Scanning double-beam laser interferometer for area measurement of thickness piezoelectric coefficient
SKALSKÝ, M. FIALKA, J. HAVRÁNEK, Z.
Double-beam laser interferometer (DBLI) facilitating surface scanning with a very high resolution and active compensation of the optical path-length drift. Compared to previous solutions utilizing a piezo-driven mirror to introduce the desired phase shift, we employ a fast electro-optical phase modulator (EOM) having flat frequency response up to several MHz. Instead of direct measurement of the phase shift introduced by the sample displacement, we use the EOM to feed this signal back to nullify the total phase shift. The amplitude detected by the lock-in amplifier thus serves as an error signal maintained at the lowest possible level by a PI controller. Therefore, the signal induced by the sample displacement and the noise background can be effectively distinguished, decreasing residual noise down to 0.01 pm. Thanks to this closed-loop measurement technique, the output signal, proportional to the feedback one, is independent from laser power as well as sample reflectivity, and the instrument may be used for scanning. Moreover, we use another feedback to stabilize the working point at π/2 difference, providing the highest sensitivity. We thus considerably suppress the influence of spurious environmental effects, which is considered a substantial disadvantage of DBLI-based methods. DBLI can be used for scanning the sample thickness displacement along the sample surface. A resolution better than 0.1 pm within the bandwidth of over 1 MHz can be achieved.
piezoelectric film characterization, thickness piezoelectric coefficient measurement, laser scanning interferometry, double-beam
Vysoké učení technické v Brně, CEITEC VUT Laboratoř pokročilých senzorů, B1.04 Purkyňova 656/123 612 00 Brno