Page 52 - Optical Communications Essentials

The Behavior of Light

42 Chapter Three

Figure 3.7. Only the vertical polarization
component passes through this polarizer.

the refracted light is polarized depends on the angle at which the light approaches
the surface and on the material itself.
These polarization characteristics of light are important when we examine
the behavior of components such as optical isolators and light filters, which are
described in Chap. 9. Here we look at three polarization-sensitive materials or
devices that are used in such components. These are polarizers, Faraday rota-
tors, and birefringent crystals.

3.6.2. Polarizers
A polarizer is a material or device that transmits only one polarization compo-
nent and blocks the other. For example, when unpolarized light enters a polar-
izer that has a vertical transmission axis as shown in Fig. 3.7, only the vertical
polarization component passes through the device. As noted earlier, a familiar
application is the use of polarizing sunglasses. To see the polarization property
of the sunglasses you are wearing, tilt your head sideways. A number of glare
spots will then appear. The sunglasses block out the polarized light from these
spots when you hold your head normally.

3.6.3. Faraday rotators
A Faraday rotator is a device that rotates the state of polarization (SOP) of light
passing through it by a specific amount. For example, a popular device rotates
the SOP clockwise by 45° or one-quarter wavelength, as shown in Fig. 3.8. This
rotation is independent of the SOP of input light, but the rotation angle is dif-
ferent depending on the direction in which the light passes through the device.
That is, the rotation is not reciprocal. In addition, the SOP of the input light is
maintained after the rotation; for example, if the input light to a 45° Faraday
rotator is linearly polarized in a vertical direction, then the rotated light exiting
the crystal also is linearly polarized at a 45° angle. The material is usually some
type of asymmetric crystal such as yttrium iron garnet (YIG), and the degree of
angular rotation is proportional to the thickness of the device.

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