CN113938193A - 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 invention discloses a mode diversity spatial laser communication system and method combining single PD detection and K-K light field recovery, belonging to the technical field of spatial laser communication, and comprising a single-wavelength laser module, a signal generation module, an electro-optical modulation module, an analog atmospheric turbulence channel module, a mode demultiplexing module, an electro-optical detection module and a digital signal processing module; the system utilizes a method combining mode diversity and Kramers-Kroning 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 and K-K light field recovery only needs 1 3dB coupler at the single mode end of each path, and can receive I, Q two paths of orthogonal component information by 1 photoelectric detector and 1 analog-to-digital converter. Saving 18 3dB couplers, 6 90 phase shifters, 18 photodetectors and 6 digital-to-analog converters. Greatly simplifying the complexity of the system and reducing the cost and the power consumption.
Description
Technical Field
The invention 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 and K-K light field recovery.
Background
The space laser communication has the advantages of high transmission rate, convenience in erection, no need of band permission, good confidentiality, good directivity and the like, and is widely applied to the fields of ground communication, satellite communication, interplanetary communication and the like. Since the transmission channel of the spatial laser communication is the atmosphere, the optical signal is susceptible to the atmospheric turbulence during the free space transmission process, and the communication quality is seriously degraded. Therefore, how to compensate the adverse effect of the atmospheric turbulence on the spatial laser communication is a very important task.
Various atmospheric turbulence compensation techniques exist, such as aperture averaging techniques, spatial diversity techniques, and the like. The aperture averaging technique can suppress the atmospheric turbulence flicker effect by increasing the receiving aperture at the receiving end, but the large aperture receiving means an increase in both the size and weight of the receiver. The space diversity technology transmits signals carrying the same information through a plurality of mutually independent communication links, can effectively overcome the multipath fading phenomenon caused by turbulent atmosphere, but relates to multi-beam transmission and multi-path reception. Most of the existing methods are very complex, high in cost, large in size, limited in practicability and difficult to meet the requirements of low cost, portability and low power consumption in the practical application process.
In recent years, an atmospheric turbulence compensation technology based on mode diversity has appeared, and the basic idea is to use mode orthogonality in few-mode optical fibers, use different modes as independent spatial channels, perform mode diversity reception on spatial light transmitted through the atmospheric channel, and then implement compensation for atmospheric turbulence through a diversity combining algorithm. At present, the method uses a balanced coherent detection technology to detect the multi-path optical signals after mode diversity, a plurality of optical mixers and balanced detectors are needed to respectively detect a plurality of modes, and when the number of the modes is large, the structure is very complex, and the cost is very high. For example, if a six-mode balanced coherent detection system is used, 6 optical mixers and 12 balanced detectors are required.
Disclosure of Invention
Aiming at the problems of overhigh 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 invention provides a mode diversity space laser communication system and a method combining single PD detection and K-K light field recovery. The system is simple to realize, a six-mode diversity system is also adopted, only 63 dB couplers and 6 photoelectric detectors are needed, the hardware realization difficulty is greatly simplified, the system is more beneficial to practical application, and the system has the advantages of small size, low cost, low power consumption and the like.
The invention is realized by the following technical scheme:
a mode diversity space laser communication system combining single PD detection and K-K light field recovery comprises a single-wavelength laser module 1, a signal generation module 2, an electro-optical modulation module 3, an atmospheric turbulence simulation 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, the output port of the radio-frequency signal generation 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 simulated atmospheric turbulence channel module 4, the output port of the simulated atmospheric turbulence channel module 4 is connected with an input port of the mode demultiplexing module 5, the output port of the mode demultiplexing module 5 is connected with an input port of the photoelectric detection module 6, and the 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 includes 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-optical modulation module 3 is connected with the input port of the transmitting end fiber collimator 41, the output port of the transmitting end fiber collimator 41 emits modulated signal light to a free space, then the modulated signal light is reflected and distorted by the turbulence screen 42, the distorted light beam is collimated by the receiving end fiber collimator 44 and then coupled to the few-mode fiber 43, and the output port of the few-mode fiber 43 is connected with the few-mode fiber input port of the mode demultiplexing module 5; the single-mode fiber output port 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 phase-only spatial light modulator; the few-mode optical fiber 43 adopts a six-mode (LP01, LP11a, LP11b, LPo2, LP21a and LP21b) step-index few-mode optical fiber.
