Page 54 - Fiber Optic Communications Fund

2

Optical Fiber Transmission

2.1 Introduction

Until the mid-1970s, communication systems transmitted information over copper cables or free space. In
1966, Charles Kao and George Hockham working at Standard Telecommunications in the UK proposed that
an optical fiber might be used as a means of communication provided the signal loss could be much less than
20 dB/km [1]. They also illustrated that the attenuation in fibers available at that time was caused by impurities
which could be removed. At Corning Glass Works, Robert Maurer, Douald Keck, and Peter Schultz worked
with fused silica, a material that can be made extremely pure. In 1970, they developed a single-mode fiber with
attenuation below 20 dB/km [2]. In 1977, the first optical telecommunication system was installed about 1.5
miles under downtown Chicago and each optical fiber carried the equivalent of 672 voice channels. In 1979,
single-mode fibers with a loss of only 0.2 dB/km at 1550 nm were fabricated [3]. The availability of low-loss
fibers combined with the advent of semiconductor lasers led to a new era of optical fiber communication.
Today, more than 80% of the world’s long-distance traffic is carried over optical fiber cable and about 25
million kilometers of optical fiber has been installed worldwide.
This chapter deals with light propagation in optical fibers. Multi-mode and single-mode fibers are discussed
using a ray-optics description in Section 2.3. A rigorous solution of the wave equation is derived in Section 2.4,
and a wave-optics description of the single-mode and multi-mode fibers is presented. Pulse propagation in
single-mode fibers is discussed in Section 2.5. The comparison between single-mode fibers and multi-mode
fibers is made in Section 2.6. Section 2.7 focuses on the design of single-mode fibers.

2.2 Fiber Structure
An optical fiber consists of a central core clad with a material of slightly lower refractive index, as shown in
Fig. 2.1. If the refractive index of the core is constant, such a fiber is called a step-index fiber. Most of the
fibers are made from glass, i.e., silica. The refractive index of the core is increased by doping the silica with
GeO . The cladding is pure silica. A polymer jacket is used to protect the fiber from moisture and abrasion.
2
For short-distance (<1 km) and low-bit-rate (∼ Mb/s) transmission systems, plastic fibers can be used. They
are: (i) inexpensive, (ii) flexible, and (iii) easy to install and connect. However, they do not transmit light
efficiently because of high absorption. For long-distance and high-bit-rate systems, glass fibers are typically
used. Optical fibers have the following advantages over copper cable.
(i) Bandwidth: To transmit more bits of information in a given time period, the transmission medium should
have a high bandwidth. Typically, the bandwidth is of the order of the carrier frequency. In the case of

Fiber Optic Communications: Fundamentals and Applications, First Edition. Shiva Kumar and M. Jamal Deen.
© 2014 John Wiley & Sons, Ltd. Published 2014 by John Wiley & Sons, Ltd.