Isolators, rotators and Faraday rotators are components used in optical isolator and polarization changing devices in order to allow the transmission of light along one specific direction whilst blocking it on all others.

In this article, we’re going to take a look at the working principle of a high power faraday rotator and isolator.

Faraday Effect

As a magneto-optical effect, the Faraday effect produces circular birefringence, with polarized waves propagating at different speeds as they pass through an external magnetic field. In this way, the plane of polarization rotates linearly proportionally to the magnetic field component in the propagation direction. Calculating the total rotation angle β is as follows:

β=VBd

In this equation, β is the angle of rotation (in radians), B is the magnetic flux density (in Teslas), d is the path length from the point of interaction between the light and magnetic field, and V is the Verdet constant.

The empirical proportionality constant (in units of radians per tesla per meter, rad/(T·m)) is a measure of the intensity of radiation that is incident on an object. It varies with wavelength and temperature, and it is tabulated for various materials.

The change of polarization direction is defined only by the magnetic field direction and sign of the Verdet constant. If a linearly polarized beam is sent through a Faraday rotator and back again after reflection at a mirror, their two passes add up rather than canceling each other. This non-reciprocal behavior distinguishes Faraday rotators from arrangements of waveplates and polarizers.

When you consider the physical origin of polarization rotation, you may think of a linearly polarized beam as a superposition of two circularly polarized components. The magnetic field causes a difference in phase velocity between these circularly polarized components. The resulting relative phase shift corresponds to a change in the linear polarization direction.

Construction Details

The magnetic field is generated by an assembly of permanent magnets and ferromagnetic materials, which is optimized such that the following goals are met:

  • The field strength should be high enough to achieve a certain rotation angle (e.g., 45°) with a short-period medium, which reduces detrimental effects related to parasitic absorption and nonlinearities of the medium.
  • To produce a uniform rotation angle, the magnetic flux density should be as uniform as possible over the region where light is sent through the medium.

The design of a magnetic device involves trade-offs between different aspects of the device. For example, a larger geometric cross section requires stronger magnets or reduces the achievable field strength. The goal is to optimize for different purposes. There are heavy and expensive high-power devices with large apertures as well as much cheaper miniature devices for low power levels.

In the case of linearly polarized light, it can be thought of as a superposition of two circularly polarized beams. The magnetic field causes a difference in phase velocity between these circularly polarized components, resulting in a relative phase shift corresponding to a change in the linear polarization direction.