CN113938193B - Mode diversity space laser communication system and method combining single PD detection with K-K light field recovery - Google Patents
Mode diversity space laser communication system and method combining single PD detection with K-K light field recovery Download PDFInfo
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Abstract
The application discloses a mode diversity space laser communication system and a mode diversity space laser communication method combining single PD detection with K-K light field recovery, which belong to the technical field of space laser communication and comprise a single wavelength laser module, a signal generation module, an electro-optic modulation module, an analog atmosphere turbulence channel module, a mode demultiplexing module, a photoelectric detection module and a digital signal processing module; the system utilizes a method of combining mode diversity and Kramers-kroming detection to carry out diversity reception on divergent space light at a receiving end, thereby realizing the maximization of coupling efficiency. The mode diversity space laser communication system combining single PD detection with K-K light field recovery can realize the reception of I, Q two paths of orthogonal component information by only 1 3dB coupler, 1 photoelectric detector and 1 analog-to-digital converter at the single mode end of each path. 18 3dB couplers, 6 90 DEG phase shifters, 18 photodetectors and 6 digital-to-analog converters are saved. The complexity of the system is greatly simplified, and the cost and the power consumption are reduced.
Description
Technical Field
The application belongs to the technical field of space laser communication, and particularly relates to a mode diversity space laser communication system and method combining single PD detection with K-K light field recovery.
Background
The space laser communication has the advantages of high transmission rate, convenient erection, no need of frequency band license, good confidentiality, good directivity and the like, and has wide application in the fields of ground communication, satellite communication, interplanetary communication and the like. Since the transmission channel of the space laser communication is the atmosphere, the optical signal is easily affected by the atmospheric turbulence in the free space transmission process, and the communication quality is seriously deteriorated. Therefore, it is a very important task how to compensate for the adverse effects of atmospheric turbulence on spatial laser communication.
There are a number of atmospheric turbulence compensation techniques such as aperture averaging techniques, spatial diversity techniques, etc. Aperture averaging techniques can suppress the effect of atmospheric turbulence flicker by increasing the receiving aperture at the receiving end, but large aperture reception implies an increase in the size and weight of the receiver at the same time. The space diversity technique can effectively overcome the multipath fading phenomenon caused by turbulent atmosphere by transmitting signals carrying the same information through a plurality of mutually independent communication links, but involves multiple beam transmission and multiple beam reception. Most of the existing methods are very complex, high in cost, large in size and limited in practicability, and the requirements of low cost, portability and low power consumption in the practical application process are difficult to meet.
In recent years, an atmospheric turbulence compensation technology based on mode diversity has emerged, and the basic idea is to use mode orthogonality in a few-mode optical fiber, take different modes as independent spatial channels, perform mode diversity reception on spatial light transmitted through the atmospheric channels, and then implement the compensation on the atmospheric turbulence through a diversity combining algorithm. At present, the method uses a balanced coherent detection technology to detect multipath optical signals after mode diversity, and a plurality of optical mixers and balanced detectors are needed to realize the respective detection of a plurality of mode lights. For example, if a balanced coherent detection system with six modes is used, 6 optical mixers, 12 balanced detectors are required.
Disclosure of Invention
Aiming at the problems of high system cost, complex structure and the like caused by using a balanced detection technology in a mode diversity space laser communication system based on balanced detection in the prior art, the application provides a mode diversity space laser communication system and a mode diversity space laser communication method combining single PD detection with K-K optical field recovery. The system is simple to realize, and also adopts a six-mode diversity system, and the system only needs 63 dB couplers and 6 photoelectric detectors, so that the hardware implementation difficulty is greatly simplified, the practical application is facilitated, and the system has the advantages of small size, low cost, low power consumption and the like.
The application is realized by the following technical scheme:
a single PD detects and combines the space laser communication system of mode diversity of K-K light field recovery, including single wavelength laser module 1, signal generation module 2, electro-optic modulation module 3, imitate the atmospheric turbulence channel module 4, mode de-multiplexing module 5, photoelectric detection module 6 and digital signal processing module 7 to make up; the output end of the single-wavelength laser module 1 is connected with an optical signal input port of the electro-optical modulator module 3, an output port of the radio frequency signal generating module 2 is connected with an electrical signal input port of the electro-optical modulator module 3, an output port of the electro-optical modulator module 3 is connected with an input port of the analog atmosphere turbulence channel module 4, an output port of the analog atmosphere turbulence channel module 4 is connected with an input port of the mode demultiplexing module 5, an output port of the mode demultiplexing module 5 is connected with an input port of the photoelectric detection module 6, and an output port of the photoelectric detection module 6 is connected with an input port of the digital signal processing module 7.
