Research On Spatial Light Coupling Technology Based On Liquid Crystal Correction

The technology of free space optical communication uses laser as a carrier for long-distance information transmission.Laser transmission has high rate and small divergence angle.Compared with the traditional communication mode,it has the advantages of high speed,large capacity and high confidentiality.With the expansion of the range of space laser communication,the demand for the receiving terminal with high sensitivity,high transmission rate and high stability is more and more intense,which makes the single-mode fiber coupling technology emerged.In general,the core's radius of a single-mode fiber is only several microns,so accurate alignment of light beam position and good incident facula pattern are the keys to ensure efficient coupling.In the process of space transmission of laser signals,atmospheric effect,thermal effect,platform vibration and other factors will cause laser some phenomena like residual turbulent scintillation,pattern fragmentation and phase fluctuation,resulting in the reduction of optical fiber coupling efficiency of communication terminals and the increase of bit error rate,which seriously affects the performance of the communication system.In order to suppress the influence of atmospheric effect on the link of space laser communication,this paper studies the calibration technology of liquid crystal special light modulator for broken facula patterns.Compared with the traditional mechanical calibration device,the liquid crystal spatial light modulator has the advantages of more driver elements,no mechanical motion and stable response.At present,liquid crystal calibration technology has been applied in the field of astronomical observation,but in the field of space laser communication that needs to maintain long-term efficient and stable communication,it is still at the initial stage.This paper combines with the demand of space laser coupling technology,and conducts specific researches on the liquid crystal calibration technology.Firstly,conducted theoretical analysis of the space laser coupling.According to the time and space description of the physical structure of atmospheric turbulence,analyzed the influence of atmospheric disturbance on the performance of the communication system and performed theoretical simulation of its phase.According to the definition of coupling efficiency and the distribution of light field,set up single-mode fiber coupling model and analyzed the influence of the laser beam displacement and phase distortion on the coupling efficiency.Theoretical analysis shows that calibration devices with high distinguishability and high efficiency and high precision detection algorithm are the key to ensure the performance of the calibration coupling system.Optimize the performance of liquid crystal spatial light modulator.As the key component of space laser system,the phase modulation characteristics and response speed of liquid crystal can directly affect the effect of calibration and coupling.According to Fraunhofer diffraction grating model,established the connection between the phase modulation and the control signal.According to the theory of continuous elasticity,analyzed the effect of the relaxation characteristics of liquid crystal molecules on the response speed of the device and adopted the method of overdrive to improve the response speed of the liquid crystal in the end.Simulation and experimental results show that the overdriving method can effectively shorten the response time of the liquid crystal under the condition that the hardware's underlying base of the device is limited,which provides favorable experimental conditions for liquid crystal calibration.High precision neural network detection algorithm.Aberration detection and wavefront reconstruction are the core problems of the liquid crystal correction system.The neural network contains sufficient computing units,which can realize phase recovery by learning far-field distorted spot.The neural network phase retrieval model uses the light intensity detector to collect the distorted facula.The detection is stable and has a certain application prospect under the condition of strong turbulence distortion.Combining with the control mode of gray wave surface of liquid crystal spatial light modulator,this paper proposed a new deconvolution neural network model.The model can input the distorted spot image and output the phase wavefront,which completely retain the high-order aberration information.In addition,the direct output of control wave surface can save calculation time,which effectively improves the detection precision and algorithm efficiency.Improve the performance of liquid crystal correction system.In the calibration system of this paper,the time delay of the liquid crystal device caused the mismatch between the loaded calibration wave surface and the actual distortion phase at the current moment,which eventually led to the calibration residual of the system.According to the frozen turbulence hypothesis,the motion of atmospheric turbulence at the receiver of the telescope has a certain time connection over a certain time scale.Therefore,the motion vectors between continuous sampling frames can be estimated by image prediction algorithm.This paper adopted the search motion estimation algorithm of adaptive root pattern to estimate and predict the gray level information detected by deconvolution neural network.Simulation and experimental results showed that this method had certain predictive compensation effect for different atmospheric conditions,and effectively improved the real-time calibration performance of the liquid crystal system.In order to verify the method proposed in this paper,we set up a spatial light coupling experimental system based on liquid crystal calibration,restored the wavefront phase by using the deconvolution neural network model,and compensated the phase by using liquid crystal spatial light modulator.For static wavefront aberration with a root mean square error of 0.67 ?(? =850nm),the neural network detected an average root mean square error of 0.1107 ?.However,the system bandwidth was only 1Hz due to the limitation of the execution speed of the current calibration system.Within the bandwidth range of the system,combined with the search algorithm of adaptive root mode,the average coupling efficiency of the corrected terminal was improved to more than 6d B.In conclusion,the liquid crystal calibration spatial light coupling system adopted in this paper has obvious advantages under the static and quasi-dynamic phase distortion conditions.This paper studies the atmospheric turbulence problem of free space laser communication system,and systematically describes the calibration technology based on liquid crystal.The coupling performance of space laser communication is obviously improved under static and quasi-dynamic atmospheric turbulence conditions by means of improving the performance of liquid crystal devices,designing a new artificial neural network detection model and estimating and predicting turbulence phase.The research results of this paper provide theoretical basis and new technical support for the application of liquid crystal calibration technology in space laser communication,and effectively promote the practical process.Although the current bandwidth of the system cannot meet the needs of space laser communication,with the further development of liquid crystal devices and hardware equipment,liquid crystal calibration technology has a great application prospect in space laser communication field.