Study On The Performance Of Modes Diversity Receipt Of Few-Mode Fiber Based Free-Space Optical Communication System

In recent years,research on free space optical communication（FSO）has mainly focused on obtaining faster communication rates and improving system communication performance under the influence of external factors such as atmospheric turbulence and pointing errors.With the development of the fiber-based mode demultiplexer,the receiving end of a few-mode fiber（FMF）based FSO system can be well compatible with the current single-mode fiber（SMF）based system:the optical signal in an FMF that supporting several modes is converted into optical signals in several SMFs with low loss and low crosstalk.In this context,as a new dimension,the spatial mode can not only improve the communication rate of the FSO system through mode multiplexing technology when the link condition is good,but also can improve the communication performance of the system in the presence of external factors through mode diversity technology when the link condition is poor.Therefore,the spatial mode-based FSO system has become a potential solution to improve communication rate and guarantee communication quality,which has good practical significance and research value.The two technologies:mode multiplexing and mode diversity,have similar technical means,the difference lies in whether different modes transmit different signals or the same signal.This feature enables the same system to implement two functions on demand,that is,to improve the system communication capacity when the link condition is good,and to ensure the system communication quality when the link condition is poor.FMF-based mode diversity reception is such a new technology that can improve the system performance in the presence of external factors.On the one hand,the optical signal will excite high-order modes other than the fundamental mode in the presence of external factors,such as atmospheric turbulence.The FMF which supports multiple modes can obtain higher coupling efficiency under the same conditions,to further obtain better performance of the FSO system.On the other hand,the traditional photodetector-based receiver has gradually fallen into a performance bottleneck due to sensitivity and other defects.Mode diversity receipt can convert a beam of few-mode optical signals into a set of single-mode optical signals,which can effectively utilize the existing mature single-mode-based optical pre-amplification technology to improve the signal intensity.Both high value of coupling efficiency and signal intensity can further improve the communication performance of the system.However,most of the current research is based on experimental results,and the theoretical analysis of FMF-based mode diversity reception is still rarely reported.Because of the lack of theoretical research,this paper proposes a theoretical model of the coupling efficiency from FSO link to an FMF in the presence of atmospheric turbulence or pointing errors.Then,based on the coupling efficiency model,the performance of modes diversity receipt of FMF-based FSO system is studied.The details are as follows:（1）For the FSO system with mode diversity reception,a mode approximation method for FMF using the Laguerre Gaussian mode group is proposed,and the influence of atmospheric turbulence on the coupling efficiency of FMF is analyzed.It is found that the coupling efficiency of different modes of FMF varies with increasing turbulence intensity.In addition,the coupling parameterplays a crucial role in the coupling efficiency.Different coupling parameters can lead to different coupling efficiencies of each high-order mode under atmospheric turbulence.This characteristic is very important for optimizing the communication performance of the system.The optimal coupling parameter is₃=1.12 for three-mode FMF,while the coupling parameter of the maximum coupling efficiency of six-mode FMF is₆=1.69.The proposed model can be verified by current experimental results.（2）In view of the influence of pointing error on coupling efficiency of FMF,a theoretical model of the effect of beam offset on coupling efficiency of FMF is established.It is found that the two coupling efficiency models are consistent in the presence of beam offset or atmospheric turbulence with ideal conditions.Besides,both the coupling parameterand the incident beam radiuscan play a key role in the coupling efficiency of FMF.（3）In view of the fluctuation of instantaneous coupling efficiency under atmospheric turbulence and the crosstalk caused by the mode demultiplexer,a truncated multivariate Gaussian function is proposed to describe the power distribution of the SMFs end of the mode demultiplexer,so as to further analyze the communication performance of FSO system with FMF-based reception in the presence of turbulence.Two schemes,equal gain combining（EGC）and maximal ratio combining（MRC）,are considered.Due to the extremely high mathematical complexity,two approximate methods are proposed to obtain the series solution of the unconditional average bit error rate when the system uses the MRC scheme,which is verified by Monte Carlo simulation.Besides,for the MRC scheme,an estimation method for estimating the weight coefficient of the corresponding SMF by using the average coupling efficiency is also proposed.The simulation results show that this estimation has high accuracy in the case of weak turbulence.（4）Aiming at the respective applicable backgrounds of zero boresight offset pointing error and non-zero boresight offset pointing error,the effect of pointing error on the communication performance of FSO system with FMF-based reception is studied.When the jitter variance is small,the increase of the boresight can significantly reduce the communication performance of the system.However,when the jitter variance is large,the increase of the boresight has little effect on the communication performance of the system.This work provides a theoretical basis for FMF-based FSO system,and provides a detailed performance analysis and theoretical reference for system analysis,design and optimization.