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Research On Compensation Methods Of Receiving Sensitivity Degradation Insatellite Optical Coherentcommunications

Posted on:2016-10-28Degree:DoctorType:Dissertation
Country:ChinaCandidate:P ZhangFull Text:PDF
GTID:1108330473456092Subject:Detection Technology and Automation
Abstract/Summary:PDF Full Text Request
Satellites optical coherent communication has the advantages of high sensitivity, high capacity and ability of anti-interference from background light, that make it one of the main research directions for long distance, high rate satellite laser communications. High sensitivity is obtained by exploiting coherent effect of local oscillator and signal light. In a practical system, optical field mismatches between local oscillator and signal light decreasethe coherent efficiency. The receiving sensitivity will degradeseriously and even be far less than the theoretical limit values. How to reduce the optical field mismatches and obtain high receiving sensitivity is an important issue to be resolved in satellites optical coherent communication.In this paper, the research aiming to resolve the receiving sensitivity degradation in satellite optical coherent communications is carried out. From phase mismatch, power distribution mismatch and optical axis mismatch of coherent optical field, we explore the degradation mechanism caused by noise or errors in optical communication system. The models of optical phase-locked loop error and receiving sensitivity are modified with optical field mismatching factors. Based on the theoretical research, the compensation methods are proposed. Feasibility of those methods is validated by mathematical simulation and experiments.The phase-lock error of optical phase-lock loop and errors coming from optical subsystem are internal factors of coherent receivers which bring receiving sensitivity degradation. we found that the Doppler frequency shift coming from fast rotating mirror is the third source which brought phase-lock error, except phase noise of lasers and shot noise inside phase-lock loop. In this paper, the characteristics of this Doppler frequency shift are derived, and the suppression method is provided too. The receiving sensitivity degradation coming from wavefront aberration of optical subsystem, phase-shift error of 90 ° space-optical hybrid and mismatch of optical front end is discussed and corresponding compensation methods are provide also.The quantitative analysis of receiving sensitivity degradation coming from the phase-shift error of 90° space-optical hybrid has not been publicly reported. In this paper, the principle of influences of phase-shift error induced by a 90o space-optical hybrid on the performance of homodyne coherent receivers is clarified. The mathematical relationship between the phase-shift error and receiving sensitivity degradation is deduced. This resolves the quantitative design-basis of 90o space-optical hybrid.In this paper, the common mode suppression ratio(CMRR) of balanced receivers is extended to that of balanced optical receiver front end and is used to characterize the noise suppressing ability of balanced optical front end with balanced mismatches. The research shows that, the mismatches of balanced optical receiver front end defined in this paper are important sources of receiving sensitivity degradation. COSTAS optical phase-lock has low pass characteristics, which make it be affected by the power mismatch only and shield off influences of transmission time mismatch.The compensation methods of receiving sensitivity degradation caused by error of beam tracking system and atmospheric looking as factors outside coherent receivers are give in this paper.The axis misalignment between the optical axes of the tracking optical path and the communication optical path would degrade the sensitivity of an optical communication system with an independent tracking optical path. This kind of degradation is compensated by introducing a nutation tracking scheme which integrate the optical tracking path and communication path together to remove the static optical axes misalignment, the tracking error is extracted from the fluctuation of output signal of photodetector. This method can increase the tracking precision and the sensitivity the same time. The mathematical model of nutation tracking, tracking error extraction method and the calculation of nutation angle are investigated.In this paper, the mathematical relationship of coherent reception power, average receiving power and coherent efficiency is deduced. By transferring the path integral into the angle integral, simulation model which can be applied to many kinds of coherent links is build. Based on this method, we investigated the variation of mean entrance power, coherent efficiency, and coherent receiving power in four scenarios of satellite to ground, near space to ground, near space to satellite and near space.The theoretical results obtained in this paper can be used to decrease the receiving sensitivity degradation of satellite optical coherent communications. The results provide basis for the receivers design and satellites optical coherent links design. The conclusions of this paper can also be widely used in coherent receivers used in terrestrial mobile platform and stratospheric airship.
Keywords/Search Tags:satellite laser communication, free-space laser communication, optical imperfections, pointing error, atmospheric turbulence
PDF Full Text Request
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