Further, the single-wavelength laser module 1 generates continuous optical waves with 1550nm wavelength, and the output power is 10 mw;
the radio frequency signal generated by the signal generating module 2 is output to the electro-optical modulation 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 to the optical carrier 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 average insertion loss is 3.5dB, the output port of the simulated atmospheric 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 photodetection module 6 includes a 3dB coupler 61, a photodetector 62, a local oscillator 63, and a digital oscilloscope 64; an output port of the mode demultiplexing module 5 is connected with an input port of the 3dB coupler 61, an output port of the local oscillator 63 is connected with an input port of the 3dB coupler 61, an output port of the 3dB coupler 61 is connected with an input port of the photodetector 62, an output port of the photodetector 62 is connected with an input port of the digital oscilloscope 64, and an output port of the digital oscilloscope 64 is connected with an input port of the digital signal processing module 7.
Further, the local oscillator 63 adopts a laser with a line width of 100kHz as a local oscillation light source for coherent detection;
the photoelectric detector 62 receives the signal light at the single-mode end of the mode demultiplexing module 5 in a direct detection mode, and converts the signal light into an electric signal to be output;
the digital oscilloscope 64 adopts the sampling with the bandwidth of 7GHz and the highest sampling frequency of 40 GS/s.
Further, the digital signal processing module 7 is configured to measure the electrical signal output by the photodetection module 6, and perform optical field reconstruction and digital signal processing on the electrical signal according to the measured related parameters such as the amplitude.
The working principle and the realization 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, radio-frequency signals are modulated on optical carriers in the form of amplitude and phase information by an electro-optical modulator, and the modulated optical signals are transmitted to a free space through a transmitting end optical fiber collimating mirror. In the space transmission link part, atmospheric turbulence is simulated through a turbulence screen to enable the refractive index of a channel to generate random fluctuation, light beams are reflected and distorted, distorted light beams are collimated through a receiving end collimating mirror and then coupled into a few-mode optical fiber. Then, the input light is separated by a mode demultiplexer according to an LP mode, 6 single-mode output ends of the mode demultiplexer are respectively connected to a 3dB coupler, frequency mixing is carried out on the 3dB coupler and local oscillation light generated by a local oscillator, photoelectric conversion is completed through a photoelectric detector, light field reconstruction, signal compensation and recovery are finally completed in a digital signal processing module, and digital signal processing mainly comprises a plurality of steps of clock recovery, frequency offset compensation, carrier recovery, judgment and the like.
Another objective of the present invention is to provide a mode diversity spatial laser communication method combining single PD detection and K-K optical field recovery, which specifically includes the following steps:
the method comprises the following steps: the method comprises the steps of placing a turbulence screen on a space transmission path of a space laser beam, introducing a distorted phase by adjusting phase information loaded on the turbulence screen, reflecting and distorting the beam by the turbulence screen to cause optical wavefront distortion, and selecting pure phase space light for the turbulence screenModulator (SLM) and introduces D/r0 to characterize the effect of turbulence on the spatial laser beam, where D is the beam diameter and r is the beam diameter0Is an atmospheric coherence length parameter;
the phase information loaded on the turbulent flow screen is phase screen information, and can be obtained through formula (1):
wherein, the turbulence model adopts a Von Karman turbulence model, and the power spectrum formula under the mode is
Wherein f ism＝5.92/l0/2π：f0＝1/L0，
The method comprises the following steps of (1) taking M sampling points in the X direction and the Y direction respectively as phase screen information and the size of a phase screen D; h is a complex Gaussian random matrix with a mean value of 0 and a variance of 1; σ (m, n) is the power spectral density based on a model, f is the spatial frequency, fmAs frequency, f, corresponding to the inner scale0To correspond to the frequency of the outer scale,/0Is an inner scale, L0Is an outer dimension, r0Is the atmospheric coherence length, and the atmospheric structure constant
