基于飞秒时间门选通信号光的瞬态显微成像技术

Femtosecond time-gated imaging technique with optic Kerr gate has a high temporal-spatial resolution and the ability of eliminating the diffusive photons, so it has very important significantances for the observations of the dynamics of the targets embedded in turbid media in the studies of physics and materials. Up to now, however, the spatial resolution for this technique is only dozens of microns, and can not be applied to the observations of the formation dynamics of the microstructures in the femtosecond laser micromachining. Therefore, in this project we propose a microscopic imaging technique based on optical Kerr gate, in which the image-bearing photons can be selected efficiently and the diffusive photons can be eliminated, in order to extend the ability of the femtosecond time-gated imaging technique to observing the dynamic behaviors of the targets inside the microdomain of turbid media.The study includes: 1) Eliminating the influence of coherent effect and improving the signal-to-noise ratio by controlling the competition between the coherent effect and optical Kerr effect in femtosecond optic Kerr gate experiments; 2）Improving the signal-to-noise ratio of the images by combining the time-gated imaging technique and spatial frequency filtering technique; 3)Investigate the temporal and spatial propagation properties of femtosecond pulses in the turbid media. The final goal is to developing the microscopic imaging system based on the optical Kerr gate which is suitable for observating the dynamic behaviors of the targets inside the microdomain of turbid media. The temporal and spatial resolutions of the iamging system are smaller than 200 fs and smaller than 10 μm, respectively, and the maximum optical depth is higher than 12.

飞秒时间门选通成像技术具有很高的时间分辨率和极强的抑制散射光的能力，因而适于观测高密度散射介质内目标物体或结构的动态过程，在物理和材料科学领域中具有重要的应用价值。然而，目前该技术成像空间分辨率仅为数十微米量级，无法测量飞秒激光烧蚀靶材等微区内微结构改变的动态过程。为此，本项目提出一种基于飞秒时间门选通信号光、抑制散射光的显微成像技术，使飞秒时间门选通成像技术的应用范围扩展至能够观测微区内超快动力学过程。主要研究内容包括：通过调控飞秒时域光克尔门中相干效应与克尔效应之间的竞争，抑制相干效应对信号光的干扰，提高成像信噪比；通过将时间门选通与空间频谱滤波有机结合，提高成像信噪比；研究飞秒脉冲在散射介质中传输的时空演化特性；研制适于散射介质微区内动态目标观测的飞秒时间门选通显微成像系统，成像系统时间分辨率优于200fs，空间分辨率优于10μm，散射介质最大光学密度大于12。

飞秒时间门选通成像技术具有很高的时间分辨率和极强的抑制散射光的能力，因而在观测高密度散射介质内目标物体或结构的动态过程等物理和材料科学领域具有重要的应用价值。本项目主要研究基于飞秒光克尔门选通显微成像技术。建立了飞秒光克尔门选通弹道光成像技术研究平台，提出并研究了飞秒外差光克尔门选通成像和差分光克尔门选通成像两种改进技术，解决了泵浦脉冲在光克尔介质中形成瞬态光阑具有低通滤波影响，导致光克尔门选通成像的图像边缘模糊，锐度下降的问题，我们研究了外差角度对飞秒外差光克尔门选通成像质量的影响，提出最优外差角度的概念。实验表明，最优外差角会随着散射体光学深度的增加而改变。开展了光克尔门选通显微成像技术研究，提出并研究了差分光克尔门选通显微成像和长工作距离光克尔门选通显微成像方法，使用这种技术可以大大提高图像对比度以及分辨率。研究结果表明在25cm的工作距离下，成像系统空间分辨率7μm，其综合性能优于以往研究结果。针对散射环境中目标物体激光成像时，存在的激光散斑干扰，提出并研究了飞秒激光诱导超连续谱辅助光克尔门选通成像和飞秒激光诱导超连续谱直接成像两种抑制散斑的成像技术。当采用飞秒激光诱导超连续谱辅助的光克尔门选通成像时，探测光的相干性大大降低，有效地抑制单脉冲光克尔门选通成像中散斑，提升成像信噪比和辨识度。实现了时间分辨率优于200fs、空间分辨率优于5μm、光学深度大于12的飞秒时间门选通显微成像。本项目共发表的标注本项目资助的SCI收录论文12篇，研究成果有望应用于燃料喷雾场微结构的成像诊断等方面。