Pulsed optoacoustic characterization of layered media

Thermoelastic waves generated by absorption of short laser pulses are used to characterize the layer structure of materials. The method is based on th e analysis of the distribution of absorbed laser energy from temporal profi les of recorded acoustic signals. Particularly in view of noninvasive medic al applications, optoacoustic front surface transducers are investigated in this study, where irradiation of the surface and detection of the acoustic wave take place on the same side of the sample. Front surface detection of optoacoustic waves is studied theoretically and experimentally, with speci al emphasis on acoustic diffraction and the differences between measurement s in the acoustic near and far field. In the experiments, samples with step wise and continuously varying depth profiles of absorption coefficient were irradiated with laser pulses of 6-8 ns duration. For the detection of the acoustic waves either an optical ultrasound sensor or an annular piezoelect ric film was used. Generating the optoacoustic waves with a flat top laser beam profile and detecting the signals in the acoustic near field yields op timal conditions for direct measurements of the distribution of absorbed en ergy and the absorption coefficient in the medium. Far field measurements a re advantageous for detecting and imaging layer boundaries at large depths in the sample. (C) 2000 American Institute of Physics. [S0021-8979(00)06015 -1].