Research On Measurement Technology Of Laser Atmospheric Transmission Characteristics Under Misalignment Conditions

Free space optical communication(FSOC)system has become a research hotspot in recent years due to its wide bandwidth,high speed and strong confidentiality.It has broad application prospects.However,atmospheric turbulence and atmospheric attenuation in the transmission channel are the main factors that restrict its development,which are characterized by the atmospheric refractive index structure constant(C_n~2)and the atmospheric extinction coefficient,respectively.By studying the atmospheric channel in depth we can accurately grasp its influence on the performance of optical communication,and develop the atmospheric effect suppression means in a targeted manner.In recent years,in order to meet the requirements of FSOC channel measurement,a synchronous measurement method for C_n~2 and extinction coefficient has been developed,which requires precise alignment of the laser beam transmission optical axis and the projection optical axis.However,it is difficult to achieve accurate optical alignment in an actual optical communication system,resulting in a relatively large measurement error.In order to solve the synchronization measurement problem under misalignment conditions and make the channel measurement easier to use,this paper proposes two measurement methods:irradiance fitting method and irradiance integration method.These two measurement methods treat the misaligned distance as a parameter,so it is possible to overcome the problem that the laser transmission optical axis and the projection optical axis are misaligned in practical applications.The principle of the irradiance fitting method is to fit the measured long-term mean irradiance profile with the theoretical long-term mean irradiance profile,and then synchronously calculate the atmospheric transmission characteristic parameters and the misaligned distance.The principle of the irradiance integration method is based on the long-term mean irradiance profile of a certain aperture including the atmospheric transmission characteristic parameter and the misaligned distance.By calculating the power in three concentric circles of different sizes,the atmospheric transmission characteristic parameters and misaligned distance are simultaneously measured.Both methods can simultaneously measure the C_n~2,the atmospheric extinction coefficient and the misaligned distance by using only the partial separation axis irradiance on the receiving aperture.The aperture constraints of the two measurement methods are also discussed.In order to verify the feasibility and accuracy of the proposed methods,phase screen simulation results and measurement results were compared and analyzed.For simulation:simulate the long-term mean irradiance profile on the receiving aperture by using the phase screen theory.The two proposed methods are used to measured the atmospheric transmission characteristic parameters separately,and the results are analyzed to get the measurement range and resolution.For measurement:measure the long-term mean irradiance profile and calculate the measurement accuracy by taking different aperture positions and misaligned distances.Besides,the effects of misaligned distance and background light were discussed.The above researches overcome the drawbacks of synchronous measurement of C_n~2and atmospheric extinction coefficient under misalignment conditions.The feasibility and accuracy of the two methods are verified by simulation and experiment,and the effective measurement range and measurement resolution are discussed in detail.The proposed methods greatly reduce the influence of misalignment on the measurement results,and makes it possible to measure the atmospheric transmission characteristic parameters under misalignment conditions.