Design of an optical system for high power CO2 laser cutting

The results of a design study for the optical system for cutting with high power CO2 lasers (6 kW and up) will be presented. As transparent materials cannot be used for these power levels, mirrors have been applied. A coaxial cutting gas supply has been designed with a laser beam entrance into the pressured gas chamber, by means of an intermediate focus. It allows the beam to enter the gas chamber through a small nozzle opening. The final optical system consists of two off-axis aspherical mirrors. In general aspherical elements are very sensitive for positional or angular errors, resulting in considerable astigmatism (basically coma). In addition, unavoidable manufacturing tolerances contribute to deteriorate the optical quality. However, it is shown that the particular combination of two aspherical elements compensate each other, leading to a high quality optical performance, which is virtually independent of the entering beam angle or position. Manufacturing tolerances have been compensated in a one time adjustment during the assembly of the optical system. Preliminary cutting results in 13 mm thick steel in a shipyard application show a significant improvement in the cutting performance.