HOME > Coatings > Polarisation

Tips for using Phase Retarders

The phase retarder 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 if possible, ideally inside the laser cabinet

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

Used "in reverse", a 90 degree phase retarder converts circular polarisation to linear polarisation.

Reflective Phase retarder mirrors for CO2 lasers

Quarter wave phase retarder mirrors solve the problem of laser cutting metals with a CO2 laser. High power CO2 lasers often emit a beam that is linearly polarised. Linear or plane polarised CO2 laser beams, will give uneven kerf width and cut quality, depending on the motion of the metal relative to the plane of polarisation.

90 degree phase retarder mirrors convert the linear (or plane polarised laser beam) typically emitted by high power CO2 lasers, to a circularly polarised laser beam. A circular polarised beam is needed to ensure even and consistent metal cutting in different axes of motion.

To convert linear polarisation to circular polarisation, a phase retarding mirror with an exact 90 degree phase shift at 10.6um wavelength is used. Often called polarisers, ECQ mirrors, 90 degree phase retarders, or lambda/4 mirrors, these reflectors introduce a phase shift of exactly 90 degrees to the beam. The 90 degree phase shift is acheived by using a carefully controlled dielectric coating deposited on a copper, or sometimes silicon, mirror substrate

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 mirror coating is needed to ensure high quality circular polarisation for consistent laser cutting. LBP phase retarders typically measure 90 degrees, +/- 1 degree. Each mirror is individually tested, and the value noted on the mirror.

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). As a simple metal layer Gold coating is very reliable, and not dependent on coating conditions. Gold mirrors can be used for "fault" finding where polarisation problems are suspected, as they have a consistent near zero phase shift.