Further, the simulated atmospheric turbulence channel module 4 comprises a transmitting end optical fiber collimating lens 41, a turbulence screen 42, a few-mode optical fiber 43 and a receiving end optical fiber collimating lens 44; the output port of the electro-optical modulation module 3 is connected with the input port of the transmitting end optical fiber collimating mirror 41, the output port of the transmitting end optical fiber collimating mirror 41 transmits the modulated signal light to free space, then the modulated signal light is reflected and distorted by the turbulence screen 42, the distorted light is collimated by the receiving end optical fiber collimating mirror 44 and then coupled into the few-mode optical fiber 43, and the output port of the few-mode optical fiber 43 is connected with the few-mode optical fiber input port of the mode demultiplexing module 5; the output port of the single-mode optical fiber of the mode demultiplexing module 5 is connected with the input port of the photoelectric detection module 6, and the output port of the photoelectric detection module 6 is connected with the input port of the digital signal processing module 7.
Further, the turbulence screen 42 is a 1920×1080 pixel pure phase spatial light modulator; the few-mode fiber 43 is a six-mode (LP 01, LP11a, LP11b, LPo2, and LP21a, LP21 b) step-index few-mode fiber.
Further, the single-wavelength laser module 1 generates light waves with continuous 1550nm wavelength, and the output power is 10mw;
the radio frequency signal generated by the signal generating module 2 is output to the electro-optic modulating module 3 through a radio frequency line and then modulated;
the electro-optical modulation module 3 adopts an IQ modulator to modulate the radio frequency signal of the signal generation module 2 onto the optical carrier wave generated by the single-wavelength laser module 1 in the form of amplitude and phase information;
the mode demultiplexing module 5 adopts a mode selection type photon lantern to separate input light according to an LP mode; the length of the few-mode tail fiber of the photon lantern is 3m, the insertion loss is 3.5dB averagely, the output port of the simulated atmosphere turbulence channel module 4 is connected with the few-mode input port of the photon lantern, and each single-mode output port of the photon lantern is respectively connected with the input port of the photoelectric detection module 6.
Further, the photo-detection module 6 includes a 3dB coupler 61, a photo-detector 62, a local oscillator 63 and a digital oscilloscope 64; the output port of the mode demultiplexing module 5 is connected with the input port of the 3dB coupler 61, the output port of the local oscillator 63 is connected with the input port of the 3dB coupler 61, then the output port of the 3dB coupler 61 is connected with the input port of the photodetector 62, the output port of the photodetector 62 is connected with the input port of the digital oscilloscope 64, and the output port of the digital oscilloscope 64 is connected with the input port of the digital signal processing module 7.
Further, the local oscillator 63 uses a laser with a linewidth of 100kHz as a local oscillator light source for coherent detection;
the photodetector 62 adopts a direct detection mode, receives the signal light at the single-mode end of the mode demultiplexing module 5, and converts the signal light into an electric signal for output;
the digital oscilloscope 64 employs a bandwidth of 7GHz with a maximum sampling frequency of 40GS/s sampling.
Further, the digital signal processing module 7 is configured to measure the electrical signal output by the photoelectric detection module 6, and perform optical field reconstruction and digital signal processing on the electrical signal according to the measured relevant parameters such as the amplitude value.
The working principle and the implementation process of the mode diversity space laser communication system combining single PD detection and K-K light field recovery are as follows:
the single wavelength laser generates continuous light with the wavelength of 1550nm, the radio frequency signal is modulated on the optical carrier wave by the electro-optical modulator in the form of amplitude and phase information, and the modulated optical signal is transmitted to the free space through the optical fiber collimating mirror at the transmitting end. In the space transmission link part, the refraction index of the channel is randomly fluctuated by simulating atmospheric turbulence through a turbulence screen, the light beam is reflected and distorted, and the distorted light beam is collimated by a receiving end collimating mirror and then coupled into a few-mode optical fiber. Then the mode de-multiplexer separates the input light according to the LP mode, 6 single-mode output ends of the mode de-multiplexer are respectively connected into a 3dB coupler, the frequency mixing is carried out on the input light and the local oscillation light generated by a local oscillator, the photoelectric conversion is finished through a photoelectric detector, finally, the light field reconstruction is finished in a digital signal processing module, the compensation and the recovery of the signal are finished, and the digital signal processing mainly comprises a plurality of steps such as clock recovery, frequency offset compensation, carrier recovery and judgment.
