Prism Pairs

Author: the photonics expert Dr. Rüdiger Paschotta (RP)

Definition: combinations of two prisms, mostly used for dispersion compensation

More specific term: anamorphic prism pairs

Categories: article belongs to category general optics general optics, article belongs to category light pulses light pulses, article belongs to category methods methods

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DOI: 10.61835/d8q Cite the article: BibTex plain text HTML Link to this page! LinkedIn

Pairs of (typically Brewster-angled) prisms can be used as beam expanders – see the article on anamorphic prism pairs. Another application, discussed in this article, is either for optical filtering or for introducing anomalous chromatic dispersion e.g. into a laser resonator without introducing significant power losses. A first prism refracts different wavelength components to slightly different angles. A second prism then refracts all components again to let them propagate in parallel directions after that prism (see Figure 1), but with a wavelength-dependent position (which is sometimes called a spatial chirp).

With a second prism pair, or simply by reflecting the beams back through the original prism pair (possibly with a small vertical offset for easier separation from the input beam), all wavelength components can later be spatially recombined; the total chromatic dispersion (see below) is then twice that for a single pass through the prism pair.

The spatial separation of different wavelength (or frequency) components can be utilized in different ways:

An optical filter can be realized e.g. by inserting a knife edge from one side, attenuating primarily the short- or the long-wavelength components. This can be used e.g. for wavelength tuning of lasers by placing such a prism pair within the laser resonator.
The wavelength-dependent optical path length of such a dispersive delay line leads to anomalous chromatic dispersion [2], which may be partly offset by material dispersion in the prisms. The overall chromatic dispersion can be adjusted by varying the insertion of one or both prisms into the beam. This technique is often used to provide adjustable dispersion compensation in mode-locked lasers (Figure 2) (e.g. for soliton mode locking) and for dispersive compression (or stretching) of optical pulses. For calculating the obtained amount of chromatic dispersion (second and higher orders), one may use Ref. [2] or some suitable software.

cavity of mode-locked laser — Figure 2: Resonator setup of a mode-locked laser. A prism pair is used for dispersion compensation. The overall anomalous chromatic dispersion allows for soliton mode locking, and can be adjusted via the prism insertion.

Typical amounts of anomalous group delay dispersion from prism pairs are up to a few thousand fs². This is often sufficient for dispersion compensation in mode-locked bulk lasers, but usually not for chirped-pulse amplification, for example. For larger amounts of dispersion, a pair of diffraction gratings may be required; these exhibit far greater angular dispersion and thus also greater chromatic dispersion. The attraction of using a prism pair, however, is that anomalous dispersion can be provided without introducing substantial losses into a laser resonator, assuming operation with p-polarized beams close to Brewster's angle.

group delay dispersion of silica and SF10 prism pairs — Figure 3: Single-pass group delay dispersion of prism pairs: comparison of a setup with silica prisms, 50 cm spacing, and another one with SF10 prisms, 20 cm spacing. At 800 nm, the sum of the prism insertions is 4 mm and the beams are at Brewster's angle. The SF10 prisms can generate more dispersion, but the higher-order dispersion is significantly higher. The calculations have been done with the RP Fiber Power software.

Fig. 4 shows the chromatic dispersion of a pair of SF10 prisms for different values of the prism insertion; this demonstrates how the dispersion can be adjusted simply by translating a prism.

Figure 4: Single-pass group delay dispersion of SF10 prism pairs with different amounts of beam insertion from 2 mm to 6 mm in steps of 1 mm. Stronger insertion leads to the higher curves, which also extend more towards shorter wavelengths. In practice, the prisms cannot be operated close to the left end of such a curve due to beam clipping.

For the compression of ultrashort pulses in the few-cycle region, prisms with a fairly small apex angle (and anti-reflection coatings) are sometimes used. Such configurations can achieve a lower residual chirp from higher-order dispersion. However, it is often necessary to compensate the higher-order dispersion with other means, e.g. with additional dispersive mirrors.

More to Learn

Prisms

Dispersion compensation

Anamorphic prism pairs

Diffraction gratings

Suppliers

The RP Photonics Buyer's Guide contains 191 suppliers for prisms. Among them:

UM Optics

UM OPTICS offers right-angle prisms based on calcium fluoride(CaF₂), silicon (Si) and zinc selenide (ZnSe) in a variety of sizes.

IRD Glass

IRD Glass manufactures custom optical prisms for nearly any application – from laser optics to imaging devices. We produce basic right-angle prisms, rhomboid prisms, wedge prisms, penta prisms, dove prisms, roof prisms and many other challenging variations and assemblies – even completely new optical prisms for new applications as our customers’ industries require them.

