Standard bending mirrors are used to direct the laser beam towards the work piece. A 90o bend (45o angle of incidence) is most common, but other angles can be used. Reflectivities approaching 100% at the desired wavelength are common.
Circular polarising mirrors are a special type of bending mirror which convert a linearly polarized beam into a circularly polarized beam. Simply speaking, every laser beam has an associated electric field. The orientation of this electric field with respect to the direction of beam propagation can greatly affect how the focused beam cuts or welds a particular material. In most CO2 laser systems, the electric field of the output beam is oriented in a plane at 45o from the horizontal or vertical. This is known as ‘S’ or ‘P’ linear polarization. A linearly polarized beam will interact differently with the work material depending on the direction of travel. For instance, it may cut nicely in the X direction but yield excessive burring or a non-perpendicular kerf in the Y direction. For consistent cutting or welding in all directions, a circularly polarized beam is required. Quarter-wave and eighth-wave circular polarizing mirrors were developed for this purpose.
Phase Shift Mirrors
Zero phase shift mirrors are used when additional bends are required in the beam path after a quarter-wave mirror. Standard bending mirrors should not be used after a quarter-wave mirror because they will “shift the phase” or alter the beam’s polarization phase angle. These mirrors are usually made from silicon substrates (copper is used in very high-power applications). Zero phase shift coatings are the highest reflectivity, lowest loss coatings available for CO2 laser optics. Polished Molybdenum is a natural zero phase shift mirror and does not require coating.