Phase Retarder Assemblies

Introductionphase-retarder-assemblies
Most industrial CO2 lasers emit a beam of linearly polarized radiation. (Electric field in a fixed
direction). It is well known that linearly polarized radiation will cut, or otherwise process metals with
excellent performance in one direction and with poor performance in the orthogonal direction. In
order to achieve equal processing performance in all directions of workpiece movement relative to
the focus, a compromise may be established whereby the electric field of the radiation is ‘spun’ once
every wavelength. That is, at about 3 x 1013 Hz. This is ‘circular’ polarization. In this way the
preferential processing ability of the electric field is used evenly. Details of reflective components,
phase retarders, used to convert linear in to circular polarization are given in Technical Data Section
12. Technical Data Sections (71.xx) describe assemblies incorporating phase retarding components.
Available models
FPR25 – Section 71.1
FPR25 is an in-line phase retarder assembly containing one λ/4 copper reflective phase retarder and
three zero-phase mirrors. It is water-cooled, factory aligned and has a clear aperture of 33mm
diameter. Model FPR25 is for use at 10.6μm wavelength. Water cooling is distributed internally.
FPR25-BB – Section 71.2
FPR25-BB is externally identical to model FPR25, but contains a specially developed phase
retardation mechanism which functions over a broad wavelength range. The unit is water cooled,
factory aligned, and has a clear aperture of 33mm diameter. This model can be supplied in two
Umicore Laser Optics Ltd. Page 71.02
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versions FPR25-BB5 for 4.5-6.0μm (CO lasers), FPR25-BB10 for 9-11μm (tunable CO2 lasers).
Water cooling is distributed internally.
OPR25 – Section 71.3
OPR25 is a simple phase retarder assembly with a copper λ/4 phase retarder mirror plus a zero-phase
periscope mirror. The mirrors are separately water cooled and the mirrors may be user-aligned. The
clear aperture is 33mm diameter.
Notes on polarization
1) Some small industrial CO2 lasers with a single tube and no special method of forcing linear
polarization emit a randomly polarized beam. Phase retarders are not used in these circumstances.
2) In order to function correctly, a reflective phase retarder must be at 45° incidence to the beam, and
at 45° azimuth angle to the electric field Most industrial lasers emit either vertically or horizontally
polarized radiation, in which case the unit, for example FPR25, must be rotated to 45° from the
horizontal. Some lasers are arranged to emit a beam linearly polarized at 45° to the vertical. In such
cases the unit must be vertical (or horizontal).
3) Other mirrors, downstream from the phase retarder assembly, can modify the state of polarization.
In principle the phase retarder unit has introduced 90° retardation. Metal mirrors used at 45°
incidence may modify this by perhaps 1 or 2° (each). Dielectric-enhanced Silicon mirrors can
introduce perhaps 5 or even 10° retardation (each) unless “zero-phase” types are specified.