Imaging of electrostatic electric fields of SAW structures using electro-optic effects

The design and analysis of SAW interdigital transducers (IDTs) requires the accurate translation of transducer weighting information to the IDT structural geometry. The electrostatic electric field is the dominant excitation mechanism of the IDT for the generation of surface waves. Measurement of the actual charge distribution and wave generation sites enables the verification of the implemented impulse response weighting. To date, the measurement of the wave generation and propagation requires elaborate laser probing stations. This paper investigates a method to obtain images of the field distribution between the electrodes of an interdigital transducer in a SAW device. This method is based on the birefringence effects found in piezoelectric materials. Using an optical microscope with appropriate linear polarization of a non-coherent visible light source, the electric field induced by the applied DC terminal voltage may be viewed. The technique is based on the rotation of polarization of the transmitted light and resulting modulation of the light intensity, which occurs in the presence of an applied electric field. The visible modulated light intensity is a mapping of the tangential electric field between the IDT electrodes. The results of the measurement can be used to identify the SAW structure´s radiation locations and relative strengths for accurate modeling of the device response. Optical measurement of the electric field strength for a SAW split electrode transducer on YZ-LiNbO₃ is presented and compared to theoretical predictions