Free-space orbital angular momentum broadcast communication system based on all-optical chaotic modulation

Abstract

The present invention discloses a free-space orbital angular momentum broadcast communication system based on all-optical chaotic modulation. A main laser in an encrypted transmitter generates a broadband chaotic carrier signal through a conventional optical feedback structure. The optical information is then encrypted by hiding it in the chaotic optical carrier through intensity modulation. A specially designed multi-order multiplexing phase plate is loaded on the spatial light modulator in the optical field control module to control the Gaussian encrypted optical signal aligned in the spatial optical path into a multiplexed light beam with multiple OAM modes. After transmission through a free-space link, multiple opposite single-order phase plates are loaded on the spatial light modulator in the OAM broadcast module to achieve demodulation of the correlated optical field modes, corresponding to multiple users at the receiving end. In the receiving module, users achieve local chaotic synchronization using a unidirectional injection-locking mechanism. The original information is then obtained by subtracting the chaotic optical carrier carrying the information from the local synchronization chaotic signal, ultimately completing secure broadcast communication.

Description

Technical Field

[0001] The present invention belongs to the field of optical communication technology, and more specifically, relates to a free-space orbital angular momentum broadcast communication system based on all-optical chaotic modulation. Background Art

[0002] In recent years, the rapid development of technologies such as 4G / 5G mobile communications, the Internet of Things, cloud computing, big data, and artificial intelligence has brought greater convenience to people's lives, but also posed greater challenges to communication systems. Optical communications, one of the mainstream methods of information transmission, is evolving from multi-channel, high-speed optical communications to ultra-high-speed, ultra-large-capacity, and ultra-long-distance optical communications.

[0003] Space laser communication boasts outstanding advantages such as high speed, strong resistance to electromagnetic interference, and no spectrum restrictions. It is a core communication system for scenarios such as "last mile communication" and satellite-to-ground / intersatellite communications. Furthermore, vortex beams carrying different orbital angular momentum (OAM) modes are orthogonal to each other, which can further enhance the transmission capacity of space laser communication systems through multiplexing and broadcasting. However, with the rapid increase in transmission capacity demand, information security issues are becoming increasingly serious. In particular, when laser beams are transmitted through long-distance free-space channels, the divergence of the beam causes the spot size at the receiving end to expand, greatly increasing the risk of information eavesdropping.

[0004] Currently, traditional information security technologies mostly use cryptographic algorithms to encrypt data at the media access control layer and its upper layers. With the advent of ultra-fast quantum computers, these algorithm-based encryption methods face the threat of brute force attacks. Physical layer security, as the primary barrier to the security of the entire communication system, is of paramount importance. Therefore, improving the physical layer information security of free-space optical communication systems has become a hot topic in academic research.

[0005] Chaotic signals generated based on the nonlinear dynamics of lasers exhibit initial value sensitivity and broadband noise-like properties. This allows information to be concealed within broadband chaotic optical carriers for secure optical transmission, making it a key technology for enhancing the physical layer security of optical communication systems. However, after in-depth research, researchers have discovered that the inherent relaxation oscillations of semiconductor lasers result in chaotic laser bandwidths of only a few GHz. In chaotic optical communications, chaotic lasers serve as carriers to conceal transmitted optical information. This limited chaotic carrier bandwidth limits the transmission rate of chaotic optical communications. While some schemes have been proposed to enhance laser chaos bandwidth, their complexity makes them difficult to apply to existing optical communication systems. Summary of the Invention

[0006] The purpose of the present invention is to overcome the shortcomings of the existing technology and provide a free-space orbital angular momentum broadcast communication system based on all-optical chaotic modulation. While ensuring the security of optical information space link transmission, it utilizes the spatial dimension of the chaotic carrier and improves the transmission capacity of the chaotic communication system.

