Wideband performance of the optically scanned hydrophone

A report is presented on recent advances in the performance of the optically scanned hydrophone (OSH). The OSH achieves quasi-real-time images of ultrashort acoustic pulses in water, typically across an aperture 10³ λ² in area. Accurate temporal waveforms of acoustic pulses with a center frequency around 20 MHz and pulse length as short as 50 ns can be achieved. The measured performance of the OSH is compared with that of the spot-poled hydrophone and interferometric systems for recording very short acoustic pulses in water. It is shown that the OSH offers a better overall performance which can be further extended by improvements to the design of the piezoelectric-semiconductor sandwich. The main factor limiting the spatial resolution and the sensitivity of the OSH is the finite thickness of the silicon photoconductive layer. By incorporating a novel silicon structure, the range of operation of the OSH can be extended above 30 MHz, providing a powerful diagnostic and calibrating tool for medical ultrasound transducers using very-wideband acoustic pulses