CN221177722U - Free space laser chaotic synchronization system - Google Patents
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Abstract
The application provides a free space laser chaotic synchronization system, which can reduce the influence of turbulent effect in a space channel on the system by means of the turbulence resistance characteristic of a vector light field structure and improve the synchronization performance of the system under various turbulence intensities. The application provides a free space laser chaotic synchronization system, which comprises a driving module, an encryption module, a modulation module, a transmission module, a demodulation module, a decryption module and a signal acquisition module.
Description
Technical Field
The application relates to the technical field of communication, in particular to a free space laser chaotic synchronization system.
Background
With the advent of the big data age, there has been a proliferation of many new network services, such as artificial intelligence, unmanned driving, cloud computing, etc. The advent of these new services has also led to explosive growth of internet data. However, current microwave communications are limited by limited frequency resources, and it is difficult to meet the requirements of future broadband data transmission. Free space optical communication (FSO) is a wireless communication mode for transmitting information in free space by taking laser as a carrier wave, has the advantages of high transmission rate, electromagnetic interference resistance, abundant frequency spectrum resources and the like, and has become an important technology for the construction of a new generation of information network. In practical applications of free space optical communication, the drift and flicker of the transmission laser will be caused by the atmospheric turbulence effect existing in the channel, which seriously affects the communication performance, so that it is necessary to research the effective atmospheric turbulence effect suppression technology. On the other hand, the security of free-space optical communications is susceptible to atmospheric scattering and beam expansion, greatly increasing the risk of interception eavesdropping.
The laser chaotic signal has the characteristics of initial value sensitivity, wide frequency spectrum, noise-like and the like, so that the physical layer encryption of the information transmitted in the free space can be realized by taking the parameters of the laser chaotic signal as a secret key, an upper layer encryption mechanism can be effectively compatible, and the laser chaotic signal has important application value in the field of free space optical communication. In practical application, the principle of laser chaotic secret communication is to hide a small-amplitude information signal in a broadband chaotic carrier signal, and realize the hidden safe transmission of information by utilizing chaotic synchronization and chaotic filtering characteristics at a receiving end. Therefore, the precondition and core foundation for chaotic secret communication during high-quality and stable laser chaotic synchronization are realized.
2023, The Federal administration institute of technology, together with Paris Sac Lei Da, introduced adaptive optics technology at the receiving end, realized information transmission (DOI: 10.1038/s 41377-023-01201-7) with single carrier communication rate up to 0.94Tbit/s in a free space channel of 53.42 km, however, the adaptive optics technology has the disadvantages of high cost, poor compatibility, etc. In addition, researches on the laser chaotic secret communication technology are mainly focused on an optical fiber transmission link. Chinese patent CN206060781U discloses a technology for implementing long-distance laser chaos synchronization by using an optical fiber environment based on the optical fiber environment. However, the research on free space laser chaotic light communication is less at present, so the laser chaotic synchronization technology based on the free space channel is slow to develop.
Disclosure of Invention
Based on the problems in the prior art, the application provides a free space laser chaotic synchronization system, and by means of the turbulence resistance characteristic of a vector light field structure, the influence of turbulence effect in a space channel on the system can be reduced, and the synchronization performance of the system under various turbulence intensities is improved.
In order to achieve the technical purpose, the application adopts the following technical scheme:
A free space laser chaos synchronous system comprises a driving module, an encryption module, a modulation module, a transmission module, a demodulation module, a decryption module and a signal acquisition module;
The driving module can generate at least a first signal and a second signal, the first signal is injected into the encryption module to drive the encryption module to generate a third signal, and the third signal is a chaotic encryption signal;
The second signal is of a Gaussian light field structure, is modulated into a fourth signal after being injected into the modulation module, and is of a vector light field structure;
The fourth signal is injected into a transmission module for transmission, and the transmission module comprises a free space channel;
The demodulation module has decoupling property with the space structure of the modulation module, and can receive a fourth signal and demodulate the fourth signal into a second signal;
The second signal is injected into the decryption module, and the decryption module is driven to generate a fifth signal, wherein the fifth signal is a chaotic decryption signal synchronous with the third signal;
The signal acquisition module can acquire and analyze a third signal and a fifth signal.
As a preferred solution, the driving module includes a main laser (DRIVE LASER, DL), a first polarization controller (Polarization Controller, PC 1), a first Fiber Coupler (FC 1), a second Fiber Coupler (FC 2), a first tunable optical attenuator (Variable Optical Attenuator, VOA 1), a Fiber Mirror (Mirror, M), an erbium-doped Fiber amplifier (Erbium Doped Fiber Amplifier, EDFA);
The driving module comprises a main laser, a first polarization controller, a first optical fiber coupler, a second optical fiber coupler, a first adjustable optical attenuator and an optical fiber reflector
The main laser is connected with a first polarization controller, the first polarization controller is connected with a first optical fiber coupler through optical fibers, the first optical fiber coupler is divided into two paths, one path is connected with a first adjustable optical attenuator and an optical fiber reflector, the other path is connected with a second optical fiber coupler, the second optical fiber coupler is divided into two paths, one path is connected with an erbium-doped optical fiber amplifier, and the other path is connected with an encryption module.
