Page 90 - Fiber Optic Communications Fund

Optical Fiber Transmission 71

direction. The loss coefficient due to Rayleigh scattering can be written as
1
∝ . (2.178)
R 4

Because of the strong wavelength dependence of Rayleigh scattering, short wavelengths (blue) are scattered
more than long wavelengths (red). The scattering at 400 nm is 9.2 times as great as that at 700 nm for equal
incident intensity. Rayleigh scattering of sunlight in a clear atmosphere is the reason why the sky is blue. This
also explains why the sun looks red in the morning/evening. The distance between the sun and an observer is
large in the morning/evening and the light has to go through a thicker atmosphere, causing the lower wave-
lengths (violet, blue) to suffer higher losses (see Eq. (2.178)) and their intensities would be too low to detect.
The dominant contributions to the fiber loss come from Rayleigh scattering in the wavelength range of
practical interest, 1550–1620 nm. One of the reasons why fiber-optic communication systems operate in the
infrared region instead of the visible region (400–700 nm) is that the loss due to Rayleigh scattering is much
smaller in the former region because of its −4 dependence. Fig. 2.34 shows the measured loss spectrum
of a single-mode fiber with 9.4 μm core diameter and Δ= 0.0019 [3]. As can be seen, the lowest fiber loss
occurs at 1.55 μm wavelength. For the silica fiber, at = 1.55 μm, the loss due to Rayleigh scattering alone
is = 0.1559 dB∕km. Thus, for the fiber shown in Fig. 2.34, 77% of the total loss at 1.55 μm comes from
R
Rayleigh scattering.
Conventional optical fibers are fabricated by doping the silica with GeO . The addition of a small amount
2
of GeO increases the refractive index and, therefore, enhancement of the core refractive index relative to
2
the cladding index is achieved. However, the addition of GeO increases the Rayleigh scattering. Therefore,
2
efforts have been made to fabricate pure silica core fibers (PSCFs) in which the core is pure silica [21, 22]. The
refractive index of cladding is reduced relative to the core index by adding a small amount of flourine. Since
most of the light is confined to the core, the PSCF has lower Rayleigh scattering coefficient than the conven-
tional silica–GeO core fiber. The attenuation of 1570 nm for PSCFs is 0.154 dB/km [21], which is the lowest
2
attenuation reported, whereas the lowest attenuation for the silica–GeO core fiber is about 0.19 dB/km.
2

2.7.2.2 Material Absorption
Material absorption can be divided into two types: (a) intrinsic absorption and (b) extrinsic absorption.

100
50
Loss (dB/km) 10 5

0.5 1

0.1
0.8 1.0 1.2 1.4 1.6
Wavelength (μm)
Figure 2.34 The measured loss spectrum of a single-mode fiber. Source: (After Ref. [3]. (c) IET. Reprinted with per-
mission from [3]. Copyright (1979) IET.