| Free Space Optical (FSO) communication is a wireless communication system withlight as information carrier and atmosphere as channel. It is widely applied to many fieldssuch as satellite-to-earth communication, coastal defense and civil communication for itsbroad bandwidth, high capacity and transmission rate, convenient build, no spectrumlicensing and resistance to interference. However, atmospheric turbulence seriously affectsthe performance of FSO, mainly causes the beam ‘tip and tilt’ and wave-front aberration,which results in worse bit error rate (BER), lower coupling efficiency and unsatisfyingcommunication. Adaptive optics (AO) can effectively compensate the atmosphericturbulence, and its key techniques contain pointing errors correction and wave-frontaberration measurement. Usually pointing errors is analyzed on pupil plane, which isdifficult to realize. And the traditional wave-front sensors as Shack-Hartmann (S-H) sufferfrom lower energy efficiency and low sensing efficiency due to its overmuch beam split, sothey are limited in FSO. To solve above problems, firstly we analyze the atmosphericchannel, then study AO-based FSO system. To overcome the impact of turbulence, wepropose new techniques of pointing errors correction and wave-front aberration sensing.The main works and creations are as follows:(1) Illustrated the cause of atmospheric turbulence and the model of Kolmogorovatmospheric turbulence, we study the impact on BER and coupling efficiency of FSO. Toimprove the performance, FSO is analyzed based on AO, and the system structure isillustrated, including the key techniques of pointing errors, wave-front sensing, control andcompensation. We focus on pointing errors correction and wave-front sensing, point outthe limitations in the traditional zonal wave-front sensing.(2) For a deeper description of the channel in FSO, we propose a new method toevaluate the atmosphere channel with the coupling efficiency. Based on the existingresearches, we derive the relationship between coupling efficiency and atmosphericcoherent length; indirectly obtain the atmospheric refractive index structure parameter.Numeral simulations verify its feasibility, the proposed method does not change theoriginal structure, has the advantages of simple structure, easy realization and low cost.(3) To solve the problems of low energy utilization, and difficult implementation inthe traditional pointing errors correction on the pupil plane, a new method is proposed which analyses the pointing errors on the focus plane. Analyzed basic theory of pointingerrors correction, we derive the relationship between the pointing errors and BER, and setup the corresponding close loop correction system. We obtain the results under differentconditions: under the weak disturbance, BER decreases from10-5to below10-9, while itdecreases from10-1to10-4under the strong disturbance. The experiment results indicatethat the pointing errors correcting system improves the communication performance inFSO, especially under the weak disturbance.(4) For higher energy utilization, a wave-front measurement method based on focalplane in FSO is proposed. We mainly study the phase difference (PD) based focal planewave-front sensing technique, and analyze the BER and coupling efficiency in FSO bytheory and numerical simulation. The experiment results indicate that the wave-frontmeasurement accuracy is nearly0.004; under the weak disturbance, BER decreases from10-5to below10-13and the coupling efficiency increases from10.32%to63.82%. PDbased method effectively restrains the impact of scintillation on wave-front sensing,significantly improves the performance.(5) To improve the wave-front sensing efficiency, we propose a new wave-frontmeasurement method based on holography in FSO. Based on holographic theory, theinfluence of holographic wave-front measurement on coupling efficiency is studied. Thesimulation results indicate that under the weak disturbance, the wave-front sensingaccuracy is nearly0.04, the coupling efficiency increases from30%to above70%., andunder the strong disturbance, wave-front sensing accuracy is nearly0.07, the couplingefficiency increases from10%to about50%. The holographic wave-front measurementimproves the communication performance, and has the higher detection precision under theweak disturbance. Compare the two wave-front sensing techniques, the following resultsare obtained: the wave-front measurement based on PD has higher detection precision andappreciable improves the system performance, while has more computation; holographybased wave-front measurement is more applicable to FSO for its less computation, highdetection efficiency and good real-time performance, although with the lower detectionprecision.The main works in this thesis not only improve theories of channel estimation in FSO,but also develop new ideas for pointing error correction and wave-front sensing in FSObased on AO; our work is significant for theory and practice of FSO. |
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