CN112436934B - Self-focusing optimized coupling space chaotic laser secret communication system and method - Google Patents
Self-focusing optimized coupling space chaotic laser secret communication system and method Download PDFInfo
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- CN112436934B CN112436934B CN202011254656.8A CN202011254656A CN112436934B CN 112436934 B CN112436934 B CN 112436934B CN 202011254656 A CN202011254656 A CN 202011254656A CN 112436934 B CN112436934 B CN 112436934B
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- H04L9/00—Cryptographic mechanisms or cryptographic arrangements for secret or secure communications; Network security protocols
- H04L9/001—Cryptographic mechanisms or cryptographic arrangements for secret or secure communications; Network security protocols using chaotic signals
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/07—Arrangements for monitoring or testing transmission systems; Arrangements for fault measurement of transmission systems
- H04B10/075—Arrangements for monitoring or testing transmission systems; Arrangements for fault measurement of transmission systems using an in-service signal
- H04B10/079—Arrangements for monitoring or testing transmission systems; Arrangements for fault measurement of transmission systems using an in-service signal using measurements of the data signal
- H04B10/0795—Performance monitoring; Measurement of transmission parameters
- H04B10/07955—Monitoring or measuring power
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
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- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/50—Transmitters
- H04B10/501—Structural aspects
- H04B10/503—Laser transmitters
- H04B10/505—Laser transmitters using external modulation
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- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/60—Receivers
- H04B10/66—Non-coherent receivers, e.g. using direct detection
- H04B10/69—Electrical arrangements in the receiver
- H04B10/691—Arrangements for optimizing the photodetector in the receiver
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- H—ELECTRICITY
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- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/80—Optical aspects relating to the use of optical transmission for specific applications, not provided for in groups H04B10/03 - H04B10/70, e.g. optical power feeding or optical transmission through water
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Abstract
The invention discloses a self-focusing optimal coupling space chaotic laser secret communication system, which comprises a chaotic encryption transmitting module, a focal length self-adaptive module, a chaotic demodulation receiving module and a synchronous module, wherein the chaotic encryption transmitting module is used for transmitting a chaotic signal to a receiving module; the focal length self-adaptive module automatically coarsely adjusts the equivalent focal length according to the coupling efficiency of the signal coupled into the optical fiber when the planar light is transmitted on the ground, and finely adjusts the system according to the error rate of the error code tester when the satellite transmits a test signal, so that the optimal coupling of the space chaotic laser secure communication system under the influence of the pointing error is realized; the invention also discloses a self-focusing optimized coupling method, which reduces the manual debugging cost through the focal length of the secondary self-adjusting variable focus lens group, can effectively reduce the mismatching caused by the pointing error of the laser signal in the transmission, improves the communication quality and has important significance for the development and application of the spatial chaotic laser communication technology.
Description
Technical Field
The invention relates to the technical field of laser communication, in particular to a self-focusing optimal coupling space chaotic laser secret communication system and a method.
Background
The space laser communication technology has the advantages of large transmission bandwidth, small divergence angle, strong anti-interference capability and the like, and also has the advantages of small volume, light weight and low energy consumption of a communication system in the aspect of space application, thereby having a very promising market, and therefore, how to efficiently realize the system and correspondingly research the characteristics of the system also become a hot spot in the field.
In the space chaotic laser communication, coherent communication is adopted to realize high-speed free space chaotic laser communication with high quality, and meanwhile, a free space optical signal needs to be introduced into an optical fiber at a demodulation subsystem part of a receiving end, which brings the problem of the coupling efficiency of the optical fiber. Due to the influence of factors such as noise of the tracking and pointing system and vibration of the satellite launching device, pointing errors are generated in the optical power coupled into the optical fiber, and communication quality is greatly influenced.
The most important thing is to realize chaos synchronization when realizing space chaos laser secret communication of free self-focusing optimized coupling, which concerns whether the signal can be demodulated in the receiving system. Due to the influence of coupling of the optical signal into the optical fiber, the optical power entering one photoelectric detector in the receiving system is subjected to random jitter, while the optical power entering the other photoelectric detector is not influenced due to the power control of the constant-receiving laser, so that mismatch between the received optical signals is generated, the chaotic carrier of the communication system is not completely eliminated in the demodulation process, extra mismatch noise is brought, and the communication quality is reduced. The existing space chaotic laser communication system does not well process the mismatch caused by the coupling of the optical fibers, and has poor communication quality, so that the space chaotic laser communication system has defects.
Disclosure of Invention
Aiming at the problems, the invention provides a self-focusing optimal coupling space chaotic laser secret communication system and a self-focusing optimal coupling space chaotic laser secret communication method.
In order to realize the aim of the invention, the invention provides a self-focusing optimal coupling space chaotic laser secret communication system, which comprises a chaotic encryption transmitting module, a focal length self-adapting module, a chaotic demodulation receiving module and a synchronizing module;
the focal length self-adaptive module comprises a first optical power detector, a second optical power detector, a variable focus lens group, an error code tester, a focal length control system and an electric control slide rail;
the focal length control system is respectively connected with the first optical power detector, the second optical power detector, the variable-focus lens group, the optical fiber lens group distance control system and the error code tester;
when the ground is roughly adjusted, the variable-focus lens group couples spatial signal light into the end face of an optical fiber, a first optical power detector detects the optical power entering the variable-focus lens group in front of the variable-focus lens group, a second optical power detector detects the optical power coupled into the optical fiber behind the end face of the optical fiber, a focal length control system receives the detection data of the first optical power detector and the second optical power detector, corresponding operation is carried out to obtain coupling efficiency, corresponding electric signals are generated to serve as control signals to act on the variable-focus lens group and an electric control slide rail, the variable-focus lens group changes the equivalent focal length according to the electric signals by utilizing the electro-optical effect, the electric control slide rail keeps consistent with the equivalent focal length of the lens group according to the distance between the electric signal control lens group and the end face of the optical fiber, the focal length control system gives out control signals for continuously reducing the equivalent focal length of the variable-focus lens group according to the coupling efficiency, and stops reducing the equivalent focal length of the variable-focus lens group when the coupling efficiency is reduced, the focal length is adjusted back to enable the coupling efficiency to reach the maximum value, and at the moment, the space chaotic laser secure communication system achieves the optimal coupling without the influence of pointing errors;
when satellite communication is finely adjusted, the variable focus lens group couples signal light transmitted from the chaotic encryption transmitting module to the space and enters the optical fiber, the optical power detector II detects the optical power coupled to the optical fiber after the end face of the optical fiber and transmits the optical power to the synchronizing module, the error code tester is connected with the chaotic demodulation receiving module, the error code rate of the system is obtained by comparing the demodulation signal and the transmission test signal of the chaotic demodulation receiving module, the focal length control system gives a control signal for continuously reducing the equivalent focal length of the variable focus lens group according to the error code rate signal, when the error code rate begins to increase, the equivalent focal length of the variable focus lens group is stopped to be reduced, the focal length is adjusted back to enable the error code rate to reach the minimum value, and at the moment, the space chaotic laser communication system achieves optimal coupling under the influence of pointing errors.
