The single mode optical isolator is a passive magneto-optical device which uses the Faraday effect of magneto-optic crystal to isolate the reflected light and only allows the light to transmit in a single direction. The optical fiber isolators are used to protect light sources from adverse effects caused by back-reflection or signal.  

       1). Minimize Feedback into Optical Systems.

       2). Low insertion loss and high-power handling capability.

       3). Polarization independent structure.

1). The principle of polarization independent isolator

Wedge type polarization independent isolator: The isolator of this structure is usually suitable for 1064~2000nm, which have a small Faraday material and use for low power.

 

 The structure and optical path of polarization independent optical isolator are shown in Figure 2, the isolator is composed of two collimators (Not shown in the figure, one in and one out), one magnet ring, one Faraday rotator (FR, Faraday Rotation) and two wedge birefringent crystals. The optical axes of the two wedge s form an angle of 45° (Figure 2). The light from the input collimator would be divide into o light and e light to transmission by the Wedge 1. When they pass the Faraday rotator, the polarization direction is rotated 45° counterclockwise( Observe in the direction of forward light propagation, below would be the same), and there are no change with o and e light when they pass through the Wedge 2, so the polarization states of this two light in the two wedge are: o→o and e→e. The combination of the Wedge acts as a parallel plate to the forward light, the lights pass in the same direction and coupling into the output collimator; The light from the input collimator would be divide into o light and e light to transmission by the Wedge 2,when they pass the passing through the Faraday rotator, the polarization rotates 45 degrees counterclockwise, the o and e light would be change. So the polarization states of this two light in the two wedge are: o→e and e→o, this two combination of the Wedge would be a Wollaston polarizer this time, the backward light would deviate from original direction after passing, and they would not coupling into the input collimator. 

 

    Insertion loss refers to the additional loss caused by adding an optical isolator, and it is defined as the ratio of the optical power of the input and output ports of the optical passive components:

As shown in the formula above, Pout is the optical power of the output port, Pin is the optical power of the input port. The performance of isolator requires the insertion loss of forward light to be as small as possible. (Note: Generally, the calculation result is negative, but the negative sign is often omitted in practice.)

 

Take the red light as the example, the power for the input port Pin =100 mW, and the power of output port Pout=93 mW, so the IL of the channel 1 is:

IL = 10 × log (100/93)

= 10 × 0.032

= 0.32 dB

    Isolation refers to the isolation ability of the optical isolator to reverse reflected light. It is defined as the decibel ratio of the power value of the reverse incident optical signal to the power value of the reverse output optical signal:

As shown in the formula above, PRin is the power of the inverted input, PRout is the power of the inverted output. The performance of the device requires the bigger isolation, the greater the isolation value of reflected light, the better.

 

 

Take the red light as the example, power of the inverted input PRin =100 mW, and thepower of the inverted output PRout=0.6 mW, so the IL of the channel 1 is:

ISO = 10 × log (100/0.6)

= 10 ×2.22

= 22.2 dB