How to Select a Window

How to Select a Window
Windows are used to transmit light while maintaining
isolation of two environments. They may be used at the
ends of a laser cavity, or to obtain optical access to sample
and vacuum chambers. They also have applications
in beam manipulation, including splitting, sampling,
and routing, and are the foundation for several coated
products like plate beamsplitters, output couplers,
harmonic separators, dichroic mirrors, and partial
reflectors.
Window vs. Etalon What is the difference between a window
and a solid etalon? Both have low transmitted wavefront error
and high surface quality to minimize beam distortion and scatter,
but a solid etalon has near-zero wedge to maximize the amount
of interference between its two surfaces. The wedge on our laser
grade windows is ≤ 10 arc seconds, while our solid etalons have
≤ 1 arc second of wedge.
Factors to consider when selecting a window include the
material, transmission, scatter, and wavefront distortion, as
well as power handling, shape, and wedge needed. CVI
Laser Optics offers catalog windows in both N-BK7 and
UV-grade fused silica, and custom substrates are available.
Typical transmitted wavefront distortion is λ/10 @ 633 nm,
but can be λ/4 for thin substrates of larger dimension.
Together with high surface quality (10-5 scratch and dig),
this results in low scatter, low distortion and high laser
damage threshold (15 J/cm² at 1064 nm narrowband,
20 nsec, 20 Hz). All windows are precision-polished.
Thickness is controlled to ± 0.25 mm, and dimensions
are controlled to + 0/- 0.25 mm, while maintaining a clear
aperture that is ≥85% of the central dimension. They can
be used at any angle of incidence, though AOI should be
considered when using a coated window at non-normal
incidence.
Parallel Windows
Parallel windows have very low wedge, finding use as
output couplers for high-gain lasers, and as sample cell
and vacuum chamber windows. Our parallel windows
maintain ≤ 10 arc seconds of wedge for circular substrates,
and ≤ 5 arc minutes of wedge for square or rectangular
substrates. Using a window with low wedge helps to
Fig 1. Devation and reflection of a beam by a wedged window of
wedge ∝
Fig 2. A beamsteering wedge formed from two wedge prisms.
minimize the angular deviation of the transmitted beam,
and significantly reduces alignment errors if the window is
repeatedly inserted and removed.
Achromatic beam sampling and splitting can be
accomplished with a parallel window by using the Fresnel
reflection from an uncoated surface. Reflection at
normal incidence from an uncoated surface is ~4% at all
wavelengths, while at 45° angle of incidence, Rs ~ 10% and
Rp ~ 1%. An uncoated window can even be used to obtain
a purely s-polarized reflected beam (albeit at only ~15%
efficiency) by working at the Brewster angle.
Wedged windows
Wedged windows are free of interference effects and
minimize back reflections as compared to parallel
windows. They can be used for this purpose, or to deviate
an incident beam by a precise angle. The reflected beams
from both surfaces can be used separately (provided that
AR coatings have not been applied). CVI Laser Optics
wedged windows are available with low wedge (known
as interferometer flats), and large wedge (1 or 3°), all
controlled to within 6 arc minutes.
Our interferometer flats have a wedge of 30 ± 5 arc
minutes to minimize interference effects between the
surfaces. They can be used in laboratory interferometers
or to verify the flatness of another optic. When placed in
contact with an optic of unknown flatness and illuminated
with monochromatic light, a series of interference fringes
can be seen due to the small air gap between the two
optics. Fringes that are straight, parallel, and equally
spaced indicate a test surface of high flatness. If curved,
the fringe pattern can be used to calculate the flatness of
the test surface. Antireflection coated interferometer flats
are available as part of the laser window product line,
W1-IF and W2-IF.
The wedged second surface of our large wedge windows
is very effective in preventing interference due to stray
back reflection. This is particularly important for output
coupling in high gain and sensitive lasers, in which case
the front surface is given a partially reflective coating
and the second surface an antireflection coating. Large
wedge windows also serve to deviate the incoming beam
by a specific angle, θd (θd = (ns/na) α, where α is the wedge
angle). This is very useful for beam-steering, as two
windows of equal wedge can be placed in tandem to allow
continuously variable tuning of the deflection angle by
rotating one window relative to the other. When oriented
at 180° to one another, the net deviation is 0°, creating a
parallel, displaced beam.
square and rectangular windows are available from the
catalog, all with the same λ/10 flatness as our round
windows, but custom sizing is available. Square and
rectangular windows are typically used in laser window
and beamsplitting applications.
Window coatings
Any of our windows can be coated on one or both sides
with one of our many standard antireflection, dielectric, or
metal coatings. The W1 and W2 series of laser windows
are available as catalog parts with antireflection coatings;
even the wedged surface can be coated. These electron
beam multilayer dielectric coatings are available at
wavelengths from 193 – 1550 nm and are designed for
single/dual laser lines or with up to 430 nm bandwidth.
We allow you to specify 0° or 45° angle of incidence, and
can optimize the antireflection coating for s-polarized,
p-polarized, or unpolarized light. Other angles of
incidence or coating designs are available on a
custom basis.
Laser damage threshold
All of our windows are inspected to 10-5 scratch and dig
standards, and are manufactured from quality N-BK7 and
UV-grade fused silica material, making them well-suited
to high power applications. Antireflection coatings and
use of UV-grade fused silica is recommended for any high
power optic, and allows our W1 and W2 laser windows to
achieve a laser damage threshold of 15 J/cm², 20nsec,
20 Hz for a narrowband 1064 nm pulsed laser.
Making the final decision
Selecting the right window is a fairly straightforward
process once you understand the capabilities of parallel
windows vs. low and large wedge windows. At that point
the material, shape, and type of coating needed will guide
you quickly to the right solution. CVI Laser Optics’ flexible
product codes allow you to specify the window parameters
in thousands of different ways, and our many custom
options guarantee the right fit for your application.
Window shape
CVI Laser Optics offers windows in three popular shapes:
round, square, and rectangular. A wide variety of round
windows are offered from 10.0 – 152.4 mm in diameter,
with thicknesses of 1.0 – 25.4 mm. A smaller range of
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Product Code
Description
Wedge
Additional Features
W1, W2
Antireflection Coated Windows
≤ 10 arc sec OR
30 ± 5 arc min
▪
▪
▪
▪
PW1
Laser Grade Circular Windows
≤ 10 arc sec
▪ N-BK7 or UV-grade fused silica
SQW
Laser Grade Square Windows
≤ 5 arc min
▪ N-BK7 or UV-grade fused silica
▪ Custom dielectric or metal coating options
RW
Laser Grade Rectangular Windows ≤ 5 arc min
▪ N-BK7 or UV-grade fused silica
▪ Custom dielectric or metal coating options
IF
Interferometer Flats
30 ± 5 arc min
▪ N-BK7 or UV-grade fused silica
▪ For coated options, see W1-IF & W2-IF
LW
Large Wedge Windows
1° ± 6 arc min OR
3° ± 6 arc min
▪ N-BK7 or UV-grade fused silica
▪ 1° or 3° wedge, others custom
▪ Custom antireflection coating options
UV-grade fused silica, other materials custom
Parallel surface option: ≤ 10 arc sec wedge
Interfermeter flat option: 30 ± 5 arc minutes wedge
Single/ dual λ or broadband AR coating options
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