The application further aims to provide a mode diversity space laser communication method combining single PD detection and K-K optical field recovery, which specifically comprises the following steps:
step one: placing a turbulence screen on a space transmission path of a space laser beam, introducing a distortion phase by adjusting phase information loaded on the turbulence screen, reflecting and distorting the light beam by the turbulence screen to lead to optical wavefront distortion, wherein the turbulence screen adopts a pure phase Space Light Modulator (SLM), and introducing D/r0 to characterize the influence of turbulence on the space laser beam, wherein D is the diameter of an emitted light beam, r 0 Is the length parameter of the atmosphere;
the phase information loaded on the turbulence screen is the phase screen information and can be obtained through the formula (1):
wherein, the turbulence model adopts a Von Karman turbulence model, and the power spectrum formula under the model is as follows
Wherein f m ＝5.92/l 0 /2π：f 0 ＝1/L 0 ，Taking M sampling points in the X direction and the Y direction respectively as phase screen information, wherein the size of the phase screen is D; h is a complex Gaussian random matrix with a mean value of 0 and a variance of 1; sigma (m, n) is the power spectral density based on a model, f is the spatial frequency, f m For frequencies corresponding to the inner scale, f 0 For frequencies corresponding to the outer scale, l 0 Is of internal dimension, L 0 Is of external scale, r 0 Is the atmospheric coherence length, and the atmospheric structural constant +.>Inversely proportional;
step two: mixing the signal light distorted in the first step with local oscillation light generated by a local oscillator when the phase matching condition is met; the phase matching condition is that the center frequency f2 of the local oscillation optical power is adjusted to the position of 2GHz of the center frequency f1 of the signal light;
step three: mixing the signal light of each single-mode output end of the mode demultiplexer with the local oscillation light after certain frequency shift in a 3dB coupler to obtain a signal meeting the minimum phase condition; the optical front end of the digital signal processing module directly receives the coupled signals by adopting a single PD, and adds light field reconstruction in a digital domain after the optical front end obtains amplitude information of the signals so as to reconstruct I, Q components of the signals; the received signal amplitude and phase satisfy the Kramers-kroming relation (3):
wherein E (t) is the minimum phase signal, P.V. is the Cauchy principal value, phi (t) is the result of Hilbert transformation of ln (|E (t) |), and the phase information is obtained through amplitude information operation by utilizing the Kramers-Kroming relation, so as to obtain the complete signal;
step four: and under the diversity receiving mode, the signals of different branches are finally compensated and recovered in a digital signal processing module, wherein the compensation and recovery comprises clock recovery, frequency offset compensation, carrier recovery and judgment, and the equal gain or maximum ratio combination is carried out after the relative time delay and the phase compensation are carried out on the processed signals.
Further, the size of each pixel of the turbulence screen in the first step is 8um×8um; each pixel has 256 gray scales, and the corresponding phase modulation depth range is 0-2n;
compared with the prior art, the application has the following advantages:
compared with a mode diversity space laser communication system based on balanced detection, the mode diversity space laser communication system and the mode diversity space laser communication method based on the combination of single PD detection and K-K light field restoration, provided by the application, realize photoelectric detection by adopting a single PD coherent receiver based on a Kramers-Kroming relation, and only need 1 3dB coupler at a single-mode end of each path based on a mode diversity scheme of combining single PD detection with a Kramers-Kroming algorithm, and can realize the reception of I, Q two paths of orthogonal component information by 1 photoelectric detector and 1 analog-digital converter;
compared with a mode diversity space laser communication system based on balanced detection, under the condition of adopting six-mode few-mode optical fibers to couple space light beams and the optical fibers, 18 3dB couplers, 6 90-degree phase shifters, 18 photodetectors and 6 digital-analog converters are saved, and the structure of a coherent detection part is greatly simplified. The complexity of the mode diversity system is greatly simplified, and the cost and the power consumption are reduced.
