Faraday Rotator & Isolator
1. Faraday rotator
A Faraday rotator is an optical device that rotates the polarization of light due to the Faraday Effect.
The Faraday rotator works because one polarization of the input light is in ferromagnetic resonance with the material which causes its phase velocity to be higher than the other. Specifically, given two rays of circularly polarized light, one with left-hand and the other with right-hand polarization, the phase velocity of the one with the polarization in the same sense as the precessing magnetization is greater.
Linearly polarized light can always be represented as a linear combination of circularly polarized light of left and right-handedness and equal amplitude. (See Jones calculus).
In other words, the plane of linearly polarized light is rotated when a magnetic field is applied parallel to the propagation direction. The empirical angle of rotation is given by:
Where β is the angle of rotation (in minutes of arc).
B is the magnetic flux density in the direction of propagation (in gauss).
d is the length of the path (in cm) where the light and magnetic field interact.
Then is the Verdet constant for the material. This empirical proportionality constant (in units of minutes of arc per gauss per cm of path, or in SI units, radians per tesla per metre) varies with wavelength and temperature and is tabulated for various materials.
Faraday rotators are used in Faraday isolators to prevent undesired back propagation of light from disrupting or damaging an optical system.
2. Faraday isolator
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 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.