Page 179 - Introduction to Information Optics

164 3. Communication with Optics

This extremely high carrier frequency makes fiber-optic communication suit-
able for broadband communications.
2. Very low loss

Fiber-optic communication can have a very low loss. The loss can be as low
as 0.2 dB/Km, which makes long-distance communications possible. For
example, the total loss for 100-km fiber is as low as 20 dB, so there is no need
for amplification or regeneration for 100-km fiber. Note that at high frequency,
traditional copper wire has a very large loss.

3. Light weight and compact size
For high-speed communication, coaxial cable is very heavy and cumber-
some. However, fiber cable has a very compact size and light weight. The
diameter of optical fiber (core plus cladding) is as small as 125^m, which is
about twice the diameter of human hair.

4. Highly secure and immune to the external electromagnetic interference

The dielectric nature of optical fiber makes it immune to external elec-
tromagnetic interference and highly secure.

Example 3.1. Assume that an optical fiber has a length L = 1 km and attenu-
ation constant a = — 0.2 dB/km. What is the output power of the optical fiber
if the input power is 1 W?
Solve: Based on the definition of attenuation, we have
0 02
0 2 1 10
^om = *V W LI1 ° = 1 • KT - ' / - lO" - « 0.96 (W)
From Example 3.1, we can see that more than 95% of power energy can be
reserved in the optical fiber after propagating 1 km. Again, this shows the
low-attenuation feature of optical fiber.

3.2. LIGHT PROPAGATION IN OPTICAL FIBERS

3.2.1. GEOMETRIC OPTICS APPROACH
The basic principle of light propagating in optic fiber can be illustrated by
the simple geometric optics principle, i.e., total internal reflection.
The basic structure of optical fiber can be described by Fig. 3.1, which
includes three layers: (1) core layer; (2) cladding layer; and (3) protection jacket
layer. The core and cladding have refractive index n, and « 2, respectively. To