Development of a high-speed, phase-sensitive laser probe system for RF surface/bulk acoustic wave devices with an autofocus function for long time continuous operation

This paper describes the implementation of the autofocus function for the laser beam into the high-speed, phase-sensitive laser probe system for RF SAW/BAW devices. This implementation can compensate defocus caused during continuous measurements that take dozens of hours. After a brief explanation of the system used in this work, detailed discussion is given on an employed evaluation function indicating focus status, which is a key factor determining autofocus reliability. It is shown that the sum of energy of Laplacians is suitable as the evaluation function, which can be calculated by the image of the probing laser spot captured by a build-in CCD camera. Then, the implementation of the autofocus function into the current system is detailed. It is confirmed that this function can adjust the focus within almost ±20 μm defocus conditions. Finally, it is confirmed how the implemented autofocus function works effectively to keep just-in-focus under the disturbance.