Research On Performances Of The OAM Multiplexing Communication System In The Case Of Atmospheric Turbulence

With the rapid developments of communication and information industries,highspeed,large-capacity and higher-data of information transmission are an inevitable trend.However,the existing communication technology has not been able to meet those requirements.Recently,the optical vortex carrying orbital angular momentum(OAM)has been proposed for enhancing the communication capacity and increasing information data rate due to its orthogonal properties.One of the serious challenges encountered by the transmission of OAM-based FSO(free-space optical)communication link is the atmospheric turbulence(AT)because it introduces intermodal crosstalk,thus degrading the performance of the communication system.The main purpose is to propose some solutions to correct the distortion brought by the AT during the propagation of the information.The main content of the work is as follow:1)Propose a novel method of retrieving the OAM states affected by differenttype ATs with GS(Gerchberg-Saxton)Algorithm.We have investigated the influences of the AT levels to the transmitting OAM beams by changing the refractive-index structural parameter and the number of turbulence random phase screens in simulation environment.The GS algorithm is then adopted to compute the pre-compensation phase and correct the distorted OAM beams.The theoretical analysis and numerical simulations show that,when the transmission distance is fixed,not only the refractive index structure parameter will affect the AT intensity,but the number of random phase screens will also affect the AT intensity level.Although in the case of strong-strength AT,due to the limitations of the GS algorithm,the pre-compensation phase cannot be calculated properly to correct the distorted beam,but under weak and medium AT intensity levels,the GS algorithm can effectively compensate the phase.At the same time,we confirmed that when the topological charge(TC)value is small and the type is single,the performance of the OAM beams recovered by the GS algorithm is better than that of the case with a large TC value and more types.2)Enhance OAM mode recognition based on joint scheme of combining GS algorithm and convolutional neural network(CNN).To achieve the efficiency recognition of the multiplexing LG beams under AT environments,we have proposed a joint scheme of combining GS algorithm and CNN(GS-CNN).The feasibility of the scheme is verified by investigating the recognition performances under various AT strength levels and mode spacings.Moreover,we also discuss the performances of recognizing LG beams with different radial indices.The numerical simulation shows that after initial restoration of the beams' intensity distribution by the GS algorithm,the following CNN can effectively extract the features of retrieved intensity distribution and achieve higher accuracy on recognizing multiplexing LG beams in severe AT environment compared to the above-mentioned solution that only used the GS algorithm,and the good performance can also be achieved in recognizing LG beams with different radial indices.Further,the scheme is improved with Shuffle Net V2 to achieve a recognition of accuracy of both single and multiplexing hybrid Bessel Gaussian beam(HBGB)under AT environment.The simulation results show that before compensation of the beams by the GS algorithm,the improved CNN achieve a higher accuracy recognition than the traditional CNN.Since both the traditional CNN and the improved CNN can achieve high accuracy in strong ATs when combined with GS algorithm,GSCNN has great application potential in OAM-based practical communication system.3)Propose a high-efficiency and self-recovery m BGBs(multiplexing Bessel Gaussian beams)communication system in ATs.In order to further study the reliability of the OAM-FSO communication system in the actual AT channel,we propose an OAMFSO communication system in ATs based on of self-recovery m BGBs.At the receiver,the transmitting m BGBs have been transferred into the perfect optical vortex(POV)with the same size for detections,and meanwhile,the transmitting m BGBs in ATs can also be compensated by the GS algorithm for achieving higher recognition.In addition,we have also designed a multiplexing phase hologram(MPH)to enhance the detectable capacity for the generated POV modes,in which multiplexing channels' information can be identified simultaneously.Finally,the detecting performances under various AT strength levels for single and multiplexing OAMs,have been investigated and discussed in details.The simulation results show that the system can achieve higher transmission performance in severe AT environment for both single and multiplexing OAM beams.