In inverse proportion;
step two: mixing the signal light distorted in the first step with local oscillator light generated by a local oscillator when the phase matching condition is met; the phase matching condition is that the central frequency f2 of the local oscillator optical power is adjusted to be 2GHz of the central frequency f1 of the signal light;
step three: mixing signal light at each single-mode output end of the mode demultiplexer with local oscillation light after certain frequency shift in a 3dB coupler to obtain a signal meeting a minimum phase condition; the optical front end of the digital signal processing module adopts a single PD to directly receive the coupled signals, and the optical front end increases light field reconstruction in a digital domain after obtaining amplitude information of the signals and is used for reconstructing I, Q components of the signals; the received signal amplitude and phase satisfy Kramers-Kroning relation (3):
wherein, E (t) is a minimum phase signal, P.V. is a Cauchy principal value, phi (t) is ln (| E (t) |) to perform Hilbert transformation, and phase information is obtained through amplitude information operation by utilizing a Kramers-Kroning relation to obtain a complete signal;
step four: and finally completing the compensation and recovery of signals in the digital signal processing module in the diversity reception mode of different branch signals, wherein the compensation and recovery of the signals comprise clock recovery, frequency offset compensation, carrier recovery and judgment, and after the relative time delay and phase compensation are carried out on the processed signals, equal gain or maximum ratio combination is carried out.
Further, the size of each pixel of the turbulence screen in the step one is 8um × 8 um; each pixel has 256 gray scales, and the corresponding phase modulation depth range is 0-2 n;
compared with the prior art, the invention 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 method based on combination of single PD detection and K-K light field recovery adopt a single PD coherent receiver based on the Kramers-Kroning relation to realize photoelectric detection, and a mode diversity scheme based on combination of single PD detection and the Kramers-Kroning algorithm only needs 1 3dB coupler at a single mode end of each path, and can realize the reception of I, Q two paths of orthogonal component information by 1 photoelectric detector and 1 analog-to-digital converter;
compared with a mode diversity space laser communication system based on balanced detection, under the condition that the six-mode few-mode optical fiber is adopted for coupling between space light beams and the optical fiber, 18 3dB couplers, 6 90-degree phase shifters, 18 photoelectric detectors and 6 digital-to-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.
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In order to more clearly illustrate the detailed description of the invention or the technical solutions in the prior art, the drawings that are needed in the detailed description of the invention or the prior art will be briefly described below. Throughout the drawings, like elements or portions are generally identified by like reference numerals. In the drawings, elements or portions 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 proposed by the present invention;
FIG. 2 is a schematic diagram of an experimental structure of the space transmission and fiber coupling section of the present invention;
FIG. 3 is a block diagram of a digital signal processing flow of the KK optical receiver of the present invention;
FIG. 4 is a graph of interruption probability of a single-mode receiving space laser communication system combining single PD detection with K-K optical field recovery under three different turbulent flow conditions of strong, medium and weak according to the present invention;
FIG. 5 is a bit error rate curve of a single-mode receiving space laser communication system combining single PD detection and K-K light field recovery under three different turbulent flow conditions of strong, medium and weak.
In the figure: a single wavelength laser module 1, a signal generating 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, a digital signal processing module 7,
A transmitting end optical fiber collimating mirror 41, a turbulence screen 42, a few-mode optical fiber 43 and a receiving end optical fiber collimating mirror 44;
Detailed Description
The following embodiments are only used for illustrating the technical solutions of the present invention more clearly, and therefore, the following embodiments are only used as examples, and the protection scope of the present invention is not limited thereby.
It is to be noted that, unless otherwise specified, technical or scientific terms used herein shall have the ordinary meaning as understood by those skilled in the art to which the invention pertains.