In one embodiment, the chaotic encryption transmitting module comprises a first laser and a transmitting end oscillation starting loop;
the transmitting end oscillation starting loop is respectively connected with the first laser and a signal to be transmitted, the transmitting end oscillation starting loop receives constant-power laser which is emitted by the first laser and has the same wavelength as the signal to be transmitted to generate chaotic carrier waves, meanwhile, the transmission signal enters the transmitting end oscillation starting loop, a coupler in the transmitting end oscillation starting loop is coupled with the chaotic carrier waves to achieve chaotic encryption of the signal, and the encrypted signal enters an atmosphere channel and is transmitted to the focal length self-adaptive module.
Specifically, the synchronization module comprises an adjustable attenuator and an attenuator control system;
when satellite communication is precisely adjusted, the attenuator control system is connected with the second optical power detector, receives detection data of the second optical power detector, and gives an electric signal for controlling the attenuation coefficient of the adjustable attenuator according to the preset power of the first laser, so that the attenuation coefficient of the attenuator is kept at a ratio of twice the power of the first laser to the detection value of the second optical power detector;
the adjustable attenuator receives a control signal of an attenuator control system and is respectively connected with the focal length adaptive module and the chaotic demodulation receiving module, the signal enters the adjustable attenuator after being coupled into an optical fiber through a variable focus lens group of the focal length adaptive module, and then enters a receiving end oscillation starting loop and a photoelectric detector I of the chaotic demodulation receiving module after being attenuated by the adjustable attenuator, so that the theoretical input synchronization of the optical power of feedback signals entering the receiving end oscillation starting loop and the transmitting end oscillation starting loop is ensured, and the mismatch of the spatial chaotic laser secret communication system is reduced.
Specifically, the chaotic demodulation receiving module comprises a second laser, a receiving end oscillation starting loop, a first photoelectric detector, a second photoelectric detector and a demodulator;
the receiving end oscillation starting loop is connected with the second laser, the second photoelectric detector and the adjustable attenuator respectively, the parameters of the receiving end oscillation starting loop and the parameters of the transmitting end oscillation starting loop are completely the same, the receiving end oscillation starting loop receives the constant-power laser which is emitted by the second laser and has the same wavelength as the signal to be transmitted, the signal transmitted from the adjustable attenuator is used as a feedback signal, and chaotic carrier waves in the same state as the transmitting end are generated;
the demodulator is connected with the first photoelectric detector, the second photoelectric detector and the error code tester, the first photoelectric detector converts an optical signal transmitted into the chaotic demodulation receiving module from the synchronous module into a corresponding electric signal, the second photoelectric detector converts an optical signal transmitted by a receiving end oscillation starting loop into a corresponding electric signal, the electric signals output by the first photoelectric detector and the second photoelectric detector are input into the demodulator, and the demodulator performs subtraction operation on the two paths of electric signals and then recovers a signal to be transmitted.
A self-focusing optimized coupling space chaotic laser secret communication method comprises the following steps:
step one, carrying out rough ground adjustment, starting a ground surface light source, emitting constant-power parallel light with the same wavelength as a signal to be transmitted, adjusting optical centers of a variable-focus lens group on an electric control slide rail and an optical fiber to the same horizontal line, observing the readings of a second optical power detector, and considering that light is primarily coupled into the optical fiber when the readings of the second optical power detector appear;
step two, starting the first optical power detector and the second optical power detector, transmitting the detected optical power data to a focal length control system, comparing the data by the focal length control system to obtain a coupling efficiency so as to obtain a control signal for continuously reducing the equivalent focal length of the variable-focus lens group, controlling the distance between the lens group and the end face of the optical fiber to be equal to the equivalent focal length by the electric control slide rail according to the signal, and reducing the focal length by the variable-focus lens group according to the signal; when the coupling efficiency is reduced, stopping reducing the equivalent focal length of the variable-focus lens group, and adjusting back the focal length to enable the coupling efficiency to reach the maximum value, at the moment, achieving the optimal coupling without the influence of the pointing error, and considering that the ground rough adjustment is finished;
thirdly, performing satellite communication fine adjustment, closing a ground surface light source and the first optical power photoelectric detector, transmitting detection data of the second optical power photoelectric detector to an attenuator control system, and starting a chaos encryption transmitting module on the satellite and a chaos demodulation receiving module on the ground;
and step four, the chaotic encryption transmitting module transmits chaotic carrier signals, the attenuator control system controls the attenuation coefficient of the adjustable attenuator to be twice of the power of the first laser device compared with the power of the second laser device according to the power data of the second optical power detector and the power of the first laser device, the optical power amplitudes entering the first and second optical power detectors are ensured to be matched, the parameter matching of the transmitting end oscillation starting loop and the receiving end oscillation starting loop is adjusted, and chaotic synchronization is realized.
Fifthly, transmitting the known pseudo-random signal serving as a signal to be transmitted into a chaotic encryption transmitting module for encryption, transmitting the encrypted signal into a chaotic demodulation receiving module through an atmospheric channel, then passing through a focal length self-adaption module and a synchronization module, demodulating the signal after performing subtraction operation on output signals of the first photoelectric detector and the second photoelectric detector by a demodulator, and comparing the known pseudo-random signal with the demodulated signal by an error code tester to obtain the error code rate of the system; the focal length control system gives a control signal for continuously reducing the equivalent focal length of the variable-focus lens group according to the error rate signal, the equivalent focal length of the variable-focus lens group is stopped to be reduced when the error rate begins to increase, the focal length is adjusted back to enable the error rate to reach the minimum value, and at the moment, the system achieves the optimal coupling of the coupling efficiency under the influence of the pointing error.
