| To investigate the properties of piezoelectric thin films,it is often required to measure the electric field-induced displacements in them. The displacements lie in the A range due to the low driving voltage. The driving voltage can't be too high because of the small thickness of the films. Too high voltage may destroy them. Interferometric technique must be applied to measure so low displacements. Michelson interferometer has a simple structure,and is applied earliest. But the bending effect of the substrate can't be avoided because only one surface of the sample is monitored in this method.So a double beam laser interferometer based on Mach-Zender structure is built up to study the piezoelectric responses of piezoelectric thin films. The bending effect of the substrate is eliminated effectively because both major surfaces of the sample are detected by the probing laser beam in this method. To compensate for low-frequency drift of the optical path-length difference and to keep the operation point on the most sensitive point,the reference mirror is adjusted with a actuator driven by a feedback loop. A lock-in amplifier is used to supply the driving signal and to detect the output signal corresponding to the sample's strain. A computer controls the whole process of measurements by communicating with the lock-in amplifier via RS232 interface. And programs are developed for piezoelectric coefficient measurement and butterfly curve measurement. So the system is an automatic test system,with operation frequencies between 1KHz and 102KHz,and the minimum displacement that can be detected is as small as 0.3pm. |
