Focus adjustment system for a fast-scanning and phase-sensitive laser probe for radio frequency surface and bulk acoustic wave devices

This paper describes a focus adjustment system particularly designed for the fast-mechanical-scanning and phase-sensitive laser probe for radio frequency (RF) surface and bulk acoustic wave (SAW/BAW) devices. When high spatial resolution is necessary for the observation, one needs an objective lens of large magnifying power with extremely shallow focal depth. Then a tiny inclination of a measurement device may cause severe defocus resulting in blurred images. We installed the focus adjustment system in the laser probe, and showed that even there is the inclination, high quality information of the wave field can be acquired without slowing down the scanning speed. The laser probe with the focus adjustment function is applied to the characterization of RF SAW devices operated in 2 GHz range. It is shown the spatial resolution better than 0.5 mum is obtainable.