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
The Faraday effect or Faraday rotation, sometimes referred to as the magneto-optic Faraday effect (MOFE), is a physical magneto-optical phenomenon. The Faraday effect causes a polarization rotation which is proportional to the projection of the ...
, on
birefringence, or on
total internal reflection
Total internal reflection (TIR) is the optical 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 reflect ...
.
[ 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" is an acronym for "light amplification by stimulated emission of radiation". The fi ...
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
In electrodynamics, circular polarization of an electromagnetic wave is a polarization state in which, at each point, the electromagnetic field of the wave has a constant magnitude and is rotating at a constant rate in a plane perpendicular to ...
waves to propagate with slightly different phase velocities. 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
An optical isolator, or optical diode, is an optical component which allows the transmission of light in only one direction. It is typically used to prevent unwanted feedback into an optical oscillator, such as a laser cavity.
The operation ...
.
Birefringent rotators
Half-wave plate
A waveplate or retarder is an optical device that alters the polarization state of a light wave travelling through it. Two common types of waveplates are the ''half-wave plate'', which shifts the polarization direction of linearly polarized ligh ...
s and quarter-wave plate
A waveplate or retarder is an optical device that alters the polarization state of a light wave travelling through it. Two common types of waveplates are the ''half-wave plate'', which shifts the polarization direction of linearly polarized ligh ...
s alter the polarization of light due to the principle of birefringence. Their performance is wavelength-specific; a fact that may be a limitation. Switchable wave plates can also be manufactured out of liquid crystals, ferro-electric liquid crystals, or magneto-optic crystals. 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 (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, causing ...
(PMD) and polarization dependent loss (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 for PMD compensation 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, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats.
It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, t ...
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 inc ...
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
References
{{Reflist
Optical devices
Polarization (waves)