Investigation On Measurement Technique Of Wavefront Temporal And Spatial Characteristics Distorted By Atmospheric Turbulence

Through measuring and correcting the dynamic aberrated wavefront on real time, adaptive optics system could remove the external disturbance and obtain near diffractive limit image.To evaluate the performence of adaptive optics system against atmospheric turbulence, the measurement of atmospheric turbulence intensity has become an important issue in adaptive optics. Greenwood frequency and atmospheric coherent length ro can describe the coherence of beam transmitting through atmospheric turbulence and the global intensity of turbulence from both time domain and space domain. Hence, in this thesis, we study the mensurement techniques and statistical characteristics of these two parameters mentioned above.A method to design a Shack-Hartmann wavefront sensor with small aperture used to measure the time-space characteristics of atmospheric turbulence was presented at first.According to the Huygens-Fresnel diffraction theory, the microlens diffraction intensity distribution along propagation direction was simulated, and the diameter of the diffraction spot was obtained reasonably. And then, In order to improve detection accuracy,this paper presents a threshold selection method. This method effectively eliminates the noise, and improves the WoC (weight of centroid) calculation precision of the Shack-Hartmann wavefront sensor. The transfer lens of Shack-Hartmann wavefront sensor and optic system were designed with Zemax, and they will be installed on500mm diameter telescope for atmospheric turbulence measurement.The trembling problem when tracking celetial object with telescope was solved with dynamic tracking technology. Light with waveband400nm-750nm was used for distorted wavefront detection, while light waveband above750nm was used for dynamic tracking.This thesis analyzed the limitation of traditional star speckle moving difference method using only one pair of points to calculate atmospheric coherence length.We increased the statistical number of light spots pair from one to twelve aligned in a square, which was completed on a Shack-Hartmann light spots array. It spatially increases the statistical number and the spatial symmetry of ro measurement so that decreased the statistical time. The results show that only three hundred of wavefront samples got in300ms is enough to obtain ro with the variation less than3%. The presented method solves the large error problem in the traditional ro measurement for a long time.The statistical characteristics of the temporal domain parameter, Greenwood frequency, of atmospheric turbulence was studied. In order to shorten the statistical time, Terry J.Brennan increased spacial statistics substantially, and calculate the frequency through the difference between two adjacent wavefront. The results show that it is still not enough. Therefore, in my new method, the statistical sample time length is increased, and300distortion wavefront are grabbed contineously. The requirements on both time and space are satisfied, and the variation of obtained Greenwood frequency is±2%, and the sampling time is only300ms. The presented method is real-time and accurate, which solved the problem of poor reliability of measuring turbulence frequency since a long time.After the continuous improvement of the statistical methods of measuring atmospheric turbulence spatial and temporal characteristics, I obtain accurate statistical results.In this thesis, atmospheric turbulence theory and diffractive optics are involved; optical system design, computer technique are utilized, these research achievements integrate lots of subjects.