Perkins Precision Developments

Perkins Precision Developments (PPD) manufactures custom, high-precision prisms and prism assemblies of all shapes and sizes and in a variety of materials including fused silica, fused quartz, N-BK7, YAG, ZnSe, SF11 and other high index glasses. Prisms are available uncoated or coated with our low-loss, high energy IBS thin film coatings for applications in the UV, visible and near-infrared (NIR). Typical coatings include anti-reflection (AR), high-reflector (HR), dichroic, beam splitter, and polarizing designs.

Common types of thin film coated prisms that we produce for both laser and imaging applications include right angle turning prisms, Risley prisms, equilateral dispersion prisms and penta prisms. Precision optical assemblies such as polarizing beam splitter cubes and dichroic prism beam splitters are also an option.

Our technical sales team will work with you at the design and quotation phases to ensure the optimal balance of performance, manufacturability and cost.

Ecoptik

Ecoptik produces optical prisms for various applications. We can make many different geometrical shapes, e.g. roof prisms, tetrahedral, wedge, octagon, penta, Powell and right angle prisms. We are happy to develop custom versions for you.

EKSMA OPTICS

The EKSMA Optics optical prism product line includes wedge prisms, laser dispersing prisms, Pellin–Broca prisms, right angle prisms, corner cubes and non-polarizing cube beamsplitters.

Artifex Engineering

Artifex Engineering offers a wide variety of custom prisms, including all typical designs tailored to your requirements (right angle prism, dove prism, pentaprism, corner cube reflector, wedge prism). Prisms are used for reflection, deviation and image manipulation. Visit our product page for more information. We look forward to your inquiry.

Sinoptix

We offer custom optical prisms from 0.3 mm to 500 mm with up to λ/10 flatness and custom coatings.

Knight Optical

Knight Optical offers an extensive range of stock prisms which are suitable for a wide array of projects and applications. Our range of prisms includes Amici, corner cube, Dove, equilateral, Fresnel, homogenising light pipe, Pellin–Broca, penta, Porro, rhomboid, right angle, as well as prisms manufactured from UV and infrared substrates. Custom prisms are also available in different substrates and with applied coatings.

Shalom EO

With decades of professional experience and a profound engineering background, Hangzhou Shalom EO delivers a full portfolio of optical prisms ideal for light deviation, rotation/inversion of the image, beam steering/aligning, etc. where versatile characteristic of our prisms allows for their widespread applications in periscopes, binoculars, lasers, interferometers, spectroscopes, cameras, laser rangefinders.

Shalom EO supplies diverse prism types, including: right angle prisms, corner cube retroreflectors, wedge prisms, Brewster angle prisms, and equilateral prisms, in addition to Penta prisms, dove prisms, roof prisms, and Porro prisms. There is a wide selection of materials: BK7, N-BK7, UV-grade fused silica, N-SF11, CaF₂, Ge and ZnSe. Coating options include uncoated, MgF₂ AR coating, broadband AR coating, and laser line wavelength V-coating.

Shalom EO is an expert supplier of optical prisms, we implement both in-process quality control and optical inspection for finished products, where Shalom EO leverages state-of-the-art metrological equipment, such as Zygo interferometers and PerkinElmer Lambda 1050+ spectroscopes, to uphold the consistent reliability of our optical components.

Laserton

Laserton has a wide range of optical prisms, including penta prisms, corner cube retroreflectors, anamorphic prisms, Dove prisms, right angled prisms, pyramid prisms, beam splitter penta prisms, half penta prisms, roof prisms, wedge prisms, equilaterla prisms, Porro prisms, rhomboid prisms, Brewster prisms and Pellin—Broca prisms. Also, we can produce custom prisms of other forms.

Avantier

An optical prism is a solid, transparent optic with flat surfaces which refract light. Light enters through one surface, and may be rotated, deviated, or dispersed before leaving through another surface. These surfaces are highly polished, and the angles between them, the number of surfaces, and the position of them determine the function of the prism.

Shanghai Optics

Shanghai Optics can provide custom high-precision prisms with arbitrary angles, sizes below 300 mm and angular accuracy of 1 arcsec. Customized coatings are available for different UV, visible and infrared wavelengths according to customer requirements.

Bibliography

[1]	O. E. Martinez, J. P. Gordon and R. L. Fork, “Negative group-velocity dispersion using refraction”, J. Opt. Soc. Am. A 1 (10), 1003 (1984); https://doi.org/10.1364/JOSAA.1.001003
[2]	R. L. Fork et al., “Negative dispersion using pairs of prisms”, Opt. Lett. 9 (5), 150 (1984); https://doi.org/10.1364/OL.9.000150
[3]	J. D. Kafka and T. Baer, “Prism-pair dispersive delay lines in optical pulse compression”, Opt. Lett. 12 (6), 401 (1987); https://doi.org/10.1364/OL.12.000401

(Suggest additional literature!)

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