Applications of Faraday Devices

The most common application for a Faraday rotator is when coupled with input and output polarisers to form an isolator. The rotation is then set at 45°. The key property of a Faraday rotator which distinguishes it from say a quartz rotation plate, is that the effect is uni-directional. A device which produces a clockwise rotation of the optical beam's polarisation axis through 45° when the beam propagates in one direction, will not reverse this effect for a beam passing back in the opposite direction, will produce an additional rotation of 45° for the reverse beam. This will then be at 90° to the input polarisation axis. This is demonstrated in the illustration below:

In the form of isolator shown, the rejected beams is not used and is simply discarded (in most applications it is not necessary to use a polariser with a side exit face, scattering of the rejected beam is sufficient). This would for example be suitable when only rejection of back-reflected radiation is required. One could however make use of these beams to form an optical circulator by the addition of a few extra components.

The commonest use of Faraday isolators is in preventing laser oscillators from being damaged by strong back reflections from subsequent optics and especially from certain types of amplifiers where a significantly enhanced back reflection may occur. They are also useful in preventing unwanted feedback from reducing the stability of line narrowed single frequency sources through mode pulling. Rotators are also used for example in ring laser systems to introduce a loss mechanism (in conjunction with some other intra-cavity polarisation selective element) which is greater for one direction of propagation than for the other. This ensures uni-directional response. Usually such devices are not required to work at such high rotation angles as the 45° found in isolators so may be made much smaller.

So if you think your laser system is suffering from the effects of back reflection, give us a call to discuss what a Faraday isolator can do for your system.