM mode crosstalk groups with different coefficients. A photodetector PD is used to convert the received optical signal with crosstalk phase into an electrical signal, and a useful signal is obtained from the obtained electrical signal. The electrical signal includes a signal jump signal SSB between signals, a DC signal PPB and an intermediate frequency signal. After the signal jump signal SSB and the DC signal PPB are filtered out by filters respectively, the obtained intermediate frequency signal is the useful signal.

2. The method for using accompanying vortex light to offset orbital angular momentum channel crosstalk according to claim 1, characterized in that: The data is loaded onto the carrier to form the signal Gaussian light GDB and the accompanying Gaussian light GPB respectively, specifically including: A pseudo-random sequence data transmitter is used to generate random serial PRBS sequence data simulating user information; After serial-to-parallel conversion, the generated serial PRBS sequence data is distributed with a length of 4 bits to form hexadecimal data, which are mapped into digital 16-QAM signals through orthogonal amplitude modulators QAM; The digital 16-QAM signal generated by the arbitrary wavelength generator AWG is converted into a 16-QAM analog signal; The signal Gaussian light GDB carrier generated by laser 1 and the analog signal generated by AWG drive the in-phase orthogonal modulator together, so that the data is loaded onto the carrier to form the signal Gaussian light GDB; The Gaussian light GPB carrier generated by the second laser and the analog signal generated by the AWG drive the in-phase quadrature modulator together, so that the data is loaded onto the carrier to form the accompanying Gaussian light GPB.

3. The method for using accompanying vortex light to offset orbital angular momentum channel crosstalk according to claim 2, characterized in that: The Gaussian light signal GDB is expressed as: The accompanying Gaussian light GPB is expressed as: Among them, d(t,f d ), p(t,f p ), f d and f p They respectively represent data signal, accompanying light signal, Gaussian light signal, the frequency of signal data and the frequency of accompanying signal; LG represents Laguerre Gaussian light, t represents the time component, and x and y represent the components of Laguerre Gaussian light on the x and y coordinate axes respectively.

4. The method for using accompanying vortex light to offset orbital angular momentum channel crosstalk according to claim 3, characterized in that: The transmitted GPB and GDB are modulated into n vortex companion lights OPB and n vortex signal lights ODB respectively, including: First, the vortex companion light OPB is expressed as: The vortex signal light ODB is expressed as: Where p represents the radial mode coefficient, if p = 0, it represents the basic radial mode; LG represents Laguerre-Gaussian light; l represents the topological charge number of the vortex light; Secondly, the OAM modes carried by OPB and ODB are consistent with the topological charge value of VPP, thus forming l1~l n OAM channels.

5. The method for using accompanying vortex light to offset orbital angular momentum channel crosstalk according to claim 4, characterized in that: The structure of the space division multiplexer SDM is a cascade of n-1 optical separators BS, specifically: The signal light passing through l1 and l2 is input into the optical separator BS1, and its output and the signal light passing through l3 are input into the optical separator BS2 until the optical separator BS n-2 The output and the n The signal light is simultaneously input to the optical splitter BS n-1 The final output is multiplexed into multiplexed light carrying the photon orbital angular momentum OAM.

6. The method for using accompanying vortex light to offset orbital angular momentum channel crosstalk according to claim 5, characterized in that: The atmospheric turbulence disturbance AT simulation channel is expressed as: in, u x ,u y and u z They represent the coordinate values ​​in the Cartesian coordinate system, ξ x and y Respectively represent the anisotropic coordinate values ​​in two directions, u d and u0 are respectively represented as u d =c(β)d0 and u0=2πD0, d0 and D0 represent the inner and outer diameters of the model respectively, c(β) and B(β) are respectively expressed as c(β)={B(β)Γ[(5-β)2]2π3} 1(β-5) and B(β)=Γ(β-1)cos(πβ2)4π 2 , Γ represents the gamma function, β represents the spectral coefficient of the Kolmogorov model, is the model refractive structure coefficient, which indicates the intensity of turbulent disturbance. The higher the value, the stronger the turbulence.