[0007] To achieve the above-mentioned object of the invention, the present invention provides a free-space orbital angular momentum broadcast communication system based on all-optical chaotic modulation, characterized by comprising: an encrypted transmitter, an optical field control module, a free-space link, an OAM broadcast module, and a receiving user end;

[0008] The encrypted transmitting end includes: a main laser DL, a polarization controller PC, a fiber coupler FC1, a variable optical attenuator VOA1, a fiber reflector M1, an optical isolator ISO, a Mach-Zehnder modulator MZM and an arbitrary waveform generator AWG;

[0009] The main laser DL generates a laser signal, which is input into the fiber coupler FC1 after passing through the polarization controller PC. The fiber coupler FC1 splits the laser into two paths according to the set optical splitting ratio. One path of light is attenuated by the variable optical attenuator VOA1 and reflected back to the main laser DL by the fiber reflector M1. The other path of light is output as a chaotic optical carrier. After passing through the optical isolator ISO, the chaotic optical carrier enters the Mach-Zehnder modulator MZM. At the same time, the information generated by the arbitrary waveform generator AWG is modulated onto the chaotic optical carrier by the MZM to obtain a chaotic optical carrier carrying information. The chaotic optical carrier is then collimated into a spatial Gaussian beam by a fiber collimator COL1 and enters the optical field control module.

[0010] The light field control module includes a linear polarizer LP and a reflective spatial light modulator SLM1;

[0011] The chaotic optical carrier carrying information passes through the linear polarizer LP and is transmitted to the reflective spatial light modulator SLM1 for OAM n-order multiplexing optical field control. The controlled optical signal is reflected by the reflective spatial light modulator SLM1 to the free space link for transmission and then enters the OAM broadcast module.

[0012] The OAM broadcast module includes a spatial light modulator SLM2, n 45-degree reflectors M2-1 to M2-n, and n fiber collimators COL2-1 to COL2-n;

[0013] The liquid crystal display of the spatial light modulator SLM2 is divided into n equal parts, each corresponding to a receiving user. Each user loads a vortex phase plate of opposite order on the corresponding part of the liquid crystal display to demodulate its own OAM. After demodulation, n Gaussian beams are obtained. The first Gaussian beam is collimated by the reflector M2-1 and the fiber collimator COL2-1 and then enters the optical fiber for transmission to receiving user 1. The second demodulated Gaussian beam is collimated by the reflector M2-2 and the fiber collimator COL2-2 and then enters the optical fiber for transmission to receiving user 2. And so on, thus realizing OAM-based broadcasting for n users.

[0014] The receiving user end includes n receiving users with the same structure, wherein the i-th receiving user includes an optical fiber isolator ISO, a small signal amplifier EDFA, a 2×2 optical fiber coupler FC2-i, three variable optical attenuators VOA2-i / VOA3-i / VOA4-i, a polarization controller PC, and a slave laser SL. i , and two photodetectors PD1-i / PD2-i, i=1,2,3,…,n;

[0015] In the i-th receiving user, the optical signal coupled by the collimator is input into the small signal amplifier EDFA after passing through the optical fiber isolator ISO. After the small signal amplifier EDFA compensates for the spatial transmission damage, it is split into two paths by the optical fiber coupler FC2-i. One of the optical signals is unidirectionally injected into the slave laser SL through the variable optical attenuator VOA2-i and the polarization controller PC. i , at this time from the laser SL i A local synchronized chaotic signal is generated and received by the photodetector PD2-i after passing through the fiber coupler FC2-i and the variable optical attenuator VOA4-i. Another optical signal passes through the variable optical attenuator VOA3-i and is received by the photodetector PD1-i. Finally, the signals received by the two photodetectors are subtracted to obtain the original optical signal.

