湍流大气中双层共轭系统激光成像研究

【作者】 丁学科；

【导师】 荣健；

【作者基本信息】 电子科技大学 ， 光学， 2008， 硕士

【摘要】 大气成像是诸如宇航卫星,航空测绘和成像雷达等工作于湍流大气环境中的光学系统必然遇到的问题,光学系统的角分辨率一般是由其入瞳衍射极限所限制（λ/D）,然而,大气成像系统的角分辨率主要由通常小于光学系统入瞳的大气相干长度r₀决定（λ/r₀）。一般情况下,大孔径望远镜系统的长曝光像的角分辨率仅相当于一个0.1m孔径的望远镜的角分辨率。为了弥补这种的缺陷,人们采用自适应光学技术（AO）,它能实时感知由大气扰动、环境温度起伏等因素造成的波面畸变,并通过调整光学系统而实现实时畸变的补偿,使现代大型望远镜系统轴上分辨率达到近衍射极限。然而,光束在湍流大气传播中存在的“波前位错”现象阻碍了自适应光学系统对畸变波波前位相的精确重建,加上大气非等晕的影响,使一般自适应光学系统只能在一个很小的视场内进行高清晰度成像,在典型情况下只有微弧度量级。特别是当观察目标稍微偏离光轴时,分辨率便明显下降,这就大大限制了AO系统推广应用。为了能在大范围获得更清晰的像,本文采用一种新的校正方法一双层共轭自适应光学（DCAO）,它是多层共轭自适应光学（MCAO）的简单系统,属于自适应光学范畴。MCAO的基本思想是将大气湍流分成若干层,在每层的共轭位置上设置1块变形镜（DM）以校正该层大气引起的波形畸变。对大气成像系统的各光学传递函数（理想、湍流、传统AO和DCAO）进行分析并成像仿真,得出DCAO系统的成像比传统AO清晰很多,分辨力几乎接近衍射极限水平,是传统AO的好几倍。通过对校正性能指标（等晕角、相干长度、闪烁指数和斯特列尔比）的数值分析,同样得到,DCAO系统在大气湍流校正方面优势非常明显。DCAO技术是目前自适应光学技术发展的趋势,也是国际光学校正研究热点之一,而将来必定向MCAO方向（三层、四层…）发展。更多还原

【Abstract】 An atmospheric imaging is unavoidable for the optical systems,such as satellite remote sensing,navigation mapping and radar imaging,working under the environment of atmospheric turbulence.Commonly,the angular resolution of the optical system is decided by the diffraction limitation of the pupil（λ/D）.However,the angular resolution of the atmospheric imaging systems is mostly limited by the coherence length （λ/r₀） which is ordinarily smaller than the pupil aperture.As a rule,the angular resolution of the long-time exposal imaging of the big-aperture systems is only equal to which of a 0.1m aperture telescope.To resolve the problem,an adaptive optics system （AOI） is adopted.The AO technology can be subtle to the wavefront aberrance caused by atmospheric distortion,the fluctuation of the environment temperature etc.and allows to perform a real-time correction through adjusting the optical system,which makes the large telescope equipped with AO offer high resolution to the diffraction limitation.It is great,however,that a wavefront dislocation influences more accurate reconstruction of the aberrant wavefront with AO,when a beam is propagating in atmospheric turbulence.The same time,there is atmospheric anisoplanatic effects.The very small field of view（FOV）,ordinarily it is about several arcseconds,characterized by the isoplanatic angle remains a serious limitation,if keeping high resolution imaging with AO.And when the observed target slightly deviates the light axis,the resolution begins to degrade rapidly.Those will deteriorate the AO performance severely,limit application extension of AO systems consumedly.To attain more large FOV of high resolution imaging,a new correcting way, named dual-conjugate adaptive optics（DCAO）,is adopted,which is a simple system of multi-conjugate adaptive optics（MCAO） belonging to the category of AO.The base theory of the MCAO system is that the atmospheric turbulence is divided into lots of layers,and a deformable mirror（DM） is displayed the conjugate height of every layer so that the wavefront aberrance caused by that turbulent layer is corrected. Through the simulation of atmospheric imaging with optical transfer function （OTF） of optical systems（ideal,turbulence,classical AO and DCAO）,the results show the imaging of DCAO system is more legible than classical AO,whose resolution is up to the diffraction limitation.And with the analysis of the performance parameters such as isoplanatic angle,coherence length,scintillation index and strehl ratio,a conclusion is that the DCAO system has a great advantage in atmospheric turbulence correctionThe DCAO,presently,is a trend of AO technology,which is one of hot spots of optical correction.And in the future,there is a stream to multi-conjugate AO（3,4 layers, and so on）.更多还原