Study On The Polarization-maintaining Optical System For Inter-satellites Coherent Laser Communication System

According to the communication modes,the space laser communication system is divided into two modes: direct detection and coherent detection.The coherent detection mode is becoming the main development of the long-distance spatial laser communication because of its high detection sensitivity,flexible modulation mode and large communication capacity.In the inter-satellite laser communication system,the optical system is the key component to realize the long-distance laser communication.In the direct detection mode,the wavefront aberration of optical system is an important parameter which affects the system performance.In the coherent detection mode,in addition to the wavefront aberration,the polarization aberrations of optical system also affect the performance of the communication system.The optical system of inter-satellite coherent laser communication belongs to the polarization sensitive system.When the signals passing through the optical system,the polarization aberrations will change the polarization states of the signals and further affect the heterodyne efficiency between the signals and local oscillators(LOs).The main purpose of the coherent communication is to realize the efficient heterodyne efficiency between the signals and LOs.Therefore,it is instructive to study the effects the polarization aberrations of optical system have on the polarization states of signals and the heterodyne efficiency,which can provide guidance for designing coherent systems with high heterodyne efficiency.By studying the polarization aberrations of the optical system and studying how to reduce the effects of polarization aberrations,it is significant to guide the design of high polarization-maintaining optical system.This paper is mainly focused on the polarization aberrations of the optical system and their effects on the heterodyne efficiency for the coherent laser communication system.The research is as follows:Based on the conventional coherent detection mode,this paper derives the heterodyne efficiency when the signals and LOs have the same polarization states,and further analyzes the influences of the amplitude distributions of the signals and LOs on the heterodyne efficiency.On the basis of the conventional coherent detection mode,a coherent detection model is proposed by considering the mismatching of the polarization states of the signals and the LOs.By introducing the polarization efficiency parameter,the theoretical relationship between the polarization aberrations of the optical system and the heterodyne efficiency of the coherent communication system is established.This analytical expressions help to analyze the effects of the polarization aberrations on the coherent communication system quantitatively.For an optical antenna terminal system,this paper analyzes its polarization characteristics by utilizing the three-dimensional polarization ray-tracing calculus.The antenna terminal system consists of two parts: the periscopic scanner system and the off-axis antenna optical system.For the periscopic scanner system,the analytical polarization transformation matrix is deduced,and the polarization aberration of this scanner at different scan angles are analyzed in detailed.When the incident signals are linear polarized or circular polarized,the output polarization states of the signals after passing through this scanner are calculated by using the derived polarization transformation matrix.After comparing the results,it is concluded that when the signals are circular polarized,the signals can be immune to the rotations of the azimuth axis.Based on the polarization ray-tracing calculus,the polarization characteristics of the combined system of the periscopic scanner and the off-axis system are analyzed.The results show that the polarization aberrations of the combined system are not uniform distributed across the pupils,and the distributions of the aberrations are related with the rotations of the scanner.For a laser communication detection platform system,by using the same ray-tracing calculus,the polarization characteristics of the communication system are analyzed by modeling the whole components of the communication systems.The results indicate that the communication system has the diattenuation and retardance,and these two aberrations are not uniform distributed across the pupil.In order to quantitatively analyze the influence of the polarization aberrations on the coherent communication,a method by combining the SVD decompositions and Pauli-Zernike decompositions is proposed to analyze the polarization aberrations of optical system.The optical properties of the optical system are decomposed into four independent parts by the SVD decompositions of the Jones matrix.The four parts correspond to the apodization,the waverfront aberration,the diattenuation and the retardance,respectively.Based on the Pauli-Zernike decompositions,the distributions of the diattenuation and retardance acorss the pupils are obtained,and the coefficients of the corresponding Pauli-Zernike polynomials are calculated.The influences of the first 9 Pauli-Zernike polynomials coefficients on the polarization states of the incident signals and the corresponding heterodyne efficiencies are calculated by using the proposed polarization aberration method.The results show that the effects of the polarization aberrations of the optical system on the coherent communication are related to the polarization states of the incident signals.In each polarization aberration coefficient,the uniform distribution of the diattenuation and retardance are the main aberrations that affect the polarization states of the signals and the corresponding heterodyne efficiencies.The distributions of the polarization aberrations of the antenna terminal system and communication platform are analyzed by using the proposed polarization aberration theory.For the antenna terminal system,the polarization efficiencies vary in the range [0.49 0.66].Through the comparisons of the polarization aberration of the antenna terminal system and the Pauli-Zernike coefficients at different scan angles,it is found that the periscopic scanner produces the major aberrations of the system's polarization aberrations.The effects of the polarization aberrations on the heterodyne efficiency induced by the periscopic scanner are larger than that of off-axis antenna system.Based on this situation,two kinds of polarization compensation methods are proposed: the passive polarization-maintaining and the active polarization-maintaining.After the compensation of the polarization aberrations,the polarization efficiency maintains the theoretical value 0.5.For the communication system of the platform,the polarization efficiency is 46%.According to the amplitudes of each Pauli-Zernike coefficients,it is found that the diattenuation and retardance along the y-axis direction are the main aberrations that affect the polarization states of the signals.Based on its polarization characteristics,an active polarization compensation method is proposed to improve the system's polarization-maintaining performance.The polarization efficiency becomes 49.8% when the compensation method is used.For the periscopic scanner system and the communication system of the platform system,the experimental results show that the polarization aberrations of the two systems are verified.The model and the analysis method proposed in this paper provide a theoretical basis for the development of coherent communication optical system with high polarization-maintaining ability,and have great engineering application value.