CN113660041A

CN113660041A - An OAM transmission device and method for enhanced safety and turbulence mitigation

Info

Publication number: CN113660041A
Application number: CN202110930993.2A
Authority: CN
Inventors: 毛雅亚; 刘博�; 忻向军; 任建新; 黄景瑞; 王瑞春; 沈磊; 吴泳锋; 孙婷婷; 赵立龙
Original assignee: Nanjing University of Information Science and Technology
Current assignee: Nanjing University of Information Science and Technology
Priority date: 2021-08-13
Filing date: 2021-08-13
Publication date: 2021-11-16

Abstract

The invention discloses an OAM transmission device and method with enhanced security and turbulence mitigation, including a transmitting end structure, an MPLC system and a receiving end structure. The transmitting end structure is used to encrypt and modulate user data to form an OAM beam. The OAM After the light beam passes through the atmospheric turbulence area, it enters the MPLC system. The MPLC system includes multiple phase planes with different phase modes, mirrors, collimators, feedback loops and controllers. The feedback loop is connected to the connection between the MPLC system and the receiving end structure. On the channel, the crosstalk between the separated beams is monitored, and the controller can receive the signal of the feedback loop and adjust at least one phase mode of the phase plane to correct the wavefront of the distorted beam and reduce the degree of crosstalk. The present invention has the advantage of both improving safety and reducing the effects of turbulence.

Description

OAM transmission device and method for security enhancement and turbulence mitigation

Technical Field

The invention relates to an optical transmission technology in the technical field of communication, in particular to an OAM transmission device and method for enhancing safety and relieving turbulence.

Background

In the multimedia and big data era of the current high-speed development, along with the emergence of the emerging fields of big data, artificial intelligence, internet of things and the like, high speed, low energy consumption, intellectualization and security are inevitable trends of the future communication technology development. The increasing and popularizing of information transmission media such as video conference, network live broadcast, remote education and the like and the continuous and rapid increase of communication service volume and user number lead to the frequent and urgent need of traditional radio frequency RF resources, and the development of high-capacity and high-safety information transmission modes is urgent. Free space optical communication FSO technology has recently been widely studied worldwide because of its advantages such as high electromagnetic interference resistance, good security, and low manufacturing cost.

However, atmospheric turbulence can severely affect the performance of free-space optical communication systems, which can cause absorption scattering, beam spreading, drift, phase distortion, etc. of the transmitted vortex beam. Atmospheric turbulence, i.e., the movement of gas molecules in the atmosphere, temperature fluctuations, pressure variations, etc., can cause the refractive index of atmospheric channels to randomly change. In fact, some laser beams carry not only spin angular momentum, but also orbital angular momentum, OAM. In recent years, the introduction of a vortex beam with orbital angular momentum into an FSO system has been one of the hot spots of research. Due to the special spatial distribution characteristic of vortex light, vortex optical rotation light waves are dispersed by atmospheric turbulence, an OAM mode carried by the vortex light is diffused to an adjacent OAM state, so that crosstalk occurs between the OAM modes, and when the received light power is low, a light beam is distorted, and thus an error code occurs in a system. Therefore, it is of great significance to study and overcome the influence of atmospheric turbulence on high-order free-space coherent optical communications. In recent years, with the development of multimedia technology, a lot of information is publicly spread on the internet, but some sensitive information is not encrypted in the spreading process. Therefore, protection of information from unauthorized access during dissemination has attracted increasing attention during the last 20 years. However, in the current research, there are few directions for both improving safety and reducing the effect of turbulence. Designing a transmission method that both improves safety and mitigates the effects of turbulence has become a hot issue in many areas of research.

Disclosure of Invention

An object of the present invention is to provide an OAM transmission apparatus and method with enhanced security and turbulence mitigation, in order to solve the problems mentioned in the background art.

In order to achieve the technical purpose, the technical scheme adopted by the invention is as follows:

an OAM transmission device with enhanced security and turbulence mitigation, wherein: comprises a sending end structure, an MPLC system and a receiving end structure, wherein the sending end structure is used for encrypting and modulating user data to form an OAM light beam, the OAM beam after passing through the atmospheric turbulence zone enters an MPLC system, the MPLC system includes a plurality of phase planes having different phase patterns, mirrors, collimators, a feedback loop, and a controller, the OAM beam affected by the turbulence is reflected back and forth between the phase planes and the mirrors to spatially separate the OAM beam, and inclined at different angles, coupled by a collimator, and then enters a receiving end structure, a feedback loop is connected on a connecting channel between the MPLC system and the receiving end structure, crosstalk between the separated light beams is monitored, a controller can receive signals of the feedback loop and adjust at least one phase mode of a phase plane, the wave front of the distorted light beam is corrected, the crosstalk degree is reduced, and the receiving end structure can implement the inverse process of the encryption and modulation of the transmitting end structure.

