Research On Plenoptic Wavefront Sensing Technique Of Free Space Laser Communication Under Strong Turbulence

Free space laser communication has become a research hotspot in recent years due to its advantages of large capacity,strong confidentiality,low power consumption,and anti-electromagnetic interference.However,free space laser communication links are subject to atmospheric turbulence.In order to suppress atmospheric turbulence,researchers at home and abroad have carried out research on the application of adaptive optics in laser communication systems.Adaptive optics integrates various disciplines such as optics,mechanics,and electricity.It can effectively compensate for wavefront distortion caused by atmospheric turbulence through real-time detection of aberrations and continuous closed-loop correction of aberrations,thereby improving communication quality.At present,the most widely used wavefront sensor in adaptive optics system is the Hartmann wavefront sensor,which has a better wavefront detection effect under the condition of moderate and weak turbulence.However,there is usually strong atmospheric turbulence in laser communication links,and it is difficult for the Hartmann wavefront sensor to accurately reconstruct the wavefront distortion.Therefore,the study of wavefront sensing technology under strong atmospheric turbulence is of great significance for expanding the application scenarios of adaptive optics and free space laser communication.This paper focuses on the plenoptic sensor,and studies the wavefront sensing technique of free space laser communication under strong atmospheric turbulence.The main contents of this paper are as follows:(1)The influence of atmospheric turbulence on the laser communication system is studied and analyzed,the limitations of the Hartmann wavefront sensor under the condition of strong air turbulence are analyzed,and the structure and principle of the wavefront sensor under strong turbulence,plenoptic sensor,are introduced.The correctness of the imaging principle analysis is proved by numerical simulation.(2)The wavefront reconstruction algorithm of the plenoptic sensor,the checkerboard algorithm,is studied.Aiming at the problem that the algorithm is insufficient in the reconstruction of the residual wavefront,a local threshold checkerboard algorithm is proposed,which can improve the wavefront reconstruction accuracy of the wavefront sensor to the residual wavefront.The checkerboard algorithm is compared with the recently proposed improved checkerboard algorithm and local threshold checkerboard algorithm in terms of reconstruction and correction accuracy and operation speed,and a wavefront restoration scheme of "checkerboard +local threshold checkerboard" is designed to realize high-precision wavefront reconstruction under strong atmospheric turbulence.(3)The wavefront correction technique of the plenoptic sensor is studied,and the distorted wavefront is corrected by the direct gradient method.Through numerical simulation and calibration experiments,two common calibration methods,push-pull method and mode method,are compared and analyzed.The mode method is selected for calibration,and based on the existing mode method,a system response matrix mode calibration and control method based on Zernike aberrations is proposed,and the convergence speed of wavefront correction is fast without other control parameters.In addition,the aberration fitting ability of the deformable mirror is analyzed by numerical simulation,which provides a theoretical basis for the subsequent wavefront correction experiments.(4)The experiments were carried out using the adaptive optics desktop experimental platform based on the plenoptic sensor,and the continuous closed-loop correction of the aberration was realized by the software programming on the basis of the experimental platform.Wavefront reconstruction experiments with different aberrations are carried out using the checkerboard algorithm and the local threshold checkerboard algorithm respectively.The results show that the local threshold checkerboard algorithm can reconstruct the residual aberrations with higher accuracy.Then,wavefront correction experiments were carried out for each single aberration,strong atmospheric turbulence and actual wavefront distortion information in laser communication links.The experimental results show that under the condition that the Hartmann wavefront sensor cannot guarantee the wavefront detection accuracy,plenoptic sensor can still perform effective wavefront reconstruction and wavefront correction.After using the local threshold checkerboard algorithm,the wavefront correction accuracy is significantly improved,which can meet the requirements of free space laser communication for wavefront correction accuracy.