【作者】 李聪；

【导师】 周昕； 李晓峰；

【作者基本信息】 四川大学 ， 光学工程（专业学位）， 2021， 硕士

【摘要】 由于介质散射效应的存在,采用传统光学成像方法很难从完全杂乱的像面散斑中直接获取成像目标的结构信息,因此在近半个世纪的时间里,各种新的散射成像原理和改善成像效果的技术被不断提出并应用到散射介质成像中。其中,散射介质光学记忆效应的发现和光学散斑统计理论的提出,极大促进了散射介质成像研究的发展。研究表明,在光学记忆效应范围内,成像系统的点扩散函数具有空间平移不变性;而根据散斑光学的统计理论可知,介质的散射效应在像面所形成散斑的功率谱密度与成像目标的自相关函数成正比。因此,当成像目标的几何尺寸位于散射介质的光学记忆效应范围内时,利用散斑自相关技术结合相位恢复算法可以对隐藏在像面散斑图样中的成像目标进行重建。但是在实际的成像系统中,特别是散射效应十分严重的情况下,噪声项的存在将会对成像目标的重建产生很大的影响。本文分析了散射介质成像过程中噪声项对散斑重建的影响,并着重讨论了对非时变噪声项的处理方法。主要工作如下:(1)详细介绍有辅助参考物和无辅助参考物两种单帧散斑自相关重建技术。有辅助参考物的散斑自相关重建技术,是在成像目标的物平面引入一已知的近似点脉冲参考物体,直接使用像面散斑的自相关运算来获取成像目标的信息,并通过数值仿真实验进行验证。不过使用有辅助参考物体的散斑自相关重建是一种侵入式成像技术,而且对于复杂的成像目标,像面散斑的自相关运算不能有效地恢复成像目标的细节部分,重建成像目标的高频信息会被噪声项淹没,因此在实际应用中有一定的局限性。而对于无辅助参考物的散斑自相关重建技术,则是根据Wiener-Khinchin定理从像面的散斑图样中获取成像目标的傅里叶幅值信息,然后通过相位迭代算法计算成像目标的傅里叶相位信息实现对成像目标的重建。无辅助参考物的散斑自相关重建技术是一种非侵入式成像技术,虽然增加了重建过程中计算的复杂性,但是,通过数值仿真与有辅助参考物的散斑自相关重建结果对比可知,其重建质量相对较好,可以有效地提高散斑相关技术的实用性。(2)针对散斑自相关重建过程中存在的特定非时变噪声项问题,提出一种基于数字离轴全息技术改善散斑自相关重建效果的方法。在成像目标的几何尺寸小于散射介质光学记忆效应范围时,利用数字离轴全息技术获取成像目标经过散射介质后的全息干涉图,通过空间光调制器使参考光束引入一次相位调制,得到另一幅全息干涉图,然后对相移前后的两幅全息干涉图进行差值运算,以消除成像过程中非时变噪声项的干扰,再通过衍射的逆运算直接重建无干扰的物面散斑光场,最后通过散斑自相关技术与相位恢复算法实现对成像目标的重建。在数值仿真实验中对比使用离轴全息去噪前后的重建效果,采用结构相似度对重建效果进行定量评估,证实所提方法能够有效地改善重建结果的质量。而且所提出方法的成像步骤简单,只需要记录两幅全息散斑图样就能够实现对噪声项的有效去除。更多还原

【Abstract】 Due to the existence of scattering effect of the medium,it is difficult to obtain the structural information of imaging target directly from the completely messy image speckles using the traditional optical imaging methods.Therefore,in the past half century,various new scattering imaging principles and improvements of imaging techniques have been continuously proposed and applied to the imaging of scattering media.Among them,the discovery of the optical memory effect of scattering medium and the proposal of the statistical theory of optical speckle have greatly promoted the development of imaging research of the scattering medium.Studies have shown that,within the range of optical memory effects,the point spread function of imaging system has spatial translation invariance;and according to the statistical theory of speckle optics,it can be known that the scattering effect of medium forms the power spectral density of speckle on the image surface and the imaging target is proportional to the autocorrelation function.Therefore,when the geometric size of imaging target is within the optical memory effect range of scattering medium,the imaging target hidden in the speckle pattern of the image surface can be reconstructed by using the speckle autocorrelation technique combined with the phase recovery algorithm.But in an actual imaging system,especially when the scattering effect is very serious,the existence of noise term will have a great influence on the reconstruction of imaging target.Therefore,this article analyzes the influence of noise items on speckle reconstruction in the imaging process of scattering media,and focuses on the treatment of time-invariant static noise items in the imaging process.The main work is as follows:(1)The principle of using single-frame speckle for imaging target reconstruction is explained in detail,and two single-frame speckle autocorrelation reconstruction techniques are introduced,namely,speckle autocorrelation reconstruction technology with and without auxiliary reference object.For the speckle autocorrelation reconstruction technology with auxiliary reference object,a known approximate point pulse reference object is introduced into the object plane of the imaging target,and the autocorrelation calculation of the image surface speckle can be used to obtain the information of the imaging target.Numerical simulation experiments are verified.However,speckle autocorrelation reconstruction using auxiliary reference objects is an invasive imaging technique,and for complex imaging targets,the autocorrelation calculation of image speckles cannot effectively restore the detailed information of the imaging target,and the high frequency information is overwhelmed by noise items and cannot achieve clear imaging,so it has certain limitations in practical applications.For speckle autocorrelation reconstruction technology without auxiliary reference objects,the Fourier amplitude information of the imaging target is obtained from the speckle pattern of the image surface according to the Wiener-Khinchin theorem,and then the Fourier amplitude information of the imaging target is calculated by the phase iterative algorithm,and the reconstruction of the imaging target is realized.Speckle autocorrelation reconstruction technology without auxiliary reference object is a non-invasive imaging technology.Although it increases the calculation complexity in the reconstruction process,the reconstruction result through numerical simulation is much better compared with the speckle auto-correlation reconstruction technology with auxiliary reference object,and effectively improve the practicability of speckle related technology in scattering medium imaging.(2)Aiming at the specific time-invariant static noise problem in the reconstruction process of speckle autocorrelation,a method based on digital off-axis holography to improve the effect of speckle autocorrelation reconstruction image is proposed.When the geometric size of imaging object is within the range of optical memory effect of the scattering medium,the digital off-axis holography technology is used to obtain the holographic interferogram of the imaging target after passing through the scattering medium;a spatial light modulator introduces the reference beam into a phase modulation to obtain another a holographic interferogram,and the obtained holographic interferogram is subjected to the difference calculation before and after the phase shift to eliminate the interference of the time-invariant noise term in the imaging process;then the inverse diffraction calculation is used to directly reconstruct the interference-free speckle light field on the object surface,And finally achieve the reconstruction of the imaging target through speckle autocorrelation technology and phase recovery algorithm.Through simulation experiments,the target reconstruction effects before and after the use of off-axis holographic denoising are compared,and the structural similarity parameter is used to quantitatively analyze the reconstruction effect,which proves that the proposed method can effectively improve the quality of reconstruction target.Moreover,the imaging steps of the proposed method are simple,and only two holographic speckle patterns need to be recorded to effectively remove noise items and achieve the purpose of rapid imaging.更多还原