Research On The Control Of Adaptive Optical System For Space Laser Communication

Space laser communication uses laser as the carrier to transmit data in space through the atmospheric channel.It combines the advantages of wireless communication and optical fiber communication.Comparing to the RF communication,it has many advantages including high communication rate,wide bandwidth,large capacity,good confidentiality,anti-electromagnetic interference,and small size.The advantages of low power and simple construction are becoming the core of the next generation of high-speed wireless communication.Except for laser communication in space,the transmission channel of space laser communication must pass through the atmosphere,the main problems faced by space laser communication are the effects of laser energy attenuation,beam arrival angle fluctuation,spot centroid drift and expansion caused by atmospheric turbulence,which seriously reduce the performance of space laser communication.In order to alleviate the effects caused by atmospheric turbulence,this thesis intends to introduce adaptive optics technology into Space laser communication systems to study the control problems associated with adaptive optics technology and Space laser communication system.At present,there is no adaptive optics technology adaptation to space laser communication system.This thesis mainly quantitatively analyzes the influence of the control performance of adaptive optics system on the performance of space laser communication,and provides the basis for the design of adaptive optics system controller for space laser communication.The fast steering mirror and deformable mirror in the adaptive optical system is studied for space laser communication.The control method proposes a high-bandwidth and high-robust to realize space laser communication over long distances,long time,multiple occasions with low error rate.In order to quantitatively analyze the influence of the control performance of the adaptive optics system on the performance of space laser communication,this paper starts from the composition of atmospheric turbulence,and obtains the atmospheric refractive index fluctuation model based on the Kolmogrov turbulence theory,and then obtains the time and space description method of atmospheric turbulence.Based on the quantitative description of the spatiotemporal intensity of atmospheric turbulence,the relationship between the adaptive optical system control bandwidth,the corrected Zernike mode number and the atmospheric turbulence correction effect is established with different turbulence intensities.Combined with the communication system model,the relationship between coupling efficiency,communication error rate and atmospheric turbulence correction effect is established,and then the quantitative relationship between the bandwidth of the adaptive optical control system,the corrected Zernike mode number,the communication coupling efficiency and the bit error rate is obtained.The correctness of the formula for deducing the quantitative relationship is verified by experiments.The requirements of various indicators of the adaptive optical control system for space laser communication are clarified,which provides a theoretical basis for the design of adaptive optics system controller for space laser communication.According to the analysis of the Zernike modes of atmospheric turbulence on the performance of laser communication,the tilt term error is the main factor affecting the performance of laser communication.Increasing the suppression of tilt disturbances will significantly improve laser communication performance.The traditional PID control algorithm reduces the error from the low frequency to the high frequency within the error rejection bandwidth.If the disturbed frequency band is outside the error rejection bandwidth of the PID algorithm,the algorithm has no suppression ability for this part of the disturbance.At the same time,the use environment of space laser communication is not fixed.In order to ensure long-term stable operation under various conditions,it is necessary to design a controller with better performance for the laser communication system.In this paper,the subspace identification method based on frequency domain correction is proposed to improve the identification accuracy of the tilt correction system model.The application of this model to the time delay compensation control strategy can significantly improve the control system bandwidth.Experimental results show that the control system error is applied after applying this method.The rejection bandwidth is increased from 69 Hz to 76 Hz.In order to further increase the bandwidth,the high-sampling frequency position feedback sensor of the fast steering mirror is used to pre-close the fast steering mirror,reduce the nonlinearity and phase lag of the fast steering mirror response,and then use the optical wavefront detector to form a double closed loop.The loop tilt correction system further improves the time delay compensation method under the double closed loop structure.The error rejection bandwidth of the tilt control system is increased from 76 Hz to 85 Hz,which can meet the tilt correction requirements of the laser communication system.According to the relationship between space laser communication and atmospheric corherent length,in order to achieve higher communication performance,in addition to the tilt term,it is still necessary to correct a certain number of zernike modes.In order to ensure the robustness of high-order correction,the system overshoot problem caused by the hysteresis characteristic of integral term in traditional PI controller is optimized,and the attenuation factor of e-index to the integral term is added to derive the control with attenuation factor PI.According to the new controller's Bode diagram,the optimized PI controller improves the system's response performance,widens the width of the mid-band,and the controller adapts to changes in the external environment.At the same time,the premise of not affecting the bandwidth of the closed-loop system,the resonance peak of the system is partially suppressed,and the overshooting characteristics and response time of the system are obviously improved.In order to verify the theoretical analysis and controller design method proposed in this paper,an adaptive optical system for laser communication is built in this paper.The atmospheric turbulence simulator is used to simulate atmospheric turbulence,and the atmospheric turbulence is adaptively corrected by adaptive optics system.The performance of the laser communication before and after correction is improved a lot.For the distortion of the static wavefront aberration of 0.2-0.4???=808 nm?,the corrected RMS value is less than 0.1?,the coupling efficiency is increased to more than80%;for the atmospheric wave corherent length is 1 cm,the Greenwood frequency is50 Hz dynamic wave before the aberration,the adaptive optical system is corrected,the average coupling efficiency of the fiber is increased to more than 70%,and the bit error rate is reduced from 10^-5 to 10^-7,and the improvement of bit error rate is about 2 orders of magnitude.In order to verify the effectiveness of the designed control method,we conducted a one-week field experiment in Lvshun,Dalian,and achieved the first adaptive optics correction experiment of horizontal space laser communication at sea level of 9 km.The bit error rate is reduced to below 10-6,and the weather conditions of 24 hours are tested.The influence of adaptive optics on laser communication performance under different weather conditions is verified,and the importance of adaptive optics system in laser communication system is revealed.This paper is a pioneering work on adaptive optics of free space optics which analyzed on the design and evaluation of AO system applied to laser communication system.The first time in China we experimented the 9km horizontal path crossing the sea,the datum of atmospheric turbulence and AO correction will provide a theoretical basis and experimental experience on the development of adaptive optics.This thesis aims to study the control method of adaptive optics system in space laser communication system.By optimizing the design of the tilt mirror and the deformable mirror controller,the correction ability of the adaptive optics system in space laser communication is improved,which greatly enhances the performance of space laser communication.The research results of this thesis provides theoretical basis and technical support for the application of adaptive optics technology in space laser communication,which promotes the practical process of space laser communication technology.