Research On The Influence Of Laser Spatial Coherence Degree On Performance Of Receiving Systerms Based On Polarization Shift Key Modulation Technology

To meet the ever-increasing demand for communication capacity and speed,free space optical communication is widely used in various fields due to its high communication speed,large capacity,small size,good confidentiality,and simple installation,and so on.It could be the best supplementary scheme for modern microwave communication technology.However,through space optical communication link,atmospheric turbulence formed due to the influence of temperature,pressure,wind and other factors will cause fluctuations in the atmospheric refractive index,leading to a series of atmospheric turbulence effect s,such as receiving optical intensity scintillation and phase fluctuations,which seriously affect system communication performance.Therefore,to alleviate the impact of atmospheric turbulence on the performance of optical communication systems,many effective methods have been proposed,including spatial diversity technology,adaptive optics technology,and partially coherent light.The application of spatial diversity technology and adaptive optics technology greatly increases the system complexity and manufacturing cost,while the application of part of the spatially coherent light as the source of the spatial optical communication system is the simplest and cost-effective method.Partially coherent light can also produce the same far-field distribution as fully coherent light.Partially coherent light represented by Gaussian-Schell Model(GSM)beams is less affected by atmospheric turbulence than fully spatially coherent light.Therefore,a large number of researchers focus on the application of partial ly coherent sources to reduce the bit error rate of the system.At the same time,an emerging polarization shift keying technology(Pol SK)is adopted to partially alleviate the impact of atmospheric turbulence on the performance of space optical communication systems.It uses the polarization state of the beam to encode information on the carrier.And it has a 3d B performance advantage over the on-off keying(OOK)modulation method.However,the proposed Pol SK modulation based space laser communication syst em needs to be further improved,and its communication performance and communication data rate need to be improved.Therefore,this paper creatively combines the advantages of partially coherent light and polarization shift keying technology,and focuses on the impact of laser spatial coherence on the performance of GSM beam-based polarization shift keying optical communication systems.The specific research content is as follows:Based on the unified theory of polarization coherence and Huygens-Fresnel principle,the polarization transmission characteristics model of space-to-ground downlink based on GSM beam is established,and the sufficient conditions for keeping the polarization transmission characteristics unchanged are obtained.The influence of laser spatial coherence on polarization transmission characteristics is analyzed by simulation,and two sufficient conditions for keeping polarization transmission characteristics unchanged are verified.In the space-to-ground downlink,the influence of source parameters on polarization transmission characteristics is the dominant factor.When the source is linearly polarized light or the spatial coherent width components of the source are equal,the polarization transmission characteristics remain unchanged whether in the ground horizontal link or in the space-to-ground link,which provides an important basis for ensuring the communication performance of the system.Regarding the influence of spatial coherence of source on the performance of circular polarization shift keying(CPol SK)modulation optical communication system based on GSM beam,the probability density distribution and scintillation index models of optical intensity based on GSM beam in atmospheric turbulence environment are established,and the BER model of CPol SK modulation optical communication system based on GSM beam is also established.The influence of laser spatial coherence on them is given as well.In the horizontal link on the ground,the GSM beam can reduce the scintillation index of optical intensity under the condition of weak turbulence and medium turbulence,and has the optimal spatial coherence of source,so that the BER of the system reaches the lowest.The optimal performance can be achieved by using fully coherent light in the space-to-ground link.Considering the complexity and adaptability of the system,a rotation-invariant quaternary polarization shift keying(4Pol SK)modulation optical communication system is designed.The optimal modulation phase of equal distance receiving optical intensity difference is given,and the influence of polarization axis rotation deviation on the receiving optical intensity of the system is analyzed.At the same time,the receiving BER model of the 4Pol SK modulation optical communication system based on GSM beam is established.In the ground horizontal link with a transmission distance of 10 km,4Pol SK modulation optical communication system with GSM beam has 2.68 d B performance advantage over quaternary amplitude shift keying(4ASK)modulation system with a fully coherent beam.To verify the correctness of the above theory and numerical simulation results,the experiment of partially coherent optical transmission characteristics of a long-distance urban horizontal link is designed and carried out.In the experiment,the spatial coherence of the transmitted linearly polarized light was used.After long-distance transmission,the receiving optical intensity,the fluctuation of the angle of arrival,and the polarization characteristics are measured.The probability density distribution of the optical intensity and the variation of the polarization characteristics under the spatial coherence of the laser are analyzed.The theory of atmospheric transmission characteristics of partially coherent light is par tially verified.At the same time,the Pol SK modulation system is simulated by Opti System software to verify its advantages.The research work of this paper is about the fudamental application of GSM beam propagation characteristics and its influence on Pol SK modulation optical communication system.It provides theoretical and experimental basis for the optimization design and engineering application of Pol SK modulation optical communication system that based on GSM beam.