| Laser communication technology is a communication method using laser as the carrier wave.It is a key communication technology in the fields of aerospace,national defense and military industry,and is gradually being integrated into the civilian field.When laser communication technology is applied in the atmospheric environment,the coupling efficiency and stability of the space optical signal to the single-mode fiber of the receiving terminal are significantly reduced due to the interference of atmospheric turbulence,which seriously affects the communication quality.Efficient and stable fiber coupling efficiency is the prerequisite and guarantee for high-speed atmospheric laser communication.The coupling efficiency of space light to single-mode fiber has become a technical bottleneck restricting atmospheric laser communication technology to be solved urgently.Adaptive optics technology is currently the best method to solve the interference of atmospheric turbulence on optical signals and improve the coupling efficiency.Since the original purpose of adaptive optics technology is to solve the problem of the influence of atmospheric turbulence on imaging quality in astronomical observations,traditional adaptive optics systems are mostly designed for imaging requirements,and relatively few designs and studies are specifically designed for the needs of laser communication systems.Therefore,in order to study the influence mechanism of atmospheric turbulence on the coupling efficiency of single-mode optical fibers in laser communication systems,and explore ways to suppress the influence of atmospheric turbulence on the coupling efficiency,aiming at the laser communication link,this paper analyzes the influence of the spatial frequency and time frequency characteristics of atmospheric turbulence on the coupling efficiency.Based on the coupling efficiency,the laser communication system's demand for the correction capability of the adaptive optics system is analyzed,and the design method of the number of correction modes of the adaptive optics system,the system bandwidth and the residual margin is given.The optimization effect of the adaptive optics system on the coupling efficiency and communication quality is verified through experiments.This article mainly carried out the following work:1.Analyze the causes of atmospheric turbulence and the effect of refractive index fluctuations based on classical theory,compare several classical atmospheric turbulence models,and analyze the characteristics of atmospheric turbulence in the typical laser communication band based on the HV model.2.Deduced a mathematical model that can quickly calculate the effect of aberration spatial modes on the coupling efficiency,and analyzed the influence of optical system parameters on the coupling efficiency.Based on the Noll Zernike sequence,the coupling efficiency of different types of aberration modes was discussed.For the influence of the specific coupling efficiency threshold,the margin range of the adaptive optical system tilt and high-order correction residual are analyzed.The analysis results of the influence of the aberration space mode on the coupling efficiency are verified through experiments.3.The influence of the wavefront overall tilt aberration on the coupling efficiency is analyzed,and the device characteristics and correction bandwidth of the overall tilt correction system are studied according to the requirements of the laser communication system.The experimental environment is established to verify the correction ability of the overall tilt correction system under the simulated turbulence disturbance of different Taylor frequencies.The experimental results show that for the system with static noise closed-loop,the G-tilt STD value is less than 0.3 ? rad,and the maximum rejection ratio is more than-30 d B in the dynamic simulation of turbulence.4.The influence of the key parameters and bandwidth of the high-order aberration correction system on the coupling efficiency is analyzed.The relationship between the scale of the correction system,the number of correction aberrations,the system bandwidth and coupling efficiency are given.An adaptive optics experimental system for laser communication was established,and the optimizing effect of the correction system on the coupling efficiency was verified under simulated turbulence with different intensities.Under the simulated turbulence disturbance of 120 Hz Greenwood frequency,the average coupling efficiency is 40.83%,and the optical power jitter is 0.48 d Bm.The laser communication experiment is carried out in the simulated turbulent channel,and the 100% frame synchronization in the statistical time is realized.The average bit error rate is 4.6*10E-5 without interleaving coding.The above research content can provide theoretical basis and technical support for the research and design of adaptive optics system based on the evaluation of singlemode fiber coupling efficiency,and provide an important reference for the further research of laser communication adaptive optics technology. |
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