An optical isolator is transparent in one direction and it blocks light in opposite direction. The major function of an optical isolator is the protection of optical devices from unwanted back reflection.

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Faraday-rotator type optical isolator

Faraday-rotator type optical isolator consists of two polarizers and a 45-degree Faraday rotator. The angle between axis of the polarizers is 45 degrees.

Figure 1 shows light propagation through the isolator in the forward and backward directions.

In the forward direction:

The polarization of light rotates in the direction towards the axis of the output polarizer. Therefore, at the output polarizer the polarization of light is along the axis of the output polarizer and the light fully passes through it.

In the backward direction:

The polarization of light rotates in the direction opposite to the axis of the output polarizer. Therefore, at the output polarizer the polarization of light is perpendicular to the axis of the output polarizer and the light is fully blocked.