Drawings
In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below. Like elements or portions are generally identified by like reference numerals throughout the several figures. In the drawings, elements or portions thereof are not necessarily drawn to scale.
FIG. 1 is a schematic diagram of a mode diversity spatial laser communication system combining single PD detection and K-K optical field recovery according to the present application;
FIG. 2 is a schematic diagram of the experimental structure of the spatial transmission and fiber coupling section of the present application;
FIG. 3 is a block diagram of the digital signal processing of the KK optical receiver of the present application;
FIG. 4 is a graph showing the outage probability of a single-mode received spatial laser communication system combining K-K optical field recovery with single PD detection in the case of three different turbulences, namely strong, medium and weak;
FIG. 5 is a graph showing error rate curves of a single-mode receiving space laser communication system combining K-K optical field recovery with single PD detection under three different turbulence conditions of strong, medium and weak.
In the figure: a single wavelength laser module 1, a signal generation module 2, an electro-optical modulation module 3, an analog atmospheric turbulence channel module 4, a mode demultiplexing module 5, a photoelectric detection module 6, a digital signal processing module 7,
A transmitting end optical fiber collimating lens 41, a turbulence screen 42, a few-mode optical fiber 43 and a receiving end optical fiber collimating lens 44;
a 3dB coupler 61, a photodetector 62, a local oscillator 63, a digital oscilloscope 64.
Detailed Description
The following embodiments of the present application will be described in detail with reference to the accompanying drawings, which are only used to more clearly illustrate the technical solution of the present application, and therefore are only used as examples, and are not to be construed as limiting the scope of the present application.
It is noted that unless otherwise indicated, technical or scientific terms used herein should be given the ordinary meaning as understood by one of ordinary skill in the art to which this application belongs.
Example 1
The embodiment establishes a mode diversity space laser communication system combining single PD detection with K-K light field recovery, and the structural block diagram is shown in figure 1, and consists of a single wavelength laser module 1, a signal generation module 2, an electro-optical modulation module 3, an analog atmosphere turbulence channel module 4, a mode demultiplexing module 5, a photoelectric detection module 6 and a digital signal processing module 7; the output end of the single-wavelength laser module 1 is connected with an optical signal input port of the electro-optical modulator module 3, an output port of the signal generation module 2 is connected with an electrical signal input port of the electro-optical modulator module 3, an output port of the electro-optical modulator module 3 is connected with an input port of the analog atmosphere turbulence channel module 4, an output port of the analog atmosphere turbulence channel module 4 is connected with an input port of the mode demultiplexing module 5, an output port of the mode demultiplexing module 5 is connected with an input port of the photoelectric detection module 6, and an output port of the photoelectric detection module 6 is connected with an input port of the digital signal processing module 7.
The experimental structure of the spatial transmission and optical fiber coupling part of this example is shown in fig. 2, the digital signal processing flow of the KK optical receiver is shown in fig. 3, and the working flow is as follows:
firstly, setting the output power of a single-wavelength laser, and providing an optical carrier for a system; the DAC generates a radio frequency signal, and the radio frequency signal is modulated onto an optical carrier wave through the IQ modulator to realize conversion from an electric signal to an optical signal. The modulated optical signal is emitted to free space through the optical fiber collimating mirror at the emitting end and is incident on the turbulence screen. The beam is reflected and distorted by simulating atmospheric turbulence through the turbulence screen. The distorted light beam is collimated by a receiving-end collimator and then coupled into a few-mode optical fiber, thereby completing space transmission and optical fiber coupling. Then the input light is separated into six modes by a mode demultiplexer, each single-mode output end of the mode demultiplexer is respectively connected into a 3dB coupler, the frequency mixing is carried out with local oscillation light generated by a local oscillator, the photoelectric conversion is finished through a photoelectric detector, finally, in a digital signal processing module, light field reconstruction is firstly carried out, then signal compensation and recovery are carried out, the digital signal processing mainly comprises a plurality of steps of clock recovery, frequency offset compensation, carrier phase recovery, judgment and the like, and the digital signal processing of the KK optical receiver is finished. In the mode diversity space laser communication system combining single PD detection with K-K light field recovery, only 1 3dB coupler is needed at a single mode end of each path, and 1 photoelectric detector and 1 analog-to-digital converter can receive I, Q two paths of orthogonal component information.
