How to Select a Prism
Prisms can bend and deviate light in many different ways,
depending on how they are cut and how they are oriented
relative to the input beam. A beam encountering a prism
at an angle is refracted upon entry and exit, dispersing
the light by wavelength and altering its path. If the prism
is cut such that the entrance and exit angles differ, the
prism will also impact the size of the beam in the plane
of its travel. Light enters a prism at normal incidence
experiences no refraction. When the next surface within
the prism is encountered, however, there is a possibility of
total internal reflection (TIR). A prism of refractive index >
1.414 will exhibit TIR at 45°, a convenient fact that allows
right-angle prisms to be used to bend light by 90° or 180°.
Some prisms like the Pellin Broca prism make use of both
wavelength dispersion and total internal reflection.
To select a prism, it is first important to understand the
different types available. Once that is achieved, the
decision comes down to factors like degree of dispersion
needed, amount of deviation required or tolerated,
material type, power handling, and choice of antireflection
or other coatings.
Prism Types
CVI Laser Optics offers multiple prism types for deviating,
turning, dispersing, and magnifying light. A wedge prism
or window is free of interference effects and minimizes
back reflections as compared to a parallel window. It
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 wedge windows
are available with low wedge (known as interferometer
flats), and large wedge (1°- 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
Large Wedge Prism
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, W1IF 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, vd (vd {ans/na, where a 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.
Right-angle prisms are most often used for image rotation,
redirecting light, and as components for beamsplitter
cubes. This is achieved through TIR of light within the
prism. Their symmetric 45-45-90 design and high-quality
surface allows them to be used as high-power 90° bending
prisms or 180° folding prisms. TIR is independent of
wavelength, therefore right-angle prisms are good highenergy reflectors for broadband applications for which
metal mirrors are too absorbing and dielectric mirrors
do not reflect a wide enough bandwidth. Our precision
right-angle bending prisms are AR coated on the entrance
and exit faces (legs), while TIR occurs at the hypotenuse
to deflect the beam by 90°. Our precision right-angle
folding prisms are AR coated on the hypotenuse, which
acts as both the entrance and exit face, while TIR occurs
at each leg to deflect the beam by 180° (retroreflection).
The resulting image is inverted, a property which is useful
in some imaging applications. Application of AR coatings
increases transmission and eliminates back reflections.
Since TIR can fail if surfaces are not kept extremely clean,
we offer metal coatings for these surfaces in place of TIR
for applications where handling is frequent or convergent/
divergent beams are used. Metal or dielectric coatings
can also be applied to the hypotenuse to allow the prism
to be used as an external mirror.
Porro prisms are 180° right-angle prisms cut in a circular
section from the center of the hypotenuse face. The
rounded edges of this design minimize breakage and
facilitate assembly. Like a standard folding prism, a Porro
prism inverts the image and displaces it. Porro prisms are
most often used in pairs, forming a double Porro prism,
to offset a beam but keep its direction and orientation
constant. In this case the second prism is rotated 90°
relative to first so that the image is rotated 180° relative to
input image (not just inverted). Double Porro prisms are
used in small optical telescopes to reorient an inverted
image and in many binoculars to both re-orient the
image and provide a longer, folded distance between the
objective lenses and eyepieces.
Dispersing prisms are used to separate a beam of white
light into its component colors, a technique often used to
separate two laser wavelengths following the same beam
path. Each ray is refracted twice as it passes through the
prism, and how much they are separated upon output will
depend on the refractive index of the prism material at
each wavelength and the distance travelled through the
prism. An equilateral dispersing prism possesses three
equal 60° angles, and is the classic shape for wavelength
separating applications. When the incident beam is
Deviation and reflection of a beam by a window of wedge a
A beamsteering wedge formed from two wedged prisms
A Right-angle folding prism
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oriented such that it travels through the prism parallel
to its base, “minimum deviation” is said to occur; the
incident and exit angles are equal, the prism magnification
is one, and reflection losses are low. Dispersing prisms are
therefore often used at the minimum deviation angle. If
the base angles of a dispersing prism are chosen such that
the ray enters and exits at the Brewster angle for a given
design wavelength, p-polarization losses in transmission
will be nearly eliminated. This design is called an isosceles
Brewster prism.
