Towards the real-time measurement of Ultrasound Fields by combining Schlieren Tomography and Wavefront Sensing

Abstract

Recent advances in ultrasound generation such as ultrasonic holography and acoustic tweezers require methods for the fast characterization of pressure fields. Typically, this can be achieved by using a hydrophone, but the measurement of the three-dimensional (3D) pressure distribution in a few cm region is extremely time-consuming, with typical times ranging from hours to even days. Alternatively, visual methods like Schlieren techniques offer a rapid assessment of the pressure field, but they remain largely qualitative. In this work, we combine Schlieren tomography with wavefront sensing to fill this void and quantitatively reconstruct 3D ultrasonic fields within seconds. Our method is based on the simultaneous acquisition of intensity images with a Schlieren setup and phase maps with a Wavefront Sensor. Because optical phase differences are related to changes in refractive index in the medium and, at the same time, to changes in pressure, we can convert phase values into pressure maps. By feeding this information into the Schlieren sinograms, we obtain quasi-real-time 3D pressure fields with sub-millimetric resolution. This new optical method is a significant step forward toward the real-time and precise characterization of ultrasound