Research On High Sensitivity Detection Technology Of Ground Optical Terminal For Deep Space Laser Communication System

Deep Space Exploration refers to the exploration of the Solar System Space and the Cosmic Space beyond Earth's gravitational pull.It is the inevitable choice for the continuous development and continuation of human beings.With the increasing detection distance of deep space probes,the traditional microwave/radio communication system has become increasingly unable to meet the requirements of data transmission rate.In recent years,free space optical communication technology has attracted wide attention in the world due to its advantages of wide bandwidth,good confidentiality,strong anti-electromagnetic interference ability,small size,low power consumption and low cost.In 2013,NASA have successfully completed the demonstration of high-speed optical communication between the Moon and the Earth and it shows that the technology also has great potential in deep space TT&C(telemetry telecontrol & communication)system.Deep Space Optical Communication is still a frontier technology,but the United States and Europe are in the forefront of this field by virtue of their own technological advantages.However,there are still many technical problems need to be solved when the technology is mature and applied to future deep space exploration missions.In Deep Space Optical Communication system,the downlink optical signal will be attenuated by the distant space distance and the atmospheric channel,so that the received light intensity is extremely weak,just a few photons.Effective detection of photon-level signals is a key step for the ground terminal,which directly determines the success of DSOC system.Therefor,the high sensitivity detection technology of deep space optical communication ground terminal is studied in this dissertation.The dissertation contains the following four major parts:Firstly,the influence of atmospheric channel on the downlink optical signal of DSOC was analyzed.The effects of atmospheric channel average power attenuation,atmospheric turbulence scintillation,beam spread and speckle were introduced.The broadening effect and the delayed effect of pulsed optical signal which caused by clouds,aerosol particles and atmospheric turbulence were analyzed in detail.The mathematical relationship between the optical communication rate and the broadening and delayed effects under pulse position modulation was established,and the influences of the two effects on the communication rate were evaluated.The characteristics of sky background light were analyzed.Secondly,in order to effectively detect the extreme weak signals,a high sensitivity detection unit for deep space optical communication based on Superconducting Nanowire Single Photon Detector was established.Based on the SNSPD's dark count rate fitting model,the Double Generalized Gamma's atmospheric turbulence scintillation model and pulse position modulation,an BER expression of DSOC which expressed by FOX-H function was derived.The BER performance under specific temperature and bias current was simulated and analyzed.According to the high background noise of deep space communication link,a up-down algorithm based on photon array counter was used to recognize and eliminate the strong background photons.The performance of SNSPD was tested.The background photons remove performance was verified by experiment,and the BER of the high sensitivity detection unit was also tested.Thirdly,an improved adaptive coupling method for optical communication system based on coarse-fine laser nutation technique was presented to solve the lower coupling efficiency problem of space light to signal mode fiber.This method can greatly improve the coupling efficiency.Based on the basic theory of spatial light to single mode fiber coupling,the effects of radial deviations between spot and fiber and the residual aberration of atmospheric turbulence on coupling efficiency were analyzed in detail.A coarse-fine composite scanning and acquisition algorithm based on laser nutation was proposed.The radial offset bias between beam spot and SMF center can be compensated in real-time by detecting the energy which was coupled into single-mode optical fiber.The validity of the above research content was verified by the experimental system.Finally,an adaptive optical system based on binary mode intensity modulation wavefront sensing technology was proposed by using single photon counting arrays according to the characteristic of the extreme weak signal of DSOC.The principle of wavefront reconstruction of the adaptive optical system was described,and the direct linear relationship between the driving voltage matrix of the deformable mirror actuators and the constructed coefficient matrix of Zernike polynomials was established.The distorted wavefront reconstruction accuracy of the AO system in strong and weak turbulence environments were studied by simulation method.The distortion wavefront correction ability of the adaptive optical system was analyzed by simulation.The results obtained in this dissertation have important reference value for the design of DOSC system.