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Prisms

Definition: transparent optical devices which refract or reflect light

German: Prismen

Category: general optics

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Optical prisms are transparent devices, in most cases consisting of some type of glass, through which light can be sent. As the end faces are not parallel to each other, refraction (a change of 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 sometimes the output beam direction is not 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.

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

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:

prism pair
Figure 1: A prism pair for spatially dispersing different wavelength components and thus also introducing wavelength-dependent phase changes and chromatic dispersion.

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 of 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°.

Retroreflector Prisms

prism retroreflector
Figure 2: A prism retroreflector. Even with some tilt of the prism, the direction of the reflected beam is not changed.

Right-angle prisms can also be 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 tilted. Only the beam offset can be somewhat changed.

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.

Wavelength-dependent refraction at the input/output prism surface is not relevant in this configuration, as the beam angles are approximately perpendicular to the surface.

Anamorphic Prisms

anamorphic prism
Figure 3: An anamorphic prism. The output beam is substantially narrower than the input beam.

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 of 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 in order 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.

Suppliers

The RP Photonics Buyer's Guide contains 60 suppliers for prisms. Among them:

See also: prism pairs, anamorphic prism pairs, polarizers, thin-film polarizers, spectrometers
and other articles in the category general optics

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