Example 1
The embodiment establishes a mode diversity spatial laser communication system combining single PD detection and K-K light field recovery, and the structural block diagram of the system 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 atmospheric turbulence channel module 4, a mode demultiplexing module 5, an electro-optical 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, the output port of the signal generation 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 simulated atmospheric turbulence channel module 4, the output port of the simulated atmospheric turbulence channel module 4 is connected with an input port of the mode demultiplexing module 5, the output port of the mode demultiplexing module 5 is connected with an input port of the photoelectric detection module 6, and the 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 space transmission and optical fiber coupling part of the embodiment 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 to provide an optical carrier for a system; and generating a radio frequency signal by the DAC, and modulating the radio frequency signal onto an optical carrier through the IQ modulator to realize the conversion of the electric signal into the optical signal. The modulated optical signal is transmitted to a free space through a transmitting end optical fiber collimating mirror and is incident on a turbulent flow screen. The light beam is reflected and distorted by simulating atmospheric turbulence through the turbulence screen. The distorted light beams are collimated by a collimator at the receiving end and then coupled into the few-mode optical fiber to complete the space transmission and optical fiber coupling parts. Then the mode demultiplexer separates the input light into six modes, each single mode output end of the mode demultiplexer is respectively connected to a 3dB coupler, the frequency mixing is carried out on the 3dB coupler and local oscillation light generated by a local oscillator, photoelectric conversion is completed through a photoelectric detector, finally, in a digital signal processing module, optical field reconstruction is carried out firstly, then signal compensation and recovery are carried out, digital signal processing mainly comprises a plurality of steps of clock recovery, frequency offset compensation, carrier phase recovery, judgment and the like, and digital signal processing of the KK optical receiver is completed. In the mode diversity spatial laser communication system combining single PD detection and K-K light field recovery, only 1 3dB coupler is needed at the single mode end of each path, and reception of I, Q paths of orthogonal component information can be realized by 1 photoelectric detector and 1 analog-to-digital converter.
The connection mode is as follows:
the output end of the single-wavelength laser module 1 is connected with the optical signal input port of the electro-optical modulator module 3, the output port of the signal generating module 2 is connected with the electrical signal input port of the electro-optical modulation module 3, 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 emits modulated signal light to a 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 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; an output port of the mode demultiplexing module 5 is connected with an input port of the 3dB coupler 61, an output port of the local oscillator 63 is connected with an input port of the 3dB coupler 61, an output port of the 3dB coupler 61 is connected with an input port of the photodetector 62, an output port of the photodetector 62 is connected with an input port of the digital oscilloscope 64, and an output port of the digital oscilloscope 64 is connected with an 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 bobble source optical electrical technology limited company, the output wavelength is set to 1550nm, the output power is set to 10mw, the linewidth is 10kHz, the narrow-linewidth laser has low phase noise, and the influence on the performance of the mode diversity spatial laser communication system combining single PD detection and K-K optical field recovery is small;
the signal generating module 2 is a DAC development board of Fujitsu LEIA-DK type, in this example, a radio frequency signal with an output frequency of 2GHz is set, the radio frequency signal generated by the DAC development board is connected to the electrical signal input port of the electro-optical modulator module 3 through a cable and loaded onto an optical carrier, so that conversion from an electrical signal to an optical signal is realized, and QPSK signal light of 4Gbps is generated;
the electro-optical modulator module 3 is a lithium niobate modulation I/Q modulator of model FTM7962 EP. And the optical carrier signal passes through the simulated atmospheric turbulence channel module 4 to complete wave front distortion.
In the mode demultiplexing module 5 of the embodiment, a mode selection type photon lantern is selected as a mode demultiplexer; the photonic lantern chosen is an all-fiber 6-mode selective multiplexer from OLKIN OPTICS, which in this example selects all six modes, i.e., LP01 mode, LP11a mode, LP11b mode, LP21a mode, and LP21b mode.
In the embodiment, the photoelectric detection module 6 selects a multimode photoelectric detector of Beijing Corona, the central frequency f2 of the local oscillator optical power is adjusted to be 2GHz of the central frequency f1 of the signal light, and the mixed signal is converted into an electric signal by the multimode photoelectric detector.
