Research On Receiving Performance Of Satellite-to-ground Downlink Coherent Laser Communication Systems

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.