Laser ultrasonics

Laser-ultrasonics uses lasers to generate and detect ultrasonic waves. It is a non-contact technique used to measure materials thickness, detect flaws and materials characterisation. The basic components of a laser-ultrasonic system are a generation laser, a detection laser and a detector.

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Ultrasound Generation by Laser

The generation lasers are short pulse (from tens of nanoseconds to femtoseconds) and high peak power lasers. Most common lasers used for ultrasound generation are solid state Q-Switched Nd:YAG and gas lasers (CO₂ or Excimers). The physical principle is of thermal expansion (also called thermoelastic regime) or ablation. In the thermoelastic regime the ultrasound is generated by the sudden thermal expansion due to the heating of a tiny surface of the material by the laser pulse. If the laser power is sufficient to heat the surface above the material boiling point, some material is evaporated (typically some nanometers) and ultrasound is generated by the recoil effect of the expanding material evaporated. In the ablation regime, a plasma is often formed above the material surface and its expansion can make a substation contribution to the ultrasonic generation.

Ultrasound Detection by Laser

Detection lasers are continuous or long pulse (typically of tens of microseconds) and with long coherence length. The detection laser light reflected (or scattered) by the material surface is perturbed (e.g. Doppler effect) by the arrival of ultrasonic waves. To detect this perturbation, interferometric techniques are usually used. Most industrial laser-ultrasonics applications uses confocal Fabry-Perot or photorefractive interferometers to demodulate the detection light.

Industrial Applications

Well established applications of laser-ultrasonics are composite inspections for the aerospace industry and on-line hot tube thickness measurements for the metallurgical industry.

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