Rasearch On Adaptive Optics Technology In Orbital Angular Momentum Based Free Space Optical Communication System

Free Space Optical(FSO)has been widely used due to its advantages of large bandwidth,high rate,fast link deployment and good security.Simultaneously,researching on high-speed and large-capacity communication technology to cope with the trend of global network growth has always been a hot topic for researchers in various countries.Orbital angular momentum(OAM),as a new dimension of light beam,has attracted more attention in optical communication technology due to its unique optical characteristics.The OAM beams with different azimuth indices are orthogonal to each other,and the azimuth indices of OAM beams can theoretically be arbitrary integers.Therefore,whether using OAM state coding or using OAM beam as carrier to realize OAM mode division multiplexing,the introduction of OAM can effectively improve the capacity of communication system and meet the modern communication needs of high rate and large capacity.However,in the actual communication link,the turbulence environment(atmospheric turbulence or oceanic turbulence)will cause the wavefront distortion of OAM beam,resulting in power loss and serious crosstalk between OAM modes,thus reducing the performance of FSO communication link.To reduce the susceptibility of OAM beam to turbulence,this paper explores the methods for improving the communication performance of OAM beams in turbulent environments on the basis of fully considering the unique transmission characteristics and structural characteristics of OAM beams.For the requirements of the establishment of turbulent environment transmission link,the design of AO and the system simulation and verification in OAM optical communication system,this paper mainly studies on the phase retrieval algorithm-based adaptive optics(PRA-AO)approaches,the optimal design of probe beam in AO-based OAM-FSO link,the PRA-AO approaches in OAM-based underwater wireless optical communication(UWOC)link and the PRA-AO pre-compensation approach.The main work and innovations of this paper are as follows:1.Research on adaptive optics approach based on single intensity measurement in OAM-FSO linkIn order to overcome the wavefront reconstruction error caused by the phase singularity of OAM beam,an single-intensity measurement phase retrival algorithm-based AO(SPRA-AO)is proposed.The SPRA-AO approach only needs the Fourier intensity distribution of the probe beam to reconstruct the wavefront information of the OAM beam,and then correct of OAM beam.This approach realizes the application of the simplified AO which appiles for the OAM-FSO communication link.According to the OAM beam propagation and distribution characteristics,the iterative constraints of SPRA are set,and the compensation performance of the distorted OAM beam by the SPRA-AO approach is studied and analyzed.Simulation results show that the SPRA-AO approach can effectively compensate distorted OAM beams for both single-channel and multiplexed OAM links and improve bit error rate(BER)performance.In the case of both background noise and detector noise,compared with the Gerchberg-Saxton-based AO(GSA-AO)approach,the SPRA-AO approach has better robustness for detector readout noise.Compared with the GSA-AO,the SPRA-AO approach can achieve similar correction performance with the GSA-AO which needs double intensity input.2.The optimal design of probe beam in OAM-FSO linkBased on the characteristics of the OAM-FSO link and the transmission characteristics of the OAM beam,the selection of the probe beam and the theory of the beam waist control under the OAM-FSO link are discussed in depth.On the one hand,it is proposed to use OAM beam as the probe to compensate for the distorted ONM information beam.Simulation results show that using an OAM beam with state l=1 as a probe beam can yield better correction effects than a Gaussian beam.On the other hand,using the correlation coefficient of the intensity of the signal beam and the probe beam as an evaluation index,an analytical expression of the beam size of the probe beam is derived.The simulation results show that it is important to properly expand the probe beam to ensure the correction performance of the AO scheme.A wider or narrower probe beam will affect the compensation effect of the AO scheme.3.Research on adaptive optics approach based on double intensity measurement PRA in OAM-FSO linkThe double-intensity detection PRA-based adaptive optics(DPRA-AO)approaches under the OAM-FSO communication link are proposed.They are spatial phase pertubation Gerchberg-Saxton algorithm-based adaptive optics(SPP GSA-AO)approach and hybrid-input-output algorithm-based adative optics(HIOA-AO)approach.The DPRA-AO approach overcomes the limites of wavefront reconstruction accuracy in SPRA-AO and introduces more prior information for PRA iteration by detecting the intensity distribution of probe beam in spatial and frequency domains.Simulation results show that the correction effect of the SPP GSA-AO approach is better than that of the traditional GSA-AO approach under the OAM-FSO link.Due to the introduction of the phase perturbation,the SPP GS A requires more iteration to convergence which costs time to obtain the correction effect.The HIOA-AO approach can quickly and effectively correct distorted OAM beams and improve the BER performance of OAM-FSO communication links.In addition,it is verified that the HIOA-AO approach is also effective for OAM multiplexed links and can effectively compensate distorted OAM multiplexed beams.In terms of robustness of AO approaches,whether SPRA-AO or DPRA-AO,the presence of background noise sould reduce the correction effect.At the same time,the proposed AO approached in this paper have strong robustness to the shot noise of the detector.In the case of both background noise and detector noise,compared with SPP GSA-AO approach,the HIOA-AO approach is more robust to detector readout noise.4.Research on post-and pre-adaptive optics compensation approches for OAM-UWOC linksThe PRA-AO approach under the OAM-UWOC link is proposed.In this approach,the influence of ocean turbulence on the propagation performance of OAM beam is alleviated by introducing PRA-AO approached.In order to solve the complexity and diversity requirements of ocean communication,pre-compensation AO approach under OAM-UWOC link is proposed.The AO module is placed at the transmitter to effectively reduce the system complexity of the receiver and increases the flexibility of the UWOC link on the basis of ensuring the performance of the communication link.Simulation results show that compared with GSA-AO,the HIOA-AO approach can effectively compensate the OAM beam under the OAM-UWOC link and improve the BER performance of the link.The pre-compensation AO approach based on HIOA could achieve similar compensation effect compared to the traditional post-compensation AO approach.