High-power CO2 laser design “from resonator end mirror to cutting lens focal point”

Abstract

An algorithm is derived that allows calculating all optical parameters relevant to the design of a high-power CO2 laser, taking into account the practical needs of a flying-optics type of laser cutting machine. The novelty of the approach taken is that the resonator and the beam delivery optics are treated simultaneously, which allows for greater flexibility in design. To begin with, the often-used optical configuration of a stable resonator combined with a beam expander for collimation is modeled and optimized. The remaining focal shift over the cutting table, mainly due to the fact that the part of the beam close to the resonator still has appreciable wavefront curvature variation, is calculated. The resulting function is programmed into the machine CNC, which continuously adjusts the lens position depending on the actual beam length, to compensate for this effect. Next, attention is paid to the well-known problem of changing beam parameters with thermal load of the output coupler. It is found necessary to model both mechanical deformation of the ZnSe window and lensing due to the temperature dependent index of refraction. The former is derived from a finite element model, for the latter, the formula of Miyamoto et al. [Proc. SPIE 1276, 112 (1990)] is used. Finally, as an example of novel resonator designs studied using this approach, the characteristics of an intracavity reflective telescope are determined for two different goals: first, to increase the fundamental mode spot size for the given resonator (and hence to improve beam quality without sacrificing power), and second to preserve beam quality when extra resonator layers are added to increase power. The beam is modeled using the concept of the “embedded Gaussian” and the “complex radius of curvature” due to Siegman [Lasers (University Science Books, CA, 1986)].