[0016] The object of the invention of the present invention is achieved like this:

[0017] The present invention is based on a free-space orbital angular momentum broadcast communication system using all-optical chaotic modulation. The main laser in the encrypted transmitter generates a broadband chaotic carrier signal through a conventional optical feedback structure. The optical information is then encrypted by hiding it in the chaotic optical carrier through intensity modulation. A specially designed multi-order multiplexing phase plate is loaded on the spatial light modulator in the optical field control module to control the Gaussian encrypted optical signal aligned in the spatial optical path into a multiplexed light beam with multiple OAM modes. After transmission through the free-space link, multiple opposite single-order phase plates are loaded on the spatial light modulator in the OAM broadcast module to achieve correlated optical field mode demodulation, corresponding to multiple users at the receiving end. In the receiving module, users achieve local chaotic synchronization using a unidirectional injection-locking mechanism. The original information is then obtained by subtracting the chaotic optical carrier carrying the information from the local synchronization chaotic signal, ultimately completing secure broadcast communication.

[0018] At the same time, the free-space orbital angular momentum broadcast communication system based on all-optical chaotic modulation of the present invention also has the following beneficial effects:

[0019] (1) The all-optical chaotic OAM broadcast secure communication scheme designed by the present invention can effectively realize the encryption and decryption of information, ensuring the communication security and privacy of users in the broadcast network;

[0020] (2) The optical field control module at the transmitting end of the OAM broadcast network only needs one spatial optical phase modulator to realize the generation of OAM multiplexed optical fields, which greatly reduces the system cost;

[0021] (3) Compared with the conventional broadband chaotic signal generation scheme, this scheme improves the transmission capacity of the chaotic secure communication system by regulating the spatial pattern of the chaotic signal. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 This is a schematic diagram of the free-space orbital angular momentum broadcast communication system based on all-optical chaotic modulation of the present invention;

[0023] Figure 2 are the time domain waveforms and spectrum diagrams of the output signals of the master laser and the slave laser; where (a1) is the time domain waveform diagram of the output signal of the master laser; (a2) is the time domain waveform diagram of the output signal of the slave laser; (b1) is the spectrum diagram of the output signal of the master laser; (b2) is the spectrum diagram of the output signal of the slave laser; (c) is the cross-correlation diagram between the output signals of the communicating parties;

[0024] Figure 3 Taking user 1 as an example: (a) is the time domain waveform of the information demodulated by the legitimate receiving user; (b) is the time domain waveform of the information demodulated by the illegal receiving user; (c) is the eye diagram of the information demodulated by the legitimate receiving user; (d) is the eye diagram of the information demodulated by the illegal receiving user. DETAILED DESCRIPTION

[0025] The following describes the specific embodiments of the present invention in conjunction with the accompanying drawings so that those skilled in the art can better understand the present invention. It should be noted that in the following description, when detailed descriptions of known functions and designs may dilute the main content of the present invention, such descriptions will be omitted here.

[0026] Example

[0027] Figure 1 This is a schematic diagram of the free-space orbital angular momentum broadcast communication system based on all-optical chaotic modulation of the present invention.

[0028] In this embodiment, if Figure 1 As shown, the present invention provides a free-space orbital angular momentum broadcast communication system based on all-optical chaotic modulation, comprising: an encrypted transmitter, an optical field control module, a free-space link, an OAM broadcast module, and a receiving user terminal;

[0029] The encrypted transmitter includes: main laser DL, polarization controller PC, fiber coupler FC1, variable optical attenuator VOA1, fiber reflector M1, optical isolator ISO, Mach-Zehnder modulator MZM and arbitrary waveform generator AWG;

[0030] The main laser DL generates a laser signal, which is input into the fiber coupler FC1 after passing through the polarization controller PC. The fiber coupler FC1 splits the laser into two paths according to the set optical splitting ratio. In this embodiment, the optical splitting ratio is set to 2:8. Then, one path containing 20% ​​of the laser light is attenuated by the variable optical attenuator VOA1 and then reflected back to the main laser DL by the fiber reflector M1. The other path containing 80% of the laser light is output as a chaotic optical carrier. After passing through the optical isolator ISO, the chaotic optical carrier enters the Mach-Zehnder modulator MZM. At the same time, the information generated by the arbitrary waveform generator AWG is modulated onto the chaotic optical carrier by the MZM to obtain a chaotic optical carrier carrying information. The chaotic optical carrier is then collimated into a spatial Gaussian beam by a fiber collimator COL1 and enters the optical field control module.

