90ֲ° Phase Shift Mirror
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 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).