基于近场微波测振的频域激光超声雷达研究

Abstract: Laser ultrasound (LU) is an important noncontact and nondestructive testing (NDT) technique and potentially useful in the analysis of bulky/surface defects, elastic integrity of industrial components and biomedical features. Based on commercial continuous-wave laser diode, the modality of harmonically modulated LU generation shares the features of convenience, flexibility, and low optical power requirement and has become a valuable research topic in NDT. Practically, LU signal generated from single frequency modulation unfavorably suffers the shortage of frequency band and the difficulty of quantification of physical properties. The extremely low optic-to-thermoelastic conversion coefficient makes the detection of LU signal even harder. In this proposal, general radar technique that involves pulse compression and match filtering are applied in our new LU system to sufficiently broaden the signal band-width. Instead of lock-in demodulation, radar technique cross-correlates received signal with the reference and retrieves time-domain LU signal sequence without using pulse laser. In the detection of laser-induced ultrasonic vibration, a near-field microwave microscopic (NFMM) configuration is proposed. With ultra-high spatial resolution, NFMM is supposed to be used as a displacement sensor to probe the subtle vibration with very high accuracy. Dimension of the near-field microwave probe needs to be optimized with the help of finite element method. The electromagnetic-acoustic interaction will be analyzed based on a near-field radiation theory combined with elastic wave model, and tested with designed experiments. The proposed interdisciplinary research emphasizes on the combination of radar signal processing, microwave, and photoacoustic NDT, improves the performance of frequency domain LU by enhancing the detectability and signal-to-noise ratio of the system.

激光超声作为一种重要的非接触无损检测技术，在弹性测量、缺陷检测、表面特性分析和生物医学成像中均发挥着重要作用。基于连续激光器频率调制的超声激发模式以其简便、灵活、所需光能量低、信号易探测等优势，成为激光超声技术的重要研究方向。然而实践中采用的谐波调制激发模式信号带宽小，定量分析困难，且对激光激励微小超声振动的检测难点突出。针对以上问题，本项目结合雷达技术中脉冲压缩和匹配滤波信号调制解调方法，将单频调制锁相检测拓展为连续频率扫描互相关时域反演模式，在避免使用脉冲源的前提下扩展带宽，实现时域测量。在超声探测方面，研究基于近场微波的微振动高灵敏检测方法，建立弹性振动与电磁波的相互作用理论，设计并优化天线结构及探测方案，结合实验开展检测机理研究和材料参数的定量分析。通过将通信领域的信号处理方法与电磁波测量技术引入无损检测，开展光电声交叉学科研究，探索新型高性能激光超声检测方法。

激光超声技术在工业无损检测领域具有重要的潜在应用价值。在激光超声检测实践中，受制于脉冲激光能量与脉宽和微弱超声探测之间的固有矛盾，难以实现高超声频率和高空间分辨率激光超声检测。为攻克难题，本项目结合雷达技术中脉冲压缩和匹配滤波信号调制解调方法，并引入近场微波探测微弱超声信号，在避免使用脉冲源的前提下扩展带宽，丰富对激光诱导超声高精度表征的技术内涵。.研究内容：开展光声线性频率调制理论建模，建立热弹定量解析模型，实现对激光超声雷达信号解析；完成对0.1MHz亚微米量级超声振动的压电换能器建模仿真，获得换能器模态与定量测量参考；利用水浸式超声换能器建立激光超声雷达系统，对深处孔型缺陷开展扫描实验；建立腔微扰理论，分析金属材料表面超声振动对近域电磁场的定量影响及场近似条件；开展高频共轴谐振腔探针设计，对探针尺寸和电磁场模式进行优化，在实验室中制备探针并验证其电磁性能；搭建近场扫描系统，开展对幅度、频率可控的压电换能器超声振动探测实验，获得扫描图像。.重要成果：理论研究方面，建立了一维与三维复合定量光-热-声多场耦合模型，定量解释了激光诱导“超声雷达波“在金属材料中的传播特征，并通过匹配滤波运算反演出时域超声回波信号，应用于缺陷回波检测的定量分析；通过腔微扰和准静态分析方法建立近场微波谐振腔共振频率与超声振动的定量关系，验证线性近似模型；基于有限元分析法验证了上述理论模型的正确性和可用性。实验层面，基于激光超声雷达实验系统，通过B扫描实现对5-20mm深度缺陷的回波测量，定位精度达到40um，与商用接触式超声检测方法测量精度相当；在近场微波扫描测振实验中，制备了18GHz近场探针，在Ku波段实现对0.02~0.2MHz超声振动的非接触精确检测，探测灵敏度达到0.1um，与光学测振技术相当。.项目贡献：实现激光超声雷达技术的检测带宽与信噪比动态调节，验证了其与商用接触式超声相当的探测能力，验证近场微波技术用于亚微米幅度超声的测量，另辟蹊径实验激光诱导超声探测。