High frequency ultrasonic imaging using optoacoustic arrays

Optical generation and detection of ultrasound is a promising alternative to piezoelectricity for high frequency arrays. We have used the thermoelastic effect to produce high frequency, broadband ultrasound in water. A pulsed diode laser, followed by an erbium doped fiber amplifier, is focused onto a light absorbing film deposited on a transparent substrate. Conversion efficiency was improved by 18 dB using a polydimethylsiloxane (PDMS) film instead of a more commonly used metallic one. Radiation pattern measurements with an 85 MHz spherically focused transducer show that our array element is 20 /spl mu/m in diameter. Measurements using our optoacoustic detector show the drawback of a glass substrate, where leaky Rayleigh waves launched from the PDMS/glass interface degrade the radiated field. Replacing the glass with a clear PDMS substrate produces a broad and smooth radiation pattern suitable for a 75 MHz array. Our work in optoacoustic detection has relied on interferometric techniques. We have previously shown a 2-D synthetic receive array producing images at 10-50 MHz with near optimal resolution and wide fields of view. We are currently exploring a more sensitive technique where the incident ultrasound modulates the thickness of an etalon. Preliminary experiments demonstrate improved sensitivity using a high finesse etalon. These results demonstrate the potential of optoacoustic generation and detection for producing high frequency ultrasound arrays.