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Faraday Rotators

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Definition: devices which can rotate the polarization state of light, exploiting the Faraday effect

German: Faraday-Rotatoren

Categories: general optics, photonic devices

How to cite the article; suggest additional literature

A Faraday rotator is a magneto-optic device, where light is transmitted through a transparent medium which is exposed to a magnetic field. The magnetic field lines have approximately the same direction as the beam direction, or the opposite direction. If the light is linearly polarized in some direction, this polarization direction is continuously rotated during the passage through the medium. The total rotation angle β can be calculated as

Faraday rotation angle

where V is the Verdet constant of the material, B is the magnetic flux density (in the direction of propagation), and L is the length of the rotator medium. Note that the Verdet “constant” usually exhibits a substantial wavelength dependence: it is smaller for longer wavelengths.

An important aspect is that the change of polarization direction is defined only by the magnetic field direction and the sign of the Verdet constant. If some linearly polarized beam is sent through a Faraday rotator and back again after reflection at a mirror, the polarization changes of the two passes add up, rather than canceling each other. This non-reciprocal behavior distinguishes Faraday rotators e.g. from arrangements of waveplates and polarizers.

Concerning the physical origin of the polarization rotation, one may consider a linearly polarized beam as a superposition of two circularly polarized beams. The magnetic field causes a difference in phase velocity between these circularly polarized components. The resulting relative phase shift corresponds to a change in the linear polarization direction.

Construction Details

The magnetic field is usually generated with an assembly of permanent magnets and ferromagnetic materials, which is optimized such that the following goals are more or less achieved:

These goals involve certain design trade-offs. In particular, a larger geometric cross section with good field homogeneity tends to require stronger magnets or to reduce the achievable field strength. For such reasons, devices are optimized for different purposes. There are heavy and expensive high-power devices with a large aperture as well as much cheaper miniature devices for low power levels.

Apart from a high Verdet constant, the Faraday medium should exhibit a high transparency in the spectral region of interest, a high optical quality, and sometimes also a high optical damage threshold. Often used Faraday media for the near-infrared spectral region are terbium–gallium garnet crystals (TGG) and terbium-doped borosilicate glass, which exhibit relatively large Verdet constants. A common choice for telecom components operating in the 1.3-μm or 1.5-μm spectral region is yttrium iron garnet (YIG).

For operation with high optical average powers, parasitic absorption in a Faraday rotator can lead to substantial internal heating and consequently to thermal beam distortions. In particular, thermal lensing can occur. Both the power-dependence and the significant aberrations of the thermal lens can be very disturbing. Additional aspects of high-power operation are discussed in the article on Faraday isolators.

In most cases, reflection losses on the input and output surface of a Faraday rotator are minimized with anti-reflection coatings, designed for the intended range of operation wavelengths. Note that the operation bandwidth can be limited not only by the coatings, but also by the wavelength dependence of the Verdet constant.

Applications

Faraday rotators find many applications in laser technology:

double-pass amplifier with Faraday mirror

Figure 1: Setup of a double-pass laser amplifier. The Faraday mirror on the right side ensures that the polarization state of light is not distorted after a double pass through the amplifier medium.

A variant of isolators are Faraday circulators, having three optical ports.

See also: Faraday isolators
and other articles in the categories general optics, photonic devices

In the RP Photonics Buyer's Guide, 37 suppliers for Faraday rotators are listed.

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