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The acoustic wave field generated in front of a submerged fiber tip by short laser pulses is theoretically and experimentally studied by fast imaging and optical pressure measurements. It is shown that the finite size of the fiber causes strong tensile stress leading to cavitation. Depending on the absorption coefficient of the laser radiation, cavitation-induced bubble formation occurs inside (low absorption) or outside (high absorption) the volume of heat deposition. The results are used to characterize the cavitation bubble formation mechanisms and to predict possible consequences for applications of fiber-guided short-pulsed laser sources in medicine.

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Thermoelastic waves generated by absorption of short laser pulses are used to characterize the layer structure of materials. The method is based on the analysis of the distribution of absorbed laser energy from temporal profiles of recorded acoustic signals. Particularly in view of noninvasive medical 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 special emphasis on acoustic diffraction and the differences between measurements in the acoustic near and far field. In the experiments, samples with stepwise 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 piezoelectric film was used. Generating the optoacoustic waves with a flat top laser beam profile and detecting the signals in the acoustic near field yields optimal conditions for direct measurements of the distribution of absorbed energy and the absorption coefficient in the medium. Far field measurements are advantageous for detecting and imaging layer boundaries at large depths in the sample.

Two-dimensional detection of ultrasonic waves is based on pressure-induced changes of optical reflectance at a glass–liquid interface, imaged with a time-gated video camera. The method is used to record optoacoustic waves generated after irradiation of optically absorbing targets with 6 ns long laser pulses. Measurements of absolute pressure values with high temporal and spatial resolution (in the range of 10 ns and 10 μm, respectively) is demonstrated. The sensitivity is varied between 0.19% and 0.81% gray level modulation per bar. The detector plane is optically transparent, making it possible to irradiate the sample through the detector without disturbing the acoustic measurement. Two-dimensional recording of ultrasonic waves is ideally suited for the analysis of acoustic emission from small sources and for optoacoustic imaging of optical absorption differences in an opaque material.