Propagation Properties And Beam Optimization In Slant Atmospheric Optical Links

With the rapid advancement of mobile Internet,high-definition image sensor and cloud computing and storage,there have been higher demands on the speed and quality of wireless interconnections between network nodes.As an important complement to radio-frequency techniques,free-space optical communications(FSOC)is capable of providing ultrahigh data transmission rate in line-of-sight(LOS)links in backbone network relay,base station interconnection,tall building communication,air-ground/space-ground/inter-satellite communications,tactical communication and emergency communications.It is supposed to be a key constituent of the future heterogeneous communication network infrastructure.Nonetheless,the turbulence effects along the atmospheric channel imposes wavefront distortions on the carrier laser beam,which lead to beam wander,extra spreading and irradiance fluctuations at the receiver end,and thereby deteriorating the optical signal to a great extent.Therefore,mitigating the turbulent effects on beam propagation through better characterization of the atmospheric channel and optimization of the optical transceiver is essential to improve the reliability of FSOC systems.Up to date,the modeling theories for horizontal atmospheric turbulence path have been well developed.The scintillation models based on Kolmogorov type power spectrum functions and extended Huygens-Fresnel principle and extended Rytov theory are serving as powerful tools for link performance prediction.Upon proper understanding of the link properties and the integration of high-precision tracking mechanism,optimization of beam parameters,spatial diversity,adaptive optics and novel partially coherent beams,the performance of near-ground horizontal optical links has been well acknowledged and many optical transceiver have been commercially deployed.On the other side,slant atmospheric paths exhibit altitude-dependent refractive index structure constant,adding to the difficulty in calculating the scintillation index with analytic approach,especially for the case of finite inner and outer scales and Gaussian beam propagation.Consequently,related researches are currently trailing behind.The presented thesis proceeds with channel modeling as well as novel partially coherent source,dedicated to the study of the fluctuation properties and the corresponding counter-measures in slant-path atmospheric optical beam propagation.The atmospheric model is essential for evaluation and analysis of link performance,which may serve as a theoretical guidance for beam parameter optimization at the optical transmitter.Accounting for the fact that the present slant-path scintillation model is only applicable to weak fluctuating links,in this thesis the integral expressions for the cutoff frequencies of the large scale and small scale effects on the turbulence power spectrum are derived according to the extended Rytov theory,and a modified scintillation model for general fluctuations is thus obtained.The modified model features a unified mathematical form in the uplink and the downlink.It not only can be used to calculating the scintillation index under large zenith angle,but also incorporates inner and outer scale parameters as well as the aperture size,so the effects of multiple factors on irradiance fluctuations can be jointly examined.With the developed model,investigations has been carried out on the scintillation characteristics affected by systematic and environmental parameters such as inner and outer scale effects,source width,receiver aperture averaging,psudo-wind speed and spatial coherence of the source are evaluated.On basis of the obtained results,the asymmetry of satellite-to-ground uplink and downlink is explained qualitatively upon the hypothesis of conservation of the turbulence fluctuating energy.Partially coherent beams(PCB)such as the Gaussian Schell-model(GSM)beams trade extra spreading for reduced scintillation at the receiver end,which can be very useful in the implementation of FSOC.However,it is proved that the optical link between a high-altitude platform and a ground-level station can barely benefit from the use of PCB.For that reason,this study focuses on the application of PCBs in short slant paths.Particularly,for the first time a type of radial partially coherent beams with convex-shaped distribution of degree of coherence(CPCB)is proposed,which is generated by spatial modulating uniformly correlated phase screens.Experimental results reveal that,CPCB will self-focus one to multiple times during propagation,by which the more optical power can be collected by the receiving aperture and the aperture averaging effects are enhanced as well,resulting in further reduced scintillation.After optimizing the modulation parameter of the degree of coherence by wave optics simulation,CPCB can boost the on-axis signal-to-noise ratio(SNR)by 1 to 2 dB,as compared to conventional GSM beams.While for the off-axis case where pointing errors exist,CPCB can offer up to 5 or 6 dB SNR gain.Most remarkably,these performance improvements come with little extra hardware cost.As regards the relative integration time which is a vital issue for PCB-related applications,analyses on the turbulence propagation of CPCB have been made in this thesis.It is concluded that,CPCB is not influenced by insufficient integration time of the photonic detector as greatly as the GSM beams.So on condition that the spatial modulator is not switching fast enough,CPCB is a better choice for the optical source.Having established a complete physical image of beam propagation in slant satellite-/air-to-ground atmospheric path,the work in this thesis provides an expansion and improvement to the present turbulence optics theory and optical field manipulation techniques.Combined with theoretical dependability and practical feasibility,it is hoped this work will benefit the development of the next generation FSOC systems.