High power second harmonic generation with free running Nd : YAG slab laser for micro-machining applications

Frequency doubled Nd:YAG lasers represent an attractive alternative to other laser tools for many material processing applications, but frequency doubling with pulsed Nd:YAG lasers has been performed until now only with pulses of tens of nanoseconds. In material processing with longer pulses (10-1000 mus), such as encountered in typical 1.06 mum industrial Nd:YAG applications, the laser-material interaction is different and, in particular, higher material ablation rates are performed. Furthermore, the green light material processing permits a better focusability and a higher absorption in most materials. However, frequency doubling with long pulse lasers is much more difficult and less efficient up to now. The main problems are the generation of a fundamental 1.064 mum beam of high quality necessary for the non-linear process, and the low damage threshold of the non linear materials in the long pulse regime. Therefore, a zigzag slab laser, which has a high beam quality and an inherently linear polarisation of the beam, is an ideal candidate for non-linear processes. The optical damage threshold in the non-linear materials is the main limiting parameter. The 140 W instantaneous power obtained for a 200 mus pulse duration in extra-cavity configuration allows us to finely process sheets up to 300 mum thick.