Visualization of Propagation of Surface Acoustic Waves in GHz Range by Laser Probing

声表面波(SAW)器件以其体积小、性能高等优点在目前的移动通信系统中得到了广泛的应用。本研究探讨了利用激光探针法对声表面波在GHz范围内的传播进行可视化，并将其应用于高性能声表面波器件的诊断工具。首先，搭建了激光探测系统。为了快速、准确地测量声表面波的幅度和相位，我们采用了所谓的刀口法。对于GHz频段的工作，我们采用棱镜镜和高速光电二极管相结合的方式来代替刀口或位置敏感光电二极管。信号检测采用相干检波。经测试，该系统具有高灵敏度、宽频带、大动态范围大于60dB的特点。该系统在精确测量相位响应、对地面振动不敏感等方面比基于迈克尔逊干涉仪的系统更具吸引力。利用相位和幅度信息，声表面波的传播可以在波数域上表征为空间分布的傅里叶变换(谱)。光谱分析用于提取各种属性。例如，可以识别和分离各种共振模式，并且可以通过评估场分布函数来补偿激光反射系数的变化。此外，通过构造SAW动画电影，可以很容易地理解SAW的传播。这些结果清楚地表明了激光探针法作为高性能声表面波器件诊断工具的有效性。

Surface-Acoustic-Wave (SAW) devices are widely used in present mobile communication systems because of their compactness and high performances. This research discussed use of the laser probe method for the visualization of the SAW propagation in GHz range and its applicability to the diagnosis tool for high performance SAW devices. First, the laser probing system was constructed. For fast and precise measurement for both SAW amplitude and phase, we employed so called the knife edge method. For the operation in GHz range, we use the combination of prism mirror and high-speed photo diode instead of the knife-edge or the position-sensitive photo diode. The coherent detection was employed for the signal detection. Through the measurement, it was shown that the system is highly sensitive, wideband and large dynamic range more than 60 dB. The present system is more attractive than the system based on the Michelson interferometer from various aspects, such as precise measurement the phase response, insensitivity to ground vibration, etc. Then the extraction of SAW properties from the measured data was also investigated. By the use of phase and amplitude information, the SAW propagation can be characterized in the wavenumber domain as the Fourier transform (spectrum) of the spatial distribution. The spectral analysis was applied to extract various properties. For example, various resonance modes can be identified and separated, and variation of the laser reflection coefficient can be compensated through the evaluation of the field distribution function. Furthermore, it was shown that the SAW propagation can be easily understandable by constructing its animation movie. These results clearly shows effectiveness of the laser probe method as the diagnosis tool for high performance SAW devices.