Compared with the prior art, the self-focusing optimized coupled space chaotic laser secret communication system has the following technical effects:
(1) the focal length of the zoom lens group is secondarily self-adjusted, so that the manual debugging cost is reduced, and meanwhile, the accuracy is very high;
(2) mismatch caused by pointing error of laser signals in transmission can be effectively reduced, communication quality can be improved, and the method has great significance for development and application of a space chaotic laser communication technology;
(3) the required modules are not complex and the implementation cost is low.
Drawings
FIG. 1 is a schematic structural diagram of a self-focusing optimally coupled spatial chaotic laser secure communication system according to an embodiment;
FIG. 2 is a diagram of a chaotic start-up loop configuration according to one embodiment;
FIG. 3 is a schematic diagram of an operating environment of a self-focusing optimally coupled spatial chaotic laser secure communication system according to an embodiment;
FIG. 4 is a diagram of an embodiment of a free-space light incoupling single mode fiber model;
FIG. 5 is a graph illustrating coupling efficiency versus equivalent focal length for coarse tuning, according to one embodiment;
FIG. 6 is a diagram illustrating a relationship between the total average ber and the equivalent focal length of the system during fine tuning according to an embodiment.
Detailed Description
In order to make the objects, technical solutions and advantages of the present application more apparent, the present application is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the present application and are not intended to limit the present application.
Reference herein to "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the application. The appearances of the phrase in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments. It is explicitly and implicitly understood by one skilled in the art that the embodiments described herein can be combined with other embodiments.
Referring to fig. 1, fig. 1 is a spatial chaotic secret laser communication system with self-focusing optimized coupling according to an embodiment, including a chaotic encryption transmitting module, a focal length adaptive module, a chaotic demodulation receiving module, and a synchronization module;
the focal length self-adaptive module comprises a first optical power detector, a second optical power detector, a variable focus lens group, an error code tester, a focal length control system and an electric control slide rail;
the focal length control system is respectively connected with the first optical power detector, the second optical power detector, the variable-focus lens group and the error code tester;
when the ground is roughly adjusted, the variable-focus lens group couples spatial signal light into the end face of an optical fiber, a first optical power detector detects the optical power entering the variable-focus lens group in front of the variable-focus lens group, a second optical power detector detects the optical power coupled into the optical fiber behind the end face of the optical fiber, a focal length control system receives the detection data of the first optical power detector and the second optical power detector, corresponding operation is carried out to obtain coupling efficiency, corresponding electric signals are generated to serve as control signals to act on the variable-focus lens group and an electric control slide rail, the variable-focus lens group changes the equivalent focal length according to the electric signals by utilizing the electro-optical effect, the electric control slide rail keeps consistent with the equivalent focal length of the lens group according to the distance between the electric signal control lens group and the end face of the optical fiber, the focal length control system gives out control signals for continuously reducing the equivalent focal length of the variable-focus lens group according to the coupling efficiency, and stops reducing the equivalent focal length of the variable-focus lens group when the coupling efficiency is reduced, the focal length is adjusted back to enable the coupling efficiency to reach the maximum value, and at the moment, the space chaotic laser secure communication system achieves the optimal coupling without the influence of pointing errors;
when satellite communication is finely adjusted, the variable focus lens group couples signal light transmitted from the chaotic encryption transmitting module to the space and enters the optical fiber, the optical power detector II detects the optical power coupled to the optical fiber after the end face of the optical fiber and transmits the optical power to the synchronizing module, the error code tester is connected with the chaotic demodulation receiving module, the error code rate of the system is obtained by comparing the demodulation signal and the transmission test signal of the chaotic demodulation receiving module, the focal length control system gives a control signal for continuously reducing the equivalent focal length of the variable focus lens group according to the error code rate signal, when the error code rate begins to increase, the equivalent focal length of the variable focus lens group is stopped to be reduced, the focal length is adjusted back to enable the error code rate to reach the minimum value, and at the moment, the space chaotic laser communication system achieves optimal coupling under the influence of pointing errors.
In the space chaotic laser secret communication system with self-focusing optimized coupling, when the ground is roughly adjusted, the variable focus lens group couples space signal light into the end face of an optical fiber, the first optical power detector detects the optical power entering the variable focus lens group in front of the variable focus lens group, the second optical power detector detects the optical power coupled into the optical fiber behind the end face of the optical fiber, the focal length control system receives the detection data of the first optical power detector and the second optical power detector, performs corresponding operation to obtain the coupling efficiency, generates corresponding electric signals as control signals to act on the variable focus lens group and an electric control slide rail, the variable focus lens group changes the equivalent focal length according to the electric signal light effect, the electric control slide rail controls the distance between the lens group and the end face of the optical fiber according to the electric signals to keep the equivalent focal length of the lens group, and the control system gives out control signals for continuously reducing the equivalent focal length of the variable focus lens group according to the coupling efficiency, when the coupling efficiency is reduced, stopping reducing the equivalent focal length of the variable-focus lens group, and adjusting back the focal length to enable the coupling efficiency to reach the maximum value, so that the space chaotic laser secret communication system achieves the optimal coupling without the influence of pointing errors; when satellite communication is finely adjusted, the variable focal length lens group couples the signal light transmitted from the chaotic encryption transmitting module to the space into the optical fiber, the optical power detector II detects the optical power coupled into the optical fiber after the end surface of the optical fiber and transmits the optical power to the synchronizing module, the error code tester is connected with the chaotic demodulation receiving module, the error code rate of the system is obtained by comparing the demodulation signal and the transmission test signal of the chaotic demodulation receiving module, the focal length control system gives a control signal for continuously reducing the equivalent focal length of the variable focal length lens group according to the error code rate signal, when the error code rate begins to increase, the equivalent focal length of the variable focal length lens group is stopped to be reduced, the focal length is adjusted back to enable the error code rate to reach the minimum value, at the moment, the space chaotic laser communication system achieves the optimal coupling under the influence of pointing error, wherein the focal length of the variable focal length lens group is secondarily adjusted, and related parameters are adjusted at the same time, the method has the advantages of reducing the manual debugging cost, having high accuracy, effectively reducing the mismatching caused by the pointing error of the laser signal in the transmission, improving the communication quality and having important significance for the development and application of the space chaotic laser communication technology.
