Research On Atmospheric Laser Communication Based On Mode Diversity

Atmospheric laser communication is a communication technology that loads the signal onto a laser beam and then transmits the signal in the atmospheric environment.It has the advantages of high transmission rate,good confidentiality,no frequency band license required,good directivity,and easy to set up.It can be applied in the fields of ground communication,satellite communication,satellite-to-ground communication,and other fields.However,when the optical signal is transmitted in the atmospheric channel,it is easily affected by the atmospheric turbulence effect,resulting in power attenuation,beam drift,beam broadening,and wavefront distortion,which seriously degrades the communication quality.Therefore,it is significant to compensate the signal damage caused by atmospheric turbulence.In this paper,the atmospheric laser communication based on mode diversity is deeply studied,the diversity reception by using multiple orthogonal modes of few mode fiber can realize atmospheric turbulence compensation.In order to explore the atmospheric laser communication system with low structural complexity,a mode diversity system based on intradyne coherent reception and KK coherent reception was experimentally built,and the communication performance of the system was studied under different turbulence intensity respectively.The main contents are as follows:Firstly,the basic characteristics of atmospheric laser communication based on mode diversity are discussed.The causes of the atmospheric attenuation effect and atmospheric turbulence effect as well as the mechanism of these two effects on the transmitted beam are expounded.In order to better study the atmospheric turbulence effect,the refractive index distribution model of atmospheric turbulence and common atmospheric turbulence simulation methods are introduced.The existing form of modes and the orthogonality of modes in few mode fiber are discussed,and several commonly used mode multi/demultiplexers for mode conversion and mode diversity are introduced.Then the coherent detection technology is briefly introduced.Secondly,the mode diversity atmospheric laser communication based on intradyne coherent reception is studied.The basic working principle and signal damage compensation algorithm of intradyne coherent receiving technology are discussed.The phase screen simulation method of atmospheric turbulence is described in detail,the mode coupling mechanism and coupling efficiency of space light to the few mode fiber under different turbulence intensities are discussed,and the mode diversity method and diversity combining algorithm at the receiving end are studied.The bit error performance and interruption probability performance of the mode diversity system are studied under weak,medium,and strong turbulence intensity respectively.The experimental results show that when the average bit error rate is 3×10^-3,the power budget of the mode diversity system based on intradyne coherent reception can be saved about 3.5d B,5.6d B,and 4.6d B compared with the single mode fiber receiving system in the case of weak,medium and strong turbulence,the signal damage caused by atmospheric turbulence can be effectively compensated.Thirdly,in order to reduce the complexity of the system structure,a mode diversity atmospheric laser communication based on Kramers-Kronig(KK)coherent reception is studied.The working principle of the KK coherent receiving technology based on the single photodetector structure is discussed,the structure and receiving performance of the KK coherent receiving technology and the intradyne coherent receiving technology are analyzed theoretically,and the light field reconstruction algorithm of the KK coherent receiving technology is described in detail.A mode diversity atmospheric laser communication system based on KK coherent reception was built experimentally.The error bit performance and interruption probability performance of the mode diversity system were studied under weak,medium,and strong turbulence intensity respectively.The performance of the mode diversity system is compared with that of the single mode fiber receiving system without diversity.The experimental results show that when the average bit error rate is 3×10^-3,compared with the corresponding single mode fiber receiving system,the mode diversity system based on KK coherent reception can save about 3.2d B,5.2d B,and4.8d B power budget,can effectively compensate atmospheric turbulence under the condition of weak,medium and strong turbulence.Each channel of the mode diversity atmospheric laser communication system based on KK coherent reception only needs to use one optical mixer,one photodetector,and one analog/digital converter.Compared with the mode diversity atmospheric laser communication system based on intradyne coherent reception,the system complexity is greatly simplified,and the cost and power consumption are reduced.Finally,the main work of this paper is summarized,the effectiveness of using mode diversity technology for atmospheric laser communication and the rationality of using KK coherent receiving technology to simplify the complexity of the system structure are analyzed.The optimization and communication performance improvement of atmospheric laser communication system based on mode diversity are prospected.