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

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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.