The further optimization scheme of the invention is as follows:

the phase plane described above has six different phase modes.

A transmission method of an OAM transmission apparatus with enhanced security and turbulence mitigation, comprising the steps of:

a. a sending end step: distributing user data to channels with different optical wavelengths simultaneously, encrypting the data by an optical switch under the control of a frequency hopping sequence, further transmitting the encrypted data by randomly distributed channels with different OAM states, generating the frequency hopping sequence by a Chua's chaotic model, modulating binary data to be transmitted in a DSP flow, carrying out electro-optical modulation on the binary data and an optical signal in a Mach-Zehnder modulator MZM after modulation and constellation encryption are carried out on the binary data, accessing the modulated signal to an OAM generator, and modulating the data to an OAM light beam;

b. the transmission process comprises the following steps: the OAM light beam is incident on the simulated turbulence plate, the vortex light beam influenced by turbulence enters the MPLC system, the MPLC system receives a signal of a feedback loop by using a controller, adjusts the phase mode of a phase plane, performs turbulence mitigation and mode de-multiplexing on the light beam,

c. a receiving end step: vortex light carrying information is decomposed and multiplexed to be changed into a Gaussian beam, the Gaussian beam is converted into an electric signal by a photoelectric detector, and then the electric signal is subjected to decryption and demodulation opposite to the encryption and modulation of a sending end to obtain decrypted data.

The specific method for encrypting data by the optical switch under the control of the frequency hopping sequence comprises the following steps:

an NxN optical switch selector is adopted to encrypt the physical layer, a chaos sequence generated by a Chua model is used as a key, and a recursive frequency hopping method is adopted: LDi → MZMi; LDi → MZMi + 1; ③ LDi → MZMi + 2; … …, respectively; wherein the equation for the Chua model is:

wherein

Is a constant value, and is characterized in that,

are variables.

The DSP flow is divided into three steps: a constellation mapping step, an up-sampling step and a shaping filtering step;

the constellation mapping step is as follows: adopting a 16QAM constellation diagram, using a Chua model as chaotic mapping to generate a masking vector, and masking 16QAM constellation points to obtain an encrypted constellation point diagram;

the up-sampling part comprises the following steps: inserting numerical values after the constellation point coordinates, wherein the numerical value of N is the number of the inserted numerical values;

the shaping and filtering steps are as follows: the coordinates of two dimensions of the data after up-sampling are respectively sent to two filters which are orthogonal with each other for shaping and filtering, and then two paths of signals are combined by an adding unit and finally sent to an Arrayed Waveguide Grating (AWG);

in the step of receiving end, the process of deciphering and demodulating electric signal includes inverse DSP flow, the inverse DSP flow is composed of matched filter part, down sampling part and constellation demapping part, each part is corresponding to DSP flow and has opposite action.

In the transmission process, a crosstalk threshold value of the controller (25) during each measurement is given, crosstalk during each measurement is considered, a Particle Swarm Optimization (PSO) is applied to optimize graphs of the first two planes of the phase plane to correct the wavefront of a distorted beam, the first two graphs are defined as a particle of the PSO, the particle can be continuously adjusted according to the optimal position and the global extreme value during operation, then the optimal solution is obtained, and Mean Square Error (MSE) is used as a fitness function of the particle. To measure the crosstalk matrix between the two channels, the mode on the transmitter side would be switched between ℓ =0 and + 1. After the MPLC mode is optimized, signals carried by the two transmitting OAMs are received and detected with low crosstalk.

The OAM transmission method for enhancing safety and relieving turbulence has the following advantages:

1. by utilizing the MPLC system, crosstalk information among signals influenced by atmospheric turbulence is obtained in time through a feedback loop, and the phase mode of a phase plane is adjusted by utilizing a controller so as to correct the wave front of a distorted light beam and effectively reduce the crosstalk problem caused by the turbulence.

