Research On Disturbance Compensation Technology For Space Optical Communication

The space optical communication which replaces microwave communicaton will be a main space communication manner, because of its high data transmission rate, great communication capability, low transmitting power and strong security. But when the beam propagates through atmosphere channel, atmosphere turbulence will induce intensity fluctuation, phase angle fluctuation, arrival angle fluctuation, beam drift and spreading, and satellite platform vibration and noise disturbance, which reduce received optical power, increase bit error rate and affect seriously the natural performance of laser communication. For assuring the successfully propagation of laser signal, this paper detailedly analyses the principle of the disturbance and the effect on optical communication, and especially researches the disturbance compensation technology. The main contents of this thesis are as follow:The principle of the atmosphere turbulence is researched. The math models are constituted for intensity fluctuation, phase angle fluctuation, arrival angle fluctuation, beam drift and beam spreading which are induced by astmosphere turbulence and the effect on received optical power is also analysed. The source, species, model and characteristics of satellite platform vibration are researched. Finally the effect on bit error rate by astmosphere turbulence and satellite platform vibration is analysed, which provides the theory foundation for the latter disturbance compensation.The adaptive optic technology is researched for wavefront phase distortion which is induced by atomosphere turbulence. The working principle and basic composing of optical system are described. For the problem that the calculation of tradition reconstruction algorithm is large, and real time compensation difficultly achieves, the wavefront reconstruction algorithm based on thresholding wavelet transform is proposed, and the wavefront phase distortion is projected to 4-dimension wavelet basis, which changes the pseudo-inverse matrix to sparse matrix and reduces effectively computational complexity. Then the threshold method is adopted, which removes several wavelet basis coefficients and enhances ulteriorly sparse degree of matrix and impoves the computational efficiency. Finally the reconstruction algorithm which is used by this paper is compared with least square, Zernike and iterative method and prove that the algorithm possesses the characteristics of fast convergence from theory and simulation. The simplex algorithm is introduced and makes the control input signals mapping with Zernike polynomial because the amount of calculation is huge and real time operation can not realize when liquid crystal space light modulator directly compensates wavefront phase distortion. The performance metric is optimized and the excellent Zernike polynomial is obtained. The simplex algorithm is modified because the traditional simplex algorithm easily converges to the local minimum and the global optimization is unavailable. The simulation results show that the modified simplex method initializes to achieve the global optimization over again. The characteristics of modified algorithms is analyzed,, which provides basis for practical application.The disturbance compensation scheme based on piezo-electric steering system is proposed, and the modified LMS algorithm is designed according to beam drift from satellite platform vibration and noise perturbation. The Filtered-X LMS is used to ensure the systemic stability. Then the choice of step size is discussed and adaptive delay filter is introduced to improve the convergence rate under the condition of not adding step size. Meanwhile the adaptive delay filter solved the problem of phase delay. Filally, the adaptive bias filter is design to eliminate the bias disturbance which is high frequency and low amplitude.The novel liquid crystal steering system is put forward to implement disturbance compensation because of the coupling between piezo-electic steering and satellite platform, complex dynamics and more consumption. The adaptive controller based on RLS lattice filter is designed. It realizes fast convergence and achieves the optimized steady disturbance compensation. The variable-order algorithm is used to remove the great transient response of high order RLS in the condition of no adding calculation. The simulation results indicate that the method is valid.