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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