Preferably, the central wavelength of the continuous laser output by the main laser is 1550nm.
Preferably, the adjustable range of the EDFA is 10 dBm-20 dBm.
As a preferred solution, the encryption module includes a first slave laser (SLAVE LASER, SL) and a first Fiber isolator (Optical Isolator, OI 1), a second variable optical attenuator (Variable Optical Attenuator, VOA 2), a second polarization controller (Polarization Controller, PC 2), and a third Fiber Coupler (FC 3);
The first slave laser is connected with a third optical fiber coupler, the third optical fiber coupler is divided into two paths, one path is sequentially connected with a second polarization controller, a second adjustable optical attenuator and a first optical fiber isolator according to the direction of an optical path, and the first optical fiber isolator is connected with the second optical fiber coupler;
the other path is connected with the signal acquisition module.
As a preferred embodiment, the modulation module includes a first collimator (Collimator, col.1), a first linear polarizer (Linear Polarizer, LP 1), and a first-order Vortex plate (VPP 1) sequentially disposed along the light transmission direction.
Preferably, the first-order vortex wave plate is parallel to the fast axis direction of the first linear polarizer.
As a preferable scheme, the transmission module comprises a beam expander, a free space channel and a beam shrinking mirror which are sequentially arranged along the light transmission direction.
Preferably, the beam expander is a 20-time beam expander.
Preferably, the beam shrinking lens is a 10-time beam shrinking lens.
As a preferable scheme, the demodulation module comprises a second first-order vortex wave plate, a second linear polaroid and a second collimator which are sequentially arranged along the light transmission direction, wherein the second first-order vortex wave plate is parallel to the fast axis direction of the second linear polaroid and is parallel to the fast axis directions of the first-order vortex wave plate and the first linear polaroid in the modulation module.
As a preferred solution, the decryption module includes a second slave laser (SLAVE LASER, SL 2), a second Fiber isolator (Optical Isolator, OI 2), a third variable optical attenuator (Variable Optical Attenuator, VOA 3), a third polarization controller (Polarization Controller, PC 3), and a fourth Fiber Coupler (FC 4);
The second slave laser is connected with a fourth optical fiber coupler, the fourth optical fiber coupler is divided into two paths, and one path is sequentially connected with a third polarization controller, a third adjustable optical attenuator and a second optical fiber isolator according to the direction of an optical path; the other path is connected with the data acquisition module;
the second slave laser is consistent with the setting parameters of the first slave laser.
Preferably, the setting parameters include control temperature, drive current.
As a preferred embodiment, the signal acquisition module includes a tunable fiber delay line (DELAY LINES, DL), a fourth tunable optical attenuator (Variable Optical Attenuator, VOA 4), a fifth tunable optical attenuator (Variable Optical Attenuator, VOA 5), a first photodetector (Photodetector, PD 1)
A second photodetector (Photodetector, PD 2), a high-speed oscilloscope (Oscilloscope, OSC),
The input end of the fourth adjustable optical attenuator is connected with the third optical fiber coupler, the output end of the fourth adjustable optical attenuator is connected with the first photoelectric detector, and the output end of the first photoelectric detector is connected with the high-speed oscilloscope;
One end of the optical fiber delay line is connected with the fourth optical fiber coupler, the other end of the optical fiber delay line is connected with the input end of the fifth adjustable optical attenuator, the output end of the fifth adjustable optical attenuator is connected with the second photoelectric detector, and the output end of the second photoelectric detector is connected with the high-speed oscilloscope.
As a preferable scheme, the free space laser chaotic synchronization system further comprises a digital signal processing module, and the digital signal processing module is used for calculating the cross correlation coefficient of the third signal and the fifth signal.
Preferably, the digital signal processing module comprises a filtering algorithm.
The application has the advantages that: the application is provided with the modulation module, modulates the light signal of the Gaussian light field structure into the light signal of the vector light field structure, can reduce the influence of turbulence effect in a space channel on the system by means of the turbulence resistance characteristic of the vector light field structure, improves the synchronization performance of the system under various turbulence intensities, and provides a potential technical scheme for free space optical communication with high reliability and high safety.
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In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings that are needed in the embodiments will be briefly described below, it being understood that the following drawings only illustrate some embodiments of the present application and therefore should not be considered as limiting the scope, and other related drawings may be obtained according to these drawings without inventive effort for a person skilled in the art.
Fig. 1 is a schematic structural diagram of a free space laser chaotic synchronization system provided in an embodiment of the present application;
Fig. 2 is a time domain waveform of a chaotic encryption signal provided in one embodiment of the present application;
FIG. 3 is a power spectrum of a chaotic encryption signal provided in one embodiment of the present application;
fig. 4 is a time domain waveform of a chaotic decryption signal provided in one embodiment of the present application;
FIG. 5 is a power spectrum of a chaotic decryption signal provided in one embodiment of the present application;
FIG. 6 is a plot of chaotic synchronization scatter of a chaotic encryption signal and a chaotic decryption signal provided in one embodiment of the present application;
FIG. 7 is a schematic diagram of the optical field structure of a second signal with a Gaussian optical field structure at different turbulence intensities provided in an embodiment of the application;
FIG. 8 is a schematic representation of the optical field structure at different turbulence intensities for a fourth signal having a radial vector optical field structure provided in an embodiment of the present application;
FIG. 9 is a plot of chaotic synchronization coefficients for a second signal having a Gaussian light field structure at different turbulence intensities provided in one embodiment of the application;
FIG. 10 is a plot of chaotic synchronization coefficients for a fourth signal having a radial vector light field structure at different turbulence intensities, provided in one embodiment of the present application.