The signal processing module 7 in this example combines single PD detection and Kramers-Kroning relationship, the optical front end only needs to obtain amplitude information of the signal, and adds light field reconstruction in the digital domain for reconstructing I, Q components of the signal; and mixing the signal light at each single-mode output end of the mode demultiplexer with the local oscillator light after certain frequency shift in the 3dB coupler to obtain a signal meeting the minimum phase condition, and obtaining phase information through amplitude information operation to obtain a complete signal. And finally completing the compensation and recovery of signals in the digital signal processing module under the diversity reception mode of different branch signals, wherein the steps comprise 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 a single PD detection combined K-K optical field recovery single mode-diversity spatial laser communication system and a single PD detection combined K-K optical field recovery single mode interfaceThe interruption probability curve of the space-receiving laser communication system under three different turbulent flow conditions of strong, medium and weak is obtained. When the interruption probability is 0.05, at D/r0Under 16 turbulence intensity conditions, the mode diversity K-K detection structure has obvious compensation effect on turbulence, and is improved by 6dB compared with a corresponding single-mode receiving structure. At D/r0Under 9 turbulent intensity circumstances, mode diversity K-K surveys structure and has obvious compensation effect to the torrent, surveys receiving structure and has promoted 5.8dB compared with corresponding single mode K-K. At D/r0With a 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 error rate 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 receiving spatial laser communication system under strong, medium and weak three different turbulence conditions; under the condition of strong turbulence, the mode diversity K-K detection structure has an obvious compensation effect on the turbulence, and compared with a corresponding single-mode receiving structure, the mode diversity K-K detection structure is improved by 6.1 dB. Under the condition of medium turbulence, the mode diversity K-K detection structure has an obvious compensation effect on the turbulence, and compared with a corresponding single-mode K-K detection receiving structure, the mode diversity K-K detection receiving structure is improved by 5 dB. In weak turbulence conditions, 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 spatial laser communication method combining single PD detection and K-K light field recovery, which specifically comprises the following steps:
the method comprises the following steps: the method comprises the steps of placing a turbulence screen on a spatial transmission path of a spatial laser beam, introducing a distortion phase by adjusting phase information loaded on the turbulence screen, reflecting and distorting the beam by the turbulence screen to cause optical wavefront distortion, selecting a pure phase Spatial Light Modulator (SLM) for the turbulence screen, and introducing D/r0 to represent the influence of turbulence on the spatial laser beam, wherein D is the diameter of an emission beam, and r is the diameter of the emission beam0Is an atmospheric coherence length parameter;
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. And filtering a Gaussian random matrix by the power spectral density, carrying out Fourier inverse transformation on the result to obtain space domain sampling values, wherein the space domain sampling values represent distortion phases, and a plurality of sampling values form a phase screen.
The size of each pixel of the turbulence screen is 8um multiplied by 8 um; each pixel has 256 gray scales, and the corresponding phase modulation depth range is 0-2 n;
the phase information loaded on the turbulent flow screen is phase screen information, and can be obtained through formula (1):
wherein, the turbulence model adopts a Von Karman turbulence model, and the power spectrum formula under the mode is
Wherein f ism＝5.92/l0/2π；f0＝1/L0，
The method comprises the following steps of (1) taking M sampling points in the X direction and the Y direction respectively as phase screen information and the size of a phase screen D; h is a complex Gaussian random matrix with a mean value of 0 and a variance of 1; σ (m, n) is the power spectral density based on a model, f is the spatial frequency, fmAs frequency, f, corresponding to the inner scale0To correspond to the frequency of the outer scale,/0Is an inner scale, L0Is an outer dimension, r0Is the atmospheric coherence length, and the atmospheric structure constant
In inverse proportion;
step two: mixing the signal light distorted in the first step with local oscillator light generated by a local oscillator when the phase matching condition is met; the phase matching condition is that the central frequency f2 of the local oscillator optical power is adjusted to be 2GHz of the central frequency f1 of the signal light; in the embodiment, the output frequency is set to be 2GHz radio frequency signals, and the central frequency f2 of the local oscillator optical power is adjusted to be 2GHz of the central frequency f1 of the signal light; the mixed signal is converted into an electric signal by a multimode photoelectric detector;
step three: mixing signal light at each single-mode output end of the mode demultiplexer with local oscillation light after certain frequency shift in a 3dB coupler to obtain a signal meeting a minimum phase condition; the optical front end of the digital signal processing module adopts a single PD to directly receive the coupled signals, and the optical front end increases light field reconstruction in a digital domain after obtaining amplitude information of the signals and is used for reconstructing I, Q components of the signals; the received signal amplitude and phase satisfy Kramers-Kroning relation (3):
wherein, E (t) is a minimum phase signal, P.V. is a Cauchy principal value, phi (t) is ln (| E (t) |) to perform Hilbert transformation, and phase information is obtained through amplitude information operation by utilizing a Kramers-Kroning relation to obtain a complete signal;
step four: and finally completing the compensation and recovery of signals in the digital signal processing module in the diversity reception mode of different branch signals, wherein the compensation and recovery of the signals comprise clock recovery, frequency offset compensation, carrier recovery and judgment, and after the relative time delay and phase compensation are carried out on the processed signals, equal gain or maximum ratio combination is carried out.
