Editing Optical isolator (section)

==The Faraday rotator==
{{main|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 (optics)|absorption]] coefficient, low [[non-linear]] [[refractive index]] and high [[laser damage threshold|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&nbsp;nm range are [[terbium doped borosilicate]] glass and [[terbium gallium garnet]] crystal (TGG). For long distance fibre communication, typically at 1310&nbsp;nm or 1550&nbsp;nm, [[yttrium iron garnet]] crystals are used (YIG). Commercial YIG based Faraday isolators reach isolations higher than 30 [[Decibel|dB]].

Optical isolators are different from 1/4 [[wave plate]] based isolators{{dubious|date=October 2016}}{{clarify|date=October 2016}} 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.