Detailed Description
For the purpose of making the objects, technical solutions and advantages of the embodiments of the present application more apparent, the technical solutions of the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present application, and it is apparent that the described embodiments are some embodiments of the present application, but not all embodiments of the present application. The components of the embodiments of the present application generally described and illustrated in the figures herein may be arranged and designed in a wide variety of different configurations.
Thus, the following detailed description of the embodiments of the application, as presented in the figures, is not intended to limit the scope of the application, as claimed, but is merely representative of selected embodiments of the application. All other embodiments, which can be made by those skilled in the art based on the embodiments of the application without making any inventive effort, are intended to be within the scope of the application.
It should be noted that: like reference numerals and letters denote like items in the following figures, and thus once an item is defined in one figure, no further definition or explanation thereof is necessary in the following figures.
In the description of the present application, it should be noted that, directions or positional relationships indicated by terms such as "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer", etc., are directions or positional relationships based on those shown in the drawings, or are directions or positional relationships conventionally put in use of the inventive product, are merely for convenience of describing the present application and simplifying the description, and are not indicative or implying that the apparatus or element to be referred to must have a specific direction, be constructed and operated in a specific direction, and thus should not be construed as limiting the present application. Furthermore, the terms "first," "second," "third," and the like are used merely to distinguish between descriptions and should not be construed as indicating or implying relative importance.
Furthermore, the terms "horizontal," "vertical," "overhang," and the like do not denote a requirement that the component be absolutely horizontal or overhang, but rather may be slightly inclined. As "horizontal" merely means that its direction is more horizontal than "vertical", and does not mean that the structure must be perfectly horizontal, but may be slightly inclined.
In the description of the present application, it should also be noted that, unless explicitly specified and limited otherwise, the terms "disposed," "mounted," "connected," and "connected" are to be construed broadly, and may be, for example, fixedly connected, detachably connected, or integrally connected; can be mechanically or electrically connected; can be directly connected or indirectly connected through an intermediate medium, and can be communication between two elements. The specific meaning of the above terms in the present application will be understood in specific cases by those of ordinary skill in the art.
It should be noted that, in the case of no conflict, different features in the embodiments of the present application may be combined with each other.
Free space optical communication (FREE SPACE optical communication, FSO for short) is a communication technology using light as a carrier and free space as a transmission medium, but when a light beam is transmitted in the atmosphere, the light beam is easy to be influenced by atmospheric turbulence, so that the wave front of the transmitted light beam is randomly fluctuated, light spot drift, light intensity fluctuant (flickering) and the like are caused, the quality of the transmitted light beam is seriously reduced, the error rate of a free space optical communication system is increased, the channel capacity is reduced, even communication is interrupted, the stability and the reliability of a communication link are seriously influenced, and how to reduce the influence of atmospheric turbulence effect on free space optical communication is a key problem in the field.
In view of this, the present application provides a free space laser chaotic synchronization system, which modulates an optical signal with a gaussian structure into a vector optical signal through an optical modulation module, so as to effectively resist the influence of atmospheric turbulence, improve the stability and reliability of communication, and details the scheme provided in this embodiment are described below.
In one embodiment of the application, a free space laser chaotic synchronization system is provided, which comprises a driving module, an encryption module, a modulation module, a transmission module, a demodulation module, a decryption module and a signal acquisition module;
The driving module can generate at least a first signal and a second signal, the first signal is injected into the encryption module to drive the encryption module to generate a third signal, and the third signal is a chaotic encryption signal;
The driving module is mainly used for generating a laser chaotic driving signal which can be respectively injected into the encryption module and the decryption module to drive the corresponding slave lasers to generate chaotic encryption signals and chaotic decryption signals;
The second signal is in a Gaussian light field structure, and is modulated into a fourth signal after being injected into the modulation module, and the fourth signal is in a vector light field structure.
The cross section of the light beam is a Gaussian light beam, the cross section of the light beam is a vector light beam, the vector light beam can be a column vector light beam or a full poincare light beam, and the modulated light beam characteristics are selected according to the requirements of actual working conditions.
The gaussian beam, i.e. the beam whose cross-section amplitude distribution obeys the gaussian function, is normally distributed in all directions, with the highest intensity in the centre of the beam and dissipates when reaching the periphery of the beam;
The vector light field has vortex phase carrying OAM state and anisotropic polarization state distribution with different space positions on the cross section. Of the vector beams, a beam having a spatially polarized state that is cylindrically symmetric is also called a cylinder vector beam. Typical column vector beams have radially polarized light and angularly polarized light.
Poincare light beams are another type of vector light beam with different spatial polarization and phase distribution. The poincare light beam can be obtained by superposing two orthogonal polarized basis vector vortex light beams with different topological charges.
