Page 191 - Introduction to Information Optics

176 3. Communication with Optics

3.2.3. OTHER ISSUES RELATED TO LIGHT PROPAGATING IN
OPTICAL FIBER

3.2.3.1. Attenuations in Optical Fiber
As discussed in Sees. 3.2.1 and 3.2.2, we know that light can be kept in
optical fiber based on the principle of total internal reflections. However, some
mechanisms can cause losses in optical fiber. Figure 3.5 shows attenuation as
a function of wavelength. When the operating wavelength / < 1.3 /mi, the loss
4
mainly conies from the Rayleigh scattering; that is, ocl/A . However, when
A > 1.6 /mi, the loss will become bigger and bigger due to infrared absorption.
There is also a loss peak around /I = 1.4/mi that is mainly due to the
absorption of —OH. Thus, to minimize the loss, current communication
systems are operated in the low-loss windows centered at 1,3/urn and/or
1.55/mi.
Note that recently it has been reported that this water absorption feature
has been almost completely removed for Lucent All Wave fiber, so a much
wider communication window (from 1.3 to 1.5 mm) can be used.

3.2.3.2. Fabricating Optical Fibers
Optical fibers are made by drawing a fiber mother rod that is called a
preform, as shown in Fig. 3.6, which has a typical diameter of a few centimeters.
The central part of the rod has a higher refractive index, obtained by doping
GeO 2. This part corresponds to the fiber core. The rod is heated to about

0.2

1.3 1.4 1.5

Fig. 3.5. Optical loss (or attenuation) in optical fiber.