Also known as the Faraday rotator mirror, the Faraday mirror is made by combining a Faraday rotator with a mirror. Here, the rotation is 45 degrees. The functioning of a Faraday mirror is simple yet difficult for many people to understand. The light hits the reflector device after passing through the Faraday rotator, and then the light reflects at the rotator and finally passes through the Faraday mirror for a second time.

As the Faraday mirror applies a single-pass rotation by 45 degrees, the polarization state at the output always remains orthogonal. The only thing is the difference in the input.

The output polarization will be linear if the initial polarization is linear, but with an orthogonal direction. Here, the rotation is done twice by 45 degrees, leading to a total of 90 degrees. On the other hand, the output polarization will be circular if the initial polarization is circular. Here, the rotation is in the opposite sense.

The double-pass rotation of the devices is the major imperfection. This rotation deviates from exactly 90 degrees, apart from the insertion loss.

Applications of Faraday Mirrors or Faraday rotator mirrors

These devices improve the performance of fiber interferometers along with fiber amplifiers and fiber lasers. Faraday mirror placed at the end of a length of single mode fiber reverses any state of polarization perturbation to the polarization state caused by the fiber during the return trip. The return polarization is rotated to be 90 degrees, orthogonal to the input polarization.

This function is useful in a fiber interferometer, especially where undesired changes in polarization must be controlled for successful interference. The Faraday mirrors neutralize the undesired polarization effects.

Regarding fiber amplifiers, the light passes through a length of the fiber, such as an erbium-doped fiber amplifier, reflected by the Faraday mirror. The reflected or returning beam has a polarization direction orthogonal to that of the input beam. The polarizer is used to separate the counter-propagating beams. Not only for fiber amplifiers, but the principle is also applicable to fiber lasers.

You will find Faraday mirrors in fiber optics as well. Here, the input and output light goes through an optical fiber. The optical fiber is usually a single-mode fiber. The fiber optic Faraday mirrors exhibit a low insertion loss of below 1 dB. But, some devices exhibit higher losses of several decibels.

Often fiber-optic Faraday mirrors are used for similar purposes such as bulk-optic devices. They exploit the compensation of random birefringence in some length of passive or active fiber.

Applications of Faraday mirrors are vast. They are ruling many industries other than fiber optics. You just need to know your requirements and find out how Faraday mirrors can be utilized for the same.