Ultrafast ultrasonic techniques using picosecond/femtosecond lasers show promise for the nondestructive evaluation at fine spatial resolutions because the induced ultrasonic waves have an extremely high frequency range, from GHz to THz. However, most existing applications are based on the linear feature variation of the measured ultrafast ultrasonic waves; this is limited by the linearity assumptions in lock-in detection schemes adopted for ultrafast ultrasonic measurement. This study proposes a technique to measure ultrafast nonlinear ultrasonic waves using a femtosecond laser and a modified lock-in detection scheme. The advantages of the proposed technique are as follows. (1) The developed technique can measure nonlinear ultrasonic waves induced by a femtosecond laser; (2) linear and nonlinear ultrasonic responses can be decomposed and measured using a modified lock-in detection scheme; and (3) the decomposed nonlinear ultrasonic waves can be used for effective micro defect detection and microstructure characterization at sub-micrometer scale. The proposed technique was used to successfully perform validation tests on micro crack detection in a silicon wafer and microstructure characterization of an additively manufactured Ti-6Al-4V sample.

使用皮秒/飞秒激光器的超快超声技术显示出在精细空间分辨率下进行无损评估的前景，因为感应超声波具有从 GHz 到 THz 的极高频率范围。然而，现有的大多数应用都是基于被测超快超声波的线性特征变化；这受到超快超声波测量采用的锁定检测方案中的线性假设的限制。本研究提出了一种使用飞秒激光器和改进的锁定检测方案来测量超快非线性超声波的技术。所提出的技术的优点如下。(1) 所开发的技术可以测量由飞秒激光引起的非线性超声波；(2) 线性和非线性超声响应可以使用改进的锁定检测方案进行分解和测量；(3) 分解后的非线性超声波可用于亚微米尺度的有效微缺陷检测和微观结构表征。所提出的技术用于成功地对硅晶片中的微裂纹检测和增材制造的 Ti-6Al-4V 样品的微观结构表征进行验证测试。