Available online at www.sciencedirect.com Optics & Laser Technology 36 (2004) 125–130 www.elsevier.com/locate/optlastec Preliminarystudiesofmaterialsurfacecleaningwithamulti-pulse passivelyQ-switchedNd:YAGlaser ConstantinFenic * ,RazvanDabu,AurelStratan,ConstantinBlanaru, ConstantinUngureanu,CatalinLuculescu Department of Plasma and Radiation Physics, National Institute for Lasers, Atomistilor 111, P.O. Box MG 36, Bucharest 76900, Romania Received 18 February 2003; accepted 14 July 2003 Abstract A multi-pulse Q-switched (MPQ) Nd:YAG laser system for surface material cleaning has been developed. Trains of pulses having total output energy of as much as 1:05 J, 10–30 pulses/train, 20–70 ns individual pulse-width, and about 100 s whole duration were generated. Ablation threshold uence and ablation rate for limestone substrate have been measured and compared with the theoretical calculations. Safe surface cleaning of limestone samples was demonstrated thanks to the high gap between the ablation threshold uence of the substrate and black crust. Tests of surface cleaning for samples of stone, ceramics, and metal were performed. ? 2003 Elsevier Ltd. All rights reserved. Keywords: Multi-pulse passively Q-switched laser; Surface cleaning; Ablation rate 1. Introduction TheapplicationofthepulsedNd:YAGlasersinmaterial surfacecleaning,particularlyforartworksrestoration,iswell known.ThreemodesofoperationofNd:YAGlaserdevices werededicatedtotheseaims:theQ-switchedmode,thefree running(FR)modeandtheshortfreerunning(SFR)mode. Usually, an electro-optically Q-switched laser generates a single nanosecond pulse (100–400 mJ pulse energy, 5– 10nspulseduration)foreachpumpingshot[1,2].Duetoa veryhighandwell-denedlevelofthepulsepowerdensity, the Q-switched technique provides a laser ablation of high precision and selectivity, without signicant thermal-like damage of the substrate. The main drawbacks are the me- chanicalstressinducedinthesurfacelayersofthesubstrate and the relatively slow ablation rate. In addition, the coup- ling of single Q-switched pulsed laser radiation through optical bers is very dicult because of the high intensity reachedattheinputfaceofthebers.Bundlesofberswere usedtopreventthisexceedingoftheberdamagethreshold [3].Vacuumcouplingisnecessarytoavoidthebreak-down eect if the laser radiation is focused to get the required ∗ Corresponding author. Tel./fax: 40-21-4574467. E-mail address: fenic@in.nipne.ro (C. Fenic). numericalapertureoftheincidentbeamontheinputfaceof the ber. IntheFRmodeofoperation,thelasergeneratesanirregu- lartrainofspikes(100–200 stypicaltrainduration)witha signicantthermalstressforthematerial.Toreducethelevel of thermal-like damages induced in the substrate, the over- all spiking duration was limited to about 20 sintheSFR modeoflaseroperation[4,5].Thisworkingregimecanpro- vide a higher ablation rate, as compared to the Q-switched technique, without mechanical damage of the substrate, but acertainthermal-likedamageeectforsubstrateispossible to result. To reduce the mentioned drawbacks, we propose a “multi-pulse Q-switched” (MPQ) Nd:YAG laser to be used for material surface cleaning. In the MPQ mode of operation, the laser generates a train of pulses with tempo- ral structure determined by the optical pumping level and the initial transmission of a saturable absorber (Cr 4+ :YAG crystal). It is characterized by a high level of intensity modulation. The relative short duration of the MPQ pulses (20–70 ns), combined with a large time separation (4– 10 s) between adjacent pulses in the train, signicantly reducethelevelofthermaldamageofthesubstrateincom- parison with free-running lasers. It is also expected to gen- erate less mechanical stress compared to the single-pulse Q-switched lasers. Unlike the single-pulse Q-switched lasers, the radiation emitted by MPQ lasers could be easier 0030-3992/$-see front matter ? 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0030-3992(03)00144-0