Optical ultrasound generators and detectors

Ultrasonic-laser instruments can use short-pulse lasers (a few nanoseconds long) to generate ultrasound and long-pulse (tens of microseconds long) or continuous lasers, coupled to an optical interferometer for the detection of US (I.e. the corresponding mechanical displacements) [22]. 

Unfortunately, laser-ultrasonics is inadequately sensitive for some material and prooess oombinations. Averaging signals compensates for this laok of sensitivity at the expenses of inoreased inspection times and decreased savings. 

The detectors oommonly used in laser ultrasonics include various types of interferometers, which are sensitive to the displacement or velocity of the surface, and of knife-edge e.g. photoelectromotive force reoeivers) detectors, which are sensitive to the tilt of the sample surface [27]. 

Worthy of speoial mention is the use of laser for phonon detection, as in Brillouin speotrosoopy which is especially suitable for deteoting surface acoustic waves (SAWs). Generalized SAWs can be used to obtain Brillouin speotra with a 5-pass Fabry-Perot interferometer [29] on the other hand, pseudo-SAWs, which are weaker than the previous ones, require a tandem (3 -r 3)-pass Fabry-Perot interferometer system [30]. 

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