Above: An ultrafast laser dispersing (ULD) prism pair with apex angle α and separation d can be used to spatially disperse different wavelength components and introduce wavelength-dependent phase changes. To learn more, please see the Tutorial tab.

Features

• Designed Spectral Range: 700 - 900 nm
• Apex Angle Tolerance: ±15 arcmin
• Unpolished Sides: Flat and Ground
• Surface Flatness: λ/4 at 633 nm
• Use to Compensate for Dispersion
• Fabricated from CaF₂, Fused Silica, SF10, or N-SF14

These Dispersion-Compensating Prism Pairs are designed to compensate for the pulse broadening effects that occur in ultrafast laser systems without introducing significant power losses. Four large-index-of-refraction prism materials are offered to provide various group velocity dispersion (GVD) values, thereby compensating for different pulse dispersion conditions.

Applications
Prism paris such as these have multiple applications. They can be used to create an optical filter; simply insert a knife edge to attenuate the short- or long-wavelength components. If the prism pair is placed inside a laser resonator, they can be used for wavelength tuning. Alternatively, prism pairs can be used for dispersion compensation. Each time light passes through the optical elements within a laser cavity, positive (commonly referred to as normal) dispersion occurs. The prism pair also contributes positive dispersion. However, this positive dispersion is somewhat offset by the extra distance that the longer wavelengths travel through the second prism (refer to the figure below). With a careful choice of prism pair geometery it is possible to create negative (commonly referred to as anomalous) dispersion that compensates for all the other elements within the cavity.

Specifications (Example Calculations)

^a Group Velocity Dispersion (see the Graphs tab for additional information)
^b Third-Order Dispersion for Prism Pairs
^c Apex Angle Tolerance: ±15 arcmin
^d Brewster's Angle

The broadband frequency spectrum of ultrashort laser pulses brings rise to the necessity for dispersion compensation. Whenever an ultrashort pulse passes through a positive dispersive material, the pulse becomes “stretched” due to the frequency dependency of the refractive index of a material. In order to maintain pulse shape and duration, a negative dispersive system is required. While there are various methods for producing dispersion control, such as grating pairs or chirped mirrors; prism pairs offer a simple, tunable, and low loss solution for dispersion compensation. Through adjusting the relative geometry of the prism pair, the dispersion can be adjusted from positive through negative values.

The apex angle in the ULD prism pairs is chosen such that the input and output angles are both near Brewster's Angle (Θ_B) in order to minimize the reflection of P-polarized light. The accumulated phase φ(ω) in a prism pair is equal to

The variables ω, d, Θ_f, and Θ_f^short correspond to frequency, prism separation, exit angle (frequency dependent), and exit angle of the shortest transmitted wavelength, respectively.

The variables n, Θ_B, and α correspond to the frequency dependent refractive index of the prism material, angle of incidence with the surface of the first prism, and apex angle of the prism, respectively. The second and third derivatives of the accumulated phase with respect to frequency are defined to be the Group Velocity Dispersion (GVD) and the Third-Order Dispersion (TOD), respectively.

As can be demonstrated through Fourier analysis, ultrashort pulses (small Δt) innately possess a broadband frequency spectrum (large Δf). However, due to the wavelength-dependent nature of the index of refraction, longer wavelengths travel faster through an optical material than short wavelengths. Consequently, femtosecond pulses are subject to pulse broadening as a result of group velocity dispersion (GVD). These effects can be reversed by using a prism pair such as those offered here to provide a negative dispersive system in order to shorten the duration of an ultrashort laser pulse.

To achieve the specified GVD values while also maximizing transmission, light should be incident on the first prism at Brewster's Angle (Θ_B). As shown in the schematic above, the first prism is used to separate the various wavelength components. Then, a second prism is positioned such that the various wavelengths of refracted light rays will propagate parallel to each other upon exiting. Furthermore, by including another pair of prisms (4 prisms total), the final rays can be made to propagate collinearly with each other as well as with the path of the incident rays [1]. This feature can be particularly useful when installing a dispersion compensator into an existing optical system.

[1] R. L. Fork, O. E. Martinez, and J. P. Gordon, "Negative dispersion using pairs of prisms," Opt. Lett. 9, 150-152 (1984).

Selection Guide for Prisms

Thorlabs offers a wide variety of prisms, which can be used to reflect, invert, rotate, disperse, steer, and collimate light. Prisms are available in N-BK7, UV Fused Silica, F2, N-SF11, α-BBO, N-KZFS8, Ge, and CaF₂. For prisms and substrates not listed below, please contact tech support.

Beam Steering Prisms

Prism	Material	Deviation	Invert	Reverse or Rotate	Illustration	Applications
Right Angle Prisms	N-BK7, UV Fused Silica, Germanium, or Calcium Fluoride	90°	90°	No		90° reflector, independent of entrance beam angle. Used in optical systems such as telescopes and periscopes.
		180°	180°	No		180° reflector, independent of entrance beam angle. Acts as a non-reversing mirror and can be used in binocular configurations.
Retroreflectors and Mounted Retroreflectors	N-BK7	180°	180°	No		180° reflector, independent of entrance beam angle. Beam alignment and beam delivery. Substitute for mirror in applications where orientation is difficult to control.
Penta Prisms and Mounted Penta Prisms	N-BK7	90°	No	No		90° reflector, without inversion or reversal of the beam profile. Can be used for alignment and optical tooling.
Roof Prisms	N-BK7	90°	90°	180o Rotation		90° reflector, inverted and rotated (deflected left to right and top to bottom). Can be used for alignment and optical tooling.
Dove Prisms and Mounted Dove Prisms	N-BK7	No	180°	2x Prism Rotation		Dove prisms may invert, reverse, or rotate an image based on which face the light is incident on. Prism in a beam rotator orientation.
		180°	180°	No		Prism acts as a non-reversing mirror. Same properties as a retro-reflector or right angle (180° orientation) prism in an optical setup.
Wedge Prisms	N-BK7	Models Available from 2° to 10°	No	No		Beam steering applications. By rotating one wedged prism, light can be steered to trace the circle defined by 2 times the specified deviation angle.
			No	No		Variable beam steering applications. When both wedges are rotated, the beam can be moved anywhere within the circle defined by 4 times the specified deviation angle.
Coupling Prisms	Rutile (TiO2) or GGG	Variable*	No	No		High index of refraction substrate used to couple light into films. Rutile used for nfilm > 1.8 GGG used for nfilm < 1.8