The mode diversity spatial laser communication system combining single PD detection and K-K light field recovery is introduced in detail, and the introduction is mainly used for further understanding the method and the core idea thereof; while the invention has been described with reference to specific embodiments and applications, it will be apparent to those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.
Claims (9)
1. A mode diversity spatial laser communication system combining single PD detection and K-K light field recovery is characterized by comprising a single-wavelength laser module (1), a signal generation module (2), an electro-optical modulation module (3), an atmosphere turbulence simulation 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 the optical signal input port of the electro-optical modulator module (3), the output port of the radio-frequency signal generation module (2) is connected with the electrical signal input port of the electro-optical modulator module (3), the output port of the electro-optical modulator module (3) is connected with the input port of the simulated atmospheric turbulence channel module (4), the output port of the simulated atmospheric turbulence channel module (4) is connected with the 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 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).
2. The mode diversity spatial laser communication system combining single PD detection and K-K optical field recovery according to claim 1, wherein said analog atmospheric turbulence channel module (4) includes a transmitting end fiber collimator (41), a turbulence screen (42), few-mode fibers (43) and a receiving end fiber collimator (44); an output port of the electro-optical modulation module (3) is connected with an input port of an emitting end optical fiber collimating mirror (41), the output port of the emitting end optical fiber collimating mirror (41) emits modulated signal light to a free space, then the modulated signal light is reflected and twisted by a turbulence screen (42), distorted light beams are collimated by a receiving end optical fiber collimating mirror (44) and then coupled into a few-mode optical fiber (43), and an output port of the few-mode optical fiber (43) is connected with a few-mode optical fiber input port of the mode demultiplexing module (5); the single-mode fiber output port 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).
3. The mode diversity spatial laser communication system combining single PD detection and K-K optical field recovery according to claim 2, characterized in that said turbulence screen (42) is a phase-only spatial light modulator of 1920 x 1080 pixels; the few-mode optical fiber (43) adopts a six-mode step-index few-mode optical fiber.
4. The mode diversity spatial laser communication system of single PD detection combined with K-K optical field recovery according to claim 1, wherein said single wavelength laser module (1) generates continuous 1550nm wavelength optical waves with output power of 10 mw;
the radio frequency signal generated by the signal generating module (2) is output to the electro-optical modulation 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 to the optical carrier 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 average insertion loss is 3.5dB, the output port of the simulated atmospheric 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).
5. The mode diversity spatial laser communication system combining single PD detection and K-K optical field recovery according to claim 1, wherein said photodetection module (6) comprises (3) dB coupler (61), photodetector (62), local oscillator (63) and digital oscilloscope (64); an output port of the mode demultiplexing module (5) is connected with an input port of the 3dB coupler (61), an output port of the local oscillator (63) is connected with an input port of the 3dB coupler (61), then an output port of the (3) dB coupler (61) is connected with an input port of the photoelectric detector (62), an output port of the photoelectric detector (62) is connected with an input port of the digital oscilloscope (64), and an output port of the digital oscilloscope (64) is connected with an input port of the digital signal processing module (7).
