| With the growing demand for high-capacity information transmission,the traditional modulation and multiplexing technologies could no longer fulfill the future demand for high-capacity and high-speed communication.Orbital Angular Momentum(OAM)multiplexing,one of the important technical means to develop free-space optical communication and optical fiber communication,could greatly improve channel capacity and spectrum utilization.However,vortex beams are affected by the preparation method and free-space atmospheric turbulence,and the phase information carried by vortex beams could be distorted,resulting in higher communication BER.In order to improve the transmission performance of vortex beams in free space,in this paper,the fabrication principles and fabrication results of two common vortex beam preparation methods,liquid crystal spatial light modulator and spiral phase plate,are firstly investigated;then a new intensity-weighted hybrid input-output(HIO)algorithm phase recovery algorithm is proposed to recover the spatial phase information damaged by vortex beams in atmospheric turbulence in time;finally,considering the effect of dynamic atmospheric turbulence on the vortex beam,a dynamic atmospheric turbulence model that could be loaded on the liquid crystal spatial light modulator is proposed.Combining the dynamic atmospheric turbulence model with the intensity-weighted HIO algorithm,a set of multiplexed vortex beam dynamic transmission simulation device is proposed,and the main work of this paper is as follows.(1)In order to research the effects caused by different preparation methods on the vortex beam transmission,the vortex beams are fabricated by using liquid crystal spatial light modulator(LC-SLM)and spiral phase plate(SPP)respectively.The fabricated results are also analyzed numerically.Before the experiments,the intensity and phase of the vortex beam under ideal conditions were studied,and the principles of LC-SLM and SPP were analyzed and the fabrication optical paths were designed respectively.In the experiments,the light intensity pictures of eight topological charge values under the two preparation methods were acquired separately.After the experiments,the light intensity pictures were optimized by using image processing.Then the inner and outer circularity of the light intensities with topological charge values of 4,6,and 8 were calculated separately for the two preparation methods.The inner and outer circularity of the beams prepared by SPP were similar,and the circularity values were above 0.86.The inner circularity of the beams prepared by LC-SLM was worse,with the average value of 0.8134 for the inner circularity and0.8553 for the outer circularity.Although the vortex beam prepared by SPP is more effective,it is much less flexible than LC-SLM due to the limited OAM modes.Therefore,the subsequent vortex beam transmission experiments under atmospheric turbulence are chosen to be carried out by double LC-SLMs.(2)The intensity-weighted HIO algorithm is proposed to recover the spatial phase for the phase distortion problem caused by atmospheric turbulence on the vortex beam transmission.In this paper,a new weighted HIO algorithm is proposed by analyzing the principle of the input-output algorithm and the type of intensity-weighted optimization scheme,and the effect of its phase recovery is verified by using numerical simulation in MATLAB.In the simulations,vortex beams with topological charge values of +2,+4,+6,+8,and +5 multiplexed with-5 are simulated to be transmitted under atmospheric turbulence and their phase distortion is recovered using the weighted HIO algorithm.The results show that the normalized power of the weighted optimized vortex beam is improved by more than 10% on average,with a maximum enhancement effect of 18.5%.Finally,the vortex beam transmission experiment under atmospheric turbulence is simulated indoors using double LC-SLMs,and its original vortex phase is successfully recovered by applying the weighted HIO algorithm with the acquired distorted light intensity pictures as input.(3)Considering that the dynamic atmospheric turbulence will have a continuous effect on the vortex beam transmission,a set of multiplexed vortex beam dynamic transmission simulation device is proposed in this paper.The device could be divided into three parts: vortex beam multiplexing at the preparation end,using the dual LC-SLM with beam splitting mirrors to achieve beam multiplexing;dynamic turbulence simulation in the transmission medium,by analyzing the vortex beam transmission under static atmospheric turbulence and Taylor’s frozen turbulence theory,a dynamic atmospheric turbulence model could be loaded on the LC-SLM to simulate dynamic atmospheric turbulence with time and space dependence;and distorted atmospheric turbulence at the receiving end.Atmospheric turbulence;distorted phase recovery at the receiver side,and intensity-weighted HIO calculation at the receiver side are introduced.In summary,in order to improve the performance of vortex optical communication in free space,vortex beam preparation,vortex phase recovery after turbulence and dynamic atmospheric turbulence simulation are investigated in this paper,respectively.It provides a new solution for the study of improving the quality of vortex optical communication in free space. |
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