ZnSe Brewster windows are generally rectangular in form, and used at a specific angle of incidence
to the laser beam (the Brewster angle). Incident radiation polarized such that the electric field is in
the plane of incidence/reflection is fully transmitted. This radiation is called ‘P-polarized’.
Radiation with an electric field at 90 degrees to this plane is termed ‘S-polarized’. It is partially
reflected at each surface of the Brewster window. In the case of ZnSe plates, approximately 50% of
S-pol. is reflected at each surface. Standard ZnSe Brewster windows have both surfaces uncoated. Sizes/tolerances
Standard sizes of uncoated Brewster plates are listed in table 5.001.
Custom-made Brewster plates may be made in sizes up to 200mm length as long as the specified
thickness is appropriate to the size. Design tolerances for standard plates are:
Length : +0 / -0.1mm
Width : +0 / -0.1mm
Thickness : ± 0.1mm
Flatness : Better than λ/80 over any 25mm dia test area.
Parallelism : Wedged 2.0 ± 1.0 minutes of arc.
Surface roughness : Better than 15Å RMS.
LIDT cw : >6000W/mm (>6kW per mm of beam dia. 1/e2)
The Brewster Angle
The Brewster angle for a material is determined by the index of refraction for the wavelength
concerned. Where the refractive index is ‘n’ the Brewster angle of incidence ‘Ob’ is given by:
θb = TAN-1 (n)
In the case of ZnSe at 10.6μm, n = 2.4028, and θb = 67 24′.
When specifying Brewster windows having some required value for optical clear aperture, it is
necessary to make appropriate allowances on the width and length of the plate for the mechanical
mounting arrangement. Applications Polarization-forcing
ZnSe Brewster windows may be used within a laser resonator so as to force the laser to emit linearly
polarized radiation. Polarizers/analysers
Outside a laser cavity, plates may be used singly or in ‘stacks’to perform the role of polarizers or
A particularly useful function of Brewster plates occurs in the laser-processing of highly reflective
metals, where Brewster-based ‘isolator’ assemblies (section 79) and phase-retarder units (section
71) may be used to prevent back-reflected radiation from re-entering the laser resonator. Wedge Angle
If the level of parallelism of the Brewster window is high, then multiple interference between beams
reflected from the two surfaces can affect the intended function of the plate. Umicore Laser Optics Brewster
plates are deliberately wedged to between 1 and 3 arcminutes in order to overcome this problem.
Plates with special coatings to enhance the polarizing function are dealt with in technical data section 5.10.
Standard products (uncoated type)
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