Non-scanning measurements for determining in-plane mode shapes in piezoelectric devices with polished surfaces

Based on laser speckle interference, we developed a non-mechanical scanning method of visualizing the mode shapes of piezoelectric devices with polished surfaces. The best way to obtain positive proof of reliability is by comparing vibrational distribution predicted by analysis and distribution obtained experimentally. This is why a various of methods have been developed and reported for plotting the vibrational distribution of piezoelectric devices. At the 2002 international frequency control symposium, we proposed a method of in-plane mode shape visualization, for bulk and surface acoustic wave devices that combined laser speckle interferometry and two-dimensional correlation filtering. We demonstrated that very clear vibrational patterns of in-plane and out-of-plane shapes in high-frequency AT-cut quartz resonators with roughly finished surfaces could quickly be obtained by using this technique. This paper discusses improvements to our laser speckle-correlation method to measure the in-plane vibrational distributions in piezoelectric devices with polished surfaces. We applied this method to measure the fundamental thickness modes in mesa-shaped AT-cut quartz resonators with polished surfaces, and verified that the mesa structure provides a superior energy-trapping effect that anticipated from computation.