In one embodiment, the chaotic encryption transmitting module comprises a first laser and a transmitting end oscillation starting loop;
the transmitting end oscillation starting loop is respectively connected with the first laser and a signal to be transmitted, the transmitting end oscillation starting loop receives constant-power laser which is emitted by the first laser and has the same wavelength as the signal to be transmitted to generate chaotic carrier waves, meanwhile, the transmission signal enters the transmitting end oscillation starting loop, a coupler in the transmitting end oscillation starting loop is coupled with the chaotic carrier waves to achieve chaotic encryption of the signal, and the encrypted signal enters an atmosphere channel and is transmitted to the focal length self-adaptive module.
Specifically, the synchronization module comprises an adjustable attenuator and an attenuator control system;
when satellite communication is precisely adjusted, the attenuator control system is connected with the second optical power detector, receives detection data of the second optical power detector, and gives an electric signal for controlling the attenuation coefficient of the adjustable attenuator according to the preset power of the first laser, so that the attenuation coefficient of the attenuator is kept at a ratio of twice the power of the first laser to the detection value of the second optical power detector;
the adjustable attenuator receives a control signal of an attenuator control system and is respectively connected with the focal length adaptive module and the chaotic demodulation receiving module, the signal enters the adjustable attenuator after being coupled into an optical fiber through a variable focus lens group of the focal length adaptive module, and then enters a receiving end oscillation starting loop and a photoelectric detector I of the chaotic demodulation receiving module after being attenuated by the adjustable attenuator, so that the theoretical input synchronization of the optical power of feedback signals entering the receiving end oscillation starting loop and the transmitting end oscillation starting loop is ensured, and the mismatch of the spatial chaotic laser secret communication system is reduced.
Specifically, the chaotic demodulation receiving module comprises a second laser, a receiving end oscillation starting loop, a first photoelectric detector, a second photoelectric detector and a demodulator;
the receiving end oscillation starting loop is connected with the second laser, the second photoelectric detector and the adjustable attenuator respectively, the parameters of the receiving end oscillation starting loop and the parameters of the transmitting end oscillation starting loop are completely the same, the receiving end oscillation starting loop receives the constant-power laser which is emitted by the second laser and has the same wavelength as the signal to be transmitted, the signal transmitted from the adjustable attenuator is used as a feedback signal, and chaotic carrier waves in the same state as the transmitting end are generated;
the demodulator is connected with the first photoelectric detector, the second photoelectric detector and the error code tester, the first photoelectric detector converts an optical signal transmitted into the chaotic demodulation receiving module from the synchronous module into a corresponding electric signal, the second photoelectric detector converts an optical signal transmitted by a receiving end oscillation starting loop into a corresponding electric signal, the electric signals output by the first photoelectric detector and the second photoelectric detector are input into the demodulator, and the demodulator performs subtraction operation on the two paths of electric signals and then recovers a signal to be transmitted.
In the chaotic demodulation receiving module, a signal to be demodulated is divided into two parts, one part of the signal enters a photoelectric detector I and is changed into an electric signal from an optical signal (the signal of the path is chaotic carrier wave + a signal to be detected), the other part of the signal enters a receiving end oscillation starting loop to be used as a feedback signal, a laser emitting laser of a laser device II is input into a receiving end oscillation starting loop to generate a chaotic carrier wave, the chaotic carrier wave is transmitted to a photoelectric detector II and is changed into an electric signal from an optical signal (the signal of the path is chaotic carrier wave), and then the two electric signals are transmitted to a demodulator to be subjected to electric phase subtraction to restore the signal to be detected.
Preferably, the first laser and the second laser are constant power lasers with the same wavelength as the signal to be transmitted.
Preferably, the transmitting end oscillation starting loop and the receiving end oscillation starting loop are chaotic oscillation starting loops with matched parameters. The structure of the chaotic oscillation starting loop can be shown in fig. 2, and includes a 2 × 2 coupler, an FDL (fiber delay line), an APD (avalanche photo diode), an RF (radio frequency amplifier), and an MZ (mach-zehnder modulator), and the chaotic carrier is generated by utilizing the nonlinear effect of the mach-zehnder modulator and through optical feedback of the loop, and when a signal to be transmitted is coupled into the carrier, the chaotic carrier plays a role in signal masking.
Preferably, the focal length adaptive module comprises a first optical power detector, a second optical power detector, a variable focus lens group, an error code tester and a focal length control system; the working principle of each part in the focal length self-adaptive module comprises the following steps:
and the first optical power detector is used for detecting the optical power entering the variable-focus lens group and transmitting the detection data to the focal length control system.
And the second optical power detector detects the optical power after entering the optical fiber and transmits detection data to the focal length control system and the attenuator control system.
And the error code tester is used for detecting the error code rate of the signal demodulated by the chaotic decryption receiving module and transmitting the error code rate data to the focal length control system.
The focal length control system receives the detection data of the first optical power detector and the second optical power detector when the ground is coarsely adjusted, performs corresponding operation to obtain the coupling efficiency, and generates corresponding electric signals to act on the variable-focus lens group and the electric control slide rail as control signals; when satellite communication is precisely adjusted, generating corresponding electric signals according to error rate data given by an error code tester to serve as control signals to act on the variable-focus lens group and the electric control slide rail;
the variable-focus lens group receives a control signal from the focal length control system to change the equivalent focal length of the lens group; the control signal acts on the variable-focus lens group, and parameters such as the refractive index of the variable-focus lens group and the like are changed through an electro-optical effect, so that the change of the focal length is realized.
And the electric control slide rail receives a control signal from the focal length control system, controls the distance between the lens group and the optical fiber end face to be consistent with the equivalent focal length of the lens group, and ensures that the lens group on the slide rail and the optical center of the optical fiber end face are on the same horizontal plane and the distance between the lens group and the optical center of the optical fiber end face is consistent with the equivalent focal length of the lens group.
In practical application, the chaotic encryption transmitting module is used for carrying out chaotic encryption on signals and transmitting the signals to free space for transmission. And the chaotic decryption receiving module is used for carrying out chaotic demodulation on the encrypted signal so as to recover the original signal.
