Zoom lens designs for use in sheet metal cutting by high power CO2-lasers

For best results and the highest speed of cutting sheet metal by high power lasers, the numerical aperture of the focussed beam must be properly matched to the material thickness. To alleviate the need for frequent changes of fixed-focal-length lenses a zoom lens system which allows fast and continuous adaptation is desirable. Requirements include near diffraction limited performance in a range of at least f/3 to f/8 and a back focus in excess of the focal length for lens protection against fumes and spatters. Four zoom lens designs for use with CO2-lasers in the 1 to 2 kW range are presented, which differ in technical complexity (use of an aspheric surface versus an additional lens element) and in the ranges of numerical apertures (f/2.8 to f/8 and f/2 to f/7). Only one of these has so far been built and measured, and cutting tests have shown slightly better performance at the short focal lengths over that of single (ZnSe) lenses of (optimized) meniscus shape.