Vector beams with anisotropic spatial polarization states are less affected by atmospheric disturbances when transmitted in atmospheric turbulence than conventional gaussian beams. The polarization components of the vector beams are mutually orthogonal and mutually non-interfering, and the propagation of the beams is not affected, so that the vector beams have certain turbulence resistance, and the influence of atmospheric turbulence effect in a space channel on the optical field structure of the laser chaotic driving signal is weakened.
The modulation module may modulate the second signal, which is originally a gaussian light field, into a fourth signal of a column vector light field or poincare light field. The corresponding light field structure is generated by using corresponding optical devices, the column vector light beam is mainly generated by using linear polaroid and vortex wave plate modulation, and the Poincare light beam can be generated by using linear polaroid, a spatial light modulator and 1/4 wave plate modulation.
The fourth signal is injected into a transmission module for transmission, and the transmission module comprises a free space channel;
The free space channel is a state simulating a real atmospheric environment, and the channel length can be set according to the simulated actual situation, and in this embodiment, the FSO length is selected to be 10m.
The demodulation module has decoupling property with the space structure of the modulation module, and can receive a fourth signal and demodulate the fourth signal into a second signal;
the demodulation module is a decoupling process of the adjustment module, namely, demodulating the column vector beam or the poincare beam into a Gaussian beam, and the demodulation structure of the corresponding light field is symmetrical with the modulation structure, namely, the column vector beam is mainly demodulated by using a vortex wave plate and a linear polaroid, and the poincare beam can be demodulated by using a 1/4 wave plate, a spatial light modulator and the linear polaroid.
The second signal is injected into the decryption module, and the decryption module is driven to generate a fifth signal, wherein the fifth signal is a chaotic decryption signal synchronous with the third signal;
The signal acquisition module is used for acquiring and analyzing a third signal and a fifth signal, is mainly used for acquiring and analyzing and calculating a cross-correlation function CCF to analyze the synchronization quality between chaotic signals, and generally selects the maximum value of the CCF absolute value, namely a cross-correlation coefficient CC to represent the cross-correlation degree of the chaotic signals, wherein the larger the CC value is, the higher the correlation of the two signals is, the better the synchronization quality is, and the worse the synchronization quality is, the closer the CC value is to 1. When the injection intensity is within a certain range, high-quality chaotic synchronization can be formed.
In this embodiment, the third signal (chaotic encryption signal) is I 1 (t), and the fifth signal (chaotic decryption signal) is I 2 (t);
The signal acquisition module acquires the I 1(t)、I2 (t) in real time and analyzes a Cross correlation coefficient CC (Cross-correlation Coefficient, CC), wherein the Cross correlation coefficient CC is the maximum value of the absolute value of a Cross correlation function (Cross-correlation Function, CCF), the CCF is calculated in a mode shown in a formula 1,
The intensity of the third signal of I 1 (t) < I 1 (t) > is the average value of the intensity of the third signal, I 2 (t) is the intensity of the fifth signal, and < I 2 (t) > is the average value of the intensity of the fifth signal. The closer the CC value is to 1, the better the synchronization quality, and the closer to 0 the worse the synchronization quality. When the injection intensity is within a certain range, high-quality chaotic synchronization can be formed.
In one embodiment of the application, the drive module comprises a main laser (DRIVE LASER, DL), a first polarization controller (Polarization Controller, PC 1), a first Fiber Coupler (FC 1), a second Fiber Coupler (FC 2), a first tunable optical attenuator (Variable Optical Attenuator, VOA 1), a Fiber Mirror (Mirror, M), an erbium-doped Fiber amplifier (Erbium Doped Fiber Amplifier, EDFA);
The driving module comprises a main laser, a first polarization controller, a first optical fiber coupler, a second optical fiber coupler, a first adjustable optical attenuator and an optical fiber reflector
The main laser is connected with a first polarization controller, the first polarization controller is connected with a first optical fiber coupler through optical fibers, the first optical fiber coupler is divided into two paths, one path is connected with a first adjustable optical attenuator and an optical fiber reflector, the other path is connected with a second optical fiber coupler, the second optical fiber coupler is divided into two paths, one path is connected with an erbium-doped optical fiber amplifier, and the other path is connected with an encryption module.
In this embodiment, the primary laser is a distributed feedback laser.
Here, the first polarization controller is configured to adjust a polarization state of the continuous laser output by the main laser during transmission, so as to generate a chaotic driving signal;
The first optical fiber coupler is used for dividing continuous laser into two beams, one beam passes through the VOA1 and then is incident into M, the VOA1 is used for controlling laser power injected into the main laser, namely feedback intensity, the M reflects the incident laser into an active area of the main laser again, so that the steady state of the main laser is disturbed, the main laser enters a chaotic state to generate a chaotic driving signal, and then the chaotic driving signal is emitted again through the first optical fiber coupler and is incident into the second optical fiber coupler.
The second optical fiber coupler branches the chaotic driving signal to generate a first signal and a second signal, the first signal is used as a driving signal of the chaotic encryption signal to be injected into the encryption module, and the second signal is injected into the modulation module to modulate the optical field structure after power adjustment and compensation are carried out on the second signal through the erbium-doped optical fiber amplifier.
