In most laser machines, one or several mirrors are used to forward the laser beam from the cavity to the working head. Usually, each mirror deflects the laser beam by an angle of 90°, corresponding to an angle of incidence of 45°. At these mirrors, reflectance should be as high as possible in order to minimize losses of laser power; in addition, phase shift between the s- and p-polarized components of the reflected beam should be as low as possible in order to avoid disturbing the polarization of the laser beam. Mirrors with such properties are called zero-phase-mirrors.

Most CO2 lasers produce a laser beam which has linear polarization. For cutting metal sheets, however, a beam with circular polarization is needed if the cutting properties have to be independent of cutting direction. For converting a beam with linear polarization into a beam with circular polarization, a 90°-phase-retarding mirror (also called lambda/4-mirror) can be used. This mirror has a special coating which produces a phase shift of 90° between the s-and p-polarized components of the reflected beam. If the incoming beam has both components with same intensity and phase (corresponding to linear polarization), the reflected beam has phase shift of 90° between both components (corresponding to linear polarization).

Windows for CO2 lasers are used either for protecting sensitive and/or expensive optics, or for separating areas with different gas pressures. In all applications, high transmittance and low absorption are needed in order to minimize distortion of the laser beam. Therefore, such windows usually consist of ZnSe substrates with AR-coatings on both surfaces (same as focussing lenses).

In many applications, the small diameter of the laser beam produced in the 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 telescope consisting of two mirrors – one with a convex surface and one with a concave surface. Such telescope mirrors are usually made of copper.

In some laser cutting processes, there is considerable risk that some portion of the laser beam is reflected at the workpiece and returned into the laser cavity. There, it might disturb laser operation. In order to avoid this problem, a so-called ATFR mirror can be inserted in the beamline. Such a mirror has high reflectance (typically 99%) for s-polarized radiation and low reflectance (typically less than 1%) for p-polarized radiation.