【作者】 亓波；

【导师】 陈洪斌；

【作者基本信息】 中国科学院研究生院（光电技术研究所） ， 光学工程， 2014， 博士

【摘要】 量子密钥分发是量子通信中发展最为迅速、最为成熟的技术，具有量子力学基本原理保证的绝对安全性。受到光纤的损耗影响以及单光子探测器的特性限制，基于光纤的量子密钥分发距离已基本达到了极限。受地球曲率和远距离可视等条件的限制，地对地自由空间量子密钥分发也很难实现更远的距离，因此人们将目光投向卫星平台，通过建立卫星平台与光学地面站之间的高稳定低损耗量子信道，可实现超远距离的量子密钥分发。通过卫星中继，将有可能实现覆盖全球的量子通信网络，这也是目前国际公认的最为可行的方案之一。高稳定低损耗量子信道的构建依赖于光学地面站和星载载荷的持续跟踪和对准，要求光学地面站ATP系统的跟踪精度要达到几微弧度，同时量子编码的特殊性又要求ATP系统具有高的偏振保持能力。本文正是在这样的背景需求下，开展星地量子通信系统中光学地面站捕获跟踪瞄准(ATP)关键技术的研究，旨在地面站ATP系统设计、理论仿真分析和技术验证等方面有所突破，为量子通信光学地面站的研制奠定技术基础。论文对量子通信光学地面站和ATP技术的国内外发展现状进行了回顾与分析，指出了ATP系统要实现高稳定低损耗量子信道所面临的技术难点：偏振保持技术、大动态范围高精度跟踪技术和高效耦合接收技术。对星地量子通信系统进行了全面的分析，确定了星地链路建立流程。系统分析了整个通信链路的链路裕量，并以此确定了ATP系统的接收口径、跟踪精度等关键技术指标。分析了大气信道对ATP性能的影响，重点分析了大气湍流导致的到达角起伏效应，并指出该效应是影响ATP系统精度的主要扰动源，同时从到达角起伏的幅值和频率分析，整个ATP系统闭环带宽要达到70Hz～100Hz。分析了光束偏振对比度变化对通信误码率的影响，并推导出星地全链路偏振对比度要优于100：1，对应地面站ATP系统的偏振对比度要优于250:1。从光学元件对偏振对比度的影响展开分析，提出了通过单镜片高保偏设计加工、光学元件小角度入射和改变光学元件间的空间布局关系等措施可以提高ATP系统的偏振对比度。在传统复合轴ATP系统跟踪方法的基础上，提出了实现大动态范围和高精度跟踪的新方法——双快速偏转反射镜串联的复合轴跟踪方法，并以偏振对比度和跟踪精度的双重约束完成了整个ATP系统的设计，针对两个FSM互相耦合问题，提出了相应的解耦策略。开展了提高ATP系统跟踪精度及光纤耦合效率的方法研究，建立了随机并行梯度下降（SPGD）算法的理论框架，分析该算法用于提高成像质量、进而提高跟踪精度的可行性；提出了利用SPGD算法提高量子通信光纤耦合效率的方法，通过仿真分析，进一步表明SPGD优化算法可以提高光纤通信的耦合效率和远场光斑质量。给出了单模光纤最优的耦合效率与光纤耦合几何参量a、大气湍流强度r0、波前校正器单元数N的关系；并指出采用多模光纤的耦合效率与其芯径尺寸密切相关，提高耦合效率应优先考虑增加光纤芯径，其次考虑增加校正器单元数，为实现具有SPGD优化功能的ATP系统分析设计提供了参考。为了验证ATP系统跟踪策略的可行性，建立了700mmATP系统实验平台，通过水平50km和96km两个链路开展了多次信标光跟踪实验，验证了强湍流条件下的ATP系统跟踪策略。同时还开展了100km量子纠缠分发实验和96km量子密钥分发实验，实验中ATP系统均采用双FSM实现了复合轴闭环跟踪。研制了一套ATP后光路系统，对该系统的各项指标进行了室内测试，测试结果表明其偏振对比度优于300：1，接收效率为34％@850nm。并将该系统与国家天文台的1m天文望远镜主光路进行集成，完成了外场工作条件下的跟踪精度测试，跟踪精度优于5urad。建立了SPGD算法静态像差补偿实验系统，望远镜系统的静态像差校正后，光斑的分布形态也有明显改善。为了进一步测试SPGD算法对跟踪精度的优化效果，开展了3km水平链路校正实验，结果表明：SPGD系统闭环后，远场光斑的脱靶量方差明显减小。更多还原