In one embodiment of the application, the central wavelength of the continuous laser output of the primary laser is 1550nm.
The erbium-doped fiber amplifier is used for adjusting the power of the second signal so as to pre-compensate the power loss caused by the space channel and ensure that the second slave laser at the receiving end has enough injection power.
In one embodiment of the application, the tunable range of the EDFA is 10dBm to 20dBm.
In one embodiment of the present application, the specific steps of the driving module generating the first signal and the second signal are:
The main laser outputs continuous laser by adopting a distributed feedback laser, the continuous laser is divided into two paths after passing through the FC1, one path of continuous laser is incident into M after passing through the bidirectional VOA1, the laser is fed back into an active area of the main laser after being reflected, the steady state of the laser is disturbed, the main laser enters a chaotic state, at the moment, the feedback intensity can be regulated by controlling the VOA1, a generated laser chaotic driving signal is divided into two paths after passing through the FC2, one path of laser chaotic driving signal is directly injected into an encryption module to generate a laser chaotic encryption signal, and the other path of laser chaotic driving signal is transmitted in a free space channel after being subjected to power amplification by an EDFA.
In one embodiment of the application, the encryption module comprises a first slave laser (SLAVE LASER, SL) a first Fiber isolator (Optical Isolator, OI 1), a second variable optical attenuator (Variable Optical Attenuator, VOA 2), a second polarization controller (Polarization Controller, PC 2), a third Fiber Coupler (FC 3);
The first slave laser is connected with a third optical fiber coupler, the third optical fiber coupler is divided into two paths, one path is sequentially connected with a second polarization controller, a second adjustable optical attenuator and a first optical fiber isolator according to the direction of an optical path, and the first optical fiber isolator is connected with the second optical fiber coupler; the other path is connected with the signal acquisition module.
The first slave laser generates a continuous laser signal, in this embodiment, the first slave laser selects a distributed feedback laser;
the first optical fiber isolator is used for controlling the chaos driving signal to pass through in one direction;
the second adjustable optical attenuator is used for controlling the optical power of the chaotic driving signal injected into the first slave laser, namely the magnitude of the injection power;
The second polarization controller is used for adjusting the polarization state of the chaotic driving signal in the transmission process, so that the subsequent high-quality chaotic synchronization is realized;
the third optical fiber coupler is used for dividing the received signal into two beams, wherein one beam is used for receiving the signal of the driving module, and the other beam is injected into the signal acquisition module.
In one embodiment of the present application, the step of generating the third signal by the encryption module is:
The first signal generated by the branching of the second optical fiber coupler is sequentially injected into the transmitting end slave laser SL1 after passing through the OI1, the VOA2, the PC2 and the optical fiber coupler FC3, and a laser chaotic encryption signal is generated;
Wherein, OI1 only allows unidirectional laser to pass through, thus block reverse laser from returning to DL; the VOA2 is used to adjust the magnitude of the optical power used to control the injection into the SL 1; the PC2 is used to adjust the polarization state of the injected chaotic driving signal.
According to the actual working condition, the control temperature and the driving current of the SL1 and the size of the injected light power are reasonably set, namely the injection intensity is reasonably set, so that the SL1 can generate a laser chaotic encryption signal I 1 (t), namely a third signal.
In one embodiment of the application, the modulation module comprises a first collimator (Collimator, col.1), a first linear polarizer (Linear Polarizer, LP 1), a first-order Vortex plate (VPP 1) arranged in order along the light transmission direction.
Preferably, the first-order vortex wave plate is parallel to the fast axis direction of the first linear polarizer.
Col.1 is used for collimating the second signal emitted by the exit drive module, VPP1 is used for modulating the light field polarization distribution. Setting the directions of the fast axes of LP1 and VPP1 to be parallel, due to the nature of VPP1 itself, when the polarization direction of the incident ray polarized light is parallel to the fast axis of the m=1 vortex wave plate, the outgoing beam is a radial polarized beam.
The modulation module is used for modulating the laser chaos driving signal (namely the second signal) which is originally in a Gaussian light field structure into a vector light field, and can be modulated into column vector light, including radial vector light and angular vector light, or can be modulated into full poincare light, including lemon vector light or star vector light, so that the influence of the atmospheric turbulence effect in a space channel on the light field structure of the laser chaos driving signal is weakened by means of the turbulence resistance characteristic of the vector light field structure.
Here, different optics are needed to be used to modulate the light into different vector modes. In this embodiment, a linear polarizer and a first-order vortex wave plate are used to modulate the Gaussian light field into a column vector light field. When the fast axis direction of the linear polarizer is parallel to the fast axis direction of the first-order vortex wave plate, radial vector light is correspondingly generated, and when the fast axis direction and the fast axis direction of the first-order vortex wave plate are mutually perpendicular, angular vector light is correspondingly generated. The Gaussian light field can be modulated into Quan Pangjia Lai field using a linear polarizer, a spatial light modulator, and a 1/4 wave plate. Setting the fast axis direction of the linear polaroid to be 45 degrees (facing the light propagation direction), setting the OAM phase plate with the topological charge number of first order to load the spatial light modulator, correspondingly generating a lemon vector light field when the fast axis direction of the 1/4 wave plate is set to be 45 degrees, and correspondingly generating a star vector light field when the fast axis direction of the 1/4 wave plate is set to be 135 degrees.