The connection mode is as follows:
the output end of the single-wavelength laser module 1 is connected with an optical signal input port of the electro-optical modulator module 3, the output port of the signal generating module 2 is connected with an electrical signal input port of the electro-optical modulator module 3, the output port of the electro-optical modulator module 3 is connected with an input port of the transmitting-end optical fiber collimating mirror 41, the output port of the transmitting-end optical fiber collimating mirror 41 transmits modulated signal light to free space, then the light beam is reflected and distorted through the turbulence screen 42, the distorted light beam is collimated through the receiving-end optical fiber collimating mirror 44 and then is coupled into the few-mode optical fiber 43, and the output port of the few-mode optical fiber 43 is connected with the few-mode optical fiber input port of the mode demultiplexing module 5; the output port of the mode demultiplexing module 5 is connected with the input port of the 3dB coupler 61, the output port of the local oscillator 63 is connected with the input port of the 3dB coupler 61, then the output port of the 3dB coupler 61 is connected with the input port of the photodetector 62, the output port of the photodetector 62 is connected with the input port of the digital oscilloscope 64, and the output port of the digital oscilloscope 64 is connected with the input port of the digital signal processing module 7.
In this embodiment, the single wavelength laser module 1 is a semiconductor narrow linewidth laser of the photo-tech company of the feibop source, the output wavelength is 1550nm, the output power is 10mw optical signal, the linewidth is 10kHz, the phase noise of the narrow linewidth laser is low, and the influence on the performance of the mode diversity space laser communication system recovered by combining single PD detection with K-K optical field is small;
the signal generating module 2 is a DAC development board of the model Fujitsu LEIA-DK, in this example, the output frequency is set to be 2GHz radio frequency signal, the radio frequency signal generated by the DAC development board is connected to the electrical signal input port of the electro-optic modulator module 3 through a cable, and is loaded onto an optical carrier, so as to realize the conversion from electrical signal to optical signal, and generate QPSK signal light of 4 Gbps;
the electro-optic modulator module 3 is a lithium niobate modulated I/Q modulator of the model FTM7962 EP. The optical carrier signal passes through the analog atmospheric turbulence channel module 4 to complete wave front distortion.
The mode selection type photon lantern is selected as a mode demultiplexer in the mode demultiplexing module 5 in the embodiment; the selected photon lantern is an all-fiber 6-mode selective multiplexer from OLKIN OPTICS, and all six modes, namely, LP01 mode, LP11a mode, LP11b mode, LP21a mode and LP21b mode are selected in this example.
The photoelectric detection module 6 of the embodiment selects a multimode photoelectric detector of Beijing Kangguan corporation, the center frequency f2 of the local oscillation optical power is adjusted to the 2GHz position of the center frequency f1 of the signal light, and the signals after mixing are converted into electric signals by the multimode photoelectric detector.
The signal processing module 7 of the embodiment combines single PD detection with Kramers-Kroming relation, the optical front end only needs to obtain the amplitude information of the signal, and the optical field reconstruction is added in the digital domain for reconstructing I, Q component of the signal; the signal light of each single-mode output end of the mode demultiplexer and the local oscillation light after certain frequency shift are mixed in the 3dB coupler to obtain a signal meeting the minimum phase condition, and phase information is obtained through amplitude information operation to obtain a complete signal. And under the diversity receiving mode, the different branch signals finally complete signal compensation and recovery in the digital signal processing module, and the method comprises a plurality of steps such as clock recovery, frequency offset compensation, carrier recovery, judgment and the like. And after the relative time delay and the phase compensation are carried out on the processed signals, equal gain or maximum ratio combination is carried out.
Fig. 4 shows outage probability curves of a single PD detection combined K-K optical field recovery mode diversity spatial laser communication system and a single PD detection combined K-K optical field recovery single mode reception spatial laser communication system under three different turbulent conditions, namely strong, medium and weak. When the interrupt probability is 0.05, the value is D/r 0 In the case of turbulent intensity of =16, the mode diversity K-K detection structure has a significant compensation effect on turbulence, which is improved by 6dB compared to the corresponding single-mode reception structure. At D/r 0 In the case of the turbulence intensity of =9, the mode diversity K-K detection structure has a significant compensation effect on turbulence, which is improved by 5.8dB compared with the corresponding single mode K-K detection reception structure. At D/r 0 In case of turbulence intensity of =3, the mode diversity K-K detection structure is improved by 3dB compared to the corresponding single-mode reception structure.
