Research On Beam Shaping Technology In Space Laser Communication System

The space laser communication different from traditional radio frequency(RF)communication is that laser beam is used as a carrier of information.The divergence angle of the laser beam is close to the diffraction limit.Therefore,it has good direction and solves the problems of high power and low security of traditional RF communication.Laser has coherence and rich spectrum resources.The high-sensitivity coherent reception with the help of optical mixer can achieve ultra-high-capacity information communication.Compare with the commonly used intensity modulation / direct detection methods,it has huge advantages.Laser communication involves many key technologies,and there are still many problems to be solved in the processing of laser beam.This paper analyzes several key technologies in space laser communication systems,present existing problems,and uses aspheric beam shape lens groups to improve these problems.a)The optical antenna generally uses the Cassegrain optical system,which is a reflective telescope.The secondary mirror will block the central beam.The laser beam has a Gaussian distribution,with high intermediate intensity and small edge intensity.The beam is shaped by an aspheric shaping mirror and then transmitted through an optical antenna to reduce the energy loss caused by the shielding of the antenna secondary mirror.Change the laser beam amplitude distribution mode from Gaussian to uniform,and the transmission efficiency can be improved by 11% compared with the optical antenna without the shaping function.b)The space beam needs to be coupled to the single-mode fiber after entering the optical system.However,due to the mismatch between the incident beam and the single-mode fiber mode,the beam after long-distance propagation can't be efficiently coupled to single-mode fiber even after excluding atmospheric turbulence and aberrations at the receiving end,the space beam can be changed to a mode supported by single-mode fiber with the help of a reflective beam-shaping unit.By studying the coupling theory of space beam and single-mode fiber,a reflective beam-shaping lens group is designed to improve the level of mode matching between space beam and single-mode fiber and realizes higher efficiency single-mode fiber coupling than uniform beam.c)The optical mixer in coherent optical communication system performs coherent mixing of the signal beam and the local oscillator beam to obtain intermediate frequency signal.How to improve the mixing efficiency of signal beam and local oscillator beam is the research focus of optical mixer.In this paper,a set of laser beam expanding and shaping lens groups is designed,which are separately used for the local oscillator beam processing and signal beam processing of the optical mixer,so that the geometric size and amplitude distribution of the two beams are close,and then can be obtained nearly 100% mixing efficiency.Based on the research of beam processing technology and aspheric beam shaping mirror in laser communication,this paper proposes an improved method of space laser communication technology,which can provide a reference to the system design of laser communication.