节点文献
星地下行相干激光通信系统接收性能研究
Research on Receiving Performance of Satellite-to-ground Downlink Coherent Laser Communication Systems
【作者】 杨清波；
【导师】 谭立英；
【作者基本信息】 哈尔滨工业大学 ， 物理电子学， 2012， 博士
【摘要】 卫星激光通信技术被视为解决卫星通信领域通信数据率“瓶颈”的有效手段之一，目前该技术得到了国际上的广泛重视，对该领域的研究已成为热门。美国、欧洲、日本和中国等国已相继进行了卫星激光通信在轨空间试验验证工作，并取得了星间和星地激光通信实验的成功，各国正积极准备将卫星光通信技术推向实用化。早期研制的卫星激光通信终端受元器件限制多采用强度调制/直接探测(IM/DD)体制，与直接探测技术相比，基于自差、外差和零差探测方式的相干激光通信具有较高的探测灵敏度，成为高速卫星激光通信潜在的解决方案之一。然而作为卫星激光通信网络中一个重要组成部分的星地下行激光通信，由于信号光从卫星到地面的传输过程中受到大气湍流的影响，使得接收到的信号光场的空间相干性受到破坏，地面接收到的信号光束的波前相位和振幅发生随机起伏。因相干激光通信系统对入射光场的波前相位畸变极为敏感，大气湍流引起的波前相位随机起伏会限制激光通信系统的相干效率(在基于光纤耦合的星地自差相干激光通信系统中，相干效率即为空间激光至单模光纤的光纤耦合效率)。此外，在星地下行链路中，地面光学系统的波前畸变和角度跟踪系统中跟踪误差也会造成相干效率的降低。本文针对大气湍流对星地下行相干激光通信系统接收性能的影响进行了研究，重点对星地下行链路中单模光纤耦合效率特性进行了理论和实验研究，对采取光场波前模式补偿和光场子集接收方式下相干激光通信系统通信性能进行了分析，并针对光学系统的波前畸变和接收机跟踪误差对星地下行相干激光通信系统通信性能的影响进行了分析。具体内容如下：研究了大气湍流对星地下行链路中空间光到单模光纤耦合效率概率分布的影响。通过数值仿真湍流畸变波前，给出了不同散斑数下空间光至单模光纤耦合效率的概率分布函数，提出了利用单模光纤后向传输模场为加权函数的孔径内正交多项式进行波前相位模式补偿的方法，对该方法的优势进行了分析，给出了该方法下空间光至单模光纤耦合效率的概率分布函数。针对星地长距离链路大气湍流对空间光至单模光纤耦合效率的影响问题，建立了大气湍流导致的波前相位畸变对自差相干激光通信系统通信性能影响模型。分析了大口径条件下，利用单模光纤后向传输模场为加权函数的孔径内正交多项式，得到了波前相位模式补偿下的概率分布，并对补偿后的星地下行自差激光通信系统接收信号衰落统计特性和系统平均误码率进行了分析，同时也对采用光场子集接收方式的系统接收信号衰落统计特性和系统平均误码率进行了分析。研究了地面接收光学系统初级像差对相干效率的影响，给出了相干效率随各阶初级像差的均方根误差的变化关系，并分析了望远镜遮挡比对相干效率的影响。修正了角度跟踪系统中大气湍流和接收机跟踪误差共同影响下波前残余相位方差模型，给出了星地下行相干激光通信系统误码率受大气湍流及跟踪误差综合影响的变化关系。进行了大气湍流对空间光至单模光纤耦合效率影响的实验研究，通过该实验对空间光至单模光纤耦合效率的概率密度、衰落概率、起伏功率谱等统计特性进行了分析，所得实验结果与数值模拟结果吻合较好。本文的研究工作将为星地下行相干激光通信系统性能的分析打下理论基础，对星地下行相干光通信系统的优化设计提供理论和实验依据。
【Abstract】 Satellite optical communication, which has been attracted much attentionthroughout the world, is considered to be a potential approach to resolve limitationof data rate in satellite communication. Space experiments in orbit have beenconducted by America, Europe, Japan, China, and so on, then the satellite-to-groundand intersatellite laser communication experiments have been successful. Countriesin the world are promoting this technology to real utility.Because of components and parts limitation, early satellite lasercommunication terminals usually adopted intensity modulation/direct detection(IM/DD) scheme. Compare with direct detection, the coherent laser communication,which has advantage of high sensitivity, is recognized as a potential choice of thehigh-speed satellite laser communication.The satellite-to-ground coherent laser communication is an important part ofthe satellite laser communication network. Optical downlinks from satellites toground involve beam paths passing through the atmosphere. Spatial coherence ofthe signal optical field is destroyed because of phase fluctuation induced by theatmospheric turbulence. The amplitude and phase fluctuate of the signal opticalfield over time. Coherent efficiency (or fiber coupling efficiency in self-homodyingscheme) of the satellite-to-ground coherent laser communication system is limitedbecause of system susceptivity for phase aberration of signal optical field.Futhermore, wavefront aberrations of ground optical system and tracking errors ofangle tracking mechanism decrease the coherent efficiency in the satellite-to-grounddownlink, too.This dissertation presents the research of atmospheric turbulence influence onthe receiving performance of coherent laser communication systems in thesatellite-to-ground downlink. The studies of coupling efficiency from space tosingle-mode fiber are emphasized. The coherent system performance in wavefrontphase modal compensation and space optical field diversity-combining methods isanalyzed. The effects of wavefront aberration and tracking errors of receiver onperformance of satellite-to-ground downlink coherent laser communication systemare analyzed. First of all, the influence of atmospheric turbulence on probability distributionof coupling efficiency in process of coupling space light into single-mode fiber isstudied. The probability distribution of coupling efficiency for different numbers ofspeckles is obtained by numerical simulation of atmospheric wavefront. Wavefrontphase modal compensation method using orthonormal polynomials on pupilweighted by the backpropagated fiber mode field is brought forward, and itsadvantage is analyzed. Based on this method, the probability distribution ofcoupling efficiency is given.Performace model of self-homodying coherent laser communication system isestablished in allusion to atmospheric wavefront aberration effect on fiber couplingefficiency in the long satellite-to-ground downlink. The fade characteristics ofreceiving signals and average bit error rate of system are investigated by usingwavefront modal compensation method based on orthonormal polynomials. Thefade characteristics of receiving signals and the average bit error rate are analyzedby using optical field diversity-combining method, too.The influence of wavefront aberrations of ground optical system on coherentefficiency is studied. The variation rules of coherent efficiency with root meansqure error of primary aberrations and obstruction ratio of telescopes are found. Wemodified residual phase variance model, taking into consideration the combinedeffects of tracking error and atmospheric wavefront aberrations, and the bit errorrate of coherent laser communication system in satellite-to-ground downlink isgiven.An experiment about the influence of turbulence on coupling efficiency ofsingle-mode fiber is done. Probability density function, fade statistic and powerspectrum of fiber coupling efficiency are researched based on the analysis of theexperimental data. The experimental results tally with the numerical results.This dissertation can benefit the perpormance analysis for thesatellite-to-ground downlink coherent laser communication system, and providetheoretical and experimental foundation for the system design.