Isolator

An isolator is a passive non-reciprocal device. It allows transmission in one direction through it and blocks all transmission in other direction.
Isolator are used in systems before optical amplifiers and lasers mainly to prevent reflections from entering these devices otherwise performance will degrade.
Important parameters of an isolator are its insertion loss (in forward direction) and isolation (in reverse direction). The insertion loss should be as small as possible while isolation should be as large as possible. The typical insertion loss is around 1 dB and isolation is around 40 to 50 dB.

Isolator works on the principle of state of polarization (SOP) of light in a single mode The state of polarization (SOP) refers to the orientation of its electric field vector on a plane that is orthogonal to its direction of propagation. The electric field can be expressed as linear combination of two orthogonal linear polarization supported by fiber. These two polarization modes are horizontal and vertical modes. The principle of operation is illustrated in Fig. 7.4.1.

Let input light signal has vertical state of polarization (SOP) and blocks energy in horizontal SOP, The polarizer is followed by Faraday
Faraday rotator is an asymmetric device which rotates the SOP clockwise by 45^o in both direction of propagation. The polarizer after Faraday rotator passes only SOPs with 45^o In this way light signal from left to right is passed through the device without any loss.
Light entering the device from right due to reflection, with same 45^o SOP orientation, is rotated another 45^o by the Faraday and blocked by the next polarizer.