Faraday Rotation

A linearly polarized wave propagates through the ferrite in the direction of bias,
and the polarization undergoes rotation proportional to the length of the ferrite. This
phenomenon is called Faraday rotation. Faraday rotation is a non-reciprocal effect.

Isolator

The isolator is a two-port non-reciprocal lossy device having unidirectional transmission characteristics:

The essential aspects of this passive device are
1. There is no attenuation when the wave propagates from port 1 to port 2.
2. When the wave propagates from port 1 to port 1 the attenuation is infinity.

The input card is in y-z plane¸ the dominant mode wave wherein the E-field vector is vertical traveling from left to right passes through resistive vane without
attenuation and enters the ferrite rod where it undergoes Faraday rotation of 450
clockwise. The wave again undergoes a rotation 450 in the anti-clockwise direction due to the twisted waveguide and the E-field vector at the output is vertical. The horizontal resistive vane does not affect the E-field as the same is vertical to its plane. Therefore the wave traveling from left to right pass though without any attenuation. Whereas the dominant mode wave entering from the right and traveling to the left undergoes a rotation 450 in the anti-clockwise direction due to the twisted waveguide. As it passes through the ferrite rod it again undergoes a rotation 450 in the anticlockwise direction and the E-field vector becomes horizontal. The resistive vane at the output which is in the horizontal plane opposes the energy as the E-filed vector is parallel to it. Therefore there is no output.

Gyrator

A gyrator is defined as a two-port device that has a relative difference in phase shift of 180o for transmission from port 1 to port 2 as compared with phase shift for transmission from port 2 to port 1. A gyrator may be obtained by employing the nonreciprocal property of Faraday rotation. The figure given below illustrates a typical microwave gyrator. It consists of a rectangular guide with a 90o twist connected to a circular guide. This in turn is connected to another rectangular guide at the other end. The two rectangular guides have the same orientation at the input ports. The circular guide contains a thin cylindrical rod of ferrite with the ends tapered to reduce reflections. A static axial magnetic field is applied so as to produce 90o Faraday rotation to the TE11 dominant mode in the circular guide.

Consider a wave propagating from left to right. In passing through the twist the plane of polarization is rotated by 90o in a counterclockwise direction. If the ferrite produced an additional 90o of rotation, the total angle of rotation will be 180o, as indicated in the figure above. For a wave propagating from right to left, the Faraday rotation is still 90o in the same sense. However, in passing through the twist, the next 90o of rotation is in a direction to cancel the Faraday rotation. Thus for transmission from port 2 to port1, there is no phase shift.