| Résumé | To maximize the signal-to-noise ratio of a laser-ultrasound system, one may choose to reduce the noise by using a better interferometer(and associated laser, optics, etc.), or to increase the signal by selecting a better generation laser. Given that interferometers have reached the detection limit of photon statistics, we focus our attention on ultrasound generation. To do so, we define an optoacoustic transfer function that describes the ultrasound generation efficiency of a pulsed laser on a specific material in a predetermined frequency bandwidth. On this basis, we compare the generation efficiency of various lasers on metals in the ablation regime. It is found that, at constant fluence, shorter pulses (ps) and shorter radiation wavelengths (UV) generate ultrasound more efficiently than longer pulses (ns) and longer radiation wavelengths (IR). It is also found that the generation efficiency is inversely proportional to frequency, as it should be if the generated ultrasonic pulse is a step function. |
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