Research On Measurement Method Of Atmospheric Characteristic Parameters Of Wireless Optical Communication Link

The performance of free space optical communication(FSOC)is subject to atmospheric effects,including atmospheric attenuation and atmospheric turbulence,which are characterized by atmospheric extinction coefficient and atmospheric refractive index structure constant,respectively.Therefore,the accurate measurement of atmospheric characteristic parameters is of great significance for guiding the design and performance research of FSOC system and the targeted development of atmospheric effect suppression methods.Most traditional methods of measuring atmospheric characteristic parameters measure atmospheric extinction coefficient and atmospheric refractive index structure constant independently,but FSOC needs to measure atmospheric extinction coefficient and atmospheric refractive index structure constant at the same time,and analyze the comprehensive influence on performance.In recent years,aiming at the research requirements of fixed-platform FSOC,a synchronous measurement method of atmospheric characteristic parameters based on long-term average irradiance has been proposed,an experimental system has been built and experimental studies have been carried out.However,none of the above methods can be applied to mobile platform FSOC.Because mobile platform FSOC needs a servo tracking mechanism to establish and maintain the link stability,the pointing error of the servo tracking mechanism will remodulate the center position of laser atmospheric transmission speckle,to change the average irradiance,so that the existing synchronous measurement method is not suitable for mobile platform FSOC application scenario.To solve this problem,in this dissertation,the average irradiance of dynamic optical link under the combined influence of pointing error and atmospheric effects is established,which lays a theoretical foundation for synchronous measurement of atmospheric characteristic parameters under the condition of relative motion.Secondly,a dynamic optical link phase screen simulation method is developed to verify the correctness of the derived theoretical results.Then,a system based on the servo tracking mechanism is constructed to locally sample the average irradiance of dynamic optical link,and two methods of measuring atmospheric characteristic parameters based on the local sampling of the average irradiance of dynamic optical link are proposed.Finally,the application of atmospheric characteristic parameter measurement in the case of non-Gaussian beam transmission is studied.The main innovation achievements are as follows:1.The average irradiance of dynamic optical link is established for the first time.Aiming at the problem that the traditional long-term average irradiance only considers the atmospheric effects and cannot accurately describe the joint influence of pointing error and atmospheric effects.Using statistical averaging method,combined with the long-term average irradiance throry and the pointing error theory,the average irradiance of dynamic optical link is established which lays a theoretical foundation for the measurement of atmospheric characteristic parameters in FSOC on mobile platforms.2.The simulation method of single-layer phase screen under dynamic optical link is developed.To solve the problem that the traditional phase screen simulation method cannot be applied in the pointing error,a phase shift factor representing the pointing error was added to the single-layer phase screen transmission process to realize the speckle simulation of optical transmission under the influence of atmospheric effects and pointing error.Then the speckle irradiance statistics are used to verify the theoretical correctness of the average irradiance of dynamic optical link.In addition,the method can also be used to simulate the irradiance fluctuation probability density function,power fluctuation probability density function and average bit-error rate(BER)under dynamic optical link,which provides guidance for the performance analysis of FSOC system.3.The detection system of average irradiance of dynamic optical link is constructed.A small aperture projection imaging system is added to the servo tracking mechanism,and the alignment and debugging of optical path,the calibration of pixel size and spot size of projection imaging detector,and the calibration of irradiation response of projection imaging detector are completed.The average irradiance of dynamic optical link can be locally sampled by small aperture projection imaging system under the condition of relative motion,which provides the original data for the subsequent measurement of atmospheric characteristic parameters.4.Two methods of measuring atmospheric characteristic parameters under dynamic optical link are proposed.Aiming at the problem that the traditional measurement methods cannot measure the atmospheric characteristic parameters under the condition of relative motion,based on the local sampling data of the average irradiance of the actual dynamic optical link,two kinds of atmospheric characteristic parameter measurement methods are proposed,named LM inversion measurement method and average power solution measurement method,respectively.LM inversion measurement method is based on the local sampling results approximating the average light intensity distribution of dynamic optical link,and the atmospheric characteristic parameters are taken as the parameters to be measured by Levenberg-Marquardt(LM)inversion algorithm.The average power measurement method selects two or more equivalent receiving apertures of different sizes and the corresponding average received power to form nonlinear equations,and uses the least square method to solve the characteristic parameters of the atmosphere.Experimental results show that both methods can accurately measure atmospheric characteristic parameters.5.The application of non-Gaussian beam in the measurement of atmospheric characteristic parameters is studied.Recent studies show that non-Gaussian beams,such as Hermite-Gaussian beams and flat-topped beams,have the potential to improve the performance of FSOC.The average irradiance of dynamic optical link of Hermite-Gaussian and flat-topped beams is derived by using statistical average method,combined with the long-term average irradiance theory of Hermite-Gaussian and flat-topped beams and the pointing error theory,and the correctness of the average irradiance of dynamic optical link is verified by dynamic optical link phase screen simulation method.Finally,LM inversion measurement method is used to measure atmospheric characteristic parameters under non-Gaussian beam.The above research solves the difficult problem of measuring atmospheric characteristic parameters under the condition of relative motion,and can measure atmospheric characteristic parameters synchronously in mobile FSOC links,guiding the design and performance optimization of FSOC system,and providing support for improving the performance of FSOC system in atmospheric environment.