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A Faraday isolator or optical isolator is an optical component which allows the transmission of polarised light in only one direction. They are typically used to prevent unwanted feedback into an optical oscillator (A laser cavity is a good example.) The operation of the device depends on the Faraday effect (which in turn is produced by magneto-optic effects) which is used in the main component, the Faraday rotator.

An isolator is made of three parts, an input polarizer (for this discussion we will assume it's polarized up and down), a Faraday rotator, and an output polarizer (we will assume this one is 45° to the right.)

Light traveling in the forward direction becomes polarized (vertically in our case) by the input polarizer. The Faraday rotator will rotate the polarization 45° to the right. The output polarizer will allow all the light to escape and continue.

Light traveling in the backward direction becomes polarized (45°; to the right in this case) by the output polarizer. The Faraday Rotator will rotate the polarization 45° more to the right so that it is horizontally polarized (the rotation is insensitive to direction of propagation) and the input polarizer, which is vertically aligned, will block this light.

Faraday isolators are different from 1/4 wave plate based isolators because it can provide non-reciprocal rotation while maintaining linear polarization which allows higher isolation to be achieved.

A polarisation-independent isolator can be constructed by the use of a polarisation beam splitter to split the incoming light, followed by a pair Faraday isolators, each configured to operate on the light from their respective output of the beam splitter, followed by a polarisation beam combiner to reconstruct an output signal. This form of optical isolator is in wide-spread use in the optical communications industry where the polarisation of the signal arriving at the isolator cannot be controlled.

Obviously the most important optical element in a Faraday isolator is the Faraday rotator. The characteristics that one looks for in a Faraday rotator optic include a high Verdet constant, low absorption coefficient, low non-linear refractive index and high damage threshold. Also, to prevent self-focusing and other thermal related effects, the optic should be as short as possible. The two most commonly used materials for the 700-1100nm range are terbium doped borosillicate glass and terbium gallium garnet crystal (TGG). For long distance fiber communication, typically at 1310 nm or 1550 nm, yttrium iron garnet crystals are used (YIG). Commercial YIG based Faraday isolators reach isolations higher than 30 dB.

It might seem at first glance that a device that allows light to flow in only one direction would violate Kirchhoff's law and the second law of thermodynamics, by allowing light energy to flow from a cold object to a hot object and blocking it in the other direction, but this is not in fact a violation.^[1][2]

1. ^ Mungan, C.E. (1999). Faraday Isolators and Kirchhoff’s Law: A Puzzle (pdf). Retrieved on 2006-07-18.
2. ^ Rayleigh, "On the magnetic rotation of light and the second law of thermodynamics", Nature (London), Vol. 64, p. 577 (Oct. 10, 1901).

• Telecommunication isolators, good pictures