| Orbital angular momentum(OAM)multiplexing technology can greatly improve the information transmission capacity and spectral efficiency of communication system.It has great potential in free space OAM multiplexing optical communication,it is one of the important trends and directions of future communication development.However,in free space OAM optical communication,the transmission medium is atmospheric turbulence,which is dynamic.The dynamic atmospheric turbulence will cause the dynamic distortion of OAM beam intensity and phase,which will cause the crosstalk between different OAM modes,widen the OAM channel and increase the communication error rate.In the past,some scholars mainly studied the impact of static atmospheric turbulence on OAM optical communication,but less on the impact of dynamic atmospheric turbulence on OAM optical communication and how to reduce this impact.Therefore,based on the original free space OAM multiplexing optical communication model,this article introduces the dynamic atmospheric turbulence.It improves the aberration compensation algorithm,and obtains a better dynamic probe compensation algorithm,which is applied to the OAM multiplexing.The main work is as follows:(1)The dynamic transmission model of OAM multiplexing optical communication is established.The OAM theory is introduced to study the free space propagation.According to the actual situation of OAM multiplexed beam propagation,the dynamic atmospheric turbulence theory is introduced,the propagation model of OAM multiplexed beam in dynamic atmospheric turbulence is established.The OAM optical transmission simulation platform in dynamic atmospheric turbulence is built.According to the OAM optical transmission process,the changes of light intensity and phase of OAM beam in vacuum transmission are simulated step by step,the transmission characteristics of OAM beam in vacuum transmission are obtained.(2)The optical intensity and phase distortion of OAM beams propagating in dynamic atmospheric turbulence are simulated.The propagation distortion of OAM beam in dynamic atmospheric turbulence is obtained.Three methods of generating static atmospheric turbulence are summarized,and the static atmospheric turbulence is simulated.The method of generating dynamic atmospheric turbulence is studied,the dynamic atmospheric turbulence is simulated under different wind speed and turbulence intensity.When propagating in dynamic atmospheric turbulence,there is not only diffraction phenomenon,but also the intensity and phase distortion of OAM beam change with the dynamic change of atmospheric turbulence.Specifically,when the turbulence becomes stronger,the intensity and phase distortion of OAM beam will become more and more serious,while when the turbulence becomes weaker,the intensity and phase distortion of OAM beam will gradually reduce.(3)A better dynamic probe compensation algorithm is obtained.The principle and fabrication method of classical aberrations are studied.On this basis,an innovative step is taken.Instead of using Gaussian beam as the initial light source,OAM beam carrying the transmission information is used as the initial light source.The dynamic probe compensation algorithm is obtained.The aberration compensation algorithm and dynamic probe compensation algorithm are comprehensively compared by using three indicators of light intensity,phase and spiral power spectrum effect.The results show that the compensation effect of dynamic probe compensation algorithm for single OAM beam is10%higher than that of aberration compensation algorithm,and the compensation effect for multiplexed OAM beam is 16%.(4)The compensation model of OAM communication in dynamic atmospheric turbulence is established,and the compensation effect of OAM beam before and after compensation in two different dynamic turbulence The experimental results show that whether it is a single OAM beam or a multiplexed OAM beam,the atmospheric refractive index structure function is less than 1×10-15m-2/3 when propagating in the atmospheric dynamic turbulence.When the atmosphere changes continuously from weak turbulence to medium turbulence,the bit error rate can maintain the limit of forward error correction(1×10-3)when using the dynamic probe compensation algorithm.However,when it is larger than 1×10-15m-2/3,even if the dynamic probe compensation algorithm is used,the BER will exceed the FEC limit.On the contrary,when the turbulence changes from medium intensity to weak turbulence,if the BER after compensation is lower than the limit of forward error correction,then the BER per communication will always be lower than the limit of FEC with the decrease of turbulence.In conclusion,using the dynamic probe compensation algorithm to compensate the beam distortion caused by dynamic atmospheric turbulence is of great significance to reduce the distortion of OAM optical transmission system caused by dynamic atmospheric turbulence,which provides a new idea and theoretical reference for the study of how to compensate the distortion of OAM optical transmission system in dynamic atmospheric turbulence. |
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