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Optics

Phase Retarders

Introduction

Phase retarder components are used in laser systems to convert a linearly polarized beam into a
circularly polarized beam. This is often necessary in X-Y cutting systems so as to provide equal cut
quality in all directions of relative motion of the workpiece. In order to convert linear into circular
polarization a total of one-quarter wave, or λ/4 retardation (90°) is required. This may be achieved
using a single λ/4 retarder, or by a combination of two λ/8 retarders.
Technical background
The majority of phase retarder components used in high power CO2 laser systems are mirrors, treated
with a special coating.

[Transmissive phase retarders are available (they are often called ‘λ/4 plates’) but are of small
aperture and very high cost].
A reflective phase retarder component needs to be arranged relative to the laser beam so that the beam
is incident at 45° to the mirror.
Importantly, the plane of polarization of the beam must be at 45° azimuth angle to the plane of
incidence/reflection. (Fig 12.001).
Phase retarder coatings are sensitive to errors in incidence and azimuth angles, and to errors in laser
wavelength.

Most folded-cavity industrial lasers emit
linearly polarized radiation. The
direction of polarization is the direction
of the vector describing the electric
field . This is usually either vertical or
horizontal, although some laser
manufacturers (eg: Rofin-Sinar) arrange
the linear polarization to be at 45° to the
base of the laser.
The effect of λ/4 phase retardation is to
cause the electric field direction to ‘spin’
with the frquency of the radiation, or
about 3 x 1013 times per second.

Product types, components
Silicon phase retarders
Si phase retarders are available in both λ/8 and λ/4 types (45° and 90° retardation).
These products are described in data section 12.10.
Copper phase retarders
Cu phase retarders are available as λ/4 type only. They have a higher laser damage threshold than
the Si type. Cu phase retarders are described in data section 12.20.
Prism phase retarders
Special prism phase retarders can be designed and made by Umicore Laser Optics.
There is no standard product. Each type will be made to custom requirements. Prism phase retarders
are not reliant upon special coatings and can be made to have high accuracy of phase retardation.
They will operate over a broad band of wavelengths.
Phase retarder assemblies
Phase retarder assemblies are described in section (71) of Umicore Laser Optics technical data.
Available assemblies include:
Offset phase retarders (2 reflectors)
In-line phase retarders (4 reflectors)
Broad-band in-line phase retarders