An optodynamic characterisation of a laser-cleaning process

摘要

Laser cleaning is an optodynamic process in which the optically induced removal of a liquid or a solid from a substrate is accompanied by a detectable acoustic signal. In our experiments we used both excimer and Q-switched Nd:YAG lasers and we examined both dry and steam laser-cleaning techniques. For the samples we used various metal, glass and marble specimens that were contaminated with particles, rust or a layer of paint. An acoustic wave, produced by the abrupt heating and detachment of the contaminants, was observed with a probe-beam deflection technique. We determined two characteristic parameters: the amplitude and the time-of-flight of the acoustic signal. The decrease of the amplitude and the velocity of propagation, which approaches sonic speed, indicate that the dynamics during the laser-cleaning process progressively weaken. According to this assumption the cleaning process is over when both parameters reach a constant value. If the irradiation with the laser pulses continues, the amplitude begins to rise again and time-of-flight decreases; and this is accompanied by a perceivable surface damage. No amplitude decrease and propagation-time prolongation were observed when the intensity of the incoming laser pulses was above the damage threshold for the particular substrate.