The last decades have seen the rapid development of laser ultrasonic (LU) techniques for remote inspection for composite structures. However, it is still challenging for current LU approaches to characterize the sub-mm layers in a multilayer structure. This study aims to apply the all-optical (AO) LU technique for the characterization of the sub-mm layers in a multilayer structure. A specific AOLU system is developed to acquire the transmitted longitudinal waves from the metallic composites. Following the acquisition of the transmitted signals, the spectrum analysis is utilized to overcome the ultrasonic signal aliasing in the time domain. The least-squares method is subsequently adopted to estimate the thicknesses of the layers simultaneously. This study has carried out a successful application of the AOLU technique for spectrum analysis of the sub-mm multilayer structure with the advantages of non-contact remote detection, reliable single-mode acquisition, and overcoming waveform aliasing.

在过去的几十年里，用于复合结构远程检测的激光超声 (LU) 技术得到了快速发展。然而，对于当前的 LU 方法来表征多层结构中的亚毫米层仍然具有挑战性。本研究旨在应用全光 (AO) LU 技术来表征多层结构中的亚毫米层。开发了一个特定的 AOLU 系统来从金属复合材料中获取传输的纵波。在获取传输信号之后，利用频谱分析来克服时域中的超声信号混叠。随后采用最小二乘法同时估计层的厚度。