Cover glass influence on high power fiber laser induced focus shift

By: D. Reitemeyer1, T. Seefeld1, F. Vollertsen1, J. P. Bergmann2
1 BIAS – Bremer Institut für angewandte Strahltechnik GmbH, Bremen, 28359, Germany
2 Jenoptik Automatisierungstechnik GmbH, Jena, 07745, Germany

In materials processing with solid state laser sources a fiber optical waveguide is used to transport the
beam to the work station where the beam is focused on the work piece by a processing head. The high
beam quality of state of the art diode pumped solid state lasers allows concentrating the high output
power in a small focus diameter. This forwards the advantage of laser beam welding, the locally
concentrated energy input which enables precise processing.
Along with today’s high beam quality new challenges arise in the systems technology with increasing
requirements for the processing head. The high energy density in the laser beam path heats up the beam
guiding and forming optics, despite the fact that they have an absorption level below 1%. The
temperature increases when fume and spatter from the welding process are locally enhancing the
absorption on the cover glass which protects the beam outlet of the welding head. Figure 1a shows the
measured temperature distribution on the surface, at 5 kW a maximum rise of 170 K was detected.
This temperature rise influences the refractive index and the surface shape. Figure 1b shows the surface
deformation of a contaminated cover glass at 5 kW laser power measured by the fringe reflexion
technique. This influences the optical properties of the optics and thus the beam geometry at the work
piece. Figure 2 shows the influence on the focus’ position and diameter in dependence of the laser
power. The so called focus shift effect may influence the precision of the process.
At ICALEO 2009 a new way to analyze the influences on the focus shift will be presented. This work is
part of a project where BIAS and Jenoptik Automatisierungstechnik GmbH are working on new
approaches for systems technology for material processing with high brightness laser sources.

The above brief overview was extracted from its original abstract and paper presented at The International Congress on Applications of Lasers & Electro-Optics (ICALEO) in Orlando, FL. To order a copy of the complete proceedings from this conference click here