Chapter 2
Optical Fibers
The phenomenon oftotal internal reflection, responsible for guiding of light in opti-
cal fibers, has been known since 1854 [1]. Although glass fibers were made in the
1920s [2]–[4], their use became practical only in the 1950s, when the use of a cladding
layer led to considerable improvement in their guiding characteristics [5]–[7]. Before
1970, optical fibers were used mainly for medical imaging over short distances [8].
Their use for communication purposes was considered impractical because of high
losses (∼1000 dB/km). However, the situation changed drastically in 1970 when, fol-
lowing an earlier suggestion [9], the loss of optical fibers was reduced to below 20
dB/km [10]. Further progress resulted by 1979 in a loss of only 0.2 dB/km near the
1.55-μm spectral region [11]. The availability of low-loss fibers led to a revolution
in the field of lightwave technology and started the era of fiber-optic communications.
Several books devoted entirely to optical fibers cover numerous advances made in their
design and understanding [12]–[21]. This chapter focuses on the role of optical fibers
as a communication channel in lightwave systems. In Section 2.1 we use geometrical-
optics description to explain the guiding mechanism and introduce the related basic
concepts. Maxwell’s equations are used in Section 2.2 to describe wave propagation
in optical fibers. The origin of fiber dispersion is discussed in Section 2.3, and Section
2.4 considers limitations on the bit rate and the transmission distance imposed by fiber
dispersion. The loss mechanisms in optical fibers are discussed in Section 2.5, and
Section 2.6 is devoted to a discussion of the nonlinear effects. The last section covers
manufacturing details and includes a discussion of the design of fiber cables.
2.1 Geometrical-Optics Description
In its simplest form an optical fiber consists of a cylindrical core of silica glass sur-
rounded by a cladding whose refractive index is lower than that of the core. Because of
an abrupt index change at the core–cladding interface, such fibers are calledstep-index
fibers. In a different type of fiber, known asgraded-index fiber, the refractive index
decreases gradually inside the core. Figure 2.1 shows schematically the index profile
and the cross section for the two kinds of fibers. Considerable insight in the guiding