Two-dimensional recording of optoacoustic waves

An optical method is used to measure optoacoustic waves that are generated by irradiating absorbing targets with nanosecond-laserpulses. Pressure-induced changes of optical reflectance on the surface of a glass prism are used to capture the stress distribution in a plane at a certain time with a gated video camera. The temporal resolution is limited by the exposure time of the camera, which was 5 ns in our experiments. With this technique we imaged optoacoustic waves that were generated in an absorbing liquid in front of an optical fiber tip after transmission of pulses from an optical parametric oscillator (OPO, 6 ns pulse duration). The absolute pressure amplitudes and the temporal development of positive and negative stress in the detector plane could be obtained from the images. In a second series of experiments crossed hairs were irradiated with laser pulses passing through the detector plane. The optoacoustic waves traveling back in opposite direction of the laser radiation were recorded. These measurements yielded absolute pressure values in the detector plane and the location of the absorbing targets. Two-dimensional recording of acoustic waves can be used for the analysis of optoacoustic emission from small absorbing structures and for imaging of buried absorbers in tissue.