【作者】 陈浩；

【导师】 宣丽；

【作者基本信息】 中国科学院研究生院（长春光学精密机械与物理研究所） ， 光学， 2012， 博士

【摘要】 自适应光学通过对动态波前畸变的实时探测、校正,使光学系统能够克服大气湍流的动态扰动,保持良好的成像效果。为了评价自适应波前校正系统应对大气湍流的能力,大气湍流强度的测量一直是自适应光学领域的重要课题。Greenwood频率和大气相干长度r0能够从时间域和空间域表达光束受湍流扰动的综合强度。因此,本论文对上述两个参数的测量统计方法进行了深入研究。本论文首先设计了一种适用于测量大气湍流时空特性的微小孔径Shack-Hartmann波前探测器。根据惠更斯-菲涅尔衍射理论模拟出微透镜后的衍射光斑分布图,并在1/e2信噪临界原则的基础上合理地设定了衍射光斑的直径。针对探测噪声,分析了传统单一阂值法在弱信号探测时存在的问题,提出逐点扣除阂值噪声的办法,较好地消除了噪声影响,提高了Shack-Hartmann波前探测器质心计算精度。应用Zemax光学设计软件设计了与500mm口径天文望远镜匹配的Shack-Hartmann波前探测器转接镜头及其测量大气湍流时空特性的光学系统。利用400nm-750nm波段进行畸变波面探测,750nm以上的波段用于动态跟踪,可以解决天文望远镜跟踪天体目标时的抖动问题。分析了传统差分星点像运动法(DIMM)在波面上采用一对点对大气相干长度r0统计的缺陷。提出畸变波面上正方形分布的十二对点的差分星点像运动统计方法,从空间角度上增加了大气相干长度r0采样的统计数量,同时从空间群的概念来说增加了中心对称统计,大幅降低了统计时间,仅使用300ms采样300幅畸变波面即可完成对大气相干长度r0的统计,得出稳定的ro值,其波动范围只有±3%,解决了长期以来大气相干长度r0测定误差大的问题。研究了大气湍流时间域参数Greenwood频率。分析了Terry J.Brennan等人为缩短统计时间在空间上大幅增加统计量、仅在相邻两幅畸变波面上选取多点的位相差值进行频率成分统计的方法,实验证明其统计量仍然不够。因此,从时间上增加统计样本长度,连续采集300幅畸变波面,兼顾了时间和空间对湍流扰动统计量上的要求,获得的Greenwood频率波动范围只有±2%,而采样时间仅需300ms,兼具即时性、准确性,解决了长期以来湍流频率测定的可信度问题。本论文经过对测量大气湍流时空特性统计方法的不断改进,获得了准确的统计结果。本论文涉及了大气湍流基本理论、衍射光学、光学系统设计及计算机技术的广泛知识,是多学科交织集成的研究结果。更多还原

【Abstract】 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.更多还原