| The networking of the laser communication system(LCS)has become more and more necessary for its wide and high-speed applications in the aerospace filed.The bidirectional link in the same-frequency is an important method to realize freely networking of space LCS in the future.In particular,the bidirectional LCS terminals with the shared-aperture same-frequency transceiver and polarizing switching has the advantages of low system complexity,small size,light weight,and fast network switching.Generally,the terminal contains both of the high-power transmitting and high-sensitivity receiving systems.However,the received signal is easy to be submerged by the light with the same wavelength from the transmitting system.Therefore,to avoid the submergence of received signal,multi-level opt-electric isolation is very necessary in which the optical isolation plays the key role in the development of the LCS and also is the difficult point for the bidirectional sharedaperture LCS.To satisfy the requirements of stray light suppression for LCS,the transmission principle and the mode of the stray light are analyzed,and the space background radiation characteristics as well as the internal stray light transmission characteristics of the inter-satellite laser communication terminal are also studied in this dissertation,respectively.Taking the links of the medium and high orbit satellites as a case,the requirements of transmit-receive isolation is analyzed on the basis of the laser signal power demands and the sensitivity index level of the detector.With the optimization of the system configuration of LCS and the investigation of the methods for internal stray light suppression,the key components of optical antenna that may affect the optical isolation are pointed and the approaches to using quarter-wave plate(QWP)to suppress stray light are proposed.One of the important affecting factors on the optical isolation is the scattering characteristics of optical surface.According to measured data of the optical surface,the bidirectional reflection distribution function models of the optical surface with different roughness based on the two models of ABg and Harvey-Shack are established.Thereafter,the isolation of the optical antennas and the laser communication terminals are simulated from the above models.Meanwhile,the experiment platforms for the isolation testing are build up to prove the correctness of the simulations.Furthermore,a design scheme of the laser communication terminal based on dual wave plates is proposed by considering the polarization characteristics of the light scattering of the optical surface.This scheme is an important way to improve the reliability of accessibility of the laser communication terminal at present and also has important implications for the design of bidirectional shared-aperture LCS.This dissertation includes the main research works and innovations as following:(1)The radiation spectrum characteristics of stars are systematically analyzed,and the filter parameters are designed that can not only effectively receive the beacon light of the laser communication system,but also select certain number of stars based on the requirements.Therefore,it may provide important conditions for star-based pointing calibration.In comparison with the filtering design with full background light suppression,the proposed design is conducive to the miniaturization and functional integration of the laser terminal,and can reduce the dependence of on-orbit calibration of the terminal on ground station resources.(2)A method for evaluating and iterating the stray light isolation level of off-axis optical antennas is proposed.The YNI value of cold reflection in the infrared system is used for reference.The wave aberration of the optical antenna and the optimal solution of the stray light isolation index that meets the requirements of the profile envelope are obtained.Stray light modeling and simulation verification show that the stray light isolation index of the optimized optical antenna is improved by 2?3 d B.This method is suitable for the design of shared-aperture transceiver optical antennas for laser communication terminals,and is of great significance to the engineering application of off-axis reflective structures in LCSs.(3)The polarization bidirectional scattering distribution function model of the smooth surface is established.Based on the polarization characteristics of the scattered light of the smooth surface,an optimized design for the same frequency and sharedaperture laser terminal with high stray light isolation is proposed.This design uses a large-aperture QWP as the window of the optical antenna,and installs a rotatable and switchable small-aperture half-wave plate(HWP)on the receiving end of the optical antenna.Through the combination design of the two wave plates,a laser communication terminal has been realized,with an improvement of the optical isolation from 70 d B to 78 d B,and the freely networking in the polarization switching terminal state being ensured.This method has a significant suggestion for the design of the LCS that has the requirements of long-distance,ultra-high-speed,co-frequency,bi-direction and high isolation.The research in this dissertation provides a theoretical foundation and reference for the engineering realization of stray light suppression in the inter-satellite laser communication system at the same frequency,and it also makes significative exploration for the data transmission of communication network system using samefrequency laser in the future. |
PDF Full Text Request