In one embodiment of the present application, the transmission module includes a beam expander, a free space channel, and a beam contractor sequentially disposed along the optical transmission direction. After the vector light field signal subjected to light field modulation passes through the beam expander, the vector light field signal starts to be transmitted in a free space channel. An atmospheric turbulence simulation device is placed in the free space channel to simulate the atmospheric turbulence effect. And then at the receiving end, the transmitted vector signals are condensed by a beam condensing lens, so that the vector signals are conveniently received by a second collimator at the rear end.
In one embodiment of the application, the beam expander is a 20-time beam expander, the beam contractor is a 10-time beam contractor, and the multiples of the beam expander and the beam contractor are determined according to actual working conditions.
In one embodiment of the present application, the demodulation module includes a second first-order Vortex wave plate (Vortex Phase-plate, VPP 2), a second linear polarizer (Linear Polarizer, LP 2), and a second collimator (Collimator, col.2) sequentially disposed along the light transmission direction, the second first-order Vortex wave plate being parallel to the fast axis direction of the second linear polarizer and being parallel to the fast axis directions of the first-order Vortex wave plate and the first linear polarizer in the modulation module.
The demodulation module demodulates the corresponding radial vector optical field structure into a Gaussian optical field, and only Gaussian modes can be coupled into the single-mode optical fiber for transmission due to the mode filtering effect of the single-mode optical fiber. The spatial structures of the vector light field modulation module and the vector light field demodulation module have decoupling property. The laser chaos driving signal (namely, the second signal) with the space structure demodulated into the Gaussian mode passes through the second collimator and then enters the optical fiber.
In one embodiment of the application, the decryption module comprises a second slave laser (SLAVE LASER, SL 2), a second Fiber isolator (Optical Isolator, OI 2), a third variable optical attenuator (Variable Optical Attenuator, VOA 3), a third polarization controller (Polarization Controller, PC 3), a fourth Fiber Coupler (FC 4);
The second slave laser is connected with a fourth optical fiber coupler, the fourth optical fiber coupler is divided into two paths, and one path is sequentially connected with a third polarization controller, a third adjustable optical attenuator and a second optical fiber isolator according to the direction of an optical path; the other path is connected with the signal acquisition module;
the second slave laser is consistent with the setting parameters of the first slave laser.
The second optical fiber isolator is used for controlling the chaos driving signal to pass through in one direction;
The third adjustable optical attenuator is used for controlling the optical power of the chaotic driving signal injected into the second slave laser, namely the magnitude of the injection power;
the third polarization controller is used for adjusting the polarization state of the chaotic driving signal in the transmission process, and is beneficial to realizing high-quality chaotic synchronization subsequently.
Preferably, the setting parameters include control temperature, drive current.
In one embodiment of the present application, the step of generating the fifth signal by the decryption module is: the laser chaotic driving signal (namely, the second signal) sequentially passes through one path of the devices OI2, VOA3, PC3 and FC4 and then is injected into the receiving end from the laser SL2 to generate a laser chaotic decryption signal. The functions of the devices are the same as those of the corresponding devices of the laser chaotic encryption signal module. Parameters of the slave laser SL2, including control temperature, drive current and the like, are consistent with the SL1 setting, and the injection intensity of the SL2 is reasonably set to generate a laser chaotic decryption signal I 2 (t), namely a fifth signal.
In one embodiment of the application, the signal acquisition module includes a fiber optic delay line (DELAY LINES, DL), a fourth variable optical attenuator (Variable Optical Attenuator, VOA 4), a fifth variable optical attenuator (Variable Optical Attenuator, VOA 5), a first photodetector (Photodetector, PD 1), a second photodetector (Photodetector, PD 2), a high-speed oscilloscope (Oscilloscope, OSC),
The input end of the fourth adjustable optical attenuator is connected with the third optical fiber coupler, the output end of the fourth adjustable optical attenuator is connected with the first photoelectric detector, and the output end of the first photoelectric detector is connected with the high-speed oscilloscope;
One end of the optical fiber delay line is connected with the fourth optical fiber coupler, the other end of the optical fiber delay line is connected with the input end of the fifth adjustable optical attenuator, the output end of the fifth adjustable optical attenuator is connected with the second photoelectric detector, and the output end of the second photoelectric detector is connected with the high-speed oscilloscope.
Since the optical paths of the laser signals generated from the laser SL1 and the laser signals generated from the laser SL2 are different when reaching the PD1 and the PD2, a larger synchronization delay is caused, so that a section of single-mode fiber needs to be introduced into the fifth signal path as an optical fiber delay line to compensate the synchronization delay at two ends,
The fourth variable optical attenuator and the fifth variable optical attenuator are used for controlling the light power incident to the photoelectric detector;
the first photoelectric detector is used for receiving the chaotic encryption signal I 1 (t), namely a third signal, and the second photoelectric detector is used for receiving the laser chaotic decryption signal I 2 (t), namely a fifth signal;
The high-speed oscilloscope collects the I 1(t)、I2 (t) in real time and analyzes the Cross correlation coefficient CC (Cross-correlation Coefficient),
The third signal of the transmitting end passes through the other path of the optical fiber coupler FC3 and is detected by the photoelectric detector PD1 through the adjustable optical attenuator VOA 4; the fifth signal is detected by PD2 after passing through VOA 5. The two detected time domain waveforms are acquired and displayed in real time by a high-speed oscilloscope OSC.
