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.