In one embodiment, the synchronization module includes an adjustable attenuator and an attenuator control system;
the attenuator control system gives a signal for changing the adjustable attenuator according to the optical power data of the optical power photoelectric detector II and the power of the laser I, so that the attenuation coefficient of the attenuator is twice of the ratio of the power of the laser I to the detection value of the optical power photoelectric detector II;
the adjustable attenuator receives a control signal of an attenuator control system to reduce the mismatch of the self-focusing optimization coupled space chaotic laser secure communication system and synchronize the theoretical input optical power of the transmitting end oscillation starting loop and the receiving end oscillation starting loop.
Specifically, the self-focusing optimized coupling space chaotic laser secret communication method comprises the following steps:
step one, carrying out rough ground adjustment, starting a ground surface light source, emitting constant-power parallel light with the same wavelength as a signal to be transmitted, adjusting optical centers of a variable-focus lens group on an electric control slide rail and an optical fiber to the same horizontal line, observing the readings of a second optical power detector, and considering that light is primarily coupled into the optical fiber when the readings of the second optical power detector occur;
step two, starting the first optical power detector and the second optical power detector, transmitting the detected optical power data to a focal length control system, comparing the data by the focal length control system to obtain a coupling efficiency so as to obtain a control signal for continuously reducing the equivalent focal length of the variable-focus lens group, controlling the distance between the lens group and the end face of the optical fiber to be equal to the equivalent focal length by the electric control slide rail according to the signal, and reducing the focal length by the variable-focus lens group according to the signal; when the coupling efficiency is reduced, stopping reducing the equivalent focal length of the variable-focus lens group, and adjusting back the focal length to enable the coupling efficiency to reach the maximum value, at the moment, achieving the optimal coupling without the influence of the pointing error, and considering that the ground rough adjustment is finished;
thirdly, performing satellite communication fine adjustment, closing a ground surface light source and the first optical power photoelectric detector, transmitting detection data of the second optical power photoelectric detector to an attenuator control system, and starting a chaos encryption transmitting module on the satellite and a chaos demodulation receiving module on the ground;
and step four, the chaotic encryption transmitting module transmits chaotic carrier signals, the attenuator control system controls the attenuation coefficient of the adjustable attenuator to be twice of the power of the first laser device compared with the power of the second laser device according to the power data of the second optical power detector and the power of the first laser device, the optical power amplitudes entering the first and second optical power detectors are ensured to be matched, the parameter matching of the transmitting end oscillation starting loop and the receiving end oscillation starting loop is adjusted, and chaotic synchronization is realized.
Fifthly, transmitting the known pseudo-random signal serving as a signal to be transmitted into a chaotic encryption transmitting module for encryption, transmitting the encrypted signal into a chaotic demodulation receiving module through an atmospheric channel, then passing through a focal length self-adaption module and a synchronization module, demodulating the signal after performing subtraction operation on output signals of the first photoelectric detector and the second photoelectric detector by a demodulator, and comparing the known pseudo-random signal with the demodulated signal by an error code tester to obtain the error code rate of the system; the focal length control system gives a control signal for continuously reducing the equivalent focal length of the variable-focus lens group according to the error rate signal, the equivalent focal length of the variable-focus lens group is stopped to be reduced when the error rate begins to increase, the focal length is adjusted back to enable the error rate to reach the minimum value, and at the moment, the system achieves the optimal coupling of the coupling efficiency under the influence of the pointing error.
Compared with the prior art, the self-focusing optimized coupled space chaotic laser secret communication system has the following technical effects:
(1) the focal length of the zoom lens group is secondarily self-adjusted, so that the manual debugging cost is reduced, and meanwhile, the accuracy is very high;
(2) mismatch caused by pointing error of laser signals in transmission can be effectively reduced, communication quality can be improved, and the method has great significance for development and application of a space chaotic laser communication technology;
(3) the required modules are not complex and the implementation cost is low.
In one embodiment, the working process of the self-focusing optimized coupled spatial chaotic laser secure communication system is described in detail by taking the working environment shown in fig. 3 as an example. A self-focusing optimized coupling space chaotic laser secret communication system comprises a chaotic encryption transmitting module, a focal length self-adapting module, a chaotic demodulation receiving module and a synchronizing module;
the chaotic encryption transmitting module comprises a first laser and a transmitting end oscillation starting loop, laser generated by the first laser enters the transmitting end oscillation starting loop to generate a chaotic carrier, the structure of the oscillation starting loop can be shown in figure 2 and comprises a 2 multiplied by 2 coupler, an FDL (fiber delay line), an APD (avalanche photodiode), an RF (radio frequency amplifier) and an MZ (Mach-Zehnder modulator), the chaotic carrier is generated through optical feedback of the loop by utilizing the nonlinear effect of the Mach-Zehnder modulator, and when a signal to be transmitted is coupled into the carrier, the chaotic carrier plays an encryption role in signal masking.
The focal length self-adaptive module comprises a first optical power detector, a second optical power detector, a variable-focus lens set, an error code tester, a focal length control system, an optical fiber lens set distance control system and an electric control slide rail, wherein when the ground is coarsely adjusted, the focal length control system adjusts the equivalent focal length of the variable-focus lens set and the distance between the lens set and the optical fiber end face by comparing the optical power of the first optical power detector and the optical power detector, so that the maximum fiber-entering coupling efficiency of signal light is realized without being influenced by pointing errors; when satellite-to-ground laser communication is started, the influence of the coupling-in-first on the system is reduced by further reducing the focus after coarse adjustment, so that the optimization of the system performance is realized;
the chaotic demodulation receiving module comprises a second laser, a receiving end oscillation starting loop, a first photoelectric detector and a second photoelectric detector, wherein light entering the chaotic demodulation receiving module is divided into two paths by the demodulator, one path of light signal comprises a transmission signal and a chaotic carrier wave, enters the first photoelectric detector to be converted into an electric signal, the other path of light signal enters the receiving end oscillation starting loop, and the loop is completely the same as the transmitting end oscillation starting loop and is used for generating the chaotic carrier wave in the same state as the transmitting end. The generated chaotic carrier enters a second photoelectric detector and is converted into an electric signal, output signals of the two detectors are subtracted through a demodulator, the chaotic carrier is eliminated, and a transmission signal is demodulated.
