ZnSe Enhanced Brewster Windows
Enhanced ZnSe Brewster windows are rectangular in form, and used at a specific angle of incidence
(the Brewster angle) to the laser beam. Incident radiation polarized such that the electric field is in
the plane of incidence/reflection is fully transmitted. This radiation is termed ‘P-polarized’.
Radiation with an electric field at 90 degrees to this plane is called ‘S-polarized’.
Enhanced Brewster windows are treated with a complex coating stack on one surface which has the
effect of enhancing the ‘natural’ polarizing effect to the extent that almost all of the S-polarized
radiation is reflected.
No anti-reflection coating is needed at the second surface since the uncoated surface will anyway
transmit 100% of the P-polarized incident radiation.
Standard sizes of enhanced Brewster plates are listed in table 5.11. Custom-made Brewster plates
may be made in sizes up to 200mm length as long as the specified thickness is appropriate to the size.
NB: the complex coating stack makes custom-made plates expensive in small quantities!
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.
Parallelism : Wedged 2.0 ± 1.0 minutes of arc.
Surface roughness : Better than 15Å RMS.
LIDT cw : 200W/mm
Max. power density : <500W/cm2 recommended *
T% P-pol : >99%
R% S-pol : >98%
* NB: refers to limit recommended to minimize thermal-lensing.
The Brewster Angle
The Brewster angle for an enhanced ZnSe plate is 67 24′. The special enhancing coatings are sensitive
to changes of wavelength, and so this product type cannot be used in broad-band applications. The
coating stack is sensitive to errors in mounting angle. When specifying Brewster windows to have
some known 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.
Outside a laser cavity, uncoated plates may be used singly or in ‘stacks’to perform the role of
polarizers or analysers. An enhanced plate can often be used where otherwise a multiple stack of
plates would be needed. This can have beneficial effects in elimination of multiple reflections from
the use of many uncoated surfaces.
Enhanced Brewster plates are particularly useful in specialized laser test equipment, where it is
necessary to use beams of CO2 laser radiation of strictly linear polarization. A typical example occurs
in equipment used for checking mirrors and phase retarders for phase change on reflection at 45
Enhanced Brewster plates are used in isolator devices (data section 79) to stop unwanted backreflected
laser energy from reaching the laser.
Multiple reflections are not a problem when enhanced Brewster plates are used. The reason for the
nominal 2 arcminute wedge in the standard range of enhanced plates is due to their selection from
batches of standard (uncoated) windows. Custom-made plates can be specified to higher levels of
parallelism if required.