Determination of the thermally induced focal shift of processing optics for ultrafast lasers with average powers of up to 525 W.

The continuous increase of the average laser power of ultrafast lasers is a challenge with respect to the thermal load of the processing optics. The power which is absorbed in an optical element leads to a temperature increase, temperature gradients, changing refractive index and shape, and finally causes distortions of the transmitted beam. In a first-order approximation this results in a change of the focal position, which may lead to an uncon-trolled change of the laser machining process.