Laser-ultrasonics uses lasers to generate and detect ultrasonic waves. It is a non-contact technique used to measure materials thickness, detect flaws and carry out materials characterization. The basic components of a laser-ultrasonic system are a generation laser, a detection laser and a detector.
Ultrasound generation by laser The generation lasers are short pulse (from tens of nanoseconds to femtoseconds) and high peak power lasers. 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 nanometres) 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 substantial contribution to the ultrasonic generation. consequently the emissivity patterns and modal content are different for the two different mechanisms. The frequency content of the generated ultrasound is partially determined by the frequency content of the laser pulses with shorter pulses giving higher frequencies. For very high frequency generation (up to 100sGHz) femtosecond lasers are used often in a pump-probe configuration with the detection system (see picosecond ultrasonics). Historically, fundamental research into the nature of laser-ultrasonics was started in 1979, by Richard J Dewhurst and Stuart B Palmer. They set up a new laboratory in the Department of Applied Physics, University of Hull. Dewhurst provided the laser-matter expertise and Palmer the ultrasound expertise. Investigations were directed towards the development of a scientific insight into physical processes converting laser-matter interaction into ultrasound. The studies were also aimed at assessing the characteristics of the ultrasound propagating from the near field into the far field. Importantly, quantitative measurements were performed between 1979 and 1982. In solids, the measurements included amplitudes of longitudinal and shear waves in absolute terms. Ultrasound generation by a laser pulse for both the thermoelastic regime and the transition to the plasma regime was examined. By comparing measurements with theoretical predictions, a description of the magnitude and direction of stresses leading to ultrasonic generation was presented for the first time. It led to the proposition that laser-generated ultrasound could be regarded as a standard acoustic source. Additionally, they showed that surface modification can sometimes be used to amplify the magnitude of ultrasonic signals. Their research also included the first quantitative studies of laser induced Rayleigh waves, which can dominate ultrasonic surface waves. In studies beyond 1982, surface waves were shown to have a potential use in non-destructive testing. One type of investigation included surface–breaking crack depth estimations in metals, using artificial cracks. Crack sizing was demonstrated, using wideband laser-ultrasonics. Findings were first reported at a Royal Society meeting in London with detailed publications elsewhere. Important features of laser ultrasonics were summarised in 1990.
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