Double Porro prism results in beam parallel to, but displaced
from its original position, with the image rotated 180°
Dispersing prism
A Pellin Broca prism is often called a constant deviation
prism, and is used for to deflect a single wavelength by
90° or for wavelength separation in a beam. It can be
imagined as an ordinary dispersing prism split in half
along the bisector of the apex angle, then rejoined using
a right-angle prism, creating a dispersing prism with an
internal right angle bend obtained by TIR. Light enters
one of the half prisms, is deviated, then travels through the
right-angle prism for TIR at its hypotenuse, is presented to
the second “half” dispersing prism in minimum deviation,
and then exits the prism deviated at exactly 90° to its initial
direction for that single wavelength. A simple dispersing
prism always deviates the longer wavelength less than
the shorter wavelength. In a Pellin Broca prism, whether
the longer wavelength is deviated more or less depends
on the orientation of the prism. The wavelength that is
deviated by exactly 90° changes as the prism is rotated
around an axis located on the side where TIR occurs,
making this prism ideal for selecting a specific wavelength
in beam-separation applications.
Application tip: For best performance, use only collimated
light when working with prisms.
Prism Materials
Equilateral dispersing prism
How a prism is cut and how the beam enters it will in large
part determine its effect on a beam, but the degree of
that effect is governed by the material from which it is
made. Refractive index determines the angle at which
TIR will occur, and how much deviation a prism will impart
to a beam. The dispersion of the material governs the
resolving power of the prism, as well as its effectiveness
in dispersion correction applications. Transmission
properties, laser damage threshold, thermal coefficient,
durability, weight, and cost should also be considered. CVI
Laser Optics utilizes six different materials to manufacture
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our catalog prisms, ranging in wavelength range, refractive
index, and dispersion. The Abbé constant is a value to
gauge dispersion in the visible wavelength range, with
lower values indicating higher dispersion.
N-BK7 is a lead- and arsenic-free borosilicate crown glass
that is used widely in the optics industry. It has excellent
transmission from 350 nm – 2.0 μm, good thermal
expansion coefficient, moderate laser damage threshold,
and is relatively low in cost. It is a hard glass that is robust
to handling, with good chemical resistance.
UV-grade fused silica is a synthetic form of fused silica
manufactured by flame hydrolysis to extremely high
standards. Its ultra-low impurity content is evident in
the wide transmission range of 180 nm – 2 μm and its
high laser damage threshold. It does not fluoresce in
response to wavelengths longer than 290 nm, and in
general exhibits good resistance to radiation darkening
from ultraviolet, x-rays, gamma rays, and neutrons. It also
boasts excellent thermal properties, including a wide
operating temperature range, low thermal coefficient,
and resistance to thermal shock. UV-grade fused silica
prisms from CVI Laser Optics have increased hardness and
resistance to scratching, resulting in better surface quality,
higher surface figure, and tighter tolerance focal lengths
than their N-BK7 equivalents.
Suprasil 1 is a type of fused silica with high chemical purity.
Almost all properties are a direct match with fused silica
(transmission range, Abbé constant, CTE, hardness), but it
has even better UV transmission and less fluorescence due
to the very low metal content (< 8 ppm). It is often used
for low fluorescence UV windows, lenses, and prisms. It is
the material of choice for use with excimer lasers in the 180
– 240 nm region.
Crystal quartz is a positive uniaxial birefringent crystal
Pellin Broca prism
grown using a hydrothermal process. Our crystal quartz
is selected to minimize inclusions and refractive index
variation. With transmission down to 170 nm and good
solarization-resistance, it is well-suited for UV beam
separation applications using a Pellin Broca prism, albeit
at higher cost. In fact, it is the material recommended for
use with high power 266 nm Q-switched pulsed lasers at
powers exceeding 50 mJ/cm2, as fused silica prisms tend
to track (i.e., suffer catastrophic damage) above this power,
likely due to self-focusing.
N-F2 is a type of Schott glass with higher refractive
index than N-BK7, and much greater dispersion. It
has similar thermal characteristics, but is not as hard as
N-BK7, reducing surface quality slightly. It exhibits good
transmission from 400 nm – 2.0 μm and good chemical
resistance at a cost similar to N-BK7 and fused silica. It is
used often in dispersing prisms.
