| Résumé | The optical generation and detection of ultrasound, often called laser-ultrasonics, has attracted considerable interest and presents numerous advantages for scientific and industrial applications, compared to classical piezoelectric generation and detection. These advantages follow essentially from generation without contact and at a distance by using lasers for generation and detection. Therefore, ultrasonic measurements and ultrasonic inspection on specimens at elevated temperature are readily feasible and the conventional uses of buffer rods, special bonds or momentary contact are eliminated. Such a feature is very important for industrial applications since materials like metals and ceramics are processed at elevated temperature. Also important for industrial applications is the ease of inspecting curved parts. Unlike conventional techniques which require precise transducer orientation to follow a complex surface contour, transduction of ultrasound with laser-ultrasonics is performed by the surface itself. A third limitation of conventional piezoelectric technology, unless special techniques are used, is its limited bandwidth at emission and reception. In contrast, lasers enable the production of broadband ultrasonic pulses extending from zero frequency to 50 MHz and even more, and interferometric receivers can be made with a bandwidth extending from a minimum value of 10 to 50 KHz (limited by ambient vibrations) to a maximum value given by the cutoff frequency of the detector (50, 100 MHz and above). |
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