Investigation Of Stray Light Suppression For Free Space Optical Communication System

The next step in the development of satellite communication requires the realization of high-bit rate transmission,and communication loads must also be suitable for space platform applications,that is,it must be miniaturized,lightweight,and energy-efficient.The only communication means that can meet these requirements is free space laser communication.Satellite laser communication terminals are faced with complex spatial background radiation during in-orbit operation,and usually need to meet the digital transmission distance of several thousand kilometers or even tens of thousands of kilometers.Strong laser power experiences serious energy loss after long-distance transmission,and satellite optical communication systems require weak-light detection,so high stray light rejection ratio and transceiver isolation constitute the key to long-distance,full-duplex laser communication in satellite optical communication systems.In this paper,the typical spatial background radiation characteristics of a satellite optical communication system in orbit and the stray light inside the optical system are theoretically studied and analyzed.From the overall design level,this paper also explores the methods of calculating the sun outage avoiding angle to be designed for the satellite optical communication terminal,and the level of stray radiation to laser terminals by other typical stars,thus providing a theoretical basis for the requirements of the satellite optical communication system for the stray light suppression level,the transmission and reception isolation and other indexes.Mainly with regard to the assembly of the optical system of the satellite optical communication terminal,that is,the 90° optical mixing phase shifter in the periscope coarse pointing mechanism,telescope,transceiver channel and coherent receiver,this paper performs stray light suppression phase shifting design,theoretical analysis,simulation calculation,experimental verification and other work.Finally,all the single machines are integrated to establish the semi-physical simulation analysis and calculation of the optical system level of the satellite optical communication terminal,which verifies that the out-of-field stray light suppression and transmission and isolation improvement design of each single/part component is obviously effective for improving the stray light suppression and transmission and reception isolation outside the field of view.The main research work and innovations of this paper are as follows:(1)A method of punching holes and setting light traps in the secondary mirror is proposed to improve the transmission and isolation of the Cassegrain telescope,and the theoretical relationship among the aperture ratio of the secondary mirror aperture,the emission efficiency and the isolation is derived.The simulation results show that when the aperture ratio K of the secondary mirror is 0.1~0.3,the transmission efficiency of the telescope is not affected,and the transceiver isolation is improved from 21.73 dB to 41.4 dB.The improved Cassegrain telescope has a transmission isolation of 41.56 dB and an unaffected transmission efficiency of 76.9%,which is consistent with the simulation results.The experimental results verify the accuracy of the theoretical model and the effectiveness of the improved method,which is of great significance for improving the transmission and isolation of the Cassegrain telescope.(2)A new single-wavelength optical transceiver channel based on the same-band inter-frequency scheme is designed,and the optical isolation mathematical model in the transceiver channel is established.The deflection of the quarter-wavelength and the polarization beam splitting are adopted,and,optimization measures,such as setting the optical trap and canceling the analyzer,are developed;the measured isolation of the prototype is 78.3 dB.Compared with the traditional multi-wavelength optical system,the scheme has the advantages of reducing the number of system lasers and the number of transceiver branches,and can realize automatic switching between different terminals to facilitate networking.The derived mathematical model for isolation has some guiding significance for optimizing the isolation of optical channels under the same-band inter-frequency scheme.(3)Based on the previous theoretical research,combined with the parameters of the optical system of the satellite optical communication terminal,the system design using the combination of the periscope catching mechanism,the off-axis telescope,the single-wavelength transceiver channel and the coherent detection is proposed.According to simulation calculation,the spurious radiated power of the communication receiving branch upon sun outage reaches-119 dBm,thus realizing immunity from sun outage.The internal transmission and reception isolation of the system reaches 100 dB or more.This form of satellite optical communication system can be widely used in satellites networking.