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Metal cutting has long been known to be sensitive to the polarization of the laser beam used. Circular polarization is used to ensure even metal cutting in different axes. Unfortunately most high power lasers have a linearly polarized output.  

Tips for using Phase Retarders

The mirror needs to be accurately positioned at 45° angle of incidence to the beam, so beam is turned through 90°.

The mirror needs to be accurately positioned at 45° to the plane of polarization.

As the most expensive mirror, location should be away from the cutting area, often inside the laser cabinet.

The mirror diameter should be 2.5 or 3 times the beam diameter.

To convert linear polarization to circular polarization a 1/4 wave phase retarding mirror is needed. Often these are known as polarizers, ECQ mirrors, or 90 degree retarders.
Laser Beam Products stock a range of standard reflective phase retarder mirrors both from copper and silicon, as well as offering custom designs.
Spec for Phase Retarders:
Reflectivity 10.6μm, 45° AOI 98% min
Phase Shift 10.6μm, 45° AOI 90° +/- 3°
Copper or Silicon Substrate Both  
Accurate control of the phase shift value of the coating is needed to ensure high quality circular polarization for consistent laser cutting. It is essential that mirrors before and after the polarizing mirror do not affect the polarization of the beam (i.e they have zero phase shift). Gold coated mirrors are recommended for this purpose having one of the lowest phase shifts known (< +/-1 degree), and as a simple metal layer coating are very reliable. Gold mirrors can be used for "fault" finding where polarization problems are suspected.

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