High Precision Angle Measurement Of Beam Deflection With Four-quadrant Detector

For free space laser communication,accurate measurement of incident beam deflection angle is the prerequisite for the acquisition and tracking of laser links,which directly determines the stability of the communication link.When laser beams undergo long-distance atmospheric transmission,light spots received by four-quadrant detectors(QD)tend to have low signal-to-noise ratios and possess an irregular energy distribution,leading to insufficient detection accuracy for the deflection angle.To meet this challenge,the precise measurement of the light deflection angle based on QD is investigated.The main contents are as follows.(1)The fundamental principles of spot position detection using a QD were illustrated,and the position detection error is theoretically analyzed.Besides,mathematical model expressing QD's position detecting accuracy was established and numerical simulation analysis is carried out on the factors influencing QD's position detecting accuracy.(2)Atmospheric turbulence usually causes the laser spot to be distorted into irregular shaped spots,to meet the challenge of insufficient angle measurement accuracy with four-quadrant detector(QD)for irregular light spot in space optical communication,we demonstrate a high precision deflection angle measurement method based on multi-source information fusion.The proposed method improves the measurement accuracy of the irregular spot deflection angle by integrating the data of QD and the image of charge coupled device(CCD),while maintain high measurement resolution and high response speed of QD-based measurement method.Compared with the conventional QD algorithm,the results of both simulation and experiment show that with the fusion algorithm,the root mean square error and the peak-to-peak measurement error is reduce by 55.56% and 42.98%,respectively.(3)To meet the chanllenge of weak-beacon measurement for the acquisition and tracking of laser links,the PPM method is used to generate the communication signal while carrying the characteristic signal of a certain frequency,and then the cross-correlation calculation was employed to to realize accurate beam deflection angle measurement when there was no independent beacon light and the signal-to-noise ratio was low.The experimental result showed that the root mean square error in the angle measurement was only 7.06 ?rad when the signal-to-noise ratio was-14.75 dB.