Research On Beam Steering And Wavefront Distortion Compensation For Space Laser Communication

Free space laser communication systems are line of sight communication,and there is usually relative motion between communication terminals.On the one hand,when there is distortion or error on the surface of the antenna,the wavefront of the output light field will be distorted,degrading the directivity of the beam and reducing the antenna gain.On the other hand,the pointing,acquisition and tracking(PAT)of the communication system requires beam steering with angular resolution beyond micro-radians to reduce beam pointing loss.In this paper,from the perspective of beam control in space laser communication applications,research is carried out from both beam steering with microradian resolution and wavefront control.The main research contents are as follows:(1)The requirements of beam steering and wavefront distortion for the space laser communication system are analyzed.The overall schemes of the beam steering with microradian resolution and beam wavefront control are carried out.(2)A liquid crystal optical phased array(LC-OPA)cascaded double-grating method is proposed for high-angular resolution beam steering.The LC-OPA performs coarse deflection at an angle of tens of micro-radians.The double-grating performs multi-stage angular compression on the LC-OPA diffracted beam,thereby realizing beam steering resolution of micro-radian.Numerical simulations verify that the double-grating structure can significantly compress the deflection angle of the beam,and the larger the grating constant,the smaller the angular compression ratio.Simulations also show that the beam can be diffracted 4 times in the double-grating configuration when the incident angle of the beam is between-43.5 mrad and 26.8 mrad.The experiment verifies that the angular resolution of the deflected beam can be enhanced by six times,and beam steering with micro-radian resolution can be realized with our proposed setup.(3)Based on liquid crystal spatial light modulator(LCoS-SLM),a scheme for simultaneous wavefront detection and wavefront compensation for optical antennas is designed.The algorithm of eliminating tilt aberration based on least squares plane fitting and the algorithm of eliminating aberration based on Zernike polynomial wavefront fitting are studied and deduced to eliminate extra phase errors.The experimental platform of wavefront distortion detection and compensation was built.For the 1550 nm beam,the extra phase error was eliminated by the algorithm,and the phase peak-valley(PV)value was reduced from 30.1209 rad to 2.8241 rad.The RMS value of the wavefront distortion after compensation by LCoS-SLM is ?/23.