In the beam delivery section of a Co2 laser catting machine, the laser beam is transferred from the laser cavity to the working head. In principle, two Co2 laser moveable mirrors would be sufficient in a 2D-machine for guiding the laser beam to any point on the worksheet. In modern 2D-machines and especially in 3Dmachines, however, the beam delivery section has additional functions which require additional Co2 optical mirrors with specific properties. In order to optimize function of these mirrors, different substrate materials are used - the most common ones are silicon(Si) and copper (Cu). Silicon mirrors have light weight and are therefore preferred in flying optics where high accelerations are needed. Copper has high thermal conductivity, and channels for cooling water can be included directly into the Co2 laser mirrors. Therefore, copper mirrors are preferred if best-possible cooling is important, for example in Co2 laser machines with very high laser power. - The optical properties of a mirror (reflectance, phase shift, etc) are determined by its coating. So in order to realize different mirror functions, different coatings are needed. As the output power of the Co2 laser is getting higher throughout the years, the technical demands and damage threshold the Co2 laser optics need to withstand is more challenging. As a solution to those challenges optics manufacturers added cooling system to the Co2 laser Cu mirror, by that the mirror will be less inclined to thermal expansion, and a proper cooling of the optical surface is ensured. The cooling of the Co2 laser optic surface is achieved by drilling of channels close to the surface, by that there is less damage to the coating as in is exposed to less heat. This drilling is not applicable to Si mirrors due to the brittleness of the material . In many applications, the small diameter of the laser beam produced in the Co2 laser cavity is not convenient because the beam has high divergence and high power density. In order to avoid subsequent problems, the beam diameter can be increased by using a Co2 laser optics telescope consisting of two mirrors – one with a convex surface and the other with a concave surface. Such Co2 laser optics telescopic mirrors are usually made of copper.