Study On Acquisition, Tracking And Pointing Key Technology Of Optical Ground Station For Quantum Communication

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.