超精密加工元件亚表面缺陷飞秒激光-超声耦合测量机理研究

This project aims to study the transient thermal and photoacoustic coupling effects in generation and propagation of ultrahigh-frequency ultrasound induced by femtosecond laser with application to imaging subsurface defects. Specifically, the mechanism of interaction between the femtosecond-laser-induced ultrasound and the subsurface defects is studied. Then the multi-modal conversion process of ultrahigh-frequency ultrasound is investigated in the presence of subsurface micro structures of ultra-precision devices. The model of the interaction between ultrahigh-frequency phonon and micro structures, micro morphology and subsurface micro defects is established. Furthermore, the process of multi-photon diffusion and asymmetric propagation caused by the micro defects is studied and modeled. The multiple material properties of the ultra-precision devices are calculated online based on different model for different defects, according to which the key parameters in the model of femtosecond-laser-induced ultrasound are updated in real-time. Second, the relationship between the information of defects and the ultrasonic echoes is studied and quantitatively established. A new measurement principle based on multilayer-recursive quadratic optimal representation is proposed. With the simulation model and huge amounts of data, the Boltzmann machine based inter-layer optimal representation is used to adaptively map the ultrasonic echoes to the sparse space of the material characteristics and defects information. Quantitative characterization of the materials and detection of subsurface defects are realized based on this optimal measurement model. Finally, a new microscopic imaging and measurement technique is proposed based on femtosecond-laser-induced ultrahigh-frequency ultrasound and applied to nondestructive detection of subsurface defects in ultra-precision devices.

本项目旨在研究飞秒激光和超声转换之间的瞬态热结构耦合物理过程，从飞秒激光超声与材料中缺陷相互作用机理入手，研究精密加工器件的亚表面微结构形貌造成的高频超声多模态转换过程，建立超高频声子与微结构、微形貌和表面、亚表面微缺陷之间的相互作用和微缺陷影响下的声子多扩散效应模型和非对称传播机制；根据不同的缺陷模型在线计算材料特性值，并作为反馈更新光声作用模型中的关键参数；在明确表面缺陷及亚表面缺陷检测机理的前提下，建立具有不同类型、大小和位置关系的缺陷信息和输出波形之间的对应关系，提出多层逐层二次最优表示的测量原理框架，在已有仿真模型和数据的指导下，采用玻尔兹曼机实现层间抽象化最优表示，自适应地实现将回波数据对应到被测材料的定量化材料特性参数和缺陷信息空间中，通过最优化模型实现对精密加工元件亚表面缺陷的定量表征和检测，形成一套全新的适用于超精密加工器件的亚表面缺陷的飞秒激光诱导超声显微测量方法。

研究超精密加工元件亚表面缺陷飞秒激光-超声耦合测量机理，其主要成果包括：“飞秒脉冲激光-瞬态热-高频超声”多物理场转换机理物理模型的建立，完成了飞秒激光的相控空间调制，搭建了法布里珀罗多普勒干涉仪的连续探测激光光路，并完成了法布里珀罗腔PID控制系统、多级放大滤波电路等多普勒振动干涉仪系统，完成了全光学检测系统的软硬件设计与集成；采用二维热栅将点光源信号辐射为阵列入射光源，灵活实现相控激光束形成的动态延时、动态聚焦和动态变孔径、变迹，实现高精度波束形成和接收，通过面激光源取代原有的线激光和点激光源并使其有效可控，有效的提高了飞秒激光的信噪比；研究了激光超声信号转换机理和光声场与亚表面缺陷的作用机理，根据全波形超声回波信号以及其时频谱特性来精确反演材料特性参数和缺陷信息（包括缺陷种类、大小、位置等）的方法，并结合了智能优化算法实现多维空间下材料多参数同时反演。实现了适用于超精密加工元件的亚表面缺陷的一体化检测。

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