Optical isolator
It is typically used to prevent unwanted feedback into an optical oscillator, such as a laser cavity.
However, integrated isolators which do not rely on magnetism have been made in recent years too.
Specifically for an optical isolator, the values are chosen to give a rotation of 45°.
Figure 2 shows a Faraday rotator with an input polarizer, and an output analyzer.
Polarization dependent isolators are typically used in free space optical systems.
[6][7] Light traveling in the forward direction is split by the input birefringent wedge into its vertical (0°) and horizontal (90°) components, called the ordinary ray (o-ray) and the extraordinary ray (e-ray) respectively.
Light traveling in the backward direction is separated into the o-ray at 45, and the e-ray at −45° by the birefringent wedge.
In the transmitted direction the beam is split and then combined and focused into the output collimator.
Figure 3 shows the propagation of light through a polarization independent isolator.
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.
Optical isolators are different from 1/4 wave plate based isolators[dubious – discuss][clarification needed] because the Faraday rotator provides non-reciprocal rotation while maintaining linear polarization.
This is due to the change in the relative magnetic field direction, positive one way, negative the other.
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 the violation is avoided because the isolator must absorb (not reflect) the light from the hot object and will eventually reradiate it to the cold one.