FIG. 5 shows bit error rate curves of a single PD detection combined K-K optical field recovery mode diversity space laser communication system and a single PD detection combined K-K optical field recovery single mode receiving space laser communication system under three different turbulence conditions of strong, medium and weak; under the condition of strong turbulence, the mode diversity K-K detection structure has obvious compensation effect on turbulence, and is improved by 6.1dB compared with the corresponding single-mode receiving structure. Under the condition of medium turbulence, the mode diversity K-K detection structure has obvious compensation effect on the turbulence, and is improved by 5dB compared with the corresponding single-mode K-K detection receiving structure. In the case of weak turbulence, the mode diversity K-K detection structure is improved by 3.2dB compared to the corresponding single mode reception structure.
Example 2
The embodiment provides a mode diversity space laser communication method combining single PD detection and K-K light field recovery, which comprises the following steps:
step one: placing a turbulence screen on a space transmission path of a space laser beam, introducing a distortion phase by adjusting phase information loaded on the turbulence screen, reflecting and distorting the light beam by the turbulence screen to lead to optical wavefront distortion, wherein the turbulence screen adopts a pure phase Space Light Modulator (SLM), and introducing D/r0 to characterize the influence of turbulence on the space laser beam, wherein D is the diameter of an emitted light beam, r 0 Is the length parameter of the atmosphere;
the method for generating the phase screen information adopts a spectrum inversion method, and the principle is two-dimensional discrete Fourier transform in Fourier optics. The power spectrum density filters Gaussian random matrix, and the result is subjected to Fourier inverse transformation to obtain space domain sampling values which represent distorted phases, and a plurality of sampling values form a phase screen.
The size of each pixel of the turbulence screen is 8um multiplied by 8um; each pixel has 256 gray scales, and the corresponding phase modulation depth range is 0-2n;
the phase information loaded on the turbulence screen is the phase screen information and can be obtained through the formula (1):
wherein, the turbulence model adopts a Von Karman turbulence model, and the power spectrum formula under the model is as follows
Wherein f m ＝5.92/l 0 /2π；f 0 ＝1/L 0 ，Taking M sampling points in the X direction and the Y direction respectively as phase screen information, wherein the size of the phase screen is D; h is a complex Gaussian random matrix with a mean value of 0 and a variance of 1; sigma (m, n) is the power spectral density based on a model, f is the spatial frequency, f m For frequencies corresponding to the inner scale, f 0 For frequencies corresponding to the outer scale, l 0 Is of internal dimension, L 0 Is of external scale, r 0 Is the atmospheric coherence length, and the atmospheric structural constant +.>Inversely proportional;
step two: mixing the signal light distorted in the first step with local oscillation light generated by a local oscillator when the phase matching condition is met; the phase matching condition is that the center frequency f2 of the local oscillation optical power is adjusted to the position of 2GHz of the center frequency f1 of the signal light; the example sets the output frequency as a 2GHz radio frequency signal, and adjusts the center frequency f2 of the local oscillation optical power to the 2GHz position of the signal optical center frequency f 1; the mixed signals are converted into electric signals by a multimode photoelectric detector;
step three: mixing the signal light of each single-mode output end of the mode demultiplexer with the local oscillation light after certain frequency shift in a 3dB coupler to obtain a signal meeting the minimum phase condition; the optical front end of the digital signal processing module directly receives the coupled signals by adopting a single PD, and adds light field reconstruction in a digital domain after the optical front end obtains amplitude information of the signals so as to reconstruct I, Q components of the signals; the received signal amplitude and phase satisfy the Kramers-kroming relation (3):
wherein E (t) is the minimum phase signal, P.V. is the Cauchy principal value, phi (t) is the result of Hilbert transformation of ln (|E (t) |), and the phase information is obtained through amplitude information operation by utilizing the Kramers-Kroming relation, so as to obtain the complete signal;
step four: and under the diversity receiving mode, the signals of different branches are finally compensated and recovered in a digital signal processing module, wherein the compensation and recovery comprises clock recovery, frequency offset compensation, carrier recovery and judgment, and the equal gain or maximum ratio combination is carried out after the relative time delay and the phase compensation are carried out on the processed signals.
