Experimental Study On Far-field Spot Image Detection For Wireless Optical Communication

Wireless laser communication(WLC)is a wireless communication technology that uses laser as a carrier to transmit information in space.In the field experiment of wireless laser communication,due to the serious influence of atmospheric turbulence,the laser spot appears the phenomenon of light intensity fluctuation,expansion,drift,damage,etc,and irregular speckles appear in the far field,resulting in the spot centroid and the geometric center are not coincident.As a result,the automatic acquisition,pointing,and tracking(APT)system cannot accurately locate the detection spot.Therefore,a spot image detection technology with strong anti-interference ability and high-precision is the key to APT system,which can effectively ensure the smooth communication link.The main work of this article is as follows:1.The effect of atmospheric turbulence on the positioning of the far-field spot image is analyzed,the direct imaging method of the Xencis camera is introduced.The spot detection principle is used to complete the beam acquisition,pointing,and focusing to identify the target spot.The diffuse reflection screen imaging is used to acquire the far-field spot image and the location algorithm is used to track the laser beam.2.Aiming at the broken and irregular degradation spot image on the diffuse reflection screen,and based on the analysis of the spatial distribution characteristics of the spot light intensity,a far-field laser spot detection algorithm with strong anti-interference ability and high accuracy is designed.This algorithm uses Open Source Computer Vision Library(OpenCV)graphics and mathematical morphology to achieve the filling of connected fields of irregular speckle images;the spot contour is extracted by Laplace operator;finally,the center coordinates and radius of the spot are obtained by two-dimensional vector space circle fitting.3.The upper computer software for spot image detection is designed,and the field experiments(10.2km,100km)platform is built for many times to verify the effectiveness of the far-field spot detection algorithm under the APT system without beacon light.At last,the paper draw a graph of the position of the spot drift under different weather conditions,the results show that the detection threshold of 10.2km far-field spot image is 50pixel-110pixel,and a single pixel represents 0.34375 cm-0.425 cm;compared with other algorithms,the root mean square error(RMSE)of spot detection algorithm is smaller,and the actual spot deviation is 17.875-46.75cm.In the 100km experiment,the two-dimensional aiming platform achieve spot realignment with an angular resolution of 17.45μrad,and the drift of spot is about 180pixel-260pixel.