[0031] The light field control module includes a linear polarizer LP and a reflective spatial light modulator SLM1;

[0032] The chaotic optical carrier carrying information passes through the linear polarizer LP and is transmitted to the reflective spatial light modulator SLM1 for OAM n-order multiplexing optical field control. The controlled optical signal is reflected by the reflective spatial light modulator SLM1 to the free space link for transmission and then enters the OAM broadcast module.

[0033] The OAM broadcast module includes a spatial light modulator SLM2, n 45-degree reflectors M2-1 to M2-n, and n fiber collimators COL2-1 to COL2-n;

[0034] The liquid crystal display of the spatial light modulator SLM2 is divided into n equal parts, each corresponding to a receiving user. Each user loads a vortex phase plate of opposite order on the corresponding part of the liquid crystal display to demodulate its own OAM. After demodulation, n Gaussian beams are obtained. The first Gaussian beam is collimated by the reflector M2-1 and the fiber collimator COL2-1 and then enters the optical fiber for transmission to receiving user 1. The second demodulated Gaussian beam is collimated by the reflector M2-2 and the fiber collimator COL2-2 and then enters the optical fiber for transmission to receiving user 2. And so on, thus realizing OAM-based broadcasting for n users.

[0035] The receiving user end includes n receiving users with the same structure, among which the i-th receiving user includes an optical fiber isolator ISO, a small signal amplifier EDFA, a 2×2 optical fiber coupler FC2-i, three variable optical attenuators VOA2-i / VOA3-i / VOA4-i, a polarization controller PC, and a slave laser SL. i , and two photodetectors PD1-i / PD2-i, i=1,2,3,…,n;

[0036] Taking user i as an example, in the i-th receiving user, the optical signal after collimator coupling passes through the optical fiber isolator ISO and is input to the small signal amplifier EDFA. After the small signal amplifier EDFA compensates for the spatial transmission damage, it is split into two paths by the optical fiber coupler FC2-i. One of the optical signals passes through the variable optical attenuator VOA2-i and the polarization controller PC and is unidirectionally injected into the slave laser SL. i , at this time from the laser SL i A local synchronized chaotic signal is generated and received by the photodetector PD2-i after passing through the fiber coupler FC2-i and the variable optical attenuator VOA4-i. Another optical signal passes through the variable optical attenuator VOA3-i and is received by the photodetector PD1-i. Finally, the signals received by the two photodetectors are subtracted to obtain the original optical signal.

[0037] Next, we will present the experimental verification results. As a proof-of-principle experiment, we have successfully verified the OAM broadcast secure transmission of a single-user signal with a bit rate of 6 Gbps and an OOK modulation format in a 1.2-meter free-space link. Taking receiving user 1 as an example, the attached Figure 2The chaotic carrier generated by the master laser DL is shown in the time domain waveform (a1), spectrum (b1), and correlation plot (c) of the local chaotic signal generated by user 1's slave laser SL1. The cross-correlation coefficient (CC) is used to quantify the correlation between the output signals. After the proposed OAM broadcasting, the cross-correlation coefficient reaches as high as 0.95. The chaotic signals generated at the receiver are highly correlated with those generated at the transmitter, ensuring correct demodulation.

[0038] Attachment Figure 3 The figure shows user 1's original information signal (a), encrypted signal waveform (b), and corresponding eye diagrams (c) and (d). Compared to the original information signal's open eye diagram, the encrypted information waveform exhibits random fluctuations, making it impossible to discern the original information. The communication eye diagram is tightly closed, and the calculated encryption bit error rate is close to 0.5, verifying the security of the OAM broadcast secure communication scheme.