The mode diversity space laser communication system combining single PD detection and K-K optical field recovery is described in detail, and the description is mainly used for further understanding the method and the core idea of the application; in addition, since modifications will readily occur to those skilled in the art, it is to be understood that the application is not limited to the details of the preferred embodiment and the scope of the application, and that various obvious modifications may be made therein without departing from the spirit of the method of the application and the scope of the appended claims.
Claims (9)
1. A communication method of a mode diversity space laser communication system combining single PD detection and K-K light field recovery is characterized in that,
the method specifically comprises the following steps:
step one: placing a turbulence screen on a space transmission path of a space laser beam, introducing a distortion phase by adjusting phase information loaded on the turbulence screen, reflecting and distorting the light beam by the turbulence screen to lead to optical wavefront distortion, wherein the turbulence screen adopts a pure-phase space light modulator, and introducing D/r0 to characterize the influence of turbulence on the space laser beam, wherein D is the diameter of an emitted light beam, and r is 0 Is the length parameter of the atmosphere;
the phase information loaded on the turbulence screen is the phase screen information and can be obtained through the formula (1):
wherein, the turbulence model adopts a Von Karman turbulence model, and the power spectrum formula under the model is as follows
Wherein f m ＝5.92/l 0 /2π：f 0 ＝1/L 0 ，Taking M sampling points in the X direction and the Y direction respectively as phase screen information, wherein the size of the phase screen is D; h is a complex Gaussian random matrix with a mean value of 0 and a variance of 1; sigma (m, n) is the power spectral density based on a model, f is the spatial frequency, f m For frequencies corresponding to the inner scale, f 0 For frequencies corresponding to the outer scale, l 0 Is of internal dimension, L 0 Is of external scale, r 0 Is the atmospheric coherence length, and the atmospheric structural constant +.>Inversely proportional;
step two: mixing the signal light distorted in the first step with local oscillation light generated by a local oscillator when the phase matching condition is met; the phase matching condition is that the center frequency f2 of the local oscillation optical power is adjusted to the position of 2GHz of the center frequency f1 of the signal light;
step three: mixing the signal light with local oscillation light; the local oscillator light needs to meet the phase matching condition, and the local oscillator generates local oscillator light with a frequency spectrum positioned at one side of the signal frequency spectrum, and can be positioned at the right side or the right side of the signal frequency spectrum; the frequency interval is the interval between the local oscillation optical spectrum and the signal spectrum edge, and is called as the protection bandwidth;
step four: mixing the signal light of each single-mode output end of the mode demultiplexer with the local oscillation light after certain frequency shift in a 3dB coupler to obtain a signal meeting the minimum phase condition; the optical front end of the digital signal processing module directly receives the coupled signals by adopting a single PD, and adds light field reconstruction in a digital domain after the optical front end obtains amplitude information of the signals so as to reconstruct I, Q components of the signals; the received signal amplitude and phase satisfy the Kramers-kroming relation (3):
wherein E (t) is the minimum phase signal, P.V. is the Cauchy principal value, phi (t) is the result of Hilbert transformation of ln (|E (t) |), and the phase information is obtained through amplitude information operation by utilizing the Kramers-Kroming relation, so as to obtain the complete signal;
step five: and under the diversity receiving mode, the signals of different branches are finally compensated and recovered in a digital signal processing module, wherein the compensation and recovery comprises clock recovery, frequency offset compensation, carrier recovery and judgment, and the equal gain or maximum ratio combination is carried out after the relative time delay and the phase compensation are carried out on the processed signals.
2. The communication method of a single PD detection combined with K-K optical field restoration mode diversity spatial laser communication system according to claim 1, wherein the size of each pixel of the turbulence screen in step one is 8um x 8um; each pixel has 256 gray scales and the corresponding phase modulation depth range is 0-2n.
