A polarization rotator is an optical device that rotates the
polarization axis of a
linearly polarized light beam by an angle of choice. Such devices can be based on the
Faraday effect, on
birefringence
Birefringence, also called double refraction, is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light. These optically anisotropic materials are described as birefrin ...
, or on
total internal reflection
In physics, total internal reflection (TIR) is the phenomenon in which waves arriving at the interface (boundary) from one medium to another (e.g., from water to air) are not refracted into the second ("external") medium, but completely refl ...
.
[ F. J. Duarte]
''Tunable Laser Optics''
2nd edition (CRC, New York, 2015). Chapter 5. Rotators of linearly polarized light have found widespread applications in modern optics since
laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word ''laser'' originated as an acronym for light amplification by stimulated emission of radi ...
beams tend to be linearly polarized and it is often necessary to rotate the original polarization to its orthogonal alternative.
[
]
Faraday rotators
A Faraday rotator consists of an optical material in a magnetic field. When light propagates in the material, interaction with the magnetic field causes left- and right-handed circularly polarized waves to propagate with slightly different phase velocities
The phase velocity of a wave is the rate at which the wave propagates in any medium. This is the velocity at which the phase of any one frequency component of the wave travels. For such a component, any given phase of the wave (for example, t ...
. Since a linearly-polarized wave can be described as a superposition of left- and right-handed circularly polarized waves, the difference in phase velocity causes the polarization direction of a linearly-polarized wave to rotate as it propagates through the material. The direction of the rotation depends on whether the light is propagating with or against the direction of the magnetic field: a rotation induced by passing through the material is not undone by passing through it in the opposite direction. This can be used to make an optical isolator.
Birefringent rotators
Half-wave plates and quarter-wave plates alter the polarization of light due to the principle of birefringence
Birefringence, also called double refraction, is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light. These optically anisotropic materials are described as birefrin ...
. Their performance is wavelength-specific; a fact that may be a limitation. Switchable wave plates can also be manufactured out of liquid crystal
Liquid crystal (LC) is a state of matter whose properties are between those of conventional liquids and those of solid crystals. For example, a liquid crystal can flow like a liquid, but its molecules may be oriented in a common direction as i ...
s, ferro-electric liquid crystals, or magneto-optic crystal
A magneto-optic effect is any one of a number of phenomena in which an electromagnetic wave propagates through a medium that has been altered by the presence of a quasistatic magnetic field. In such a medium, which is also called gyrotropic or g ...
s. These devices can be used to rapidly change the angle of polarization in response to an electric signal, and can be used for rapid polarization state generation (PSG) or polarization state analysis
Polarization or polarisation may refer to:
Mathematics
*Polarization of an Abelian variety, in the mathematics of complex manifolds
*Polarization of an algebraic form, a technique for expressing a homogeneous polynomial in a simpler fashion by ...
(PSA) with high accuracy. In particular, the PSG and PSA made with magneto-optic (MO) switches have been successfully used to analyze polarization mode dispersion
Polarization mode dispersion (PMD) is a form of modal dispersion where two different polarizations of light in a waveguide, which normally travel at the same speed, travel at different speeds due to random imperfections and asymmetries, causi ...
(PMD) and polarization dependent loss
Polarization or polarisation may refer to:
Mathematics
*Polarization of an Abelian variety, in the mathematics of complex manifolds
*Polarization of an algebraic form, a technique for expressing a homogeneous polynomial in a simpler fashion by ...
(PDL) with accuracies not obtainable with rotating waveplate methods, thanks to the binary nature of the MO switches. Furthermore, MO switches have also been successfully adopted to generate differential group delay
In signal processing, group delay and phase delay are functions that describe in different ways the delay times experienced by a signal’s various sinusoidal frequency components as they pass through a linear time-invariant (LTI) system (such as ...
for PMD compensation PMD may refer to:
Government and diplomacy
* Pakistan Meteorological Department
* Performance Management Division, of the Cabinet Office, Government of India
* Possible Military Dimensions of a nuclear program (specifically that of Iran)
Heal ...
and PMD emulation applications.
Prism rotators
Prism rotators use multiple internal reflections to produce beams with rotated polarization. Because they are based on total internal reflection, they are ''broadband''—they work over a broad range of wavelength
In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats.
In other words, it is the distance between consecutive corresponding points of the same ''phase (waves ...
s.[
;Double Fresnel rhomb: A double ]Fresnel rhomb
A Fresnel rhomb is an optical prism that introduces a 90° phase difference between two perpendicular components of polarization, by means of two total internal reflections. If the incident beam is linearly polarized at 45° to the plane of in ...
rotates the linear polarization axis by 90° using four internal reflections. A disadvantage may be a low ratio of useful optical aperture to length.[ J. M. Bennett and H. E. Bennett, Polarization, in ''Handbook of Optics'', W. G. Driscoll and W. Vaughan (eds.) (McGraw-Hill, New York, 1978). Chapter 10.]
;Broadband prismatic rotator: A broadband prismatic rotator rotates the linear polarization by 90° using seven internal reflections to induce collinear rotation, as shown in the diagram.
See also
* Optical rotation
Optical rotation, also known as polarization rotation or circular birefringence, is the rotation of the orientation of the plane of polarization about the optical axis of linearly polarized light as it travels through certain materials. Circul ...
References
{{Reflist
Optical devices
Polarization (waves)