Page 45 - Optical Communications Essentials

Source: Optical Communications Essentials

Chapter
3

The Behavior of Light

The concepts of how light travels along an optical fiber and how it interacts
with matter are essential to understanding why certain components are needed
and what their functions are in an optical fiber communication system. In this
chapter discussions on the properties of light cover the dual wave-particle
nature of light, the speed of light in different materials, reflection, refraction,
and polarization. These concepts relate to optical phenomena that we see every
day, such as light traveling through a solid (e.g., glass), reflection, and refrac-
tion. Obviously these factors also play a major role in optical fiber communica-
tions. So, let’s get “enlightened” with the following discussions.

3.1. The Dual Wave-Particle Nature of Light
The fundamental behavior of light is somewhat mysterious since some phe-
nomena can be explained by using a wave theory whereas in other cases light
behaves as though it is composed of miniature particles. This results in a dual
wave-particle nature of light. The wave nature is necessary to explain how light
travels through an optical fiber, but the particle theory is needed to explain how
optical sources generate signals and how photodetectors change these optical
signals to electric signals.
Light particles are known as photons, which have a certain energy associated
with them. As described in Sec. 3.3, the most common measure of photon
energy is the electron volt (eV), which is the energy a photon gains when mov-
ing through a 1-V electric field. Photons travel in straight lines called rays and
are used to explain certain light phenomena using the so-called ray theory or
geometric optics approach. This approach is valid when the object with which
the light interacts is much larger than the wavelength of the light. This theory
explains large-scale optical effects such as reflection and refraction (which are
described in Sec. 3.4) and describes how devices such as light sources, photode-
tectors, and optical amplifiers function.

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