The synchronous module comprises an adjustable attenuator and an attenuator control system, wherein the attenuator control system is used for controlling the attenuation coefficient of the adjustable attenuator when the satellite-ground laser communication is started, so that the optical power amplitudes entering the first photoelectric detector and the second photoelectric detector are ensured to be matched, and chaotic synchronization is realized.
The space chaotic laser secret communication system based on the self-focusing optimized coupling reduces the influence of the fiber-entering coupling on the communication performance of the system through the following steps.
Starting a ground debugging light source, emitting constant-power parallel light with the same wavelength as a signal to be transmitted, adjusting the optical centers of a variable-focus lens group on an electric control slide rail and an optical fiber to the same horizontal line, observing the reading of a second optical power detector, and ensuring that light is primarily coupled into the optical fiber;
step two, starting the first optical power detector and the second optical power detector, transmitting the detected optical power data to a focal length control system, comparing the data by the focal length control system to obtain a coupling efficiency eta, wherein the coupling efficiency eta at the moment can be obtained by coupling free space optical coupler shown in figure 4 into a single mode fiber model, and the formula is as follows:
where η (r, f) refers to the coupling efficiency under the influence of pointing errors, and is related to the lateral offset of the focusing field from the fiber end face and the equivalent focal length f. D is the diameter of the receiving aperture, λ is the wavelength of the light, ω0Is the mode field radius of single-mode fiber, epsilon is the central masking ratio, f is the equivalent focal length, r is the lateral offset of the focusing field relative to the end face of the fiber, J0(x) Is a zero order bessel function of the first kind. Since the coarse adjustment is done at ground level, it can be considered that the lateral offset of the focusing field from the fiber end face due to pointing error is zero, so the expression for coupling efficiency becomes:
where η (f) refers to the coupling efficiency without considering the influence of pointing error, and is related to the equivalent focal length f only. Under the condition that other parameters are fixed, the coupling efficiency is a function of f, so that the coupling efficiency can be continuously increased by the focal length control system by giving a control signal for continuously reducing the equivalent focal length of the variable-focus lens group, meanwhile, the optical fiber lens group distance control system controls the electric control slide rail according to the signal to ensure that the distance between the lens group and the end surface of the optical fiber is equal to the equivalent focal length, the model condition shown in figure 4 is met, when the coupling efficiency is reduced, the equivalent focal length of the variable-focus lens group is stopped to be reduced, the focal length is adjusted back to enable the coupling efficiency to reach the maximum value, at the moment, the optimal coupling without the influence of pointing errors is achieved, and the rough adjustment of the system is considered to be finished;
step three, turning off the ground debugging light source and the first optical power detector, transmitting the detection data of the second optical power detector to an attenuator control system, and starting a chaos encryption transmitting module on the satellite and a chaos demodulation receiving module on the ground;
and step four, the chaotic encryption transmitting module transmits chaotic carrier signals, the attenuator control system controls the attenuation coefficient of the adjustable attenuator to be twice of the power of the first laser device compared with the power of the second laser device according to the power data of the second optical power detector and the power of the first laser device, the optical power amplitudes entering the first and second optical power detectors are ensured to be matched, the parameter matching of the transmitting end oscillation starting loop and the receiving end oscillation starting loop is adjusted, and chaotic synchronization is realized.
And step five, the chaotic encryption transmitting module transmits a known pseudo-random signal, the demodulator demodulates the output signals of the first photodetector and the second photodetector after addition operation, the error code tester compares the pseudo-random signal with the demodulated signal to obtain the error code rate of the system, the error code rate of the chaotic communication system at the moment is the error code rate influenced by fiber-in coupling, and the calculation formula is as follows:
wherein d is the mask ratio, K1(r, f) is the current signal amplitude obtained by the photodetector one, and the calculation formula is as follows:
wherein P is1Power of the first laser, gain coefficient of the first and second photodetectors, point charge amount, etaeTo quantum efficiency, hpIs the Planck constant, v is the optical frequency (obtained by dividing the speed of light c by the wavelength λ), TsIs the signal light pulse time, is the inverse of the signal bit rate;
<n2(r,f)>the chaotic mismatch noise is chaotic mismatch noise, and the calculation formula is as follows:
where delta phi is the offset phase mismatch,<ξ2>for the synchronization error variance, the calculation formula is as follows:
wherein Δ T represents the mismatch of time delay, τ is the high cut-off response time, and Δ τ is the high cut-off response time mismatch;
since the lateral shift of the focusing field with respect to the fiber end face is caused by pointing deviation, there is the same probability density function F (r, F) that follows a Rician distribution
I0(x) For zero order correction of Bessel function, A is the fiber position lateral offset error, σrFor jitter errors, the jitter error depends on the accuracy σ of the tracking systemθAnd the equivalent focal length f of the receiving optical coupling system:
σr(f)＝σθf (8)
therefore, the overall average bit error rate of the system affected by the coupling of the incoming fibers can be expressed as
Therefore, after the coarse adjustment of the two focal lengths in the step, the error rate of the system can be effectively optimized by further fine adjustment of the focal lengths, the influence of fiber-entering coupling is reduced, the focal length control system gives a control signal for continuously reducing the equivalent focal length of the variable focal lens group according to the error rate signal, the equivalent focal length of the variable focal lens group is stopped being reduced when the error rate begins to increase, the focal length is adjusted back to enable the error rate to reach the minimum value, and at the moment, the system achieves the optimal coupling of the coupling efficiency under the influence of the pointing error.
Further, a method for reducing the influence of fiber-in coupling by self-adjusting focal length is described by taking a space chaotic laser communication system using a geosynchronous satellite as an example. Because the construction of the actual space chaotic laser communication system relates to contents in various aspects, the system of the actual system is utilized to carry out test conditionsThe deficiency is therefore illustrated by numerical simulations. In the embodiment, a space chaotic laser communication system using a geostationary satellite has the satellite height of about 36000km away from the ground, adopts a binary amplitude keying (OOK) modulation mode and has a communication rate of 1Gb/s, so that the signal light pulse time T issIs 10-9s, the mask ratio d (the ratio of the signal to the carrier amplitude) is 1.2, the optical wavelength λ of the signal to be measured is 1550nm, and as a transmission type optical system is adopted, the central masking ratio epsilon is 0, the quantum efficiency is 0.75, the time delay mismatch is 1ps, the high-cut-off response time is 25ps, the high-cut-off response time mismatch Δ τ meets the condition that Δ τ/τ is 0.01, and the offset phase mismatch is 0.02 rad. Several important system parameters are as follows: the chaos encryption transmitting module laser has one power: 4mW, photodetector gain coefficient: 100, receiving aperture diameter: 0.14m, the mode field radius of the single mode fiber used in the system: 10 μm, fiber position lateral offset error: 1 μm, tracking accuracy of the system: 1.3 μ rad. When a certain parameter is analyzed, other parameters are not changed.
