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Tips for using Phase Retarders

The phase shift 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° incidence to the plane of polarisation

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

The phase retarder diameter should be 2.5 or 3 times the beam diameter.

Phase retarder mirror for CO2 laser

Reflective quarter wave phase retarder mirrors solve the problem of metal laser cutting with a CO2 laser. High power CO2 lasers often emit a beam that is linearly polarised. The linear polarisation is due to internal cavity fold mirrors, which have a small anisotropy in reflection when used at 45 degree incidence. ( %R for S plane polarisation is slightly better than %R for P plane polarsition. Linearly polarised laser light couples in to the metal being cut variably, depending on the alignment of the plane of polarisation with the axis of motion. CO2 lasers with no 45 degree fold mirrors internally, or very shallow angle fold mirrors like the Ferranti MF400/1200 laser are likely to be randomly polarised in their output.

90 degree phase shift mirrors convert the linear (or plane polarised laser beam) to a circularly polarised laser beam. It's this circular polarisation that is needed to ensure even metal cutting in different axes of motion.

To convert linear polarisation to circular polarisation a ¼ wave phase retarding mirror is needed. Often these are known as polarisers, ECQ mirrors, 90 degree retarders, or lambda/4 mirrors.

Laser Beam Products stock a range of standard reflective phase retarder mirrors both from copper and silicon mirror substrates, as well as offering custom designed reflective phase retarder mirrors.

Accurate control of the phase shift value of the coating is needed to ensure high quality circular polarisation for consistent laser cutting.

It is essential that mirrors before and after the polarising mirror do not affect the polarisation 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. Using Gold mirrors can be used for "fault" finding where polarisation problems are suspected