The Faraday Rotator
The most important optical element in an 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 700–1100 nm range are terbium doped borosilicate glass and terbium gallium garnet crystal (TGG). For long distance fibre 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.
Optical isolators are different from 1/4 wave plate based isolators because the Faraday rotator provides non-reciprocal rotation while maintaining linear polarization. That is, the polarization rotation due to the Faraday rotator is always in the same relative direction. So in the forward direction, the rotation is positive 45°. In the reverse direction, the rotation is −45°. This is due to the change in the relative magnetic field direction, positive one way, negative the other. This then adds to a total of 90° when the light travels in the forward direction and then the negative direction. This allows the higher isolation to be achieved.
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