Optical breakdown as a broadband underwater acoustic source

Abstract

Current underwater transducers provide little support for remote measurement of material properties. Such measurements are often limited by high impedance contrast, and target inhomogeneities that arise from biofouling and corrosion. In the presence of these limitations, a sensing system would benefit from broadband, spatially-localized target excitation. The high bandwidth would allow a return signal to encode broader information content about the target, while spatial localization would provide stronger target excitation and higher spatial resolution for measurements. Although these source properties are not achievable with traditional transducer designs, they can be produced through laser-induced optical breakdown in water. During optical breakdown, focused laser light excites a localized plasma, which expands and produces a mechanical shock wave. The shock properties are set by both the medium hydrodynamics and the laser parameters, allowing the acoustic source to be optically tuned. In laboratory experiments, breakdown from a 4 mJ/ 4ns laser pulse emits an exponential pressure wave with a decay constant under 250 ns and peak pressure near 600 kPa at 2 cm range. Here, I discuss the physics and fundamental limitations of acoustic transduction via underwater optical breakdown. Experimental results illustrate how the acoustic source properties can be tuned through optical control.