7. The method for using accompanying vortex light to offset orbital angular momentum channel crosstalk according to claim 6, characterized in that: The damaged ODB and OPB become a series of OAM mode crosstalk groups with different coefficients, expressed as: Among them, m l and n l represent the complex coefficients of each OAM mode in the crosstalk group of different damaged ODBs and OPBs, W(x,y) represents the crosstalk mode group formed by ODB and OPB interfered by AT, or In theory, since ODB and OPB suffer from the same AT effect, m l With n l are equal, if we ignore the higher-order radial modes p ≥ 1 and almost all the light is received, then 8. The method for using accompanying vortex light to offset orbital angular momentum channel crosstalk according to claim 7, characterized in that: The method of converting the received optical signal with crosstalk phase into an electrical signal by using a photodetector PD comprises: At the receiving end, the square-rate hybrid detection method is used to convert the received optical signal into an electrical signal, and its photocurrent is expressed as: I∝∫∫[d(t,f d )+p(t,f p )]·W(x,y)·W(x,y) * ·[d(t,f d )+p(t,f p )] * dxdy =[d(t,f d )+p(t,f p )]·[d(t,f d )+p(t,f p )] * ·∫∫W(x,y)·W(x,y) * dxdy ={|d(t,f d )| 2 +|p(t,f p )| 2 +2Re[d(t,f d )·p(t,f p ) * ]}·∫∫W| 2 dxdy The symbols *, |·| and Re(·) represent conjugate operation, modular operation and real number operation respectively; |d(t,f d )| 2 ,|p(t,f p )| 2 and 2Re[d(t,f d )·p(t,f p ) * ] represent the inter-signal jump signal SSB, DC signal PPB and intermediate frequency signal, Δf(f d -f p ) represents the frequency difference between the detection light and the signal light; Indicates the OAM mode group of crosstalk; W(x,y) * represents the conjugated crosstalk OAM mode, due to W(x,y) * and W(x,y) is a conjugate mode group, so ∫∫|W| 2 dxdy can be simplified to because So The above formula is further simplified to: I∝|d(t,f d )| 2 +|p(t,f p )| 2 +2Re[d(t,f d )·p(t,f p ) * ] The last intermediate frequency signal in the formula is the useful signal that needs to be obtained, and the other two signals are filtered out separately through filters.

9. A device for verifying a method of using accompanying vortex light to offset orbital angular momentum channel crosstalk, characterized in that: The device includes: First, according to the polynomial f(x)=x 31 +x 28 +1, the PRBS-31 module generates a pseudo-random sequence for simulating user data. Through the 16-QAM modulation module, the random sequence is first divided into multiple groups of 4-bit sequences, which are mapped into digital 16-QAM signals respectively; using an arbitrary wavelength generator AWG, the digital 16-QAM signal is converted into an analog 16-QAM signal; The Gaussian optical carrier generated by the laser ECL1 drives the orthogonal modulator I / Q together with the simulated 16-QAM signal to load data onto the optical carrier to form a GDB signal. After being amplified and filtered by the Er-doped fiber amplifier EDFA and the bandpass filter BPF, the GDB is evenly divided into upper and lower GDB paths by the fiber coupler OC. After delayed transmission through single-mode optical fibers SMF of different lengths, they form two independent upper and lower GDB branch channels. After the upper branch GDB is regulated by the polarization controller PC2 and the collimator C2, the GDB in the optical fiber is converted into a linearly polarized free-space GDB. The Gaussian optical carrier output by the laser ECL2 is used as the GPB of the upper branch Ch1. After conditioning by PC1 and C1, the upper branch GPB is adjusted to the linearly polarized free-space GPB of Ch1. The reflector M1 is used to adjust the transmission direction of the light beam. In the upper branch, the free-space GDB and GPB are converged into one light beam through the action of the optical separator BS1. Two prisms L1 and L2 with different focal lengths form a beam expansion system. The focal length of L1 is f, and the focal length of L2 is 2f. When the mixed GDB and GPB pass through this beam expansion system, the beam size is increased. A larger beam size is conducive to improving the efficiency of conversion between Gaussian light and vortex light. The polarization controller composed of the polarizer P1 and the half-wave plate HWP1 modulates the GPB and GDB transmitted in the free space into linear polarized light again. Similarly, the lower branch GDBCh2 output by the OC is converted into linearly polarized free-space GDB light after being regulated by the polarization controller PC3 and the collimator C3. The Gaussian optical carrier output by the laser ECL3 is used as the GPB of the lower branch, which is also converted into the lower branch GPB of free-space linear polarization after being regulated by the polarization controller PC4 and the collimator C4. The GDB and GPB of the lower branch are synthesized by the optical separator BS2 and converged into a mixed light. After being adjusted by the lens L3, the lens L4, the polarizer P2 and the half-wave plate HWP, the size of the mixed light of the lower branch is enlarged and the polarization is adjusted to linear polarization respectively. Secondly, after the mixed light of the upper and lower branches GDB and GPD irradiates the spiral phase plates VPP1 and VPP2 respectively, they are transformed into vortex light of the upper and lower branches respectively, among which l1 carried by the upper branches ODB and OPB is 4; l2 carried by the lower branches ODB and OPB is 6; the upper branch vortex light l1 and the lower branch vortex light l2 are passed through the optical separator BS3 to realize the spatial division multiplexing of different vortex lights. In order to simulate the AT channel, the spatial light modulator SLM realizes different degrees of AT interference on the incident light by loading AT phase diagrams of different intensities. When the spatially multiplexed vortex lights l1 and l2 are irradiated on the SLM, their spiral phases are disturbed by turbulence to different degrees, and the spiral phases are distorted to different degrees, generating multiple OAM crosstalk modes; Finally, the vortex light beam after turbulent disturbance is divided into two groups by the light separator BS4. One group of light is captured by a charge coupled device CCD to observe the energy profile of the vortex light beam after turbulent disturbance and the influence of different intensities AT on the donut-shaped energy profile. The other group of light is focused by lens L5 and incident on a PD to generate relevant electrical signals. The oscilloscope OSC and the linear processing module are used to capture the demodulated electrical signals and the corresponding system performance analysis, respectively.

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