2. A turbulent flow relieving frequency hopping system based on OAM multiplexing is realized by using an optical switch and a multi-plane optical converter. When data is transmitted in a physical layer, encryption processing is sequentially carried out in two stages of optical frequency hopping and constellation modulation, and the safety of user data transmission is effectively improved. Therefore, on one hand, the influence of atmospheric turbulence on high-order free space coherent optical communication is effectively overcome, and the OAM multiplexing technology is realized; on the other hand, new safety encryption is introduced to the traditional optical frequency hopping system based on wavelength or polarization, the capacity of a space optical communication channel and the safety of a physical layer are improved simultaneously, and a new way is provided for exploring the communication safety of OAM multiplexing in free space optical communication.

Drawings

Fig. 1 is a flow diagram of an OAM transmission system with security enhancement and turbulence mitigation;

FIG. 2 is a flow chart of a DSP and inverse DSP system;

fig. 3 is a conventional 16QAM constellation;

fig. 4 is a constellation diagram after masking 16QAM constellation points;

FIG. 5 is a decrypted binary bit stream diagram;

fig. 6 is a flowchart of the MPLC system.

The label names in the figure: a transmitting end configuration 1, an MPLC system 2, a phase plane 21, a mirror 22, a collimator 23, a feedback loop 24, a controller 25, and a receiving end configuration 3.

Detailed Description

Embodiments of the present invention are described in further detail below with reference to the accompanying drawings.

In the OAM transmission method with enhanced security and turbulence mitigation according to this embodiment, a system flowchart is shown in fig. 1. The whole system can be divided into three parts, namely a sending end step, a transmission process step and a receiving end step.

The specific workflow of various aspects of the system is as follows:

detailed description of optical frequency hopping:

the patent adopts NXN optical switch selector to encrypt the physical layer, theoretically, if transmitting laser with N wavelengths, there can be N! The selection mode greatly improves the transmission safety. We use the chaos sequence generated by Chua model as the key to select different frequency hopping modes. We use the recursive frequency hopping method: phi LD_i→MZM_i ；②LD_i→MZM_i+1 ；③LD_i→MZM_i+2(ii) a … … are provided. Wherein the equation for the Chua model is:

wherein

Is a constant value, and is characterized in that,

are variables.

Details of the DSP flow:

the DSP and inverse DSP flow in the system is shown in FIG. 2:

the DSP flow can be divided into three parts: a constellation mapping part, an up-sampling part and a shaping filtering part.

(1) Constellation mapping section

The traditional 16QAM constellation diagram adopted in this patent is shown in fig. 3, and we use a Chua model as chaotic mapping to generate a masking vector, and mask 16QAM constellation points to obtain an encrypted constellation diagram shown in fig. 4.

(2) Upsampling section

In order to make the mapped signal decision-making above the shaping filtering, a N-fold upsampling is performed before entering the filter. And inserting numerical values after the coordinates of the constellation points, wherein the numerical value of N is the number of the inserted numerical values.

(3) Shaping filter part

The coordinate of two dimensionalities of the data after up sampling is respectively sent to two filters which are orthogonal with each other for shaping and filtering, and then two paths of signals are combined by an adding unit and finally sent to an Arrayed Waveguide Grating (AWG).

The inverse DSP flow is composed of a matched filter part, a down-sampling part and a constellation demapping part. Each portion of which corresponds and acts in reverse in the DSP flow. Fig. 5 is a constellation diagram of gaussian white noise after decryption.

MPLC system flow details:

in the MPLC system, the optical fiber branches encrypted in the first two steps are fed into a customized OAM generator to generate multiple OAM beams. And the OAM light beam is incident on the simulated turbulence plate at a distance of 1 m. The light beam affected by the turbulence is then projected onto a first plane of the spatial light modulator SLM, where it bounces between the mirror and the same SLM, through 6 different phase modes, after passing through a 6-plane MPLC, the two beams will be spatially separated and tilted at different angles. The two beams are then coupled to an array of optical fibers using collimators. After the two ports collect power, a feedback loop is designed to monitor the crosstalk between the two channels. Given the crosstalk at each measurement, a particle swarm algorithm is applied to update the patterns of the first two planes to correct the wavefront of the distorted beam.

The above is only a preferred embodiment of the present invention, and the protection scope of the present invention is not limited to the above-mentioned embodiments, and all technical solutions belonging to the idea of the present invention belong to the protection scope of the present invention. It should be noted that modifications and embellishments within the scope of the invention may be made by those skilled in the art without departing from the principle of the invention.