The simulation process comprises the following steps:
when the rough adjustment is carried out on the ground, the coupling efficiency is obtained by the focal length control system through comparing the data of the first optical power detector and the second optical power detector, because the rough adjustment is carried out on the ground, the influences of factors such as noise of a tracking and aiming system, vibration of a satellite transmitting device and the like do not exist, the transverse offset of a focusing field relative to the end face of the optical fiber at the moment can be considered to be zero, and the relation between the coupling efficiency and the equivalent focal length of the variable-focus lens group under the condition can be obtained by a formula (2) and is shown in fig. 5. It can be seen that there is a maximum where the coupling efficiency increases first and then decreases as the focal length decreases. The curve is numerically calculated to find that the optimal focal length is 126.06cm, that is, the focal length control system continuously reduces the equivalent focal length of the variable focus lens set to achieve optimal coupling when reaching 126.06cm, and when the equivalent focal length is less than the value, the coupling efficiency is reduced. After coarse adjustment on the ground, the system can achieve the optimal coupling without the influence of the pointing error.
When the satellite system is started to perform fine adjustment, at this time, the whole system is influenced by pointing errors, so that the coupling efficiency of the optical fiber is randomly fluctuated, the optical power entering the first photoelectric detector and the second photoelectric detector is mismatched, the error rate of the system is influenced as shown in formula (2), the jitter error can be reduced due to further reduction of the focal length, but the coupling efficiency is reduced, and a relation graph of the overall average error rate of the system and the equivalent focal length under the influence of coupling efficiency jitter is obtained by simulating formula (9) and is shown in fig. 6. It can be seen that further reduction of the focal length, and with coarse adjustment, optimizes the error rate of the system, which is minimized at a focal length of 121.21m, and below this value, the error rate of the system increases and the performance of the system deteriorates. The influence of the coupling efficiency of the incoming fiber is reduced by further adjusting the focal length by the focal length control system, so that the system achieves the optimal coupling under the influence of the pointing error.
In summary, the present invention utilizes the self-adjustment of the equivalent focal length of the optical coupling system to reduce the influence of spatial light-to-fiber coupling, and can effectively improve the communication quality of the system. In an actual system, the optimal focal length of the system can be determined through the algorithm according to different system parameters, the performance optimization degree is different after fine adjustment according to different system designs, but if the system device parameters are unreasonable, the optimization effect will be poor.
The technical features of the above embodiments can be arbitrarily combined, and for the sake of brevity, all possible combinations of the technical features in the above embodiments are not described, but should be considered as the scope of the present specification as long as there is no contradiction between the combinations of the technical features.
It should be noted that the terms "first \ second \ third" referred to in the embodiments of the present application merely distinguish similar objects, and do not represent a specific ordering for the objects, and it should be understood that "first \ second \ third" may exchange a specific order or sequence when allowed. It should be understood that "first \ second \ third" distinct objects may be interchanged under appropriate circumstances such that the embodiments of the application described herein may be implemented in an order other than those illustrated or described herein.
The terms "comprising" and "having" and any variations thereof in the embodiments of the present application are intended to cover non-exclusive inclusions. For example, a process, method, apparatus, product, or device that comprises a list of steps or modules is not limited to the listed steps or modules but may alternatively include other steps or modules not listed or inherent to such process, method, product, or device.
The above-mentioned embodiments only express several embodiments of the present application, and the description thereof is more specific and detailed, but not construed as limiting the scope of the invention. It should be noted that, for a person skilled in the art, several variations and modifications can be made without departing from the concept of the present application, which falls within the scope of protection of the present application. Therefore, the protection scope of the present patent shall be subject to the appended claims.
Claims (5)
1. A self-focusing optimized coupling space chaotic laser secret communication system is characterized by comprising a chaotic encryption transmitting module, a focal length self-adapting module, a chaotic demodulation receiving module and a synchronizing module;
the focal length self-adaptive module comprises a first optical power detector, a second optical power detector, a variable focus lens group, an error code tester, a focal length control system and an electric control slide rail;
the focal length control system is respectively connected with the first optical power detector, the second optical power detector, the variable-focus lens group, the optical fiber lens group distance control system and the error code tester;
when the ground is roughly adjusted, the variable-focus lens group couples spatial signal light into the end face of an optical fiber, a first optical power detector detects the optical power entering the variable-focus lens group in front of the variable-focus lens group, a second optical power detector detects the optical power coupled into the optical fiber behind the end face of the optical fiber, a focal length control system receives the detection data of the first optical power detector and the second optical power detector, corresponding operation is carried out to obtain coupling efficiency, corresponding electric signals are generated to serve as control signals to act on the variable-focus lens group and an electric control slide rail, the variable-focus lens group changes the equivalent focal length according to the electric signals by utilizing the electro-optical effect, the electric control slide rail keeps consistent with the equivalent focal length of the lens group according to the distance between the electric signal control lens group and the end face of the optical fiber, the focal length control system gives out control signals for continuously reducing the equivalent focal length of the variable-focus lens group according to the coupling efficiency, and stops reducing the equivalent focal length of the variable-focus lens group when the coupling efficiency is reduced, the focal length is adjusted back to enable the coupling efficiency to reach the maximum value, and at the moment, the space chaotic laser secure communication system achieves the optimal coupling without the influence of pointing errors;
when satellite communication is finely adjusted, the variable focus lens group couples signal light transmitted from the chaotic encryption transmitting module to the space and enters the optical fiber, the optical power detector II detects the optical power coupled to the optical fiber after the end face of the optical fiber and transmits the optical power to the synchronizing module, the error code tester is connected with the chaotic demodulation receiving module, the error code rate of the system is obtained by comparing the demodulation signal and the transmission test signal of the chaotic demodulation receiving module, the focal length control system gives a control signal for continuously reducing the equivalent focal length of the variable focus lens group according to the error code rate signal, when the error code rate begins to increase, the equivalent focal length of the variable focus lens group is stopped to be reduced, the focal length is adjusted back to enable the error code rate to reach the minimum value, and at the moment, the space chaotic laser communication system achieves optimal coupling under the influence of pointing errors.
