Optical prism : a complete guide

Most of us remember the famous rock band Pink Floyd’s “Dark side of the moon” album  cover showing a light beam been diffracted in a “rainbow” through a triangular prism. That is a very good summary of how light beam travel through a prism, below you’ll find a complete guide to look further in optical prisms.

If you want to learn more, please visit our website optec.

What is a optical prism ?

Definition

An optical prism is a bloc of optical material cut in geometrical shape for the specific purpose of modifying the incident light beam by affecting it’s direction, rotational angle or diffraction.

In optical systems they are used to turn a signal or separate the signal according to it’s wavelengths.

Specifications

Many different optical materials can be used to manufacture a prism, depending of the type of prisms and the wavelength range they will be used in. Optical prisms can be provided with below materials  :

• Optical glass : B270, H-K9L / NBK7, F2, N-SF11
• Fused silica
• IR material : CaF2, BaF2, MgF2, Silicon, Ge
• Sapphire
• Many other references

Below table shows the typical specifications / tolerances for optical prism manufacturing :

Specification Detail Dimensional range 0.3mm to 300mm Dimensional tolerances +/- 0.1mm (normal) or +/-0.05mm Surface flatness up to Lambda /10 Surface quality S&D 40/20 Angle tolerance up to +/- 1 arcmin

Prisms can be coated, AR to reduce signal reflection on the surface or Reflective coatings to enhance the signal reflection at specific wavelengths.

Different types of optical prisms

Type of prism Description Usage Image Right angle prism Prism with a right angle triangular cut shape with both sides opposite to the hypotenuse of equal dimension. They are used to deviate a light path to 90° or 180° by TIR (Total Internal Reflection) Penta prism Prism with a pentagone cut shape, with specific angles. Penta prisms are used to redirect a signal by 90° without inverting nor reversing the image. Retroreflector Round base prism with three face pyramidal sphape. Return the light at 180° even without alignment of the incident light and the retroreflector. Used in topograply and other applications. Dove prism Parallelepiped which correspond at a truncated right angle prism. Rotate the image with no deviation of the beam. The rotation angle is equal to twice the prism optical axis angle compare to the incident beam. Roof prism Roof shaped prism. Used to deviate the signal at 90° while inverting it. Equilateral prism Prism made of high diffractive Abbe number glass in a shape of an equilateral triangle. Used to disperse the light with different angles for each wavelength. (think of the Pink Floyd album cover). Pellin Broca prism Quadrilateral prism with angles of 90°, 75°, 135° and 60°. Deviate the light by 90°, used in spectroscopy and for harmonic separations. Wedge prism Prism made of a plan surface and and other tilt surface. Used to deviate light signal (mostly for laser beams) Micro prism Prism with dimensions between 0,3 and 3mm of side. Used in optical fiber sysems and endoscopy. Glued prism Assembly of custom prism made by gluing / cementing, can include air space in between parts. Custom made to fit customer applications.

Where to buy optical prism ?

If you are looking for prisms to be used in optical application, you should look for precision optics suppliers, with experience in the optical material you are looking for.

Please note that as you can notice above in the description of the manufacturing process, optical prisms can be manufactured in parallel, for almost each step, therefore order quantities are a key point in manufacturing cost valuation.

Prepare all the below information :

• Material
• Technical drawing
• Coating request
• Expecting quantities

And then send your request to your favorite precision optics supplier. By click on the below you can contact SINOPTIX directly for your request.

Definition: transparent optical devices which refract or reflect light

More specific terms: anamorphic prism pairs, corner cube prisms, wedge prisms

Category: general optics

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For purchasing prisms, use the RP Photonics Buyer's Guide – an expert-curated directory for finding all relevant suppliers, which also offers advanced purchasing assistance.

Optical prisms are transparent devices, in most cases consisting of some type of optical glass, through which light can be sent. As the end faces are not parallel to each other, refraction (a change in beam direction) occurs, which is somewhat wavelength-dependent due to the chromatic dispersion of the material. In some cases, however, one uses total internal reflection, and the output beam direction may then not be wavelength-dependent.

Reflections at prism surfaces are often unwanted. In some cases, they are suppressed at least for p polarization by having a beam angle close to Brewster's angle. In other cases, one applies anti reflection coatings to the surfaces. However, there are also reflecting prisms (see below), having mirror coatings on some surfaces.

Prisms find many different applications in optics; some of them are discussed below.

