Research On Phase Compensation Technology In Orbital Angular Momentum Free Space Optical Communication

Free Space Optical(FSO)communication technology has attracted significant attention and research due to its advantages,which include large transmission capacity,strong anti-interference capabilities,and good confidentiality.The orbital angular momentum(OAM)beams are mutually orthogonal between different modes.Therefore,by utilizing OAM keying and multiplexing techniques,the capacity of the FSO communication system can be increased.However,atmospheric turbulence can distort the helical phase of the OAM beam,resulting in crosstalk in the receiving end mode and adversely affecting the overall performance of the communication link.This paper introduces two phase compensation techniques based on wavefront sensorless to enhance the performance of the OAM FSO system.These techniques are applied to the OAM FSO system to improve its overall performance.The primary research topics covered in this paper are as follows:The paper first addresses the issue of twin images during the phase retrieval process in the Hybrid-Input-Output Algorithm(HIOA).To resolve this issue,a phase compensation technique based on the Complementary Binary Mask Hybrid-Input-Output Algorithm(CBM-HIOA)is introduced.Simulation results indicate that twin images can arise during the phase retrieval process in HIOA.However,the introduced CBM-HIOA technique is effective in mitigating the impact of twin images.The paper compares and evaluates the phase compensation capabilities of HIOA and CBM-HIOA for OAM beams.The study examines the compensation accuracy and speed of the two techniques under varying levels of turbulence intensity.The results show that CBM-HIOA outperforms HIOA in terms of compensation accuracy and speed under different turbulence intensities.Furthermore,the study finds that the performance of CBM-HIOA in terms of escaping local optima can be further enhanced by modifying the shape of the complementary binary mask.Specifically,the use of three-sector and five-sector masks yields better compensation results compared to semicircular masks in the simulations.Secondly,as CBM-HIOA requires iterative operations,a technique based on self-referencing interferometry(SRI)is proposed to enhance the speed of phase compensation.Under weak and moderate turbulence,SRI can effectively correct the phase of the OAM beam.However,under strong turbulence conditions,stable compensation cannot be achieved using this technique.The study also finds that the pinhole size has a significant impact on the compensation performance of SRI.Specifically,a smaller pinhole size yields better compensation performance under weak turbulence conditions,while increasing the pinhole size can enhance the compensation effect under strong turbulence conditions.Finally,the paper applies HIOA,CBM-HIOA,and SRI to simulate image transmission in the OAM FSO system.This provides a more intuitive way to demonstrate the effectiveness of the three phase compensation technologies in enhancing the transmission reliability of the OAM FSO system.The simulation results indicate that CBM-HIOA performs better than SRI and HIOA in terms of communication performance.However,SRI outperforms CBM-HIOA and HIOA in terms of operating speed.