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Depolarizer (optics)
Encyclopedia
A depolarizer or depolariser is an optical device used to scramble the polarization of light. An ideal depolarizer would output randomly polarized light whatever its input, but all practical depolarizers produce pseudo-random output polarization.
Optical systems are often sensitive to the polarization of light reaching them (for example grating
-based spectrometer
s). Unwanted polarization of the input to such a system may cause errors in the system's output.
The Cornu depolarizer was one of the earliest designs, named after its inventor Marie Alfred Cornu
. It consists of a pair of 45 degree prisms of quartz crystal, optically contacted to form a cuboid. The fast axes
are 90 degrees apart and 45 degrees from the sides of the depolarizer (see figure). Any ray entering the prism effectively passes through two wave plate
s. The thickness of these wave plates and therefore their retardance varies across the beam. The phase shift is given by
.
For an input beam of uniform polarization the output polarization will be periodic in
. The phase shift is also dependent on wavelength
due to dispersion
.
The use of two prisms means that the output is essentially coaxial with the input. At the interface between the prisms refraction does take place, as the refractive indices
are exchanged. There is therefore some separation of the components of the output beam.
This device is not commonly used today, but similar designs are commercially available.
The Lyot depolarizer is another early design. It was invented by Bernard Lyot
. It consists of two wave plates with their fast axes 45 degrees apart, with the second plate twice the thickness of the first. The output is periodic as a function of wavelength and as a function of the wave-plates' thickness. Special considerations are needed when this depolarizer is to be used for a particular application, because the optimal wave-plate thicknesses depend on the signal wavelength and optical spectrum with which it is to be used. It is commercially available for broadband visible applications.
This device is especially attractive in fiber-optics where two pieces of correct length of polarization-maintaining optical fiber
spliced together at 45% angle are used instead of the wave-plates, thus no other components such as beam splitter
s are required.
The quartz-silica wedge depolarizer is a common commercial design, and is similar to the Cornu depolarizer, however the angle between the two components is much smaller (2° is typical) and only the first component is birefringent
. The second component is made of fused silica, which has a very similar refractive index to quartz, but is not birefringent. The fast axis of the quartz element is generally at 45 degrees to the wedge. The whole device is much more compact than a Cornu depolarizer (for the same aperture).
As with the Cornu depolarizer, there is some separation of the output as a function of polarization, as well as some beam deviation due to the imperfect match in refractive index between quartz and silica. The output is periodic across the depolarizer. Because the wedge angle is so much smaller than in a Cornu depolarizer the period is larger, often around 6 mm. This depolarizer also has a preferred orientation because of its single defined fast axis. In commercial wedge depolarizers this is usually marked.
Other birefringent materials can be used in place of quartz in the above designs.
Wedge depolarizers exhibit some small beam deviation. This is true even if the faces of the optic are exactly parallel. Because each half of the optic is a wedge, and the two halves do not have exactly the same refractive index (for a particular polarization), the depolarizer is effectively very slightly wedged (optically).
to start with. It is also possible to use a rotating half-wave plate to produce polarization which is periodic in time, and therefore effectively scrambled for sufficiently slow responses. Other methods have been demonstrated, such as the use of Faraday rotator
s and liquid crystal
s. It is also possible to depolarize light using fiber optics.
Optical systems are often sensitive to the polarization of light reaching them (for example grating
Diffraction grating
In optics, a diffraction grating is an optical component with a periodic structure, which splits and diffracts light into several beams travelling in different directions. The directions of these beams depend on the spacing of the grating and the wavelength of the light so that the grating acts as...
-based spectrometer
Spectrometer
A spectrometer is an instrument used to measure properties of light over a specific portion of the electromagnetic spectrum, typically used in spectroscopic analysis to identify materials. The variable measured is most often the light's intensity but could also, for instance, be the polarization...
s). Unwanted polarization of the input to such a system may cause errors in the system's output.
Cornu depolarizer
Marie Alfred Cornu
Marie Alfred Cornu was a French physicist. The French generally refer to him as Alfred Cornu.Cornu was born at Orléans and was educated at the École polytechnique and the École des mines...
