Picosecond Laser Induced Colour Centres: Stress-free Markings and Choice of Guiding Optics

It is known that ionising radiation (X-ray, gamma rays, electrons) can induce numerous changes in the physical properties of glasses. The most obvious effect is visible colouration, which is caused by the accumulation of colour centres (defects) in the irradiated volume. The application of such induced colour centres has prompted a renewed interest since they can be generated and bleached reversibly. We present results on laser induced colour centres in different types of glasses using picosecond laser pulses. For example, an unexpectedly low peak intensity threshold of 4*10 14 W/m for the laser induced colouring in BK7 glass is observed at a wavelength of 355 nm and a pulse width of 6 ps. A strong absorption of laser induced colour centres between 330 and 800 nm for BK7 is shown. Picosecond laser pulses at a longer wavelength of 532 nm and even 1064 nm induce colour centres in many different glasses, making marking application more feasible. By applying different focusing optics the influence of changing focal length and energy flux density on the volume colouring effect have been analysed. The changes in the optical transmission compared to the unperturbed state are significant. Non-linear optical effects and possible implications for future optics guiding ultra-short laser pulses are addressed.