12.5   ISOLATORS

In many applications it is desired to have an optical fiber component through which
light should be able to propagate along one direction but not in the reverse direction.

Such devices are called isolators, as they isolate the input portion and from the output
portion from any reflections that may be occurring in the path of the output arm. Such
isolators are a very important component in any EDFA (Chapter 9).

The basic principle used in the realization of such a component is the Faraday
effect, discussed in Chapter 14. In such an effect, when linearly polarized light
propagates through an optical element that is subjected to a magnetic field applied
along the direction of propagation of the light beam, the plane of polarization of the
light beam rotates (Fig. 12.10). The angle of rotation depends on the material and on
the magnitude of the magnetic field and the length of the medium. The Faraday effect
is referred to as a nonreciprocal effect since if we now send a beam in the reverse
direction, then since the light wave is now propagating opposite to the direction of
the applied magnetic field, the rotation of the plane of polarization takes place along
the same direction and the output beam from the input side is now not parallel to the
input polarization (Fig. 12.10).

Now if the applied magnetic field is such as to cause a rotation of the plane of
polarization by 45^o, any reflected wave as it traverses through the medium will exit
with its state of polarization perpendicular to the input polarization state. If we place
a polarizer in the input path, the reflected light, having its polarization state normal
to the polarizer, will not be able to pass through, and thus any reflection from the
output end will be blocked. Figure 12.11 shows an optical isolator that allows light
to propagate from left to right but stops any beam traveling from right to left. Such
isolators are usually provided with fiber optic input and output ports (pigtails) and

are also very compact. They are usually characterized by a loss of 0.2 to 2 dB in the
forward direction, while the loss in the reverse direction could be 20 to 40 dB.

The principle used in making an isolator can also be used to build a circulator,
in which light entering one port (say, port 1) leaves the device via another port (say,
port 2), and light that enters from port 2 exits from another port (say, port 3)
(Fig. 12.12). Circulators are very useful devices in many applications, such as in
dispersion compensation using fiber Bragg gratings (Chapter 11).