N-SF10 is a type of Schott glass with even higher refractive
index than N-BK7 and N-F2, and offers greater dispersion.
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Its transmission is best in the visible and near-infrared,
from 400 nm – 2.4 μm. Though thermal characteristics
are similar to N-BK7, it is a slightly softer glass, resulting
in lenses with lower surface quality than their N-BK7
equivalents. Its high refractive index maximizes the
deviation of an input beam, while its strong dispersing
power facilitates wavelength separation and correction
of group delay dispersion in femtosecond laser systems,
making it well worth its increased cost.
Prism Quality & Laser Damage Threshold
Prism quality is important to consider when working with
high power lasers, or in performance-critical applications.
All of our prisms are manufactured to high quality
standards, though this can vary slightly from one material
to another or between prism types. Transmitted wavefront
error or surface figure ranges from λ/4 to λ/10 for all of
our N-BK7 prisms, while prisms made from other materials
maintain consistent λ/10 performance. Surface quality
is 20-10 scratch and dig for N-BK7 and 30-10 for N-F2 &
N-SF10, while fused silica, Suprasil 1, and crystal quartz are
delivered to 10-5 scratch and dig. Our single wavelength
AR coatings reduce stray reflections to ≤ 0.25% per surface
(R ≤ 0.50% per surface for broadband AR coatings).
These stringent specifications combine to yield high laser
damage thresholds for all our prisms.
Making the final decision
Much of the final decision in choosing a prism comes
down to material and coating options, but angular
deviation or wedge tolerance should also be considered.
Prisms perform best when working with collimated
beams, but if you are working with a slightly converging
or diverging beam, be sure to model the impact this may
have on total internal reflection or ability of the prism
to separate wavelengths spatially. Broadband metal
coatings can be used to mitigate TIR failure for imperfectly
collimated beams, in which case they will yield a small but
equal amount of loss for all wavelengths and angles. The
deep product knowledge available through CVI Laser
Optics technical support can help you to navigate some of
the more sophisticated considerations in prism selection
and help you to find the solution you need.
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Selection Guide:
Product Code Description
Angular deviation /wedge
Additional Features
IF
Interferometer Flats
30 ± 5 arc min
▪ N-BK7 or UV-grade fused silica
▪ For AR-coated options, see W1-IF & W2-IF
LW
Large Wedge Windows
1° ± 6 arc min
▪ N-BK7 or UV-grade fused silica
▪ 1° or 3° wedge, others custom
▪ Custom antireflection coating options
OR
3° ± 6 arc min
RAP
Uncoated Right-Angle
Prisms
± 3 arc min
▪ N-BK7 or UV-grade fused silica
▪ Custom dielectric or metal coating options
P90
Precision Right-Angle
Prisms
± 3 arc min
▪ N-BK7 or UV-grade fused silica
▪ Square faces AR coated (legs)
▪ Custom dielectric or metal coating options
P180
Precision Folding Prisms
± 3 arc min
▪ N-BK7 or UV-grade fused silica
▪ Hypotenuse AR coated
▪ Custom dielectric or metal coating options
PLBC
Pellin Broca Prisms
a: ± 30 arc min
▪
▪
▪
▪
β: ± 2°
Suprasil 1, UV-grade fused silica, or crystal quartz
Turns beam by 90°
Wavelength dispersion for beam separation
Uncoated; custom coatings available
PORR
Porro Prisms
± 10 arc sec
▪ N-BK7 or UV-grade fused silica
▪ Hypotenuse AR coated (round face)
▪ Custom dielectric or metal coating options
EDP
High-Precision Equilateral
Dispersing Prisms
± 3 arc min
▪ N-BK7, N-F2 or UV-grade fused silica
▪ Minimum loss for rays parallel to bottom of prism
▪ Custom antireflection coating options
IB
Isosceles Brewster Prisms
± 2 arc min
▪ Suprasil 1, N-SF10 glass or UV-grade fused silica
▪ Lower dispersion than equilateral prisms
▪ Custom antireflection coating options
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