to 10 MHz. In practice the choice of technique is often determined by the physical optics and the sample (surface) condition. Many techniques fail to work well on rough surfaces (e.g. simple interferometers) and there are many different schemes to overcome this problem. For instance, photorefractive crystals and four wave mixing are used in an interferometer to compensate for the effects of surface roughness. These techniques are usually expensive in terms of monetary cost and in terms of light budget (thus requiring more laser power to achieve the same signal to noise under ideal conditions). At low to moderate frequencies (say < 1 GHz), the mechanism for detection is the movement of the surface of the sample. 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. This is because, in all these techniques, interferometry is used to linearize the detection transfer function and when linearized, maximum sensitivity is achieved. Under these conditions, photon shot noise dominates the sensitivity and this is fundamental to all the optical detection techniques. However, the ultimate limit is determined by the phonon shot noise. Since the phonon frequency is many orders of magnitude lower than the photon frequency, the ultimate sensitivity of ultrasonic detection can be much higher. The usual method for increasing the sensitivity of optical detection is to use more optical power. However, the shot noise limited SNR is proportional to the square root of the total detection power. Thus, increasing optical power has limited effect, and damaging power levels are easily reached before achieving an adequate SNR. Consequently, optical detection frequent has lower SNR than non-optical contacting techniques. Optical generation (at least in the firmly thermodynamic regime) is proportional to the optical power used and it is generally more efficient to improve the generation rather than the detection (again the limit is the damage threshold). Techniques like CHOTs (cheap optical transducers) can overcome the limit of optical detection sensitivity by passively amplifying the amplitude of vibration before optical detection and can result in an increase in sensitivity by several orders of magnitude. Ultrasonic laser technique operation The "Laser Ultrasonic" technique is part of those measurement techniques known as "non-destructive techniques or NDT", that is, methods which do