The fabrication of piezoelectric ultrasound arrays which can directly deliver spatially two-dimensional images, remains an unsolved problem in spite of tremendous progress in the fabrication technology of ultrasound transducers. For this reason, in PTB's Ultrasonics and Medicine Working Group an optical alternative technique has been developed which enables the determination of spatially two-dimensional sound pressure distributions of ultrasound fields in water. It thereby utilizes a dielectric optical layer as sensitive element, the reflectivity of which is changed through the incident sound. With the aid of a scanned laser beam the sound pressure can be probed successively. This serial technique is supplemented by parallel probing. Here, the whole sensor is illuminated simultaneously and the reflection change is detected with the aid of a CCD camera. In this way, instantaneous images of the sound pressure can be obtained in two dimensions. A spatial resolution of under 100 µm and a bandwidth of up to 50 MHz are attained, which has not yet been possible using conventional techniques. An advantage over a scanner-hydrophone measurement setup also lies in the fact that practically no mechanically moved parts are used. Moreover, optical measuring methods are not susceptible to electromagnetic radiation.
The optical multilayer hydrophone has so far been employed for the investigation of short ultrasound pulses in the MHz range. Such pulses are utilized e.g. in medical diagnostic devices - which one will find nowadays in every doctor's practice, instruments and systems for material testing and ultrasound process controls in the broadest sense. The video, which you will find on this page, demonstrates this. Shown in colour is a representation of the sound pressure in dependance on time (red: high pressure, blue: low pressure) in a sectional plane vertical to the direction of sound propagation. Single oscillations and the spatial focussing effect of the transducer are distinctly visible. Sound field measurements are needed for purposes of research and development, for the specification of technical data, for declaration and approval and for quality assurance. Here, the new technology can be employed to advantage. Furthermore, ultrasound pulses find application in many fields of research. For special fields, e.g. in domains with high electromagnetic fields, such as MRT devices or transformers, an optical ultrasound reception technique is especially suitable.
