Clark M, Sharples S D, Somekh M G
Department of Electrical and Electronic Engineering, University of Nottingham, University Park, Nottingham NG7 2RD, UK.
IEEE Trans Ultrason Ferroelectr Freq Control. 2000;47(1):65-74. doi: 10.1109/58.818749.
A fast, non-contact Rayleigh wave scanning microscope is demonstrated, which is capable of scan rates of up to a maximum of 1000 measurements/s with typical speeds of up to 250 measurements/s on real samples. The system uses a mode-locked, Q-switched Nd:YAG laser operating at a mode-locked frequency of 82 MHz and a Q-switch frequency of 1 kHz. The Q-switch frequency determines the upper limit of the scanning rate. The generating laser illumination is delivered and controlled by a computer-generated hologram (CGH). The generating laser produces around 30 pulses at 82 MHz and additional harmonics at 164 and 246 MHz and above. The microscope can operate at these harmonics provided the spatial bandwidth of the optics and the temporal bandwidth of the electronics are suitable. The ultrasound is detected with a specialized knife-edge detector. The microscope has been developed for imaging on isotropic materials. Despite this, the system can be used on anisotropic materials, but imaging and interpreting images can be difficult. The anisotropy and grain structure of the material can distort the Rayleigh wavefront, leading to signal loss. A model has been developed to simulate polycrystalline-anisotropic materials; this is discussed along with possible solutions that would overcome the problems associated with anisotropy. Rayleigh wave amplitude images are demonstrated on silicon nitride at 82 and 164 MHz and on polycrystalline aluminium at 82 MHz.
展示了一种快速、非接触式瑞利波扫描显微镜，其在实际样品上的扫描速率最高可达1000次测量/秒，典型速度可达250次测量/秒。该系统使用锁模、调Q的Nd:YAG激光器，锁模频率为82 MHz，调Q频率为1 kHz。调Q频率决定了扫描速率的上限。产生激光照明由计算机生成全息图（CGH）提供和控制。产生激光在82 MHz产生约30个脉冲，并在164 MHz和246 MHz及以上产生额外谐波。只要光学器件的空间带宽和电子器件的时间带宽合适，显微镜就可以在这些谐波下工作。超声用专门的刀口探测器进行检测。该显微镜是为各向同性材料成像而开发的。尽管如此，该系统也可用于各向异性材料，但成像和解释图像可能会很困难。材料的各向异性和晶粒结构会使瑞利波前畸变，导致信号损失。已开发出一个模型来模拟多晶各向异性材料；将对此进行讨论，并探讨可能克服与各向异性相关问题的解决方案。展示了在82 MHz和164 MHz下氮化硅以及在82 MHz下多晶铝上的瑞利波振幅图像。
