Experimental Study On Polarization Control In Coherent Optical Communication Systems

Coherent optical communication has received extensive attention due to its advantages in detection sensitivity,communication capacity and communication range.However,it also faces enormous challenges.especially in polarization control.Specifically the atmospheric turbulence gives rise to random variation of the polarization state of the signal light,and the fluctuation of temperature also causes the polarization state to drift slowly,on the other hand,the optical mixer is rather sensitive to the polarization state of the signal and local oscillator light and the changes of the polarization would reduce the optical mixing efficiency and the performance of the communication system as well.Therefore,the polarization control technology is necessary to take control of the polarization state of the signal light and improve the performance of the coherent optical communication system.In this dissertation,the polarization-mixing efficiency of the coherent optical communication is studied,and a polarization control algorithm is realized to control the polarization state of signal light.The main contributions are listed as follows:1.According to the basic theory of polarized light and the structure of optical mixer,we derive the relationship among signal light,local oscillator polarization,IF signal and mixing efficiency.The limitations of several polarization control schemes are investigated.In addition,a polarization control scheme using the intermediate frequency signal as feedback signal is proposed.2.Through numerical simulation,the control effect is evaluated in terms of simulated annealing algorithm and particle swarm algorithm,which are commonly used in polarization control.A new control algorithm,called single particle optimization algorithm,which is suitable for coherent optical communication,is designed based on simulated annealing algorithm,particle swarm optimization and genetic algorithm.In this algorithm,we determine the parameters by theoretical simulation,observe the control effect and make comparison with the three algorithms.3.The limitations of several commonly used reset algorithms in polarization control are analyzed,a small step back-reset algorithm is also designed.The effects of small step back reset and direct reset on the performance of coherent optical communication system are studied by theoretical simulation.We determine the reset method that can be used in this system and conduct experimental research.4.The single particle optimization algorithm and the direct reset method are applied to the polarization control system.The specific changes of the polarization state of the signal light and the influence of polarization control on the performance of the coherent optical communication system are studied.The results are as follows:The amplitude of the intermediate frequency signal can be selected as the feedback signal of the polarization control when the communication distance is 600m,1.3km,10.2km.The single particle optimization algorithm designed in this dissertation is far superior to simulated annealing algorithm and particle swarm optimization algorithm.The reset process of direct reset mode will seriously affect the amplitude of signal,and the reset problem needs to be further solved.The changes in frequency of the signal light polarization state is slow when communication distance is 600m and 1.3km,it significantly faster as the communication distance increases to 10.2km.However,after the closed loop of the polarization control system,the amplitude of intermediate frequency signal increases rapidly,fluctuation variance decreases and mixing efficiency increases,improving the performance of the coherent optical communication system.