| Free-space optical communication combines the advantages of convenient construction and flexible networking of microwave communication and high transmission rate,abundant frequency band resources and high security of optical fiber communication,which has a wide range of research and engineering application values.However,for a laser communication system whose communication channel is the atmosphere,the unreliability of the link will decrease the quality of the communication,and even cause the link broken.Therefore,it is essential for the reliable laser communication to introduce the adaptive optics processing technology into the free-space laser communication system to suppress the effect of atmospheric turbulence.The dissertation focuses on the key technologies of spatial adaptive processing for laser communication channels,and conducts important research on the adaptive optical system model and the wavefront correction algorithm.The main research contents are as follows:First of all,aiming for the closed-loop adaptive optical system composed of the micromechanical membrane deformable mirror(DM)and the Shack-Hartmann wavefront sensor(SHWS),the dissertation proposes a complete second-order DM-SHWS model to characterize the conversion relationship of the normalized command of the deformable mirror and the offset of the light spot.Unlike traditional modeling methods that only contain second-order term coefficient,the first-order and constant term coefficients are additionally introduced to characterize the experimental measurement error of the reference surface response and optimize the fitting results of the function curve between the spot offset and the mirror command,respectively.Subsequently,in view of the nonlinear response and strong coupling of the control channels of the micromechanical membrane deformable mirror,the calibration method of the system model was elaborated.The experiment verifies that compared with the traditional calibration method of the system model,the fitting curve between the spot offset and the mirror command obtained by the complete secondorder fitting method is more accurate with the actual measurement data,especially at the upper and lower borders of the normalized command domain.Meanwhile,we apply the complete secondorder system response model to reconstruct the wavefront offset surface of the random voltage command.The accuracy of the reconstruction situations that the offset error rate is less than 10%and the wavefront RMS value is less than 1/8 wavelength is more than 95%,which indicates the good resistance to the noise jitter.Secondly,for the wavefront correction process of the wavefront aberration,the dissertation proposes an iterative wavefront correction algorithm based on the complete second-order system response model.The algorithm uses an optimization method to obtain the normalized command of the deformable mirror that can construct a reflection surface,of which the phase is conjugated with the wavefront aberration,and obtain the optimal correction result through the nonlinear iterative residual aberration correction.Different optimization methods are simulated and analyzed by MATLAB and we select the Gauss-Newton method as the realization method of obtaining the conjugation command of the wavefront correction algorithm according to the number of iterations and time-consuming situation.In addition,the performance of the iterative wavefront correction algorithm has been verified on the adaptive optics experimental platform,which shows that the correction result can converge to the optimal or approximately optimal in two iterations and the output wavefront of system is less than 1/8 wavelength.As for the adaptability of the dynamic atmospheric turbulence,due to the limited correction ability of the deformable mirror,the output wavefront of the adaptive optical system can converge to less than 1/8 wavelength,as long as the RMS value of the input wavefront is less than 1.8 wavelengths. |
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