Defocus And Off-axis Performance Analysis Of Cassegrain Antenna System

In recent years,satellite laser communication has attracted great attention in the field of satellite communication due to its small size,large capacity,high data rate,and good security performance.As an important part of satellite communication systems,the optical antenna system is also a hot research topic.The Cassegrain antenna system has the advantages of a wide application range,strong aberration cancellation ability,and compact structure,and is widely used in free-space optical communication systems.At present,the research on the Cassegrain antenna system mainly adopts two methods: geometric optics and diffraction theory.Geometric optics is simpler than diffraction theory and can save time and cost in the optimization and analysis process of the system,so it has become the first choice of researchers and engineers.Based on basic experimental laws such as the linear propagation of light and the theorem of refraction and reflection,geometric optics discusses the propagation of light beams.It does not physically involve the properties of light,so the results obtained will inevitably have errors.In this thesis,by combining geometric optics and diffraction theory,the Cassegrain optical antenna system is analyzed,and the applicability of the geometric optics method to different optical antenna systems is judged by defining the parameter ?.At the same time,based on the theory of diffraction,this thesis analyzes the performance of a space optical communication system composed of a transmitting antenna,a receiving antenna,and a fiber coupling system,involving several actual situations such as defocus,off-axis,and fiber deviation of the antenna system.The main research contents of the thesis are as follows:(1)The Cassegrain optical antenna system is analyzed by the geometrical optics analysis method and diffraction theory analysis method respectively.By calculating the transmission efficiency of the system under different transmission distances,the differences between different methods of geometric optics and diffraction theory are obtained.(2)The ratio of diffraction loss to geometric loss is defined as the loss ratio ?,and ?is used to judge the error of the results obtained by the geometrical optics method,thereby judging the applicability of the geometrical optics method to the transmission system.At the same time,the application of parameter ? in an optical system is given.(3)The whole Cassegrain optical system is analyzed by diffraction theory,and the influence on the transmission and coupling of the system when the antenna system is defocused is analyzed.According to the simulation results,an optimization scheme to improve the coupling efficiency is proposed: the coupling efficiency can be improved by introducing the defocusing parameters of the main mirror of the transmitting and receiving antennas;Adjusting the position of the fiber to improve the coupling efficiency.(4)Diffraction theory is used to analyze the effect on the performance of the offaxial antenna system.The effect of the off-axis distance ? on the fiber coupling is calculated.Under the ideal state,the spatial light-fiber mode field matching degree is the highest.With the increase in off-axis distance,the coupling efficiency appeared at multiple peaks,and the peaks showed a decreasing trend.