3. A single-PD detection and K-K optical field recovery combined mode diversity space laser communication system for realizing the communication method as claimed in claim 1, which is characterized by comprising a single-wavelength laser module (1), a signal generation module (2), an electro-optical modulation module (3), an analog atmosphere turbulence channel module (4), a mode demultiplexing module (5), a photoelectric detection module (6) and a digital signal processing module (7); the output end of the single-wavelength laser module (1) is connected with an optical signal input port of the electro-optical modulator module (3), an output port of the radio frequency signal generating module (2) is connected with an electric signal input port of the electro-optical modulator module (3), an output port of the electro-optical modulator module (3) is connected with an input port of the analog atmosphere turbulence channel module (4), an output port of the analog atmosphere turbulence channel module (4) is connected with an input port of the mode demultiplexing module (5), an output port of the mode demultiplexing module (5) is connected with an input port of the photoelectric detection module (6), and an output port of the photoelectric detection module (6) is connected with an input port of the digital signal processing module (7).
4. A single PD detection combined K-K optical field restoration mode diversity spatial laser communication system according to claim 3, wherein said simulated atmospheric turbulence channel module (4) comprises a transmitting end fiber collimator (41), a turbulence screen (42), a few-mode fiber (43) and a receiving end fiber collimator (44); the output port of the electro-optic modulation module (3) is connected with the input port of the transmitting-end optical fiber collimating mirror (41), the output port of the transmitting-end optical fiber collimating mirror (41) transmits the modulated signal light to a free space, then the modulated signal light is reflected and distorted through the turbulence screen (42), the distorted light is collimated through the receiving-end optical fiber collimating mirror (44) and then is coupled into the few-mode optical fiber (43), and the output port of the few-mode optical fiber (43) is connected with the few-mode optical fiber input port of the mode demultiplexing module (5); the output port of the single-mode optical fiber of the mode demultiplexing module (5) is connected with the input port of the photoelectric detection module (6), and the output port of the photoelectric detection module (6) is connected with the input port of the digital signal processing module (7).
5. A single PD detection in combination with K-K optical field restoration mode diversity spatial laser communication system according to claim 3, wherein said turbulence screen (42) employs 1920 x 1080 pixel phase-only spatial light modulator; the few-mode optical fiber (43) adopts a six-mode step-index few-mode optical fiber.
6. A single PD detection in combination with K-K optical field restoration mode diversity spatial laser communication system according to claim 3, wherein said single wavelength laser module (1) generates optical waves of continuous 1550nm wavelength with an output power of 10mw;
the radio frequency signals generated by the signal generating module (2) are output to the electro-optic modulation module 3 through radio frequency lines and then modulated;
the electro-optical modulation module (3) adopts an IQ modulator to modulate the radio frequency signal of the signal generation module 2 onto the optical carrier wave generated by the single-wavelength laser module 1 in the form of amplitude and phase information;
the mode demultiplexing module (5) adopts a mode selection type photon lantern to separate input light according to an LP mode; the length of the few-mode tail fiber of the photon lantern is 3m, the insertion loss is 3.5dB averagely, the output port of the simulated atmosphere turbulence channel module (4) is connected with the few-mode input port of the photon lantern, and each single-mode output port of the photon lantern is respectively connected with the input port of the photoelectric detection module (6).
7. A single PD detection in combination with K-K optical field restoration mode diversity spatial laser communication system according to claim 3, wherein said photodetection module (6) comprises (3) a dB coupler (61), a photodetector (62), a local oscillator (63) and a digital oscilloscope (64); the output port of the mode demultiplexing module (5) is connected with the input port of the 3dB coupler (61), the output port of the local oscillator (63) is connected with the input port of the 3dB coupler (61), then the output port of the 3dB coupler (61) is connected with the input port of the photoelectric detector (62), the output port of the photoelectric detector (62) is connected with the input port of the digital oscilloscope (64), and the output port of the digital oscilloscope (64) is connected with the input port of the digital signal processing module (7).
8. The mode diversity spatial laser communication system combining single PD detection with K-K optical field restoration according to claim 7, wherein the local oscillator (63) uses a laser with a linewidth of 100kHz as a local oscillator light source for coherent detection;
the photoelectric detector (62) adopts a direct detection mode, receives signal light of a single-mode end of the mode demultiplexing module 5, and converts the signal light into an electric signal to be output;
the digital oscilloscope (64) employs a bandwidth of 7GHz with a maximum sampling frequency of 40 GS/s.
9. A single PD detection and K-K optical field restoration combined mode diversity spatial laser communication system according to claim 3, wherein the digital signal processing module (7) is configured to measure the electrical signal output by the photoelectric detection module (6), and perform optical field reconstruction and digital signal processing on the electrical signal according to the measured relevant parameters such as amplitude.
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