Laser ultrasonics

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.

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.

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.

Investigations were directed towards the development of a scientific insight into physical processes converting laser-matter interaction into ultrasound.

[5] 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.

A significant breakthrough for the application of laser ultrasonics came in 1986, when the first optical interferometer capable of reasonable detection sensitivity on rough industrial surfaces was demonstrated.

Monchalin et al.[14][15] at the National Research Council of Canada in Boucherville showed that a Fabry–Pérot interferometer system could assess optical speckle returning from rough surfaces.

Some techniques (notably conventional Fabry–Pérot detectors) require high frequency stability and this usually implies long coherence length.

For instance, photorefractive crystals and four wave mixing are used in an interferometer to compensate for the effects of surface roughness.

At high frequencies (say >1 GHz), other mechanisms may come into play (for instance modulation of the sample refractive index with stress).

Under ideal circumstances most detection techniques can be considered theoretically as interferometers and, as such, their ultimate sensitivities are all roughly equal.

However, the shot noise limited SNR is proportional to the square root of the total detection power.

The use of a high power laser, with consequent vaporization of the material, is the optimal way to obtain an ultrasonic response from the object.

However, to fall within the scope of non-destructive measurements, it is preferred to avoid this phenomenon by using low power lasers.

[20] Optical generation and detection of ultrasound offers scanning techniques to produce ultrasonic images known as B- and C-scans, and for TOFD (time-of-flight-diffraction) studies.