08-11-2012, 05:57 PM
Optical Waveguides and Fibers
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The dramatic reduction of transmission loss in optical fibers coupled with equally important
developments in the area of light sources and detectors has brought about a phenomenal growth
of the fiber optic industry during the past two decades. The birth of optical fiber communication
coincided with the fabrication of low-loss optical fibers and room-temperature operation of
semiconductor lasers in 1970. Ever since, the scientific and technological progress in this field
has been so phenomenal that we are already in the fifth generation of optical fiber
communication systems within a brief span of 30 years. Recent developments in optical
amplifiers and wavelength division multiplexing (WDM) are taking us to a communication
system with almost “zero” loss and “infinite” bandwidth. Indeed, optical fiber communication
systems are fulfilling the increased demand on communication links, especially with the
proliferation of the Internet. This module, Optical Waveguides and Fibers, is an introduction to
the basics of fiber optics, discussing especially the characteristics of optical fibers as regards
their application to telecommunication (to be covered in Module 1.8, Fiber Optic
Telecommunication) and fiber optic sensors.
Objectives
The objective of the module is to present the basic characteristics of optical fibers that are
important from the point of view of their applications in telecommunication and sensing.
Characteristics such as attenuation, pulse dispersion, single-mode and multimode fibers, gradedindex,
fibers and zero-dispersion wavelength are covered.
When you finish this module, you will be able to
• Describe how light is guided through optical fibers.
• Differentiate between multimode and single-mode fibers.
• Calculate the numerical aperture (NA), intermodal dispersion, and material dispersion.
• Understand zero-dispersion wavelength and dispersion-shifted fibers.
• Know the importance of plastic optical fibers with regard to their application in
communication.
• Describe how optical fibers can be used in sensing.
Opening Demonstrations
Fiber optics is an exciting field, and the demonstration exercises that follow are sure to kindle
one’s interest in this fascinating area.
1. Demonstration of light guidance through optical fibers. Take a 2-meter length or so of
optical fiber—preferably a large-diameter plastic optical fiber—and make perpendicular
cuts at both the ends. Couple light from a laser pointer into one of the ends and see for
HISTORICAL INTRODUCTION
Communication implies transfer of information from one point to another. When it is necessary
to transmit information, such as speech, images, or data, over a distance, one generally uses the
concept of carrier wave communication. In such a system, the information to be sent modulates
an electromagnetic wave such as a radio wave, microwave, or light wave, which acts as a
carrier. This modulated wave is then transmitted to the receiver through a channel and the
receiver demodulates it to retrieve the imprinted signal. The carrier frequencies associated with
TV broadcast (∼ 50–900 MHz) are much higher than those associated with AM radio broadcast
(∼ 600 kHz–20 MHz). This is due to the fact that, in any communication system employing
electromagnetic waves as the carrier, the amount of information that can be sent increases as
the frequency of the carrier is increased.1 Obviously, TV broadcast has to carry much more
information than AM broadcasts. Since optical beams have frequencies in the range of
1014 to 1015 Hz, the use of such beams as the carrier would imply a tremendously large increase
in the information-transmission capacity of the system as compared to systems employing radio
waves or microwaves.