In one embodiment of the present application, the free space laser chaotic synchronization system further includes a digital signal processing module for calculating a cross correlation coefficient of the third signal and the fifth signal.
In one embodiment of the application, the digital signal processing module includes a filtering algorithm. The filtering algorithm is an algorithm processing mode, and has the functions of mainly eliminating the influence of devices and space disturbance on laser chaos in the transmission process of an optical fiber link and a space channel and filtering real noise in chaotic laser similar to noise. And taking a signal acquired by the oscilloscope as an initial input, setting a low-pass filter, namely filtering high-frequency noise in the signal, and allowing a chaotic signal with lower frequency to pass through to obtain a filtered signal. And then, calculating the filtered I 1 (t) and I 2 (t) to obtain the cross-correlation coefficient.
A typical structure of a free space laser chaotic synchronization system provided in an embodiment of the present application is shown in fig. 1.
The technical scheme of the invention is further described below with reference to the accompanying drawings and examples.
In one embodiment of the application the main laser DL output power is set to 7.5dBm, the edfa is set to 15dBm, and the receive end coupling efficiency is fixed to 20%. The drive currents for SL1 and SL2 were set to 20mA, the control temperature was set to 25 ℃, and the output powers for SL1 and SL2 were 2.21dBm and 4.45dBm, respectively. The center wavelengths of the intrinsic laser spectra output by SL1 and SL2 are 1549.48nm and 1549.66nm, respectively. The signal power injected into SL1 and SL2 is 0.18dBm and 2.34dBm, respectively, i.e., the injection strength is-2.03 dB and-2.11 dB, respectively.
When vector light field modulation and demodulation are not performed, the chaotic synchronization of the two signals is regulated under the condition of no turbulence, and the time domain waveform and the power spectrum of the obtained chaotic encryption signal are shown in fig. 2 and 3; the time domain waveform and the power spectrum shape of the chaotic decryption signal are shown in fig. 4 and 5; the two show a high degree of similarity.
The chaotic synchronization scatter diagram of the chaotic encryption signal and the chaotic decryption signal obtained through offline digital signal processing is shown in fig. 6, and the synchronization coefficient of the chaotic encryption signal and the chaotic decryption signal is 0.97.
Then a vector light field modulation module and a vector light field demodulation module are introduced to modulate the light field structure of the laser chaotic driving signal, and radial vector light modulation is carried out in the application.
In this embodiment, the vector modulation module includes a linear polarizer LP and a first-order vortex wave plate VPP (m=1). The laser chaos driving signal emitted by the collimator Col.1 has a light field structure in a linear Gaussian state, and the diameter D=3mm of a light spot. And rotating the LP to maximize the transmitted light power, then ensuring that the directions of the fast axes of the LP and the VPP are parallel, and because of the nature of the VPP, when the polarization direction of the incident ray polarized light is parallel to the fast axis of the m=1 vortex wave plate, the emergent light beam is a radial polarized light beam. After the optical field structure is modulated into radial polarization, the laser chaotic driving signal is injected into a space channel for transmission.
In the embodiment, an atmospheric turbulence simulation device is adopted to simulate FSO free space channel transmission, the lengths r0 of the atmospheric coherence are respectively 1mm, 1.5mm and 3mm, and the turbulence intensity of the space channel is represented by D/r0, and the larger the D/r0 is, the larger the corresponding turbulence intensity is. A beam quality analyzer is placed at the end of the spatial channel before the vector optical field demodulation module to observe the optical field structure affected by the turbulence effect,
In the embodiment, the states of the gaussian optical field signal (i.e. the second signal) and the radial vector optical field signal (i.e. the fourth signal) affected by turbulence are compared, as shown in fig. 7 and 8. In the process of gradually increasing the turbulence intensity, the light field structures of the Gaussian light field and the radial vector light field are degraded to different degrees, and the light spot shape is irregular and the light intensity distribution is uneven. The radial vector light is not degraded to the extent of gaussian field severity at the same turbulence intensity. In particular, under moderate and strong turbulence conditions, the gaussian field has degraded to an irregular shape, but the radial vector field still retains the annular structure.
Under different turbulence intensities, recording 50 groups of synchronization coefficients of Gaussian light field and radial vector light field laser chaotic encryption signals and decryption signals, and obtaining the chaotic synchronization coefficient scatter diagrams shown in fig. 9 and 10. In the Gaussian light field state, the synchronization performance of the laser chaotic encryption signal and the laser chaotic decryption signal is influenced in a weak turbulence environment, and in a medium and strong turbulence environment, the chaotic synchronization performance is seriously reduced. For the radial vector light field, the synchronous performance of the system is also affected by the turbulence effect, but the overall performance is better than that of the Gaussian light field, which shows that the vector structure reduces the influence of the turbulence effect on the chaotic synchronization to a certain extent.