Claims

1. an OAM transmission device of enhanced safety and turbulent flow mitigation, is characterized in that: comprise a transmitting end structure (1), an MPLC system (2) and a receiving end structure (3), and the described transmitting end structure (1) uses After encrypting and modulating the user's data, an OAM beam is formed, and the OAM beam enters the MPLC system (2) after passing through the atmospheric turbulence region, and the MPLC system (2) includes a plurality of phase planes (21) with different phase patterns. ), a mirror (22), a collimator (23), a feedback loop (24) and a controller (25), the OAM beam affected by the turbulence is reflected back and forth between the phase plane (21) and the mirror (22), The OAM beams are spatially separated and inclined at different angles, and after being coupled by the collimator (23), they enter the receiving end structure (3), and the feedback loop (24) is connected to the MPLC system (2) and the receiving end structure (3). On the connection channel of the end structure (3), the crosstalk between the separated beams is monitored, and the controller (25) can receive the signal of the feedback loop (24) and adjust at least one phase mode of the phase plane (21) to correct The wavefront of the beam is distorted to reduce the degree of crosstalk, and the receiving end structure (3) can implement the inverse process of encryption and modulation in the transmitting end structure (1).

2. An OAM transmission device for enhanced safety and turbulence mitigation according to claim 1, characterized in that: the phase plane (21) has six different phase modes.

3. the transmission method of the OAM transmission device of a kind of safety enhancement and turbulence mitigation as claimed in claim 1, is characterized in that: comprise the following steps:

a. Steps at the sending end: Distribute the user's data to channels of different optical wavelengths at the same time, and then the optical switch encrypts the data under the control of the frequency hopping sequence, and the encrypted data is further transmitted by randomly assigned channels with different OAM states, so The frequency hopping sequence described above is generated by Chua's chaotic model. After the binary data to be transmitted is modulated and constellation encrypted in the DSP process, it is electro-optically modulated with the optical signal in the Mach-Zehnder modulator MZM, and the modulated signal is connected to the OAM generator. data modulation to the OAM beam;

b. Transmission process steps: the OAM beam is incident on the simulated turbulent plate, the vortex beam affected by the turbulence enters the MPLC system (2), and the MPLC system (2) uses the controller (25) to receive the signal from the feedback loop (24), Adjust the phase mode of the phase plane (21), turbulence mitigation and mode demultiplexing of the beam,

c. Steps at the receiving end: the vortex light carrying the information becomes a Gaussian beam after demultiplexing, and the Gaussian beam is converted into an electrical signal by a photodetector, and then the electrical signal is decrypted and demodulated in the opposite way of encryption and modulation at the sending end. Obtain decrypted data.

4. the OAM transmission method of a kind of security enhancement according to claim 3 and turbulent flow alleviation, it is characterized in that: the concrete method that optical switch encrypts data under the control of frequency hopping sequence is:

The N×N optical switch selector is used for encryption at the physical level, the chaotic sequence generated by the Chua model is used as the key, and the recursive frequency hopping method is adopted: ①LDi→MZMi; ②LDi→MZMi+1; ③LDi→MZMi+2;… ...; where the equation of the Chua model is:

where is a constant,

for the variable.

5. the OAM transmission method of a kind of security enhancement and turbulence mitigation according to claim 3, is characterized in that: DSP flow process is divided into three steps: constellation mapping step, upsampling step and shaping filtering step;

The constellation mapping steps are: adopting the 16QAM constellation diagram, using the Chua model as the chaotic mapping to generate a masking vector, and masking the 16QAM constellation points to obtain an encrypted constellation point diagram;

The steps of the upsampling part are: insert a value after the coordinates of the constellation point, and the value of N is the number of insertions;

The shaping and filtering steps are as follows: the up-sampled data sends the coordinates in the two dimensions to two mutually orthogonal filters for shaping and filtering, and then an adding unit combines the two signals, and finally sends them to the arrayed waveguide. grating AWG;

In the step of the receiving end, the process of decrypting and demodulating the electrical signal includes an inverse DSP process, and the inverse DSP process consists of a matched filter part, a downsampling part and a constellation demapping part, each part of which corresponds to and is in the DSP process. The opposite effect.

6. The OAM transmission method of a kind of safety enhancement and turbulence mitigation according to claim 3, is characterized in that: in the transmission process step, given the crosstalk threshold value of the controller (25) when measuring each time, taking into account each For the crosstalk during the second measurement, the particle swarm algorithm PSO is used to optimize the graph of the first two planes of the phase plane (21) to correct the wavefront of the distorted beam.