2. The self-focusing optimal coupling space chaotic laser secret communication system according to claim 1, wherein the chaotic encryption transmitting module comprises a first laser and a transmitting end oscillation starting loop;
the transmitting end oscillation starting loop is respectively connected with the first laser and a signal to be transmitted, receives constant-power laser which is emitted by the first laser and has the same wavelength as the signal to be transmitted, generates chaotic carrier waves, simultaneously transmits the signal to the transmitting end oscillation starting loop, is coupled with the chaotic carrier waves in the coupler so as to realize chaotic encryption of the signal, and transmits the encrypted signal to the focal length self-adaptive module after entering an atmosphere channel.
3. The self-focusing optimally coupled space chaotic laser secure communication system according to claim 2, characterized in that the synchronization module comprises an adjustable attenuator and an attenuator control system;
when satellite communication is precisely adjusted, the attenuator control system is connected with the second optical power detector, receives detection data of the second optical power detector, and gives an electric signal for controlling the attenuation coefficient of the adjustable attenuator according to the preset power of the first laser, so that the attenuation coefficient of the attenuator is kept at a ratio of twice the power of the first laser to the detection value of the second optical power detector;
the adjustable attenuator receives a control signal of an attenuator control system and is respectively connected with the focal length adaptive module and the chaotic demodulation receiving module, the signal enters the adjustable attenuator after being coupled into an optical fiber through a variable focus lens group of the focal length adaptive module, and then enters a receiving end oscillation starting loop and a photoelectric detector I of the chaotic demodulation receiving module after being attenuated by the adjustable attenuator, so that the theoretical input synchronization of the optical power of feedback signals entering the receiving end oscillation starting loop and the transmitting end oscillation starting loop is ensured, and the mismatch of the spatial chaotic laser secret communication system is reduced.
4. The self-focusing optimized-coupling space chaotic laser secret communication system according to claim 3, characterized in that the chaotic demodulation receiving module comprises a second laser, a receiving end oscillation starting loop, a first photoelectric detector, a second photoelectric detector and a demodulator;
the receiving end oscillation starting loop is connected with the second laser, the second photoelectric detector and the adjustable attenuator respectively, the parameters of the receiving end oscillation starting loop and the parameters of the transmitting end oscillation starting loop are completely the same, the receiving end oscillation starting loop receives the constant-power laser which is emitted by the second laser and has the same wavelength as the signal to be transmitted, the signal transmitted from the adjustable attenuator is used as a feedback signal, and chaotic carrier waves in the same state as the transmitting end are generated;
the demodulator is connected with the first photoelectric detector, the second photoelectric detector and the error code tester, the first photoelectric detector converts an optical signal transmitted into the chaotic demodulation receiving module from the synchronous module into a corresponding electric signal, the second photoelectric detector converts an optical signal transmitted by a receiving end oscillation starting loop into a corresponding electric signal, the electric signals output by the first photoelectric detector and the second photoelectric detector are input into the demodulator, and the demodulator performs subtraction operation on the two paths of electric signals and then recovers a signal to be transmitted.
5. A self-focusing optimized coupling space chaotic laser secret communication method is characterized by comprising the following steps:
step one, carrying out rough ground adjustment, starting a ground surface light source, emitting constant-power parallel light with the same wavelength as a signal to be transmitted, adjusting optical centers of a variable-focus lens group on an electric control slide rail and an optical fiber to the same horizontal line, observing the readings of a second optical power detector, and considering that light is primarily coupled into the optical fiber when the readings of the second optical power detector occur;
step two, starting the first optical power detector and the second optical power detector, transmitting the detected optical power data to a focal length control system, comparing the data by the focal length control system to obtain a coupling efficiency so as to obtain a control signal for continuously reducing the equivalent focal length of the variable-focus lens group, controlling the distance between the lens group and the end face of the optical fiber to be equal to the equivalent focal length by the electric control slide rail according to the signal, and reducing the focal length by the variable-focus lens group according to the signal; when the coupling efficiency is reduced, stopping reducing the equivalent focal length of the variable-focus lens group, and adjusting back the focal length to enable the coupling efficiency to reach the maximum value, at the moment, achieving the optimal coupling without the influence of the pointing error, and considering that the ground rough adjustment is finished;
thirdly, performing satellite communication fine adjustment, closing a ground surface light source and the first optical power photoelectric detector, transmitting detection data of the second optical power photoelectric detector to an attenuator control system, and starting a chaos encryption transmitting module on the satellite and a chaos demodulation receiving module on the ground;
fourthly, the chaotic encryption transmitting module transmits a chaotic carrier signal, the attenuator control system controls the attenuation coefficient of the adjustable attenuator to be twice of the power of the first laser device compared with the power of the second laser device according to the power data of the second optical power detector and the power of the first laser device, the optical power amplitudes entering the first and second optical power detectors are ensured to be matched, the parameter matching of the transmitting end oscillation starting loop and the receiving end oscillation starting loop is adjusted, and chaotic synchronization is realized;
fifthly, transmitting the known pseudo-random signal serving as a signal to be transmitted into a chaotic encryption transmitting module for encryption, transmitting the encrypted signal into a chaotic demodulation receiving module through an atmospheric channel, then passing through a focal length self-adaption module and a synchronization module, demodulating the signal after performing subtraction operation on output signals of the first photoelectric detector and the second photoelectric detector by a demodulator, and comparing the known pseudo-random signal with the demodulated signal by an error code tester to obtain the error code rate of the system; the focal length control system gives a control signal for continuously reducing the equivalent focal length of the variable-focus lens group according to the error rate signal, the equivalent focal length of the variable-focus lens group is stopped to be reduced when the error rate begins to increase, the focal length is adjusted back to enable the error rate to reach the minimum value, and at the moment, the system achieves the optimal coupling of the coupling efficiency under the influence of the pointing error.
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