A promising alternative to piezoelectricity for high frequency array applications is optical generation and detection of ultrasound. An array element is defined by the size and location of a laser beam focused onto a suitable surface. We've built a two-dimensional synthetic receive array, where a HeNe laser probes the surface displacements of a thin reflective membrane. Using a conventional transducer as the ultrasound source, images with near optimal resolution and wide fields of view have been produced at 10 - 50 MHz. We are currently exploring a different form of optical detection where the incident ultrasound modulates the thickness of an etalon (a Fabry-Perot interferometer). Preliminary experiments demonstrate improved sensitivity using a high finesse etalon. Our work in optical generation of ultrasound uses the thermoelastic effect. A major drawback to thermoelastic generation has been poor conversion efficiency. We obtained an increase in conversion efficiency of nearly 20 dB using an optical absorbing film consisting of a mixture of polydimethylsiloxane (PDMS) and carbon black. Radiation pattern measurements indicate that we have produced a 75 MHz two-dimensional array element. These results demonstrate the potential of optoacoustic arrays for high frequency ultrasound imaging.
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