【Abstract】 Quantum Key Distribution (QKD), the fastest growing and most technologicallysophisticated quantum communication technology, has become the pioneer in the futurepractical quantum communications because of its unconditional security guaranteed bybasic principles of quantum mechanics. Restricted by fiber loss and imperfect featuresof detectors, the distance of fiber-channel QKD has basically reached the limit currently.With the limitation of earth curvature, long-distance visualization and other conditions,it is also difficult for ground-to-ground free-space QKD to achieve longer distances.Therefore, people attempt to find a breakthrough on satellite platforms. By establishingstable low-loss quantum channels between ground and satellite platforms,ultra-long-distance QKD can be achieved. Transmitting via satellite platforms, globalquantum communication network could be established, which is one of the mostfeasible solutions recognized internationally at present. The establishment of stable,low-loss channels for quantum communications relies on continuous tracking andpointing. Accordingly, the tracking accuracy of the ATP system in optical groundstations should reach the level of several urads. And the ATP system should have theability of high polarization maintaining due to the particularity of quantum coding.Under this background, researches on key technologies of quantum communicationsystem—Acquisition, Tracking and Pointing (ATP) technologies of telescopes in theoptical ground stations were done in the first time in order that we can achieve abreakthrough in the design theory, system design, performance simulation and technicalverification of ATP system, laying a technical foundation for establishing globalquantum communication network.The domestic and international research situations and trends of quantumcommunications in optical ground stations and ATP technologies were reviewed andanalyzed, pointing out the technical difficulties faced to achieve stable, low-lossquantum channels: high tracking accuracy in wide dynamic ranges and efficientcoupling receiving technology.A comprehensive analysis of satellite-to-ground quantum communication systemwas conducted and the process of establishing the satellite-to-ground links wasdetermined. The link margin of the entire communication link was analyzedsystematically. Receiving aperture, tracking accuracy and other key technical indicatorsof the ATP system were determined with the analysis. The impacts of atmosphericchannels on communication links were studied. The angular fluctuation response caused by atmospheric turbulence was analyzed in details, which is pointed out as the mainerror source influencing tracking accuracy of the ATP system. Meanwhile, theclosed-loop bandwidth of the entire system should reach70Hz~100Hz on the basis ofanalyzing the amplitude and frequency of the angular fluctuation.The effect of the variation of polarization contrast on the transmission error ratewas analyzed. From the analysis, we can conclude that the polarization contrast of theentire satellite link should be better than100:1, and polarization contrast of thecorresponding ATP system in ground stations should be better than250:1. By analyzingthe impact of the optical components on polarization contrast, some practical solutionsto improve the polarization contrast of the ATP system were proposed such as the highpolarization maintaining design and processing for monolithic lens, small angle ofincidence for optical components, changing the spatial layout among the opticalcomponents and so on.Based on the traditional compound-axis ATP system, a novel method of achievingwide motion range and high tracking accuracy—double compound-axis trackingmethod with two FSMs (Fast Steering Mirrors) was presented. The design of the entireATP system with the two constraints of dual polarization contrast and tracking accuracywas accomplished. The optical layout of the whole system was completed. Thecorresponding decoupling strategies for mutual coupling problems of the two FSMswere proposed.The tracking accuracy and coupling efficiency of the ATP system were studied andthe theoretical framework of stochastic parallel gradient descent (SPGD) algorithms wasestablished. The feasibility of improving the quality of images to achieve high trackingaccuracy by SPGD algorithms was analyzed. A method of improving the fiber-couplingefficiency of quantum communications by SPGD algorithms was illustrated andsimulated. The analysis of simulation shows that the coupling efficiency of fibercommunications and the quality of far-field spots can be boosted greatly by adoptingSPGD algorithms. The relationship of optimal coupling efficiency and parametersincluding geometric parameters a, atmospheric turbulence intensity r0, unit numbers ofwave front corrector N was given. And it was pointed out that multi-mode fibercoupling efficiency was closely related to its core diameter. And to improve thecoupling efficiency, increasing core diameter and unit numbers of correctors of fibershould be considered. The analysis above provided a reference for analyzing anddesigning the ATP system with an optimized function of SPGD. A700mm test platform for the verification of telescope technologies wasestablished to verify tracking strategies of the ATP system. A number of beacon-lighttracking experiments by two links in50km and96km levels were carried out to verifytracking strategies of the ATP system under strong atmosphere turbulence conditions.The quantum entanglement distributing experiment in100km and quantum keydistribution experiment in96km were done at the same time. A double compound-axisclosed-loop tracking strategy with two FSMs series connection was adopted in bothexperiments and mutual tracking and pointing with the transmitters were achieved.An ATP back-end optical path system was developed and the various indicators ofthe system were tested indoors. The test shows that the polarization contrast is betterthan300:1and the receiving efficiency was34%. The ATP back-end optical path systemwas integrated into the main optical circuits of the1m telescope in the NationalObservatory. The test of tracking accuracy under operating conditions outdoors wascompleted and the tracking accuracy achieved was better than5urad.The experimental system of the static aberration compensation based on SPGDalgorithms was established. After revising the static aberration of the whole telescopesystem, there was an obvious improvement for the distribution pattern of spot. Theexperiment of3km horizontal-link correction was carried out to test the optimizingeffect of tracking accuracy by SPGD algorithms. The results show that the jitter of thefar-field spots was decreased significantly in a close-looped SPGD system.更多还原