Laser Mirrors – Laser Optics
ZERO PHASE SHIFT LASER MIRRORS
In most laser machines, one or several laser 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.
90° Phase Shift Laser Mirrors
Most CO2 lasers produce a laser beam which has linear polarization. For cutting metal sheets, however, a beam with circular polarization is required if the cutting properties are independent of the cutting direction. To convert a beam from linear to circular polarization, a 90°-phase retarding laser mirror (also called a 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 these components have the same intensity and phase (corresponding to linear polarization), the reflected beam undergoes a 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 (the same as focusing lenses).
Telescopic Laser Mirror
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 laser mirrors – one with a convex surface and the other with a concave surface. Such telescopic mirrors are usually made of copper.
ATFR (Absorbing Thin-Film Reflector)
In some laser cutting processes, a portion of the beam may reflect off the work piece and into the laser cavity, where it might disturb laser operation. In order to avoid such problems, an ATFR laser mirror can be inserted into the beam line. An ATFR mirror is characterized by high reflectance (typically 99%) for s-polarized radiation and low reflectance (typically less than 1%) for p-polarized radiation.