6. The mode diversity spatial laser communication system combining single PD detection and K-K optical field recovery according to claim 5, characterized in that said local oscillator (63) uses a laser with a line width of 100kHz as a local oscillator light source for coherent detection;
the photoelectric detector (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 to be output;
the digital oscilloscope (64) adopts the sampling with the bandwidth of 7GHz and the highest sampling frequency of 40 GS/s.
7. The mode diversity spatial laser communication system combining single PD detection and K-K optical field recovery according to claim 1, wherein said digital signal processing module (7) is configured to measure the electrical signal output by the photodetection module (6), perform optical field reconstruction and digital signal processing on the electrical signal according to the measured related parameters such as amplitude.
8. The communication method of the mode diversity spatial laser communication system with single PD detection combined with K-K optical field recovery according to claim 1,
the method specifically comprises the following steps:
the method comprises the following steps: 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 beam by the turbulence screen to cause optical wavefront distortion, selecting a pure-phase spatial light modulator for the turbulence screen, and introducing D/r0 to represent the influence of turbulence on the space laser beam, wherein D is the diameter of an emission beam, and r is the diameter of the emission beam0Is an atmospheric coherence length parameter;
the phase information loaded on the turbulent flow screen is phase screen information, and can be obtained through formula (1):
wherein, the turbulence model adopts a Von Karman turbulence model, and the power spectrum formula under the mode is
Wherein f ism＝5.92/l0/2π；f0＝1/L0，
The method comprises the following steps of (1) taking M sampling points in the X direction and the Y direction respectively as phase screen information and the size of a phase screen D; h is a complex Gaussian random matrix with a mean value of 0 and a variance of 1; σ (m, n) is the power spectral density based on a model, f is the spatial frequency, fmAs frequency, f, corresponding to the inner scale0To correspond to the frequency of the outer scale,/0Is an inner scale, L0Is an outer dimension, r0Is the atmospheric coherence length, and the atmospheric structure constant
In inverse proportion;
step two: mixing the signal light distorted in the first step with local oscillator light generated by a local oscillator when the phase matching condition is met; the phase matching condition is that the central frequency f2 of the local oscillator optical power is adjusted to be 2GHz of the central frequency f1 of the signal light;
step two: and mixing the signal light and the local oscillator light. The local oscillator light needs to meet the phase matching condition, and the local oscillator generates the local oscillator light of which the frequency spectrum is positioned on one side of the signal frequency spectrum, and the local oscillator light can be positioned on the right side or the right side of the signal frequency spectrum; the frequency interval is the interval between the local oscillator optical spectrum and the signal spectrum edge, and is called as the guard bandwidth.
Step three: mixing signal light at each single-mode output end of the mode demultiplexer with local oscillation light after certain frequency shift in a 3dB coupler to obtain a signal meeting a minimum phase condition; the optical front end of the digital signal processing module adopts a single PD to directly receive the coupled signals, and the optical front end increases light field reconstruction in a digital domain after obtaining amplitude information of the signals and is used for reconstructing I, Q components of the signals; the received signal amplitude and phase satisfy Kramers-Kroning relation (3):
wherein, E (t) is a minimum phase signal, P.V. is a Cauchy principal value, phi (t) is ln (| E (t) |) to perform Hilbert transformation, and phase information is obtained through amplitude information operation by utilizing a Kramers-Kroning relation to obtain a complete signal;
step four: and finally completing the compensation and recovery of signals in the digital signal processing module in the diversity reception mode of different branch signals, wherein the compensation and recovery of the signals comprise clock recovery, frequency offset compensation, carrier recovery and judgment, and after the relative time delay and phase compensation are carried out on the processed signals, equal gain or maximum ratio combination is carried out.
9. The communication method of the mode diversity spatial laser communication system combining single PD detection and K-K optical field recovery according to claim 8, wherein the size of each pixel of the turbulence screen in the first step is 8um x 8 um; each pixel has 256 gray levels, corresponding to a phase modulation depth range of 0-2 n.
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