* Depends on angle of incidence and index of refraction

Dispersive Prisms

Prism	Material	Deviation	Invert	Reverse or Rotate	Illustration	Applications
Equilateral Prisms	F2, N-SF11, Germanium, or Calcium Flouride	Variable*	No	No		Dispersion prisms are a substitute for diffraction gratings. Use to separate white light into visible spectrum.
Pellin Broca Prisms	N-BK7, UV Fused Silica, or CaF2	90°	90°	No		Ideal for wavelength separation of a beam of light, output at 90°. Used to separate harmonics of a laser or compensate for group velocity dispersion.
Dispersion Compensating Prism Pairs	Fused Silica, CaF2, SF10, or N-SF14	Variable Vertical Offset	No	No		Compensate for pulse broadening effects in ultrafast laser systems. Can be used as an optical filter, for wavelength tuning, or dispersion compensation.

* Depends on angle of incidence and index of refraction

Beam Manipulating Prisms

Prism	Material	Deviation	Invert	Reverse or Rotate	Illustration	Applications
Anamorphic Prism Pairs	N-KZFS8 or N-SF11	Variable Vertical Offset	No	No		Variable magnification along one axis. Collimating elliptical beams (e.g., laser diodes) Converts an elliptical beam into a circular beam by magnifying or contracting the input beam in one axis.

Polarization Altering Prisms

Prism	Material	Deviation	Invert	Reverse or Rotate	Illustration	Applications
Glan-Taylor, Glan-Laser, and α-BBO Glan-Laser Polarizers	Glan-Taylor: Calcite Glan-Laser: α-BBO or Calcite	p-pol. - 0° s-pol. - 112°*	No	No		Double prism configuration and birefringent calcite produce extremely pure linearly polarized light. Total Internal Reflection of s-pol. at the gap between the prism while p-pol. is transmitted.
Rutile Polarizers	Rutile (TiO2)	s-pol. - 0° p-pol. absorbed by housing	No	No		Double prism configuration and birefringent rutile (TiO2) produce extremely pure linearly polarized light. Total Internal Reflection of p-pol. at the gap between the prisms while s-pol. is transmitted.
Double Glan-Taylor Polarizers	Calcite	p-pol. - 0° s-pol. absorbed by housing	No	No		Triple prism configuration and birefringent calcite produce maximum polarized field over a large half angle. Total Internal Reflection of s-pol. at the gap between the prism while p-pol. is transmitted.
Glan Thompson Polarizers	Calcite	p-pol. - 0° s-pol. absorbed by housing	No	No		Double prism configuration and birefringent calcite produce a polarizer with the widest field of view while maintaining a high extinction ratio. Total Internal Reflection of s-pol. at the gap between the prism while p-pol. is transmitted.
Wollaston Prisms Wollaston Polarizers	Calcite	Symmetric p-pol. and s-pol. deviation angle	No	No		Double prism configuration and birefringent calcite produce the widest deviation angle of beam displacing polarizers. s-pol. and p-pol. deviate symmetrically from the prism. Wollaston prisms are used in spectrometers and polarization analyzers.
Beam Displacing Prisms	Calcite	2.7 or 4.0 mm Beam Displacement	No	No		Single prism configuration and birefringent calcite separate an input beam into two orthogonally polarized output beams. s-pol. and p-pol. are displaced by 2.7 or 4.0 mm. Beam displacing prisms can be used as polarizing beamsplitters where 90o separation is not possible.
Fresnel Rhomb Retarders	N-BK7	Linear to circularly polarization Vertical Offset	No	No		λ/4 Fresnel Rhomb Retarder turns a linear input into circularly polarized output. Uniform λ/4 retardance over a wider wavelength range compared to birefringent wave plates.
		Rotates linearly polarized light 90°	No	No		λ/2 Fresnel Rhomb Retarder rotates linearly polarized light 90°. Uniform λ/2 retardance over a wider wavelength range compared to birefringent wave plates.

* s-polarized light is not pure and contains some p-polarized reflections.

Beamsplitter Prisms

Prism	Material	Deviation	Invert	Reverse or Rotate	Illustration	Applications
Beamsplitter Cube and Mounted Beamsplitter Cube	N-BK7 - Grade A 400-700 nm 700-1100 nm 1100-1600 nm	50:50 splitting ratio, 0° and 90° s- and p- pol. within 10% of each other	No	No		Double prism configuration and dielectric coating provide 50:50 beamsplitting nearly independent of polarization. Non-polarizing beamsplitter over the specified wavelength range.
Polarizing Beamsplitter Cube and Mounted Polarizing Beamsplitter Cube	SF2 420-680 nm 620-1000 nm 900-1300 nm 1200-1600 nm	p-pol. - 0° s-pol. - 90°	No	No		Double prism configuration and dielectric coating transmit p-pol. light and reflect s-pol. light. For highest polarization use the transmitted beam.