利用位相恢复-波前调制反馈系统透过散射介质动态高分辨成像

In practical applications of imaging techniques, the heterogeneous medium between the target object and observer scatters the light emitted or reflected from the object and influences the imaging effect of object seriously. Thus, it is quite important to eliminate the scattering effect in the field of imaging. Recently, various kinds of techniques, such as adaptive optics, wavefront shaping, non-invasive imaging and time-reversal ultrasonic encoding, have been proposed and applied to eliminate the scattering effect and make the target object which is hidden by scattering medium visible again. However, the speed of these imaging methods is slow because of the time-consuming processes such as iterative optimization, post-processing and scanning. It means that these techniques tend to work only if the scattering medium and target object is perfectly frozen in time. Therefore, we will propose and build a kind of phase retrieval-wavefront modulation feedback system in this project to image through dynamic scattering medium in high resolution. By modulating the illuminating light quickly based on the change of dynamic scattering medium, this system produces phase-conjugate light to compensate for the scattering effect and make an image of target object even when the scattering medium and the object itself changes with time. By using this system, we plan to get the imaging speed down to millisecond-scale and achieve image with high-resolution when the object is smaller than 50 μm at the same time. In addition, this system will be used in dynamic imaging through biological tissue even when the distance between the target object and the surface of the dynamic tissue is over 2 mm.

在成像技术被实际应用的过程中，存在于待观测物体和观察者之间的非均匀介质使得物体发出或反射的光被散射，对成像效果产生严重影响。因此，消除散射介质的影响对成像技术的发展有着十分重要的意义。近年来，自适应光学，波前整形，非侵入式成像和时间反演超声编码等技术的提出和应用，能在一定程度上消除散射，使得原本被散射层遮盖的物体为观察者所见。但是这些方法由于成像过程中需要反复优化，后处理或扫描等原因，成像时间过长，无法对迅速变化的散射介质和物体起作用。因此申请人拟建立一套位相恢复-波前调制反馈系统对被散射层遮盖的物体进行动态高分辨成像。该系统能根据动态散射介质的变化迅速调制照明光，产生相应的位相共轭波抵消散射作用，在散射介质层和物体均随时间变化的情况下成像。成像时间达到毫秒级，同时分辨率小于50μm。并将此成像技术运用于对深层生物组织内的物体进行成像，动态成像深度大于2mm。

基于傅里叶光学理论，如果一个物体本身对光场没有吸收，则当用物光的共轭光照射物体的时候，物体对光场的扰动会被抵消。本项目就是基于这一原理，生成可以随物体或者散射介质变化的共轭光，从而消除物体或散射介质的散射作用。.本项目设计并制作了一套位相恢复-波前调制反馈系统，用于消除物体或散射介质对光场的扰动，进而对包裹在散射介质中的目标物体进行成像。在该反馈系统中，利用位相恢复迭代算法，根据物光场光强分布反推物光的全部信息，并计算出共轭光的复振幅分布。根据反馈的共轭光信息，利用计算全息技术生成位相型计算全息图，将计算全息图输入空间光调制器（SLM）中，使得照明光通过SLM的调制之后产生物体的共轭光，从而抵消物体对光场的扰动。利用这样一套反馈系统，数值模拟并实验实现了有选择性地消除宏观动态物体对光场的扰动，反馈系统可以在多波长照明下工作。将散射介质看成是需要被消除的物体，我们可以将此反馈系统运用于对被散射介质遮盖的目标物体进行成像。通过比较不同的位相恢复迭代算法选择合适的算法并进行优化，以减少位相恢复过程的耗时，使得在散射介质和物体发生变化的时候，该反馈系统能做出相应的反应。利用我们设计的位相恢复-波前调制反馈系统，能透过动态散射层对分辨率小于50μm的物体进行成像，成像深度大于2mm。为了优化成像效果，本项目还讨论了散射介质运动速度、目标物体运动速度、目标物体深度、散射介质颗粒大小等条件对成像效果的影响，为后续与生物成像相关的研究提供思路。