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 at 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 loss 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.