[0039] Although the above describes the illustrative specific embodiments of the present invention to facilitate understanding of the present invention by those skilled in the art, it should be clear that the present invention is not limited to the scope of the specific embodiments. For those skilled in the art, as long as various changes are within the spirit and scope of the present invention as defined and determined by the appended claims, these changes are obvious, and all inventions and creations using the concepts of the present invention are protected.

Claims

1. A free-space orbital angular momentum broadcast communication system based on all-optical chaotic modulation, characterized in that: include: Encrypted transmitter, optical field control module, free space link, OAM broadcast module, and receiving user end; The encrypted transmitting end includes: a main laser DL, a polarization controller PC, a fiber coupler FC1, a variable optical attenuator VOA1, a fiber reflector M1, an optical isolator ISO, a Mach-Zehnder modulator MZM and an arbitrary waveform generator AWG; The main laser DL generates a laser signal, which is input into the fiber coupler FC1 after passing through the polarization controller PC. The fiber coupler FC1 splits the laser into two paths according to the set optical splitting ratio. One path of light is attenuated by the variable optical attenuator VOA1 and reflected back to the main laser DL by the fiber reflector M1. The other path of light is output as a chaotic optical carrier. After passing through the optical isolator ISO, the chaotic optical carrier enters the Mach-Zehnder modulator MZM. At the same time, the information generated by the arbitrary waveform generator AWG is modulated onto the chaotic optical carrier by the MZM to obtain a chaotic optical carrier carrying information. The chaotic optical carrier is then collimated into a spatial Gaussian beam by a fiber collimator COL1 and enters the optical field control module. The light field control module includes a linear polarizer LP and a reflective spatial light modulator SLM1; The chaotic optical carrier carrying information passes through the linear polarizer LP and is transmitted to the reflective spatial light modulator SLM1 for OAM n-order multiplexing optical field control. The controlled optical signal is reflected by the reflective spatial light modulator SLM1 to the free space link for transmission and then enters the OAM broadcast module. The OAM broadcast module includes a spatial light modulator SLM2, n 45-degree reflectors M2-1 to M2-n, and n fiber collimators COL2-1 to COL2-n; The liquid crystal display of the spatial light modulator SLM2 is divided into n equal parts, each corresponding to a receiving user. Each user loads a vortex phase plate of opposite order on the corresponding part of the liquid crystal display to demodulate its own OAM. After demodulation, n Gaussian beams are obtained. The first Gaussian beam is collimated by the reflector M2-1 and the fiber collimator COL2-1 and then enters the optical fiber for transmission to receiving user 1. The second demodulated Gaussian beam is collimated by the reflector M2-2 and the fiber collimator COL2-2 and then enters the optical fiber for transmission to receiving user 2. And so on, thus realizing OAM-based broadcasting for n users. The receiving user end includes n receiving users with the same structure, wherein the i-th receiving user includes an optical fiber isolator ISO, a small signal amplifier EDFA, a 2×2 optical fiber coupler FC2-i, three variable optical attenuators VOA2-i / VOA3-i / VOA4-i, a polarization controller PC, and a slave laser SL. i , and two photodetectors PD1-i / PD2-i, i=1,2,3,…,n; In the i-th receiving user, the optical signal coupled by the collimator is input into the small signal amplifier EDFA after passing through the optical fiber isolator ISO. After the small signal amplifier EDFA compensates for the spatial transmission damage, it is split into two paths by the optical fiber coupler FC2-i. One of the optical signals is unidirectionally injected into the slave laser SL through the variable optical attenuator VOA2-i and the polarization controller PC. i , at this time from the laser SL i A local synchronized chaotic signal is generated and received by the photodetector PD2-i after passing through the fiber coupler FC2-i and the variable optical attenuator VOA4-i. Another optical signal passes through the variable optical attenuator VOA3-i and is received by the photodetector PD1-i. Finally, the signals received by the two photodetectors are subtracted to obtain the original optical signal.

Images