Surface acoustic wave measurements in ultra wideband acoustic devices using a heterodyne interferometer
Résumé
Ultra wideband (UWB) acoustic transducers provide improved measurement resolution as passive sensors, and find applications in various fields like electronics and telecommunications, non-destructive evaluation, signal processing, and biomedical research. The signal strength of such devices can be controlled by changing the number of fingers (electrodes) of the inter- digital transducers (IDTs), the spacing between them, and the area of the sensor depending on the application requirements. Optical interferometry is a well-known and an effective tool for measurements using interference phenomena. The objective of this work is to obtain optical measurements of propagating short SAW pulses generated by UWB transducers.
For a given UWB acoustic device with input (chirp) and output (receiving) IDTs, electrical measurements are first performed. The amplitude and the phase information of the surface acoustic waves is measured at the position of the output IDT and serves as a reference. Optical measurements are then made by replacing the output (receiving) IDT with a heterodyne interferometer and recording the beating signal in time. It is first shown that using a heterodyne interferometer for the measurements allow us to relieve the constraints on stabilization of the optical interferometer, making measurements more robust. Second, use of a heterodyne interferometer allows us to not only measure the response at the position of the output IDT, but also at different positions in between the input and output IDTs. This allows us to investigate the changes in the propagation of surface acoustic waves within the device and especially their dispersion.