Coherent Fiber-Optic Method for Spatially-Resolved, Time-Dependent, Picometer-Scale Displacement Amplitude Measurements of Surface Acoustic Waves

Surface acoustic wave (SAW) devices are widely used in communications, sensing and quantum systems due to their small footprint and high sensitivity. Better understanding of SAW propagation theory and loss mechanisms could benefit device design for highly efficient SAW sensors. Here, we propose a technique for absoluteamplitude measurements of vertical SAW displacement, using a fiber point-sensor through optical-acoustic wave interaction. This is achieved by a curved Fabry-Pérot interferometer formed by a single-mode fiber endface and the curved SAW surface. A theoretical model is implemented to relate the sensor response to SAW amplitude and to eliminate the need for focusing lenses, all while maintaining high spatial resolution. The 4x larger SAW wavelength compared to the fiber endface dimension and the high optical sampling speed, ensure coherent optical and acoustic wave interaction. Results show amplitudes on the order of a few picometers for a lithium niobate-based SAW at 100 MHz, consistent with theoretical expectations and experimental validation. Through scanning the sensor in the horizontal direction, we can image the quasi-standing waves formed in the SAW cavity. The proposed setup is simpler, more compact, non-contact, applicable to more substrate materials compared to previous approaches, and can provide insights to improve SAW device design to enhance performance. It opens a new door for applications such as material surface defect detection, bio-sample concentration sensing, or coherent coupling to quantum systems.