Coloured marking inside glass by laser radiation

Laser labelling inside glass induces micro-cracks by high energy densities in the focus. The micro-cracks reduce the mechanical stability of glass. Light scattering allows the observer to perceive the cracks as white pixels. Coloured marking of glass in this manner is not possible. Coloured marking inside glass by changing the oxidation state of the metal ions locally in the focus does not weaken the mechanical properties of the glass. Two kind of glass systems, lime-natron-silicate and borosilicate with 0.5 % mass-content of doping are investigated. The simultaneous presence of donators and acceptors allows a transition of electrons between polyvalent ions, and can lead to permanent colour-centres inside the glass, due to the fact that the absorption of the polyvalent ions is changed by the laser-induced conversion process. For this purpose a 3 omega Nd:YAG (wavelength lambdaL = 355 nm, pulse duration t = 10 to 80 ns) and a Ti:Sapphire solid-state laser (wavelength lambdaL = 810 nm, pulse duration t = 200 fs) are used. The radiation parameters and the chemical composition of the glass (mainly doping) are the dominant factors to generate coloured marking. The transmittance as a function of the fluence and the change of the absorption coefficient is measured and gives a statement of the colourshade. Further the difference between lime-natron-silicate and borosilicate glass (same doping variety) is examined. Actually mauve, yellow, red-brown an grey colouring can be produced. Cracks in the microstructure of glass can also be the cause for brown colour-centres generating