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Dispersive Prisms

If a laser beam propagates through a prism, where the end faces are not parallel to each other, there is a beam deflection, the magnitude of which depends on the refractive index. Due to the chromatic dispersion of the material, the deflection angle becomes wavelength-dependent. This phenomenon is exploited in dispersive prisms as used for various purposes:

• One can use a prism as a polychromator, i.e., to separate wavelength components with substantially different wavelengths in a beam. For example, one can separate a frequency-doubled beam from residual fundamental light. Also, one may use that effect in a spectrometer, but only with a poor wavelength resolution, as the angular dispersion is not very high.
• Similarly, one may combine beams at different wavelengths (→ spectral beam combining). (If the wavelengths are relatively close, diffraction gratings are better suited, as they offer a much higher angular dispersion.)
• An intracavity prism in a laser can be used for wavelength tuning.
• Dispersive prism pairs are used to generate chromatic dispersion which is not just that of the prism material, as the path length in the whole setup also becomes wavelength-dependent (see Figure 1). This methods is used, for example, for dispersion compensation in mode-locked lasers. Anomalous dispersion can be obtained even if the prism dispersion is normal.

Typically, one uses a symmetric configuration, where the input and output beams have approximately the same angle against the corresponding surface. This allows one to have Brewster's angle at both surfaces, provided that the prism angle is chosen appropriately. Also, one avoids changes in the beam size. A prism is easily aligned to that symmetric configuration, as it leads to the smallest deflection angle.

If one uses Schott F10 glass as an example of a highly dispersive flint glass, a prism angle of 60° as obtained in a equilateral triangle is quite suitable, as it allows for an approximately symmetric configuration with input and output angles close to Brewster's angle, which is also close to 60°.

Reflecting Prisms

Reflecting prisms are optical prisms where one exploits the reflection of light at at least one surface. The reflection may either be caused by a coating applied to the surface – a dielectric coating or a metal coating – or one may use total internal reflection, if the angle of incidence is large enough.

Prisms for Image Rotation, Polarization Manipulation and Beam Shifting

Prisms with multiple internal reflections (e.g. pentaprisms) are often used in imaging systems, for example to perform image rotations in the viewfinders of photo cameras and in binoculars. Other devices are used for manipulating the polarization state of light.

Different kinds of rotations are reflections can be achieved with prisms of different geometries, causing different numbers of reflections. Also, prisms can be used for achieving transverse offsets of images or laser beams, where the alignment tolerances are less strict than for mirror arrangements.

Retroreflector Prisms

Some reflecting prisms are used as retroreflectors, where one exploits total internal reflection at two different locations (Figure 2). The reflected beam is parallel to the incoming beam, if the angle between the reflecting surfaces is 90° – even if the prism is somewhat rotated around an axis perpendicular to the drawing plane; only the beam offset can be somewhat changed. For a prism with two reflections as shown, this principle does not work for a prism rotation around an axis in the plane. There are corner cube prisms where reflections on three mutually perpendicular services occur, so that slight rotations of the prism around any axis will not change the direction of the outgoing beam.

Note that a mirror would be different in that respect: a tilt of the mirror would change the beam direction by twice the tilt angle. Prism retroreflectors are much simpler to align, as their exact orientation does not matter. The crucial advantage of prisms results from the fact that any rotation affects the direction of more than one relevant reflecting surface, but maintaining their relative orientations.

Wavelength-dependent refraction at the input/output prism surface is not relevant in the shown configuration.

Note that phase changes for total internal reflection in prisms are polarization-dependent. Therefore, arbitrary polarization states can generally not be preserved.

Anamorphic Prisms

Anamorphic prisms are used for modifying the beam size in one direction. Here, one uses substantially different angles of the input and output beam with respect to the corresponding surfaces – for example, normal incidence at the input (see Figure 3). The beam size is changed only in one direction – not due to any kind of focusing, but simply due to the geometry.

As at least one of the beams is far from Brewster's angle, one often uses anti-reflection coatings.

If the change in beam direction is disturbing, one can use a prism pair which is oriented such that there is only a parallel shift of the beam.

A typical application of anamorphic prisms is for symmetrizing the output beam of a laser diode. One often uses anamorphic prism pairs for that purpose to keep the beam direction unchanged.

Compound Prisms

Compound prisms are made by contacting two or more prisms consisting of different materials. For example, a double-Amici prism is made such that the refraction at the internal surface leads to an overall zero deflection angle, but to a wavelength-dependent beam offset. It can be used in simple low-resolution spectrometers.

Prism Polarizers

Polarizers are often made in the forms of prisms, e.g. Glan–Taylor prisms and Wollaston prisms. The article on polarizers gives more details.

Conical Prisms

There are prisms hearing a conical surface; these are called axicons.

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