. It consists of a pair of 45 degree prisms of quartz crystal, optically contacted to form a cuboid. The fast axes
Birefringence
Birefringence, or double refraction, is the decomposition of a ray of light into two rays when it passes through certain anisotropic materials, such as crystals of calcite or boron nitride. The effect was first described by the Danish scientist Rasmus Bartholin in 1669, who saw it in calcite...
are 90 degrees apart and 45 degrees from the sides of the depolarizer (see figure). Any ray entering the prism effectively passes through two wave plate
Wave plate
A wave plate or retarder is an optical device that alters the polarization state of a light wave travelling through it.- Operation :A wave plate works by shifting the phase between two perpendicular polarization components of the light wave. A typical wave plate is simply a birefringent crystal...
s. The thickness of these wave plates and therefore their retardance varies across the beam. The phase shift is given by
.For an input beam of uniform polarization the output polarization will be periodic in
. The phase shift is also dependent on wavelengthWavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
due to dispersion
Dispersion (optics)
In optics, dispersion is the phenomenon in which the phase velocity of a wave depends on its frequency, or alternatively when the group velocity depends on the frequency.Media having such a property are termed dispersive media...
.
The use of two prisms means that the output is essentially coaxial with the input. At the interface between the prisms refraction does take place, as the refractive indices
Refractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....
are exchanged. There is therefore some separation of the components of the output beam.
This device is not commonly used today, but similar designs are commercially available.
Lyot depolarizer
Bernard Lyot
Bernard Ferdinand Lyot was a French astronomer.His interest in astronomy started in 1914. He soon acquired a telescope and soon upgraded to a . From graduation in 1918 until 1929, he worked as a demonstrator at the Ecole Polytechnique...
. It consists of two wave plates with their fast axes 45 degrees apart, with the second plate twice the thickness of the first. The output is periodic as a function of wavelength and as a function of the wave-plates' thickness. Special considerations are needed when this depolarizer is to be used for a particular application, because the optimal wave-plate thicknesses depend on the signal wavelength and optical spectrum with which it is to be used. It is commercially available for broadband visible applications.
This device is especially attractive in fiber-optics where two pieces of correct length of polarization-maintaining optical fiber
Polarization-maintaining optical fiber
In fiber optics, polarization-maintaining optical fiber is optical fiber in which the polarization of linearly polarized light waves launched into the fiber is maintained during propagation, with little or no cross-coupling of optical power between the polarization modes...
spliced together at 45% angle are used instead of the wave-plates, thus no other components such as beam splitter
Beam splitter
A beam splitter is an optical device that splits a beam of light in two. It is the crucial part of most interferometers.In its most common form, a rectangle, it is made from two triangular glass prisms which are glued together at their base using Canada balsam...
s are required.
Quartz-silica
Birefringence
Birefringence, or double refraction, is the decomposition of a ray of light into two rays when it passes through certain anisotropic materials, such as crystals of calcite or boron nitride. The effect was first described by the Danish scientist Rasmus Bartholin in 1669, who saw it in calcite...
. The second component is made of fused silica, which has a very similar refractive index to quartz, but is not birefringent. The fast axis of the quartz element is generally at 45 degrees to the wedge. The whole device is much more compact than a Cornu depolarizer (for the same aperture).
As with the Cornu depolarizer, there is some separation of the output as a function of polarization, as well as some beam deviation due to the imperfect match in refractive index between quartz and silica. The output is periodic across the depolarizer. Because the wedge angle is so much smaller than in a Cornu depolarizer the period is larger, often around 6 mm. This depolarizer also has a preferred orientation because of its single defined fast axis. In commercial wedge depolarizers this is usually marked.
Quartz-quartz
Quartz-quartz wedge depolarizers are commercially available, though not common. They are similar to Cornu depolarizers, but with the small wedge angle of the silica-compensated wedge.Other birefringent materials can be used in place of quartz in the above designs.
Wedge depolarizers exhibit some small beam deviation. This is true even if the faces of the optic are exactly parallel. Because each half of the optic is a wedge, and the two halves do not have exactly the same refractive index (for a particular polarization), the depolarizer is effectively very slightly wedged (optically).
Other ways to produce depolarized light
In many applications it is possible to use a quarter-wave plate to produce circularly polarized light, but this is only possible for light of a limited range of wavelengths which is linearly polarizedLinear polarization
In electrodynamics, linear polarization or plane polarization of electromagnetic radiation is a confinement of the electric field vector or magnetic field vector to a given plane along the direction of propagation...
to start with. It is also possible to use a rotating half-wave plate to produce polarization which is periodic in time, and therefore effectively scrambled for sufficiently slow responses. Other methods have been demonstrated, such as the use of Faraday rotator
Faraday rotator
A Faraday rotator is an optical device that rotates the polarization of light due to the Faraday effect, which in turn is based on a magneto-optic effect....
s and liquid crystal
Liquid crystal
Liquid crystals are a state of matter that have properties between those of a conventional liquid and those of a solid crystal. For instance, an LC may flow like a liquid, but its molecules may be oriented in a crystal-like way. There are many different types of LC phases, which can be...
s. It is also possible to depolarize light using fiber optics.