Design And Test Of High Efficient Space Optical Hybrid In Homodyne Coherent Laser Communication System

Homodyne coherent laser communication has the advantages of high optical gain,large communication capacity and strong anti-interception capability,which is considered to be one of the most competitive technology to solve the problem of long-distance,high-rate communications.The realization of homodyne coherent laser communication needs to satisfy a series of harsh conditions,and the high efficient space optical hybrid is one of the conditions.In this paper space optical hybrid's key performance—phase difference,insertion loss,energy uniformity of branches and splitting ratio—are analyzed and simulated,and the corresponding compensation and optimization method is proposed and verified,which can increase the efficiency of space optical hybrid and sensitivity of the system.Firstly,the relationship between sensitivity of homodyne coherent detection and space optical hybrid's splitting ratio is analyzed,the result show that the sensitivity of optimum splitting ratio is 2.55 dB higher than uniform splitting,the result of the verification experiment is consistent with the result of the analysis.Secondly,the mathematical model of 90° space optical hybrid is built,and insertion loss,energy uniformity of branches and phase difference of space optical hybrid is studied respectively.Thirdly,the splitting ratio adjustment and 90° phase difference compensation method is proposed,and it was simulated by mathematical method and verified by experiment.The results show that the phase difference can be accurately compensated and expectant splitting ratio can be obtained by rotating 1/2 wave plate and 1/4 wave plate.Fourthly,an aspheric beam shaper is designed to convert uniform distribution of signal beam amplitude to Gaussian distribution.The amplitude distribution shaped is similar to ideal distribution,and the maximum hybriding efficiency is 14.2% larger than not shaping.Finally,experiments were performed for insertion loss,uniformity of branches,phase accuracy and hybriding efficiency of 90° space optical hybrid.And an experiment was performed for splitting ratio adjustment and 90° phase difference compensation method.In addition,an experiment was performed for the relationship between sensitivity of homodyne coherent detection and space optical hybrid's splitting ratio.The experimental results agree with the theoretical analysis results.