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 μm 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.