Research On Adaptive Compensation Method For Orbital Angular Momentum Space Optical Communication System

Free Space Optical(FSO)communication systems have become a research hotspot in the field of communication in recent years due to their high speed,and low cost.With the rapid development of information industry,the increase of information transmission speed and channel capacity has become an irreversible trend.Orbital angular momentum(OAM)provides a new research dimension for beam multiplexing.Because OAM beams with different topological loads are orthogonal to each other,they are widely used to enhance communication capability and improve information rate.However,in FSO communication systems,atmospheric turbulence(AT)can easily cause distortion of OAM beams,leading to energy diffusion to surrounding modes,thereby degrading the communication quality of OAM-FSO communication systems.Therefore,compensating for the distortion of OAM beams caused by AT has become a research hotspot both domestically and internationally.This thesis focuses on the adaptive compensation technology for OAM beams in FSO systems,proposing residual network based adaptive optics(Resnet-AO),modified phase diversity algorithm based adaptive optics(MPDA-AO),and intelligent phase diversity algorithm based adaptive optics(IPDA-AO)for OAM beams in OAM-FSO links,achieving high-precision correction of distorted OAM beams in AT channels.The main research work is as follows:1.Propose a residual network based adaptive optics(Resnet-AO)scheme for OAM beam correction based on residual neural networks.This scheme only requires the collection of distorted OAM beam intensity maps and corresponding atmospheric turbulence phase screens.After extensive training,Resnet can accurately extract atmospheric turbulence information from OAM intensity maps,thereby compensating for OAM signal beams.The correction performance of the scheme under OAM-FSO link is simulated,and compared with the adaptive optics scheme based on the Single Intensity Measurement Phase Retrieval Algorithm(SPRA).After compensation by Resnet-AO scheme,the strength distribution of OAM recovers from uneven shape to uniform ring.When Cn2=1×10-14m-2/3,the compensation effect of Resnet-AO was 24.56%higher than that of SPRAAO.The research results indicate that the Resnet-AO scheme achieves effective compensation for distorted OAM beams in the OAM-FSO link,and the effect is better than SPRA-AO.2.Propose a modified phase diversity algorithm based adaptive optics(MPDA-AO)scheme in the OAM-FSO link.This scheme provides more prior information for the phase difference algorithm by detecting the intensity distribution of the probe beam on the back focal plane and different defocusing planes,and effectively improves the correction effect of AO.The compensation performance of this scheme for distorted OAM beams under different conditions was simulated and analyzed.The research results indicate that MPDA-AO can effectively correct distorted OAM beams,improving the compensation effect for OAM beams.3.Propose an intelligent phase diversity algorithm based adaptive optics(IPDA-AO)scheme under OAM-FSO link.This scheme collects the intensity distribution of the probe beam at two different positions as input to the Convolutional Neural Network(CNN).After extensive training,CNN can predict the atmospheric turbulence phase screen based on the intensity distribution of the front focal plane and defocus plane positions,thereby achieving correction of OAM beams.The research results indicate that the IPDA-AO scheme can effectively compensate for the influence of atmospheric turbulence on OAM beams,and the compensation time can be greatly reduced,improving real-time performance.