Purpose of an Isolator
Isolator is composed of a Farady rotator,which consists of a magneto-optic material contained in a magnetic field, and two suitably arrayed polarizers.The purpose of an isolator is to reduce or eliminate the effects of optical feedback, which could cause frequencey shift, limitation of modulation bandwidth, amplitude fluctuation,noise and even damage.It allows a light to pass through in one direction only.

Operation of an isolator
The forward mode
The incident light,whether or not polarized, passes through the input polarizer and becomes a linearly polarized light. When entering the Faraday rotator,the polarization plane of the linearly polarized light is rotated at 45o. It then enters the output polarizer which is tilted 45o. The light passes through the isolator and makes reflections, which constitute feedback.
The reverse mode
When re-entering the isolator,the feedback passes through the the output polarizer, where the feedback is re-polarized at 45o. It then enters the Faraday rotator and is rotated 45o in the same direction as the initial tilt. By making a total of 90o with respect to the input polarizer, the light is completely shut out here.