An optical isolator, or optical diode, is an optical component which allows the transmission of light in only one direction. It is typically used to prevent unwanted feedback into an optical oscillator, such as a laser cavity. The operation of the device depends on the Faraday effect (which in turn is produced by magneto-optic effect), which is used in the main component, the Faraday rotator.
The main component of the optical isolator is the Faraday rotator. The magnetic field, B, applied to the Faraday rotator causes a rotation in the polarization of the light due to the Faraday effect. The angle of rotation, β, is given by,
where, ν is the Verdet constant of the material (amorphous or crystalline; solid, liquid, or gaseous) of which the rotator is made, and d is the length of the rotator. This is shown in Figure 2. Specifically for an optical isolator, the values are chosen to give a rotation of 45 degrees.
Polarization dependent isolator
The polarization dependent isolator, or Faraday isolator, is made of three parts, an input polarizer (polarized vertically), a Faraday rotator, and an output polarizer, called an analyser (polarized at 45 degrees)
Light travelling in the forward direction becomes polarized vertically by the input polarizer. The Faraday rotator will rotate the polarization by 45 degrees. The analyser then enables the light to be transmitted through the isolator.
Light travelling in the backward direction becomes polarized at 45 degrees by the analyser. The Faraday rotator will again rotate the polarization by 45 degrees. This means the light is polarized horizontally (the rotation is sensitive to direction of propagation). Since the polarizer is vertically aligned, the light will be extinguished.
Figure 2 shows a Faraday rotator with an input polarizer, and an output analyser. For a polarization dependent isolator, the angle between the polarizer and the analyser, β, is set to 45 degrees. The Faraday rotator is chosen to give a 45 degree rotation.
Polarization dependent isolators are typically used in free space optical systems. This is because the polarization of the source is typically maintained by the system. In optical fibre systems, the polarization direction is typically dispersed in non polarization maintaining systems. Hence the angle of polarization will lead to a loss.
Polarization independent isolator
The polarization independent isolator is made of three parts, an input birefringent wedge (with its ordinary polarization direction vertical and its extraordinary polarization direction horizontal), a Faraday rotator, and an output birefringent wedge (with its ordinary polarization direction at 45 degrees, and its extraordinary polarization direction at -45 degrees).
Light travelling in the forward direction is split by the input birefringent wedge into its vertical (0 degrees) and horizontal (90 degrees) components, called the ordinary ray (o-ray) and the extraordinary ray (e-ray) respectively. The Faraday rotator rotates both the o-ray and e-ray by 45 degrees. This means the o-ray is now at 45 degrees, and the e-ray is at -45 degrees. The output birefringent wedge then recombines the two components.
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