The above description is only of the preferred embodiments of the present application and is not intended to limit the present application, but various modifications and variations can be made to the present application by those skilled in the art. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application should be included in the protection scope of the present application.
Claims (12)
1. A free space laser chaos synchronous system is characterized in that: the synchronous system comprises a driving module, an encryption module, a modulation module, a transmission module, a demodulation module, a decryption module and a signal acquisition module;
The driving module can generate at least a first signal and a second signal, the first signal is injected into the encryption module to drive the encryption module to generate a third signal, and the third signal is a chaotic encryption signal;
The second signal is of a Gaussian light field structure, is modulated into a fourth signal after being injected into the modulation module, and is of a vector light field structure;
The fourth signal is injected into a transmission module for transmission, and the transmission module comprises a free space channel;
The demodulation module has decoupling property with the space structure of the modulation module, and can receive a fourth signal and demodulate the fourth signal into a second signal;
The second signal is injected into the decryption module, and the decryption module is driven to generate a fifth signal, wherein the fifth signal is a chaotic decryption signal synchronous with the third signal;
The signal acquisition module can acquire and analyze a third signal and a fifth signal.
2. The free space laser chaotic synchronization system according to claim 1, wherein: the driving module comprises a main laser, a first polarization controller, a first optical fiber coupler, a second optical fiber coupler, a first adjustable optical attenuator, an optical fiber reflector and an erbium-doped optical fiber amplifier;
The main laser is connected with a first polarization controller, the first polarization controller is connected with a first optical fiber coupler through optical fibers, the first optical fiber coupler is divided into two paths, one path is connected with a first adjustable optical attenuator and an optical fiber reflector, the other path is connected with a second optical fiber coupler, the second optical fiber coupler is divided into two paths, one path is connected with an erbium-doped optical fiber amplifier, and the other path is connected with an encryption module.
3. The free space laser chaotic synchronization system according to claim 2, wherein: the central wavelength of the continuous laser output by the main laser is 1550 nm.
4. The free space laser chaotic synchronization system according to claim 2, wherein: the encryption module comprises a first slave laser, a first optical fiber isolator, a second adjustable optical attenuator, a second polarization controller and a third optical fiber coupler,
The first slave laser is connected with a third optical fiber coupler which is divided into two paths,
One path is sequentially connected with a second polarization controller, a second adjustable optical attenuator and a first optical fiber isolator according to the direction of the light path, and the first optical fiber isolator is connected with a second optical fiber coupler;
the other path is connected with the signal acquisition module.
5. The free space laser chaotic synchronization system according to claim 1, wherein: the modulation module comprises a first collimator, a first linear polaroid and a first-order vortex wave plate which are sequentially arranged along the light transmission direction, and the first-order vortex wave plate is parallel to the fast axis direction of the first linear polaroid.
6. The free space laser chaotic synchronization system according to claim 1, wherein: the transmission module comprises a beam expander, a free space channel and a beam shrinking lens which are sequentially arranged along the light transmission direction.
7. The free space laser chaotic synchronization system according to claim 6, wherein: the beam expander is 20 times of beam expander, and the beam contractor is 10 times of beam contractor.
8. The free space laser chaotic synchronization system according to claim 5, wherein: the demodulation module comprises a second first-order vortex wave plate, a second linear polaroid and a second collimator which are sequentially arranged along the light transmission direction, wherein the second first-order vortex wave plate is parallel to the fast axis direction of the second linear polaroid and is parallel to the fast axis directions of the first-order vortex wave plate and the first linear polaroid in the modulation module.
9. The free space laser chaotic synchronization system according to claim 1, wherein: the decryption module comprises a second slave laser, a second optical fiber isolator, a third adjustable optical attenuator, a third polarization controller and a fourth optical fiber coupler;
The second slave laser is connected with a fourth optical fiber coupler, the fourth optical fiber coupler is divided into two paths, and one path is sequentially connected with a third polarization controller, a third adjustable optical attenuator and a second optical fiber isolator according to the direction of an optical path; the other path is connected with the data acquisition module;
the second slave laser is consistent with the setting parameters of the first slave laser.
10. The free space laser chaotic synchronization system according to claim 9, wherein: the setting parameters include control temperature, drive current.
11. The free space laser chaotic synchronization system according to claim 1, wherein: the signal acquisition module comprises an adjustable optical fiber delay line, a fourth adjustable optical attenuator, a fifth adjustable optical attenuator, a first photoelectric detector, a second photoelectric detector and a high-speed oscilloscope,
The input end of the fourth adjustable optical attenuator is connected with the third optical fiber coupler, the output end of the fourth adjustable optical attenuator is connected with the first photoelectric detector, and the output end of the first photoelectric detector is connected with the high-speed oscilloscope;
One end of the optical fiber delay line is connected with the fourth optical fiber coupler, the other end of the optical fiber delay line is connected with the input end of the fifth adjustable optical attenuator, the output end of the fifth adjustable optical attenuator is connected with the second photoelectric detector, and the output end of the second photoelectric detector is connected with the high-speed oscilloscope.
12. The free-space laser chaotic synchronization system according to claim 11, wherein: the free space laser chaos synchronous system also comprises a digital signal processing module which is used for calculating the cross-correlation coefficient of the third signal and the fifth signal.
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