Research On Improving The Coupling Efficiency Of Space Optical Communication System

As space communication technology plays an increasingly important role in social service,scientific application and so on,the traditional microwave communication has been difficult to meet the needs of some communication links.Because of its large communication capacity,fast transmission rate and strong anti-interference ability,free space optical communication(FSO)has become a hot research field in communication.In order to improve the transmission rate and stability of the communication link,the FSO based on optical fiber communication technology has been widely studied and applied.For the above communication system,it is necessary to couple the received space light into the single-mode fiber(SMF)firstly.However,the coupling is always difficult,and it is constrained by two aspects: as the core size of SMF is small,the focal spot cannot be aligned with SMF under the influence of alignment error,thermal effect and other factors;the transmission quality of space light is easy to be influenced by the atmospheric turbulence,and the phase distortion of beam is serious.All these factors will lead to the increase of bit error rate of communication system,which will seriously affect the normal operation of communication link.Therefore,it is essential to ensure the beam coupled into the SMF efficiently and stably.Adaptive optics(AO)technique can be used to measure and correct the wavefront aberration caused by atmospheric turbulence effectively.However,the vibration of the platform and the noise of the system still lead to the deviation of the focal spot from the SMF.In order to give full play to the advantages of AO technology and further improve the coupling efficiency,it is necessary to introduce a correction mechanism in the system that combines the AO to form a secondary calibration system.Based on the theoretical model of SMF coupled with space light,the influence of static alignment error and random jitter on the coupling efficiency are analyzed.The results show that the existence of both can lead to serious coupling loss,they must be compensated to obtain high coupling efficiency.Then,the changes of coupling efficiency with different turbulence intensity was studied.The results show that the stronger the turbulence intensity,the smaller the coupling efficiency,so we need to correct more higher order aberration to improve the coupling efficiency.In order to further improve the coupling efficiency in AO system,the single-mode fiber coupling method is studied: simulated annealing algorithm and stochastic parallel gradient descent algorithm is applied to the SMF coupling method,respectively.The basic principle of the two algorithms and their application flow in improving the coupling efficiency are discussed respectively.And their optimization of the coupling efficiency and the convergence speed are analyzed by numerical simulation;On this basis,a SMF coupling method based on raster spiral scanning and SPGD algorithm is proposed.For the case of random jitter,the optimum iteration process and convergence effect of coupling efficiency are analyzed.The results show that: by setting the optimal scanning step,the initial alignment error can be efficiently compensated by raster spiral scanning,the success probability of initial correction can be higher than 99%,and the residual error is only 0.5?m 6.5?m;by adopting the SPGD algorithm,the random deviation can be eliminated,and the coupling efficiency also promote to 0.81 which approach the theoretical limit.According to the above method,the experimental platform is built.The experimental results show: the coupling scheme can effectively correct the lateral deviation,and the coupling power can quickly converge to the extreme value.